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Sample records for 1-palmitoyl-2-oleoyl-phosphatidylcholine popc bilayers

  1. A molecular dynamics study of the lateral free energy profile of a pair of cholesterol molecules as a function of their distance in phospholipid bilayers

    NASA Astrophysics Data System (ADS)

    Andoh, Yoshimichi; Oono, Kimiko; Okazaki, Susumu; Hatta, Ichiro

    2012-04-01

    Free energy profile of a pair of cholesterol molecules in a leaflet of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers in the liquid-crystalline phase has been calculated as a function of their lateral distance using a combination of NPT-constant atomistic molecular dynamics calculations (P = 1 atm and T = 310.15 K) and the thermodynamic integration method. The calculated free energy clearly shows that the two cholesterol molecules form a dimer separated by a distance of 1.0-1.5 nm in POPC bilayers. Well depth of the free energy profile is about 3.5 kJ/mol, which is comparable to the thermal energy kBT at 310.15 K. This indicates that the aggregation of cholesterol molecules in the bilayers depends on the temperature as well as the concentration of the system. The free energy function obtained here may be used as a reference when coarse grained potential model is investigated for this two-component system. Local structure of POPC molecules around two cholesterol molecules has also been investigated.

  2. Influence of Hydrophobic Mismatch on Structures and Dynamics of Gramicidin A and Lipid Bilayers

    PubMed Central

    Kim, Taehoon; Lee, Kyu Il; Morris, Phillip; Pastor, Richard W.; Andersen, Olaf S.; Im, Wonpil

    2012-01-01

    Gramicidin A (gA) is a 15-amino-acid antibiotic peptide with an alternating L-D sequence, which forms (dimeric) bilayer-spanning, monovalent cation channels in biological membranes and synthetic bilayers. We performed molecular dynamics simulations of gA dimers and monomers in all-atom, explicit dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dioleoylphosphatidylcholine (DOPC), and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers. The variation in acyl chain length among these different phospholipids provides a way to alter gA-bilayer interactions by varying the bilayer hydrophobic thickness, and to determine the influence of hydrophobic mismatch on the structure and dynamics of both gA channels (and monomeric subunits) and the host bilayers. The simulations show that the channel structure varied little with changes in hydrophobic mismatch, and that the lipid bilayer adapts to the bilayer-spanning channel to minimize the exposure of hydrophobic residues. The bilayer thickness, however, did not vary monotonically as a function of radial distance from the channel. In all simulations, there was an initial decrease in thickness within 4–5 Å from the channel, which was followed by an increase in DOPC and POPC or a further decrease in DLPC and DMPC bilayers. The bilayer thickness varied little in the monomer simulations—except one of three independent simulations for DMPC and all three DLPC simulations, where the bilayer thinned to allow a single subunit to form a bilayer-spanning water-permeable pore. The radial dependence of local lipid area and bilayer compressibility is also nonmonotonic in the first shell around gA dimers due to gA-phospholipid interactions and the hydrophobic mismatch. Order parameters, acyl chain dynamics, and diffusion constants also differ between the lipids in the first shell and the bulk. The lipid behaviors in the first shell around gA dimers are more complex than predicted from a simple mismatch

  3. Influence of hydrophobic mismatch on structures and dynamics of gramicidin a and lipid bilayers.

    PubMed

    Kim, Taehoon; Lee, Kyu Il; Morris, Phillip; Pastor, Richard W; Andersen, Olaf S; Im, Wonpil

    2012-04-01

    Gramicidin A (gA) is a 15-amino-acid antibiotic peptide with an alternating L-D sequence, which forms (dimeric) bilayer-spanning, monovalent cation channels in biological membranes and synthetic bilayers. We performed molecular dynamics simulations of gA dimers and monomers in all-atom, explicit dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dioleoylphosphatidylcholine (DOPC), and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers. The variation in acyl chain length among these different phospholipids provides a way to alter gA-bilayer interactions by varying the bilayer hydrophobic thickness, and to determine the influence of hydrophobic mismatch on the structure and dynamics of both gA channels (and monomeric subunits) and the host bilayers. The simulations show that the channel structure varied little with changes in hydrophobic mismatch, and that the lipid bilayer adapts to the bilayer-spanning channel to minimize the exposure of hydrophobic residues. The bilayer thickness, however, did not vary monotonically as a function of radial distance from the channel. In all simulations, there was an initial decrease in thickness within 4-5 Å from the channel, which was followed by an increase in DOPC and POPC or a further decrease in DLPC and DMPC bilayers. The bilayer thickness varied little in the monomer simulations-except one of three independent simulations for DMPC and all three DLPC simulations, where the bilayer thinned to allow a single subunit to form a bilayer-spanning water-permeable pore. The radial dependence of local lipid area and bilayer compressibility is also nonmonotonic in the first shell around gA dimers due to gA-phospholipid interactions and the hydrophobic mismatch. Order parameters, acyl chain dynamics, and diffusion constants also differ between the lipids in the first shell and the bulk. The lipid behaviors in the first shell around gA dimers are more complex than predicted from a simple mismatch model

  4. Macro-ripple phase formation in bilayers composed of galactosylceramide and phosphatidylcholine.

    PubMed Central

    Brown, R E; Anderson, W H; Kulkarni, V S

    1995-01-01

    As determined by freeze fracture electron microscopy, increasing levels of bovine brain galactosylceramide (GalCer) altered the surface structure of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers by inducing a striking "macro-ripple" phase in the larger, multilamellar lipid vesicles at GalCer mole fractions between 0.4 and 0.8. The term "macro-ripple" phase was used to distinguish it from the P beta' ripple phase observed in saturated, symmetric-chain length phosphatidylcholines. Whereas the P beta' ripple phase displays two types of corrugations, one with a wavelength of 12-15 nm and the other with a wavelength of 25-35 nm, the macro-ripple phase occurring in GalCer/POPC dispersions was of one type with a wavelength of 100-110 nm. Also, in contrast to the extended linear arrays of adjacent ripples observed in the P beta' ripple phase, the macro-ripple phase of GalCer/POPC dispersions was interrupted frequently by packing defects resulting from double dislocations and various disclinations and, thus, appeared to be continuously twisting and turning. Control experiments verified that the macro-ripple phase was not an artifact of incomplete lipid mixing or demixing during preparation. Three different methods of lipid mixing were compared: a spray method of rapid solvent evaporation, a sublimation method of solvent removal, and solvent removal using a rotary evaporation apparatus. Control experiments also revealed that the macro-ripple phase was observed regardless of whether lipid specimens were prepared by either ultra-rapid or manual plunge freezing methods as well as either in the presence or absence of the cryo-protectant glycerol. The macro-ripple phase was always observed in mixtures that were fully annealed by incubation above the main thermal transition of both POPC and bovine brain GalCer before rapid freezing. If the GalCer mixed with POPC contained only nonhydroxy acyl chains or only 2-hydroxy acyl chains, then the occurrence of macro

  5. Macro-ripple phase formation in bilayers composed of galactosylceramide and phosphatidylcholine.

    PubMed

    Brown, R E; Anderson, W H; Kulkarni, V S

    1995-04-01

    As determined by freeze fracture electron microscopy, increasing levels of bovine brain galactosylceramide (GalCer) altered the surface structure of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers by inducing a striking "macro-ripple" phase in the larger, multilamellar lipid vesicles at GalCer mole fractions between 0.4 and 0.8. The term "macro-ripple" phase was used to distinguish it from the P beta' ripple phase observed in saturated, symmetric-chain length phosphatidylcholines. Whereas the P beta' ripple phase displays two types of corrugations, one with a wavelength of 12-15 nm and the other with a wavelength of 25-35 nm, the macro-ripple phase occurring in GalCer/POPC dispersions was of one type with a wavelength of 100-110 nm. Also, in contrast to the extended linear arrays of adjacent ripples observed in the P beta' ripple phase, the macro-ripple phase of GalCer/POPC dispersions was interrupted frequently by packing defects resulting from double dislocations and various disclinations and, thus, appeared to be continuously twisting and turning. Control experiments verified that the macro-ripple phase was not an artifact of incomplete lipid mixing or demixing during preparation. Three different methods of lipid mixing were compared: a spray method of rapid solvent evaporation, a sublimation method of solvent removal, and solvent removal using a rotary evaporation apparatus. Control experiments also revealed that the macro-ripple phase was observed regardless of whether lipid specimens were prepared by either ultra-rapid or manual plunge freezing methods as well as either in the presence or absence of the cryo-protectant glycerol. The macro-ripple phase was always observed in mixtures that were fully annealed by incubation above the main thermal transition of both POPC and bovine brain GalCer before rapid freezing. If the GalCer mixed with POPC contained only nonhydroxy acyl chains or only 2-hydroxy acyl chains, then the occurrence of macro

  6. Nanomechanics of phospholipid bilayer failure under strip biaxial stretching using molecular dynamics

    NASA Astrophysics Data System (ADS)

    Murphy, M. A.; Horstemeyer, M. F.; Gwaltney, Steven R.; Stone, Tonya; LaPlaca, Michelle; Liao, Jun; Williams, Lakiesha; Prabhu, R.

    2016-06-01

    The current study presents a nanoscale in silico investigation of strain rate dependency of membrane (phospholipid bilayer) failure when placed under strip biaxial tension with two planar areas. The nanoscale simulations were conducted in the context of a multiscale modelling framework in which the macroscale damage (pore volume fraction) progression is delineated into pore nucleation (number density of pores), pore growth (size of pores), and pore coalescence (inverse of nearest neighbor distance) mechanisms. As such, the number density, area fraction, and nearest neighbor distances were quantified in association with the stress–strain behavior. Deformations of a 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer were performed using molecular dynamics to simulate mechanoporation of a neuronal cell membrane due to injury, which in turn can result in long-term detrimental effects that could ultimately lead to cell death. Structures with 72 and 144 phospholipids were subjected to strip biaxial tensile deformations at multiple strain rates. Formation of a water bridge through the phospholipid bilayer was the metric to indicate structural failure. Both the larger and smaller bilayers had similar behavior regarding pore nucleation and the strain rate effect on pore growth post water penetration. The applied strain rates, planar area, and cross-sectional area had no effect on the von Mises strains at which pores greater than 0.1 nm2 were detected (0.509  ±  7.8%) or the von Mises strain at failure (ε failure  =  0.68  ±  4.8%). Additionally, changes in bilayer planar and cross-sectional areas did not affect the stress response. However, as the strain rate increased from 2.0  ×  108 s‑1 to 1.0  ×  109 s‑1, the yield stress increased from 26.5 MPa to 66.7 MPa and the yield strain increased from 0.056 to 0.226.

  7. Nanomechanics of phospholipid bilayer failure under strip biaxial stretching using molecular dynamics

    NASA Astrophysics Data System (ADS)

    Murphy, M. A.; Horstemeyer, M. F.; Gwaltney, Steven R.; Stone, Tonya; LaPlaca, Michelle; Liao, Jun; Williams, Lakiesha; Prabhu, R.

    2016-06-01

    The current study presents a nanoscale in silico investigation of strain rate dependency of membrane (phospholipid bilayer) failure when placed under strip biaxial tension with two planar areas. The nanoscale simulations were conducted in the context of a multiscale modelling framework in which the macroscale damage (pore volume fraction) progression is delineated into pore nucleation (number density of pores), pore growth (size of pores), and pore coalescence (inverse of nearest neighbor distance) mechanisms. As such, the number density, area fraction, and nearest neighbor distances were quantified in association with the stress-strain behavior. Deformations of a 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer were performed using molecular dynamics to simulate mechanoporation of a neuronal cell membrane due to injury, which in turn can result in long-term detrimental effects that could ultimately lead to cell death. Structures with 72 and 144 phospholipids were subjected to strip biaxial tensile deformations at multiple strain rates. Formation of a water bridge through the phospholipid bilayer was the metric to indicate structural failure. Both the larger and smaller bilayers had similar behavior regarding pore nucleation and the strain rate effect on pore growth post water penetration. The applied strain rates, planar area, and cross-sectional area had no effect on the von Mises strains at which pores greater than 0.1 nm2 were detected (0.509  ±  7.8%) or the von Mises strain at failure (ɛ failure  =  0.68  ±  4.8%). Additionally, changes in bilayer planar and cross-sectional areas did not affect the stress response. However, as the strain rate increased from 2.0  ×  108 s-1 to 1.0  ×  109 s-1, the yield stress increased from 26.5 MPa to 66.7 MPa and the yield strain increased from 0.056 to 0.226.

  8. Structure and dynamics of POPC bilayers in water solutions of room temperature ionic liquids

    SciTech Connect

    Benedetto, Antonio; Bingham, Richard J.; Ballone, Pietro

    2015-03-28

    Molecular dynamics simulations in the NPT ensemble have been carried out to investigate the effect of two room temperature ionic liquids (RTILs), on stacks of phospholipid bilayers in water. We consider RTIL compounds consisting of chloride ([bmim][Cl]) and hexafluorophosphate ([bmim][PF{sub 6}]) salts of the 1-buthyl-3-methylimidazolium ([bmim]{sup +}) cation, while the phospholipid bilayer is made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Our investigations focus on structural and dynamical properties of phospholipid and water molecules that could be probed by inelastic and quasi-elastic neutron scattering measurements. The results confirm the fast incorporation of [bmim]{sup +} into the lipid phase already observed in previous simulations, driven by the Coulomb attraction of the cation for the most electronegative oxygens in the POPC head group and by sizeable dispersion forces binding the neutral hydrocarbon tails of [bmim]{sup +} and of POPC. The [bmim]{sup +} absorption into the bilayer favours the penetration of water into POPC, causes a slight but systematic thinning of the bilayer, and further stabilises hydrogen bonds at the lipid/water interface that already in pure samples (no RTIL) display a lifetime much longer than in bulk water. On the other hand, the effect of RTILs on the diffusion constant of POPC (D{sub POPC}) does not reveal a clearly identifiable trend, since D{sub POPC} increases upon addition of [bmim][Cl] and decreases in the [bmim][PF{sub 6}] case. Moreover, because of screening, the electrostatic signature of each bilayer is only moderately affected by the addition of RTIL ions in solution. The analysis of long wavelength fluctuations of the bilayers shows that RTIL sorption causes a general decrease of the lipid/water interfacial tension and bending rigidity, pointing to the destabilizing effect of RTILs on lipid bilayers.

  9. Structure and dynamics of POPC bilayers in water solutions of room temperature ionic liquids

    NASA Astrophysics Data System (ADS)

    Benedetto, Antonio; Bingham, Richard J.; Ballone, Pietro

    2015-03-01

    Molecular dynamics simulations in the NPT ensemble have been carried out to investigate the effect of two room temperature ionic liquids (RTILs), on stacks of phospholipid bilayers in water. We consider RTIL compounds consisting of chloride ([bmim][Cl]) and hexafluorophosphate ([bmim][PF6]) salts of the 1-buthyl-3-methylimidazolium ([bmim]+) cation, while the phospholipid bilayer is made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Our investigations focus on structural and dynamical properties of phospholipid and water molecules that could be probed by inelastic and quasi-elastic neutron scattering measurements. The results confirm the fast incorporation of [bmim]+ into the lipid phase already observed in previous simulations, driven by the Coulomb attraction of the cation for the most electronegative oxygens in the POPC head group and by sizeable dispersion forces binding the neutral hydrocarbon tails of [bmim]+ and of POPC. The [bmim]+ absorption into the bilayer favours the penetration of water into POPC, causes a slight but systematic thinning of the bilayer, and further stabilises hydrogen bonds at the lipid/water interface that already in pure samples (no RTIL) display a lifetime much longer than in bulk water. On the other hand, the effect of RTILs on the diffusion constant of POPC (DPOPC) does not reveal a clearly identifiable trend, since DPOPC increases upon addition of [bmim][Cl] and decreases in the [bmim][PF6] case. Moreover, because of screening, the electrostatic signature of each bilayer is only moderately affected by the addition of RTIL ions in solution. The analysis of long wavelength fluctuations of the bilayers shows that RTIL sorption causes a general decrease of the lipid/water interfacial tension and bending rigidity, pointing to the destabilizing effect of RTILs on lipid bilayers.

  10. Structure and dynamics of POPC bilayers in water solutions of room temperature ionic liquids.

    PubMed

    Benedetto, Antonio; Bingham, Richard J; Ballone, Pietro

    2015-03-28

    Molecular dynamics simulations in the NPT ensemble have been carried out to investigate the effect of two room temperature ionic liquids (RTILs), on stacks of phospholipid bilayers in water. We consider RTIL compounds consisting of chloride ([bmim][Cl]) and hexafluorophosphate ([bmim][PF6]) salts of the 1-buthyl-3-methylimidazolium ([bmim](+)) cation, while the phospholipid bilayer is made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Our investigations focus on structural and dynamical properties of phospholipid and water molecules that could be probed by inelastic and quasi-elastic neutron scattering measurements. The results confirm the fast incorporation of [bmim](+) into the lipid phase already observed in previous simulations, driven by the Coulomb attraction of the cation for the most electronegative oxygens in the POPC head group and by sizeable dispersion forces binding the neutral hydrocarbon tails of [bmim](+) and of POPC. The [bmim](+) absorption into the bilayer favours the penetration of water into POPC, causes a slight but systematic thinning of the bilayer, and further stabilises hydrogen bonds at the lipid/water interface that already in pure samples (no RTIL) display a lifetime much longer than in bulk water. On the other hand, the effect of RTILs on the diffusion constant of POPC (DPOPC) does not reveal a clearly identifiable trend, since DPOPC increases upon addition of [bmim][Cl] and decreases in the [bmim][PF6] case. Moreover, because of screening, the electrostatic signature of each bilayer is only moderately affected by the addition of RTIL ions in solution. The analysis of long wavelength fluctuations of the bilayers shows that RTIL sorption causes a general decrease of the lipid/water interfacial tension and bending rigidity, pointing to the destabilizing effect of RTILs on lipid bilayers. PMID:25833602

  11. Capsaicin interaction with TRPV1 channels in a lipid bilayer: molecular dynamics simulation.

    PubMed

    Hanson, Sonya M; Newstead, Simon; Swartz, Kenton J; Sansom, Mark S P

    2015-03-24

    Transient receptor potential vanilloid subtype 1 (TRPV1) is a heat-sensitive ion channel also involved in pain sensation, and is the receptor for capsaicin, the active ingredient of hot chili peppers. The recent structures of TRPV1 revealed putative ligand density within the S1 to S4 voltage-sensor-like domain of the protein. However, questions remain regarding the dynamic role of the lipid bilayer in ligand binding to TRPV1. Molecular dynamics simulations were used to explore behavior of capsaicin in a 1-palmitoyl-2-oleoyl phosphatidylcholine bilayer and with the target S1-S4 transmembrane helices of TRPV1. Equilibrium simulations reveal a preferred interfacial localization for capsaicin. We also observed a capsaicin molecule flipping from the extracellular to the intracellular leaflet, and subsequently able to access the intracellular TRPV1 binding site. Calculation of the potential of mean force (i.e., free energy profile) of capsaicin along the bilayer normal confirms that it prefers an interfacial localization. The free energy profile indicates that there is a nontrivial but surmountable barrier to the flipping of capsaicin between opposing leaflets of the bilayer. Molecular dynamics of the S1-S4 transmembrane helices of the TRPV1 in a lipid bilayer confirm that Y511, known to be crucial to capsaicin binding, has a distribution along the bilayer normal similar to that of the aromatic group of capsaicin. Simulations were conducted of the TRPV1 S1-S4 transmembrane helices in the presence of capsaicin placed in the aqueous phase, in the lipid, or docked to the protein. No stable interaction between ligand and protein was seen for simulations initiated with capsaicin in the bilayer. However, interactions were seen between TRPV1 and capsaicin starting from the cytosolic aqueous phase, and capsaicin remained stable in the majority of simulations from the docked pose. We discuss the significance of capsaicin flipping from the extracellular to the intracellular

  12. Capsaicin Interaction with TRPV1 Channels in a Lipid Bilayer: Molecular Dynamics Simulation

    PubMed Central

    Hanson, Sonya M.; Newstead, Simon; Swartz, Kenton J.; Sansom, Mark S.P.

    2015-01-01

    Transient receptor potential vanilloid subtype 1 (TRPV1) is a heat-sensitive ion channel also involved in pain sensation, and is the receptor for capsaicin, the active ingredient of hot chili peppers. The recent structures of TRPV1 revealed putative ligand density within the S1 to S4 voltage-sensor-like domain of the protein. However, questions remain regarding the dynamic role of the lipid bilayer in ligand binding to TRPV1. Molecular dynamics simulations were used to explore behavior of capsaicin in a 1-palmitoyl-2-oleoyl phosphatidylcholine bilayer and with the target S1–S4 transmembrane helices of TRPV1. Equilibrium simulations reveal a preferred interfacial localization for capsaicin. We also observed a capsaicin molecule flipping from the extracellular to the intracellular leaflet, and subsequently able to access the intracellular TRPV1 binding site. Calculation of the potential of mean force (i.e., free energy profile) of capsaicin along the bilayer normal confirms that it prefers an interfacial localization. The free energy profile indicates that there is a nontrivial but surmountable barrier to the flipping of capsaicin between opposing leaflets of the bilayer. Molecular dynamics of the S1–S4 transmembrane helices of the TRPV1 in a lipid bilayer confirm that Y511, known to be crucial to capsaicin binding, has a distribution along the bilayer normal similar to that of the aromatic group of capsaicin. Simulations were conducted of the TRPV1 S1–S4 transmembrane helices in the presence of capsaicin placed in the aqueous phase, in the lipid, or docked to the protein. No stable interaction between ligand and protein was seen for simulations initiated with capsaicin in the bilayer. However, interactions were seen between TRPV1 and capsaicin starting from the cytosolic aqueous phase, and capsaicin remained stable in the majority of simulations from the docked pose. We discuss the significance of capsaicin flipping from the extracellular to the intracellular

  13. Capsaicin interaction with TRPV1 channels in a lipid bilayer: molecular dynamics simulation.

    PubMed

    Hanson, Sonya M; Newstead, Simon; Swartz, Kenton J; Sansom, Mark S P

    2015-03-24

    Transient receptor potential vanilloid subtype 1 (TRPV1) is a heat-sensitive ion channel also involved in pain sensation, and is the receptor for capsaicin, the active ingredient of hot chili peppers. The recent structures of TRPV1 revealed putative ligand density within the S1 to S4 voltage-sensor-like domain of the protein. However, questions remain regarding the dynamic role of the lipid bilayer in ligand binding to TRPV1. Molecular dynamics simulations were used to explore behavior of capsaicin in a 1-palmitoyl-2-oleoyl phosphatidylcholine bilayer and with the target S1-S4 transmembrane helices of TRPV1. Equilibrium simulations reveal a preferred interfacial localization for capsaicin. We also observed a capsaicin molecule flipping from the extracellular to the intracellular leaflet, and subsequently able to access the intracellular TRPV1 binding site. Calculation of the potential of mean force (i.e., free energy profile) of capsaicin along the bilayer normal confirms that it prefers an interfacial localization. The free energy profile indicates that there is a nontrivial but surmountable barrier to the flipping of capsaicin between opposing leaflets of the bilayer. Molecular dynamics of the S1-S4 transmembrane helices of the TRPV1 in a lipid bilayer confirm that Y511, known to be crucial to capsaicin binding, has a distribution along the bilayer normal similar to that of the aromatic group of capsaicin. Simulations were conducted of the TRPV1 S1-S4 transmembrane helices in the presence of capsaicin placed in the aqueous phase, in the lipid, or docked to the protein. No stable interaction between ligand and protein was seen for simulations initiated with capsaicin in the bilayer. However, interactions were seen between TRPV1 and capsaicin starting from the cytosolic aqueous phase, and capsaicin remained stable in the majority of simulations from the docked pose. We discuss the significance of capsaicin flipping from the extracellular to the intracellular

  14. A Critical Comparison of Biomembrane Force Fields: Structure and Dynamics of Model DMPC, POPC, and POPE Bilayers.

    PubMed

    Pluhackova, Kristyna; Kirsch, Sonja A; Han, Jing; Sun, Liping; Jiang, Zhenyan; Unruh, Tobias; Böckmann, Rainer A

    2016-04-28

    Atomistic molecular dynamics simulations have become an important source of information for the structure and dynamics of biomembranes at molecular detail difficult to access in experiments. A number of force fields for lipid membrane simulations have been derived in the past; the choice of the most suitable force field is, however, frequently hampered by the availability of parameters for specific lipids. Additionally, the comparison of different quantities among force fields is often aggravated by varying simulation parameters. Here, we compare four atomistic lipid force fields, namely, the united-atom GROMOS54a7 and the all-atom force fields CHARMM36, Slipids, and Lipid14, for a broad range of structural and dynamical properties of saturated and monounsaturated phosphatidylcholine bilayers (DMPC and POPC) as well as for monounsaturated phosphatidylethanolamine bilayers (POPE). Additionally, the ability of the different force fields to describe the gel-liquid crystalline phase transition is compared and their computational efficiency estimated. Moreover, membrane properties like the water flux across the lipid bilayer and lipid acyl chain protrusion probabilities are compared.

  15. The structure of the CD3 ζζ transmembrane dimer in POPC and raft-like lipid bilayer: a molecular dynamics study.

    PubMed

    Petruk, Ariel Alcides; Varriale, Sonia; Coscia, Maria Rosaria; Mazzarella, Lelio; Merlino, Antonello; Oreste, Umberto

    2013-11-01

    Plasma membrane lipids significantly affect assembly and activity of many signaling networks. The present work is aimed at analyzing, by molecular dynamics simulations, the structure and dynamics of the CD3 ζζ dimer in palmitoyl-oleoyl-phosphatidylcholine bilayer (POPC) and in POPC/cholesterol/sphingomyelin bilayer, which resembles the raft membrane microdomain supposed to be the site of the signal transducing machinery. Both POPC and raft-like environment produce significant alterations in structure and flexibility of the CD3 ζζ with respect to nuclear magnetic resonance (NMR) model: the dimer is more compact, its secondary structure is slightly less ordered, the arrangement of the Asp6 pair, which is important for binding to the Arg residue in the alpha chain of the T cell receptor (TCR), is stabilized by water molecules. Different interactions of charged residues with lipids at the lipid-cytoplasm boundary occur when the two environments are compared. Furthermore, in contrast to what is observed in POPC, in the raft-like environment correlated motions between transmembrane and cytoplasmic regions are observed. Altogether the data suggest that when the TCR complex resides in the raft domains, the CD3 ζζ dimer assumes a specific conformation probably necessary to the correct signal transduction.

  16. Model-free estimation of the effective correlation time for C-H bond reorientation in amphiphilic bilayers: 1H-13C solid-state NMR and MD simulations

    NASA Astrophysics Data System (ADS)

    Ferreira, Tiago Mendes; Ollila, O. H. Samuli; Pigliapochi, Roberta; Dabkowska, Aleksandra P.; Topgaard, Daniel

    2015-01-01

    Molecular dynamics (MD) simulations give atomically detailed information on structure and dynamics in amphiphilic bilayer systems on timescales up to about 1 μs. The reorientational dynamics of the C-H bonds is conventionally verified by measurements of 13C or 2H nuclear magnetic resonance (NMR) longitudinal relaxation rates R1, which are more sensitive to motional processes with correlation times close to the inverse Larmor frequency, typically around 1-10 ns on standard NMR instrumentation, and are thus less sensitive to the 10-1000 ns timescale motion that can be observed in the MD simulations. We propose an experimental procedure for atomically resolved model-free estimation of the C-H bond effective reorientational correlation time τe, which includes contributions from the entire range of all-atom MD timescales and that can be calculated directly from the MD trajectories. The approach is based on measurements of 13C R1 and R1ρ relaxation rates, as well as 1H-13C dipolar couplings, and is applicable to anisotropic liquid crystalline lipid or surfactant systems using a conventional solid-state NMR spectrometer and samples with natural isotopic composition. The procedure is demonstrated on a fully hydrated lamellar phase of 1-palmitoyl-2-oleoyl-phosphatidylcholine, yielding values of τe from 0.1 ns for the methyl groups in the choline moiety and at the end of the acyl chains to 3 ns for the g1 methylene group of the glycerol backbone. MD simulations performed with a widely used united-atom force-field reproduce the τe-profile of the major part of the acyl chains but underestimate the dynamics of the glycerol backbone and adjacent molecular segments. The measurement of experimental τe-profiles can be used to study subtle effects on C-H bond reorientational motions in anisotropic liquid crystals, as well as to validate the C-H bond reorientation dynamics predicted in MD simulations of amphiphilic bilayers such as lipid membranes.

  17. Model-free estimation of the effective correlation time for C–H bond reorientation in amphiphilic bilayers: {sup 1}H–{sup 13}C solid-state NMR and MD simulations

    SciTech Connect

    Ferreira, Tiago Mendes; Ollila, O. H. Samuli; Pigliapochi, Roberta; Dabkowska, Aleksandra P.; Topgaard, Daniel

    2015-01-28

    Molecular dynamics (MD) simulations give atomically detailed information on structure and dynamics in amphiphilic bilayer systems on timescales up to about 1 μs. The reorientational dynamics of the C–H bonds is conventionally verified by measurements of {sup 13}C or {sup 2}H nuclear magnetic resonance (NMR) longitudinal relaxation rates R{sub 1}, which are more sensitive to motional processes with correlation times close to the inverse Larmor frequency, typically around 1-10 ns on standard NMR instrumentation, and are thus less sensitive to the 10-1000 ns timescale motion that can be observed in the MD simulations. We propose an experimental procedure for atomically resolved model-free estimation of the C–H bond effective reorientational correlation time τ{sub e}, which includes contributions from the entire range of all-atom MD timescales and that can be calculated directly from the MD trajectories. The approach is based on measurements of {sup 13}C R{sub 1} and R{sub 1ρ} relaxation rates, as well as {sup 1}H−{sup 13}C dipolar couplings, and is applicable to anisotropic liquid crystalline lipid or surfactant systems using a conventional solid-state NMR spectrometer and samples with natural isotopic composition. The procedure is demonstrated on a fully hydrated lamellar phase of 1-palmitoyl-2-oleoyl-phosphatidylcholine, yielding values of τ{sub e} from 0.1 ns for the methyl groups in the choline moiety and at the end of the acyl chains to 3 ns for the g{sub 1} methylene group of the glycerol backbone. MD simulations performed with a widely used united-atom force-field reproduce the τ{sub e}-profile of the major part of the acyl chains but underestimate the dynamics of the glycerol backbone and adjacent molecular segments. The measurement of experimental τ{sub e}-profiles can be used to study subtle effects on C–H bond reorientational motions in anisotropic liquid crystals, as well as to validate the C–H bond reorientation dynamics predicted in MD

  18. 25-Hydroxycholesterol Increases the Availability of Cholesterol in Phospholipid Membranes

    SciTech Connect

    Olsen, Brett N.; Schlesinger, Paul H.; Ory, Daniel S.; Baker, Nathan A.

    2011-02-01

    Side-chain oxysterols are enzymatically generated oxidation products of cholesterol that serve a central role in mediating cholesterol homeostasis. Recent work has shown that side-chain oxysterols, such as 25-hydroxycholesterol (25-HC), alter membrane structure in very different ways from cholesterol, suggesting a possible mechanism for how these oxysterols regulate cholesterol homeostasis. Here we extend our previous work, using molecular dynamics simulations of 25-HC and cholesterol mixtures in 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers to examine interactions between 25-HC and cholesterol in the same bilayer. When added to cholesterol-containing membranes, 25-HC causes larger changes in membrane structure than when added to cholesterol-free membranes, demonstrating interactions between the two sterols. We also find that the presence of 25-HC changes the position, orientation, and solvent accessibility of cholesterol, shifting it into the water interface and therefore its availability to external acceptors. This is consistent with experimental results showing that oxysterols can trigger cholesterol trafficking from the plasma membrane to the endoplasmic reticulum. These interactions provide a potential mechanism for 25-HC-mediated regulation of cholesterol trafficking and homeostasis through direct modulation of cholesterol availability.

  19. Metastability of a Supercompressed Fluid Monolayer

    PubMed Central

    Smith, Ethan C.; Crane, Jonathan M.; Laderas, Ted G.; Hall, Stephen B.

    2003-01-01

    Previous studies showed that monomolecular films of extracted calf surfactant collapse at the equilibrium spreading pressure during quasi-static compressions but become metastable at much higher surface pressures when compressed faster than a threshold rate. To determine the mechanism by which the films become metastable, we studied single-component films of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC). Initial experiments confirmed similar metastability of POPC if compressed above a threshold rate. Measurements at different surface pressures then showed that rates of collapse, although initially increasing above the equilibrium spreading pressure, reached a sharply defined maximum and then slowed considerably. When heated, rapidly compressed films recovered their ability to collapse with no discontinuous change in area, arguing that the metastability does not reflect transition of the POPC film to a new phase. These observations indicate that in several respects, the supercompression of POPC monolayers resembles the supercooling of three-dimensional liquids toward a glass transition. PMID:14581205

  20. Formation of stable nanodiscs by bihelical apolipoprotein A-I mimetic peptide.

    PubMed

    Kariyazono, Hirokazu; Nadai, Ryo; Miyajima, Rin; Takechi-Haraya, Yuki; Baba, Teruhiko; Shigenaga, Akira; Okuhira, Keiichiro; Otaka, Akira; Saito, Hiroyuki

    2016-02-01

    Nanodiscs are composed of scaffold protein or peptide such as apolipoprotein A-I (apoA-I) and phospholipids. Although peptide-based nanodiscs have an advantage to modulate the size of nanodiscs by changing phospholipid/peptide ratios, they are usually less stable than apoA-I-based nanodiscs. In this study, we designed a novel nanodisc scaffold peptide (NSP) that has proline-punctuated bihelical amphipathic structure based on apoA-I mimetic peptides. NSP formed α-helical structure on 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) nanodiscs prepared by cholate dialysis method. Dynamic light scattering measurements demonstrated that diameters of NSP nanodiscs vary depending upon POPC/NSP ratios. Comparison of thermal unfolding of nanodiscs monitored by circular dichroism measurements demonstrated that NSP forms much more stable nanodiscs with POPC than monohelical peptide, 4F, exhibiting comparable stability to apoA-I-POPC nanodiscs. Intrinsic Trp fluorescence measurements showed that Trp residues of NSP exhibit more hydrophobic environment than that of 4 F on nanodiscs, suggesting the stronger interaction of NSP with phospholipids. Thus, the bihelical structure of NSP appears to increase the stability of nanodiscs because of the enhanced interaction of peptides with phospholipids. In addition, NSP as well as 4F spontaneously solubilized POPC vesicles into nanodiscs without using detergent. These results indicate that bihelical NSP forms nanodiscs with comparable stability to apoA-I and has an ability to control the size of nanodiscs simply by changing phospholipid/peptide ratios.

  1. Membrane Pore Formation by Amyloid beta (25-35) Peptide

    NASA Astrophysics Data System (ADS)

    Kandel, Nabin; Tatulian, Suren

    Amyloid (A β) peptide contributes to Alzheimer's disease by a yet unidentified mechanism. One of the possible mechanisms of A β toxicity is formation of pores in cellular membranes. We have characterized the formation of pores in phospholipid membranes by the Aβ25 - 35 peptide (GSNKGAIIGLM) using fluorescence, Fourier transform infrared spectroscopy (FTIR) and circular dichroism (CD) techniques. CD and FTIR identified formation of β-sheet structure upon incubation of the peptide in aqueous buffer for 2 hours. Unilamellar vesicles composed of a zwitterionic lipid, 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC), and 70 % POPC plus 30 % of an acidic lipid, 1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG), are made in 30 mM CaCl2. Quin-2, a fluorophore that displays increased fluorescence upon Ca2+ binding, is added to the vesicles externally. Peptide addition results in increased Quin-2 fluorescence, which is interpreted by binding of the peptide to the vesicles, pore formation, and Ca2+ leakage. The positive and negative control measurements involve addition of a detergent, Triton X-100, which causes vesicle rupture and release of total calcium, and blank buffer, respectively.

  2. Poly-3-hydroxybutyrate/polyphosphate complexes form voltage-activated Ca2+ channels in the plasma membranes of Escherichia coli.

    PubMed

    Reusch, R N; Huang, R; Bramble, L L

    1995-09-01

    The lipidic polymer, poly-3-hydroxybutyrate (PHB), is found in the plasma membranes of Escherichia col complexed to calcium polyphosphate (CaPPi). The composition, location, and putative structure of the polymer salt complexes led Reusch and Sadoff (1988) to propose that the complexes function as Ca2+ channels. Here we use bilayer patch-clamp techniques to demonstrate that voltage-activated Ca2+ channels composed of PHB and CaPPi are in the plasma membranes of E. coli. Single channel calcium currents were observed in vesicles of plasma membranes incorporated into planar bilayers of synthetic 1-palmitoyl, 2-oleoyl phosphatidylcholine. The channels were extracted from cells and incorporated into bilayers, where they displayed many of the signal characteristics of protein Ca2+ channels: voltage-activated selective for divalent over monovalent cations, permeant to Ca2+, manner by La3+, Co2+, Cd2+, and Mg2+, in that order. The channel-active extract, purified by size exclusion chromatography, was found to contain only PHB and CaPPi. This composition was confirmed by the observation of comparable single channel currents with complexes reconstituted from synthetic CaPPi and PHB, isolated from E. coli. This is the first report of a biological non-proteinaceous calcium channel. We suggest that poly-3-hydroxybutyrate/calcium polyphosphate complexes are evolutionary antecedents of protein Ca2+ channels. PMID:8519976

  3. 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.

  4. Cholesterol in condensed and fluid phosphatidylcholine monolayers studied by epifluorescence microscopy.

    PubMed Central

    Worthman, L A; Nag, K; Davis, P J; Keough, K M

    1997-01-01

    Epifluorescence microscopy was used to investigate the effect of cholesterol on monolayers of dipalmitoylphosphatidylcholine (DPPC) and 1 -palmitoyl-2-oleoyl phosphatidylcholine (POPC) at 21 +/- 2 degrees C using 1 mol% 1-palmitoyl-2-[12-[(7-nitro-2-1, 3-benzoxadizole-4-yl)amino]dodecanoyl]phosphatidylcholine (NBD-PC) as a fluorophore. Up to 30 mol% cholesterol in DPPC monolayers decreased the amounts of probe-excluded liquid-condensed (LC) phase at all surface pressures (pi), but did not effect the monolayers of POPC, which remained in the liquid-expanded (LE) phase at all pi. At low pi (2-5 mN/m), 10 mol% or more cholesterol in DPPC induced a lateral phase separation into dark probe-excluded and light probe-rich regions. In POPC monolayers, phase separation was observed at low pi when > or =40 mol% or more cholesterol was present. The lateral phase separation observed with increased cholesterol concentrations in these lipid monolayers may be a result of the segregation of cholesterol-rich domains in ordered fluid phases that preferentially exclude the fluorescent probe. With increasing pi, monolayers could be transformed from a heterogeneous dark and light appearance into a homogeneous fluorescent phase, in a manner that was dependent on pi and cholesterol content. The packing density of the acyl chains may be a determinant in the interaction of cholesterol with phosphatidylcholine (PC), because the transformations in monolayer surface texture were observed in phospholipid (PL)/sterol mixtures having similar molecular areas. At high pi (41 mN/m), elongated crystal-like structures were observed in monolayers containing 80-100 mol% cholesterol, and these structures grew in size when the monolayers were compressed after collapse. This observation could be associated with the segregation and crystallization of cholesterol after monolayer collapse. Images FIGURE 3 FIGURE 4 PMID:9168032

  5. Small-Angle X-Ray Scattering of the Cholesterol Incorporation into Human ApoA1-POPC Discoidal Particles

    PubMed Central

    Midtgaard, Søren Roi; Pedersen, Martin Cramer; Arleth, Lise

    2015-01-01

    Structural and functional aspects of high-density lipoproteins have been studied for over half a century. Due to the plasticity of this highly complex system, new aspects continue to be discovered. Here, we present a structural study of the human Apolipoprotein A1 (ApoA1) and investigate the role of its N-terminal domain, the so-called globular domain of ApoA1, in discoidal complexes with phospholipids and increasing amounts of cholesterol. Using a combination of solution-based small-angle x-ray scattering (SAXS) and molecular constrained data modeling, we show that the ApoA1-1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)-based particles are disk shaped with an elliptical cross section and composed by a central lipid bilayer surrounded by two stabilizing ApoA1 proteins. This structure is very similar to the particles formed in the so-called nanodisc system, which is based on N-terminal truncated ApoA1 protein. Although it is commonly agreed that the nanodisc is plain disk shaped, several more advanced structures have been proposed for the full-length ApoA1 in combination with POPC and cholesterol. This prompted us to make a detailed comparative study of the ApoA1 and nanodisc systems upon cholesterol uptake. Based on the presented SAXS analysis it is found that the N-terminal domains of ApoA1-POPC-cholesterol particles are not globular but instead an integrated part of the protein belt stabilizing the particles. Upon incorporation of increasing amounts of cholesterol, the presence of the N-terminal domain allows the bilayer thickness to increase while maintaining an overall flat bilayer structure. This is contrasted by the energetically more strained and less favorable lens shape required to fit the SAXS data from the N-terminal truncated nanodisc system upon cholesterol incorporation. This suggests that the N-terminal domain of ApoA1 actively participates in the stabilization of the ApoA1-POPC-cholesterol discoidal particle and allows for a more optimal

  6. Comparative Computer Simulation Study of Cholesterol in Hydrated Unary and Binary Lipid Bilayers and in an Anhydrous Crystal

    PubMed Central

    Plesnar, Elzbieta; Subczynski, Witold K.; Pasenkiewicz-Gierula, Marta

    2013-01-01

    Models created with molecular dynamics simulations are used to compare the organization and dynamics of cholesterol (Chol) molecules in three different environments: (1) a hydrated pure Chol bilayer that models the Chol bilayer domain, which is a pure Chol domain embedded in the bulk membrane; (2) a 2-palmitoyl-3-oleoyl-D-glycerol-1-phosphorylcholine bilayer saturated with cholesterol (POPC-Chol50) that models the bulk membrane; and (3) a Chol crystal. The computer model of the hydrated pure Chol bilayer is stable on the μs time scale. Some structural characteristics of Chol molecules in the Chol bilayer are similar to those in the POPC-Chol50 bilayer (e.g., tilt of Chol rings and chains), while others are similar to those in Chol crystals (e.g., surface area per Chol, bilayer thickness). The key result of this study is that the Chol bilayer has, unexpectedly, a dynamic structure, with Chol mobility similar to that in the POPC-Chol50 bilayer though slower. This is the major difference compared to Chol crystals, where Chol molecules are immobile. Also, water accessibility to Chol-OH groups in the Chol bilayer is not limited. On average, each Chol molecule makes 2.3 hydrogen bonds with water in the Chol bilayer, as compared with 1.7 hydrogen bonds in the POPC-Col50 bilayer. PMID:23848956

  7. Binding Orientations and Lipid Interactions of Human Amylin at Zwitterionic and Anionic Lipid Bilayers

    PubMed Central

    Qian, Zhenyu; Jia, Yan; Wei, Guanghong

    2016-01-01

    Increasing evidence suggests that the interaction of human islet amyloid polypeptide (hIAPP) with lipids may facilitate hIAPP aggregation and cause the death of pancreatic islet β-cells. However, the detailed hIAPP-membrane interactions and the influences of lipid compositions are unclear. In this study, as a first step to understand the mechanism of membrane-mediated hIAPP aggregation, we investigate the binding behaviors of hIAPP monomer at zwitterionic palmitoyloleoyl-phosphatidylcholine (POPC) bilayer by performing atomistic molecular dynamics simulations. The results are compared with those of hIAPP at anionic palmitoyloleoyl-phosphatidylglycerol (POPG) bilayers. We find that the adsorption of hIAPP to POPC bilayer is mainly initiated from the C-terminal region and the peptide adopts a helical structure with multiple binding orientations, while the adsorption to POPG bilayer is mostly initiated from the N-terminal region and hIAPP displays one preferential binding orientation, with its hydrophobic residues exposed to water. hIAPP monomer inserts into POPC lipid bilayers more readily than into POPG bilayers. Peptide-lipid interaction analyses show that the different binding features of hIAPP at POPC and POPG bilayers are attributed to different magnitudes of electrostatic and hydrogen-bonding interactions with lipids. This study provides mechanistic insights into the different interaction behaviors of hIAPP with zwitterionic and anionic lipid bilayers. PMID:26649316

  8. Self-Induced Docking Site of a Deeply Embedded Peripheral Membrane Protein

    PubMed Central

    Jaud, Simon; Tobias, Douglas J.; Falke, Joseph J.; White, Stephen H.

    2007-01-01

    As a first step toward understanding the principles of the targeting of C2 domains to membranes, we have carried out a molecular dynamics simulation of the C2 domain of cytosolic phospholipase A2 (cPLA2-C2) in a 1-palmitoyl-2-oleoyl-phosphatidylcholine bilayer at constant pressure and temperature (NPT, 300 K and 1 atm). Using the high-resolution crystal structure of cPLA2-C2 as a starting point, we embedded two copies of the C2 domain into a preequilibrated membrane at the depth and orientation previously defined by electron paramagnetic resonance (EPR). Noting that in the membrane-bound state the three calcium binding loops are complexed to two calcium ions, we initially restrained the calcium ions at the membrane depth determined by EPR. But the depth and orientation of the domains remained within EPR experimental errors when the restraints were later removed. We find that the thermally disordered, chemically heterogeneous interfacial zones of phosphatidylcholine bilayers allow local lipid remodeling to produce a nearly perfect match to the shape and polarity of the C2 domain, thereby enabling the C2 domain to assemble and optimize its own lipid docking site. The result is a cuplike docking site with a hydrophobic bottom and hydrophilic rim. Contrary to expectations, we did not find direct interactions between the protein-bound calcium ions and lipid headgroups, which were sterically excluded from the calcium binding cleft. Rather, the lipid phosphate groups provided outer-sphere calcium coordination through intervening water molecules. These results show that the combined use of high-resolution protein structures, EPR measurements, and molecular dynamics simulations provides a general approach for analyzing the molecular interactions between membrane-docked proteins and lipid bilayers. PMID:17071664

  9. The helical structure of surfactant peptide KL4 when bound to POPC

    PubMed Central

    Mills, Frank D.; Antharam, Vijay C.; Ganesh, Omjoy K.; Elliott, Doug W.; McNeill, Seth A.; Long, Joanna R.

    2009-01-01

    KL4 is a 21-residue peptide employed as a functional mimic of lung surfactant protein B which successfully lowers surface tension in the alveoli. A mechanistic understanding of how KL4 affects lipid properties has proven elusive as the secondary structure of KL4 in lipid preparations has not been determined at high resolution. The sequence of KL4 is based on the C-terminus of SP-B, a naturally occurring helical protein that binds to lipid interfaces. The spacing of the lysine residues in KL4 precludes the formation of a canonical amphipathic α-helix; qualitative measurements using Raman, CDc and FTIR spectroscopies have given conflicting results as to the secondary structure of the peptide as well as its orientation in the lipid environment. Here, we present a structural model of KL4 bound to lipid bilayers based on solid state NMR data. Double-quantum correlation experiments employing 13C-enriched peptides were used to quantitatively determine the backbone torsion angles in KL4 at several positions. These measurements, coupled with CD experiments, verify the helical nature of KL4 when bound to lipids, with (Φ, Ψ) angles that differ substantially from common values for α-helices of (-60, -45). The average torsion angles found for KL4 bound to POPC: POPG lipid vesicles are (-105, -30); this deviation from ideal α-helical structure allows KL4 to form an amphipathic helix at the lipid interface. PMID:18636713

  10. The impact of cell-penetrating peptides on membrane bilayer structure during binding and insertion.

    PubMed

    Hirst, Daniel J; Lee, Tzong-Hsien; Kulkarni, Ketav; Wilce, Jacqueline A; Aguilar, Marie-Isabel

    2016-08-01

    We have studied the effect of penetratin and a truncated analogue on the bilayer structure using dual polarisation interferometry, to simultaneously measure changes in mass per unit area and birefringence (an optical parameter representing bilayer order) with high sensitivity during the binding and dissociation from the membrane. Specifically, we studied penetratin (RQIKIWFQNRRMKWKK), along with a shortened and biotinylated version known as R8K-biotin (RRMKWKKK(Biotin)-NH2). Overall both peptides bound only weakly to the neutral DMPC and POPC bilayers, while much higher binding was observed for the anionic DMPC/DMPG and POPC/POPG. The binding of penetratin to gel-phase DMPC/DMPG was adequately represented by a two-state model, whereas on the fluid-phase POPC/POPG it exhibited a distinctly different binding pattern, best represented by a three-state kinetic model. However, R8K-biotin did not bind well to DMPC/DMPG and showed a more transitory and superficial binding to POPC/POPG. Comparing the modelling results for both peptides binding to POPC/POPG suggests an important role for a securely bound intermediate prior to penetratin insertion and translocation. Overall these results further elucidate the mechanism of penetratin, and provide another example of the significance of the ability of DPI to measure structural changes and the use of kinetic analysis to investigate the stages of peptide-membrane interactions. PMID:27163492

  11. Self-consistent mean-field model for palmitoyloleoylphosphatidylcholine-palmitoyl sphingomyelin–cholesterol lipid bilayers

    PubMed Central

    Tumaneng, Paul W.; Pandit, Sagar A.; Zhao, Guijun; Scott, H. L.

