Creation of localized skyrmion bubbles in Co/Pt bilayers using a spin-valve nanopillar

NASA Astrophysics Data System (ADS)

Grab, Jennifer L.; Rugar, Alison E.; Ralph, Daniel C.

2018-05-01

We fabricate devices in which a magnetic nanopillar spin valve makes contact to a Co/Pt bilayer thin film with perpendicular magnetic anisotropy, to achieve local control of domains in the Co/Pt bilayer underneath the nanopillar. The goal is to develop the ability to nucleate, detect, and annihilate magnetic skyrmions in the Co/Pt using spin-polarized currents from the nanopillar. We demonstrate the ability to distinguish the local behavior of the Co/Pt film beneath the nanopillar from the extended film and show that the two can switch independently of each other. This allows us to isolate a localized domain under the pillar that can be controlled separately from the rest of the Co/Pt film using applied currents and magnetic fields. Micromagnetic simulations indicate that this localized domain has skyrmion symmetry. Our results represent a first step toward controlling room-temperature skyrmions using localized spin-transfer torque.

Coexistence of a two-states organization for a cell-penetrating peptide in lipid bilayer.

PubMed

Plénat, Thomas; Boichot, Sylvie; Dosset, Patrice; Milhiet, Pierre-Emmanuel; Le Grimellec, Christian

2005-12-01

Primary amphipathic cell-penetrating peptides transport cargoes across cell membranes with high efficiency and low lytic activity. These primary amphipathic peptides were previously shown to form aggregates or supramolecular structures in mixed lipid-peptide monolayers, but their behavior in lipid bilayers remains to be characterized. Using atomic force microscopy, we have examined the interactions of P(alpha), a primary amphipathic cell-penetrating peptide which remains alpha-helical whatever the environment, with dipalmitoylphosphatidylcholine (DPPC) bilayers. Addition of P(alpha) at concentrations up to 5 mol % markedly modified the supported bilayers topography. Long and thin filaments lying flat at the membrane surface coexisted with deeply embedded peptides which induced a local thinning of the bilayer. On the other hand, addition of P(alpha) only exerted very limited effects on the corresponding liposome's bilayer physical state, as estimated from differential scanning calorimetry and diphenylhexatriene fluorescence anisotropy experiments. The use of a gel-fluid phase separated supported bilayers made of a dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine mixture confirmed both the existence of long filaments, which at low peptide concentration were preferentially localized in the fluid phase domains and the membrane disorganizing effects of 5 mol % P(alpha). The simultaneous two-states organization of P(alpha), at the membrane surface and deeply embedded in the bilayer, may be involved in the transmembrane carrier function of this primary amphipathic peptide.

Direct atomic force microscopic evidence of hydrogen bonding interaction in phosphatidic acid Langmuir-Blodgett bilayer

NASA Astrophysics Data System (ADS)

Chunbo, Yuan; Ying, Wu; Yueming, Sun; Zuhong, Lu; Juzheng, Liu

1997-12-01

Molecularly resolved atomic force microscopic images of phosphatidic acid Langmuir-Blodgett bilayers show that phosphate groups in polar region of the films are packing in a distorted hexagonal organization with long-range orientational and positional order. Intermolecular hydrogen bonding interactions, which should be responsible for the ordering and stability of bilayers, are visualized directly between adjacent phosphate groups in the polar region of the bilayer. Some adjacent phosphatidic acid molecules link each other through the formation of intermolecular hydrogen bonds between phosphate groups in polar region to form local supramolecules, which provide the bilayer's potential as a functionized film in the investigation on the lateral conductions of protons in the biological bilayers.

Amphipathic amines affect membrane excitability in paramecium: role for bilayer couple.

PubMed Central

Browning, J L; Nelson, D L

1976-01-01

Amphipathic amines and local anesthetics stimulated reversal of the ciliary beating direction in wild-type Paramecium. Ca++ influx across the surface membrane and the consequent increase in internal Ca++ causes ciliary reversal and backward swimming. Mutant cells of the "Pawn" class, which lack a "gating" mechanism for regulating Ca++ influx, did not swim backwards in the presence of local anesthetics. Local anesthetics stimulated the passive efflux of K+ but had no effect on the active transport of K+ or Ca++. Apparently passive influx of Ca++ also was stimulated by local anesthetics as evidenced by their effects on swimming direction. These data can be interpreted in terms of the "bilayer couple" hypothesis of Sheetz and Singer [(1974) Proc. Nat. Acad. Sci. USA 71, 4457-4461]: amphipathic drugs affect cells by asymmetric insertion into one face of the lipid bilayer. As predicted by this hypothesis, the drugs' effects were seen only after a short time lag, and quaternary amines were less effective than tertiary amines. The effect on behavior was caused by any of several amphipathic cations, and the relative potency was a function of their hydrophobicity. Amphipathic anions, which according to the hypothesis would insert into the opposite face of the lipid bilayer, had little effect on ciliary reversal. Asymmetric perturbation of the lipid bilayer with amphipathic cations may trigger the opening of the Ca++ gate. Images PMID:1061147

Penetration of HIV-1 Tat47-57 into PC/PE Bilayers Assessed by MD Simulation and X-ray Scattering.

PubMed

Neale, Chris; Huang, Kun; García, Angel E; Tristram-Nagle, Stephanie

2015-09-22

The interactions of the basic, cell-penetrating region (Y47GRKKRRQRRR57) of the HIV-1 Tat protein with dioleoylphosphatidylcholine (DOPC) bilayers were previously assessed by comparing experimental X-ray diffuse scattering with atomistic molecular dynamics simulations. Here, we extend this investigation by evaluating the influence of phosphatidylethanolamine (PE) lipids. Using experimental bilayer form factors derivedfrom X-ray diffuse scattering data as a guide, our simulations indicate that Tat peptides localize close to the carbonyl-glycerol group in the headgroup region of bilayers composed of either DOPC or DOPC:DOPE (1:1) lipid. Our results also suggest that Tat peptides may more frequently insert into the hydrophobic core of bilayers composed of PC:PE (1:1) lipids than into bilayers composed entirely of PC lipids. PE lipids may facilitate peptide translocation across a lipid bilayer by stabilizing intermediate states in which hydrated peptides span the bilayer.

Accelerating Convergence in Molecular Dynamics Simulations of Solutes in Lipid Membranes by Conducting a Random Walk along the Bilayer Normal.

PubMed

Neale, Chris; Madill, Chris; Rauscher, Sarah; Pomès, Régis

2013-08-13

All molecular dynamics simulations are susceptible to sampling errors, which degrade the accuracy and precision of observed values. The statistical convergence of simulations containing atomistic lipid bilayers is limited by the slow relaxation of the lipid phase, which can exceed hundreds of nanoseconds. These long conformational autocorrelation times are exacerbated in the presence of charged solutes, which can induce significant distortions of the bilayer structure. Such long relaxation times represent hidden barriers that induce systematic sampling errors in simulations of solute insertion. To identify optimal methods for enhancing sampling efficiency, we quantitatively evaluate convergence rates using generalized ensemble sampling algorithms in calculations of the potential of mean force for the insertion of the ionic side chain analog of arginine in a lipid bilayer. Umbrella sampling (US) is used to restrain solute insertion depth along the bilayer normal, the order parameter commonly used in simulations of molecular solutes in lipid bilayers. When US simulations are modified to conduct random walks along the bilayer normal using a Hamiltonian exchange algorithm, systematic sampling errors are eliminated more rapidly and the rate of statistical convergence of the standard free energy of binding of the solute to the lipid bilayer is increased 3-fold. We compute the ratio of the replica flux transmitted across a defined region of the order parameter to the replica flux that entered that region in Hamiltonian exchange simulations. We show that this quantity, the transmission factor, identifies sampling barriers in degrees of freedom orthogonal to the order parameter. The transmission factor is used to estimate the depth-dependent conformational autocorrelation times of the simulation system, some of which exceed the simulation time, and thereby identify solute insertion depths that are prone to systematic sampling errors and estimate the lower bound of the amount of sampling that is required to resolve these sampling errors. Finally, we extend our simulations and verify that the conformational autocorrelation times estimated by the transmission factor accurately predict correlation times that exceed the simulation time scale-something that, to our knowledge, has never before been achieved.

Studies of the Superconducting Transition in the Mo/Au-Bilayer Thin Films

NASA Technical Reports Server (NTRS)

Sadleir, John; Smith, Stephen; Iyomoto, naoko; Bandler, Simon; Chervenak, Jay; Brown, Ari; Brekowsky, Regis; Kilbourne, Caroline; Robinson, Ian

2007-01-01

At NASA Goddard, microcalorimeter arrays using superconducting transition edge sensor thermometers (TESs) are under development for high energy resolution X-ray astrophysics applications. We report on our studies of the superconducting transition in our Mo/Au-bilayer TES films including: low current measurements of the superconducting bilayer's resistance transition versus temperature on pixels with different normal metal absorber attachment designs and measured temperature scaling of the critical current and critical magnetic field.

Proline kink angle distributions for GWALP23 in lipid bilayers of different thicknesses.

PubMed

Rankenberg, Johanna M; Vostrikov, Vitaly V; DuVall, Christopher D; Greathouse, Denise V; Koeppe, Roger E; Grant, Christopher V; Opella, Stanley J

2012-05-01

By using selected (2)H and (15)N labels, we have examined the influence of a central proline residue on the properties of a defined peptide that spans lipid bilayer membranes by solid-state nuclear magnetic resonance (NMR) spectroscopy. For this purpose, GWALP23 (acetyl-GGALW(5)LALALALALALALW(19)LAGA-ethanolamide) is a suitable model peptide that employs, for the purpose of interfacial anchoring, only one tryptophan residue on either end of a central α-helical core sequence. Because of its systematic behavior in lipid bilayer membranes of differing thicknesses [Vostrikov, V. V., et al. (2010) J. Biol. Chem. 285, 31723-31730], we utilize GWALP23 as a well-characterized framework for introducing guest residues within a transmembrane sequence; for example, a central proline yields acetyl-GGALW(5)LALALAP(12)ALALALW(19)LAGA-ethanolamide. We synthesized GWALP23-P12 with specifically placed (2)H and (15)N labels for solid-state NMR spectroscopy and examined the peptide orientation and segmental tilt in oriented DMPC lipid bilayer membranes using combined (2)H GALA and (15)N-(1)H high-resolution separated local field methods. In DMPC bilayer membranes, the peptide segments N-terminal and C-terminal to the proline are both tilted substantially with respect to the bilayer normal, by ~34 ± 5° and 29 ± 5°, respectively. While the tilt increases for both segments when proline is present, the range and extent of the individual segment motions are comparable to or smaller than those of the entire GWALP23 peptide in bilayer membranes. In DMPC, the proline induces a kink of ~30 ± 5°, with an apparent helix unwinding or "swivel" angle of ~70°. In DLPC and DOPC, on the basis of (2)H NMR data only, the kink angle and swivel angle probability distributions overlap those of DMPC, yet the most probable kink angle appears to be somewhat smaller than in DMPC. As has been described for GWALP23 itself, the C-terminal helix ends before Ala(21) in the phospholipids DMPC and DLPC yet remains intact through Ala(21) in DOPC. The dynamics of bilayer-incorporated, membrane-spanning GWALP23 and GWALP23-P12 are less extensive than those observed for WALP family peptides that have more than two interfacial Trp residues.

Proline Kink Angle Distributions for GWALP23 in Lipid Bilayers of Different Thickness†

PubMed Central

Rankenberg, Johanna M.; Vostrikov, Vitaly V.; DuVall, Christopher D.; Greathouse, Denise V.; Koeppe, Roger E.; Grant, Christopher V.; Opella, Stanley J.

2013-01-01

By using selected 2H and 15N labels, we have examined the influence of a central proline residue upon the properties of a defined peptide that spans lipid bilayer membranes by solid-state NMR spectroscopy. For this purpose, GWALP23 (acetyl-GGALW5LALALALALALALW19LAGA-ethanolamide) is a suitable model peptide that employs—for the purpose of interfacial anchoring—only one tryptophan residue on either end of a central alpha-helical core sequence. Because of its systematic behavior in lipid bilayer membranes of differing thickness (see J. Biol. Chem. 285, 31723), we utilize GWALP23 as a well-characterized framework for introducing guest residues within a transmembrane sequence; for example, a central proline yields acetyl-GGALW5LALALAP12ALALALW19LAGA-ethanolamide. We synthesized the GWALP23-P12 with specifically placed 2H and 15N labels for solid-state NMR spectroscopy, and examined the peptide orientation and segmental tilt in oriented DMPC lipid bilayer membranes using combined (2H)-GALA and (15N-1H) high resolution separated local field methods. In DMPC bilayer membranes, the peptide segments N-terminal and C-terminal to the proline are both tilted substantially with respect to the bilayer normal, by about 34° and 29° (± 5°), respectively. While the tilt increases for both segments when proline is present, the range and extent of the individual segment motions are comparable or less than those of the entire GWALP23 peptide in bilayer membranes. In DMPC, the proline induces a kink of about 30° (± 5°), with an apparent helix unwinding or “swivel” angle of about 70°. In DLPC and DOPC, based on 2H NMR data only, the kink angle and swivel angle probability distributions overlap those of DMPC, yet the most probable kink angle appears somewhat smaller than in DMPC. As has been described for GWALP23 itself, the C-terminal helix ends before Ala-21 in the phospholipids DMPC and DLPC, yet remains intact through Ala-21 in DOPC. The dynamics of bilayer-incorporated, membrane-spanning GWALP23 and GWALP23-P12 are less extensive than observed for WALP-family peptides that have more than two interfacial Trp residues. PMID:22489564

Molecular structure of the dioctadecyldimethylammonium bromide (DODAB) bilayer.

PubMed

Jamróz, Dorota; Kepczynski, Mariusz; Nowakowska, Maria

2010-10-05

Dioctadecyldimethylammonium bromide (DODAB) is a double-chained quaternary ammonium surfactant that assembles in water into bilayer structures. This letter reports the molecular dynamics (MD) computer simulations of the DODAB bilayer at 25 °C. The simulations show that the surfactant membrane arranges spontaneously into the rippled phase (P(β)(')) at that temperature. The ordering within the chain fragment closest to the hydrophilic head (carbon atoms 1-5) is relatively low. It grows significantly for the carbon atoms located in the center of the membrane (atoms 6-17). The C6-C17 chain fragments are well aligned and tilted by ca. 15° with respect to the bilayer normal.

Lipid vesicles and other colloids as drug carriers on the skin.

PubMed

Cevc, Gregor

2004-03-27

Colloids from an aqueous suspension can cross the skin barrier only through hydrophilic pathways. Various colloids have a different ability to do this by penetrating narrow pores of fixed size in the skin, or the relevant nano-pores in barriers modelling the skin. Such ability is governed by colloid adaptability, which must be high enough to allow penetrant deformation to the size of a pore in such barrier: for a 100 nm colloid trespassing the skin this means at least 5-fold deformation/elongation. (Lipid) Bilayer vesicles are normally more adaptable than the comparably large (lipid coated) fluid droplets. One of the reasons for this, and an essential condition for achieving a high bilayer adaptability and pore penetration, is a high bilayer membrane elasticity. The other reason is the relaxation of changing colloid's volume-to-surface constraint during pore penetration; it stands to reason that such relaxation requires a concurrent, but only transient and local, bilayer permeabilisation. Both these phenomena are reflected in bilayer composition sensitivity, which implies non-linear pressure dependency of the apparent barrier penetrability, for example. Amphipats that acceptably weaken a membrane (surfactants, (co)solvents, such as certain alcohols, etc.) consequently facilitate controlled, local bilayer destabilisation and increase lipid bilayer flexibility. When used in the right quantity, such additives thus lower the energetic expense for elastic bilayer deformation, associated with pore penetration. Another prerequisite for aggregate transport through the skin is the colloid-induced opening of the originally very narrow ( approximately 0.4 nm) gaps between cells in the barrier to pores with diameter above 30 nm. Colloids incapable of enforcing such widening-and simultaneously of self-adapting to the size of 20-30 nm without destruction-are confined to the skin surface. All relatively compact colloids seem to fall in this latter category. This includes mixed lipid micelles, solid (nano)particles, nano-droplets, biphasic vesicles, etc. Such colloids, therefore, merely enter the skin through the rare wide gaps between groups of skin cells near the organ surface. Transdermal drug delivery systems based on corresponding drug formulations, therefore, rely on simple drug diffusion through the skin; the colloid then, at best, can modulate drug transport through the barrier. In contrast, the adaptability-and stability-optimised mixed lipid vesicles (Transfersomes, a trademark of IDEA AG) can trespass much narrower pathways between most cells in the skin; such highly adaptable colloids thus mediate drug transport through the skin. Sufficiently stable ultra-adaptable carriers, therefore, can ensure targeted drug delivery deep below the application site. This has already been shown in numerous preclinical tests and several phase I and phase II clinical studies. Drug delivery by means of highly adaptable drug carriers, moreover, allows highly efficient and well-tolerated drug targeting into the skin proper. Sustained drug release through the skin into systemic blood circulation is another field of ultradeformable drug carrier application.

Continuity of monolayer-bilayer junctions for localization of lipid raft microdomains in model membranes

DOE PAGES

Ryu, Yong -Sang; Wittenberg, Nathan J.; Suh, Jeng -Hun; ...

2016-05-27

We show that the selective localization of cholesterol-rich domains and associated ganglioside receptors prefer to occur in the monolayer across continuous monolayer-bilayer junctions (MBJs) in supported lipid membranes. For the MBJs, glass substrates were patterned with poly(dimethylsiloxane) (PDMS) oligomers by thermally-assisted contact printing, leaving behind 3 nm-thick PDMS patterns. The hydrophobicity of the transferred PDMS patterns was precisely tuned by the stamping temperature. Lipid monolayers were formed on the PDMS patterned surface while lipid bilayers were on the bare glass surface. Due to the continuity of the lipid membranes over the MBJs, essentially free diffusion of lipids was allowed betweenmore » the monolayer on the PDMS surface and the upper leaflet of the bilayer on the glass substrate. The preferential localization of sphingomyelin, ganglioside GM1 and cholesterol in the monolayer region enabled to develop raft microdomains through coarsening of nanorafts. Furthermore, our methodology provides a simple and effective scheme of non-disruptive manipulation of the chemical landscape associated with lipid phase separations, which leads to more sophisticated applications in biosensors and as cell culture substrates.« less

Continuity of Monolayer-Bilayer Junctions for Localization of Lipid Raft Microdomains in Model Membranes

PubMed Central

Ryu, Yong-Sang; Wittenberg, Nathan J.; Suh, Jeng-Hun; Lee, Sang-Wook; Sohn, Youngjoo; Oh, Sang-Hyun; Parikh, Atul N.; Lee, Sin-Doo

2016-01-01

We show that the selective localization of cholesterol-rich domains and associated ganglioside receptors prefer to occur in the monolayer across continuous monolayer-bilayer junctions (MBJs) in supported lipid membranes. For the MBJs, glass substrates were patterned with poly(dimethylsiloxane) (PDMS) oligomers by thermally-assisted contact printing, leaving behind 3 nm-thick PDMS patterns. The hydrophobicity of the transferred PDMS patterns was precisely tuned by the stamping temperature. Lipid monolayers were formed on the PDMS patterned surface while lipid bilayers were on the bare glass surface. Due to the continuity of the lipid membranes over the MBJs, essentially free diffusion of lipids was allowed between the monolayer on the PDMS surface and the upper leaflet of the bilayer on the glass substrate. The preferential localization of sphingomyelin, ganglioside GM1 and cholesterol in the monolayer region enabled to develop raft microdomains through coarsening of nanorafts. Our methodology provides a simple and effective scheme of non-disruptive manipulation of the chemical landscape associated with lipid phase separations, which leads to more sophisticated applications in biosensors and as cell culture substrates. PMID:27230411

Strain and curvature induced evolution of electronic band structures in twisted graphene bilayer.

PubMed

Yan, Wei; He, Wen-Yu; Chu, Zhao-Dong; Liu, Mengxi; Meng, Lan; Dou, Rui-Fen; Zhang, Yanfeng; Liu, Zhongfan; Nie, Jia-Cai; He, Lin

2013-01-01

It is well established that strain and geometry could affect the band structure of graphene monolayer dramatically. Here we study the evolution of local electronic properties of a twisted graphene bilayer induced by a strain and a high curvature, which are found to strongly affect the local band structures of the twisted graphene bilayer. The energy difference of the two low-energy van Hove singularities decreases with increasing lattice deformation and the states condensed into well-defined pseudo-Landau levels, which mimic the quantization of massive chiral fermions in a magnetic field of about 100 T, along a graphene wrinkle. The joint effect of strain and out-of-plane distortion in the graphene wrinkle also results in a valley polarization with a significant gap. These results suggest that strained graphene bilayer could be an ideal platform to realize the high-temperature zero-field quantum valley Hall effect.

Interplay of local structure, charge, and spin in bilayered manganese perovskites

NASA Astrophysics Data System (ADS)

Rybicki, Damian; Sikora, Marcin; Przewoznik, Janusz; Kapusta, Czesław; Mitchell, John F.

2018-03-01

Chemical doping is a reliable method of modification of the electronic properties of transition metal compounds. In manganese perovskites, it leads to charge transfer and peculiar ordering phenomena. However, depending on the interplay of the local crystal structure and electronic properties, synthesis of stable compounds in the entire doping range is often impossible. Here, we show results of high-energy resolution x-ray absorption and emission spectroscopies on a La2 -2 xSr1 +2 xMn2O7 family of bilayered manganites in a broad doping range (0.5 ≤x ≤1 ). We established a relation between local Mn charge and Mn-O distances as a function of doping. Based on a comparison of such relation with other manganites, we suggest why stable structures cannot be realized for certain doping levels of bilayered compounds.

Proton Diffusion through Bilayer Pores

DOE PAGES

McDaniel, Jesse G.; Yethiraj, Arun

2017-09-26

The transport of protons through channels in complex environments is important in biology and materials science. In this work, we use multistate empirical valence bond simulations to study proton transport within a well-defined bilayer pore in a lamellar L β phase lyotropic liquid crystal (LLC). The LLC is formed from the self-assembly of dicarboxylate gemini surfactants in water, and a bilayer-spanning pore of radius of approximately 3–5 Å results from the uneven partitioning of surfactants between the two leaflets of the lamella. Local proton diffusion within the pore is significantly faster than diffusion at the bilayer surface, which is duemore » to the greater hydrophobicity of the surfactant/water interface within the pore. Proton diffusion proceeds by surface transport along exposed hydrophobic pockets at the surfactant/water interface and depends on the continuity of hydronium–water hydrogen bond networks. At the bilayer surface, there is a reduced fraction of the “Zundel” intermediates that are central to the Grotthuss transport mechanism, whereas the fraction of these species within the bilayer pore is similar to that in bulk water. Our results demonstrate that the chemical nature of the confining interface, in addition to confinement length scale, is an important determiner of local proton transport in nanoconfined aqueous environments.« less

Elastic energy of polyhedral bilayer vesicles

PubMed Central

Haselwandter, Christoph A.; Phillips, Rob

2011-01-01

In recent experiments the spontaneous formation of hollow bilayer vesicles with polyhedral symmetry has been observed. On the basis of the experimental phenomenology it was suggested that the mechanism for the formation of bilayer polyhedra is minimization of elastic bending energy. Motivated by these experiments, we study the elastic bending energy of polyhedral bilayer vesicles. In agreement with experiments, and provided that excess amphiphiles exhibiting spontaneous curvature are present in sufficient quantity, we find that polyhedral bilayer vesicles can indeed be energetically favorable compared to spherical bilayer vesicles. Consistent with experimental observations we also find that the bending energy associated with the vertices of bilayer polyhedra can be locally reduced through the formation of pores. However, the stabilization of polyhedral bilayer vesicles over spherical bilayer vesicles relies crucially on molecular segregation of excess amphiphiles along the ridges rather than the vertices of bilayer polyhedra. Furthermore, our analysis implies that, contrary to what has been suggested on the basis of experiments, the icosahedron does not minimize elastic bending energy among arbitrary polyhedral shapes and sizes. Instead, we find that, for large polyhedron sizes, the snub dodecahedron and the snub cube both have lower total bending energies than the icosahedron. PMID:21797397

Proximity Effects and Nonequilibrium Superconductivity in Transition-Edge Sensors

NASA Technical Reports Server (NTRS)

Sadleir, John E.; Smith, Stephen J.; Robinson, Ian K.; Finkbeiner, Fred M.; Chervenak, James A.; Bandler, Simon R.; Eckart, Megan E.; Kilbourne, Caroline A.

2011-01-01

We have recently shown that normal-metal/superconductor (N/S) bilayer TESs (superconducting Transition-Edge Sensors) exhibit weak-link behavior.l Here we extend our understanding to include TESs with added noise-mitigating normal-metal structures (N structures). We find TESs with added Au structures also exhibit weak-link behavior as evidenced by exponential temperature dependence of the critical current and Josephson-like oscillations of the critical current with applied magnetic field. We explain our results in terms of an effect converse to the longitudinal proximity effect (LoPE) 1, the lateral inverse proximity effect (LaiPE), for which the order parameter in the N/S bilayer is reduced due to the neighboring N structures. Resistance and critical current measurements are presented as a function of temperature and magnetic field taken on square Mol Au bilayer TESs with lengths ranging from 8 to 130 {\\mu}m with and without added N structures. We observe the inverse proximity effect on the bilayer over in-plane distances many tens of microns and find the transition shifts to lower temperatures scale approximately as the inverse square of the in- plane N-structure separation distance, without appreciable broadening of the transition width. We also present evidence for nonequilbrium superconductivity and estimate a quasiparticle lifetime of 1.8 \\times 10-10 s for the bilayer. The LoPE model is also used to explain the increased conductivity at temperatures above the bilayer's steep resistive transition.

Indole Localization in an Explicit Bilayer Revealed via Molecular Dynamics

NASA Astrophysics Data System (ADS)

Norman, Kristen

2005-11-01

It is well known that the amino-acid tryptophan is particularly stable in the interfacial region of biological membranes, and this preference is a property of the tryptophan side-chain. Analogues of this side-chain, such as indole, strongly localize in the interfacial region, especially near the glycerol moiety of the lipids in the bilayer. Using molecular dynamics calculations, we determine the potential of mean force (PMF) for indoles in the bilayer. We compare the calculated PMF for indole with that of benzene to show that exclusion from the center of the lipid bilayer does not occur in all aromatics, but is strong in indoles. We find three minima in the PMF. Indole is most stabilized near the glycerol moiety. A weaker binding location is found near the choline groups of the lipid molecules. An even weaker binding side is found near the center of the lipid hydrocarbon core. Comparisions between uncharged, weakly charged, and highly charged indoles demonstrate that the exclusion is caused by the charge distribution on the indole rather than the ``lipo-phobic'' effect. High temperature simulations are used to determine the relative contribution of enthalpy and entropy to indole localization. The orientation of indole is found to be largely charge independent and is a strong function of depth within the bilayer. We find good agreement between simulated SCD order parameters for indole and experimentally determined order parameters.

Longitudinal Proximity Effect, Lateral Inverse Proximity Effect, and Nonequilibrium Superconductivity in Transition-edge Sensors

NASA Technical Reports Server (NTRS)

Sadleir, John E.

2010-01-01

We have recently shown that normal-metal/superconductor (N/S) bilayer TESs (superconducting Transition-Edge Sensors) exhibit weak-link behavior. Our measurements were explained in terms of a longitudinal proximity effect model in which superconducting order from the higher transition temperature leads is induced into the TES bilayer plane over remarkably long distances (up to 290 micron). Here we extend our understanding to include TESs with added noise-mitigating normal-metal structures (N structures). We explain our results of an effect converse to the longitudinal proximity effect (LoPE), the lateral inverse proximity effect (LaiPE), for which the order parameter in the N/S bilayer is reduced due to the neighboring N structures. We present resistance and critical current measurements as a function of temperature and magnetic field taken on square Mo/Au bilayer TESs with lengths ranging from 8 to 130 micron with and without added N structures. We observe the inverse proximity effect on the bilayer over in-plane distances many tens of microns and find the transition shifts to lower temperature scale approximately as the inverse square of the in-plane N-structure separation distance, without appreciable broadening of the transition width. We find TESs with added Au structures exhibit weak-link behavior as evidenced by exponential temperature dependence of the critical current and Josephson-like oscillations of the critical current with applied magnetic field. We also present evidence for nonequilbrium superconductivity and estimate a quasiparticle lifetime of 1.8 x 10(exp -10) s for the bilayer. The LoPE model is also used to explain the increased conductivity at temperatures above the bilayer's steep resistive transition

Longitudinal Proximity Effect, Lateral Inverse Proximity Effect, and Nonequilibrium Superconductivity in Transition-Edge Sensors

NASA Technical Reports Server (NTRS)

Sadleir, John E.

2010-01-01

We have recently shown that normal-metal/superconductor (N /S) bilayer TESs (superconducting Transition-Edge Sensors) exhibit weak-link behavior. Our measurements were explained in terms of a longitudinal proximity effect model in which superconducting order from the higher transition temperature leads is induced into the TES bilayer plane over remarkably long distances (up to 290 micron). Here we extend our understanding to include TESs with added noise-mitigating normal-metal structures (N structures). We explain our results in terms of an effect converse to the longitudinal proximity effect (LoPE), the lateral inverse proximity effect (LaiPE), for which the order parameter in the N /S bilayer is reduced due to the neighboring N structures. We present resistance and critical current measurements as a function of temperature and magnetic field taken on square Mo/Au bilayer TESs with lengths ranging from 8 to 130 micron with and without added N structures. We observe the inverse proximity effect on the bilayer over in-plane distances many tens of microns and find the transition shifts to lower temperatures scale approximately as the inverse square of the in-plane N-structure separation distance, without appreciable broadening of the transition width. We find TESs with added Au structures exhibit weak-link behavior as evidenced by exponential temperature dependence of the critical current and Josephson-like oscillations of the critical current with applied magnetic field. We also present evidence for nonequilbrium superconductivity and estimate a quasiparticle lifetime of 1.8 x 10(exp -10) s for the bilayer. The LoPE model is also used to explain the increased conductivity at temperatures above the bilayer's steep resistive transition.

Charge 2e/3 Superconductivity and Topological Degeneracies without Localized Zero Modes in Bilayer Fractional Quantum Hall States.

PubMed

Barkeshli, Maissam

2016-08-26

It has been recently shown that non-Abelian defects with localized parafermion zero modes can arise in conventional Abelian fractional quantum Hall (FQH) states. Here we propose an alternate route to creating, manipulating, and measuring topologically protected degeneracies in bilayer FQH states coupled to superconductors, without the creation of localized parafermion zero modes. We focus mainly on electron-hole bilayers, with a ±1/3 Laughlin FQH state in each layer, with boundaries that are proximity coupled to a superconductor. We show that the superconductor induces charge 2e/3 quasiparticle-pair condensation at each boundary of the FQH state, and that this leads to (i) topologically protected degeneracies that can be measured through charge sensing experiments and (ii) a fractional charge 2e/3 ac Josephson effect. We demonstrate that an analog of non-Abelian braiding is possible, despite the absence of a localized zero mode. We discuss several practical advantages of this proposal over previous work, and also several generalizations.

Interplay of local structure, charge, and spin in bilayered manganese perovskites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rybicki, Damian; Sikora, Marcin; Przewoznik, Janusz

Chemical doping is a reliable method of modification of the electronic properties of transition metal compounds. In manganese perovskites, it leads to charge transfer and peculiar ordering phenomena. However, depending on the interplay of the local crystal structure and electronic properties, synthesis of stable compounds in the entire doping range is often impossible. In this paper, we show results of high-energy resolution x-ray absorption and emission spectroscopies on amore » $${\\mathrm{La}}_{2{-}2x}{\\mathrm{Sr}}_{1+2x}{\\mathrm{Mn}}_{2}{\\mathrm{O}}_{7}$$ family of bilayered manganites in a broad doping range $$(0.5{\\le}x{\\le}1)$$. We established a relation between local Mn charge and Mn-O distances as a function of doping. Finally, based on a comparison of such relation with other manganites, we suggest why stable structures cannot be realized for certain doping levels of bilayered compounds.« less

Interplay of local structure, charge, and spin in bilayered manganese perovskites

DOE PAGES

Rybicki, Damian; Sikora, Marcin; Przewoznik, Janusz; ...

2018-03-27

Chemical doping is a reliable method of modification of the electronic properties of transition metal compounds. In manganese perovskites, it leads to charge transfer and peculiar ordering phenomena. However, depending on the interplay of the local crystal structure and electronic properties, synthesis of stable compounds in the entire doping range is often impossible. In this paper, we show results of high-energy resolution x-ray absorption and emission spectroscopies on amore » $${\\mathrm{La}}_{2{-}2x}{\\mathrm{Sr}}_{1+2x}{\\mathrm{Mn}}_{2}{\\mathrm{O}}_{7}$$ family of bilayered manganites in a broad doping range $$(0.5{\\le}x{\\le}1)$$. We established a relation between local Mn charge and Mn-O distances as a function of doping. Finally, based on a comparison of such relation with other manganites, we suggest why stable structures cannot be realized for certain doping levels of bilayered compounds.« less

Controlling the layer localization of gapless states in bilayer graphene with a gate voltage

NASA Astrophysics Data System (ADS)

Jaskólski, W.; Pelc, M.; Bryant, Garnett W.; Chico, Leonor; Ayuela, A.

2018-04-01

Experiments in gated bilayer graphene with stacking domain walls present topological gapless states protected by no-valley mixing. Here we research these states under gate voltages using atomistic models, which allow us to elucidate their origin. We find that the gate potential controls the layer localization of the two states, which switches non-trivially between layers depending on the applied gate voltage magnitude. We also show how these bilayer gapless states arise from bands of single-layer graphene by analyzing the formation of carbon bonds between layers. Based on this analysis we provide a model Hamiltonian with analytical solutions, which explains the layer localization as a function of the ratio between the applied potential and interlayer hopping. Our results open a route for the manipulation of gapless states in electronic devices, analogous to the proposed writing and reading memories in topological insulators.

Raman imaging of lipid bilayer membrane by surface enhanced Raman scattering

NASA Astrophysics Data System (ADS)

Mori, Motoaki; Abe, Shunsuke; Kondo, Takahiro; Saito, Yuika

2018-04-01

We investigated two-dimensional lipid bilayers by spectroscopic imaging with surface enhanced Raman spectroscopy (SERS). A DSPC lipid bilayer incubated on a glass substrate was coated with a thin layer of silver. Due to the strong electromagnetic enhancement of the silver film and the affinity to lipid molecules, the Raman spectrum of a single bilayer was obtained in a 1 s exposure time with 0.1 mW of incident laser power. In the C-H vibrational region of the spectra, which is sensitive to bilayer configurations, a randomly stacked area was dominated by the CH3 asymmetric-stretch mode, whereas flat areas including double bilayers showed typical SERS spectra. The spectral features of the randomly stacked area are explained by the existence of many free lipid molecules, which is supported by DFT calculations of paired DSPC molecules. Our method can be applied to reveal the local crystallinity of single lipid bilayers, which is difficult to assess by conventional Raman imaging.

Interactions of the local anesthetic tetracaine with membranes containing phosphatidylcholine and cholesterol: a /sup 2/H NMR study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Auger, M.; Jarrell, H.C.; Smith, I.C.P.

1988-06-28

The interactions of local anesthetic tetracaine with multilamellar dispersions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol have been investigated by deuterium nuclear magnetic resonance of specifically deuteriated tetracaines, DMPC and cholesterol. Experiments were performed at pH 5.5, when the anesthetic is primarily charged, and at pH 9.5, when it is primarily uncharged. The partition coefficients of the anesthetic in the membrane have been measured at both pH values for phosphatidylcholine bilayers with and without cholesterol. The higher partition coefficients obtained at pH 9.5 reflect the hydrophobic interactions between the uncharged form of the anesthetic and the hydrocarbon region of the bilayer. Themore » lower partition coefficients for the DMPC/cholesterol system at both pH values suggest that cholesterol, which increases the order of the lipid chains, decreases the solubility of tetracaine into the bilayer. For phosphatidylcholine bilayers, it has been proposed that the charged tetracaine at low pH is located mostly at the phospholipid headgroup level while the uncharged tetracaine intercalates more deeply into the bilayer. The present study suggests that the location of tetracaine in the cholesterol-containing system is different from that in pure phosphatidylcholine bilayers: the anesthetic sits higher in the membrane. An increase in temperature results in a deeper penetration of the anesthetic into the bilayer. Moreover, the incorporation of the anesthetic into DMPC bilayers with or without cholesterol results in a reduction of the lipid order parameters both in the plateau and in the tail regions of the acyl chains, this effect being greater with the charged form of the anesthetic.« less

Interaction of PLGA and trimethyl chitosan modified PLGA nanoparticles with mixed anionic/zwitterionic phospholipid bilayers studied using molecular dynamics simulations

NASA Astrophysics Data System (ADS)

Novak, Brian; Astete, Carlos; Sabliov, Cristina; Moldovan, Dorel

2012-02-01

Poly(lactic-co-glycolic acid) (PLGA) is a biodegradable polymer. Nanoparticles of PLGA are commonly used for drug delivery applications. The interaction of the nanoparticles with the cell membrane may influence the rate of their uptake by cells. Both PLGA and cell membranes are negatively charged, so adding positively charged polymers such as trimethyl chitosan (TMC) which adheres to the PLGA particles improves their cellular uptake. The interaction of 3 nm PLGA and TMC-modified-PLGA nanoparticles with lipid bilayers composed of mixtures of phosphatidylcholine and phosphatidylserine lipids was studied using molecular dynamics simulations. The free energy profiles as function of nanoparticles position along the normal direction to the bilayers were calculated, the distribution of phosphatidylserine lipids as a function of distance of the particle from the bilayer was calculated, and the time scale for particle motion in the directions parallel to the bilayer surface was estimated.

Fluorescent molecular probes based on excited state prototropism in lipid bilayer membrane

NASA Astrophysics Data System (ADS)

Mohapatra, Monalisa; Mishra, Ashok K.

2012-03-01

Excited state prototropism (ESPT) is observed in molecules having one or more ionizable protons, whose proton transfer efficiency is different in ground and excited states. The interaction of various ESPT molecules like naphthols and intramolecular ESPT (ESIPT) molecules like hydroxyflavones etc. with different microheterogeneous media have been studied in detail and excited state prototropism as a probe concept has been gaining ground. The fluorescence of different prototropic forms of such molecules, on partitioning to an organized medium like lipid bilayer membrane, often show sensitive response to the local environment with respect to the local structure, physical properties and dynamics. Our recent work using 1-naphthol as an ESPT fluorescent molecular probe has shown that the incorporation of monomeric bile salt molecules into lipid bilayer membranes composed from dipalmitoylphosphatidylcholine (DPPC, a lung surfactant) and dimyristoylphosphatidylcholine (DMPC), in solid gel and liquid crystalline phases, induce appreciable wetting of the bilayer up to the hydrocarbon core region, even at very low (<= 1 mM) concentrations of the bile salts. The incorporation and location of fisetin, an ESIPT molecule having antioxidant properties, in lipid bilayer membrane has been sensitively monitored from its intrinsic fluorescence behaviour.

Addition and subtraction of spin pumping voltages in magnetic hybrid structures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Azevedo, A., E-mail: aac@df.ufpe.br; Alves Santos, O.; Cunha, R. O.

2014-04-14

We report an investigation of the spin pumping voltage generated in bilayers of ferromagnetic/normal metal in which the ferromagnetic layer is yttrium iron garnet or Permalloy and the normal-metal layer is Pt or Ta. We also investigated a special case in which the voltage is detected in single layer of Permalloy under ferromagnetic resonance condition. It is shown that the spin pumping voltage generated in metallic bilayers have contributions from both layers and the resulting voltage depends on the relative signs of charge currents generated by the inverse spin Hall effect. For instance, the spin pumping voltage generated in Tamore » has the same sign as the one generate in single layer of Permalloy, but contrary to the voltage generated in Pt. When the voltage is measured in shunted metallic bilayers, the resulting voltage can be a sum or a subtraction of the voltages generated in both layers.« less

Helical edge states and fractional quantum Hall effect in a graphene electron-hole bilayer

NASA Astrophysics Data System (ADS)

Sanchez-Yamagishi, Javier D.; Luo, Jason Y.; Young, Andrea F.; Hunt, Benjamin M.; Watanabe, Kenji; Taniguchi, Takashi; Ashoori, Raymond C.; Jarillo-Herrero, Pablo

2017-02-01

Helical 1D electronic systems are a promising route towards realizing circuits of topological quantum states that exhibit non-Abelian statistics. Here, we demonstrate a versatile platform to realize 1D systems made by combining quantum Hall (QH) edge states of opposite chiralities in a graphene electron-hole bilayer at moderate magnetic fields. Using this approach, we engineer helical 1D edge conductors where the counterpropagating modes are localized in separate electron and hole layers by a tunable electric field. These helical conductors exhibit strong non-local transport signals and suppressed backscattering due to the opposite spin polarizations of the counterpropagating modes. Unlike other approaches used for realizing helical states, the graphene electron-hole bilayer can be used to build new 1D systems incorporating fractional edge states. Indeed, we are able to tune the bilayer devices into a regime hosting fractional and integer edge states of opposite chiralities, paving the way towards 1D helical conductors with fractional quantum statistics.

Periodic minimal surface organizations of the lipid bilayer at the lung surface and in cubic cytomembrane assemblies.

PubMed

Larsson, Marcus; Larsson, Kåre

2014-03-01

The existence of infinite periodic lipid bilayer structures in biological systems was first demonstrated in cell membrane assemblies. Such periodicity is only possible in symmetric bilayers, and their occurrence is discussed here in relation to the asymmetry of cell membranes in vivo. A periodic membrane conformation in the prolamellar body of plants corresponds to a dormant state without photosynthesis. A similar reversible formation of a dormant state has also been observed in the mitochondria of the amoeba Chaos. In these cases the energy production has become insufficient to maintain the membrane asymmetry. Formation of membranes that are symmetric over the bilayer is proposed to be a principal mechanism behind formation of cubic membrane systems. Another type of bicontinuous minimal surface structure is considered to form the alveolar lining of mammals at normal breathing conditions. The CLP surface corresponds to such a tetragonal surface phase. It is also a symmetric bilayer and in a state of zero energy expenditure. Structural alternatives of the bilayer conformation in this latter system are also discussed here. © 2013 Elsevier B.V. All rights reserved.

Topological mosaics in moiré superlattices of van der Waals heterobilayers

NASA Astrophysics Data System (ADS)

Tong, Qingjun; Yu, Hongyi; Zhu, Qizhong; Wang, Yong; Xu, Xiaodong; Yao, Wang

2017-04-01

Van der Waals (vdW) heterostructures formed by two-dimensional atomic crystals provide a powerful approach towards designer condensed matter systems. Incommensurate heterobilayers with small twisting and/or lattice mismatch lead to the interesting concept of moiré superlattices, where the atomic registry is locally indistinguishable from commensurate bilayers but has local-to-local variation over long range. Here we show that such moiré superlattices can lead to periodic modulation of local topological order in vdW heterobilayers formed by two massive Dirac materials. By tuning the vdW heterojunction from normal to the inverted type-II regime via an interlayer bias, the commensurate heterobilayer can become a topological insulator (TI), depending on the interlayer hybridization controlled by the atomic registry between the vdW layers. This results in a mosaic pattern of TI regions and normal insulator (NI) regions in moiré superlattices, where topologically protected helical modes exist at the TI/NI phase boundaries. By using symmetry-based k .p and tight-binding models, we predict that this topological phenomenon can be present in inverted transition metal dichalcogenides heterobilayers. Our work points to a new means of realizing programmable and electrically switchable topological superstructures from two-dimensional arrays of TI nano-dots to one-dimensional arrays of TI nano-stripes.

Diffusion of dihydropyridine calcium channel antagonists in cardiac sarcolemmal lipid multibilayers.

PubMed Central

Chester, D W; Herbette, L G; Mason, R P; Joslyn, A F; Triggle, D J; Koppel, D E

1987-01-01

A membrane bilayer pathway model has been proposed for the interaction of dihydropyridine (DHP) calcium channel antagonists with receptors in cardiac sarcolemma (Rhodes, D.G., J.G. Sarmiento, and L.G. Herbette. 1985. Mol. Pharmacol. 27:612-623) involving drug partition into the bilayer with subsequent receptor binding mediated (though probably not rate-limited) by diffusion within the bilayer. Recently, we have characterized the partition step, demonstrating that DHPs reside, on a time-average basis, near the bilayer hydrocarbon core/water interface. Drug distribution about this interface may define a plane of local concentration for lateral diffusion within the membrane. The studies presented herein examine the diffusional dynamics of an active rhodamine-labeled DHP and a fluorescent phospholipid analogue (DiIC16) in pure cardiac sarcolemmal lipid multibilayer preparations as a function of bilayer hydration. At maximal bilayer hydration, the drug diffuses over macroscopic distances within the bilayer at a rate identical to that of DiI (D = 3.8 X 10(-8) cm2/s), demonstrating the overall feasibility of the membrane diffusion model. The diffusion coefficients for both drug and lipid decreased substantially as the bilayers were dehydrated. While identical at maximal hydration, drug diffusion was significantly slower than that of DiIC16 in partially dehydrated bilayers, probably reflecting differences in mass distribution of these probes in the bilayer. Images FIGURE 1 FIGURE 2 FIGURE 3 FIGURE 4 PMID:2447967

Thermodynamic study of benzocaine insertion into different lipid bilayers

NASA Astrophysics Data System (ADS)

Cascales, J. J. López; Costa, S. D. Oliveira; Porasso, R. D.

2011-10-01

Despite the general consensus concerning the role played by sodium channels in the molecular mechanism of local anesthetics, the potency of anaesthetic drugs also seems to be related with their solubility in lipid bilayers. In this respect, this work represents a thermodynamic study of benzocaine insertion into lipid bilayers of different compositions by means of molecular dynamics simulation. Thus, the free energy profiles associated with benzocaine insertion into symmetric lipid bilayers composed of different proportions of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylserine were studied. From the simulation results, a maximum in the free energy (ΔG) profile was measured in the region of the lipid/solution interface. This free energy barrier appears to be very much dependent on the lipid composition of the membrane. On the other hand, the minimum free energy (ΔG) within the bilayer remained almost independent of the lipid composition of the bilayer. By repeating the study at different temperatures, it was seen how the spontaneity of benzocaine insertion into the lipid bilayer is due to an increase in the entropy associated with the process.

Thermodynamic study of benzocaine insertion into different lipid bilayers.

PubMed

Cascales, J J López; Costa, S D Oliveira; Porasso, R D

2011-10-07

Despite the general consensus concerning the role played by sodium channels in the molecular mechanism of local anesthetics, the potency of anaesthetic drugs also seems to be related with their solubility in lipid bilayers. In this respect, this work represents a thermodynamic study of benzocaine insertion into lipid bilayers of different compositions by means of molecular dynamics simulation. Thus, the free energy profiles associated with benzocaine insertion into symmetric lipid bilayers composed of different proportions of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylserine were studied. From the simulation results, a maximum in the free energy (ΔG) profile was measured in the region of the lipid/solution interface. This free energy barrier appears to be very much dependent on the lipid composition of the membrane. On the other hand, the minimum free energy (ΔG) within the bilayer remained almost independent of the lipid composition of the bilayer. By repeating the study at different temperatures, it was seen how the spontaneity of benzocaine insertion into the lipid bilayer is due to an increase in the entropy associated with the process. © 2011 American Institute of Physics

Probing Lipid Bilayers under Ionic Imbalance.

PubMed

Lin, Jiaqi; Alexander-Katz, Alfredo

2016-12-06

Biological membranes are normally under a resting transmembrane potential (TMP), which originates from the ionic imbalance between extracellular fluids and cytosols, and serves as electric power storage for cells. In cell electroporation, the ionic imbalance builds up a high TMP, resulting in the poration of cell membranes. However, the relationship between ionic imbalance and TMP is not clearly understood, and little is known about the effect of ionic imbalance on the structure and dynamics of biological membranes. In this study, we used coarse-grained molecular dynamics to characterize a dipalmitoylphosphatidylcholine bilayer system under ionic imbalances ranging from 0 to ∼0.06 e charges per lipid (e/Lip). We found that the TMP displayed three distinct regimes: 1) a linear regime between 0 and 0.045 e/Lip, where the TMP increased linearly with ionic imbalance; 2) a yielding regime between ∼0.045 and 0.060 e/Lip, where the TMP displayed a plateau; and 3) a poration regime above ∼0.060 e/Lip, where we observed pore formation within the sampling time (80 ns). We found no structural changes in the linear regime, apart from a nonlinear increase in the area per lipid, whereas in the yielding regime the bilayer exhibited substantial thinning, leading to an excess of water and Na + within the bilayer, as well as significant misalignment of the lipid tails. In the poration regime, lipid molecules diffused slightly faster. We also found that the fluid-to-gel phase transition temperature of the bilayer dropped below the normal value with increased ionic imbalances. Our results show that a high ionic imbalance can substantially alter the essential properties of the bilayer, making the bilayer more fluid like, or conversely, depolarization of a cell could in principle lead to membrane stiffening. Copyright © 2016 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Effect of quantum tunneling on spin Hall magnetoresistance

NASA Astrophysics Data System (ADS)

Ok, Seulgi; Chen, Wei; Sigrist, Manfred; Manske, Dirk

2017-02-01

We present a formalism that simultaneously incorporates the effect of quantum tunneling and spin diffusion on the spin Hall magnetoresistance observed in normal metal/ferromagnetic insulator bilayers (such as Pt/Y3Fe5O12) and normal metal/ferromagnetic metal bilayers (such as Pt/Co), in which the angle of magnetization influences the magnetoresistance of the normal metal. In the normal metal side the spin diffusion is known to affect the landscape of the spin accumulation caused by spin Hall effect and subsequently the magnetoresistance, while on the ferromagnet side the quantum tunneling effect is detrimental to the interface spin current which also affects the spin accumulation. The influence of generic material properties such as spin diffusion length, layer thickness, interface coupling, and insulating gap can be quantified in a unified manner, and experiments that reveal the quantum feature of the magnetoresistance are suggested.

Testing Pullout Strength of Pedicle Screw Using Synthetic Bone Models: Is a Bilayer Foam Model a Better Representation of Vertebra?

PubMed

Varghese, Vicky; Krishnan, Venkatesh; Saravana Kumar, Gurunathan

2018-06-01

A biomechanical study. A new biomechanical model of the vertebra has been developed that accounts for the inhomogeneity of bone and the contribution of the pedicle toward the holding strength of a pedicle screw. Pullout strength studies are typically carried out on rigid polyurethane foams that represent the homogeneous vertebral framework of the spine. However, the contribution of the pedicle region, which contributes to the inhomogeneity in this framework, has not been considered in previous investigations. Therefore, we propose a new biomechanical model that can account for the vertebral inhomogeneity, especially the contribution of the pedicles toward the pullout strength of the pedicle screw. A bilayer foam model was developed by joining two foams representing the pedicle and the vertebra. The results of the pullout strength tests performed on the foam models were compared with those from the tests performed on the cadaver lumbar vertebra. Significant differences ( p <0.05) were observed between the pullout strength of the pedicle screw in extremely osteoporotic (0.18±0.11 kN), osteoporotic (0.37±0.14 kN), and normal (0.97±0.4 kN) cadaver vertebra. In the monolayer model, significant differences ( p <0.05) were observed in pullout strength between extremely osteoporotic (0.3±0.02 kN), osteoporotic (0.65±0.12 kN), and normal (0.99±0.04 kN) bone model. However, the bilayer foam model exhibited no significant differences ( p >0.05) in the pullout strength of pedicle screws between osteoporotic (0.85±0.08 kN) and extremely osteoporotic bone models (0.94±0.08 kN), but there was a significant difference ( p <0.05) between osteoporotic (0.94±0.08 kN) and normal bone models (1.19±0.05 kN). There were no significant differences ( p >0.05) in pullout strength between cadaver and bilayer foam model in normal bones. The new synthetic bone model that reflects the contribution of the pedicles to the pullout strength of the pedicle screws could provide a more efficacious means of testing pedicle-screw pullout strength. The bilayer model can match the pullout strength value of normal lumbar vertebra bone whereas the monolayer foam model was able to match that of the extremely osteoporotic lumbar vertebra.

Development of a slip sensor using separable bilayer with Ecoflex-NBR film

NASA Astrophysics Data System (ADS)

Kim, Sung Joon; Moon, Hyungpil; Choi, Hyouk Ryeol; Koo, Ja Choon

2017-04-01

Polymer film-type slip sensor is presented by using novel working principle rather than measuring micro-vibration. The sensor is comprised of bilayer with Ecoflex and NBR(acrylonitrile butadiene rubber) films divided by di-electric. When slip occur on surface, bilayer have relative displacement from each other because friction-induced vibration make a clearance between two layers. This displacement can be obtained by capacitance difference. CNT(carbon nanotube) was employed for electrode because of flexible and stretchable characteristics. Also normal and shear force can be decoupled by the working principle. To verify developed sensor, slip test apparatus was designed and experiments were conducted.

Lipopolysaccharide Membrane Building and Simulation

PubMed Central

Jo, Sunhwan; Wu, Emilia L.; Stuhlsatz, Danielle; Klauda, Jeffery B.; Widmalm, Göran; Im, Wonpil

2015-01-01

Summary While membrane simulations are widely employed to study the structure and dynamics of various lipid bilayers and membrane proteins in the bilayers, simulations of lipopolysaccharides (LPS) in membrane environments have been limited due to its structural complexity, difficulties in building LPS-membrane systems, and lack of appropriate molecular force field. In this work, as a first step to extend CHARMM-GUI Membrane Builder to incorporate LPS molecules and to explore their structures and dynamics in membrane environments using molecular dynamics simulations, we describe step-by-step procedures to build LPS bilayer systems using CHARMM and the recently developed CHARMM carbohydrate and lipid force fields. Such procedures are illustrated by building various bilayers of Escherichia coli O6 LPS and their preliminary simulation results are given in terms of per-LPS area and density distributions of various components along the membrane normal. PMID:25753722

Performance of an X-ray Microcalorimeter with a 240 μm Absorber and a 50 μm TES Bilayer

NASA Astrophysics Data System (ADS)

Miniussi, Antoine R.; Adams, Joseph S.; Bandler, Simon R.; Chervenak, James A.; Datesman, Aaron M.; Eckart, Megan E.; Ewin, Audrey J.; Finkbeiner, Fred M.; Kelley, Richard L.; Kilbourne, Caroline A.; Porter, Frederick S.; Sadleir, John E.; Sakai, Kazuhiro; Smith, Stephen J.; Wakeham, Nicholas A.; Wassell, Edward J.; Yoon, Wonsik

2018-05-01

Superconducting transition-edge sensor (TES) microcalorimeters are being developed for a variety of potential astrophysics missions, including Athena. The X-ray integral field unit instrument on this mission requires close-packed pixels on a 0.25 mm pitch, and high quantum efficiency between 0.2 and 12 keV. In this work, we describe a new approach with 50 μm square TESs consisting of a Mo/Au bilayer, deposited on silicon nitride membranes to provide a weak thermal conductance to a 50 mK heat bath. Larger TESs usually have additional normal metal stripes on top of the bilayer to reduce the noise. However, we have found that excellent spectral performance can be achieved without the need for any normal metal stripes on top of the TES. A spectral performance of 1.58 ± 0.12 eV at 5.9 keV has been achieved, the best resolution seen in any of our devices with this pixel size.

Structure and Thermotropic phase Behavior of Fluorinated Phospholipid Bilayers: A combined Attenuated Total Reflection FTIR Spectroscopy and Imaging Ellipsometry Study

PubMed Central

Schuy, Steffen; Faiss, Simon; Yoder, Nicholas C.; Kalsani, Venkateshwarlu; Kumar, Krishna; Janshoff, Andreas; Vogel, Reiner

2008-01-01

Lipid bilayers consisting of lipids with terminally perfluoroalkylated chains have remarkable properties. They exhibit increased stability and phase-separated nanoscale patterns in mixtures with nonfluorinated lipids. In order to understand the bilayer properties that are responsible for this behavior, we have analyzed the structure of solid-supported bilayers composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and of a DPPC analogue with 6 terminal perfluorinated methylene units (F6-DPPC). Polarized attenuated total reflection Fourier-transform infrared spectroscopy indicates that for F6-DPPC, the tilt of the lipid acyl chains to the bilayer normal is increased to 39° as compared to 21° for native DPPC, for both lipids in the gel phase. This substantial increase of the tilt angle is responsible for a decrease of the bilayer thickness from 5.4 nm for DPPC to 4.5 nm for F6-DPPC, as revealed by temperature-controlled imaging ellipsometry on microstructured lipid bilayers and solution atomic force microscopy. During the main phase transition from the gel to the fluid phase, both the relative bilayer thickness change and the relative area change are substantially smaller for F6-DPPC than for DPPC. In light of these structural and thermotropic data, we propose a model in which the higher acyl-chain tilt angle in F6-DPPC is the result of a conformational rearrangement to minimize unfavorable fluorocarbon–hydrocarbon interactions in the center of the bilayer due to chain staggering. PMID:18563929

Observing a model ion channel gating action in model cell membranes in real time in situ: membrane potential change induced alamethicin orientation change.

PubMed

Ye, Shuji; Li, Hongchun; Wei, Feng; Jasensky, Joshua; Boughton, Andrew P; Yang, Pei; Chen, Zhan

2012-04-11

Ion channels play crucial roles in transport and regulatory functions of living cells. Understanding the gating mechanisms of these channels is important to understanding and treating diseases that have been linked to ion channels. One potential model peptide for studying the mechanism of ion channel gating is alamethicin, which adopts a split α/3(10)-helix structure and responds to changes in electric potential. In this study, sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), has been applied to characterize interactions between alamethicin (a model for larger channel proteins) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers in the presence of an electric potential across the membrane. The membrane potential difference was controlled by changing the pH of the solution in contact with the bilayer and was measured using fluorescence spectroscopy. The orientation angle of alamethicin in POPC lipid bilayers was then determined at different pH values using polarized SFG amide I spectra. Assuming that all molecules adopt the same orientation (a δ distribution), at pH = 6.7 the α-helix at the N-terminus and the 3(10)-helix at the C-terminus tilt at about 72° (θ(1)) and 50° (θ(2)) versus the surface normal, respectively. When pH increases to 11.9, θ(1) and θ(2) decrease to 56.5° and 45°, respectively. The δ distribution assumption was verified using a combination of SFG and ATR-FTIR measurements, which showed a quite narrow distribution in the angle of θ(1) for both pH conditions. This indicates that all alamethicin molecules at the surface adopt a nearly identical orientation in POPC lipid bilayers. The localized pH change in proximity to the bilayer modulates the membrane potential and thus induces a decrease in both the tilt and the bend angles of the two helices in alamethicin. This is the first reported application of SFG to the study of model ion channel gating mechanisms in model cell membranes. © 2012 American Chemical Society

The Probable Explanation for the Low Friction of Natural Joints.

PubMed

Pawlak, Zenon; Urbaniak, Wieslaw; Hagner-Derengowska, Magdalena; Hagner, Wojciech

2015-04-01

The surface of an articular cartilage, coated with phospholipid (PL) bilayers, plays an important role in its lubrication and movement. Intact (normal) and depleted surfaces of the joint were modelled and the pH influence on the surface interfacial energy, wettability and friction were investigated. In the experiments, the deterioration of the PL bilayer was controlled by its wettability and the applied friction. The surrounding fluid of an undamaged articular cartilage, the synovial fluid, has a pH value of approximately 7.4. Buffer solutions were formulated to represent the synovial fluid with various pH values. It was found that the surface interfacial energy was stabilised at its lowest values when the pH varied between 6.5 and 9.5. These results suggested that as the PL bilayers deteriorated, the hydration repulsion mechanism became less effective as friction increased. The decreased number of bilayers changed the wettability and lowered PL lubricant properties.

Maximally asymmetric transbilayer distribution of anionic lipids alters the structure and interaction with lipids of an amyloidogenic protein dimer bound to the membrane surface

PubMed Central

Cheng, Sara Y.; Chou, George; Buie, Creighton; Vaughn, Mark W.; Compton, Campbell; Cheng, Kwan H.

2016-01-01

We used molecular dynamics simulations to explore the effects of asymmetric transbilayer distribution of anionic phosphatidylserine (PS) lipids on the structure of a protein on the membrane surface and subsequent protein–lipid interactions. Our simulation systems consisted of an amyloidogenic, beta-sheet rich dimeric protein (D42) absorbed to the phosphatidylcholine (PC) leaflet, or protein-contact PC leaflet, of two membrane systems: a single-component PC bilayer and double PC/PS bilayers. The latter comprised of a stable but asymmetric transbilayer distribution of PS in the presence of counterions, with a 1-component PC leaflet coupled to a 1-component PS leaflet in each bilayer. The maximally asymmetric PC/PS bilayer had a non-zero transmembrane potential (TMP) difference and higher lipid order packing, whereas the symmetric PC bilayer had a zero TMP difference and lower lipid order packing under physiologically relevant conditions. Analysis of the adsorbed protein structures revealed weaker protein binding, more folding in the N-terminal domain, more aggregation of the N- and C-terminal domains and larger tilt angle of D42 on the PC leaflet surface of the PC/PS bilayer versus the PC bilayer. Also, analysis of protein-induced membrane structural disruption revealed more localized bilayer thinning in the PC/PS versus PC bilayer. Although the electric field profile in the non-protein-contact PS leaflet of the PC/PS bilayer differed significantly from that in the non-protein-contact PC leaflet of the PC bilayer, no significant difference in the electric field profile in the protein-contact PC leaflet of either bilayer was evident. We speculate that lipid packing has a larger effect on the surface adsorbed protein structure than the electric field for a maximally asymmetric PC/PS bilayer. Our results support the mechanism that the higher lipid packing in a lipid leaflet promotes stronger protein–protein but weaker protein–lipid interactions for a dimeric protein on membrane surfaces. PMID:26827904

Layer Anti-Ferromagnetism on Bilayer Honeycomb Lattice

PubMed Central

Tao, Hong-Shuai; Chen, Yao-Hua; Lin, Heng-Fu; Liu, Hai-Di; Liu, Wu-Ming

2014-01-01

Bilayer honeycomb lattice, with inter-layer tunneling energy, has a parabolic dispersion relation, and the inter-layer hopping can cause the charge imbalance between two sublattices. Here, we investigate the metal-insulator and magnetic phase transitions on the strongly correlated bilayer honeycomb lattice by cellular dynamical mean-field theory combined with continuous time quantum Monte Carlo method. The procedures of magnetic spontaneous symmetry breaking on dimer and non-dimer sites are different, causing a novel phase transition between normal anti-ferromagnet and layer anti-ferromagnet. The whole phase diagrams about the magnetism, temperature, interaction and inter-layer hopping are obtained. Finally, we propose an experimental protocol to observe these phenomena in future optical lattice experiments. PMID:24947369

Focused-ion-beam induced interfacial intermixing of magnetic bilayers for nanoscale control of magnetic properties.

PubMed

Burn, D M; Hase, T P A; Atkinson, D

2014-06-11

Modification of the magnetic properties in a thin-film ferromagnetic/non-magnetic bilayer system by low-dose focused ion-beam (FIB) induced intermixing is demonstrated. The highly localized capability of FIB may be used to locally control magnetic behaviour at the nanoscale. The magnetic, electronic and structural properties of NiFe/Au bilayers were investigated as a function of the interfacial structure that was actively modified using focused Ga(+) ion irradiation. Experimental work used MOKE, SQUID, XMCD as well as magnetoresistance measurements to determine the magnetic behavior and grazing incidence x-ray reflectivity to elucidate the interfacial structure. Interfacial intermixing, induced by low-dose irradiation, is shown to lead to complex changes in the magnetic behavior that are associated with monotonic structural evolution of the interface. This behavior may be explained by changes in the local atomic environment within the interface region resulting in a combination of processes including the loss of moment on Ni and Fe, an induced moment on Au and modifications to the spin-orbit coupling between Au and NiFe.

SARS E protein in phospholipid bilayers: an anomalous X-ray reflectivity study

NASA Astrophysics Data System (ADS)

Khattari, Z.; Brotons, G.; Arbely, E.; Arkin, I. T.; Metzger, T. H.; Salditt, T.

2005-02-01

We report on an anomalous X-ray reflectivity study to locate a labelled residue of a membrane protein with respect to the lipid bilayer. From such experiments, important constraints on the protein or peptide conformation can be derived. Specifically, our aim is to localize an iodine-labelled phenylalanine in the SARS E protein, incorporated in DMPC phospholipid bilayers, which are deposited in the form of thick multilamellar stacks on silicon surfaces. Here, we discuss the experimental aspects and the difficulties associated with the Fourier synthesis analysis that gives the electron density profile of the membranes.

Tunneling Spectroscopy of Quantum Hall States in Bilayer Graphene

NASA Astrophysics Data System (ADS)

Wang, Ke; Harzheim, Achim; Watanabe, Kenji; Taniguchi, Takashi; Kim, Philip

In the quantum Hall (QH) regime, ballistic conducting paths along the physical edges of a sample appear, leading to quantized Hall conductance and vanishing longitudinal magnetoconductance. These QH edge states are often described as ballistic compressible strips separated by insulating incompressible strips, the spatial profiles of which can be crucial in understanding the stability and emergence of interaction driven QH states. In this work, we present tunneling transport between two QH edge states in bilayer graphene. Employing locally gated device structure, we guide and control the separation between the QH edge states in bilayer graphene. Using resonant Landau level tunneling as a spectroscopy tool, we measure the energy gap in bilayer graphene as a function of displacement field and probe the emergence and evolution of incompressible strips.

Lipid bilayer thickness determines cholesterol's location in model membranes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marquardt, Drew; Heberle, Frederick A.; Greathouse, Denise V.

Cholesterol is an essential biomolecule of animal cell membranes, and an important precursor for the biosynthesis of certain hormones and vitamins. It is also thought to play a key role in cell signaling processes associated with functional plasma membrane microdomains (domains enriched in cholesterol), commonly referred to as rafts. In all of these diverse biological phenomena, the transverse location of cholesterol in the membrane is almost certainly an important structural feature. Using a combination of neutron scattering and solid-state 2H NMR, we have determined the location and orientation of cholesterol in phosphatidylcholine (PC) model membranes having fatty acids of differentmore » lengths and degrees of unsaturation. The data establish that cholesterol reorients rapidly about the bilayer normal in all the membranes studied, but is tilted and forced to span the bilayer midplane in the very thin bilayers. The possibility that cholesterol lies flat in the middle of bilayers, including those made from PC lipids containing polyunsaturated fatty acids (PUFAs), is ruled out. Finally, these results support the notion that hydrophobic thickness is the primary determinant of cholesterol's location in membranes.« less

Lipid bilayer thickness determines cholesterol's location in model membranes

DOE PAGES

Marquardt, Drew; Heberle, Frederick A.; Greathouse, Denise V.; ...

2016-10-11

Cholesterol is an essential biomolecule of animal cell membranes, and an important precursor for the biosynthesis of certain hormones and vitamins. It is also thought to play a key role in cell signaling processes associated with functional plasma membrane microdomains (domains enriched in cholesterol), commonly referred to as rafts. In all of these diverse biological phenomena, the transverse location of cholesterol in the membrane is almost certainly an important structural feature. Using a combination of neutron scattering and solid-state 2H NMR, we have determined the location and orientation of cholesterol in phosphatidylcholine (PC) model membranes having fatty acids of differentmore » lengths and degrees of unsaturation. The data establish that cholesterol reorients rapidly about the bilayer normal in all the membranes studied, but is tilted and forced to span the bilayer midplane in the very thin bilayers. The possibility that cholesterol lies flat in the middle of bilayers, including those made from PC lipids containing polyunsaturated fatty acids (PUFAs), is ruled out. Finally, these results support the notion that hydrophobic thickness is the primary determinant of cholesterol's location in membranes.« less

Lipid domains control myelin basic protein adsorption and membrane interactions between model myelin lipid bilayers

PubMed Central

Lee, Dong Woog; Banquy, Xavier; Kristiansen, Kai; Kaufman, Yair; Boggs, Joan M.; Israelachvili, Jacob N.

2014-01-01

The surface forces apparatus and atomic force microscope were used to study the effects of lipid composition and concentrations of myelin basic protein (MBP) on the structure of model lipid bilayers, as well as the interaction forces and adhesion between them. The lipid bilayers had a lipid composition characteristic of the cytoplasmic leaflets of myelin from “normal” (healthy) and “disease-like” [experimental allergic encephalomyelitis (EAE)] animals. They showed significant differences in the adsorption mechanism of MBP. MBP adsorbs on normal bilayers to form a compact film (3–4 nm) with strong intermembrane adhesion (∼0.36 mJ/m2), in contrast to its formation of thicker (7–8 nm) swelled films with weaker intermembrane adhesion (∼0.13 mJ/m2) on EAE bilayers. MBP preferentially adsorbs to liquid-disordered submicron domains within the lipid membranes, attributed to hydrophobic attractions. These results show a direct connection between the lipid composition of membranes and membrane–protein adsorption mechanisms that affects intermembrane spacing and adhesion and has direct implications for demyelinating diseases. PMID:24516125

Simulation studies of structure and edge tension of lipid bilayer edges: effects of tail structure and force-field.

PubMed

West, Ana; Ma, Kevin; Chung, Jonathan L; Kindt, James T

2013-08-15

Molecular dynamics simulations of lipid bilayer ribbons have been performed to investigate the structures and line tensions associated with free bilayer edges. Simulations carried out for dioleoyl phosphatidylcholine with three different force-field parameter sets yielded edge line tensions of 45 ± 2 pN, over 50% greater than the most recently reported experimentally determined value for this lipid. Edge tensions obtained from simulations of a series of phosphatidylcholine lipid bilayer ribbons with saturated acyl tails of length 12-16 carbons and with monounsaturated acyl tails of length 14-18 carbons could be correlated with the excess area associated with forming the edge, through a two-parameter fit. Saturated-tail lipids underwent local thickening near the edge, producing denser packing that correlated with lower line tensions, while unsaturated-tail lipids showed little or no local thickening. In a dipalmitoyl phosphatidylcholine ribbon initiated in a tilted gel-phase structure, lipid headgroups tended to tilt toward the nearer edge producing a herringbone pattern, an accommodation that may account for the reported edge-induced stabilization of an ordered structure at temperatures near a lipid gel-fluid phase transition.

Influence of chain ordering on the selectivity of dipalmitoylphosphatidylcholine bilayer membranes for permeant size and shape.

PubMed Central

Xiang, T X; Anderson, B D

1998-01-01

The effects of lipid chain packing and permeant size and shape on permeability across lipid bilayers have been investigated in gel and liquid crystalline dipalmitoylphosphatidylcholine (DPPC) bilayers by a combined NMR line-broadening/dynamic light scattering method using seven short-chain monocarboxylic acids (formic acid, acetic acid, propionic acid, butyric acid, valeric acid, isovaleric acid, and trimethylacetic acid) as permeants. The experimental permeability coefficients are compared with the predictions of a bulk solubility diffusion model in which the bilayer membrane is represented as a slab of bulk hexadecane. Deviations of the observed permeability coefficients (Pm) from the values predicted from solubility diffusion theory (Po) lead to the determination of a correction factor, the permeability decrement f (= Pm/Po), to account for the effects of chain ordering. The natural logarithm of f has been found to correlate linearly with the inverse of the bilayer free surface area with slopes of 25 +/- 2, 36 +/- 3, 45 +/- 8, 32 +/- 12, 33 +/- 4, 49 +/- 12, and 75 +/- 6 A2 for formic acid, acetic acid, propionic acid, butyric acid, valeric acid, isovaleric acid, and trimethylacetic acid, respectively. The slope, which measures the sensitivity of the permeability coefficient of a given permeant to bilayer chain packing, exhibits an excellent linear correlation (r = 0.94) with the minimum cross-sectional area of the permeant and a poor correlation (r = 0.59) with molecular volume, suggesting that in the bilayer interior the permeants prefer to move with their long principal axis along the bilayer normal. Based on these studies, a permeability model combining the effects of bilayer chain packing and permeant size and shape on permeability across lipid membranes is developed. PMID:9826590

Nontrivial behavior of water in the vicinity and inside lipid bilayers as probed by molecular dynamics simulations.

PubMed

Krylov, Nikolay A; Pentkovsky, Vladimir M; Efremov, Roman G

2013-10-22

The atomic-scale diffusion of water in the presence of several lipid bilayers mimicking biomembranes is characterized via unconstrained molecular dynamics (MD) simulations. Although the overall water dynamics corresponds well to literature data, namely, the efficient braking near polar head groups of lipids, a number of interesting and biologically relevant details observed in this work have not been sufficiently discussed so far; for instance, the fact that waters "sense" the membrane unexpectedly early, before water density begins to decrease. In this "transitional zone" the velocity distributions of water and their H-bonding patterns deviate from those in the bulk solution. The boundaries of this zone are well preserved even despite the local (<1 nm size) perturbation of the lipid bilayer, thus indicating a decoupling of the surface and bulk dynamics of water. This is in excellent agreement with recent experimental data. Near the membrane surface, water movement becomes anisotropic, that is, solvent molecules preferentially move outward the bilayer. Deep in the membrane interior, the velocities can even exceed those in the bulk solvent and undergo large-scale fluctuations. The analysis of MD trajectories of individual waters in the middle part of the acyl chain region of lipids reveals a number of interesting rare phenomena, such as the fast (ca. 50 ps) breakthrough across the membrane or long-time (up to 750 ps) "roaming" between lipid leaflets. The analysis of these events was accomplished to delineate the mechanisms of spontaneous water permeation inside the hydrophobic membrane core. It was shown that such nontrivial dynamics of water in an "alien" environment is driven by the dynamic heterogeneities of the local bilayer structure and the formation of transient atomic-scale "defects" in it. The picture observed in lipid bilayers is drastically different from that in a primitive membrane mimic, a hydrated cyclohexane slab. The possible biological impact of such phenomena in equilibrated lipid bilayers is discussed.

Phytochemicals perturb membranes and promiscuously alter protein function.

PubMed

Ingólfsson, Helgi I; Thakur, Pratima; Herold, Karl F; Hobart, E Ashley; Ramsey, Nicole B; Periole, Xavier; de Jong, Djurre H; Zwama, Martijn; Yilmaz, Duygu; Hall, Katherine; Maretzky, Thorsten; Hemmings, Hugh C; Blobel, Carl; Marrink, Siewert J; Koçer, Armağan; Sack, Jon T; Andersen, Olaf S

2014-08-15

A wide variety of phytochemicals are consumed for their perceived health benefits. Many of these phytochemicals have been found to alter numerous cell functions, but the mechanisms underlying their biological activity tend to be poorly understood. Phenolic phytochemicals are particularly promiscuous modifiers of membrane protein function, suggesting that some of their actions may be due to a common, membrane bilayer-mediated mechanism. To test whether bilayer perturbation may underlie this diversity of actions, we examined five bioactive phenols reported to have medicinal value: capsaicin from chili peppers, curcumin from turmeric, EGCG from green tea, genistein from soybeans, and resveratrol from grapes. We find that each of these widely consumed phytochemicals alters lipid bilayer properties and the function of diverse membrane proteins. Molecular dynamics simulations show that these phytochemicals modify bilayer properties by localizing to the bilayer/solution interface. Bilayer-modifying propensity was verified using a gramicidin-based assay, and indiscriminate modulation of membrane protein function was demonstrated using four proteins: membrane-anchored metalloproteases, mechanosensitive ion channels, and voltage-dependent potassium and sodium channels. Each protein exhibited similar responses to multiple phytochemicals, consistent with a common, bilayer-mediated mechanism. Our results suggest that many effects of amphiphilic phytochemicals are due to cell membrane perturbations, rather than specific protein binding.

Phytochemicals Perturb Membranes and Promiscuously Alter Protein Function

PubMed Central

2015-01-01

A wide variety of phytochemicals are consumed for their perceived health benefits. Many of these phytochemicals have been found to alter numerous cell functions, but the mechanisms underlying their biological activity tend to be poorly understood. Phenolic phytochemicals are particularly promiscuous modifiers of membrane protein function, suggesting that some of their actions may be due to a common, membrane bilayer-mediated mechanism. To test whether bilayer perturbation may underlie this diversity of actions, we examined five bioactive phenols reported to have medicinal value: capsaicin from chili peppers, curcumin from turmeric, EGCG from green tea, genistein from soybeans, and resveratrol from grapes. We find that each of these widely consumed phytochemicals alters lipid bilayer properties and the function of diverse membrane proteins. Molecular dynamics simulations show that these phytochemicals modify bilayer properties by localizing to the bilayer/solution interface. Bilayer-modifying propensity was verified using a gramicidin-based assay, and indiscriminate modulation of membrane protein function was demonstrated using four proteins: membrane-anchored metalloproteases, mechanosensitive ion channels, and voltage-dependent potassium and sodium channels. Each protein exhibited similar responses to multiple phytochemicals, consistent with a common, bilayer-mediated mechanism. Our results suggest that many effects of amphiphilic phytochemicals are due to cell membrane perturbations, rather than specific protein binding. PMID:24901212

Single DNA molecules on freestanding and supported cationic lipid bilayers: diverse conformational dynamics controlled by the local bilayer properties

NASA Astrophysics Data System (ADS)

Herold, Christoph; Schwille, Petra; Petrov, Eugene P.

2016-02-01

We present experimental results on the interaction of DNA macromolecules with cationic lipid membranes with different properties, including freestanding membranes in the fluid and gel state, and supported lipid membranes in the fluid state and under conditions of fluid-gel phase coexistence. We observe diverse conformational dynamics of membrane-bound DNA molecules controlled by the local properties of the lipid bilayer. In case of fluid-state freestanding lipid membranes, the behaviour of DNA on the membrane is controlled by the membrane charge density: whereas DNA bound to weakly charged membranes predominantly behaves as a 2D random coil, an increase in the membrane charge density leads to membrane-driven irreversible DNA collapse and formation of subresolution-sized DNA globules. On the other hand, electrostatic binding of DNA macromolecules to gel-state freestanding membranes leads to completely arrested diffusion and conformational dynamics of membrane-adsorbed DNA. A drastically different picture is observed in case of DNA interaction with supported cationic lipid bilayers: When the supported bilayer is in the fluid state, membrane-bound DNA molecules undergo 2D translational Brownian motion and conformational fluctuations, irrespectively of the charge density of the supported bilayer. At the same time, when the supported cationic membrane shows fluid-gel phase coexistence, membrane-bound DNA molecules are strongly attracted to micrometre-sized gel-phase domains enriched with the cationic lipid, which results in 2D compaction of the membrane-bound macromolecules. This DNA compaction, however, is fully reversible, and disappears as soon as the membrane is heated above the fluid-gel coexistence. We also discuss possible biological implications of our experimental findings.

Enhanced Circular Dichroism of Gold Bilayered Slit Arrays Embedded with Rectangular Holes.

PubMed

Zhang, Hao; Wang, Yongkai; Luo, Lina; Wang, Haiqing; Zhang, Zhongyue

2017-01-01

Gold bilayered slit arrays with rectangular holes embedded into the metal surface are designed to enhance the circular dichroism (CD) effect of gold bilayered slit arrays. The rectangular holes in these arrays block electric currents and generate localized surface plasmons around these holes, thereby strengthening the CD effect. The CD enhancement factor depends strongly on the rotational angle and the structural parameters of the rectangular holes; this factor can be enhanced further by drilling two additional rectangular holes into the metal surfaces of the arrays. These results help facilitate the design of chiral structures to produce a strong CD effect and large electric fields.

Experimental Investigation of the Electronic Properties of Twisted Bilayer Graphene by STM and STS

NASA Astrophysics Data System (ADS)

Yin, Longjing; Qiao, Jiabin; Wang, Wenxiao; Zuo, Weijie; He, Lin

The electronic properties of graphene multilayers depend sensitively on their stacking order. A twisted angle is treated as a unique degree of freedom to tune the electronic properties of graphene system. Here we study electronic structures of the twisted bilayers by scanning tunneling microscopy (STM) and spectroscopy (STS). We demonstrate that the interlayer coupling strength affects both the Van Hove singularities and the Fermi velocity of twisted bilayers dramatically. This removes the discrepancy about the Fermi velocity renormalization in the twisted bilayers and provides a consistent interpretation of all current data. Moreover, we report the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers by STM and STS. At a magic twisted angle, about 1.11°, a pronounced sharp peak is observed in the tunnelling spectra due to the action of the non-Abelian gauge fields. Because of the effective non-Abelian gauge fields, the rotation angle could transfer the charge carriers in the twisted bilayers from massless Dirac fermions into well localized electrons, or vice versa, efficiently. This provides a new route to tune the electronic properties of graphene systems, which will be essential in future graphene nanoelectronics.

Design and evaluation of bilayered buccal film preparations for local administration of lidocaine hydrochloride.

PubMed

Preis, Maren; Woertz, Christina; Schneider, Katharina; Kukawka, Jennifer; Broscheit, Jens; Roewer, Norbert; Breitkreutz, Jörg

2014-04-01

Bilayered oromucosal film preparations (buccal films) offer a promising way to enable drug administration via the oral cavity. Adding a non-soluble or slowly eroding/dissolving backing layer to a mucoadhesive drug-loaded layer enables unidirectional drug delivery. The aim of this study was to investigate different approaches to the manufacture of bilayered films and to examine their properties by applying different characterization methods including an optimized experimental setup for the study of drug release from bilayered films. A solvent suitability study was performed screening over 15 polymers with respect to their feasibility for viscous film formation for film preparation by solvent casting method. Two methods (double-casting and pasting) were found as suitable methods for bilayered film manufacturing. Results from drug release experiments indicated that slowly eroding hypromellose backing layer films revealed the best shielding of the drug-loaded layer to enable unidirectional drug release. In summary, manufacturing of bilayered films using the described methods was feasible. Furthermore, the use of an optimized experimental setup for drug dissolution studies enabled monitoring of drug release without delays in sampling. Copyright © 2014 Elsevier B.V. All rights reserved.

Normal metal - insulator - superconductor thermometers and coolers with titanium-gold bilayer as the normal metal

NASA Astrophysics Data System (ADS)

Räisänen, I. M. W.; Geng, Z.; Kinnunen, K. M.; Maasilta, I. J.

2018-03-01

We have fabricated superconductor - insulator - normal metal - insulator - superconductor (SINIS) tunnel junctions in which Al acts as the superconductor, AlOx is the insulator, and the normal metal consists of a thin Ti layer (5 nm) covered with a thicker Au layer (40 nm). We have characterized the junctions by measuring their current-voltage curves between 60 mK and 750 mK. For comparison, the same measurements have been performed for a SINIS junction pair whose normal metal is Cu. The Ti-Au bilayer decreases the SINIS tunneling resistance by an order of magnitude compared to junctions where Cu is used as normal metal, made with the same oxidation parameters. The Ti-Au devices are much more robust against chemical attacks, and their lower tunneling resistance makes them more robust against static charge. More significantly, they exhibit significantly stronger electron cooling than Cu devices with identical fabrication steps, when biased close to the energy gap of the superconducting Al. By using a self-consistent thermal model, we can fit the current-voltage characteristics well, and show an electron cooling from 200 mK to 110 mK, with a non-optimized device.

Maximally asymmetric transbilayer distribution of anionic lipids alters the structure and interaction with lipids of an amyloidogenic protein dimer bound to the membrane surface.

PubMed

Cheng, Sara Y; Chou, George; Buie, Creighton; Vaughn, Mark W; Compton, Campbell; Cheng, Kwan H

2016-03-01

We used molecular dynamics simulations to explore the effects of asymmetric transbilayer distribution of anionic phosphatidylserine (PS) lipids on the structure of a protein on the membrane surface and subsequent protein-lipid interactions. Our simulation systems consisted of an amyloidogenic, beta-sheet rich dimeric protein (D42) absorbed to the phosphatidylcholine (PC) leaflet, or protein-contact PC leaflet, of two membrane systems: a single-component PC bilayer and double PC/PS bilayers. The latter comprised of a stable but asymmetric transbilayer distribution of PS in the presence of counterions, with a 1-component PC leaflet coupled to a 1-component PS leaflet in each bilayer. The maximally asymmetric PC/PS bilayer had a non-zero transmembrane potential (TMP) difference and higher lipid order packing, whereas the symmetric PC bilayer had a zero TMP difference and lower lipid order packing under physiologically relevant conditions. Analysis of the adsorbed protein structures revealed weaker protein binding, more folding in the N-terminal domain, more aggregation of the N- and C-terminal domains and larger tilt angle of D42 on the PC leaflet surface of the PC/PS bilayer versus the PC bilayer. Also, analysis of protein-induced membrane structural disruption revealed more localized bilayer thinning in the PC/PS versus PC bilayer. Although the electric field profile in the non-protein-contact PS leaflet of the PC/PS bilayer differed significantly from that in the non-protein-contact PC leaflet of the PC bilayer, no significant difference in the electric field profile in the protein-contact PC leaflet of either bilayer was evident. We speculate that lipid packing has a larger effect on the surface adsorbed protein structure than the electric field for a maximally asymmetric PC/PS bilayer. Our results support the mechanism that the higher lipid packing in a lipid leaflet promotes stronger protein-protein but weaker protein-lipid interactions for a dimeric protein on membrane surfaces. Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.

Effects of Normal Metal Features on Superconducting Transition-Edge Sensors

NASA Astrophysics Data System (ADS)

Wakeham, N. A.; Adams, J. S.; Bandler, S. R.; Chervenak, J. A.; Datesman, A. M.; Eckart, M. E.; Finkbeiner, F. M.; Kelley, R. L.; Kilbourne, C. A.; Miniussi, A. R.; Porter, F. S.; Sadleir, J. E.; Sakai, K.; Smith, S. J.; Wassell, E. J.; Yoon, W.

2018-04-01

In transition-edge sensors (TESs), the addition of normal metal stripes on top of the superconducting bilayer, perpendicular to the current direction, is known to globally alter the sensitivity of the resistance R to changes in temperature T and current I. Here, we describe measurements of the dependence of the TES current on magnetic field B, bath temperature and voltage bias in devices with various numbers of stripes. We show that the normal metal features have a profound effect on the appearance of localized regions of very large (T/R) dR/dT . We associate this with changes in the current distribution and corresponding changes in the oscillatory pattern of I(B). 140 μm TESs with no stripes are found to have a relatively smooth resistive transition and sufficiently low noise that the measured energy resolution is 1.6 eV for X-rays of 1.5 keV. The predicted energy resolution at 6 keV is better than 2 eV, once the heat capacity is optimized for these higher energies.

Room temperature ordering of dipalmitoyl phosphatidylserine bilayers induced by short chain alcohols.

PubMed

Wachtel, E; Bach, D; Miller, I R

2013-01-01

Using differential scanning calorimetry and small and wide angle X-ray diffraction, we show that, following extended incubation at room temperature, methanol, propanol, and three of the isomers of butanol can induce ordering in dipalmitoyl phosphatidylserine (DPPS) gel phase bilayers. The organization of the bilayers in the presence of ethanol, described previously, is now observed to be a general effect of short chain alcohols. Evidence is presented for tilting of the acyl chains with respect to the bilayer normal in the presence of ethanol or propanol. However, the different chain lengths of the alcohols, and isomeric form, influence the thermal stability of the ordered gel to different extents. This behavior is unlike that of the gel state phosphatidylcholine analog which, in the presence of short chain alcohols, undergoes hydrocarbon chain interdigitation. Dipalmitoyl phosphatidylcholine added to DPPS in the presence of 20 vol% ethanol, acts to suppress the ordered gel phase. Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.

Stacked bilayer phosphorene: strain-induced quantum spin Hall state and optical measurement

PubMed Central

Zhang, Tian; Lin, Jia-He; Yu, Yan-Mei; Chen, Xiang-Rong; Liu, Wu-Ming

2015-01-01

Bilayer phosphorene attracted considerable interest, giving a potential application in nanoelectronics owing to its natural bandgap and high carrier mobility. However, very little is known regarding the possible usefulness in spintronics as a quantum spin Hall (QSH) state of material characterized by a bulk energy gap and gapless spin-filtered edge states. Here, we report a strain-induced topological phase transition from normal to QSH state in bilayer phosphorene, accompanied by band-inversion that changes number from 0 to 1, which is highly dependent on interlayer stacking. When the bottom layer is shifted by 1/2 unit-cell along zigzag/armchair direction with respect to the top layer, the maximum topological bandgap 92.5 meV is sufficiently large to realize QSH effect even at room-temperature. An optical measurement of QSH effect is therefore suggested in view of the wide optical absorption spectrum extending to far infra-red, making bilayer phosphorene a promising candidate for opto-spintronic devices. PMID:26370771

Spectral properties of excitons in the bilayer graphene

NASA Astrophysics Data System (ADS)

Apinyan, V.; Kopeć, T. K.

2018-01-01

In this paper, we consider the spectral properties of the bilayer graphene with the local excitonic pairing interaction between the electrons and holes. We consider the generalized Hubbard model, which includes both intralayer and interlayer Coulomb interaction parameters. The solution of the excitonic gap parameter is used to calculate the electronic band structure, single-particle spectral functions, the hybridization gap, and the excitonic coherence length in the bilayer graphene. We show that the local interlayer Coulomb interaction is responsible for the semimetal-semiconductor transition in the double layer system, and we calculate the hybridization gap in the band structure above the critical interaction value. The formation of the excitonic band gap is reported as the threshold process and the momentum distribution functions have been calculated numerically. We show that in the weak coupling limit the system is governed by the Bardeen-Cooper-Schrieffer (BCS)-like pairing state. Contrary, in the strong coupling limit the excitonic condensate states appear in the semiconducting phase, by forming the Dirac's pockets in the reciprocal space.

Confining and repulsive potentials from effective non-Abelian gauge fields in graphene bilayers

NASA Astrophysics Data System (ADS)

González, J.

2016-10-01

We investigate the effect of shear and strain in graphene bilayers, under conditions where the distortion of the lattice gives rise to a smooth one-dimensional modulation in the stacking sequence of the bilayer. We show that strain and shear produce characteristic Moiré patterns which can have the same visual appearance on a large scale, but representing graphene bilayers with quite different electronic properties. The different features in the low-energy electronic bands can be ascribed to the effect of a fictitious non-Abelian gauge field mimicking the smooth modulation of the stacking order. Strained and sheared bilayers show a complementary behavior, which can be understood from the fact that the non-Abelian gauge field acts as a repulsive interaction in the former, expelling the electron density away from the stacking domain walls, while behaving as a confining interaction leading to localization of the electronic states in the sheared bilayers. In this latter case, the presence of the effective gauge field explains the development of almost flat low-energy bands, resembling the form of the zeroth Landau level characteristic of a Dirac fermion field. The estimate of the gauge field strength in those systems gives a magnitude of the order of several tens of tesla, implying a robust phenomenology that should be susceptible of being observed in suitably distorted bilayer samples.

Dynamic patterns in a supported lipid bilayer driven by standing surface acoustic waves.

PubMed

Hennig, Martin; Neumann, Jürgen; Wixforth, Achim; Rädler, Joachim O; Schneider, Matthias F

2009-11-07

In the past decades supported lipid bilayers (SLBs) have been an important tool in order to study the physical properties of biological membranes and cells. So far, controlled manipulation of SLBs is very limited. Here we present a new technology to create lateral patterns in lipid membranes controllable in both space and time. Surface acoustic waves (SAWs) are used to generate lateral standing waves on a piezoelectric substrate which create local "traps" in the lipid bilayer and lead to a lateral modulation in lipid concentration. We demonstrate that pattern formation is reversible and does not affect the integrity of the lipid bilayer as shown by extracting the diffusion constant of fluid membranes. The described method could possibly be used to design switchable interfaces for the lateral transport and organization of membrane bound macromolecules to create dynamic bioarrays and control biofilm formation.

Ultraviolet absorption spectrum of the half-filled bilayer graphene

NASA Astrophysics Data System (ADS)

Apinyan, V.; Kopeć, T. K.

2018-07-01

We consider the optical properties of the half-filled AB-stacked bilayer graphene with the excitonic pairing and condensation between the layers. Both intra and interlayer local Coulomb interaction effects have been taken into account and the role of the exact Fermi energy has been discussed in details. We have calculated the absorption coefficient, refractive index, dielectric response functions and the electron energy loss spectrum for different interlayer Coulomb interaction regimes and for different temperatures. Considering the full four-band model for the interacting AB bilayer graphene, a good agreement is achieved with other theoretical and experimental works on the subject, in particular, limiting cases of the theory. The calculations, presented here, permit to estimate accurately the effects of excitonic pairing and condensation on the optical properties of the bilayer graphene. The modifications of the plasmon excitation spectrum are discussed in details for a very large interval of the interlayer interaction parameter.

CURVATURE-DRIVEN MOLECULAR FLOW ON MEMBRANE SURFACE*

PubMed Central

MIKUCKI, MICHAEL; ZHOU, Y. C.

2017-01-01

This work presents a mathematical model for the localization of multiple species of diffusion molecules on membrane surfaces. Morphological change of bilayer membrane in vivo is generally modulated by proteins. Most of these modulations are associated with the localization of related proteins in the crowded lipid environments. We start with the energetic description of the distributions of molecules on curved membrane surface, and define the spontaneous curvature of bilayer membrane as a function of the molecule concentrations on membrane surfaces. A drift-diffusion equation governs the gradient flow of the surface molecule concentrations. We recast the energetic formulation and the related governing equations by using an Eulerian phase field description to define membrane morphology. Computational simulations with the proposed mathematical model and related numerical techniques predict (i) the molecular localization on static membrane surfaces at locations with preferred mean curvatures, and (ii) the generation of preferred mean curvature which in turn drives the molecular localization. PMID:29056778

Diffusional dynamics of an active rhodamine-labeled 1,4-dihydropyridine in sarcolemmal lipid multibilayers.

PubMed Central

Mason, R P; Chester, D W

1989-01-01

A "membrane bilayer pathway" model, involving ligand partition into the bilayer, lateral diffusion, and receptor binding has been invoked to describe the 1,4-dihydropyridine (DHP) calcium channel antagonist receptor binding mechanism. In an earlier study (Chester et al. 1987. Biophys. J. 52:1021-1030), the diffusional component of this model was examined using an active fluorescence labeled DHP calcium channel antagonist, nisoldipine-lissamine rhodamine B (Ns-R), in purified cardiac sarcolemmal (CSL) lipid multibilayers. Diffusion coefficient measurements on membrane-bound drug and phospholipid at maximum bilayer hydration yielded similar values (3.8 x 10(-8) cm2/s). However, decreases in bilayer hydration resulted in dramatically reduced diffusion coefficient values for both probes with substantially greater impact on Ns-R diffusion. These data suggested that hydration dependent diffusional differences could be a function of relative probe location along the bilayer normal. In this communication, we have addressed the relative effect of the rhodamine substituent on Ns-R diffusion complex by examining the diffusional dynamics of free rhodamine B under the same conditions used to evaluate Ns-R complex and phospholipid diffusion. X-ray diffraction studies were performed to determine the Ns-R location in the membrane and model the CSL lipid bilayer profile structure to give a rationale for the differences in probe diffusional dynamics as a function of interbilayer water space. PMID:2611332

Does fluoride disrupt hydrogen bond network in cationic lipid bilayer? Time-dependent fluorescence shift of Laurdan and molecular dynamics simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pokorna, Sarka; Jurkiewicz, Piotr; Hof, Martin, E-mail: martin.hof@jh-inst.cas.cz

2014-12-14

Time-dependent fluorescence shift (TDFS) of Laurdan embedded in phospholipid bilayers reports on hydration and mobility of the phospholipid acylgroups. Exchange of H{sub 2}O with D{sub 2}O prolongs the lifetime of lipid-water and lipid-water-lipid interactions, which is reflected in a significantly slower TDFS kinetics. Combining TDFS measurements in H{sub 2}O and D{sub 2}O hydrated bilayers with atomistic molecular dynamics (MD) simulations provides a unique tool for characterization of the hydrogen bonding at the acylgroup level of lipid bilayers. In this work, we use this approach to study the influence of fluoride anions on the properties of cationic bilayers composed of trimethylammonium-propanemore » (DOTAP). The results obtained for DOTAP are confronted with those for neutral phosphatidylcholine (DOPC) bilayers. Both in DOTAP and DOPC H{sub 2}O/D{sub 2}O exchange prolongs hydrogen-bonding lifetime and does not disturb bilayer structure. These results are confirmed by MD simulations. TDFS experiments show, however, that for DOTAP this effect is cancelled in the presence of fluoride ions. We interpret these results as evidence that strongly hydrated fluoride is able to steal water molecules that bridge lipid carbonyls. Consequently, when attracted to DOTAP bilayer, fluoride disrupts the local hydrogen-bonding network, and the differences in TDFS kinetics between H{sub 2}O and D{sub 2}O hydrated bilayers are no longer observed. A distinct behavior of fluoride is also evidenced by MD simulations, which show different lipid-ion binding for Cl{sup −} and F{sup −}.« less

Confocal Raman Microscopy for in Situ Measurement of Phospholipid-Water Partitioning into Model Phospholipid Bilayers within Individual Chromatographic Particles.

PubMed

Kitt, Jay P; Bryce, David A; Minteer, Shelley D; Harris, Joel M

2018-06-05

The phospholipid-water partition coefficient is a commonly measured parameter that correlates with drug efficacy, small-molecule toxicity, and accumulation of molecules in biological systems in the environment. Despite the utility of this parameter, methods for measuring phospholipid-water partition coefficients are limited. This is due to the difficulty of making quantitative measurements in vesicle membranes or supported phospholipid bilayers, both of which are small-volume phases that challenge the sensitivity of many analytical techniques. In this work, we employ in situ confocal Raman microscopy to probe the partitioning of a model membrane-active compound, 2-(4-isobutylphenyl) propionic acid or ibuprofen, into both hybrid- and supported-phospholipid bilayers deposited on the pore walls of individual chromatographic particles. The large surface-area-to-volume ratio of chromatographic silica allows interrogation of a significant lipid bilayer area within a very small volume. The local phospholipid concentration within a confocal probe volume inside the particle can be as high as 0.5 M, which overcomes the sensitivity limitations of making measurements in the limited membrane areas of single vesicles or planar supported bilayers. Quantitative determination of ibuprofen partitioning is achieved by using the phospholipid acyl-chains of the within-particle bilayer as an internal standard. This approach is tested for measurements of pH-dependent partitioning of ibuprofen into both hybrid-lipid and supported-lipid bilayers within silica particles, and the results are compared with octanol-water partitioning and with partitioning into individual optically trapped phospholipid vesicle membranes. Additionally, the impact of ibuprofen partitioning on bilayer structure is evaluated for both within-particle model membranes and compared with the structural impacts of partitioning into vesicle lipid bilayers.

Ionizable Nitroxides for Studying Local Electrostatic Properties of Lipid Bilayers and Protein Systems by EPR.

PubMed

Voinov, Maxim A; Smirnov, Alex I

2015-01-01

Electrostatic interactions are known to play a major role in the myriad of biochemical and biophysical processes. Here, we describe biophysical methods to probe local electrostatic potentials of proteins and lipid bilayer systems that are based on an observation of reversible protonation of nitroxides by electron paramagnetic resonance (EPR). Two types of probes are described: (1) methanethiosulfonate derivatives of protonatable nitroxides for highly specific covalent modification of the cysteine's sulfhydryl groups and (2) spin-labeled phospholipids with a protonatable nitroxide tethered to the polar head group. The probes of both types report on their ionization state through changes in magnetic parameters and degree of rotational averaging, thus, allowing the electrostatic contribution to the interfacial pKa of the nitroxide, and, therefore, the local electrostatic potential to be determined. Due to their small molecular volume, these probes cause a minimal perturbation to the protein or lipid system. Covalent attachment secures the position of the reporter nitroxides. Experimental procedures to characterize and calibrate these probes by EPR, and also the methods to analyze the EPR spectra by simulations are outlined. The ionizable nitroxide labels and the nitroxide-labeled phospholipids described so far cover an exceptionally wide range of ca. 2.5-7.0 pH units, making them suitable to study a broad range of biophysical phenomena, especially at the negatively charged lipid bilayer surfaces. The rationale for selecting proper electrostatically neutral interface for probe calibration, and examples of lipid bilayer surface potential studies, are also described. © 2015 Elsevier Inc. All rights reserved.

Effect of intrinsic curvature and edge tension on the stability of binary mixed-membrane three-junctions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gardner, Jasmine M.; Abrams, Cameron F.; Deserno, Markus

We use a combination of coarse-grained molecular dynamics simulations and theoretical modeling to examine three-junctions in mixed lipid bilayer membranes. These junctions are localized defect lines in which three bilayers merge in such a way that each bilayer shares one monolayer with one of the other two bilayers. The resulting local morphology is non-lamellar, resembling the threefold symmetric defect lines in inverse hexagonal phases, but it regularly occurs during membrane fission and fusion events. We realize a system of junctions by setting up a honeycomb lattice, which in its primitive cell contains two hexagons and four three-line junctions, permitting usmore » to study their stability as well as their line tension. We specifically consider the effects of lipid composition and intrinsic curvature in binary mixtures, which contain a fraction of negatively curved lipids in a curvature-neutral background phase. Three-junction stability results from a competition between the junction and an open edge, which arises if one of the three bilayers detaches from the other two. We show that the stable phase is the one with the lower defect line tension. The strong and opposite monolayer curvatures present in junctions and edges enhance the mole fraction of negatively curved lipids in junctions and deplete it in edges. This lipid sorting affects the two line tensions and in turn the relative stability of the two phases. It also leads to a subtle entropic barrier for the transition between junction and edge that is absent in uniform membranes.« less

Interaction of saponin 1688 with phase separated lipid bilayers.

PubMed

Chen, Maohui; Balhara, Vinod; Jaimes Castillo, Ana Maria; Balsevich, John; Johnston, Linda J

2017-07-01

Saponins are a diverse family of naturally occurring plant triterpene or steroid glycosides that have a wide range of biological activities. They have been shown to permeabilize membranes and in some cases membrane disruption has been hypothesized to involve saponin/cholesterol complexes. We have examined the interaction of steroidal saponin 1688-1 with lipid membranes that contain cholesterol and have a mixture of liquid-ordered (L o ) and liquid-disordered (L d ) phases as a model for lipid rafts in cellular membranes. A combination of atomic force microscopy (AFM) and fluorescence was used to probe the effect of saponin on the bilayer. The results demonstrate that saponin forms defects in the membrane and also leads to formation of small aggregates on the membrane surface. Although most of the membrane damage occurs in the liquid-disordered phase, fluorescence results demonstrate that saponin localizes in both ordered and disordered membrane phases, with a modest preference for the disordered regions. Similar effects are observed for both direct incorporation of saponin in the lipid mixture used to make vesicles/bilayers and for incubation of saponin with preformed bilayers. The results suggest that the initial sites of interaction are at the interface between the domains and surrounding disordered phase. The preference for saponin localization in the disordered phase may reflect the ease of penetration of saponin into a less ordered membrane, rather than the actual cholesterol concentration in the membrane. Dye leakage assays indicate that a high concentration of saponin is required for membrane permeabilization consistent with the supported lipid bilayer experiments. Crown Copyright © 2017. Published by Elsevier B.V. All rights reserved.

Role of Transmembrane Potential and Defects on the Permeabilization of Lipid Bilayers by Alamethicin, an Ion-Channel-Forming Peptide.

PubMed

Su, ZhangFei; Shodiev, Muzaffar; Leitch, J Jay; Abbasi, Fatemeh; Lipkowski, Jacek

2018-05-29

The insertion and ion-conducting channel properties of alamethicin reconstituted into a 1,2-di- O-phytanyl- sn-glycero-3-phosphocholine bilayer floating on the surface of a gold (111) electrode modified with a 1-thio-β-d-glucose (β-Tg) self-assembled monolayer were investigated using a combination of electrochemical impedance spectroscopy (EIS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The hydrophilic β-Tg monolayer separated the bilayer from the gold substrate and created a water-rich spacer region, which better represents natural cell membranes. The EIS measurements acquired information about the membrane resistivity (a measure of membrane porosity), and the PM-IRRAS experiments provided insight into the conformation and orientation of the membrane constituents as a function of the transmembrane potential. The results showed that the presence of alamethicin had a small effect on the conformation and orientation of phospholipid molecules within the bilayer for all studied potentials. In contrast, the alamethicin peptides assumed a surface state, where the helical axes adopted a large tilt angle with respect to the surface normal, at small transmembrane potentials, and inserted into the bilayer at sufficiently negative transmembrane potentials forming pores, which behaved as barrel-stave ion channels for ionic transport across the membrane. The results indicated that insertion of alamethincin peptides into the bilayer was driven by the dipole-field interactions and that the transitions between the inserted and surface states were electrochemically reversible. Additionally, the EIS measurements performed on phospholipid bilayers without alamethicin also showed that the application of negative transmembrane potentials introduces defects into the bilayer. The membrane resistances measured in both the absence and presence of alamethicin show similar dependencies on the electrode potential, suggesting that the insertion of the peptide may also be assisted by the electroporation of the membrane. The findings in this study provide new insights into the mechanism of alamethicin insertion into phospholipid bilayers.

Model-based Approaches for the Determination of Lipid Bilayer Structure from Small-Angle Neutron and X-ray Scattering Data

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heberle, Frederick A; Pan, Jianjun; Standaert, Robert F

2012-01-01

Some of our recent work has resulted in the detailed structures of fully hydrated, fluid phase phosphatidylcholine (PC) and phosphatidylglycerol (PG) bilayers. These structures were obtained from the joint refinement of small-angle neutron and X-ray data using the scattering density profile (SDP) models developed by Ku erka et al. (Ku erka et al. 2012; Ku erka et al. 2008). In this review, we first discuss models for the standalone analysis of neutron or X-ray scattering data from bilayers, and assess the strengths and weaknesses inherent in these models. In particular, it is recognized that standalone data do not contain enoughmore » information to fully resolve the structure of inherently disordered fluid bilayers, and therefore may not provide a robust determination of bilayer structural parameters, including the much sought after area per lipid. We then discuss the development of matter density-based models (including the SDP model) that allow for the joint refinement of different contrast neutron and X-ray data sets, as well as the implementation of local volume conservation in the unit cell (i.e., ideal packing). Such models provide natural definitions of bilayer thicknesses (most importantly the hydrophobic and Luzzati thicknesses) in terms of Gibbs dividing surfaces, and thus allow for the robust determination of lipid areas through equivalent slab relationships between bilayer thickness and lipid volume. In the final section of this review, we discuss some of the significant findings/features pertaining to structures of PC and PG bilayers as determined from SDP model analyses.« less

Selective differential human blood bilayer media for isolation of Gardnerella (Haemophilus) vaginalis.

PubMed Central

Totten, P A; Amsel, R; Hale, J; Piot, P; Holmes, K K

1982-01-01

New selective and differential human blood bilayer agar media with Tween 80 (HBT medium) or without Tween 80 (HB medium), developed for the isolation of Gardnerella (Haemophilus) vaginalis, permitted significantly higher G. vaginalis isolation rates than have been obtained for other media used for this purpose. HB medium consists of a basal layer of Columbia agar base containing colistin and naladixic acid with added amphotericin B and an overlayer of the same composition plus 5% human blood. HBT agar also contains Proteose Peptone No. 3 (Difco Laboratories) and Tween 80 in the basal layer and the overlayer. Both Tween 80 and the bilayer composition enhanced G. vaginalis production of human blood hemolysis, permitting detection of this organism even in the presence of heavy growth of other vaginal flora. The use of HB or HBT medium thus permitted the demonstration that G. vaginalis was present in vaginal fluid from a large percentage (up to 68%) of normal women. However, the concentration of G. vaginalis was found by semiquantitative analysis to be significantly higher in vaginal fluid from women with nonspecific vaginitis than in fluid from normal women. Images PMID:6764766

A Dry-Etch Process for Low Temperature Superconducting Transition Edge Sensors for Far Infrared Bolometer Arrays

NASA Technical Reports Server (NTRS)

Allen, Christine A.; Chervenak, James A.; Hsieh, Wen-Ting; McClanahan, Richard A.; Miller, Timothy M.; Mitchell, Robert; Moseley, S. Harvey; Staguhn, Johannes; Stevenson, Thomas R.

2003-01-01

The next generation of ultra-low power bolometer arrays, with applications in far infrared imaging, spectroscopy and polarimetry, utilizes a superconducting bilayer as the sensing element to enable SQUID multiplexed readout. Superconducting transition edge sensors (TES s) are being produced with dual metal systems of superconductinghormal bilayers. The transition temperature (Tc) is tuned by altering the relative thickness of the superconductor with respect to the normal layer. We are currently investigating MoAu and MoCu bilayers. We have developed a dry-etching process for MoAu TES s with integrated molybdenum leads, and are working on adapting the process to MoCu. Dry etching has the advantage over wet etching in the MoAu system in that one can achieve a high degree of selectivity, greater than 10, using argon ME, or argon ion milling, for patterning gold on molybdenum. Molybdenum leads are subsequently patterned using fluorine plasma.. The dry-etch technique results in a smooth, featureless TES with sharp sidewalls, no undercutting of the Mo beneath the normal metal, and Mo leads with high critical current. The effects of individual processing parameters on the characteristics of the transition will be reported.

Behavior of Bilayer Leaflets in Asymmetric Model Membranes: Atomistic Simulation Studies

DOE PAGES

Tian, Jianhui; Nickels, Jonathan; Katsaras, John; ...

2016-04-27

Spatial organization within lipid bilayers is an important feature for a range of biological processes. Leaflet compositional asymmetry and lateral lipid organization are just two of the ways in which membrane structure appears to be more complex than initially postulated by the fluid mosaic model. This raises the question of how the phase behavior in one bilayer leaflet may affect the apposing leaflet and how one begins to construct asymmetric model systems to investigate these interleaflet interactions. In this paper, we report on all-atom molecular dynamics simulations (a total of 4.1 μs) of symmetric and asymmetric bilayer systems composed ofmore » liquid-ordered (Lo) or liquid-disordered (Ld) leaflets, based on the nanodomain-forming POPC/DSPC/cholesterol system. We begin by analyzing an asymmetric bilayer with leaflets derived from simulations of symmetric Lo and Ld bilayers. In this system, we observe that the properties of the Lo and Ld leaflets are similar to those of the Lo and Ld leaflets in corresponding symmetric systems. However, it is not obvious that mixing the equilibrium structures of their symmetric counterparts is the most appropriate way to construct asymmetric bilayers nor that these structures will manifest interleaflet couplings that lead to domain registry/antiregistry. We therefore constructed and simulated four additional asymmetric bilayer systems by systematically adding or removing lipids in the Ld leaflet to mimic potential density fluctuations. We find that the number of lipids in the Ld leaflet affects its own properties, as well as those of the apposing Lo leaflet. Collectively, the simulations reveal the presence of weak acyl chain interdigitation across bilayer leaflets, suggesting that interdigitation alone does not contribute significantly to the interleaflet coupling in nonphase-separated bilayers of this chemical composition. Finally, however, the properties of both leaflets appear to be sensitive to changes in in-plane lipid packing, possibly providing a mechanism for interleaflet coupling by modulating local density and/or curvature fluctuations.« less

Proximity-induced superconductivity in all-silicon superconductor /normal-metal junctions

NASA Astrophysics Data System (ADS)

Chiodi, F.; Duvauchelle, J.-E.; Marcenat, C.; Débarre, D.; Lefloch, F.

2017-07-01

We have realized laser-doped all-silicon superconducting (S)/normal metal (N) bilayers of tunable thickness and dopant concentration. We observed a strong reduction of the bilayers' critical temperature when increasing the normal metal thickness, a signature of the highly transparent S/N interface associated to the epitaxial sharp laser doping profile. We extracted the interface resistance by fitting with the linearized Usadel equations, demonstrating a reduction of 1 order of magnitude from previous superconductor/doped Si interfaces. In this well-controlled crystalline system we exploited the low-resistance S/N interfaces to elaborate all-silicon lateral SNS junctions with long-range proximity effect. Their dc transport properties, such as the critical and retrapping currents, could be well understood in the diffusive regime. Furthermore, this work led to the estimation of important parameters in ultradoped superconducting Si, such as the Fermi velocity, the coherence length, or the electron-phonon coupling constant, fundamental to conceive all-silicon superconducting electronics.

Permeability of acetic acid across gel and liquid-crystalline lipid bilayers conforms to free-surface-area theory.

PubMed Central

Xiang, T X; Anderson, B D

1997-01-01

Solubility-diffusion theory, which treats the lipid bilayer membrane as a bulk lipid solvent into which permeants must partition and diffuse across, fails to account for the effects of lipid bilayer chain order on the permeability coefficient of any given permeant. This study addresses the scaling factor that must be applied to predictions from solubility-diffusion theory to correct for chain ordering. The effects of bilayer chemical composition, temperature, and phase structure on the permeability coefficient (Pm) of acetic acid were investigated in large unilamellar vesicles by a combined method of NMR line broadening and dynamic light scattering. Permeability values were obtained in distearoylphosphatidylcholine, dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and dilauroylphosphatidylcholine bilayers, and their mixtures with cholesterol, at various temperatures both above and below the gel-->liquid-crystalline phase transition temperatures (Tm). A new scaling factor, the permeability decrement f, is introduced to account for the decrease in permeability coefficient from that predicted by solubility-diffusion theory owing to chain ordering in lipid bilayers. Values of f were obtained by division of the observed Pm by the permeability coefficient predicted from a bulk solubility-diffusion model. In liquid-crystalline phases, a strong correlation (r = 0.94) between f and the normalized surface density sigma was obtained: in f = 5.3 - 10.6 sigma. Activation energies (Ea) for the permeability of acetic acid decreased with decreasing phospholipid chain length and correlated with the sensitivity of chain ordering to temperature, [symbol: see text] sigma/[symbol: see text](1/T), as chain length was varied. Pm values decreased abruptly at temperatures below the main phase transition temperatures in pure dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine bilayers (30-60-fold) and below the pretransition in dipalmitoylphosphatidylcholine bilayers (8-fold), and the linear relationship between in f and sigma established for liquid-crystalline bilayers was no longer followed. However, in both gel and liquid-crystalline phases in f was found to exhibit an inverse correlation with free surface area (in f = -0.31 - 29.1/af, where af is the average free area (in square angstroms) per lipid molecule). Thus, the lipid bilayer permeability of acetic acid can be predicted from the relevant chain-packing properties in the bilayer (free surface area), regardless of whether chain ordering is varied by changes in temperature, lipid chain length, cholesterol concentration, or bilayer phase structure, provided that temperature effects on permeant dehydration and diffusion and the chain-length effects on bilayer barrier thickness are properly taken into account. PMID:8994607

Mechanical properties of electrospun bilayer fibrous membranes as potential scaffolds for tissue engineering.

PubMed

Pu, Juan; Komvopoulos, Kyriakos

2014-06-01

Bilayer fibrous membranes of poly(l-lactic acid) (PLLA) were fabricated by electrospinning, using a parallel-disk mandrel configuration that resulted in the sequential deposition of a layer with fibers aligned across the two parallel disks and a layer with randomly oriented fibers, both layers deposited in a single process step. Membrane structure and fiber alignment were characterized by scanning electron microscopy and two-dimensional fast Fourier transform. Because of the intricacies of the generated electric field, bilayer membranes exhibited higher porosity than single-layer membranes consisting of randomly oriented fibers fabricated with a solid-drum collector. However, despite their higher porosity, bilayer membranes demonstrated generally higher elastic modulus, yield strength and toughness than single-layer membranes with random fibers. Bilayer membrane deformation at relatively high strain rates comprised multiple abrupt microfracture events characterized by discontinuous fiber breakage. Bilayer membrane elongation yielded excessive necking of the layer with random fibers and remarkable fiber stretching (on the order of 400%) in the layer with fibers aligned in the stress direction. In addition, fibers in both layers exhibited multiple localized necking, attributed to the nonuniform distribution of crystalline phases in the fibrillar structure. The high membrane porosity, good mechanical properties, and good biocompatibility and biodegradability of PLLA (demonstrated in previous studies) make the present bilayer membranes good scaffold candidates for a wide range of tissue engineering applications. Copyright © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Single-component supported lipid bilayers probed using broadband nonlinear optics.

PubMed

Olenick, Laura L; Chase, Hilary M; Fu, Li; Zhang, Yun; McGeachy, Alicia C; Dogangun, Merve; Walter, Stephanie R; Wang, Hong-Fei; Geiger, Franz M

2018-01-31

Broadband SFG spectroscopy is shown to offer considerable advantages over scanning systems in terms of signal-to-noise ratios when probing well-formed single-component supported lipid bilayers formed from zwitterionic lipids with PC headgroups. The SFG spectra obtained from bilayers formed from DOPC, POPC, DLPC, DMPC, DPPC and DSPC show a common peak at ∼2980 cm -1 , which is subject to interference between the C-H and the O-H stretches from the aqueous phase, while membranes having transition temperatures above the laboratory temperature produce SFG spectra with at least two additional peaks, one at ∼2920 cm -1 and another at ∼2880 cm -1 . The results validate spectroscopic and structural data from SFG experiments utilizing asymmetric bilayers in which one leaflet differs from the other in the extent of deuteration. Differences in H 2 O-D 2 O exchange experiments reveal that the lineshapes of the broadband SFG spectra are significantly influenced by interference from OH oscillators in the aqueous phase, even when those oscillators are not probed by the incident infrared light in our broadband setup. In the absence of spectral interference from the OH stretches of the solvent, the alkyl chain terminal methyl group of the bilayer is found to be tilted at an angle of 15° to 35° from the surface normal.

electric dipole superconductor in bilayer exciton system

NASA Astrophysics Data System (ADS)

Sun, Qing-Feng; Jiang, Qing-Dong; Bao, Zhi-Qiang; Xie, X. C.

Recently, it was reported that the bilayer exciton systems could exhibit many new phenomena, including the large bilayer counterflow conductivity, the Coulomb drag, etc. These phenomena imply the formation of exciton condensate superfluid state. On the other hand, it is now well known that the superconductor is the condensate superfluid state of the Cooper pairs, which can be viewed as electric monopoles. In other words, the superconductor state is the electric monopole condensate superfluid state. Thus, one may wonder whether there exists electric dipole superfluid state. In this talk, we point out that the exciton in a bilayer system can be considered as a charge neutral electric dipole. And we derive the London-type and Ginzburg-Landau-type equations of electric dipole superconductivity. From these equations, we discover the Meissner-type effect (against spatial variation of magnetic fields), and the dipole current Josephson effect. The frequency in the AC Josephson effect of the dipole current is equal to that in the normal (monopole) superconductor. These results can provide direct evidence for the formation of exciton superfluid state in the bilayer systems and pave new ways to obtain the electric dipole current. We gratefully acknowledge the financial support by NBRP of China (2012CB921303 and 2015CB921102) and NSF-China under Grants Nos. 11274364 and 11574007.

The central element of the synaptonemal complex in mice is organized as a bilayered junction structure.

PubMed

Hernández-Hernández, Abrahan; Masich, Sergej; Fukuda, Tomoyuki; Kouznetsova, Anna; Sandin, Sara; Daneholt, Bertil; Höög, Christer

2016-06-01

The synaptonemal complex transiently stabilizes pairing interactions between homologous chromosomes during meiosis. Assembly of the synaptonemal complex is mediated through integration of opposing transverse filaments into a central element, a process that is poorly understood. We have, here, analyzed the localization of the transverse filament protein SYCP1 and the central element proteins SYCE1, SYCE2 and SYCE3 within the central region of the synaptonemal complex in mouse spermatocytes using immunoelectron microscopy. Distribution of immuno-gold particles in a lateral view of the synaptonemal complex, supported by protein interaction data, suggest that the N-terminal region of SYCP1 and SYCE3 form a joint bilayered central structure, and that SYCE1 and SYCE2 localize in between the two layers. We find that disruption of SYCE2 and TEX12 (a fourth central element protein) localization to the central element abolishes central alignment of the N-terminal region of SYCP1. Thus, our results show that all four central element proteins, in an interdependent manner, contribute to stabilization of opposing N-terminal regions of SYCP1, forming a bilayered transverse-filament-central-element junction structure that promotes synaptonemal complex formation and synapsis. © 2016. Published by The Company of Biologists Ltd.

Anesthetics mechanism on a DMPC lipid membrane model: Insights from molecular dynamics simulations.

PubMed

Saeedi, Marzieh; Lyubartsev, Alexander P; Jalili, Seifollah

2017-07-01

To provide insight into the molecular mechanisms of local anesthetic action, we have carried out an extensive investigation of two amide type local anesthetics, lidocaine and articaine in both charged and uncharged forms, interacting with DMPC lipid membrane. We have applied both standard molecular dynamics simulations and metadynamics simulations to provide a detailed description of the free energy landscape of anesthetics embedded in the lipid bilayer. The global minimum of the free energy surface (equilibrium position of anesthetics in the lipid membrane) occurred around 1nm of the bilayer center. The uncharged anesthetics show more affinity to bind to this region compared to the charged drugs. The binding free energy of uncharged lidocaine in the membrane (-30.3kJ/mol) is higher than uncharged articaine (-24.0kJ/mol), which is in good agreement with higher lipid solubility of lidocaine relative to the articaine. The octanol/water partition coefficient of uncharged drugs was also investigated using expanded ensemble simulations. In addition, complementary standard MD simulations were carried out to study the partitioning behavior of multiple anesthetics inside the lipid bilayer. The results obtained here are in line with previously reported simulations and suggest that the different forms of anesthetics induce different structural modifications in the lipid bilayer, which can provide new insights into their complex membrane translocation phenomena. Copyright © 2017 Elsevier B.V. All rights reserved.

Experimental study of the bending elasticity of charged lipid bilayers in aqueous solutions with pH5

NASA Astrophysics Data System (ADS)

Mitkova, D.; Stoyanova-Ivanova, A.; Ermakov, Yu A.; Vitkova, V.

2012-12-01

Exposure to high concentrations of contaminations due to air polluting gases, vapours and aerosols and possibly altering the normal pH in the body could lead to undesirable changes in the properties of biological cells. Here, we study experimentally the mechanical properties of synthetic phospholipid bilayers containing increasing molar fractions (up to 0.15) of charged lipid (synthetic phosphatidylserine) in aqueous solutions with controlled ionic strength and at pH 5, which is slightly lower than the physiological values of pH. Our observations in phase contrast and fluorescence testified to the coexistence of two phases in membranes for temperatures below 29°C. Micro-sized inhomogeneities in vesicle membranes were systematically observed at temperatures lower than 29°C and for molar fractions of phosphatidylserine in the bilayer higher than 0.1. For the quantitative determination of the membrane bending rigidity, we applied thermal fluctuation analysis of the shape of quasispherical lipid vesicles. As far as the liquid-crystalline state of the bilayer is a necessary condition for the application of the experimental method, only vesicles satisfying this requirement were processed for determination of their membrane bending rigidity. The value obtained for the bending modulus of bilayers with 0.15 molar content of charged lipid is about two times higher than the bending modulus of uncharged membranes in the same bathing solution. These findings are in qualitative agreement with our previous results for the bending rigidity of charged bilayers, measured by vesicle micromanipulation.

Ionizable Nitroxides for Studying Local Electrostatic Properties of Lipid Bilayers and Protein Systems by EPR

PubMed Central

Voinov, Maxim A.; Smirnov, Alex I.

2016-01-01

Electrostatic interactions are known to play one of the major roles in the myriad of biochemical and biophysical processes. In this Chapter we describe biophysical methods to probe local electrostatic potentials of proteins and lipid bilayer systems that is based on an observation of reversible protonation of nitroxides by EPR. Two types of the electrostatic probes are discussed. The first one includes methanethiosulfonate derivatives of protonatable nitroxides that could be used for highly specific covalent modification of the cysteine’s sulfhydryl groups. Such spin labels are very similar in magnetic parameters and chemical properties to conventional MTSL making them suitable for studying local electrostatic properties of protein-lipid interfaces. The second type of EPR probes is designed as spin-labeled phospholipids having a protonatable nitroxide tethered to the polar head group. The probes of both types report on their ionization state through changes in magnetic parameters and a degree of rotational averaging, thus, allowing one to determine the electrostatic contribution to the interfacial pKa of the nitroxide, and, therefore, determining the local electrostatic potential. Due to their small molecular volume these probes cause a minimal perturbation to the protein or lipid system while covalent attachment secure the position of the reporter nitroxides. Experimental procedures to characterize and calibrate these probes by EPR and also the methods to analyze the EPR spectra by least-squares simulations are also outlined. The ionizable nitroxide labels and the nitroxide-labeled phospholipids described so far cover an exceptionally wide pH range from ca. 2.5 to 7.0 pH units making them suitable to study a broad range of biophysical phenomena especially at the negatively charged lipid bilayer surfaces. The rationale for selecting proper electrostatically neutral interface for calibrating such probes and example of studying surface potential of lipid bilayer is also described. PMID:26477252

Simulations of a Membrane-Anchored Peptide: Structure, Dynamics, and Influence on Bilayer Properties

PubMed Central

Jensen, Morten Ø.; Mouritsen, Ole G.; Peters, Günther H.

2004-01-01

A three-dimensional structure of a model decapeptide is obtained by performing molecular dynamics simulations of the peptide in explicit water. Interactions between an N-myristoylated form of the folded peptide anchored to dipalmitoylphosphatidylcholine fluid phase lipid membranes are studied at different applied surface tensions by molecular dynamics simulations. The lipid membrane environment influences the conformational space explored by the peptide. The overall secondary structure of the anchored peptide is found to deviate at times from its structure in aqueous solution through reversible conformational transitions. The peptide is, despite the anchor, highly mobile at the membrane surface with the peptide motion along the bilayer normal being integrated into the collective modes of the membrane. Peptide anchoring moderately alters the lateral compressibility of the bilayer by changing the equilibrium area of the membrane. Although membrane anchoring moderately affects the elastic properties of the bilayer, the model peptide studied here exhibits conformational flexibility and our results therefore suggest that peptide acylation is a feasible way to reinforce peptide-membrane interactions whereby, e.g., the lifetime of receptor-ligand interactions can be prolonged. PMID:15189854

Cholesterol orientation and tilt modulus in DMPC bilayers

PubMed Central

Khelashvili, George; Pabst, Georg; Harries, Daniel

2010-01-01

We performed molecular dynamics (MD) simulations of hydrated bilayers containing mixtures of dimyristoylphosphatidylcholine (DMPC) and Cholesterol at various ratios, to study the effect of cholesterol concentration on its orientation, and to characterize the link between cholesterol tilt and overall phospholipid membrane organization. The simulations show a substantial probability for cholesterol molecules to transiently orient perpendicular to the bilayer normal, and suggest that cholesterol tilt may be an important factor for inducing membrane ordering. In particular, we find that as cholesterol concentration increases (1%–40% cholesterol) the average cholesterol orientation changes in a manner strongly (anti)correlated with the variation in membrane thickness. Furthermore, cholesterol orientation is found to be determined by the aligning force exerted by other cholesterol molecules. To quantify this aligning field, we analyzed cholesterol orientation using, to our knowledge, the first estimates of the cholesterol tilt modulus χ from MD simulations. Our calculations suggest that the aligning field that determines χ is indeed strongly linked to sterol composition. This empirical parameter (χ) should therefore become a useful quantitative measure to describe cholesterol interaction with other lipids in bilayers, particularly in various coarse-grained force fields. PMID:20518573

Bi-layered nanocomposite bandages for controlling microbial infections and overproduction of matrix metalloproteinase activity.

PubMed

Anjana, J; Mohandas, Annapoorna; Seethalakshmy, S; Suresh, Maneesha K; Menon, Riju; Biswas, Raja; Jayakumar, R

2018-04-15

Chronic diabetic wounds is characterised by increased microbial contamination and overproduction of matrix metalloproteases that would degrade the extracellular matrix. A bi-layer bandage was developed, that promotes the inhibition of microbial infections and matrix metalloprotease (MMPs) activity. Bi-layer bandage containing benzalkonium chloride loaded gelatin nanoparticles (BZK GNPs) in chitosan-Hyaluronic acid (HA) as a bottom layer and sodium alendronate containing chitosan as top layer was developed. We hypothesized that the chitosan-gelatin top layer with sodium alendronate could inhibit the MMPs activity, whereas the chitosan-HA bottom layer with BZK GNPs (240±66nm) would enable the elimination of microbes. The porosity, swelling and degradation nature of the prepared Bi-layered bandage was studied. The bottom layer could degrade within 4days whereas the top layer remained upto 7days. The antimicrobial activity of the BZK NPs loaded bandage was determined using normal and clinical strains. Gelatin zymography shows that the proteolytic activity of MMP was inhibited by the bandage. Copyright © 2017 Elsevier B.V. All rights reserved.

Confocal Raman Microscopy for In-situ Measurement of Phospholipid-Water Partitioning into Model Phospholipid Bilayers within Individual Chromatographic Particles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kitt, Jay P.; Bryce, David A.; Minteer, Shelley D.

The phospholipid-water partition coefficient is a commonly measured parameter that correlates with drug efficacy, small-molecule toxicity, and accumulation of molecules in biological systems in the environment. Despite the utility of this parameter, methods for measuring phospholipid-water partition coefficients are limited. This is due to the difficulty of making quantitative measurements in vesicle membranes or supported phospholipid bilayers, both of which are small-volume phases that challenge the sensitivity of many analytical techniques. In this paper, we employ in-situ confocal Raman microscopy to probe the partitioning of a model membrane-active compound, 2-(4-isobutylphenyl) propionic acid or ibuprofen, into both hybrid- and supported-phospholipid bilayersmore » deposited on the pore walls of individual chromatographic particles. The large surface-area-to-volume ratio of chromatographic silica allows interrogation of a significant lipid bilayer area within a very small volume. The local phospholipid concentration within a confocal probe volume inside the particle can be as high as 0.5 M, which overcomes the sensitivity limitations of making measurements in the limited membrane areas of single vesicles or planar supported bilayers. Quantitative determination of ibuprofen partitioning is achieved by using the phospholipid acyl-chains of the within-particle bilayer as an internal standard. This approach is tested for measurements of pH-dependent partitioning of ibuprofen into both hybrid-lipid and supported-lipid bilayers within silica particles, and the results are compared with octanol-water partitioning and with partitioning into individual optically-trapped phospholipid vesicle membranes. Finally and additionally, the impact of ibuprofen partitioning on bilayer structure is evaluated for both within-particle model membranes and compared with the structural impacts of partitioning into vesicle lipid bilayers.« less

Confocal Raman Microscopy for In-situ Measurement of Phospholipid-Water Partitioning into Model Phospholipid Bilayers within Individual Chromatographic Particles

DOE PAGES

Kitt, Jay P.; Bryce, David A.; Minteer, Shelley D.; ...

2018-05-14

The phospholipid-water partition coefficient is a commonly measured parameter that correlates with drug efficacy, small-molecule toxicity, and accumulation of molecules in biological systems in the environment. Despite the utility of this parameter, methods for measuring phospholipid-water partition coefficients are limited. This is due to the difficulty of making quantitative measurements in vesicle membranes or supported phospholipid bilayers, both of which are small-volume phases that challenge the sensitivity of many analytical techniques. In this paper, we employ in-situ confocal Raman microscopy to probe the partitioning of a model membrane-active compound, 2-(4-isobutylphenyl) propionic acid or ibuprofen, into both hybrid- and supported-phospholipid bilayersmore » deposited on the pore walls of individual chromatographic particles. The large surface-area-to-volume ratio of chromatographic silica allows interrogation of a significant lipid bilayer area within a very small volume. The local phospholipid concentration within a confocal probe volume inside the particle can be as high as 0.5 M, which overcomes the sensitivity limitations of making measurements in the limited membrane areas of single vesicles or planar supported bilayers. Quantitative determination of ibuprofen partitioning is achieved by using the phospholipid acyl-chains of the within-particle bilayer as an internal standard. This approach is tested for measurements of pH-dependent partitioning of ibuprofen into both hybrid-lipid and supported-lipid bilayers within silica particles, and the results are compared with octanol-water partitioning and with partitioning into individual optically-trapped phospholipid vesicle membranes. Finally and additionally, the impact of ibuprofen partitioning on bilayer structure is evaluated for both within-particle model membranes and compared with the structural impacts of partitioning into vesicle lipid bilayers.« less

Fusion of raft-like lipid bilayers operated by a membranotropic domain of the HSV-type I glycoprotein gH occurs through a cholesterol-dependent mechanism.

PubMed

Vitiello, Giuseppe; Falanga, Annarita; Petruk, Ariel Alcides; Merlino, Antonello; Fragneto, Giovanna; Paduano, Luigi; Galdiero, Stefania; D'Errico, Gerardino

2015-04-21

A wealth of evidence indicates that lipid rafts are involved in the fusion of the viral lipid envelope with the target cell membrane. However, the interplay between these sterol- and sphingolipid-enriched ordered domains and viral fusion glycoproteins has not yet been clarified. In this work we investigate the molecular mechanism by which a membranotropic fragment of the glycoprotein gH of the Herpes Simplex Virus (HSV) type I (gH625) drives fusion of lipid bilayers formed by palmitoyl oleoyl phosphatidylcholine (POPC)-sphingomyelin (SM)-cholesterol (CHOL) (1 : 1 : 1 wt/wt/wt), focusing on the role played by each component. The comparative analysis of the liposome fusion assays, Dynamic Light Scattering (DLS), spectrofluorimetry, Neutron Reflectivity (NR) and Electron Spin Resonance (ESR) experiments, and Molecular Dynamics (MD) simulations shows that CHOL is fundamental for liposome fusion to occur. In detail, CHOL stabilizes the gH625-bilayer association by specific interactions with the peptide Trp residue. The interaction with gH625 causes an increased order of the lipid acyl chains, whose local rotational motion is significantly hampered. SM plays only a minor role in the process, favoring the propagation of lipid perturbation to the bilayer inner core. The stiffening of the peptide-interacting bilayer leaflet results in an asymmetric perturbation of the membrane, which is locally destabilized thus favoring fusion events. Our results show that viral fusion glycoproteins are optimally suited to exert a high fusogenic activity on lipid rafts and support the relevance of cholesterol as a key player of membrane-related processes.

Identification of trapped and boundary lipid binding sites in M13 coat protein/lipid complexes by deuterium NMR spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Van Gorkom, L.C.; Horvath, L.I.; Hemminga, M.A.

The major coat protein of M13 bacteriophage has been incorporated into bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine, deuterated in the trimethyl segments of the choline headgroup (DMPC-d9). Two-component deuterium and phosphorus-31 NMR spectra have been observed from bilayer complexes containing the coat protein, indicating slow exchange (on the deuterium quadrupole anisotropy and phosphorus-31 chemical shift averaging time scales) of lipid molecules of less than 10(3) Hz between two motionally distinct environments in the complexes. The fraction of the isotropic spectral component increases with increasing M13 protein concentration, and this component is attributed to lipid headgroups, which are disordered relative to their order inmore » protein-free bilayers. The activation energy of the fast local motions of the trimethyl groups of the choline residue in the headgroup decreases from 23 kJ mol-1 in the pure lipid bilayers to 20 kJ mol-1 for the protein-associated lipid headgroups. The chemical exchange rate of lipid molecules between the two motionally distinct environments has been estimated to be 20-50 Hz by steady-state line-shape simulations of the deuterium spectra of DMPC-d9/M13 coat protein complexes using exchange-coupled modified Bloch equations. The off-rate was, as expected from one-to-one exchange, independent of the L/P ratio; tau off -1 = 0.23 kHz. It is suggested that the protein-associated lipid may be trapped between closely packed parallel aggregates of M13 coat protein and that the high local concentration of protein in a one-dimensional arrangement in lipid bilayers may be required for the fast reassembly of phage particles before release from an infected cell.« less

Effect of the interfacial tension and ionic strength on the thermodynamic barrier associated to the benzocaine insertion into a cell membrane.

PubMed

López Cascales, J J; Oliveira Costa, S D

2013-02-01

The insertion of local anaesthetics into a cell membrane is a key aspect for explaining their activity at a molecular level. It has been described how the potency and response time of local anaesthetics is improved (for clinical applications) when they are dissolved in a solution of sodium bicarbonate. With the aim of gaining insight into the physico-chemical principles that govern the action mechanism of these drugs at a molecular level, simulations of benzocaine in binary lipid bilayers formed by DPPC/DPPS were carried out for different ionic strengths of the aqueous solution. From these molecular dynamic simulations, we observed how the thermodynamic barrier associated with benzocaine insertion into the lipid bilayers diminished exponentially as the fraction of DPPS in the bilayer increased, especially when the ionic strength of the aqueous solution increased. In line with these results, we also observed how this thermodynamic barrier diminished exponentially with the phospholipid/water interfacial tension. Copyright © 2012 Elsevier B.V. All rights reserved.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ryu, Yong -Sang; Wittenberg, Nathan J.; Suh, Jeng -Hun

We show that the selective localization of cholesterol-rich domains and associated ganglioside receptors prefer to occur in the monolayer across continuous monolayer-bilayer junctions (MBJs) in supported lipid membranes. For the MBJs, glass substrates were patterned with poly(dimethylsiloxane) (PDMS) oligomers by thermally-assisted contact printing, leaving behind 3 nm-thick PDMS patterns. The hydrophobicity of the transferred PDMS patterns was precisely tuned by the stamping temperature. Lipid monolayers were formed on the PDMS patterned surface while lipid bilayers were on the bare glass surface. Due to the continuity of the lipid membranes over the MBJs, essentially free diffusion of lipids was allowed betweenmore » the monolayer on the PDMS surface and the upper leaflet of the bilayer on the glass substrate. The preferential localization of sphingomyelin, ganglioside GM1 and cholesterol in the monolayer region enabled to develop raft microdomains through coarsening of nanorafts. Furthermore, our methodology provides a simple and effective scheme of non-disruptive manipulation of the chemical landscape associated with lipid phase separations, which leads to more sophisticated applications in biosensors and as cell culture substrates.« less

Rationalization of reduced penetration of drugs through ceramide gel phase membrane.

PubMed

Paloncýová, Markéta; DeVane, Russell H; Murch, Bruce P; Berka, Karel; Otyepka, Michal

2014-11-25

Since computing resources have advanced enough to allow routine molecular simulation studies of drug molecules interacting with biologically relevant membranes, a considerable amount of work has been carried out with fluid phospholipid systems. However, there is very little work in the literature on drug interactions with gel phase lipids. This poses a significant limitation for understanding permeation through the stratum corneum where the primary pathway is expected to be through a highly ordered lipid matrix. To address this point, we analyzed the interactions of p-aminobenzoic acid (PABA) and its ethyl (benzocaine) and butyl (butamben) esters with two membrane bilayers, which differ in their fluidity at ambient conditions. We considered a dioleoylphosphatidylcholine (DOPC) bilayer in a fluid state and a ceramide 2 (CER2, ceramide NS) bilayer in a gel phase. We carried out unbiased (100 ns long) and biased z-constraint molecular dynamics simulations and calculated the free energy profiles of all molecules along the bilayer normal. The free energy profiles converged significantly slower for the gel phase. While the compounds have comparable affinities for both membranes, they exhibit penetration barriers almost 3 times higher in the gel phase CER2 bilayer. This elevated barrier and slower diffusion in the CER2 bilayer, which are caused by the high ordering of CER2 lipid chains, explain the low permeability of the gel phase membranes. We also compared the free energy profiles from MD simulations with those obtained from COSMOmic. This method provided the same trends in behavior for the guest molecules in both bilayers; however, the penetration barriers calculated by COSMOmic did not differ between membranes. In conclusion, we show how membrane fluid properties affect the interaction of drug-like molecules with membranes.

High Reflectance Nanoscale V/Sc Multilayer for Soft X-ray Water Window Region.

PubMed

Huang, Qiushi; Yi, Qiang; Cao, Zhaodong; Qi, Runze; Loch, Rolf A; Jonnard, Philippe; Wu, Meiyi; Giglia, Angelo; Li, Wenbin; Louis, Eric; Bijkerk, Fred; Zhang, Zhong; Wang, Zhanshan

2017-10-10

V/Sc multilayer is experimentally demonstrated for the first time as a high reflectance mirror for the soft X-ray water window region. It primarily works at above the Sc-L edge (λ = 3.11 nm) under near normal incidence while a second peak appears at above the V-L edge (λ = 2.42 nm) under grazing incidence. The V/Sc multilayer fabricated with a d-spacing of 1.59 nm and 30 bilayers has a smaller interface width (σ = 0.27 and 0.32 nm) than the conventional used Cr/Sc (σ = 0.28 and 0.47 nm). For V/Sc multilayer with 30 bilayers, the introduction of B 4 C barrier layers has little improvement on the interface structure. As the number of bilayers increasing to 400, the growth morphology and microstructure of the V/Sc layers evolves with slightly increased crystallization. Nevertheless, the surface roughness remains to be 0.25 nm. A maximum soft X-ray reflectance of 18.4% is measured at λ = 3.129 nm at 9° off-normal incidence using the 400-bilayers V/Sc multilayer. According to the fitted model, an s-polarization reflectance of 5.2% can also be expected at λ = 2.425 nm under 40° incidence. Based on the promising experimental results, further improvement of the reflectance can be achieved by using a more stable deposition system, exploring different interface engineering methods and so on.

Electronic transport of bilayer graphene with asymmetry line defects

NASA Astrophysics Data System (ADS)

Zhao, Xiao-Ming; Wu, Ya-Jie; Chen, Chan; Liang, Ying; Kou, Su-Peng

2016-11-01

In this paper, we study the quantum properties of a bilayer graphene with (asymmetry) line defects. The localized states are found around the line defects. Thus, the line defects on one certain layer of the bilayer graphene can lead to an electric transport channel. By adding a bias potential along the direction of the line defects, we calculate the electric conductivity of bilayer graphene with line defects using the Landauer-Büttiker theory, and show that the channel affects the electric conductivity remarkably by comparing the results with those in a perfect bilayer graphene. This one-dimensional line electric channel has the potential to be applied in nanotechnology engineering. Project supported by the National Basic Research Program of China (Grant Nos. 2011CB921803 and 2012CB921704), the National Natural Science Foundation of China (Grant Nos. 11174035, 11474025, 11504285, and 11404090), the Specialized Research Fund for the Doctoral Program of Higher Education, China, the Fundamental Research Funds for the Central Universities, China, the Scientific Research Program Fund of the Shaanxi Provincial Education Department, China (Grant No. 15JK1363), and the Young Talent Fund of University Association for Science and Technology in Shaanxi Province, China.

A New Route to Liposil Formation by an Interfacial Sol-Gel Process Confined by Lipid Bilayer.

PubMed

Shen, Shukun; Yang, Lu; Lu, Yaxing; Chen, Jian-Gang; Song, Shaofei; Hu, Daodao; Parikh, Atul

2015-11-18

We report a new and simple approach to prepare a class of silica-reinforced liposomes with hybrid core-shell nanostructures. The amphiphilic natural structure of lipids was exploited to sequester hydrophobic molecules, namely precursor TEOS and pyrene, in the hydrophobic midplane of liposomal bilayer assemblies in the aqueous phase. Subsequent interfacial hydrolysis of TEOS at the bilayer/water interface and ensuing condensation within the hydrophobic interstices of the lipid bilayer drives silica formation in situ, producing a novel class of silica-lipid hybrid liposils. Structural characterization by scanning- and transmission electron microscopy confirm that the liposils so generated preserve closed topologies and size-monodipersity of the parent lecithin liposomes, and DSC-TGA and XRD measurements provide evidence for the silica coating. Monitoring fluorescence measurements using embedded pyrene yield detailed information on microenvironment changes, which occur during sol-gel process and shed light on the structural evolution during silica formation. We envisage that liposils formed by this simple, new approach, exploiting the hydrophobic core of the lipid bilayer to spatially localize silica-forming precursors enables preparation of stable liposils exhibiting capacity for cargo encapsulation, bicompatibility, and fluorescence monitoring, more generally opening a window for construction of stable, functional hybrid materials.

Modulated phases of phospholipid bilayers induced by tocopherols.

PubMed

Kamal, Md Arif; Raghunathan, V A

2012-11-01

The influence of α-, γ- and δ-tocopherols on the structure and phase behavior of dipalmitoyl phosphatidylcholine (DPPC) bilayers has been determined from X-ray diffraction studies on oriented multilayers. In all the three cases the main-transition temperature (T(m)) of DPPC was found to decrease with increasing tocopherol concentration up to around 25 mol%. Beyond this the main transition is suppressed in the case of γ-tocopherol, whereas T(m) becomes insensitive to composition in the other two cases. The pre-transition is found to be suppressed over a narrow tocopherol concentration range between 7.5 and 10 mol% in DPPC-γ-tocopherol and DPPC-δ-tocopherol bilayers, and the ripple phase occurs down to the lowest temperature studied. In all the three cases a modulated phase is observed above a tocopherol concentration of about 10 mol%, which is similar to the P(β) phase reported in DPPC-cholesterol bilayers. This phase is found to occur even in excess water conditions at lower tocopherol concentrations, and consists of bilayers with periodic height modulation. These results indicate the ability of tocopherols to induce local curvature in membranes, which could be important for some of their biological functions. Copyright © 2012 Elsevier B.V. All rights reserved.

Functional liposomes and supported lipid bilayers: towards the complexity of biological archetypes.

PubMed

Berti, Debora; Caminati, Gabriella; Baglioni, Piero

2011-05-21

This perspective paper provides some illustrative examples on the interplay between information gathered on planar supported lipid bilayers (SLB) and unilamellar lipid vesicles (ULV) to get an integrated description of phenomena occurring at the nanoscale that involve locally bilayered structures. Similarities and differences are underlined and critically compared in terms of biomimetic fidelity and instrumental accessibility to structural and dynamical parameters, focusing on some recent reports that either explicitly address this comparison or introducing some studies that separately investigate the same process in SLB and lipid vesicles. Despite the structural similarity on the nanoscale, the different topology implies radically different characterization techniques that have evolved in sectorial and separated approaches. The quest for increasing levels of compositional complexity for bilayered systems should not result in a loss of structural and dynamical control: this is the central challenge of future research in this area, where the integrated approach highlighted in this contribution would enable improved levels of understanding. © The Owner Societies 2011

Absorption and folding of melittin onto lipid bilayer membranes via unbiased atomic detail microsecond molecular dynamics simulation.

PubMed

Chen, Charles H; Wiedman, Gregory; Khan, Ayesha; Ulmschneider, Martin B

2014-09-01

Unbiased molecular simulation is a powerful tool to study the atomic details driving functional structural changes or folding pathways of highly fluid systems, which present great challenges experimentally. Here we apply unbiased long-timescale molecular dynamics simulation to study the ab initio folding and partitioning of melittin, a template amphiphilic membrane active peptide. The simulations reveal that the peptide binds strongly to the lipid bilayer in an unstructured configuration. Interfacial folding results in a localized bilayer deformation. Akin to purely hydrophobic transmembrane segments the surface bound native helical conformer is highly resistant against thermal denaturation. Circular dichroism spectroscopy experiments confirm the strong binding and thermostability of the peptide. The study highlights the utility of molecular dynamics simulations for studying transient mechanisms in fluid lipid bilayer systems. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova. Copyright © 2014. Published by Elsevier B.V.

The anomalously high melting temperature of bilayer ice.

PubMed

Kastelowitz, Noah; Johnston, Jessica C; Molinero, Valeria

2010-03-28

Confinement of water usually depresses its melting temperature. Here we use molecular dynamics simulations to determine the liquid-crystal equilibrium temperature for water confined between parallel hydrophobic or mildly hydrophilic plates as a function of the distance between the surfaces. We find that bilayer ice, an ice polymorph in which the local environment of each water molecule strongly departs from the most stable tetrahedral structure, has the highest melting temperature (T(m)) of the series of l-layer ices. The melting temperature of bilayer ice is not only unusually high compared to the other confined ices, but also above the melting point of bulk hexagonal ice. Recent force microscopy experiments of water confined between graphite and a tungsten tip reveal the formation of ice at room temperature [K. B. Jinesh and J. W. M. Frenken, Phys. Rev. Lett. 101, 036101 (2008)]. Our results suggest that bilayer ice, for which we compute a T(m) as high as 310 K in hydrophobic confinement, is the crystal formed in those experiments.

Interlayer couplings, Moiré patterns, and 2D electronic superlattices in MoS2/WSe2 hetero-bilayers

PubMed Central

Zhang, Chendong; Chuu, Chih-Piao; Ren, Xibiao; Li, Ming-Yang; Li, Lain-Jong; Jin, Chuanhong; Chou, Mei-Yin; Shih, Chih-Kang

2017-01-01

By using direct growth, we create a rotationally aligned MoS2/WSe2 hetero-bilayer as a designer van der Waals heterostructure. With rotational alignment, the lattice mismatch leads to a periodic variation of atomic registry between individual van der Waals layers, exhibiting a Moiré pattern with a well-defined periodicity. By combining scanning tunneling microscopy/spectroscopy, transmission electron microscopy, and first-principles calculations, we investigate interlayer coupling as a function of atomic registry. We quantitatively determine the influence of interlayer coupling on the electronic structure of the hetero-bilayer at different critical points. We show that the direct gap semiconductor concept is retained in the bilayer although the valence and conduction band edges are located at different layers. We further show that the local bandgap is periodically modulated in the X-Y direction with an amplitude of ~0.15 eV, leading to the formation of a two-dimensional electronic superlattice. PMID:28070558

Ultrafast lithium diffusion in bilayer graphene

NASA Astrophysics Data System (ADS)

Kühne, Matthias; Paolucci, Federico; Popovic, Jelena; Ostrovsky, Pavel M.; Maier, Joachim; Smet, Jurgen H.

2017-09-01

Solids that simultaneously conduct electrons and ions are key elements for the mass transfer and storage required in battery electrodes. Single-phase materials with a high electronic and high ionic conductivity at room temperature are hard to come by, and therefore multiphase systems with separate ion and electron channels have been put forward instead. Here we report on bilayer graphene as a single-phase mixed conductor that demonstrates Li diffusion faster than in graphite and even surpassing the diffusion of sodium chloride in liquid water. To measure Li diffusion, we have developed an on-chip electrochemical cell architecture in which the redox reaction that forces Li intercalation is localized only at a protrusion of the device so that the graphene bilayer remains unperturbed from the electrolyte during operation. We performed time-dependent Hall measurements across spatially displaced Hall probes to monitor the in-plane Li diffusion kinetics within the graphene bilayer and measured a diffusion coefficient as high as 7 × 10-5 cm2 s-1.

Recoil hysteresis of Sm -Co/Fe exchange-spring bilayers

NASA Astrophysics Data System (ADS)

Kang, K.; Lewis, L. H.; Jiang, J. S.; Bader, S. D.

2005-12-01

The exchange-spring behavior found in Sm-Co (20nm)/Fe epitaxial bilayer films was investigated by analyzing major hysteresis and recoil curves as a function of anneal conditions. The hard layer consists of nanocrystalline intermetallic Sm-Co hexagonal phases (majority phase Sm2Co7 with SmCo3 and SmCo5). Recoil curves, obtained from the successive removal to remanence and reapplication of an increasingly negative field from the major demagnetization curve, reveal the reversible and irreversible components of the magnetization. The Sm-Co thickness was fixed at 20nm while the Fe thicknesses of 10 and 20nm were studied, with ex situ annealing carried out in evacuated, sealed silica tubes at different temperatures. The peak in the recoil curve area is associated with the coercivity of the hard phase. The development of the soft component magnetization is revealed by the departure of the recoil area from zero with application of a reverse field. These two features together confirm that annealing stabilizes the 10nm Fe bilayer sample against local magnetic reversal while it weakens the 20nm bilayer sample. Furthermore, in both its as-deposited and annealed states the Sm -Co/Fe bilayer of 10nm Fe thickness always displays a higher exchange field and smaller recoil loop areas than the bilayer of 20nm Fe thickness, consistent with a stronger exchange response and more reversible magnetization in the former.

Erythrocyte membrane model with explicit description of the lipid bilayer and the spectrin network.

PubMed

Li, He; Lykotrafitis, George

2014-08-05

The membrane of the red blood cell (RBC) consists of spectrin tetramers connected at actin junctional complexes, forming a two-dimensional (2D) sixfold triangular network anchored to the lipid bilayer. Better understanding of the erythrocyte mechanics in hereditary blood disorders such as spherocytosis, elliptocytosis, and especially, sickle cell disease requires the development of a detailed membrane model. In this study, we introduce a mesoscale implicit-solvent coarse-grained molecular dynamics (CGMD) model of the erythrocyte membrane that explicitly describes the phospholipid bilayer and the cytoskeleton, by extending a previously developed two-component RBC membrane model. We show that the proposed model represents RBC membrane with the appropriate bending stiffness and shear modulus. The timescale and self-consistency of the model are established by comparing our results with experimentally measured viscosity and thermal fluctuations of the RBC membrane. Furthermore, we measure the pressure exerted by the cytoskeleton on the lipid bilayer. We find that defects at the anchoring points of the cytoskeleton to the lipid bilayer (as in spherocytes) cause a reduction in the pressure compared with an intact membrane, whereas defects in the dimer-dimer association of a spectrin filament (as in elliptocytes) cause an even larger decrease in the pressure. We conjecture that this finding may explain why the experimentally measured diffusion coefficients of band-3 proteins are higher in elliptocytes than in spherocytes, and higher than in normal RBCs. Finally, we study the effects that possible attractive forces between the spectrin filaments and the lipid bilayer have on the pressure applied on the lipid bilayer by the filaments. We discover that the attractive forces cause an increase in the pressure as they diminish the effect of membrane protein defects. As this finding contradicts with experimental results, we conclude that the attractive forces are moderate and do not impose a complete attachment of the filaments to the lipid bilayer. Copyright © 2014 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Erythrocyte Membrane Model with Explicit Description of the Lipid Bilayer and the Spectrin Network

PubMed Central

Li, He; Lykotrafitis, George

2014-01-01

The membrane of the red blood cell (RBC) consists of spectrin tetramers connected at actin junctional complexes, forming a two-dimensional (2D) sixfold triangular network anchored to the lipid bilayer. Better understanding of the erythrocyte mechanics in hereditary blood disorders such as spherocytosis, elliptocytosis, and especially, sickle cell disease requires the development of a detailed membrane model. In this study, we introduce a mesoscale implicit-solvent coarse-grained molecular dynamics (CGMD) model of the erythrocyte membrane that explicitly describes the phospholipid bilayer and the cytoskeleton, by extending a previously developed two-component RBC membrane model. We show that the proposed model represents RBC membrane with the appropriate bending stiffness and shear modulus. The timescale and self-consistency of the model are established by comparing our results with experimentally measured viscosity and thermal fluctuations of the RBC membrane. Furthermore, we measure the pressure exerted by the cytoskeleton on the lipid bilayer. We find that defects at the anchoring points of the cytoskeleton to the lipid bilayer (as in spherocytes) cause a reduction in the pressure compared with an intact membrane, whereas defects in the dimer-dimer association of a spectrin filament (as in elliptocytes) cause an even larger decrease in the pressure. We conjecture that this finding may explain why the experimentally measured diffusion coefficients of band-3 proteins are higher in elliptocytes than in spherocytes, and higher than in normal RBCs. Finally, we study the effects that possible attractive forces between the spectrin filaments and the lipid bilayer have on the pressure applied on the lipid bilayer by the filaments. We discover that the attractive forces cause an increase in the pressure as they diminish the effect of membrane protein defects. As this finding contradicts with experimental results, we conclude that the attractive forces are moderate and do not impose a complete attachment of the filaments to the lipid bilayer. PMID:25099803

Impact of amphiphilic molecules on the structure and stability of homogeneous sphingomyelin bilayer: Insights from atomistic simulations

NASA Astrophysics Data System (ADS)

Kumari, Pratibha; Kaur, Supreet; Sharma, Shobha; Kashyap, Hemant K.