    2012-01-01

    The connection between membrane inhomogeneity and the structural basis of lipid rafts has sparked interest in the lateral organization of model lipid bilayers of two and three components. In an effort to investigate anisotropic lipid distribution in mixed bilayers, a self-consistent mean-field theoretical model is applied to palmitoyloleoylphosphatidylcholine (POPC)–palmitoyl sphingomyelin (PSM)–cholesterol mixtures. The compositional dependence of lateral organization in these mixtures is mapped onto a ternary plot. The model utilizes molecular dynamics simulations to estimate interaction parameters and to construct chain conformation libraries. We find that at some concentration ratios the bilayers separate spatially into regions of higher and lower chain order coinciding with areas enriched with PSM and POPC, respectively. To examine the effect of the asymmetric chain structure of POPC on bilayer lateral inhomogeneity, we consider POPC-lipid interactions with and without angular dependence. Results are compared with experimental data and with results from a similar model for mixtures of dioleoylphosphatidylcholine, steroyl sphingomyelin, and cholesterol. PMID:21517541

  12. Self-consistent mean-field model for palmitoyloleoylphosphatidylcholine-palmitoyl sphingomyelin-cholesterol lipid bilayers

    NASA Astrophysics Data System (ADS)

    Tumaneng, Paul W.; Pandit, Sagar A.; Zhao, Guijun; Scott, H. L.

    2011-03-01

    The connection between membrane inhomogeneity and the structural basis of lipid rafts has sparked interest in the lateral organization of model lipid bilayers of two and three components. In an effort to investigate anisotropic lipid distribution in mixed bilayers, a self-consistent mean-field theoretical model is applied to palmitoyloleoylphosphatidylcholine (POPC)-palmitoyl sphingomyelin (PSM)-cholesterol mixtures. The compositional dependence of lateral organization in these mixtures is mapped onto a ternary plot. The model utilizes molecular dynamics simulations to estimate interaction parameters and to construct chain conformation libraries. We find that at some concentration ratios the bilayers separate spatially into regions of higher and lower chain order coinciding with areas enriched with PSM and POPC, respectively. To examine the effect of the asymmetric chain structure of POPC on bilayer lateral inhomogeneity, we consider POPC-lipid interactions with and without angular dependence. Results are compared with experimental data and with results from a similar model for mixtures of dioleoylphosphatidylcholine, steroyl sphingomyelin, and cholesterol.

  13. Octyl-beta-D-glucopyranoside partitioning into lipid bilayers: thermodynamics of binding and structural changes of the bilayer.

    PubMed

    Wenk, M R; Alt, T; Seelig, A; Seelig, J

    1997-04-01

    The interaction of the nonionic detergent octyl-beta-D-glucopyranoside (OG) with lipid bilayers was studied with high-sensitivity isothermal titration calorimetry (ITC) and solid-state 2H-NMR spectroscopy. The transfer of OG from the aqueous phase to lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) can be investigated by employing detergent at concentrations below the critical micellar concentration; it can be defined by a surface partition equilibrium with a partition coefficient of K = 120 +/- 10 M-1, a molar binding enthalpy of delta H degrees D = 1.3 +/- 0.15 kcal/mol, and a free energy of binding of delta G degrees D = -5.2 kcal/mol. The heat of transfer is temperature dependent, with a molar heat capacity of delta CP = -75 cal K-1 mol-1. The large heat capacity and the near-zero delta H are typical for a hydrophobic binding equilibrium. The partition constant K decreased to approximately 100 M-1 for POPC membranes mixed with either negatively charged lipids or cholesterol, but was independent of membrane curvature. In contrast, a much larger variation was observed in the partition enthalpy. delta H degrees D increased by about 50% for large vesicles and by 75% for membranes containing 50 mol% cholesterol. Structural changes in the lipid bilayer were investigated with solid-state 2H-NMR. POPC was selectively deuterated at the headgroup segments and at different positions of the fatty acyl chains, and the measurement of the quadrupolar splittings provided information on the conformation and the order of the bilayer membrane. Addition of OG had almost no influence on the lipid headgroup region, even at concentrations close to bilayer disruption. In contrast, the fluctuations of fatty acyl chain segments located in the inner part of the bilayer increased strongly with increasing OG concentration. The 2H-NMR results demonstrate that the headgroup region is the most stable structural element of the lipid membrane, remaining intact

  14. Nanoparticle-lipid bilayer interactions studied with lipid bilayer arrays

    NASA Astrophysics Data System (ADS)

    Lu, Bin; Smith, Tyler; Schmidt, Jacob J.

    2015-04-01

    can provide insight into the nature of the particle-membrane interaction through variation of membrane and solution properties not possible with cell-based assays. However, the scope of these studies can be limited because of the low throughput characteristic of lipid bilayer platforms. We have recently described an easy to use, parallel lipid bilayer platform which we have used to electrically investigate the activity of 60 nm diameter amine and carboxyl modified polystyrene nanoparticles (NH2-NP and COOH-NP) with over 1000 lipid bilayers while varying lipid composition, bilayer charge, ionic strength, pH, voltage, serum, particle concentration, and particle charge. Our results confirm recent studies finding activity of NH2-NP but not COOH-NP. Detailed analysis shows that NH2-NP formed pores 0.3-2.3 nm in radius, dependent on bilayer and solution composition. These interactions appear to be electrostatic, as they are regulated by NH2-NP surface charge, solution ionic strength, and bilayer charge. The ability to rapidly measure a large number of nanoparticle and membrane parameters indicates strong potential of this bilayer array platform for additional nanoparticle bilayer studies. Electronic supplementary information (ESI) available: Impact of ionic strength on particle-membrane interaction in POPC : POPE : Chol : POPS (3 : 1 : 1 : 1) bilayers; impact of voltage magnitude, bilayer charge, voltage sign and ionic strength on pore size. See DOI: 10.1039/c4nr06892k

  15. Kinetics and thermodynamics of chlorpromazine interaction with lipid bilayers: effect of charge and cholesterol.

    PubMed

    Martins, Patrícia T; Velazquez-Campoy, Adrian; Vaz, Winchil L C; Cardoso, Renato M S; Valério, Joana; Moreno, Maria João

    2012-03-01

    Passive transport across cell membranes is the major route for the permeation of xenobiotics through tight endothelia such as the blood–brain barrier. The rate of passive permeation through lipid bilayers for a given drug is therefore a critical step in the prediction of its pharmacodynamics. We describe a detailed study on the kinetics and thermodynamics for the interaction of chlorpromazine (CPZ), an antipsychotic drug used in the treatment of schizophrenia, with neutral and negatively charged lipid bilayers. Isothermal titration calorimetry was used to study the partition and translocation of CPZ in lipid membranes composed of pure POPC, POPC:POPS (9:1), and POPC:Chol:POPS (6:3:1). The membrane charge due to the presence of POPS as well as the additional charge resulting from the introduction of CPZ in the membrane were taken into account, allowing the calculation of the intrinsic partition coefficients (K(P)) and the enthalpy change (ΔH) associated with the process. The enthalpy change upon partition to all lipid bilayers studied is negative, but a significant entropy contribution was also observed for partition to the neutral membrane. Because of the positive charge of CPZ, the presence of negatively charged lipids in the bilayer increases both the observed amount of CPZ that partitions to the membrane (KP(obs)) and the magnitude of ΔH. However, when the electrostatic effects are discounted, the intrinsic partition coefficient was smaller, indicating that the hydrophobic contribution was less significant for the negatively charged membrane. The presence of cholesterol strongly decreases the affinity of CPZ for the bilayer in terms of both the amount of CPZ that associates with the membrane and the interaction enthalpy. A quantitative characterization of the rate of CPZ translocation through membranes composed of pure POPC and POPC:POPS (9:1) was also performed using an innovative methodology developed in this work based on the kinetics of the heat evolved

  16. Viscoelastic deformation of lipid bilayer vesicles.

    PubMed

    Wu, Shao-Hua; Sankhagowit, Shalene; Biswas, Roshni; Wu, Shuyang; Povinelli, Michelle L; Malmstadt, Noah

    2015-10-01

    Lipid bilayers form the boundaries of the cell and its organelles. Many physiological processes, such as cell movement and division, involve bending and folding of the bilayer at high curvatures. Currently, bending of the bilayer is treated as an elastic deformation, such that its stress-strain response is independent of the rate at which bending strain is applied. We present here the first direct measurement of viscoelastic response in a lipid bilayer vesicle. We used a dual-beam optical trap (DBOT) to stretch 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) giant unilamellar vesicles (GUVs). Upon application of a step optical force, the vesicle membrane deforms in two regimes: a fast, instantaneous area increase, followed by a much slower stretching to an eventual plateau deformation. From measurements of dozens of GUVs, the average time constant of the slower stretching response was 0.225 ± 0.033 s (standard deviation, SD). Increasing the fluid viscosity did not affect the observed time constant. We performed a set of experiments to rule out heating by laser absorption as a cause of the transient behavior. Thus, we demonstrate here that the bending deformation of lipid bilayer membranes should be treated as viscoelastic.

  17. Molecular dynamics simulations of the rotary motor F(0) under external electric fields across the membrane.

    PubMed

    Lin, Yang-Shan; Lin, Jung-Hsin; Chang, Chien-Cheng

    2010-03-17

    The membrane-bound component F(0), which is a major component of the F(0)F(1)-ATP synthase, works as a rotary motor and plays a central role in driving the F(1) component to transform chemiosmotic energy into ATP synthesis. We conducted molecular dynamics simulations of b(2)-free F(0) in a 1-palmitoyl-2-oleoyl-phosphatidylcholine lipid bilayer for tens of nanoseconds with two different protonation states of the cAsp-61 residue at the interface of the a-c complex in the absence of electric fields and under electric fields of +/-0.03 V/nm across the membrane. To our surprise, we observed that the upper half of the N-terminal helix of the c(1) subunit rotated about its axis clockwise by 30 degrees . An energetic analysis revealed that the electrostatic repulsion between this N-terminal helix and subunit c(12) was a major contributor to the observed rotation. A correlation map analysis indicated that the correlated motions of residues in the interface of the a-c complex were significantly reduced by external electric fields. The deuterium order parameter (S(CD)) profile calculated by averaging all the lipids in the F(0)-bound bilayer was not very different from that of the pure bilayer system, in agreement with recent (2)H solid-state NMR experiments. However, by delineating the lipid properties according to their vicinity to F(0), we found that the S(CD) profiles of different lipid shells were prominently different. Lipids close to F(0) formed a more ordered structure. Similarly, the lateral diffusion of lipids on the membrane surface also followed a shell-dependent behavior. The lipids in the proximity of F(0) exhibited very significantly reduced diffusional motion. The numerical value of S(CD) was anticorrelated with that of the diffusion coefficient, i.e., the more ordered lipid structures led to slower lipid diffusion. Our findings will help elucidate the dynamics of F(0) depending on the protonation state and electric field, and may also shed some light on the

  18. Physical properties of the hybrid lipid POPC on micrometer-sized domains in mixed lipid membranes.

    PubMed

    Shimokawa, Naofumi; Nagata, Mariko; Takagi, Masahiro

    2015-08-28

    Macro-phase separation in mixed lipid membranes containing the hybrid lipid palmitoyloleoylphosphatidylcholine (POPC) was observed by fluorescent and confocal laser scanning microscopy. In a binary system consisting of the saturated lipid dipalmitoylphosphatidylcholine (DPPC) and the hybrid lipid POPC, the hybrid lipid forms a liquid-disordered (Ld) phase. In a ternary system consisting of this binary system and an unsaturated lipid dioleoylphosphatidylcholine (DOPC), three-phase coexistence is observed. The POPC-rich phase appears around DPPC-rich domains, and the hybrid lipid is expected to behave like a line-active agent (linactant). Finally, phase separation in a four-component system, composed of this ternary system and cholesterol, was examined. Domains with a size that is smaller than 1 μm are found, and domain-induced budding is also observed. To explain small domain formation and domain-induced budding, chain ordering was evaluated based on Laurdan generalized polarization measurements. Our observations revealed that the hybrid lipid acted like a linactant to solid domains and disturbed chain ordering in liquid-ordered (Lo) domains. In both cases, the hybrid lipid reduced line tension at the domain boundary.

  19. On the microscopic and mesoscopic perturbations of lipid bilayers upon interaction with the MPER domain of the HIV glycoprotein gp41.

    PubMed

    Oliva, Rosario; Emendato, Alessandro; Vitiello, Giuseppe; De Santis, Augusta; Grimaldi, Manuela; D'Ursi, Anna Maria; Busi, Elena; Del Vecchio, Pompea; Petraccone, Luigi; D'Errico, Gerardino

    2016-08-01

    The effect of the 665-683 fragment of the HIV fusion glycoprotein 41, corresponding to the MPER domain of the protein and named gp41MPER, on the microscopic structure and mesoscopic arrangement of palmitoyl oleoyl phosphatidylcholine (POPC) and POPC/sphingomyelin (SM)/cholesterol (CHOL) lipid bilayers is analyzed. The microscopic structuring of the bilayers has been studied by Electron Spin Resonance (ESR) spectroscopy, using glycerophosphocholines spin-labelled in different positions along the acyl chain. Transitions of the bilayer liquid crystalline state have been also monitored by Differential Scanning Calorimetry (DSC). Changes of the bilayers morphology have been studied by determining the dimension of the liposomes through Dynamic Light Scattering (DLS) measurements. The results converge in showing that the sample preparation procedure, the bilayer composition and the peptide/lipid ratio critically tune the lipid response to the peptide/membrane interaction. When gp41MPER is added to preformed liposomes, it positions at the bilayer interface and the lipid perturbation is limited to the more external segments. In contrast, if the peptide is mixed with the lipids during the liposome preparation, it assumes a trans-membrane topology. This happens at all peptide/lipid ratios for fluid POPC bilayers, while in the case of rigid POPC/SM/CHOL membranes a minimum ratio has to be reached, thus suggesting peptide self-aggregation to occur. Peptide insertion results in a dramatic increase of the lipid ordering and bilayer stiffening, which reflect in significant changes in liposome average dimension and distribution. The biological implications of these findings are discussed. PMID:27179640

  20. Effect of Sodium Chloride on a Lipid Bilayer

    PubMed Central

    Böckmann, Rainer A.; Hac, Agnieszka; Heimburg, Thomas; Grubmüller, Helmut

    2003-01-01

    Electrostatic interactions govern structural and dynamical properties of membranes and can vary considerably with the composition of the aqueous buffer. We studied the influence of sodium chloride on a pure POPC lipid bilayer by fluorescence correlation spectroscopy experiments and molecular dynamics simulations. Increasing sodium chloride concentration was found to decrease the self-diffusion of POPC lipids within the bilayer. Self-diffusion coefficients calculated from the 100 ns simulations agree with those measured on a millisecond timescale, suggesting that most of the relaxation processes relevant for lipid diffusion are faster than the simulation timescale. As the dominant effect, the molecular dynamics simulations revealed a tight binding of sodium ions to the carbonyl oxygens of on average three lipids leading to larger complexes with reduced mobility. Additionally, the bilayer thickens by ∼2 Å, which increases the order parameter of the fatty acyl chains. Sodium binding alters the electrostatic potential, which is largely compensated by a changed polarization of the aqueous medium and a lipid dipole reorientation. PMID:12944279

  1. Topologies, structures and parameter files for lipid simulations in GROMACS with the OPLS-aa force field: DPPC, POPC, DOPC, PEPC, and cholesterol.

    PubMed

    Kulig, Waldemar; Pasenkiewicz-Gierula, Marta; Róg, Tomasz

    2015-12-01

    In this data article we provide topologies and force field parameters files for molecular dynamics simulations of lipids in the OPLS-aa force field using the GROMACS package. This is the first systematic parameterization of lipid molecules in this force field. Topologies are provided for four phosphatidylcholines: saturated DPPC, mono-cis unsaturated POPC and DOPC, and mono-trans unsaturated PEPC. Parameterization of the phosphatidylcholines was achieved in two steps: first, we supplemented the OPLS force field parameters for DPPC with new parameters for torsion angles and van der Waals parameters for the carbon and hydrogen atoms in the acyl chains, as well as new partial atomic charges and parameters for torsion angles in the phosphatidylcholine and glycerol moieties [1]. Next, we derived parameters for the cis and trans double bonds and the neighboring them single bonds [2]. Additionally, we provide GROMACS input files with parameters describing simulation conditions (md.mdp), which are strongly recommended to be used with these lipids models. The data are associated with the research article "Cis and trans unsaturated phosphatidylcholine bilayers: a molecular dynamics simulation study" [2] and provided as supporting materials. PMID:26568975

  2. Topologies, structures and parameter files for lipid simulations in GROMACS with the OPLS-aa force field: DPPC, POPC, DOPC, PEPC, and cholesterol.

    PubMed

    Kulig, Waldemar; Pasenkiewicz-Gierula, Marta; Róg, Tomasz

    2015-12-01

    In this data article we provide topologies and force field parameters files for molecular dynamics simulations of lipids in the OPLS-aa force field using the GROMACS package. This is the first systematic parameterization of lipid molecules in this force field. Topologies are provided for four phosphatidylcholines: saturated DPPC, mono-cis unsaturated POPC and DOPC, and mono-trans unsaturated PEPC. Parameterization of the phosphatidylcholines was achieved in two steps: first, we supplemented the OPLS force field parameters for DPPC with new parameters for torsion angles and van der Waals parameters for the carbon and hydrogen atoms in the acyl chains, as well as new partial atomic charges and parameters for torsion angles in the phosphatidylcholine and glycerol moieties [1]. Next, we derived parameters for the cis and trans double bonds and the neighboring them single bonds [2]. Additionally, we provide GROMACS input files with parameters describing simulation conditions (md.mdp), which are strongly recommended to be used with these lipids models. The data are associated with the research article "Cis and trans unsaturated phosphatidylcholine bilayers: a molecular dynamics simulation study" [2] and provided as supporting materials.

  3. Covalent attachment of functionalized lipid bilayers to planar waveguides for measuring protein binding to biomimetic membranes.

    PubMed Central

    Heyse, S.; Vogel, H.; Sänger, M.; Sigrist, H.

    1995-01-01

    A new method is presented for measuring sensitively the interactions between ligands and their membrane-bound receptors in situ using integrated optics, thus avoiding the need for additional labels. Phospholipid bilayers were attached covalently to waveguides by a novel protocol, which can in principle be used with any glass-like surface. In a first step, phospholipids carrying head-group thiols were covalently immobilized onto SiO2-TiO2 waveguide surfaces. This was accomplished by acylation of aminated waveguides with the heterobifunctional crosslinker N-succinimidyl-3-maleimidopropionate, followed by the formation of thioethers between the surface-grafted maleimides and the synthetic thiolipids. The surface-attached thiolipids served as hydrophobic templates and anchors for the deposition of a complete lipid bilayer either by fusion of lipid vesicles or by lipid self-assembly from mixed lipid/detergent micelles. The step-by-step lipid bilayer formation on the waveguide surface was monitored in situ by an integrated optics technique, allowing the simultaneous determination of optical thickness and one of the two refractive indices of the adsorbed organic layers. Surface coverages of 50-60% were calculated for thiolipid layers. Subsequent deposition of POPC resulted in an overall lipid layer thickness of 45-50 A, which corresponds to the thickness of a fluid bilayer membrane. Specific recognition reactions occurring at cell membrane surfaces were modeled by the incorporation of lipid-anchored receptor molecules into the supported bilayer membranes. (1) The outer POPC layer was doped with biotinylated phosphatidylethanolamine. Subsequent specific binding of streptavidin was optically monitored. (2) A lipopeptide was incorporated in the outer POPC monolayer. Membrane binding of monoclonal antibodies, which were directed against the peptide moiety of the lipopeptide, was optically detected. The specific antibody binding correlated well with the lipopepitde

  4. Transmembrane peptides influence the affinity of sterols for phospholipid bilayers.

    PubMed

    Nyström, Joel H; Lönnfors, Max; Nyholm, Thomas K M

    2010-07-21

    Cholesterol is distributed unevenly between different cellular membrane compartments, and the cholesterol content increases from the inner bilayers toward the plasma membrane. It has been suggested that this cholesterol gradient is important in the sorting of transmembrane proteins. Cholesterol has also been to shown play an important role in lateral organization of eukaryotic cell membranes. In this study the aim was to determine how transmembrane proteins influence the lateral distribution of cholesterol in phospholipid bilayers. Insight into this can be obtained by studying how cholesterol interacts with bilayer membranes of different composition in the presence of designed peptides that mimic the transmembrane helices of proteins. For this purpose we developed an assay in which the partitioning of the fluorescent cholesterol analog CTL between LUVs and mbetaCD can be measured. Comparison of how cholesterol and CTL partitioning between mbetaCD and phospholipid bilayers with different composition suggests that CTL sensed changes in bilayer composition similarly as cholesterol. Therefore, the results obtained with CTL can be used to understand cholesterol distribution in lipid bilayers. The effect of WALP23 on CTL partitioning between DMPC bilayers and mbetaCD was measured. From the results it was clear that WALP23 increased both the order in the bilayers (as seen from CTL and DPH anisotropy) and the affinity of the sterol for the bilayer in a concentration dependent way. Although WALP23 also increased the order in DLPC and POPC bilayers the effects on CTL partitioning was much smaller with these lipids. This indicates that proteins have the largest effect on sterol interactions with phospholipids that have longer and saturated acyl chains. KALP23 did not significantly affect the acyl chain order in the phospholipid bilayers, and inclusion of KALP23 into DMPC bilayers slightly decreased CTL partitioning into the bilayer. This shows that transmembrane proteins can

  5. Phospholipid bilayer relaxation dynamics as revealed by the pulsed electron-electron double resonance of spin labels

    NASA Astrophysics Data System (ADS)

    Syryamina, V. N.; Dzuba, S. A.

    2012-10-01

    Electron paramagnetic resonance (EPR) spectroscopy in the form of pulsed electron-electron double resonance (ELDOR) was applied to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) phospholipid bilayers containing lipids that were spin-labeled at different carbon positions along the lipid acyl chain. Pulsed ELDOR detects motionally induced spin flips of nitrogen nuclei in the nitroxide spin labels, which manifests itself as magnetization transfer (MT) in the nitroxide EPR spectrum. The MT effect was observed over a wide temperature range (100-225 K) on a microsecond time scale. In line with a previous study on molecular glasses [N. P. Isaev and S. A. Dzuba, J. Chem. Phys. 135, 094508 (2011), 10.1063/1.3633241], the motions that induce MT effect were suggested to have the same nature as those in dielectric secondary (β) Johari-Goldstein fast relaxation. The results were compared with literature dielectric relaxation data for POPC bilayers, revealing some common features. Molecular motions resulting in MT are faster for deeper spin labels in the membrane interior. The addition of cholesterol to the bilayer suppresses the lipid motions near the steroid nucleus and accelerates the lipid motions beyond the steroid nucleus, in the bilayer interior. This finding was attributed to the lipid acyl chains being more ordered near the steroid nucleus and less ordered in the bilayer interior. The motions are absent in dry lipids, indicating that the motions are determined by intermolecular interactions in the bilayer.

  6. Effect of hydrostatic pressure on water penetration and rotational dynamics in phospholipid-cholesterol bilayers.

    PubMed Central

    Bernsdorff, C; Wolf, A; Winter, R; Gratton, E

    1997-01-01

    The effect of high hydrostatic pressure on the lipid bilayer hydration, the mean order parameter, and rotational dynamics of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) cholesterol vesicles has been studied by time-resolved fluorescence spectroscopy up to 1500 bar. Whereas the degree of hydration in the lipid headgroup and interfacial region was assessed from fluorescence lifetime data using the probe 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), the corresponding information in the upper acyl chain region was estimated from its effect on the fluorescence lifetime of and 3-(diphenylhexatrienyl)propyl-trimethylammonium (TMAP-DPH). The lifetime data indicate a greater level of interfacial hydration for DPPC bilayers than for POPC bilayers, but there is no marked difference in interchain hydration of the two bilayer systems. The addition of cholesterol at levels from 30 to 50 mol% to DPPC has a greater effect on the increase of hydrophobicity in the interfacial region of the bilayer than the application of hydrostatic pressure of several hundred to 1000 bar. Although the same trend is observed in the corresponding system, POPC/30 mol% cholesterol, the observed effects are markedly less pronounced. Whereas the rotational correlation times of the fluorophores decrease in passing the pressure-induced liquid-crystalline to gel phase transition of DPPC, the wobbling diffusion coefficient remains essentially unchanged. The wobbling diffusion constant of the two fluorophores changes markedly upon incorporation of 30 mol% cholesterol, and increases at higher pressures, also in the case of POPC/30 mol% cholesterol. The observed effects are discussed in terms of changes in the rotational characteristics of the fluorophores and the phase-state of the lipid mixture. The results demonstrate the ability of cholesterol to adjust the structural and dynamic properties of membranes composed of

  7. Formation of pit-spanning phospholipid bilayers on nanostructured silicon dioxide surfaces for studying biological membrane events.

    PubMed

    Pfeiffer, Indriati; Zäch, Michael

    2013-01-01

    Zwitterionic phospholipid vesicles are known to adsorb and ultimately rupture on flat silicon dioxide (SiO2) surfaces to form supported lipid bilayers. Surface topography, however, alters the kinetics and mechanistic details of vesicles adsorption, which under certain conditions may be exploited to form a suspended bilayer. Here we describe the use of nanostructured SiO2 surfaces prepared by the colloidal lithography technique to scrutinize the formation of suspended 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers from a solution of small unilamellar lipid vesicles (SUVs). Atomic force microscopy (AFM) and quartz crystal microbalance with dissipation monitoring (QCM-D) were employed to characterize nanostructure fabrication and lipid bilayer assembly on the surface.

  8. Effects of cholesterol and saturated sphingolipids on acyl chain order in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers--a comparative study with phase-selective fluorophores.

    PubMed

    Engberg, Oskar; Nurmi, Henrik; Nyholm, Thomas K M; Slotte, J Peter

    2015-04-14

    Saturated sphingolipids have high acyl chain order. Our aim was to study how palmitoylated sphingomyelin (PSM), ceramide (PCer), glucosyl (GlcPCer)-, and galactosylceramide (GalPCer) were able to order the bulk acyl chains of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), in comparison with cholesterol. For this reason, we used lipid probes which had preferred phases that were either the disordered phase (1-oleoyl-2-propionyl[DPH-sn-glycero-3-phosphcholine (18:1-DPH-PC) or the ordered phase (trans parinaric acid (tPA). DPH was also used, although it has no clear phase preference. We measured steady-state anisotropy (all probes) and performed fluorescence lifetime analysis (tPA) as a function of composition and temperature. At concentrations where the saturated sphingolipids were not aggregated into ordered domains (and 23 °C), they did not increase POPC acyl chain order as determined from 18:1-DPH-PC anisotropy. As expected, cholesterol increased the POPC acyl chain order linearly as a function of concentration (0-28 mol %). Since PCer already forms ordered domains below 5 mol % (at 23 °C), we measured the acyl chain ordering effect of PCer at 50 °C (0-13 mol %) and observed that PCer ordered POPC acyl chains as efficiently as cholesterol. We conclude that the bulk acyl chain order of POPC was not markedly affected in bilayers where disordered and ordered domains coexist.

  9. Naratriptan aggregation in lipid bilayers: perspectives from molecular dynamics simulations.

    PubMed

    Wood, Irene; Pickholz, Mónica

    2016-09-01

    In order to understand the interaction between naratriptan and a fully hydrated bilayer of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyl-choline (POPC), we carried out molecular dynamics simulations. The simulations were performed considering neutral and protonated ionization states, starting from different initial conditions. At physiological pH, the protonated state of naratriptan is predominant. It is expected that neutral compounds could have larger membrane partition than charged compounds. However, for the specific case of triptans, it is difficult to study neutral species in membranes experimentally, making computer simulations an interesting tool. When the naratriptan molecules were originally placed in water, they partitioned between the bilayer/water interface and water phase, as has been described for similar compounds. From this condition, the drugs displayed low access to the hydrophobic environment, with no significant effects on bilayer organization. The molecules anchored in the interface, due mainly to the barrier function of the polar and oriented lipid heads. On the other hand, when placed inside the bilayer, both neutral and protonated naratriptan showed self-aggregation in the lipid tail environment. In particular, the protonated species exhibited a pore-like structure, dragging water through this environment. Graphical Abstract Different behaviour of Naratriptan and Sumatriptan, when the drugs were originally placed in the lipid core. PMID:27558798

  10. Interaction of 3β-amino-5-cholestene with phospholipids in binary and ternary bilayer membranes

    PubMed Central

    Lönnfors, Max; Engberg, Oskar; Peterson, Blake R.; Slotte, J. Peter

    2011-01-01

    3β-Amino-5-cholestene (aminocholesterol) is a synthetic sterol whose properties in bilayer membranes have been examined. In fluid palmitoyl sphingomyelin (PSM) bilayers, aminocholesterol and cholesterol were equally effective in increasing acyl chain order, based on changes in diphenylhexatriene (DPH) anisotropy. In fluid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, aminocholesterol ordered acyl chains, but slightly less efficiently than cholesterol. Aminocholesterol eliminated the PSM and DPPC gel-to-liquid crystalline phase transition enthalpy linearly with concentration, and the enthalpy approached zero at 30 mol% sterol. Whereas cholesterol was able to increase the thermostability of ordered PSM domains in a fluid bilayer, aminocholesterol under equal conditions failed to do this, suggesting that its interaction with PSM was not as favorable as cholesterol’s. In ternary mixed bilayers, containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), PSM or DPPC, and cholesterol at proportions to contain a liquid-ordered phase (60:40 by mol of POPC and PSM or DPPC, and 30 mol% cholesterol), the average life-time of trans parinaric acid (tPA) was close to 20 ns. When cholesterol was replaced with aminocholesterol in such mixed bilayers, the average life-time of tPA was only marginally shorter (about 18 ns). This observation, together with acyl chain ordering data, clearly shows that aminocholesterol was able to form a liquid-ordered phase with saturated PSM or DPPC. We conclude that aminocholesterol should be a good sterol replacement in model membrane systems for which a partial positive charge is deemed beneficial. PMID:22128897

  11. Acyl chain composition and coexisting fluid phases in lipid bilayers

    NASA Astrophysics Data System (ADS)

    Gu, Yongwen; Bradley, Miranda; Mitchell, Drake

    2011-10-01

    At room temperature phospholipid bilayers enriched in sphingolipids and cholesterol may form a solid phase as well as two coexisting fluid phases. These are the standard fluid phase, or the liquid-disordered phase, ld, and the liquid-ordered phase, lo, which is commonly associated with lipid rafts. Ternary mixtures of palmitoyl-oleoyl-phosphocholine (POPC; 16:0,18:1 PC), sphingomyelin (SPM), and cholesterol (Chol) form coexisting lo, ld and solid phases over a wide range of molar ratios. We are examining the ability of two fluorescent probes to detect these 2 phases: NBD linked to di-16:0 PE which partitions strongly into the lo phase and NBD linked to di-18:1 PE which partitions strongly into the ld phase. We are also examining the effect of the highly polyunsaturated phospholipid stearoyl-docosahexanoyl-phosphocholine (SDPC; 18:0, 22:6 PC) on the ternary phase diagram of POPC/SPM/Chol with particular focus on the functionally important lo/ld coexistence region. We report on the fluorescence lifetime and anisotropy decay dynamics of these two fluorescent probes.

  12. Interactions of the C-terminus of lung surfactant protein B with lipid bilayers are modulated by acyl chain saturation.

    PubMed

    Antharam, Vijay C; Farver, R Suzanne; Kuznetsova, Anna; Sippel, Katherine H; Mills, Frank D; Elliott, Douglas W; Sternin, Edward; Long, Joanna R

    2008-11-01

    Lung surfactant protein B (SP-B) is critical to minimizing surface tension in the alveoli. The C-terminus of SP-B, residues 59-80, has much of the surface activity of the full protein and serves as a template for the development of synthetic surfactant replacements. The molecular mechanisms responsible for its ability to restore lung compliance were investigated with circular dichroism, differential scanning calorimetry, and (31)P and (2)H solid-state NMR spectroscopy. SP-B(59-80) forms an amphipathic helix which alters lipid organization and acyl chain dynamics in fluid lamellar phase 4:1 DPPC:POPG and 3:1 POPC:POPG MLVs. At higher levels of SP-B(59-80) in the POPC:POPG lipid system a transition to a nonlamellar phase is observed while DPPC:POPG mixtures remain in a lamellar phase. Deuterium NMR shows an increase in acyl chain order in DPPC:POPG MLVs on addition of SP-B(59-80); in POPC:POPG MLVs, acyl chain order parameters decrease. Our results indicate SP-B(59-80) penetrates deeply into DPPC:POPG bilayers and binds more peripherally to POPC:POPG bilayers. Similar behavior has been observed for KL(4), a peptide mimetic of SP-B which was originally designed using SP-B(59-80) as a template and has been clinically demonstrated to be successful in treating respiratory distress syndrome. The ability of these helical peptides to differentially partition into lipid lamellae based on their degree of monounsaturation and subsequent changes in lipid dynamics suggest a mechanism for lipid organization and trafficking within the dynamic lung environment. PMID:18694722

  13. Hydration lubrication and shear-induced self-healing of lipid bilayer boundary lubricants in phosphatidylcholine dispersions.

    PubMed

    Sorkin, Raya; Kampf, Nir; Zhu, Linyi; Klein, Jacob

    2016-03-14

    Measurements of normal and shear (frictional) forces between mica surfaces across small unilamellar vesicle (SUV) dispersions of the phosphatidylcholine (PC) lipids DMPC (14:0), DPPC (16:0) and DSPC (18:0) and POPC (16:0, 18:1), at physiologically high pressures, are reported. We have previously studied the normal and shear forces between two opposing surfaces bearing PC vesicles across pure water and showed that liposome lubrication ability improved with increasing acyl chain length, and correlated strongly with the SUV structural integrity on the substrate surface (DSPC > DPPC > DMPC). In the current study, surprisingly, we discovered that this trend is reversed when the measurements are conducted in SUV dispersions, instead of pure water. In their corresponding SUV dispersion, DMPC SUVs ruptured and formed bilayers, which were able to provide reversible and reproducible lubrication with extremely low friction (μ < 10(-4)) up to pressures of 70-90 atm. Similarly, POPC SUVs also formed bilayers which exhibited low friction (μ < 10(-4)) up to pressures as high as 160 atm. DPPC and DSPC SUVs also provided good lubrication, but with slightly higher friction coefficients (μ = 10(-3)-10(-4)). We believe these differences originate from fast self-healing of the softer surface layers (which are in their liquid disordered phase, POPC, or close to it, DMPC), which renders the robustness of the DPPC or DSPC (both in their solid ordered phase) less important in these conditions. Under these circumstances, the enhanced hydration of the less densely packed POPC and DMPC surface layers is now believed to play an important role, and allows enhanced lubrication via the hydration lubrication mechanism. Our findings may have implications for the understanding of complex biological systems such us biolubrication of synovial joints. PMID:26861851

  14. General model of phospholipid bilayers in fluid phase within the single chain mean field theory.

    PubMed

    Guo, Yachong; Pogodin, Sergey; Baulin, Vladimir A

    2014-05-01

    Coarse-grained model for saturated phospholipids: 1,2-didecanoyl-sn-glycero-3-phosphocholine (DCPC), 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and unsaturated phospholipids: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1,2- dioleoyl-sn-glycero-3-phosphocholine (DOPC) is introduced within the single chain mean field theory. A single set of parameters adjusted for DMPC bilayers gives an adequate description of equilibrium and mechanical properties of a range of saturated lipid molecules that differ only in length of their hydrophobic tails and unsaturated (POPC, DOPC) phospholipids which have double bonds in the tails. A double bond is modeled with a fixed angle of 120°, while the rest of the parameters are kept the same as saturated lipids. The thickness of the bilayer and its hydrophobic core, the compressibility, and the equilibrium area per lipid correspond to experimentally measured values for each lipid, changing linearly with the length of the tail. The model for unsaturated phospholipids also fetches main thermodynamical properties of the bilayers. This model is used for an accurate estimation of the free energies of the compressed or stretched bilayers in stacks or multilayers and gives reasonable estimates for free energies. The proposed model may further be used for studies of mixtures of lipids, small molecule inclusions, interactions of bilayers with embedded proteins.

  15. Estimation of activation energy for electroporation and pore growth rate in liquid crystalline and gel phases of lipid bilayers using molecular dynamics simulations.

    PubMed

    Majhi, Amit Kumar; Kanchi, Subbarao; Venkataraman, V; Ayappa, K G; Maiti, Prabal K

    2015-11-28

    Molecular dynamics simulations of electroporation in POPC and DPPC lipid bilayers have been carried out at different temperatures ranging from 230 K to 350 K for varying electric fields. The dynamics of pore formation, including threshold field, pore initiation time, pore growth rate, and pore closure rate after the field is switched off, was studied in both the gel and liquid crystalline (Lα) phases of the bilayers. Using an Arrhenius model of pore initiation kinetics, the activation energy for pore opening was estimated to be 25.6 kJ mol(-1) and 32.6 kJ mol(-1) in the Lα phase of POPC and DPPC lipids respectively at a field strength of 0.32 V nm(-1). The activation energy decreases to 24.2 kJ mol(-1) and 23.7 kJ mol(-1) respectively at a higher field strength of 1.1 V nm(-1). At temperatures below the melting point, the activation energy in the gel phase of POPC and DPPC increases to 28.8 kJ mol(-1) and 34.4 kJ mol(-1) respectively at the same field of 1.1 V nm(-1). The pore closing time was found to be higher in the gel than in the Lα phase. The pore growth rate increases linearly with temperature and quadratically with field, consistent with viscosity limited growth models.

  16. Possible Domain Formation In PE/PC Bilayers Containing High Cholesterol

    NASA Astrophysics Data System (ADS)

    Hein, Matthew; Hussain, Fazle; Huang, Juyang

    2015-03-01

    Cholesterol is a significant component of animal cell membranes, and its presence has the effects of not only adding rigidity to the lipid bilayer, but also leading to the formation of lipid domains. Two other lipids of interest are phosphatidylethanolamine (PE), which constitutes about 45 percent of the phospholipids found in human nervous tissues, and phosphatidylcholine (PC), which is found in every cell of the human body. The maximum solubility of cholesterol is the highest mole fraction of cholesterol that the lipid bilayer can retain, at which point cholesterol begins to precipitate out to form cholesterol monohydrate crystals. We have measured the maximum solubility of cholesterol in mixtures of 16:0-18:1PE and 16:0-18:1PC using a new light scattering technique, which utilizes the anisotropic nature of light scattering by cholesterol crystals. This new method is highly accurate and reproducible. Our results show that the maximum solubility of cholesterol increases linearly as a function of the molar ratio POPC/(POPE+POPC), which suggests possible domain formation in mixtures of PE and PC containing maximum amount of cholesterol.

  17. The Lipid Bilayer Provides a Site for Cortisone Crystallization at High Cortisone Concentrations

    PubMed Central

    Alsop, Richard J.; Khondker, Adree; Hub, Jochen S.; Rheinstädter, Maikel C.

    2016-01-01

    Cortisone is an injected anti-inflammatory drug that can cause painful side effects known as “steroid flares” which are caused by cortisone crystallizing at the injection site. We used molecular dynamics simulations and X-ray diffraction to study the interaction of cortisone with model lipid membranes made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) at drug concentrations from 0 mol% to 50 mol%. Cortisone was found to partition in the lipid bilayer and locate in the hydrophilic to hydrophobic interface of the membranes. Cortisone strongly affects the integrity of the membrane, as quantified by a decreased membrane thickness, increased area per lipid, and decreased lipid tail order parameters. At cortisone concentrations of more than 20 mol%, signals from crystallized cortisone were observed. These crystallites are embedded in the bilayers and orient with the membranes. While the cortisone molecules align parallel to the bilayers at low concentrations, they start to penetrate the hydrophobic core at higher concentrations. Trans-membrane crystallites start to nucleate when the membrane thickness has decreased such that cortisone molecules in the different leaflets can find partners from the opposite leaflet resulting in a non-zero density of cortisone molecules in the bilayer center. We suggest that the lipid bilayer provides a site for cortisone crystallization. PMID:26936102

  18. Importance of phospholipid bilayer integrity in the analysis of protein–lipid interactions

    SciTech Connect

    Drücker, Patrick; Gerke, Volker; Galla, Hans-Joachim

    2014-10-10

    Highlights: • We show long-term mechanical stabilization of solid supported bilayers. • Bilayer integrity is essential for the investigation of protein–lipid interactions. • Protein adsorption to a bilayer containing defects causes membrane destruction. - Abstract: The integrity of supported phospholipid bilayer membranes is of crucial importance for the investigation of lipid–protein interactions. Therefore we recorded the formation of supported membranes on SiO{sub 2} and mica by quartz crystal microbalance and controlled the integrity by atomic force microscopy. This study aims to analyze how membrane defects affect protein–lipid interactions. The experiments focused on a lipid mixture of POPC/DOPC/Chol/POPS/PI(4,5)P{sub 2} (37:20:20:20:3) and the binding of the peripheral membrane associated protein annexin A2. We found that formation of a continuous undisturbed bilayer is an indispensable precondition for a reliable determination and quantification of lipid–protein-interactions. If membrane defects were present, protein adsorption causes membrane disruption and lipid detachment on a support thus leading to false determination of binding constants. Our results obtained for PI(4,5)P{sub 2} and cholesterol containing supported membranes yield new knowledge to construct functional surfaces that may cover nanoporous substrates, form free standing membranes or may be used for lab-on-a-chip applications.

  19. Structural characterization of the voltage sensor domain and voltage-gated K+- channel proteins vectorially-oriented within a single bilayer membrane at the solid/vapor and solid/liquid interfaces via neutron interferometry

    PubMed Central

    Gupta, S.; Dura, J.A.; Freites, J.A.; Tobias, D.J.; Blasie, J. K.

    2012-01-01

    The voltage-sensor domain (VSD) is a modular 4-helix bundle component that confers voltage sensitivity to voltage-gated cation channels in biological membranes. Despite extensive biophysical studies and the recent availability of x-ray crystal structures for a few voltage-gated potassium (Kv-) channels and a voltage-gate sodium (Nav-) channel, a complete understanding of the cooperative mechanism of electromechanical coupling, interconverting the closed-to-open states (i.e. non-conducting to cation conducting) remains undetermined. Moreover, the function of these domains is highly dependent on the physical-chemical properties of the surrounding lipid membrane environment. The basis for this work was provided by a recent structural study of the VSD from a prokaryotic Kv-channel vectorially-oriented within a single phospholipid (POPC; 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) membrane investigated by x-ray interferometry at the solid/moist He (or solid/vapor) and solid/liquid interfaces thus achieving partial to full hydration, respectively (Gupta et. al. Phys. Rev E. 2011, 84). Here, we utilize neutron interferometry to characterize this system in substantially greater structural detail at the sub-molecular level, due to its inherent advantages arising from solvent contrast variation coupled with the deuteration of selected sub-molecular membrane components, especially important for the membrane at the solid/liquid interface. We demonstrate the unique vectorial orientation of the VSD and the retention of its molecular conformation manifest in the asymmetric profile structure of the protein within the profile structure of this single bilayer membrane system. We definitively characterize the asymmetric phospholipid bilayer solvating the lateral surfaces of the VSD protein within the membrane. The profile structures of both the VSD protein and phospholipid bilayer depend upon the hydration state of the membrane. We also determine the distribution of water and

  20. On the application of the MARTINI coarse-grained model to immersion of a protein in a phospholipid bilayer

    NASA Astrophysics Data System (ADS)

    Mustafa, Ghulam; Nandekar, Prajwal P.; Yu, Xiaofeng; Wade, Rebecca C.

    2015-12-01

    An important step in the simulation of a membrane protein in a phospholipid bilayer is the correct immersion of the protein in the bilayer. Crystal structures are determined without the bilayer. Particularly for proteins with monotopic domains, it can be unclear how deeply and in which orientation the protein is being inserted in the membrane. We have previously developed a procedure combining coarse-grain (CG) with all-atom (AA) molecular dynamics (MD) simulations to insert and simulate a cytochrome P450 (CYP) possessing an N-terminal transmembrane helix connected by a flexible linker region to a globular domain that dips into the membrane. The CG simulations provide a computationally efficient means to explore different orientations and conformations of the CYP in the membrane. Converged configurations obtained in the CG simulations are then refined in AA simulations. Here, we tested different variants of the MARTINI CG model, differing in the water model, the treatment of long-range non-bonded interactions, and the implementation (GROMACS 4.5.5 vs 5.0.4), for this purpose. We examined the behavior of the models for simulating a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer in water and for the immersion of CYP3A4 in a POPC bilayer, and compared the CG-MD results with the previously reported experimental and simulation results. We also tested the methodology on a set of four other CYPs. Finally, we propose an optimized protocol for modeling such protein-membrane systems that provides the most plausible configurations and is computationally efficient; this incorporates the standard non-polar water model and the GROMACS 5.0.4 implementation with a reaction field treatment of long-range interactions.

  1. On the application of the MARTINI coarse-grained model to immersion of a protein in a phospholipid bilayer

    SciTech Connect

    Mustafa, Ghulam E-mail: rebecca.wade@h-its.org; Nandekar, Prajwal P.; Yu, Xiaofeng; Wade, Rebecca C. E-mail: rebecca.wade@h-its.org

    2015-12-28

    An important step in the simulation of a membrane protein in a phospholipid bilayer is the correct immersion of the protein in the bilayer. Crystal structures are determined without the bilayer. Particularly for proteins with monotopic domains, it can be unclear how deeply and in which orientation the protein is being inserted in the membrane. We have previously developed a procedure combining coarse-grain (CG) with all-atom (AA) molecular dynamics (MD) simulations to insert and simulate a cytochrome P450 (CYP) possessing an N-terminal transmembrane helix connected by a flexible linker region to a globular domain that dips into the membrane. The CG simulations provide a computationally efficient means to explore different orientations and conformations of the CYP in the membrane. Converged configurations obtained in the CG simulations are then refined in AA simulations. Here, we tested different variants of the MARTINI CG model, differing in the water model, the treatment of long-range non-bonded interactions, and the implementation (GROMACS 4.5.5 vs 5.0.4), for this purpose. We examined the behavior of the models for simulating a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer in water and for the immersion of CYP3A4 in a POPC bilayer, and compared the CG-MD results with the previously reported experimental and simulation results. We also tested the methodology on a set of four other CYPs. Finally, we propose an optimized protocol for modeling such protein-membrane systems that provides the most plausible configurations and is computationally efficient; this incorporates the standard non-polar water model and the GROMACS 5.0.4 implementation with a reaction field treatment of long-range interactions.

  2. Improved Coarse-Grained Modeling of Cholesterol-Containing Lipid Bilayers

    PubMed Central

    2015-01-01

    Cholesterol trafficking, which is an essential function in mammalian cells, is intimately connected to molecular-scale interactions through cholesterol modulation of membrane structure and dynamics and interaction with membrane receptors. Since these effects of cholesterol occur on micro- to millisecond time scales, it is essential to develop accurate coarse-grained simulation models that can reach these time scales. Cholesterol has been shown experimentally to thicken the membrane and increase phospholipid tail order between 0 and 40% cholesterol, above which these effects plateau or slightly decrease. Here, we showed that the published MARTINI coarse-grained force-field for phospholipid (POPC) and cholesterol fails to capture these effects. Using reference atomistic simulations, we systematically modified POPC and cholesterol bonded parameters in MARTINI to improve its performance. We showed that the corrections to pseudobond angles between glycerol and the lipid tails and around the oleoyl double bond particle (the “angle-corrected model”) slightly improves the agreement of MARTINI with experimentally measured thermal, elastic, and dynamic properties of POPC membranes. The angle-corrected model improves prediction of the thickening and ordering effects up to 40% cholesterol but overestimates these effects at higher cholesterol concentration. In accordance with prior work that showed the cholesterol rough face methyl groups are important for limiting cholesterol self-association, we revised the coarse-grained representation of these methyl groups to better match cholesterol-cholesterol radial distribution functions from atomistic simulations. In addition, by using a finer-grained representation of the branched cholesterol tail than MARTINI, we improved predictions of lipid tail order and bilayer thickness across a wide range of concentrations. Finally, transferability testing shows that a model incorporating our revised parameters into DOPC outperforms other

  3. D IR Line Shapes for Determining the Structure of a Peptide in a Bilayer

    NASA Astrophysics Data System (ADS)

    Woys, Ann Marie; Lin, Y. S.; Skinner, J. S.; Zanni, M. T.; Reddy, A. S.; de Pablo, J. J.

    2010-06-01

    Structure of the antimicrobial peptide, ovispirin, on a lipid bilayer was determined using 2D IR spectroscopy and spectra calculated from molecular dynamics simulations. Ovispirin is an 18 residue amphipathic peptide that binds parallel to the membrane in a mostly alpha helical conformation. 15 of the 18 residues were ^1^3C^1^8O isotopically labeled on the backbone to isolate the amide I vibration at each position. 2D IR spectra were collected for each labeled peptide in 3:1 POPC/POPG vesicles, and peak width along the diagonal was measured. The diagonal line width is sensitive to the vibrator's electrostatic environment, which varies through the bilayer. We observe an oscillatory line width spanning 10 to 24 cm-1 and with a period of nearly 3.6 residues. To further investigate the position of ovispirin in a bilayer, molecular dynamics simulations determined the peptide depth to be just below the lipid headgroups. The trajectory of ovispirin at this depth was used to calculate 2D IR spectra, from which the diagonal line width is measured. Both experimental and simulated line widths are similar in periodicity and suggest a kink in the peptide backbone and the tilt in the bilayer. A. Woys, Y. S. Lin, A. S. Reddy, W. Xiong, J. J. de Pablo, J. S. Skinner, and M. T. Zanni, JACS 132, 2832-2838 (2010).