2018-04-01

Modulation of lipid membrane properties due to the permeation of amphiphiles is an important biological process pertaining to many applications in the field of pharmaceutics, toxicology, and biotechnology. Sphingolipids are both structural and functional lipids that constitute an important component of mechanically stable and chemically resistant outer leaflets of plasma membranes. Here, we present an atomistic molecular dynamics simulation study to appreciate the concentration-dependent effects of small amphiphilic molecules, such as ethanol, acetone, and dimethyl sulfoxide (DMSO), on the structure and stability of a fully hydrated homogeneous N-palmitoyl-sphingomyelin (PSM) bilayer. The study reveals an increase in the lateral expansion of the bilayer along with disordering of the hydrophobic lipid tails on increasing the concentration of ethanol. At higher concentrations of ethanol, rupturing of the bilayer is quite evident through the analysis of partial electron density profiles and lipid tail order parameters. For ethanol containing systems, permeation of water molecules in the hydrophobic part of the bilayer is allowed through local defects made due to the entry of ethanol molecules via ethanol-ethanol and ethanol-PSM hydrogen bonds. Moreover, the extent of PSM-PSM hydrogen bonding decreases with increasing ethanol concentration. On the other hand, acetone and DMSO exhibit minimal effects on the stability of the PSM bilayer at their lower concentrations, but at higher concentrations they tend to enhance the stability of the bilayer. The simulated potential of mean force (PMF) profiles for the translocation of the three solutes studied reveal that the free-energy of transfer of an ethanol molecule across the PSM lipid head region is lower than that for acetone and DMSO molecules. However, highest free-energy rise in the core hydrophobic part of the bilayer is observed for the DMSO molecule, whereas the ethanol and acetone PMF profiles show a lower barrier in the hydrophobic region of the bilayer.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tian, Jianhui; Nickels, Jonathan; Katsaras, John

Spatial organization within lipid bilayers is an important feature for a range of biological processes. Leaflet compositional asymmetry and lateral lipid organization are just two of the ways in which membrane structure appears to be more complex than initially postulated by the fluid mosaic model. This raises the question of how the phase behavior in one bilayer leaflet may affect the apposing leaflet and how one begins to construct asymmetric model systems to investigate these interleaflet interactions. In this paper, we report on all-atom molecular dynamics simulations (a total of 4.1 μs) of symmetric and asymmetric bilayer systems composed ofmore » liquid-ordered (Lo) or liquid-disordered (Ld) leaflets, based on the nanodomain-forming POPC/DSPC/cholesterol system. We begin by analyzing an asymmetric bilayer with leaflets derived from simulations of symmetric Lo and Ld bilayers. In this system, we observe that the properties of the Lo and Ld leaflets are similar to those of the Lo and Ld leaflets in corresponding symmetric systems. However, it is not obvious that mixing the equilibrium structures of their symmetric counterparts is the most appropriate way to construct asymmetric bilayers nor that these structures will manifest interleaflet couplings that lead to domain registry/antiregistry. We therefore constructed and simulated four additional asymmetric bilayer systems by systematically adding or removing lipids in the Ld leaflet to mimic potential density fluctuations. We find that the number of lipids in the Ld leaflet affects its own properties, as well as those of the apposing Lo leaflet. Collectively, the simulations reveal the presence of weak acyl chain interdigitation across bilayer leaflets, suggesting that interdigitation alone does not contribute significantly to the interleaflet coupling in nonphase-separated bilayers of this chemical composition. Finally, however, the properties of both leaflets appear to be sensitive to changes in in-plane lipid packing, possibly providing a mechanism for interleaflet coupling by modulating local density and/or curvature fluctuations.« less

Control of defect localization in crystalline wrinkling by curvature and topology

NASA Astrophysics Data System (ADS)

Lopez Jimenez, Francisco

We investigate the influence of curvature and topology on crystalline wrinkling patterns in generic elastic bilayers. Our numerical analysis predicts that the total number of defects created by adiabatic compression exhibits universal quadratic scaling for spherical, ellipsoidal and toroidal surfaces over a wide range of system sizes. However, both the localization of individual defects and the orientation of defect chains depend strongly on the local Gaussian curvature and its gradients across a surface. Our results imply that curvature and topology can be utilized to pattern defects in elastic materials, thus promising improved control over hierarchical bending, buckling or folding processes. Generally, this study suggests that bilayer systems provide an inexpensive yet valuable experimental test-bed for exploring the effects of geometrically induced forces on assemblies of topological charges. Joint work with Norbert Stoop, Romain Lagrange, Jorn Dunkel and Pedro M. Reis.

Unidirectional magnetoresistance in magnetic thin films with non-uniform thickness

NASA Astrophysics Data System (ADS)

Jia, M. W.; Zhou, C.; Zeng, F. L.; Wu, Y. Z.

2018-05-01

The magnetoresistance (MR) of Co film and Co/Pt bilayers was studied systematically as a function of Co and Pt thickness at room temperature. In the samples with the wedge shape, we found the unidirectional MR which has the characteristics of R (Mz )≠R (-Mz ) with the magnetization normal to the film. The measured unidirectional MR is attributed to the differential anomalous Hall resistance due to the thickness difference at the electrodes for the longitudinal resistance measurements. The unidirectional MR effect in the Co/Pt bilayers can be greatly suppressed by a non-magnetic Cu inserting layer.

Aromatic Side Chain Water-to-Lipid Transfer Free Energies Show a Depth Dependence across the Membrane Normal.

PubMed

McDonald, Sarah K; Fleming, Karen G

2016-06-29

Quantitating and understanding the physical forces responsible for the interactions of biomolecules are fundamental to the biological sciences. This is especially challenging for membrane proteins because they are embedded within cellular bilayers that provide a unique medium in which hydrophobic sequences must fold. Knowledge of the energetics of protein-lipid interactions is thus vital to understand cellular processes involving membrane proteins. Here we used a host-guest mutational strategy to calculate the Gibbs free energy changes of water-to-lipid transfer for the aromatic side chains Trp, Tyr, and Phe as a function of depth in the membrane. This work reveals an energetic gradient in the transfer free energies for Trp and Tyr, where transfer was most favorable to the membrane interfacial region and comparatively less favorable into the bilayer center. The transfer energetics follows the concentration gradient of polar atoms across the bilayer normal that naturally occurs in biological membranes. Additional measurements revealed nearest-neighbor coupling in the data set are influenced by a network of aromatic side chains in the host protein. Taken together, these results show that aromatic side chains contribute significantly to membrane protein stability through either aromatic-aromatic interactions or placement at the membrane interface.

Permeability of DOPC bilayers under photoinduced oxidation: Sensitivity to photosensitizer.

PubMed

Bacellar, Isabel O L; Baptista, Mauricio S; Junqueira, Helena C; Wainwright, Mark; Thalmann, Fabrice; Marques, Carlos M; Schroder, André P

2018-06-07

The modification of lipid bilayer permeability is one of the most striking yet poorly understood physical transformations that follow photoinduced lipid oxidation. We have recently proposed that the increase of permeability of photooxidized 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers is controlled by the time required by the oxidized lipid species to diffuse and aggregate into pores. Here we further probe this mechanism by studying photosensitization of DOPC membranes by methylene blue (MB) and DO15, a more hydrophobic phenothiazinium photosensitizer, under different irradiation powers. Our results not only reveal the interplay between the production rate and the diffusion of the oxidized lipids, but highlight also the importance of photosensitizer localization in the kinetics of oxidized membrane permeability. Copyright © 2018. Published by Elsevier B.V.

Superconducting order parameter fluctuations in NbN/NiCu and NbTiN/NiCu bilayer nanostripes for photon detection

NASA Astrophysics Data System (ADS)

Aichner, Bernd; Jausner, Florian; Zechner, Georg; Mühlgassner, Rita; Lang, Wolfgang; Klimov, Andrii; Puźniak, Roman; Słysz, Wojciech; Guziewicz, Marek; Kruszka, Renata; Wegrzecki, Maciej; Sobolewski, Roman

2017-05-01

Thermodynamic fluctuations of the superconducting order parameter in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) thin bilayers patterned to microbridges are investigated. Plain NbN and NbTiN films served as reference materials for the analyses. The samples were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the superconducting films were coated with NiCu overlays, using co-sputtering. The positive magnetoresistance of the superconducting single layers is very small in the normal state but with a sharp upturn close to the superconducting transition, a familiar signature of superconducting fluctuations. The fluctuation-enhanced conductivity (paraconductivity) of the NbN and NbTiN single layer films is slightly larger than the prediction of the parameter-free Aslamazov-Larkin theory for order-parameter fluctuations in two-dimensional superconductors. The addition of a ferromagnetic top layer, however, changes the magnetotransport properties significantly. The S/F bilayers show a negative magnetoresistance up to almost room temperature, while the signature of fluctuations is similar to that in the plain films, demonstrating the relevance of both ferromagnetic and superconducting effects in the S/F bilayers. The paraconductivity is reduced below theoretical predictions, in particular in the NbTiN/NiCu bilayers. Such suppression of the fluctuation amplitude in S/F bilayers could be favorable to reduce dark counts in superconducting photon detectors and lead the way to enhance their performance.

Lipid Clustering Correlates with Membrane Curvature as Revealed by Molecular Simulations of Complex Lipid Bilayers

PubMed Central

Koldsø, Heidi; Shorthouse, David; Hélie, Jean; Sansom, Mark S. P.

2014-01-01

Cell membranes are complex multicomponent systems, which are highly heterogeneous in the lipid distribution and composition. To date, most molecular simulations have focussed on relatively simple lipid compositions, helping to inform our understanding of in vitro experimental studies. Here we describe on simulations of complex asymmetric plasma membrane model, which contains seven different lipids species including the glycolipid GM3 in the outer leaflet and the anionic lipid, phosphatidylinositol 4,5-bisphophate (PIP2), in the inner leaflet. Plasma membrane models consisting of 1500 lipids and resembling the in vivo composition were constructed and simulations were run for 5 µs. In these simulations the most striking feature was the formation of nano-clusters of GM3 within the outer leaflet. In simulations of protein interactions within a plasma membrane model, GM3, PIP2, and cholesterol all formed favorable interactions with the model α-helical protein. A larger scale simulation of a model plasma membrane containing 6000 lipid molecules revealed correlations between curvature of the bilayer surface and clustering of lipid molecules. In particular, the concave (when viewed from the extracellular side) regions of the bilayer surface were locally enriched in GM3. In summary, these simulations explore the nanoscale dynamics of model bilayers which mimic the in vivo lipid composition of mammalian plasma membranes, revealing emergent nanoscale membrane organization which may be coupled both to fluctuations in local membrane geometry and to interactions with proteins. PMID:25340788

Lipid clustering correlates with membrane curvature as revealed by molecular simulations of complex lipid bilayers.

PubMed

Koldsø, Heidi; Shorthouse, David; Hélie, Jean; Sansom, Mark S P

2014-10-01

Cell membranes are complex multicomponent systems, which are highly heterogeneous in the lipid distribution and composition. To date, most molecular simulations have focussed on relatively simple lipid compositions, helping to inform our understanding of in vitro experimental studies. Here we describe on simulations of complex asymmetric plasma membrane model, which contains seven different lipids species including the glycolipid GM3 in the outer leaflet and the anionic lipid, phosphatidylinositol 4,5-bisphophate (PIP2), in the inner leaflet. Plasma membrane models consisting of 1500 lipids and resembling the in vivo composition were constructed and simulations were run for 5 µs. In these simulations the most striking feature was the formation of nano-clusters of GM3 within the outer leaflet. In simulations of protein interactions within a plasma membrane model, GM3, PIP2, and cholesterol all formed favorable interactions with the model α-helical protein. A larger scale simulation of a model plasma membrane containing 6000 lipid molecules revealed correlations between curvature of the bilayer surface and clustering of lipid molecules. In particular, the concave (when viewed from the extracellular side) regions of the bilayer surface were locally enriched in GM3. In summary, these simulations explore the nanoscale dynamics of model bilayers which mimic the in vivo lipid composition of mammalian plasma membranes, revealing emergent nanoscale membrane organization which may be coupled both to fluctuations in local membrane geometry and to interactions with proteins.

Can the Lateral Proximity Effect Be Used to Create the Superconducting Transition of a Micron-Sized TES?

NASA Technical Reports Server (NTRS)

Barrentine, E. M.; Brandl, D. E.; Brown, A. D.; Denis, K. L.; Fionkbeiner, F. M.; Hsieh, W. T.; Nagler, P. C.; Stevenson, T. R.; Timble, P. T.; U-Yen, K.

2012-01-01

Recent measurements of micron-sized Mo/Au bilayer Transition Edge Sensors (TESs) have demonstrated that the TES can behave like an S-S'-S weak link due to the lateral proximity effect from superconducting leads. In this regime the Tc is a function of bias current, and the effective Tc shifts from the bilayer Tc towards the lead Tc. We explore the idea that a micron-sized S-N-S weak link could provide a new method to engineer the TES Tc. This method would be particularly useful when small size requirements for a bilayer TES (such as for a hot-electron microbolometer) lead to undesirable shifts in the bilayer Te. We present measurements of a variety of micron-sized normal Au 'TES' devices with Nb leads. We find no evidence of a superconducting transition in the Au film of these devices, in dramatic contrast to the strong lateral proximity effect seen in micron-sized Mo/Au bilayer devices. The absence of a transition in these devices is also in disagreement with theoretical predictions for S-N-S weak links. We hypothesize that a finite contact resistance between the Nb and Au may be weakening the effect. We conclude that the use of the lateral proximity effect to create a superconducting transition will be difficult given current fabrication procedures.

Probing microscopic material properties inside simulated membranes through spatially resolved three-dimensional local pressure fields and surface tensions

PubMed Central

Kasson, Peter M.; Hess, Berk; Lindahl, Erik

2013-01-01

Cellular lipid membranes are spatially inhomogeneous soft materials. Materials properties such as pressure and surface tension thus show important microscopic-scale variation that is critical to many biological functions. We present a means to calculate pressure and surface tension in a 3D-resolved manner within molecular-dynamics simulations and show how such measurements can yield important insight. We also present the first corrections to local virial and pressure fields to account for the constraints typically used in lipid simulations that otherwise cause problems in highly oriented systems such as bilayers. Based on simulations of an asymmetric bacterial ion channel in a POPC bilayer, we demonstrate how 3D-resolved pressure can probe for both short-range and long-range effects from the protein on the membrane environment. We also show how surface tension is a sensitive metric for inter-leaflet equilibrium and can be used to detect even subtle imbalances between bilayer leaflets in a membrane-protein simulation. Since surface tension is known to modulate the function of many proteins, this effect is an important consideration for predictions of ion channel function. We outline a strategy by which our local pressure measurements, which we make available within a version of the GROMACS simulation package, may be used to design optimally equilibrated membrane-protein simulations. PMID:23318532

Dynamic nuclear polarization enhanced nuclear magnetic resonance and electron spin resonance studies of hydration and local water dynamics in micelle and vesicle assemblies.

PubMed

McCarney, Evan R; Armstrong, Brandon D; Kausik, Ravinath; Han, Songi

2008-09-16

We present a unique analysis tool for the selective detection of local water inside soft molecular assemblies (hydrophobic cores, vesicular bilayers, and micellar structures) suspended in bulk water. Through the use of dynamic nuclear polarization (DNP), the (1)H NMR signal of water is amplified, as it interacts with stable radicals that possess approximately 658 times higher spin polarization. We utilized stable nitroxide radicals covalently attached along the hydrophobic tail of stearic acid molecules that incorporate themselves into surfactant-based micelle or vesicle structures. Here, we present a study of local water content and fluid viscosity inside oleate micelles and vesicles and Triton X-100 micelles to serve as model systems for soft molecular assemblies. This approach is unique because the amplification of the NMR signal is performed in bulk solution and under ambient conditions with site-specific spin labels that only detect the water that is directly interacting with the localized spin labels. Continuous wave (cw) electron spin resonance (ESR) analysis provides rotational dynamics of the spin-labeled molecular chain segments and local polarity parameters that can be related to hydration properties, whereas we show that DNP-enhanced (1)H NMR analysis of fluid samples directly provides translational water dynamics and permeability of the local environment probed by the spin label. Our technique therefore has the potential to become a powerful analysis tool, complementary to cw ESR, to study hydration characteristics of surfactant assemblies, lipid bilayers, or protein aggregates, where water dynamics is a key parameter of their structure and function. In this study, we find that there is significant penetration of water inside the oleate micelles with a higher average local water viscosity (approximately 1.8 cP) than in bulk water, and Triton X-100 micelles and oleate vesicle bilayers mostly exclude water while allowing for considerable surfactant chain motion and measurable water permeation through the soft structure.

Interactions of the GM2 activator protein with phosphatidylcholine bilayers: a site-directed spin-labeling power saturation study.

PubMed

Mathias, Jordan D; Ran, Yong; Carter, Jeffery D; Fanucci, Gail E

2009-09-02

The GM2 activator protein (GM2AP) is an accessory protein that is an essential component in the catabolism of the ganglioside GM2. A function of GM2AP is to bind and extract GM2 from intralysosomal vesicles, forming a soluble protein-lipid complex, which interacts with the hydrolase Hexosaminidase A, the enzyme that cleaves the terminal sugar group of GM2. Here, we used site-directed spin labeling with power saturation electron paramagnetic resonance to determine the surface-bound orientation of GM2AP upon phosphatidylcholine vesicles. Because GM2AP extracts lipid ligands from the vesicle and is undergoing exchange on and off the vesicle surface, we utilized a nickel-chelating lipid to localize the paramagnetic metal collider to the lipid bilayer-aqueous interface. Spin-labeled sites that collide with the lipid-bound metal relaxing agent provide a means for mapping sites of the protein that interact with the lipid bilayer interface. Results show that GM2AP binds to lipid bilayers such that the residues lining the lipid-binding cavity lie on the vesicle surface. This orientation creates a favorable microenvironment that can allow for the lipid tails to flip out of the bilayer directly into the hydrophobic pocket of GM2AP.

Effects of Stretching Speed on Mechanical Rupture of Phospholipid/Cholesterol Bilayers: Molecular Dynamics Simulation

PubMed Central

Shigematsu, Taiki; Koshiyama, Kenichiro; Wada, Shigeo

2015-01-01

Rupture of biological cell membrane under mechanical stresses is critical for cell viability. It is triggered by local rearrangements of membrane molecules. We investigated the effects of stretching speed on mechanical rupture of phospholipid/cholesterol bilayers using unsteady molecular dynamics simulations. We focused on pore formation, the trigger of rupture, in a 40 mol% cholesterol-including bilayer. The unsteady stretching was modeled by proportional and temporal scaling of atom positions at stretching speeds from 0.025 to 30 m/s. The effects of the stretching speed on the critical areal strain, where the pore forms, is composed of two regimes. At low speeds (<1.0 m/s), the critical areal strain is insensitive to speed, whereas it significantly increases at higher speeds. Also, the strain is larger than that of a pure bilayer, regardless of the stretching speeds, which qualitatively agrees with available experimental data. Transient recovery of the cholesterol and phospholipid molecular orientations was evident at lower speeds, suggesting the formation of a stretch-induced interdigitated gel-like phase. However, this recovery was not confirmed at higher speeds or for the pure bilayer. The different responses of the molecular orientations may help explain the two regimes for the effect of stretching speed on pore formation. PMID:26471872

Direct Imaging of Individual Intrinsic Hydration Layers on Lipid Bilayers at Ångstrom Resolution

PubMed Central

Fukuma, Takeshi; Higgins, Michael J.; Jarvis, Suzanne P.

2007-01-01

The interactions between water and biological molecules have the potential to influence the structure, dynamics, and function of biological systems, hence the importance of revealing the nature of these interactions in relation to the local biochemical environment. We have investigated the structuring of water at the interface of supported dipalmitoylphosphatidylcholine bilayers in the gel phase in phosphate buffer solution using frequency modulation atomic force microscopy (FM-AFM). We present experimental results supporting the existence of intrinsic (i.e., surface-induced) hydration layers adjacent to the bilayer. The force versus distance curves measured between the bilayer and the AFM tip show oscillatory force profiles with a peak spacing of 0.28 nm, indicative of the existence of up to two hydration layers next to the membrane surface. These oscillatory force profiles reveal the molecular-scale origin of the hydration force that has been observed between two apposing lipid bilayers. Furthermore, FM-AFM imaging at the water/lipid interface visualizes individual hydration layers in three dimensions, with molecular-scale corrugations corresponding to the lipid headgroups. The results demonstrate that the intrinsic hydration layers are stable enough to present multiple energy barriers to approaching nanoscale objects, such as proteins and solvated ions, and are expected to affect membrane permeability and transport. PMID:17325013

Dislocations in bilayer graphene

NASA Astrophysics Data System (ADS)

Butz, Benjamin; Dolle, Christian; Niekiel, Florian; Weber, Konstantin; Waldmann, Daniel; Weber, Heiko B.; Meyer, Bernd; Spiecker, Erdmann

2014-01-01

Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating ABAC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and few-layer graphene.

Dislocations in bilayer graphene.

PubMed

Butz, Benjamin; Dolle, Christian; Niekiel, Florian; Weber, Konstantin; Waldmann, Daniel; Weber, Heiko B; Meyer, Bernd; Spiecker, Erdmann

2014-01-23

Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating AB B[Symbol: see text]AC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and few-layer graphene.

Kinetic Defects Induced by Melittin in Model Lipid Membranes: A Solution Atomic Force Microscopy Study.

PubMed

Pan, Jianjun; Khadka, Nawal K

2016-05-26

Quantitative characterization of membrane defects (pores) is important for elucidating the molecular basis of many membrane-active peptides. We study kinetic defects induced by melittin in vesicular and planar lipid bilayers. Fluorescence spectroscopy measurements indicate that melittin induces time-dependent calcein leakage. Solution atomic force microscopy (AFM) is used to visualize melittin-induced membrane defects. After initial equilibration, the most probable defect radius is ∼3.8 nm in 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) bilayers. Unexpectedly, defects become larger with longer incubation, accompanied by substantial shape transformation. The initial defect radius is ∼4.7 nm in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers. Addition of 30 mol % cholesterol to DOPC bilayers suppresses defect kinetics, although the inhibitory impact is negated by longer incubation. Overall, the kinetic rate of defect development follows DLPC > DOPC > DOPC/cholesterol. Kinetic defects are also observed when anionic lipids are present. Based on the observation that defects can occupy as large as 40% of the bilayer surface, we propose a kinetic defect growth model. We also study the effect of melittin on the phase behavior of DOPC/egg-sphingomyelin/cholesterol bilayers. We find that melittin initially suppresses or eliminates liquid-ordered (Lo) domains; Lo domains gradually emerge and become the dominant species with longer incubation; and defects in phase-coexisting bilayers have a most probable radius of ∼5 nm and are exclusively localized in the liquid-disordered (Ld) phase. Our experimental data highlight that melittin-induced membrane defects are not static; conversely, spontaneous defect growth is intrinsically associated with membrane permeabilization exerted by melittin.

Membrane Protein Mobility and Orientation Preserved in Supported Bilayers Created Directly from Cell Plasma Membrane Blebs.

PubMed

Richards, Mark J; Hsia, Chih-Yun; Singh, Rohit R; Haider, Huma; Kumpf, Julia; Kawate, Toshimitsu; Daniel, Susan

2016-03-29

Membrane protein interactions with lipids are crucial for their native biological behavior, yet traditional characterization methods are often carried out on purified protein in the absence of lipids. We present a simple method to transfer membrane proteins expressed in mammalian cells to an assay-friendly, cushioned, supported lipid bilayer platform using cell blebs as an intermediate. Cell blebs, expressing either GPI-linked yellow fluorescent proteins or neon-green fused transmembrane P2X2 receptors, were induced to rupture on glass surfaces using PEGylated lipid vesicles, which resulted in planar supported membranes with over 50% mobility for multipass transmembrane proteins and over 90% for GPI-linked proteins. Fluorescent proteins were tracked, and their diffusion in supported bilayers characterized, using single molecule tracking and moment scaling spectrum (MSS) analysis. Diffusion was characterized for individual proteins as either free or confined, revealing details of the local lipid membrane heterogeneity surrounding the protein. A particularly useful result of our bilayer formation process is the protein orientation in the supported planar bilayer. For both the GPI-linked and transmembrane proteins used here, an enzymatic assay revealed that protein orientation in the planar bilayer results in the extracellular domains facing toward the bulk, and that the dominant mode of bleb rupture is via the "parachute" mechanism. Mobility, orientation, and preservation of the native lipid environment of the proteins using cell blebs offers advantages over proteoliposome reconstitution or disrupted cell membrane preparations, which necessarily result in significant scrambling of protein orientation and typically immobilized membrane proteins in SLBs. The bleb-based bilayer platform presented here is an important step toward integrating membrane proteomic studies on chip, especially for future studies aimed at understanding fundamental effects of lipid interactions on protein activity and the roles of membrane proteins in disease pathways.

Energetics of Mg incorporation at GaN(0001) and GaN(0001¯) surfaces

NASA Astrophysics Data System (ADS)

Sun, Qiang; Selloni, Annabella; Myers, T. H.; Doolittle, W. Alan

2006-04-01

By using density functional calculations in the generalized gradient approximation, we investigate the energetics of Mg adsorption and incorporation at GaN(0001) and GaN(0001¯) surfaces under various Ga and Mg coverage conditions as well as in presence of light or electron beam-induced electronic excitation. We find significant differences in Mg incorporation between Ga- and N-polar surfaces. Mg incorporation is easier at the Ga-polar surface, but high Mg coverages are found to cause important distortions which locally change the polarity from Ga to N polar. At the N-rich and moderately Ga-rich GaN(0001) surface, 0.25 ML of Mg substituting Ga in the top bilayer strongly reduce the surface diffusion barriers of Ga and N adatoms, in agreement with the surfactant effect observed in experiments. As the Mg coverage exceeds 0.5 ML, partial incorporation in the subsurface region (second bilayer) becomes favorable. A surface structure with 0.5 ML of incorporated Mg in the top bilayer and 0.25 ML in the second bilayer is found to be stable over a wide range of Ga chemical potential. At the Ga bilayer-terminated GaN(0001) surface, corresponding to Ga-rich conditions, configurations where Mg is incorporated in the interface region between the metallic Ga bilayer and the underlying GaN bilayer appear to be favored. At the N-polar surface, Mg is not incorporated under N-rich or moderately Ga-rich conditions, whereas incorporation in the adlayer may take place under Ga-rich conditions. In the presence of light or electron beam induced excitation, energy differences between Mg incorporated at the surface and in deeper layers are reduced so that the tendency toward surface segregation is also reduced.

Capacitance of graphenes

NASA Astrophysics Data System (ADS)

Young, Andrea; Dean, Cory; Meric, Inanc; Hone, Jim; Shepard, Ken; Kim, Philip

2010-03-01

Using a transfer procedure and single crystal hexagonal Boron Nitride gate dielectric, we are able to fabricate high mobility graphene devices with local top and back gates. The novel geometry of these devices allows us to measure the spatially averaged compressibility of mono- and bilayer graphene using the ``penetration field'' technique [Eisenstein, J.P. et al. Phys. Rev. Lett. 68, 674 (1992)]. In particular, we analyze the the effects of strong transverse electric fields on the compressibility of graphenes, especially as pertains to charged impurity scattering in single layer graphene and the opening of an energy gap in bilayer.

The evolutionary origin of the need to sleep: an inevitable consequence of synaptic neurotransmission?

PubMed

Cantor, Robert S

2015-01-01

It is proposed that the evolutionary origin of the need to sleep is the removal of neurotransmitters (NTs) that escape reuptake and accumulate in brain interstitial fluid (ISF). Recent work suggests that the activity of ionotropic postsynaptic receptors, rapidly initiated by binding of NTs to extracellular sites, is modulated over longer times by adsorption of these NTs to the lipid bilayers in which the receptors are embedded. This bilayer-mediated mechanism is far less molecularly specific than binding, so bilayer adsorption of NTs that have diffused into synapses for other receptors would modulate their activity as well. Although NTs are recycled by membrane protein reuptake, the process is less than 100% efficient; a fraction escapes the region in which these specific reuptake proteins are localized and eventually diffuses throughout the ISF. It is estimated that even if only 0.1% of NTs escape reuptake, they would accumulate and adsorb to bilayers in synapses of other receptors sufficiently to affect receptor activity, the harmful consequences of which are avoided by sleep: a period of efficient convective clearance of solutes together with greatly reduced synaptic activity.

Effective Brownian ratchet separation by a combination of molecular filtering and a self-spreading lipid bilayer system.

PubMed

Motegi, Toshinori; Nabika, Hideki; Fu, Yingqiang; Chen, Lili; Sun, Yinlu; Zhao, Jianwei; Murakoshi, Kei

2014-07-01

A new molecular manipulation method in the self-spreading lipid bilayer membrane by combining Brownian ratchet and molecular filtering effects is reported. The newly designed ratchet obstacle was developed to effectively separate dye-lipid molecules. The self-spreading lipid bilayer acted as both a molecular transport system and a manipulation medium. By controlling the size and shape of ratchet obstacles, we achieved a significant increase in the separation angle for dye-lipid molecules compared to that with the previous ratchet obstacle. A clear difference was observed between the experimental results and the simple random walk simulation that takes into consideration only the geometrical effect of the ratchet obstacles. This difference was explained by considering an obstacle-dependent local decrease in molecular diffusivity near the obstacles, known as the molecular filtering effect at nanospace. Our experimental findings open up a novel controlling factor in the Brownian ratchet manipulation that allow the efficient separation of molecules in the lipid bilayer based on the combination of Brownian ratchet and molecular filtering effects.

Brillouin light scattering study of spin waves in NiFe/Co exchange spring bilayer films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Haldar, Arabinda; Banerjee, Chandrima; Laha, Pinaki

2014-04-07

Spin waves are investigated in Permalloy(Ni{sub 80}Fe{sub 20})/Cobalt(Co) exchange spring bilayer thin films using Brillouin light scattering (BLS) experiment. The magnetic hysteresis loops measured by magneto-optical Kerr effect show a monotonic decrease in coercivity of the bilayer films with increasing Py thickness. BLS study shows two distinct modes, which are modelled as Damon-Eshbach and perpendicular standing wave modes. Linewidths of the frequency peaks are found to increase significantly with decreasing Py layer thickness. Interfacial roughness causes to fluctuate exchange coupling at the nanoscale regimes and the effect is stronger for thinner Py films. A quantitative analysis of the magnon linewidthsmore » shows the presence of strong local exchange coupling field which is much larger compared to macroscopic exchange field.« less

Lipid nanoparticle interactions and assemblies

NASA Astrophysics Data System (ADS)

Preiss, Matthew Ryan

Novel liposome-nanoparticle assemblies (LNAs) provide a biologically inspired route for designing multifunctional bionanotheranostics. LNAs combine the benefits of lipids and liposomes to encapsulate, transport, and protect hydrophilic and hydrophobic therapeutics with functional nanoparticles. Functional nanoparticles endow LNAs with additional capabilities, including the ability to target diseases, triggered drug release, controlled therapeutic output, and diagnostic capabilities to produce a drug delivery system that can effectively and efficiently deliver therapeutics while reducing side effects. Not only could LNAs make existing drugs better, they could also provide an avenue to allow once promising non-approved drugs (rejected due to harmful side effects, inadequate pharmacokinetics, and poor efficacy) to be safely used through targeted and controlled delivery directly to the diseased site. LNAs have the potential to be stimuli responsive, delivering drugs on command by external (ultrasound, RF heating, etc.) or internal (pH, blood sugar, heart rate, etc.) stimuli. Individually, lipids and nanoparticles have been clinically approved for therapy, such as Doxil (a liposomal doxorubicin for cancer treatment), and diagnosis, such as Feridex (an iron oxide nanoparticle an MRI contrast enhancement agent for liver tumors). In order to engineer these multifunctional LNAs for theranostic applications, the interactions between nanoparticles and lipids must be better understood. This research sought to explore the formation, design, structures, characteristics, and functions of LNAs. To achieve this goal, different types of LNAs were formed, specifically magnetoliposomes, bilayer decorated LNAs (DLNAs), and lipid-coated magnetic nanoparticles (LMNPs). A fluorescent probe was embedded in the lipid bilayer of magnetoliposomes allowing the local temperature and membrane fluidity to be observed. When subjected to an electromagnetic field that heated the encapsulated iron oxide nanoparticles encapsulated in the lipid bilayer, the local temperature and membrane fluidity could be observed. DLNAs were encapsulated with different sized nanoparticles and concentrations in order to observe the effect of the bilayer nanoparticles on the lipid bilayer's phase behavior and leakage. Two different sized nanoparticles were used, a 2 nm gold nanoparticle (GNP) much smaller than the thickness of the bilayer and a 4 nm GNP near the thickness of the lipid bilayer. The 2 nm GNPs were shown to affect the lipid bilayer differently than the 4 nm GNP. Specifically, the two nanoparticles altered the phase behavior and leakage differently in a temperature dependent fashion, demonstrating that embedded nanoparticle size can be used induce or inhibit bilayer leakage. A dual solvent exchange method was used to control the lipid surface composition of an iron oxide nanoparticle with a cationic lipid and a polyethylene glycol (PEG) lipid to produce lipid coated magnetic nanoparticles (LMNPs). PEG is well known for its ability to enhance the pharmacokinetics of nanostructures by preventing uptake by the immune system. By controlling the lipid surface composition, the surface charge and PEG conformation can be controlled which allowed the LMNPs to be used as an MRI contrast agent and a delivery system for siRNA that could be triggered with temperature.

Interaction of Local Anesthetics with Biomembranes Consisting of Phospholipids and Cholesterol: Mechanistic and Clinical Implications for Anesthetic and Cardiotoxic Effects

PubMed Central

2013-01-01

Despite a long history in medical and dental application, the molecular mechanism and precise site of action are still arguable for local anesthetics. Their effects are considered to be induced by acting on functional proteins, on membrane lipids, or on both. Local anesthetics primarily interact with sodium channels embedded in cell membranes to reduce the excitability of nerve cells and cardiomyocytes or produce a malfunction of the cardiovascular system. However, the membrane protein-interacting theory cannot explain all of the pharmacological and toxicological features of local anesthetics. The administered drug molecules must diffuse through the lipid barriers of nerve sheaths and penetrate into or across the lipid bilayers of cell membranes to reach the acting site on transmembrane proteins. Amphiphilic local anesthetics interact hydrophobically and electrostatically with lipid bilayers and modify their physicochemical property, with the direct inhibition of membrane functions, and with the resultant alteration of the membrane lipid environments surrounding transmembrane proteins and the subsequent protein conformational change, leading to the inhibition of channel functions. We review recent studies on the interaction of local anesthetics with biomembranes consisting of phospholipids and cholesterol. Understanding the membrane interactivity of local anesthetics would provide novel insights into their anesthetic and cardiotoxic effects. PMID:24174934

Performance of an X-Ray Microcalorimeter with a 240 Micron Absorber and a 50 Micron TES Bilayer

NASA Technical Reports Server (NTRS)

Miniussi, Antoine R.; Adams, Joseph S.; Bandler, Simon R.; Chervenak, James A.; Datesman, Aaron M.; Eckart, Megan E.; Ewin, Audrey J.; Finkbeiner, Fred M.; Kelley, Richard L.; Kilbourne, Caroline A.;

Inductive detection of fieldlike and dampinglike ac inverse spin-orbit torques in ferromagnet/normal-metal bilayers

NASA Astrophysics Data System (ADS)

Berger, Andrew J.; Edwards, Eric R. J.; Nembach, Hans T.; Karenowska, Alexy D.; Weiler, Mathias; Silva, Thomas J.

2018-03-01

Functional spintronic devices rely on spin-charge interconversion effects, such as the reciprocal processes of electric field-driven spin torque and magnetization dynamics-driven spin and charge flow. Both dampinglike and fieldlike spin-orbit torques have been observed in the forward process of current-driven spin torque and dampinglike inverse spin-orbit torque has been well studied via spin pumping into heavy metal layers. Here, we demonstrate that established microwave transmission spectroscopy of ferromagnet/normal metal bilayers under ferromagnetic resonance can be used to inductively detect the ac charge currents driven by the inverse spin-charge conversion processes. This technique relies on vector network analyzer ferromagnetic resonance (VNA-FMR) measurements. We show that in addition to the commonly extracted spectroscopic information, VNA-FMR measurements can be used to quantify the magnitude and phase of all ac charge currents in the sample, including those due to spin pumping and spin-charge conversion. Our findings reveal that Ni80Fe20/Pt bilayers exhibit both dampinglike and fieldlike inverse spin-orbit torques. While the magnitudes of both the dampinglike and fieldlike inverse spin-orbit torque are of comparable scale to prior reported values for similar material systems, we observed a significant dependence of the dampinglike magnitude on the order of deposition. This suggests interface quality plays an important role in the overall strength of the dampinglike spin-to-charge conversion.

Annealing-Induced Bi Bilayer on Bi2Te3 Investigated via Quasi-Particle-Interference Mapping.

PubMed

Schouteden, Koen; Govaerts, Kirsten; Debehets, Jolien; Thupakula, Umamahesh; Chen, Taishi; Li, Zhe; Netsou, Asteriona; Song, Fengqi; Lamoen, Dirk; Van Haesendonck, Chris; Partoens, Bart; Park, Kyungwha

2016-09-27

Topological insulators (TIs) are renowned for their exotic topological surface states (TSSs) that reside in the top atomic layers, and hence, detailed knowledge of the surface top atomic layers is of utmost importance. Here we present the remarkable morphology changes of Bi2Te3 surfaces, which have been freshly cleaved in air, upon subsequent systematic annealing in ultrahigh vacuum and the resulting effects on the local and area-averaging electronic properties of the surface states, which are investigated by combining scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and Auger electron spectroscopy (AES) experiments with density functional theory (DFT) calculations. Our findings demonstrate that the annealing induces the formation of a Bi bilayer atop the Bi2Te3 surface. The adlayer results in n-type doping, and the atomic defects act as scattering centers of the TSS electrons. We also investigated the annealing-induced Bi bilayer surface on Bi2Te3 via voltage-dependent quasi-particle-interference (QPI) mapping of the surface local density of states and via comparison with the calculated constant-energy contours and QPI patterns. We observed closed hexagonal patterns in the Fourier transform of real-space QPI maps with secondary outer spikes. DFT calculations attribute these complex QPI patterns to the appearance of a "second" cone due to the surface charge transfer between the Bi bilayer and the Bi2Te3. Annealing in ultrahigh vacuum offers a facile route for tuning of the topological properties and may yield similar results for other topological materials.

Sensitive imaging of magnetization structure and dynamics using picosecond laser heating

NASA Astrophysics Data System (ADS)

Bartell, Jason; Jermain, Colin; Aradhya, Sriharsha; Brangham, Jack; Yang, Fengyuan; Ralph, Daniel; Fuchs, Gregory

We demonstrate the time-resolved longitudinal spin Seebeck effect (TRLSSE) as the basis for an ultrafast, high-resolution, and sensitive microscope for imaging ferromagnetic insulator/normal metal spintronic devices. By focusing a picosecond laser to 0.7 μm, we generate a sub-100 ps electrical signal from the combination of the TRLSSE and the inverse spin Hall effect in yittrium iron garnet (YIG)/platinum (Pt) bilayers. This signal is a spatiotemporal measurement of the local, in-plane magnetic orientation of YIG with outstanding sensitivity better than 0.3° /√{ Hz } in samples with 20 nm of YIG. Static imaging of YIG/Pt devices reveals variations in the local magnetic anisotropy on a few micron scale. Phase-sensitive ferromagnetic resonance imaging reveals corresponding variations in the resonance field, amplitude, phase, and linewidth. These results show the TRLSSE is a powerful tool for static and dynamic studies of spintronic devices made with ferromagnetic insulators. This research was supported by the AFOSR (FA9550-14-1-0243) and by NSF (DMR-1406333, DMR-1507274, and DMR-1120296).

Position-Dependent Diffusion Tensors in Anisotropic Media from Simulation: Oxygen Transport in and through Membranes.

PubMed

Ghysels, An; Venable, Richard M; Pastor, Richard W; Hummer, Gerhard

2017-06-13

A Bayesian-based methodology is developed to estimate diffusion tensors from molecular dynamics simulations of permeants in anisotropic media, and is applied to oxygen in lipid bilayers. By a separation of variables in the Smoluchowski diffusion equation, the multidimensional diffusion is reduced to coupled one-dimensional diffusion problems that are treated by discretization. The resulting diffusivity profiles characterize the membrane transport dynamics as a function of the position across the membrane, discriminating between diffusion normal and parallel to the membrane. The methodology is first validated with neat water, neat hexadecane, and a hexadecane slab surrounded by water, the latter being a simple model for a lipid membrane. Next, a bilayer consisting of pure 1-palmitoyl 2-oleoylphosphatidylcholine (POPC), and a bilayer mimicking the lipid composition of the inner mitochondrial membrane, including cardiolipin, are investigated. We analyze the detailed time evolution of oxygen molecules, in terms of both normal diffusion through and radial diffusion inside the membrane. Diffusion is fast in the more loosely packed interleaflet region, and anisotropic, with oxygen spreading more rapidly in the membrane plane than normal to it. Visualization of the propagator shows that oxygen enters the membrane rapidly, reaching its thermodynamically favored center in about 1 ns, despite the free energy barrier at the headgroup region. Oxygen transport is quantified by computing the oxygen permeability of the membranes and the average radial diffusivity, which confirm the anisotropy of the diffusion. The position-dependent diffusion constants and free energies are used to construct compartmental models and test assumptions used in estimating permeability, including Overton's rule. In particular, a hexadecane slab surrounded by water is found to be a poor model of oxygen transport in membranes because the relevant energy barriers differ substantially.

Fabry-Pérot Interference in Gapped Bilayer Graphene with Broken Anti-Klein Tunneling

NASA Astrophysics Data System (ADS)

Varlet, Anastasia; Liu, Ming-Hao; Krueckl, Viktor; Bischoff, Dominik; Simonet, Pauline; Watanabe, Kenji; Taniguchi, Takashi; Richter, Klaus; Ensslin, Klaus; Ihn, Thomas

2014-09-01

We report the experimental observation of Fabry-Pérot interference in the conductance of a gate-defined cavity in a dual-gated bilayer graphene device. The high quality of the bilayer graphene flake, combined with the device's electrical robustness provided by the encapsulation between two hexagonal boron nitride layers, allows us to observe ballistic phase-coherent transport through a 1-μm-long cavity. We confirm the origin of the observed interference pattern by comparing to tight-binding calculations accounting for the gate-tunable band gap. The good agreement between experiment and theory, free of tuning parameters, further verifies that a gap opens in our device. The gap is shown to destroy the perfect reflection for electrons traversing the barrier with normal incidence (anti-Klein tunneling). The broken anti-Klein tunneling implies that the Berry phase, which is found to vary with the gate voltages, is always involved in the Fabry-Pérot oscillations regardless of the magnetic field, in sharp contrast with single-layer graphene.

Double Barriers and Magnetic Field in Bilayer Graphene

NASA Astrophysics Data System (ADS)

Redouani, Ilham; Jellal, Ahmed; Bahlouli, Hocine

2015-12-01

We study the transmission probability in an AB-stacked bilayer graphene of Dirac fermions scattered by a double-barrier structure in the presence of a magnetic field. We take into account the full four bands structure of the energy spectrum and use the suitable boundary conditions to determine the transmission probability. Our numerical results show that for energies higher than the interlayer coupling, four ways for transmission are possible while for energies less than the height of the barrier, Dirac fermions exhibit transmission resonances and only one transmission channel is available. We show that, for AB-stacked bilayer graphene, there is no Klein tunneling at normal incidence. We find that the transmission displays sharp peaks inside the transmission gap around the Dirac point within the barrier regions while they are absent around the Dirac point in the well region. The effect of the magnetic field, interlayer electrostatic potential, and various barrier geometry parameters on the transmission probabilities is also discussed.

Sign reversal of Hall signals in Tm3Fe5O12 /Pt with perpendicular magnetic anisotropy

NASA Astrophysics Data System (ADS)

Liu, Yawen; Tang, Chi; Xu, Yadong; Shi, Zhong; Shi, Jing

Robust interface strain-induced perpendicular magnetic anisotropy is produced in atomically flat ferromagnetic insulator Tm3Fe5O12 (TIG) films grown with pulsed laser deposition on both substituted-Gd3Ga5O12 and Nd3Ga5O12 (NGG). In TIG/Pt bilayers, we observe large hysteresis loops over a wide range of Pt thicknesses and temperatures. Both the ordinary Hall effect and anomalous Hall effect undergo a sign reversal as the temperature is lowered. The temperature dependence of the Hall signals in bilayers with different thickness of Pt indicates the existence of exchange interaction at the interface. Our results provide a clue to further understand the origin of the anomalous Hall effect in ferromagnetic insulator/normal metal bilayer systems. The work was supported as part of the SHINES, an Energy Frontier Research Center funded by the US Department of Energy, BES under Award No. SC0012670.

Buckling failure of square ice-nanotube arrays constrained in graphene nanocapillaries.

PubMed

Zhu, YinBo; Wang, FengChao; Wu, HengAn

2016-08-07

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates that the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the "buckling failure" of the square-ice-nanotube columns, which is dominated by the lateral pressure.

Shubnikov-de Haas oscillations of high mobility holes in monolayer and bilayer WSe2: spin-valley locking, effective mass, and inter-layer coupling

NASA Astrophysics Data System (ADS)

Fallahazad, Babak; Movva, Hema Chandra Prakash; Kim, Kyounghwan; Larentis, Stefano; Taniguchi, Takashi; Watanabe, Kenji; Banerjee, Sanjay K.; Tutuc, Emanuel

We study the magnetotransport properties of high mobility holes in monolayer and bilayer WSe2, measured in dual-gated samples with top and bottom hexagonal boron-nitride dielectrics, and using platinum bottom contacts. Thanks to the Pt high work-function combined with the a high hole density induced electrostatically by an applied top gate bias, the contacts remain ohmic down to low (1.5 K) temperatures. The samples display well defined Shubnikov-de Haas (SdH) oscillations, and quantum Hall states (QHS) in high magnetic fields. In both mono and bilayer WSe2, the SdH oscillations and the QHSs occur predominantly at even filling factors, evincing a two-fold Landau level degeneracy consistent with spin-valley locking. The Fourier transform analysis of the SdH oscillations in dual-gated bilayer WSe2 reveal the presence of two subbands, each localized in the top or the bottom layer, as well as negative compressibility. From the temperature dependence of the SdH oscillation amplitude we determine a hole effective mass of 0.45me for both mono and bilayer WSe2. The top and bottom layer densities can be independently tuned using the top and bottom gates, respectively, evincing a weak interlayer coupling. This work has been supported by NRI-SWAN and Intel corporation.

Molecular Dynamics Studies of Liposomes as Carriers for Photosensitizing Drugs: Development, Validation, and Simulations with a Coarse-Grained Model.

PubMed

Jämbeck, Joakim P M; Eriksson, Emma S E; Laaksonen, Aatto; Lyubartsev, Alexander P; Eriksson, Leif A

2014-01-14

Liposomes are proposed as drug delivery systems and can in principle be designed so as to cohere with specific tissue types or local environments. However, little detail is known about the exact mechanisms for drug delivery and the distributions of drug molecules inside the lipid carrier. In the current work, a coarse-grained (CG) liposome model is developed, consisting of over 2500 lipids, with varying degrees of drug loading. For the drug molecule, we chose hypericin, a natural compound proposed for use in photodynamic therapy, for which a CG model was derived and benchmarked against corresponding atomistic membrane bilayer model simulations. Liposomes with 21-84 hypericin molecules were generated and subjected to 10 microsecond simulations. Distribution of the hypericins, their orientations within the lipid bilayer, and the potential of mean force for transferring a hypericin molecule from the interior aqueous "droplet" through the liposome bilayer are reported herein.

Mutual adaptation of a membrane protein and its lipid bilayer during conformational changes.

PubMed

Sonntag, Yonathan; Musgaard, Maria; Olesen, Claus; Schiøtt, Birgit; Møller, Jesper Vuust; Nissen, Poul; Thøgersen, Lea

2011-01-01

The structural elucidation of membrane proteins continues to gather pace, but we know little about their molecular interactions with the lipid environment or how they interact with the surrounding bilayer. Here, with the aid of low-resolution X-ray crystallography, we present direct structural information on membrane interfaces as delineated by lipid phosphate groups surrounding the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) in its phosphorylated and dephosphorylated Ca(2+)-free forms. The protein-lipid interactions are further analysed using molecular dynamics simulations. We find that SERCA adapts to membranes of different hydrophobic thicknesses by inducing local deformations in the lipid bilayers and by undergoing small rearrangements of the amino-acid side chains and helix tilts. These mutually adaptive interactions allow smooth transitions through large conformational changes associated with the transport cycle of SERCA, a strategy that may be of general nature for many membrane proteins.

Effect of tension and curvature on the chemical potential of lipids in lipid aggregates.

PubMed

Grafmüller, Andrea; Lipowsky, Reinhard; Knecht, Volker

2013-01-21

Understanding the factors that influence the free energy of lipids in bilayer membranes is an essential step toward understanding exchange processes of lipids between membranes. In general, both lipid composition and membrane geometry can affect lipid exchange rates between bilayer membranes. Here, the free energy change ΔG(des) for the desorption of dipalmitoyl-phosphatidylcholine (DPPC) lipids from different lipid aggregates has been computed using molecular dynamics simulations and umbrella sampling. The value of ΔG(des) is found to depend strongly on the local properties of the aggregate, in that both tension and curvature lead to an increase in ΔG(des). A detailed analysis shows that the increased desorption free energy for tense bilayers arises from the increased conformational entropy of the lipid tails, which reduces the favorable component -TΔS(L) of the desorption free energy.

Electronic properties of bilayer graphenes strongly coupled to interlayer stacking and an external field

DOE PAGES

Park, Changwon; Ryou, Junga; Hong, Suklyun; ...

2015-07-02

Bilayer graphene (BLG) with a tunable band gap appears interesting as an alternative to graphene for practical applications; thus, its transport properties are being actively pursued. Using density functional theory and perturbation analysis, we investigated, under an external electric field, the electronic properties of BLG in various stackings relevant to recently observed complex structures. We established the first phase diagram summarizing the stacking-dependent gap openings of BLG for a given field. Lastly, we further identified high-density midgap states, localized on grain boundaries, even under a strong field, which can considerably reduce the overall transport gap.

Spin-Swapping Transport and Torques in Ultrathin Magnetic Bilayers

NASA Astrophysics Data System (ADS)

Saidaoui, Hamed Ben Mohamed; Manchon, A.

2016-07-01

Planar spin transport in disordered ultrathin magnetic bilayers comprising a ferromagnet and a normal metal (typically used for spin pumping, spin Seebeck and spin-orbit torque experiments) is investigated theoretically. Using a tight-binding model that puts the extrinsic spin Hall effect and spin swapping on equal footing, we show that the nature of spin-orbit coupled transport dramatically depends on the ratio between the layer thickness d and the mean free path λ . While the spin Hall effect dominates in the diffusive limit (d ≫λ ), spin swapping dominates in the Knudsen regime (d ≲λ ). A remarkable consequence is that spin swapping induces a substantial fieldlike torque in the Knudsen regime.

Topological gapped edge states in fractional quantum Hall-superconductor heterostructures

NASA Astrophysics Data System (ADS)

Cook, Ashley; Repellin, Cécile; Regnault, Nicolas; Neupert, Titus

We propose and implement a numerical setup for studying edge states of fractional quantum Hall droplets with a superconducting instability. We focus on a time-reversal symmetric bilayer fractional quantum Hall system of Laughlin ν = 1 / 3 states. The fully gapped edges carry a topological parafermionic degree of freedom that can encode quantum information protected against local perturbations. We numerically simulate such a system using exact diagonalization by restricting the calculation to the Laughlin quasihole subspace. We study the quantization of the total charge on each edge and show that the ground states are permuted by spin flux insertion and the parafermionic Josephson effect, evidencing their topological nature and the Cooper pairing of fractionalized quasiparticles. The full affiliation for Author 3 is: Laboratoire Pierre Aigrain, Ecole Normale Supérieure-PSL Research University, CNRS, Université Pierre et Marie Curie-Sorbonne Universités, Université Paris Diderot-Sorbonne Paris Cité, 24 rue Lhomond, 75231 Paris.

Two-Point Microrheology of Phase-Separated Domains in Lipid Bilayers

PubMed Central

Hormel, Tristan T.; Reyer, Matthew A.; Parthasarathy, Raghuveer

2015-01-01

Though the importance of membrane fluidity for cellular function has been well established for decades, methods for measuring lipid bilayer viscosity remain challenging to devise and implement. Recently, approaches based on characterizing the Brownian dynamics of individual tracers such as colloidal particles or lipid domains have provided insights into bilayer viscosity. For fluids in general, however, methods based on single-particle trajectories provide a limited view of hydrodynamic response. The technique of two-point microrheology, in which correlations between the Brownian dynamics of pairs of tracers report on the properties of the intervening medium, characterizes viscosity at length-scales that are larger than that of individual tracers and has less sensitivity to tracer-induced distortions, but has never been applied to lipid membranes. We present, to our knowledge, the first two-point microrheological study of lipid bilayers, examining the correlated motion of domains in phase-separated lipid vesicles and comparing one- and two-point results. We measure two-point correlation functions in excellent agreement with the forms predicted by two-dimensional hydrodynamic models, analysis of which reveals a viscosity intermediate between those of the two lipid phases, indicative of global fluid properties rather than the viscosity of the local neighborhood of the tracer. PMID:26287625

Ionophores at work: Exploring the interaction of guanosine-based amphiphiles with phospholipid membranes.

PubMed

Vitiello, Giuseppe; Musumeci, Domenica; Koutsioubas, Alexandros; Paduano, Luigi; Montesarchio, Daniela; D'Errico, Gerardino

2017-12-01

An amphiphilic derivative of guanosine, carrying a myristoyl group at the 5'-position and two methoxy(triethylene glycol) appendages at the 2' and 3'-positions (1), endowed with high ionophoric activity, has been here studied in its interaction mode with a model lipid membrane along with its 5'-spin-labelled analogue 2, bearing the 5-doxyl-stearic in lieu of the myristic residue. Electron spin resonance spectra, carried out on the spin-labelled nucleolipid 2 in mixture with a DOPC/DOPG phospholipid bilayer, on one side, and on spin-labelled lipids mixed with 1, on the other, integrated with dynamic light scattering and neutron reflectivity measurements, allowed getting an in-depth picture of the effect of the ionophores on membrane structure, relevant to clarify the ion transport mechanism through lipid bilayers. Particularly, dehydration of lipid headgroups and lowering of both the local polarity and acyl chains order across the bilayer, due to the insertion of the oligo(ethylene glycol) chains in the bilayer hydrophobic core, have been found to be the main effects of the amphiphilic guanosines interaction with the membrane. These results furnish directions to rationally implement future ionophores design. Copyright © 2017 Elsevier B.V. All rights reserved.

Gate-defined quantum confinement in suspended bilayer graphene

NASA Astrophysics Data System (ADS)

Allen, M. T.; Martin, J.; Yacoby, A.

2012-07-01

Quantum-confined devices that manipulate single electrons in graphene are emerging as attractive candidates for nanoelectronics applications. Previous experiments have employed etched graphene nanostructures, but edge and substrate disorder severely limit device functionality. Here we present a technique that builds quantum-confined structures in suspended bilayer graphene with tunnel barriers defined by external electric fields that open a bandgap, thereby eliminating both edge and substrate disorder. We report clean quantum dot formation in two regimes: at zero magnetic field B using the energy gap induced by a perpendicular electric field and at B>0 using the quantum Hall ν=0 gap for confinement. Coulomb blockade oscillations exhibit periodicity consistent with electrostatic simulations based on local top-gate geometry, a direct demonstration of local control over the band structure of graphene. This technology integrates single electron transport with high device quality and access to vibrational modes, enabling broad applications from electromechanical sensors to quantum bits.

Topological induced valley polarization in bilayer graphene/Boron Nitride

NASA Astrophysics Data System (ADS)

Basile, Leonardo; Idrobo, Juan C.

2015-03-01

Novel electronic devices relay in our ability to control internal quantum degrees of freedom of the electron e.g., its spin. The valley number degree of freedom is a pseudospin that labels degenerate eigenstates at local maximum/minimum on the valence/conduction band. Valley polarization, that is, selective electronic localization in a momentum valley and its manipulation can be achieved by means of circular polarized light (CPL) in a system with strong spin-orbit coupling (SOC). In this talk, we will show theoretically that despite the fact that neither graphene or BN have a strong SOC, a bilayer of graphene on BN oriented at a twist angle has different absorption for right- and left- CPL. This induced polarization occurs due to band folding of the electronic bands, i.e., it has a topological origin. This research was supported EPN multidisciplinary grant and by DOE SUFD MSED.

Atomistic Monte Carlo Simulation of Lipid Membranes

PubMed Central

Wüstner, Daniel; Sklenar, Heinz

2014-01-01

Biological membranes are complex assemblies of many different molecules of which analysis demands a variety of experimental and computational approaches. In this article, we explain challenges and advantages of atomistic Monte Carlo (MC) simulation of lipid membranes. We provide an introduction into the various move sets that are implemented in current MC methods for efficient conformational sampling of lipids and other molecules. In the second part, we demonstrate for a concrete example, how an atomistic local-move set can be implemented for MC simulations of phospholipid monomers and bilayer patches. We use our recently devised chain breakage/closure (CBC) local move set in the bond-/torsion angle space with the constant-bond-length approximation (CBLA) for the phospholipid dipalmitoylphosphatidylcholine (DPPC). We demonstrate rapid conformational equilibration for a single DPPC molecule, as assessed by calculation of molecular energies and entropies. We also show transition from a crystalline-like to a fluid DPPC bilayer by the CBC local-move MC method, as indicated by the electron density profile, head group orientation, area per lipid, and whole-lipid displacements. We discuss the potential of local-move MC methods in combination with molecular dynamics simulations, for example, for studying multi-component lipid membranes containing cholesterol. PMID:24469314

Low-temperature sup 2 H NMR spectroscopy of phospholipid bilayers containing docosahexaenoyl (22:6. omega. 3) chains

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barry, J.A.; Trouard, T.P.; Salmon, A.

1991-08-27

Polyunsaturated fatty acids are widely distributed components of biological membranes and are believed to be involved in many biological functions. However, the mechanisms by which they act on a molecular level are not understood. To further investigate the unique properties of {omega}3 polyunsaturated phospholipid bilayers, deuterium nuclear ming tic resonance ({sup 2}H NMR) studies have been made of the liquid-crystalline (L{sub {alpha}}) and gel phases of a homologous series of mixed-chain phosphatidylcholines containing docosahexaenoic acid: (per-{sup 2}H-n:0) (22:6) PC, where n = 12, 14, 16, and 18. The moments of the {sup 2}H NMR lineshapes have been evaluated, and formmore » these the warming and cooling main phase transition temperatures were determined. The transition temperatures of the mixed-chain series were found to be significantly lower ing those of the corresponding lipids in the disaturated series, di(per-{sup 2}H-n:0)PC, with hystereses ranging form 2 to 14 C. Distinct effects of the docosahexaenoyl chain on bilayer order were found, though these effects varied across the mixed-chain series. In evaluating the moment data, an empirical method for normalizing the moments with respect to differences in temperature was applied, in addition to using the reduced temperature method. For the systems studied here, the method of normalization haing significant effect on the interpretation of the moment data.« less

Asymmetric distribution of charged lipids between the leaflets of a vesicle bilayer induced by melittin and alamethicin

DOE Office of Scientific and Technical Information (OSTI.GOV)

Qian, Shuo; Heller, William T

2011-01-01

Cellular membranes are complex mixtures of lipids, proteins, and other small molecules that provide functional, dynamic barriers between the cell and its environment, as well as between environments within the cell. The lipid composition of the membrane is highly specific and controlled in terms of both content and lipid localization. The membrane structure results from the complex interplay between the wide varieties of molecules present. Here, small-angle neutron scattering and selective deuterium labeling were used to probe the impact of the membrane-active peptides melittin and alamethicin on the structure of lipid bilayers composed of a mixture of the lipids dimyristoylmore » phosphatidylglycerol (DMPG) and chain-perdeuterated dimyristoyl phosphatidylcholine (DMPC). We found that both peptides enriched the outer leaflet of the bilayer with the negatively charged DMPG, creating an asymmetric distribution of lipids. The level of enrichment is peptide concentration-dependent and is stronger for melittin than it is for alamethicin. The enrichment between the inner and outer bilayer leaflets occurs at very low peptide concentrations and increases with peptide concentration, including when the peptide adopts a membrane-spanning, pore-forming state. The results suggest that these membrane-active peptides may have a secondary stressful effect on target cells at low concentrations that results from a disruption of the lipid distribution between the inner and outer leaflets of the bilayer that is independent of the formation of transmembrane pores.« less

Buckling failure of square ice-nanotube arrays constrained in graphene nanocapillaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, YinBo; Wang, FengChao, E-mail: wangfc@ustc.edu.cn; Wu, HengAn

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates thatmore » the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the “buckling failure” of the square-ice-nanotube columns, which is dominated by the lateral pressure.« less

Dendrimer Interactions with Lipid Bilayer: Comparison of Force Field and Effect of Implicit vs Explicit Solvation.

PubMed

Kanchi, Subbarao; Gosika, Mounika; Ayappa, K G; Maiti, Prabal K

2018-06-13

The understanding of dendrimer interactions with cell membranes has great importance in drug/gene delivery based therapeutics. Although molecular simulations have been used to understand the nature of dendrimer interactions with lipid membranes, its dependency on available force field parameters is poorly understood. In this study, we have carried out fully atomistic molecular dynamics (MD) simulations of a protonated G3 poly(amido amine) (PAMAM) dendrimer-dimyristoylphosphatidylcholine (DMPC) lipid bilayer complex using three different force fields (FFs) namely, CHARMM, GAFF, and GROMOS in the presence of explicit water to understand the structure of the lipid-dendrimer complex and nature of their interaction. CHARMM and GAFF dendrimers initially in contact with the lipid head groups were found to move away from the lipid bilayer during the course of simulation; however, the dendrimer remained strongly bound to the lipid head groups with the GROMOS FF. Potential of the mean force (PMF) computations of the dendrimer along the bilayer normal showed a repulsive barrier (∼20 kcal/mol) between dendrimer and lipid bilayer in the case of CHARMM and GAFF force fields. In contrast, an attractive interaction (∼40 kcal/mol) is obtained with the GROMOS force field, consistent with experimental observations of membrane binding observed with lower generation G3 PAMAM dendrimers. This difference with the GROMOS dendrimer is attributed to the strong dendrimer-lipid interaction and lowered surface hydration of the dendrimer. Assessing the role of solvent, we find that the CHARMM and GAFF dendrimers strongly bind to the lipid bilayer with an implicit solvent (Generalized Born) model, whereas binding is not observed with explicit water (TIP3P). The opposing nature of dendrimer-membrane interactions in the presence of explicit and implicit solvents demonstrates that hydration effects play an important role in modulating the dendrimer-lipid interaction warranting a case for refinement of the existing dendrimer/lipid force fields.

Self-assembling siloxane bilayer directly on SiO2 surface of micro-cantilevers for long-term highly repeatable sensing to trace explosives.

PubMed

Chen, Ying; Xu, Pengcheng; Li, Xinxin

2010-07-02

This paper presents a novel sensing layer modification technique for static micro-cantilever sensors that detect trace explosives by measuring specific adsorption-induced surface stress. For the first time, a method of directly modifying a siloxane sensing bilayer on an SiO(2) surface is proposed to replace the conventional self-assembled monolayers (SAMs) of thiols on Au to avoid the trouble from long-term unstable Au-S bonds. For modifying the long-term reliable sensing bilayer on the piezoresistor-integrated micro-cantilevers, a siloxane-head bottom layer is self-assembled directly on the SiO(2) cantilever surface, which is followed by grafting another explosive-sensing-group functionalized molecule layer on top of the siloxane layer. The siloxane-modified sensor has experimentally exhibited a highly resoluble response to 0.1 ppb TNT vapor. More importantly, the repeated detection results after 140 days show no obvious attenuation in sensing signal. Also observed experimentally, the specific adsorption of the siloxane sensing bilayer to TNT molecules causes a tensile surface stress on the cantilever. Herein the measured tensile surface stress is in contrast to the compressive surface stress normally measured from conventional cantilever sensors where the sensitive thiol-SAMs are modified on an Au surface. The reason for this newly observed phenomenon is discussed and preliminarily analyzed.

Ultrasensitive two-color fluorescence probes for dipole potential in phospholipid membranes

PubMed Central

Klymchenko, Andrey S.; Duportail, Guy; Mély, Yves; Demchenko, Alexander P.

2003-01-01

The principle of electrochromic modulation of excited-state intramolecular proton-transfer reaction was applied for the design of fluorescence probes with high two-color sensitivity to dipole potential, Ψd, in phospholipid bilayers. We report on the effect of Ψd variation on excitation and fluorescence spectra of two new 3-hydroxyflavone probes, which possess opposite orientations of the fluorescent moiety in the lipid bilayer. The dipole potential in the bilayer was modulated by the addition of 6-ketocholestanol or phloretin and by substitution of dimyristoyl phosphatidylcholine lipid with its ether analog 1,2-di-o-tetradecyl-sn-glycero-3-phosphocholine, and its value was estimated by the reference styryl dye 1-(3-sulfonatopropyl)-4-{β[2-(di-n-octylamino)-6-naphthyl]vinyl}pyridinium betaine. We demonstrate that after Ψd changes, the probe orienting in the bilayer similarly to the reference dye shows similar shifts in the excitation spectra, whereas the probe with the opposite orientation shows the opposite shifts. The new observation is that the response of 3-hydroxyflavone probes to Ψd in excitation spectra is accompanied by and quantitatively correlated with dramatic changes of relative intensities of the two well separated emission bands that belong to the initial normal and the product tautomer forms of the excited-state intramolecular proton-transfer reaction. This provides a strong response to Ψd by change in emission color. PMID:12972636

Broadband angle-independent antireflection coatings on nanostructured light trapping solar cells

NASA Astrophysics Data System (ADS)

Vázquez-Guardado, Abraham; Boroumand, Javaneh; Franklin, Daniel; Chanda, Debashis

2018-03-01

Backscattering from nanostructured surfaces greatly diminishes the efficacy of light trapping solar cells. While the analytical design of broadband, angle-independent antireflection coatings on nanostructured surfaces proved inefficient, numerical optimization proves a viable alternative. Here, we numerically design and experimentally verify the performance of single and bilayer antireflection coatings on a 2D hexagonal diffractive light trapping pattern on crystalline silicon substrates. Three well-known antireflection coatings, aluminum oxide, silicon nitride, and silicon oxide, which also double as high-quality surface passivation materials, are studied in the 400-1000 nm band. By varying thickness and conformity, the optimal parameters that minimize the broadband total reflectance (specular and scattering) from the nanostructured surface are obtained. The design results in a single-layer antireflection coating with normal-angle wavelength-integrated reflectance below 4% and a bilayer antireflection coating demonstrating reflection down to 1.5%. We show experimentally an angle-averaged reflectance of ˜5.2 % up to 60° incident angle from the optimized bilayer antireflection-coated nanostructured surface, paving the path toward practical implementation of the light trapping solar cells.

Fluorescence Spectroscopy in Thermodynamic and Kinetic Analysis of pH-Dependent Membrane Protein Insertion

PubMed Central

Ladokhin, Alexey S.

2016-01-01

Experimental determination of the free energy stabilizing the structure of membrane proteins in their native lipid environment is undermined by a lack of appropriate methods and suitable model systems. Here, we demonstrate how fluorescence correlation spectroscopy can be used to characterize thermodynamics of pH-triggered bilayer insertion of nonconstitutive membrane proteins (e.g., bacterial toxins, colicins). The experimental design is guided by the appropriate thermodynamic scheme which considers two independent processes: pH-dependent formation of a membrane-competent form and its insertion into the lipid bilayer. Measurements of a model protein annexin B12 under conditions of lipid saturation demonstrate that protonation leading to the formation of the membrane-competent state occurs near membrane interface. Lipid titration experiments demonstrate that the free energy of transfer to the intermediate interfacial state is especially favorable, while the free energy of final insertion is modulated by interplay of hydrophobic and electrostatic interactions on the bilayer interface. The general principles of kinetic measurements along the insertion pathway containing interfacial intermediate are discussed and practical examples emphasizing appropriate fitting and normalization procedures are presented. PMID:21609856

Study of the benzocaine transfer from aqueous solution to the interior of a biological membrane.

PubMed

Porasso, Rodolfo D; Bennett, W F Drew; Oliveira-Costa, S D; López Cascales, J J

2009-07-23

The precise molecular mechanism of general anesthetics remains unknown. It is therefore important to understand where molecules with anesthetic properties localize within biological membranes. We have determined the free energy profile of a benzocaine molecule (BZC) across a biological membrane using molecular dynamics simulation. We use an asymmetric phospholipid bilayer with DPPS in one leaflet of a DPPC bilayer (Lopez Cascales et al. J. Phys. Chem. B 2006, 110, 2358-2363) to model a biological bilayer. From the free energy profile, we predict the zone of actuation of a benzocaine is located in the hydrocarbon region or at the end of the lipid head, depending of the presence of charged lipids (DPPS) in the leaflet. We observe a moderate increase in the disorder of the membrane and in particular an increase in the disorder of DPPS. Static and dynamic physicochemical properties of the benzocaine, such as its dipole orientation, translational diffusion coefficient, and rotational relaxation time were measured.

Incorporating isolated molybdenum (Mo) atoms into Bilayer Epitaxial Graphene on 4H-SiC(0001)

NASA Astrophysics Data System (ADS)

Huang, Han; Wan, Wen; Li, Hui; Wong, Swee Liang; Lv, Lu; Gao, Yongli; Wee, Andrew T. S.

2014-03-01

The atomic structures and electronic properties of isolated Mo atoms in bilayer epitaxial graphene (BLEG) on 4H-SiC(0001) are investigated by low temperature scanning tunneling microscopy (LT-STM). LT-STM results reveal that isolated Mo dopants prefer to substitute C atoms at α-sites, and preferentially locate between the graphene bilayers. First-principles calculations confirm that the embedding of single Mo dopants within BLEG is energetically favorable as compared to monolayer graphene. The calculated bandstructures show that Mo-doped BLEG is n-doped, and each Mo atom introduces a local magnetic moment of 1.81 μB. Our findings demonstrate a simple and stable method to incorporate single transition metal dopants into the graphene lattice to tune its electronic and magnetic properties for possible use in graphene spin devices. NRF-CRP (Singapore) grants R-143-000-360-281and R-144-000-295-281. ``Shenghua Professorship'' startup funding from CSU and the support from the NSF of China (Grant No.11304398).

Pressure effects on dipalmitoylphosphatidylcholine bilayers measured by sub 2 H nuclear magnetic resonance

DOE Office of Scientific and Technical Information (OSTI.GOV)

Driscoll, D.A.; Samarasinghe, S.; Adamy, S.

1991-04-02

The effects of pressure, up to 5 kbar, on multilamellar vesicles of 1,2-dipalmitoyl-sn-phosphatidylcholine perdeuterated in the acyl chains (DPPC-d{sub 62}) were examined by using high-pressure NMR techniques. A deuterium probe was built, and the quadrupole splitting was measured against pressure at various temperatures. The experiments were performed on pure lipid bilayers in the liquid-crystalline state and on bilayers in the liquid-crystalline state containing the local anesthetic tetracaine. The results show that the order parameter of all segments of the acyl chains increases with pressure in the liquid-crystalline state. The more highly ordered regions of the chains are affected slightly moremore » than the regions near the methyl ends. The addition of tetracaine increases the disorder of the chains, and pressure reverses the effect of anesthetic on the lipid as seen by the reversal of the changes in line shape and the measured order parameter.« less

Lipid packing determines protein-membrane interactions: challenges for apolipoprotein A–I and High Density Lipoproteins

PubMed Central

Sánchez, Susana A.; Tricerri, M. Alejandra; Ossato, Giulia; Gratton, Enrico

2010-01-01

Summary Protein and protein-lipid interactions, with and within specific areas in the cell membrane, are critical in order to modulate the cell signaling events required to maintain cell functions and viability. Biological bilayers are complex, dynamic platforms, and thus in vivo observations usually need to be preceded by studies on model systems that simplify and discriminate the different factors involved in lipid-protein interactions. Fluorescence microscopy studies using giant unilamellar vesicles (GUVs) as membrane model systems provide a unique methodology to quantify protein binding, interaction and lipid solubilization in artificial bilayers. The large size of lipid domains obtainable on GUVs, together with fluorescence microscopy techniques, provides the possibility to localize and quantify molecular interactions. FCS (Fluorescence Correlation Spectroscopy) can be performed using the GUV model to extract information on mobility and concentration. Two-photon Laurdan GP (Generalized Polarization) reports on local changes in membrane water content (related to membrane fluidity) due to protein binding or lipid removal from a given lipid domain. In this review, we summarize the experimental microscopy methods used to study the interaction of human apolipoprotein A–I (apoA-I) in lipid-free and lipid-bound conformations with bilayers and natural membranes. Results described here help us to understand cholesterol homeostasis, and offer a methodological design suited to different biological systems. PMID:20347719

Fatty acyl chain order in lecithin model membranes determined from proton magnetic resonance.

PubMed

Bloom, M; Burnell, E E; MacKay, A L; Nichol, C P; Valic, M I; Weeks, G

1978-12-26

Proton magnetic resonance (1H NMR) has been used to compare the local orientational order of acyl chains in phospholipid bilayers of multilamellar and small sonicated vesicular membranes of dipalmitoyllecithin (DPL) at 50 degrees C and egg yolk lecithin (EYL) at 31 degrees C. The orientational order of the multilamellar systems was characterized using deuterium magnetic resonance order parameters and 1H NMR second moments. 1H NMR line shapes in the vesicle samples were calculated using vesicle size distributions, determined directly using electron microscopy, and a theory of motional narrowing, which takes into account the symmetry properties of the bilayer systems. The predicted non-Lorentzian line shapes and widths were found to be in good agreement with experimental results, indicating that the local orientational order (called "packing" by many workers) in the bilayers of small vesicles and in multilamellar membranes is substantially the same. This results was found to be true not only for the largest 1H NMR line associated with the nonterminal methylene protons but also for the resolved 1H NMR lines due to the alpha-CH2 and the terminal CH3 positions on the acyl chain. Analysis of the vesicle 1H NMR spectra of EYL taken with different medium viscosities yielded a value of approximately 4 X 10(-8) cm2 s-1 for the lateral diffusion constant of the phospholipid molecules at 31 degrees C.

Twisted MoSe 2 bilayers with variable local stacking and interlayer coupling revealed by low-frequency Raman spectroscopy

DOE PAGES

Puretzky, Alexander A.; Liang, Liangbo; Li, Xufan; ...

2016-01-14

Unique twisted bilayers of MoSe 2 with multiple stacking orientations and interlayer couplings in the narrow range of twist angles, 60 ± 3°, are revealed by low-frequency Raman spectroscopy and theoretical analysis. The slight deviation from 60 allows the concomitant presence of patches featuring all three high-symmetry stacking configurations (2H or AA', AB', A'B) in one unique bilayer system. In this case, the periodic arrangement of the patches and their size strongly depend on the twist angle. Ab initio modeling predicts significant changes in frequencies and intensities of low-frequency modes versus stacking and twist angle. Experimentally, the variable stacking andmore » coupling across the interface is revealed by the appearance of two breathing modes corresponding to the mixture of the high-symmetry stacking configurations and unaligned regions of monolayers. Only one breathing mode is observed outside the narrow range of twist angles. This indicates a stacking transition to unaligned monolayers with mismatched atom registry without the in-plane restoring force required to generate a shear mode. As a result, the variable interlayer coupling and spacing in transition metal dichalcogenide bilayers revealed in this study may provide a new platform for optoelectronic applications of these materials.« less

Twisted MoSe 2 bilayers with variable local stacking and interlayer coupling revealed by low-frequency Raman spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Puretzky, Alexander A.; Liang, Liangbo; Li, Xufan

Unique twisted bilayers of MoSe 2 with multiple stacking orientations and interlayer couplings in the narrow range of twist angles, 60 ± 3°, are revealed by low-frequency Raman spectroscopy and theoretical analysis. The slight deviation from 60 allows the concomitant presence of patches featuring all three high-symmetry stacking configurations (2H or AA', AB', A'B) in one unique bilayer system. In this case, the periodic arrangement of the patches and their size strongly depend on the twist angle. Ab initio modeling predicts significant changes in frequencies and intensities of low-frequency modes versus stacking and twist angle. Experimentally, the variable stacking andmore » coupling across the interface is revealed by the appearance of two breathing modes corresponding to the mixture of the high-symmetry stacking configurations and unaligned regions of monolayers. Only one breathing mode is observed outside the narrow range of twist angles. This indicates a stacking transition to unaligned monolayers with mismatched atom registry without the in-plane restoring force required to generate a shear mode. As a result, the variable interlayer coupling and spacing in transition metal dichalcogenide bilayers revealed in this study may provide a new platform for optoelectronic applications of these materials.« less

Molecular dynamics study of lipid bilayers modeling the plasma membranes of mouse hepatocytes and hepatomas.

PubMed

Andoh, Yoshimichi; Aoki, Noriyuki; Okazaki, Susumu

2016-02-28

Molecular dynamics (MD) calculations of lipid bilayers modeling the plasma membranes of normal mouse hepatocytes and hepatomas in water have been performed under physiological isothermal-isobaric conditions (310.15 K and 1 atm). The changes in the membrane properties induced by hepatic canceration were investigated and were compared with previous MD calculations included in our previous study of the changes in membrane properties induced by murine thymic canceration. The calculated model membranes for normal hepatocytes and hepatomas comprised 23 and 24 kinds of lipids, respectively. These included phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, sphingomyelin, lysophospholipids, and cholesterol. We referred to previously published experimental values for the mole fraction of the lipids adopted in the present calculations. The calculated structural and dynamic properties of the membranes such as lateral structure, order parameters, lateral self-diffusion constants, and rotational correlation times all showed that hepatic canceration causes plasma membranes to become more ordered laterally and less fluid. Interestingly, this finding contrasts with the less ordered structure and increased fluidity of plasma membranes induced by thymic canceration observed in our previous MD study.

Molecular dynamics study of lipid bilayers modeling the plasma membranes of mouse hepatocytes and hepatomas

NASA Astrophysics Data System (ADS)

Andoh, Yoshimichi; Aoki, Noriyuki; Okazaki, Susumu

2016-02-01

Molecular dynamics (MD) calculations of lipid bilayers modeling the plasma membranes of normal mouse hepatocytes and hepatomas in water have been performed under physiological isothermal-isobaric conditions (310.15 K and 1 atm). The changes in the membrane properties induced by hepatic canceration were investigated and were compared with previous MD calculations included in our previous study of the changes in membrane properties induced by murine thymic canceration. The calculated model membranes for normal hepatocytes and hepatomas comprised 23 and 24 kinds of lipids, respectively. These included phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, sphingomyelin, lysophospholipids, and cholesterol. We referred to previously published experimental values for the mole fraction of the lipids adopted in the present calculations. The calculated structural and dynamic properties of the membranes such as lateral structure, order parameters, lateral self-diffusion constants, and rotational correlation times all showed that hepatic canceration causes plasma membranes to become more ordered laterally and less fluid. Interestingly, this finding contrasts with the less ordered structure and increased fluidity of plasma membranes induced by thymic canceration observed in our previous MD study.

Implications of Weak-Link Behavior on the Performance of Mo/Au Bilayer Transition-Edge Sensors

NASA Technical Reports Server (NTRS)

Smith, Stephen J.