  4. Probing the Huntingtin 1-17 Membrane Anchor on a Phospholipid Bilayer by Using All-Atom Simulations

    PubMed Central

    Côté, Sébastien; Binette, Vincent; Salnikov, Evgeniy S.; Bechinger, Burkhard; Mousseau, Normand

    2015-01-01

    Mislocalization and aggregation of the huntingtin protein are related to Huntington’s disease. Its first exon—more specifically the first 17 amino acids (Htt17)—is crucial for the physiological and pathological functions of huntingtin. It regulates huntingtin’s activity through posttranslational modifications and serves as an anchor to membrane-containing organelles of the cell. Recently, structure and orientation of the Htt17 membrane anchor were determined using a combined solution and solid-state NMR approach. This prompted us to refine this model by investigating the dynamics and thermodynamics of this membrane anchor on a POPC bilayer using all-atom, explicit solvent molecular dynamics and Hamiltonian replica exchange. Our simulations are combined with various experimental measurements to generate a high-resolution atomistic model for the huntingtin Htt17 membrane anchor on a POPC bilayer. More precisely, we observe that the single α-helix structure is more stable in the phospholipid membrane than the NMR model obtained in the presence of dodecylphosphocholine detergent micelles. The resulting Htt17 monomer has its hydrophobic plane oriented parallel to the bilayer surface. Our results further unveil the key residues interacting with the membrane in terms of hydrogen bonds, salt-bridges, and nonpolar contributions. We also observe that Htt17 equilibrates at a well-defined insertion depth and that it perturbs the physical properties—order parameter, thickness, and area per lipid—of the bilayer in a manner that could favor its dimerization. Overall, our observations reinforce and refine the NMR measurements on the Htt17 membrane anchor segment of huntingtin that is of fundamental importance to its biological functions. PMID:25762330

  5. Terminal lipophilization of a unique DNA dodecamer by various nucleolipid headgroups: Their incorporation into artificial lipid bilayers and hydrodynamic properties

    PubMed Central

    Werz, Emma

    2015-01-01

    Summary A series of six cyanine-5-labeled oligonucleotides (LONs 10–15), each terminally lipophilized with different nucleolipid head groups, were synthesized using the recently prepared phosphoramidites 4b–9b. The insertion of the LONs within an artificial lipid bilayer, composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), was studied by single molecule fluorescence spectroscopy and microscopy with the help of an optically transparent microfluidic sample carrier with perfusion capabilities. The incorporation of the lipo-oligonucleotides into the bilayer was studied with respect to efficiency (maximal bilayer brightness) as well as stability against perfusion (final stable bilayer brightness). Attempts to correlate these parameters with the log P values of the corresponding nucleolipid head groups failed, a result which clearly demonstrates that not only the lipophilicity but mainly the chemical structure and topology of the head group is of decisive importance for the optimal interaction of a lipo-oligonucleotide with an artificial lipid bilayer. Moreover, fluorescence half-live and diffusion time values were measured to determine the diffusion coefficients of the lipo-oligonucleotides. PMID:26124894

  6. Effect of Sterol Structure on Chain Ordering of an Unsaturated Phospholipid: A 2H-NMR Study of POPC/Sterol Membranes

    NASA Astrophysics Data System (ADS)

    Shaghaghi, Mehran; Thewalt, Jenifer; Zuckermann, Martin

    2012-10-01

    The physical properties of biological membranes are considerably altered by the presence of sterols. In particular, sterols help to maintain the integrity of the cell by adjusting the fluidity of the plasma membrane. Cholesterol is in addition an important component of lipid rafts which are hypothesized to compartmentalize the cell membrane surface thereby making it possible for certain proteins to function. Using 2H-NMR spectroscopy, we studied the effect of a series of different sterols on the chain ordering of POPC, an unsaturated phospholipid present in eukaryotic cell membranes. We were able to assigned specific roles to the structural differences between the sterols by comparing the manner in which they affect the average lipid chain conformation of POPC.

  7. AFM Investigations of Phase Separation in Supported Membranes of Binary Mixtures of POPC and an Eicosanyl-based Bisphosphocholine Bolalipid

    PubMed Central

    Mulligan, Kirk; Brownholland, David; Carnini, Anna; Thompson, David H.; Johnston, Linda J.

    2010-01-01

    Supported membranes prepared from binary mixtures of DOPC and the bolalipid C20BAS have been examined by atomic force microscopy (AFM). The supported membranes are phase separated to give a thicker DOPC-rich phase and a thinner bolalipid-rich phase for a range of lipid compositions. These results confirm an earlier prediction from mean field theory that phase separation is the thermodynamically stable state for membranes containing approximately equimolar C20BAS and double chain monopolar lipids with chain lengths exceeding fifteen carbons. Hydrophobic mismatch between the monopolar lipid hydrocarbon chains and the membrane spanning bolalipid chains was suggested to provide the driving force for phase separation. The AFM results also show that the morphology of the mixed POPC:C20BAS supported membranes varies significantly with the conditions used to prepare the vesicles and supported membrane samples. The complex membrane morphologies observed are attributed to the interplay of several factors, including a compositionally heterogeneous vesicle population, exchange of lipid between the vesicle solution and solid substrate during formation of the supported membrane, and slow equilibration of domains due to pinning of the lipids to the solid support. PMID:20099816

  8. Oxidative Degradation of the Monolayer of 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-Phosphocholine (POPC) in Low-Level Ozone.

    PubMed

    Qiao, Lin; Ge, Aimin; Liang, Yimin; Ye, Shen

    2015-11-01

    Ambient ozone is a common pollutant in the atmosphere that has an extremely high oxidative ability, can dramatically change the structure and functionality of biomolecules, and is harmful to public health. However, the knowledge about the influence of low-level ozone is still very limited at a molecular level. In the present study, the monolayer of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC, 16:0-18:1 PC) as well as its binary mixed monolayer with 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC, 16:0 PC), which are widely found in many biological systems, have been systematically investigated in a low-level ozone environment (20 ± 10 ppb), by π-A isotherm, sum frequency generation (SFG) vibrational spectroscopy, and atomic force microscopy (AFM). Our results demonstrate that the POPC monolayer is unstable and the C═C moieties in the oleyl chain are selectively oxidized by the low-level ozone. The oxidized lipids from POPC initially remain and reorientate the hydrophilic portion to the water surface and gradually dissolve into the aqueous solution. One should take great caution when using unsaturated lipid molecules to avoid their possible oxidation in the ambient environment. The present study expands and deepens our insights into the oxidation mechanism of unsaturated lipids at a molecular level.

  9. Reconstitution of KCNE1 into lipid bilayers: comparing the structural, dynamic, and activity differences in micelle and vesicle environments.

    PubMed

    Coey, Aaron T; Sahu, Indra D; Gunasekera, Thusitha S; Troxel, Kaylee R; Hawn, Jaclyn M; Swartz, Max S; Wickenheiser, Marilyn R; Reid, Ro-jay; Welch, Richard C; Vanoye, Carlos G; Kang, Congbao; Sanders, Charles R; Lorigan, Gary A

    2011-12-20

    KCNE1 (minK), found in the human heart and cochlea, is a transmembrane protein that modulates the voltage-gated potassium KCNQ1 channel. While KCNE1 has previously been the subject of extensive structural studies in lyso-phospholipid detergent micelles, key observations have yet to be confirmed and refined in lipid bilayers. In this study, a reliable method for reconstituting KCNE1 into lipid bilayer vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho(1'-rac-glycerol) (sodium salt) (POPG) was developed. Microinjection of the proteoliposomes into Xenopus oocytes expressing the human KCNQ1 (K(V)7.1) voltage-gated potassium channel led to nativelike modulation of the channel. Circular dichroism spectroscopy demonstrated that the percent helicity of KCNE1 is significantly higher for the protein reconstituted in lipid vesicles than for the previously described structure in 1.0% 1-myristoyl-2-hydroxy-sn-glycero-3-phospho(1'-rac-glycerol) (sodium salt) (LMPG) micelles. SDSL electron paramagnetic resonance spectroscopic techniques were used to probe the local structure and environment of Ser28, Phe54, Phe57, Leu59, and Ser64 of KCNE1 in both POPC/POPG vesicles and LMPG micelles. Spin-labeled KCNE1 cysteine mutants at Phe54, Phe57, Leu59, and Ser64 were found to be located inside POPC/POPG vesicles, whereas Ser28 was found to be located outside the membrane. Ser64 was shown to be water inaccessible in vesicles but found to be water accessible in LMPG micelle solutions. These results suggest that key components of the micelle-derived structure of KCNE1 extend to the structure of this protein in lipid bilayers but also demonstrate the need to refine this structure using data derived from the bilayer-reconstituted protein to more accurately define its native structure. This work establishes the basis for such future studies.

  10. Bilayer Graphene Electromechanical Systems

    NASA Astrophysics Data System (ADS)

    Champagne, Alexandre; Storms, Matthew; Yigen, Serap; Reulet, Bertrand

    Bilayer graphene is an outstanding electromechanical system, and its electronic and mechanical properties, as well as their coupling, are widely tunable. To the best of our knowledge, simultaneous charge transport and mechanical spectroscopy (via RF mixing) has not been realized in bilayer graphene. We present data showing clear electromechanical resonances in three suspended bilayer devices whose length range from 1 to 2 microns. We first describe the low-temperature current annealing of the devices which is crucial to achieve the transconductance, I -VG , necessary to implement a RF mixing detection method. We describe our RF mixing circuit and data. We measure clear mechanical resonances ranging in frequency from 50 to 140 MHz. We show that we can smoothly tune the resonance frequencies of our bilayer resonators with mechanical strain applied via a backgate voltage. We measure quality factors up to 4000. We briefly discuss the effects of the RF driving power on the dispersion of the mechanical resonance. We aim to use these high quality mechanical resonance as a mechanical sensor of the bilayer quantum Hall phase transitions. We show initial data of a bilayer mechanical resonance as a function of magnetic field and quantum Hall phase transitions.

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

    DOE PAGES

    Tian, Jianhui; Nickels, Jonathan; Katsaras, John; Cheng, Xiaolin

    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

  12. Two-Dimensional Potentials of Mean Force of Nile Red in Intact and Damaged Model Bilayers. Application to Calculations of Fluorescence Spectra.

    PubMed

    Singh, Gurpreet; Chamberlin, Adam C; Zhekova, Hristina R; Noskov, Sergei Y; Tieleman, D Peter

    2016-01-12

    Fluorescent dyes revolutionized and expanded our understanding of biological membranes. The interpretation of experimental fluorescence data in terms of membrane structure, however, requires detailed information about the molecular environment of the dyes. Nile red is a fluorescent molecule whose excitation and emission maxima depend on the polarity of the solvent. It is mainly used as a probe to study lipid microenvironments, for example in imaging the progression of damage to the myelin sheath in multiple sclerosis. In this study, we determine the position and orientation of Nile red in lipid bilayers by calculating two-dimensional Potential of Mean Force (2D-PMF) profiles in a defect-free 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer and in damaged bilayers containing two mixtures of the oxidized lipid 1-palmitoyl-2-(9'-oxo-nonanoyl)-sn-glycero-3-phosphocholine and POPC. From 2D-PMF simulations we obtain positions and orientations of Nile Red corresponding to the minimum on the binding free energy surface in three different membrane environments with increasing amounts of water, mimicking damage in biological tissue. Using representative snapshots from the simulations, we use combined quantum mechanical/molecular mechanical (QM/MM) models to calculate the emission spectrum of Nile red as a function of its local solvation environment. The results of QM and QM/MM computations are in qualitative agreement with the experimentally observed shift in fluorescence for the dye moving from aqueous solution to the more hydrophobic environment of the lipid interiors. The range of the conformation dependent values of the computed absorption-emission spectra and the lack of solvent relaxation effects in the QM/MM calculations made it challenging to delineate specific differences between the intact and damaged bilayers.

  13. 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.

  14. Twisting bilayer graphene superlattices.

    PubMed

    Lu, Chun-Chieh; Lin, Yung-Chang; Liu, Zheng; Yeh, Chao-Hui; Suenaga, Kazu; Chiu, Po-Wen

    2013-03-26

    Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decomposition of SiC; (ii) chemical vapor deposition (CVD) on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layers one atop the other, the latter of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycrystalline structures. The present study investigates bilayer graphene grown by ambient pressure CVD on polycrystalline Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes are shown to result from the twist, with the angle determined by transmission electron microscopy. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.

  15. Nanoparticle-lipid bilayer interactions studied with lipid bilayer arrays.

    PubMed

    Lu, Bin; Smith, Tyler; Schmidt, Jacob J

    2015-05-01

    The widespread environmental presence and commercial use of nanoparticles have raised significant health concerns as a result of many in vitro and in vivo assays indicating toxicity of a wide range of nanoparticle species. Many of these assays have identified the ability of nanoparticles to damage cell membranes. These interactions can be studied in detail using artificial lipid bilayers, which can provide insight into the nature of the particle-membrane interaction through variation of membrane and solution properties not possible with cell-based assays. However, the scope of these studies can be limited because of the low throughput characteristic of lipid bilayer platforms. We have recently described an easy to use, parallel lipid bilayer platform which we have used to electrically investigate the activity of 60 nm diameter amine and carboxyl modified polystyrene nanoparticles (NH2-NP and COOH-NP) with over 1000 lipid bilayers while varying lipid composition, bilayer charge, ionic strength, pH, voltage, serum, particle concentration, and particle charge. Our results confirm recent studies finding activity of NH2-NP but not COOH-NP. Detailed analysis shows that NH2-NP formed pores 0.3-2.3 nm in radius, dependent on bilayer and solution composition. These interactions appear to be electrostatic, as they are regulated by NH2-NP surface charge, solution ionic strength, and bilayer charge. The ability to rapidly measure a large number of nanoparticle and membrane parameters indicates strong potential of this bilayer array platform for additional nanoparticle bilayer studies.

  16. Partitioning of 2,6-Bis(1H-Benzimidazol-2-yl)pyridine Fluorophore into a Phospholipid Bilayer: Complementary Use of Fluorescence Quenching Studies and Molecular Dynamics Simulations

    PubMed Central

    Kyrychenko, Alexander; Sevriukov, Igor Yu.; Syzova, Zoya A.; Ladokhin, Alexey S.; Doroshenko, Andrey O.

    2014-01-01

    Successful use of fluorescence sensing in elucidating the biophysical properties of lipid membranes requires knowledge of the distribution and location of an emitting molecule in the bilayer. We report here that 2,6-bis(1H-benzimidazol-2-yl)pyridine (BBP), which is almost non-fluorescent in aqueous solutions, reveals a strong emission enhancement in a hydrophobic environment of a phospholipid bilayer, making it interesting for fluorescence probing of water content in a lipid membrane. Comparing the fluorescence behavior of BBP in a wide variety of solvents with those in phospholipid vesicles, we suggest that the hydrogen bonding interactions between a BBP fluorophore and water molecules play a crucial role in the observed “light switch effect”. Therefore, the loss of water-induced fluorescence quenching inside a membrane are thought to be due to deep penetration of BBP into the hydrophobic, water-free region of a bilayer. Characterized by strong quenching by transition metal ions in solution, BBP also demonstrated significant shielding from the action of the quencher in the presence of phospholipid vesicles. We used the increase in fluorescence intensity, measured upon titration of probe molecules with lipid vesicles, to estimate the partition constant and the Gibbs free energy (ΔG) of transfer of BBP from aqueous buffer into a membrane. Partitioning BBP revealed strongly favorable ΔG, which depends only slightly on the lipid composition of a bilayer, varying in a range from -6.5 to -7.0 kcal/mol. To elucidate the binding interactions of the probe with a membrane on the molecular level, a distribution and favorable location of BBP in a POPC bilayer were modeled via atomistic molecular dynamics (MD) simulations using two different approaches: (i) free, diffusion-driven partitioning of the probe molecules into a bilayer and (ii) constrained umbrella sampling of a penetration profile of the dye molecule across a bilayer. Both of these MD approaches agreed with

  17. General aspects of peptide selectivity towards lipid bilayers and cell membranes studied by variation of the structural parameters of amphipathic helical model peptides.

    PubMed

    Dathe, Margitta; Meyer, Jana; Beyermann, Michael; Maul, Björn; Hoischen, Christian; Bienert, Michael

    2002-02-01

    Model compounds of modified hydrophobicity (Eta), hydrophobic moment (mu) and angle subtended by charged residues (Phi) were synthesized to define the general roles of structural motifs of cationic helical peptides for membrane activity and selectivity. The peptide sets were based on a highly hydrophobic, non-selective KLA model peptide with high antimicrobial and hemolytic activity. Variation of the investigated parameters was found to be a suitable method for modifying peptide selectivity towards either neutral or highly negatively charged lipid bilayers. Eta and mu influenced selectivity preferentially via modification of activity on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) bilayers, while the size of the polar/hydrophobic angle affected the activity against 1-palmitoyl-2-oleoylphosphatidyl-DL-glycerol (POPG). The influence of the parameters on the activity determining step was modest in both lipid systems and the activity profiles were the result of the parameters' influence on the second less pronounced permeabilization step. Thus, the activity towards POPC vesicles was determined by the high permeabilizing efficiency, however, changes in the structural parameters preferentially influenced the relatively moderate affinity. In contrast, intensive peptide accumulation via electrostatic interactions was sufficient for the destabilization of highly negatively charged POPG lipid membranes, but changes in the activity profile, as revealed by the modification of Phi, seem to be preferentially caused by variation of the low permeabilizing efficiency. The parameters proved very effective also in modifying antimicrobial and hemolytic activity. However, their influence on cell selectivity was limited. A threshold value of hydrophobicity seems to exist which restricted the activity modifying potential of mu and Phi on both lipid bilayers and cell membranes.

  18. Observation of the simultaneous transport of Asian mineral dust aerosols with anthropogenic pollutants using a POPC during a long-lasting dust event in late spring 2014

    NASA Astrophysics Data System (ADS)

    Pan, Xiaole; Uno, Itsushi; Hara, Yukari; Kuribayashi, Masatoshi; Kobayashi, Hiroshi; Sugimoto, Nobuo; Yamamoto, Shigekazu; Shimohara, Takaaki; Wang, Zifa

    2015-03-01

    We observed a long-lasting dust event from 25 May to 2 June 2014, using a polarization optical particle counter (POPC). The transport of dust plumes over East Asia was verified on the basis of observations of Moderate Resolution Imaging Spectroradiometer, a lidar network, and surface synoptic observation stations. Mixing of dust and anthropogenic pollutants was investigated according to the variation in the depolarization ratio as a function of particle size. The nonsphericity of dust particles varied due to the impact of anthropogenic pollutants on their pathway. In the coarse mode, dust particles always had a clear nonspherical configuration, although large amounts of nitrate were also present. Supermicron particles are occasionally present in a spherical configuration, possibly due to the complex mixing of natural dust and anthropogenic particles. Statistically, ~64% of the total nitrate mass was deemed to be transported from outside of Japan due to a trapping effect in the dust plume.

  19. Hydration behaviour of POPC/C(12)-Bet mixtures investigated by sorption gravimetry, (31)P NMR spectroscopy and X-ray diffraction.

    PubMed

    Pfeiffer, H; Weichert, H; Klose, G; Heremans, K

    2012-02-01

    The hydration behaviour of mixtures of the zwitterionic phospholipid 1-palmitoyl-2-oleolyl-sn-glycero-3-phosphocholine (POPC) and the zwitterionic surfactant N,N-dimethyl-N-dodecyl-betain (C(12)-Bet) was investigated by sorption gravimetry, solid-state (31)P NMR-spectroscopy and small angle X-ray diffraction (SAXD). Negative excess hydration (dehydration) was found for almost all hydration degrees investigated. This behaviour is explained by the formation of an inner salt between the dipoles of phospholipid and surfactant headgroups that show a reverse sequence of partial charges with respect to the hydrocarbon backbone. The formation of an inner-salt most probably reduces potential water binding sites. Moreover, NMR data suggest that the incorporation of the zwitterionic surfactant into the phospholipid membrane is correlated with reorientation of the phosphate axis towards the membrane director as well as with reduced lateral and wobbling diffusion. PMID:22285958

  20. 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.

  1. Effect of Supporting Polyelectrolyte Multilayers and Deposition Conditions on the Formation of 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine Lipid Bilayers.

    PubMed

    Wlodek, Magdalena; Szuwarzynski, Michal; Kolasinska-Sojka, Marta

    2015-09-29

    The formation of complete supported lipid bilayers by vesicle adsorption and rupture was studied in relation to deposition conditions of vesicles and underlying cushion formed from various polyelectrolytes. Lipid vesicles were formed from zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and negatively charged 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) in phosphate buffer of various pH with or without NaCl addition. Polyelectrolyte multilayer films (PEM) were constructed by sequential adsorption of alternately charged polyelectrolytes from their solutions-layer-by-layer deposition (LBL). The mechanism of the formation of supported lipid bilayer on polyelectrolyte films was studied by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). QCM-D allowed following the adsorption kinetics while AFM measurements verified the morphology of lipid vesicles and isolated bilayer patches on the PEM cushions providing local topological images in terms of lateral organization. Additionally, polyelectrolyte cushions were characterized with ellipsometry to find thickness and swelling properties, and their roughness was determined using AFM. It has been demonstrated that the pH value and an addition of NaCl in the buffer solution as well as the type of the polyelectrolyte cushion influence the kinetics of bilayer formation and the quality of formed bilayer patches. PMID:26334376

  2. Bursting Bubbles and Bilayers

    PubMed Central

    Wrenn, Steven P.; Dicker, Stephen M.; Small, Eleanor F.; Dan, Nily R.; Mleczko, Michał; Schmitz, Georg; Lewin, Peter A.

    2012-01-01

    This paper discusses various interactions between ultrasound, phospholipid monolayer-coated gas bubbles, phospholipid bilayer vesicles, and cells. The paper begins with a review of microbubble physics models, developed to describe microbubble dynamic behavior in the presence of ultrasound, and follows this with a discussion of how such models can be used to predict inertial cavitation profiles. Predicted sensitivities of inertial cavitation to changes in the values of membrane properties, including surface tension, surface dilatational viscosity, and area expansion modulus, indicate that area expansion modulus exerts the greatest relative influence on inertial cavitation. Accordingly, the theoretical dependence of area expansion modulus on chemical composition - in particular, poly (ethylene glyclol) (PEG) - is reviewed, and predictions of inertial cavitation for different PEG molecular weights and compositions are compared with experiment. Noteworthy is the predicted dependence, or lack thereof, of inertial cavitation on PEG molecular weight and mole fraction. Specifically, inertial cavitation is predicted to be independent of PEG molecular weight and mole fraction in the so-called mushroom regime. In the “brush” regime, however, inertial cavitation is predicted to increase with PEG mole fraction but to decrease (to the inverse 3/5 power) with PEG molecular weight. While excellent agreement between experiment and theory can be achieved, it is shown that the calculated inertial cavitation profiles depend strongly on the criterion used to predict inertial cavitation. This is followed by a discussion of nesting microbubbles inside the aqueous core of microcapsules and how this significantly increases the inertial cavitation threshold. Nesting thus offers a means for avoiding unwanted inertial cavitation and cell death during imaging and other applications such as sonoporation. A review of putative sonoporation mechanisms is then presented, including those

  3. 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

  4. Lipid bilayers on nano-templates

    DOEpatents

    Noy, Aleksandr; Artyukhin, Alexander B.; Bakajin, Olgica; Stoeve, Pieter

    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

  5. Cholesterol's location in lipid bilayers

    DOE PAGES

    Marquardt, Drew; Kučerka, Norbert; Wassall, Stephen R.; Harroun, Thad A.; Katsaras, John

    2016-04-04

    It is well known that cholesterol modifies the physical properties of lipid bilayers. For example, the much studied liquid-ordered Lo phase contains rapidly diffusing lipids with their acyl chains in the all trans configuration, similar to gel phase bilayers. Moreover, the Lo phase is commonly associated with cholesterol-enriched lipid rafts, which are thought to serve as platforms for signaling proteins in the plasma membrane. Cholesterol's location in lipid bilayers has been studied extensively, and it has been shown – at least in some bilayers – to align differently from its canonical upright orientation, where its hydroxyl group is in themore » vicinity of the lipid–water interface. In this study we review recent works describing cholesterol's location in different model membrane systems with emphasis on results obtained from scattering, spectroscopic and molecular dynamics studies.« less

  6. The topology of lysine-containing amphipathic peptides in bilayers by circular dichroism, solid-state NMR, and molecular modeling.

    PubMed Central

    Vogt, B; Ducarme, P; Schinzel, S; Brasseur, R; Bechinger, B

    2000-01-01

    In order to better understand the driving forces that determine the alignment of amphipathic helical polypeptides with respect to the surface of phospholipid bilayers, lysine-containing peptide sequences were designed, prepared by solid-phase chemical synthesis, and reconstituted into membranes. CD spectroscopy indicates that all peptides exhibit a high degree of helicity in the presence of SDS micelles or POPC small unilamellar vesicles. Proton-decoupled (31)P-NMR solid-state NMR spectroscopy demonstrates that in the presence of peptides liquid crystalline phosphatidylcholine membranes orient well along glass surfaces. The orientational distribution and dynamics of peptides labeled with (15)N at selected sites were investigated by proton-decoupled (15)N solid-state NMR spectroscopy. Polypeptides with a single lysine residue adopt a transmembrane orientation, thereby locating this polar amino acid within the core region of the bilayer. In contrast, peptides with > or = 3 lysines reside along the surface of the membrane. With 2 lysines in the center of an otherwise hydrophobic amino acid sequence the peptides assume a broad orientational distribution. The energy of lysine discharge, hydrophobic, polar, and all other interactions are estimated to quantitatively describe the polypeptide topologies observed. Furthermore, a molecular modeling algorithm based on the hydrophobicities of atoms in a continuous hydrophilic-hydrophobic-hydrophilic potential describes the experimentally observed peptide topologies well. PMID:11053137

  7. Experimental support for a novel compount motion model for the time-resolved fluorescence anisotropy decay of TMA-PDH in lipid vesicle bilayers

    NASA Astrophysics Data System (ADS)

    Muller, Johan M.; van Faassen, Ernst E.; van Ginkel, Gijsbert

    1994-08-01

    Many structural studies of lipid membrane systems employ fluorescence anisotropy experiments on lipid soluble dyes that have been embedded in the lipid bilayers as optical probes. The conventional models for the interpretation of the anisotropy decay curves have a number of conceptual problems related to the form of the effective potential experienced by the probe molecules due to the interaction with the surrounding lipid environment. Therefore, a new model recently proposed by van der Sijs. (Chem. Phys. Letters 216 (1993) 559). In this paper we test this new compound model in a reanalysis of time-resolved fluorescence anisotropy experiments using TMA-DPH as a fluorescent probe. The orientational order and reorientational dynamics of TMA-DPH in small unilamellar vesicles (SUV) of POPC, DOPC, EGGPC, DLPC, EGGPG, DOPG, SQDG and DGDG was studied. We find that the new model improves the description of the underlying motional processes at short time scales and lacks certain unphysical aspects of previous models, e.g. a population of TMA-DPH probe molecules at and beyond 90° with the local bilayer normal. In contrast with previous models the compound motion model yields a good agreement between our vesicle data and previously published results from oriental lipid bilayers using ESR and AFD.

  8. Lipid-Protein Correlations in Nanoscale Phospholipid Bilayers by Solid-State NMR

    PubMed Central

    Kijac, Aleksandra; Shih, Amy Y.; Nieuwkoop, Andrew J.; Schulten, Klaus; Sligar, Stephen G.; Rienstra, Chad M.

    2011-01-01

    Nanodiscs are an example of discoidal nanoscale lipid/protein particles that have been extremely useful for the biochemical and biophysical characterization of membrane proteins. They are discoidal lipid bilayer fragments encircled and stabilized by two amphipathic helical proteins named membrane scaffolding protein (MSP), ~10 nm in size. Nanodiscs are homogeneous, easily prepared with reproducible success, amenable to preparations with a variety of lipids, and stable under a range of temperatures. Here we present solid-state NMR (SSNMR) studies on lyophilized, rehydrated POPC Nanodiscs prepared with uniformly 13C, 15N-labeled MSP1D1 (Δ1-11 truncated MSP). Under these conditions, by SSNMR we directly determine the gel-to-liquid crystal lipid phase transition to be at 3 ± 2 °C. Above this phase transition, the lipid 1H signals have slow transverse relaxation, enabling filtering experiments as previously demonstrated for lipid vesicles. We incorporate this approach into two- and three-dimensional heteronuclear SSNMR experiments to examine the MSP1D1 residues interfacing with the lipid bilayer. These 1H-13C and 1H-13C-13C correlation spectra are used to identify and quantify the number of lipid-correlated and solvent-exposed residues by amino acid type, which furthermore is compared with molecular dynamics studies of MSP1D1 in Nanodiscs. This study demonstrates the utility of SSNMR experiments with Nanodiscs for examining lipid-protein interfaces and has important applications for future structural studies of membrane proteins in physiologically relevant formulations. PMID:20804175

  9. Effect of repeated apoA-IMilano/POPC infusion on lipids, (apo)lipoproteins, and serum cholesterol efflux capacity in cynomolgus monkeys.

    PubMed

    Kempen, Herman J; Gomaraschi, Monica; Bellibas, S Eralp; Plassmann, Stephanie; Zerler, Brad; Collins, Heidi L; Adelman, Steven J; Calabresi, Laura; Wijngaard, Peter L J

    2013-09-01

    MDCO-216, a complex of dimeric recombinant apoA-IMilano (apoA-IM) and palmitoyl-oleoyl-phosphatidylcholine (POPC), was administered to cynomolgus monkeys at 30, 100, and 300 mg/kg every other day for a total of 21 infusions, and effects on lipids, (apo)lipoproteins, and ex-vivo cholesterol efflux capacity were monitored. After 7 or 20 infusions, free cholesterol (FC) and phospholipids (PL) were strongly increased, and HDL-cholesterol (HDL-C), apoA-I, and apoA-II were strongly decreased. We then measured short-term effects on apoA-IM, lipids, and (apo)lipoproteins after the first or the last infusion. After the first infusion, PL and FC went up in the HDL region and also in the LDL and VLDL regions. ApoE shifted from HDL to LDL and VLDL regions, while ApoA-IM remained located in the HDL region. On day 41, ApoE levels were 8-fold higher than on day 1, and FC, PL, and apoE resided mostly in LDL and VLDL regions. Drug infusion quickly decreased the endogenous cholesterol esterification rate. ABCA1-mediated cholesterol efflux on day 41 was markedly increased, whereas scavenger receptor type B1 (SRB1) and ABCG1-mediated effluxes were only weakly increased. Strong increase of FC is due to sustained stimulation of ABCA1-mediated efflux, and drop in HDL and formation of large apoE-rich particles are due to lack of LCAT activation. PMID:23828780

  10. Effect of repeated apoA-IMilano/POPC infusion on lipids, (apo)lipoproteins, and serum cholesterol efflux capacity in cynomolgus monkeys[S

    PubMed Central

    Kempen, Herman J.; Gomaraschi, Monica; Bellibas, S. Eralp; Plassmann, Stephanie; Zerler, Brad; Collins, Heidi L.; Adelman, Steven J.; Calabresi, Laura; Wijngaard, Peter L. J.

    2013-01-01

    MDCO-216, a complex of dimeric recombinant apoA-IMilano (apoA-IM) and palmitoyl-oleoyl-phosphatidylcholine (POPC), was administered to cynomolgus monkeys at 30, 100, and 300 mg/kg every other day for a total of 21 infusions, and effects on lipids, (apo)lipoproteins, and ex-vivo cholesterol efflux capacity were monitored. After 7 or 20 infusions, free cholesterol (FC) and phospholipids (PL) were strongly increased, and HDL-cholesterol (HDL-C), apoA-I, and apoA-II were strongly decreased. We then measured short-term effects on apoA-IM, lipids, and (apo)lipoproteins after the first or the last infusion. After the first infusion, PL and FC went up in the HDL region and also in the LDL and VLDL regions. ApoE shifted from HDL to LDL and VLDL regions, while ApoA-IM remained located in the HDL region. On day 41, ApoE levels were 8-fold higher than on day 1, and FC, PL, and apoE resided mostly in LDL and VLDL regions. Drug infusion quickly decreased the endogenous cholesterol esterification rate. ABCA1-mediated cholesterol efflux on day 41 was markedly increased, whereas scavenger receptor type B1 (SRB1) and ABCG1-mediated effluxes were only weakly increased. Strong increase of FC is due to sustained stimulation of ABCA1-mediated efflux, and drop in HDL and formation of large apoE-rich particles are due to lack of LCAT activation. PMID:23828780

  11. Fundamental Studies of Assembly and Mechanical Properties of Lipid Bilayer Membranes and Unilamellar Vesicles

    NASA Astrophysics Data System (ADS)

    Wang, Xi

    This dissertation work focuses on: (i) obtaining a phospholipid bilayer membrane (LBM)/conducting electrode system with low defect density and optimized rigidity; (ii) investigating vesicle stability and mechanical properties. LBM is a simplified yet representative cell membrane model. LBMs assembled on conductive surfaces can probe protein-LBM interactions activities electrochemically. Sterically stabilized vesicles could be used as cell models or for drug delivery. The main challenges for LBM assembly on gold are vesicles do not spontaneously rupture to form LBMs on gold and the roughness of the gold substrate has considerable influence on molecular film defect density. In this study, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) vesicles were functionalized with 1,2-distearoyl-sn-glycero-3-phosphoethanolamine- N-poly(ethylene glycol)-2000-N-[3-(2-pyridyldithio)propionate] (DSPE-PEG-PDP) to yield stable LBMs on gold without surface modification. A template-stripping method was used to obtain atomically flat and pristine gold surfaces. The critical force to initiate vesicle rupture decreases with increasing DSPE-PEG-PDP concentration, indicating that gold-thiolate bonding between DSPE-PEG-PDP and gold substrates promotes LBM formation. Mechanical properties of LBMs and vesicles were investigated as a function of DSPE-PEG-PDP concentration via Atomic Force Microscopy. The elastic moduli of LBMs were determined with DSPE-PEG-PDP concentration ranging from 0mol% to 24mol% and were found to depend on PEG chain conformation. Incorporating DSPE-PEG-PDP molecules with PEG in mushroom conformation results in a decrease of LBM rigidity, while incorporating PEG in brush conformation leads to LBM stiffening. Contrarily, mechanical properties of functionalized vesicles did not vary significantly by varying DSPE-PEG-PDP concentration. LBM with tunable rigidity by adjusting DSPE-PEG-PDP concentration provides a versatile cell membrane model for studying protein or

  12. Current switching in superconductor semiconductor bilayers

    NASA Astrophysics Data System (ADS)

    Rahman, F.; Thornton, T. J.; Huber, R.

    2006-09-01

    We describe results of electrical transport experiments on niobium-on-indium arsenide and aluminium-on-indium arsenide bilayers. The temperature-dependent properties of electrical conduction in these bilayers is examined first in order to characterize the quality of super-semi interfaces. Next, we look at the differential resistance of the bilayers as a function of bias current. The switching of current between the metal and semiconductor components of the bilayer gives rise to a quasi-inductive effect as it causes voltage spikes in the composite system. Also described is the variation of critical current for these bilayers with temperature and magnetic field.

  13. Phosphatydylglycerol promotes bilayer insertion of salmon calcitonin.

    PubMed Central

    Bradshaw, J P

    1997-01-01

    Neutron diffraction from oriented multibilayers has been used to study the bilayer interaction of the amphipathic peptide salmon calcitonin. Penetration of calcitonin into bilayers composed of dioleoylphosphatidylcholine increases with the addition of 15% (mol) of the anionic phospholipid dioleoylphosphatidylglycerol. Neutron scattering profiles of water distribution in stacked bilayers show a continuous band of deuterons across each bilayer, consistent with the suggestion that the hormone forms transbilayer alpha-helixes under these conditions. These experiments add to the growing body of data on the role of phosphatidylglycerol in bilayer insertion of protein helices and suggests a possible evolutionary history for calcitonin. Images FIGURE 5 PMID:9129820

  14. Bilayer Effects of Antimalarial Compounds

    PubMed Central

    Ramsey, Nicole B.; Andersen, Olaf S.

    2015-01-01

    Because of the perpetual development of resistance to current therapies for malaria, the Medicines for Malaria Venture developed the Malaria Box to facilitate the drug development process. We tested the 80 most potent compounds from the box for bilayer-mediated effects on membrane protein conformational changes (a measure of likely toxicity) in a gramicidin-based stopped flow fluorescence assay. Among the Malaria Box compounds tested, four compounds altered membrane properties (p< 0.05); MMV007384 stood out as a potent bilayer-perturbing compound that is toxic in many cell-based assays, suggesting that testing for membrane perturbation could help identify toxic compounds. In any case, MMV007384 should be approached with caution, if at all. PMID:26551613

  15. Profile structures of the voltage-sensor domain and the voltage-gated K+-channel vectorially oriented in a single phospholipid bilayer membrane at the solid-vapor and solid-liquid interfaces determined by x-ray interferometry

    PubMed Central

    Gupta, S.; Liu, J.; Strzalka, J.; Blasie, J. K.

    2011-01-01

    One subunit of the prokaryotic voltage-gated potassium ion channel from Aeropyrum pernix (KvAP) is comprised of six transmembrane α helices, of which S1–S4 form the voltage-sensor domain (VSD) and S5 and S6 contribute to the pore domain (PD) of the functional homotetramer. However, the mechanism of electromechanical coupling interconverting the closed-to-open (i.e., nonconducting-to-K+-conducting) states remains undetermined. Here, we have vectorially oriented the detergent (OG)-solubilized VSD in single monolayers by two independent approaches, namely “directed-assembly” and “self-assembly,” to achieve a high in-plane density. Both utilize Ni coordination chemistry to tether the protein to an alkylated inorganic surface via its C-terminal His6 tag. Subsequently, the detergent is replaced by phospholipid (POPC) via exchange, intended to reconstitute a phospholipid bilayer environment for the protein. X-ray interferometry, in which interference with a multilayer reference structure is used to both enhance and phase the specular x-ray reflectivity from the tethered single membrane, was used to determine directly the electron density profile structures of the VSD protein solvated by detergent versus phospholipid, and with either a moist He (moderate hydration) or bulk aqueous buffer (high hydration) environment to preserve a native structure conformation. Difference electron density profiles, with respect to the multilayer substrate itself, for the VSD-OG monolayer and VSD-POPC membranes at both the solid-vapor and solid-liquid interfaces, reveal the profile structures of the VSD protein dominating these profiles and further indicate a successful reconstitution of a lipid bilayer environment. The self-assembly approach was similarly extended to the intact full-length KvAP channel for comparison. The spatial extent and asymmetry in the profile structures of both proteins confirm their unidirectional vectorial orientation within the reconstituted membrane and

  16. Effect of benzyl alcohol on lipid bilayers. A comparisons of bilayer systems.

    PubMed Central

    Ebihara, L; Hall, J E; MacDonald, R C; McIntosh, T J; Simon, S A

    1979-01-01

    The effect of the small anesthetic molecule, benzyl alcohol, on the structure of various bilayer system has been studied by optical, electrical, and x-ray diffraction techniques. We find that the modifications in bilayer thickness caused by benzyl alcohol differ dramatically for planar (or black lipid) bilayers containing solvent, planar bilayers containing little or no solvent, and vesicular bilayers. Benzyl alcohol increases the thickness of planar bilayers containing n-alkane solvents, yet decreases the thickness of "solvent-free" planar bilayers. The effect of benzyl alcohol on vesicular bilayers below the phase transition temperature also depends on whether solvent is present in the bilayers. Without solvent, gel-state bilayers are reduced in thickness by benzyl alcohol, whereas in the presence of solvent, the thickness is unchanged. Above the phase transition temperature, benzyl alcohol has no measurable effect on vesicular bilayer thickness, whether solvent is present or not. These results indicate that different model membrane systems respond quite differently to a particular anesthetic. PMID:263698

  17. Helium Microbeam Mixing of Bilayers.

    NASA Astrophysics Data System (ADS)

    Davis, John Baran

    This study is an experimental and theoretical investigation of room-temperature mixing of bilayers by micron-width He^+ ion beams. Bilayer targets, including Cu/Al, Cu/Si and Sb/Si, were irradiated at room temperature in the University at Albany's Dynamitron particle accelerator with 2-MeV He^+ ion beams ranging from 2 to 6 μm in width. At doses on the order of 10^ {19}/cm^2, RBS spectra revealed evidence of interface mixing in all targets to depths of several thousand A within the cylinder irradiated by the beam. Both RBS spectra and isometric RBS contour maps of the target also showed that mixing of the interface extends laterally well beyond the irradiated area. The interface mixing reaches a maximum in an annular region several times larger in diameter than the ion-beam. Standard theories of primary-recoil, secondary -cascade and thermal-spike mixing predicted interface widths two orders of magnitude smaller than observed for Cu/Al bilayers. Furthermore, He^+ irradiation of Cu/Al targets at liquid-nitrogen temperature did not produce interface mixing, further indicating that ballistic interpretations of the mixing are inadequate. Defect concentrations as a function of position and time were calculated by numerical solution of coupled rate equations for vacancies and interstitials in aluminum. The results of these calculations show that room-temperature He^+ mixing of Cu/Al results almost exclusively from interstitial migration. The numerically calculated concentration of interstitials within the damage cylinder was used to derive an approximate expression for interface width as a function of dose. Comparisons of these predicted values with the experimentally determined interface width as a function of dose agree, within uncertainties. In addition, the annular region observed on RBS maps is explained by the continued presence of a non-equilibrium concentration of interstitials after the ion beam is shut off. Interface mixing in Cu/Si targets, although

  18. Cholesterol Perturbs Lipid Bilayers Nonuniversally

    SciTech Connect

    Pan Jianjun; Mills, Thalia T.; Tristram-Nagle, Stephanie; Nagle, John F.

    2008-05-16

    Cholesterol is well known to modulate the physical properties of biomembranes. Using modern x-ray scattering methods, we have studied the effects of cholesterol on the bending modulus K{sub C}, the thickness D{sub HH}, and the orientational order parameter S{sub xray} of lipid bilayers. We find that the effects are different for at least three classes of phospholipids characterized by different numbers of saturated hydrocarbon chains. Most strikingly, cholesterol strongly increases K{sub C} when both chains of the phospholipid are fully saturated but not at all when there are two monounsaturated chains.

  19. Annular bilayer magnetoelectric composites: theoretical analysis.

    PubMed

    Guo, Mingsen; Dong, Shuxiang

    2010-01-01

    The laminated bilayer magnetoelectric (ME) composites consist of magnetostrictive and piezoelectric layers are known to have giant ME coefficient due to the high coupling efficiency in bending mode. In our previous report, the bar-shaped bilayer composite has been investigated by using a magnetoelectric-coupling equivalent circuit. Here, we propose an annular bilayer ME composite, which consists of magnetostrictive and piezoelectric rings. This composite has a much lower resonance frequency of bending mode compared with its radial mode. In addition, the annular bilayer ME composite is expected to respond to vortex magnetic field as well as unidirectional magnetic field. In this paper, we investigate the annular bilayer ME composite by using impedance-matrix method and predict the ME coefficients as a function of geometric parameters of the composites. PMID:20178914

  20. Photon correlation spectroscopy of bilayer lipid membranes.

    PubMed

    Crilly, J F; Earnshaw, J C

    1983-02-01

    Light scattering by thermal fluctuations on simple monoglyceride bilayer membranes has been used to investigate the viscoelastic properties of these structures. Spectroscopic analysis of these fluctuations (capillary waves) permits the nonperturbative measurement of the interfacial tension and a shear interfacial viscosity acting normal to the membrane plane. The methods were established by studies of solvent and nonsolvent bilayers of glycerol monooleate (GMO). Changes in the tension of GMO/n-decane membranes induced by altering the composition of the parent solution were detected and quantified. In a test of the reliability of the technique controlled variations of the viscosity of the aqueous bathing solution were accurately monitored. The technique was applied to solvent-free bilayers formed from dispersions of GMO in squalane. The lower tensions observed attested to the comparative absence of solvent in such bilayers. In contrast to the solvent case, the solvent-free membranes exhibited a significant transverse shear viscosity, indicative of the enhanced intermolecular interactions within the bilayer.

  1. Cholesterol favors the emergence of a long-range autocorrelated fluctuation pattern in voltage-induced ionic currents through lipid bilayers.

    PubMed

    Corvalán, Natalia A; Kembro, Jackelyn M; Clop, Pedro D; Perillo, María A

    2013-08-01

    The present paper was aimed at evaluating the effect of cholesterol (CHO) on the voltage-induced lipid pore formation in bilayer membranes through a global characterization of the temporal dynamics of the fluctuation pattern of ion currents. The bilayer model used was black lipid membranes (BLMs) of palmitoyloleoylphosphatidylethanolamine and palmitoyloleoylphosphatidylcholine (POPE:POPC) at a 7:3 molar ratio in the absence (BLM0) or in the presence of 30 (BLM30), 40 (BLM40) or 50(BLM50)mol% of cholesterol with respect to total phospholipids. Electrical current intensities (I) were measured in voltage (ΔV) clamped conditions at ΔV ranging between 0 and ±200mV. The autocorrelation parameter α derived from detrended fluctuation analysis (DFA) on temporal fluctuation patterns of electrical currents allowed discriminating between non-correlated (α=0.5, white noise) and long-range correlated (0.5<α<1) behaviors. The increase in |ΔV| as well as in cholesterol content increased the number of conductance states, the magnitude of conductance level, the capacitance of the bilayers and increased the tendency towards the development of long-range autocorrelated (fractal) processes (0.5<α<1) in lipid channel generation. Experiments were performed above the phase transition temperature of the lipid mixtures, but compositions used predicted a superlattice-like organization. This leads to the conclusion that structural defects other than phase coexistence may promote lipid channel formation under voltage clamped conditions. Furthermore, cholesterol controls the voltage threshold that allows the percolation of channel behavior where isolated channels become an interconnected network.

  2. "Nanocrystal bilayer for tandem catalysis"

    SciTech Connect

    Yamada, Yusuke; Tsung, Chia Kuang; Huang, Wenyu; Huo, Ziyang; E.Habas, Susan E; Soejima, Tetsuro; Aliaga, Cesar E; Samorjai, Gabor A; Yang, Peidong

    2011-01-24

    Supported catalysts are widely used in industry and can be optimized by tuning the composition and interface of the metal nanoparticles and oxide supports. Rational design of metal-metal oxide interfaces in nanostructured catalysts is critical to achieve better reaction activities and selectivities. We introduce here a new class of nanocrystal tandem catalysts that have multiple metal-metal oxide interfaces for the catalysis of sequential reactions. We utilized a nanocrystal bilayer structure formed by assembling platinum and cerium oxide nanocube monolayers of less than 10 nm on a silica substrate. The two distinct metal-metal oxide interfaces, CeO2-Pt and Pt-SiO2, can be used to catalyse two distinct sequential reactions. The CeO2-Pt interface catalysed methanol decomposition to produce CO and H2, which were subsequently used for ethylene hydroformylation catalysed by the nearby Pt-SiO2 interface. Consequently, propanal was produced selectively from methanol and ethylene on the nanocrystal bilayer tandem catalyst. This new concept of nanocrystal tandem catalysis represents a powerful approach towards designing high-performance, multifunctional nanostructured catalysts

  3. Galactocerebroside-phospholipid interactions in bilayer membranes.

    PubMed

    Ruocco, M J; Shipley, G G; Oldfield, E

    1983-07-01

    Differential scanning calorimetry (DSC) and x-ray diffraction have been used to study the interaction of hydrated N-palmitoylgalactosylsphingosine (NPGS) and dipalmitoylphosphatidylcholine (DPPC). For mixtures containing less than 23 mol% NPGS, complete miscibility of NPGS into hydrated DPPC bilayers is observed in both the bilayer gel and liquid-crystal phases. X-ray diffraction data demonstrate insignificant differences in the DPPC-bilayer gel-phase parameters on incorporation of up to 23 mol% NPGS. At greater than 23 mol% NPGS, additional high-temperature transitions occur, indicating phase separation of cerebroside. For these cerebroside concentrations, at 20 degrees C, x-ray diffraction shows two lamellar phases, hydrated DPPC-NPGS gel bilayers (d = 64 A) containing 23 mol% NPGS, and NPGS "crystal" bilayers (d = 55 A). On heating to temperatures greater than 45 degrees C, the mixed DPPC-NPGS bilayer phase undergoes chain melting, and on further increasing the temperature progressively more NPGS is incorporated into the liquid-crystal DPPC-NPGS bilayer phase. At temperatures greater than 82 degrees C (the transition temperature of hydrated NPGS), complete lipid miscibility is observed at all DPPC/NPGS molar ratios.

  4. 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.

  5. 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

  6. Cholera toxin interactions with lipid bilayers.

    PubMed

    Tosteson, M T; Tosteson, D C; Rubnitz, J

    1980-01-01

    The purpose of the experiments described in this paper was to assess the binding of cholera toxin to bilayers containing its receptor, the monosialoganglioside, GMl. The assay was based on the fact that GMl confers on the bilayer a negative surface charge. The magnitude of this surface charge was estimated by measuring the electrical conductance (G) of the bilayers exposed to nonactin-K+ under conditions where G is directly proportional to the potassium concentration in the aqueous solutions immediatey adjacent to the membrane surface. When bilayers were formed from mixtures of GMl and glycerolmonooleate (GMO), it was found that the molar ratio of the lipids in the bilayer was the same as that in the membrane forming solution. It was further found that cholera toxin or the binding subunit of the toxin (choleragenoid) bind to GMO bilayers containing GMl (but not to GMO bilayers containing phosphatidyl serine or disialoganglioside GDla). The value of the apparent dissociation constant for the binding of choleragen to its receptor was found to be 10(-11) M, comparable to values found in intact cells.

  7. Fusion and fission of fluid amphiphilic bilayers.

    PubMed

    Gotter, Martin; Strey, Reinhard; Olsson, Ulf; Wennerström, Håkan

    2005-01-01

    The system water-oil (n-decane)-nonionic surfactant (C12E5) forms bilayer phases in a large concentration region, but, for a given oil-to-surfactant ratio, only in a narrow temperature range. In addition to the anisotropic lamellar phase (Lalpha) there is also, at slightly higher temperature, a sponge or L3-phase where the bilayers build up an isotropic structure extending macroscopically in three dimensions. In this phase the bilayer mid-surface has a mean curvature close to zero and a negative Euler characteristic. In this paper we study how the bilayers in the lamellar and the sponge phase respond dynamically to sudden temperature changes. The monolayer spontaneous curvature depends sensitively on temperature and a change of temperature thus provides a driving force for a change in bilayer topology. The equilibration therefore involves kinetic steps of fusion/fission of bilayers. Such dynamic processes have previously been monitored by temperature jump experiments using light scattering in the sponge phase. These experiments revealed an extraordinarily strong dependence of the relaxation time on the bilayer volume fraction phi. At phi < 0.1 the relaxation times are so slow that experiments using deuterium nuclear magnetic resonance (2H-NMR) appear feasible. We here report on the first experiments concerned with the dynamics of the macroscopic phase transition sponge-lamellae by 2H-NMR. We find that the sponge-to-lamellae transition occurs through a nucleation process followed by domain growth involving bilayer fission at domain boundaries. In contrast, the lamellae-to-sponge transformation apparently occurs through a succession of uncorrelated bilayer fusion events. PMID:15715316

  8. Negative Coulomb Drag in Double Bilayer Graphene.

    PubMed

    Li, J I A; Taniguchi, T; Watanabe, K; Hone, J; Levchenko, A; Dean, C R

    2016-07-22

    We report on an experimental measurement of Coulomb drag in a double quantum well structure consisting of bilayer-bilayer graphene, separated by few layer hexagonal boron nitride. At low temperatures and intermediate densities, a novel negative drag response with an inverse sign is observed, distinct from the momentum and energy drag mechanisms previously reported in double monolayer graphene. By varying the device aspect ratio, the negative drag component is suppressed and a response consistent with pure momentum drag is recovered. In the momentum drag dominated regime, excellent quantitative agreement with the density and temperature dependence predicted for double bilayer graphene is found. PMID:27494491

  9. Negative Coulomb Drag in Double Bilayer Graphene.

    PubMed

    Li, J I A; Taniguchi, T; Watanabe, K; Hone, J; Levchenko, A; Dean, C R

    2016-07-22

    We report on an experimental measurement of Coulomb drag in a double quantum well structure consisting of bilayer-bilayer graphene, separated by few layer hexagonal boron nitride. At low temperatures and intermediate densities, a novel negative drag response with an inverse sign is observed, distinct from the momentum and energy drag mechanisms previously reported in double monolayer graphene. By varying the device aspect ratio, the negative drag component is suppressed and a response consistent with pure momentum drag is recovered. In the momentum drag dominated regime, excellent quantitative agreement with the density and temperature dependence predicted for double bilayer graphene is found.

  10. Berry phase transition in twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Rode, Johannes C.; Smirnov, Dmitri; Schmidt, Hennrik; Haug, Rolf J.

    2016-09-01

    The electronic dispersion of a graphene bilayer is highly dependent on rotational mismatch between layers and can be further manipulated by electrical gating. This allows for an unprecedented control over electronic properties and opens up the possibility of flexible band structure engineering. Here we present novel magnetotransport data in a twisted bilayer, crossing the energetic border between decoupled monolayers and coupled bilayer. In addition a transition in Berry phase between π and 2π is observed at intermediate magnetic fields. Analysis of Fermi velocities and gate induced charge carrier densities suggests an important role of strong layer asymmetry for the observed phenomena.

  11. Negative Coulomb Drag in Double Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Li, J. I. A.; Taniguchi, T.; Watanabe, K.; Hone, J.; Levchenko, A.; Dean, C. R.

    2016-07-01

    We report on an experimental measurement of Coulomb drag in a double quantum well structure consisting of bilayer-bilayer graphene, separated by few layer hexagonal boron nitride. At low temperatures and intermediate densities, a novel negative drag response with an inverse sign is observed, distinct from the momentum and energy drag mechanisms previously reported in double monolayer graphene. By varying the device aspect ratio, the negative drag component is suppressed and a response consistent with pure momentum drag is recovered. In the momentum drag dominated regime, excellent quantitative agreement with the density and temperature dependence predicted for double bilayer graphene is found.

  12. 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.

  13. Charge renormalization of bilayer elastic properties.

    PubMed

    Sknepnek, Rastko; Vernizzi, Graziano; Olvera de la Cruz, Monica

    2012-09-14

    By combining molecular dynamics simulations and analytical arguments, we investigate the elastic properties of charged lipid bilayers. We show that electrostatic interactions between the head groups can lead to solidification of the lipid bilayer that would otherwise be in a liquid state if the charges were absent. All elastic parameters of the bilayer such as the bending rigidity κ and the two-dimensional bulk modulus λ and Young's modulus Y are found to depend on the values of the charges assigned to the lipid head groups. To extract κ and λ, we fit the molecular dynamics data to a standard elastic model for lipid bilayers. Moreover, we analytically obtain the dependence of the Young modulus Y on the relative strengths of electrostatic and van der Waals interactions in the zero temperature limit.

  14. Resonant Tunneling in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Fallahazad, Babak; Lee, Kayoung; Kang, Sangwoo; Xue, Jiamin; Larentis, Stefano; Corbet, Christopher; Kim, Kyounghwan; Movva, Hema; Taniguchi, Takashi; Watanabe, Kenji; Register, Leonard; Banerjee, Sanjay; Tutuc, Emanuel

    2015-03-01

    We present the realization and characterization of independently contacted and rotationally aligned double bilayer graphene heterostructures, that show gate-tunable tunneling resonances and negative differential resistance in their interlayer current-voltage characteristics. Our devices are fabricated by successively stacking mechanically exfoliated bilayer graphene and hexagonal boron nitride dielectric using a layer-by-layer transfer technique. The bilayers are rotationally aligned during the device fabrication by selecting flakes with straight edges, and using them as a reference for alignment. We determine the heterostructure energy band alignment at the tunneling resonance using the individual layer carrier densities, and including the chemical potential dependence on the carrier density. Our analysis show that the tunneling resonances occur when the charge neutrality points of the two bilayer graphene are energetically aligned, which suggests the resonances stem from the momentum conserving tunneling. This work has been supported by NRI-SWAN, ONR, and Intel.

  15. Self healing nature of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Debroy, Sanghamitra; Pavan Kumar Miriyala, V.; Vijaya Sekhar, K.; Acharyya, Swati Ghosh; Acharyya, Amit

    2016-08-01

    The phenomenon of self healing of cracks in bilayer graphene sheet has been studied using molecular dynamics simulations. The bilayer graphene sheet was subjected to uniaxial tensile load resulting in initiation and propagation of cracks on exceeding the ultimate tensile strength. Subsequently, all forces acting on the sheet were removed and sheet was relaxed. The cracks formed in the graphene sheet healed without any external aid within 0.4 ps The phenomenon of self healing of the cracks in graphene sheet was found to be independent of the length of the crack, but occurred for critical crack opening distance less than 5 Å for AA stacked sheet and 13 Å for AB stacked bilayer graphene sheet. Self healing was observed for both AB (mixed stacking of armchair and zigzag graphene sheet) and AA (both sheets of similar orientation i.e. either armchair-armchair or zigzag-zigzag) stacking of bilayer graphene sheet.

  16. Liquid to quasicrystal transition in bilayer water

    NASA Astrophysics Data System (ADS)

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

    2010-10-01

    The phase behavior of confined water is a topic of intense and current interest due to its relevance in biology, geology, and materials science. Nevertheless, little is known about the phases that water forms even when confined in the simplest geometries, such as water confined between parallel surfaces. Here we use molecular dynamics simulations to compute the phase diagram of two layers of water confined between parallel non hydrogen bonding walls. This study shows that the water bilayer forms a dodecagonal quasicrystal, as well as two previously unreported bilayer crystals, one tiled exclusively by pentagonal rings. Quasicrystals, structures with long-range order but without periodicity, have never before been reported for water. The dodecagonal quasicrystal is obtained from the bilayer liquid through a reversible first-order phase transition and has diffusivity intermediate between that of the bilayer liquid and ice phases. The water quasicrystal and the ice polymorphs based on pentagons are stabilized by compression of the bilayer and are not templated by the confining surfaces, which are smooth. This demonstrates that these novel phases are intrinsically favored in bilayer water and suggests that these structures could be relevant not only for confined water but also for the wetting and properties of water at interfaces.

  17. Dynamic morphologies of microscale droplet interface bilayers.

    PubMed

    Mruetusatorn, Prachya; Boreyko, Jonathan B; Venkatesan, Guru A; Sarles, Stephen A; Hayes, Douglas G; Collier, C Patrick

    2014-04-21

    Droplet interface bilayers (DIBs) are a powerful platform for studying the dynamics of synthetic cellular membranes; however, very little has been done to exploit the unique dynamical features of DIBs. Here, we generate microscale droplet interface bilayers (μDIBs) by bringing together femtoliter-volume water droplets in a microfluidic oil channel, and characterize morphological changes of the μDIBs as the droplets shrink due to evaporation. By varying the initial conditions of the system, we identify three distinct classes of dynamic morphology. (1) Buckling and fission: when forming μDIBs using the lipid-out method (lipids in oil phase), lipids in the shrinking monolayers continually pair together and slide into the bilayer to conserve their mass. As the bilayer continues to grow, it becomes confined, buckles, and eventually fissions one or more vesicles. (2) Uniform shrinking: when using the lipid-in method (lipids in water phase) to form μDIBs, lipids uniformly transfer from the monolayers and bilayer into vesicles contained inside the water droplets. (3) Stretching and unzipping: finally, when the droplets are pinned to the wall(s) of the microfluidic channel, the droplets become stretched during evaporation, culminating in the unzipping of the bilayer and droplet separation. These findings offer a better understanding of the dynamics of coupled lipid interfaces. PMID:24647872

  18. Dynamic Morphologies of Microscale Droplet Interface Bilayers

    SciTech Connect

    Mruetusatorn, Prachya; Boreyko, Jonathan B; Sarles, Stephen A; Venkatesan, Guru; Hayes, Douglas G; Collier, Pat

    2014-01-01

    Droplet interface bilayers (DIBs) are a powerful platform for studying the dynamics of synthetic cellular membranes; however, very little has been done to exploit the unique dynamical features of DIBs. Here, we generate microscale droplet interface bilayers ( DIBs) by bringing together femtoliter-volume water droplets in a microfluidic oil channel, and characterize morphological changes of the DIBs as the droplets shrink due to evaporation. By varying the initial conditions of the system, we identify three distinct classes of dynamic morphology. (1) Buckling and Fission: When forming DIBs using the lipid-out method (lipids in oil phase), lipids in the shrinking monolayers continually pair together and slide into the bilayer to conserve their mass. As the bilayer continues to grow, it becomes confined, buckles, and eventually fissions one or more vesicles. (2) Uniform Shrinking: When using the lipid-in method (lipids in water phase) to form DIBs, lipids uniformly transfer from the monolayers and bilayer into vesicles contained inside the water droplets. (3) Stretching and Unzipping: Finally, when the droplets are pinned to the wall(s) of the microfluidic channel, the droplets become stretched during evaporation, culminating in the unzipping of the bilayer and droplet separation. These findings offer a better understanding of the dynamics of coupled lipid interfaces.

  19. Lipid bilayers covalently anchored to carbon nanotubes.

    PubMed

    Dayani, Yasaman; Malmstadt, Noah

    2012-05-29

    The unique physical and electrical properties of carbon nanotubes make them an exciting material for applications in various fields such as bioelectronics and biosensing. Due to the poor water solubility of carbon nanotubes, functionalization for such applications has been a challenge. Of particular need are functionalization methods for integrating carbon nanotubes with biomolecules and constructing novel hybrid nanostructures for bionanoelectronic applications. We present a novel method for the fabrication of dispersible, biocompatible carbon nanotube-based materials. Multiwalled carbon nanotubes (MWCNTs) are covalently modified with primary amine-bearing phospholipids in a carbodiimide-activated reaction. These modified carbon nanotubes have good dispersibility in nonpolar solvents. Fourier transform infrared (FTIR) spectroscopy shows peaks attributable to the formation of amide bonds between lipids and the nanotube surface. Simple sonication of lipid-modified nanotubes with other lipid molecules leads to the formation of a uniform lipid bilayer coating the nanotubes. These bilayer-coated nanotubes are highly dispersible and stable in aqueous solution. Confocal fluorescence microscopy shows labeled lipids on the surface of bilayer-modified nanotubes. Transmission electron microscopy (TEM) shows the morphology of dispersed bilayer-coated MWCNTs. Fluorescence quenching of lipid-coated MWCNTs confirms the bilayer configuration of the lipids on the nanotube surface, and fluorescence anisotropy measurements show that the bilayer is fluid above the gel-to-liquid transition temperature. The membrane protein α-hemolysin spontaneously inserts into the MWCNT-supported bilayer, confirming the biomimetic membrane structure. These biomimetic nanostructures are a promising platform for the integration of carbon nanotube-based materials with biomolecules.

  20. Molecular dynamics simulations of unsaturated lipid bilayers

    NASA Astrophysics Data System (ADS)

    Rabinovich, Alexander L.; Balabaev, Nikolay K.

    2001-02-01

    Molecular dynamics simulations were carried out for bilayers of lipid molecules having stearic acid (C18:0) chain in position '3-D' (using the nomenclature of M. Sundaralingam, 1972) and fatty acid chain C18:0, C18:1(omega 9), C18:2(omega 6), C18:3(omega 3), C20:4(omega 6) or C22:6(omega 3) in position '2-D'. To investigate the properties of the bilayers two models were considered. In the first model, the simulation cells of the bilayers consisted of 96 phosphatidylcholine (PC) molecules and 2304 water molecules: 48 lipid molecules per layer and 24 H2O molecules per lipid. The water was modeled by explicit TIP3P water molecules. In the second model, the head group of the lipid molecules was treated as an effective sphere -- diacylglycerolipids (DGs) were considered, the interface of each monolayer was modeled by a flat surface; no water molecules were present explicitly. The bilayers consisted of 48 X 2 equals 96 glycerolipids arranged in a rectangular simulation cell. Various properties of the bilayers -- the C-H bond order parameter -SCH profiles of the hydrocarbon tails, the root-mean-square values of the positional fluctuations of the lipid chain carbons, mass density distributions of lipid molecules and water along the normals were investigated.

  1. Molecular dynamics simulations of unsaturated lipid bilayers

    NASA Astrophysics Data System (ADS)

    Rabinovich, Alexander L.; Balabaev, Nikolay K.

    2000-02-01

    Molecular dynamics simulations were carried out for bilayers of lipid molecules having stearic acid (C18:0) chain in position '3-D' (using the nomenclature of M. Sundaralingam, 1972) and fatty acid chain C18:0, C18:1(omega 9), C18:2(omega 6), C18:3(omega 3), C20:4(omega 6) or C22:6(omega 3) in position '2-D'. To investigate the properties of the bilayers two models were considered. In the first model, the simulation cells of the bilayers consisted of 96 phosphatidylcholine (PC) molecules and 2304 water molecules: 48 lipid molecules per layer and 24 H2O molecules per lipid. The water was modeled by explicit TIP3P water molecules. In the second model, the head group of the lipid molecules was treated as an effective sphere -- diacylglycerolipids (DGs) were considered, the interface of each monolayer was modeled by a flat surface; no water molecules were present explicitly. The bilayers consisted of 48 X 2 equals 96 glycerolipids arranged in a rectangular simulation cell. Various properties of the bilayers -- the C-H bond order parameter -SCH profiles of the hydrocarbon tails, the root-mean-square values of the positional fluctuations of the lipid chain carbons, mass density distributions of lipid molecules and water along the normals were investigated.

  2. Piezoelectricity in asymmetrically strained bilayer graphene

    NASA Astrophysics Data System (ADS)

    Van der Donck, M.; De Beule, C.; Partoens, B.; Peeters, F. M.; Van Duppen, B.

    2016-09-01

    We study the electronic properties of commensurate faulted bilayer graphene by diagonalizing the one-particle Hamiltonian of the bilayer system in a complete basis of Bloch states of the individual graphene layers. Our novel approach is very general and can be easily extended to any commensurate graphene-based heterostructure. Here, we consider three cases: (i) twisted bilayer graphene, (ii) bilayer graphene where triaxial stress is applied to one layer and (iii) bilayer graphene where uniaxial stress is applied to one layer. We show that the resulting superstructures can be divided into distinct classes, depending on the twist angle or the magnitude of the induced strain. The different classes are distinguished from each other by the interlayer coupling mechanism, resulting in fundamentally different low-energy physics. For the cases of triaxial and uniaxial stress, the individual graphene layers tend to decouple and we find significant charge transfer between the layers. In addition, this piezoelectric effect can be tuned by applying a perpendicular electric field. Finally, we show how our approach can be generalized to multilayer systems.

  3. Magnetoacoustic resonance in magnetoelectric bilayers

    NASA Astrophysics Data System (ADS)

    Filippov, D. A.; Bichurin, M. I.; Petrov, V. M.; Srinivasan, G.

    2004-03-01

    Layered composites of ferrite and ferroelectric single crystal thin films are of interest for studies on magnetoelectric interactions [1,2]. Such interactions result in unique and novel effects that are absent in single phase materials. For example, in a single crystal composite it is possible to control the ferromagnetic resonance (FMR) parameters for the ferrite by means of hypersonic oscillations induced in the ferroelectric phase. The absorption of acoustic oscillations by the ferrite results in variation in FMR line shape and power absorbed. One anticipates resonance absorption of elastic waves when the frequency of elastic waves coincides with the precession frequency of magnetization vector. This work is concerned with the nature of FMR under the influence of acoustic oscillations with the same frequency as FMR. Bilayers of ferrite and piezoelectric single crystals are considered. Hypersonic waves induced in the piezoelectric phase transmit acoustic power into ferrite due to mechanical connectivity between the phases. That transmission depends strongly on interface coupling [3]. We estimate the resulting variations in ferromagnetic resonance line shape. Estimates of magnetoelectric effect at magnetoacoustic resonance are also given. In addition, dependence of absorption of acoustic power on sample dimensions and compliances, electric and magnetic susceptibilities, piezoelectric and magnetostriction coefficients is discussed. The theory provided here is important for an understanding of interface coupling and the nature of magnetoelastic interactions in the composites. 1. M. I. Bichurin and V. M. Petrov, Zh. Tekh. Fiz. 58, 2277 (1988) [Sov. Phys. Tech. Phys. 33, 1389 (1988)]. 2. M.I. Bichurin, I. A. Kornev, V. M. Petrov, A. S. Tatarenko, Yu. V. Kiliba, and G. Srinivasan. Phys. Rev. B 64, 094409 (2001). 3. M. I. Bichurin, V. M. Petrov, and G. Srinivasan, J. Appl. Phys. 92, 7681 (2002). This work was supported by grants from the Russian Ministry of Education (

  4. Probing Structural Dynamics and Topology of the KCNE1 Membrane Protein in Lipid Bilayers via Site-Directed Spin Labeling and Electron Paramagnetic Resonance Spectroscopy.

    PubMed

    Sahu, Indra D; Craig, Andrew F; Dunagan, Megan M; Troxel, Kaylee R; Zhang, Rongfu; Meiberg, Andrew G; Harmon, Corrinne N; McCarrick, Robert M; Kroncke, Brett M; Sanders, Charles R; Lorigan, Gary A

    2015-10-20

    KCNE1 is a single transmembrane protein that modulates the function of voltage-gated potassium channels, including KCNQ1. Hereditary mutations in the genes encoding either protein can result in diseases such as congenital deafness, long QT syndrome, ventricular tachyarrhythmia, syncope, and sudden cardiac death. Despite the biological significance of KCNE1, the structure and dynamic properties of its physiologically relevant native membrane-bound state are not fully understood. In this study, the structural dynamics and topology of KCNE1 in bilayered lipid vesicles was investigated using site-directed spin labeling (SDSL) and electron paramagnetic resonance (EPR) spectroscopy. A 53-residue nitroxide EPR scan of the KCNE1 protein sequence including all 27 residues of the transmembrane domain (45-71) and 26 residues of the N- and C-termini of KCNE1 in lipid bilayered vesicles was analyzed in terms of nitroxide side-chain motion. Continuous wave-EPR spectral line shape analysis indicated the nitroxide spin label side-chains located in the KCNE1 TMD are less mobile when compared to the extracellular region of KCNE1. The EPR data also revealed that the C-terminus of KCNE1 is more mobile when compared to the N-terminus. EPR power saturation experiments were performed on 41 sites including 18 residues previously proposed to reside in the transmembrane domain (TMD) and 23 residues of the N- and C-termini to determine the topology of KCNE1 with respect to the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) lipid bilayers. The results indicated that the transmembrane domain is indeed buried within the membrane, spanning the width of the lipid bilayer. Power saturation data also revealed that the extracellular region of KCNE1 is solvent-exposed with some of the portions partially or weakly interacting with the membrane surface. These results are consistent with the previously published solution NMR

  5. Possible mechanism of adhesion in a mica supported phospholipid bilayer

    SciTech Connect

    Pertsin, Alexander; Grunze, Michael

    2014-05-14

    Phospholipid bilayers supported on hydrophilic solids like silica and mica play a substantial role in fundamental studies and technological applications of phospholipid membranes. In both cases the molecular mechanism of adhesion between the bilayer and the support is of primary interest. Since the possibilities of experimental methods in this specific area are rather limited, the methods of computer simulation acquire great importance. In this paper we use the grand canonical Monte Carlo technique and an atomistic force field to simulate the behavior of a mica supported phospholipid bilayer in pure water as a function of the distance between the bilayer and the support. The simulation reveals a possible adhesion mechanism, where the adhesion is due to individual lipid molecules that protrude from the bilayer and form widely spaced links with the support. Simultaneously, the bilayer remains separated from the bilayer by a thin water interlayer which maintains the bilayer fluidity.

  6. Expression and recovery of biologically active recombinant Apolipoprotein AI(Milano) from transgenic safflower (Carthamus tinctorius) seeds.

    PubMed

    Nykiforuk, Cory L; Shen, Yin; Murray, Elizabeth W; Boothe, Joseph G; Busseuil, David; Rhéaume, Eric; Tardif, Jean-Claude; Reid, Alexandra; Moloney, Maurice M

    2011-02-01

    Apolipoprotein AI Milano (ApoAI(Milano) ) was expressed as a fusion protein in transgenic safflower seeds. High levels of expression corresponding to 7 g of ApoAI(Milano) per kilogram of seed have been identified in a line selected for commercialization. The ApoAI(Milano) fusion protein was extracted from seed using an oilbody-based process and matured in vitro prior to final purification. This yielded a Des-1,2-ApoAI(Milano) product which was confirmed by biochemical characterization including immunoreactivity against ApoAI antibodies, isoelectric point, N-terminal sequencing and electrospray mass spectrometry. Purified Des-1,2-ApoAI(Milano) readily associated with dimyristoylphosphatidylcholine in clearance assays comparable to Human ApoAI. Its biological activity was assessed by cholesterol efflux assays using Des-1,2-ApoAI(Milano) :1-palmitoyl-2-oleoyl phosphatidylcholine complexes in vitro and in vivo. This study has established that high levels of biologically functional ApoAI(Milano) can be produced using a plant-based expression system.

  7. Investigation of phospholipids of the pulmonary extracellular lining by electron paramagnetic resonance. The effects of phosphatidylglycerol and unsaturated phosphatidylcholines on the fluidity of dipalmitoyl phosphatidylcholine.

    PubMed Central

    Hook, G E; Spalding, J W; Ortner, M J; Tombropoulos, E G; Chignell, C F

    1984-01-01

    The membranous structures of the pulmonary extracellular lining were removed from the lungs of rabbits by pulmonary lavage and isolated by differential centrifugation. This membranous fraction contained 93% of the total extracellular phospholipids present in lavage effluents and consisted of membranous vesicles, membrane fragments, tubular myelin and secreted lamellar bodies. The fraction was rich in phosphatidylcholine (79.4%) containing 85.2% palmitic acid in the 1-position and 57.4% palmitic acid in the 2-position. Phosphatidylglycerol was the next most abundant phospholipid, accounting for 9.4% of the total. E.p.r. spectra, obtained by using 5-doxylmethylstearate as a probe, showed that the extracellular phospholipids of the pulmonary lining were organized into structures which were much more fluid than erythrocyte-ghost membranes. The fluidity of phosphatidylcholine isolated from the membranous fraction was similar to that of the fraction itself, indicating that the minor phospholipids had very little influence on the fluidity of the major phospholipid. At physiological temperature, the fluidity of dipalmitoyl phosphatidylcholine was relatively low, but could be markedly increased by the presence of 1-palmitoyl-2-oleoyl phosphatidylcholine or phosphatidylglycerol (10%). Protein present in the extracellular phospholipid fraction did not affect the fluidity of the fraction. These studies indicate that the unsaturated phosphatidylcholines could play a major role in determining the fluidity of the important surface-tension-lowering phospholipids such as dipalmitoyl phosphatidylcholine. Images Fig. 1. PMID:6093777

  8. Characterization of a novel thermophilic phospholipase B from Thermotoga lettingae TMO: applicability in enzymatic degumming of vegetable oils.

    PubMed

    Wei, Tao; Xu, Chunping; Yu, Xuan; Jia, Weiwei; Yang, Kunpeng; Jia, Chunxiao; Mao, Duobin

    2015-04-01

    A novel phospholipase B (TLPLB) from Thermotoga lettingae TMO has been cloned, functionally overexpressed in Escherichia coli and purified to homogeneity. Gas chromatography indicated that the enzyme could efficiently hydrolyze both the sn-1 and sn-2 ester bonds of 1-palmitoyl-2-oleoyl phosphatidylcholine as phospholipase B. TLPLB was optimally active at 70 °C and pH 5.5, respectively. Its thermostability is relatively high with a half-life of 240 min at 90 °C. TLPLB also displayed remarkable organic solvent tolerance and maintained approximately 91-161 % of its initial activity in 20 and 50 % (v/v) hydrophobic organic solvents after incubation for 168 h. Furthermore, TLPLB exhibited high degumming activity towards rapeseed, soybean, peanut and sunflower seed oils, where the phosphorus contents were decreased from 225.2, 189.3, 85.6 and 70.4 mg/kg to 4.9, 4.7, 3.2 and 2.2 mg/kg within 5 h, respectively. TLPLB could therefore be used for the degumming of vegetable oils.

  9. Droplet Microfluidics for Artificial Lipid Bilayers

    NASA Astrophysics Data System (ADS)

    Punnamaraju, Srikoundinya; Steckl, Andrew

    2012-02-01

    Droplet interface bilayer is a versatile approach that allows formation of artificial lipid bilayer membrane at the interface of two lipid monolayer coated aqueous droplets in a lipid filled oil medium. Versatility exists in the form of voltage control of DIB area, ability of forming networks of DIBs, volume control of droplets and lipid-oil, and ease of reformation. Significant effect of voltage on the area and capacitance of DIB as well as DIB networks are characterized using simultaneous optical and electrical recordings. Mechanisms behind voltage-induced effects on DIBs are investigated. Photo induced effect on the DIB membrane porosity is obtained by incorporating UVC-sensitive photo-polymerizable lipids in DIB. Photo-induced effects can be extended for in-vitro studies of triggered release of encapsulated contents across membranes. A droplet based low voltage digital microfluidic platform is developed to automate DIB formation, which could potentially be used for forming arrays of lipid bilayer membranes.

  10. Layer resolved capacitive probing of graphene bilayers

    NASA Astrophysics Data System (ADS)

    Zibrov, Alexander; Parmentier, François; Li, Jia; Wang, Lei; Hunt, Benjamin; Dean, Cory; Hone, James; Taniguchi, Takashi; Watanabe, Kenji; Young, Andrea

    Compared to single layer graphene, graphene bilayers have an additional ``which-layer'' degree of freedom that can be controlled by an external electric field in a dual-gated device geometry. We describe capacitance measurements capable of directly probing this degree of freedom. By performing top gate, bottom gate, and penetration field capacitance measurements, we directly extract layer polarization of both Bernal and twisted bilayers. We will present measurements of hBN encapsulated bilayers at both zero and high magnetic field, focusing on the physics of the highly degenerate zero-energy Landau level in the high magnetic field limit where spin, valley, and layer degeneracy are all lifted by electronic interactions.

  11. Magnetic ratchet effect in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kheirabadi, Narjes; McCann, Edward; Fal'ko, Vladimir I.

    2016-10-01

    We consider the orbital effect of an in-plane magnetic field on electrons in bilayer graphene, deriving linear-in-field contributions to the low-energy Hamiltonian arising from the presence of either skew interlayer coupling or interlayer potential asymmetry, the latter being tunable by an external metallic gate. To illustrate the relevance of such terms, we consider the ratchet effect in which a dc current results from the application of an alternating electric field in the presence of an in-plane magnetic field and inversion-symmetry breaking. By comparison with recent experimental observations in monolayer graphene [C. Drexler et al., Nat. Nanotechnol. 8, 104 (2013), 10.1038/nnano.2012.231], we estimate that the effect in bilayer graphene can be two orders of magnitude greater than that in monolayer graphene, illustrating that the bilayer is an ideal material for the realization of optoelectronic effects that rely on inversion-symmetry breaking.

  12. Spin precession modulation in a magnetic bilayer

    SciTech Connect

    Stupakiewicz, A.; Maziewski, A.; Pashkevich, M.; Stognij, A.; Novitskii, N.

    2012-12-24

    We report on modulation of the spin precession in a Co/garnet bilayer by femtosecond laser excitation using time-resolved magneto-optical tools. Damped oscillations in the Faraday rotation transients representing precessional motion of the magnetization vector are observed in both the 2 nm Co layer and 1.8 {mu}m garnet of the bilayer with distinct frequencies differing by about a factor of two. The excitation efficiency of these precessions strongly depends on the out-of-plane magnetic field. The modulation effect with the coupling in a magnetic bilayer can be useful for non-thermally controlling the magnetization of nanomagnets and ultrafast switching in magnetic nanodevices.

  13. Supported lipid bilayer/carbon nanotube hybrids

    NASA Astrophysics Data System (ADS)

    Zhou, Xinjian; Moran-Mirabal, Jose M.; Craighead, Harold G.; McEuen, Paul L.

    2007-03-01

    Carbon nanotube transistors combine molecular-scale dimensions with excellent electronic properties, offering unique opportunities for chemical and biological sensing. Here, we form supported lipid bilayers over single-walled carbon nanotube transistors. We first study the physical properties of the nanotube/supported lipid bilayer structure using fluorescence techniques. Whereas lipid molecules can diffuse freely across the nanotube, a membrane-bound protein (tetanus toxin) sees the nanotube as a barrier. Moreover, the size of the barrier depends on the diameter of the nanotube-with larger nanotubes presenting bigger obstacles to diffusion. We then demonstrate detection of protein binding (streptavidin) to the supported lipid bilayer using the nanotube transistor as a charge sensor. This system can be used as a platform to examine the interactions of single molecules with carbon nanotubes and has many potential applications for the study of molecular recognition and other biological processes occurring at cell membranes.

  14. Design of Asymmetric Peptide Bilayer Membranes.

    PubMed

    Li, Sha; Mehta, Anil K; Sidorov, Anton N; Orlando, Thomas M; Jiang, Zhigang; Anthony, Neil R; Lynn, David G

    2016-03-16

    Energetic insights emerging from the structural characterization of peptide cross-β assemblies have enabled the design and construction of robust asymmetric bilayer peptide membranes. Two peptides differing only in their N-terminal residue, phosphotyrosine vs lysine, coassemble as stacks of antiparallel β-sheets with precisely patterned charged lattices stabilizing the bilayer leaflet interface. Either homogeneous or mixed leaflet composition is possible, and both create nanotubes with dense negative external and positive internal solvent exposed surfaces. Cross-seeding peptide solutions with a preassembled peptide nanotube seed leads to domains of different leaflet architecture within single nanotubes. Architectural control over these cross-β assemblies, both across the bilayer membrane and along the nanotube length, provides access to highly ordered asymmetric membranes for the further construction of functional mesoscale assemblies.

  15. Structure and Orientational Texture of Self-Organizing Lipid Bilayers

    SciTech Connect

    Watkins, E. B.; Miller, C. E.; Majewski, J.; Mulder, D. J.; Kuhl, T. L.

    2009-06-12

    The structure of single supported dipalmitoyl-phosphatidylcholine bilayers prepared by vesicle fusion or Langmuir-Blodgett-Schaeffer (LBS) deposition techniques was characterized by x-ray reflectivity and grazing incidence diffraction in bulk water. LBS bilayers display symmetric leaflets similar to monolayer structures, while vesicle fusion yields more inhomogeneous bilayers. Diffraction establishes that lipids are always coupled across the bilayer even when leaflets are deposited independently and suggests the existence of orientational texture.

  16. Bilayer graphene quantum dot defined by topgates

    SciTech Connect

    Müller, André; Kaestner, Bernd; Hohls, Frank; Weimann, Thomas; Pierz, Klaus; Schumacher, Hans W.

    2014-06-21

    We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantum dot devices. At temperatures below 500 mK, the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantum dot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.

  17. Temperature effect on plasmons in bilayer graphene

    SciTech Connect

    Patel, Digish K. Sharma, A. C.; Ashraf, S. S. Z.; Ambavale, S. K.

    2015-06-24

    We have theoretically investigated the plasmon dispersion and damping rate of doped bilayer graphene (BLG) at finite temperatures within the random phase approximation. Our computed results on plasmon dispersion show that plasmon frequency enhances with increasing temperatures in contrast to single layer graphene where it is suppressed. This can be attributed to the fact that the dynamic response of the electron gas or screening in bilayer graphene is different from that of single layer graphene. Further the temperature effect on damping rate is also discussed.

  18. Electronic properties of graphene-based bilayer systems

    NASA Astrophysics Data System (ADS)

    Rozhkov, A. V.; Sboychakov, A. O.; Rakhmanov, A. L.; Nori, Franco

    2016-08-01

    This article reviews the theoretical and experimental work related to the electronic properties of bilayer graphene systems. Three types of bilayer stackings are discussed: the AA, AB, and twisted bilayer graphene. This review covers single-electron properties, effects of static electric and magnetic fields, bilayer-based mesoscopic systems, spin-orbit coupling, dc transport and optical response, as well as spontaneous symmetry violation and other interaction effects. The selection of the material aims to introduce the reader to the most commonly studied topics of theoretical and experimental research in bilayer graphene.

  19. Electromagnetic resonant modes of dielectric sphere bilayers

    SciTech Connect

    Andueza, A. Pérez-Conde, J.; Sevilla, J.

    2015-05-28

    Sphere bilayers have been proposed as promising structures for electromagnetic management in photonic crystal devices. These arrangements are made of two intertwined subsets of spheres of different size and refractive index, one subset filling the interstitial sites of the other. We present a systematic study of the electromagnetic resonant modes of the bilayers, in comparison with those of the constituent subsets of spheres. Three samples were built with glass and Teflon spheres and their transmission spectra measured in the microwave range (10–25 GHz). Simulations with finite integration time-domain method are in good agreement with experiments. Results show that the bilayer presents the same resonances as one of the subsets but modified by the presence of the other in its resonant frequencies and in the electric field distributions. As this distortion is not very large, the number of resonances in a selected spectral region is determined by the dominant subset. The degree of freedom that offers the bilayer could be useful to fine tune the resonances of the structure for different applications. A map of modes useful to guide this design is also presented. Scale invariance of Maxwell equations allows the translation of these results in the microwave range to the visible region; hence, some possible applications are discussed in this framework.

  20. Electronic properties of a biased graphene bilayer.

    PubMed

    Castro, Eduardo V; Novoselov, K S; Morozov, S V; Peres, N M R; Lopes dos Santos, J M B; Nilsson, Johan; Guinea, F; Geim, A K; Castro Neto, A H

    2010-05-01

    We study, within the tight-binding approximation, the electronic properties of a graphene bilayer in the presence of an external electric field applied perpendicular to the system-a biased bilayer. The effect of the perpendicular electric field is included through a parallel plate capacitor model, with screening correction at the Hartree level. The full tight-binding description is compared with its four-band and two-band continuum approximations, and the four-band model is shown to always be a suitable approximation for the conditions realized in experiments. The model is applied to real biased bilayer devices, made out of either SiC or exfoliated graphene, and good agreement with experimental results is found, indicating that the model is capturing the key ingredients, and that a finite gap is effectively being controlled externally. Analysis of experimental results regarding the electrical noise and cyclotron resonance further suggests that the model can be seen as a good starting point for understanding the electronic properties of graphene bilayer. Also, we study the effect of electron-hole asymmetry terms, such as the second-nearest-neighbour hopping energies t' (in-plane) and γ(4) (inter-layer), and the on-site energy Δ.

  1. 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.

  2. Capillary wrinkling of thin bilayer polymeric sheets

    NASA Astrophysics Data System (ADS)

    Chang, Jooyoung; Menon, Narayanan; Russell, Thomas

    We have investigated capillary force induced wrinkling on a floated polymeric bilayer thin sheet. The origin of the wrinkle pattern is compressional hoop stress caused by the capillary force of a water droplet placed on the floated polymeric thin sheet afore investigated. Herein, we study the effect of the differences of surface energy arising from the hydrophobicity of Polystyrene (PS Mw: 97 K, Contact Angle: 88 º) and the hydrophilicity of Poly(methylmethacrylate) (PMMA Mw: 99K, Contact Angle: 68 º) on two sides of a bilayer film. We measure the number and the length of the wrinkles by broadly varying the range of thicknesses of top (9 nm to 550 nm) and bottom layer (25 nm to 330 nm). At the same, there is only a small contrast in mechanical properties of the two layers (PS E = 3.4 GPa, and PMMA E = 3 GPa). The number of the wrinkles is not strongly affected by the composition (PS(Top)/PMMA(Bottom) or PMMA(Top)/PS(Bottom)) and the thickness of each and overall bilayer system. However, the length of the wrinkle is governed by the contact angle of the drop on the top layer of bilayer system. We also compare this to the wrinkle pattern obtained in monolayer systems over a wide range of thickness from PS and PMMA (7 nm to 1 μm). W.M. Keck Foundation.

  3. Simulation Studies of Alamethicin-Bilayer Interactions

    PubMed Central

    Biggin, P. C.; Breed, J.; Son, H. S.; Sansom, M. S. P.

    1997-01-01

    Alamethicin is an α-helical peptide that forms voltage-activated ion channels. Experimental data suggest that channel formation occurs via voltage-dependent insertion of alamethicin helices into lipid bilayers, followed by self-assembly of inserted helices to form a parallel helix bundle. Changes in the kink angle of the alamethicin helix about its central proline residue have also been suggested to play a role in channel gating. Alamethicin helices generated by simulated annealing and restrained molecular dynamics adopt a kink angle similar to that in the x-ray crystal structure, even if such simulations start with an idealized unkinked helix. This suggests that the kinked helix represents a stable conformation of the molecule. Molecular dynamics simulations in the presence of a simple bilayer model and a transbilayer voltage difference are used to explore possible mechanisms of helix insertion. The bilayer is represented by a hydrophobicity potential. An alamethicin helix inserts spontaneously in the absence of a transbilayer voltage. Application of a cis positive voltage decreases the time to insertion. The helix kink angle fluctuates during the simulations. Insertion of the helix is associated with a decrease in the mean kink angle, thus helping the alamethicin molecule to span the bilayer. The simulation results are discussed in terms of models of alamethicin channel gating. ImagesFIGURE 1FIGURE 6 PMID:9017192

  4. Phase Behavior and Domain Size in Sphingomyelin-Containing Lipid Bilayers

    SciTech Connect

    Petruzielo, Robin S; Heberle, Frederick A; Drazba, Paul; Katsaras, John; Feigenson, Gerald

    2013-01-01

    Membrane raft size measurements are crucial to understanding the stability and functionality of rafts in cells. The challenge of accurately measuring raft size is evidenced by the disparate reports of domain sizes, which range from nanometers to microns for the ternary model membrane system sphingomyelin (SM)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/cholesterol (Chol). Using F rster resonance energy transfer (FRET) and differential scanning calorimetry (DSC), we established phase diagrams for porcine brain SM (bSM)/dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Chol and bSM/POPC/Chol at 15 and 25 C. By combining two techniqueswith different spatial sensitivities, namely FRET and small-angle neutron scattering (SANS),we have significantly narrowed the uncertainty in domain size estimates for bSM/POPC/Chol mixtures. Compositional trends in FRET data revealed coexisting domains at 15 and 25 C for bothmixtures, while SANS measurements detected no domain formation for bSM/POPC/Chol. Together these results indicate that liquid domains in bSM/POPC/Chol are between 2 and 7 nmin radius at 25 C: that is, domains must be on the order of the 2 6 nmF rster distance of the FRET probes, but smaller than the ~7 nm minimum cluster size detectable with SANS. However, for palmitoyl SM (PSM)/POPC/Chol at a similar composition, SANS detected coexisting liquid domains. This increase in domain size upon replacing the natural SMcomponent (which consists of amixture of chain lengths) with synthetic PSM, suggests a role for SM chain length in modulating raft size in vivo.

  5. Investigating the Structure of Multicomponent Gel-Phase Lipid Bilayers.

    PubMed

    Hartkamp, Remco; Moore, Timothy C; Iacovella, Christopher R; Thompson, Michael A; Bulsara, Pallav A; Moore, David J; McCabe, Clare

    2016-08-23

    Single- and multicomponent lipid bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC), isostearyl isostearate, and heptadecanoyl heptadecanoate in the gel phase are studied via molecular dynamics simulations. It is shown that the structural properties of multicomponent bilayers can deviate strongly from the structures of their single-component counterparts. Specifically, the lipid mixtures are shown to adopt a compact packing by offsetting the positioning depths at which different lipid species are located in the bilayer. This packing mechanism affects the area per lipid, the bilayer height, and the chain tilt angles and has important consequences for other bilayer properties, such as interfacial hydrogen bonding and bilayer permeability. In particular, the simulations suggest that bilayers containing isostearyl isostearate or heptadecanoyl heptadecanoate are less permeable than pure 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine or DSPC bilayers. Furthermore, hydrogen-bond analysis shows that the residence times of lipid-water hydrogen bonds depend strongly on the bilayer composition, with longer residence times for bilayers that have a higher DSPC content. The findings illustrate and explain the fundamental differences between the properties of single- and multicomponent bilayers. PMID:27558724

  6. Repetitive formation of optically-observable planar lipid bilayers by rotating chambers on a microaperture.

    PubMed

    Tomoike, Fumiaki; Tonooka, Taishi; Osaki, Toshihisa; Takeuchi, Shoji

    2016-07-01

    Optical observation of a planar lipid bilayer is an effective method of lipid bilayer characterization. However, previous methods for optically observable lipid bilayer formation are unsuitable for repetitive formation of lipid bilayers. In this paper, we propose a system that facilitates repetitive formation of horizontal lipid bilayers via mechanical rotation of the rotating part. We show that multiple bilayers can be observed within a short period, and that the electrical and optical characteristics of a bilayer can be analyzed simultaneously. PMID:27256329

  7. The interaction of polyphenols with bilayers: conditions for increasing bilayer adhesion.

    PubMed Central

    Huh, N W; Porter, N A; McIntosh, T J; Simon, S A

    1996-01-01

    Because proteins and other molecules with a high polyphenol content are commonly involved in adhesion processes, we are investigating the interactions between polyphenols and biological materials. A naturally occurring polyphenol that binds a variety of proteins and lipids is tannic acid (TA), which contains five digallic acid residues covalently linked to a central D-glucose. A previous study has shown that TA increases the adhesion between apposing phosphatidylcholine (PC) bilayers and over a very narrow concentration range collapses the interbilayer fluid space from about 15 A to 5 A. To determine the chemical requirements a polyphenolic molecule must possess to increase bilayer adhesion, we have synthesized several simpler TA analogs that vary in their size, shape, and number of gallic acid and hydroxyl groups. X-ray diffraction, absorbance, binding, and differential scanning calorimetry measurements were used to investigate the interaction of these polyphenolic molecules with egg PC (EPC) and dipalmitoyl PC (DPPC) bilayers. Of these synthetic polyphenols, only penta-O-galloyl-alpha-D-glucose (PGG) was able to completely mimic the effects of TA by collapsing the interbilayer fluid space from 15 A to 5 A, decreasing the dipole potential by about 300 mV, increasing the transition enthalpy of DPPC liposomes, and inducing an interdigitated phase in DPPC. Binding studies indicated that the fluid space was reduced to 5 A at an EPC:PGG mole ratio of 5:1. We conclude that these polyphenols collapse the fluid space of PC bilayers because they 1) are amphipathic and partition into the bilayers interfacial region, 2) are long enough to span the interbilayer space, 3) contain several gallic acids distributed so that they can partition simultaneously into apposing bilayers, and 4) have sufficient gallic acid residues to interact with all lipid headgroups and cover the bilayer surface. Under these conditions we conclude that the polyphenols from interbilayer bridges. We

  8. Topological valley transport at bilayer graphene domain walls

    NASA Astrophysics Data System (ADS)

    Ju, Long; Shi, Zhiwen; Nair, Nityan; Lv, Yinchuan; Jin, Chenhao; Velasco, Jairo; Ojeda-Aristizabal, Claudia; Bechtel, Hans A.; Martin, Michael C.; Zettl, Alex; Analytis, James; Wang, Feng

    2015-04-01

    Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. A tunable bandgap can be induced in bilayer graphene by an external electric field, and such gapped bilayer graphene is predicted to be a topological insulating phase protected by no-valley mixing symmetry, featuring quantum valley Hall effects and chiral edge states. Observation of such chiral edge states, however, is challenging because inter-valley scattering is induced by atomic-scale defects at real bilayer graphene edges. Recent theoretical work has shown that domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. Here we report an experimental observation of ballistic (that is, with no scattering of electrons) conducting channels at bilayer graphene domain walls. We employ near-field infrared nanometre-scale microscopy (nanoscopy) to image in situ bilayer graphene layer-stacking domain walls on device substrates, and we fabricate dual-gated field effect transistors based on the domain walls. Unlike single-domain bilayer graphene, which shows gapped insulating behaviour under a vertical electrical field, bilayer graphene domain walls feature one-dimensional valley-polarized conducting channels with a ballistic length of about 400 nanometres at 4 kelvin. Such topologically protected one-dimensional chiral states at bilayer graphene domain walls open up opportunities for exploring unique topological phases and valley physics in graphene.

  9. Interactions between charged, uncharged, and zwitterionic bilayers containing phosphatidylglycerol.

    PubMed Central

    McIntosh, T J; Magid, A D; Simon, S A

    1990-01-01

    Pressure vs. distance relationships have been obtained for phosphatidylglycerol bilayers, in both charged and uncharged states. Water was removed from the lipid multilayers by the application of osmotic pressures in the range of 0-2.7 x 10(9) dyn/cm2, and the distance between adjacent bilayers was obtained from Fourier analysis of lamellar x-ray diffraction data. For phosphatidylglycerol bilayers made electrically neutral either by lowering the pH or by adding equimolar concentrations of the positively charged lipid stearylamine, the pressure-distance data could be fit with a single exponential. The measured decay lengths were 1.1 A at low pH and 1.5 A with stearylamine, which are similar to decay lengths of the hydration pressure found for gel phases of other neutral bilayers. In addition, the magnitude of this repulsive pressure was proportional to the square of the Volta potential (equivalent to the dipole potential for electrically neutral bilayers) measured in monolayers in equilibrium with bilayers, in agreement with results previously found for the hydration pressure between phosphatidylcholine bilayers. For charged phosphatidylglycerol bilayers, the pressure-distance relation had two distinct regions. For bilayer separations greater than 10 A, the pressure-distance data had an exponential decay length (11 A) and a magnitude consistent with that expected for electrostatic repulsion from double-layer theory. For bilayer separations of 2-10 A, the pressure decayed much more rapidly with increasing bilayer separation (decay length less than 1 A). We interpret these data at low bilayer separations in terms of a combination of hydration repulsion and steric hindrance between the lipid head groups and the sodium ions trapped between apposing bilayers. PMID:2393704

  10. High pressure volumetric measurements in dipalmitoylphosphatidylcholine bilayers.

    PubMed

    Tosh, R E; Collings, P J

    1986-07-10

    The one previously reported high pressure volumetric experiment on a phospholipid bilayer investigated a region of pressure between 0 and 25 MPa and obtained isothermal compressibility values for the liquid crystal and intermediate phases which differed by more than a factor of ten. We report new volumetric measurements around the main transition in dipalmitoylphosphatidylcholine (DPPC) from 0 to 100 MPa. The isothermal compressibility data for the two phases are of the same order of magnitude, and the experimentally determined coexistence curve, specific volume dependence, and volume discontinuity values are compared with the predictions of the phenomenological theory according to Sugar and Tarjan ((1982) Sov. Phys. Crystallogr. 27, 4-5). Significant discrepancies between this theory and experiment are found. Finally, the data indicate that steric interactions play a more dominant role in the main transition of phospholipid bilayers than in transitions in most thermotropic liquid crystals.