2011-01-01

Understanding the physical properties of the superconducting-to-normal transition is fundamental for optimizing the design and performance of transition-edge sensors (TESs). Recent critical current measurements of Mol Au bilayer test structures show that they act as weak superconducting links, exhibiting oscillatory, Fraunhofer-like behavior with applied magnetic field. In this paper we investigate the implications of this behavior for TES X-ray detectors, under operational bias conditions. These devices include normal metal features used for absorber attachment and unexplained noise suppression, which result in modifications to the previously reported critical current behavior. We present measurements of the logarithmic resistance sensitivity with temperature, a, and current, b, as a function of applied magnetic field and bias point within the resistive transition. Results show that these important device parameters exhibit similar oscillatory behavior with applied magnetic field, which in turn affects the signal responsivity and noise, and hence the energy resolution. These results show the significance of the critical current in determining the performance of TESs and hold promise to improve future.

Changes in lipid bilayer structure caused by the helix-to-sheet transition of an HIV-1 gp41 fusion peptide derivative

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heller, William T.; Rai, Durgesh K.

HIV-1, like other enveloped viruses, undergoes fusion with the cell membrane to infect it. Viral coat proteins are thought to bind the virus to the membrane and actively fuse the viral and cellular membranes together. The actual molecular mechanism of fusion is challenging to visualize, resulting in the use of model systems. In this paper, the bilayer curvature modifying properties of a synthetic variant of the HIV-1 gp41 fusion peptide with lipid bilayer vesicles composed of a mixture of dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylserine (DMPS) were studied. In 7:3 DMPC:DMPS vesicles made with deuterium-labeled DMPC, the peptide was observedmore » to undergo a concentration-dependent conformational transition between an α-helix and an antiparallel β-sheet. Through the use of small-angle neutron scattering (SANS) and selective deuterium labeling, it was revealed that conformational transition of the peptide is also accompanied by a transition in the structure of the lipid bilayer. In addition to changes in the distribution of the lipid between the leaflets of the vesicle, the SANS data are consistent with two regions having different thicknesses. Finally, of the two different bilayer structures, the one corresponding to the smaller area fraction, being ~8% of the vesicle area, is much thicker than the remainder of the vesicle, which suggests that there are regions of localized negative curvature similar to what takes place at the point of contact between two membranes immediately preceding fusion.« less

Changes in lipid bilayer structure caused by the helix-to-sheet transition of an HIV-1 gp41 fusion peptide derivative

DOE PAGES

Heller, William T.; Rai, Durgesh K.

2017-01-16

HIV-1, like other enveloped viruses, undergoes fusion with the cell membrane to infect it. Viral coat proteins are thought to bind the virus to the membrane and actively fuse the viral and cellular membranes together. The actual molecular mechanism of fusion is challenging to visualize, resulting in the use of model systems. In this paper, the bilayer curvature modifying properties of a synthetic variant of the HIV-1 gp41 fusion peptide with lipid bilayer vesicles composed of a mixture of dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylserine (DMPS) were studied. In 7:3 DMPC:DMPS vesicles made with deuterium-labeled DMPC, the peptide was observedmore » to undergo a concentration-dependent conformational transition between an α-helix and an antiparallel β-sheet. Through the use of small-angle neutron scattering (SANS) and selective deuterium labeling, it was revealed that conformational transition of the peptide is also accompanied by a transition in the structure of the lipid bilayer. In addition to changes in the distribution of the lipid between the leaflets of the vesicle, the SANS data are consistent with two regions having different thicknesses. Finally, of the two different bilayer structures, the one corresponding to the smaller area fraction, being ~8% of the vesicle area, is much thicker than the remainder of the vesicle, which suggests that there are regions of localized negative curvature similar to what takes place at the point of contact between two membranes immediately preceding fusion.« less

Adsorption and magnetism of bilayer graphene on the MnO polar surface with oxygen vacancies in the interface: First principles study

NASA Astrophysics Data System (ADS)

Ilyasov, Victor V.; Ershov, Igor V.; Popova, Inna G.; Pham, Khang D.; Nguyen, Chuong V.

2018-05-01

In this paper, we investigate systematically the structural, electronic, magnetic and adsorption properties of Bernal-stacked bilayer graphene on MnO(111) surface terminated by an oxygen atom, as a function of nonstoichiometric composition of the BLG/MnOx(111) interface. For additional functionalization of the BLG/MnOx(111) system, we also studied the adsorption properties of oxygen adsorbed on the BLG/MnOx(111) interface. Our results showed that the BLG is bound to the MnOx(111) substrate by the weak interaction for both spin-up and spin-down. Furthermore, we found that BLG adsorbed on the MnOx(111) substrate with a reduced oxygen symmetry in the interface is accompanied with a downshift of the Fermi level, which identifies the band structure of BLG as a p-type semiconductor. Upon interaction between BLG and MnOx(111) substrate, a forbidden gap of about 350 meV was opened between its bonding and antibonding π bands. A forbidden gap and the local magnetic moments in bilayer graphene can be controlled by changing the oxygen nonstoichometry or by oxygen adsorption. Additionally, magnetism has been predicted in the bilayer graphene adsorbed on the polar MnOx(111) surface with oxygen vacancies in the BLG/MnOx(111) interface, and its nature has also been discussed in this work. These results showed that the adsorption of bilayer graphene on the MnO(111) substrate can be used for developing novel generation of electronic and spintronic devices.

pH sensing by lipids in membranes: The fundamentals of pH-driven migration, polarization and deformations of lipid bilayer assemblies.

PubMed

Angelova, Miglena I; Bitbol, Anne-Florence; Seigneuret, Michel; Staneva, Galya; Kodama, Atsuji; Sakuma, Yuka; Kawakatsu, Toshihiro; Imai, Masayuki; Puff, Nicolas

2018-03-06

Most biological molecules contain acido-basic groups that modulate their structure and interactions. A consequence is that pH gradients, local heterogeneities and dynamic variations are used by cells and organisms to drive or regulate specific biological functions including energetic metabolism, vesicular traffic, migration and spatial patterning of tissues in development. While the direct or regulatory role of pH in protein function is well documented, the role of hydrogen and hydroxyl ions in modulating the properties of lipid assemblies such as bilayer membranes is only beginning to be understood. Here, we review approaches using artificial lipid vesicles that have been instrumental in providing an understanding of the influence of pH gradients and local variations on membrane vectorial motional processes: migration, membrane curvature effects promoting global or local deformations, crowding generation by segregative polarization processes. In the case of pH induced local deformations, an extensive theoretical framework is given and an application to a specific biological issue, namely the structure and stability of mitochondrial cristae, is described. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo. Copyright © 2018 Elsevier B.V. All rights reserved.

Pseudogap in normal underdoped phase of Bi2212: LDA + DMFT + Σk

NASA Astrophysics Data System (ADS)

Nekrasov, I. A.; Kuchinskii, E. Z.; Pchelkina, Z. V.; Sadovskii, M. V.

2007-09-01

Pseudogap phenomena are observed for normal underdoped phase of different high- Tc cuprates. Among others Bi 2Sr 2CaCu 2O 8- δ (Bi2212) compound is one of the most studied experimentally [A. Damascelli, Z. Hussain, Z.-X. Shen, Rev. Mod. Phys. 75 (2003) 473; J.C. Campuzano, M.R. Norman, M. Randeria, in: K.H. Bennemann, J.B. Ketterson (Eds.), Physics of Superconductors, vol. 2, Springer, Berlin, 2004, p. 167; J. Fink et al., cond-mat/0512307; X.J. Zhou et al., cond-mat/0604284]. To describe pseudogap regime in Bi2212, we employ novel generalized DMFT + Σk approach [E.Z. Kuchinskii, I.A. Nekrasov, M.V. Sadovskii, JETP Lett. 82 (2005) 198; M.V. Sadovskii et al., Phys. Rev. B 72 (2005) 155105, and these proceedings, doi:10.1016/j.physc.2007.03.367]. This approach gives possibility to preserve conventional dynamical mean-field theory (DMFT) equations [A. Georges et al., Rev. Mod. Phys. 68 (1996) 13] and include an additional (momentum dependent) self-energy Σk. In the present case, Σk describes non-local dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations [M.V. Sadovskii, Physics-Uspekhi 44 (2001) 515, cond-mat/0408489]. The effective single impurity problem in the DMFT + Σk is solved by numerical renormalization group (NRG) [R. Bulla, A.C. Hewson, Th. Pruschke, J. Phys. Cond. Mat. 10 (1998) 8365; R. Bulla, Phys. Rev. Lett. 83 (1999) 136]. To take into account material specific properties of two neighboring CuO 2 layers of Bi2212 we employ local density approximation (LDA) to calculate necessary model parameters, e.g. the values of intra- and interlayer hopping integrals between Cu-sites. Onsite Coulomb interaction U for x2- y2 orbital was calculated in constrained LDA method [O. Gunnarsson et al., Phys. Rev. B 39 (1989) 1708]. The value of pseudogap potential Δ was obtained within DMFT(NRG) [E.Z. Kuchinskii, I.A. Nekrasov, M.V. Sadovskii, JETP Lett. 82 (2005) 198; M.V. Sadovskii et al., Phys. Rev. B 72 (2005) 155105, and these proceedings, doi:10.1016/j.physc.2007.03.367]. Here, we report theoretical LDA + DMFT + Σk quasiparticle bands dispersion, Fermi surface (FS) and angular resolved photoemission (ARPES) spectra accounting for pseudogap and bilayer splitting effects for normal underdoped Bi2212 ( δ = 0.15). We show that LDA-calculated value of bilayer splitting (BS) is too small to describe experimentally observed peak-dip-hump structure. Fermi surface in presence of the pseudogap fluctuations is almost insensitive to the BS value. Results obtained are in good agreement with recent ARPES experiments.

Dynamics, Surface Electrostatics and Phase Properties of Nanoscale Curved Lipid Bilayers

NASA Astrophysics Data System (ADS)

Koolivand, Amir

Surface electrostatic potential of a lipid bilayer governs many vital functions of living cells. Several classes of proteins are known of exhibiting strong binding preferences to curved lipid bilayer surfaces. In this project we employed electron paramagnetic resonance (EPR) of a recently introduced phospholipid (IMTSL-PTE) bearing a pH-sensitive nitroxide covalently attached to the lipid head group to measure the surface electrostatics of the lipid membrane and nanopore-confined lipid bilayers as a function of the bilayer curvature. The pKa of the ionizable group of this lipid-based spin probe is reporting on the bilayer surface electrostatics potential by changes in the EPR spectra. Specifically, both rotational dynamics and magnetic parameters of the nitroxide are affected by the probe protonation. Effect of curvature on the surface electrostatic potential and dynamics of lipid bilayer was studied for POPG and DMPG unilamellar vesicles (ULVs). It was found that the magnitude of the negative surface electrostatic potential increased upon decrease in the vesicle diameter for the bilayers in the fluid phase; however, no significant changes were observed for DMPG ULVs in a gel phase. We speculate that biologically relevant fluid bilayer phase allows for a larger variability in the lipid packing density in the lipid polar head group region than a more ordered gel phase and it is likely that the lipid flip-flop is responsible for pH equilibration of IMTSL-PTE. The kinetic EPR study of nitroxide reduction showed that the rate of flip-flop is in the order of 10-5 s-1. The flip-flop rate constant increases when vesicle size deceases. Oxygen permeability measured by X-ban EPR decreases in higher curved vesicles---an observation that is consistent with a tighter packing in smaller vesicles. Partitioning of a small nitroxide molecule TEMPO into ULVs was measured by X-band (9 GHz) and W-band (95 GHz) EPR spectroscopy. The partitioning coefficient of this probe in the lipid phase of the bilayer was higher in smaller vesicles likely due to a larger number of defects in smaller vesicles allowing more water soluble molecules partitioning into lipid bilayers. However, the rotational correlation time for TEMPO slows down in smaller vesicles indicating an increase in the lipid packing. Pulsed EPR techniques, HYSCORE and ESEEM spectroscopy, were used to detect local water concentration and distinguish the hydrogen bonded water to the nitroxide from the bulk one. HYSCORE was then employed to investigate the effect of bilayer curvature on the water penetration into lipid bilayer and it was found that the higher curved lipids allow more water to penetrate into lipid bilayer as a result of more defects in the highly curved lipid vesicles. Nanopore-confined lipid bilayers formed inside ordered nanochannels of anodic aluminum oxide (AAO) have found many practical applications, serving as thermodynamically stable biophysical models of cellular membranes of concave curvature and allowing for stabilization of membrane proteins in functional conformations. It was found that surface potential of POPG lipids inside the AAO pores are higher than that of vesicles---the effect that is attributed to highly ordered and packed lipids inside the AAO nanopores. At pH=7.0 the AAO zeta potential was found to be -29+/-0.64 mV. Cytochrome C and poly glutamic acid as positively and negatively charged macromolecules in physiological pH (7.4) were used to prepare multilayer protein nanotubes and cytochrome c interaction with AAO was studied by CD and UV-Vis spectroscopy. Lipid nanotube arrays containing a transmembrane WALP peptide were also formed and these macroscopically aligned lipid nanotubes were studied by CD spectroscopy. The lipid phase transition of DMPC and binding of melittin, an antibacterial peptide model, were observed from a frequency change for the QCM quartz-AAO-Lipid as a promising "biosensor".

Molecular organization, localization and orientation of antifungal antibiotic amphotericin B in a single lipid bilayer

PubMed Central

Grudzinski, Wojciech; Sagan, Joanna; Welc, Renata; Luchowski, Rafal; Gruszecki, Wieslaw I.

2016-01-01

Amphotericin B is a popular antifungal antibiotic, a gold standard in treatment of systemic mycotic infections, due to its high effectiveness. On the other hand, applicability of the drug is limited by its considerable toxicity to patients. Biomembranes are a primary target of physiological activity of amphotericin B and both the pharmacologically desired and toxic side effects of the drug relay on its molecular organization in the lipid phase. In the present work, molecular organization, localization and orientation of amphotericin B, in a single lipid bilayer system, was analysed simultaneously, thanks to application of a confocal fluorescence lifetime imaging microscopy of giant unilamellar vesicles. The results show that the presence of sterols, in the lipid phase, promotes formation of supramolecular structures of amphotericin B and their penetration into the membrane hydrophobic core. The fact that such an effect is substantially less pronounced in the case of cholesterol than ergosterol, the sterol of fungal membranes, provides molecular insight into the selectivity of the drug. PMID:27620838

Generation and detection of pure valley current by electrically induced Berry curvature in bilayer graphene

NASA Astrophysics Data System (ADS)

Shimazaki, Y.; Yamamoto, M.; Borzenets, I. V.; Watanabe, K.; Taniguchi, T.; Tarucha, S.

2015-12-01

The field of `Valleytronics’ has recently been attracting growing interest as a promising concept for the next generation electronics, because non-dissipative pure valley currents with no accompanying net charge flow can be manipulated for computational use, akin to pure spin currents. Valley is a quantum number defined in an electronic system whose energy bands contain energetically degenerate but non-equivalent local minima (conduction band) or maxima (valence band) due to a certain crystal structure. Specifically, spatial inversion symmetry broken two-dimensional honeycomb lattice systems exhibiting Berry curvature is a subset of possible systems that enable optical, magnetic and electrical control of the valley degree of freedom. Here we use dual-gated bilayer graphene to electrically induce and control broken inversion symmetry (or Berry curvature) as well as the carrier density for generating and detecting the pure valley current. In the insulating regime, at zero-magnetic field, we observe a large nonlocal resistance that scales cubically with the local resistivity, which is evidence of pure valley current.

Probing topology and dynamics of the second transmembrane domain (M2δ) of the acetyl choline receptor using magnetically aligned lipid bilayers (bicelles) and EPR spectroscopy.

PubMed

Sahu, Indra D; Mayo, Daniel J; Subbaraman, Nidhi; Inbaraj, Johnson J; McCarrick, Robert M; Lorigan, Gary A

2017-08-01

Characterizing membrane protein structure and dynamics in the lipid bilayer membrane is very important but experimentally challenging. EPR spectroscopy offers a unique set of techniques to investigate a membrane protein structure, dynamics, topology, and distance constraints in lipid bilayers. Previously our lab demonstrated the use of magnetically aligned phospholipid bilayers (bicelles) for probing topology and dynamics of the membrane peptide M2δ of the acetyl choline receptor (AchR) as a proof of concept. In this study, magnetically aligned phospholipid bilayers and rigid spin labels were further utilized to provide improved dynamic information and topology of M2δ peptide. Seven TOAC-labeled AchR M2δ peptides were synthesized to demonstrate the utility of a multi-labeling amino acid substitution alignment strategy. Our data revealed the helical tilts to be 11°, 17°, 9°, 17°, 16°, 11°, 9°±4° for residues I7TOAC, Q13TOAC, A14TOAC, V15TOAC, C16TOAC, L17TOAC, and L18TOAC, respectively. The average helical tilt of the M2δ peptide was determined to be ∼13°. This study also revealed that the TOAC labels were attached to the M2δ peptide with different dynamics suggesting that the sites towards the C-terminal end are more rigid when compared to the sites towards the N-terminus. The dynamics of the TOAC labeled sites were more resolved in the aligned samples when compared to the randomly disordered samples. This study highlights the use of magnetically aligned lipid bilayer EPR technique to determine a more accurate helical tilt and more resolved local dynamics of AchR M2δ peptide. Copyright © 2017 Elsevier B.V. All rights reserved.

The role of lipid composition for insertion and stabilization of amino acids in membranes

NASA Astrophysics Data System (ADS)

Johansson, Anna C. V.; Lindahl, Erik

2009-05-01

While most membrane protein helices are clearly hydrophobic, recent experiments have indicated that it is possible to insert marginally hydrophobic helices into bilayers and have suggested apparent in vivo free energies of insertion for charged residues that are low, e.g., a few kcals for arginine. In contrast, a number of biophysical simulation studies have predicted that the bilayer interior is close to a pure hydrophobic environment with large penalties for hydrophilic amino acids—and yet the experimental scales do significantly better at predicting actual membrane proteins from sequence. Here, we have systematically studied the dependence of the free energy profiles on lipid properties, including tail length, saturation, headgroup hydrogen bond strength, and charge, both to see to whether the in vivo insertion can be explained in whole or part from lipid composition of the endoplasmic reticulum (ER) membranes, and if the solvation properties can help interpret how protein function depends on the lipids. We find that lipid charge is important to stabilize charged amino acids inside the bilayer (with implications, e.g., for ion channels), that thicker bilayers have higher solvation costs for hydrophilic side chains, and that headgroup hydrogen bond strength determines how adaptive the lipids are as a hydrophobic/hydrophilic solvent. None of the different free energy profiles are even close to the low apparent in vivo insertion cost, which suggests that regardless of the specific ER membrane composition the current experimental results cannot be explained by normal lipid-type variation.

Graphene-polydimethylsiloxane/chromium bilayer-based flexible, reversible, and large bendable photomechanical actuators

NASA Astrophysics Data System (ADS)

Leeladhar; Raturi, Parul; Kumar, Ajeet; Singh, J. P.

2017-09-01

We demonstrate the fabrication of highly versatile photomechanical actuators based on graphene-polymer/metal bilayers that offers fast, low-cost fabrication, large deflection, reversible actuation under zero applied pre-strain, and wavelength-selective response. The photomechanical actuator consists of a graphene nanoplatelet (GNP)-polydimethylsiloxane (PDMS) nanocomposite with a thin chromium metal coating of 35 nm thickness on the backside of the structure. The photomechanical response of the GNP-PDMS/Cr photomechanical actuator was measured by recording the variation of the bending angle upon infrared (IR) light illumination. The bending in the bilayer actuator is caused by the generation of thermal stress due to the large mismatch (the ratio being 1/20) of the thermal expansion coefficient between the two layers as a result of IR absorption by GNPs and a subsequent increase in the local temperature. The maximum bending angle was found to be about 40 degrees with a corresponding large deflection value of about 6-7 mm within 6 s for IR illumination with an intensity of 550 mW cm-2. The corresponding actuation response and relaxation times were about 1 and 3 s, respectively. The GNP-PDMS/Cr bilayer combination when integrated with the standard surface micromachining technique of micro-electromechanical system fabrication can find useful applications in the realization of micro soft-robotics, controlled drug delivery, and light-driven micro switches i.e. micro-optomechanical systems.

Controllable poly-crystalline bilayered and multilayered graphene film growth by reciprocal chemical vapor deposition

NASA Astrophysics Data System (ADS)

Wu, Qinke; Jung, Seong Jun; Jang, Sung Kyu; Lee, Joohyun; Jeon, Insu; Suh, Hwansoo; Kim, Yong Ho; Lee, Young Hee; Lee, Sungjoo; Song, Young Jae

2015-06-01

We report the selective growth of large-area bilayered graphene film and multilayered graphene film on copper. This growth was achieved by introducing a reciprocal chemical vapor deposition (CVD) process that took advantage of an intermediate h-BN layer as a sacrificial template for graphene growth. A thin h-BN film, initially grown on the copper substrate using CVD methods, was locally etched away during the subsequent graphene growth under residual H2 and CH4 gas flows. Etching of the h-BN layer formed a channel that permitted the growth of additional graphene adlayers below the existing graphene layer. Bilayered graphene typically covers an entire Cu foil with domain sizes of 10-50 μm, whereas multilayered graphene can be epitaxially grown to form islands a few hundreds of microns in size. This new mechanism, in which graphene growth proceeded simultaneously with h-BN etching, suggests a potential approach to control graphene layers for engineering the band structures of large-area graphene for electronic device applications.We report the selective growth of large-area bilayered graphene film and multilayered graphene film on copper. This growth was achieved by introducing a reciprocal chemical vapor deposition (CVD) process that took advantage of an intermediate h-BN layer as a sacrificial template for graphene growth. A thin h-BN film, initially grown on the copper substrate using CVD methods, was locally etched away during the subsequent graphene growth under residual H2 and CH4 gas flows. Etching of the h-BN layer formed a channel that permitted the growth of additional graphene adlayers below the existing graphene layer. Bilayered graphene typically covers an entire Cu foil with domain sizes of 10-50 μm, whereas multilayered graphene can be epitaxially grown to form islands a few hundreds of microns in size. This new mechanism, in which graphene growth proceeded simultaneously with h-BN etching, suggests a potential approach to control graphene layers for engineering the band structures of large-area graphene for electronic device applications. Electronic supplementary information (ESI) available: The growth conditions, statistical studies of OM images and high-resolution STM/TEM measurements for multi-/bi-layered graphene are discussed in detail. See DOI: 10.1039/c5nr02716k

DC conductivity of twisted bilayer graphene: Angle-dependent transport properties and effects of disorder

NASA Astrophysics Data System (ADS)

Andelković, M.; Covaci, L.; Peeters, F. M.

2018-03-01

The in-plane dc conductivity of twisted bilayer graphene is calculated using an expansion of the real-space Kubo-Bastin conductivity in terms of Chebyshev polynomials. We investigate within a tight-binding approach the transport properties as a function of rotation angle, applied perpendicular electric field, and vacancy disorder. We find that for high-angle twists, the two layers are effectively decoupled, and the minimum conductivity at the Dirac point corresponds to double the value observed in monolayer graphene. This remains valid even in the presence of vacancies, hinting that chiral symmetry is still preserved. On the contrary, for low twist angles, the conductivity at the Dirac point depends on the twist angle and is not protected in the presence of disorder. Furthermore, for low angles and in the presence of an applied electric field, we find that the chiral boundary states emerging between AB and BA regions contribute to the dc conductivity, despite the appearance of localized states in the AA regions. The results agree qualitatively with recent transport experiments in low-angle twisted bilayer graphene.

Geometrical aspects of the frustration in the cubic phases of lyotropic liquid crystals.

PubMed Central

Anderson, D M; Gruner, S M; Leibler, S

1988-01-01

Bicontinuous cubic phases, composed of bilayers arranged in the geometries of periodic minimal surfaces, are found in a variety of different lipid/water systems. It has been suggested recently that these cubic structures arrive as the result of competition between two free-energy terms: the curvature energy of each monolayer and the stretching energy of the lipid chains. This scenario, closely analogous to the one that explains the origin of the hexagonal phases, is investigated here by means of simple geometrical calculations. It is first assumed that the lipid bilayer is of constant thickness and the distribution of the (local) mean curvature of the phospholipid-water interfaces is calculated. Then, assuming the mean curvature of these interfaces is constant, the distribution of the bilayer's thickness is calculated. Both calculations quantify the fact that the two energy terms are frustrated and cannot be satisfied simultaneously. However, the amount of the frustration can be smaller for the cubic phase than for the lamellar and hexagonal structures. Therefore, this phase can appear in the phase diagram between the other two, as observed in many recent experiments. PMID:3399497

In Vivo Assessment of Printed Microvasculature in a Bilayer Skin Graft to Treat Full-Thickness Wounds

PubMed Central

Yanez, Maria; Rincon, Julio; Dones, Aracely; De Maria, Carmelo; Gonzales, Raoul

2015-01-01

Chronic wounds such as diabetic foot ulcers and venous leg ulcers are common problems in people suffering from type 2 diabetes. These can cause pain, and nerve damage, eventually leading to foot or leg amputation. These types of wounds are very difficult to treat and sometimes take months or even years to heal because of many possible complications during the process. Allogeneic skin grafting has been used to improve wound healing, but the majority of grafts do not survive several days after being implanted. We have been studying the behavior of fibroblasts and keratinocytes in engineered capillary-like endothelial networks. A dermo-epidermal graft has been implanted in an athymic nude mouse model to assess the integration with the host tissue as well as the wound healing process. To build these networks into a skin graft, a modified inkjet printer was used, which allowed the deposit of human microvascular endothelial cells. Neonatal human dermal fibroblast cells and neonatal human epidermal keratinocytes were manually mixed in the collagen matrix while endothelial cells printed. A full-thickness wound was created at the top of the back of athymic nude mice and the area was covered by the bilayered graft. Mice of the different groups were followed until completion of the specified experimental time line, at which time the animals were humanely euthanized and tissue samples were collected. Wound contraction improved by up to 10% when compared with the control groups. Histological analysis showed the neoskin having similar appearance to the normal skin. Both layers, dermis and epidermis, were present with thicknesses resembling normal skin. Immunohistochemistry analysis showed favorable results proving survival of the implanted cells, and confocal images showed the human cells' location in the samples that were collocated with the bilayer printed skin graft. PMID:25051339

Structure and functional dynamics characterization of the ion channel of the human respiratory syncytial virus (hRSV) small hydrophobic protein (SH) transmembrane domain by combining molecular dynamics with excited normal modes.

PubMed

Araujo, Gabriela C; Silva, Ricardo H T; Scott, Luis P B; Araujo, Alexandre S; Souza, Fatima P; de Oliveira, Ronaldo Junio

2016-12-01

The human respiratory syncytial virus (hRSV) is the major cause of lower respiratory tract infection in children and elderly people worldwide. Its genome encodes 11 proteins including SH protein, whose functions are not well known. Studies show that SH protein increases RSV virulence degree and permeability to small compounds, suggesting it is involved in the formation of ion channels. The knowledge of SH structure and function is fundamental for a better understanding of its infection mechanism. The aim of this study was to model, characterize, and analyze the structural behavior of SH protein in the phospholipids bilayer environment. Molecular modeling of SH pentameric structure was performed, followed by traditional molecular dynamics (MD) simulations of the protein immersed in the lipid bilayer. Molecular dynamics with excited normal modes (MDeNM) was applied in the resulting system in order to investigate long time scale pore dynamics. MD simulations support that SH protein is stable in its pentameric form. Simulations also showed the presence of water molecules within the bilayer by density distribution, thus confirming that SH protein is a viroporin. This water transport was also observed in MDeNM studies with histidine residues of five chains (His22 and His51), playing a key role in pore permeability. The combination of traditional MD and MDeNM was a very efficient protocol to investigate functional conformational changes of transmembrane proteins that act as molecular channels. This protocol can support future investigations of drug candidates by acting on SH protein to inhibit viral infection. Graphical Abstract The ion channel of the human respiratory syncytial virus (hRSV) small hydrophobic protein (SH) transmembrane domainᅟ.

In vivo assessment of printed microvasculature in a bilayer skin graft to treat full-thickness wounds.

PubMed

Yanez, Maria; Rincon, Julio; Dones, Aracely; De Maria, Carmelo; Gonzales, Raoul; Boland, Thomas

2015-01-01

Chronic wounds such as diabetic foot ulcers and venous leg ulcers are common problems in people suffering from type 2 diabetes. These can cause pain, and nerve damage, eventually leading to foot or leg amputation. These types of wounds are very difficult to treat and sometimes take months or even years to heal because of many possible complications during the process. Allogeneic skin grafting has been used to improve wound healing, but the majority of grafts do not survive several days after being implanted. We have been studying the behavior of fibroblasts and keratinocytes in engineered capillary-like endothelial networks. A dermo-epidermal graft has been implanted in an athymic nude mouse model to assess the integration with the host tissue as well as the wound healing process. To build these networks into a skin graft, a modified inkjet printer was used, which allowed the deposit of human microvascular endothelial cells. Neonatal human dermal fibroblast cells and neonatal human epidermal keratinocytes were manually mixed in the collagen matrix while endothelial cells printed. A full-thickness wound was created at the top of the back of athymic nude mice and the area was covered by the bilayered graft. Mice of the different groups were followed until completion of the specified experimental time line, at which time the animals were humanely euthanized and tissue samples were collected. Wound contraction improved by up to 10% when compared with the control groups. Histological analysis showed the neoskin having similar appearance to the normal skin. Both layers, dermis and epidermis, were present with thicknesses resembling normal skin. Immunohistochemistry analysis showed favorable results proving survival of the implanted cells, and confocal images showed the human cells' location in the samples that were collocated with the bilayer printed skin graft.

Magnetic anisotropies in ultrathin fcc Fe(001) films grown on Cu(001) substrates

NASA Astrophysics Data System (ADS)

Cochran, J. F.; Rudd, J. M.; From, M.; Heinrich, B.; Bennett, W.; Schwarzacher, W.; Egelhoff, W. F., Jr.

1992-03-01

Ferromagnetic resonance absorption measurements at 36.3 GHz and at room temperature have been used to determine the g factor and anisotropy parameters for a series of bilayers composed of two 3-ML-thick fcc Fe (001) films separated by a variable thickness of fcc Cu(001). The resonance field and linewidth were measured versus the out-of-plane magnetic-field angle, θH. The magnetic properties of these ten coupled bilayer films were found to be remarkably similar from specimen to specimen, despite the fact that each member of the bilayer was only 3 ML thick. The average g factor was found to be =2.08+/-0.02, and the average effective magnetization was found to be -5.5+/-0.5 kOe; i.e., the specimens were magnetized normal to the specimen plane in zero applied magnetic field. If the effective field along the specimen normal can be attributed to a second-order surface anisotropy energy of the form Fs=-KU1 sin2θM, then =1.25+/-0.06 ergs/cm2, assuming a value 4πMs=21.6 kOe for the saturation magnetization and using d=5.4 Å for each film thickness. (This energy includes both sides of the film; the energy corresponding to a single Fe-Cu interface is 0.63 erg/cm2.) These specimens exhibited no measurable in-plane anisotropy. The linewidth was found to exhibit a sharp decrease for θH near 20°. This decrease could be explained in terms of the angular dependence of inhomogeneous line broadening due to a 1% variation in the perpendicular effective field from place to place in the sample plane.

Enhanced converse magnetoelectric effect in cylindrical piezoelectric-magnetostrictive composites

NASA Astrophysics Data System (ADS)

Wu, Gaojian; Zhang, Ru; Zhang, Ning

2016-10-01

Enhanced converse magnetoelectric (ME) effect has been experimentally observed in cylindrical PZT-Terfenol-D piezoelectric-magnetostrictive bilayered composites, where the piezoelectric and magnetostrictive components are coupled through normal stresses instead of shear stresses that act in most of previous multiferroic composites. A theoretical model based on elastodynamics analysis has been proposed to describe the frequency response of converse ME effect for axial and radial modes in the bilayered cylindrical composites. The theory shows good agreement with the experimental results. The different variation tendency of resonant converse ME coefficient, as well as different variation rate of resonance frequency with bias magnetic field for axial and radial modes is interpreted in terms of demagnetizing effect. This work is of theoretical and technological significance for the application of converse ME effect as magnetic sensor, transducers, coil-free flux switch, etc.

Sputtering growth of Y3Fe5O12/Pt bilayers and spin transfer at Y3Fe5O12/Pt interfaces

NASA Astrophysics Data System (ADS)

Chang, Houchen; Liu, Tao; Reifsnyder Hickey, Danielle; Janantha, P. A. Praveen; Mkhoyan, K. Andre; Wu, Mingzhong

2017-12-01

For the majority of previous work on Y3Fe5O12 (YIG)/normal metal (NM) bi-layered structures, the YIG layers were grown on Gd3Ga5O12 first and were then capped by an NM layer. This work demonstrates the sputtering growth of a Pt/YIG structure where the Pt layer was grown first and the YIG layer was then deposited on the top. The YIG layer shows well-oriented (111) texture, a surface roughness of 0.15 nm, and an effective Gilbert damping constant less than 4.7 × 10-4, and the YIG/Pt interface allows for efficient spin transfers. This demonstration indicates the feasibility of fabricating high-quality NM/YIG/NM tri-layered structures for new physics studies.

Bi-layer structure of counterstreaming energetic electron fluxes: a diagnostic tool of the acceleration mechanism in the Earth's magnetotail

NASA Astrophysics Data System (ADS)

Sarafopoulos, D. V.

2010-02-01

For the first time we identify a bi-layer structure of energetic electron fluxes in the Earth's magnetotail and establish (using datasets mainly obtained by the Geotail Energetic Particles and Ion Composition (EPIC/ICS) instrument) that it actually provides strong evidence for a purely spatial structure. Each bi-layer event is composed of two distinct layers with counterstreaming energetic electron fluxes, parallel and antiparallel to the local ambient magnetic field lines; in particular, the tailward directed fluxes always occur in a region adjacent to the lobes. Adopting the X-line as a standard reconnection model, we determine the occurrence of bi-layer events relatively to the neutral point, in the substorm frame; four (out of the shown seven) events are observed earthward and three tailward, a result implying that four events probably occurred with the substorm's local recovery phase. We discuss the bi-layer events in terms of the X-line model; they add more constraints for any candidate electron acceleration mechanism. It should be stressed that until this time, none proposed electron acceleration mechanism has discussed or predicted these layered structures with all their properties. Then we discuss the bi-layer events in terms of the much promising "akis model", as introduced by Sarafopoulos (2008). The akis magnetic field topology is embedded in a thinned plasma sheet and is potentially causing charge separation. We assume that as the Rc curvature radius of the magnetic field line tends to become equal to the ion gyroradius rg, then the ions become non-adiabatic. At the limit Rc=rg the demagnetization process is also under way and the frozen-in magnetic field condition is violated by strong wave turbulence; hence, the ion particles in this geometry are stochastically scattered. In addition, ion diffusion probably takes place across the magnetic field, since an intense pressure gradient is directed earthward; hence, ions are ejected tailward of akis. This way, in front of akis an "ion capsule region" is formed with net positive charge. In between them a distinct region with an electric field E⊥ orthogonal to the magnetic field is emerged; E⊥ in front of akis is directed earthward. The field-aligned and highly anisotropic energetic electron populations have probably resulted via spatially separated antiparallel and field-aligned electric fields being the very heart of the acceleration source. We assume that the ultimate cause for the field-aligned electric fields are the net positive capsule charge and the net negative charge trapped at the tip of akis; both charges will be eventually neutralized through field aligned currents, but they remain unshielded for sufficient time to produce the observed events.

Effects of imidazolium-based ionic surfactants on the size and dynamics of phosphatidylcholine bilayers with saturated and unsaturated chains.

PubMed

Lee, Hwankyu

2015-07-01

Imidazolium-based ionic surfactants of different sizes were simulated with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers. Regardless of the phospholipid type, larger surfactants at higher concentrations more significantly insert into the bilayer and increase the bilayer-surface size, in agreement with experiments and previous simulations. Insertion of surfactants only slightly decreases the bilayer thickness, as also observed in experiments. Although the surfactant insertion and its effect on the bilayer size and thickness are similar in different types of bilayers, the volume fractions of surfactants in the bilayer are higher for DMPC bilayers than for POPC and DOPC bilayers. In particular, ionic surfactants with four hydrocarbons yield their volume fractions of 4.6% and 8.7%, respectively, in POPC and DMPC bilayers, in quantitative agreement with experimental values of ∼5% and ∼10%. Also, the inserted surfactants increase the lateral diffusivity of the bilayer, which depends on the bilayer type. These findings indicate that although the surfactant insertion does not depend on the bilayer type, the effects of surfactants on the volume fraction and bilayer dynamics occur more significantly in the DMPC bilayer because of the smaller area per lipid and shorter saturated tails, which helps explain the experimental observations regarding different volume fractions of surfactants in POPC and DMPC bilayers. Copyright © 2015 Elsevier Inc. All rights reserved.

How drugs get into cells: tested and testable predictions to help discriminate between transporter-mediated uptake and lipoidal bilayer diffusion

PubMed Central

Kell, Douglas B.; Oliver, Stephen G.

2014-01-01

One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest. We use this strategy to compare the intellectual status and available evidence for two models or views of mechanisms of transmembrane drug transport into intact biological cells. One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose “natural” biological roles, and substrates are based in intermediary metabolism. Despite a recent review elsewhere, we can find no evidence able to support BDII as we can find no experiments in intact cells in which phospholipid bilayer diffusion was either varied independently or measured directly (although there are many papers where it was inferred by seeing a covariation of other dependent variables). By contrast, we find an abundance of evidence showing cases in which changes in the activities of named and genetically identified transporters led to measurable changes in the rate or extent of drug uptake. PBIN also has considerable predictive power, and accounts readily for the large differences in drug uptake between tissues, cells and species, in accounting for the metabolite-likeness of marketed drugs, in pharmacogenomics, and in providing a straightforward explanation for the late-stage appearance of toxicity and of lack of efficacy during drug discovery programmes despite macroscopically adequate pharmacokinetics. Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive. PMID:25400580

Reconciling structural and thermodynamic predictions using all-atom and coarse-grain force fields: the case of charged oligo-arginine translocation into DMPC bilayers.

PubMed

Hu, Yuan; Sinha, Sudipta Kumar; Patel, Sandeep

2014-10-16

Using the translocation of short, charged cationic oligo-arginine peptides (mono-, di-, and triarginine) from bulk aqueous solution into model DMPC bilayers, we explore the question of the similarity of thermodynamic and structural predictions obtained from molecular dynamics simulations using all-atom and Martini coarse-grain force fields. Specifically, we estimate potentials of mean force associated with translocation using standard all-atom (CHARMM36 lipid) and polarizable and nonpolarizable Martini force fields, as well as a series of modified Martini-based parameter sets. We find that we are able to reproduce qualitative features of potentials of mean force of single amino acid side chain analogues into model bilayers. In particular, modifications of peptide-water and peptide-membrane interactions allow prediction of free energy minima at the bilayer-water interface as obtained with all-atom force fields. In the case of oligo-arginine peptides, the modified parameter sets predict interfacial free energy minima as well as free energy barriers in almost quantitative agreement with all-atom force field based simulations. Interfacial free energy minima predicted by a modified coarse-grained parameter set are -2.51, -4.28, and -5.42 for mono-, di-, and triarginine; corresponding values from all-atom simulations are -0.83, -3.33, and -3.29, respectively, all in units of kcal/mol. We found that a stronger interaction between oligo-arginine and the membrane components and a weaker interaction between oligo-arginine and water are crucial for producing such minima in PMFs using the polarizable CG model. The difference between bulk aqueous and bilayer center states predicted by the modified coarse-grain force field are 11.71, 14.14, and 16.53 kcal/mol, and those by the all-atom model are 6.94, 8.64, and 12.80 kcal/mol; those are of almost the same order of magnitude. Our simulations also demonstrate a remarkable similarity in the structural aspects of the ensemble of configurations generated using the all-atom and coarse-grain force fields. Both resolutions show that oligo-arginine peptides adopt preferential orientations as they translocate into the bilayer. The guiding theme centers on charged groups maintaining coordination with polar and charged bilayer components as well as local water. We also observe similar behaviors related with membrane deformations.

A strategy to stabilise the local structure of Ti{sup 4+} and Zn{sup 2+} species against aging in TiO{sub 2}/aluminium-doped ZnO bi-layers for applications in hybrid solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pellegrino, Giovanna; La Magna, Antonino; Bongiorno, Corrado

We explore a strategy to counteract aging issues in TiO{sub 2}/aluminium-doped ZnO bi-layers used in hybrid solar cells photo-anodes, mainly related to Zn diffusion in the TiO{sub 2} matrix. Different Ti{sup 4+} and Zn{sup 2+} local structures within the anatase grains and along the film thickness were found as a function of post-deposition annealing treatments in the range between 200 °C and 500 °C by synchrotron radiation extended x-ray absorption fine structure analyses. In particular, in the 500 °C-treated sample, diffusion of zinc species along the TiO{sub 2} grain-boundaries has been observed with aging (3 years). In contrast, a mild thermal budget at 200 °Cmore » favours a proper atomic arrangement of the zinc-containing anatase lattice which reduces Zn diffusion, thus guaranteeing a good stability with aging.« less

Husimi function and phase-space analysis of bilayer quantum Hall systems at ν = 2/λ

NASA Astrophysics Data System (ADS)

Calixto, M.; Peón-Nieto, C.

2018-05-01

We propose localization measures in phase space of the ground state of bilayer quantum Hall systems at fractional filling factors , to characterize the three quantum phases (shortly denoted by spin, canted and ppin) for arbitrary -isospin λ. We use a coherent state (Bargmann) representation of quantum states, as holomorphic functions in the 8-dimensional Grassmannian phase-space (a higher-dimensional generalization of the Haldane’s 2-dimensional sphere ). We quantify the localization (inverse volume) of the ground state wave function in phase-space throughout the phase diagram (i.e. as a function of Zeeman, tunneling, layer distance, etc, control parameters) with the Husimi function second moment, a kind of inverse participation ratio that behaves as an order parameter. Then we visualize the different ground state structure in phase space of the three quantum phases, the canted phase displaying a much higher delocalization (a Schrödinger cat structure) than the spin and ppin phases, where the ground state is highly coherent. We find a good agreement between analytic (variational) and numeric diagonalization results.

Gate-defined Quantum Confinement in Suspended Bilayer Graphene

NASA Astrophysics Data System (ADS)

Allen, Monica

2013-03-01

Quantum confined devices in carbon-based materials offer unique possibilities for applications ranging from quantum computation to sensing. In particular, nanostructured carbon is a promising candidate for spin-based quantum computation due to the ability to suppress hyperfine coupling to nuclear spins, a dominant source of spin decoherence. Yet graphene lacks an intrinsic bandgap, which poses a serious challenge for the creation of such devices. We present a novel approach to quantum confinement utilizing tunnel barriers defined by local electric fields that break sublattice symmetry in suspended bilayer graphene. This technique electrostatically confines charges via band structure control, thereby eliminating the edge and substrate disorder that hinders on-chip etched nanostructures to date. We report clean single electron tunneling through gate-defined quantum dots in two regimes: at zero magnetic field using the energy gap induced by a perpendicular electric field and at finite magnetic fields using Landau level confinement. The observed Coulomb blockade periodicity agrees with electrostatic simulations based on local top-gate geometry, a direct demonstration of local control over the band structure of graphene. This technology integrates quantum confinement with pristine device quality and access to vibrational modes, enabling wide applications from electromechanical sensors to quantum bits. More broadly, the ability to externally tailor the graphene bandgap over nanometer scales opens a new unexplored avenue for creating quantum devices.

Coupling Field Theory with Mesoscopic Dynamical Simulations of Multicomponent Lipid Bilayers

PubMed Central

McWhirter, J. Liam; Ayton, Gary; Voth, Gregory A.

2004-01-01

A method for simulating a two-component lipid bilayer membrane in the mesoscopic regime is presented. The membrane is modeled as an elastic network of bonded points; the spring constants of these bonds are parameterized by the microscopic bulk modulus estimated from earlier atomistic nonequilibrium molecular dynamics simulations for several bilayer mixtures of DMPC and cholesterol. The modulus depends on the composition of a point in the elastic membrane model. The dynamics of the composition field is governed by the Cahn-Hilliard equation where a free energy functional models the coupling between the composition and curvature fields. The strength of the bonds in the elastic network are then modulated noting local changes in the composition and using a fit to the nonequilibrium molecular dynamics simulation data. Estimates for the magnitude and sign of the coupling parameter in the free energy model are made treating the bending modulus as a function of composition. A procedure for assigning the remaining parameters in the free energy model is also outlined. It is found that the square of the mean curvature averaged over the entire simulation box is enhanced if the strength of the bonds in the elastic network are modulated in response to local changes in the composition field. We suggest that this simulation method could also be used to determine if phase coexistence affects the stress response of the membrane to uniform dilations in area. This response, measured in the mesoscopic regime, is already known to be conditioned or renormalized by thermal undulations. PMID:15347594

Sensing small molecule interactions with lipid membranes by local pH modulation.

PubMed

Huang, Da; Zhao, Tao; Xu, Wei; Yang, Tinglu; Cremer, Paul S

2013-11-05

Herein, we utilized a label-free sensing platform based on pH modulation to detect the interactions between tetracaine, a positively charged small molecule used as a local anesthetic, and planar supported lipid bilayers (SLBs). The SLBs were patterned inside a flow cell, allowing for various concentrations of tetracaine to be introduced over the surface in a buffer solution. Studies with membranes containing POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) yielded an equilibrium dissociation constant value of Kd = 180 ± 47 μm for this small molecule-membrane interaction. Adding cholesterol to the SLBs decreased the affinity between tetracaine and the bilayers, while this interaction tightened when POPE (1-hexadecanoyl-2-(9-Z-octadecenoyl)-sn-glycero-3-phosphoethanolamine) was added. Studies were also conducted with three negatively charged membrane lipids, POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt)), POPS (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (sodium salt)), and ganglioside GM1. All three measurements gave rise to a similar tightening of the apparent Kd value compared with pure POPC membranes. The lack of chemical specificity with the identity of the negatively charged lipid indicated that the tightening was largely electrostatic. Through a direct comparison with ITC measurements, it was found that the pH modulation sensor platform offers a facile, inexpensive, highly sensitive, and rapid method for the detection of interactions between putative drug candidates and lipid bilayers. As such, this technique may potentially be exploited as a screen for drug development and analysis.

Structure of Sphingomyelin Bilayers: A Simulation Study

PubMed Central

Chiu, S. W.; Vasudevan, S.; Jakobsson, Eric; Mashl, R. Jay; Scott, H. Larry

2003-01-01

We have carried out a molecular dynamics simulation of a hydrated 18:0 sphingomyelin lipid bilayer. The bilayer contained 1600 sphingomyelin (SM) molecules, and 50,592 water molecules. After construction and initial equilibration, the simulation was run for 3.8 ns at a constant temperature of 50°C and a constant pressure of 1 atm. We present properties of the bilayer calculated from the simulation, and compare with experimental data and with properties of dipalmitoyl phosphatidylcholine (DPPC) bilayers. The SM bilayers are significantly more ordered and compact than DPPC bilayers at the same temperature. SM bilayers also exhibit significant intramolecular hydrogen bonding between phosphate ester oxygen and hydroxyl hydrogen atoms. This results in a decreased hydration in the polar region of the SM bilayer compared with DPPC. Since our simulation system is very large we have calculated the power spectrum of bilayer undulation and peristaltic modes, and we compare these data with similar calculations for DPPC bilayers. We find that the SM bilayer has significantly larger bending modulus and area compressibility compared to DPPC. PMID:14645055

Particle-based simulations of bilayer membranes: self-assembly, structural analysis, and shock-wave damage

NASA Astrophysics Data System (ADS)

Steinhauser, Martin O.; Schindler, Tanja

2017-01-01

We report on the results of particle-based, coarse-grained molecular dynamics simulations of amphiphilic lipid molecules in aqueous environment where the membrane structures at equilibrium are subsequently exposed to strong shock waves, and their damage is analyzed. The lipid molecules self-assemble from unbiased random initial configurations to form stable bilayer membranes, including closed vesicles. During self-assembly of lipid molecules, we observe several stages of clustering, starting with many small clusters of lipids, gradually merging together to finally form one single bilayer membrane. We find that the clustering of lipids sensitively depends on the hydrophobic interaction h_c of the lipid tails in our model and on temperature T of the system. The self-assembled bilayer membranes are quantitatively analyzed at equilibrium with respect to their degree of order and their local structure. We also show that—by analyzing the membrane fluctuations and using a linearized theory— we obtain area compression moduli K_A and bending stiffnesses κ _B for our bilayer membranes which are within the experimental range of in vivo and in vitro measurements of biological membranes. We also discuss the density profile and the pair correlation function of our model membranes at equilibrium which has not been done in previous studies of particle-based membrane models. Furthermore, we present a detailed phase diagram of our lipid model that exhibits a sol-gel transition between quasi-solid and fluid domains, and domains where no self-assembly of lipids occurs. In addition, we present in the phase diagram the conditions for temperature T and hydrophobicity h_c of the lipid tails of our model to form closed vesicles. The stable bilayer membranes obtained at equilibrium are then subjected to strong shock waves in a shock tube setup, and we investigate the damage in the membranes due to their interaction with shock waves. Here, we find a transition from self-repairing membranes (reducing their damage after impact) and permanent (irreversible) damage, depending on the shock front speed. The here presented idea of using coarse-grained (CG) particle models for soft matter systems in combination with the investigation of shock-wave effects in these systems is a quite new approach.

Moiré edge states in twisted graphene nanoribbons

NASA Astrophysics Data System (ADS)

Fleischmann, M.; Gupta, R.; Weckbecker, D.; Landgraf, W.; Pankratov, O.; Meded, V.; Shallcross, S.

2018-05-01

The edge physics of graphene based systems is well known to be highly sensitive to the atomic structure at the boundary, with localized zero mode edge states found only on the zigzag-type termination of the lattice. Here we demonstrate that the graphene twist bilayer supports an additional class of edge states, that (i) are found for all edge geometries and thus are robust against edge roughness, (ii) occur at energies coinciding with twist induced Van Hove singularities in the bulk and (iii) possess an electron density strongly modulated by the moiré lattice. Interestingly, these "moiré edge states" exist only for certain lattice commensurations and thus the edge physics of the twist bilayer is, in dramatic contrast to that of the bulk, not uniquely determined by the twist angle.

Direct Measurement of Cyclic Current-Voltage Responses of Integral Membrane Proteins at a Self-Assembled Lipid-Bilayer-Modified Electrode: Cytochrome f and Cytochrome c Oxidase

NASA Astrophysics Data System (ADS)

Salamon, Z.; Hazzard, J. T.; Tollin, G.

1993-07-01

Direct cyclic voltage-current responses, produced in the absence of redox mediators, for two detergent-solubilized integral membrane proteins, spinach cytochrome f and beef heart cytochrome c oxidase, have been obtained at an optically transparent indium oxide electrode modified with a self-assembled lipid-bilayer membrane. The results indicate that both proteins interact with the lipid membrane so as to support quasi-reversible electron transfer redox reactions at the semiconductor electrode. The redox potentials that were obtained from analysis of the cyclic "voltammograms," 365 mV for cytochrome f and 250 and 380 mV for cytochrome c oxidase (vs. normal hydrogen electrode), compare quite well with the values reported by using conventional titration methods. The ability to obtain direct electrochemical measurements opens up another approach to the investigation of the properties of integral membrane redox proteins.

Colloidal layers in magnetic fields and under shear flow

NASA Astrophysics Data System (ADS)

Löwen, H.; Messina, R.; Hoffmann, N.; Likos, C. N.; Eisenmann, C.; Keim, P.; Gasser, U.; Maret, G.; Goldberg, R.; Palberg, T.

2005-11-01

The behaviour of colloidal mono- and bilayers in external magnetic fields and under shear is discussed and recent progress is summarized. Superparamagnetic colloidal particles form monolayers when they are confined to a air-water interface in a hanging water droplet. An external magnetic field allows us to tune the strength of the mutual dipole-dipole interaction between the colloids and the anisotropy of the interaction can be controlled by the tilt angle of the magnetic field relative to the surface normal of the air-water interface. For sufficiently large magnetic field strength crystalline monolayers are found. The role of fluctuations in these two-dimensional crystals is discussed. Furthermore, clustering phenomena in binary mixtures of superparamagnetic particles forming fluid monolayers are predicted. Finally, we address sheared colloidal bilayers and find that the orientation of confined colloidal crystals can be tailored by a previously applied shear direction.

Theory of in-plane current induced spin torque in metal/ferromagnet bilayers

NASA Astrophysics Data System (ADS)

Sakanashi, Kohei; Sigrist, Manfred; Chen, Wei

2018-05-01

Using a semiclassical approach that simultaneously incorporates the spin Hall effect (SHE), spin diffusion, quantum well states, and interface spin–orbit coupling (SOC), we address the interplay of these mechanisms as the origin of the spin–orbit torque (SOT) induced by in-plane currents, as observed in the normal metal/ferromagnetic metal bilayer thin films. Focusing on the bilayers with a ferromagnet much thinner than its spin diffusion length, such as Pt/Co with  ∼10 nm thickness, our approach addresses simultaneously the two contributions to the SOT, namely the spin-transfer torque (SHE-STT) due to SHE-induced spin injection, and the inverse spin Galvanic effect spin–orbit torque (ISGE-SOT) due to SOC-induced spin accumulation. The SOC produces an effective magnetic field at the interface, hence it modifies the angular momentum conservation expected for the SHE-STT. The SHE-induced spin voltage and the interface spin current are mutually dependent and, hence, are solved in a self-consistent manner. The result suggests that the SHE-STT and ISGE-SOT are of the same order of magnitude, and the spin transport mediated by the quantum well states may be an important mechanism for the experimentally observed rapid variation of the SOT with respect to the thickness of the ferromagnet.

The structure of an integral membrane peptide: a deuterium NMR study of gramicidin.

PubMed Central

Prosser, R S; Daleman, S I; Davis, J H

1994-01-01

Solid state deuterium NMR was employed on oriented multilamellar dispersions consisting of 1,2-dilauryl-sn-glycero-3-phosphatidylcholine and deuterium (2H) exchange-labeled gramicidin D, at a lipid to protein molar ratio (L/P) of 15:1, in order to study the dynamic structure of the channel conformation of gramicidin in a liquid crystalline phase. The corresponding spectra were used to discriminate between several structural models for the channel structure of gramicidin (based on the left- and right-handed beta 6.3 LD helix) and other models based on a structure obtained from high resolution NMR. The oriented spectrum is complicated by the fact that many of the doublets, corresponding to the 20 exchangeable sites, partially overlap. Furthermore, the asymmetry parameter, eta, of the electric field gradient tensor of the amide deuterons is large (approximately 0.2) and many of the amide groups are involved in hydrogen bonding, which is known to affect the quadrupole coupling constant. In order to account for these complications in simulating the spectra in the fast motional regime, an ab initio program called Gaussian 90 was employed, which permitted us to calculate, by quantum mechanical means, the complete electric field gradient tensor for each residue in gramicidin (using two structural models). Our results indicated that the left-handed helical models were inconsistent with our observed spectra, whereas a model based on the high-resolution structure derived by Arseniev and coworkers, but relaxed by a simple energy minimization procedure, was consistent with our observed spectra. The molecular order parameter was then estimated from the motional narrowing assuming the relaxed (right-handed) Arseniev structure. Our resultant order parameter of SZZ = 0.91 translates into an rms angle of 14 degrees, formed by the helix axis and the local bilayer normal. The strong resemblance between our spectra (and also those reported for gramicidin in 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) multilayers) and the spectra of the same peptide incorporated in a lyotropic nematic phase, suggests that the lyotropic nematic phase simulates the local environment of the lipid bilayer. PMID:7520293

Lateral organization, bilayer asymmetry, and inter-leaflet coupling of biological membranes

DOE PAGES

Smith, Jeremy C.; Cheng, Xiaolin; Nickels, Jonathan D.

2015-07-29

Understanding of cell membrane organization has evolved significantly from the classic fluid mosaic model. It is now recognized that biological membranes are highly organized structures, with differences in lipid compositions between inner and outer leaflets and in lateral structures within the bilayer plane, known as lipid rafts. These organizing principles are important for protein localization and function as well as cellular signaling. However, the mechanisms and biophysical basis of lipid raft formation, structure, dynamics and function are not clearly understood. One key question, which we focus on in this review, is how lateral organization and leaflet compositional asymmetry are coupled.more » Detailed information elucidating this question has been sparse because of the small size and transient nature of rafts and the experimental challenges in constructing asymmetric bilayers. Resolving this mystery will require advances in both experimentation and modeling. We discuss here the preparation of model systems along with experimental and computational approaches that have been applied in efforts to address this key question in membrane biology. Furthermore, we seek to place recent and future advances in experimental and computational techniques in context, providing insight into in-plane and transverse organization of biological membranes.« less

A new functional membrane protein microarray based on tethered phospholipid bilayers.

PubMed

Chadli, Meriem; Maniti, Ofelia; Marquette, Christophe; Tillier, Bruno; Cortès, Sandra; Girard-Egrot, Agnès

2018-04-30

A new prototype of a membrane protein biochip is presented in this article. This biochip was created by the combination of novel technologies of peptide-tethered bilayer lipid membrane (pep-tBLM) formation and solid support micropatterning. Pep-tBLMs integrating a membrane protein were obtained in the form of microarrays on a gold chip. The formation of the microspots was visualized in real-time by surface plasmon resonance imaging (SPRi) and the functionality of a GPCR (CXCR4), reinserted locally into microwells, was assessed by ligand binding studies. In brief, to achieve micropatterning, P19-4H, a 4 histidine-possessing peptide spacer, was spotted inside microwells obtained on polystyrene-coated gold, and Ni-chelating proteoliposomes were injected into the reaction chamber. Proteoliposome binding to the peptide was based on metal-chelate interaction. The peptide-tethered lipid bilayer was finally obtained by addition of a fusogenic peptide (AH peptide) to promote proteoliposome fusion. The CXCR4 pep-tBLM microarray was characterized by surface plasmon resonance imaging (SPRi) throughout the building-up process. This new generation of membrane protein biochip represents a promising method of developing a screening tool for drug discovery.

Erythrocyte agglutination by wheat germ agglutinin: ionic strength dependence of the contact seam topology.

PubMed

Rolfe, M; Parmar, A; Hoy, T G; Coakley, W T

2001-01-01

The topology of the cell-cell contact seam formed when normal or pronase pre-treated (PPT) erythrocytes are exposed to wheat germ agglutinin (WGA) in isotonic media of different ionic strengths was examined here. Lectin uptake and cell agglutination were also quantified. Agglutination of normal cells was gradually and significantly inhibited as ionic strength (IS) was reduced from 0.15 (buffered 145 mm NaCl) to 0.105. Agglutination was less inhibited in PPT cells, even when IS was reduced to 0.09. Cell contact seams formed during agglutination showed patterns of localized contacts. The scale of the patterns, i.e. the average lateral separation distance of contact regions, was 0.62 microm for normal cells and was significantly shorter, at 0.44 microm, for PPT cells at an IS of 0.15. The scale increased significantly for both cell types when the IS was reduced to 0.09. Flow cytometry measurements showed that WGA uptake by normal cells increased slightly, whilst that for PPT cells was unchanged, as IS was decreased from 0.15 to 0.09. The results imply that, whilst ionic strength change does not exert a strong influence on intermolecular WGA-ligand binding, physico-chemical modification of the interaction between cells modulates not only the extent and progression of the biospecific lectin-induced cell-cell agglutination but also the topology of the contact seam. The IS dependence of contact separation in WGA-agglutinated cells is contrasted here with that reported for cells adhering in dextran solutions. The influence of IS change and pronase pre-treatment on contact pattern are consistent with predictions, from interfacial instability theory, of punctuate thinning of the aqueous layer separating bilayer membranes in close apposition.

Lipid bilayers on nano-templates

DOEpatents

Noy, Aleksandr [Belmont, CA; Artyukhin, Alexander B [Menlo Park, CA; Bakajin, Olgica [San Leandro, CA; Stoeve, Pieter [Davis, CA

2009-08-04

A lipid bilayer on a nano-template comprising a nanotube or nanowire and a lipid bilayer around the nanotube or nanowire. One embodiment provides a method of fabricating a lipid bilayer on a nano-template comprising the steps of providing a nanotube or nanowire and forming a lipid bilayer around the polymer cushion. One embodiment provides a protein pore in the lipid bilayer. In one embodiment the protein pore is sensitive to specific agents

Modelling of noble anaesthetic gases and high hydrostatic pressure effects in lipid bilayers

DOE PAGES

Moskovitz, Yevgeny; Yang, Hui

2015-01-08

Our objective was to study molecular processes that might be responsible for inert gas narcosis and high-pressure nervous syndrome. The classical molecular dynamics trajectories (200 ns-long) of dioleoylphosphatidylcholine (DOPC) bilayers simulated by the Berger force field were evaluated for water and the atomic distribution of noble gases around DOPC molecules at a pressure range of 1 - 1000 bar and temperature of 310 Kelvin. Xenon and argon have been tested as model gases for general anesthetics, and neon has been investigated for distortions that are potentially responsible for neurological tremor at hyperbaric conditions. The analysis of stacked radial pair distributionmore » functions of DOPC headgroup atoms revealed the explicit solvation potential of gas molecules, which correlates with their dimensions. The orientational dynamics of water molecules at the biomolecular interface should be considered as an influential factor; while excessive solvation effects appearing in the lumen of membrane-embedded ion channels could be a possible cause of inert gas narcosis. All the noble gases tested exhibit similar patterns of the order parameter for both DOPC acyl chains, which is opposite to the patterns found for the order parameter curve at high hydrostatic pressures in intact bilayers. This finding supports the ‘critical volume’ hypothesis of anesthesia pressure reversal. The irregular lipid headgroup-water boundary observed in DOPC bilayers saturated with neon in the pressure range of 1 - 100 bar could be associated with the possible manifestation of neurological tremor at the atomic scale. The non-immobilizer neon also demonstrated the highest momentum impact on the normal component of the DOPC diffusion coefficient representing monolayers undulations rate, which indicates enhanced diffusivity, rather than atom size, as the key factor.« less

A theoretical study of diffusional transport over the alveolar surfactant layer.

PubMed

Aberg, Christoffer; Sparr, Emma; Larsson, Marcus; Wennerström, Håkan

2010-10-06

In this communication, we analyse the passage of oxygen and carbon dioxide over the respiratory membrane. The lung surfactant membrane at the alveolar interface can have a very special arrangement, which affects the diffusional transport. We present a theoretical model for the diffusion of small molecules in membranes with a complex structure, and we specifically compare a membrane composed of a tubular bilayer network with a membrane consisting of a stack of bilayers. Oxygen and carbon dioxide differ in terms of their solubility in the aqueous and the lipid regions of the membrane, and we show that this difference clearly influences their transport properties in the different membrane structures. During normal respiration, the rate-limiting step for carbon dioxide transport is in the gas phase of the different compartments in the lung. For oxygen, on the other hand, the rate is limited by the transport between alveoli and the capillary blood vessels, including the lung surfactant membrane. In a membrane with a structure of a continuous tubular lipid network, oxygen transport is facilitated to a significant extent compared with the structure of aligned lipid bilayers. The model calculations in the present study show that transport of oxygen through the tubular structure is indeed ca 30 per cent faster than transport through a membrane composed of stacked bilayers. The tubular network will also facilitate the transport of apolar substances between the gas phase and the blood. Important examples are ethanol and other volatile liquids that can leave the blood through the lungs, and gaseous anaesthetics or volatile solvents that are inhaled. This exemplifies a new physiological role of a tubular lipid network in the lung surfactant membrane.

In vitro effects of the anti-Alzheimer drug memantine on the human erythrocyte membrane and molecular models

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zambrano, Pablo; Suwalsky, Mario; Villena, Fernando

Memantine is a NMDA antagonist receptor clinically used for treating Alzheimer's disease. NMDA receptors are present in the human neurons and erythrocyte membranes. The aim of the present study was to investigate the effects of memantine on human erythrocytes. With this purpose, the drug was developed to in vitro interact with human red cells and bilayers built-up of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE). The latter represent lipids respectively present in both outer and inner monolayers of the red cell membrane. Results obtained by scanning electron microscopy (SEM) showed that memantine changed the normal biconcave shape of red cells to cup-shaped stomatocytes.more » According to the bilayer-couple hypothesis the drug intercalated into the inner monolayer of the erythrocyte membrane. Experimental results obtained by X-ray diffraction on multibilayers of DMPC and DMPE, and by differential scanning calorimetry on multilamellar vesicles indicated that memantine preferentially interacted with DMPC in a concentration-dependent manner. Thus, it can be concluded that in the low therapeutic plasma concentration of circa 1 μM memantine is located in NMDA receptor channel without affecting the erythrocyte shape. However, at higher concentrations, once the receptors became saturated excess of memantine molecules (20 μM) would interact with phosphoinositide lipids present in the inner monolayer of the erythrocyte membrane inducing the formation of stomatocytes. However, 40–50 μM memantine was required to interact with isolated phosphatidylcholine bilayers. - Highlights: • The interaction of memantine with human erythrocytes and lipid bilayers were assessed. • Memantine induced morphological changes to human erythrocytes. • Memantine interacted with classes of phospholipids present in the erythrocyte membrane. • Results support the hypothesis that memantine interacts with NMDA receptors.« less

Eicosapentaenoic acid reduces membrane fluidity, inhibits cholesterol domain formation, and normalizes bilayer width in atherosclerotic-like model membranes.

PubMed

Mason, R Preston; Jacob, Robert F; Shrivastava, Sandeep; Sherratt, Samuel C R; Chattopadhyay, Amitabha

2016-12-01

Cholesterol crystalline domains characterize atherosclerotic membranes, altering vascular signaling and function. Omega-3 fatty acids reduce membrane lipid peroxidation and subsequent cholesterol domain formation. We evaluated non-peroxidation-mediated effects of eicosapentaenoic acid (EPA), other TG-lowering agents, docosahexaenoic acid (DHA), and other long-chain fatty acids on membrane fluidity, bilayer width, and cholesterol domain formation in model membranes. In membranes prepared at 1.5:1 cholesterol-to-phospholipid (C/P) mole ratio (creating pre-existing domains), EPA, glycyrrhizin, arachidonic acid, and alpha linolenic acid promoted the greatest reductions in cholesterol domains (by 65.5%, 54.9%, 46.8%, and 45.2%, respectively) compared to controls; other treatments had modest effects. EPA effects on cholesterol domain formation were dose-dependent. In membranes with 1:1 C/P (predisposing domain formation), DHA, but not EPA, dose-dependently increased membrane fluidity. DHA also induced cholesterol domain formation without affecting temperature-induced changes in-bilayer unit cell periodicity relative to controls (d-space; 57Å-55Å over 15-30°C). Together, these data suggest simultaneous formation of distinct cholesterol-rich ordered domains and cholesterol-poor disordered domains in the presence of DHA. By contrast, EPA had no effect on cholesterol domain formation and produced larger d-space values relative to controls (60Å-57Å; p<0.05) over the same temperature range, suggesting a more uniform maintenance of lipid dynamics despite the presence of cholesterol. These data indicate that EPA and DHA had different effects on membrane bilayer width, membrane fluidity, and cholesterol crystalline domain formation; suggesting omega-3 fatty acids with differing chain length or unsaturation may differentially influence membrane lipid dynamics and structural organization as a result of distinct phospholipid/sterol interactions. Copyright © 2016. Published by Elsevier B.V.

Binding of peptides with basic and aromatic residues to bilayer membranes: phenylalanine in the myristoylated alanine-rich C kinase substrate effector domain penetrates into the hydrophobic core of the bilayer.

PubMed

Zhang, Wenyi; Crocker, Evan; McLaughlin, Stuart; Smith, Steven O

2003-06-13

Electrostatic interactions with positively charged regions of membrane-associated proteins such as myristoylated alanine-rich C kinase substrate (MARCKS) may have a role in regulating the level of free phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in plasma membranes. Both the MARCKS protein and a peptide corresponding to the effector domain (an unstructured region that contains 13 basic residues and 5 phenylalanines), MARCKS-(151-175), laterally sequester the polyvalent lipid PI(4,5)P2 in the plane of a bilayer membrane with high affinity. We used high resolution magic angle spinning NMR to establish the location of MARCKS-(151-175) in membrane bilayers, which is necessary to understand the sequestration mechanism. Measurements of cross-relaxation rates in two-dimensional nuclear Overhauser enhancement spectroscopy NMR experiments show that the five Phe rings of MARCKS-(151-175) penetrate into the acyl chain region of phosphatidylcholine bilayers containing phosphatidylglycerol or PI(4,5)P2. Specifically, we observed strong cross-peaks between the aromatic protons of the Phe rings and the acyl chain protons of the lipids, even for very short (50 ms) mixing times. The position of the Phe rings implies that the adjacent positively charged amino acids in the peptide are close to the level of the negatively charged lipid phosphates. The deep location of the MARCKS peptide in the polar head group region should enhance its electrostatic sequestration of PI(4,5)P2 by an "image charge" mechanism. Moreover, this location has interesting implications for membrane curvature and local surface pressure effects and may be relevant to a wide variety of other proteins with basic-aromatic clusters, such as phospholipase D, GAP43, SCAMP2, and the N-methyl-d-aspartate receptor.

Basal-plane dislocations in bilayer graphene - Peculiarities in a quasi-2D material

NASA Astrophysics Data System (ADS)

Butz, Benjamin

2015-03-01

Dislocations represent one of the most fascinating and fundamental concepts in materials science. First and foremost, they are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly alter the local electronic or optical properties of semiconductors and ionic crystals. In layered crystals like graphite dislocation movement is restricted to the basal plane. Thus, those basal-plane dislocations cannot escape enabling their confinement in between only two atomic layers of the material. So-called bilayer graphene is the thinnest imaginable quasi-2D crystal to explore the nature and behavior of dislocations under such extreme boundary conditions. Robust graphene membranes derived from epitaxial graphene on SiC provide an ideal platform for their investigation. The presentation will give an insight in the direct observation of basal-plane partial dislocations by transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. The investigation reveals striking size effects. First, the absence of stacking fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern, which corresponds to an alternating AB <--> BA change of the stacking order. Most importantly, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane, which directly results from accommodation of strain. In fact, the buckling completely changes the strain state of the bilayer graphene and is of key importance for its electronic/spin transport properties. Due to the high degree of disorder in our quasi-2D material it is one of the very few examples for a perfect linear magnetoresistance, i.e. the linear dependency of the in-plane electrical resistance on a magnetic field applied perpendicular to the graphene sheet up to field strengths of more than 60 T. This research is financed by the German Research Foundation through the SFB 953 ``Synthetic Carbon Allotropes.''

High-resolution orientation and depth of insertion of the voltage-sensing S4 helix of a potassium channel in lipid bilayers.

PubMed

Doherty, Tim; Su, Yongchao; Hong, Mei

2010-08-27

The opening and closing of voltage-gated potassium (Kv) channels are controlled by several conserved Arg residues in the S4 helix of the voltage-sensing domain. The interaction of these positively charged Arg residues with the lipid membrane has been of intense interest for understanding how membrane proteins fold to allow charged residues to insert into lipid bilayers against free-energy barriers. Using solid-state NMR, we have now determined the orientation and insertion depth of the S4 peptide of the KvAP channel in lipid bilayers. Two-dimensional (15)N correlation experiments of macroscopically oriented S4 peptide in phospholipid bilayers revealed a tilt angle of 40 degrees and two possible rotation angles differing by 180 degrees around the helix axis. Remarkably, the tilt angle and one of the two rotation angles are identical to those of the S4 helix in the intact voltage-sensing domain, suggesting that interactions between the S4 segment and other helices of the voltage-sensing domain are not essential for the membrane topology of the S4 helix. (13)C-(31)P distances between the S4 backbone and the lipid (31)P indicate a approximately 9 A local thinning and 2 A average thinning of the DMPC (1,2-dimyristoyl-sn-glycero-3-phosphochloline)/DMPG (1,2-dimyristoyl-sn-glycero-3-phosphatidylglycerol) bilayer, consistent with neutron diffraction data. Moreover, a short distance of 4.6 A from the guanidinium C(zeta) of the second Arg to (31)P indicates the existence of guanidinium phosphate hydrogen bonding and salt bridges. These data suggest that the structure of the Kv gating helix is mainly determined by protein-lipid interactions instead of interhelical protein-protein interactions, and the S4 amino acid sequence encodes sufficient information for the membrane topology of this crucial gating helix. Copyright (c) 2010 Elsevier Ltd. All rights reserved.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cheng, Chi-Yuan; Kausik, Ravinath; Han, Songi, E-mail: songi@chem.ucsb.edu

Elucidating the physical effect of cholesterol (Chol) on biological membranes is necessary towards rationalizing their structural and functional role in cell membranes. One of the debated questions is the role of hydration water in Chol-embedding lipid membranes, for which only little direct experimental data are available. Here, we study the hydration dynamics in a series of Chol-rich and depleted bilayer systems using an approach termed {sup 1}H Overhauser dynamic nuclear polarization (ODNP) NMR relaxometry that enables the sensitive and selective determination of water diffusion within 5–10 Å of a nitroxide-based spin label, positioned off the surface of the polar headgroupsmore » or within the nonpolar core of lipid membranes. The Chol-rich membrane systems were prepared from mixtures of Chol, dipalmitoyl phosphatidylcholine and/or dioctadecyl phosphatidylcholine lipid that are known to form liquid-ordered, raft-like, domains. Our data reveal that the translational diffusion of local water on the surface and within the hydrocarbon volume of the bilayer is significantly altered, but in opposite directions: accelerated on the membrane surface and dramatically slowed in the bilayer interior with increasing Chol content. Electron paramagnetic resonance (EPR) lineshape analysis shows looser packing of lipid headgroups and concurrently tighter packing in the bilayer core with increasing Chol content, with the effects peaking at lipid compositions reported to form lipid rafts. The complementary capability of ODNP and EPR to site-specifically probe the hydration dynamics and lipid ordering in lipid membrane systems extends the current understanding of how Chol may regulate biological processes. One possible role of Chol is the facilitation of interactions between biological constituents and the lipid membrane through the weakening or disruption of strong hydrogen-bond networks of the surface hydration layers that otherwise exert stronger repulsive forces, as reflected in faster surface water diffusivity. Another is the concurrent tightening of lipid packing that reduces passive, possibly unwanted, diffusion of ions and water across the bilayer.« less

Antimicrobial Peptide Simulations and the Influence of Force Field on the Free Energy for Pore Formation in Lipid Bilayers.

PubMed

Bennett, W F Drew; Hong, Chun Kit; Wang, Yi; Tieleman, D Peter

2016-09-13

Due to antimicrobial resistance, the development of new drugs to combat bacterial and fungal infections is an important area of research. Nature uses short, charged, and amphipathic peptides for antimicrobial defense, many of which disrupt the lipid membrane in addition to other possible targets inside the cell. Computer simulations have revealed atomistic details for the interactions of antimicrobial peptides and cell-penetrating peptides with lipid bilayers. Strong interactions between the polar interface and the charged peptides can induce bilayer deformations - including membrane rupture and peptide stabilization of a hydrophilic pore. Here, we performed microsecond-long simulations of the antimicrobial peptide CM15 in a POPC bilayer expecting to observe pore formation (based on previous molecular dynamics simulations). We show that caution is needed when interpreting results of equilibrium peptide-membrane simulations, given the length of time single trajectories can dwell in local energy minima for 100's of ns to microseconds. While we did record significant membrane perturbations from the CM15 peptide, pores were not observed. We explain this discrepancy by computing the free energy for pore formation with different force fields. Our results show a large difference in the free energy barrier (ca. 40 kJ/mol) against pore formation predicted by the different force fields that would result in orders of magnitude differences in the simulation time required to observe spontaneous pore formation. This explains why previous simulations using the Berger lipid parameters reported pores induced by charged peptides, while with CHARMM based models pores were not observed in our long time-scale simulations. We reconcile some of the differences in the distance dependent free energies by shifting the free energy profiles to account for thickness differences between force fields. The shifted curves show that all the models describe small defects in lipid bilayers in a consistent manner, suggesting a common physical basis.

Gramicidin ion channels in a lipid bilayer supported on polyelectrolyte multilayer films: an electrochemical impedance study.

PubMed

Diamanti, Eleftheria; Gutiérrez-Pineda, Eduart; Politakos, Nikolaos; Andreozzi, Patrizia; Rodriguez-Presa, María José; Knoll, Wolfgang; Azzaroni, Omar; Gervasi, Claudio A; Moya, Sergio E

2017-12-06

Supported membranes on polymer cushions are of fundamental interest as models for cell membranes. The use of polyelectrolyte multilayers (PEMs) assembled by the layer by layer (LbL) technique as supports for a bilayer allows for easy integration of the lipid bilayer on surfaces and devices and for nanoscale tunable spacing of the lipid bilayer. Controlling ionic permeability in lipid bilayers supported on PEMs triggers potential applications in sensing and as models for transport phenomena in cell membranes. Lipid bilayers displaying gramicidin channels are fabricated on top of polyallylamine hydrochloride (PAH) and polystyrene sulfonate (PSS) multilayer films, by the assembly of vesicles of phosphatidylcholine and phosphatidylserine, 50 : 50 M/M, carrying gramicidin (GA). Quartz crystal microbalance with dissipation shows that the vesicles with GA fuse into a bilayer. Atomic force microscopy reveals that the presence of GA alters the bilayer topography resulting in depressions in the bilayer of around 70 nm in diameter. Electrochemical impedance spectroscopy (EIS) studies show that supported bilayers carrying GA have smaller resistances than the bilayers without GA. Lipid layers carrying GA display a higher conductance for K + than for Na + and are blocked in the presence of Ca 2+ .

Coulomb drag in electron-hole bilayer: Mass-asymmetry and exchange correlation effects

NASA Astrophysics Data System (ADS)

Arora, Priya; Singh, Gurvinder; Moudgil, R. K.

2018-04-01

Motivated by a recent experiment by Zheng et al. [App. Phys. Lett. 108, 062102 (2016)] on coulomb drag in electron-hole and hole-hole bilayers based on GaAs/AlGaAs semiconductor heterostructure, we investigate theoretically the influence of mass-asymmetry and temperature-dependence of correlations on the drag rate. The correlation effects are dealt with using the Vignale-Singwi effective inter-layer interaction model which includes correlations through local-field corrections to the bare coulomb interactions. However, in this work, we have incorporated only the intra-layer correlations using the temperature-dependent Hubbard approximation. Our results display a reasonably good agreement with the experimental data. However, it is crucial to include both the electron-hole mass-asymmetry and temperature-dependence of correlations. Mass-asymmetry and correlations are found to result in a substantial enhancement of drag resistivity.