  11. Space charge and screening in bilayer graphene.

    PubMed

    Kolomeisky, Eugene B; Straley, Joseph P; Abrams, Daniel L

    2016-11-30

    Undoped bilayer graphene is a two-dimensional semimetal with a low-energy excitation spectrum that is parabolic in the momentum. As a result, the screening of an arbitrary external charge Ze is accompanied by a reconstruction of the ground state: valence band electrons (for Z  >  0) are promoted to form a space charge around the charge while the holes leave the physical picture. The outcome is a flat neutral object resembling the regular atom except that for [Formula: see text] it is described by a strictly linear Thomas-Fermi theory. This theory also predicts that the bilayer's static dielectric constant is the same as that of a two-dimensional electron gas in the long-wavelength limit. PMID:27636158

  12. Vortex Physics in the Quantum Hall Bilayer

    NASA Astrophysics Data System (ADS)

    Fertig, H. A.; Murthy, Ganpathy

    2013-06-01

    There exists a strong analogy between the quantum Hall bilayer system at total filling factor ν = 1 and a thin film superfluid, in which the groundstate is described as a condensate of particle-hole pairs. The analogy draws support from experiments which display near dissipationless transport properties at low temperatures. However dissipation is always present at any accessible temperature, suggesting that in a proper description, unpaired vortex-like excitations must be present. The mechanism by which this happens remains poorly understood. A key difference between the quantum Hall bilayer and simpler thin-film superfluids is that the vortices, more properly called merons in the former context, are charged objects. We demonstrate that a model in which disorder induces merons in the groundstate, through coupling to this charge, can naturally explain many of the observed imperfect superfluid properties...

  13. Epitaxial oxide bilayer on Pt (001) nanofacets

    NASA Astrophysics Data System (ADS)

    Hennessy, Daniel; Komanicky, Vladimir; Iddir, Hakim; Pierce, Michael S.; Menzel, Andreas; Chang, Kee-Chul; Barbour, Andi; Zapol, Peter; You, Hoydoo

    2012-01-01

    We observed an epitaxial, air-stable, partially registered (2 × 1) oxide bilayer on Pt (001) nanofacets [V. Komanicky, A. Menzel, K.-C. Chang, and H. You, J. Phys. Chem. 109, 23543 (2005)]. The bilayer is made of two half Pt layers; the top layer has four oxygen bonds and the second layer two. The positions and oxidation states of the Pt atoms are determined by analyzing crystal truncation rods and resonance scattering data. The positions of oxygen atoms are determined by density functional theory (DFT) calculations. Partial registry on the nanofacets and the absence of such registry on the extended Pt (001) surface prepared similarly are explained in DFT calculations by strain relief that can be accommodated only by nanoscale facets.

  14. Driving skyrmions in a composite bilayer

    NASA Astrophysics Data System (ADS)

    Wang, Zidong; Grimson, Malcolm J.

    2016-07-01

    Magnetic skyrmions and multiferroics are the most interesting objects in nanostructure science that have great potential in future spin-electronic technology. The study of multiferroic skyrmions has attracted much interest in recent years. This article reports magnetic Bloch skyrmions induced by an electric driving field in a composite bilayer (chiral-magnetic/ferroelectric bilayer) lattice. By using the spin dynamics method, we use a classical magnetic spin model and an electric pseudospin model, which are coupled by a strong magnetoelectric coupling in the dynamical simulations. Interestingly, we observe some skyrmionlike objects in the electric component either during the switching process or by applying a magnetic field, which is due to the connection between the electric and the magnetic structures.

  15. Bilayer Graphene: An Electrically Tunable Semiconductor

    NASA Astrophysics Data System (ADS)

    Min, Hongki; Sahu, Bhagawan; Banerjee, Sanjay; MacDonald, Allan

    2007-03-01

    Using ab initio density functional theory calculations, we verify [1,2] that the energy band structure of bilayer graphene can be tuned by applying an external electric field. As the strength of the external electric field increases, the electronic spectrum of bilayer graphene changes from a that of a zero-gap semiconductor to that of a gapped semiconductor. From the ab initio calculations the external field dependence of the screened interlayer potential difference and tunneling amplitudes are extracted by fitting to a tight-binding model. We discuss the role of interlayer correlations in determining the size of the gap and the accuracy of local density approximation. [1] Edward McCann and Vladimir I. Fal'ko, Phys. Rev. Lett. 96, 086805 (2006). [2] Taisuke Ohta, Aaron Bostwick,, Thomas Seyller, Karsten Horn, and Eli Rotenberg, Science 313, 951 (2006).

  16. Hydrodynamic trapping of molecules in lipid bilayers

    PubMed Central

    Jönsson, Peter; McColl, James; Clarke, Richard W.; Ostanin, Victor P.; Jönsson, Bengt; Klenerman, David

    2012-01-01

    In this work we show how hydrodynamic forces can be used to locally trap molecules in a supported lipid bilayer (SLB). The method uses the hydrodynamic drag forces arising from a flow through a conical pipette with a tip radius of 1–1.5 μm, placed approximately 1 μm above the investigated SLB. This results in a localized forcefield that acts on molecules protruding from the SLB, yielding a hydrodynamic trap with a size approximately given by the size of the pipette tip. We demonstrate this concept by trapping the protein streptavidin, bound to biotin receptors in the SLB. It is also shown how static and kinetic information about the intermolecular interactions in the lipid bilayer can be obtained by relating how the magnitude of the hydrodynamic forces affects the accumulation of protein molecules in the trap. PMID:22699491

  17. How Bilayer Graphene Got a Bandgap

    SciTech Connect

    Feng Wang

    2009-06-02

    Graphene is the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. But theres a catch: graphene has no bandgap. Now Feng Wang and his colleagues at Berkeley Lab and UC Berkeley have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts, which is the energy of infrared radiation.

  18. How Bilayer Graphene Got a Bandgap

    ScienceCinema

    Wang, Feng

    2016-07-12

    Graphene is the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. But theres a catch: graphene has no bandgap. Now Feng Wang and his colleagues at Berkeley Lab and UC Berkeley have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts, which is the energy of infrared radiation.

  19. Integral Membrane Proteins and Bilayer Proteomics

    PubMed Central

    Whitelegge, Julian P.

    2013-01-01

    Integral membrane proteins reside within the bilayer membranes that surround cells and organelles, playing critical roles in movement of molecules across them and the transduction of energy and signals. While their extreme amphipathicity presents technical challenges, biological mass spectrometry has been applied to all aspects of membrane protein chemistry and biology, including analysis of primary, secondary, tertiary and quaternary structure, as well as the dynamics that accompany functional cycles and catalysis. PMID:23301778

  20. Factors Influencing Efficacy of Bilayered Cell Therapy

    PubMed Central

    Allam, Reynald C.; Van Driessche, Freya; Zhu, Yiliang

    2014-01-01

    Objective: Diabetic foot ulcers (DFUs) that fail to heal with standard care should be treated with advanced wound care products. Efficacy of advanced therapies is dependent on many factors. A secondary analysis of pivotal trial data for a bilayered cellular construct used in the treatment of DFU was undertaken to determine if glycemic control and other factors had an effect on time to healing. Approach: We analyzed the effect of age, gender, diabetes type, insulin usage, body mass index, smoking, initial and ending glycohemoglobin (HgbA1c), Charcot deformity, and wound area, duration, and location on likelihood of healing for wounds treated with bilayered cellular construct (BLCC). Results: In those treated with BLCC, initial wound area (cm2), age, and history of Charcot deformity were found to significantly affect healing. Neither initial HgbA1c nor change in HgbA1c was associated with healing. The bilayered product was found to be equally effective regardless of initial or change in HgbA1c levels (p-values 0.94 and 0.44, respectively). In the control group, initial HgbA1c, insulin usage, female gender, and wound location at the toes significantly influenced healing. Innovation: BLCC subgroup analysis to elucidate selection criteria allowing for targeted use of advanced products on those more likely to respond as well as direct further research into prognostic indicators for BLCC-treated patients. Conclusion: The bilayered cellular construct product remains equally effective regardless of initial or change in HgbA1c levels. Further specific research into the effect of glucose control and other factors on the effectiveness of different advanced DFU treatment products is recommended. PMID:24940555

  1. How Bilayer Graphene Got a Bandgap

    ScienceCinema

    Feng Wang

    2010-01-08

    Graphene is the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. But theres a catch: graphene has no bandgap. Now Feng Wang and his colleagues at Berkeley Lab and UC Berkeley have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts, which is the energy of infrared radiation.

  2. Bilayer membrane interactions with nanofabricated scaffolds

    SciTech Connect

    Collier, C. Patrick

    2015-07-29

    Membrane function is facilitated by lateral organization within the lipid bilayer, including phase-separation of lipids into more ordered domains (lipid rafts) and anchoring of the membrane to a cytoskeleton. These features have proven difficult to reproduce in model membrane systems such as black lipid membranes, unilamellar vesicles and supported bilayers. However, advances in micro/nanofabrication have resulted in more realistic synthetic models of membrane-cytoskeleton interactions that can help uncover the design rules responsible for biological membrane formation and organization. This review will focus on describing micro-/nanostructured scaffolds that can emulate the connections of a cellular membrane to an underlying “cytoskeleton”. This includes molecular-based scaffolds anchored to a solid substrate through surface chemistry, solid-state supports modified by material deposition, lithography and etching, the creation of micro/nanoporous arrays, integration with microfluidics, and droplet-based bilayers at interfaces. Lastly, model systems such as these are increasing our understanding of structure and organization in cell membranes, and how they result in the emergence of functionality at the nanoscale.

  3. Bilayer membrane interactions with nanofabricated scaffolds

    DOE PAGES

    Collier, C. Patrick

    2015-07-29

    Membrane function is facilitated by lateral organization within the lipid bilayer, including phase-separation of lipids into more ordered domains (lipid rafts) and anchoring of the membrane to a cytoskeleton. These features have proven difficult to reproduce in model membrane systems such as black lipid membranes, unilamellar vesicles and supported bilayers. However, advances in micro/nanofabrication have resulted in more realistic synthetic models of membrane-cytoskeleton interactions that can help uncover the design rules responsible for biological membrane formation and organization. This review will focus on describing micro-/nanostructured scaffolds that can emulate the connections of a cellular membrane to an underlying “cytoskeleton”. Thismore » includes molecular-based scaffolds anchored to a solid substrate through surface chemistry, solid-state supports modified by material deposition, lithography and etching, the creation of micro/nanoporous arrays, integration with microfluidics, and droplet-based bilayers at interfaces. Lastly, model systems such as these are increasing our understanding of structure and organization in cell membranes, and how they result in the emergence of functionality at the nanoscale.« less

  4. Lipid bilayer vesicle generation using microfluidic jetting.

    PubMed

    Coyne, Christopher W; Patel, Karan; Heureaux, Johanna; Stachowiak, Jeanne; Fletcher, Daniel A; Liu, Allen P

    2014-01-01

    Bottom-up synthetic biology presents a novel approach for investigating and reconstituting biochemical systems and, potentially, minimal organisms. This emerging field engages engineers, chemists, biologists, and physicists to design and assemble basic biological components into complex, functioning systems from the bottom up. Such bottom-up systems could lead to the development of artificial cells for fundamental biological inquiries and innovative therapies(1,2). Giant unilamellar vesicles (GUVs) can serve as a model platform for synthetic biology due to their cell-like membrane structure and size. Microfluidic jetting, or microjetting, is a technique that allows for the generation of GUVs with controlled size, membrane composition, transmembrane protein incorporation, and encapsulation(3). The basic principle of this method is the use of multiple, high-frequency fluid pulses generated by a piezo-actuated inkjet device to deform a suspended lipid bilayer into a GUV. The process is akin to blowing soap bubbles from a soap film. By varying the composition of the jetted solution, the composition of the encompassing solution, and/or the components included in the bilayer, researchers can apply this technique to create customized vesicles. This paper describes the procedure to generate simple vesicles from a droplet interface bilayer by microjetting. PMID:24637415

  5. Lipid Bilayer Vesicle Generation Using Microfluidic Jetting

    PubMed Central

    Coyne, Christopher W.; Patel, Karan; Heureaux, Johanna; Stachowiak, Jeanne; Fletcher, Daniel A.; Liu, Allen P.

    2014-01-01

    Bottom-up synthetic biology presents a novel approach for investigating and reconstituting biochemical systems and, potentially, minimal organisms. This emerging field engages engineers, chemists, biologists, and physicists to design and assemble basic biological components into complex, functioning systems from the bottom up. Such bottom-up systems could lead to the development of artificial cells for fundamental biological inquiries and innovative therapies1,2. Giant unilamellar vesicles (GUVs) can serve as a model platform for synthetic biology due to their cell-like membrane structure and size. Microfluidic jetting, or microjetting, is a technique that allows for the generation of GUVs with controlled size, membrane composition, transmembrane protein incorporation, and encapsulation3. The basic principle of this method is the use of multiple, high-frequency fluid pulses generated by a piezo-actuated inkjet device to deform a suspended lipid bilayer into a GUV. The process is akin to blowing soap bubbles from a soap film. By varying the composition of the jetted solution, the composition of the encompassing solution, and/or the components included in the bilayer, researchers can apply this technique to create customized vesicles. This paper describes the procedure to generate simple vesicles from a droplet interface bilayer by microjetting. PMID:24637415

  6. Lipid bilayer vesicle generation using microfluidic jetting.

    PubMed

    Coyne, Christopher W; Patel, Karan; Heureaux, Johanna; Stachowiak, Jeanne; Fletcher, Daniel A; Liu, Allen P

    2014-02-21

    Bottom-up synthetic biology presents a novel approach for investigating and reconstituting biochemical systems and, potentially, minimal organisms. This emerging field engages engineers, chemists, biologists, and physicists to design and assemble basic biological components into complex, functioning systems from the bottom up. Such bottom-up systems could lead to the development of artificial cells for fundamental biological inquiries and innovative therapies(1,2). Giant unilamellar vesicles (GUVs) can serve as a model platform for synthetic biology due to their cell-like membrane structure and size. Microfluidic jetting, or microjetting, is a technique that allows for the generation of GUVs with controlled size, membrane composition, transmembrane protein incorporation, and encapsulation(3). The basic principle of this method is the use of multiple, high-frequency fluid pulses generated by a piezo-actuated inkjet device to deform a suspended lipid bilayer into a GUV. The process is akin to blowing soap bubbles from a soap film. By varying the composition of the jetted solution, the composition of the encompassing solution, and/or the components included in the bilayer, researchers can apply this technique to create customized vesicles. This paper describes the procedure to generate simple vesicles from a droplet interface bilayer by microjetting.

  7. Self-folding graphene-polymer bilayers

    NASA Astrophysics Data System (ADS)

    Deng, Tao; Yoon, ChangKyu; Jin, Qianru; Li, Mingen; Liu, Zewen; Gracias, David H.

    2015-05-01

    In order to incorporate the extraordinary intrinsic thermal, electrical, mechanical, and optical properties of graphene with three dimensional (3D) flexible substrates, we introduce a solvent-driven self-folding approach using graphene-polymer bilayers. A polymer (SU-8) film was spin coated atop chemically vapor deposited graphene films on wafer substrates and graphene-polymer bilayers were patterned with or without metal electrodes using photolithography, thin film deposition, and etching. After patterning, the bilayers were released from the substrates and they self-folded to form fully integrated, curved, and folded structures. In contrast to planar graphene sensors on rigid substrates, we assembled curved and folded sensors that are flexible and they feature smaller form factors due to their 3D geometry and large surface areas due to their multiple rolled architectures. We believe that this approach could be used to assemble a range of high performance 3D electronic and optical devices of relevance to sensing, diagnostics, wearables, and energy harvesting.

  8. Self-folding graphene-polymer bilayers

    SciTech Connect

    Deng, Tao; Yoon, ChangKyu; Jin, Qianru; Li, Mingen; Liu, Zewen; Gracias, David H.

    2015-05-18

    In order to incorporate the extraordinary intrinsic thermal, electrical, mechanical, and optical properties of graphene with three dimensional (3D) flexible substrates, we introduce a solvent-driven self-folding approach using graphene-polymer bilayers. A polymer (SU-8) film was spin coated atop chemically vapor deposited graphene films on wafer substrates and graphene-polymer bilayers were patterned with or without metal electrodes using photolithography, thin film deposition, and etching. After patterning, the bilayers were released from the substrates and they self-folded to form fully integrated, curved, and folded structures. In contrast to planar graphene sensors on rigid substrates, we assembled curved and folded sensors that are flexible and they feature smaller form factors due to their 3D geometry and large surface areas due to their multiple rolled architectures. We believe that this approach could be used to assemble a range of high performance 3D electronic and optical devices of relevance to sensing, diagnostics, wearables, and energy harvesting.

  9. 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.

  10. 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.

  11. Molecular dynamics modelling of EGCG clusters on ceramide bilayers

    SciTech Connect

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

    2015-12-31

    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.

  12. Automatable lipid bilayer formation for ion channel studies

    NASA Astrophysics Data System (ADS)

    Poulos, Jason L.; Bang, Hyunwoo; Jeon, Tae-Joon; Schmidt, Jacob J.

    2008-08-01

    Transmembrane proteins and ion channels are important drug targets and have been explored as single molecule sensors. For these proteins to function normally they must be integrated within lipid bilayers; however, the labor and skill required to create artificial lipid bilayers have the limited the possible applications utilizing these proteins. In order to reduce the complexity and cost of lipid bilayer formation and measurement, we have modified a previously published lipid bilayer formation technique using mechanically contacted monolayers so that the process is automatable, requiring minimal operator input. Measurement electronics are integrated with the fluid handling system, greatly reducing the time and operator feedback characteristically required of traditional bilayer experiments. To demonstrate the biological functionality of the resultant bilayers and the system's capabilities as a membrane platform, the ion channel gramicidin A was incorporated and measured with this system.

  13. Bilayer splitting and c-axis coupling in CMR bilayer manganites

    SciTech Connect

    Jozwiak, Chris; Graf, Jeff; Zhou, Shuyun; Bostwick, Aaron; Rotenberg, Eli; Zheng, Hong; Mitchell, John; Lanzara, Alessandra

    2009-09-03

    By performing angle-resolved photoemission spectroscopy of the bilayer colossal magnetoresistive (CMR) manganite, La2-2xSr1+2xMn2O7, we provide the complete mapping of the Fermi-level spectral weight topology. Clear and unambiguous bilayer splitting of the in-plane 3dx2-y2 band, mapped throughout the Brillouin zone, and the full mapping of the 3d3z2-r2 band are reported. Peculiar doping and temperature dependencies of these bands imply that as transition from the ferromagnetic metallic phase approaches, either as a function of doping or temperature, coherence along the c-axis between planes within the bilayer is lost, resulting in reduced interplane coupling. These results suggest that interplane coupling plays a large role in the CMR transition.

  14. Intercalation of small hydrophobic molecules in lipid bilayers containing cholesterol

    SciTech Connect

    Worcester, D.L.; Hamacher, K.; Kaiser, H.; Kulasekere, R.; Torbet, J.

    1994-12-31

    Partitioning of small hydrophobic molecules into lipid bilayers containing cholesterol has been studied using the 2XC diffractometer at the University of Missouri Research Reactor. Locations of the compounds were determined by Fourier difference methods with data from both deuterated and undeuterated compounds introduced into the bilayers from the vapor phase. Data fitting procedures were developed for determining how well the compounds were localized. The compounds were found to be localized in a narrow region at the center of the hydrophobic layer, between the two halves of the bilayer. The structures are therefore intercalated structures with the long axis of the molecules in the plane of the bilayer.

  15. Functional One-Dimensional Lipid Bilayers on Carbon Nanotube Templates

    SciTech Connect

    Artyukhin, A; Shestakov, A; Harper, J; Bakajin, O; Stroeve, P; Noy, A

    2004-07-23

    We present one-dimensional (1-D) lipid bilayer structures that integrate carbon nanotubes with a key biological environment-phospholipid membrane. Our structures consist of lipid bilayers wrapped around carbon nanotubes modified with a hydrophilic polymer cushion layer. Despite high bilayer curvature, the lipid membrane maintains its fluidity and can sustain repeated damage-recovery cycles. We also present the first evidence of spontaneous insertion of pore-forming proteins into 1-D lipid bilayers. These structures could lead to the development of new classes of biosensors and bioelectronic devices.

  16. Incubation of MDCO-216 (ApoA-IMilano/POPC) with Human Serum Potentiates ABCA1-Mediated Cholesterol Efflux Capacity, Generates New Prebeta-1 HDL, and Causes an Increase in HDL Size.

    PubMed

    Kempen, Herman J; Schranz, Dorota B; Asztalos, Bela F; Otvos, James; Jeyarajah, Elias; Drazul-Schrader, Denise; Collins, Heidi L; Adelman, Steven J; Wijngaard, Peter L J

    2014-01-01

    MDCO-216 is a complex of dimeric ApoA-IMilano and palmitoyl oleoyl phosphatidylcholine (POPC), previously shown to reduce atherosclerotic plaque burden. Here we studied the effect of incubation of human plasma or serum with MDCO-216 on cholesterol efflux capacity from J774 cells, on prebeta-1 high density lipoprotein (prebeta-1 HDL) and on HDL size assessed by proton nuclear magnetic resonance ((1)H-NMR). MDCO-216 incubated in buffer containing 4% human serum albumin stimulated both ABCA1-mediated efflux and ABCA1-independent cholesterol efflux from J774 macrophages. When incubated with human serum a dose- and time-dependent synergistic increase of the ABCA1-mediated efflux capacity were observed. Using a commercially available ELISA for prebeta-1 HDL, MDCO-216 as such was poorly detected (12-15% of nominal amount of protein). Prebeta-1 HDL was rapidly lost when human plasma alone is incubated at 37°C. In contrast, incubation of human plasma with MDCO-216 at 37°C produced a large amount of new prebeta-1 HDL. Native 2D electrophoresis followed by immunoblotting with an apoA-I antibody, which also detects ApoA-I Milano, confirmed the increase in prebeta-1 HDL upon incubation at 37°C. With the increase of prebeta-1 HDL, the concomitant disappearance of the small alpha-3 and alpha-4 HDL and MDCO-216 and an increase in the large alpha-1 and alpha-2 HDL were observed. Immunoblotting with Mab 17F3 specific for ApoA-I Milano showed the appearance of ApoA-I Milano in alpha-1 and alpha-2, but not in prebeta-1 HDL. (1)H-NMR analysis of plasma incubated with MDCO-216 confirmed rapid disappearance of small-sized HDL particles and increase of medium- and large-sized HDL particles accompanied with a decrease in total HDL particle number. In conclusion, incubation of human plasma or serum with MDCO-216 strongly enhanced ABCA1-mediated cholesterol efflux, caused a strong increase of prebeta-1 HDL, and drastically changed the distribution of HDL subpopulations. Overall, the

  17. Incubation of MDCO-216 (ApoA-IMilano/POPC) with Human Serum Potentiates ABCA1-Mediated Cholesterol Efflux Capacity, Generates New Prebeta-1 HDL, and Causes an Increase in HDL Size

    PubMed Central

    Schranz, Dorota B.; Asztalos, Bela F.; Otvos, James; Drazul-Schrader, Denise; Wijngaard, Peter L. J.

    2014-01-01

    MDCO-216 is a complex of dimeric ApoA-IMilano and palmitoyl oleoyl phosphatidylcholine (POPC), previously shown to reduce atherosclerotic plaque burden. Here we studied the effect of incubation of human plasma or serum with MDCO-216 on cholesterol efflux capacity from J774 cells, on prebeta-1 high density lipoprotein (prebeta-1 HDL) and on HDL size assessed by proton nuclear magnetic resonance (1H-NMR). MDCO-216 incubated in buffer containing 4% human serum albumin stimulated both ABCA1-mediated efflux and ABCA1-independent cholesterol efflux from J774 macrophages. When incubated with human serum a dose- and time-dependent synergistic increase of the ABCA1-mediated efflux capacity were observed. Using a commercially available ELISA for prebeta-1 HDL, MDCO-216 as such was poorly detected (12–15% of nominal amount of protein). Prebeta-1 HDL was rapidly lost when human plasma alone is incubated at 37°C. In contrast, incubation of human plasma with MDCO-216 at 37°C produced a large amount of new prebeta-1 HDL. Native 2D electrophoresis followed by immunoblotting with an apoA-I antibody, which also detects ApoA-I Milano, confirmed the increase in prebeta-1 HDL upon incubation at 37°C. With the increase of prebeta-1 HDL, the concomitant disappearance of the small alpha-3 and alpha-4 HDL and MDCO-216 and an increase in the large alpha-1 and alpha-2 HDL were observed. Immunoblotting with Mab 17F3 specific for ApoA-I Milano showed the appearance of ApoA-I Milano in alpha-1 and alpha-2, but not in prebeta-1 HDL. 1H-NMR analysis of plasma incubated with MDCO-216 confirmed rapid disappearance of small-sized HDL particles and increase of medium- and large-sized HDL particles accompanied with a decrease in total HDL particle number. In conclusion, incubation of human plasma or serum with MDCO-216 strongly enhanced ABCA1-mediated cholesterol efflux, caused a strong increase of prebeta-1 HDL, and drastically changed the distribution of HDL subpopulations. Overall, the results

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

    PubMed

    Taylor, Graham J; Venkatesan, Guru A; Collier, C Patrick; Sarles, Stephen A

    2015-10-14

    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 for 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.

  19. Effect of monoglycerides and fatty acids on a ceramide bilayer.

    PubMed

    Akinshina, Anna; Das, Chinmay; Noro, Massimo G

    2016-07-14

    Monoglycerides and unsaturated fatty acids, naturally present in trace amounts in the stratum corneum (top layer of skin) lipid matrix, are commonly used in pharmaceutical, cosmetic and health care formulations. However, a detailed molecular understanding of how the oil additives get incorporated into the skin lipids from topical application and, once incorporated, how they affect the properties and integrity of the lipid matrix remains unexplored. Using ceramide 2 bilayers as skin lipid surrogates, we use a series of molecular dynamics simulations with six different natural oil ingredients at multiple concentrations to investigate the effect of the oils on the properties and stability of the bilayers. The six oils: monoolein, monostearin, monoelaidin, oleic acid, stearic acid and linoleic acid - all having the same length of the alkyl chain, C18, but a varying degree of saturation, allow us to systematically address the effect of unsaturation in the additives. Our results show that at low oil concentration (∼5%) the mixed bilayers containing any of the oils and ceramide 2 (CER2) become more rigid than pure CER2 bilayers due to more efficient lipid packing. Better packing also results in the formation of larger numbers of hydrogen bonds between the lipids, which occurs at the expense of the hydrogen bonds between lipids and water. The mixed bilayers with saturated or trans-unsaturated oils remain stable over the whole range of oil concentration. In contrast, the presence of the oils with at least one cis-double bond leads to bilayer instability and complete loss of bilayer structure at the oil content of about 50-65%. Two cis-double bonds in the lipid tail induce bilayer disruption at even lower concentration (∼30%). The mixed bilayers remain in the gel phase (without melting to a fluid phase) until the phase transition to a non-bilayer phase occurs. We also demonstrate that the stability of the bilayer strongly correlates with the order parameter of the lipid

  20. Bilayer-thickness-mediated interactions between integral membrane proteins.

    PubMed

    Kahraman, Osman; Koch, Peter D; Klug, William S; Haselwandter, Christoph A

    2016-04-01

    Hydrophobic thickness mismatch between integral membrane proteins and the surrounding lipid bilayer can produce lipid bilayer thickness deformations. Experiment and theory have shown that protein-induced lipid bilayer thickness deformations can yield energetically favorable bilayer-mediated interactions between integral membrane proteins, and large-scale organization of integral membrane proteins into protein clusters in cell membranes. Within the continuum elasticity theory of membranes, the energy cost of protein-induced bilayer thickness deformations can be captured by considering compression and expansion of the bilayer hydrophobic core, membrane tension, and bilayer bending, resulting in biharmonic equilibrium equations describing the shape of lipid bilayers for a given set of bilayer-protein boundary conditions. Here we develop a combined analytic and numerical methodology for the solution of the equilibrium elastic equations associated with protein-induced lipid bilayer deformations. Our methodology allows accurate prediction of thickness-mediated protein interactions for arbitrary protein symmetries at arbitrary protein separations and relative orientations. We provide exact analytic solutions for cylindrical integral membrane proteins with constant and varying hydrophobic thickness, and develop perturbative analytic solutions for noncylindrical protein shapes. We complement these analytic solutions, and assess their accuracy, by developing both finite element and finite difference numerical solution schemes. We provide error estimates of our numerical solution schemes and systematically assess their convergence properties. Taken together, the work presented here puts into place an analytic and numerical framework which allows calculation of bilayer-mediated elastic interactions between integral membrane proteins for the complicated protein shapes suggested by structural biology and at the small protein separations most relevant for the crowded membrane

  1. Effect of monoglycerides and fatty acids on a ceramide bilayer.

    PubMed

    Akinshina, Anna; Das, Chinmay; Noro, Massimo G

    2016-07-14

    Monoglycerides and unsaturated fatty acids, naturally present in trace amounts in the stratum corneum (top layer of skin) lipid matrix, are commonly used in pharmaceutical, cosmetic and health care formulations. However, a detailed molecular understanding of how the oil additives get incorporated into the skin lipids from topical application and, once incorporated, how they affect the properties and integrity of the lipid matrix remains unexplored. Using ceramide 2 bilayers as skin lipid surrogates, we use a series of molecular dynamics simulations with six different natural oil ingredients at multiple concentrations to investigate the effect of the oils on the properties and stability of the bilayers. The six oils: monoolein, monostearin, monoelaidin, oleic acid, stearic acid and linoleic acid - all having the same length of the alkyl chain, C18, but a varying degree of saturation, allow us to systematically address the effect of unsaturation in the additives. Our results show that at low oil concentration (∼5%) the mixed bilayers containing any of the oils and ceramide 2 (CER2) become more rigid than pure CER2 bilayers due to more efficient lipid packing. Better packing also results in the formation of larger numbers of hydrogen bonds between the lipids, which occurs at the expense of the hydrogen bonds between lipids and water. The mixed bilayers with saturated or trans-unsaturated oils remain stable over the whole range of oil concentration. In contrast, the presence of the oils with at least one cis-double bond leads to bilayer instability and complete loss of bilayer structure at the oil content of about 50-65%. Two cis-double bonds in the lipid tail induce bilayer disruption at even lower concentration (∼30%). The mixed bilayers remain in the gel phase (without melting to a fluid phase) until the phase transition to a non-bilayer phase occurs. We also demonstrate that the stability of the bilayer strongly correlates with the order parameter of the lipid

  2. How antimicrobial peptides disrupt lipid bilayers?

    NASA Astrophysics Data System (ADS)

    Sengupta, Durba

    2011-03-01

    The molecular basis for the activity of cyclic and linear antimicrobial peptides is analysed. We performed multi-scale molecular dynamics simulations and biophysical measurements to probe the interaction of antimicrobial peptides with model membranes. Two linear antimicrobial peptides, magainin and melittin and a cyclic one, BPC194 have been studied. We test different models to determine the generic and specific forces that lead to bilayer disruption. We probe whether interfacial stress or local membrane perturbation is more likely to lead to the porated state. We further analyse the reasons that determine specificity and increase of activity in antimicrobial peptides. The results provide detailed insight in the mode of action of antimicrobial peptides.

  3. Bilayer avalanche spin-diode logic

    SciTech Connect

    Friedman, Joseph S. Querlioz, Damien; Fadel, Eric R.; Wessels, Bruce W.; Sahakian, Alan V.

    2015-11-15

    A novel spintronic computing paradigm is proposed and analyzed in which InSb p-n bilayer avalanche spin-diodes are cascaded to efficiently perform complex logic operations. This spin-diode logic family uses control wires to generate magnetic fields that modulate the resistance of the spin-diodes, and currents through these devices control the resistance of cascaded devices. Electromagnetic simulations are performed to demonstrate the cascading mechanism, and guidelines are provided for the development of this innovative computing technology. This cascading scheme permits compact logic circuits with switching speeds determined by electromagnetic wave propagation rather than electron motion, enabling high-performance spintronic computing.

  4. Self-assembled lipid bilayer materials

    DOEpatents

    Sasaki, Darryl Y.; Waggoner, Tina A.; Last, Julie A.

    2005-11-08

    The present invention is a self-assembling material comprised of stacks of lipid bilayers formed in a columnar structure, where the assembly process is mediated and regulated by chemical recognition events. The material, through the chemical recognition interactions, has a self-regulating system that corrects the radial size of the assembly creating a uniform diameter throughout most of the structure. The materials form and are stable in aqueous solution. These materials are useful as structural elements for the architecture of materials and components in nanotechnology, efficient light harvesting systems for optical sensing, chemical processing centers, and drug delivery vehicles.

  5. Collective Excitations in Electron-Hole Bilayers

    SciTech Connect

    Kalman, G. J.; Hartmann, P.; Donko, Z.; Golden, K. I.

    2007-06-08

    We report a combined analytic and molecular dynamics analysis of the collective mode spectrum of a bipolar (electron-hole) bilayer in the strong coupling classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles. In the in-phase spectra we identify longitudinal and transverse acoustic modes wholly maintained by correlations. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed.

  6. Rich Polymorphic Behavior of Wigner Bilayers.

    PubMed

    Antlanger, Moritz; Kahl, Gerhard; Mazars, Martial; Šamaj, Ladislav; Trizac, Emmanuel

    2016-09-01

    Self-assembly into target structures is an efficient material design strategy. Combining analytical calculations and computational techniques of evolutionary and Monte Carlo types, we report about a remarkable structural variability of Wigner bilayer ground states, when charges are confined between parallel charged plates. Changing the interlayer separation, or the plate charge asymmetry, a cascade of ordered patterns emerges. At variance with the symmetric case phenomenology, the competition between commensurability features and charge neutralization leads to long range attraction, appearance of macroscopic charges, exotic phases, and nonconventional phase transitions with distinct critical indices, offering the possibility of a subtle, but precise and convenient control over patterns. PMID:27661720

  7. Bilayer avalanche spin-diode logic

    NASA Astrophysics Data System (ADS)

    Friedman, Joseph S.; Fadel, Eric R.; Wessels, Bruce W.; Querlioz, Damien; Sahakian, Alan V.

    2015-11-01

    A novel spintronic computing paradigm is proposed and analyzed in which InSb p-n bilayer avalanche spin-diodes are cascaded to efficiently perform complex logic operations. This spin-diode logic family uses control wires to generate magnetic fields that modulate the resistance of the spin-diodes, and currents through these devices control the resistance of cascaded devices. Electromagnetic simulations are performed to demonstrate the cascading mechanism, and guidelines are provided for the development of this innovative computing technology. This cascading scheme permits compact logic circuits with switching speeds determined by electromagnetic wave propagation rather than electron motion, enabling high-performance spintronic computing.

  8. Rich Polymorphic Behavior of Wigner Bilayers

    NASA Astrophysics Data System (ADS)

    Antlanger, Moritz; Kahl, Gerhard; Mazars, Martial; Šamaj, Ladislav; Trizac, Emmanuel

    2016-09-01

    Self-assembly into target structures is an efficient material design strategy. Combining analytical calculations and computational techniques of evolutionary and Monte Carlo types, we report about a remarkable structural variability of Wigner bilayer ground states, when charges are confined between parallel charged plates. Changing the interlayer separation, or the plate charge asymmetry, a cascade of ordered patterns emerges. At variance with the symmetric case phenomenology, the competition between commensurability features and charge neutralization leads to long range attraction, appearance of macroscopic charges, exotic phases, and nonconventional phase transitions with distinct critical indices, offering the possibility of a subtle, but precise and convenient control over patterns.

  9. Supported Lipid Bilayer/Carbon Nanotube Hybrids

    NASA Astrophysics Data System (ADS)

    Zhou, Xinjian; Moran-Mirabal, Jose; Craighead, Harold; McEuen, Paul

    2007-03-01

    We form supported lipid bilayers on single-walled carbon nanotubes and use this hybrid structure to probe the properties of lipid membranes and their functional constituents. We first demonstrate membrane continuity and lipid diffusion over the nanotube. A membrane-bound tetanus toxin protein, on the other hand, sees the nanotube as a diffusion barrier whose strength depends on the diameter of the nanotube. Finally, we present results on the electrical detection of specific binding of streptavidin to biotinylated lipids with nanotube field effect transistors. Possible techniques to extract dynamic information about the protein binding events will also be discussed.

  10. Giant magnetoresistance in bilayer graphene nanoflakes

    NASA Astrophysics Data System (ADS)

    Farghadan, Rouhollah; Farekiyan, Marzieh

    2016-09-01

    Coherent spin transport through bilayer graphene (BLG) nanoflakes sandwiched between two electrodes made of single-layer zigzag graphene nanoribbon was investigated by means of Landauer-Buttiker formalism. Application of a magnetic field only on BLG structure as a channel produces a perfect spin polarization in a large energy region. Moreover, the conductance could be strongly modulated by magnetization of the zigzag edge of AB-stacked BLG, and the junction, entirely made of carbon, produces a giant magnetoresistance (GMR) up to 100%. Intestinally, GMR and spin polarization could be tuned by varying BLG width and length. Generally, MR in a AB-stacked BLG strongly increases (decreases) with length (width).

  11. Twisted bi-layer graphene: microscopic rainbows.

    PubMed

    Campos-Delgado, J; Algara-Siller, G; Santos, C N; Kaiser, U; Raskin, J-P

    2013-10-11

    Blue, pink, and yellow colorations appear from twisted bi-layer graphene (tBLG) when transferred to a SiO2 /Si substrate (SiO2 = 100 nm-thick). Raman and electron microscope studies reveal that these colorations appear for twist angles in the 9-15° range. Optical contrast simulations confirm that the observed colorations are related to the angle-dependent electronic properties of tBLG combined with the reflection that results from the layered structure tBLG/100 nm-thick SiO2 /Si. PMID:23606323

  12. Effects of ion interactions with a cholesterol-rich bilayer.

    PubMed

    Mao, Lingxue; Yang, Linlin; Zhang, Qiansen; Jiang, Hualiang; Yang, Huaiyu

    Previous molecular dynamics (MD) simulations of ion-lipid interactions have focused on pure phospholipid bilayers. Many functional microdomains in membranes have a complex composition of cholesterol and phospholipids. Here, we reveal the distinctiveness of the interactions and the effects of the ions on a cholesterol-rich bilayer by performing MD simulations of a cholesterol-rich bilayer with a Na(+)/K(+) mixture or a Na(+)/K(+)/Ca(2+)/Mg(2+) mixture. The simulations reveal that Ca(2+) maintains its dominant role in the interaction with the cholesterol-rich bilayer, but the binding affinity of Mg(2+) to the cholesterol-rich bilayer is even weaker than the affinities of Na(+) and K(+), whereas its interaction with pure phospholipid bilayers is strong and is only slightly weaker than that of Ca(2+). Additionally, it was found that the presence of additional divalent cations induces the headgroups of phospholipids to be more perpendicular to the membrane surface, reducing the lateral movement of lipids and slightly altering the ordering and packing of the cholesterol-rich bilayer, different from divalent cations, which strongly influence that ordering and packing of pure phospholipid bilayers. Therefore, this study indicates that cholesterol in the membrane could affect the interactions between membrane and cations. The findings could be helpful in understanding the biological processes relevant to regulation of cations in cholesterol-rich regions.

  13. Fabrication of a planar zwitterionic lipid bilayer on titanium oxide.

    PubMed

    Cho, Nam-Joon; Frank, Curtis W

    2010-10-19

    There is great demand to fabricate planar phospholipid bilayers on biocompatible materials. The preferred method of forming bilayers on these substrates is the spontaneous adsorption and rupture of phospholipid vesicles. However, in the case of titanium oxide, model vesicles composed solely of zwitterionic phospholipids do not follow this self-assembly pathway under physiological conditions, prompting the use of complex bilayer materials and less-facile methods. Herein, we report a novel pH-based strategy for fabricating zwitterionic bilayers on titanium oxide in a simple and robust manner. Depending on the pH conditions under which lipid vesicles adsorb onto titanium oxide, quartz crystal microbalance-dissipation (QCM-D) monitoring demonstrated that the self-assembly pathway can in fact result in planar bilayer formation. The pH of the solution could then be adjusted to physiological levels with no effect on the mass and viscoelastic properties of the bilayer. Moreover, fluorescence recovery after photobleaching (FRAP) measurements indicated a high degree of lateral lipid diffusivity within the bilayer at physiological pH, commensurate with its role as a cell membrane mimic. Compared to existing protocols, this strategy permits the fabrication of a more diverse array of planar bilayers on titanium oxide by tuning the self-assembly pathway of lipid vesicle adsorption onto solid substrates.

  14. Asymmetric heat transfer from nanoparticles in lipid bilayers

    NASA Astrophysics Data System (ADS)

    Potdar, Dipti; Sammalkorpi, Maria

    2015-12-01

    Here, we use molecular dynamics simulations to characterize the heat transfer properties of lipid bilayer - gold nanoparticle systems in which the nanoparticle acts as a heat source. The focus is on dipalmitoylphosphatidylcholine (DPPC) lipid bilayers and thiolated alcohol and alkyl functionalized nanoparticles as prototype hydrophilic and hydrophobic nanoparticles. We find hydrophilic nanoparticles which are partly in contact with the surrounding water environment are more efficient in transferring heat to the system than hydrophobic ones which reside surrounded by the membrane. This is because of the hydrogen bonding capability of the hydroxy pentanethiol and the more efficient heat conductivity through water than the lipid bilayer. Additionally, we find the heat conductance is strongly asymmetric and has a discontinuity between the bilayer leaflets. In total, the findings provide understanding on heat transport from localized heat sources in lipid bilayers and could bear significance, e.g., in engineering and controlling photoactivated triggering of liposomal systems.

  15. Lipid mobility in supported lipid bilayers by single molecule tracking

    NASA Astrophysics Data System (ADS)

    Kohram, Maryam; Shi, Xiaojun; Smith, Adam

    2015-03-01

    Phospholipid bilayers are the main component of cell membranes and their interaction with biomolecules in their immediate environment is critical for cellular functions. These interactions include the binding of polycationic polymers to lipid bilayers which affects many cell membrane events. As an alternative method of studying live cell membranes, we assemble a supported lipid bilayer and investigate its binding with polycationic polymers in vitro by fluorescently labeling the molecules of the supported lipid bilayer and tracking their mobility. In this work, we use single molecule tracking total internal reflection fluorescence microscopy (TIRF) to study phosphatidylinositol phosphate (PIP) lipids with and without an adsorbed polycationic polymer, quaternized polyvinylpyridine (QPVP). Individual molecular trajectories are obtained from the experiment, and a Brownian diffusion model is used to determine diffusion coefficients through mean square displacements. Our results indicate a smaller diffusion coefficient for the supported lipid bilayers in the presence of QPVP in comparison to its absence, revealing that their binding causes a decrease in lateral mobility.

  16. Surface Viscosity, Diffusion, and Intermonolayer Friction: Simulating Sheared Amphiphilic Bilayers

    PubMed Central

    Shkulipa, S. A.; den Otter, W. K.; Briels, W. J.

    2005-01-01

    The flow properties of an amphiphilic bilayer are studied in molecular dynamics simulations, by exposing a coarse grained model bilayer to two shear flows directed along the bilayer surface. The first field, with a vorticity perpendicular to the bilayer, induces a regular shear deformation, allowing a direct calculation of the surface viscosity. In experiments this property is measured indirectly, by relating it to the diffusion coefficient of a tracer particle through the Saffman-Einstein expression. The current calculations provide an independent test of this relation. The second flow field, with a vorticity parallel to the bilayer, causes the two constituent monolayers to slide past one another, yielding the interlayer friction coefficient. PMID:15894643

  17. Band gap opening in bilayer silicene by alkali metal intercalation.

    PubMed

    Liu, Hongsheng; Han, Nannan; Zhao, Jijun

    2014-11-26

    Recently, bilayer and multilayer silicene have attracted increased attention following the boom of silicene, which holds great promise for future applications in microelectronic devices. Herein we systematically investigate all stacking configurations of bilayer silicene and the corresponding electronic properties. Strong coupling is found between two silicene layers, which destroys the Dirac cones in the band structures of pristine silicene and makes bilayer silicene sheets metallic. However, intercalation of alkali metal (especially potassium) can effectively decouple the interaction between two silicene layers. In the K-intercalated bilayer silicene (KSi4), the Dirac cones are recovered with a small band gap of 0.27 eV located about 0.55 eV below the Fermi level. Furthermore, intercalation of K(+) cations in bilayer silicene (K(+)Si4) results in a semiconductor with a moderate band gap of 0.43 eV, making it ideal for microelectronic applications.

  18. Millimeter-area, free standing, phospholipid bilayers.

    PubMed

    Beltramo, Peter J; Van Hooghten, Rob; Vermant, Jan

    2016-05-11

    Minimal model biomembrane studies have the potential to unlock the fundamental mechanisms of cellular function that govern the processes upon which life relies. However, existing methods to fabricate free-standing model membranes currently have significant limitations. Bilayer sizes are often tens of micrometers, decoupling curvature or substrate effects, orthogonal control over tension, and solvent exchange combined with microscopy techniques is not possible, which restricts the studies that can be performed. Here, we describe a versatile platform to generate free standing, planar, phospholipid bilayers with millimeter scale areas. The technique relies on an adapted thin-film balance apparatus allowing for the dynamic control of the nucleation and growth of a planar black lipid membrane in the center of an orifice surrounded by microfluidic channels. Success is demonstrated using several different lipid types, including mixtures that show the same temperature dependent phase separation as existing protocols, moreover, membranes are highly stable. Two advantages unique to the proposed method are the dynamic control of the membrane tension and the possibility to make extremely large area membranes. We demonstrate this by showing how a block polymer, F68, used in drug delivery increases the membrane compliance. Together, the results demonstrate a new paradigm for studying the mechanics, structure, and function of model membranes.

  19. The electron-hole bilayer tunnel FET

    NASA Astrophysics Data System (ADS)

    Lattanzio, Livio; De Michielis, Luca; Ionescu, Adrian M.

    2012-08-01

    We propose a novel tunnel field-effect transistor (TFET) concept called the electron-hole bilayer TFET (EHBTFET). This device exploits the carrier tunneling through a bias-induced electron-hole bilayer in order to achieve improved switching and higher drive currents when compared to a lateral p-i-n junction TFET. The device principle and performances are studied by 2D numerical simulations. Output and transfer characteristics, as well as the impact of back gate bias, silicon thickness and gate length on the device behavior are evaluated. Device performances are compared for Si and Ge implementations. Nearly ideal average subthreshold slope (SSAVG ∼ 10 mV/dec over 7 decades of current) and ION/IOFF > 108 at VD = VG = 0.5 V are obtained, due to the OFF-ON binary transition which leads to the abrupt onset of the band-to-band tunneling inside the semiconductor channel. Remarkably, for Ge EHBTFETs the ION (∼11 μA/μm at VDD = 0.5 V) is 10× larger than in Ge tunnel FETs and 380× larger than in Si EHBTFETs.

  20. Engineering Lipid Bilayer Membranes for Protein Studies

    PubMed Central

    Khan, Muhammad Shuja; Dosoky, Noura Sayed; Williams, John Dalton

    2013-01-01

    Lipid membranes regulate the flow of nutrients and communication signaling between cells and protect the sub-cellular structures. Recent attempts to fabricate artificial systems using nanostructures that mimic the physiological properties of natural lipid bilayer membranes (LBM) fused with transmembrane proteins have helped demonstrate the importance of temperature, pH, ionic strength, adsorption behavior, conformational reorientation and surface density in cellular membranes which all affect the incorporation of proteins on solid surfaces. Much of this work is performed on artificial templates made of polymer sponges or porous materials based on alumina, mica, and porous silicon (PSi) surfaces. For example, porous silicon materials have high biocompatibility, biodegradability, and photoluminescence, which allow them to be used both as a support structure for lipid bilayers or a template to measure the electrochemical functionality of living cells grown over the surface as in vivo. The variety of these media, coupled with the complex physiological conditions present in living systems, warrant a summary and prospectus detailing which artificial systems provide the most promise for different biological conditions. This study summarizes the use of electrochemical impedance spectroscopy (EIS) data on artificial biological membranes that are closely matched with previously published biological systems using both black lipid membrane and patch clamp techniques. PMID:24185908

  1. Controlling the Electronic Structure of Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Ohta, Taisuke; Bostwick, Aaron; McChesney, Jessica; Seyller, Thomas; Horn, Karsten; Rotenberg, Eli

    2007-03-01

    Carbon-based materials such as carbon nanotubes, graphite intercalation compounds, fullerenes, and ultrathin graphite films exhibit many exotic phenomena such as superconductivity and an anomalous quantum Hall effect. These findings have caused renewed interest in the electronic structure of ultrathin layers of graphene: a single honeycomb carbon layer that is the building block for these materials. There is a strong motivation to incorporate graphene multilayers into atomic-scale devices, spurred on by rapid progress in their fabrication and manipulation. We have synthesized bilayer graphene thin films deposited on insulating silicon carbide and characterized their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands [1]. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic scale electronic devices. [1] T. Ohta, A. Bostwick, T. Seyller, K. Horn, E. Rotenberg, Science, 313, 951 (2006).

  2. Millimeter-area, free standing, phospholipid bilayers.

    PubMed

    Beltramo, Peter J; Van Hooghten, Rob; Vermant, Jan

    2016-05-11

    Minimal model biomembrane studies have the potential to unlock the fundamental mechanisms of cellular function that govern the processes upon which life relies. However, existing methods to fabricate free-standing model membranes currently have significant limitations. Bilayer sizes are often tens of micrometers, decoupling curvature or substrate effects, orthogonal control over tension, and solvent exchange combined with microscopy techniques is not possible, which restricts the studies that can be performed. Here, we describe a versatile platform to generate free standing, planar, phospholipid bilayers with millimeter scale areas. The technique relies on an adapted thin-film balance apparatus allowing for the dynamic control of the nucleation and growth of a planar black lipid membrane in the center of an orifice surrounded by microfluidic channels. Success is demonstrated using several different lipid types, including mixtures that show the same temperature dependent phase separation as existing protocols, moreover, membranes are highly stable. Two advantages unique to the proposed method are the dynamic control of the membrane tension and the possibility to make extremely large area membranes. We demonstrate this by showing how a block polymer, F68, used in drug delivery increases the membrane compliance. Together, the results demonstrate a new paradigm for studying the mechanics, structure, and function of model membranes. PMID:27050618

  3. Theoretical study on stability of hybrid bilayers

    NASA Astrophysics Data System (ADS)

    Silva, Thiago S.; de Lima Bernardo, Bertúlio; Azevedo, Sèrgio

    2015-04-01

    Motivated by the recent experimental realization of the hybrid nanostructure of graphene and boron nitride (h-BN) sheet, and studies of gap modulation by strain, we use first principles calculations based on density functional theory to investigate the effects of strain in hybrid bilayers composed of two monolayers of graphene with a nanodomain of {{B}3}{{N}3}. The calculations were made with two different approximations for the functional exchange-correlation, GGA and VDW-DF. We investigate the modification in the electronic structure and structural properties of various configurations of the hybrid bilayers. Among the configurations, those with Bernal stacking are found to be more stable when compared to the others. Studies of the compressive strain influence were made only in the structure that has been shown to be the most stable. We have found that the two approximations used in the calculations exhibit the same results for the electronic properties of all structures. The opening of the energy gap due to strain was possible in the calculations by using the GGA approximation, but the same does not happen in the calculations using the VDW-DF approximation. Our analysis shows that the VDW-DF approximation is better suited for studies involving surfaces.

  4. Phase transitions in planar bilayer membranes.

    PubMed

    White, S H

    1975-02-01

    Temperature-dependent structural changes in planar bilayer membranes formed from glycerol monooleate (GMO) dispersed in various n-alkane solvents (C12-C17) have been studied using precise measurements of specific geometric capacitance (Cg). Cg generally increases as temperature (T) decreases. A change in the slope of Cg(T) occurs between 15 and 18 degrees C for all solvent systems examined. Measurements of the interfacial tension (gamma) of the bulk GMO-alkane dispersions against 0.1 M NaCl show that gamma generally decreases with decreasing temperature. The data can be fitted with two straight lines of different slope which intersect on the average at 17 degrees C. Pagano et al. (1973, Science (Wash. D.C.). 181:557) have shown using calorimetry that GMO has a phase transition at about 15 degrees C. Thus, the changes in Cg and gamma with temperature are likely to result from a GMO phase transition. A second structural change is observed to occur between 5 and 10 degrees C which has not been detected calorimetrically. Calculations of Cg based on various estimates of the hydrocarbon dielectric coefficient (epsilon-b) and/or hydrocarbon thickness (delta-b) leads to models for the structure of the bilayer above and below the phase transition temperature.

  5. Phospholipid surface bilayers at the air-water interface. II. Water permeability of dimyristoylphosphatidylcholine surface bilayers.

    PubMed Central

    Ginsberg, L; Gershfeld, N L

    1985-01-01

    Dispersions of dimyristoylphosphatidylcholine (DMPC) in water have been reported to form a structure at 29 degrees C at the equilibrium air/water surface with a molecular density equal to that of a typical bilayer. In this study, the water permeability of this structure has been evaluated by measuring the rate of water evaporation from DMPC dispersions in water in the temperature range where the surface film density exceeds that of a monolayer. Evaporation rates for the lipid dispersions did not deviate from those for lipid-free systems throughout the entire temperature range examined (20-35 degrees C) except at 29 degrees C, where a barrier to evaporation was detected. This strengthens the view that the structure that forms at this temperature has the properties of a typical bilayer. PMID:3978199

  6. The electrical response of bilayers to the bee venom toxin melittin: Evidence for transient bilayer permeabilization

    PubMed Central

    Wiedman, Gregory; Herman, Katherine; Searson, Peter; Wimley, William C.; Hristova, Kalina

    2014-01-01

    Melittin is a 26-residue bee venom peptide that folds into amphipathic α-helix and causes membrane permeabilization via a mechanism that is still disputed. While an equilibrium transmembrane pore model has been a central part of the mechanistic dialogue for decades, there is growing evidence that a transmembrane pore is not required for melittin’s activity. In part, the controversy is due to limited experimental tools to probe the bilayer’s response to melittin. Electrochemical impedance spectroscopy (EIS) is a technique that can reveal details of molecular mechanism of peptide activity, as it yields direct, real-time measurements of membrane resistance and capacitance of supported bilayers. In this work, EIS was used in conjunction with vesicle leakage studies to characterize the response of bilayers of different lipid compositions to melittin. Experiments were carried out at low peptide to lipid ratios between 1:5000 and 1:100. The results directly demonstrate that the response of the bilayer to melittin at these concentrations cannot be explained by an equilibrium transmembrane pore model. PMID:23384418

  7. Controlled permeability of phospholipid bilayers and bilayer-templated polymer nanocapsules

    NASA Astrophysics Data System (ADS)

    Danila, Delia Cezara

    The work presented focuses on the study of the permeability of the phospholipid bilayer membranes of liposomes---a model of biological membranes---by the use of carrier molecules as well as on the study of the permeability of polymer nanocapsules when using liposomes as a template for their synthesis. The results of this work can lead to the development of novel biomedical devices and to the development of novel classes of polymer nanomaterials. A series of carrier molecules, alpha-aminophosphonates, was synthesized in order to transport a polar molecule, an amino acid (alanine), across the lipid bilayer of synthetic liposomal membranes. Aminophosphonates were prepared by the Kabachnik-Fields reaction from a primary amine, a phosphite, and a carbonyl compound. Transport of alanine, a model polar molecule, was studied by liposome efflux experiments coupled with enzymatic assays. All carriers exhibited moderate transport properties, which makes them suitable for construction of controlled released devices. A second project involved the use of liposomes as a template for the synthesis of polymer nanocapsules. Porous polymer nanocapsules were prepared by co-dissolving hydrophobic monomers and porogens in the interior of bilayer membranes followed by polymerization and subsequent removal of lipid scaffold and porogens. Structural characterization was performed by electron microscopy. Permeability of polymer nanocapsules was studied by analyzing the retention of molecules of various sizes in the capsule interior. Selective pores were produced that were capable of differentiating molecules with a relatively small size difference. This work describes a new class of polymer nanomaterials previously unavailable.

  8. Superconductivity of bilayer phosphorene under interlayer compression

    NASA Astrophysics Data System (ADS)

    Gui-Qin, Huang; Zhong-Wen, Xing

    2016-02-01

    According to first-principles calculations, it is our prediction that bilayer phosphorene (BLP) will become a quasi-two-dimensional superconductor under a certain degree of interlayer compression. A decreasing interlayer distance may realize the transition in the BLP from a semiconducting phase to a metallic phase. On the other hand, a severe vertical compression may make the BLP lattice become dynamically unstable. It is found that in the stable metallic phase of the BLP, interlayer phonon modes dominate the electron-phonon coupling λ. The obtained λ can be greater than 1 and the superconducting temperature Tc can be higher than 10 K. Project supported by the State Key Program for Basic Researches of China (Grant No. 2014CB921103) and the Natural Science Foundation of Jiangsu Province, China (Grant Nos. BK20141441 and BK2010012).

  9. Combinatorics of giant hexagonal bilayer hemoglobins.

    PubMed

    Hanin, L G; Vinogradov, S N

    2000-01-01

    The paper discusses combinatorial and probabilistic models allowing to characterize various aspects of spacial symmetry and structural heterogeneity of the giant hexagonal bilayer hemoglobins (HBL Hb). Linker-dodecamer configurations of HBL are described for two and four linker types (occurring in the two most studied HBL Hb of Arenicola and Lumbricus, respectively), and the most probable configurations are found. It is shown that, for HBL with marked dodecamers, the number of 'normal-marked' pairs of dodecamers in homological position follows a binomial distribution. The group of symmetries of the dodecamer substructure of HBL is identified with the dihedral group D6. Under natural symmetry assumptions, the total dipole moment of the dodecamer substructure of HBL is shown to be zero. Biological implications of the mathematical findings are discussed.

  10. Terahertz conductivity of twisted bilayer graphene.

    PubMed

    Zou, Xingquan; Shang, Jingzhi; Leaw, Jianing; Luo, Zhiqiang; Luo, Liyan; La-o-Vorakiat, Chan; Cheng, Liang; Cheong, S A; Su, Haibin; Zhu, Jian-Xin; Liu, Yanpeng; Loh, Kian Ping; Castro Neto, A H; Yu, Ting; Chia, Elbert E M

    2013-02-01

    Using terahertz time-domain spectroscopy, the real part of optical conductivity [σ(1)(ω)] of twisted bilayer graphene was obtained at different temperatures (10-300 K) in the frequency range 0.3-3 THz. On top of a Drude-like response, we see a strong peak in σ(1)(ω) at ~2.7 THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, which is caused by the presence of a van Hove singularity arising from a commensurate twisting of the two graphene layers. PMID:23432306

  11. Space charge and screening in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kolomeisky, Eugene B.; Straley, Joseph P.; Abrams, Daniel L.

    2016-11-01

    Undoped bilayer graphene is a two-dimensional semimetal with a low-energy excitation spectrum that is parabolic in the momentum. As a result, the screening of an arbitrary external charge Ze is accompanied by a reconstruction of the ground state: valence band electrons (for Z  >  0) are promoted to form a space charge around the charge while the holes leave the physical picture. The outcome is a flat neutral object resembling the regular atom except that for Z\\gg 1 it is described by a strictly linear Thomas-Fermi theory. This theory also predicts that the bilayer’s static dielectric constant is the same as that of a two-dimensional electron gas in the long-wavelength limit.

  12. Molecular interactions of quinidine with phospholipid bilayers.

    PubMed

    Suwalsky, M; Villena, F; Bagnara, M; Sotomayor, C P

    1995-01-01

    Quinidine (QUIN) is one of the most important and efficient antiarrhythmic drugs (AAD). It belongs to class I, which are the drugs that exert their action at the level of the sodium channels in the membrane of the myocard. Several hypotheses support the idea that the molecular mechanism of action of the AAD is via nonspecific interactions with phospholipids sited in the neighborhood of the channels. In order to probe the validity of these hypotheses, QUIN was made to interact with the phospholipids dimyristoylphosphadidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE). These interactions were performed in a hydrophobic and a hydrophilic medium under a wide range of molar ratios. The resulting products were analyzed by X-ray diffraction. QUIN solutions were also made to interact with DMPC liposomes, which were studied by fluorescent spectroscopy. Finally, human erythrocytes which were incubated with QUIN solutions were observed by scanning electron microscopy. The results of these experiments proved that QUIN indeed interacted with phospholipid bilayers. PMID:7546041

  13. Tuning quantum properties in bilayer ruthenates

    NASA Astrophysics Data System (ADS)

    Ke, Xianglin

    The mutual coupling among spin, charge, lattice and orbital degrees of freedom in transition-metal oxide materials often leads to the competition of various types of energetic states. This makes such materials dramatically susceptible to external parameters, giving rise to novel physical properties and rich phase diagrams. In this talk, I shall use a bilayer ruthenate, Ca3Ru2O7, as an example to discuss the emergent phenomena achieved by systematically tuning materials magnetic and electronic properties via chemical doping, magnetic field, and pressure. I shall show that this system provides a rare opportunity to investigate the interplay between correlated metal and Mott insulator. This work was done in collaboration with M. Zhu, T. Tao, S. D. Mahanti, Z. Q. Mao, J. Peng, T. Hong, W. Tian, H. Cao, C. R. dela Cruz, D. Singh, and K. Prokes.

  14. Graphene-Templated Supported Lipid Bilayer Nanochannels.

    PubMed

    Li, Wan; Chung, Jean K; Lee, Young Kwang; Groves, Jay T

    2016-08-10

    The use of patterned substrates to impose geometrical restriction on the lateral mobility of molecules in supported lipid membranes has found widespread utility in studies of cell membranes. Here, we template-pattern supported lipid membranes with nanopatterned graphene. We utilize focused ion beam milling to pattern graphene on its growth substrate, then transfer the patterned graphene to fresh glass substrates for subsequent supported membrane formation. We observe that graphene functions as an excellent lateral diffusion barrier for supported lipid bilayers. Additionally, the observed diffusion dynamics of lipids in nanoscale graphene channels reveal extremely low boundary effects, a common problem with other materials. We suggest this is attributable to the ultimate thinness of graphene. PMID:27362914

  15. Collective Dynamics of Complex Plasma Bilayers

    SciTech Connect

    Hartmann, P.; Donko, Z.; Kalman, G. J.; Kyrkos, S.; Golden, K. I.; Rosenberg, M.

    2009-12-11

    A classical dusty plasma experiment was performed using two different dust grain sizes to form a strongly coupled asymmetric bilayer (two closely spaced interacting monolayers) of two species of charged dust particles. The observation and analysis of the thermally excited particle oscillations revealed the collective mode structure and dispersion (wave propagation) in this system; in particular, the existence of the theoretically predicted k=0 energy (frequency) gap was verified. Equilibrium molecular-dynamics simulations were performed to emulate the experiment, assuming Yukawa-type interparticle interaction. The simulations and analytic calculations based both on lattice summation and on the quasilocalized charge approximation approach are in good agreement with the experimental findings and help in identifying and characterizing the observed phenomena.

  16. Main phase transitions in supported lipid single-bilayer.

    PubMed

    Charrier, A; Thibaudau, F

    2005-08-01

    We have studied the phase transitions of a phospholipidic single-bilayer supported on a mica substrate by real-time temperature-controlled atomic force microscopy. We show the existence of two phase transitions in this bilayer that we attribute to two gel (L(beta))/fluid (L(alpha)) transitions, corresponding to the independent melting of each leaflet of the bilayer. The ratio of each phase with temperature and the large broadening of the transitions' widths have been interpreted through a basic thermodynamic framework in which the surface tension varies during the transitions. The experimental data can be fit with such a model using known thermodynamic parameters. PMID:15879467

  17. A circuit model for defective bilayer graphene transistors

    NASA Astrophysics Data System (ADS)

    Umoh, Ime J.; Moktadir, Zakaria; Hang, Shuojin; Kazmierski, Tom J.; Mizuta, Hiroshi

    2016-05-01

    This paper investigates the behaviour of a defective single-gate bilayer graphene transistor. Point defects were introduced into pristine graphene crystal structure using a tightly focused helium ion beam. The transfer characteristics of the exposed transistors were measured ex-situ for different defect concentrations. The channel peak resistance increased with increasing defect concentration whilst the on-off ratio showed a decreasing trend for both electrons and holes. To understand the electrical behaviour of the transistors, a circuit model for bilayer graphene is developed which shows a very good agreement when validated against experimental data. The model allowed parameter extraction of bilayer transistor and can be implemented in circuit level simulators.

  18. Modeling liquid crystal bilayer structures with minimal surfaces.

    PubMed

    Enlow, J D; Enlow, R L; McGrath, K M; Tate, M W

    2004-01-22

    This paper describes a new convenient and accurate method of calculating x-ray diffraction integrated intensities from detailed cubic bilayer structures. The method is employed to investigate the structure of a particular surfactant system (didodecyldimethylammonium bromide in a solution of oil and heavy water), for which single-crystal experimental data have recently been collected. The diffracted peak intensities correlate well with theoretical structures based on mathematical minimal surfaces. Optimized electron density profiles of the bilayer are presented, providing new insight into key features of the bilayer structure.

  19. Theory of periodic structures in lipid bilayer membranes

    PubMed Central

    Falkovitz, Meira S.; Seul, Michael; Frisch, Harry L.; McConnell, Harden M.

    1982-01-01

    An approximate, new model for the structure of the periodic, undulated Pβ′, phase of phosphatidylcholine bilayers is proposed. The properties of this phase are deduced by minimizing a Landau-de Gennes expression for the bilayer free energy when this free energy contains a term favoring a spontaneous curvature of the membrane. The theoretical calculation leads to a model for the Pβ′ phase of phosphatidylcholine bilayers having a number of novel physical properties, including periodic variations in membrane “fluidity.” PMID:16593202

  20. Topological valley transport at bilayer graphene domain walls

    NASA Astrophysics Data System (ADS)

    Ju, Long; Shi, Zhiwen; Nair, Nityan; Lv, Yinchuan; Jin, Chenhao; Velasco, Jairo, Jr.; Ojeda-Aristizabal, Claudia; Bechtel, Hans; Martin, Michael; Zettl, Alex; Analytis, James; McEuen, Paul; Wang, Feng

    Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. An external electric field can induce a tunable bandgap in bilayer graphene, and domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. In this talk, I will present our efforts on revealing the topologically protected edge states at AB-BA domain walls by combining near field infrared nanoscopy with electrical transport measurement. These one-dimensional valley-polarized conducting channels feature a ballistic length of about 400 nanometres at 4 kelvin.

  1. 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; Sarles, Stephen A.

    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

  2. Tethered and Polymer Supported Bilayer Lipid Membranes: Structure and Function

    PubMed Central

    Andersson, Jakob; Köper, Ingo

    2016-01-01

    Solid supported bilayer lipid membranes are model systems to mimic natural cell membranes in order to understand structural and functional properties of such systems. The use of a model system allows for the use of a wide variety of analytical tools including atomic force microscopy, impedance spectroscopy, neutron reflectometry, and surface plasmon resonance spectroscopy. Among the large number of different types of model membranes polymer-supported and tethered lipid bilayers have been shown to be versatile and useful systems. Both systems consist of a lipid bilayer, which is de-coupled from an underlying support by a spacer cushion. Both systems will be reviewed, with an emphasis on the effect that the spacer moiety has on the bilayer properties. PMID:27249006

  3. Effect of impurity doping in gapped bilayer graphene

    SciTech Connect

    Han, Qi; Yan, Baoming; Jia, Zhenzhao; Niu, Jingjing; Yu, Dapeng; Wu, Xiaosong

    2015-10-19

    Impurity doping plays a pivotal role in semiconductor electronics. We study the doping effect in a two-dimensional semiconductor, gapped bilayer graphene. By employing in situ deposition of calcium on the bilayer graphene, dopants are controllably introduced. Low temperature transport results show a variable range hopping conduction near the charge neutrality point persisting up to 50 K, providing evidence for the impurity levels inside the gap. Our experiment confirms a predicted peculiar effect in the gapped bilayer graphene, i.e., formation of in-gap states even if the bare impurity level lies in the conduction band. The result provides perspective on the effect of doping and impurity levels in semiconducting bilayer graphene.

  4. Structural studies of polymer-cushioned lipid bilayers.

    PubMed

    Majewski, J; Wong, J Y; Park, C K; Seitz, M; Israelachvili, J N; Smith, G S

    1998-11-01

    The structure of softly supported polymer-cushioned lipid bilayers, prepared in two different ways at the quartz-solution interface, were determined using neutron reflectometry. The polymer cushion consisted of a thin layer of branched, cationic polyethyleneimine (PEI), and the bilayers were formed by adsorption of small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles. When vesicles were first allowed to adsorb to a bare quartz substrate, an almost perfect bilayer formed. When the polymer was then added to the aqueous solution, it appeared to diffuse beneath this bilayer, effectively lifting it from the substrate. In contrast, if the polymer layer is adsorbed first to the bare quartz substrate followed by addition of vesicles to the solution, there is very little interaction of the vesicles with the polymer layer, and the result is a complex structure most likely consisting of patchy multilayers or adsorbed vesicles.

  5. Light Driven Formation and Rupture of Droplet Bilayers

    PubMed Central

    Dixit, Sanhita S.; Kim, Hanyoup; Vasilyev, Arseny; Eid, Aya; Faris, Gregory W.

    2010-01-01

    We demonstrate optical manipulation of nanoliter aqueous droplets containing surfactant or lipid molecules and immersed in an organic liquid using near infrared light. The resulting emulsion droplets are manipulated using both the thermocapillary effect and convective fluid motion. Droplet pair-interactions induced in the emulsion upon optical initiation and control provide direct observations of the coalescence steps in intricate detail. Droplet-droplet adhesion (bilayer formation) is observed under several conditions. Selective bilayer rupture is also realized using the same infrared laser. The technique provides a novel approach to study thin film drainage and interface stability in emulsion dynamics. The formation of stable lipid bilayers at the adhesion interface between interacting water droplets can provide an optical platform to build droplet-based lipid bilayer assays. The technique also has relevance for understanding and improving microfluidics applications by devising Petri dish based droplet assays requiring no substrate fabrication. PMID:20361732

  6. The role of lamellate phospholipid bilayers in lubrication of joints.

    PubMed

    Pawlak, Zenon; Urbaniak, Wiesław; Gadomski, Adam; Yusuf, Kehinde Q; Afara, Isaac O; Oloyede, Adekunle

    2012-01-01

    This study aims to determine the effect of progressive loss of the surface active phospholipids on the characteristics, and hence tribological function of articular cartilage. In accordance to Hill's hypothesis, 3-7 lipid bilayers at pH 7.4 operate as the solid lubricant in the cartilage-cartilage interface during physiological function. These bilayers are known to be depleted during cartilage degeneration. This study models this loss of phospholipid bilayers, studying experimentally both wet and dry cartilage surfaces, measuring surface wettability, and friction coefficient under a constant stress of 1.2 MPa. The results demonstrate that the friction coefficient increases gradually with loss of the phospholipid bilayers, and gains in value with decrease in wettability.

  7. Bilayer properties of hydroxytyrosol- and tyrosol-phosphatidylcholine lipids

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Tyrosol and hydroxytyrosol are the phytochemicals abundantly found in olive oil. Transphosphatidylation of tyrosol and hydroxytyrosol with dioleoylphosphocholine resulted in phospholipids with antioxidant properties. The ability of these phyto-phospholipids to form liposomes and supported bilayers w...

  8. Visualizing Atomic-Scale Negative Differential Resistance in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Kim, Keun Su; Kim, Tae-Hwan; Walter, Andrew L.; Seyller, Thomas; Yeom, Han Woong; Rotenberg, Eli; Bostwick, Aaron

    2013-01-01

    We investigate the atomic-scale tunneling characteristics of bilayer graphene on silicon carbide using the scanning tunneling microscopy. The high-resolution tunneling spectroscopy reveals an unexpected negative differential resistance (NDR) at the Dirac energy, which spatially varies within the single unit cell of bilayer graphene. The origin of NDR is explained by two near-gap van Hove singularities emerging in the electronic spectrum of bilayer graphene under a transverse electric field, which are strongly localized on two sublattices in different layers. Furthermore, defects near the tunneling contact are found to strongly impact on NDR through the electron interference. Our result provides an atomic-level understanding of quantum tunneling in bilayer graphene, and constitutes a useful step towards graphene-based tunneling devices.

  9. Visualizing atomic-scale negative differential resistance in bilayer graphene.

    PubMed

    Kim, Keun Su; Kim, Tae-Hwan; Walter, Andrew L; Seyller, Thomas; Yeom, Han Woong; Rotenberg, Eli; Bostwick, Aaron

    2013-01-18

    We investigate the atomic-scale tunneling characteristics of bilayer graphene on silicon carbide using the scanning tunneling microscopy. The high-resolution tunneling spectroscopy reveals an unexpected negative differential resistance (NDR) at the Dirac energy, which spatially varies within the single unit cell of bilayer graphene. The origin of NDR is explained by two near-gap van Hove singularities emerging in the electronic spectrum of bilayer graphene under a transverse electric field, which are strongly localized on two sublattices in different layers. Furthermore, defects near the tunneling contact are found to strongly impact on NDR through the electron interference. Our result provides an atomic-level understanding of quantum tunneling in bilayer graphene, and constitutes a useful step towards graphene-based tunneling devices. PMID:23373943

  10. Sub-wavelength antenna enhanced bilayer graphene tunable photodetector

    DOEpatents

    Beechem, III, Thomas Edwin; Howell, Stephen W.; Peters, David W.; Davids, Paul; Ohta, Taisuke

    2016-03-22

    The integration of bilayer graphene with an absorption enhancing sub-wavelength antenna provides an infrared photodetector capable of real-time spectral tuning without filters at nanosecond timescales.

  11. Electronic properties of asymmetrically doped twisted graphene bilayers

    NASA Astrophysics Data System (ADS)

    Trambly de Laissardière, Guy; Namarvar, Omid Faizy; Mayou, Didier; Magaud, Laurence

    2016-06-01

    Rotated graphene bilayers form an exotic class of nanomaterials with fascinating electronic properties governed by the rotation angle θ . For large rotation angles, the electron eigenstates are restricted to one layer and the bilayer behaves like two decoupled graphene layers. At intermediate angles, Dirac cones are preserved but with a lower velocity and van Hove singularities are induced at energies where the two Dirac cones intersect. At very small angles, eigenstates become localized in peculiar moiré zones. We analyze here the effect of an asymmetric doping for a series of commensurate rotated bilayers on the basis of tight-binding calculations of their band dispersions, density of states, participation ratio, and diffusive properties. While a small doping level preserves the θ dependence of the rotated bilayer electronic structure, larger doping induces a further reduction of the band velocity in the same way as a further reduction of the rotation angle.

  12. Tethered and Polymer Supported Bilayer Lipid Membranes: Structure and Function.

    PubMed

    Andersson, Jakob; Köper, Ingo

    2016-01-01

    Solid supported bilayer lipid membranes are model systems to mimic natural cell membranes in order to understand structural and functional properties of such systems. The use of a model system allows for the use of a wide variety of analytical tools including atomic force microscopy, impedance spectroscopy, neutron reflectometry, and surface plasmon resonance spectroscopy. Among the large number of different types of model membranes polymer-supported and tethered lipid bilayers have been shown to be versatile and useful systems. Both systems consist of a lipid bilayer, which is de-coupled from an underlying support by a spacer cushion. Both systems will be reviewed, with an emphasis on the effect that the spacer moiety has on the bilayer properties. PMID:27249006

  13. An engineered dimeric protein pore that spans adjacent lipid bilayers

    PubMed Central

    Mantri, Shiksha; Sapra, K. Tanuj; Cheley, Stephen; Sharp, Thomas H.; Bayley, Hagan

    2013-01-01

    The bottom-up construction of artificial tissues is an underexplored area of synthetic biology. An important challenge is communication between constituent compartments of the engineered tissue and between the engineered tissue and additional compartments, including extracellular fluids, further engineered tissue and living cells. Here we present a dimeric transmembrane pore that can span two adjacent lipid bilayers and thereby allow aqueous compartments to communicate. Two heptameric staphylococcal α-hemolysin (αHL) pores were covalently linked in an aligned cap-to-cap orientation. The structure of the dimer, (α7)2, was confirmed by biochemical analysis, transmission electron microscopy (TEM) and single-channel electrical recording. We show that one of two β barrels of (α7)2 can insert into the lipid bilayer of a small unilamellar vesicle, while the other spans a planar lipid bilayer. (α7)2 pores spanning two bilayers were also observed by TEM. PMID:23591892

  14. Chronopotentiometric studies of phosphatidylcholine bilayers modified by ergosterol.

    PubMed

    Naumowicz, Monika; Petelska, Aneta Dorota; Figaszewski, Zbigniew Artur

    2011-01-01

    We have monitored the effect of ergosterol on electrical capacitance and electrical resistance of the phosphatidylcholine bilayer membranes using chronopotentiometry method. The chronopotentiometric characteristic of the bilayers depends on constant-current flow through the membranes. For low current values, no electroporation takes place and the membrane voltage rises exponentially to a constant value described by the Ohm's law. Based on these kinds of chronopotentiometric curves, a method of the membrane capacitance and the membrane resistance calculations is presented.

  15. 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.

  16. Subterahertz excitations and magnetoelectric effects in hexaferrite-piezoelectric bilayers

    SciTech Connect

    Ustinov, Alexey B.; Srinivasan, G.

    2008-10-06

    A frequency-agile hexaferrite-piezoelectric composite for potential device applications at subterahertz frequencies is studied. The bilayer is composed of aluminum substituted barium hexagonal ferrite (BaAl{sub 2}Fe{sub 10}O{sub 19}) and lead zirconate titanate (PZT). A dc electric field applied to PZT results in mechanical deformation of the ferrite, leading to a frequency shift in ferromagnetic resonance. The bilayer demonstrates magnetoelectric interaction coefficient of about 0.37 Oe cm/kV.

  17. Global phase diagram of bilayer quantum Hall ferromagnets

    NASA Astrophysics Data System (ADS)

    Abolfath, M.; Radzihovsky, L.; MacDonald, A. H.

    2002-06-01

    We present a microscopic study of the interlayer spacing d versus in-plane magnetic field B|| phase diagram for bilayer quantum Hall (QH) pseudoferromagnets. In addition to the interlayer charge balanced commensurate and incommensurate states analyzed previously, we address the corresponding interlayer charge unbalanced ``canted'' QH states. We predict a large anomaly in the bilayer capacitance at the canting transition and the formation of dipole stripe domains with periods exceeding 1 micron in the canted state.

  18. Spontaneous interlayer superfluidity in bilayer systems of cold polar molecules

    SciTech Connect

    Lutchyn, Roman M.; Das Sarma, S.; Rossi, Enrico

    2010-12-15

    Recent experimental progress in producing ultracold polar molecules with a net electric dipole moment opens up possibilities for realizing quantum phases governed by the long-range and anisotropic dipole-dipole interactions. In this work we predict the existence of experimentally observable broken-symmetry states with spontaneous interlayer coherence in cold polar molecule bilayers. These exotic states, which are manifestations of collective bilayer quantum entanglement, appear due to strong repulsive interlayer interactions and exhibit properties of superfluids, ferromagnets, and excitonic condensates.

  19. Nanoparticles Evading The Reticuloendothelial System: Role of The Supported Bilayer

    PubMed Central

    Li, Shyh-Dar; Huang, Leaf

    2009-01-01

    Summary We have previously shown that the PEGylated LPD (liposome-polycation-DNA) nanoparticles were highly efficient in delivering siRNA to the tumor with low liver uptake. Its mechanism of evading the reticuloendothelial system (RES) is reported here. In LPD, nucleic acids were condensed with protamine into a compact core, which was then coated by two cationic lipid bilayers with the inner bilayer stabilized by charge-charge interaction (also called the supported bilayer). Finally, a detergent-like molecule, polyethylene glycol (PEG)-phospholipid is post-inserted into the lipid bilayer to modify the surface of LPD. The dynamic light scattering (DLS) data showed that LPD had improved stability compared to cationic liposomes after incubation with a high concentration of DSPE-PEG2000, which is known to disrupt the bilayer. LPD prepared with a multivalent cationic lipid, DSGLA, had enhanced stability compared to those containing DOTAP, a monovalent cationic lipid, suggesting that stronger charge-charge interaction in the supported bilayer contributed to a higher stability. Distinct nanoparticle structure was found in the PEGylated LPD by transmission electron microscopy, while the cationic liposomes were transformed into tubular micelles. Size exclusion chromatography data showed that approximately 60% of the total cationic lipids, which were located in the outer bilayer of LPD, were stripped off during the PEGylation; and about 20% of the input DSPE-PEG2000 was incorporated into the inner bilayer with about 10.6 mol% of DSPE-PEG2000 presented on the particle surface. This led to complete charge shielding, low liver sinusoidal uptake, and 32.5% injected dose delivered to the NCI-H460 tumor in a xenograft model. PMID:19595666

  20. Mechanism of unassisted ion transport across membrane bilayers

    NASA Technical Reports Server (NTRS)

    Wilson, M. A.; Pohorille, A.

    1996-01-01

    To establish how charged species move from water to the nonpolar membrane interior and to determine the energetic and structural effects accompanying this process, we performed molecular dynamics simulations of the transport of Na+ and Cl- across a lipid bilayer located between two water lamellae. The total length of molecular dynamics trajectories generated for each ion was 10 ns. Our simulations demonstrate that permeation of ions into the membrane is accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby polar lipid head groups and water penetrate the nonpolar membrane interior. Once the ion crosses the midplane of the bilayer the deformation "switches sides"; the initial defect slowly relaxes, and a defect forms in the outgoing side of the bilayer. As a result, the ion remains well solvated during the process; the total number of oxygen atoms from water and lipid head groups in the first solvation shell remains constant. A similar membrane deformation is formed when the ion is instantaneously inserted into the interior of the bilayer. The formation of defects considerably lowers the free energy barrier to transfer of the ion across the bilayer and, consequently, increases the permeabilities of the membrane to ions, compared to the rigid, planar structure, by approximately 14 orders of magnitude. Our results have implications for drug delivery using liposomes and peptide insertion into membranes.

  1. Improved collagen bilayer dressing for the controlled release of drugs.

    PubMed

    Sripriya, Ramasamy; Kumar, Muthusamy Senthil; Sehgal, Praveen Kumar

    2004-08-15

    A novel bilayer dressing has been developed from bovine succinylated collagen. The dressing contains an antibiotic, Ciprofloxacin, for both immediate and time-regulated release for controlling the infection, as the infected open wounds need special care. The dressing consists of a sponge and a film, both prepared from succinylated bovine collagen. The sponge has a smooth surface on one side; its rough surface on the other side forms the bilayer system with the film. Both sponge and film act as an anionic reservoir to hold the cationic Ciprofloxacin. The drug, after dispersing in poly (N-vinyl-2-pyrrolidione) (PVP) solution is allowed to spread in the bilayer system by diffusion. The drug stays in the bilayer system because of ionic binding, but starts releasing when comes in contact with the wound. Release of the drug is immediate, but it is regulated by ionic binding between the drug and succinylated collagen. The wound exudates, and there is a polarity-controlled release of the drug from the bilayer system. The PVP and bilayer system permits only time-regulated release, and the system lasts up to 5 days with therapeutically sufficient drug availability.

  2. Robustly Engineering Thermal Conductivity of Bilayer Graphene by Interlayer Bonding.

    PubMed

    Zhang, Xiaoliang; Gao, Yufei; Chen, Yuli; Hu, Ming

    2016-02-25

    Graphene and its bilayer structure are the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. Their realistic applications in emerging nanoelectronics usually call for thermal transport manipulation in a controllable and precise manner. In this paper we systematically studied the effect of interlayer covalent bonding, in particular different interlay bonding arrangement, on the thermal conductivity of bilayer graphene using equilibrium molecular dynamics simulations. It is revealed that, the thermal conductivity of randomly bonded bilayer graphene decreases monotonically with the increase of interlayer bonding density, however, for the regularly bonded bilayer graphene structure the thermal conductivity possesses unexpectedly non-monotonic dependence on the interlayer bonding density. The results suggest that the thermal conductivity of bilayer graphene depends not only on the interlayer bonding density, but also on the detailed topological configuration of the interlayer bonding. The underlying mechanism for this abnormal phenomenon is identified by means of phonon spectral energy density, participation ratio and mode weight factor analysis. The large tunability of thermal conductivity of bilayer graphene through rational interlayer bonding arrangement paves the way to achieve other desired properties for potential nanoelectronics applications involving graphene layers.

  3. Robustly Engineering Thermal Conductivity of Bilayer Graphene by Interlayer Bonding

    PubMed Central

    Zhang, Xiaoliang; Gao, Yufei; Chen, Yuli; Hu, Ming

    2016-01-01

    Graphene and its bilayer structure are the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. Their realistic applications in emerging nanoelectronics usually call for thermal transport manipulation in a controllable and precise manner. In this paper we systematically studied the effect of interlayer covalent bonding, in particular different interlay bonding arrangement, on the thermal conductivity of bilayer graphene using equilibrium molecular dynamics simulations. It is revealed that, the thermal conductivity of randomly bonded bilayer graphene decreases monotonically with the increase of interlayer bonding density, however, for the regularly bonded bilayer graphene structure the thermal conductivity possesses unexpectedly non-monotonic dependence on the interlayer bonding density. The results suggest that the thermal conductivity of bilayer graphene depends not only on the interlayer bonding density, but also on the detailed topological configuration of the interlayer bonding. The underlying mechanism for this abnormal phenomenon is identified by means of phonon spectral energy density, participation ratio and mode weight factor analysis. The large tunability of thermal conductivity of bilayer graphene through rational interlayer bonding arrangement paves the way to achieve other desired properties for potential nanoelectronics applications involving graphene layers. PMID:26911859

  4. Microporous device for local electric recordings on model lipid bilayers

    NASA Astrophysics Data System (ADS)

    Kaufeld, Theresa; Steinem, Claudia; Schmidt, Christoph F.

    2015-01-01

    A powerful approach for characterizing lipid membranes and embedded proteins is the reconstitution of model lipid bilayers. The extreme fragility of 5 nm thick bilayers is a challenge for device design and requires a trade off of stability against accessibility. We here present a microporous lab-on-chip device that allows us to form stable, solvent-free lipid bilayers from giant unilamellar vesicles (GUVs) in a geometry that provides a unique set of access possibilities. The device is constructed around a micro-fabricated silicon chip with clusters of 1 µm-diameter pores and provides optical access to the lipid bilayers for high-NA epifluorescence imaging. At the same time, solvent exchange is possible on both sides of the lipid bilayer. Complete coverage can be achieved with GUVs, so that voltages can be applied across the lipid bilayer and single-channel currents can be measured using external or integrated silver/silver chloride electrodes. We describe the micro-fabrication by standard cleanroom techniques and the characterization of the device by atomic force microscopy, scanning electron microscopy and impedance spectroscopy. In proof-of-concept experiments we demonstrate that the device is capable of low-noise, single-ion-channel recordings. Electronic Supplementary Information (ESI) available: See DOI: 10.1039/b000000x/

  5. Predicting proton titration in cationic micelle and bilayer environments

    SciTech Connect

    Morrow, Brian H.; Shen, Jana K.; Eike, David M.; Murch, Bruce P.; Koenig, Peter H.

    2014-08-28

    Knowledge of the protonation behavior of pH-sensitive molecules in micelles and bilayers has significant implications in consumer product development and biomedical applications. However, the calculation of pK{sub a}’s in such environments proves challenging using traditional structure-based calculations. Here we apply all-atom constant pH molecular dynamics with explicit ions and titratable water to calculate the pK{sub a} of a fatty acid molecule in a micelle of dodecyl trimethylammonium chloride and liquid as well as gel-phase bilayers of diethyl ester dimethylammonium chloride. Interestingly, the pK{sub a} of the fatty acid in the gel bilayer is 5.4, 0.4 units lower than that in the analogous liquid bilayer or micelle, despite the fact that the protonated carboxylic group is significantly more desolvated in the gel bilayer. This work illustrates the capability of all-atom constant pH molecular dynamics in capturing the delicate balance in the free energies of desolvation and Coulombic interactions. It also shows the importance of the explicit treatment of ions in sampling the protonation states. The ability to model dynamics of pH-responsive substrates in a bilayer environment is useful for improving fabric care products as well as our understanding of the side effects of anti-inflammatory drugs.

  6. Predicting proton titration in cationic micelle and bilayer environments

    NASA Astrophysics Data System (ADS)

    Morrow, Brian H.; Eike, David M.; Murch, Bruce P.; Koenig, Peter H.; Shen, Jana K.

    2014-08-01

    Knowledge of the protonation behavior of pH-sensitive molecules in micelles and bilayers has significant implications in consumer product development and biomedical applications. However, the calculation of pKa's in such environments proves challenging using traditional structure-based calculations. Here we apply all-atom constant pH molecular dynamics with explicit ions and titratable water to calculate the pKa of a fatty acid molecule in a micelle of dodecyl trimethylammonium chloride and liquid as well as gel-phase bilayers of diethyl ester dimethylammonium chloride. Interestingly, the pKa of the fatty acid in the gel bilayer is 5.4, 0.4 units lower than that in the analogous liquid bilayer or micelle, despite the fact that the protonated carboxylic group is significantly more desolvated in the gel bilayer. This work illustrates the capability of all-atom constant pH molecular dynamics in capturing the delicate balance in the free energies of desolvation and Coulombic interactions. It also shows the importance of the explicit treatment of ions in sampling the protonation states. The ability to model dynamics of pH-responsive substrates in a bilayer environment is useful for improving fabric care products as well as our understanding of the side effects of anti-inflammatory drugs.

  7. Predicting proton titration in cationic micelle and bilayer environments

    PubMed Central

    Morrow, Brian H.; Eike, David M.; Murch, Bruce P.; Koenig, Peter H.; Shen, Jana K.

    2014-01-01

    Knowledge of the protonation behavior of pH-sensitive molecules in micelles and bilayers has significant implications in consumer product development and biomedical applications. However, the calculation of pKa’s in such environments proves challenging using traditional structure-based calculations. Here we apply all-atom constant pH molecular dynamics with explicit ions and titratable water to calculate the pKa of a fatty acid molecule in a micelle of dodecyl trimethylammonium chloride and liquid as well as gel-phase bilayers of diethyl ester dimethylammonium chloride. Interestingly, the pKa of the fatty acid in the gel bilayer is 5.4, 0.4 units lower than that in the analogous liquid bilayer or micelle, despite the fact that the protonated carboxylic group is significantly more desolvated in the gel bilayer. This work illustrates the capability of all-atom constant pH molecular dynamics in capturing the delicate balance in the free energies of desolvation and Coulombic interactions. It also shows the importance of the explicit treatment of ions in sampling the protonation states. The ability to model dynamics of pH-responsive substrates in a bilayer environment is useful for improving fabric care products as well as our understanding of the side effects of anti-inflammatory drugs. PMID:25173037

  8. Mechanism of unassisted ion transport across membrane bilayers.

    PubMed

    Wilson, M A; Pohorille, A

    1996-07-17

    To establish how charged species move from water to the nonpolar membrane interior and to determine the energetic and structural effects accompanying this process, we performed molecular dynamics simulations of the transport of Na+ and Cl- across a lipid bilayer located between two water lamellae. The total length of molecular dynamics trajectories generated for each ion was 10 ns. Our simulations demonstrate that permeation of ions into the membrane is accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby polar lipid head groups and water penetrate the nonpolar membrane interior. Once the ion crosses the midplane of the bilayer the deformation "switches sides"; the initial defect slowly relaxes, and a defect forms in the outgoing side of the bilayer. As a result, the ion remains well solvated during the process; the total number of oxygen atoms from water and lipid head groups in the first solvation shell remains constant. A similar membrane deformation is formed when the ion is instantaneously inserted into the interior of the bilayer. The formation of defects considerably lowers the free energy barrier to transfer of the ion across the bilayer and, consequently, increases the permeabilities of the membrane to ions, compared to the rigid, planar structure, by approximately 14 orders of magnitude. Our results have implications for drug delivery using liposomes and peptide insertion into membranes.

  9. Electrical perturbations of ultrathin bilayers: role of ionic conductive layer.

    PubMed

    Nazaripoor, Hadi; Koch, Charles R; Bhattacharjee, Subir

    2014-12-16

    The effect of electrostatic force on the dynamics, morphological evolution, and drainage time of ultrathin liquid bilayers (<100 nm) are investigated for perfect dielectric-perfect dielectric (PD-PD) and ionic liquid-perfect dielectric (IL-PD) bilayers. The weakly nonlinear "thin film" equation is solved numerically to obtain spatiotemporal evolution of the liquid-liquid interface responses to transverse electric field. In order to predict the electrostatic component of conjoining/disjoining pressure acting on the interface for IL-PD bilayers, an analytical model is developed using the nonlinear Poisson-Boltzmann equation. It is found that IL-PD bilayers with electric permittivity ratio of layers (lower to top), εr, greater than one remain stable under an applied electric field. An extensive numerical study is carried out to generate a map based on εr and the initial mean thickness of the lower layer. This map is used to predict the formation of various structures on PD-PD bilayer interface and provides a baseline for unstable IL-PD bilayers. The use of an ionic liquid (IL) layer is found to reduce the size of the structures, but results in polydispersed and disordered pillars spread over the domain. The numerical predictions follow similar trend of experimental observation of Lau and Russel. (Lau, C. Y.; Russel, W. B. Fundamental Limitations on Ordered Electrohydrodynamic Patterning; Macromolecules 2011, 44, 7746-7751). PMID:25419880

  10. Effect of insulin on lipid bilayer viscoelasticity.

    PubMed

    Hianik, T; Zórad, S; Kavecanský, J; Macho, L

    1987-04-01

    Changes in the Young elasticity modulus in perpendicular direction to the membrane surface E perpendicular, in the coefficient of dynamic viscosity eta, in the electric capacitance C, in the surface charge U1, in the conductivity g and in the coefficient of non-linearity beta of current-voltage characteristic caused by insulin were studied in bilayer lipid membranes (BLM) prepared from a mixture of egg lecithin and cholesterol (4:1, w/w) in n-heptane. Even relatively small concentrations of insulin in electrolyte (ci approximately 4.8 x 10(-11) mol/l) caused a diminution in parameters E perpendicular and eta. Negative surface charge emerged on the membrane due to the insulin absorption, and U1 gradually increased depending on the concentration of the hormone in the electrolyte. Addition of insulin was also followed by an increase in membrane conductivity and affected the value of the coefficient of non-linearity beta of current-voltage characteristic. The effect of insulin on the BLM structure was discussed on the basis of the results obtained.

  11. Rotational disorder in twisted bilayer graphene.

    PubMed

    Beechem, Thomas E; Ohta, Taisuke; Diaconescu, Bogdan; Robinson, Jeremy T

    2014-02-25

    Conventional means of stacking two-dimensional (2D) crystals inevitably leads to imperfections. To examine the ramifications of these imperfections, rotational disorder and strain are quantified in twisted bilayer graphene (TBG) using a combination of Raman spectroscopic and low-energy electron diffraction imaging. The twist angle between TBG layers varies on the order of 2° within large (50-100 μm) single-crystalline grains, resulting in changes of the emergent Raman response by over an order of magnitude. Rotational disorder does not evolve continuously across the large grains but rather comes about by variations in the local twist angles between differing contiguous subgrains, ∼ 1 μm in size, that themselves exhibit virtually no twist angle variation (ΔΘ ∼ 0.1°). Owing to weak out-of-plane van der Waals bonding between azimuthally rotated graphene layers, these subgrains evolve in conjunction with the 0.3% strain variation observed both within and between the atomic layers. Importantly, the emergent Raman response is altered, but not removed, by these extrinsic perturbations. Interlayer interactions are therefore resilient to strain and rotational disorder, a fact that gives promise to the prospect of designer 2D solid heterostructures created via transfer processes.

  12. Terahertz conductivity of twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Chia, Elbert E. M.; Zou, Xingquan; Shang, Jingzhi; Leaw, Jianing; Luo, Zhiqiang; Luo, Liyan; Cheong, Siew Ann; Su, Haibin; Zhu, Jian-Xin; Castro Neto, A. H.; Yu, Ting

    2013-03-01

    Using terahertz time-domain spectroscopy, the real part of optical conductivity [σ1 (ω) ] of twisted bilayer graphene was obtained at different temperatures (10 - 300 K) in the frequency range 0.3 - 3 THz. On top of a Drude-like response, we see a strong and narrow peak in σ1 (ω) at ~2.7 THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, that is caused by the presence of van Hove singularities arising from a commensurate twisting of the two graphene layers. Singapore MOE AcRF Tier 2 (ARC 23/08), NRF-CRP (NRF-CRP4-2008-04), NNSA of the U.S. DOE at LANL (DE-AC52-06NA25396), LANL LDRD Program, NRF-CRP (R-144-000-295-281), DOE DE-FG02-08ER46512, ONR MURI N00014-09-1-1063.