Dynamics and stability of lipid bilayers modulated by thermosensitive polypeptides, cholesterols, and PEGylated lipids.

PubMed

Lee, Hwankyu; Kim, Hyun Ryoung; Park, Jae Chan

2014-02-28

Lipid bilayers, which consist of dipalmitoylglycerophosphocholines (DPPCs), PEGylated lipids, cholesterols, and elastin-like polypeptides (ELPs; [VPGVG]3) at different molar ratios, were simulated. Simulations were carried out for 2 μs using the coarse-grained (CG) model that had captured the experimentally observed phase behavior of PEGylated lipids and lateral diffusivity of DPPC bilayers. Starting with the initial position of ELPs on the bilayer surface, ELPs insert into the hydrophobic region of the bilayer because of their interaction with lipid tails, consistent with previous all-atom simulations. Lateral diffusion coefficients of DPPCs significantly increase in the bilayer composed of more ELPs and less cholesterols, showing their opposite effects on the bilayer dynamics. In particular, ELPs modulate the dynamics and phase for the disordered liquid bilayer, but not for the ordered gel bilayer, indicating that ELPs can destabilize only the disordered bilayer. In the ordered bilayer, ELP chains tend to have a spherical shape and slowly diffuse, while they are extended and diffuse faster in the disordered bilayer, indicating the effect of the bilayer phase on the conformation and diffusivity of ELPs. These findings explain the experimental observation that the ELP-conjugated liposomes are stable at 310 K (ordered phase) but become unstable and release the encapsulated drugs at 315 K (disordered phase), which suggests the effects of ELPs and cholesterols. Since the cholesterol-stabilized bilayer can be destabilized by the extended shaped ELPs only in the disordered phase (not in the ordered phase), the inclusion of cholesterols is required to safely shield drugs at 310 K as well as allow ELPs to disrupt lipids and destabilize the liposomes at 315 K.

Improved free-energy landscape reconstruction of bacteriorhodopsin highlights local variations in unfolding energy.

PubMed

Heenan, Patrick R; Yu, Hao; Siewny, Matthew G W; Perkins, Thomas T

2018-03-28

Precisely quantifying the energetics that drive the folding of membrane proteins into a lipid bilayer remains challenging. More than 15 years ago, atomic force microscopy (AFM) emerged as a powerful tool to mechanically extract individual membrane proteins from a lipid bilayer. Concurrently, fluctuation theorems, such as the Jarzynski equality, were applied to deduce equilibrium free energies (ΔG 0 ) from non-equilibrium single-molecule force spectroscopy records. The combination of these two advances in single-molecule studies deduced the free-energy of the model membrane protein bacteriorhodopsin in its native lipid bilayer. To elucidate this free-energy landscape at a higher resolution, we applied two recent developments. First, as an input to the reconstruction, we used force-extension curves acquired with a 100-fold higher time resolution and 10-fold higher force precision than traditional AFM studies of membrane proteins. Next, by using an inverse Weierstrass transform and the Jarzynski equality, we removed the free energy associated with the force probe and determined the molecular free-energy landscape of the molecule under study, bacteriorhodopsin. The resulting landscape yielded an average unfolding free energy per amino acid (aa) of 1.0 ± 0.1 kcal/mol, in agreement with past single-molecule studies. Moreover, on a smaller spatial scale, this high-resolution landscape also agreed with an equilibrium measurement of a particular three-aa transition in bacteriorhodopsin that yielded 2.7 kcal/mol/aa, an unexpectedly high value. Hence, while average unfolding ΔG 0 per aa is a useful metric, the derived high-resolution landscape details significant local variation from the mean. More generally, we demonstrated that, as anticipated, the inverse Weierstrass transform is an efficient means to reconstruct free-energy landscapes from AFM data.

Localized Vibrations of Bi Bilayer Leading to Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Weak Topological Insulator n-Type BiSe.

PubMed

Samanta, Manisha; Pal, Koushik; Pal, Provas; Waghmare, Umesh V; Biswas, Kanishka

2018-05-02

Realization of high thermoelectric performance in n-type semiconductors is of imperative need on account of the dearth of efficient n-type thermoelectric materials compared to the p-type counterpart. Moreover, development of efficient thermoelectric materials based on Te-free compounds is desirable because of the scarcity of Te in the Earth's crust. Herein, we report the intrinsic ultralow thermal conductivity and high thermoelectric performance near room temperature in n-type BiSe, a Te-free solid, which recently has emerged as a weak topological insulator. BiSe possesses a layered structure consisting of a bismuth bilayer (Bi 2 ) sandwiched between two Bi 2 Se 3 quintuple layers [Se-Bi-Se-Bi-Se], resembling natural heterostructure. High thermoelectric performance of BiSe is realized through the ultralow lattice thermal conductivity (κ lat of ∼0.6 W/mK at 300 K), which is significantly lower than that of Bi 2 Se 3 (κ lat of ∼1.8 W/mK at 300 K), although both of them belong to the same layered homologous family (Bi 2 ) m (Bi 2 Se 3 ) n . Phonon dispersion calculated from first-principles and the experimental low-temperature specific heat data indicate that soft localized vibrations of bismuth bilayer in BiSe are responsible for its ultralow κ lat . These low energy optical phonon branches couple strongly with the heat carrying acoustic phonons, and consequently suppress the phonon mean free path leading to low κ lat . Further optimization of thermoelectric properties of BiSe through Sb substitution and spark plasma sintering (SPS) results in high ZT ∼ 0.8 at 425 K along the pressing direction, which is indeed remarkable among Te-free n-type thermoelectric materials near room temperature.

Clinical efficacy and mechanism of bilayered living human skin equivalent (HSE) in treatment of diabetic foot ulcers.

PubMed

Brem, Harold; Young, Jan; Tomic-Canic, Marjana; Isaacs, Cary; Ehrlich, H Paul

2003-01-01

Bilayered living human skin equivalent (HSE) consists of cultured keratinocytes residing on the surface of a fibroblast-populated collagen lattice. Although HSE is FDA-approved for treatment of diabetic foot and venous stasis ulcers, its clinical efficacy remains limited, because the molecular mechanisms underlying its therapeutic effect are not fully understood. It is, therefore, often applied mistakenly as a skin graft. In this report, we delineate a mechanism of HSE biological effect and consequent optimal clinical use in accelerating closure of diabetic foot ulcers. HSE was grafted onto nude mice and the release of various growth factors was evaluated by reverse transcription-polymerase chain reaction (RT-PCR) and immunochemistry. Clinical: HSE was grafted onto 11 consecutive patients with diabetes who had 13 non-ischemic foot ulcers and healing was measured as time to 100% closure (e.g., no drainage and 100% epithelialized). HSE cellular components were determined to express 15 different growth factors/cytokine genes known to promote wound healing. Histological evidence from the nude mice showed that the collagen component of HSE underwent remodeling within the first seven days of grafting. Clinical: All diabetic foot ulcers healed in 31.8 12.4 days. Local release of a unique combination of 15 growth factors expressed by HSE keratinocyte and fibroblast components generates closure of diabetic foot ulcers. HSE should be applied with the same surgical conditions for a skin graft (i.e., no cellulitis, no drainage, and negligible bacteria). We hypothesize that bilayered HSE generates its effect by way of the local synthesis and release of multiple growth factors in specific combination and concentration, which improves the impaired reparative process of chronic wounds.

Improved free-energy landscape reconstruction of bacteriorhodopsin highlights local variations in unfolding energy

NASA Astrophysics Data System (ADS)

Heenan, Patrick R.; Yu, Hao; Siewny, Matthew G. W.; Perkins, Thomas T.

2018-03-01

Precisely quantifying the energetics that drive the folding of membrane proteins into a lipid bilayer remains challenging. More than 15 years ago, atomic force microscopy (AFM) emerged as a powerful tool to mechanically extract individual membrane proteins from a lipid bilayer. Concurrently, fluctuation theorems, such as the Jarzynski equality, were applied to deduce equilibrium free energies (ΔG0) from non-equilibrium single-molecule force spectroscopy records. The combination of these two advances in single-molecule studies deduced the free-energy of the model membrane protein bacteriorhodopsin in its native lipid bilayer. To elucidate this free-energy landscape at a higher resolution, we applied two recent developments. First, as an input to the reconstruction, we used force-extension curves acquired with a 100-fold higher time resolution and 10-fold higher force precision than traditional AFM studies of membrane proteins. Next, by using an inverse Weierstrass transform and the Jarzynski equality, we removed the free energy associated with the force probe and determined the molecular free-energy landscape of the molecule under study, bacteriorhodopsin. The resulting landscape yielded an average unfolding free energy per amino acid (aa) of 1.0 ± 0.1 kcal/mol, in agreement with past single-molecule studies. Moreover, on a smaller spatial scale, this high-resolution landscape also agreed with an equilibrium measurement of a particular three-aa transition in bacteriorhodopsin that yielded 2.7 kcal/mol/aa, an unexpectedly high value. Hence, while average unfolding ΔG0 per aa is a useful metric, the derived high-resolution landscape details significant local variation from the mean. More generally, we demonstrated that, as anticipated, the inverse Weierstrass transform is an efficient means to reconstruct free-energy landscapes from AFM data.

Clinical concentrations of chemically diverse general anesthetics minimally affect lipid bilayer properties.

PubMed

Herold, Karl F; Sanford, R Lea; Lee, William; Andersen, Olaf S; Hemmings, Hugh C

2017-03-21

General anesthetics have revolutionized medicine by facilitating invasive procedures, and have thus become essential drugs. However, detailed understanding of their molecular mechanisms remains elusive. A mechanism proposed over a century ago involving unspecified interactions with the lipid bilayer known as the unitary lipid-based hypothesis of anesthetic action, has been challenged by evidence for direct anesthetic interactions with a range of proteins, including transmembrane ion channels. Anesthetic concentrations in the membrane are high (10-100 mM), however, and there is no experimental evidence ruling out a role for the lipid bilayer in their ion channel effects. A recent hypothesis proposes that anesthetic-induced changes in ion channel function result from changes in bilayer lateral pressure that arise from partitioning of anesthetics into the bilayer. We examined the effects of a broad range of chemically diverse general anesthetics and related nonanesthetics on lipid bilayer properties using an established fluorescence assay that senses drug-induced changes in lipid bilayer properties. None of the compounds tested altered bilayer properties sufficiently to produce meaningful changes in ion channel function at clinically relevant concentrations. Even supra-anesthetic concentrations caused minimal bilayer effects, although much higher (toxic) concentrations of certain anesthetic agents did alter lipid bilayer properties. We conclude that general anesthetics have minimal effects on bilayer properties at clinically relevant concentrations, indicating that anesthetic effects on ion channel function are not bilayer-mediated but rather involve direct protein interactions.

Self-consistent study of local and nonlocal magnetoresistance in a YIG/Pt bilayer

NASA Astrophysics Data System (ADS)

Wang, Xi-guang; Zhou, Zhen-wei; Nie, Yao-zhuang; Xia, Qing-lin; Guo, Guang-hua

2018-03-01

We present a self-consistent study of the local spin Hall magnetoresistance (SMR) and nonlocal magnon-mediated magnetoresistance (MMR) in a heavy-metal/magnetic-insulator heterostructure at finite temperature. We find that the thermal fluctuation of magnetization significantly affects the SMR. It appears unidirectional with respect to the direction of electrical current (or magnetization). The unidirectionality of SMR originates from the asymmetry of creation or annihilation of thermal magnons induced by the spin Hall torque. Also, a self-consistent model can well describe the features of MMR.

Improvement of Ion/Ioff for h-BN encapsulated bilayer graphene by graphite local back gate electrode

NASA Astrophysics Data System (ADS)

Uwanno, Teerayut; Taniguchi, Takashi; Watanabe, Kenji; Nagashio, Kosuke

The critical issue for bilayer graphene (BLG) devices is low Ion/Ioff even at the band gap of 0.3eV. Band gap in BLG can be formed by creating potential difference between the two layers of BLG. This can be done by applying external electric field perpendicularly to BLG to induce different carrier densities in the two layers. Due to such origin, the spatial uniformity of band gap in the channel is quite sensitive to charge inhomogeneity in BLG. In order to apply electric field of 3V/nm to open the maximum band gap of 0.3eV, high- k gate stack has been utilized so far. However, oxide dielectrics usually have large charge inhomogeneity causing in-plane potential fluctuation in BLG channel. Due to surface flatness and small charge inhomogeneity, h-BN has been used as dielectrics to achieve high quality graphene devices, however, Ion/Iofffor BLG/ h-BN heterostuctures has not been reported yet. In this study, we used graphite as local back gate electrode to BLG encapsulated with h-BN. This resulted in much higher Ion/Ioff, indicating the importance of screening of charge inhomogeneity from SiO2 substrate surface by local graphite back gate electrode. This research was partly supported by JSPS Core-to-Core Program, A. Advanced Research Networks.

Effect of low levels of lipid oxidation on the curvature, dynamics, and permeability of lipid bilayers and their interactions with cationic nanoparticles

NASA Astrophysics Data System (ADS)

Lee, Hwankyu; Malmstadt, Noah

2018-04-01

Lipid bilayers composed of saturated and unsaturated lipids, oxidized lipids, and cholesterol at concentrations of 0–18 mol% oxidized lipid were simulated, showing that the presence of oxidized lipid increases bilayer disorder, curvature, and lateral dynamics at low oxidized-lipid concentrations of 18 mol% or less. The aldehyde terminal of a shortened oxidized-lipid tail tends to interact with water and thus bends toward the bilayer-water interface, in agreement with previous experiments and simulations. In particular, water molecules pass through the oxidized bilayer without pore formation, implying passive permeability. A single nanoparticle, which consists of 300 polystyrene (PS) chains with cationic terminals, added to this bilayer simulation induces negative bilayer curvature and inserts to the bilayer, regardless of the oxidized-lipid concentration. Hydrophobic monomers and cationic terminals of the PS particle interact respectively with lipid tails and headgroups, leading to the wrapping of either lipid monolayer or bilayer along the particle surface. These results indicate that lipid oxidation increases membrane curvature and permeability even at such a low concentration of oxidized lipid, which supports the experimental observations regarding the passive permeability of oxidized bilayer, and also that oxidized lipids of low concentration do not significantly influence the insertion of a cationic PS particle to the bilayer.

Computing Curvature Sensitivity of Biomolecules in Membranes by Simulated Buckling.

PubMed

Elías-Wolff, Federico; Lindén, Martin; Lyubartsev, Alexander P; Brandt, Erik G

2018-03-13

Membrane curvature sensing, where the binding free energies of membrane-associated molecules depend on the local membrane curvature, is a key factor to modulate and maintain the shape and organization of cell membranes. However, the microscopic mechanisms are not well understood, partly due to absence of efficient simulation methods. Here, we describe a method to compute the curvature dependence of the binding free energy of a membrane-associated probe molecule that interacts with a buckled membrane, which has been created by lateral compression of a flat bilayer patch. This buckling approach samples a wide range of curvatures in a single simulation, and anisotropic effects can be extracted from the orientation statistics. We develop an efficient and robust algorithm to extract the motion of the probe along the buckled membrane surface, and evaluate its numerical properties by extensive sampling of three coarse-grained model systems: local lipid density in a curved environment for single-component bilayers, curvature preferences of individual lipids in two-component membranes, and curvature sensing by a homotrimeric transmembrane protein. The method can be used to complement experimental data from curvature partition assays and provides additional insight into mesoscopic theories and molecular mechanisms for curvature sensing.

Surface functionalization of a polymeric lipid bilayer for coupling a model biological membrane with molecules, cells, and microstructures.

PubMed

Morigaki, Kenichi; Mizutani, Kazuyuki; Saito, Makoto; Okazaki, Takashi; Nakajima, Yoshihiro; Tatsu, Yoshiro; Imaishi, Hiromasa

2013-02-26

We describe a stable and functional model biological membrane based on a polymerized lipid bilayer with a chemically modified surface. A polymerized lipid bilayer was formed from a mixture of two diacetylene-containing phospholipids, 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DiynePC) and 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphoethanolamine (DiynePE). DiynePC formed a stable bilayer structure, whereas the ethanolamine headgroup of DiynePE enabled functional molecules to be grafted onto the membrane surface. Copolymerization of DiynePC and DiynePE resulted in a robust bilayer. Functionalization of the polymeric bilayer provided a route to a robust and biomimetic surface that can be linked with biomolecules, cells, and three-dimensional (3D) microstructures. Biotin and peptides were grafted onto the polymeric bilayer for attaching streptavidin and cultured mammalian cells by molecular recognition, respectively. Nonspecific adsorption of proteins and cells on polymeric bilayers was minimum. DiynePE was also used to attach a microstructure made of an elastomer (polydimethylsiloxan: PDMS) onto the membrane, forming a confined aqueous solution between the two surfaces. The microcompartment enabled us to assay the activity of a membrane-bound enzyme (cyochrome P450). Natural (fluid) lipid bilayers were incorporated together with membrane-bound proteins by lithographically polymerizing DiynePC/DiynePE bilayers. The hybrid membrane of functionalized polymeric bilayers and fluid bilayers offers a novel platform for a wide range of biomedical applications including biosensor, bioassay, cell culture, and cell-based assay.

Clinical concentrations of chemically diverse general anesthetics minimally affect lipid bilayer properties

PubMed Central

Herold, Karl F.; Sanford, R. Lea; Lee, William; Andersen, Olaf S.; Hemmings, Hugh C.

2017-01-01

General anesthetics have revolutionized medicine by facilitating invasive procedures, and have thus become essential drugs. However, detailed understanding of their molecular mechanisms remains elusive. A mechanism proposed over a century ago involving unspecified interactions with the lipid bilayer known as the unitary lipid-based hypothesis of anesthetic action, has been challenged by evidence for direct anesthetic interactions with a range of proteins, including transmembrane ion channels. Anesthetic concentrations in the membrane are high (10–100 mM), however, and there is no experimental evidence ruling out a role for the lipid bilayer in their ion channel effects. A recent hypothesis proposes that anesthetic-induced changes in ion channel function result from changes in bilayer lateral pressure that arise from partitioning of anesthetics into the bilayer. We examined the effects of a broad range of chemically diverse general anesthetics and related nonanesthetics on lipid bilayer properties using an established fluorescence assay that senses drug-induced changes in lipid bilayer properties. None of the compounds tested altered bilayer properties sufficiently to produce meaningful changes in ion channel function at clinically relevant concentrations. Even supra-anesthetic concentrations caused minimal bilayer effects, although much higher (toxic) concentrations of certain anesthetic agents did alter lipid bilayer properties. We conclude that general anesthetics have minimal effects on bilayer properties at clinically relevant concentrations, indicating that anesthetic effects on ion channel function are not bilayer-mediated but rather involve direct protein interactions. PMID:28265069

Simulation study of the structure and phase behavior of ceramide bilayers and the role of lipid head group chemistry

PubMed Central

Guo, Shan; Moore, Timothy C.; Iacovella, Christopher R.; Strickland, L. Anderson; McCabe, Clare

2014-01-01

Ceramides are known to be a key component of the stratum corneum, the outermost protective layer of the skin that controls barrier function. In this work, molecular dynamics simulations are used to examine the behavior of ceramide bilayers, focusing on non-hydroxy sphingosine (NS) and non-hydroxy phytosphingosine (NP) ceramides. Here, we propose a modified version of the CHARMM force field for ceramide simulation, which is directly compared to the more commonly used GROMOS-based force field of Berger (Biophys. J. 1997, 72); while both force fields are shown to closely match experiment from a structural standpoint at the physiological temperature of skin, the modified CHARMM force field is better able to capture the thermotropic phase transitions observed in experiment. The role of ceramide chemistry and its impact on structural ordering is examined by comparing ceramide NS to NP, using the validated CHARMM-based force field. These simulations demonstrate that changing from ceramide NS to NP results in changes to the orientation of the OH groups in the lipid headgroups. The arrangement of OH groups perpendicular to the bilayer normal for ceramide NP, verse parallel for NS, results in the formation of a distinct hydrogen bonding network, that is ultimately responsible for shifting the gel-to-liquid phase transition to higher temperature, in direct agreement with experiment. PMID:24501589

Reconciling Structural and Thermodynamic Predictions Using All-Atom and Coarse-Grain Force Fields: The Case of Charged Oligo-Arginine Translocation into DMPC Bilayers

PubMed Central

2015-01-01

Using the translocation of short, charged cationic oligo-arginine peptides (mono-, di-, and triarginine) from bulk aqueous solution into model DMPC bilayers, we explore the question of the similarity of thermodynamic and structural predictions obtained from molecular dynamics simulations using all-atom and Martini coarse-grain force fields. Specifically, we estimate potentials of mean force associated with translocation using standard all-atom (CHARMM36 lipid) and polarizable and nonpolarizable Martini force fields, as well as a series of modified Martini-based parameter sets. We find that we are able to reproduce qualitative features of potentials of mean force of single amino acid side chain analogues into model bilayers. In particular, modifications of peptide–water and peptide–membrane interactions allow prediction of free energy minima at the bilayer–water interface as obtained with all-atom force fields. In the case of oligo-arginine peptides, the modified parameter sets predict interfacial free energy minima as well as free energy barriers in almost quantitative agreement with all-atom force field based simulations. Interfacial free energy minima predicted by a modified coarse-grained parameter set are −2.51, −4.28, and −5.42 for mono-, di-, and triarginine; corresponding values from all-atom simulations are −0.83, −3.33, and −3.29, respectively, all in units of kcal/mol. We found that a stronger interaction between oligo-arginine and the membrane components and a weaker interaction between oligo-arginine and water are crucial for producing such minima in PMFs using the polarizable CG model. The difference between bulk aqueous and bilayer center states predicted by the modified coarse-grain force field are 11.71, 14.14, and 16.53 kcal/mol, and those by the all-atom model are 6.94, 8.64, and 12.80 kcal/mol; those are of almost the same order of magnitude. Our simulations also demonstrate a remarkable similarity in the structural aspects of the ensemble of configurations generated using the all-atom and coarse-grain force fields. Both resolutions show that oligo-arginine peptides adopt preferential orientations as they translocate into the bilayer. The guiding theme centers on charged groups maintaining coordination with polar and charged bilayer components as well as local water. We also observe similar behaviors related with membrane deformations. PMID:25290376

Role of the array geometry in multi-bilayer hair cell sensors

NASA Astrophysics Data System (ADS)

Tamaddoni, Nima J.; Sarles, Stephen A.

2014-03-01

Recently, a bio-inspired, synthetic membrane-based hair cell sensor was fabricated and characterized. This sensor generates current in response to mechanical stimuli, such as airflow or free vibration, which perturb the sensor's hair. Vibration transferred from the hair to a lipid membrane (lipid bilayer) causes a voltage-dependent time rate of change in electrical capacitance of the membrane, which produces measurable current. Studies to date have been performed on systems containing only two droplets and a single bilayer, even though an array of multiple bilayers can be formed with more than 2 droplets. Thus, it is yet to be determined how multiple lipid bilayers affect the sensing response of a membrane-based hair cell sensor. In this work, we assemble serial droplet arrays with more than 1 bilayer to experimentally study the current generated by each membrane in response to perturbation of a single hair element. Two serial array configurations are studied: The first consists of a serial array of 3 bilayers formed using 4 droplets with the hair positioned in an end droplet. The second configuration consists of 3 droplets and 2 bilayers in series with the hair positioned in the central droplet. In serial arrays of up to four droplets, we observe that mechanotransduction of the hair's motion into a capacitive current occurs at every membrane, with bilayers positioned adjacent to the droplet containing the hair generating the largest sensing current. The measured currents suggest the total current generated by all bilayers in a 4-droplet, 3-bilaye array is greater than the current produced by a single-membrane sensor and similar in magnitude to the sum of currents output by 3, single-bilayer sensors operated independently. Moreover, we learned that bilayers positioned on the same side of the hair produce sensing currents that are in-phase, whereas bilayers positioned on opposite sides of the droplet containing the hair generate out-of-phase responses.

Similarities and differences of serotonin and its precursors in their interactions with model membranes studied by molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Wood, Irene; Martini, M. Florencia; Pickholz, Mónica

2013-08-01

In this work, we report a molecular dynamics (MD) simulations study of relevant biological molecules as serotonin (neutral and protonated) and its precursors, tryptophan and 5-hydroxy-tryptophan, in a fully hydrated bilayer of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyl-choline (POPC). The simulations were carried out at the fluid lamellar phase of POPC at constant pressure and temperature conditions. Two guest molecules of each type were initially placed at the water phase. We have analyzed, the main localization, preferential orientation and specific interactions of the guest molecules within the bilayer. During the simulation run, the four molecules were preferentially found at the water-lipid interphase. We found that the interactions that stabilized the systems are essentially hydrogen bonds, salt bridges and cation-π. None of the guest molecules have access to the hydrophobic region of the bilayer. Besides, zwitterionic molecules have access to the water phase, while protonated serotonin is anchored in the interphase. Even taking into account that these simulations were done using a model membrane, our results suggest that the studied molecules could not cross the blood brain barrier by diffusion. These results are in good agreement with works that show that serotonin and Trp do not cross the BBB by simple diffusion.

Surface protection in bio-shields via a functional soft skin layer: Lessons from the turtle shell.

PubMed

Shelef, Yaniv; Bar-On, Benny

2017-09-01

The turtle shell is a functional bio-shielding element, which has evolved naturally to provide protection against predator attacks that involve biting and clawing. The near-surface architecture of the turtle shell includes a soft bi-layer skin coating - rather than a hard exterior - which functions as a first line of defense against surface damage. This architecture represents a novel type of bio-shielding configuration, namely, an inverse structural-mechanical design, rather than the hard-coated bio-shielding elements identified so far. In the current study, we used experimentally based structural modeling and FE simulations to analyze the mechanical significance of this unconventional protection architecture in terms of resistance to surface damage upon extensive indentations. We found that the functional bi-layer skin of the turtle shell, which provides graded (soft-softer-hard) mechanical characteristics to the bio-shield exterior, serves as a bumper-buffer mechanism. This material-level adaptation protects the inner core from the highly localized indentation loads via stress delocalization and extensive near-surface plasticity. The newly revealed functional bi-layer coating architecture can potentially be adapted, using synthetic materials, to considerably enhance the surface load-bearing capabilities of various engineering configurations. Copyright © 2017 Elsevier Ltd. All rights reserved.

Molecular dynamics modelling of EGCG clusters on ceramide bilayers

NASA Astrophysics Data System (ADS)

Yeo, Jingjie; Cheng, Yuan; Li, Weifeng; Zhang, Yong-Wei

2015-12-01

A novel method of atomistic modelling and characterization of both pure ceramide and mixed lipid bilayers is being developed, using only the General Amber ForceField. Lipid bilayers modelled as pure ceramides adopt hexagonal packing after equilibration, and the area per lipid and bilayer thickness are consistent with previously reported theoretical results. Mixed lipid bilayers are modelled as a combination of ceramides, cholesterol, and free fatty acids. This model is shown to be stable after equilibration. Green tea extract, also known as epigallocatechin-3-gallate, is introduced as a spherical cluster on the surface of the mixed lipid bilayer. It is demonstrated that the cluster is able to bind to the bilayers as a cluster without diffusing into the surrounding water.

Mapping surface charge density of lipid bilayers by quantitative surface conductivity microscopy

PubMed Central

Klausen, Lasse Hyldgaard; Fuhs, Thomas; Dong, Mingdong

2016-01-01

Local surface charge density of lipid membranes influences membrane–protein interactions leading to distinct functions in all living cells, and it is a vital parameter in understanding membrane-binding mechanisms, liposome design and drug delivery. Despite the significance, no method has so far been capable of mapping surface charge densities under physiologically relevant conditions. Here, we use a scanning nanopipette setup (scanning ion-conductance microscope) combined with a novel algorithm to investigate the surface conductivity near supported lipid bilayers, and we present a new approach, quantitative surface conductivity microscopy (QSCM), capable of mapping surface charge density with high-quantitative precision and nanoscale resolution. The method is validated through an extensive theoretical analysis of the ionic current at the nanopipette tip, and we demonstrate the capacity of QSCM by mapping the surface charge density of model cationic, anionic and zwitterionic lipids with results accurately matching theoretical values. PMID:27561322

Mixing, diffusion, and percolation in binary supported membranes containing mixtures of lipids and amphiphilic block copolymers.

PubMed

Gettel, Douglas L; Sanborn, Jeremy; Patel, Mira A; de Hoog, Hans-Peter; Liedberg, Bo; Nallani, Madhavan; Parikh, Atul N

2014-07-23

Substrate-mediated fusion of small polymersomes, derived from mixtures of lipids and amphiphilic block copolymers, produces hybrid, supported planar bilayers at hydrophilic surfaces, monolayers at hydrophobic surfaces, and binary monolayer/bilayer patterns at amphiphilic surfaces, directly responding to local measures of (and variations in) surface free energy. Despite the large thickness mismatch in their hydrophobic cores, the hybrid membranes do not exhibit microscopic phase separation, reflecting irreversible adsorption and limited lateral reorganization of the polymer component. With increasing fluid-phase lipid fraction, these hybrid, supported membranes undergo a fluidity transition, producing a fully percolating fluid lipid phase beyond a critical area fraction, which matches the percolation threshold for the immobile point obstacles. This then suggests that polymer-lipid hybrid membranes might be useful models for studying obstructed diffusion, such as occurs in lipid membranes containing proteins.

Hydrodynamic collective effects of active protein machines in solution and lipid bilayers

PubMed Central

Mikhailov, Alexander S.; Kapral, Raymond

2015-01-01

The cytoplasm and biomembranes in biological cells contain large numbers of proteins that cyclically change their shapes. They are molecular machines that can function as molecular motors or carry out various other tasks in the cell. Many enzymes also undergo conformational changes within their turnover cycles. We analyze the advection effects that nonthermal fluctuating hydrodynamic flows induced by active proteins have on other passive molecules in solution or membranes. We show that the diffusion constants of passive particles are enhanced substantially. Furthermore, when gradients of active proteins are present, a chemotaxis-like drift of passive particles takes place. In lipid bilayers, the effects are strongly nonlocal, so that active inclusions in the entire membrane contribute to local diffusion enhancement and the drift. All active proteins in a biological cell or in a membrane contribute to such effects and all passive particles, and the proteins themselves, will be subject to them. PMID:26124140

Fabrication of Ag-Au bimetallic nanoparticles by laser-induced dewetting of bilayer films

NASA Astrophysics Data System (ADS)

Oh, Yoonseok; Lee, Jeeyoung; Lee, Myeongkyu

2018-03-01

We here show that Ag-Au bimetallic nanoparticles (NPs) can be produced by dewetting an Ag/Au bilayer film coated on glass using a nanosecond-pulsed laser beam. Elemental analysis revealed that the obtained bimetallic NPs are Ag-Au alloys, with two elements well mixed over the whole volume of the particle. The composition of the produced particles was controllable by changing the relative thickness of each layer. The localized surface plasmon resonance (LSPR) peak was red-shifted with an increasing Au content and the LSPR wavelength could be tuned from 415 to 525 nm by varying the alloy composition. A film area of several square centimeters could be transformed into Ag-Au NPs by a single laser pulse of 6 ns duration. This study provides a facile and scalable route to prepare bimetallic NPs for plasmonic and other applications.

Mapping surface charge density of lipid bilayers by quantitative surface conductivity microscopy

NASA Astrophysics Data System (ADS)

Klausen, Lasse Hyldgaard; Fuhs, Thomas; Dong, Mingdong

2016-08-01

Local surface charge density of lipid membranes influences membrane-protein interactions leading to distinct functions in all living cells, and it is a vital parameter in understanding membrane-binding mechanisms, liposome design and drug delivery. Despite the significance, no method has so far been capable of mapping surface charge densities under physiologically relevant conditions. Here, we use a scanning nanopipette setup (scanning ion-conductance microscope) combined with a novel algorithm to investigate the surface conductivity near supported lipid bilayers, and we present a new approach, quantitative surface conductivity microscopy (QSCM), capable of mapping surface charge density with high-quantitative precision and nanoscale resolution. The method is validated through an extensive theoretical analysis of the ionic current at the nanopipette tip, and we demonstrate the capacity of QSCM by mapping the surface charge density of model cationic, anionic and zwitterionic lipids with results accurately matching theoretical values.

Amphiphilic naproxen prodrugs: differential scanning calorimetry study on their interaction with phospholipid bilayers.

PubMed

Giuffrida, Maria Chiara; Pignatello, Rosario; Castelli, Francesco; Sarpietro, Maria Grazia

2017-09-01

Naproxen, a nonsteroid anti-inflammatory drug studied for Alzheimer's disease, was conjugated with lipoamino acids (LAA) directly or through a diethylamine (EDA) spacer to improve the drug lipophilicity and the interaction with phospholipid bilayers. The interaction of naproxen and its prodrugs with biomembrane models consisting of dimyristoylphosphatidylcholine multilamellar vesicles was studied by differential scanning calorimetry. The transfer of prodrugs from a lipophilic carrier to a biomembrane model was also studied. Naproxen conjugation to lipoamino acids improves its interaction with biomembrane models and affects the transfer from a lipophilic carrier to biomembrane model. LAA portion may localize between the phospholipid chains; the entity of the interaction depends not only on the presence of the spacer but also on the LAA chain length. Variation of LAA portion can modulate the naproxen prodrugs affinity towards the biological membrane as well as towards the lipophilic carrier. © 2017 Royal Pharmaceutical Society.

Mapping surface charge density of lipid bilayers by quantitative surface conductivity microscopy.

PubMed

Klausen, Lasse Hyldgaard; Fuhs, Thomas; Dong, Mingdong

2016-08-26

Local surface charge density of lipid membranes influences membrane-protein interactions leading to distinct functions in all living cells, and it is a vital parameter in understanding membrane-binding mechanisms, liposome design and drug delivery. Despite the significance, no method has so far been capable of mapping surface charge densities under physiologically relevant conditions. Here, we use a scanning nanopipette setup (scanning ion-conductance microscope) combined with a novel algorithm to investigate the surface conductivity near supported lipid bilayers, and we present a new approach, quantitative surface conductivity microscopy (QSCM), capable of mapping surface charge density with high-quantitative precision and nanoscale resolution. The method is validated through an extensive theoretical analysis of the ionic current at the nanopipette tip, and we demonstrate the capacity of QSCM by mapping the surface charge density of model cationic, anionic and zwitterionic lipids with results accurately matching theoretical values.

Peptide-Lipid Interactions: Experiments and Applications

PubMed Central

Galdiero, Stefania; Falanga, Annarita; Cantisani, Marco; Vitiello, Mariateresa; Morelli, Giancarlo; Galdiero, Massimiliano

2013-01-01

The interactions between peptides and lipids are of fundamental importance in the functioning of numerous membrane-mediated cellular processes including antimicrobial peptide action, hormone-receptor interactions, drug bioavailability across the blood-brain barrier and viral fusion processes. Moreover, a major goal of modern biotechnology is obtaining new potent pharmaceutical agents whose biological action is dependent on the binding of peptides to lipid-bilayers. Several issues need to be addressed such as secondary structure, orientation, oligomerization and localization inside the membrane. At the same time, the structural effects which the peptides cause on the lipid bilayer are important for the interactions and need to be elucidated. The structural characterization of membrane active peptides in membranes is a harsh experimental challenge. It is in fact accepted that no single experimental technique can give a complete structural picture of the interaction, but rather a combination of different techniques is necessary. PMID:24036440

Immobilized lipid-bilayer materials

DOEpatents

Sasaki, Darryl Y.; Loy, Douglas A.; Yamanaka, Stacey A.

2000-01-01

A method for preparing encapsulated lipid-bilayer materials in a silica matrix comprising preparing a silica sol, mixing a lipid-bilayer material in the silica sol and allowing the mixture to gel to form the encapsulated lipid-bilayer material. The mild processing conditions allow quantitative entrapment of pre-formed lipid-bilayer materials without modification to the material's spectral characteristics. The method allows for the immobilization of lipid membranes to surfaces. The encapsulated lipid-bilayer materials perform as sensitive optical sensors for the detection of analytes such as heavy metal ions and can be used as drug delivery systems and as separation devices.

Shaped Apertures in Photoresist Films Enhance the Lifetime and Mechanical Stability of Suspended Lipid Bilayers

PubMed Central

Kalsi, Sumit; Powl, Andrew M.; Wallace, B.A.; Morgan, Hywel; de Planque, Maurits R.R.

2014-01-01

Planar lipid bilayers suspended in apertures provide a controlled environment for ion channel studies. However, short lifetimes and poor mechanical stability of suspended bilayers limit the experimental throughput of bilayer electrophysiology experiments. Although bilayers are more stable in smaller apertures, ion channel incorporation through vesicle fusion with the suspended bilayer becomes increasingly difficult. In an alternative bilayer stabilization approach, we have developed shaped apertures in SU8 photoresist that have tapered sidewalls and a minimum diameter between 60 and 100 μm. Bilayers formed at the thin tip of these shaped apertures, either with the painting or the folding method, display drastically increased lifetimes, typically >20 h, and mechanical stability, being able to withstand extensive perturbation of the buffer solution. Single-channel electrical recordings of the peptide alamethicin and of the proteoliposome-delivered potassium channel KcsA demonstrate channel conductance with low noise, made possible by the small capacitance of the 50 μm thick SU8 septum, which is only thinned around the aperture, and unimpeded proteoliposome fusion, enabled by the large aperture diameter. We anticipate that these shaped apertures with micrometer edge thickness can substantially enhance the throughput of channel characterization by bilayer lipid membrane electrophysiology, especially in combination with automated parallel bilayer platforms. PMID:24739164

Distribution of Drug Molecules in Lipid Membranes: Neutron Diffraction and MD Simulations.

NASA Astrophysics Data System (ADS)

Boggara, Mohan; Mihailescu, Ella; Krishnamoorti, Ramanan

2009-03-01

Non-steroidal anti-inflammatory drugs (NSAIDs) e.g. Aspirin and Ibuprofen, with chronic usage cause gastro intestinal (GI) toxicity. It has been shown experimentally that NSAIDs pre-associated with phospholipids reduce the GI toxicity and also increase the therapeutic activity of these drugs compared to the unmodified ones. In this study, using neutron diffraction, the DOPC lipid bilayer structure (with and without drug) as well as the distribution of a model NSAID (Ibuprofen) as a function of its position along the membrane normal was obtained at sub-nanometer resolution. It was found that the bilayer thickness reduces as the drug is added. Further, the results are successfully compared with atomistic Molecular Dynamics simulations. Based on this successful comparison and motivated by atomic details from MD, quasi-molecular modeling of the lipid membrane is being carried out and will be presented. The above study is expected to provide an effective methodology to design drug delivery nanoparticles based on a variety of soft condensed matter such as lipids or polymers.

Tuning Surface Energy Landscapes in Metallic Quantum Films using Alkali Adsorbates

NASA Astrophysics Data System (ADS)

Khajetoorians, Alexander; Qin, Shengyong; Zhu, Wenguang; Eisele, Holger; Zhang, Zhenyu; Shih, Chih-Kang

2008-03-01

Quantum confinement shows a strong interplay with growth and kinetics in thin metal systems where the Fermi wavelength has a special relationship to the surface normal lattice constant. In the case of Pb/Si(111) systems, this relationship reveals an interesting thickness-dependent bilayer oscillation in the density of states and surface energy up to a phase. In this paper, we report on a novel effect: tuning of the energy landscape of a flat-top quantum Pb mesa using Cs adsorbates. Using STM/STS, we show that depositing Cs adsorbates on a thin Pb mesa promotes quantum stable Pb nanoislands on preferentially unstable thicknesses. Thickness-dependent nanoisland densities show a strong bilayer oscillation correlating with quantum stability. By modifying the Cs coverage on the mesa surface, we can tune the lateral size distribution of the nanoislands and the overall amplitude of the island density oscillation. Nanoisland formation is linked to a step decoration of Cs adatoms along the step edge of the nanoisland.

Morphological Effects and Antioxidant Capacity of Solanum crispum (Natre) In Vitro Assayed on Human Erythrocytes.

PubMed

Suwalsky, Mario; Ramírez, Patricia; Avello, Marcia; Villena, Fernando; Gallardo, María José; Barriga, Andrés; Manrique-Moreno, Marcela

2016-06-01

In order to gain insight into the molecular mechanism of the antioxidant properties of Solanum crispum, aqueous extracts of its leaves were assayed on human erythrocytes and molecular models of its membrane. Phenolics and alkaloids were detected by HPLC-MS. Scanning electron and defocusing microscopy showed that S. crispum changed erythrocytes from the normal shape to echinocytes. These results imply that molecules present in the aqueous extracts were located in the outer monolayer of the erythrocyte membrane. Dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) were chosen as representative of phospholipid classes located in the outer and inner monolayers of the erythrocyte membrane, respectively. X-ray diffraction showed that S. crispum preferentially interacted with DMPC bilayers. Experiments regarding its antioxidant properties showed that S. crispum neutralized the oxidative capacity of HClO on DMPE bilayers; defocusing microscopy and hemolysis assays demonstrated the protective effect of S. crispum against the oxidant effects of HClO on human erythrocytes.

Lipid intermediates in membrane fusion: formation, structure, and decay of hemifusion diaphragm.

PubMed

Kozlovsky, Yonathan; Chernomordik, Leonid V; Kozlov, Michael M

2002-11-01

Lipid bilayer fusion is thought to involve formation of a local hemifusion connection, referred to as a fusion stalk. The subsequent fusion stages leading to the opening of a fusion pore remain unknown. The earliest fusion pore could represent a bilayer connection between the membranes and could be formed directly from the stalk. Alternatively, fusion pore can form in a single bilayer, referred to as hemifusion diaphragm (HD), generated by stalk expansion. To analyze the plausibility of stalk expansion, we studied the pathway of hemifusion theoretically, using a recently developed elastic model. We show that the stalk has a tendency to expand into an HD for lipids with sufficiently negative spontaneous splay, (~)J(s)< 0. For different experimentally relevant membrane configurations we find two characteristic values of the spontaneous splay. (~)J*(s) and (~)J**(s), determining HD dimension. The HD is predicted to have a finite equilibrium radius provided that the spontaneous splay is in the range (~)J**(s)< (~)J(s)<(~)J*(s), and to expand infinitely for (~)J(s)<(~)J**(s). In the case of common lipids, which do not fuse spontaneously, an HD forms only under action of an external force pulling the diaphragm rim apart. We calculate the dependence of the HD radius on this force. To address the mechanism of fusion pore formation, we analyze the distribution of the lateral tension emerging in the HD due to the establishment of lateral equilibrium between the deformed and relaxed portions of lipid monolayers. We show that this tension concentrates along the HD rim and reaches high values sufficient to rupture the bilayer and form the fusion pore. Our analysis supports the hypothesis that transition from a hemifusion to a fusion pore involves radial expansion of the stalk.

Role of lipid phase separations and membrane hydration in phospholipid vesicle fusion.

PubMed

Hoekstra, D

1982-06-08

The relationship between lipid phase separation and fusion of small unilamellar phosphatidylserine-containing vesicles was investigated. The kinetics of phase separation were monitored by following the increase of self-quenching of the fluorescent phospholipid analogue N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine, which occurs when the local concentration of the probe increases upon Ca2+-induced phase separation in phosphatidylserine (PS) bilayers [Hoekstra, D. (1982) Biochemistry 21, 1055-1061]. Fusion was determined by using the resonance energy transfer fusion assay [Struck, D. K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099], which monitors the mixing of fluorescent lipid donor and acceptor molecules, resulting in an increase in energy transfer efficiency. The results show that in the presence of Ca2+, fusion proceeds much more rapidly (t 1/2 less than 5 s) than the process of phase separation (T 1/2 congruent to 1 min). Mg2+ also induced fusion, albeit at higher concentrations than Ca2+. Mg2+-induced phase separation were not detected, however. Subthreshold concentrations of Ca2+ (0.5 mM) or Mg2+ (2 mM) induced extensive fusion of PS-containing vesicles in poly(ethylene glycol) containing media. This effect did not appear to be a poly(ethylene glycol)-facilitated enhancement of cation binding to the bilayer, and consequently Ca2+-induced phase separation was not observed. The results suggest that macroscopic phase separation may facilitate but does not induced the fusion process and is therefore, not directly involved in the actual fusion mechanism. The fusion experiments performed in the presence of poly(ethylene glycol) suggest that the degree of bilayer dehydration and the creation of "point defects" in the bilayer without rigorous structural rearrangements in the membrane are dominant factors in the initial fusion events.

High Yield Chemical Vapor Deposition Growth of High Quality Large-Area AB Stacked Bilayer Graphene

PubMed Central

Liu, Lixin; Zhou, Hailong; Cheng, Rui; Yu, Woo Jong; Liu, Yuan; Chen, Yu; Shaw, Jonathan; Zhong, Xing; Huang, Yu; Duan, Xiangfeng

2012-01-01

Bernal stacked (AB stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electrical field. Mechanical exfoliation can be used to produce AB stacked bilayer graphene flakes but typically with the sizes limited to a few micrometers. Chemical vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixture of AB and randomly stacked structures. Herein we report a rational approach to produce large-area high quality AB stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H2/CH4 ratio in a low pressure CVD process to enable the continued growth of bilayer graphene. A high temperature and low pressure nucleation step is found to be critical for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90 %) and high coverage (up to 99 %). The electrical transport studies demonstrated that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB stacked bilayer graphene with the highest carrier mobility exceeding 4,000 cm2/V·s at room temperature, comparable to that of the exfoliated bilayer graphene. PMID:22906199

Spectroscopic probing of ortho-nitrophenol localization in phospholipid bilayers.

PubMed

Sánchez, Julieta M; Turina, Anahí del V; Perillo, María A

2007-11-12

In the present study we estimated the localization of ortho-nitrophenol (ONP) within model membranes through its efficiency to quench and to modify the anisotropy of DPH and TMA-DPH fluorescence. These fluorescent probes are known to sense the hydrocarbon core and the polar head group region of membranes, respectively. TMA-DPH fluorescence in MLVs was more efficiently quenched than DPH (K(q,TMA-DPH)=2.36 and K(q,DPH)=1.07 mM ns(-1)). Moreover, these results demonstrated the interfacial localization of ONP and may contribute to understand membrane-mediated mechanisms of ONP-induced toxicity and the behavior of ONP as a product of several enzymatic reactions occurring in the presence of lipid-water interfaces.

Membrane-Surface Anchoring of Charged Diacylglycerol-Lactones Correlates with Biological Activities | Center for Cancer Research

Cancer.gov

The inside cover picture shows the molecular structure of a DAG lactone derivative on top of the inner leaflet of a DMPC bilayer. The confocal microscopy image illustrates DAG-lactone-stimulated membrane localization of PKCδ-ECFP in living cells, while the space-filling model shows the surface of the C1B domain of PKCδ, the target of the lactone.

Tip-enhanced Raman spectroscopy of lipid bilayers in water with an alumina- and silver-coated tungsten tip.

PubMed

Nakata, Atsushi; Nomoto, Tomonori; Toyota, Taro; Fujinami, Masanori

2013-01-01

Tip-enhanced Raman spectroscopy (TERS) of supported phospholipid bilayers in an aqueous environment is discussed in this paper. Two bilayer membranes were examined: 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). We fabricated alumina- and silver-coated tungsten tips that are very robust in water. There was a large time-dependence in the TERS spectra for the DOPC bilayers, whereas no such time-dependence was observed in the DPPC bilayer spectra under the probe tip. The spectral changes of DOPC bilayers are discussed in terms of the fluidity of the liquid crystalline phase. Time-resolved TERS thus has the potential to characterize inhomogeneity and diffusion in fluidic phospholipid bilayer membranes.

Method of fabricating lipid bilayer membranes on solid supports

NASA Technical Reports Server (NTRS)

Cho, Nam-Joon (Inventor); Frank, Curtis W. (Inventor); Glenn, Jeffrey S. (Inventor); Cheong, Kwang Ho (Inventor)

2012-01-01

The present invention provides a method of producing a planar lipid bilayer on a solid support. With this method, a solution of lipid vesicles is first deposited on the solid support. Next, the lipid vesicles are destabilized by adding an amphipathic peptide solution to the lipid vesicle solution. This destabilization leads to production of a planar lipid bilayer on the solid support. The present invention also provides a supported planar lipid bilayer, where the planar lipid bilayer is made of naturally occurring lipids and the solid support is made of unmodified gold or titanium oxide. Preferably, the supported planar lipid bilayer is continuous. The planar lipid bilayer may be made of any naturally occurring lipid or mixture of lipids, including, but not limited to phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinsitol, cardiolipin, cholesterol, and sphingomyelin.

Microwave control of the superconducting proximity effect and minigap in magnetic and normal metals

PubMed Central

Linder, Jacob; Amundsen, Morten; Ouassou, Jabir Ali

2016-01-01

We demonstrate theoretically that microwave radiation applied to superconducting proximity structures controls the minigap and other spectral features in the density of states of normal and magnetic metals, respectively. Considering both a bilayer and Josephson junction geometry, we show that microwaves with frequency ω qualitatively alters the spectral properties of the system: inducing a series of resonances, controlling the minigap size Emg, and even replacing the minigap with a strong peak of quasiparticle accumulation at zero energy when ω = Emg. The interaction between light and Cooper pairs may thus open a route to active control of quantum coherent phenomena in superconducting proximity structures. PMID:27982128

SFG studies on interactions between antimicrobial peptides and supported lipid bilayers.

PubMed

Chen, Xiaoyun; Chen, Zhan

2006-09-01

The mode of action of antimicrobial peptides (AMPs) in disrupting cell membrane bilayers is of fundamental importance in understanding the efficiency of different AMPs, which is crucial to design antibiotics with improved properties. Recent developments in the field of sum frequency generation (SFG) vibrational spectroscopy have made it a powerful and unique biophysical technique in investigating the interactions between AMPs and a single substrate supported planar lipid bilayer. We will review some of the recent progress in applying SFG to study membrane lipid bilayers and discuss how SFG can provide novel information such as real-time bilayer structure change and AMP orientation during AMP-lipid bilayer interactions in a very biologically relevant manner. Several examples of applying SFG to monitor such interactions between AMPs and a dipalmitoyl phosphatidylglycerol (DPPG) bilayer are presented. Different modes of actions are observed for melittin, tachyplesin I, d-magainin 2, MSI-843, and a synthetic antibacterial oligomer, demonstrating that SFG is very effective in the study of AMPs and AMP-lipid bilayer interactions.

Examining the origins of the hydration force between lipid bilayers using all-atom simulations.

PubMed

Gentilcore, Anastasia N; Michaud-Agrawal, Naveen; Crozier, Paul S; Stevens, Mark J; Woolf, Thomas B

2010-05-01

Using 237 all-atom double bilayer simulations, we examined the thermodynamic and structural changes that occur as a phosphatidylcholine lipid bilayer stack is dehydrated. The simulated system represents a micropatch of lipid multilayer systems that are studied experimentally using surface force apparatus, atomic force microscopy and osmotic pressure studies. In these experiments, the hydration level of the system is varied, changing the separation between the bilayers, in order to understand the forces that the bilayers feel as they are brought together. These studies have found a curious, strongly repulsive force when the bilayers are very close to each other, which has been termed the "hydration force," though the origins of this force are not clearly understood. We computationally reproduce this repulsive, relatively free energy change as bilayers come together and make qualitative conclusions as to the enthalpic and entropic origins of the free energy change. This analysis is supported by data showing structural changes in the waters, lipids and salts that have also been seen in experimental work. Increases in solvent ordering as the bilayers are dehydrated are found to be essential in causing the repulsion as the bilayers come together.

Lateral Diffusion of Peripheral Membrane Proteins on Supported Lipid Bilayers Is Controlled by the Additive Frictional Drags of 1) Bound Lipids and 2) Protein Domains Penetrating into the Bilayer Hydrocarbon Core

PubMed Central

Ziemba, Brian P.; Falke, Joseph J.

2013-01-01

Peripheral membrane proteins bound to lipids on bilayer surfaces play central roles in a wide array of cellular processes, including many signaling pathways. These proteins diffuse in the plane of the bilayer and often undergo complex reactions involving the binding of regulatory and substrate lipids and proteins they encounter during their 2-D diffusion. Some peripheral proteins, for example pleckstrin homology (PH) domains, dock to the bilayer in a relatively shallow position with little penetration into the bilayer. Other peripheral proteins exhibit more complex bilayer contacts, for example classical protein kinase C isoforms (PKCs) bind as many as six lipids in stepwise fashion, resulting in the penetration of three PKC domains (C1A, C1B, C2) into the bilayer headgroup and hydrocarbon regions. A molecular understanding of the molecular features that control the diffusion speeds of proteins bound to supported bilayers would enable key molecular information to be extracted from experimental diffusion constants, revealing protein-lipid and protein-bilayer interactions difficult to study by other methods. The present study investigates a range of 11 different peripheral protein constructs comprised by 1 to 3 distinct domains (PH, C1A, C1B, C2, anti-lipid antibody). By combining these constructs with various combinations of target lipids, the study measures 2-D diffusion constants on supported bilayers for 17 different protein-lipid complexes. The resulting experimental diffusion constants, together with the known membrane interaction parameters of each complex, are used to analyze the molecular features correlated with diffusional slowing and bilayer friction. The findings show that both 1) individual bound lipids and 2) individual protein domains that penetrate into the hydrocarbon core make additive contributions to the friction against the bilayer, thereby defining the 2-D diffusion constant. An empirical formula is developed that accurately estimates the diffusion constant and bilayer friction of a peripheral protein in terms of its number of bound lipids and its geometry of penetration into the bilayer hydrocarbon core, yielding an excellent global best fit (R2 of 0.97) to the experimental diffusion constants. Finally, the observed additivity of the frictional contributions suggests that further development of current theory describing bilayer dynamics may be needed. The present findings provide constraints that will be useful in such theory development. PMID:23701821

Lateral diffusion of peripheral membrane proteins on supported lipid bilayers is controlled by the additive frictional drags of (1) bound lipids and (2) protein domains penetrating into the bilayer hydrocarbon core.

PubMed

Ziemba, Brian P; Falke, Joseph J

2013-01-01

Peripheral membrane proteins bound to lipids on bilayer surfaces play central roles in a wide array of cellular processes, including many signaling pathways. These proteins diffuse in the plane of the bilayer and often undergo complex reactions involving the binding of regulatory and substrate lipids and proteins they encounter during their 2D diffusion. Some peripheral proteins, for example pleckstrin homology (PH) domains, dock to the bilayer in a relatively shallow position with little penetration into the bilayer. Other peripheral proteins exhibit more complex bilayer contacts, for example classical protein kinase C isoforms (PKCs) bind as many as six lipids in stepwise fashion, resulting in the penetration of three PKC domains (C1A, C1B, C2) into the bilayer headgroup and hydrocarbon regions. A molecular understanding of the molecular features that control the diffusion speeds of proteins bound to supported bilayers would enable key molecular information to be extracted from experimental diffusion constants, revealing protein-lipid and protein-bilayer interactions difficult to study by other methods. The present study investigates a range of 11 different peripheral protein constructs comprised by 1-3 distinct domains (PH, C1A, C1B, C2, anti-lipid antibody). By combining these constructs with various combinations of target lipids, the study measures 2D diffusion constants on supported bilayers for 17 different protein-lipid complexes. The resulting experimental diffusion constants, together with the known membrane interaction parameters of each complex, are used to analyze the molecular features correlated with diffusional slowing and bilayer friction. The findings show that both (1) individual bound lipids and (2) individual protein domains that penetrate into the hydrocarbon core make additive contributions to the friction against the bilayer, thereby defining the 2D diffusion constant. An empirical formula is developed that accurately estimates the diffusion constant and bilayer friction of a peripheral protein in terms of its number of bound lipids and its geometry of penetration into the bilayer hydrocarbon core, yielding an excellent global best fit (R(2) of 0.97) to the experimental diffusion constants. Finally, the observed additivity of the frictional contributions suggests that further development of current theory describing bilayer dynamics may be needed. The present findings provide constraints that will be useful in such theory development. Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.

Direct in situ measurement of specific capacitance, monolayer tension, and bilayer tension in a droplet interface bilayer

DOE PAGES

Taylor, Graham J.; Venkatesan, Guru A.; Collier, C. Patrick; ...

2015-08-05

In this study, thickness and tension are important physical parameters of model cell membranes. However, traditional methods to measure these quantities require multiple experiments using separate equipment. This work introduces a new multi-step procedure for directly accessing in situ multiple physical properties of droplet interface bilayers (DIB), including specific capacitance (related to thickness), lipid monolayer tension in the Plateau-Gibbs border, and bilayer tension. The procedure employs a combination of mechanical manipulation of bilayer area followed by electrowetting of the capacitive interface to examine the sensitivities of bilayer capacitance to area and contact angle to voltage, respectively. These data allow formore » determining the specific capacitance of the membrane and surface tension of the lipid monolayer, which are then used to compute bilayer thickness and tension, respectively. The use of DIBs affords accurate optical imaging of the connected droplets in addition to electrical measurements of bilayer capacitance, and it allows for reversibly varying bilayer area. After validating the accuracy of the technique with diphytanoyl phosphatidylcholine (DPhPC) DIBs in hexadecane, the method is applied herein to quantify separately the effects on membrane thickness and tension caused by varying the solvent in which the DIB is formed and introducing cholesterol into the bilayer. Because the technique relies only on capacitance measurements and optical images to determine both thickness and tension, this approach is specifically well-suited for studying the effects of peptides, biomolecules, natural and synthetic nanoparticles, and other species that accumulate within membranes without altering bilayer conductance.« less

Energetic and dynamic analysis of transport of Na + and K + through a cyclic peptide nanotube in water and in lipid bilayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Yeonho; Lee, Ji Hye; Hwang, Hoon

Potential of mean force (PMF) profiles and position-dependent diffusion coefficients of Na + and K + are calculated to elucidate the translocation of ions through a cyclic peptide nanotube, composed of 8 × cyclo[-(D-Leu-Trp) 4-] rings, in water and in hydrated DMPC bilayers. The PMF profiles and PMF decomposition analysis for the monovalent cations show that favorable interactions of the cations with the CPN as well as the lipid bilayer and dehydration free energy penalties are two major competing factors which determine the free energy surface for ion transport through CPNs both in water and lipid bilayers, and that themore » selectivity of CPNs to cations mainly arises from favorable interaction energies of cations with CPNs and lipid bilayers that are more dominant than the dehydration penalties. Calculations of the position-dependent diffusion coefficients and dynamic friction kernels of the cations indicate that the dehydration process along with the molecular rearrangements occurring outside the channel and the coupling of the ion motions with the chain-structured water movements inside the channel lead to decrease of the diffusion coefficients far away from the channel entrance and also reduced coefficients inside the channel. Here the PMF and diffusivity profiles for Na + and K + reveal that the energetics of ion transport through the CPN are governed by global interactions of ions with all the components in the system while the diffusivity of ions through the channel is mostly determined by local interactions of ions with the confined water molecules inside the channel. Comparison of Na + and K + ion distributions based on overdamped Brownian dynamics simulations based on the PMF and diffusivity profiles with the corresponding results from molecular dynamics shows good agreement, indicating accuracy of the Bayesian inference method for determining diffusion coefficients in this application. In addition this work shows that position-dependent diffusion coefficients of ions are required to explain the dynamics and conductance of ions through the CPN properly.« less

Energetic and dynamic analysis of transport of Na + and K + through a cyclic peptide nanotube in water and in lipid bilayers

DOE PAGES

Song, Yeonho; Lee, Ji Hye; Hwang, Hoon; ...

2016-11-04

Potential of mean force (PMF) profiles and position-dependent diffusion coefficients of Na + and K + are calculated to elucidate the translocation of ions through a cyclic peptide nanotube, composed of 8 × cyclo[-(D-Leu-Trp) 4-] rings, in water and in hydrated DMPC bilayers. The PMF profiles and PMF decomposition analysis for the monovalent cations show that favorable interactions of the cations with the CPN as well as the lipid bilayer and dehydration free energy penalties are two major competing factors which determine the free energy surface for ion transport through CPNs both in water and lipid bilayers, and that themore » selectivity of CPNs to cations mainly arises from favorable interaction energies of cations with CPNs and lipid bilayers that are more dominant than the dehydration penalties. Calculations of the position-dependent diffusion coefficients and dynamic friction kernels of the cations indicate that the dehydration process along with the molecular rearrangements occurring outside the channel and the coupling of the ion motions with the chain-structured water movements inside the channel lead to decrease of the diffusion coefficients far away from the channel entrance and also reduced coefficients inside the channel. Here the PMF and diffusivity profiles for Na + and K + reveal that the energetics of ion transport through the CPN are governed by global interactions of ions with all the components in the system while the diffusivity of ions through the channel is mostly determined by local interactions of ions with the confined water molecules inside the channel. Comparison of Na + and K + ion distributions based on overdamped Brownian dynamics simulations based on the PMF and diffusivity profiles with the corresponding results from molecular dynamics shows good agreement, indicating accuracy of the Bayesian inference method for determining diffusion coefficients in this application. In addition this work shows that position-dependent diffusion coefficients of ions are required to explain the dynamics and conductance of ions through the CPN properly.« less

Laser Fabrication of Polymer Ferroelectric Nanostructures for Nonvolatile Organic Memory Devices.

PubMed

Martínez-Tong, Daniel E; Rodríguez-Rodríguez, Álvaro; Nogales, Aurora; García-Gutiérrez, Mari-Cruz; Pérez-Murano, Francesc; Llobet, Jordi; Ezquerra, Tiberio A; Rebollar, Esther

2015-09-09

Polymer ferroelectric laser-induced periodic surface structures (LIPSS) have been prepared on ferroelectric thin films of a poly(vinylidene fluoride-trifluoroethylene) copolymer. Although this copolymer does not absorb light at the laser wavelength, LIPSS on the copolymer can be obtained by forming a bilayer with other light-absorbing polymers. The ferroelectric nature of the structured bilayer was proven by piezoresponse force microscopy measurements. Ferroelectric hysteresis was found on both the bilayer and the laser-structured bilayer. We show that it is possible to write ferroelectric information at the nanoscale. The laser-structured ferroelectric bilayer showed an increase in the information storage density of an order of magnitude, in comparison to the original bilayer.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yeo, Jingjie; Cheng, Yuan; Li, Weifeng

A novel method of atomistic modelling and characterization of both pure ceramide and mixed lipid bilayers is being developed, using only the General Amber ForceField. Lipid bilayers modelled as pure ceramides adopt hexagonal packing after equilibration, and the area per lipid and bilayer thickness are consistent with previously reported theoretical results. Mixed lipid bilayers are modelled as a combination of ceramides, cholesterol, and free fatty acids. This model is shown to be stable after equilibration. Green tea extract, also known as epigallocatechin-3-gallate, is introduced as a spherical cluster on the surface of the mixed lipid bilayer. It is demonstrated thatmore » the cluster is able to bind to the bilayers as a cluster without diffusing into the surrounding water.« less

Numerical simulation of terahertz transmission of bilayer metallic meshes with different thickness of substrates

NASA Astrophysics Data System (ADS)

Zhang, Gaohui; Zhao, Guozhong; Zhang, Shengbo

2012-12-01

The terahertz transmission characteristics of bilayer metallic meshes are studied based on the finite difference time domain method. The bilayer well-shaped grid, the array of complementary square metallic pill and the cross wire-hole array were investigated. The results show that the bilayer well-shaped grid achieves a high-pass of filter function, while the bilayer array of complementary square metallic pill achieves a low-pass of filter function, the bilayer cross wire-hole array achieves a band-pass of filter function. Between two metallic microstructures, the medium need to be deposited. Obviously, medium thicknesses have an influence on the terahertz transmission characteristics of metallic microstructures. Simulation results show that with increasing the thicknesses of the medium the cut-off frequency of high-pass filter and low-pass filter move to low frequency. But the bilayer cross wire-hole array possesses two transmission peaks which display competition effect.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Boreyko, Jonathan B; Mruetusatorn, Prachya; Sarles, Stephen A

Droplet interface bilayers (DIBs) are a robust platform for studying synthetic cellular membranes; however, to date no DIBs have been produced at cellular length scales. Here, we create microscale droplet interface bilayers ( DIBs) at the interface between aqueous femtoliter-volume droplets within an oil-filled microfluidic channel. The uniquely large area-to-volume ratio of the droplets results in strong evaporation effects, causing the system to transition through three distinct regimes. First, the two adjacent droplets shrink into the shape of a single spherical droplet, where an augmented lipid bilayer partitions two hemi-spherical volumes. In the second regime, the combined effects of themore » shrinking monolayers and growing bilayer force the confined bilayer to buckle to conserve its mass. Finally, at a bending moment corresponding to a critical shear stress, the buckling bilayer fissions a vesicle to regulate its shape and stress. The DIBs produced here enable evaporation-induced bilayer dynamics reminiscent of endo- and exocytosis in cells.« less

Graphene Monoxide Bilayer As a High-Performance on/off Switching Media for Nanoelectronics.

PubMed

Woo, Jungwook; Yun, Kyung-Han; Chung, Yong-Chae

2016-04-27

The geometries and electronic characteristics of the graphene monoxide (GMO) bilayer are predicted via density functional theory (DFT) calculations. All the possible sequences of the GMO bilayer show the typical interlayer bonding characteristics of two-dimensional bilayer systems with a weak van der Waals interaction. The band gap energies of the GMO bilayers are predicted to be adequate for electronic device application, indicating slightly smaller energy gaps (0.418-0.448 eV) compared to the energy gap of the monolayer (0.536 eV). Above all, in light of the band gap engineering, the band gap of the GMO bilayer responds to the external electric field sensitively. As a result, a semiconductor-metal transition occurs at a small critical electric field (EC = 0.22-0.30 V/Å). It is therefore confirmed that the GMO bilayer is a strong candidate for nanoelectronics.

Potassium-doped n-type bilayer graphene

NASA Astrophysics Data System (ADS)

Yamada, Takatoshi; Okigawa, Yuki; Hasegawa, Masataka

2018-01-01

Potassium-doped n-type bilayer graphene was obtained. Chemical vapor deposited bilayer and single layer graphene on copper (Cu) foils were used. After etching of Cu foils, graphene was dipped in potassium hydroxide aqueous solutions to dope potassium. Graphene on silicon oxide was characterized by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and Raman spectroscopy. Both XPS and EDX spectra indicated potassium incorporation into the bilayer graphene via intercalation between the graphene sheets. The downward shift of the 2D peak position of bilayer graphene after the potassium hydroxide (KOH) treatment was confirmed in Raman spectra, indicating that the KOH-treated bilayer graphene was doped with electrons. Electrical properties were measured using Hall bar structures. The Dirac points of bilayer graphene were shifted from positive to negative by the KOH treatment, indicating that the KOH-treated bilayer graphene was n-type conduction. For single layer graphene after the KOH treatment, although electron doping was confirmed from Raman spectra, the peak of potassium in the X-ray photoelectron spectroscopy (XPS) spectrum was not detected. The Dirac points of single layer graphene with and without the KOH treatment showed positive.

Structure of Carbon Nanotube Porins in Lipid Bilayers: An in Situ Small-Angle X-ray Scattering (SAXS) Study [Atomic-level structure of carbon nanotube porins in lipid bilayers: an in-situ small-angle x-ray scattering (SAXS) study

DOE PAGES

Tran, Ich C.; Tunuguntla, Ramya H.; Kim, Kyunghoon; ...

2016-06-20

Carbon nanotube porins (CNTPs), small segments of carbon nanotubes capable of forming defined pores in lipid membranes, are important future components for bionanoelectronic devices as they could provide a robust analog of biological membrane channels. Furthermore, in order to control the incorporation of these CNT channels into lipid bilayers, it is important to understand the structure of the CNTPs before and after insertion into the lipid bilayer as well as the impact of such insertion on the bilayer structure. Here we employed a noninvasive in situ probe, small-angle X-ray scattering, to study the integration of CNT porins into dioleoylphosphatidylcholine bilayers.more » These results show that CNTPs in solution are stabilized by a monolayer of lipid molecules wrapped around their outer surface. We also demonstrate that insertion of CNTPs into the lipid bilayer results in decreased bilayer thickness with the magnitude of this effect increasing with the concentration of CNTPs.« less

Understanding the Interaction of Pluronics L61 and L64 with a DOPC Lipid Bilayer: An Atomistic Molecular Dynamics Study

DOE PAGES

Ileri Ercan, Nazar; Stroeve, Pieter; Tringe, Joseph W.; ...

2016-09-13

In this paper, we investigate the interactions of Pluronics L61 and L64 with a dioleylphosphatidylcholine (DOPC) lipid bilayer by atomistic molecular dynamics simulations using the all-atom OPLS force field. Our results show that the initial configuration of the polymer with respect to the bilayer determines its final conformation within the bilayer. When the polymer is initially placed at the lipid/water interface, we observe partial insertion of the polymer in a U-shaped conformation. On the other hand, when the polymer is centered at the bilayer, it stabilizes to a transmembrane state, which facilitates water transport across the bilayer. We show thatmore » membrane thickness decreases while its fluidity increases in the presence of Pluronics. When the polymer concentration inside the bilayer is high, pore formation is initiated with L64. Finally, our results show good agreement with existing experimental data and reveal that the hydrophilic/lipophilic balance of the polymer plays a critical role in the interaction mechanisms as well as in the dynamics of Pluronics with and within the bilayer.« less

Assessment of bilayer silicene to probe as quantum spin and valley Hall effect

NASA Astrophysics Data System (ADS)

Rehman, Majeed Ur; Qiao, Zhenhua

2018-02-01

Silicene takes precedence over graphene due to its buckling type structure and strong spin orbit coupling. Motivated by these properties, we study the silicene bilayer in the presence of applied perpendicular electric field and intrinsic spin orbit coupling to probe as quantum spin/valley Hall effect. Using analytical approach, we calculate the spin Chern-number of bilayer silicene and then compare it with monolayer silicene. We reveal that bilayer silicene hosts double spin Chern-number as compared to single layer silicene and therefore accordingly has twice as many edge states in contrast to single layer silicene. In addition, we investigate the combined effect of intrinsic spin orbit coupling and the external electric field, we find that bilayer silicene, likewise single layer silicene, goes through a phase transitions from a quantum spin Hall state to a quantum valley Hall state when the strength of the applied electric field exceeds the intrinsic spin orbit coupling strength. We believe that the results and outcomes obtained for bilayer silicene are experimentally more accessible as compared to bilayer graphene, because of strong SO coupling in bilayer silicene.

Phase transition behaviors of the supported DPPC bilayer investigated by sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM).

PubMed

Wu, Heng-Liang; Tong, Yujin; Peng, Qiling; Li, Na; Ye, Shen

2016-01-21

The phase transition behaviors of a supported bilayer of dipalmitoylphosphatidyl-choline (DPPC) have been systematically evaluated by in situ sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM). By using an asymmetric bilayer composed of per-deuterated and per-protonated monolayers, i.e., DPPC-d75/DPPC and a symmetric bilayer of DPPC/DPPC, we were able to probe the molecular structural changes during the phase transition process of the lipid bilayer by SFG spectroscopy. It was found that the DPPC bilayer is sequentially melted from the top (adjacent to the solution) to bottom leaflet (adjacent to the substrate) over a wide temperature range. The conformational ordering of the supported bilayer does not decrease (even slightly increases) during the phase transition process. The conformational defects in the bilayer can be removed after the complete melting process. The phase transition enthalpy for the bottom leaflet was found to be approximately three times greater than that for the top leaflet, indicating a strong interaction of the lipids with the substrate. The present SFG and AFM observations revealed similar temperature dependent profiles. Based on these results, the temperature-induced structural changes in the supported lipid bilayer during its phase transition process are discussed in comparison with previous studies.

Interaction of the Antimicrobial Peptide Aurein 1.2 and Charged Lipid Bilayer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rai, Durgesh K.; Qian, Shuo

Aurein 1.2 is a potent antimicrobial peptide secreted by frog Litoria aurea. As a short membrane-active peptide with only 13 amino acids in sequence, it has been found to be residing on the surface of lipid bilayer and permeabilizing bacterial membranes at high concentration. However, the detail at the molecular level is largely unknown. Here in this study, we investigated the action of Aurein 1.2 in charged lipid bilayers composed of DMPC/DMPG. Oriented Circular Dichroism results showed that the peptide was on the surface of lipid bilayer regardless of the charged lipid ratio. Only at a very high peptide-to-lipid ratiomore » (~1/10), the peptide became perpendicular to the bilayer, however no pore was detected by neutron in-plane scattering. To further understand how it interacted with charged lipid bilayers, we employed Small Angle Neutron Scattering to probe lipid distribution across bilayer leaflets in lipid vesicles. The results showed that Aurein 1.2 interacted strongly with negatively charged DMPG, causing strong asymmetry in lipid bilayer. At high concentration, while the vesicles were intact, we found additional structure feature on the bilayer. Finally, our study provides a glimpse into how Aurein 1.2 disturbs anionic lipid-containing membranes without pore formation.« less

Interaction of the Antimicrobial Peptide Aurein 1.2 and Charged Lipid Bilayer

DOE PAGES

Rai, Durgesh K.; Qian, Shuo

2017-06-16

Aurein 1.2 is a potent antimicrobial peptide secreted by frog Litoria aurea. As a short membrane-active peptide with only 13 amino acids in sequence, it has been found to be residing on the surface of lipid bilayer and permeabilizing bacterial membranes at high concentration. However, the detail at the molecular level is largely unknown. Here in this study, we investigated the action of Aurein 1.2 in charged lipid bilayers composed of DMPC/DMPG. Oriented Circular Dichroism results showed that the peptide was on the surface of lipid bilayer regardless of the charged lipid ratio. Only at a very high peptide-to-lipid ratiomore » (~1/10), the peptide became perpendicular to the bilayer, however no pore was detected by neutron in-plane scattering. To further understand how it interacted with charged lipid bilayers, we employed Small Angle Neutron Scattering to probe lipid distribution across bilayer leaflets in lipid vesicles. The results showed that Aurein 1.2 interacted strongly with negatively charged DMPG, causing strong asymmetry in lipid bilayer. At high concentration, while the vesicles were intact, we found additional structure feature on the bilayer. Finally, our study provides a glimpse into how Aurein 1.2 disturbs anionic lipid-containing membranes without pore formation.« less

Nanomechanical characterization of phospholipid bilayer islands on flat and porous substrates: a force spectroscopy study.

PubMed

Nussio, Matthew R; Oncins, Gerard; Ridelis, Ingrid; Szili, Endre; Shapter, Joseph G; Sanz, Fausto; Voelcker, Nicolas H

2009-07-30

In this study, we compare for the first time the nanomechanical properties of lipid bilayer islands on flat and porous surfaces. 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) bilayers were deposited on flat (silicon and mica) and porous silicon (pSi) substrate surfaces and examined using atomic force spectroscopy and force volume imaging. Force spectroscopy measurements revealed the effects of the underlying substrate and of the lipid phase on the nanomechanical properties of bilayers islands. For mica and silicon, significant differences in breakthrough force between the center and the edges of bilayer islands were observed for both phospolipids. These differences were more pronounced for DMPC than for DPPC, presumably due to melting effects at the edges of DMPC bilayers. In contrast, bilayer islands deposited on pSi yielded similar breakthrough forces in the central region and along the perimeter of the islands, and those values in turn were similar to those measured along the perimeter of bilayer islands deposited on the flat substrates. The study also demonstrates that pSi is suitable solid support for the formation of pore-spanning phospholipid bilayers with potential applications in transmembrane protein studies, drug delivery, and biosensing.

Alcohol's Effects on Lipid Bilayer Properties

PubMed Central

Ingólfsson, Helgi I.; Andersen, Olaf S.

2011-01-01

Alcohols are known modulators of lipid bilayer properties. Their biological effects have long been attributed to their bilayer-modifying effects, but alcohols can also alter protein function through direct protein interactions. This raises the question: Do alcohol's biological actions result predominantly from direct protein-alcohol interactions or from general changes in the membrane properties? The efficacy of alcohols of various chain lengths tends to exhibit a so-called cutoff effect (i.e., increasing potency with increased chain length, which that eventually levels off). The cutoff varies depending on the assay, and numerous mechanisms have been proposed such as: limited size of the alcohol-protein interaction site, limited alcohol solubility, and a chain-length-dependent lipid bilayer-alcohol interaction. To address these issues, we determined the bilayer-modifying potency of 27 aliphatic alcohols using a gramicidin-based fluorescence assay. All of the alcohols tested (with chain lengths of 1–16 carbons) alter the bilayer properties, as sensed by a bilayer-spanning channel. The bilayer-modifying potency of the short-chain alcohols scales linearly with their bilayer partitioning; the potency tapers off at higher chain lengths, and eventually changes sign for the longest-chain alcohols, demonstrating an alcohol cutoff effect in a system that has no alcohol-binding pocket. PMID:21843475

Scattering Studies of Hydrophobic Monomers in Liposomal Bilayers: An Expanding Shell Model of Monomer Distribution

DOE Office of Scientific and Technical Information (OSTI.GOV)

Richter, Andrew; Dergunov, Sergey; Ganus, Bill

2011-01-01

Hydrophobic monomers partially phase separate from saturated lipids when loaded into lipid bilayers in amounts exceeding a 1:1 monomer/lipid molar ratio. This conclusion is based on the agreement between two independent methods of examining the structure of monomer-loaded bilayers. Complete phase separation of monomers from lipids would result in an increase in bilayer thickness and a slight increase in the diameter of liposomes. A homogeneous distribution of monomers within the bilayer would not change the bilayer thickness and would lead to an increase in the liposome diameter. The increase in bilayer thickness, measured by the combination of small-angle neutron scatteringmore » (SANS) and small-angle X-ray scattering (SAXS), was approximately half of what was predicted for complete phase separation. The increase in liposome diameter, measured by dynamic light scattering (DLS), was intermediate between values predicted for a homogeneous distribution and complete phase separation. Combined SANS, SAXS, and DLS data suggest that at a 1.2 monomer/lipid ratio approximately half of the monomers are located in an interstitial layer sandwiched between lipid sheets. These results expand our understanding of using self-assembled bilayers as scaffolds for the directed covalent assembly of organic nanomaterials. In particular, the partial phase separation of monomers from lipids corroborates the successful creation of nanothin polymer materials with uniform imprinted nanopores. Pore-forming templates do not need to span the lipid bilayer to create a pore in the bilayer-templated films.« less

Scattering Studies of Hydrophobic Monomers in Liposomal Bilayers: An Expanding Shell Model of Monomer Distribution

DOE Office of Scientific and Technical Information (OSTI.GOV)

Richter, Andrew G.; Dergunov, Sergey A.; Ganus, Bill

2011-03-10

Hydrophobic monomers partially phase separate from saturated lipids when loaded into lipid bilayers in amounts exceeding a 1:1 monomer/lipid molar ratio. This conclusion is based on the agreement between two independent methods of examining the structure of monomer-loaded bilayers. Complete phase separation of monomers from lipids would result in an increase in bilayer thickness and a slight increase in the diameter of liposomes. A homogeneous distribution of monomers within the bilayer would not change the bilayer thickness and would lead to an increase in the liposome diameter. The increase in bilayer thickness, measured by the combination of small-angle neutron scatteringmore » (SANS) and small-angle X-ray scattering (SAXS), was approximately half of what was predicted for complete phase separation. The increase in liposome diameter, measured by dynamic light scattering (DLS), was intermediate between values predicted for a homogeneous distribution and complete phase separation. Combined SANS, SAXS, and DLS data suggest that at a 1.2 monomer/lipid ratio approximately half of the monomers are located in an interstitial layer sandwiched between lipid sheets. These results expand our understanding of using self-assembled bilayers as scaffolds for the directed covalent assembly of organic nanomaterials. In particular, the partial phase separation of monomers from lipids corroborates the successful creation of nanothin polymer materials with uniform imprinted nanopores. Finally, pore-forming templates do not need to span the lipid bilayer to create a pore in the bilayer-templated films.« less

Permeation of halide anions through phospholipid bilayers occurs by the solubility-diffusion mechanism

NASA Technical Reports Server (NTRS)

Paula, S.; Volkov, A. G.; Deamer, D. W.

1998-01-01

Two alternative mechanisms are frequently used to describe ionic permeation of lipid bilayers. In the first, ions partition into the hydrophobic phase and then diffuse across (the solubility-diffusion mechanism). The second mechanism assumes that ions traverse the bilayer through transient hydrophilic defects caused by thermal fluctuations (the pore mechanism). The theoretical predictions made by both models were tested for halide anions by measuring the permeability coefficients for chloride, bromide, and iodide as a function of bilayer thickness, ionic radius, and sign of charge. To vary the bilayer thickness systematically, liposomes were prepared from monounsaturated phosphatidylcholines (PC) with chain lengths between 16 and 24 carbon atoms. The fluorescent dye MQAE (N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide) served as an indicator for halide concentration inside the liposomes and was used to follow the kinetics of halide flux across the bilayer membranes. The observed permeability coefficients ranged from 10(-9) to 10(-7) cm/s and increased as the bilayer thickness was reduced. Bromide was found to permeate approximately six times faster than chloride through bilayers of identical thickness, and iodide permeated three to four times faster than bromide. The dependence of the halide permeability coefficients on bilayer thickness and on ionic size were consistent with permeation of hydrated ions by a solubility-diffusion mechanism rather than through transient pores. Halide permeation therefore differs from that of a monovalent cation such as potassium, which has been accounted for by a combination of the two mechanisms depending on bilayer thickness.

Molecular dynamics simulation of sodium dodecylsulfate (SDS) bilayers.

PubMed

Zhang, Hongshu; Yuan, Shiling; Sun, Jichao; Liu, Jianqiang; Li, Haiping; Du, Na; Hou, Wanguo

2017-11-15

Sodium dodecylsulfate (SDS) - a simple single tailed surfactant (STS) can form stable vesicles from its micellar solution without any additives under the mediation of solid surfaces. To further understand the mechanism of this transition on the molecular level, molecular dynamics simulations are performed to study segments of SDS bilayers (as part of vesicles) in the bulk solution systematically, at the moment that the lower leaflet of bilayers already detached from solid surfaces. The SDS membrane would rather keep their bilayers structure than return to micelles when the initial interdigitated degree (δ i ) between alkyl chains is more than 8.0±1.4%. And the interdigitated degree is always approaching to 31.7±2.0% while the equilibrium is reached. The aggregates behave as curved bilayers, planar bilayers, perforated bilayers, and micelles with the increase of the lower leaflet cross-sectional area. Besides, the structures of salt bridge and water bridge structures are formed between DS - and Na + ions or water molecules, which contribute to the stability of SDS bilayers. The distribution difference of the salt bridges along the direction of S-O axis between the two leaflets leads to the asymmetry of the bilayers, which plays supplementary role to the formation of bilayers curvature. We expect that this work help to shed light on the understanding of interface phenomena and the mechanism of simple single-tailed surfactant vesicle self-assembly on the molecular level. Copyright © 2017 Elsevier Inc. All rights reserved.

Electron Transporting Semiconductor Dielectric Intramolecular

DTIC Science & Technology

2012-04-27

gate dielectric, and the capacitance times mobility was 80 nS/V (10x typical pentacene /oxide), stable to heating to 70 °C in air. Remarkably...oxide/ Pentacene Bilayer Transistors: High Mobility n-Channel, Ambipolar and Nonvolatile Devices” Adv. Funct. Mater. 18, 1832-1839 (2008) Sun, J...case of layered OSC OFETs. This proposal is somewhat different from a model by deLeeuw for amorphous OFETs13 in which carriers would be locally

Vapor deposition of water on graphitic surfaces: formation of amorphous ice, bilayer ice, ice I, and liquid water.

PubMed

Lupi, Laura; Kastelowitz, Noah; Molinero, Valeria

2014-11-14

Carbonaceous surfaces are a major source of atmospheric particles and could play an important role in the formation of ice. Here we investigate through molecular simulations the stability, metastability, and molecular pathways of deposition of amorphous ice, bilayer ice, and ice I from water vapor on graphitic and atomless Lennard-Jones surfaces as a function of temperature. We find that bilayer ice is the most stable ice polymorph for small cluster sizes, nevertheless it can grow metastable well above its region of thermodynamic stability. In agreement with experiments, the simulations predict that on increasing temperature the outcome of water deposition is amorphous ice, bilayer ice, ice I, and liquid water. The deposition nucleation of bilayer ice and ice I is preceded by the formation of small liquid clusters, which have two wetting states: bilayer pancake-like (wetting) at small cluster size and droplet-like (non-wetting) at larger cluster size. The wetting state of liquid clusters determines which ice polymorph is nucleated: bilayer ice nucleates from wetting bilayer liquid clusters and ice I from non-wetting liquid clusters. The maximum temperature for nucleation of bilayer ice on flat surfaces, T(B)(max) is given by the maximum temperature for which liquid water clusters reach the equilibrium melting line of bilayer ice as wetting bilayer clusters. Increasing water-surface attraction stabilizes the pancake-like wetting state of liquid clusters leading to larger T(B)(max) for the flat non-hydrogen bonding surfaces of this study. The findings of this study should be of relevance for the understanding of ice formation by deposition mode on carbonaceous atmospheric particles, including soot.

In vitro effects of the anti-Alzheimer drug memantine on the human erythrocyte membrane and molecular models.

PubMed

Zambrano, Pablo; Suwalsky, Mario; Villena, Fernando; Jemiola-Rzeminska, Malgorzata; Strzalka, Kazimierz

2017-01-29

Memantine is a NMDA antagonist receptor clinically used for treating Alzheimer's disease. NMDA receptors are present in the human neurons and erythrocyte membranes. The aim of the present study was to investigate the effects of memantine on human erythrocytes. With this purpose, the drug was developed to in vitro interact with human red cells and bilayers built-up of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE). The latter represent lipids respectively present in both outer and inner monolayers of the red cell membrane. Results obtained by scanning electron microscopy (SEM) showed that memantine changed the normal biconcave shape of red cells to cup-shaped stomatocytes. According to the bilayer-couple hypothesis the drug intercalated into the inner monolayer of the erythrocyte membrane. Experimental results obtained by X-ray diffraction on multibilayers of DMPC and DMPE, and by differential scanning calorimetry on multilamellar vesicles indicated that memantine preferentially interacted with DMPC in a concentration-dependent manner. Thus, it can be concluded that in the low therapeutic plasma concentration of circa 1 μM memantine is located in NMDA receptor channel without affecting the erythrocyte shape. However, at higher concentrations, once the receptors became saturated excess of memantine molecules (20 μM) would interact with phosphoinositide lipids present in the inner monolayer of the erythrocyte membrane inducing the formation of stomatocytes. However, 40-50 μM memantine was required to interact with isolated phosphatidylcholine bilayers. Copyright © 2016 Elsevier Inc. All rights reserved.

Porous nanoparticle-supported lipid bilayers (protocells) for targeted delivery and methods of using same

DOEpatents

Brinker, C. Jeffrey; Carnes, Eric C.; Ashley, Carlee Erin; Willman, Cheryl L.

2017-02-28

The present invention is directed to protocells for specific targeting of hepatocellular and other cancer cells which comprise a nanoporous silica core with a supported lipid bilayer; at least one agent which facilitates cancer cell death (such as a traditional small molecule, a macromolecular cargo (e.g. siRNA or a protein toxin such as ricin toxin A-chain or diphtheria toxin A-chain) and/or a histone-packaged plasmid DNA disposed within the nanoporous silica core (preferably supercoiled in order to more efficiently package the DNA into protocells) which is optionally modified with a nuclear localization sequence to assist in localizing protocells within the nucleus of the cancer cell and the ability to express peptides involved in therapy (apoptosis/cell death) of the cancer cell or as a reporter, a targeting peptide which targets cancer cells in tissue to be treated such that binding of the protocell to the targeted cells is specific and enhanced and a fusogenic peptide that promotes endosomal escape of protocells and encapsulated DNA. Protocells according to the present invention may be used to treat cancer, especially including hepatocellular (liver) cancer using novel binding peptides (c-MET peptides) which selectively bind to hepatocellular tissue or to function in diagnosis of cancer, including cancer treatment and drug discovery.

Communication: Effect of accidental mode degeneracy on Raman intensity in 2D materials: Hybrid functional study of bilayer phosphorene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sun, Yi-Yang; Zhang, Shengbai

2016-07-14

Bulk black phosphorus has two optical phonon modes labeled as A{sub g}{sup 2} and B{sub 2u}, respectively, that are nearly degenerate in frequency. However, density functional theory calculations using local or semi-local functionals cannot reproduce this degeneracy. Here, we propose a hybrid functional approach aided by van der Waals (vdW) force fields, which can accurately describe the lattice dynamic and electronic properties of both bulk and few-layer black phosphorus (phosphorene). Using this approach we show that in bilayer phosphorene, the two Raman modes derived from the B{sub 2u} and A{sub g}{sup 2} modes could exhibit strong resonance as a resultmore » of the accidental degeneracy so that both modes could be observed in Raman experiment. Without the mode degeneracy, however, the Raman intensity of the B{sub 2u}-derived mode would be too weak to be observed. We further show that the accidental degeneracy is correlated to the applied strain, which enables Raman spectroscopy to be a powerful tool for characterizing built-in strains in 2D materials, e.g., due to the interaction with substrates, which has emerged as an important issue in vdW epitaxy.« less

Structural and electron diffraction scaling of twisted graphene bilayers

NASA Astrophysics Data System (ADS)

Zhang, Kuan; Tadmor, Ellad B.

2018-03-01

Multiscale simulations are used to study the structural relaxation in twisted graphene bilayers and the associated electron diffraction patterns. The initial twist forms an incommensurate moiré pattern that relaxes to a commensurate microstructure comprised of a repeating pattern of alternating low-energy AB and BA domains surrounding a high-energy AA domain. The simulations show that the relaxation mechanism involves a localized rotation and shrinking of the AA domains that scales in two regimes with the imposed twist. For small twisting angles, the localized rotation tends to a constant; for large twist, the rotation scales linearly with it. This behavior is tied to the inverse scaling of the moiré pattern size with twist angle and is explained theoretically using a linear elasticity model. The results are validated experimentally through a simulated electron diffraction analysis of the relaxed structures. A complex electron diffraction pattern involving the appearance of weak satellite peaks is predicted for the small twist regime. This new diffraction pattern is explained using an analytical model in which the relaxation kinematics are described as an exponentially-decaying (Gaussian) rotation field centered on the AA domains. Both the angle-dependent scaling and diffraction patterns are in quantitative agreement with experimental observations. A Matlab program for extracting the Gaussian model parameters accompanies this paper.

Transport methods for probing the barrier domain of lipid bilayer membranes.

PubMed Central

Xiang, T X; Chen, X; Anderson, B D

1992-01-01

Two experimental techniques have been utilized to explore the barrier properties of lecithin/decane bilayer membranes with the aim of determining the contributions of various domains within the bilayer to the overall barrier. The thickness of lecithin/decane bilayers was systematically varied by modulating the chemical potential of decane in the annulus surrounding the bilayer using different mole fractions of squalene in decane. The dependence of permeability of a model permeant (acetamide) on the thickness of the solvent-filled region of the bilayer was assessed in these bilayers to determine the contribution of this region to the overall barrier. The flux of acetamide was found to vary linearly with bilayer area with Pm = (2.9 +/- 0.3) x 10(-4) cm s-1, after correcting for diffusion through unstirred water layers. The ratio between the overall membrane permeability coefficient and that calculated for diffusion through the hydrocarbon core in membranes having maximum thickness was 0.24, suggesting that the solvent domain contributes only slightly to the overall barrier properties. Consistent with these results, the permeability of acetamide was found to be independent of bilayer thickness. The relative contributions of the bilayer interface and ordered hydrocarbon regions to the transport barrier may be evaluated qualitatively by exploring the effective chemical nature of the barrier microenvironment. This may be probed by comparing functional group contributions to transport with those obtained for partitioning between water and various model bulk solvents ranging in polarity or hydrogen-bonding potential. A novel approach is described for obtaining group contributions to transport using ionizable permeants and pH adjustment. Using this approach, bilayer permeability coefficients of p-toluic acid and p-hydroxymethyl benzoic acid were determined to be 1.1 +/- 0.2 cm s-1 and (1.6 +/- 0.4) x 10(-3) cm s-1, respectively. From these values, the -OH group contribution to bilayer transport [delta(delta G0-OH)] was found to be 3.9 kcal/mol. This result suggests that the barrier region of the bilayer does not resemble the hydrogen-bonding environment found in octanol, but is somewhat less selective (more polar) than a hydrocarbon solvent. PMID:1420875

Solvent-assisted lipid bilayer formation on silicon dioxide and gold.

PubMed

Tabaei, Seyed R; Choi, Jae-Hyeok; Haw Zan, Goh; Zhdanov, Vladimir P; Cho, Nam-Joon

2014-09-02

Planar lipid bilayers on solid supports mimic the fundamental structure of biological membranes and can be investigated using a wide range of surface-sensitive techniques. Despite these advantages, planar bilayer fabrication is challenging, and there are no simple universal methods to form such bilayers on diverse material substrates. One of the novel methods recently proposed and proven to form a planar bilayer on silicon dioxide involves lipid deposition in organic solvent and solvent exchange to influence the phase of adsorbed lipids. To scrutinize the specifics of this solvent-assisted lipid bilayer (SALB) formation method and clarify the limits of its applicability, we have developed a simplified, continuous solvent-exchange version to form planar bilayers on silicon dioxide, gold, and alkanethiol-coated gold (in the latter case, a lipid monolayer is formed to yield a hybrid bilayer) and varied the type of organic solvent and rate of solvent exchange. By tracking the SALB formation process with simultaneous quartz crystal microbalance-dissipation (QCM-D) and ellipsometry, it was determined that the acoustic, optical, and hydration masses along with the acoustic and optical thicknesses, measured at the end of the process, are comparable to those observed by employing conventional fabrication methods (e.g., vesicle fusion). As shown by QCM-D measurements, the obtained planar bilayers are highly resistant to protein adsorption, and several, but not all, water-miscible organic solvents could be successfully used in the SALB procedure, with isopropanol yielding particularly high-quality bilayers. In addition, fluorescence recovery after photobleaching (FRAP) measurements demonstrated that the coefficient of lateral lipid diffusion in the fabricated bilayers corresponds to that measured earlier in the planar bilayers formed by vesicle fusion. With increasing rate of solvent exchange, it was also observed that the bilayer became incomplete and a phenomenological model was developed in order to explain this feature. The results obtained allowed us to clarify and discriminate likely steps of the SALB formation process as well as determine the corresponding influence of organic solvent type and flow conditions on these steps. Taken together, the findings demonstrate that the SALB formation method can be adapted to a continuous solvent-exchange procedure that is technically minimal, quick, and efficient to form planar bilayers on solid supports.

Quasi-two-dimensional spin correlations in the triangular lattice bilayer spin glass LuCoGaO 4

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fritsch, Katharina; Ross, Kathyrn A.; Granroth, Garrett E.

Here we present a single-crystal time-of-flight neutron scattering study of the static and dynamic spin correlations in LuCoGaO 4, a quasi-two-dimensional dilute triangular lattice antiferromagnetic spin-glass material. This system is based on Co 2+ ions that are randomly distributed on triangular bilayers within the YbFe 2O 4 type, hexagonal crystal structure. Antiferromagnetic short-range two-dimensional correlations at wave vectors Q = (1/3,1/3, L) develop within the bilayers at temperatures as high as |Θ CW| ~100 K and extend over roughly five unit cells at temperatures below T g = 19 K. These two-dimensional static correlations are observed as diffuse rods ofmore » neutron scattering intensity along c * and display a continuous spin freezing process in their energy dependence. Aside from exhibiting these typical spin-glass characteristics, this insulating material reveals a novel gapped magnetic resonant spin excitation at ΔE ~12 meV localized around Q = (1 / 3, 1 / 3,L) . The temperature dependence of the spin gap associated with this two-dimensional excitation correlates with the evolution of the static correlations into the spin-glass state ground state. Lastly, we associate it with the effect of the staggered exchange field acting on the S eff = 1/2 Ising-like doublet of the Co 2+ moments.« less

Quasi-two-dimensional spin correlations in the triangular lattice bilayer spin glass LuCoGaO 4

DOE PAGES

Fritsch, Katharina; Ross, Kathyrn A.; Granroth, Garrett E.; ...

2017-09-13

Here we present a single-crystal time-of-flight neutron scattering study of the static and dynamic spin correlations in LuCoGaO 4, a quasi-two-dimensional dilute triangular lattice antiferromagnetic spin-glass material. This system is based on Co 2+ ions that are randomly distributed on triangular bilayers within the YbFe 2O 4 type, hexagonal crystal structure. Antiferromagnetic short-range two-dimensional correlations at wave vectors Q = (1/3,1/3, L) develop within the bilayers at temperatures as high as |Θ CW| ~100 K and extend over roughly five unit cells at temperatures below T g = 19 K. These two-dimensional static correlations are observed as diffuse rods ofmore » neutron scattering intensity along c * and display a continuous spin freezing process in their energy dependence. Aside from exhibiting these typical spin-glass characteristics, this insulating material reveals a novel gapped magnetic resonant spin excitation at ΔE ~12 meV localized around Q = (1 / 3, 1 / 3,L) . The temperature dependence of the spin gap associated with this two-dimensional excitation correlates with the evolution of the static correlations into the spin-glass state ground state. Lastly, we associate it with the effect of the staggered exchange field acting on the S eff = 1/2 Ising-like doublet of the Co 2+ moments.« less

Four new species of coccidia (Apicomplexa: Eimeriidae) from Owen Stanley Skinks, Papuascincus stanleyanus (Sauria: Scincidae), from Papua New Guinea

USGS Publications Warehouse

McAllister, Chris T.; Duszynski, Donald W.; Fisher, Robert N.; Austin, Christopher C.

2014-01-01

Between September and November 1991, 12 Owen Stanley skinks, Papuascincus stanleyanus (Booulenger) were collected from various localities on Papua New Guinea and examined for coccidians. Six (50%) were found to harbour four eimerians that we describe here as new. Oocysts of Eimeria burseyi sp. n. were elongate to ellipsoidal with a bilayered wall and measured (length x width, L x W) 36.0 x 24.0 microm, with a L/W ratio of 1.5. Both micropyle and oocyst residuum were absent, but a polar granule was present. Oocysts of Eimeria goldbergi sp. n. were ellipsoidal, with a bilayered wall, and measured 21.4 x 16.1 microm; L/W ratio was 1.3. Both micropyle and oocyst residuum were absent, but a single or fragmented polar granule was present. Oocysts of Eimeria boulengeri sp. n. were spheroidal to slightly subspheroidal, with a thin, single-layered wall that readily collapses, and measured 16.0 microm, L/W ratio was 1.0. Both micropyle and oocyst residuum were absent, but usually one (sometimes two) polar granule(s) were present. Oocysts of Eimeria niuginiensis sp. n. were oblong to tapered with a bilayered wall, and measured 20.0 x 13.1 microm; L/W ratio was 1.5. A micropyle, oocyst residuum and polar granule were absent. To our knowledge, these represent the only coccidians ever described from P. stanleyanus.

Contact and Bandgap Engineering in Two Dimensional Crystal

NASA Astrophysics Data System (ADS)

Chu, Tao

At the heart of semiconductor research, bandgap is one of the key parameters for materials and determine their applications in modern technologies. For traditional bulk semiconductors, the bandgap is determined by the chemical composition and specific arrangement of the crystal lattices, and usually invariant during the device operation. Nevertheless, it is highly desirable for many optoelectronic and electronic applications to have materials with continuously tunable bandgap available. In the past decade, 2D layered materials including graphene and transition metal dichalcogenides (TMDs) have sparked interest in the scientific community, owing to their unique material properties and tremendous potential in various applications. Among many newly discovered properties that are non-existent in bulk materials, the strong in-plane bonding and weak van der Waals inter-planar interaction in these 2D layered structures leads to a widely tunable bandgap by electric field. This provides an extra knob to engineer the fundamental material properties and open a new design space for novel device operation. This thesis focuses on this field controlled dynamic bandgap and can be divided into three parts: (1) bilayer graphene is the first known 2D crystal with a bandgap can be continuously tuned by electric field. However, the electrical transport bandgaps is much smaller than both theoretical predictions and extracted bandgaps from optical measurements. In the first part of the thesis, the limiting factors of preventing achieving a large transport bandgap in bilayer graphene are investigated and different strategies to achieve a large transport bandgap are discussed, including the vertically scaling of gate oxide and patterning channel into ribbon structure. With a record large transport bandgap of ~200meV, a dual-gated semiconducting bilayer graphene P/N junction with extremely scaled gap of 20nm in-between is fabricated. A tunable local maxima feature, associated with 1D vHs DOS at the band edge of bilayer graphene, was experimentally observed in transport for the first time. (2) The bandgap of bilayer MoS2 is also predicted to be continuously tuned to zero by applying a perpendicular electric field. Here, the first experimental realization of tuning the bandgap of bilayer MoS2 by a vertical electric field is presented. An analytical approach utilizing the threshold voltages from ambipolar characteristics is employed to quantitatively extract bandgaps, which is further benchmarked by temperature dependent bandgap measurements and photoluminescence measurements. (3) Few layer graphene is employed as an example to demonstrate a novel self-aligned edge contacting scheme for layered material systems.

Bilayered, non-cross-linked collagen matrix for regeneration of facial defects after skin cancer removal: a new perspective for biomaterial-based tissue reconstruction.

PubMed

Ghanaati, Shahram; Kovács, Adorján; Barbeck, Mike; Lorenz, Jonas; Teiler, Anna; Sadeghi, Nader; Kirkpatrick, Charles James; Sader, Robert

2016-03-01

Classically skin defects are covered by split thickness skin grafts or by means of local or regional skin flaps. In the presented case series for the first time a bilayered, non-crossed-linked collagen matrix has been used in an off-label fashion in order to reconstruct facial skin defects following different types of skin cancer resection. The material is of porcine origin and consists of a spongy and a compact layer. The ratio of the two layers is 1:3 in favour of the spongy layer. The aim of the study was to investigate the potential of this matrix for skin regeneration as an alternative to the standard techniques of skin grafts or flaps. Six patients between 39 and 83 years old were included in the study based on a therapeutic trial. The collagen matrix was used in seven defects involving the nose, eyelid, forehead- and posterior scalp regions, and ranging from 1,2 to 6 cm in diameter. Two different head and neck surgeons at two different institutions performed the operations. Each used a different technique in covering the wound following surgery, i.e. with and without a latex-based sheet under the pressure dressing. In three cases cylindrical biopsies were taken after 14 days. In all cases the biomaterial application was performed without any complication and no adverse effects were observed. Clinically, the collagen matrix contributed to a tension-free skin regeneration, independent of the wound dressing used. The newly regenerated skin showed strong similarity to the adjacent normal tissue both in quality and colour. Histological analysis indicated that the spongy layer replaced the defective connective tissue, by providing stepwise integration into the surrounding implantation bed, while the compact layer was infiltrated by mononuclear cells and contributed to its epithelialization by means of a "conductive"process from the surrounding epithelial cells. The clinical and histological data demonstrate that the collagen bilayered matrix used in this series contributes to a "Guided-Integrative-Regeneration-Process", which still needs to be further understood. The biomimetic nature of this material seems to contribute to physiological matrix remodelling, which probably involves other matricellular proteins essential for soft tissue regeneration. A deeper understanding of the mechanism, involved in the tissue integration of this material and its contribution to soft tissue regeneration based on the direct and indirect effect of matricellular proteins could open new therapeutic avenues for biomaterial-based soft tissue regeneration as an alternative to traditional flap-based plastic surgery.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lupi, Laura; Kastelowitz, Noah; Molinero, Valeria, E-mail: Valeria.Molinero@utah.edu

Carbonaceous surfaces are a major source of atmospheric particles and could play an important role in the formation of ice. Here we investigate through molecular simulations the stability, metastability, and molecular pathways of deposition of amorphous ice, bilayer ice, and ice I from water vapor on graphitic and atomless Lennard-Jones surfaces as a function of temperature. We find that bilayer ice is the most stable ice polymorph for small cluster sizes, nevertheless it can grow metastable well above its region of thermodynamic stability. In agreement with experiments, the simulations predict that on increasing temperature the outcome of water deposition ismore » amorphous ice, bilayer ice, ice I, and liquid water. The deposition nucleation of bilayer ice and ice I is preceded by the formation of small liquid clusters, which have two wetting states: bilayer pancake-like (wetting) at small cluster size and droplet-like (non-wetting) at larger cluster size. The wetting state of liquid clusters determines which ice polymorph is nucleated: bilayer ice nucleates from wetting bilayer liquid clusters and ice I from non-wetting liquid clusters. The maximum temperature for nucleation of bilayer ice on flat surfaces, T{sub B}{sup max} is given by the maximum temperature for which liquid water clusters reach the equilibrium melting line of bilayer ice as wetting bilayer clusters. Increasing water-surface attraction stabilizes the pancake-like wetting state of liquid clusters leading to larger T{sub B}{sup max} for the flat non-hydrogen bonding surfaces of this study. The findings of this study should be of relevance for the understanding of ice formation by deposition mode on carbonaceous atmospheric particles, including soot.« less

Ultra-high vacuum surface analysis study of rhodopsin incorporation into supported lipid bilayers.

PubMed

Michel, Roger; Subramaniam, Varuni; McArthur, Sally L; Bondurant, Bruce; D'Ambruoso, Gemma D; Hall, Henry K; Brown, Michael F; Ross, Eric E; Saavedra, S Scott; Castner, David G

2008-05-06

Planar supported lipid bilayers that are stable under ambient atmospheric and ultra-high-vacuum conditions were prepared by cross-linking polymerization of bis-sorbylphosphatidylcholine (bis-SorbPC). X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were employed to investigate bilayers that were cross-linked using either redox-initiated radical polymerization or ultraviolet photopolymerization. The redox method yields a more structurally intact bilayer; however, the UV method is more compatible with incorporation of transmembrane proteins. UV polymerization was therefore used to prepare cross-linked bilayers with incorporated bovine rhodopsin, a light-activated, G-protein-coupled receptor (GPCR). A previous study (Subramaniam, V.; Alves, I. D.; Salgado, G. F. J.; Lau, P. W.; Wysocki, R. J.; Salamon, Z.; Tollin, G.; Hruby, V. J.; Brown, M. F.; Saavedra, S. S. J. Am. Chem. Soc. 2005, 127, 5320-5321) showed that rhodopsin retains photoactivity after incorporation into UV-polymerized bis-SorbPC, but did not address how the protein is associated with the bilayer. In this study, we show that rhodopsin is retained in supported bilayers of poly(bis-SorbPC) under ultra-high-vacuum conditions, on the basis of the increase in the XPS nitrogen concentration and the presence of characteristic amino acid peaks in the ToF-SIMS data. Angle-resolved XPS data show that the protein is inserted into the bilayer, rather than adsorbed on the bilayer surface. This is the first study to demonstrate the use of ultra-high-vacuum techniques for structural studies of supported proteolipid bilayers.

Partitioning of lysolipids, fatty acids and their mixtures in aqueous lipid bilayers: solute concentration/composition effects.

PubMed

Singh, Jasmeet; Lai, Amy Jo; Alaee, Yasmin; Ranganathan, Radha

2014-01-01

Distributions of lysopalmitoylphosphatidylcholine (LPPC), palmitic acid (PA) and their 1:1 mixtures between water and dipalmitoylphosphatidylcholine (DPPC) bilayer were determined using a fluorescence probe that selectively detects only the solutes in water. Water solute concentrations were obtained at each of several lipid concentrations. Dynamic Light Scattering experiments confirmed that the lipid/solute aggregates were vesicles in the concentration range investigated. Lipid concentration dependence of the solute component in water was fit to a thermodynamic model of solute distribution between two coexisting solvents. Water/bilayer partition coefficient and the free energy of transfer, for each of these solutes were determined from the fit. Main findings are: (1) Water/bilayer partition coefficient of solute is greater for 2 to 10% solute mole fraction than for 0 to 2%, signaling solute induced bilayer perturbation that increases bilayer solubility, beginning at 2% solute mole fraction. (2) Partition coefficients are in the order LPPC

Partitioning of Lysolipids, Fatty Acids and Their Mixtures in Aqueous Lipid Bilayers: Solute Concentration / Composition Effects

PubMed Central

Singh, Jasmeet; Lai, Amy Jo; Alaee, Yasmin; Ranganathan, Radha

2013-01-01

Distribution of lysopalmitoylphosphatidylcholine (LPPC), Palmitic acid (PA) and their 1:1 mixtures between water and dipalmitoylphosphatidylcholine (DPPC) bilayer were determined using a fluorescence probe that selectively detects only the solutes in water. Water solute concentrations were obtained at each of several lipid concentrations. Dynamic Light Scattering experiments confirmed that the lipid/solute aggregates were vesicles in the concentration range investigated. Lipid concentration dependence of the solute component in water was fit to a thermodynamic model of solute distribution between two coexisting solvents. Water/bilayer partition coefficient and the free energy of transfer, for each of these solutes were determined from the fit. Main findings are: (1) Water/bilayer partition coefficient of solute is greater for 2 to 10 % solute mole fraction than for 0 to 2 %, signaling solute induced bilayer perturbation that increases bilayer solubility, beginning at 2 % solute mole fraction. (2) Partition coefficients are in the order LPPC

Membrane Interaction of Antimicrobial Peptides Using E. coli Lipid Extract as Model Bacterial Cell Membranes and SFG Spectroscopy

PubMed Central

Soblosky, Lauren; Ramamoorthy, Ayyalusamy; Chen, Zhan

2015-01-01

Supported lipid bilayers are used as a convenient model cell membrane system to study biologically important molecule-lipid interactions in situ. However, the lipid bilayer models are often simple and the acquired results with these models may not provide all pertinent information related to a real cell membrane. In this work, we use sum frequency generation (SFG) vibrational spectroscopy to study molecular-level interactions between the antimicrobial peptides (AMPs) MSI-594, ovispirin-1 G18, magainin 2 and a simple 1,2-dipalmitoyl-d62-sn-glycero-3-phosphoglycerol (dDPPG)-1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) bilayer. We compared such interactions to those between the AMPs and a more complex dDPPG/E. coli polar lipid extract bilayer. We show that to fully understand more complex aspects of peptide-bilayer interaction, such as interaction kinetics, a heterogeneous lipid composition is required, such as the E. coli polar lipid extract. The discrepancy in peptide-bilayer interaction is likely due in part to the difference in bilayer charge between the two systems since highly negative charged lipids can promote more favorable electrostatic interactions between the peptide and lipid bilayer. Results presented in this paper indicate that more complex model bilayers are needed to accurately analyze peptide-cell membrane interactions and demonstrates the importance of using an appropriate lipid composition to study AMP interaction properties. PMID:25707312

Effect of Membrane Tension on the Electric Field and Dipole Potential of Lipid Bilayer Membrane

PubMed Central

Warshaviak, Dora Toledo; Muellner, Michael J.; Chachisvilis, Mirianas

2011-01-01

The dipole potential of lipid bilayer membrane controls the difference in permeability of the membrane to oppositely charged ions. We have combined molecular dynamics (MD) simulations and experimental studies to determine changes in electric field and electrostatic potential of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid bilayer in response to applied membrane tension. MD simulations based on CHARMM36 force field showed that electrostatic potential of DOPC bilayer decreases by ~45 mV in the physiologically relevant range of membrane tension values (0 to 15 dyn/cm). The electrostatic field exhibits a peak (~0.8×109 V/m) near the water/lipid interface which shifts by 0.9 Å towards the bilayer center at 15 dyn/cm. Maximum membrane tension of 15 dyn/cm caused 6.4% increase in area per lipid, 4.7% decrease in bilayer thickness and 1.4% increase in the volume of the bilayer. Dipole-potential sensitive fluorescent probes were used to detect membrane tension induced changes in DOPC vesicles exposed to osmotic stress. Experiments confirmed that dipole potential of DOPC bilayer decreases at higher membrane tensions. These results are suggestive of a potentially new mechanosensing mechanism by which mechanically induced structural changes in the lipid bilayer membrane could modulate the function of membrane proteins by altering electrostatic interactions and energetics of protein conformational states. PMID:21722624

High Resistivity Lipid Bilayers Assembled on Polyelectrolyte Multilayer Cushions: An Impedance Study.

PubMed

Diamanti, Eleftheria; Gregurec, Danijela; Rodríguez-Presa, María José; Gervasi, Claudio A; Azzaroni, Omar; Moya, Sergio E

2016-06-28

Supported membranes on top of polymer cushions are interesting models of biomembranes as cell membranes are supported on a polymer network of proteins and sugars. In this work lipid vesicles formed by a mixture of 30% 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 70% 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS) are assembled on top of a polyelectrolyte multilayer (PEM) cushion of poly(allylamine hydrochloride) (PAH) and poly(styrene sodium sulfonate) (PSS). The assembly results in the formation of a bilayer on top of the PEM as proven by means of the quartz crystal microbalance with dissipation technique (QCM-D) and by cryo-transmission electron microscopy (cryo-TEM). The electrical properties of the bilayer are studied by electrochemical impedance spectroscopy (EIS). The bilayer supported on the PEMs shows a high resistance, on the order of 10(7) Ω cm(2), which is indicative of a continuous, dense bilayer. Such resistance is comparable with the resistance of black lipid membranes. This is the first time that such values are obtained for lipid bilayers supported on PEMs. The assembly of polyelectrolytes on top of a lipid bilayer decreases the resistance of the bilayer up to 2 orders of magnitude. The assembly of the polyelectrolytes on the lipids induces defects or pores in the bilayer which in turn prompts a decrease in the measured resistance.

The use of virtual ground to control transmembrane voltages and measure bilayer currents in serial arrays of droplet interface bilayers

NASA Astrophysics Data System (ADS)

Sarles, Stephen A.

2013-09-01

The droplet interface bilayer (DIB) is a simple technique for constructing a stable lipid bilayer at the interface of two lipid-encased water droplets submerged in oil. Networks of DIBs formed by connecting more than two droplets constitute a new form of modular biomolecular smart material, where the transduction properties of a single lipid bilayer can affect the actions performed at other interface bilayers in the network via diffusion through the aqueous environments of shared droplet connections. The passive electrical properties of a lipid bilayer and the arrangement of droplets that determine the paths for transport in the network require specific electrical control to stimulate and interrogate each bilayer. Here, we explore the use of virtual ground for electrodes inserted into specific droplets in the network and employ a multichannel patch clamp amplifier to characterize bilayer formation and ion-channel activity in a serial DIB array. Analysis of serial connections of DIBs is discussed to understand how assigning electrode connections to the measurement device can be used to measure activity across all lipid membranes within a network. Serial arrays of DIBs are assembled using the regulated attachment method within a multi-compartment flexible substrate, and wire-type electrodes inserted into each droplet compartment of the substrate enable the application of voltage and measurement of current in each droplet in the array.

Photogeneration of Charge Carriers in Bilayer Assemblies of Conjugated Rigid-Rod Polymers

DTIC Science & Technology

1994-07-08

photoinduced electron transfer and exciplex formation at the bilayer interface. Thus photocarrier generation on photoexcitation of the conjugated rigid...rod polymers in the bilayer occurs by photoinduced electron transfer, forming intermolecular exciplexes which dissociate efficiently in electric field...photogeneration, conjugated rigid-rod polymers, is. MACI COD bilayer assemblies, electron transfer, exciplexes . 11. SEOJUTY CLASUICA 10. 51(11MIE CLASSIMIAVION

Review of bilayer tablet technology.

PubMed

Abebe, Admassu; Akseli, Ilgaz; Sprockel, Omar; Kottala, Niranjan; Cuitiño, Alberto M

2014-01-30

Therapeutic strategies based on oral delivery of bilayer (and multilayer) tablets are gaining more acceptance among brand and generic products due to a confluence of factors including advanced delivery strategies, patient compliance and combination therapy. Successful manufacturing of these ever more complex systems needs to overcome a series of challenges from formulation design to tablet press monitoring and control. This article provides an overview of the state-of-the-art of bilayer tablet technology, highlighting the main benefits of this type of oral dosage forms while providing a description of current challenges and advances toward improving manufacturing practices and product quality. Several aspects relevant to bilayer tablet manufacturing are addressed including material properties, lubrication, layer ordering, layer thickness, layer weight control, as well as first and final compression forces. A section is also devoted to bilayer tablet characterization that present additional complexities associated with interfaces between layers. The available features of the manufacturing equipment for bilayer tablet production are also described indicating the different strategies for sensing and controls offered by bilayer tablet press manufacturers. Finally, a roadmap for bilayer tablet manufacturing is advanced as a guideline to formulation design and selection of process parameters and equipment. Copyright © 2013 Elsevier B.V. All rights reserved.

Contact bubble bilayers with flush drainage.

PubMed

Iwamoto, Masayuki; Oiki, Shigetoshi

2015-03-16

Planar lipid bilayers have been used to form stable bilayers into which membrane proteins are reconstituted for measurements of their function under an applied membrane potential. Recently, a lipid bilayer membrane is formed by the apposition of two monolayers that line an oil-electrolyte interface. Here, a bilayer membrane system is developed with picoliter bubbles under mechanically and chemically manipulable conditions. A water bubble lined with a phospholipid monolayer is blown from a glass pipette into an oil phase. Two blowing pipettes are manipulated, and bubbles (each with a diameter of ~ 50 μm) are held side by side to form a bilayer, which is termed a contact bubble bilayer. With the electrode implemented in the blowing pipette, currents through the bilayer are readily measured. The intra-bubble pressure is varied with the pressure-controller, leading to various sizes of the bubble and the membrane area. A rapid solution exchange system is developed by introducing additional pressure-driven injection pipettes, and the blowing pipette works as a drain. The solution is exchanged within 20 ms. Also, an asymmetric membrane with different lipid composition of each leaflet is readily formed. Example applications of this versatile method are presented to characterize the function of ion channels.

Contact Bubble Bilayers with Flush Drainage

PubMed Central

Iwamoto, Masayuki; Oiki, Shigetoshi

2015-01-01

Planar lipid bilayers have been used to form stable bilayers into which membrane proteins are reconstituted for measurements of their function under an applied membrane potential. Recently, a lipid bilayer membrane is formed by the apposition of two monolayers that line an oil-electrolyte interface. Here, a bilayer membrane system is developed with picoliter bubbles under mechanically and chemically manipulable conditions. A water bubble lined with a phospholipid monolayer is blown from a glass pipette into an oil phase. Two blowing pipettes are manipulated, and bubbles (each with a diameter of ~ 50 μm) are held side by side to form a bilayer, which is termed a contact bubble bilayer. With the electrode implemented in the blowing pipette, currents through the bilayer are readily measured. The intra-bubble pressure is varied with the pressure-controller, leading to various sizes of the bubble and the membrane area. A rapid solution exchange system is developed by introducing additional pressure-driven injection pipettes, and the blowing pipette works as a drain. The solution is exchanged within 20 ms. Also, an asymmetric membrane with different lipid composition of each leaflet is readily formed. Example applications of this versatile method are presented to characterize the function of ion channels. PMID:25772819

X-ray diffraction evidence for myelin disorder in brain from humans with Alzheimer's disease.

PubMed

Chia, L S; Thompson, J E; Moscarello, M A

1984-09-05

Wide-angle X-ray diffraction studies revealed that the lipid phase transition temperature of myelin from brain tissue of humans with Alzheimer's disease was about 12 degrees C lower than that of normal age-matched controls, indicating differences in the physical organization of the myelin lipid bilayer. Elevated levels of malondialdehyde and conjugated diene were found in brain tissue from humans with Alzheimer's disease, indicating an increased amount of lipid peroxidation over the controls. An increase in myelin disorder and in lipid peroxidation can both be correlated with aging in human brain, but the changes in myelin from humans with Alzheimer's disease are more pronounced than in normal aging. These changes might represent severe or accelerated aging.

Spin-orbit torques in magnetic bilayers

NASA Astrophysics Data System (ADS)

Haney, Paul

2015-03-01

Spintronics aims to utilize the coupling between charge transport and magnetic dynamics to develop improved and novel memory and logic devices. Future progress in spintronics may be enabled by exploiting the spin-orbit coupling present at the interface between thin film ferromagnets and heavy metals. In these systems, applying an in-plane electrical current can induce magnetic dynamics in single domain ferromagnets, or can induce rapid motion of domain wall magnetic textures. There are multiple effects responsible for these dynamics. They include spin-orbit torques and a chiral exchange interaction (the Dzyaloshinskii-Moriya interaction) in the ferromagnet. Both effects arise from the combination of ferromagnetism and spin-orbit coupling present at the interface. There is additionally a torque from the spin current flux impinging on the ferromagnet, arising from the spin hall effect in the heavy metal. Using a combination of approaches, from drift-diffusion to Boltzmann transport to first principles methods, we explore the relative contributions to the dynamics from these different effects. We additionally propose that the transverse spin current is locally enhanced over its bulk value in the vicinity of an interface which is oriented normal to the charge current direction.

Assessment of pseudo-bilayer structures in the heterogate germanium electron-hole bilayer tunnel field-effect transistor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Padilla, J. L., E-mail: jose.padilladelatorre@epfl.ch; Alper, C.; Ionescu, A. M.

2015-06-29

We investigate the effect of pseudo-bilayer configurations at low operating voltages (≤0.5 V) in the heterogate germanium electron-hole bilayer tunnel field-effect transistor (HG-EHBTFET) compared to the traditional bilayer structures of EHBTFETs arising from semiclassical simulations where the inversion layers for electrons and holes featured very symmetric profiles with similar concentration levels at the ON-state. Pseudo-bilayer layouts are attained by inducing a certain asymmetry between the top and the bottom gates so that even though the hole inversion layer is formed at the bottom of the channel, the top gate voltage remains below the required value to trigger the formation of themore » inversion layer for electrons. Resulting benefits from this setup are improved electrostatic control on the channel, enhanced gate-to-gate efficiency, and higher I{sub ON} levels. Furthermore, pseudo-bilayer configurations alleviate the difficulties derived from confining very high opposite carrier concentrations in very thin structures.« less

Toward tunable band gap and tunable dirac point in bilayer graphene with molecular doping.

PubMed

Yu, Woo Jong; Liao, Lei; Chae, Sang Hoon; Lee, Young Hee; Duan, Xiangfeng

2011-11-09

The bilayer graphene has attracted considerable attention for potential applications in future electronics and optoelectronics because of the feasibility to tune its band gap with a vertical displacement field to break the inversion symmetry. Surface chemical doping in bilayer graphene can induce an additional offset voltage to fundamentally affect the vertical displacement field and the band gap opening in bilayer graphene. In this study, we investigate the effect of chemical molecular doping on band gap opening in bilayer graphene devices with single or dual gate modulation. Chemical doping with benzyl viologen molecules modulates the displacement field to allow the opening of a transport band gap and the increase of the on/off ratio in the bilayer graphene transistors. Additionally, Fermi energy level in the opened gap can be rationally controlled by the amount of molecular doping to obtain bilayer graphene transistors with tunable Dirac points, which can be readily configured into functional devices, such as complementary inverters.

Quantitative Raman microspectroscopy for water permeability parameters at a droplet interface bilayer.

PubMed

Braziel, S; Sullivan, K; Lee, S

2018-01-29

Using confocal Raman microspectroscopy, we derive parameters for bilayer water transport across an isolated nanoliter aqueous droplet pair. For a bilayer formed with two osmotically imbalanced and adherent nanoliter aqueous droplets in a surrounding oil solvent, a droplet interface bilayer (DIB), the water permeability coefficient across the lipid bilayer was determined from monitoring the Raman scattering from the C[triple bond, length as m-dash]N stretching mode of K 3 Fe(CN) 6 as a measure of water uptake into the swelling droplet of a DIB pair. We also derive passive diffusional permeability coefficient for D 2 O transport across a droplet bilayer using O-D Raman signal. This method provides a significant methodological advance in determining water permeability coefficients in a convenient and reliable way.

Step-wise potential development across the lipid bilayer under external electric fields

NASA Astrophysics Data System (ADS)

Majhi, Amit Kumar

2018-04-01

Pore formation across the bilayers under external electric field is an important phenomenon, which has numerous applications in biology and bio-engineering fields. However, it is not a ubiquitous event under all field applications. To initiate a pore in the bilayer a particular threshold electric field is required. The electric field alters the intrinsic potential distribution across the bilayer as we as it enhances total potential drop across the bilayer, which causes the pore formation. The intrinsic potential profile has a maximum peak value, which is 0.8 V and it gets enhanced under application of external field, 0.43 V/nm. The peak value becomes 1.4 V when a pore appears in the bilayer and it continues to evolve as along as the external electric field remains switched on.

Quantifying the bending of bilayer temperature-sensitive hydrogels

NASA Astrophysics Data System (ADS)

Dong, Chenling; Chen, Bin

2017-04-01

Stimuli-responsive hydrogels can serve as manipulators, including grippers, sensors, etc., where structures can undergo significant bending. Here, a finite-deformation theory is developed to quantify the evolution of the curvature of bilayer temperature-sensitive hydrogels when subjected to a temperature change. Analysis of the theory indicates that there is an optimal thickness ratio to acquire the largest curvature in the bilayer and also suggests that the sign or the magnitude of the curvature can be significantly affected by pre-stretches or small pores in the bilayer. This study may provide important guidelines in fabricating temperature-responsive bilayers with desirable mechanical performance.

Imaging mycobacterial growth and division with a fluorogenic probe.

PubMed

Hodges, Heather L; Brown, Robert A; Crooks, John A; Weibel, Douglas B; Kiessling, Laura L

2018-05-15

Control and manipulation of bacterial populations requires an understanding of the factors that govern growth, division, and antibiotic action. Fluorescent and chemically reactive small molecule probes of cell envelope components can visualize these processes and advance our knowledge of cell envelope biosynthesis (e.g., peptidoglycan production). Still, fundamental gaps remain in our understanding of the spatial and temporal dynamics of cell envelope assembly. Previously described reporters require steps that limit their use to static imaging. Probes that can be used for real-time imaging would advance our understanding of cell envelope construction. To this end, we synthesized a fluorogenic probe that enables continuous live cell imaging in mycobacteria and related genera. This probe reports on the mycolyltransferases that assemble the mycolic acid membrane. This peptidoglycan-anchored bilayer-like assembly functions to protect these cells from antibiotics and host defenses. Our probe, quencher-trehalose-fluorophore (QTF), is an analog of the natural mycolyltransferase substrate. Mycolyltransferases process QTF by diverting their normal transesterification activity to hydrolysis, a process that unleashes fluorescence. QTF enables high contrast continuous imaging and the visualization of mycolyltransferase activity in cells. QTF revealed that mycolyltransferase activity is augmented before cell division and localized to the septa and cell poles, especially at the old pole. This observed localization suggests that mycolyltransferases are components of extracellular cell envelope assemblies, in analogy to the intracellular divisomes and polar elongation complexes. We anticipate QTF can be exploited to detect and monitor mycobacteria in physiologically relevant environments.

Anisotropic two-gap superconductivity and the absence of a Pauli paramagnetic limit in single-crystalline LaO0.5F0.5BiS2

NASA Astrophysics Data System (ADS)

Chan, Y. C.; Yip, K. Y.; Cheung, Y. W.; Chan, Y. T.; Niu, Q.; Kajitani, J.; Higashinaka, R.; Matsuda, T. D.; Yanase, Y.; Aoki, Y.; Lai, K. T.; Goh, Swee K.

2018-03-01

Ambient-pressure-grown LaO0.5F0.5BiS2 with a superconducting transition temperature Tc˜3 K possesses a highly anisotropic normal state. By a series of electrical resistivity measurements with a magnetic-field direction varying between the crystalline c axis and the a b plane, we present datasets displaying the temperature dependence of the out-of-plane upper critical field Hc2 ⊥(T ) , the in-plane upper critical field Hc2 ∥(T ) , as well as the angular dependence of Hc 2 at fixed temperatures for ambient-pressure-grown LaO0.5F0.5BiS2 single crystals. The anisotropy of the superconductivity, Hc2 ∥/Hc2 ⊥ , reaches ˜16 on approaching 0 K, but it decreases significantly near Tc. A pronounced upward curvature of Hc2 ∥(T ) is observed near Tc, which we analyze using a two-gap model. Moreover, Hc2 ∥(0 ) is found to exceed the Pauli paramagnetic limit, which can be understood by considering the strong spin-orbit coupling associated with Bi as well as the breaking of the local inversion symmetry at the electronically active BiS2 bilayers. Hence, LaO0.5F0.5BiS2 with a centrosymmetric lattice structure is a unique platform to explore the physics associated with local parity violation in the bulk crystal.

Room-temperature giant magneto-mechanical-electric cross-coupling in Si-integrated PbZr0.52Ti0.48O3/Ni50Mn35In15 multiferroic heterostructures

NASA Astrophysics Data System (ADS)

Singh, Kirandeep; Kaur, Davinder

2017-04-01

The current study reports the strong magnetoelectric coupling (M-E) in silicon (Si)-integrated ferromagnetic shape memory alloy-based PZT/Ni-Mn-In thin-film multiferroic heterostructure. The strain-mediated nature of converse M-E coupling is reflected from the butterfly-shaped normalized magnetization (M/M s) versus electric field plots. The direct M-E properties of the heterostructure were measured with a frequency of AC magnetic field, bias magnetic field, as well as with temperature. A maximum direct M-E coupling in the bilayered thin-film multiferroic heterostructures occurred at resonance frequencies around the first-order structural transitional temperature of the bottom Ni-Mn-In layer. It was observed that the measuring temperature remarkably affects the direct M-E characteristic of the heterostructure. A large direct ME effect and converse ME effect coefficient α DME  ~  894 mV cm-1.Oe and α CME ~ 2.7  ×  10-5 s m-1, respectively, were achieved in the bilayer at room temperature. The mechanism of direct as well as converse M-E effects in the thin-film multiferroic heterostructures is discussed. The electrically driven angular dependence of normalized magnetization (M/M s) reveals the twofold symmetric magnetic anisotropy of the heterostructure, with the drastic shifting of the magnetic hard axis at E  >  E c (coercivity of PZT).

Fatty acids are required for epidermal permeability barrier function.

PubMed

Mao-Qiang, M; Elias, P M; Feingold, K R

1993-08-01

The permeability barrier is mediated by a mixture of ceramides, sterols, and free fatty acids arranged as extracellular lamellar bilayers in the stratum corneum. Whereas prior studies have shown that cholesterol and ceramides are required for normal barrier function, definitive evidence for the importance of nonessential fatty acids is not available. To determine whether epidermal fatty acid synthesis also is required for barrier homeostasis, we applied 5-(tetradecyloxy)-2-furancarboxylic acid (TOFA), an inhibitor of acetyl CoA carboxylase, after disruption of the barrier by acetone or tape stripping. TOFA inhibits epidermal fatty acid by approximately 50% and significantly delays barrier recovery. Moreover, coadministration of palmitate with TOFA normalizes barrier recovery, indicating that the delay is due to a deficiency in bulk fatty acids. Furthermore, TOFA treatment also delays the return of lipids to the stratum corneum and results in abnormalities in the structure of lamellar bodies, the organelle which delivers lipid to the stratum corneum. In addition, the organization of secreted lamellar body material into lamellar bilayers within the stratum corneum interstices is disrupted by TOFA treatment. Finally, these abnormalities in lamellar body and stratum corneum membrane structure are corrected by coapplication of palmitate with TOFA. These results demonstrate a requirement for bulk fatty acids in barrier homeostasis. Thus, inhibiting the epidermal synthesis of any of the three key lipids that form the extracellular, lipid-enriched membranes of the stratum corneum results in an impairment in barrier homeostasis.

Fatty acids are required for epidermal permeability barrier function.

PubMed Central

Mao-Qiang, M; Elias, P M; Feingold, K R

1993-01-01

The permeability barrier is mediated by a mixture of ceramides, sterols, and free fatty acids arranged as extracellular lamellar bilayers in the stratum corneum. Whereas prior studies have shown that cholesterol and ceramides are required for normal barrier function, definitive evidence for the importance of nonessential fatty acids is not available. To determine whether epidermal fatty acid synthesis also is required for barrier homeostasis, we applied 5-(tetradecyloxy)-2-furancarboxylic acid (TOFA), an inhibitor of acetyl CoA carboxylase, after disruption of the barrier by acetone or tape stripping. TOFA inhibits epidermal fatty acid by approximately 50% and significantly delays barrier recovery. Moreover, coadministration of palmitate with TOFA normalizes barrier recovery, indicating that the delay is due to a deficiency in bulk fatty acids. Furthermore, TOFA treatment also delays the return of lipids to the stratum corneum and results in abnormalities in the structure of lamellar bodies, the organelle which delivers lipid to the stratum corneum. In addition, the organization of secreted lamellar body material into lamellar bilayers within the stratum corneum interstices is disrupted by TOFA treatment. Finally, these abnormalities in lamellar body and stratum corneum membrane structure are corrected by coapplication of palmitate with TOFA. These results demonstrate a requirement for bulk fatty acids in barrier homeostasis. Thus, inhibiting the epidermal synthesis of any of the three key lipids that form the extracellular, lipid-enriched membranes of the stratum corneum results in an impairment in barrier homeostasis. Images PMID:8102380

Effects of Dimethyl Sulfoxide on Surface Water near Phospholipid Bilayers.

PubMed

Lee, Yuno; Pincus, Philip A; Hyeon, Changbong

2016-12-06

Despite much effort to probe the properties of dimethyl sulfoxide (DMSO) solution, the effects of DMSO on water, especially near plasma membrane surfaces, still remain elusive. By performing molecular dynamics simulations at varying DMSO concentrations (X DMSO ), we study how DMSO affects structural and dynamical properties of water in the vicinity of phospholipid bilayers. As proposed by a number of experiments, our simulations confirm that DMSO induces dehydration from bilayer surfaces and disrupts the H-bond structure of water. However, DMSO-enhanced water diffusivity at solvent-bilayer interfaces, an intriguing discovery reported by a spin-label measurement, is not confirmed in our simulations. To resolve this discrepancy, we examine the location of the spin label (Tempo) relative to the solvent-bilayer interface. In accord with the evidence in the literature, our simulations, which explicitly model Tempo-phosphatidylcholine, find that the Tempo moiety is equilibrated at ∼8-10 Å below the bilayer surface. Furthermore, the DMSO-enhanced surface-water diffusion is confirmed only when water diffusion is analyzed around the Tempo moiety that is immersed below the bilayer surface, which implies that the experimentally detected signal of water using Tempo stems from the interior of bilayers, not from the interface. Our analysis finds that the increase of water diffusion below the bilayer surface is coupled to the increase of area per lipid with an increasing X DMSO (≲10mol%). Underscoring the hydrophobic nature of the Tempo moiety, our study calls for careful re-evaluation of the use of Tempo in measurements on lipid bilayer surfaces. Copyright © 2016 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Binding, folding and insertion of a β-hairpin peptide at a lipid bilayer surface: Influence of electrostatics and lipid tail packing.

PubMed

Reid, Keon A; Davis, Caitlin M; Dyer, R Brian; Kindt, James T

2018-03-01

Antimicrobial peptides (AMPs) act as host defenses against microbial pathogens. Here we investigate the interactions of SVS-1 (KVKVKVKV d P l PTKVKVKVK), an engineered AMP and anti-cancer β-hairpin peptide, with lipid bilayers using spectroscopic studies and atomistic molecular dynamics simulations. In agreement with literature reports, simulation and experiment show preferential binding of SVS-1 peptides to anionic over neutral bilayers. Fluorescence and circular dichroism studies of a Trp-substituted SVS-1 analogue indicate, however, that it will bind to a zwitterionic DPPC bilayer under high-curvature conditions and folds into a hairpin. In bilayers formed from a 1:1 mixture of DPPC and anionic DPPG lipids, curvature and lipid fluidity are also observed to promote deeper insertion of the fluorescent peptide. Simulations using the CHARMM C36m force field offer complementary insight into timescales and mechanisms of folding and insertion. SVS-1 simulated at an anionic mixed POPC/POPG bilayer folded into a hairpin over a microsecond, the final stage in folding coinciding with the establishment of contact between the peptide's valine sidechains and the lipid tails through a "flip and dip" mechanism. Partial, transient folding and superficial bilayer contact are seen in simulation of the peptide at a zwitterionic POPC bilayer. Only when external surface tension is applied does the peptide establish lasting contact with the POPC bilayer. Our findings reveal the influence of disruption to lipid headgroup packing (via curvature or surface tension) on the pathway of binding and insertion, highlighting the collaborative effort of electrostatic and hydrophobic interactions on interaction of SVS-1 with lipid bilayers. Copyright © 2017 Elsevier B.V. All rights reserved.

Is the cholesterol bilayer domain a barrier to oxygen transport into the eye lens?

PubMed

Plesnar, Elzbieta; Szczelina, Robert; Subczynski, Witold K; Pasenkiewicz-Gierula, Marta

2018-02-01

In the eye lens, the oxygen partial pressure is very low and the cholesterol (Chol) content in cell membranes is very high. Disturbance of these quantities results in cataract development. In human lens membranes, both bulk phospholipid-Chol domains and the pure Chol bilayer domains (CBDs) were experimentally detected. It is hypothesized that the CBD constitutes a significant barrier to oxygen transport into the lens. Transmembrane profiles of the oxygen diffusion-concentration product, obtained with electron paramagnetic resonance spin-labeling methods, allow evaluation of the oxygen permeability (P M ) of phospholipid membranes but not the CBD. Molecular dynamics simulation can independently provide components of the product across any bilayer domain, thus allowing evaluation of the P M across the CBD. Therefore, to test the hypothesis, MD simulation was used. Three bilayers containing palmitoyl-oleoyl-phosphorylcholine (POPC) and Chol were built. The pure Chol bilayer modeled the CBD, the 1:1 POPC-Chol bilayer modeled the bulk membrane in which the CBD is embedded, and the POPC bilayer was a reference. To each model, 200 oxygen molecules were added. After equilibration, the oxygen concentration and diffusion profiles were calculated for each model and multiplied by each other. From the respective product profiles, the P M of each bilayer was calculated. Favorable comparison with experimental data available only for the POPC and POPC-Chol bilayers validated these bilayer models and allowed the conclusion that oxygen permeation across the CBD is ~10 smaller than across the bulk membrane, supporting the hypothesis that the CBD is a barrier to oxygen transport into the eye lens. Copyright © 2017 Elsevier B.V. All rights reserved.

Wood-Graphene Oxide Composite for Highly Efficient Solar Steam Generation and Desalination.

PubMed

Liu, Keng-Ku; Jiang, Qisheng; Tadepalli, Sirimuvva; Raliya, Ramesh; Biswas, Pratim; Naik, Rajesh R; Singamaneni, Srikanth

2017-03-01

Solar steam generation is a highly promising technology for harvesting solar energy, desalination and water purification. We introduce a novel bilayered structure composed of wood and graphene oxide (GO) for highly efficient solar steam generation. The GO layer deposited on the microporous wood provides broad optical absorption and high photothermal conversion resulting in rapid increase in the temperature at the liquid surface. On the other hand, wood serves as a thermal insulator to confine the photothermal heat to the evaporative surface and to facilitate the efficient transport of water from the bulk to the photothermally active space. Owing to the tailored bilayer structure and the optimal thermo-optical properties of the individual components, the wood-GO composite structure exhibited a solar thermal efficiency of ∼83% under simulated solar excitation at a power density of 12 kW/m 2 . The novel composite structure demonstrated here is highly scalable and cost-efficient, making it an attractive material for various applications involving large light absorption, photothermal conversion and heat localization.

Atomistic and coarse-grained computer simulations of raft-like lipid mixtures.

PubMed

Pandit, Sagar A; Scott, H Larry

2007-01-01

Computer modeling can provide insights into the existence, structure, size, and thermodynamic stability of localized raft-like regions in membranes. However, the challenges in the construction and simulation of accurate models of heterogeneous membranes are great. The primary obstacle in modeling the lateral organization within a membrane is the relatively slow lateral diffusion rate for lipid molecules. Microsecond or longer time-scales are needed to fully model the formation and stability of a raft in a membra ne. Atomistic simulations currently are not able to reach this scale, but they do provide quantitative information on the intermolecular forces and correlations that are involved in lateral organization. In this chapter, the steps needed to carry out and analyze atomistic simulations of hydrated lipid bilayers having heterogeneous composition are outlined. It is then shown how the data from a molecular dynamics simulation can be used to construct a coarse-grained model for the heterogeneous bilayer that can predict the lateral organization and stability of rafts at up to millisecond time-scales.

Replacement of Cetyltrimethylammoniumbromide Bilayer on Gold Nanorod by Alkanethiol Crosslinker for Enhanced Plasmon Resonance Sensitivity

PubMed Central

Casas, Justin; Venkataramasubramani, Meenakshi; Wang, Yanyan; Tang, Liang

2013-01-01

Surface modification of gold nanorods (GNRs) is often problematic due to tightly packed cetyltrimethylammoniumbromide (CTAB) bilayer. Herein, we performed a double phase transfer ligand exchange to achieve displacement of CTAB on nanorods. During the removal, 11-mercaptoundecanoic acid (MUDA) crosslinker is simultaneously assembled on nanorod surfaces to prevent aggregation. The resulting MUDA-GNRs retain the shape and position of plasmon peaks similar to CTAB-capped GNRs. The introduction of carboxyl groups allows covalent conjugation of biological receptors in a facile fashion to construct a robust, label-free biosensor based on localized surface plasmon resonance (LSPR) transduction of biomolecular interaction. More importantly, smaller MUDA layer on the GNRs reduces the distance of target binding to the plasmonic nanostructure interface, leading to a significant enhancement in LSPR assay sensitivity and specificity. Compared to modification using conventional electropolymer adsorption, MUDA-coated gold nanosensor exhibits five times lower detection limit for cardiac troponin I assay with a high selectivity. PMID:23816849

DOE Office of Scientific and Technical Information (OSTI.GOV)

Armstrong, Clare L; Barrett, M; Heiss, Arno

Inelastic neutron scattering was used to study the effect of 5 and 40 mol% cholesterol on the lateral nanoscale dynamics of phospholipid membranes. By measuring the excitation spectrum at several lateral q || values (up to q || = 3 1), complete dispersion curves were determined of gel, fluid and liquid-ordered phase bilayers. The inclusion of cholesterol had a distinct effect on the collective dynamics of the bilayer s hydrocarbon chains; specifically, we observed a pronounced stiffening of the membranes on the nanometer length scale in both gel and fluid bilayers, even though they were experiencing a higher degree ofmore » molecular disorder. Also, for the first time we determined the nanoscale dynamics in the high-cholesterol liquid-ordered phase of bilayers containing cholesterol. Namely, this phase appears to be softer than fluid bilayers, but better ordered than bilayers in the gel phase.« less

Membrane interaction of antimicrobial peptides using E. coli lipid extract as model bacterial cell membranes and SFG spectroscopy.

PubMed

Soblosky, Lauren; Ramamoorthy, Ayyalusamy; Chen, Zhan

2015-04-01

Supported lipid bilayers are used as a convenient model cell membrane system to study biologically important molecule-lipid interactions in situ. However, the lipid bilayer models are often simple and the acquired results with these models may not provide all pertinent information related to a real cell membrane. In this work, we use sum frequency generation (SFG) vibrational spectroscopy to study molecular-level interactions between the antimicrobial peptides (AMPs) MSI-594, ovispirin-1 G18, magainin 2 and a simple 1,2-dipalmitoyl-d62-sn-glycero-3-phosphoglycerol (dDPPG)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) bilayer. We compared such interactions to those between the AMPs and a more complex dDPPG/Escherichia coli (E. coli) polar lipid extract bilayer. We show that to fully understand more complex aspects of peptide-bilayer interaction, such as interaction kinetics, a heterogeneous lipid composition is required, such as the E. coli polar lipid extract. The discrepancy in peptide-bilayer interaction is likely due in part to the difference in bilayer charge between the two systems since highly negative charged lipids can promote more favorable electrostatic interactions between the peptide and lipid bilayer. Results presented in this paper indicate that more complex model bilayers are needed to accurately analyze peptide-cell membrane interactions and demonstrates the importance of using an appropriate lipid composition to study AMP interaction properties. Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.

The Interaction of Melittin with Dimyristoyl Phosphatidylcholine-Dimyristoyl Phosphatidylserine Lipid Bilayer Membranes

DOE PAGES

Rai, Durgesh K.; Qian, Shuo; Heller, William T.

2016-08-13

We report that membrane-active peptides (MAPs), which interact directly with the lipid bilayer of a cell and include toxins and host defense peptides, display lipid composition-dependent activity. Phosphatidylserine (PS) lipids are anionic lipids that are found throughout the cellular membranes of most eukaryotic organisms where they serve as both a functional component and as a precursor to phosphatidylethanolamine lipids. The inner leaflet of the plasma membrane contains more PS than the outer one, and the asymmetry is actively maintained. Here, the impact of the MAP melittin on the structure of lipid bilayer vesicles made of a mixture of phosphatidylcholine andmore » phosphatidylserine was studied. Small-angle neutron scattering of the MAP associated with selectively deuterium-labeled lipid bilayer vesicles revealed how the thickness and lipid composition of phosphatidylserine-containing vesicles change in response to melittin. The peptide thickens the lipid bilayer for concentrations up to P/L = 1/500, but membrane thinning results when P/L = 1/200. The thickness transition is accompanied by a large change in the distribution of DMPS between the leaflets of the bilayer. The change in composition is driven by electrostatic interactions, while the change in bilayer thickness is driven by changes in the interaction of the peptide with the headgroup region of the lipid bilayer. Lastly, the results provide new information about lipid-specific interactions that take place in mixed composition lipid bilayer membranes.« less

Layer configurations comparison of bilayer-films for EGFET pH sensor application

NASA Astrophysics Data System (ADS)

Rahman, R. A.; Zulkefle, M. A.; Yusof, K. A.; Abdullah, W. F. H.; Rusop, M.; Herman, S. H.

2018-05-01

The comparison between bilayer configurations were presented in this paper. TiO2 and ZnO layer configurations were manipulated in order to investigate which configuration produce highest sensing performance to be applied as EGFET pH sensor. Both of the materials were deposited together as the bilayer film. The configurations were manipulated between TiO2/ZnO and ZnO/TiO2. ITO was used as the substrate in this study and both of the materials were deposited by using sol-gel spin coating technique. After deposition process, these bilayer film then undergone for EGFET pH sensor measurement and physical characterization. The EGFET pH sensor measurement was done by dipping the fabricated bilayer film into three different pH values, which is pH4, pH7 and pH10. Bilayer film act as the pH-sensitive membrane, which connected to the commercial metal-oxide semiconductor FET (MOSFET). This MOSFET was connected to the interfacing circuit. Voltage output obtained were recorded and the graph was plotted by using the data recorded. Based on the EGFET pH sensor measurement, TiO2/ZnO bilayer film exhibit higher sensing performance, compared with ZnO/TiO2. TiO2/ZnO bilayer film produced 53.10 mV/pH with the linearity value of 0.9913. Afterwards, fabricated bilayer films then were characterized with AFM to explore their surface roughness and surface topography behavior.

The Interaction of Melittin with Dimyristoyl Phosphatidylcholine-Dimyristoyl Phosphatidylserine Lipid Bilayer Membranes.

PubMed

Rai, Durgesh K; Qian, Shuo; Heller, William T

2016-11-01

Membrane-active peptides (MAPs), which interact directly with the lipid bilayer of a cell and include toxins and host defense peptides, display lipid composition-dependent activity. Phosphatidylserine (PS) lipids are anionic lipids that are found throughout the cellular membranes of most eukaryotic organisms where they serve as both a functional component and as a precursor to phosphatidylethanolamine lipids. The inner leaflet of the plasma membrane contains more PS than the outer one, and the asymmetry is actively maintained. Here, the impact of the MAP melittin on the structure of lipid bilayer vesicles made of a mixture of phosphatidylcholine and phosphatidylserine was studied. Small-angle neutron scattering of the MAP associated with selectively deuterium-labeled lipid bilayer vesicles revealed how the thickness and lipid composition of phosphatidylserine-containing vesicles change in response to melittin. The peptide thickens the lipid bilayer for concentrations up to P/L=1/500, but membrane thinning results when P/L=1/200. The thickness transition is accompanied by a large change in the distribution of DMPS between the leaflets of the bilayer. The change in composition is driven by electrostatic interactions, while the change in bilayer thickness is driven by changes in the interaction of the peptide with the headgroup region of the lipid bilayer. The results provide new information about lipid-specific interactions that take place in mixed composition lipid bilayer membranes. Copyright © 2016 Elsevier B.V. All rights reserved.

Determination of lipid bilayer affinities and solvation characteristics by electrokinetic chromatography using polymer-bound lipid bilayer nanodiscs.

PubMed

Penny, William M; Palmer, Christopher P

2018-03-01

Styrene-maleic acid polymer-bound lipid bilayer nanodiscs have been investigated and characterized by electrokinetic chromatography. Linear solvation energy relationship analysis was employed to characterize the changes in solvation environment of nanodiscs of varied belt to lipid ratio, belt polymer chemistry and molecular weight, and lipid composition. Increases in the lipid to belt polymer ratio resulted in smaller, more cohesive nanodiscs with greater electrophoretic mobility. Nanodisc structures with belt polymers of different chemistry and molecular weight were compared and showed only minor changes in solvent characteristics and selectivity consistent with changes in structure of the lipid bilayer. Seven phospholipid and sphingomyelin nanodiscs of different lipid composition were characterized. Changes in lipid head group structure had a significant effect on bilayer-solute interactions. In most cases, changes in alkyl tail structure had no discernible effect on solvation environment aside from those explained by changes in the gel-liquid transition temperature. Comparison to vesicles of similar lipid composition show only minor differences in solvation environment, likely due to differences in lipid composition and bilayer curvature. Together these results provide evidence that the dominant solute-nanodisc interactions are with the lipid bilayer and that head group chemistry has a greater impact on bilayer-solute interactions than alkyl tail or belt polymer structure. Nanodisc electrokinetic chromatography is demonstrated to allow characterization of solute interactions with lipid bilayers of varied composition. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Blistering of langmuir-blodgett bilayers containing anionic phospholipids as observed by atomic force microscopy.

PubMed Central

Rinia, H A; Demel, R A; van der Eerden, J P; de Kruijff, B

1999-01-01

Asymmetric bilayers of different phospholipid compositions have been prepared by the Langmuir-Blodgett (L-B) method, and imaged by atomic force microscopy (AFM). Such bilayers can function as a model for biological membranes. The first leaflet consisted of zwitterionic phospholipids phosphatidylcholine (PC) or phosphatidylethanolamine (PE). The second leaflet consisted of the anionic phospholipid phosphatidylglycerol (PG), in either the condensed or liquid phase or, for comparison, of PC. Different bilayers showed different morphology. In all bilayers defects in the form of holes were present. In some bilayers with a first leaflet consisting of PC, polygonal line-shaped defects were observed, whereas when the first leaflet consisted of PE, mainly round defects were seen. Not only the shape, but also the amount of defects varied, depending on the condition and the composition of the second leaflet. In most of the PG-containing systems the defects were surrounded by elevations, which reversibly disappeared in the presence of divalent cations. This is the first time that such elevations have been observed on phospholipid bilayers. We propose that they are induced by phospholipid exchange between the two leaflets around the defects, leading to the presence of negatively charged phospholipids in the first leaflet. Because the substrate is also negatively charged, the bilayer around the edges is repelled and lifted up. Since it was found that the elevations are indeed detached from the substrate, we refer to this effect as bilayer blistering. PMID:10465778

Influence of Temperature on Transdermal Penetration Enhancing Mechanism of Borneol: A Multi-Scale Study

PubMed Central

Yin, Qianqian; Wang, Ran; Yang, Shufang; Wu, Zhimin; Guo, Shujuan; Dai, Xingxing; Qiao, Yanjiang; Shi, Xinyuan

2017-01-01

The influence of temperature on the transdermal permeation enhancing mechanism of borneol (BO) was investigated using a multi-scale method, containing a coarse-grained molecular dynamic (CG-MD) simulation, an in vitro permeation experiment, and a transmission electron microscope (TEM) study. The results showed that BO has the potential to be used as a transdermal penetration enhancer to help osthole (OST) penetrate into the bilayer. With the increasing temperature, the stratum corneum (SC) becomes more flexible, proving to be synergistic with the permeation enhancement of BO, and the lag time (TLag) of BO and OST are shortened. However, when the temperature increased too much, with the effect of BO, the structure of SC was destroyed; for example, a water pore was formed and the micelle reversed. Though there were a number of drugs coming into the SC, the normal bilayer structure was absent. In addition, through comparing the simulation, in vitro experiment, and TEM study, we concluded that the computer simulation provided some visually detailed information, and the method plays an important role in related studies of permeation. PMID:28106833

Obesity resistance and deregulation of lipogenesis in Δ6-fatty acid desaturase (FADS2) deficiency.

PubMed

Stoffel, Wilhelm; Hammels, Ina; Jenke, Britta; Binczek, Erika; Schmidt-Soltau, Inga; Brodesser, Susanne; Odenthal, Margarete; Thevis, Mario

2014-01-01

Δ-6-fatty acid desaturase (FADS2) is the key enzyme in the biosynthesis of polyunsaturated fatty acids (PUFAs), the essential structural determinants of mammalian membrane lipid-bilayers. We developed the auxotrophic fads2(-/-) mouse mutant to assess the enigmatic role of ω3- and ω6-PUFAs in lipid homeostasis, membrane structure and function. Obesity resistance is another major phenotype of the fads2(-/-) mutant, the molecular basis of which is unknown. Phospholipidomic profiling of membrane systems of fads2(-/-)mice revealed diacylglycerol-structures, deprived of PUFAs but substituted with surrogate eicosa-5,11,14-trienoic acid. ω6-Arachidonic (AA) and ω3-docosahexaenoic acid (DHA) supplemented diets transformed fads2(-/-) into AA-fads2(-/-) and DHA-fads2(-/-) mutants. Severely altered phospholipid-bilayer structures of subcellular membranes of fads2(-/-) liver specifically interfered with maturation of transcription factor sterol-regulatory-element-binding protein, the key regulator of lipogenesis and lipid homeostasis. This study strengthens the concept that specific PUFA-substituted membrane phospholipid species are critical constituents of the structural platform operative in lipid homeostasis in normal and disease conditions.

Kinematics of red cell aspiration by fluorescence-imaged microdeformation.

PubMed

Discher, D E; Mohandas, N

1996-10-01

Maps of fluorescing red cell membrane components on a pipette-aspirated projection are quantitated in an effort to elucidate and unify the heterogeneous kinematics of deformation. Transient gradients of diffusing fluorescent lipid first demonstrate the fluidity of an otherwise uniform-density bilayer and corroborate a "universal" calibration scale for relative surface density. A steep but smooth and stable gradient in the densities of the skeleton components spectrin, actin, and protein 4.1 is used to estimate large elastic strains along the aspirated skeleton. The deformation fields are argued to be an unhindered response to loading in the surface normal direction. Density maps intermediate to those of the compressible skeleton and fluid bilayer are exhibited by particular transmembrane proteins (e.g., Band 3) and yield estimates for the skeleton-connected fractions. Such connected proteins appear to occupy a significant proportion of the undeformed membrane surface and can lead to steric exclusion of unconnected integral membrane proteins from regions of network condensation. Consistent with membrane repatterning kinematics in reversible deformation, final vesiculation of the projection tip produces a cell fragment concentrated in freely diffusing proteins but depleted of skeleton.

Lipid nanotube or nanowire sensor

DOEpatents

Noy, Aleksandr [Belmont, CA; Bakajin, Olgica [San Leandro, CA; Letant, Sonia [Livermore, CA; Stadermann, Michael [Dublin, CA; Artyukhin, Alexander B [Menlo Park, CA

2009-06-09

A sensor apparatus comprising a nanotube or nanowire, a lipid bilayer around the nanotube or nanowire, and a sensing element connected to the lipid bilayer. Also a biosensor apparatus comprising a gate electrode; a source electrode; a drain electrode; a nanotube or nanowire operatively connected to the gate electrode, the source electrode, and the drain electrode; a lipid bilayer around the nanotube or nanowire, and a sensing element connected to the lipid bilayer.

Lipid nanotube or nanowire sensor

DOEpatents

Noy, Aleksandr [Belmont, CA; Bakajin, Olgica [San Leandro, CA; Letant, Sonia [Livermore, CA; Stadermann, Michael [Dublin, CA; Artyukhin, Alexander B [Menlo Park, CA

2010-06-29

A sensor apparatus comprising a nanotube or nanowire, a lipid bilayer around the nanotube or nanowire, and a sensing element connected to the lipid bilayer. Also a biosensor apparatus comprising a gate electrode; a source electrode; a drain electrode; a nanotube or nanowire operatively connected to the gate electrode, the source electrode, and the drain electrode; a lipid bilayer around the nanotube or nanowire, and a sensing element connected to the lipid bilayer.

Elasticity of bilayers containing PEG lipids

NASA Astrophysics Data System (ADS)

Bivas, I.; Winterhalter, M.; Méléard, P.; Bothorel, P.

1998-02-01

The addition of lipids with a poly(ethylene glycol) head group (Stealth or grafted or PEG lipids) to a phosphatidylcholine bilayer changes the mechanical properties of the membrane. We calculate the dependences of the bending and stretching elasticities of the bilayer on the PEG lipid concentration and on the monomer number in its polymer chain. The role of the bending elasticity at blocked flip-flop of the pure bilayer is revealed.