  13. Detection of bilayer lipid with graphene nanoribbon

    NASA Astrophysics Data System (ADS)

    Akbari, Elnaz; Buntat, Zolkafle; Afroozeh, Abdolkarim; Zeinalinezhad, Alireza; Nilashi, Mehrbakhsh

    2015-09-01

    Single-layer graphene consists of sp 2-bonded carbon atoms arranged in a two-dimensional (2D) hexagonal lattice comprising a thin layer of single carbon atoms. Owing to its special characteristics including electrical, physical, and optical properties, graphene is considered more suitable for sensor applications than other materials. Moreover, it is possible to produce biosensors using electrolyte-gated field-effect transistors based on graphene (GFETs) to identify the alterations in charged lipid membrane properties. This paper illustrates how membrane thickness and electrical charge can result in a monolayer GFET, with emphasis on conductance variation. It is proposed that the thickness and electrical charge of the lipid bilayer are functions of carrier density, and equations relating these suitable control parameters were derived. Adaptive neuro fuzzy inference system (ANFIS) has been incorporated to obtain other model for conductance characteristic. The comparison between the analytical models and ANFIS with the experimental data extracted from previous work show an acceptable agreement. [Figure not available: see fulltext.

  14. Spin Hall Magnetoresistance in Metallic Bilayers.

    PubMed

    Kim, Junyeon; Sheng, Peng; Takahashi, Saburo; Mitani, Seiji; Hayashi, Masamitsu

    2016-03-01

    Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that consist of a heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find a nearly tenfold increase of SMR in W/CoFeB compared to previously studied HM/ferromagnetic insulator systems. The SMR increases with decreasing temperature despite the negligible change in the W layer resistivity. A model is developed to account for the absorption of the longitudinal spin current to the FM layer, one of the key characteristics of a metallic ferromagnet. We find that the model not only quantitatively describes the HM layer thickness dependence of SMR, allowing accurate estimation of the spin Hall angle and the spin diffusion length of the HM layer, but also can account for the temperature dependence of SMR by assuming a temperature dependent spin polarization of the FM layer. These results illustrate the unique role a metallic ferromagnetic layer plays in defining spin transmission across the HM/FM interface. PMID:26991195

  15. Magnetic Irreversibility in VO2/Ni Bilayers

    NASA Astrophysics Data System (ADS)

    de La Venta, Jose; Lauzier, Josh; Sutton, Logan

    The temperature dependence of the coercivity and magnetization of VO2/Ni bilayers was studied. VO2 exhibits a well-known Structural Phase Transition (SPT) at 330-340 K, from a low temperature monoclinic (M) to a high temperature rutile (R) structure. The SPT of VO2 induces an inverse magnetoelastic effect that strongly modifies the coercivity and magnetization of the Ni films. In addition, the growth conditions allow tuning of the magnetic properties. Ni films deposited on top of VO2 (M) show an irreversible change in the coercivity after the first cycle through the high temperature phase, with a corresponding change in the surface morphology of VO2. On the other hand, the Ni films grown on top of VO2 (R) do not show this irreversibility. These results indicate that properties of magnetic films are strongly affected by the strain induced by materials that undergo SPT and that it is possible to control the magnetic properties by tuning the growth conditions.

  16. Bilayer fractional quantum Hall states with dipoles

    NASA Astrophysics Data System (ADS)

    Yao, N. Y.; Bennett, S. D.; Laumann, C. R.; Lev, B. L.; Gorshkov, A. V.

    2015-09-01

    Using the example of dysprosium atoms in an optical lattice, we show how dipolar interactions between magnetic dipoles can be used to obtain fractional quantum Hall states. In our approach, dysprosium atoms are trapped one atom per site in a deep optical lattice with negligible tunneling. Microwave and spatially dependent optical dressing fields are used to define an effective spin-1/2 or spin-1 degree of freedom in each atom. Thinking of spin-1/2 particles as hard-core bosons, dipole-dipole interactions give rise to boson hopping, topological flat bands with Chern number 1, and the ν =1/2 Laughlin state. Thinking of spin-1 particles as two-component hard-core bosons, dipole-dipole interactions again give rise to boson hopping, topological flat bands with Chern number 2, and the bilayer Halperin (2,2,1) state. By adjusting the optical fields, we find a phase diagram, in which the (2,2,1) state competes with superfluidity. Generalizations to solid-state magnetic dipoles are discussed.

  17. Control cell adhesion with dynamic bilayer films

    NASA Astrophysics Data System (ADS)

    Kourouklis, Andreas; Lerum, Ronald; Bermudez, Harry

    2012-02-01

    Interfacially-directed assembly of amphiphilic block copolymers was employed to create ultrathin films having the potential to correlate the dynamics of ECM cues with cell adhesion and cytoskeletally-generated forces. The mobility of the polymeric bilayer films were tuned by the incorporation of hydrophobic homopolymer chains, which are thought to reduce interlayer friction. Labeling of the block copolymer chains with an adhesive peptide ligand (RGD) provided a specific means to study integrin-mediated cellular processes and the corresponding mechanotransduction. By seeding anchorage-dependent cells on ``dynamic'' (laterally mobile) and ``static'' films that display the same amount of RGD, we have found that cells recognize the difference in RGD diffusivity and develop distinct responses over time. We intend to examine changes in cell response by controlling the extent of cytoskeletally-generated forces and the assembly dynamics of focal adhesion complexes. Such films provide a unique platform to unveil the biomechanical signals related with ECM dynamics, and may ultimately facilitate a deeper understanding of cellular processes.

  18. The ground state construction of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Giuliani, Alessandro; Jauslin, Ian

    2016-09-01

    We consider a model of half-filled bilayer graphene, in which the three dominant Slonczewski-Weiss-McClure hopping parameters are retained, in the presence of short-range interactions. Under a smallness assumption on the interaction strength U as well as on the inter-layer hopping ɛ, we construct the ground state in the thermodynamic limit, and prove that the pressure and two-point Schwinger function, away from its singularities, are analytic in U, uniformly in ɛ. The interacting Fermi surface is degenerate, and consists of eight Fermi points, two of which are protected by symmetries, while the locations of the other six are renormalized by the interaction, and the effective dispersion relation at the Fermi points is conical. The construction reveals the presence of different energy regimes, where the effective behavior of correlation functions changes qualitatively. The analysis of the crossover between regimes plays an important role in the proof of analyticity and in the uniform control of the radius of convergence. The proof is based on a rigorous implementation of fermionic renormalization group methods, including determinant estimates for the renormalized expansion.

  19. Charge instabilities in strongly correlated bilayer systems

    NASA Astrophysics Data System (ADS)

    Seibold, G.

    2003-09-01

    We investigate the charge-instabilities of the Hubbard-Holstein model with two coupled layers. In this system the scattering processes naturally separate into contributions which are either symmetric or antisymmetric combinations with respect to exchange of the layers. It turns out that the short-range strong correlations suppress finite wave-vector nesting instabilities for both symmetries but favor the occurrence of phase separation in the symmetric channel. Inclusion of a sizeable long-range Coulomb (LRC) interaction frustrates the q=0 instabilities and supports the formation of incommensurate charge-density waves (CDW). Upon reducing doping from half-filling and for small electron-phonon coupling g the CDW instability first occurs in the antisymmetric channel but both instability lines merge with increasing g. While LRC forces always suppress the phase separation instability in the symmetric channel, the CDW period in the antisymmetric sector tends to infinity (q_cto 0) for sufficiently small Coulomb interaction. This feature allows for the possibility of singular scattering over the whole Fermi surface. We discuss possible implications of our results for the bilayer high- T c cuprates.

  20. Exceptional Optoelectronic Properties of Hydrogenated Bilayer Silicene

    SciTech Connect

    Huang, Bing; Yoon, Mina; Smith, Sean C; Wei, Su-Huai; Deng, Hui-Xiong; Liu, Feng; Lee, Hoonkyung; Sumpter, Bobby G

    2014-01-01

    Silicon is arguably the best electronic material, but not as good an optoelectronic material. By employing first-principles calculations and the cluster-expansion approach, we discover that hydrogenated bilayer silicene (BS) shows promising potential as a new kind of optoelectronic material. Most significantly, hydrogenation converts the intrinsic BS, a strongly indirect semiconductor, into a direct-gap semiconductor with a widely tunable bandgap. At low hydrogen concentrations, four ground states of single- and double-side hydrogenated BS are characterized with dipole-allowed direct (or quasidirect) bandgaps in the desirable range from 1 to 1.5 eV, suitable for solar applications. At high hydrogen concentrations, three well-ordered double-side hydrogenated BS structures exhibit direct (or quasidirect) bandgaps in the color range of red, green, and blue, affording white light-emitting diodes. Our findings open opportunities to search for new silicon-based light-absorption and light-emitting materials for earth-abundant, high-efficiency, optoelectronic applications.

  1. Molecular Dynamics Simulations of Hydrophilic Pores in Lipid Bilayers

    PubMed Central

    Leontiadou, Hari; Mark, Alan E.; Marrink, Siewert J.

    2004-01-01

    Hydrophilic pores are formed in peptide free lipid bilayers under mechanical stress. It has been proposed that the transport of ionic species across such membranes is largely determined by the existence of such meta-stable hydrophilic pores. To study the properties of these structures and understand the mechanism by which pore expansion leads to membrane rupture, a series of molecular dynamics simulations of a dipalmitoylphosphatidylcholine (DPPC) bilayer have been conducted. The system was simulated in two different states; first, as a bilayer containing a meta-stable pore and second, as an equilibrated bilayer without a pore. Surface tension in both cases was applied to study the formation and stability of hydrophilic pores inside the bilayers. It is observed that below a critical threshold tension of ∼38 mN/m the pores are stabilized. The minimum radius at which a pore can be stabilized is 0.7 nm. Based on the critical threshold tension the line tension of the bilayer was estimated to be ∼3 × 10−11 N, in good agreement with experimental measurements. The flux of water molecules through these stabilized pores was analyzed, and the structure and size of the pores characterized. When the lateral pressure exceeds the threshold tension, the pores become unstable and start to expand causing the rupture of the membrane. In the simulations the mechanical threshold tension necessary to cause rupture of the membrane on a nanosecond timescale is much higher in the case of the equilibrated bilayers, as compared with membranes containing preexisting pores. PMID:15041656

  2. Elastic deformation and failure of lipid bilayer membranes containing cholesterol.

    PubMed Central

    Needham, D; Nunn, R S

    1990-01-01

    Giant bilayer vesicles were reconstituted from several lipids and lipid/cholesterol (CHOL) mixtures: stearolyloleoylphosphatidylcholine (SOPC), bovine sphingomyelin (BSM), diarachidonylphosphatidylcholine (DAPC), SOPC/CHOL, BSM/CHOL, DAPC/CHOL, and extracted red blood cell (RBC) lipids with native cholesterol. Single-walled vesicles were manipulated by micropipette suction and several membrane material properties were determined. The properties measured were the elastic area compressibility modulus K, the critical areal strain alpha c, and the tensile strength tau lys, from which the failure energy or membrane toughness Tf was calculated. The elastic area expansion moduli for these lipid and lipid/cholesterol bilayers ranged from 57 dyn/cm for DAPC to 1,734 dyn/cm for BSM/CHOL. The SOPC/CHOL series and RBC lipids had intermediate values. The results indicated that the presence of cholesterol is the single most influential factor in increasing bilayer cohesion, but only for lipids where both chains are saturated, or mono- or diunsaturated. Multiple unsaturation in both lipid chains inhibits the condensing effect of cholesterol in bilayers. The SOPC/CHOL system was studied in more detail. The area expansion modulus showed a nonlinear increase with increasing cholesterol concentration up to a constant plateau, indicating a saturation limit for cholesterol in the bilayer phase of approximately 55 mol% CHOL. The membrane compressibility was modeled by a property-averaging composite theory involving two bilayer components, namely, uncomplexed lipid and a lipid/cholesterol complex of stoichiometry 1/1.22. The area expansion modulus of this molecular composite membrane was evaluated by a combination of the expansion moduli of each component scaled by their area fractions in the bilayer. Bilayer toughness, which is the energy stored in the bilayer at failure, showed a maximum value at approximately 40 mol% CHOL. This breakdown energy was found to be only a fraction of the

  3. Structure and Stability of the Spinach Aquaporin SoPIP2;1 in Detergent Micelles and Lipid Membranes

    PubMed Central

    Plasencia, Inés; Survery, Sabeen; Ibragimova, Sania; Hansen, Jesper S.; Kjellbom, Per; Helix-Nielsen, Claus; Johanson, Urban; Mouritsen, Ole G.

    2011-01-01

    Background SoPIP2;1 constitutes one of the major integral proteins in spinach leaf plasma membranes and belongs to the aquaporin family. SoPIP2;1 is a highly permeable and selective water channel that has been successfully overexpressed and purified with high yields. In order to optimize reconstitution of the purified protein into biomimetic systems, we have here for the first time characterized the structural stability of SoPIP2;1. Methodology/Principal Finding We have characterized the protein structural stability after purification and after reconstitution into detergent micelles and proteoliposomes using circular dichroism and fluorescence spectroscopy techniques. The structure of SoPIP2;1 was analyzed either with the protein solubilized with octyl-β-D-glucopyranoside (OG) or reconstituted into lipid membranes formed by E. coli lipids, diphytanoylphosphatidylcholine (DPhPC), or reconstituted into lipid membranes formed from mixtures of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE), 1-palmitoyl-2oleoyl-phosphatidylethanolamine (POPE), 1-palmitoyl-2-oleoyl-phosphatidylserine (POPS), and ergosterol. Generally, SoPIP2;1 secondary structure was found to be predominantly α-helical in accordance with crystallographic data. The protein has a high thermal structural stability in detergent solutions, with an irreversible thermal unfolding occurring at a melting temperature of 58°C. Incorporation of the protein into lipid membranes increases the structural stability as evidenced by an increased melting temperature of up to 70°C. Conclusion/Significance The results of this study provide insights into SoPIP2;1 stability in various host membranes and suggest suitable choices of detergent and lipid composition for reconstitution of SoPIP2;1 into biomimetic membranes for biotechnological applications. PMID:21339815

  4. Bacterial RTX toxins allow acute ATP release from human erythrocytes directly through the toxin pore.

    PubMed

    Skals, Marianne; Bjaelde, Randi G; Reinholdt, Jesper; Poulsen, Knud; Vad, Brian S; Otzen, Daniel E; Leipziger, Jens; Praetorius, Helle A

    2014-07-01

    ATP is as an extracellular signaling molecule able to amplify the cell lysis inflicted by certain bacterial toxins including the two RTX toxins α-hemolysin (HlyA) from Escherichia coli and leukotoxin A (LtxA) from Aggregatibacter actinomycetemcomitans. Inhibition of P2X receptors completely blocks the RTX toxin-induced hemolysis over a larger concentration range. It is, however, at present not known how the ATP that provides the amplification is released from the attacked cells. Here we show that both HlyA and LtxA trigger acute release of ATP from human erythrocytes that preceded and were not caused by cell lysis. This early ATP release did not occur via previously described ATP-release pathways in the erythrocyte. Both HlyA and LtxA were capable of triggering ATP release in the presence of the pannexin 1 blockers carbenoxolone and probenecid, and the HlyA-induced ATP release was found to be similar in erythrocytes from pannexin 1 wild type and knock-out mice. Moreover, the voltage-dependent anion channel antagonist TRO19622 had no effect on ATP release by either of the toxins. Finally, we showed that both HlyA and LtxA were able to release ATP from ATP-loaded lipid (1-palmitoyl-2-oleoyl-phosphatidylcholine) vesicles devoid of any erythrocyte channels or transporters. Again we were able to show that this happened in a non-lytic fashion, using calcein-containing vesicles as controls. These data show that both toxins incorporate into lipid vesicles and allow ATP to be released. We suggest that both toxins cause acute ATP release by letting ATP pass the toxin pores in both human erythrocytes and artificial membranes.

  5. 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

  6. Actin Assembly at Model-Supported Lipid Bilayers

    PubMed Central

    Heath, George R.; Johnson, Benjamin R.G.; Olmsted, Peter D.; Connell, Simon D.; Evans, Stephen D.

    2013-01-01

    We report on the use of supported lipid bilayers to reveal dynamics of actin polymerization from a nonpolymerizing subphase via cationic phospholipids. Using varying fractions of charged lipid, lipid mobility, and buffer conditions, we show that dynamics at the nanoscale can be used to control the self-assembly of these structures. In the case of fluid-phase lipid bilayers, the actin adsorbs to form a uniform two-dimensional layer with complete surface coverage whereas gel-phase bilayers induce a network of randomly oriented actin filaments, of lower coverage. Reducing the pH increased the polymerization rate, the number of nucleation events, and the total coverage of actin. A model of the adsorption/diffusion process is developed to provide a description of the experimental data and shows that, in the case of fluid-phase bilayers, polymerization arises equally due to the adsorption and diffusion of surface-bound monomers and the addition of monomers directly from the solution phase. In contrast, in the case of gel-phase bilayers, polymerization is dominated by the addition of monomers from solution. In both cases, the filaments are stable for long times even when the G-actin is removed from the supernatant—making this a practical approach for creating stable lipid-actin systems via self-assembly. PMID:24268147

  7. Atomistic mechanisms for bilayer growth of graphene on metal substrates

    SciTech Connect

    Chen, Wei; Cui, Ping; Zhu, Wenguang; Kaxiras, Efthimios; Gao, Yanfei; Zhang, Zhenyu

    2015-01-08

    Epitaxial growth on metal substrates has been shown to be the most powerful approach in producing large-scale high-quality monolayer graphene, yet it remains a major challenge to realize uniform bilayer graphene growth. Here we carry out a comparative study of the atomistic mechanisms for bilayer graphene growth on the (111) surfaces of Cu and Ni, using multiscale approaches combining first-principles calculations and rate-equation analysis. We first show that the relatively weak graphene-Cu interaction enhances the lateral diffusion and effective nucleation of C atoms underneath the graphene island, thereby making it more feasible to grow bilayer graphene on Cu. In contrast, the stronger graphene-Ni interaction suppresses the lateral mobility and dimerization of C atoms underneath the graphene, making it unlikely to achieve controlled growth of bilayer graphene on Ni. We then determine the critical graphene size beyond which nucleation of the second layer will take place. Intriguingly, the critical size exhibits an effective inverse "Ehrlich-Schwoebel barrier" effect, becoming smaller for faster C migration from the Cu surface to the graphene-Cu interface sites across the graphene edge. Lastly, these findings allow us to propose a novel alternating growth scheme to realize mass production of bilayer graphene.

  8. Atomistic mechanisms for bilayer growth of graphene on metal substrates

    DOE PAGES

    Chen, Wei; Cui, Ping; Zhu, Wenguang; Kaxiras, Efthimios; Gao, Yanfei; Zhang, Zhenyu

    2015-01-08

    Epitaxial growth on metal substrates has been shown to be the most powerful approach in producing large-scale high-quality monolayer graphene, yet it remains a major challenge to realize uniform bilayer graphene growth. Here we carry out a comparative study of the atomistic mechanisms for bilayer graphene growth on the (111) surfaces of Cu and Ni, using multiscale approaches combining first-principles calculations and rate-equation analysis. We first show that the relatively weak graphene-Cu interaction enhances the lateral diffusion and effective nucleation of C atoms underneath the graphene island, thereby making it more feasible to grow bilayer graphene on Cu. In contrast,more » the stronger graphene-Ni interaction suppresses the lateral mobility and dimerization of C atoms underneath the graphene, making it unlikely to achieve controlled growth of bilayer graphene on Ni. We then determine the critical graphene size beyond which nucleation of the second layer will take place. Intriguingly, the critical size exhibits an effective inverse "Ehrlich-Schwoebel barrier" effect, becoming smaller for faster C migration from the Cu surface to the graphene-Cu interface sites across the graphene edge. Lastly, these findings allow us to propose a novel alternating growth scheme to realize mass production of bilayer graphene.« less

  9. Formation of polymersomes with double bilayers templated by quadruple emulsions.

    PubMed

    Kim, Shin-Hyun; Nam, Jin; Kim, Jin Woong; Kim, Do-Hoon; Han, Sang-Hoon; Weitz, David A

    2013-04-01

    Polymersomes, vesicles composed of bilayer membranes of amphiphilic block-copolymers, are promising delivery vehicles for long-term storage and controlled release of bioactives; enhanced stability of the membrane makes polymersomes potentially useful in a wide range of biological delivery applications by comparison with liposomes. However, unilamellar structure is intrinsically fragile when subjected to external stress. Here, we report a microfluidic approach to produce polymersomes with double bilayers, providing higher stability and lower permeability than unilamellar polymersomes. To achieve this, we developed a new design of a capillary microfluidic device to produce quadruple-emulsion drops which serve as a template for the polymersomes-in-polymersomes. When two bilayers are attracted by depletion in polymersomes-in-polymersomes, the inner polymersomes protrude and bud, forming double bilayers. We confirm these structures are indeed double bilayers using microaspiration and selective doping of the leaflets with nanoparticles. The resultant polymersomes have great potential as highly stable and biocompatible microcarriers for robust encapsulation and storage of bioactives such as drugs, cosmetics and nutrients.

  10. Hydrogel-Stabilized Droplet Bilayers for High Speed Solution Exchange

    PubMed Central

    Acharya, Shiv A.; Portman, Alexander; Salazar, Carl S.; Schmidt, Jacob J.

    2013-01-01

    Many applications utilizing artificial lipid bilayers require the ability to exchange the bilayer's solution environment. However, because of the instability of the bilayer, the rate of solution exchange is limited, which significantly hinders the measurement rate and throughput. We have developed an artificial bilayer system that can withstand high flow speeds, up to 2.1 m/s, by supporting the bilayer with a hydrogel. We demonstrated the ability to measure during flow by measuring the conductance of gramicidin-A channels while switching between solutions of two different compositions, recording a time to measure 90% change in current of approximately 2.7 seconds at a flow rate of 0.1 m/s. We also demonstrated a potential application of this system by measuring the conductance modulation of the rat TRPM8 ion channel by an agonist and antagonist at varying concentrations, obtaining 7-point IC50 and EC50 values in approximately 7 minutes and 4-point values within 4 minutes. PMID:24190577

  11. Naphthalene Derivatives Induce Acyl Chain Interdigitation in Dipalmitoylphosphatidylcholine Bilayers.

    PubMed

    Kamal, Md Arif; Raghunathan, V A

    2016-01-14

    The interdigitated phase of the lipid bilayer results when acyl chains from opposing monolayers fully interpenetrate such that the terminal methyl groups of the respective lipid chains are located at the interfacial region on the opposite sides of the bilayer. Usually, chain interdigitation is not encountered in a symmetric chain phosphatidylcholine (PC) membrane but can be induced under certain special conditions. In this article, we elucidate the contribution of small amphiphatic molecules in altering the physical properties of a symmetric chain PC bilayer membrane, which results in acyl chain interdigitation. Using small-angle X-ray scattering (SAXS), we have carried out a systematic investigation of the physical interactions of three naphthalene derivatives containing hydroxyl groups: β-naphthol, 2,3-dihydroxynaphthalene, and 2,7-dihydroxynaphthalene, with dipalmitoylphosphatidylcholine (DPPC) bilayers. On the basis of the diffraction patterns, we have determined the temperature-composition phase diagrams of these binary mixtures. The present study not only enables us to gain insight into the role played by small molecules in altering the packing arrangement of the acyl chains of the constituting PC lipids of the bilayer but also brings to light some important features that have not yet been reported hitherto. One such feature is the stabilization of the enigmatic asymmetric ripple phase over a wide temperature and concentration range. The results presented here strongly point toward a clear correlation between chain interdigitation and the stability of the ripple phase.

  12. Modeling nanoparticle wrapping or translocation in bilayer membranes

    NASA Astrophysics Data System (ADS)

    Curtis, Emily M.; Bahrami, Amir H.; Weikl, Thomas R.; Hall, Carol K.

    2015-08-01

    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 10 Å to 250 Å show that hydrophilic nanoparticles with diameters greater than 20 Å become wrapped while the nanoparticle with a diameter of 10 Å does not. Instead this smaller particle became embedded in the bilayer surface where it can interact with the hydrophilic head groups of the lipid molecules. We also investigate the interaction between a DPPC bilayer and hydrophobic nanoparticles with diameters 10 Å to 40 Å. 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.

  13. Anomalous conductivity noise in gapped bilayer graphene heterostructure

    NASA Astrophysics Data System (ADS)

    Aamir, Mohammed Ali; Karnatak, Paritosh; Sai, T. Phanindra; Ghosh, Arindam

    Bilayer graphene has unique electronic properties - it has a tunable band gap and also, valley symmetry and pseudospin degree of freedom like its single layer counterpart. In this work, we present a study of conductance fluctuations in dual gated bilayer graphene heterostructures by varying the Fermi energy and the band gap independently. At a fixed band gap, we find that the conductance fluctuations obtained by Fermi energy ensemble sampling increase rapidly as the Fermi energy is tuned to charge neutrality point (CNP) whereas the time-dependent conductance fluctuations diminish rapidly. This discrepancy is completely absent at higher number densities, where the transport is expected to be through the 2D bulk of the bilayer system. This observation indicates that near the CNP, electrical transport is highly sensitive to Fermi energy, but becomes progressively immune to time-varying disorder. A possible explanation may involve transport via edge states which becomes the dominant conduction mechanism when the bilayer graphene is gapped and Fermi energy is situated close to the CNP, thereby causing a dimensional crossover from 2D to 1D transport. Our experiment outlines a possible experimental protocol to probe intrinsic topological states in gapped bilayer graphene.

  14. Modeling nanoparticle wrapping or translocation in bilayer membranes.

    PubMed

    Curtis, Emily M; Bahrami, Amir H; Weikl, Thomas R; Hall, Carol K

    2015-09-14

    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 10 Å to 250 Å show that hydrophilic nanoparticles with diameters greater than 20 Å become wrapped while the nanoparticle with a diameter of 10 Å does not. Instead this smaller particle became embedded in the bilayer surface where it can interact with the hydrophilic head groups of the lipid molecules. We also investigate the interaction between a DPPC bilayer and hydrophobic nanoparticles with diameters 10 Å to 40 Å. 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.

  15. Hydrogel-Stabilized Droplet Bilayers for High Speed Solution Exchange

    NASA Astrophysics Data System (ADS)

    Acharya, Shiv A.; Portman, Alexander; Salazar, Carl S.; Schmidt, Jacob J.

    2013-11-01

    Many applications utilizing artificial lipid bilayers require the ability to exchange the bilayer's solution environment. However, because of the instability of the bilayer, the rate of solution exchange is limited, which significantly hinders the measurement rate and throughput. We have developed an artificial bilayer system that can withstand high flow speeds, up to 2.1 m/s, by supporting the bilayer with a hydrogel. We demonstrated the ability to measure during flow by measuring the conductance of gramicidin-A channels while switching between solutions of two different compositions, recording a time to measure 90% change in current of approximately 2.7 seconds at a flow rate of 0.1 m/s. We also demonstrated a potential application of this system by measuring the conductance modulation of the rat TRPM8 ion channel by an agonist and antagonist at varying concentrations, obtaining 7-point IC50 and EC50 values in approximately 7 minutes and 4-point values within 4 minutes.

  16. Assessment of pseudo-bilayer structures in the heterogate germanium electron-hole bilayer tunnel field-effect transistor

    SciTech Connect

    Padilla, J. L. Alper, C.; Ionescu, A. M.; Medina-Bailón, C.; Gámiz, F.

    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 the 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.

  17. Neutron diffraction studies of amphipathic helices in phospholipid bilayers

    SciTech Connect

    Bradshaw, J.P.; Gilchrist, P.J.; Duff, K.C.; Saxena, A.M.

    1994-12-31

    The structural feature which is thought to facilitate the interaction of many peptides with phospholipid bilayers is the ability to fold into an amphipathic helix. In most cases the exact location and orientation of this helix with respect to the membrane is not known, and may vary with factors such as pH and phospholipid content of the bilayer. The growing interest in this area is stimulated by indications that similar interactions can contribute to the binding of certain hormones to their cell-surface receptors. We have been using the techniques of neutron diffraction from stacked phospholipid bilayers in an attempt to investigate this phenomenon with a number of membrane-active peptides. Here we report some of our findings with three of these: the bee venom melittin; the hormone calcitonin; and a synthetic peptide representing the ion channel fragment of influenza A M2 protein.

  18. CO adsorption on a silica bilayer supported on Ru(0001)

    NASA Astrophysics Data System (ADS)

    Schlexer, Philomena; Pacchioni, Gianfranco; Włodarczyk, Radosław; Sauer, Joachim

    2016-06-01

    Silica bilayers are built up of two layers of corner sharing SiO4-tetrahedra and constitute an inert ultra-thin membrane supported on the Ru(0001) surface. We have investigated the adsorption of CO on that system using DFT with inclusion of dispersion corrections. The molecules adsorb at the interface between the SiO2 film and Ru(0001) surface. The estimated barrier for diffusion of CO through the silica bilayer is around 0.5 eV. The CO bond length, the C-O stretching frequency and the silica-ruthenium distance depend strongly on the CO coverage. The band observed at 2051 cm- 1 in previous experiments can be assigned to a CO coverage of around 0.5 ML on Ru(0001), with the silica bilayer floating above the CO molecules.

  19. Hierarchically structured porous cadmium selenide polycrystals using polystyrene bilayer templates.

    PubMed

    Park, Jin Young; Hendricks, Nicholas R; Carter, Kenneth R

    2012-09-18

    In this study, a novel approach is demonstrated to fabricate hierarchically structured cadmium selenide (CdSe) layers with size-tunable nano/microporous morphologies achieved using polystyrene (PS) bilayered templates (top layer: colloidal template) via potentiostatic electrochemical deposition. The PS bilayer template is made in two steps. First, various PS patterns (stripes, ellipsoids, and circles) are prepared as the bottom layers through imprint lithography. In a second step, a top template is deposited that consists of a self-assembled layer of colloidal 2D packed PS particles. Electrochemical growth of CdSe crystals in the voids and selective removal of the PS bilayered templates give rise to hierarchically patterned 2D hexagonal porous CdSe structures. This simple and facile technique provides various unconventional porous CdSe films, arising from the effect of the PS bottom templates.

  20. Polyunsaturated Fatty Acids in Lipid Bilayers and Tubules

    NASA Astrophysics Data System (ADS)

    Hirst, Linda S.; Yuan, Jing; Pramudya, Yohannes; Nguyen, Lam T.

    2007-03-01

    Omega-3 polyunsaturated fatty acids (PUFAs) are found in a variety of biological membranes and have been implicated with lipid raft formation and possible function, typical molecules include DHA (Docosahexanoic Acid) and AA (Alphalinoleic Acid) which have been the focus of considerable attention in recent years. We are interested in the phase behavior of these molecules in the lipid bilayer. The addition of lipid molecules with polyunsaturated chains has a clear effect on the fluidity and curvature of the membrane and we investigate the effects the addition of polyunsaturated lipids on bilayer structure and tubule formation. Self-assembled cylindrical lipid tubules have attracted considerable attention because of their interesting structures and potential technological applications. Using x-ray diffraction techniques, Atomic Force Microscopy and confocal fluorescence imaging, both symmetric and mixed chain lipids were incorporated into model membranes and the effects on bilayer structure and tubule formation investigated.

  1. Anomalous Spectral Features of a Neutral Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Cheng, C.-M.; Xie, L. F.; Pachoud, A.; Moser, H. O.; Chen, W.; Wee, A. T. S.; Castro Neto, A. H.; Tsuei, K.-D.; Özyilmaz, B.

    2015-05-01

    Graphene and its bilayer are two-dimensional systems predicted to show exciting many-body effects near the neutrality point. The ideal tool to investigate spectrum reconstruction effects is angle-resolved photoemission spectroscopy (ARPES) as it probes directly the band structure with information about both energy and momentum. Here we reveal, by studying undoped exfoliated bilayer graphene with ARPES, two essential aspects of its many-body physics: the electron-phonon scattering rate has an anisotropic k-dependence and the type of electronic liquid is non-Fermi liquid. The latter behavior is evident from an observed electron-electron scattering rate that scales linearly with energy from 100 meV to 600 meV and that is associated with the proximity of bilayer graphene to a two-dimensional quantum critical point of competing orders.

  2. Lipid bilayer microarray for parallel recording of transmembrane ion currents.

    PubMed

    Le Pioufle, Bruno; Suzuki, Hiroaki; Tabata, Kazuhito V; Noji, Hiroyuki; Takeuchi, Shoji

    2008-01-01

    This paper describes a multiwell biochip for simultaneous parallel recording of ion current through transmembrane pores reconstituted in planar lipid bilayer arrays. Use of a thin poly(p-xylylene) (parylene) film having micrometer-sized apertures (phi=15-50 microm, t=20 microm) led to formation of highly stable bilayer lipid membranes (BLMs) for incorporation of transmembrane pores; thus, a large number of BLMs could be arrayed without any skillful technique. We optically confirmed the simultaneous formation of BLMs in a 5x5 matrix, and in our durability test, the BLM lasted more than 15 h. Simultaneous parallel recording of alamethicin and gramicidin transmembrane pores in multiple contiguous recording sites demonstrated the feasibility of high-throughput screening of transmembrane ion currents in artificial lipid bilayers.

  3. Interaction driven quantum Hall effect in artificially stacked graphene bilayers.

    PubMed

    Iqbal, Muhammad Zahir; Iqbal, Muhammad Waqas; Siddique, Salma; Khan, Muhammad Farooq; Ramay, Shahid Mahmood; Nam, Jungtae; Kim, Keun Soo; Eom, Jonghwa

    2016-01-01

    The honeycomb lattice structure of graphene gives rise to its exceptional electronic properties of linear dispersion relation and its chiral nature of charge carriers. The exceptional electronic properties of graphene stem from linear dispersion relation and chiral nature of charge carries, originating from its honeycomb lattice structure. Here, we address the quantum Hall effect in artificially stacked graphene bilayers and single layer graphene grown by chemical vapor deposition. The quantum Hall plateaus started to appear more than 3 T and became clearer at higher magnetic fields up to 9 T. Shubnikov-de Hass oscillations were manifestly observed in graphene bilayers texture. These unusual plateaus may have been due to the layers interaction in artificially stacked graphene bilayers. Our study initiates the understanding of interactions between artificially stacked graphene layers. PMID:27098387

  4. Exchange bias training effect in coupled all ferromagnetic bilayer structures.

    PubMed

    Binek, Ch; Polisetty, S; He, Xi; Berger, A

    2006-02-17

    Exchange coupled bilayers of soft and hard ferromagnetic thin films show remarkable analogies to conventional antiferromagnetic/ferromagnetic exchange bias heterostructures. Not only do all these ferromagnetic bilayers exhibit a tunable exchange bias effect, they also show a distinct training behavior upon cycling the soft layer through consecutive hysteresis loops. In contrast with conventional exchange bias systems, such all ferromagnetic bilayer structures allow the observation of training induced changes in the bias-setting hardmagnetic layer by means of simple magnetometry. Our experiments show unambiguously that the exchange bias training effect is driven by deviations from equilibrium in the pinning layer. A comparison of our experimental data with predictions from a theory based upon triggered relaxation phenomena shows excellent agreement.

  5. Intermonolayer Friction and Surface Shear Viscosity of Lipid Bilayer Membranes

    PubMed Central

    den Otter, W. K.; Shkulipa, S. A.

    2007-01-01

    The flow behavior of lipid bilayer membranes is characterized by a surface viscosity for in-plane shear deformations, and an intermonolayer friction coefficient for slip between the two leaflets of the bilayer. Both properties have been studied for a variety of coarse-grained double-tailed model lipids, using equilibrium and nonequilibrium molecular dynamics simulations. For lipids with two identical tails, the surface shear viscosity rises rapidly with tail length, while the intermonolayer friction coefficient is less sensitive to the tail length. Interdigitation of lipid tails across the bilayer midsurface, as observed for lipids with two distinct tails, strongly enhances the intermonolayer friction coefficient, but hardly affects the surface shear viscosity. The simulation results are compared against the available experimental data. PMID:17468168

  6. Detection of supported lipid bilayers with carbon nanotube transistors

    NASA Astrophysics Data System (ADS)

    Zhou, Xinjian; Moran-Mirabal, Jose; Craighead, Harold; McEuen, Paul

    2005-03-01

    Supported lipid bilayers are important synthetic structures that can be used to mimic and study the properties and functions of cellular membranes, as well as to perform various bioassays which involve membrane bound receptors. The fusion of phospholipid vesicles and formation of a supported lipid bilayer can be detected in real time with high sensitivity by carbon nanotube field effect transistors which have been patterned on the same substrate. The properties of different vesicles, such as fusion rates and phospholipid composition can be distinguished by the conductance change of carbon nanotube field effect transistors. Fluorescence is used to verify the formation of a supported lipid bilayer, although the detection scheme is label-free. This demonstrates that electrical detection with carbon nanotubes can provide a powerful tool for study of lipid membranes.

  7. Equilibrium Configurations of Lipid Bilayer Membranes and Carbon Nanostructures

    NASA Astrophysics Data System (ADS)

    Ivaïlo, M. Mladenov; Peter, A. Djondjorov; Mariana, Ts. Hadzhilazova; Vassil, M. Vassilev

    2013-02-01

    The present article concerns the continuum modelling of the mechanical behaviour and equilibrium shapes of two types of nano-scale objects: fluid lipid bilayer membranes and carbon nanostructures. A unified continuum model is used to handle four different case studies. Two of them consist in representing in analytic form cylindrical and axisymmetric equilibrium configurations of single-wall carbon nanotubes and fluid lipid bilayer membranes subjected to uniform hydrostatic pressure. The third one is concerned with determination of possible shapes of junctions between a single-wall carbon nanotube and a fiat graphene sheet or another single-wall carbon nanotube. The last one deals with the mechanical behaviour of closed fluid lipid bilayer membranes (vesicles) adhering onto a fiat homogeneous rigid substrate subjected to micro-injection and uniform hydrostatic pressure.

  8. Edge State and Intrinsic Hole Doping in Bilayer Phosphorene

    NASA Astrophysics Data System (ADS)

    Osada, Toshihito

    2015-01-01

    Using a simple LCAO model by Harrison, we have qualitatively studied the edge state of bilayer phosphorene, which is a unit structure of the layered crystal of black phosphorus. This model successfully reproduces the isolated edge state in the bulk gap in monolayer phosphorene. In bilayer phosphorene, however, it shows that edge states are almost buried in the valence band and there is no isolated midgap edge state at the zigzag edge. Since the buried edge state works as acceptor, holes are doped from the edge state into the bulk. This gives a possible explanation for p-type conduction in undoped black phosphorus. Under the vertical electric field, the intrinsic hole doping is reduced because a part of edge states move into the gap. These features of bilayer phosphorene might be better suited for device application.

  9. Electro-absorption of silicene and bilayer graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Abdelsalam, Hazem; Talaat, Mohamed H.; Lukyanchuk, Igor; Portnoi, M. E.; Saroka, V. A.

    2016-07-01

    We study numerically the optical properties of low-buckled silicene and AB-stacked bilayer graphene quantum dots subjected to an external electric field, which is normal to their surface. Within the tight-binding model, the optical absorption is calculated for quantum dots, of triangular and hexagonal shapes, with zigzag and armchair edge terminations. We show that in triangular silicene clusters with zigzag edges a rich and widely tunable infrared absorption peak structure originates from transitions involving zero energy states. The edge of absorption in silicene quantum dots undergoes red shift in the external electric field for triangular clusters, whereas blue shift takes place for hexagonal ones. In small clusters of bilayer graphene with zigzag edges the edge of absorption undergoes blue/red shift for triangular/hexagonal geometry. In armchair clusters of silicene blue shift of the absorption edge takes place for both cluster shapes, while red shift is inherent for both shapes of the bilayer graphene quantum dots.

  10. Anomalous spectral features of a neutral bilayer graphene.

    PubMed

    Cheng, C-M; Xie, L F; Pachoud, A; Moser, H O; Chen, W; Wee, A T S; Castro Neto, A H; Tsuei, K-D; Özyilmaz, B

    2015-01-01

    Graphene and its bilayer are two-dimensional systems predicted to show exciting many-body effects near the neutrality point. The ideal tool to investigate spectrum reconstruction effects is angle-resolved photoemission spectroscopy (ARPES) as it probes directly the band structure with information about both energy and momentum. Here we reveal, by studying undoped exfoliated bilayer graphene with ARPES, two essential aspects of its many-body physics: the electron-phonon scattering rate has an anisotropic k-dependence and the type of electronic liquid is non-Fermi liquid. The latter behavior is evident from an observed electron-electron scattering rate that scales linearly with energy from 100 meV to 600 meV and that is associated with the proximity of bilayer graphene to a two-dimensional quantum critical point of competing orders. PMID:25985064

  11. Modifiers of the Dipole Potential of Lipid Bilayers

    PubMed Central

    Efimova, S. S.; Ostroumova, O. S.

    2015-01-01

    This paper assesses the magnitude of change in the dipole potential (φd) of membranes caused by the adsorption of modifiers on lipid bilayers of various compositions. We tested flavonoids, muscle relaxants, thyroid hormones, and xanthene and styrylpyridinium dyes in order to assess their dipole-modifying properties. A quantitative description of the modifying action of flavonoids, muscle relaxants, thyroid hormones, and xanthene dyes is shown as the ratio of the maximum change in the bilayer dipole potential upon saturation and the absolute φd value of the unmodified membrane. The slopes of the linear relationship between the increase in the dipole potential of phospholipid bilayers and the concentration of styrylpyridinium dyes in membrane-bathing solutions were found. We described the relationships between the change in φd and the chemical structure of modifiers, as well as the charge and spontaneous curvature of lipid monolayers. PMID:26798493

  12. Interaction driven quantum Hall effect in artificially stacked graphene bilayers

    PubMed Central

    Iqbal, Muhammad Zahir; Iqbal, Muhammad Waqas; Siddique, Salma; Khan, Muhammad Farooq; Ramay, Shahid Mahmood; Nam, Jungtae; Kim, Keun Soo; Eom, Jonghwa

    2016-01-01

    The honeycomb lattice structure of graphene gives rise to its exceptional electronic properties of linear dispersion relation and its chiral nature of charge carriers. The exceptional electronic properties of graphene stem from linear dispersion relation and chiral nature of charge carries, originating from its honeycomb lattice structure. Here, we address the quantum Hall effect in artificially stacked graphene bilayers and single layer graphene grown by chemical vapor deposition. The quantum Hall plateaus started to appear more than 3 T and became clearer at higher magnetic fields up to 9 T. Shubnikov-de Hass oscillations were manifestly observed in graphene bilayers texture. These unusual plateaus may have been due to the layers interaction in artificially stacked graphene bilayers. Our study initiates the understanding of interactions between artificially stacked graphene layers. PMID:27098387

  13. The mechanism of detergent solubilization of lipid bilayers.

    PubMed

    Lichtenberg, Dov; Ahyayauch, Hasna; Goñi, Félix M

    2013-07-16

    Multiple data are available on the self-assembly of mixtures of bilayer-forming amphiphiles, particularly phospholipids and micelle-forming amphiphiles, commonly denoted detergents. The structure of such mixed assemblies has been thoroughly investigated, described in phase diagrams, and theoretically rationalized in terms of the balance between the large spontaneous curvature of the curvophilic detergent and the curvophobic phospholipids. In this critical review, we discuss the mechanism of this process and try to explain the actual mechanism involved in solubilization. Interestingly, membrane solubilization by some detergents is relatively slow and the common attribute of these detergents is that their trans-bilayer movement, commonly denoted flip-flop, is very slow. Only detergents that can flip into the inner monolayer cause relatively rapid solubilization of detergent-saturated bilayers. This occurs via the following sequence of events: 1), relatively rapid penetration of detergent monomers into the outer monolayer; 2), trans-membrane equilibration of detergent monomers between the two monolayers; 3), saturation of the bilayer by detergents and consequent permeabilization of the membrane; and 4), transition of the whole bilayer to thread-like mixed micelles. When the detergent cannot flip to the inner monolayer, the outer monolayer becomes unstable due to mass imbalance between the monolayers and inclusion of the curvophilic detergent molecules in a flat surface. Consequently, the outer monolayer forms mixed micellar structures within the outer monolayer. Shedding of these micelles into the aqueous solution results in partial solubilization. The consequent leakage of detergent into the liposome results in trans-membrane equilibration of detergent and subsequent micellization through the rapid bilayer-saturation mechanism.

  14. Lipid bilayer modules as determinants of K+ channel gating.

    PubMed

    Syeda, Ruhma; Santos, Jose S; Montal, Mauricio

    2014-02-14

    The crystal structure of the sensorless pore module of a voltage-gated K(+) (Kv) channel showed that lipids occupy a crevice between subunits. We asked if individual lipid monolayers of the bilayer embody independent modules linked to channel gating modulation. Functional studies using single channel current recordings of the sensorless pore module reconstituted in symmetric and asymmetric lipid bilayers allowed us to establish the deterministic role of lipid headgroup on gating. We discovered that individual monolayers with headgroups that coat the bilayer-aqueous interface with hydroxyls stabilize the channel open conformation. The hydroxyl need not be at a terminal position and the effect is not dependent on the presence of phosphate or net charge on the lipid headgroup. Asymmetric lipid bilayers allowed us to determine that phosphoglycerides with glycerol or inositol on the extracellular facing monolayer stabilize the open conformation of the channel. This indirect effect is attributed to a change in water structure at the membrane interface. By contrast, inclusion of the positively charged lysyl-dioleoyl-phosphatidylglycerol exclusively on the cytoplasmic facing monolayer of the bilayer increases drastically the probability of finding the channel open. Such modulation is mediated by a π-cation interaction between Phe-19 of the pore module and the lysyl moiety anchored to the phosphatidylglycerol headgroup. The new findings imply that the specific chemistry of the lipid headgroup and its selective location in either monolayer of the bilayer dictate the stability of the open conformation of a Kv pore module in the absence of voltage-sensing modules.

  15. Manipulating lipid bilayer material properties using biologically active amphipathic molecules

    NASA Astrophysics Data System (ADS)

    Ashrafuzzaman, Md; Lampson, M. A.; Greathouse, D. V.; Koeppe, R. E., II; Andersen, O. S.

    2006-07-01

    Lipid bilayers are elastic bodies with properties that can be manipulated/controlled by the adsorption of amphipathic molecules. The resulting changes in bilayer elasticity have been shown to regulate integral membrane protein function. To further understand the amphiphile-induced modulation of bilayer material properties (thickness, intrinsic monolayer curvature and elastic moduli), we examined how an enantiomeric pair of viral anti-fusion peptides (AFPs)—Z-Gly-D-Phe and Z-Gly-Phe, where Z denotes a benzyloxycarbonyl group, as well as Z-Phe-Tyr and Z-D-Phe-Phe-Gly—alters the function of enantiomeric pairs of gramicidin channels of different lengths in planar bilayers. For both short and long channels, the channel lifetimes and appearance frequencies increase as linear functions of the aqueous AFP concentration, with no apparent effect on the single-channel conductance. These changes in channel function do not depend on the chirality of the channels or the AFPs. At pH 7.0, the relative changes in channel lifetimes do not vary when the channel length is varied, indicating that these compounds exert their effects primarily by causing a positive-going change in the intrinsic monolayer curvature. At pH 4.0, the AFPs are more potent than at pH 7.0 and have greater effects on the shorter channels, indicating that these compounds now change the bilayer elastic moduli. When AFPs of different anti-fusion potencies are compared, the rank order of the anti-fusion activity and the channel-modifying activity is similar, but the relative changes in anti-fusion potency are larger than the changes in channel-modifying activity. We conclude that gramicidin channels are useful as molecular force transducers to probe the influence of small amphiphiles upon lipid bilayer material properties.

  16. [Calcium-induced changes in bilayer membranes from oxidized cholesterol].

    PubMed

    Hianik, T; Miklovichova, J; Foltinova, O; Bajchi, A

    1985-01-01

    Elastic properties of oxidized cholesterol bilayers in n-octane and membrane solvent free were studied by measuring Young modulus E perpendicular in the direction perpendicular to the membrane plane as a function of concentration of calcium ions. Interaction between calcium ions and solvent free bilayers resulted in a significant increases of Young modulus E perpendicular in the concentration range 20-40 mmol/l Ca2+. It is suggested that the hardening of the membrane is caused by some structural changes in the hydrophobic region of the membrane.

  17. Prediction of superconductivity in Li-intercalated bilayer phosphorene

    SciTech Connect

    Huang, G. Q.; Xing, Z. W.; Xing, D. Y.

    2015-03-16

    It is shown that bilayer phosphorene can be transformed from a direct-gap semiconductor to a BCS superconductor by intercalating Li atoms. For the Li-intercalated bilayer phosphorene, we find that the electron occupation of Li-derived band is small and superconductivity is intrinsic. With increasing the intercalation of Li atoms, both increased metallicity and strong electron-phonon coupling are favorable for the enhancement of superconductivity. The obtained electron-phonon coupling λ can be larger than 1 and the superconducting temperature T{sub c} can be increased up to 16.5 K, suggesting that phosphorene may be a good candidate for a nanoscale superconductor.

  18. Proposed parametric cooling of bilayer cuprate superconductors by terahertz excitation.

    PubMed

    Denny, S J; Clark, S R; Laplace, Y; Cavalleri, A; Jaksch, D

    2015-04-01

    We propose and analyze a scheme for parametrically cooling bilayer cuprates based on the selective driving of a c-axis vibrational mode. The scheme exploits the vibration as a transducer making the Josephson plasma frequencies time dependent. We show how modulation at the difference frequency between the intrabilayer and interbilayer plasmon substantially suppresses interbilayer phase fluctuations, responsible for switching c-axis transport from a superconducting to a resistive state. Our calculations indicate that this may provide a viable mechanism for stabilizing nonequilibrium superconductivity even above Tc, provided a finite pair density survives between the bilayers out of equilibrium.