Undoped GaAs bilayers for exciton condensation experiments

NASA Astrophysics Data System (ADS)

Lilly, M. P.

2005-03-01

Experimental progress in transport studies of exciton condensation of in electron and hole bilayers at high magnetic fields [1,2] has shown this novel physics can be observed. Fabrication of the bipolar electron-hole bilayers for zero field studies of exciton condensation still remains elusive. We describe a series of experiments on undoped GaAs/AlGaAs heterostructures with the motivation of making electron-hole bilayers. In these undoped devices, external electric fields induce carriers rather than the traditional doping techniques. Single layer electron (or hole) devices demonstrate a high mobility over a wide range of density. More recently, fully undoped bilayers have been made where the density in each layer is independently controlled with gates on the top and bottom of the bilayer. In this talk we present high field transport of undoped electron-electron bilayers, and describe recent progress towards extending the fabrication techniques to creating electron-hole bilayers for exciton condensation studies at zero magnetic field. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. 1. M. Kellogg, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 93 036801 (2004). 2. E. Tutoc, M. Shayegan, and D. A. Huse, Phys. Rev. Lett. 93, 036802 (2004).

Pressure-induced enhancement in the thermoelectric properties of monolayer and bilayer SnSe2

NASA Astrophysics Data System (ADS)

Zou, Daifeng; Yu, Chuanbin; Li, Yuhao; Ou, Yun; Gao, Yongyi

2018-03-01

The electronic structures of monolayer and bilayer SnSe2 under pressure were investigated by using first-principles calculations including van der Waals interactions. For monolayer SnSe2, the variation of electronic structure under pressure is controlled by pressure-dependent lattice parameters. For bilayer SnSe2, the changes in electronic structure under pressure are dominated by intralayer and interlayer atomic interactions. The n-type thermoelectric properties of monolayer and bilayer SnSe2 under pressure were calculated on the basis of the semi-classical Boltzmann transport theory. It was found that the electrical conductivity of monolayer and bilayer SnSe2 can be enhanced under pressure, and such dependence can be attributed to the pressure-induced changes of the Se-Sn antibonding states in conduction band. Finally, the doping dependence of power factors of n-type monolayer and bilayer SnSe2 at three different pressures were estimated, and the results unveiled that thermoelectric performance of n-type monolayer and bilayer SnSe2 can be improved by applying external pressure. This study benefits to understand the nature of the transport properties for monolayer and bilayer SnSe2 under pressure, and it offers valuable insight for designing high-performance thermoelectric few-layered SnSe2 through strain engineering induced by external pressure.

Revealing the preferred interlayer orientations and stackings of two-dimensional bilayer gallium selenide crystals

DOE PAGES

Li, Xufan; Basile Carrasco, Leonardo A.; Yoon, Mina; ...

2015-01-21

Characterizing and controlling the interlayer orientations and stacking order of bilayer two-dimensional (2D) crystals and van der Waals (vdW) heterostructure is crucial to optimize their electrical and optoelectronic properties. The four polymorphs of layered gallium selenide (GaSe) that result from different layer stacking provide an ideal platform to study the stacking configurations in bilayer 2D crystals. Here, through a controllable vapor-phase deposition method we selectively grow bilayer GaSe crystals and investigate their two preferred 0° or 60° interlayer rotations. The commensurate stacking configurations (AA' and AB-stacking) in as-grown 2D bilayer GaSe crystals are clearly observed at the atomic scale andmore » the Ga-terminated edge structure are identified for the first time by using atomic-resolution scanning transmission electron microscopy (STEM). Theoretical analysis of the interlayer coupling energetics vs. interlayer rotation angle reveals that the experimentally-observed orientations are energetically preferred among the bilayer GaSe crystal polytypes. Here, the combined experimental and theoretical characterization of the GaSe bilayers afforded by these growth studies provide a pathway to reveal the atomistic relationships in interlayer orientations responsible for the electronic and optical properties of bilayer 2D crystals and vdW heterostructures.« less

Pressure-induced enhancement in the thermoelectric properties of monolayer and bilayer SnSe2.

PubMed

Zou, Daifeng; Yu, Chuanbin; Li, Yuhao; Ou, Yun; Gao, Yongyi

2018-03-01

The electronic structures of monolayer and bilayer SnSe 2 under pressure were investigated by using first-principles calculations including van der Waals interactions. For monolayer SnSe 2 , the variation of electronic structure under pressure is controlled by pressure-dependent lattice parameters. For bilayer SnSe 2 , the changes in electronic structure under pressure are dominated by intralayer and interlayer atomic interactions. The n -type thermoelectric properties of monolayer and bilayer SnSe 2 under pressure were calculated on the basis of the semi-classical Boltzmann transport theory. It was found that the electrical conductivity of monolayer and bilayer SnSe 2 can be enhanced under pressure, and such dependence can be attributed to the pressure-induced changes of the Se-Sn antibonding states in conduction band. Finally, the doping dependence of power factors of n -type monolayer and bilayer SnSe 2 at three different pressures were estimated, and the results unveiled that thermoelectric performance of n -type monolayer and bilayer SnSe 2 can be improved by applying external pressure. This study benefits to understand the nature of the transport properties for monolayer and bilayer SnSe 2 under pressure, and it offers valuable insight for designing high-performance thermoelectric few-layered SnSe 2 through strain engineering induced by external pressure.

Thermoelastic damping in bilayered microbar resonators with circular cross-section

NASA Astrophysics Data System (ADS)

Liang, Xiaoyao; Li, Pu

2017-11-01

It is always a challenge to determine the Thermoelastic damping (TED) in bilayered microbars precisely. In this paper, a model for TED in the bilayered and cantilevered microbar was proposed, in which the total damping was derived by calculating the energy evanished in each layer. The distribution of temperature in the bilayered microbar with a thermodynamically ideal boundary receiving a time-harmonic force is obtained. An infinite summation for the computing of TED in the bilayered slender microbars under axial loading is presented, and the convergence rate of it is discussed. There are little differences between the results computed by our model and that by finite element method (FEM).

Orientation-adjusted anomalous insulator-metal transition in NdNiO3/LaMnO3 bilayers

NASA Astrophysics Data System (ADS)

Pan, S. Y.; Shi, L.; Zhao, J. Y.; Zhou, S. M.; Xu, X. M.

2018-04-01

NdNiO3/LaMnO3 (NNO/LMO) bilayers were epitaxially grown on SrTiO3 (STO) substrates with different orientations by the polymer-assisted deposition technique. A well crystallization quality of the bilayers is confirmed by X-ray diffraction. Two consecutive transitions, an anomalous insulator-metal transition at ˜100 K followed by the typical metal-insulator transition at ˜171 K, are observed in the (001)-oriented NNO/LMO/STO bilayer. The anomalous insulator-metal transition temperature increases to 142 K for the (111)-oriented NNO/LMO/STO bilayer. Meanwhile, the magnetic properties of the NNO/LMO bilayers show an obvious difference with [100] and [111] orientations. Considering the different strain directions and the related oxygen octahedral distortion/rotation, it is suggested that the magnetic changes and the low-temperature anomalous insulator-metal transition in the NNO/LMO bilayers are attributed to the strong interlayer exchange coupling and charge transfer adjusted by the substrate orientation, which can be an effective technique to tune the properties of transition-metal oxide films.

Strain and deformations engineered germanene bilayer double gate-field effect transistor by first principles

NASA Astrophysics Data System (ADS)

Meher Abhinav, E.; Chandrasekaran, Gopalakrishnan; Kasmir Raja, S. V.

2017-10-01

Germanene, silicene, stanene, phosphorene and graphene are some of single atomic materials with novel properties. In this paper, we explored bilayer germanene-based Double Gate-Field Effect Transistor (DG-FET) with various strains and deformations using Density Functional Theory (DFT) and Green's approach by first-principle calculations. The DG-FET of 1.6 nm width, 6 nm channel length (Lch) and HfO2 as gate dielectric has been modeled. For intrinsic deformation of germanene bilayer, we have enforced minute mechanical deformation of wrap and twist (5°) and ripple (0.5 Å) on germanene bilayer channel material. By using NEGF formalism, I-V Characteristics of various strains and deformation tailored DG-FET was calculated. Our results show that rough edge and single vacancy (5-9) in bilayer germanene diminishes the current around 47% and 58% respectively as compared with pristine bilayer germanene. In case of strain tailored bilayer DG-FET, multiple NDR regions were observed which can be utilized in building stable multiple logic states in digital circuits and high frequency oscillators using negative resistive techniques.

Buckling-dependent switching behaviours in shifted bilayer germanene nanoribbons: A computational study

NASA Astrophysics Data System (ADS)

Arjmand, T.; Tagani, M. Bagheri; Soleimani, H. Rahimpour

2018-01-01

Bilayer germanene nanoribbons are investigated in different stacks like buckled and flat armchair and buckled zigzag germanene nanoribbons by performing theoretical calculations using the nonequilibrium Greens function method combined with density functional theory. In these bilayer types, the current oscillates with change of interlayer distances or intra-layer overlaps and is dependent on the type of the bilayer. Band gap of AA-stacked of shifted flat bilayer armchair germanene nanoribbon oscillates by change of interlayer distance which is in contrast to buckled bilayer armchair germanene nanoribbon. So, results show the buckling makes system tend to be a semiconductor with wide band gap. Therefore, AA-stacked of shifted flat bilayer armchair germanene nanoribbon has properties between zigzag and armchair edges, the higher current under bias voltages similar to zigzag edge and also oscillations in current like buckled armchair edges. Also, it is found that HOMO-LUMO band gap strongly affects oscillation in currents and their I-V characteristic. This kind of junction improves the switching properties at low voltages around the band gap.

Ab-initio calculation of electronic structure and optical properties of AB-stacked bilayer α-graphyne

NASA Astrophysics Data System (ADS)

Behzad, Somayeh

2016-09-01

Monolayer α-graphyne is a new two-dimensional carbon allotrope with many special features. In this work the electronic properties of AA- and AB-stacked bilayers of this material and then the optical properties are studied, using first principle plane wave method. The electronic spectrum has two Dirac cones for AA stacked bilayer α-graphyne. For AB-stacked bilayer, the interlayer interaction changes the linear bands into parabolic bands. The optical spectra of the most stable AB-stacked bilayer closely resemble to that of the monolayer, except for small shifts of peak positions and increasing of their intensity. For AB-stacked bilayer, a pronounced peak has been found at low energies under the perpendicular polarization. This peak can be clearly ascribed to the transitions at the Dirac point as a result of the small degeneracy lift in the band structure.

Microfluidic array platform for simultaneous lipid bilayer membrane formation.

PubMed

Zagnoni, M; Sandison, M E; Morgan, H

2009-01-01

In recent years, protein array technologies have found widespread applications in proteomics. However, new methods for high-throughput analysis of protein-protein and protein-compound interactions are still required. In this paper, an array of lipid bilayer membranes formed within a microfluidic system with integrated electrodes is presented. The system is comprised of three layers that are clamped together, thus rendering the device cleanable and reusable. The device microfluidics enable the simultaneous formation of an array of lipid bilayers using a previously developed air-exposure technique, thereby avoiding the need to manually form individual bilayers. The Ag/AgCl electrodes allow for ion channel measurements, each of the sites being independently addressable. Typically, a 50% yield in simultaneous lipid bilayer formation over 12 sites was obtained and ion channel recordings have been acquired over multiple sites. This system has great potential for the development of an automatable platform of suspended lipid bilayer arrays.

Electronic, Mechanical, and Dielectric Properties of Two-Dimensional Atomic Layers of Noble Metals

NASA Astrophysics Data System (ADS)

Kapoor, Pooja; Kumar, Jagdish; Kumar, Arun; Kumar, Ashok; Ahluwalia, P. K.

2017-01-01

We present density functional theory-based electronic, mechanical, and dielectric properties of monolayers and bilayers of noble metals (Au, Ag, Cu, and Pt) taken with graphene-like hexagonal structure. The Au, Ag, and Pt bilayers stabilize in AA-stacked configuration, while the Cu bilayer favors the AB stacking pattern. The quantum ballistic conductance of the noble-metal mono- and bilayers is remarkably increased compared with their bulk counterparts. Among the studied systems, the tensile strength is found to be highest for the Pt monolayer and bilayer. The noble metals in mono- and bilayer form show distinctly different electron energy loss spectra and reflectance spectra due to the quantum confinement effect on going from bulk to the monolayer limit. Such tunability of the electronic and dielectric properties of noble metals by reducing the degrees of freedom of electrons offers promise for their use in nanoelectronics and optoelectronics applications.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tran, Ich C.; Tunuguntla, Ramya H.; Kim, Kyunghoon

Carbon nanotube porins (CNTPs), small segments of carbon nanotubes capable of forming defined pores in lipid membranes, are important future components for bionanoelectronic devices as they could provide a robust analog of biological membrane channels. Furthermore, in order to control the incorporation of these CNT channels into lipid bilayers, it is important to understand the structure of the CNTPs before and after insertion into the lipid bilayer as well as the impact of such insertion on the bilayer structure. Here we employed a noninvasive in situ probe, small-angle X-ray scattering, to study the integration of CNT porins into dioleoylphosphatidylcholine bilayers.more » These results show that CNTPs in solution are stabilized by a monolayer of lipid molecules wrapped around their outer surface. We also demonstrate that insertion of CNTPs into the lipid bilayer results in decreased bilayer thickness with the magnitude of this effect increasing with the concentration of CNTPs.« less

Tunneling Plasmonics in Bilayer Graphene.

PubMed

Fei, Z; Iwinski, E G; Ni, G X; Zhang, L M; Bao, W; Rodin, A S; Lee, Y; Wagner, M; Liu, M K; Dai, S; Goldflam, M D; Thiemens, M; Keilmann, F; Lau, C N; Castro-Neto, A H; Fogler, M M; Basov, D N

2015-08-12

We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At subnanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single- and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene, yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers.

Synthesis and Characterization of Novel Anchorlipids for Tethered Bilayer Lipid Membranes.

PubMed

Andersson, Jakob; Knobloch, Jacqueline J; Perkins, Michael V; Holt, Stephen A; Köper, Ingo

2017-05-09

Tethered bilayer lipid membranes are versatile solid-supported model membrane systems. Core to these systems is an anchorlipid that covalently links a lipid bilayer to a support. The molecular structure of these lipids can have a significant impact on the properties of the resulting bilayer. Here, the synthesis of anchorlipids containing ester groups in the tethering part is described. The lipids are used to form bilayer membranes, and the resulting structures are compared with membranes formed using conventional anchorlipids or sparsely tethered membranes. All membranes showed good electrical sealing properties; the disulphide-terminated anchorlipids could be used in a sparsely tethered system without significantly reducing the sealing properties of the lipid bilayers. The sparsely tethered systems also allowed for higher ion transport across the membrane, which is in good correlation with higher hydration of the spacer region as seen by neutron scattering.

Statistical Analysis of Bending Rigidity Coefficient Determined Using Fluorescence-Based Flicker-Noise Spectroscopy.

PubMed

Doskocz, Joanna; Drabik, Dominik; Chodaczek, Grzegorz; Przybyło, Magdalena; Langner, Marek

2018-06-01

Bending rigidity coefficient describes propensity of a lipid bilayer to deform. In order to measure the parameter experimentally using flickering noise spectroscopy, the microscopic imaging is required, which necessitates the application of giant unilamellar vesicles (GUV) lipid bilayer model. The major difficulty associated with the application of the model is the statistical character of GUV population with respect to their size and the homogeneity of lipid bilayer composition, if a mixture of lipids is used. In the paper, the bending rigidity coefficient was measured using the fluorescence-enhanced flicker-noise spectroscopy. In the paper, the bending rigidity coefficient was determined for large populations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine vesicles. The quantity of obtained experimental data allows to perform statistical analysis aiming at the identification of the distribution, which is the most appropriate for the calculation of the value of the membrane bending rigidity coefficient. It has been demonstrated that the bending rigidity coefficient is characterized by an asymmetrical distribution, which is well approximated with the gamma distribution. Since there are no biophysical reasons for that we propose to use the difference between normal and gamma fits as a measure of the homogeneity of vesicle population. In addition, the effect of a fluorescent label and types of instrumental setups on determined values has been tested. Obtained results show that the value of the bending rigidity coefficient does not depend on the type of a fluorescent label nor on the type of microscope used.

Design of a novel bilayered gastric mucoadhesive system for localized and unidirectional release of lamotrigine

PubMed Central

Mohana Raghava Srivalli, K.; Lakshmi, P.K.; Balasubramaniam, J.

2012-01-01

Lamotrigine is a BCS class II drug with pH dependent solubility. The bilayered gastric mucoadhesive tablets of lamotrigine were designed such that the drug and controlled release polymers were incorporated in the upper layer and the lower layer had the mucoadhesive polymers. The major ingredients selected for the upper layer were the drug and control release polymer (either HPMC K15M or polyox) while the lower MA layer predominantly comprised of Carbopol 974P. A 23 full factorial design was constructed for this study and the tablets were optimized for parameters like tablet size, shape, ex vivo mucoadhesive properties and unidirectional drug release. Oval tablets with an average size of 14 mm diameter were set optimum. Maximum mucoadhesive bond strength of 79.3 ± 0.91 * 103 dyn/cm2 was achieved with carbopol when used in combination with a synergistic resin polymer. All the tested formulations presented a mucoadhesion time of greater than 12 h. The incorporation of methacrylic polymers in the lower layer ensured unidirectional drug release from the bilayered tablets. The unidirectional drug release was confirmed after comparing the dissolution results of paddle method with those of a modified basket method. Model independent similarity and dissimilarity factor methods were used for the comparison of dissolution results. Controlled drug release profiles with zero order kinetics were obtained with polyox and HPMC K15M which reported t90% at 6th and 12th hours, respectively. The “n” value with polyox was 0.992 and that with HPMC K15M was 0.946 indicating an approximate case II transport. These two formulations showed the potential for oral administration of lamotrigine as bilayered gastric mucoadhesive tablets by yielding highest similarity factor values, 96.06 and 92.47, respectively, between the paddle and modified basket method dissolution release profiles apart from reporting the best tablet physical properties and maximum mucoadhesive strength. PMID:24109205

A comparative study on the effect of Curcumin and Chlorin-p6 on the transport of the LDS cation across a negatively charged POPG bilayer: Effect of pH

NASA Astrophysics Data System (ADS)

Varshney, G. K.; Kintali, S. R.; Gupta, P. K.; Das, K.

2017-02-01

We report the use of interface selective Second Harmonic generation technique to investigate the transport of the LDS cation across POPG liposomes in the pH range of 4.0 to 8.0 in the presence and absence of two amphiphilic drugs, Curcumin and Chlorin-p6 (Cp6). Our results show that bilayer permeability of liposomes is significantly affected by the presence of the drugs and pH of the medium as evidenced by significant changes in the transport kinetics of the LDS. Studies carried out in the pH range 4.0-8.0 show that while Cp6 significantly enhanced the transport of LDS at pH 4.0, the transport of the cation was seen to increase with increasing pH, with maximum effect at pH 7.4 for Curcumin. The pH dependent bilayer localization of both the drugs was investigated by conducting steady state FRET studies using DPH labeled lipids as donors. The FRET results and the relative population of the various ionic/nonionic species of the drugs at different pH suggest that distance dependent interaction between the various ionic species of the drugs and polar head groups of the lipid is responsible for the observed pH dependence enhancement of the drug induced membrane permeability. Another interesting observation was that the stability of Curcumin in presence of POPG liposomes was observed to degrade significantly near physiological pH (7.4 and 8.0). Although this degradation did not affect the liposome integrity, interestingly this was observed to enhance the transport of the LDS cation across the bilayer. That the degradation products of Curcumin are equally effective as the drug itself in enhancing the membrane permeability lends additional support to the current opinion that the bioactive degradation products of the drug may have a significant contribution to its observed pharmacological effects.

Effects of cholesterol concentration on the interaction of cytarabine with lipid membranes: a molecular dynamics simulation study.

PubMed

Karami, Leila; Jalili, Seifollah

2015-01-01

Liposomal cytarabine, DepoCyt, is a chemotherapy agent which is used in cancer treatment. This form of cytarabine has more efficacy and fewer side effects relative to the other forms. Since DepoCyt contains the cytarabine encapsulated within phosphatidylcholine and the sterol molecules, we modeled dioleoylphosphatidylcholine (DOPC)/cholesterol bilayer membrane as a carrier for cytarabine to study drug-bilayer interactions. For this purpose, we performed a series of united-atom molecular dynamics (MD) simulations for 25 ns to investigate the interactions between cytarabine and cholesterol-containing DOPC lipid bilayers. Only the uncharged form of cytarabine molecule was investigated. In this study, different levels of the cholesterol content (0, 20, and 40%) were used. MD simulations allowed us to determine dynamical and structural properties of the bilayer membrane and to estimate the preferred location and orientation of the cytarabine molecule inside the bilayer membrane. Properties such as membrane thickness, area per lipid, diffusion coefficient, mass density, bilayer packing, order parameters, and intermolecular interactions were examined. The results show that by increasing the cholesterol concentration in the lipid bilayers, the bilayer thickness increases and area per lipid decreases. Moreover, in accordance with the experiments, our calculations show that cholesterol molecules have ordering effect on the hydrocarbon acyl chains. Furthermore, the cytarabine molecule preferentially occupies the polar region of the lipid head groups to form specific interactions (hydrogen bonds). Our results fully support the experimental data. Our finding about drug-bilayer interaction is crucial for the liposomal drug design.

Molecular Dynamics of a Water-Lipid Bilayer Interface

NASA Technical Reports Server (NTRS)

Wilson, Michael A.; Pohorille, Andrew

1994-01-01

We present results of molecular dynamics simulations of a glycerol 1-monooleate bilayer in water. The total length of analyzed trajectories is 5ns. The calculated width of the bilayer agrees well with the experimentally measured value. The interior of the membrane is in a highly disordered fluid state. Atomic density profile, orientational and conformational distribution functions, and order parameters indicate that disorder increases toward the center of the bilayer. Analysis of out-of-plane thermal fluctuations of the bilayer surfaces occurring at the time scale of the present calculations reveals that the distribution of modes agrees with predictions of the capillary wave model. Fluctuations of both bilayer surfaces are uncorrelated, yielding Gaussian distribution of instantaneous widths of the membrane. Fluctuations of the width produce transient thinning defects in the bilayer which occasionally span almost half of the membrane. The leading mechanism of these fluctuations is the orientational and conformational motion of head groups rather than vertical motion of the whole molecules. Water considerably penetrates the head group region of the bilayer but not its hydrocarbon core. The total net excess dipole moment of the interfacial water points toward the aqueous phase, but the water polarization profile is non-monotonic. Both water and head groups significantly contribute to the surface potential across the interface. The calculated sign of the surface potential is in agreement with that from experimental measurements, but the value is markedly overestimated. The structural and electrical properties of the water-bilayer system are discussed in relation to membrane functions, in particular transport of ions and nonelectrolytes across membranes.

Understanding the interaction of block copolymers with DMPC lipid bilayer using coarse-grained molecular dynamics simulations.

PubMed

Hezaveh, Samira; Samanta, Susruta; De Nicola, Antonio; Milano, Giuseppe; Roccatano, Danilo

2012-12-13

In this paper, we present a computational model of the adsorption and percolation mechanism of poloxamers (poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) triblock copolymers) across a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayer. A coarse-grained model was used to cope with the long time scale of the percolation process. The simulations have provided details of the interaction mechanism of Pluronics with lipid bilayer. In particular, the results have shown that polymer chains containing a PPO block with a length comparable to the DMPC bilayer thickness, such as P85, tends to percolate across the lipid bilayer. On the contrary, Pluronics with a shorter PPO chain, such as L64 and F38, insert partially into the membrane with the PPO block part while the PEO blocks remain in water on one side of the lipid bilayer. The percolation of the polymers into the lipid tail groups reduces the membrane thickness and increases the area per lipid. These effects are more evident for P85 than L64 or F38. Our findings are qualitatively in good agreement with published small-angle X-ray scattering experiments that have evidenced a thinning effect of Pluronics on the lipid bilayer as well as the role of the length of the PPO block on the permeation process of the polymer through the lipid bilayer. Our theoretical results complement the experimental data with a detailed structural and dynamic model of poloxamers at the interface and inside the lipid bilayer.

Voltage-Gated Channel Mechanosensitivity: Fact or Friction?

PubMed Central

Morris, Catherine E.

2011-01-01

The heart is a continually active pulsatile fluid pump. It generates appropriate forces by precisely timed and spaced engagement of its contractile machinery. Largely, it makes its own control signals, the most crucial of which are precisely timed and spaced fluxes of ions across the sarcolemma, achieved by the timely opening and closing of diverse voltage-gated channels (VGC). VGCs have four voltage sensors around a central ion-selective pore that opens and closes under the influence of membrane voltage. Operation of any VGC is secondarily tuned by the mechanical state (i.e., structure) of the bilayer in which it is embedded. Rates of opening and closing, in other words, vary with bilayer structure. Thus, in the intensely mechanical environment of the myocardium and its vasculature, VGCs kinetics might be routinely modulated by reversible and irreversible nano-scale changes in bilayer structure. If subtle bilayer deformations are routine in the pumping heart, VGCs could be subtly transducing bilayer mechanical signals, thereby tuning cardiac rhythmicity, collectively contributing to mechano-electric feedback. Reversible bilayer deformations would be expected with changing shear flows and tissue distension, while irreversible bilayer restructuring occurs with ischemia, inflammation, membrane remodeling, etc. I suggest that tools now available could be deployed to help probe whether/how the inherent mechanosensitivity of VGCs – an attribute substantially reflecting the dependence of voltage sensor stability on bilayer structure – contributes to cardiac rhythmicity. Chief among these tools are voltage sensor toxins (whose inhibitory efficacy varies with the mechanical state of bilayer) and arrhythmia-inducing VGC mutants with distinctive mechano-phenotypes. PMID:21660289

Atomistic-level study of the interactions between hIAPP protofibrils and membranes: Influence of pH and lipid composition.

PubMed

Qian, Zhenyu; Zou, Yu; Zhang, Qingwen; Chen, Peijie; Ma, Buyong; Wei, Guanghong; Nussinov, Ruth

2018-02-09

The pathology of type 2 diabetes mellitus is associated with the aggregation of human islet amyloid polypeptide (hIAPP) and aggregation-mediated membrane disruption. The interactions of hIAPP aggregates with lipid membrane, as well as the effects of pH and lipid composition at the atomic level, remain elusive. Herein, using molecular dynamics simulations, we investigate the interactions of hIAPP protofibrillar oligomers with lipids, and the membrane perturbation that they induce, when they are partially inserted in an anionic dipalmitoyl-phosphatidylglycerol (DPPG) membrane or a mixed dipalmitoyl-phosphatidylcholine (DPPC)/DPPG (7:3) lipid bilayer under acidic/neutral pH conditions. We observed that the tilt angles and insertion depths of the hIAPP protofibril are strongly correlated with the pH and lipid composition. At neutral pH, the tilt angle and insertion depth of hIAPP protofibrils at a DPPG bilayer reach ~52° and ~1.62 nm with respect to the membrane surface, while they become ~77° and ~1.75 nm at a mixed DPPC/DPPG membrane. The calculated tilt angle of hIAPP at DPPG membrane is consistent with a recent chiral sum frequency generation spectroscopic study. The acidic pH induces a smaller tilt angle of ~40° and a shallower insertion depth (~1.24 nm) of hIAPP at the DPPG membrane surface, mainly due to protonation of His18 near the turn region. These differences mainly result from a combination of distinct electrostatic, van der Waals, hydrogen bonding and salt-bridge interactions between hIAPP and lipid bilayers. The hIAPP-membrane interaction energy analysis reveals that besides charged residues K1, R11 and H18, aromatic residues Phe15 and Phe23 also exhibit strong interactions with lipid bilayers, revealing the crucial role of aromatic residues in stabilizing the membrane-bound hIAPP protofibrils. hIAPP-membrane interactions disturb the lipid ordering and the local bilayer thickness around the peptides. Our results provide atomic-level information of membrane interaction of hIAPP protofibrils, revealing pH-dependent and membrane-modulated hIAPP aggregation at the early stage. Copyright © 2018 Elsevier B.V. All rights reserved.

Channel Protein Engineering: A Novel Approach Towards the Molecular Dissection Determinants in Ligand-Regulated Channels

DTIC Science & Technology

1991-01-23

lidocaine , within the lumen of the pore. Specific predictions for possible experimental mutations are made which can serve to test both the proposed...to the protein from the bilayer interior. 2. Synthesis of tetrameric synthetic channel proteins and demonstration of channel blockade by a local...Congress, Vancouver, Canada. S9.4, p. 2 4 (1990). Grove, A., J. M. Tomich and M. Montal. Design, synthesis and single channel characterization of d

Folding and stacking defects of graphene flakes probed by electron nanobeam

DOE Office of Scientific and Technical Information (OSTI.GOV)

Persichetti, L.; Fanfoni, M.; Sgarlata, A.

2011-07-25

Combining nanoscale imaging with local electron spectroscopy and diffraction has provided direct information on folding and stacking defects of graphene flakes produced by unrolled multi-walled carbon nanotubes. Structural data obtained by nanoarea electron diffraction complemented with systematic electron energy loss spectroscopy measurements of the surface plasmon losses of single flakes show the presence of flat bilayer regions coexisting with folded areas where the topology of buckled graphene resembles that of warped carbon nanostructures.

Adsorption of beryllium atoms and clusters both on graphene and in a bilayer of graphite investigated by DFT.

PubMed

Ferro, Yves; Fernandez, Nicolas; Allouche, Alain; Linsmeier, Christian

2013-01-09

We herein investigate the interaction of beryllium with a graphene sheet and in a bilayer of graphite by means of periodic DFT calculations. In all cases, we find the beryllium atoms to be more weakly bonded on graphene than in the bilayer. Be(2) forms both magnetic and non-magnetic structures on graphene depending on the geometrical configuration of adsorption. We find that the stability of the Be/bilayer system increases with the size of the beryllium clusters inserted into the bilayer of graphite. We also find a charge transfer from beryllium to the graphite layers. All these results are analysed in terms of electronic structure.

Interaction of elaiophylin with model bilayer membrane

NASA Astrophysics Data System (ADS)

Genova, J.; Dencheva-Zarkova, M.

2017-01-01

Elaiophylin is a new macrodiolide antibiotic, which is produced by the Streptomyces strains [1]. It displays biological activities against Gram-positive bacteria and fungi. The mode of action of this antibiotic has been attributed to an alteration of the membrane permeability. When this antibiotic is inserted into the bilayer membranes destabilization of the membrane and formation of ion-penetrable channels is observed. The macrodiolide antibiotic forms stable cation selective ion channels in synthetic lipid bilayer membranes. The aim of this work was to study the interactions of Elaiophylin with model bilayer membranes and to get information on the mechanical properties of lipid bilayers in presence of this antibiotic. Patch-clamp technique [2] were used in the study

Mechanical properties of lipid bilayers from molecular dynamics simulation

PubMed Central

Venable, Richard M.; Brown, Frank L.H.; Pastor, Richard W.

2015-01-01

Lipid areas (Aℓ), bilayer area compressibilities (KA), bilayer bending constants (KC), and monolayer spontaneous curvatures (c0) from simulations using the CHARMM36 force field are reported for 12 representative homogenous lipid bilayers. Aℓ (or their surrogate, the average deuterium order parameter in the “plateau region” of the chain) agree very well with experiment, as do the KA. Simulated KC are in near quantitative agreement with vesicle flicker experiments, but are somewhat larger than KC from x-ray, pipette aspiration, and neutron spin echo for saturated lipids. Spontaneous curvatures of bilayer leaflets from the simulations are approximately 30% smaller than experimental values of monolayers in the inverse hexagonal phase. PMID:26238099

Theoretical study on strain induced variations in electronic properties of 2H-MoS{sub 2} bilayer sheets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dong, Liang; Dongare, Avinash M., E-mail: dongare@uconn.edu; Namburu, Raju R.

2014-02-03

The strain dependence of the electronic properties of bilayer sheets of 2H-MoS{sub 2} is studied using ab initio simulations based on density functional theory. An indirect band gap for bilayer MoS{sub 2} is observed for all variations of strain along the basal plane. Several transitions for the indirect band gap are observed for various strains for the bilayer structure. The variation of the band gap and the carrier effective masses for the holes and the electrons for the bilayer MoS{sub 2} structure under conditions of uniaxial strain, biaxial strain, as well as uniaxial stress is investigated.

Dynamic Morphologies and Stability of Droplet Interface Bilayers

NASA Astrophysics Data System (ADS)

Guiselin, Benjamin; Law, Jack O.; Chakrabarti, Buddhapriya; Kusumaatmaja, Halim

2018-06-01

We develop a theoretical framework for understanding dynamic morphologies and stability of droplet interface bilayers (DIBs), accounting for lipid kinetics in the monolayers and bilayer, and droplet evaporation due to imbalance between osmotic and Laplace pressures. Our theory quantitatively describes distinct pathways observed in experiments when DIBs become unstable. We find that when the timescale for lipid desorption is slow compared to droplet evaporation, the lipid bilayer will grow and the droplets approach a hemispherical shape. In contrast, when lipid desorption is fast, the bilayer area will shrink and the droplets eventually detach. Our model also suggests there is a critical size below which DIBs can become unstable, which may explain experimental difficulties in miniaturizing the DIB platform.

Jumping acoustic bubbles on lipid bilayers.

PubMed

Der Loughian, Christelle; Muleki Seya, Pauline; Pirat, Christophe; Inserra, Claude; Béra, Jean-Christophe; Rieu, Jean-Paul

2015-05-07

In the context of sonoporation, we use supported lipid bilayers as a model for biological membranes and investigate the interactions between the bilayer and microbubbles induced by ultrasound. Among the various types of damage caused by bubbles on the surface, our experiments exhibit a singular dynamic interaction process where bubbles are jumping on the bilayer, forming a necklace pattern of alteration on the membrane. This phenomenon was explored with different time and space resolutions and, based on our observations, we propose a model for a microbubble subjected to the combined action of van der Waals, acoustic and hydrodynamic forces. Describing the repeated jumps of the bubble, this model explains the lipid exchanges between the bubble and bilayer.

Heat capacity and transport measurements in sputtered niobium-zirconium multilayers

NASA Astrophysics Data System (ADS)

Broussard, P. R.; Mael, D.

1989-08-01

We have studied the electrical resistivity and heat capacity for multilayers of niobium and zirconium prepared by magnetron sputtering for values of the bilayer period Λ varying from 4 to 950 Å. We find a transition in the thermal part of the resistivity that correlates with the coherent-to-incoherent transition seen in earlier work. The heat capacity data for the normal state show anomalous behavior for both the electronic coefficient γ and the Debye temperature. We also study the variation in Tc and the jump in the specific heat.

Cell membrane causes the lipid bilayers to behave as variable capacitors: A resonance with self-induction of helical proteins.

PubMed

Monajjemi, Majid

2015-12-01

Cell membrane has a unique feature of storing biological energies in a physiologically relevant environment. This study illustrates a capacitor model of biological cell membrane including DPPC structures. The electron density profile models, electron localization function (ELF) and local information entropy have been applied to study the interaction of proteins with lipid bilayers in the cell membrane. The quantum and coulomb blockade effects of different thicknesses in the membrane have also been specifically investigated. It has been exhibited the quantum effects can appear in a small region of the free space within the membrane thickness due to the number and type of phospholipid layers. In addition, from the viewpoint of quantum effects by Heisenberg rule, it is shown the quantum tunneling is allowed in some micro positions while it is forbidden in other forms of membrane capacitor systems. Due to the dynamical behavior of the cell membrane, its capacitance is not fixed which results a variable capacitor. In presence of the external fields through protein trance membrane or ions, charges exert forces that can influence the state of the cell membrane. This causes to appear the charge capacitive susceptibility that can resonate with self-induction of helical coils; the resonance of which is the main reason for various biological pulses. Copyright © 2015 Elsevier B.V. All rights reserved.

Evolution and development of model membranes for physicochemical and functional studies of the membrane lateral heterogeneity.

PubMed

Morigaki, Kenichi; Tanimoto, Yasushi

2018-03-14

One of the main questions in the membrane biology is the functional roles of membrane heterogeneity and molecular localization. Although segregation and local enrichment of protein/lipid components (rafts) have been extensively studied, the presence and functions of such membrane domains still remain elusive. Along with biochemical, cell observation, and simulation studies, model membranes are emerging as an important tool for understanding the biological membrane, providing quantitative information on the physicochemical properties of membrane proteins and lipids. Segregation of fluid lipid bilayer into liquid-ordered (Lo) and liquid-disordered (Ld) phases has been studied as a simplified model of raft in model membranes, including giant unilamellar vesicles (GUVs), giant plasma membrane vesicles (GPMVs), and supported lipid bilayers (SLB). Partition coefficients of membrane proteins between Lo and Ld phases were measured to gauze their affinities to lipid rafts (raftophilicity). One important development in model membrane is patterned SLB based on the microfabrication technology. Patterned Lo/Ld phases have been applied to study the partition and function of membrane-bound molecules. Quantitative information of individual molecular species attained by model membranes is critical for elucidating the molecular functions in the complex web of molecular interactions. The present review gives a short account of the model membranes developed for studying the lateral heterogeneity, especially focusing on patterned model membranes on solid substrates. Copyright © 2018 Elsevier B.V. All rights reserved.

Cellular Localization and Trafficking of the Human ABCG1 Transporter

PubMed Central

Neufeld, Edward B.; O’Brien, Katherine; Walts, Avram D.; Stonik, John A.; Demosky, Steven J.; Malide, Daniela; Combs, Christian A.; Remaley, Alan T.

2014-01-01

We have developed a suitable heterologous cell expression system to study the localization, trafficking, and site(s) of function of the human ABCG1 transporter. Increased plasma membrane (PM) and late endosomal (LE) cholesterol generated by ABCG1 was removed by lipoproteins and liposomes, but not apoA-I. Delivery of ABCG1 to the PM and LE was required for ABCG1-mediated cellular cholesterol efflux. ABCG1 LEs frequently contacted the PM, providing a collisional mechanism for transfer of ABCG1-mobilized cholesterol, similar to ABCG1-mediated PM cholesterol efflux to lipoproteins. ABCG1-mobilized LE cholesterol also trafficked to the PM by a non-vesicular pathway. Transfer of ABCG1-mobilized cholesterol from the cytoplasmic face of LEs to the PM and concomitant removal of cholesterol from the outer leaflet of the PM bilayer by extracellular acceptors suggests that ABCG1 mobilizes cholesterol on both sides of the lipid bilayer for removal by acceptors. ABCG1 increased uptake of HDL into LEs, consistent with a potential ABCG1-mediated cholesterol efflux pathway involving HDL resecretion. Thus, ABCG1 at the PM mobilizes PM cholesterol and ABCG1 in LE/LYS generates mobile pools of cholesterol that can traffic by both vesicular and non-vesicular pathways to the PM where it can also be transferred to extracellular acceptors with a lipid surface. PMID:25405320

Local Control Models of Cardiac Excitation–Contraction Coupling

PubMed Central

Stern, Michael D.; Song, Long-Sheng; Cheng, Heping; Sham, James S.K.; Yang, Huang Tian; Boheler, Kenneth R.; Ríos, Eduardo

1999-01-01

In cardiac muscle, release of activator calcium from the sarcoplasmic reticulum occurs by calcium- induced calcium release through ryanodine receptors (RyRs), which are clustered in a dense, regular, two-dimensional lattice array at the diad junction. We simulated numerically the stochastic dynamics of RyRs and L-type sarcolemmal calcium channels interacting via calcium nano-domains in the junctional cleft. Four putative RyR gating schemes based on single-channel measurements in lipid bilayers all failed to give stable excitation–contraction coupling, due either to insufficiently strong inactivation to terminate locally regenerative calcium-induced calcium release or insufficient cooperativity to discriminate against RyR activation by background calcium. If the ryanodine receptor was represented, instead, by a phenomenological four-state gating scheme, with channel opening resulting from simultaneous binding of two Ca2+ ions, and either calcium-dependent or activation-linked inactivation, the simulations gave a good semiquantitative accounting for the macroscopic features of excitation–contraction coupling. It was possible to restore stability to a model based on a bilayer-derived gating scheme, by introducing allosteric interactions between nearest-neighbor RyRs so as to stabilize the inactivated state and produce cooperativity among calcium binding sites on different RyRs. Such allosteric coupling between RyRs may be a function of the foot process and lattice array, explaining their conservation during evolution. PMID:10051521

Exchange and correlation effects on plasmon dispersions and Coulomb drag in low-density electron bilayers

NASA Astrophysics Data System (ADS)

Badalyan, S. M.; Kim, C. S.; Vignale, G.; Senatore, G.

2007-03-01

We investigate the effect of exchange and correlation (XC) on the plasmon spectrum and the Coulomb drag between spatially separated low-density two-dimensional electron layers. We adopt a different approach, which employs dynamic XC kernels in the calculation of the bilayer plasmon spectra and of the plasmon-mediated drag, and static many-body local field factors in the calculation of the particle-hole contribution to the drag. The spectrum of bilayer plasmons and the drag resistivity are calculated in a broad range of temperatures taking into account both intra- and interlayer correlation effects. We observe that both plasmon modes are strongly affected by XC corrections. After the inclusion of the complex dynamic XC kernels, a decrease of the electron density induces shifts of the plasmon branches in opposite directions. This is in stark contrast with the tendency observed within random phase approximation that both optical and acoustical plasmons move away from the boundary of the particle-hole continuum with a decrease in the electron density. We find that the introduction of XC corrections results in a significant enhancement of the transresistivity and qualitative changes in its temperature dependence. In particular, the large high-temperature plasmon peak that is present in the random phase approximation is found to disappear when the XC corrections are included. Our numerical results at low temperatures are in good agreement with the results of recent experiments by Kellogg [Solid State Commun. 123, 515 (2002)].

Molecular modeling of biomembranes and their complexes with protein transmembrane α-helices

NASA Astrophysics Data System (ADS)

Kuznetsov, Andrey S.; Smirnov, Kirill V.; Antonov, Mikhail Yu.; Nikolaev, Ivan N.; Efremov, Roman G.

2017-11-01

Helical segments are common structural elements of membrane proteins. Dimerization and oligomerization of transmembrane (TM) α-helices provides the framework for spatial structure formation and protein-protein interactions. The membrane itself also takes part in the protein functioning. There are some examples of the mutual influence of the lipid bilayer properties and embedded membrane proteins. This work aims at the detail investigation of protein-lipid interactions using model systems: TM peptides corresponding to native protein segments. Three peptides were considered corresponding to TM domains of human glycophorin A (GpA), epidermal growth factor receptor (EGFR) and proposed TM-segment of human neuraminidase-1 (Neu1). A computational analysis of structural and dynamical properties was performed using molecular dynamics method. Monomers of peptides were considered incorporated into hydrated lipid bilayers. It was confirmed, that all these TM peptides have stable helical conformation in lipid environment, and the mutual adaptation of peptides and membrane was observed. It was shown that incorporation of the peptide into membrane results in the modulation of local and mean structural properties of the bilayer. Each peptide interacts with lipid acyl chains having special binding sites on the surface of central part of α-helix that exist for at least 200 ns. However, lipid acyl chains substitute each other faster occupying the same site. The formation of a special pattern of protein-lipid interactions may modulate the association of TM domains of membrane proteins, so membrane environment should be considered when proposing new substances targeting cell receptors.

Lipids, curvature, and nano-medicine*

PubMed Central

Mouritsen, Ole G

2011-01-01

The physical properties of the lamellar lipid-bilayer component of biological membranes are controlled by a host of thermodynamic forces leading to overall tensionless bilayers with a conspicuous lateral pressure profile and build-in curvature-stress instabilities that may be released locally or globally in terms of morphological changes. In particular, the average molecular shape and the propensity of the different lipid and protein species for forming non-lamellar and curved structures are a source of structural transitions and control of biological function. The effects of different lipids, sterols, and proteins on membrane structure are discussed and it is shown how one can take advantage of the curvature-stress modulations brought about by specific molecular agents, such as fatty acids, lysolipids, and other amphiphilic solutes, to construct intelligent drug-delivery systems that function by enzymatic triggering via curvature. Practical applications: The simple concept of lipid molecular shape and how it impacts on the structure of lipid aggregates, in particular the curvature and curvature stress in lipid bilayers and liposomes, can be exploited to construct liposome-based drug-delivery systems, e.g., for use as nano-medicine in cancer therapy. Non-lamellar-forming lysolipids and fatty acids, some of which may be designed to be prodrugs, can be created by phospholipase action in diseased tissues thereby providing for targeted drug release and proliferation of molecular entities with conical shape that break down the permeability barrier of the target cells and may hence enhance efficacy. PMID:22164124

Ultrasound, liposomes, and drug delivery: principles for using ultrasound to control the release of drugs from liposomes.

PubMed

Schroeder, Avi; Kost, Joseph; Barenholz, Yechezkel

2009-11-01

Ultrasound is used in many medical applications, such as imaging, blood flow analysis, dentistry, liposuction, tumor and fibroid ablation, and kidney stone disruption. In the past, low frequency ultrasound (LFUS) was the main method to downsize multilamellar (micron range) vesicles into small (nano scale) unilamellar vesicles. Recently, the ability of ultrasound to induce localized and controlled drug release from liposomes, utilizing thermal and/or mechanical effects, has been shown. This review, deals with the interaction of ultrasound with liposomes, focusing mainly on the mechanical mechanism of drug release from liposomes using LFUS. The effects of liposome lipid composition and physicochemical properties, on one hand, and of LFUS parameters, on the other, on liposomal drug release, are addressed. Acoustic cavitation, in which gas bubbles oscillate and collapse in the medium, thereby introducing intense mechanical strains, increases release substantially. We suggest that the mechanism of release may involve formation and collapse of small gas nuclei in the hydrophobic region of the lipid bilayer during exposure to LFUS, thereby inducing the formation of transient pores through which drugs are released. Introducing PEG-lipopolymers to the liposome bilayer enhances responsivity to LFUS, most likely due to absorption of ultrasonic energy by the highly hydrated PEG headgroups. The presence of amphiphiles, such as phospholipids with unsaturated acyl chains, which destabilize the lipid bilayer, also increases liposome susceptibility to LFUS. Application of these principles to design highly LFUS-responsive liposomes is discussed.

Topological Valley Transport at Bilayer Graphene Domain Walls

DTIC Science & Technology

2015-04-22

2015. Published online 22 April 2015. 1. McCann, E. Asymmetry gap in the electronic band structure of bilayer graphene . Phys. Rev. B 74, 161403 (2006...6. Yao, W., Yang, S. A. & Niu, Q. Edge states in graphene : from gapped flat- band to gapless chiral modes. Phys. Rev. Lett. 102, 096801 (2009). 7...induced in bilayer graphene by an external electric field1–5, and such gapped bilayer graphene is predicted to be a topo- logical insulating phase

Subterahertz Longitudinal Phonon Modes Propagating in a Lipid Bilayer Immersed in an Aqueous Medium

NASA Astrophysics Data System (ADS)

Zakhvataev, V. E.

2018-04-01

The properties of subterahertz longitudinal acoustic phonon modes in the hydrophobic region of a lipid bilayer immersed in a compressible viscous aqueous medium are investigated theoretically. An approximate expression is obtained for the Mandelstam-Brillouin components of the dynamic structure factor of a bilayer. The analysis is based on a generalized hydrodynamic model of the "two-dimensional lipid bilayer + three-dimensional fluid medium" system, as well as on known sharp estimates for the frequencies and lifetimes of long-wavelength longitudinal acoustic phonons in a free hydrated lipid bilayer and in water, obtained from inelastic X-ray scattering experiments and molecular dynamics simulations. It is shown that, for characteristic values of the parameters of the membrane system, subterahertz longitudinal phonon-like excitations in the hydrophobic part of the bilayer are underdamped. In this case, the contribution of the viscous flow of the aqueous medium to the damping of a longitudinal membrane mode is small compared with the contribution of the lipid bilayer. Quantitative estimates of the damping ratio agree well with the experimental results for the vibration mode of the enzyme lysozyme in aqueous solution [1]. It is also shown that a coupling between longitudinal phonon modes of the bilayer and relaxation processes in its fluid environment gives rise to an additional peak in the scattering spectrum, which corresponds to a non-propagating mode.

Salt-induced effects on natural and inverse DPPC lipid membranes: Molecular dynamics simulation.

PubMed

Rezaei Sani, Seyed Mojtaba; Akhavan, Mojdeh; Jalili, Seifollah

2018-08-01

Molecular dynamics (MD) simulations of a dipalmitoylphosphatidylcholine (DPPC) bilayer and its neutral inverse-phosphocholine equivalent (DPCPe) were performed to find salt-induced effects on their surface structure and the nature of ion-lipid interactions. We found that the area per lipid is not considerably affected by the inversion, but the deuterium order parameter of carbon atoms in the region of carbonyl carbons changes dramatically. MD simulations indicate that Ca 2+ ions can bind to the surface of both DPPC and DPCPe membranes, but K + ions do not bind to them. In the case of Na + , however, the ions can bind to natural lipids but not to the inverse ones. Also, our results demonstrate that the hydration level of CPe bilayers is substantially lower than PC bilayers and the averaged orientation of water dipoles in the region of CPe headgroups is effectively inverted compared to PC lipids. This might be important in the interaction of the bilayer with its biological environment. Furthermore, it was found for the CPe bilayers that the enhanced peaks of the electrostatic potential profiles shift further away from the bilayer center relative to those of PC bilayers. This behavior makes the penetration of cations into the bilayer more difficult and possibly explains the experimentally observed enhanced release rates of anionic compounds in the CPe membrane. Copyright © 2018 Elsevier B.V. All rights reserved.

The Molecular Structure of a Phosphatidylserine Bilayer Determined by Scattering and Molecular Dynamics Simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pan, Jianjun; Cheng, Xiaolin; Monticelli, Luca

2014-01-01

Phosphatidylserine (PS) lipids play essential roles in biological processes, including enzyme activation and apoptosis. We report on the molecular structure and atomic scale interactions of a fluid bilayer composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylserine (POPS). A scattering density profile model, aided by molecular dynamics (MD) simulations, was developed to jointly refine different contrast small-angle neutron and X-ray scattering data, which yielded a lipid area of 62.7 A2 at 25 C. MD simulations with POPS lipid area constrained at different values were also performed using all-atom and aliphatic united-atom models. The optimal simulated bilayer was obtained using a model-free comparison approach. Examination of themore » simulated bilayer, which agrees best with the experimental scattering data, reveals a preferential interaction between Na+ ions and the terminal serine and phosphate moieties. Long-range inter-lipid interactions were identified, primarily between the positively charged ammonium, and the negatively charged carboxylic and phosphate oxygens. The area compressibility modulus KA of the POPS bilayer was derived by quantifying lipid area as a function of surface tension from area-constrained MD simulations. It was found that POPS bilayers possess a much larger KA than that of neutral phosphatidylcholine lipid bilayers. We propose that the unique molecular features of POPS bilayers may play an important role in certain physiological functions.« less

Variations of thermoelectric performance by electric fields in bilayer MX2 (M = W, Mo; X = S, Se).

PubMed

Wang, Rui-Ning; Dong, Guo-Yi; Wang, Shu-Fang; Fu, Guang-Sheng; Wang, Jiang-Long

2017-02-22

A gate electrode is usually used to controllably tune the carrier concentrations, further modulating the electrical conductivity and the Seebeck coefficient to obtain the optimum thermoelectric figure of merit (ZT) in two-dimensional materials. On the other hand, it is necessary to investigate how an electric field induced by a gate voltage affects the electronic structures, further determining the thermoelectric properties. Therefore, by using density functional calculations in combination with Boltzmann theory, the thermoelectric properties of bilayer MX 2 (M = W, Mo; X = S, Se) with or without a 1 V nm -1 perpendicular electric field are comparatively investigated. First of all, the variations of the electrical conductivity (σ), electron thermal conductivity and Seebeck coefficient (S) with the carrier concentration are studied. Due to the trade-off relationship between S and σ, there is an optimum concentration to obtain the maximum ZT, which increases with the temperature due to the enhancement of the Seebeck coefficient. Moreover, N-type bilayers have larger optimum ZTs than P-type bilayers. In addition, the electric field results in the increase of the Seebeck coefficient in low hole-doped MS 2 bilayers and high hole-doped MSe 2 bilayers, thus leading to similar variations in ZT. The optimum ZTs are reduced from 2.11 × 10 -2 , 3.19 × 10 -2 , 2.47 × 10 -2 , and 2.58 × 10 -2 to 1.57 × 10 -2 , 1.51 × 10 -2 , 2.08 × 10 -2 , and 1.43 × 10 -2 for the hole-doped MoS 2 , MoSe 2 , and WSe 2 bilayers, respectively. For N-type bilayers, the electric field shows a destructive effect, resulting in the obvious reduction of the Seebeck coefficient in the MSe 2 layers and the low electron-doped MS 2 bilayers. In electron-doped bilayers, the optimum ZTs will decrease from 3.03 × 10 -2 , 6.64 × 10 -2 , and 6.69 × 10 -2 to 2.81 × 10 -2 , 3.59 × 10 -2 , and 4.39 × 10 -2 for the MoS 2 , MoSe 2 , and WSe 2 bilayers, respectively.

Molecular dynamics simulations of glyphosate in a DPPC lipid bilayer.

PubMed

Frigini, Ezequiel N; López Cascales, J J; Porasso, Rodolfo D

2018-07-01

Extensive molecular dynamics simulations have been performed to study the effect of glyphosate (in their neutral and charged forms, GLYP and GLYP 2- , respectively) on fully hydrated DiPalmitoylPhosphatidylCholine (DPPC) lipid bilayer. First, we calculated the free energy profile (using the Umbrella Sampling technique) for both states of charge of glyphosate. The minimum value for the free energy for GLYP is ∼-60 kJ mol -1 located at z = ±1.7 nm (from the lipid bilayer center), and there is almost no maximum at the center of the lipid bilayer. By contrast, the minimum for GLYP 2- is ∼-35 kJ mol -1 located at z = ± 1.4 nm (from the lipid bilayer center), and the maximum reaches ∼35 kJ mol -1 at the center of the lipid bilayer. Then, different lipid bilayer properties were analyzed for different glyphosate:lipid (G:L) ratios. The mean area per lipid was slightly affected, increasing only 5% (in the presence of glyphosate at high concentrations), which is in agreement with the slight decrease in deuterium order parameters. As for the thickness of the bilayer, it is observed that the state of charge produces opposite effects. On one hand, the neutral state produces an increase in the thickness of the lipid bilayer; on the other, the charged form produces a decrease in the thickness, which not depend linearly on the G:L ratios, either. The orientation of the DPPC head groups is practically unaffected throughout the range of the G:L ratios studied. Finally, the mobility of the lipids of the bilayer is strongly affected by the presence of glyphosate, considerably increasing its lateral diffusion coefficient noteworthy (one order of magnitude), with increasing G:L ratio. Copyright © 2018 Elsevier B.V. All rights reserved.

All-Atom Molecular Dynamics-Based Analysis of Membrane-Stabilizing Copolymer Interactions with Lipid Bilayers Probed under Constant Surface Tensions.

PubMed

Houang, Evelyne M; Bates, Frank S; Sham, Yuk Y; Metzger, Joseph M

2017-11-30

An all-atom phospholipid bilayer and triblock copolymer model was developed for molecular dynamics (MD) studies. These were performed to investigate the mechanism of interaction between membrane-stabilizing triblock copolymer P188 and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) lipid bilayers under applied lateral surface tension (γ) to model membrane mechanical stress. Results showed that P188 insertion is driven by the hydrophobic poly(propylene oxide) (PPO) core and dependent on bilayer area per lipid. Moreover, insertion of P188 increased the bilayer's resistance to mechanical rupture, as observed by a significant increase in the absolute lateral pressure required to disrupt the bilayer. To further investigate the specific chemical features of P188 underlying membrane stabilizer function, a series of MD simulations with triblock copolymers of the same class as P188 but of varying chemical composition and sizes were performed. Results showed that triblock copolymer insertion into the lipid bilayer is dependent on overall copolymer hydrophobicity, with higher copolymer hydrophobicity requiring a reduced bilayer area per lipid ratio for insertion. Further analysis revealed that the effect of copolymer insertion on membrane mechanical integrity was also dependent on hydrophobicity. Here, P188 insertion significantly increased the absolute apparent lateral pressure required to rupture the POPC bilayer, thereby protecting the membrane against mechanical stress. In marked contrast, highly hydrophobic copolymers decreased the lateral pressure necessary for membrane rupture and thus rendering the membrane significantly more susceptible to mechanical stress. These new in silico findings align with recent experimental findings using synthetic lipid bilayers and in muscle cells in vitro and mouse models in vivo. Collectively, these data underscore the importance of PEO-PPO-PEO copolymer chemical composition in copolymer-based muscle membrane stabilization in vitro and in vivo. All-atom modeling with MD simulations holds promise for investigating novel copolymers with enhanced membrane interacting properties.

Membrane solubilisation and reconstitution by octylglucoside: comparison of synthetic lipid and natural lipid extract by isothermal titration calorimetry.

PubMed

Krylova, Oxana O; Jahnke, Nadin; Keller, Sandro

2010-08-01

We have studied the solubilisation and reconstitution of lipid membranes composed of either synthetic phosphatidylcholine or Escherichia. coli polar lipid extract by the non-ionic detergent octylglucoside. For both lipid systems, composition-dependent transformations of unilamellar vesicles into micelles or vice versa were followed by high-sensitivity isothermal titration calorimetry. Data obtained over a range of detergent and lipid concentrations could be rationalised in terms of a three-stage phase separation model involving bilayer, bilayer/micelle coexistence, and micellar ranges, yielding the detergent/lipid phase diagrams and the bilayer-to-micelle partition coefficients of both detergent and lipid. The most notable difference between the lipids investigated was a substantial widening of the bilayer/micelle coexistence range for E. coli lipid, which was due to an increased preference of the detergent and a decreased affinity of the lipid for the micellar phase as compared with the bilayer phase. These effects on the bilayer-to-micelle partition coefficients could be explained by the high proportion in E. coli membranes of lipids possessing negative spontaneous curvature, which hampers both their transfer into strongly curved micellar structures as well as the insertion of detergent into condensed bilayers.

Cyclic and Linear Monoterpenes in Phospholipid Membranes: Phase Behavior, Bilayer Structure, and Molecular Dynamics.

PubMed

Pham, Quoc Dat; Topgaard, Daniel; Sparr, Emma

2015-10-13

Monoterpenes are abundant in essential oils extracted from plants. These relatively small and hydrophobic molecules have shown important biological functions, including antimicrobial activity and membrane penetration enhancement. The interaction between the monoterpenes and lipid bilayers is considered important to the understanding of the biological functions of monoterpenes. In this study, we investigated the effect of cyclic and linear monoterpenes on the structure and dynamics of lipids in model membranes. We have studied the ternary system 1,2-dimyristoyl-sn-glycero-3-phosphocholine-monoterpene-water as a model with a focus on dehydrated conditions. By combining complementary techniques, including differential scanning calorimetry, solid-state nuclear magnetic resonance, and small- and wide-angle X-ray scattering, bilayer structure, phase transitions, and lipid molecular dynamics were investigated at different water contents. Monoterpenes cause pronounced melting point depression and phase segregation in lipid bilayers, and the extent of these effects depends on the hydration conditions. The addition of a small amount of thymol to the fluid bilayer (volume fraction of 0.03 in the bilayer) leads to an increased order in the acyl chain close to the bilayer interface. The findings are discussed in relation to biological systems and lipid formulations.

Hematite/silica nanoparticle bilayers on mica: AFM and electrokinetic characterization.

PubMed

Morga, Maria; Adamczyk, Zbigniew; Kosior, Dominik; Oćwieja, Magdalena

2018-06-06

Quantitative studies on self-assembled hematite/silica nanoparticle (NP) bilayers on mica were performed by applying scanning electron microscopy (SEM), atomic force microscopy (AFM), and streaming potential measurements. The coverage of the supporting hematite layers was adjusted by changing the bulk concentration of the suspension and the deposition time. The coverage was determined by direct enumeration of deposited particles from AFM images and SEM micrographs. Afterward, silica nanoparticle monolayers were assembled under diffusion-controlled transport. A unique functional relationship was derived connecting the silica coverage with the hematite precursor layer coverage. The formation of the hematite monolayer and the hematite/silica bilayer was also monitored in situ by streaming potential measurements. It was confirmed that the zeta potential of the bilayers was independent of the supporting layer coverage, exceeding 0.15. These measurements were theoretically interpreted in terms of the general electrokinetic model that allowed for deriving a formula for calculating nanoparticle coverage in the bilayers. Additionally, from desorption experiments, the interactions among hematite/silica particles in the bilayers were determined using DLVO theory. These results facilitate the development of a robust method of preparing nanoparticle bilayers with controlled properties, with potential applications in catalytic processes.

Theoretical investigation of structural and optical properties of semi-fluorinated bilayer graphene

NASA Astrophysics Data System (ADS)

Xiao-Jiao, San; Bai, Han; Jing-Geng, Zhao

2016-03-01

We have studied the structural and optical properties of semi-fluorinated bilayer graphene using density functional theory. When the interlayer distance is 1.62 Å, the two graphene layers in AA stacking can form strong chemical bonds. Under an in-plane stress of 6.8 GPa, this semi-fluorinated bilayer graphene becomes the energy minimum. Our calculations indicate that the semi-fluorinated bilayer graphene with the AA stacking sequence and rectangular fluorinated configuration is a nonmagnetic semiconductor (direct gap of 3.46 eV). The electronic behavior at the vicinity of the Fermi level is mainly contributed by the p electrons of carbon atoms forming C=C double bonds. We compare the optical properties of the semi-fluorinated bilayer graphene with those of bilayer graphene stacked in the AA sequence and find that the semi-fluorinated bilayer graphene is anisotropic for the polarization vector on the basal plane of graphene and a red shift occurs in the [010] polarization, which makes the peak at the low-frequency region located within visible light. This investigation is useful to design polarization-dependence optoelectronic devices. Project supported by the Program of Educational Commission of Heilongjiang Province, China (Grant No. 12541131).

Modulating interactions between ligand-coated nanoparticles and phase-separated lipid bilayers by varying the ligand density and the surface charge.

PubMed

Chen, Xiaojie; Tieleman, D Peter; Liang, Qing

2018-02-01

The interactions between nanoparticles and lipid bilayers are critical in applications of nanoparticles in nanomedicine, cell imaging, toxicology, and elsewhere. Here, we investigate the interactions between nanoparticles coated with neutral and/or charged ligands and phase-separated lipid bilayers using coarse-grained molecular dynamics simulation. Both penetration and adsorption processes as well as the final distribution of the nanoparticles can be readily modulated by varying the ligand density and the surface charge of the nanoparticles. Completely hydrophobic (neutral) nanoparticles with larger size initially preferentially penetrate into the liquid-disordered region of the lipid bilayer and finally transfer into the liquid-ordered region; partially hydrophilic nanoparticles with low or moderate surface charge tend to either distribute in the liquid-disordered region or be adsorbed on the surface of the lipid bilayer, while strongly hydrophilic nanoparticles with high surface charge always reside on the surface of the lipid bilayer. Interactions of the nanoparticles with the lipid bilayers are affected by the surface charge of nanoparticles, hydrophobic mismatch, bending of the ligands, and the packing state of the lipids. Insight in these factors can be used to improve the efficiency of designing nanoparticles for specific applications.

Photo-Sensitivity of Large Area Physical Vapor Deposited Mono and Bilayer MoS2 (Postprint)

DTIC Science & Technology

2014-07-01

layer MoS2 without any apparent rectifying junctions , making device fabrication straightforward. For bi-layers, no such effect was present, suggesting...layer MoS2 without any apparent rectifying junctions , making device fabrication straightforward. For bi-layers, no such effect was present, suggesting...pressure below 5×10−9 Torr for atomically sharp and clean interfaces. The mono and bi-layer specimens were grown on 100 nm thick thermal oxide coated silicon

Engineering plant membranes using droplet interface bilayers.

PubMed

Barlow, N E; Smpokou, E; Friddin, M S; Macey, R; Gould, I R; Turnbull, C; Flemming, A J; Brooks, N J; Ces, O; Barter, L M C

2017-03-01

Droplet interface bilayers (DIBs) have become widely recognised as a robust platform for constructing model membranes and are emerging as a key technology for the bottom-up assembly of synthetic cell-like and tissue-like structures. DIBs are formed when lipid-monolayer coated water droplets are brought together inside a well of oil, which is excluded from the interface as the DIB forms. The unique features of the system, compared to traditional approaches (e.g., supported lipid bilayers, black lipid membranes, and liposomes), is the ability to engineer multi-layered bilayer networks by connecting multiple droplets together in 3D, and the capability to impart bilayer asymmetry freely within these droplet architectures by supplying droplets with different lipids. Yet despite these achievements, one potential limitation of the technology is that DIBs formed from biologically relevant components have not been well studied. This could limit the reach of the platform to biological systems where bilayer composition and asymmetry are understood to play a key role. Herein, we address this issue by reporting the assembly of asymmetric DIBs designed to replicate the plasma membrane compositions of three different plant species; Arabidopsis thaliana , tobacco, and oats, by engineering vesicles with different amounts of plant phospholipids, sterols and cerebrosides for the first time. We show that vesicles made from our plant lipid formulations are stable and can be used to assemble asymmetric plant DIBs. We verify this using a bilayer permeation assay, from which we extract values for absolute effective bilayer permeation and bilayer stability. Our results confirm that stable DIBs can be assembled from our plant membrane mimics and could lead to new approaches for assembling model systems to study membrane translocation and to screen new agrochemicals in plants.

Can NO-indomethacin counteract the topical gastric toxicity induced by indomethacin interactions with phospholipid bilayers?

PubMed

Pereira-Leite, Catarina; Nunes, Cláudia; Bozelli, José C; Schreier, Shirley; Kamma-Lorger, Christina S; Cuccovia, Iolanda M; Reis, Salette

2018-05-23

Nitric oxide (NO)-releasing nonsteroidal anti-inflammatory drugs (NSAIDs) have been developed to overcome the gastrointestinal and cardiovascular toxicity of NSAIDs, by chemically associating a NO-releasing moiety with commercial NSAIDs. Since increasing evidence supports that NSAIDs toxicity is related to their topical actions in membrane lipids, this work aims to evaluate the impact of adding a NO-releasing moiety to parent NSAIDs regarding their effect on lipid bilayers. Thus, the interactions of NO-indomethacin and indomethacin (parent drug) with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers were described herein at pH 3.0 and 7.4. Diverse experimental techniques were combined to characterize the partitioning and location of drugs in DMPC bilayers, and to analyze their effect on the lipid phase transition and the bilayer structure and dynamics. The partitioning of NO-indomethacin into DMPC bilayers was similar to that of charged indomethacin and smaller than that of neutral indomethacin. Both drugs were found to insert the DMPC bilayer and the membrane location of indomethacin was pH-dependent. NO-indomethacin and indomethacin induced a decrease of the main phase transition temperature of DMPC. The effect of these drugs on the bilayer structure and dynamics was dependent on diverse factors, namely drug ionization state, drug:lipid molar ratio, temperature and lipid phase. It is noteworthy that NO-indomethacin induced more pronounced alterations in the biophysical properties of DMPC bilayers than indomethacin, considering equivalent membrane concentrations. Such modifications may have in vivo implications, particularly in the gastric mucosa, where NO-NSAIDs-induced changes in the protective properties of phospholipid layers may contribute to the occurrence of adverse effects. Copyright © 2018 Elsevier B.V. All rights reserved.

Two-Phase Contiguous Supported Lipid Bilayer Model for Membrane Rafts via Polymer Blotting and Stenciling.

PubMed

Richards, Mark J; Daniel, Susan

2017-02-07

The supported lipid bilayer has been portrayed as a useful model of the cell membrane compatible with many biophysical tools and techniques that demonstrate its appeal in learning about the basic features of the plasma membrane. However, some of its potential has yet to be realized, particularly in the area of bilayer patterning and phase/composition heterogeneity. In this work, we generate contiguous bilayer patterns as a model system that captures the general features of membrane domains and lipid rafts. Micropatterned polymer templates of two types are investigated for generating patterned bilayer formation: polymer blotting and polymer lift-off stenciling. While these approaches have been used previously to create bilayer arrays by corralling bilayers patches with various types of boundaries impenetrable to bilayer diffusion, unique to the methods presented here, there are no physical barriers to diffusion. In this work, interfaces between contiguous lipid phases define the pattern shapes, with continuity between them allowing transfer of membrane-bound biomolecules between the phases. We examine effectors of membrane domain stability including temperature and cholesterol content to investigate domain dynamics. Contiguous patterning of supported bilayers as a model of lipid rafts expands the application of the SLB to an area with current appeal and brings with it a useful toolset for characterization and analysis. These combined tools should be helpful to researchers investigating lipid raft dynamics and function and biomolecule partitioning studies. Additionally, this patterning technique may be useful for applications such as bioseparations that exploit differences in lipid phase partitioning or creation of membranes that bind species like viruses preferentially at lipid phase boundaries, to name a few.

Comparison of breaking tests for the characterization of the interfacial strength of bilayer tablets.

PubMed

Castrati, Luca; Mazel, Vincent; Busignies, Virginie; Diarra, Harona; Rossi, Alessandra; Colombo, Paolo; Tchoreloff, Pierre

2016-11-20

The bilayer tableting technology is gaining more acceptance in the drug industry, due to its ability to improve the drug delivery strategies. It is currently assessed by the European Pharmacopoeia, that the mechanical strength of tablets can be evaluated using a diametral breaking tester. This device applies a force diametrically, and records the tablet breaking point. This approach has been used to measure the structural integrity of single layer tablets as well as bilayer (and multi-layer) tablets. The latter ones, however, have a much complex structure. Therefore, testing a bilayer tablet with the currently used breaking test methodology might not be appropriate. The aim of this work was to compare results from several tests that have been proposed to quantify the interfacial strength of bilayer tablets. The obtained results would provide an indication on which tests are appropriate to evaluate the robustness of a bilayer tablet. Bilayer tablets were fabricated using a model formulation: Microcrystalline Cellulose (MCC) for the first layer, and spray dried lactose (SDLac) as second layer. Each set of tablets were tested using the following tests: Diametral Test, Shear Test and Indentation Test. The tablets were examined before and after the breaking test using Scanning Electron Microscopy (SEM). When a bilayer tablet was subjected to shearing or indentation, it showed signs of clear delamination. Differently, using the diametral test system, the tablets showed no clear difference, before and after the testing. However, when examining each layer via SEM, it was clear that a fracture occurred in the layer made of SDLac. Thus, the diametral test is a measure of the strength of one of the two layers and therefore it is not suited to test the mechanical strength of bilayer tablets. Copyright © 2016 Elsevier B.V. All rights reserved.

Performance Enhancement of Small Molecular Solar Cells by Bilayer Cathode Buffer.

PubMed

Sun, Qinjun; Zhao, Huanbin; Zhou, Miao; Gao, Liyan; Hao, Yuying

2016-04-01

An effective composite bilayer cathode buffer structure is proposed for use in small molecular solar cells. CsF was doped in Alq3 to form the first cathode buffer, leading to small serial resistances. BCP was used as the second cathode buffer to block the holes to the electrode. The optimized bilayer cathode buffer significantly increased the short circuit and fill factor of devices. By integrating this bilayer cathode buffer, the CuPc/C60 small molecular heterojunction cell exhibited a power conversion efficiency of up to 0.8%, which was an improvement of 56% compared to a device with only the Alq3 cathode buffer. Meanwhile, the bilayer cathode buffer still has a good protective effect on the performance of the device.

Neutron diffraction from aligned stacks of lipid bilayers using the WAND instrument

DOE PAGES

Marquardt, Drew; Frontzek, Matthias D.; Zhao, Yu; ...

2018-02-06

Neutron diffraction from aligned stacks of lipid bilayers is examined using the Wide-Angle Neutron Diffractometer (WAND), located at the High Flux Isotope Reactor, Oak Ridge, Tennessee, USA. Data were collected at different levels of hydration and neutron contrast by varying the relative humidity (RH) and H 2O/D 2O ratio from multi-bilayers of dioleoylphosphatidylcholine and sunflower phosphatidylcholine extract aligned on single-crystal silicon substrates. This work highlights the capabilites of a newly fabricated sample hydration cell, which allows the lipid bilayers to be hydrated with varying H/D ratios from the RH generated by saturated salt solutions, and also demonstrates WAND's capability asmore » an instrument suitable for the study of aligned lipid multi-bilayers.« less

Neutron diffraction from aligned stacks of lipid bilayers using the WAND instrument

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marquardt, Drew; Frontzek, Matthias D.; Zhao, Yu

Neutron diffraction from aligned stacks of lipid bilayers is examined using the Wide-Angle Neutron Diffractometer (WAND), located at the High Flux Isotope Reactor, Oak Ridge, Tennessee, USA. Data were collected at different levels of hydration and neutron contrast by varying the relative humidity (RH) and H 2O/D 2O ratio from multi-bilayers of dioleoylphosphatidylcholine and sunflower phosphatidylcholine extract aligned on single-crystal silicon substrates. This work highlights the capabilites of a newly fabricated sample hydration cell, which allows the lipid bilayers to be hydrated with varying H/D ratios from the RH generated by saturated salt solutions, and also demonstrates WAND's capability asmore » an instrument suitable for the study of aligned lipid multi-bilayers.« less

Agar/gelatin bilayer gel matrix fabricated by simple thermo-responsive sol-gel transition method.

PubMed

Wang, Yifeng; Dong, Meng; Guo, Mengmeng; Wang, Xia; Zhou, Jing; Lei, Jian; Guo, Chuanhang; Qin, Chaoran

2017-08-01

We present a simple and environmentally-friendly method to generate an agar/gelatin bilayer gel matrix for further biomedical applications. In this method, the thermally responsive sol-gel transitions of agar and gelatin combined with the different transition temperatures are exquisitely employed to fabricate the agar/gelatin bilayer gel matrix and achieve separate loading for various materials (e.g., drugs, fluorescent materials, and nanoparticles). Importantly, the resulting bilayer gel matrix provides two different biopolymer environments (a polysaccharide environment vs a protein environment) with a well-defined border, which allows the loaded materials in different layers to retain their original properties (e.g., magnetism and fluorescence) and reduce mutual interference. In addition, the loaded materials in the bilayer gel matrix exhibit an interesting release behavior under the control of thermal stimuli. Consequently, the resulting agar/gelatin bilayer gel matrix is a promising candidate for biomedical applications in drug delivery, controlled release, fluorescence labeling, and bio-imaging. Copyright © 2017 Elsevier B.V. All rights reserved.

First-principles study on interlayer state in alkali and alkaline earth metal atoms intercalated bilayer graphene

NASA Astrophysics Data System (ADS)

Kaneko, Tomoaki; Saito, Riichiro

2017-11-01

Energetics and electronic structures of alkali metal (Li, Na, K, Rb, and Cs) and alkaline earth metal (Be, Mg, Ca, Sr, and Ba) atoms intercalated bilayer graphene are systematically investigated using first-principles calculations based on density functional theory. Formation of alkali and alkaline earth metal atoms intercalated bilayer graphene is exothermic except for Be and Mg. The interlayer state between two graphene layers is occupied for K, Rb, Cs, Ca, Sr, and Ba. We find that the energetic position of the interlayer states between bilayer graphene monotonically shifts downward with increasing of interlayer distance. The interlayer distances of more than 4.5 Å and 4.0 Å, respectively, are necessary for the occupation of the interlayer state in bilayer graphene for alkali and alkaline earth metal atoms, which is almost independent of the intercalant metal species. We discuss the relevance to occurrence of superconductivity for the metal intercalated bilayer graphene in terms of the occupation of the interlayer state and the phonon frequency of metal ions.

Consequences of ions and pH on the supramolecular organization of sphingomyelin and sphingomyelin/cholesterol bilayers.

PubMed

Chemin, Caroline; Bourgaux, Claudie; Péan, Jean-Manuel; Pabst, Georg; Wüthrich, Patrick; Couvreur, Patrick; Ollivon, Michel

2008-06-01

For drug delivery purpose the anticancer drug S12363 was loaded into ESM/Chol-liposomes using either a pH or an ammonium gradient. Association between the drug and the liposome depends markedly on the liposome membrane structure. Thus, ESM and ESM/Chol bilayer organization had been characterized by coupled DSC and XRDT as a function of both cholesterol concentration and aqueous medium composition. ESM bilayers exhibited a ripple lamellar gel phase P(beta') below the melting temperature and adopted a L(beta)-like gel phase upon Chol insertion. Supramolecular organization of ESM and ESM/Chol bilayers was not modified by citrate buffer or ammonium sulfate solution whatever the pH (3< or = pH < or =7). Nevertheless, in ESM bilayer, ammonium sulfate salt induced a peculiar organization of head groups, leading to irregular d-spacing and weakly correlated bilayers. Moreover, in the presence of salts, a weakening of van der Waals attraction forces was seen and led to a swelling of the water layer.

Fusion of single proteoliposomes with planar, cushioned bilayers in microfluidic flow cells

PubMed Central

Karatekin, Erdem; Rothman, James E.

2013-01-01

Many biological processes rely on membrane fusion, therefore assays to study its mechanisms are necessary. Here we report an assay with sensitivity to single-vesicle, even to single-molecule events using fluorescently labeled vesicle-associated v-SNARE liposomes and target-membrane-associated t-SNARE-reconstituted planar, supported bilayers (SBLs). Docking and fusion events can be detected using conventional far-field epifluorescence or total internal reflection fluorsecence microscopy. Unlike most previous attempts, fusion here is dependent on SNAP25, one of the t-SNARE subunits that is required for fusion in vivo. The success of the assay is due to the use of (i) bilayers covered with a thin layer of poly(ethylene glycol) to control bilayer-bilayer and bilayer-substrate interactions, (ii) microfluidic flow channels which presents many advantages such as the removal of non-specifically bound liposomes by flow. The protocol takes 6–8 days to complete. Analysis can take up to two weeks. PMID:22517259

Revealing the preferred interlayer orientations and stackings of two-dimensional bilayer gallium selenide crystals.

PubMed

Li, Xufan; Basile, Leonardo; Yoon, Mina; Ma, Cheng; Puretzky, Alexander A; Lee, Jaekwang; Idrobo, Juan C; Chi, Miaofang; Rouleau, Christopher M; Geohegan, David B; Xiao, Kai

2015-02-23

Characterizing and controlling the interlayer orientations and stacking orders of two-dimensional (2D) bilayer crystals and van der Waals (vdW) heterostructures is crucial to optimize their electrical and optoelectronic properties. The four polymorphs of layered gallium selenide (GaSe) crystals that result from different layer stackings provide an ideal platform to study the stacking configurations in 2D bilayer crystals. Through a controllable vapor-phase deposition method, bilayer GaSe crystals were selectively grown and their two preferred 0° or 60° interlayer rotations were investigated. The commensurate stacking configurations (AA' and AB stacking) in as-grown bilayer GaSe crystals are clearly observed at the atomic scale, and the Ga-terminated edge structure was identified using scanning transmission electron microscopy. Theoretical analysis reveals that the energies of the interlayer coupling are responsible for the preferred orientations among the bilayer GaSe crystals. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Strain, stabilities and electronic properties of hexagonal BN bilayers

NASA Astrophysics Data System (ADS)

Fujimoto, Yoshitaka; Saito, Susumu

Hexagonal boron nitride (h-BN) atomic layers have been regarded as fascinating materials both scientifically and technologically due to the sizable band gap. This sizable band-gap nature of the h-BN atomic layers would provide not only new physical properties but also novel nano- and/or opto-electronics applications. Here, we study the first-principles density-functional study that clarifies the biaxial strain effects on the energetics and the electronic properties of h-BN bilayers. We show that the band gaps of the h-BN bilayers are tunable by applying strains. Furthermore, we show that the biaxial strains can produce a transition from indirect to direct band gaps of the h-BN bilayer. We also discuss that both AA and AB stacking patterns of h-BN bilayer become feasible structures because h-BN bilayers possess two different directions in the stacking patterns. Supported by MEXT Elements Strategy Initiative to Form Core Research Center through Tokodai Institute for Element Strategy, JSPS KAKENHI Grant Numbers JP26390062 and JP25107005.

Magnetic properties of magnetic bilayer Kekulene structure: A Monte Carlo study

NASA Astrophysics Data System (ADS)

Jabar, A.; Masrour, R.

2018-06-01

In the present work, we have studied the magnetic properties of magnetic bilayer Kekulene structure with mixed spin-5/2 and spin-2 Ising model using Monte Carlo study. The magnetic phase diagrams of mixed spins Ising model have been given. The thermal total, partial magnetization and magnetic susceptibilities of the mixed spin-5/2 and spin-2 Ising model on a magnetic bilayer Kekulene structure are obtained. The transition temperature has been deduced. The effect of crystal field and exchange interactions on the this bilayers has been studied. The partial and total magnetic hysteresis cycles of the mixed spin-5/2 and spin-2 Ising model on a magnetic bilayer Kekulene structure have been given. The superparamagnetism behavior is observed in magnetic bilayer Kekulene structure. The magnetic coercive field decreases with increasing the exchange interactions between σ-σ and temperatures values and increases with increasing the absolute value of exchange interactions between σ-S. The multiple hysteresis behavior appears.

Interplay between alkyl chain asymmetry and cholesterol addition in the rigid ion pair amphiphile bilayer systems

NASA Astrophysics Data System (ADS)

Huang, Fong-yin; Chiu, Chi-cheng

2017-01-01

Ion pair amphiphile (IPA), a molecular complex composed of a pair of cationic and anionic surfactants, has been proposed as a novel phospholipid substitute. Controlling the physical stability of IPA vesicles is important for its application developments such as cosmetic and drug deliveries. To investigate the effects of IPA alkyl chain combinations and the cholesterol additive on the structural and mechanical properties of IPA vesicular bilayers, we conducted a series of molecular dynamics studies on the hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS) and dodecyltrimethylammonium-hexadecylsulfate (DTMA-HS) IPA bilayers with cholesterol. We found that both IPA bilayers are in the gel phase at 298 K, consistent with experimental observations. Compared with the HTMA-DS system, the DTMA-HS bilayer has more disordered alkyl chains in the hydrophobic region. When adding cholesterol, it induces alkyl chain ordering around its rigid sterol ring. Yet, cholesterol increases the molecular areas for all species and disturbs the molecular packing near the hydrophilic region and the bilayer core. Cholesterol also promotes the alkyl chain mismatch between the IPA moieties, especially for the DTMA-HS bilayer. The combined effects lead to non-monotonically enhancement of the membrane mechanical moduli for both IPA-cholesterol systems. Furthermore, cholesterol can form H-bonds with the alkylsulfate and thus enhance the contribution of alkylsulfate to the overall mechanical moduli. Combined results provide valuable molecular insights into the roles of each IPA component and the cholesterol on modulating the IPA bilayer properties.

Asymmetric or symmetric bilayer formation during oblique drop impact depends on rheological properties of saturated and unsaturated lipid monolayers.

PubMed

Vranceanu, Marcel; Terinte, Nicoleta; Nirschl, Hermann; Leneweit, Gero

2011-02-01

Bilayer structures are formed by approaching two liquid surfaces with phospholipid monolayers, which are brought into contact by oblique drop impact on a liquid surface. Asymmetric bilayers can be produced by the coupling of drop and target monolayers. In contrast, symmetric bilayers or multilayers are formed by collapse of the compressed target monolayer. We show that under all studied conditions bilayer/multilayer synthesis takes place. The experimental conditions for the synthesis of asymmetric or symmetric bilayers are described quantitatively in terms of the surface rheological (surface elasticity and dilational viscosity) and the hydrodynamical parameters (Weber number and impact angle). The composition and mechanical properties of the phospholipid monolayers strongly influences the patterns of drop impact and the bilayer/multilayer formation. Cholesterol stiffens unsaturated phospholipid monolayers and fluidifies saturated monolayers. All monolayers form asymmetric vesicle-like structures, which are stable in the aqueous medium. Additionally, unsaturated phospholipid monolayers without cholesterol form symmetric vesicles by folding parts of the target monolayer. Sufficient presence of cholesterol in unsaturated phospholipid monolayers inhibits the folding of the target monolayer and the subsequent formation of symmetric bilayers. The rheological properties of saturated and unsaturated phospholipid monolayers and their mixtures with cholesterol are discussed. Based on drop impact results it is shown that the state of a so far undefined region in the DPPC/cholesterol phase diagram is a fluid phase. Copyright © 2010 Elsevier Inc. All rights reserved.

Interdroplet bilayer arrays in millifluidic droplet traps from 3D-printed moulds.

PubMed

King, Philip H; Jones, Gareth; Morgan, Hywel; de Planque, Maurits R R; Zauner, Klaus-Peter

2014-02-21

In droplet microfluidics, aqueous droplets are typically separated by an oil phase to ensure containment of molecules in individual droplets of nano-to-picoliter volume. An interesting variation of this method involves bringing two phospholipid-coated droplets into contact to form a lipid bilayer in-between the droplets. These interdroplet bilayers, created by manual pipetting of microliter droplets, have proved advantageous for the study of membrane transport phenomena, including ion channel electrophysiology. In this study, we adapted the droplet microfluidics methodology to achieve automated formation of interdroplet lipid bilayer arrays. We developed a 'millifluidic' chip for microliter droplet generation and droplet packing, which is cast from a 3D-printed mould. Droplets of 0.7-6.0 μL volume were packed as homogeneous or heterogeneous linear arrays of 2-9 droplets that were stable for at least six hours. The interdroplet bilayers had an area of up to 0.56 mm(2), or an equivalent diameter of up to 850 μm, as determined from capacitance measurements. We observed osmotic water transfer over the bilayers as well as sequential bilayer lysis by the pore-forming toxin melittin. These millifluidic interdroplet bilayer arrays combine the ease of electrical and optical access of manually pipetted microdroplets with the automation and reproducibility of microfluidic technologies. Moreover, the 3D-printing based fabrication strategy enables the rapid implementation of alternative channel geometries, e.g. branched arrays, with a design-to-device time of just 24-48 hours.

Using Micropatterned Lipid Bilayer Arrays to Measure the Effect of Membrane Composition on Merocyanine 540 Binding

PubMed Central

Smith, Kathryn A.; Conboy, John C.

2011-01-01

The lipophilic dye merocyanine 540 (MC540) was used to model small molecule-membrane interactions using micropatterned lipid bilayer arrays (MLBAs) prepared using a 3D Continuous Flow Microspotter (CFM). Fluorescence microscopy was used to monitor MC540 binding to fifteen different bilayer compositions simultaneously. MC540 fluorescence was two times greater for bilayers composed of liquid-crystalline (l.c.) phase lipids (1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)) compared to bilayers in the gel phase (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)). The effect cholesterol (CHO) had on MC540 binding to the membrane was found to be dependent on the lipid component; cholesterol decreased MC540 bindingin DMPC, DPPC and DSPC bilayers while having little to no effect on the remaining l.c. phase lipids. MC540 fluorescence was also lowered when 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (sodium salt) (DOPS) was incorporated into DOPC bilayers. The increase in the surface charge density appears to decrease the occurrence of highly fluorescent monomers and increase the formation of weakly fluorescent dimers via electrostatic repulsion. This paper demonstrates that MLBAs are a useful tool for preparing high density reproducible bilayer arrays to study small molecule-membrane interactions in a high-throughput manner. PMID:21376014

Cholesterol effect on water permeability through DPPC and PSM lipid bilayers: a molecular dynamics study.

PubMed

Saito, Hiroaki; Shinoda, Wataru

2011-12-29

Water permeability of two different lipid bilayers of dipalmitoylphosphatidylcholine (DPPC) and palmitoylsphingomyelin (PSM) in the absence and presence of cholesterol (0-50 mol %) have been studied by molecular dynamics simulations to elucidate the molecular mechanism of the reduction in water leakage across the membranes by the addition of cholesterol. An enhanced free energy barrier was observed in these membranes with increased cholesterol concentration, and this was explained by the reduced cavity density around the cholesterol in the hydrophobic membrane core. There was an increase of trans conformers in the hydrophobic lipid chains adjacent to the cholesterol, which reduced the cavity density. The enhanced free energy barrier was found to be the main reason to reduce the water permeability with increased cholesterol concentration. At low cholesterol concentrations the PSM bilayer exhibited a higher free energy barrier than the DPPC bilayer for water permeation, while at greater than 30 mol % of cholesterol the difference became minor. This tendency for the PSM and DPPC bilayers to resemble each other at higher cholesterol concentrations was similar to commonly observed trends in several structural properties, such as order parameters, cross-sectional area per molecule, and cavity density profiles in the hydrophobic regions of bilayer membranes. These results demonstrate that DPPC and PSM bilayers with high cholesterol contents possess similar physical properties, which suggests that the solubility of cholesterol in these lipid bilayers has importance for an understanding of multicomponent lipid membranes with cholesterol. © 2011 American Chemical Society

Effect of intra-membrane C60 fullerenes on the modulus of elasticity and the mechanical resistance of gel and fluid lipid bilayers

NASA Astrophysics Data System (ADS)

Zhou, Jihan; Liang, Dehai; Contera, Sonia

2015-10-01

Penetration and partition of C60 to the lipid bilayer core are both relevant to C60 toxicity, and useful to realise C60 biomedical potential. A key aspect is the effect of C60 on bilayer mechanical properties. Here, we present an experimental study on the mechanical effect of the incorporation of C60 into the hydrophobic core of fluid and gel phase zwitterionic phosphatidylcholine (PC) lipid bilayers. We demonstrate its incorporation inside the hydrophobic lipid core and the effect on the packing of the lipids and the vesicle size using a combination of infrared (IR) spectroscopy, atomic force microscopy (AFM) and laser light scattering. Using AFM we measured the Young's modulus of elasticity (E) of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in the absence (presence) of intra-membranous C60 at 24.5 °C. E of fluid phase supported bilayers is not altered by C60, but E increases with incorporation of C60 in gel phase bilayers. The increase is higher for longer hydrocarbon chains: 1.6 times for DPPC and 2 times for DSPC. However the mechanical resistance of gel phase bilayers of curved bilayered structures decreases with the incorporation of C60. Our combined results indicate that C60 causes a decrease in gel phase lipid mobility, i.e. an increase in membrane viscosity.

Polymer-cushioned bilayers. II. An investigation of interaction forces and fusion using the surface forces apparatus.

PubMed Central

Wong, J Y; Park, C K; Seitz, M; Israelachvili, J

1999-01-01

We have created phospholipid bilayers supported on soft polymer "cushions" which act as deformable substrates (see accompanying paper, Wong, J. Y., J. Majewski, M. Seitz, C. K. Park, J. N. Israelachvili, and G. S. Smith. 1999. Biophys. J. 77:1445-1457). In contrast to "solid-supported" membranes, such "soft-supported" membranes can exhibit more natural (higher) fluidity. Our bilayer system was constructed by adsorption of small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles onto polyethylenimine (PEI)-supported Langmuir-Blodgett lipid monolayers on mica. We used the surface forces apparatus (SFA) to investigate the long-range forces, adhesion, and fusion of two DMPC bilayers both above and below their main transition temperature (T(m) approximately 24 degrees C). Above T(m), hemi-fusion activation pressures of apposing bilayers were considerably smaller than for solid-supported bilayers, e.g., directly supported on mica. After separation, the bilayers naturally re-formed after short healing times. Also, for the first time, complete fusion of two fluid (liquid crystalline) phospholipid bilayers was observed in the SFA. Below T(m) (gel state), very high pressures were needed for hemi-fusion and the healing process became very slow. The presence of the polymer cushion significantly alters the interaction potential, e.g., long-range forces as well as fusion pressures, when compared to solid-supported systems. These fluid model membranes should allow the future study of integral membrane proteins under more physiological conditions. PMID:10465756

DNA nanotechnology: Bringing lipid bilayers into shape

NASA Astrophysics Data System (ADS)

Howorka, Stefan

2017-07-01

Lipid bilayers form the thin and floppy membranes that define the boundary of compartments such as cells. Now, a method to control the shape and size of bilayers using DNA nanoscaffolds has been developed. Such designer materials advance synthetic biology and could find use in membrane research.

Transmembrane Polyproline Helix.

PubMed

Kubyshkin, Vladimir; Grage, Stephan L; Bürck, Jochen; Ulrich, Anne S; Budisa, Nediljko

2018-05-03

The third most abundant polypeptide conformation in nature, the polyproline-II helix, is a polar, extended secondary structure with a local organization stabilized by intercarbonyl interactions within the peptide chain. Here we design a hydrophobic polyproline-II helical peptide based on an oligomeric octahydroindole-2-carboxylic acid scaffold and demonstrate its transmembrane alignment in model lipid bilayers by means of solid-state 19 F NMR. As result, we provide a first example of a purely artificial transmembrane peptide with a structural organization that is not based on hydrogen-bonding.

Scraping and stapling of end-grafted DNA chains by a bioadhesive spreading vesicle to reveal chain internal friction and topological complexity.

PubMed

Nam, Gimoon; Hisette, Marie Laure; Sun, Yuting Liang; Gisler, Thomas; Johner, Albert; Thalmann, Fabrice; Schröder, André Pierre; Marques, Carlos Manuel; Lee, Nam-Kyung

2010-08-20

Stained end-grafted DNA molecules about 20 μm long are scraped away and stretched out by the spreading front of a bioadhesive vesicle. Tethered biotin ligands bind the vesicle bilayer to a streptavidin substrate, stapling the DNAs into frozen confinement paths. Image analysis of the stapled DNA gives access, within optical resolution, to the local stretching values of individual DNA molecules swept by the spreading front, and provides evidence of self-entanglements.

Extension of the Mott-Gurney Law for a Bilayer Gap

NASA Astrophysics Data System (ADS)

Dubinov, A. E.; Kitayev, I. N.

2018-04-01

Steady drift states of an electron flow in a planar gap filled with a bilayer dielectric have been considered. Exact mathematical formulas have been derived that describe the distributions of the electrostatic potential and space charge limited electron flow current (extended Mott-Gurney law for a bilayer diode).

Atomic structure and bonding of the interfacial bilayer between Au nanoparticles and epitaxially regrown MgAl{sub 2}O{sub 4} substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Guo-zhen; Canadian Centre of Electron Microscopy and Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1; Majdi, Tahereh

2014-12-08

A unique metal/oxide interfacial bilayer formed between Au nanoparticles and MgAl{sub 2}O{sub 4} substrates following thermal treatment is reported. Associated with the formation of the bilayer was the onset of an abnormal epitaxial growth of the substrate under the nanoparticle. According to the redistribution of atoms and the changes of their electronic structure probed across the interface by a transmission electron microscopy, we suggest two possible atomic models of the interfacial bilayer.

Quantitation of cholesterol incorporation into extruded lipid bilayers.

PubMed

Ibarguren, Maitane; Alonso, Alicia; Tenchov, Boris G; Goñi, Felix M

2010-09-01

Cholesterol incorporation into lipid bilayers, in the form of multilamellar vesicles or extruded large unilamellar vesicles, has been quantitated. To this aim, the cholesterol contents of bilayers prepared from phospholipid:cholesterol mixtures 33-75 mol% cholesterol have been measured and compared with the original mixture before lipid hydration. There is a great diversity of cases, but under most conditions the actual cholesterol proportion present in the extruded bilayers is much lower than predicted. A quantitative analysis of the vesicles is thus required before any experimental study is undertaken. 2010 Elsevier B.V. All rights reserved.

Variation of superconducting transition temperature by proximity effect in NbN/FeN bilayers

NASA Astrophysics Data System (ADS)

Hwang, Tae-Jong; Kim, Dong-Ho

2017-09-01

We report on the proximity effect in superconductor/ferromagnet bilayers made of a new combination of NbN for the superconductor and FeN for the ferromagnet. The bilayers were prepared by reactive magnetron sputtering on a thermally oxidized Si substrate. For a constant NbN layer thickness, the superconducting transition temperatures of the bilayers exhibited a nonmonotonic dependence on the thickness of the FeN layer. The results were interpreted in terms of the proximity effect between the superconductor and ferromagnetic materials.

Structure and energetics of carbon, hexagonal boron nitride, and carbon/hexagonal boron nitride single-layer and bilayer nanoscrolls

NASA Astrophysics Data System (ADS)

Siahlo, Andrei I.; Poklonski, Nikolai A.; Lebedev, Alexander V.; Lebedeva, Irina V.; Popov, Andrey M.; Vyrko, Sergey A.; Knizhnik, Andrey A.; Lozovik, Yurii E.

2018-03-01

Single-layer and bilayer carbon and hexagonal boron nitride nanoscrolls as well as nanoscrolls made of bilayer graphene/hexagonal boron nitride heterostructure are considered. Structures of stable states of the corresponding nanoscrolls prepared by rolling single-layer and bilayer rectangular nanoribbons are obtained based on the analytical model and numerical calculations. The lengths of nanoribbons for which stable and energetically favorable nanoscrolls are possible are determined. Barriers to rolling of single-layer and bilayer nanoribbons into nanoscrolls and barriers to nanoscroll unrolling are calculated. Based on the calculated barriers nanoscroll lifetimes in the stable state are estimated. Elastic constants for bending of graphene and hexagonal boron nitride layers used in the model are found by density functional theory calculations.

Modified release from lipid bilayer coated mesoporous silica nanoparticles using PEO–PPO–PEO triblock copolymers [Modified release from lipid bilayer coated mesoporous silica nanoparticles using PEO PPO PEO triblock copolymers

DOE PAGES

Rahman, Masoud; Yu, Erick; Forman, Evan; ...

2014-08-20

Triblock copolymers comprised of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO, or trade name Pluronic) interact with lipid bilayers to increase their permeability. Here we demonstrate a novel application of Pluronic L61 and L64 as modification agents in tailoring the release rate of a molecular indicator species from 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayer-coated superparamagnetic Fe 3O 4/mesoporous silica coreshell nanoparticles. Lastly, we show there is a direct relationship between Pluronic concentration and the indicator molecule release, suggesting Pluronic may be useful for the controlled release of drugs from lipid bilayer-coated carriers.

Enhanced sensitivity of a microfabricated resonator using a graphene-polystyrene bilayer membrane

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yun, Minhyuk; Lee, Eunho; Cho, Kilwon

2014-08-18

A graphene layer was synthesized using chemical vapor deposition methods and a polystyrene solution was spin-cast onto the graphene film. The graphene-polystyrene bilayer membrane was attached between the two tines of a microfabricated quartz tuning fork (QTF). The modulus of the graphene-polystyrene bilayer was measured to be twice that of a pristine polystyrene membrane. Exposure of the membrane-coated QTF to ethanol vapor decreased the resonance frequency of the microresonator. The bilayer membrane-coated QTF produced a frequency change that was three times the change obtained using a polystyrene membrane-coated QTF, with a lower degree of degradation in the Q factor. Themore » limit of detection of the bilayer membrane-coated QTF to ethanol vapor was determined to be 20 ppm.« less

Quantifying the Relationship between Curvature and Electric Potential in Lipid Bilayers.

PubMed

Bruhn, Dennis S; Lomholt, Michael A; Khandelia, Himanshu

2016-06-02

Cellular membranes mediate vital cellular processes by being subject to curvature and transmembrane electrical potentials. Here we build upon the existing theory for flexoelectricity in liquid crystals to quantify the coupling between lipid bilayer curvature and membrane potentials. Using molecular dynamics simulations, we show that headgroup dipole moments, the lateral pressure profile across the bilayer, and spontaneous curvature all systematically change with increasing membrane potentials. In particular, there is a linear dependence between the bending moment (the product of bending rigidity and spontaneous curvature) and the applied membrane potentials. We show that biologically relevant membrane potentials can induce biologically relevant curvatures corresponding to radii of around 500 nm. The implications of flexoelectricity in lipid bilayers are thus likely to be of considerable consequence both in biology and in model lipid bilayer systems.

Hot carrier response in gapped bilayer graphene

NASA Astrophysics Data System (ADS)

Aivazian, Grant; Ross, Jason; Watanabe, K.; Taniguchi, T.; Kitamura, K.; Cobden, David; Xu, Xiaodong

2013-03-01

Recently bilayer graphene has been shown to develop a bandgap upon breaking of inversion symmetry by a perpendicular electric field that is in situtunable between zero and several hundred meV (corresponding to wavelengths in the mid-IR). Such unique tunability offers bilayer graphene a niche in mid-IR optoelectronic devices where a lack of high performance photodetectors exists. In this work we have performed spatially and temporally resolved photocurrent measurements in a dual-gated bilayer graphene FET under continuous-wave and pulsed laser excitation. We find that photocurrent generation in native bilayer graphene is dominated by hot carriers, as is the case in monolayer graphene, but it behaves very differently from monolayer graphene once a bandgap has been opened. Work supported by the NSF Early Career Grant and DARPA N66001-11-1-4124.

Shear-driven motion of supported lipid bilayers in microfluidic channels.

PubMed

Jönsson, Peter; Beech, Jason P; Tegenfeldt, Jonas O; Höök, Fredrik

2009-04-15

In this work, we demonstrate how a lateral motion of a supported lipid bilayer (SLB) and its constituents can be created without relying on self-spreading forces. The force driving the SLB is instead a viscous shear force arising from a pressure-driven bulk flow acting on the SLB that is formed on a glass wall inside a microfluidic channel. In contrast to self-spreading bilayers, this method allows for accurate control of the bilayer motion by altering the bulk flow in the channel. Experiments showed that an egg yolk phosphatidylcholine SLB formed on a glass support moved in a rolling motion under these shear forces, with the lipids in the upper leaflet of the bilayer moving at twice the velocity of the bilayer front. The drift velocity of different lipid probes in the SLB was observed to be sensitive to the interactions between the lipid probe and the surrounding molecules, resulting in drift velocities that varied by up to 1 order of magnitude for the different lipid probes in our experiments. Since the method provides a so far unattainable control of the motion of all molecules in an SLB, we foresee great potential for this technique, alone or in combination with other methods, for studies of lipid bilayers and different membrane-associated molecules.

Characterization of Interactions between Curcumin and Different Types of Lipid Bilayers by Molecular Dynamics Simulation.

PubMed

Lyu, Yuan; Xiang, Ning; Mondal, Jagannath; Zhu, Xiao; Narsimhan, Ganesan

2018-03-01

Curcumin (CUR) is a natural food ingredient with known ability to target microbial cell membrane. In this study, the interactions of CUR with different types of model lipid bilayers (POPE, POPG, POPC, DOPC, and DPPE), mixtures of model lipid bilayers (POPE/POPG), and biological membrane mimics (Escherichia coli and yeast) were investigated by all-atom explicit solvent molecular dynamics (MD) simulation. CUR readily inserts into different types of model lipid bilayer systems in the liquid crystalline state, staying in the lipid tails region near the interface of lipid head and lipid tail. Parallel orientation to the membrane surface is found to be more probable than perpendicular for CUR, as indicated by the tilt angle distribution. This orientation preference is less significant as the fraction of POPE is increased in the system, likely due to the better water solvation of perpendicular orientation in the POPE bilayer. In E. coli and yeast bilayers, tilt angle distributions were similar to that for POPE/POPG mixed bilayer, with water hydration number around CUR for the former being higher. Insertion of CUR resulted in membrane thinning. The results from these simulations provide insights into the possible differences in membrane disrupting activity of CUR against different types of microorganisms.

Significant role of antiferromagnetic GdFeO3 on multiferroism of bilayer thin films

NASA Astrophysics Data System (ADS)

Shah, Jyoti; Bhatt, Priyanka; Dayas, K. Diana Diana; Kotnala, R. K.

2018-02-01

Inversion of BaTiO3 and GdFeO3 thin films in bilayer configuration has been deposited by pulsed laser deposition technique. A significant effect of strain on thin film has been observed by X-ray diffraction analysis. Tensile strain of 1.04% and 0.23% has been calculated by X-ray diffraction results. Higher polarization value 70.4 μC cm-2 has been observed by strained BaTiO3 film in GdFeO3/BaTiO3 bilayer film. Strained GdFeO3 film in BaTiO3/GdFeO3 bilayer configuration exhibited ferromagnetic behaviour showed maximum magnetization value of 50 emu gm-1. Magnetoelectric coupling coefficient of bilayer films have been carried out by dynamic method. Room temperature magnetoelectric coupling 2500 mV cm-1-Oe has been obtained for BaTiO3/GdFeO3 bilayer film. The high ME coupling of the BaTiO3/GdFeO3 bilayer film reveals strong interfacial coupling between ferroelectric and ferromagnetic dipoles. On magnetoelectric coupling coefficient effect of ferromagnetic GdFeO3 layer has a significant role. Such high value of ME coupling may be useful in realization of magnetoelectric RAM (MeRAM) application.

Surface potentials measure ion concentrations near lipid bilayers during rapid solution changes.

PubMed Central

Laver, D R; Curtis, B A

1996-01-01

We describe a puffing method for changing solutions near one surface of lipid bilayers that allows simultaneous measurement of channel activity and extent of solution change at the bilayer surface. Ion adsorption to the lipid headgroups and screening of the bilayer surface charge by mobile ions provided a convenient probe for the ionic composition of the solution at the bilayer surface. Rapid ionic changes induced a shift in bilayer surface potential that generated a capacitive transient current under voltage-clamp conditions. This depended on the ion species and bilayer composition and was accurately described by the Stern-Gouy-Chapman theory. The time course of solute concentrations during solution changes could also be modeled by an exponential exchange of bath and puffing solutions with time constants ranging from 20 to 110 ms depending on the flow pressure. During changes in [Cs+] and [Ca2+] (applied separately or together) both the mixing model and capacitive currents predicted [Cs+] and [Ca2+] transients consistent with those determined experimentally from: 1) the known Cs(+)-dependent conductance of open ryanodine receptor channels and 2) the Ca(2+)-dependent gating of ryanodine receptor Ca2+ channels from cardiac and skeletal muscle. Images FIGURE 1 FIGURE 4 FIGURE 5 FIGURE 8 PMID:8842210

Effects and Location of Coplanar and Noncoplanar PCB in a Lipid Bilayer: A Solid-State NMR Study.

PubMed

Totland, Christian; Nerdal, Willy; Steinkopf, Signe

2016-08-02

Coplanar and noncoplanar polychlorinated biphenyls (PCBs) are known to have different routes and degree of toxicity. Here, the effects of noncoplanar PCB 52 and coplanar PCB 77 present at 2 mol % in a model system consisting of POPC liposomes (50% hydrated) are investigated by solid-state (13)C and (31)P NMR at 298 K. Both PCBs intercalate horizontally in the outer part of the bilayer, near the segments of the acyl chain close to the glycerol group. Despite similar membrane locations, the coplanar PCB 77 shows little effect on the bilayer properties overall, except for the four nearest neighboring lipids, while the effect of PCB 52 is more dramatic. The first ∼2 layers of lipids around each PCB 52 in the bilayer form a high fluidity lamellar phase, whereas lipids beyond these layers form a lamellar phase with a slight increase in fluidity compared to a bilayer without PCB 52. Further, a third high mobility domain is observed. The explanation for this is the interference of several high fluidity lamellar phases caused by interactions of PCB 52 molecules in different leaflets of the model bilayer. This causes formation of high curvature toroidal region in the bilayer and might induce formation of channels.

DSC and EPR investigations on effects of cholesterol component on molecular interactions between paclitaxel and phospholipid within lipid bilayer membrane.

PubMed

Zhao, Lingyun; Feng, Si-Shen; Kocherginsky, Nikolai; Kostetski, Iouri

2007-06-29

Differential scanning calorimetry (DSC) and electron paramagnetic resonance spectroscopy (EPR) were applied to investigate effects of cholesterol component on molecular interactions between paclitaxel, which is one of the best antineoplastic agents found from nature, and dipalmitoylphosphatidylcholine (DPPC) within lipid bilayer vesicles (liposomes), which could also be used as a model cell membrane. DSC analysis showed that incorporation of paclitaxel into the DPPC bilayer causes a reduction in the cooperativity of bilayer phase transition, leading to a looser and more flexible bilayer structure. Including cholesterol component in the DPPC/paclitaxel mixed bilayer can facilitate the molecular interaction between paclitaxel and lipid and make the tertiary system more stable. EPR analysis demonstrated that both of paclitaxel and cholesterol have fluidization effect on the DPPC bilayer membranes although cholesterol has more significant effect than paclitaxel does. The reduction kinetics of nitroxides by ascorbic acid showed that paclitaxel can inhibit the reaction by blocking the diffusion of either the ascorbic acid or nitroxide molecules since the reaction is tested to be a first order one. Cholesterol can remarkably increase the reduction reaction speed. This research may provide useful information for optimizing liposomal formulation of the drug as well as for understanding the pharmacology of paclitaxel.

Anesthetic diffusion through lipid membranes depends on the protonation rate.

PubMed

Pérez-Isidoro, Rosendo; Sierra-Valdez, F J; Ruiz-Suárez, J C

2014-12-18

Hundreds of substances possess anesthetic action. However, despite decades of research and tests, a golden rule is required to reconcile the diverse hypothesis behind anesthesia. What makes an anesthetic to be local or general in the first place? The specific targets on proteins, the solubility in lipids, the diffusivity, potency, action time? Here we show that there could be a new player equally or even more important to disentangle the riddle: the protonation rate. Indeed, such rate modulates the diffusion speed of anesthetics into lipid membranes; low protonation rates enhance the diffusion for local anesthetics while high ones reduce it. We show also that there is a pH and membrane phase dependence on the local anesthetic diffusion across multiple lipid bilayers. Based on our findings we incorporate a new clue that may advance our understanding of the anesthetic phenomenon.

Influence of cost functions and optimization methods on solving the inverse problem in spatially resolved diffuse reflectance spectroscopy

NASA Astrophysics Data System (ADS)

Rakotomanga, Prisca; Soussen, Charles; Blondel, Walter C. P. M.

2017-03-01

Diffuse reflectance spectroscopy (DRS) has been acknowledged as a valuable optical biopsy tool for in vivo characterizing pathological modifications in epithelial tissues such as cancer. In spatially resolved DRS, accurate and robust estimation of the optical parameters (OP) of biological tissues is a major challenge due to the complexity of the physical models. Solving this inverse problem requires to consider 3 components: the forward model, the cost function, and the optimization algorithm. This paper presents a comparative numerical study of the performances in estimating OP depending on the choice made for each of the latter components. Mono- and bi-layer tissue models are considered. Monowavelength (scalar) absorption and scattering coefficients are estimated. As a forward model, diffusion approximation analytical solutions with and without noise are implemented. Several cost functions are evaluated possibly including normalized data terms. Two local optimization methods, Levenberg-Marquardt and TrustRegion-Reflective, are considered. Because they may be sensitive to the initial setting, a global optimization approach is proposed to improve the estimation accuracy. This algorithm is based on repeated calls to the above-mentioned local methods, with initial parameters randomly sampled. Two global optimization methods, Genetic Algorithm (GA) and Particle Swarm Optimization (PSO), are also implemented. Estimation performances are evaluated in terms of relative errors between the ground truth and the estimated values for each set of unknown OP. The combination between the number of variables to be estimated, the nature of the forward model, the cost function to be minimized and the optimization method are discussed.

Permeation of protons, potassium ions, and small polar molecules through phospholipid bilayers as a function of membrane thickness.

PubMed Central

Paula, S; Volkov, A G; Van Hoek, A N; Haines, T H; Deamer, D W

1996-01-01

Two mechanisms have been proposed to account for solute permeation of lipid bilayers. Partitioning into the hydrophobic phase of the bilayer, followed by diffusion, is accepted by many for the permeation of water and other small neutral solutes, but transient pores have also been proposed to account for both water and ionic solute permeation. These two mechanisms make distinctively different predictions about the permeability coefficient as a function of bilayer thickness. Whereas the solubility-diffusion mechanism predicts only a modest variation related to bilayer thickness, the pore model predicts an exponential relationship. To test these models, we measured the permeability of phospholipid bilayers to protons, potassium ions, water, urea, and glycerol. Bilayers were prepared as liposomes, and thickness was varied systematically by using unsaturated lipids with chain lengths ranging from 14 to 24 carbon atoms. The permeability coefficient of water and neutral polar solutes displayed a modest dependence on bilayer thickness, with an approximately linear fivefold decrease as the carbon number varied from 14 to 24 atoms. In contrast, the permeability to protons and potassium ions decreased sharply by two orders of magnitude between 14 and 18 carbon atoms, and leveled off, when the chain length was further extended to 24 carbon atoms. The results for water and the neutral permeating solutes are best explained by the solubility-diffusion mechanism. The results for protons and potassium ions in shorter-chain lipids are consistent with the transient pore model, but better fit the theoretical line predicted by the solubility-diffusion model at longer chain lengths. PMID:8770210

Theoretical study of binding and permeation of ether-based polymers through interfaces.

PubMed

Samanta, Susruta; Hezaveh, Samira; Roccatano, Danilo

2013-11-27

We present a molecular dynamics simulation study on the interactions of poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), and their ABA-type block copolymer, poloxamers, at water/n-heptane and 1,2-dimyristoyl-sn-glycero-3-phospatidycholine (DMPC) lipid bilayer/water interfaces. The partition coefficients in water/1-octanol of the linear polyethers up to three monomers were calculated. The partition coefficients evidenced a higher hydrophobicity of the PPO in comparison to PEO. At the water/n-heptane interface, the polymers tend to adopt elongated conformations in agreement with similar experimental ellipsometry studies of different poloxamers. In the case of the poloxamers at the n-heptane/water interface, the stronger preference of the PPO block for the hydrophobic phase resulted in bottle-brush-type polymer conformations. At lipid bilayer/water interface, the PEO polymers, as expected from their hydrophilic nature, are weakly adsorbed on the surface of the lipid bilayer and locate in the water phase close to the headgroups. The free energy barriers of permeation calculated for short polymer chains suggest a thermodynamics propensity for the water phase that increase with the chain length. The lower affinity of PEO for the hydrophobic interior of the lipid bilayer resulted in the spontaneous expulsion within the simulation time. On the contrary, PPO chains and poloxamers have a longer residence time inside the bilayer, and they tend to concentrate in the tail region of the bilayer near the polar headgroups. In addition, polymers with PPO unit length comparable to the thickness of the hydrophobic region of the bilayer tend to span across the bilayer.

Permeation of protons, potassium ions, and small polar molecules through phospholipid bilayers as a function of membrane thickness

NASA Technical Reports Server (NTRS)

Paula, S.; Volkov, A. G.; Van Hoek, A. N.; Haines, T. H.; Deamer, D. W.

1996-01-01

Two mechanisms have been proposed to account for solute permeation of lipid bilayers. Partitioning into the hydrophobic phase of the bilayer, followed by diffusion, is accepted by many for the permeation of water and other small neutral solutes, but transient pores have also been proposed to account for both water and ionic solute permeation. These two mechanisms make distinctively different predictions about the permeability coefficient as a function of bilayer thickness. Whereas the solubility-diffusion mechanism predicts only a modest variation related to bilayer thickness, the pore model predicts an exponential relationship. To test these models, we measured the permeability of phospholipid bilayers to protons, potassium ions, water, urea, and glycerol. Bilayers were prepared as liposomes, and thickness was varied systematically by using unsaturated lipids with chain lengths ranging from 14 to 24 carbon atoms. The permeability coefficient of water and neutral polar solutes displayed a modest dependence on bilayer thickness, with an approximately linear fivefold decrease as the carbon number varied from 14 to 24 atoms. In contrast, the permeability to protons and potassium ions decreased sharply by two orders of magnitude between 14 and 18 carbon atoms, and leveled off, when the chain length was further extended to 24 carbon atoms. The results for water and the neutral permeating solutes are best explained by the solubility-diffusion mechanism. The results for protons and potassium ions in shorter-chain lipids are consistent with the transient pore model, but better fit the theoretical line predicted by the solubility-diffusion model at longer chain lengths.

Measurement of the membrane dipole electric field in DMPC vesicles using vibrational shifts of p-cyanophenylalanine and molecular dynamics simulations.

PubMed

Shrestha, Rebika; Cardenas, Alfredo E; Elber, Ron; Webb, Lauren J

2015-02-19

The magnitude of the membrane dipole field was measured using vibrational Stark effect (VSE) shifts of nitrile oscillators placed on the unnatural amino acid p-cyanophenylalanine (p-CN-Phe) added to a peptide sequence at four unique positions. These peptides, which were based on a repeating alanine-leucine motif, intercalated into small unilamellar DMPC vesicles which formed an α-helix as confirmed by circular dichroic (CD) spectroscopy. Molecular dynamics simulations of the membrane-intercalated helix containing two of the nitrile probes, one near the headgroup region of the lipid (αLAX(25)) and one buried in the interior of the bilayer (αLAX(16)), were used to examine the structure of the nitrile with respect to the membrane normal, the assumed direction of the dipole field, by quantifying both a small tilt of the helix in the bilayer and conformational rotation of the p-CN-Phe side chain at steady state. Vibrational absorption energies of the nitrile oscillator at each position showed a systematic blue shift as the nitrile was stepped toward the membrane interior; for several different concentrations of peptide, the absorption energy of the nitrile located in the middle of the bilayer was ∼3 cm(-1) greater than that of the nitrile closest to the surface of the membrane. Taken together, the measured VSE shifts and nitrile orientations within the membrane resulted in an absolute magnitude of 8-11 MV/cm for the dipole field, at the high end of the range of possible values that have been accumulated from a variety of indirect measurements. Implications for this are discussed.

Measurement of the Membrane Dipole Electric Field in DMPC Vesicles Using Vibrational Shifts of p-Cyanophenylalanine and Molecular Dynamics Simulations

PubMed Central

Shrestha, Rebika; Cardenas, Alfredo E.; Elber, Ron; Webb, Lauren J.

2015-01-01

The magnitude of the membrane dipole field was measured using vibrational Stark effect (VSE) shifts of nitrile oscillators placed on the unnatural amino acid p-cyanophenylalanine (p-CN-Phe) added to a peptide sequence at four unique positions. These peptides, which were based on a repeating alanine-leucine motif, intercalated into small unilamellar DMPC vesicles which formed an α-helix as confirmed by circular dichroic (CD) spectroscopy. Molecular dynamics simulations of the membrane-intercalated helix containing two of the nitrile probes, one near the head-group region of the lipid (αLAX(25)) and one buried in the interior of the bilayer (αLAX(16)), were used to examine the structure of the nitrile with respect to the membrane normal, the assumed direction the dipole field, by quantifying both a small tilt of the helix in the bilayer and conformational rotation of the p-CN-Phe side chain at steady-state. Vibrational absorption energies of the nitrile oscillator at each position showed a systematic blue shift as the nitrile was stepped towards the membrane interior; for several different concentrations of peptide, the absorption energy of the nitrile located in the middle of the bilayer was ~3 cm−1 greater than that of the nitrile closest to the surface of the membrane. Taken together, the measured VSE shifts and nitrile orientations within the membrane resulted in a value of 8 – 11 MV/cm for the dipole field, at the high end of the range of possible values that have been accumulated from a variety of indirect measurements. Implications for this are discussed. PMID:25602635

Rotational behaviour of PEGylated gold nanorods in a lipid bilayer system

NASA Astrophysics Data System (ADS)

Oroskar, Priyanka A.; Jameson, Cynthia J.; Murad, Sohail

2017-06-01

PEGylated gold nanorods are widely used as nanocarriers in targeted drug delivery and other nanotechnology applications due to the special optical and photo-thermal characteristics of gold nanorods. In this work, we employ coarse-grain molecular simulations to examine the pathway by which PEGylated gold nanorods enter and exit a dipalmitoylphosphatidylcholine lipid bilayer membrane and follow the behaviour of the system to investigate the consequences. We find that PEGylated gold nanorods rotate during permeation, lying down and straightening up as they make their way through the lipid membrane. We find that this rotational behaviour, irrespective of the initial orientation of the nanorod with respect to the membrane normal, is concomitant with the changing interactions of polyethylene glycol (PEG) beads with lipid head beads in both membrane leaflets. For a nanorod with hydrophilic ligands, such as PEG, lying down appears to be driven by favourable hydrophilic interactions with the phosphate and choline groups of the lipid. Mobility of the ligands offers mechanisms for these favourable interactions and for minimising unfavourable interactions with the hydrophobic lipid tails that constitute the inner section of the membrane; the PEG ligands can stretch out to reach the phosphate and choline groups of both leaflets and they can coil in and interact with each other and avoid the alkane lipid tails. Recently developed experimental techniques for imaging, orientation, and rotation of single gold nanorods may be able to observe this predicted rotational behaviour. We find that lipid flip-flop mechanisms do not differ significantly from a spherical gold nanoparticle to a gold nanorod, and PEGylated gold nanorods like their spherical counterparts do not remove lipid molecules from the bilayer membrane. Our results should be of interest to experimentalists who plan to use functionalised gold nanorods in biomedical applications.

Structure and Dynamics of Nonionic Surfactant Aggregates in Layered Materials.

PubMed

Guégan, Régis; Veron, Emmanuel; Le Forestier, Lydie; Ogawa, Makoto; Cadars, Sylvian

2017-09-26

The aggregation of surfactants on solid surfaces as they are adsorbed from solution is the basis of numerous technological applications such as colloidal stabilization, ore flotation, and floor cleaning. The understanding of both the structure and the dynamics of surfactant aggregates applies to the development of alternative ways of preparing hybrid layered materials. For this purpose, we study the adsorption of the triethylene glycol mono n-decyl ether (C 10 E 3 ) nonionic surfactant onto a synthetic montmorillonite (Mt), an aluminosilicate clay mineral for organoclay preparation with important applications in materials sciences, catalysis, wastewater treatment, or as drug delivery. The aggregation mechanisms follow those observed in an analogous natural Mt, with the condensation of C 10 E 3 in a bilayer arrangement once the surfactant self-assembles in a lamellar phase beyond the critical micelle concentration, underlining the importance of the surfactant state in solution. Solid-state 1 H nuclear magnetic resonance (NMR) at fast magic-angle spinning (MAS) and high magnetic field combined with 1 H- 13 C correlation experiments and different types of 13 C NMR experiments selectively probes mobile or rigid moieties of C 10 E 3 in three different aggregate organizations: (i) a lateral monolayer, (ii) a lateral bilayer, and (iii) a normal bilayer. High-resolution 1 H{ 27 Al} CP- 1 H- 1 H spin diffusion experiments shed light on the proximities and dynamics of the different fragments and fractions of the intercalated surfactant molecules with respect to the Mt surface. 23 Na and 1 H NMR measurements combined with complementary NMR data, at both molecular and nanometer scales, precisely pointed out the location of the C 10 E 3 ethylene oxide hydrophilic group in close contact with the Mt surface interacting through ion-dipole or van der Waals interactions.

Plasmon dispersion and Coulomb drag in low-density electron bi-layers

NASA Astrophysics Data System (ADS)

Badalyan, S. M.; Kim, C. S.; Vignale, G.; Senatore, G.

2007-03-01

We investigate the effect of exchange and correlation (xc) on the plasmon spectrum and the Coulomb drag between spatially separated low-density two-dimensional electron layers. We adopt a new approach, which employs dynamic xc kernels in the calculation of the bi-layer plasmon spectra and of the plasmon-mediated drag, and static many-body local field factors in the calculation of the particle-hole contribution to the drag. We observe that both optical and acoustical plasmon modes are strongly affected by xc corrections and shift in opposite directions with decreasing density. This is in stark contrast with the tendency observed within the random phase approximation (RPA). We find that the introduction of xc corrections results in a significant enhancement of the transresistivity and qualitative changes in its temperature dependence. In particular, the large high-temperature plasmon peak that is present in the RPA is found to disappear when the xc corrections are included. Our numerical results are in good agreement with the results of recent experiments by M. Kellogg et al., Solid State Commun. 123, 515 (2002).

Neutron scattering to study membrane systems: from lipid vesicles to living cells.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nickels, Jonathan D.; Chatterjee, Sneha; Stanley, Christopher B.

The existence and role of lateral lipid organization in biological membranes has been studied and contested for more than 30 years. Lipid domains, or rafts, are hypothesized as scalable compartments in biological membranes, providing appropriate physical environments to their resident membrane proteins. This implies that lateral lipid organization is associated with a range of biological functions, such as protein co-localization, membrane trafficking, and cell signaling, to name just a few. Neutron scattering techniques have proven to be an excellent tool to investigate these structural features in model lipids, and more recently, in living cells. I will discuss our recent workmore » using neutrons to probe the structure and mechanical properties in model lipid systems and our current efforts in using neutrons to probe the structure and organization of the bilayer in a living cell. These efforts in living cells have used genetic and biochemical strategies to generate a large neutron scattering contrast, making the membrane visible. I will present our results showing in vivo bilayer structure and discuss the outlook for this approach.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smith, Jeremy C.; Cheng, Xiaolin; Nickels, Jonathan D.

Understanding of cell membrane organization has evolved significantly from the classic fluid mosaic model. It is now recognized that biological membranes are highly organized structures, with differences in lipid compositions between inner and outer leaflets and in lateral structures within the bilayer plane, known as lipid rafts. These organizing principles are important for protein localization and function as well as cellular signaling. However, the mechanisms and biophysical basis of lipid raft formation, structure, dynamics and function are not clearly understood. One key question, which we focus on in this review, is how lateral organization and leaflet compositional asymmetry are coupled.more » Detailed information elucidating this question has been sparse because of the small size and transient nature of rafts and the experimental challenges in constructing asymmetric bilayers. Resolving this mystery will require advances in both experimentation and modeling. We discuss here the preparation of model systems along with experimental and computational approaches that have been applied in efforts to address this key question in membrane biology. Furthermore, we seek to place recent and future advances in experimental and computational techniques in context, providing insight into in-plane and transverse organization of biological membranes.« less

Water in Inhomogeneous Nanoconfinement: Coexistence of Multilayered Liquid and Transition to Ice Nanoribbons.

PubMed

Qiu, Hu; Zeng, Xiao Cheng; Guo, Wanlin

2015-10-27

Phase behavior and the associated phase transition of water within inhomogeneous nanoconfinement are investigated using molecular dynamics simulations. The nanoconfinement is constructed by a flat bottom plate and a convex top plate. At 300 K, the confined water can be viewed as a coexistence of monolayer, bilayer, and trilayer liquid domains to accommodate the inhomogeneous confinement. With increasing liquid density, the confined water with uneven layers transforms separately into two-dimensional ice crystals with unchanged layer number and rhombic in-plane symmetry for oxygen atoms. The monolayer water undergoes the transition first into a puckered ice nanoribbon, and the bilayer water transforms into a rhombic ice nanoribbon next, followed by the transition of trilayer water into a trilayer ice nanoribbon. The sequential localized liquid-to-solid transition within the inhomogeneous confinement can also be achieved by gradually decreasing the temperature at low liquid densities. These findings of phase behaviors of water under the inhomogeneous nanoconfinement not only extend the phase diagram of confined water but also have implications for realistic nanofluidic systems and microporous materials.

Thickness-dependent surface plasmon resonance of ITO nanoparticles for ITO/In-Sn bilayer structure.

PubMed

Wei, Wenzuo; Hong, Ruijin; Jing, Ming; Shao, Wen; Tao, Chunxian; Zhang, Dawei

2018-01-05

Tuning the localized surface plasmon resonance (LSPR) in doped semiconductor nanoparticles (NPs), which represents an important characteristic in LSPR sensor applications, still remains a challenge. Here, indium tin oxide/indium tin alloy (ITO/In-Sn) bilayer films were deposited by electron beam evaporation and the properties, such as the LSPR and surface morphology, were investigated by UV-VIS-NIR double beam spectrophotometer and atomic force microscopy (AFM), respectively. By simply engineering the thickness of ITO/In-Sn NPs without any microstructure fabrications, the LSPR wavelength of ITO NPs can be tuned by a large amount from 858 to 1758 nm. AFM images show that the strong LSPR of ITO NPs is closely related to the enhanced coupling between ITO and In-Sn NPs. Blue shifts of ITO LSPR from 1256 to 1104 nm are also observed in the as-annealed samples due to the higher free carrier concentration. Meanwhile, we also demonstrated that the ITO LSPR in ITO/In-Sn NPs structures has good sensitivity to the surrounding media and stability after 30 d exposure in air, enabling its application prospects in many biosensing devices.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Seyler, Kyle L.; Zhong, Ding; Klein, Dahlia R.

Bulk chromium tri-iodide (CrI 3) has long been known as a layered van der Waals ferromagnet. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet, providing a new platform for investigating light–matter interactions and magneto-optical phenomena in the atomically thin limit. Here in this paper, we report spontaneous circularly polarized photoluminescence in monolayer CrI 3 under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI 3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI 3 bilayers. Distinct frommore » the Wannier–Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI 3. We attribute the photoluminescence to a parity-forbidden d–d transition characteristic of Cr 3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.« less

Charge renormalization and inversion of a highly charged lipid bilayer: effects of dielectric discontinuities and charge correlations.

PubMed

Taheri-Araghi, Sattar; Ha, Bae-Yeun

2005-08-01

We reexamine the problem of charge renormalization and inversion of a highly charged surface of a low dielectric constant immersed in ionic solutions. To be specific, we consider an asymmetrically charged lipid bilayer, in which only one layer is negatively charged. In particular, we study how dielectric discontinuities and charge correlations (among lipid charges and condensed counterions) influence the effective charge of the surface. When counterions are monovalent (e.g., Na+), our mean-field approach implies that dielectric discontinuities can enhance counterion condensation. A simple scaling picture shows how the effects of dielectric discontinuities and surface-charge distributions are intertwined: Dielectric discontinuities diminish condensation if the backbone charge is uniformly smeared out while counterions are localized in space; they can, however, enhance condensation when the backbone charge is discrete. In the presence of asymmetric salts such as CaCl2 , we find that the correlation effect, treated at the Gaussian level, is more pronounced when the surface has a lower dielectric constant, inverting the sign of the charge at a smaller value of Ca2+ concentration.

Imaging Magnetization Structure and Dynamics in Ultrathin Y3Fe5O12/Pt Bilayers with High Sensitivity Using the Time-Resolved Longitudinal Spin Seebeck Effect

NASA Astrophysics Data System (ADS)

Bartell, Jason M.; Jermain, Colin L.; Aradhya, Sriharsha V.; Brangham, Jack T.; Yang, Fengyuan; Ralph, Daniel C.; Fuchs, Gregory D.

2017-04-01

We demonstrate an instrument for time-resolved magnetic imaging that is highly sensitive to the in-plane magnetization state and dynamics of thin-film bilayers of yttrium iron garnet [Y3Fe5O12(YIG )]/Pt : the time-resolved longitudinal spin Seebeck (TRLSSE) effect microscope. We detect the local in-plane magnetic orientation within the YIG by focusing a picosecond laser to generate thermally driven spin current from the YIG into the Pt by the spin Seebeck effect and then use the inverse spin Hall effect in the Pt to transduce this spin current to an output voltage. To establish the time resolution of TRLSSE, we show that pulsed optical heating of patterned YIG (20 nm )/Pt (6 nm )/Ru (2 nm ) wires generates a magnetization-dependent voltage pulse of less than 100 ps. We demonstrate TRLSSE microscopy to image both static magnetic structure and gigahertz-frequency magnetic resonance dynamics with submicron spatial resolution and a sensitivity to magnetic orientation below 0.3 °/√{H z } in ultrathin YIG.

pH-dependent association of SN-38 with lipid bilayers of a novel liposomal formulation.

PubMed

Peikov, Viktor; Ugwu, Sydney; Parmar, Manjeet; Zhang, Allen; Ahmad, Imran

2005-08-11

The aim of this study was to determine the location of SN-38 molecules in a liposomal formulation as a function of pH. Steady-state fluorescence polarization anisotropy and gel filtration studies of blank (placebo) liposomes, liposomes containing SN-38 and SN-38 solutions (in some cases suspensions) were conducted before lyophilization and after re-hydration at different pH conditions. SN-38, l-(4-trimethylammoniumphenyl)-6-phenyl-l,3,5-hexatriene p-toluenesulfonate (TMA-DPH), N-((4-(6-phenyl-l,3,5-hexatrienyl)phenyl)propyl)trimethylammonium p-toluenesulfonate (TMAP-DPH) and l,6-diphenyl-l,3,5-hexatriene (DPH) were used as fluoroprobes in the polarization anisotropy measurements. The localization of SN-38 was governed by the degree of hydrophobicity of the drug molecules. At high pH, SN-38 is in its inactive, hydrophilic form and partitioned into the water phase of the liposome suspensions. In lyophilized LE-SN38 liposomes re-hydrated with low pH buffer, SN-38 was found at the water-lipid interface of the bilayer.

Selective area growth of Bernal bilayer epitaxial graphene on 4H-SiC (0001) substrate by electron-beam irradiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dharmaraj, P.; Jeganathan, K., E-mail: kjeganathan@yahoo.com; Parthiban, S.

We report selective area growth of large area homogeneous Bernal stacked bilayer epitaxial graphene (BLEG) on 4H-SiC (0001) substrate by electron-beam irradiation. Sublimation of Si occurs by energetic electron irradiations on SiC surface via breaking of Si–C bonds in the localized region, which allows the selective growth of graphene. Raman measurements ensure the formation of homogeneous BLEG with weak compressive strain of −0.08%. The carrier mobility of large area BLEG is ∼5100 cm{sup 2} V{sup −1} s{sup −1} with a sheet carrier density of 2.2 × 10{sup 13} cm{sup −2}. Current-voltage measurements reveal that BLEG on 4H-SiC forms a Schottky junction with an operation at mAmore » level. Our study reveals that the barrier height at the Schottky junction is low (∼0.58 eV) due to the Fermi-level pinning above the Dirac point.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kapp, O.H.; Mainwaring, M.G.; Vinogradov, S.N.

A fraction of the extracellular hemoglobin of Lumbricus terrestris, obtained by gel filtration at neutral pH subsequent to dissociation either at pH 9.8 or at pH 4.0 or at pH 7.0 in 10 mM sodium phosphotungstate, consisting of the three subunits D1 (31 kDa), D2 (37 kDa) and T (50 kDa), produced two peaks when subjected to FPLC on a Superose 6 column at neutral pH. Peak I, eluting at a slightly greater volume than the native hemoglobin, consisted of reassociated hexagonal bilayer structures when examined by scanning transmission electron microscopy. The dimensions of the three reassociated hexagonal bilayer structuresmore » were 25 nm x 16 nm. Although the latter are smaller than the dimensions of the native hemoglobin, 30 nm x 20 nm, the diameter of the central cavity remained unchanged. Subtraction of the digitized and averaged images of the reassociated forms from those of the native hemoglobin suggested that the spatial localization of the fourth subunit, subunit M (16.7 kDa), was limited primarily to the periphery of the molecule.« less

Morphological transitions of brain sphingomyelin are determined by the hydration protocol: ripples re-arrange in plane, and sponge-like networks disintegrate into small vesicles.

PubMed

Meyer, H W; Bunjes, H; Ulrich, A S

1999-06-01

The phase transition of hydrated brain sphingomyelin occurs at around 35 degrees C, which is close to the physiological temperature. Freeze-fracture electron microscopy is used to characterize different gel state morphologies in terms of solid-ordered and liquid-ordered phase states, according to the occurrence of ripples and other higher-dimensional bilayer deformations. Evidently, the natural mixed-chain sphingomyelin does not assume the flat L beta, phase but instead the rippled P beta, phase, with symmetric and asymmetric ripples as well as macroripples and an egg-carton pattern, depending on the incubation conditions. An unexpected difference was observed between samples that are hydrated above and below the phase transition temperature. When the lipid is hydrated at low temperature, a sponge-like network of bilayers is formed in the gel state, next to some normal lamellae. The network loses its ripples during cold-incubation, which indicates the formation of a liquid-ordered (lo) gel phase. Ripples re-appear upon warming and the sponge-like network disintegrates spontaneously and irreversibly into small vesicles above the phase transition.

Three Dimensional Architecture of Membrane-Embedded MscS in the Closed Conformation

PubMed Central

Vásquez, Valeria; Sotomayor, Marcos; Cortes, D. Marien; Roux, Benoît; Schulten, Klaus; Perozo, Eduardo

2009-01-01

The mechanosensitive channel of small conductance (MscS) is part of a coordinated response to osmotic challenges in E. coli. MscS opens as a result of membrane tension changes, thereby releasing small solutes and effectively acting as an osmotic safety valve. Both, the functional state depicted by its crystal structure and its gating mechanism remain unclear. Here, we combine site-directed spin labeling, electron paramagnetic resonance (EPR) spectroscopy, and molecular dynamics simulations with novel energy restraints based on experimental EPR data to investigate the native transmembrane and periplasmic molecular architecture of closed MscS in a lipid bilayer. In the closed conformation, MscS shows a more compact transmembrane domain than in the crystal structure, characterized by a realignment of the transmembrane segments towards the normal of the membrane. The previously unresolved NH2-terminus forms a short helical hairpin capping the extracellular ends of TM1 and TM2 and in close interaction with the bilayer interface. The present three-dimensional model of membrane-embedded MscS in the closed state represents a key step in determining the molecular mechanism of MscS gating. PMID:18343404

The behaviour of tributyl phosphate in an organic diluent

NASA Astrophysics Data System (ADS)

Leay, Laura; Tucker, Kate; Del Regno, Annalaura; Schroeder, Sven L. M.; Sharrad, Clint A.; Masters, Andrew J.

2014-09-01

Tributyl phosphate (TBP) is used as a complexing agent in the Plutonium Uranium Extraction (PUREX) liquid-liquid phase extraction process for recovering uranium and plutonium from spent nuclear reactor fuel. Here, we address the molecular and microstructure of the organic phases involved in the extraction process, using molecular dynamics to show that when TBP is mixed with a paraffinic diluent, the TBP self-assembles into a bi-continuous phase. The underlying self-association of TBP is driven by intermolecular interaction between its polar groups, resulting in butyl moieties radiating out into the organic solvent. Simulation predicts a TBP diffusion constant that is anomalously low compared to what might normally be expected for its size; experimental nuclear magnetic resonance (NMR) studies also indicate an extremely low diffusion constant, consistent with a molecular aggregation model. Simulation of TBP at an oil/water interface shows the formation of a bilayer system at low TBP concentrations. At higher concentrations, a bulk bi-continuous structure is observed linking to this surface bilayer. We suggest that this structure may be intimately connected with the surprisingly rapid kinetics of the interfacial mass transport of uranium and plutonium from the aqueous to the organic phase in the PUREX process.

Passive Membrane Permeability: Beyond the Standard Solubility-Diffusion Model.

PubMed

Parisio, Giulia; Stocchero, Matteo; Ferrarini, Alberta

2013-12-10

The spontaneous diffusion of solutes through lipid bilayers is still a challenge for theoretical predictions. Since permeation processes remain beyond the capabilities of unbiased molecular dynamics simulations, an alternative strategy is currently adopted to gain insight into their mechanism and time scale. This is based on a monodimensional description of the translocation process only in terms of the position of the solute along the normal to the lipid bilayer, which is formally expressed in the solubility-diffusion model. Actually, a role of orientational and conformational motions has been pointed out, and the use of advanced simulation techniques has been proposed to take into account their effect. Here, we discuss the limitations of the standard solubility-diffusion approach and propose a more general description of membrane translocation as a diffusion process on a free energy surface, which is a function of the translational and rotational degrees of freedom of the molecule. Simple expressions for the permeability coefficient are obtained under suitable conditions. For fast solute reorientation, the classical solubility-diffusion equation is recovered. Under the assumption that well-defined minima can be identified on the free energy landscape, a mechanistic interpretation of the permeability coefficient in terms of all possible permeation paths is given.

Quasiclassical theory for the superconducting proximity effect in Dirac materials

NASA Astrophysics Data System (ADS)

Hugdal, Henning G.; Linder, Jacob; Jacobsen, Sol H.

2017-06-01

We derive the quasiclassical nonequilibrium Eilenberger and Usadel equations to first order in quantities small compared to the Fermi energy, valid for Dirac edge and surface electrons with spin-momentum locking p .σ ¯ , as relevant for topological insulators. We discuss in detail several of the key technical points and assumptions of the derivation, and provide a Riccati parametrization of the equations. Solving first the equilibrium equations for S/N and S/F bilayers and Josephson junctions, we study the superconducting proximity effect in Dirac materials. Similarly to related works, we find that the effect of an exchange field depends strongly on the direction of the field. Only components normal to the transport direction lead to attenuation of the Cooper pair wave function inside the F. Fields parallel to the transport direction lead to phase shifts in the dependence on the superconducting phase difference for both the charge current and density of states in an S/F/S junction. Moreover, we compute the differential conductance in S/N and S/F bilayers with an applied voltage bias and determine the dependence on the length of the N and F regions and the exchange field.

Arrayed water-in-oil droplet bilayers for membrane transport analysis.

PubMed

Watanabe, R; Soga, N; Hara, M; Noji, H

2016-08-02

The water-in-oil droplet bilayer is a simple and useful lipid bilayer system for membrane transport analysis. The droplet interface bilayer is readily formed by the contact of two water-in-oil droplets enwrapped by a phospholipid monolayer. However, the size of individual droplets with femtoliter volumes in a high-throughput manner is difficult to control, resulting in low sensitivity and throughput of membrane transport analysis. To overcome this drawback, in this study, we developed a novel micro-device in which a large number of droplet interface bilayers (>500) are formed at a time by using femtoliter-sized droplet arrays immobilized on a hydrophobic/hydrophilic substrate. The droplet volume was controllable from 3.5 to 350 fL by changing the hydrophobic/hydrophilic pattern on the device, allowing high-throughput analysis of membrane transport mechanisms including membrane permeability to solutes (e.g., ions or small molecules) with or without the aid of transport proteins. Thus, this novel platform broadens the versatility of water-in-oil droplet bilayers and will pave the way for novel analytical and pharmacological applications such as drug screening.

Thermal Response Analysis of Phospholipid Bilayers Using Ellipsometric Techniques.

PubMed

González-Henríquez, Carmen M; Villegas-Opazo, Vanessa A; Sagredo-Oyarce, Dallits H; Sarabia-Vallejos, Mauricio A; Terraza, Claudio A

2017-08-18

Biomimetic planar artificial membranes have been widely studied due to their multiple applications in several research fields. Their humectation and thermal response are crucial for reaching stability; these characteristics are related to the molecular organization inside the bilayer, which is affected by the aliphatic chain length, saturations, and molecule polarity, among others. Bilayer stability becomes a fundamental factor when technological devices are developed-like biosensors-based on those systems. Thermal studies were performed for different types of phosphatidylcholine (PC) molecules: two pure PC bilayers and four binary PC mixtures. These analyses were carried out through the detection of slight changes in their optical and structural parameters via Ellipsometry and Surface Plasmon Resonance (SPR) techniques. Phospholipid bilayers were prepared by Langmuir-Blodgett technique and deposited over a hydrophilic silicon wafer. Their molecular inclination degree, mobility, and stability of the different phases were detected and analyzed through bilayer thickness changes and their optical phase-amplitude response. Results show that certain binary lipid mixtures-with differences in its aliphatic chain length-present a co-existence of two thermal responses due to non-ideal mixing.

Comparative atomic-scale hydration of the ceramide and phosphocholine headgroup in solution and bilayer environments

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gillams, Richard J.; McLain, Sylvia E., E-mail: sylvia.mclain@bioch.ox.ac.uk; Lorenz, Christian D., E-mail: chris.lorenz@kcl.ac.uk

2016-06-14

Previous studies have used neutron diffraction to elucidate the hydration of the ceramide and the phosphatidylcholine headgroup in solution. These solution studies provide bond-length resolution information on the system, but are limited to liquid samples. The work presented here investigates how the hydration of ceramide and phosphatidylcholine headgroups in a solution compares with that found in a lipid bilayer. This work shows that the hydration patterns seen in the solution samples provide valuable insight into the preferential location of hydrating water molecules in the bilayer. There are certain subtle differences in the distribution, which result from a combination of themore » lipid conformation and the lipid-lipid interactions within the bilayer environment. The lipid-lipid interactions in the bilayer will be dependent on the composition of the bilayer, whereas the restricted exploration of conformational space is likely to be applicable in all membrane environments. The generalized description of hydration gathered from the neutron diffraction studies thus provides good initial estimation for the hydration pattern, but this can be further refined for specific systems.« less

Mechanical properties of drug loaded diblock copolymer bilayers: A molecular dynamics study

NASA Astrophysics Data System (ADS)

Grillo, Damián A.; Albano, Juan M. R.; Mocskos, Esteban E.; Facelli, Julio C.; Pickholz, Mónica; Ferraro, Marta B.

2018-06-01

In this work, we present results of coarse-grained simulations to study the encapsulation of prilocaine (PLC), both neutral and protonated, on copolymer bilayers through molecular dynamics simulations. Using a previously validated membrane model, we have simulated loaded bilayers at different drug concentrations and at low (protonated PLC) and high (neutral PLC) pH levels. We have characterized key structural parameters of the loaded bilayers in order to understand the effects of encapsulation of PLC on the bilayer structure and mechanical properties. Neutral PLC was encapsulated in the hydrophobic region leading to a thickness increase, while the protonated species partitioned between the water phase and the poly(ethylene oxide)-poly(butadiene) (PBD) interface, relaxing the PBD region and leading to a decrease in the thickness. The tangential pressures of the studied systems were calculated, and their components were decomposed in order to gain insights on their compensation. In all cases, it is observed that the loading of the membrane does not significantly decrease the stability of the bilayer, indicating that the system could be used for drug delivery.

Mechanical properties of drug loaded diblock copolymer bilayers: A molecular dynamics study.

PubMed

Grillo, Damián A; Albano, Juan M R; Mocskos, Esteban E; Facelli, Julio C; Pickholz, Mónica; Ferraro, Marta B

2018-06-07

In this work, we present results of coarse-grained simulations to study the encapsulation of prilocaine (PLC), both neutral and protonated, on copolymer bilayers through molecular dynamics simulations. Using a previously validated membrane model, we have simulated loaded bilayers at different drug concentrations and at low (protonated PLC) and high (neutral PLC) pH levels. We have characterized key structural parameters of the loaded bilayers in order to understand the effects of encapsulation of PLC on the bilayer structure and mechanical properties. Neutral PLC was encapsulated in the hydrophobic region leading to a thickness increase, while the protonated species partitioned between the water phase and the poly(ethylene oxide)-poly(butadiene) (PBD) interface, relaxing the PBD region and leading to a decrease in the thickness. The tangential pressures of the studied systems were calculated, and their components were decomposed in order to gain insights on their compensation. In all cases, it is observed that the loading of the membrane does not significantly decrease the stability of the bilayer, indicating that the system could be used for drug delivery.

Tensile and shear methods for measuring strength of bilayer tablets.

PubMed

Chang, Shao-Yu; Li, Jian-Xin; Sun, Changquan Calvin

2017-05-15

Both shear and tensile measurement methods have been used to quantify interfacial bonding strength of bilayer tablets. The shear method is more convenient to perform, but reproducible strength data requires careful control of the placement of tablet and contact point for shear force application. Moreover, data obtained from the shear method depend on the orientation of the bilayer tablet. Although more time-consuming to perform, the tensile method yields data that are straightforward to interpret. Thus, the tensile method is preferred in fundamental bilayer tableting research to minimize ambiguity in data interpretation. Using both shear and tensile methods, we measured the mechanical strength of bilayer tablets made of several different layer combinations of lactose and microcrystalline cellulose. We observed a good correlation between strength obtained by the tensile method and carefully conducted shear method. This suggests that the shear method may be used for routine quality test of bilayer tablets during manufacturing because of its speed and convenience, provided a protocol for careful control of the placement of the tablet interface, tablet orientation, and blade is implemented. Copyright © 2017 Elsevier B.V. All rights reserved.

Modeling Nanoparticle Wrapping or Translocation in Bilayer Membranes

PubMed Central

Curtis, Emily M.; Bahrami, Amir H.; Weikl, Thomas R.; Hall, Carol K.

2015-01-01

The spontaneous wrapping of nanoparticles by membranes is of increasing interest as nanoparticles become more prevalent in consumer products and hence more likely to enter the human body. We introduce a simulations-based tool that can be used to visualize the molecular level interaction between nanoparticles and bilayer membranes. By combining LIME, an intermediate resolution, implicit solvent model for phospholipids, with discontinuous molecular dynamics (DMD), we are able to simulate the wrapping or embedding of nanoparticles by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer membranes. Simulations of hydrophilic nanoparticles with diameters from 10Å to 250Å show that hydrophilic nanoparticles with diameters greater than 20Å become wrapped while the nanoparticle with a diameter of 10Å does not . Instead this smaller particle became embedded in the bilayer surface where it could interact with the hydrophilic head groups of the lipid molecules. We also investigate the interaction between a DPPC bilayer and hydrophobic nanoparticles with diameters 10Å to 40Å. These nanoparticles do not undergo the wrapping process; instead they directly penetrate the membrane and embed themselves within the inner hydrophobic core of the bilayers. PMID:26260123

Neutron Reflectivity as a Tool for Physics-Based Studies of Model Bacterial Membranes.

PubMed

Barker, Robert D; McKinley, Laura E; Titmuss, Simon

2016-01-01

The principles of neutron reflectivity and its application as a tool to provide structural information at the (sub-) molecular unit length scale from models for bacterial membranes are described. The model membranes can take the form of a monolayer for a single leaflet spread at the air/water interface, or bilayers of increasing complexity at the solid/liquid interface. Solid-supported bilayers constrain the bilayer to 2D but can be used to characterize interactions with antimicrobial peptides and benchmark high throughput lab-based techniques. Floating bilayers allow for membrane fluctuations, making the phase behaviour more representative of native membranes. Bilayers of varying levels of compositional accuracy can now be constructed, facilitating studies with aims that range from characterizing the fundamental physical interactions, through to the characterization of accurate mimetics for the inner and outer membranes of Gram-negative bacteria. Studies of the interactions of antimicrobial peptides with monolayer and bilayer models for the inner and outer membranes have revealed information about the molecular control of the outer membrane permeability, and the mode of interaction of antimicrobials with both inner and outer membranes.

Calcium and zinc differentially affect the structure of lipid membranes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kučerka, Norbert; Dushanov, Ermuhammad; Kholmurodov, Kholmirzo T.

Interactions of calcium (Ca 2+) and zinc (Zn 2+) cations with biomimetic membranes made of dipalmitoylphosphatidylcholine (DPPC) were studied by small angle neutron diffraction (SAND). Experiments show that the structure of these lipid bilayers is differentially affected by the two divalent cations. Initially, both Ca 2+ and Zn 2+ cause DPPC bilayers to thicken, while further increases in Ca 2+ concentration result in the bilayer thinning, eventually reverting to having the same thickness as pure DPPC. The binding of Zn 2+, on the other hand, causes the bilayers to swell to a maximum thickness, and the addition of more Znmore » 2+ does not result in a further thickening of the membrane. Agreement between our results obtained using oriented planar membranes and those from vesicular samples implies that the effect of cations on bilayer thickness is the result of electrostatic interactions, rather than geometrical constraints due to bilayer curvature. This notion is further reinforced by MD simulations. Lastly, the radial distribution functions reveal a strong interaction between Ca 2+ and the phosphate oxygens, while Zn 2+ shows a much weaker binding specificity.« less

Effect of vacuum thermal annealing on a molybdenum bilayer back contact deposited by radio-frequency magnetron sputtering for chalcogenide- and kesterite-based solar cells

NASA Astrophysics Data System (ADS)

Liu, Xiaolei; Cui, Hongtao; Hao, Xiaojing; Huang, Shujuan; Conibeer, Gavin

2017-12-01

Molybdenum (Mo) thin films are still a dominant choice for the back contact layer of Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) solar cells. This paper presents a review of Mo back contacts for CIGS and CZTS solar cells, including the requirements for a good back contact, the reason for the choice of Mo, and post-treatment. Additionally, a Mo bilayer back contact was fabricated by varying the argon (Ar) pressure during sputtering to provide both low resistivity and good adhesion to the soda-lime glass substrate. The effects of vacuum thermal annealing on the electrical, morphological and structural properties of the Mo bilayer were also investigated. Vacuum thermal annealing was seen to densify the Mo bilayer, reduce the sheet resistance, and improve the bilayer's adhesion to the soda-lime glass. The Mo bilayer back contact with a low sheet resistance of 0.132 Ω/□ and strong adhesion was made for chalcogenide- and kesterite-based solar cells.

Single-molecule height measurements on microsomal cytochrome P450 in nanometer-scale phospholipid bilayer disks

NASA Astrophysics Data System (ADS)

Bayburt, Timothy H.; Sligar, Stephen G.

2002-05-01

The architecture of membrane proteins in their native environment of the phospholipid bilayer is critical for understanding physiological function, but has been difficult to realize experimentally. In this communication we describe the incorporation of a membrane-anchored protein into a supported phospholipid bilayer. Cytochrome P450 2B4 solubilized and purified from the hepatic endoplasmic reticulum was incorporated into phospholipid bilayer nanostructures and oriented on a surface for visualization by atomic force microscopy. Individual P450 molecules were observed protruding from the bilayer surface. Problems associated with deformation of the protein by the atomic force microscopy probe were avoided by analyzing force-dependent height measurements to quantitate the height of the protein above the bilayer surface. Measurements of the atomic force microscopy cantilever deflection as a function of probe-sample separation reveal that the top of the P450 opposite the N-terminal membrane anchor region sits 3.5 nanometers above the phospholipid-water boundary. Models of the orientation of the enzyme are presented and discussed in relation to membrane interactions and interaction with cytochrome P450 reductase.

Calcium and zinc differentially affect the structure of lipid membranes

DOE PAGES

Kučerka, Norbert; Dushanov, Ermuhammad; Kholmurodov, Kholmirzo T.; ...

2017-03-09

Interactions of calcium (Ca 2+) and zinc (Zn 2+) cations with biomimetic membranes made of dipalmitoylphosphatidylcholine (DPPC) were studied by small angle neutron diffraction (SAND). Experiments show that the structure of these lipid bilayers is differentially affected by the two divalent cations. Initially, both Ca 2+ and Zn 2+ cause DPPC bilayers to thicken, while further increases in Ca 2+ concentration result in the bilayer thinning, eventually reverting to having the same thickness as pure DPPC. The binding of Zn 2+, on the other hand, causes the bilayers to swell to a maximum thickness, and the addition of more Znmore » 2+ does not result in a further thickening of the membrane. Agreement between our results obtained using oriented planar membranes and those from vesicular samples implies that the effect of cations on bilayer thickness is the result of electrostatic interactions, rather than geometrical constraints due to bilayer curvature. This notion is further reinforced by MD simulations. Lastly, the radial distribution functions reveal a strong interaction between Ca 2+ and the phosphate oxygens, while Zn 2+ shows a much weaker binding specificity.« less

Depletion with Cyclodextrin Reveals Two Populations of Cholesterol in Model Lipid Membranes

PubMed Central

Litz, Jonathan P.; Thakkar, Niket; Portet, Thomas; Keller, Sarah L.

2016-01-01

Recent results provide evidence that cholesterol is highly accessible for removal from both cell and model membranes above a threshold concentration that varies with membrane composition. Here we measured the rate at which methyl-β-cyclodextrin depletes cholesterol from a supported lipid bilayer as a function of cholesterol mole fraction. We formed supported bilayers from two-component mixtures of cholesterol and a PC (phosphatidylcholine) lipid, and we directly visualized the rate of decrease in area of the bilayers with fluorescence microscopy. Our technique yields the accessibility of cholesterol over a wide range of concentrations (30–66 mol %) for many individual bilayers, enabling fast acquisition of replicate data. We found that the bilayers contain two populations of cholesterol, one with low surface accessibility and the other with high accessibility. A larger fraction of the total membrane cholesterol appears in the more accessible population when the acyl chains of the PC-lipid tails are more unsaturated. Our findings are most consistent with the predictions of the condensed-complex and cholesterol bilayer domain models of cholesterol-phospholipid interactions in lipid membranes. PMID:26840728

The electronic transport properties of defected bilayer sliding armchair graphene nanoribbons

NASA Astrophysics Data System (ADS)

Mohammadi, Amin; Haji-Nasiri, Saeed

2018-04-01

By applying non-equilibrium Green's functions (NEGF) in combination with tight-binding (TB) model, we investigate and compare the electronic transport properties of perfect and defected bilayer armchair graphene nanoribbons (BAGNRs) under finite bias. Two typical defects which are placed in the middle of top layer (i.e. single vacancy (SV) and stone wale (SW) defects) are examined. The results reveal that in both perfect and defected bilayers, the maximum current refers to β-AB, AA and α-AB stacking orders, respectively, since the intermolecular interactions are stronger in them. Moreover it is observed that a SV decreases the current in all stacking orders, but the effects of a SW defect is nearly unpredictable. Besides, we introduced a sequential switching behavior and the effects of defects on the switching performance is studied as well. We found that a SW defect can significantly improve the switching behavior of a bilayer system. Transmission spectrum, band structure, molecular energy spectrum and molecular projected self-consistent Hamiltonian (MPSH) are analyzed subsequently to understand the electronic transport properties of these bilayer devices which can be used in developing nano-scale bilayer systems.