  19. Interphases of chain molecules: Monolayers and lipid bilayer membranes

    PubMed Central

    Dill, Ken A.; Flory, Paul J.

    1980-01-01

    Using the lattice model for a liquid, we treat the packing of short-chain molecules in interphases such as bilayer membranes. The constant density in the interphase imposes intermolecular constraints on the configurations of the flexible chains. The statistical theory here presented predicts a diffuse distribution of chain ends near the bilayer midplane; no adjustable parameters are required. Inasmuch as some of the chains terminate relatively near the polar interface, the number of chains reaching deeper planar layers is diminished. Consequently, configurational freedom increases with depth. This is the source of the well-known disorder gradient. PMID:16592834

  20. Teflon™-coated silicon apertures for supported lipid bilayer membranes

    NASA Astrophysics Data System (ADS)

    Wilk, S. J.; Goryll, M.; Laws, G. M.; Goodnick, S. M.; Thornton, T. J.; Saraniti, M.; Tang, J.; Eisenberg, R. S.

    2004-10-01

    We present a method for microfabricating apertures in a silicon substrate using well-known cleanroom technologies resulting in highly reproducible giga-seal resistance bilayer formations. Using a plasma etcher, 150μm apertures have been etched through a silicon wafer. Teflon™ has been chemically vapor deposited so that the surface resembles bulk Teflon and is hydrophobic. After fabrication, reproducible high resistance bilayers were formed and characteristic measurements of a self-inserted single OmpF porin ion channel protein were made.

  1. Proposed parametric cooling of bilayer cuprate superconductors by terahertz excitation.

    PubMed

    Denny, S J; Clark, S R; Laplace, Y; Cavalleri, A; Jaksch, D

    2015-04-01

    We propose and analyze a scheme for parametrically cooling bilayer cuprates based on the selective driving of a c-axis vibrational mode. The scheme exploits the vibration as a transducer making the Josephson plasma frequencies time dependent. We show how modulation at the difference frequency between the intrabilayer and interbilayer plasmon substantially suppresses interbilayer phase fluctuations, responsible for switching c-axis transport from a superconducting to a resistive state. Our calculations indicate that this may provide a viable mechanism for stabilizing nonequilibrium superconductivity even above Tc, provided a finite pair density survives between the bilayers out of equilibrium. PMID:25884134

  2. Interactions between lipid bilayers and inorganic material surfaces

    NASA Astrophysics Data System (ADS)

    Mager, Morgan Douglas

    Because of their unique biological and material properties, lipid bilayers have been extensively studied for use in biosensor and drug delivery applications. In the past, these systems have mostly taken the form of bulk solutions. More recently, researchers have integrated bilayers with chip-based architectures to take advantage of advanced optical, scanning probe and electronic characterization. These applications typically involve the creation of hybrid devices with inorganic and bilayer components, both of which affect the final device performance. In particular, the properties of supported lipid bilayers (SLBs) are known to depend on the substrate chemistry and topography as well as the lipid used. In spite of the large body of work involving these systems, there is still much that remains unknown about the formation and ultimate structure of the interface between these very different materials. One outstanding question in the study of SLBs is the role that the bilayer deposition method plays in determining the bilayer properties. In this work, we have developed a new method for forming and patterning lipid bilayers: bubble collapse deposition (BCD). This method is similar to an in situ version of Langmuir-Blodgett deposition, and offers unique possibilities for the fabrication of lipid-based devices. Briefly, a lipid monolayer is "inked" onto the surface of an air bubble. This bubble is then brought down on a solid support and the air is withdrawn. This withdrawal of air shrinks the bubble, which causes the monolayer to fold over on itself and redeposit on the surface as a bilayer. With BCD, we have demonstrated the first SLB formation on alumina using uncharged lipids. Using this system, we have measured a previously unobserved enhanced hydrodynamic coupling at the alumina surface. We have also used BCD to produce a hybrid lipid-gated chemical delivery device with potentially sub-cellular spatial resolution. Because of the unique material properties of the

  3. Bilayer resist system utilizing alkali-developable organosilicon positive photoresist

    NASA Astrophysics Data System (ADS)

    Nate, Kazuo; Mizushima, Akiko; Sugiyama, Hisashi

    1991-06-01

    A bi-layer resist system utilizing an alkali-developable organosilicon positive photoresist (OSPR) has been developed. The composite prepared from an alkali-soluble organosilicon polymer, poly(p- hydroxybenzylsilsesquioxane) and naphthoquinone diazide becomes a alkali-developable positive photoresist which is sensitive to UV (i line - g line) region, and exhibited high oxygen reactive ion etching (O2 RIE) resistance. The sensitivity and the resolution of OSPR are almost the same as those of conventional novolac-based positive photoresists. The bi-layer resist system utilizing OSPR as the top imaging layer gave fine patterns of underlayers with high aspect ratio easily.

  4. Global phase diagram of bilayer quantum Hall ferromagnets

    NASA Astrophysics Data System (ADS)

    Abolfath, Ramin; Radzihovsky, Leo; MacDonald, Allan

    2002-03-01

    We present a microscopic study of the interlayer spacing d versus in-plane magnetic field B_allel phase diagram for bilayer quantum Hall (QH) pseudo-ferromagnets [1]. In addition to the interlayer charge balanced commensurate and incommensurate states studied previously [2], we nalyze microscopically the corresponding interlayer charge unbalanced canted QH states, predicted in Ref. [3]. We predict a large anomaly in the bilayer capacitance at the canting transition and the formation of dipole stripe domains with periods exceeding 1 micron in the canted state. [1] M. Abolfath et al. cond-mat/0110049. [2] Yang et al. PRL 72, 732 (1994). [3] Leo Radzihovsky PRL 87, 236802 (2001).

  5. hERG drug response measured in droplet bilayers.

    PubMed

    Portonovo, Shiva A; Salazar, Carl S; Schmidt, Jacob J

    2013-04-01

    We show measurements of the human cardiac potassium ion channel Kv11.1 (hERG) in droplet bilayers incorporated directly from commercial membrane preparations of HEK293 cells. Although we do not obtain ensemble conductance kinetics and rectification observed in patch clamp measurements of hERG, ensemble currents measured in our system showed inhibition dependent on astemizole and E-4031 concentration, with IC50 values similar to those found with patch clamp. The availability of engineered HEK cells expressing a variety of ion channels, combined with the simplicity of the inhibition measurement, suggest that droplet bilayers may have considerable technological potential for determination of ion channel drug potency. PMID:23160842

  6. Gate-Tunable Resonant Tunneling in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Fallahazad, Babak; Lee, Kayoung; Kang, Sangwoo; Xue, Jiamin; Larentis, Stefano; Corbet, Christopher; Kim, Kyounghwan; Movva, Hema C. P.; Taniguchi, Takashi; Watanabe, Kenji; Register, Leonard F.; Banerjee, Sanjay K.; Tutuc, Emanuel

    2015-01-01

    We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current-voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron-nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

  7. Scanning probe microscopy investigation of bilayered manganites

    NASA Astrophysics Data System (ADS)

    Huang, Junwei

    The bilayered manganite La2-2xSr 1+2xMn2O7, with x in the ferromagnetic compositional region, exhibits very interesting electronic and magnetic properties below the Curie temperature, such as a colossal magneto-resistance (CMR) effect. We have studied the microscopic electronic structure in the x = 0.32, 0.4 compounds at 80 K and 20 K by using a home-built low temperature scanning tunneling microscope (STM) and the evolution of the ferromagnetic domains with temperature and magnetic field in the x = 0.32 compound from 30 K to 110 K by using a home-built low temperature magnetic force microscope (MFM). STM topographic images show nano-sized patterns composed of Mn 3+-rich and Mn4+-rich regions in the mixed-valent matrix. Tunneling spectra I(V)& dIdV (V) show a gap and a tunneling asymmetry of the LDOS as a function of the sample bias voltage. By using current-imaging tunneling spectroscopy (CITS), we obtained a series of tunneling conductance maps which show the coexistence of localized electrons and itinerant electrons in this system. In the x = 0.32 compound, we observed a modulation with a wave vector of 16 A propagating along a-axis at 20K. This indicates the formation of a charge density wave as a result of Fermi surface nesting in this system. In MFM images, we observed that below 60 K, the ferromagnetic (FM) domains form stable treelike patterns and the domains are mainly oriented in the out-of-plane direction. As the temperature increases, the FM domains begin to experience a gradual change. This change becomes more and more rapid above 80 K. The FM domains change their magnetization from the out-of-plane direction to in-plane around 88 K. The in-plane FM domains completely disappear near T C. We also observed thermal hysteresis occurring in magnetic structures. We conclude that the formation of FM domains at low temperatures is determined by the energy associated with surface magnetic free poles and domain walls. At high temperatures, the two

  8. Solvent-free simulations of heterogeneous lipid bilayers

    NASA Astrophysics Data System (ADS)

    Brannigan, Grace H.

    Two molecular implicit solvent models for fluid lipid bilayers are presented. The first represents lipids by rigid, asymmetric, soft spherocylinders. The three parameter potential between pairs of lipids gives rise to micelles, fluid bilayers, and gel-like bilayers. Fluid bilayers have compressibility moduli in agreement with experimental systems but display bending moduli at least three times larger than typical biological membranes without cholesterol. The second three parameter model represents lipids by flexible chains of beads; the hydrophobic effect is mimicked through a soft pair potential localized at the interface between hydrophobic and hydrophilic beads. Fluid bilayers composed of these molecules have both compressibility moduli and elastic moduli in agreement with experimental systems, as well as realistic interfacial tensions and stress profiles. Monte Carlo simulations are used to demonstrate self-assembly for both models. Phase behavior is studied for bilayers composed of the rigid model. Regions of solid, "hexatic", and fluid bilayer behavior are established by identification of phase boundaries. The main melting transition is found to be density driven; the melting temperature scales inversely with lipid length since thermal expansion increases with lipid aspect ratio. A plausible sub-transition is identified for longer molecules. The dependence of membrane elasticity on bilayer thickness is obtained by adjusting the length of the rigid molecules at constant temperature and surface tension. The bending modulus scales with the square of the membrane thickness, as expected, but the proportionality constant is an order of magnitude smaller than expected using continuum elastic theories or measured by experiments. The proportionality constant is found to be non-monotonic for bilayers composed of multiple lipid species; the thinnest membranes are not the most flexible. This is shown to be quantitatively consistent with the random quadratic mixing

  9. Efficient tunable generic model for self-assembling fluid bilayer membranes

    NASA Astrophysics Data System (ADS)

    Deserno, Markus

    2005-03-01

    We present a new model for the simulation of generic lipid bilayers in the mesoscopic regime (between a few nanometers and many tens of nanometers), which is very robust, versatile, and extremely efficient, since it avoids the need for an embedding solvent. Based entirely on simple pair potentials, it features a wide region of unassisted self assembly into fluid bilayers without the need for careful parameter tuning. The resulting membranes display the correct continuum elastic behavior with bending constants in the experimentally relevant range. It can be readily used to study events like bilayer fusion, bilayer melting, lipid mixtures, rafts, and protein-bilayer interactions.

  10. Homogeneous immunoassay based on dihexadecyl-dimethylammonium bilayer membranes.

    PubMed

    Ihara, H; Imamura, T; Kunitake, T; Hirayama, C

    1988-06-30

    We describe a rapid, highly sensitive, simple immunoassay based on cationic bilayer membranes prepared from dihexadecyl-dimethylammonium bromide. This detergent was dispersed in water to give a slightly turbid solution which contained lamellar (not vesicular) bilayer membranes with a gel to liquid crystal phase transition (Tc, 28 degrees C). An anionic hapten (N,N'-bis(2,4-dinitrophenyl)-L-lysine) combined electrostatically with this cationic membrane to induce aggregation of membranes, making the solution turbid. This aggregation was suppressed by antibodies to the hapten. The aggregation rate was rapid at a temperature above Tc, and the amount of antibodies was quantified by measuring absorbance or light scattering of the aqueous solution with a detection limit of 0.1 ng/ml of the antibodies. Anionic and zwitterionic bilayer membranes from di(tetradecyl)phosphate and phosphocholine derivatives, respectively, were less sensitive as detectors of the hapten-antibody reaction. Ionic interactions between cationic bilayer membranes and anionic haptens are important for successful assay performance.

  11. Dynamics and instabilities of lipid bilayer membrane shapes.

    PubMed

    Shi, Zheng; Baumgart, Tobias

    2014-06-01

    Biological membranes undergo constant shape remodeling involving the formation of highly curved structures. The lipid bilayer represents the fundamental architecture of the cellular membrane with its shapes determined by the Helfrich curvature bending energy. However, the dynamics of bilayer shape transitions, especially their modulation by membrane proteins, and the resulting shape instabilities, are still not well understood. Here, we review in a unifying manner several theories that describe the fluctuations (i.e. undulations) of bilayer shapes as well as their local coupling with lipid or protein density variation. The coupling between local membrane curvature and lipid density gives rise to a 'slipping mode' in addition to the conventional 'bending mode' for damping the membrane fluctuation. This leads to a number of interesting experimental phenomena regarding bilayer shape dynamics. More importantly, curvature-inducing proteins can couple with membrane shape and eventually render the membrane unstable. A criterion for membrane shape instability is derived from a linear stability analysis. The instability criterion reemphasizes the importance of membrane tension in regulating the stability and dynamics of membrane geometry. Recent progresses in understanding the role of membrane tension in regulating dynamical cellular processes are also reviewed. Protein density is emphasized as a key factor in regulating membrane shape transitions: a threshold density of curvature coupling proteins is required for inducing membrane morphology transitions. PMID:24529968

  12. Statistical mechanics and dynamics of two supported stacked lipid bilayers.

    PubMed

    Manghi, Manoel; Destainville, Nicolas

    2010-03-16

    The statistical physics and dynamics of double supported bilayers are studied theoretically. The main goal in designing double supported lipid bilayers is to obtain model systems of biomembranes: the upper bilayer is meant to be almost freely floating, the substrate being screened by the lower bilayer. The fluctuation-induced repulsion between membranes and between the lower membrane and the wall are explicitly taken into account using a Gaussian variational approach. It is shown that the variational parameters, the "effective" adsorption strength, and the average distance to the substrate, depend strongly on temperature and membrane elastic moduli, the bending rigidity, and the microscopic surface tension, which is a signature of the crucial role played by membrane fluctuations. The range of stability of these supported membranes is studied, showing a complex dependence on bare adsorption strengths. In particular, the experimental conditions of having an upper membrane slightly perturbed by the lower one and still bound to the surface are found. Included in the theoretical calculation of the damping rates associated with membrane normal modes are hydrodynamic friction by the wall and hydrodynamic interactions between both membranes. PMID:20000797

  13. Dynamics and instabilities of lipid bilayer membrane shapes

    PubMed Central

    Shi, Zheng; Baumgart, Tobias

    2014-01-01

    Biological membranes undergo constant shape remodeling involving the formation of highly curved structures. The lipid bilayer represents the fundamental architecture of the cellular membrane with its shapes determined by the Helfrich curvature bending energy. However, the dynamics of bilayer shape transitions, especially their modulation by membrane proteins, and the resulting shape instabilities, are still not well understood. Here, we review in a unifying manner several theories that describe the fluctuations (i.e. undulations) of bilayer shapes as well as their local coupling with lipid or protein density variation. The coupling between local membrane curvature and lipid density gives rise to a ‘slipping mode’ in addition to the conventional ‘bending mode’ for damping the membrane fluctuation. This leads to a number of interesting experimental phenomena regarding bilayer shape dynamics. More importantly, curvature-inducing proteins can couple with membrane shape and eventually render the membrane unstable. A criterion for membrane shape instability is derived from a linear stability analysis. The instability criterion reemphasizes the importance of membrane tension in regulating the stability and dynamics of membrane geometry. Recent progresses in understanding the role of membrane tension in regulating dynamical cellular processes are also reviewed. Protein density is emphasized as a key factor in regulating membrane shape transitions: a threshold density of curvature coupling proteins is required for inducing membrane morphology transitions. PMID:24529968

  14. Polydimethylsiloxane bilayer films with an embedded spontaneous curvature.

    PubMed

    Egunov, A I; Korvink, J G; Luchnikov, V A

    2016-01-01

    Elastomer polydimethylsiloxane (PDMS) films with embedded in-plane gradient stress are created by making PDMS/(PDMS + silicone oil) crosslinked bilayers and extracting the oil in a suitable organic solvent bath. The collapse of the elastomer after oil extraction generates differential stress in the films that is manifested through their out-of-plane deformation. The curvature κ of narrow stripes of the bilayer, which is composed of layers of approximately equal thicknesses and elasticity moduli, is satisfactorily described by the simple relationship κ = 1.5δH(-1), where δ is the mechanical strain, and H is the total thickness of the bilayer. Curvature mapping of triangular PDMS plates reveals the existence of spherical and cylindrical types of deformation at different locations of the plates. Various 3D-shaped objects can be formed by the self-folding of appropriately designed 2D patterns that are cut from the films, or by nonuniform distribution of the collapsing layer. Thin PDMS bilayers with embedded stress roll up into microtubes of almost perfect cylindrical shape when released in a controlled manner from a substrate. PMID:26539638

  15. Magnetoresistance of heavy and light metal/ferromagnet bilayers

    SciTech Connect

    Avci, Can Onur; Garello, Kevin; Mendil, Johannes; Ghosh, Abhijit; Blasakis, Nicolas; Gabureac, Mihai; Trassin, Morgan; Fiebig, Manfred; Gambardella, Pietro

    2015-11-09

    We studied the magnetoresistance of normal metal (NM)/ferromagnet (FM) bilayers in the linear and nonlinear (current-dependent) regimes and compared it with the amplitude of the spin-orbit torques and thermally induced electric fields. Our experiments reveal that the magnetoresistance of the heavy NM/Co bilayers (NM = Ta, W, and Pt) is phenomenologically similar to the spin Hall magnetoresistance (SMR) of YIG/Pt, but has a much larger anisotropy of the order of 0.5%, which increases with the atomic number of the NM. This SMR-like behavior is absent in light NM/Co bilayers (NM = Ti and Cu), which present the standard anisotropic magnetoresistance expected from polycrystalline FM layers. In the Ta, W, and Pt/Co bilayers, we find an additional magnetoresistance directly proportional to the current and to the transverse component of the magnetization. This so-called unidirectional SMR, of the order of 0.005%, is largest in W and correlates with the amplitude of the antidamping spin-orbit torque. The unidirectional SMR is below the accuracy of our measurements in YIG/Pt.

  16. Normal and Frictional Interactions between Liposome-Bearing Biomacromolecular Bilayers.

    PubMed

    Gaisinskaya-Kipnis, Anastasia; Klein, Jacob

    2016-08-01

    Highly efficient lubricating boundary layers at biosurfaces such as cartilage have been proposed to comprise phospholipids complexed with biomacromolecules exposed at the surfaces. To gain insight into this, a systematic study on the normal and frictional forces between surfaces bearing a sequentially deposited model alginate-on-chitosan bilayer, bearing different adsorbed phosphatidylcholine (PC) liposomes, was carried out using a surface force balance. Structures of the resulting surface complexes were determined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). The liposome/lipid-polymer complexes could maintain their integrity up to high pressures in terms of both normal and shear interactions between the surfaces, which were repeatable, reproducible, and revealed very low friction (coefficient of friction μ down to 10(-3)-10(-4), depending on the PC used) up to pressures of hundreds of atm. We attribute this remarkable lubrication capability ultimately to hydration lubrication acting at the hydrated phosphocholine headgroups of the PC lipids, either exposed at the liposome surfaces or through complexation with the polyelectrolyte bilayer. Values of μ, while low, were roughly an order of magnitude higher than for the same PC vesicles adsorbed on bare mica, a difference attributed to their lower density on the bilayer; the bilayer, however, stabilized the PC-vesicles far better than bare mica against rupture and shear at high compressions and sliding. PMID:27409248

  17. Stable high conductivity ceria/bismuth oxide bilayered electrolytes

    SciTech Connect

    Wachsman, E.D.; Jayaweera, P.; Jiang, N.; Lowe, D.M.; Pound, B.G.

    1997-01-01

    The authors have developed a high conductivity bilayered ceria/bismuth oxide anolyte/electrolyte that uses the Po{sub 2} gradient to obtain stability at the anolyte-electrolyte interface and reduced electronic conduction due to the electrolyte region. Results in terms of solid oxide fuel cell (SOFC) performance and stability are presented. These results include a 90 to 160 mV increase in open-circuit potential, depending on temperature, with the bilayered structure as compared to SOFCs fabricated from a single ceria layer. An open-circuit potential of >1.0 V was obtained at 500 C with the bilayered structure. This increase in open-circuit potential is obtained without any measurable increase in cell resistance and is stable for over 1,400 h of testing, under both open-circuit and maximum power conditions. Moreover, SOFCs fabricated from the bilayered structure result in a 33% greater power density as compared to cells with a single ceria electrolyte layer.

  18. Anthrax toxin-induced rupture of artificial lipid bilayer membranes

    NASA Astrophysics Data System (ADS)

    Nablo, Brian J.; Panchal, Rekha G.; Bavari, Sina; Nguyen, Tam L.; Gussio, Rick; Ribot, Wil; Friedlander, Art; Chabot, Donald; Reiner, Joseph E.; Robertson, Joseph W. F.; Balijepalli, Arvind; Halverson, Kelly M.; Kasianowicz, John J.

    2013-08-01

    We demonstrate experimentally that anthrax toxin complexes rupture artificial lipid bilayer membranes when isolated from the blood of infected animals. When the solution pH is temporally acidified to mimic that process in endosomes, recombinant anthrax toxin forms an irreversibly bound complex, which also destabilizes membranes. The results suggest an alternative mechanism for the translocation of anthrax toxin into the cytoplasm.

  19. Lipid Bilayer Membrane Perturbation by Embedded Nanopores: A Simulation Study.

    PubMed

    Garcia-Fandiño, Rebeca; Piñeiro, Ángel; Trick, Jemma L; Sansom, Mark S P

    2016-03-22

    A macromolecular nanopore inserted into a membrane may perturb the dynamic organization of the surrounding lipid bilayer. To better understand the nature of such perturbations, we have undertaken a systematic molecular dynamics simulation study of lipid bilayer structure and dynamics around three different classes of nanopore: a carbon nanotube, three related cyclic peptide nanotubes differing in the nature of their external surfaces, and a model of a β-barrel nanopore protein. Periodic spatial distributions of several lipid properties as a function of distance from the nanopore were observed. This was especially clear for the carbon nanotube system, for which the density of lipids, the bilayer thickness, the projection of lipid head-to-tail vectors onto the membrane plane, and lipid lateral diffusion coefficients exhibited undulatory behavior as a function of the distance from the surface of the channel. Overall, the differences in lipid behavior as a function of the nanopore structure reveal local adaptation of the bilayer structure and dynamics to different embedded nanopore structures. Both the local structure and dynamic behavior of lipids around membrane-embedded nanopores are sensitive to the geometry and nature of the outer surface of the macromolecule/molecular assembly forming the pore.

  20. Negative terahertz conductivity in disordered graphene bilayers with population inversion

    SciTech Connect

    Svintsov, D.; Otsuji, T.; Ryzhii, V.; Mitin, V.; Shur, M. S.

    2015-03-16

    The gapless energy band spectra make the structures based on graphene and graphene bilayer with the population inversion to be promising media for the interband terahertz (THz) lasing. However, a strong intraband absorption at THz frequencies still poses a challenge for efficient THz lasing. In this paper, we show that in the pumped graphene bilayer, the indirect interband radiative transitions accompanied by scattering of carriers by disorder can provide a substantial negative contribution to the THz conductivity (together with the direct interband transitions). In the graphene bilayer on high-κ substrates with point charged defects, these transitions substantially compensate the losses due to the intraband (Drude) absorption. We also demonstrate that the indirect interband contribution to the THz conductivity in a graphene bilayer with the extended defects (such as the charged impurity clusters) can surpass by several times the fundamental limit associated with the direct interband transitions, and the Drude conductivity as well. These predictions can affect the strategy of the graphene-based THz laser implementation.

  1. Metal ion binding to phospholipid bilayers evaluated by microaffinity chromatography.

    PubMed

    Ross, Eric E; Hoag, Christian; Pfeifer, Zach; Lundeen, Christopher; Owens, Sarah

    2016-06-17

    Group I and II ion binding to phospholipid membranes was evaluated by affinity chromatography utilizing a new stationary phase system based on lipid bilayers supported within large-pore particles composed of Stöber silica spheres. Using an inductively coupled plasma mass spectrometer for detection, robust determination of binding selectivity within group II ions is achieved with capillary columns containing nanomole quantities of lipid and using picomoles of metal analyte. Columns with a unique lipid formulation can be prepared within three hours using a solvent-casting assembly method. The observable thermotropic phase behavior of dipalmitoylphosphatidylcholine has a significant effect on alkaline metal binding and demonstrates the dynamic nature of the supported bilayers. Of the group I ions, only lithium exhibits retention with neutral phosphatidylcholine bilayer stationary phases. A comparison of Stöber-based supports with two commercially available large-pore silicas reveals the effect that particle structure has on analyte accessibility to the bilayer surface as evaluated by retention per supported lipid mass. PMID:27189434

  2. Super-Sensitive and Robust Biosensors from Supported Polymer Bilayers

    SciTech Connect

    Paxton, Walter F.

    2015-09-01

    Biological organisms are potentially the most sensitive and selective biological detection systems known, yet we are currently severely limited in our ability to exploit biological interactions in sensory devices, due in part to the limited stability of biological systems and derived materials. This proposal addresses an important aspect of integrating biological sensory materials in a solid state device. If successful, such technology could enable entirely new classes of robust biosensors that could be miniaturized and deployed in the field. The critical aims of the proposed work were 1) the calibration of a more versatile approach to measuring pH, 2) the use of this method to monitor pH changes caused by the light-induced pumping of protons across vesicles with bacteriorhodopsin integrated into the membranes (either polymer or lipid); 3) the preparation of bilayer assemblies on platinum surfaces; 4) the enhanced detection of lightinduced pH changes driven by bR-loaded supported bilayers. I have developed a methodology that may enable that at interfaces and developed a methodology to characterize the functionality of bilayer membranes with reconstituted membrane proteins. The integrity of the supported bilayer films however must be optimized prior to the full realization of the work originally envisioned in the original proposal. Nevertheless, the work performed on this project and the encouraging results it has produced has demonstrated that these goals are challenging yet within reach.

  3. Interaction of gramicidin with DPPC/DODAB bilayer fragments.

    PubMed

    Carvalho, Camilla A; Olivares-Ortega, Constanza; Soto-Arriaza, Marco A; Carmona-Ribeiro, Ana M

    2012-12-01

    The interaction between the antimicrobial peptide gramicidin (Gr) and dipalmitoylphosphatidylcholine (DPPC)/dioctadecyldimethylammonium bromide (DODAB) 1:1 large unilamellar vesicles (LVs) or bilayer fragments (BFs) was evaluated by means of several techniques. The major methods were: 1) Gr intrinsic fluorescence and circular dichroism (CD) spectroscopy; 2) dynamic light scattering for sizing and zeta-potential analysis; 3) determination of the bilayer phase transition from extrinsic fluorescence of bilayer probes; 4) pictures of the dispersions for evaluation of coloidal stability over a range of time and NaCl concentration. For Gr in LVs, the Gr dimeric channel conformation is suggested from: 1) CD and intrinsic fluorescence spectra similar to those in trifluoroethanol (TFE); 2) KCl or glucose permeation through the LVs/Gr bilayer. For Gr in BFs, the intertwined dimeric, non-channel Gr conformation is evidenced by CD and intrinsic fluorescence spectra similar to those in ethanol. Both LVs and BFs shield Gr tryptophans against quenching by acrylamide but the Stern-Volmer quenching constant was slightly higher for Gr in BFs confirming that the peptide is more exposed to the water phase in BFs than in LVs. The DPPC/DODAB/Gr supramolecular assemblies may predict the behavior of other antimicrobial peptides in assemblies with lipids.

  4. A comparison of DMPC- and DLPE-based lipid bilayers.

    PubMed Central

    Damodaran, K V; Merz, K M

    1994-01-01

    A 250 ps molecular dynamics simulation of the dimyristoylphosphatidylcholine (DMPC)-based lipid bilayer, including explicit water molecules, is reported. The solvent environment of the head groups and other structural properties of the bilayer have been analyzed and compared with experimental results as well as our previous simulation of the dilauroylphosphatidylethanolamine (DLPE)-based bilayer. From this comparison we find that the solvent structure around the DMPC head group (clathrate shell) is significantly different than that around the DLPE head group (typical hydrogen bonding interactions). We have modeled the probable relationship between the different solvent environments around the R-N(CH3)3+ (DMPC) and R-NH3+ (DLPE) head groups and the different interlammelar distances in these systems by performing potential of mean force (PMF) simulations on two N(CH3)4+ and NH4+ ions in water. From the PMF simulations it appears that the differences in the hydration of the DMPC and DLPE head groups is not responsible for the differences in the hydration force observed for these systems. We also find that the orientational polarization of DLPE and DMPC is similar, which suggests that solvent polarization is not responsible for the differences in the hydration repulsion behavior observed in these systems. We also examined the order parameters for DMPC and found them to be in reasonable agreement with experiment. Given the different characteristics of the DLPE and DMPC head groups, we suggest an explanation of the differences in the interlammellar spacings of bilayers composed of these like-charged lipids. From our DLPE simulations we find that the R-NH3+ head groups can interact with the nonesterified oxygens of the phosphate group in an intraleaflet or an interleaflet manner. For the latter a "cross link" between two leaflets can be formed, which causes a stabilization of the interlamellar spacings at fairly short distances. Moreover, due to the strong intraleaflet

  5. Bilayer Approaches for Nanoparticle Phase Transfer

    NASA Astrophysics Data System (ADS)

    Kini, Gautam Chandrakanth

    Nanoparticles (NPs) are often synthesized in organic solvents due to advantages of superior size and shape control obtainable in a non-polar environment. However, many applications featuring NPs require them to be in aqueous media. To transfer NPs from oil to water, surfactants with amphiphilic (hydrophobic and hydrophilic) groups have been widely used. A popular phase-transfer approach involves formation of oil-in-water emulsions upon which the oil storing the NPs is boiled off. In the process, surfactants form bilayers with hydrophobic groups on the NPs rendering them water-dispersible. This transfer route however is limited in that NPs aggregate to form clusters which results in poor colloidal stability and for the specific case of quantum dots (QDs), adversely impacts optical properties. It has ever since remained a challenge to devise approaches that transfer NPs from oil to water as single particles without compromising NP stability and properties. We have discovered that by simple addition of salt to water during the step of emulsion formation, NP transfer efficiency can be greatly enhanced in "salty-micelles" of surfactants. The strength of this approach lies in its simplicity and generic nature in that the transfer scheme is valid for different NP, surfactant and salt types. Using a model system with cadmium selenide (CdSe) QDs as NPs, Aerosol-OT (AOT) as the surfactant and NaCl as the salt in water, we found >90% of CdSe QDs transferred in salty-micelles of AOT which was significantly higher than the 45-55% QDs that transferred in deionized-water (DI-water) micelles of AOT. In the salty-micelle environment, QDs were found to exist predominantly as single NPs with narrow size distribution, as established by light scattering, analytical ultracentrifugation and electron microscopy. The effects of salt were in lowering aqueous solubility of AOT through "salting-out" action and in screening repulsions between like-charged head groups of AOT molecules

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

    SciTech Connect

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

    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 for 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.

  7. Cholesterol enhances surface water diffusion of phospholipid bilayers

    SciTech Connect

    Cheng, Chi-Yuan; Kausik, Ravinath; Han, Songi; Olijve, Luuk L. C.

    2014-12-14

    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 headgroups 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

  8. Cholesterol enhances surface water diffusion of phospholipid bilayers

    NASA Astrophysics Data System (ADS)

    Cheng, Chi-Yuan; Olijve, Luuk L. C.; Kausik, Ravinath; Han, Songi

    2014-12-01

    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 1H 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 headgroups 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

  9. Evaporation-Induced Buckling and Fission of Microscale Droplet Interface Bilayers

    SciTech Connect

    Boreyko, Jonathan B; Mruetusatorn, Prachya; Sarles, Stephen A; Retterer, Scott T; Collier, Pat

    2013-01-01

    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 the 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.

  10. Masking apertures enabling automation and solution exchange in sessile droplet lipid bilayers.

    PubMed

    Portonovo, Shiva A; Schmidt, Jacob

    2012-02-01

    Reconstitution of ion channels and transmembrane proteins in planar lipid bilayer membranes allow for their scientific study in highly controlled environments. Recent work with lipid bilayers formed from mechanically joined monolayers has shown their potential for wider technological application, including automation and parallelization. However, bilayer areas are highly sensitive to variations in mechanical position and the bilayers themselves cannot withstand significant perfusion of adjacent solutions. Toward this end, here we describe use of an aperture that masks the monolayer contact area, enabling formation of highly consistent bilayer areas and significantly reducing their variation with changes in relative position of the monolayers. Further, use of the aperture enables flow of solution adjacent to the bilayer without rupture or significant change in bilayer area. The device design is scalable and compatible with SBS standard instrumentation and automation technology, potentially enabling its use for rapid, parallel automated measurements of ion channels for large scale scientific studies and pharmaceutical screening.

  11. Investigating Hydrophilic Pores in Model Lipid Bilayers Using Molecular Simulations: Correlating Bilayer Properties with Pore-Formation Thermodynamics.

    PubMed

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

    2015-06-23

    Cell-penetrating and antimicrobial peptides show a remarkable ability to translocate across physiological membranes. Along with factors such as electric-potential-induced perturbations of membrane structure and surface tension effects, experiments invoke porelike membrane configurations during the solute transfer process into vesicles and cells. The initiation and formation of pores are associated with a nontrivial free-energy cost, thus necessitating a consideration of the factors associated with pore formation and the attendant free energies. Because of experimental and modeling challenges related to the long time scales of the translocation process, we use umbrella sampling molecular dynamics simulations with a lipid-density-based order parameter to investigate membrane-pore-formation free energy employing Martini coarse-grained models. We investigate structure and thermodynamic features of the pore in 18 lipids spanning a range of headgroups, charge states, acyl chain lengths, and saturation. We probe the dependence of pore-formation barriers on the area per lipid, lipid bilayer thickness, and membrane bending rigidities in three different lipid classes. The pore-formation free energy in pure bilayers and peptide translocating scenarios are significantly coupled with bilayer thickness. Thicker bilayers require more reversible work to create pores. The pore-formation free energy is higher in peptide-lipid systems than in peptide-free lipid systems due to penalties to maintain the solvation of charged hydrophilic solutes within the membrane environment.

  12. Organization and Structure of Branched Amphipathic Oligopeptide Bilayers.

    PubMed

    Jia, Zhiguang; Whitaker, Susan K; Tomich, John M; Chen, Jianhan

    2016-09-27

    A class of self-assembling branched amphiphilic peptide capsules (BAPCs) was recently developed that could serve as a new drug delivery vehicle. BAPCs can encapsulate solutes up to ∼12 kDa during assembly, are unusually stable, and are readily taken up by cells with low cytotoxicity. Coarse-grained simulations have supported that BAPCs are defined by bilayers that resemble those formed by diacyl phospholipids. Here, atomistic simulations were performed to characterize the structure and organization of bilayers formed by three branched amphiphilic peptides (BAPs): bis(Ac-FLIVIGSII)-K-K4-CO-NH2, bis(Ac-CHA-LIVIGSII)-K-K4-CO-NH2, and bis(Ac-FLIVI)-K-K4-CO-NH2. The results show BAPs form a network of intra- and intermolecular backbone hydrogen bonds within the same leaflet in addition to hydrophobic side-chain interactions. The terminal residues of two leaflets form an interdigitation region locking two leaflets together. The phenyl groups in bis(Ac-FLIVIGSII)-K-K4-CO-NH2 and bis(Ac-FLIVI)-K-K4-CO-NH2 are tightly packed near the bilayer center but do not formed ordered structures with specific π-π stacking. Replacing phenyl groups with the cyclohexane side chain only slightly increases the level of disorder in bilayer structures and thus should not significantly affect the stability, consistent with experimental results on bis(Ac-CHA-LIVIGSII)-K-K4-CO-NH2 BAPCs. Self-assembly simulations further suggest that leaflet interdigitation likely occurs at early stages of BAPC formation. Atomistic simulations also reveal that the BAPC bilayers are highly permeable to water. This prediction was validated using fluorescence measurements of encapsulated self-quenching dye upon transferring BAPCs to buffers with different salt concentrations. Improved understanding of the organization and structure of BAPC bilayers at the atomic level will provide a basis for future rational modifications of BAP sequence to improve BAPC properties as a new class of delivery vehicle. PMID

  13. 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

  14. 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.

  15. Formation and finite element analysis of tethered bilayer lipid structures.

    PubMed

    Kwak, Kwang Joo; Valincius, Gintaras; Liao, Wei-Ching; Hu, Xin; Wen, Xuejin; Lee, Andrew; Yu, Bo; Vanderah, David J; Lu, Wu; Lee, L James

    2010-12-01

    Rapid solvent exchange of an ethanolic solution of diphytanoyl phosphatidylcholine (DPhyPC) in the presence of a mixed self-assembled monolayer (SAM) [thiolipid/β-mercaptoethanol (βME) (3/7 mol/mol) on Au] shows a transition from densely packed tethered bilayer lipid membranes [(dp)tBLMs], to loosely packed tethered bilayer lipid membranes [(lp)tBLMs], and tethered bilayer liposome nanoparticles (tBLNs) with decreasing DPhyPC concentration. The tethered lipidic constructs in the aqueous medium were analyzed by atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). Finite element analysis (FEA) was applied to interpret spectral EIS features without referring to equivalent circuit modeling. Using structural data obtained earlier from neutron reflectometry and dielectric constants of lipid bilayers, we reproduced experimentally observed features of the electrochemical impedance (EI) spectra of complex surface constructs involving small pinhole defects, large membrane-free patches, and bound liposomes. We demonstrated by FEA that highly insulating (dp)tBLMs with low-defect density exhibit EI spectra in the shape of a perfect semicircle with or without low-frequency upward "tails" in the Cole-Cole representation. Such EI spectra were observed at DPhyPC concentrations of >5 × 10(-3) mol L(-1). While AFM was not able to visualize very small lateral defects in such films, EI spectra unambiguously signaled their presence by increased low frequency "tails". Using FEA we demonstrate that films with large diameter visible defects (>25 nm by AFM) produce EI spectral features consisting of two semicircles of comparable size. Such films were typically obtained at DPhyPC concentrations of <5 × 10(-3) mol L(-1). At DPhyPC concentrations of <1.0 × 10(-3) mol L(-1) the planar bilayer structures were replaced by ellipsoidal liposomes with diameters ranging from 50 to 500 nm as observed in AFM images. Despite the distinct surface morphology change, the EI

  16. Bilayer graphene: physics and application outlook in photonics

    NASA Astrophysics Data System (ADS)

    Yan, Hugen

    2015-05-01

    Layered materials, such as graphene, transition metal dichacogenides and black phosphorus have attracted lots of attention recently. They are emerging novel materials in electronics and photonics, with tremendous potential in revolutionizing the traditional electronics and photonics industry. Marrying layered material to the nanophotonics is being proved fruitful. With the recent emphasis and development of metasurfaces in nanophotonics, atomically thin materials can find their unique position and strength in this field. In this article, I will focus on one specific two dimensional material: bilayer graphene. Basic physics will be reviewed, such as band-gap opening, electron-phonon interaction, phonon-plasmon interaction and Fano resonances in the optical response. Moreover, I will review the application of bilayer graphene as a sensitive and fast photodetector. An outlook will be given in the final part of the paper.

  17. Optical response of Al/Ti bilayer transition edge sensors

    NASA Astrophysics Data System (ADS)

    Zhang, Qing-Ya; Wang, Tian-Shun; Liu, Jian-She; Dong, Wen-Hui; He, Gen-Fang; Li, Tie-Fu; Zhou, Xing-Xiang; Chen, Wei

    2014-11-01

    We report the optical response characteristics of Al/Ti bilayer transition edge sensors (TESs), which are mainly comprised of Al/Ti bilayer thermometers and suspended SiN membranes for thermal isolation. The measurement was performed in a 3He sorption refrigerator and the device's response to optical pulses was investigated using a pulsed laser source. Based on these measurements, we obtained the effective recovery time (τeff) of the devices at different biases and discussed the dependence of τeff on the bias. The device with a 940 μm × 940 μm continuous suspended SiN membrane demonstrated a fast response speed with τeff = 3.9 μs, which indicates a high temperature sensitivity (α = T/R · dR/dT = 326). The results also showed that the TES exhibits good linearity under optical pulses of variable widths.

  18. Anomalous Hall effect in YIG|Pt bilayers

    SciTech Connect

    Meyer, Sibylle Schlitz, Richard; Geprägs, Stephan; Opel, Matthias; Huebl, Hans; Goennenwein, Sebastian T. B.; Gross, Rudolf

    2015-03-30

    We measure the ordinary and the anomalous Hall effect in a set of yttrium iron garnet|platinum (YIG|Pt) bilayers via magnetization orientation dependent magnetoresistance experiments. Our data show that the presence of the ferrimagnetic insulator YIG leads to an anomalous Hall effect like voltage in Pt, which is sensitive to both Pt thickness and temperature. Interpretation of the experimental findings in terms of the spin Hall anomalous Hall effect indicates that the imaginary part of the spin mixing conductance G{sub i} plays a crucial role in YIG|Pt bilayers. In particular, our data suggest a sign change in G{sub i} between 10 K and 300 K. Additionally, we report a higher order Hall effect contribution, which appears in thin Pt films on YIG at low temperatures.

  19. Asymmetric bilayer graphene nanoribbon MOSFETs for analog and digital electronics

    NASA Astrophysics Data System (ADS)

    Dinarvand, A.; Ahmadi, V.; Darvish, Gh.

    2016-05-01

    In this paper, a new structure was proposed for bilayer graphene nanoribbon field-effect transistor (BGNFET) mainly to enhance the electrical characteristics in analog and digital applications. The proposed device uses two metallic gates on the top and bottom of a bilayer graphene nanoribbon, which is surrounded by SiO2 and connected to heavily doped source/drain contacts. Electrical properties of the proposed device were explored using fully self-consistent solution of Poisson and Schrödinger equations based on the nonequilibrium Green's function (NEGF) formalism. Significant improvements in the electrical behavior was seen in the simulation results for gates asymmetrically biased. The comparison with graphene nanoribbon FET showed that the proposed structure benefited from higher intrinsic voltage gain and cut-off frequency and improved switching characteristics such as delay and Ion/Ioff ratio.

  20. Collective lipid bilayer dynamics excited by surface acoustic waves.

    PubMed

    Reusch, T; Schülein, F J R; Nicolas, J D; Osterhoff, M; Beerlink, A; Krenner, H J; Müller, M; Wixforth, A; Salditt, T

    2014-09-12

    We use standing surface acoustic waves to induce coherent phonons in model lipid multilayers deposited on a piezoelectric surface. Probing the structure by phase-controlled stroboscopic x-ray pulses we find that the internal lipid bilayer electron density profile oscillates in response to the externally driven motion of the lipid film. The structural response to the well-controlled motion is a strong indication that bilayer structure and membrane fluctuations are intrinsically coupled, even though these structural changes are averaged out in equilibrium and time integrating measurements. Here the effects are revealed by a timing scheme with temporal resolution on the picosecond scale in combination with the sub-nm spatial resolution, enabled by high brilliance synchrotron x-ray reflectivity.

  1. Formation and finite element analysis of tethered bilayer lipid structures.

    PubMed

    Kwak, Kwang Joo; Valincius, Gintaras; Liao, Wei-Ching; Hu, Xin; Wen, Xuejin; Lee, Andrew; Yu, Bo; Vanderah, David J; Lu, Wu; Lee, L James

    2010-12-01

    Rapid solvent exchange of an ethanolic solution of diphytanoyl phosphatidylcholine (DPhyPC) in the presence of a mixed self-assembled monolayer (SAM) [thiolipid/β-mercaptoethanol (βME) (3/7 mol/mol) on Au] shows a transition from densely packed tethered bilayer lipid membranes [(dp)tBLMs], to loosely packed tethered bilayer lipid membranes [(lp)tBLMs], and tethered bilayer liposome nanoparticles (tBLNs) with decreasing DPhyPC concentration. The tethered lipidic constructs in the aqueous medium were analyzed by atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). Finite element analysis (FEA) was applied to interpret spectral EIS features without referring to equivalent circuit modeling. Using structural data obtained earlier from neutron reflectometry and dielectric constants of lipid bilayers, we reproduced experimentally observed features of the electrochemical impedance (EI) spectra of complex surface constructs involving small pinhole defects, large membrane-free patches, and bound liposomes. We demonstrated by FEA that highly insulating (dp)tBLMs with low-defect density exhibit EI spectra in the shape of a perfect semicircle with or without low-frequency upward "tails" in the Cole-Cole representation. Such EI spectra were observed at DPhyPC concentrations of >5 × 10(-3) mol L(-1). While AFM was not able to visualize very small lateral defects in such films, EI spectra unambiguously signaled their presence by increased low frequency "tails". Using FEA we demonstrate that films with large diameter visible defects (>25 nm by AFM) produce EI spectral features consisting of two semicircles of comparable size. Such films were typically obtained at DPhyPC concentrations of <5 × 10(-3) mol L(-1). At DPhyPC concentrations of <1.0 × 10(-3) mol L(-1) the planar bilayer structures were replaced by ellipsoidal liposomes with diameters ranging from 50 to 500 nm as observed in AFM images. Despite the distinct surface morphology change, the EI

  2. Mechanical properties of lipid bilayers and regulation of mechanosensitive function

    PubMed Central

    Balleza, Daniel

    2012-01-01

    Material properties of lipid bilayers, including thickness, intrinsic curvature and compressibility regulate the function of mechanosensitive (MS) channels. This regulation is dependent on phospholipid composition, lateral packing and organization within the membrane. Therefore, a more complete framework to understand the functioning of MS channels requires insights into bilayer structure, thermodynamics and phospholipid structure, as well as lipid-protein interactions. Phospholipids and MS channels interact with each other mainly through electrostatic forces and hydrophobic matching, which are also crucial for antimicrobial peptides. They are excellent models for studying the formation and stabilization of membrane pores. Importantly, they perform equivalent responses as MS channels: (1) tilting in response to tension and (2) dissipation of osmotic gradients. Lessons learned from pore forming peptides could enrich our knowledge of mechanisms of action and evolution of these channels. Here, the current state of the art is presented and general principles of membrane regulation of mechanosensitive function are discussed. PMID:22790280

  3. Evolution of interlayer coupling in twisted molybdenum disulfide bilayers

    NASA Astrophysics Data System (ADS)

    Liu, Kaihui; Zhang, Liming; Cao, Ting; Jin, Chenhao; Qiu, Diana; Zhou, Qin; Zettl, Alex; Yang, Peidong; Louie, Steve G.; Wang, Feng

    2014-09-01

    Van der Waals coupling is emerging as a powerful method to engineer physical properties of atomically thin two-dimensional materials. In coupled graphene-graphene and graphene-boron nitride layers, interesting physical phenomena ranging from Fermi velocity renormalization to Hofstadter’s butterfly pattern have been demonstrated. Atomically thin transition metal dichalcogenides, another family of two-dimensional-layered semiconductors, can show distinct coupling phenomena. Here we demonstrate the evolution of interlayer coupling with twist angles in as-grown molybdenum disulfide bilayers. We find that the indirect bandgap size varies appreciably with the stacking configuration: it shows the largest redshift for AA- and AB-stacked bilayers, and a significantly smaller but constant redshift for all other twist angles. Our observations, together with ab initio calculations, reveal that this evolution of interlayer coupling originates from the repulsive steric effects that leads to different interlayer separations between the two molybdenum disulfide layers in different stacking configurations.

  4. Superlattice structures in twisted bilayers of folded graphene.

    PubMed

    Schmidt, Hennrik; Rode, Johannes C; Smirnov, Dmitri; Haug, Rolf J

    2014-01-01

    The electronic properties of bilayer graphene strongly depend on relative orientation of the two atomic lattices. Whereas Bernal-stacked graphene is most commonly studied, a rotational mismatch between layers opens up a whole new field of rich physics, especially at small interlayer twist. Here we report on magnetotransport measurements on twisted graphene bilayers, prepared by folding of single layers. These reveal a strong dependence on the twist angle, which can be estimated by means of sample geometry. At small rotation, superlattices with a wavelength in the order of 10 nm arise and are observed by friction atomic force microscopy. Magnetotransport measurements in this small-angle regime show the formation of satellite Landau fans. These are attributed to additional Dirac singularities in the band structure and discussed with respect to the wide range of interlayer coupling models.

  5. Electrical oscillation in Pt/VO{sub 2} bilayer strips

    SciTech Connect

    Wang, Ying; Qi, Long; Xu, Yanjun; Wu, Yihong; Chai, Jianwei; Wang, Shijie; Yang, Yumeng; Tanaka, Hidekazu

    2015-02-14

    We report on the observation of stable electrical oscillation in Pt/vanadium dioxide (VO{sub 2}) bilayer strips, in which the Pt overlayer serves the dual purposes of heating up the VO{sub 2} and weakening the electric field in the VO{sub 2} layer. Systematic measurements in an ultrahigh vacuum nanoprobe system show that the oscillation frequency increases with the bias current and/or with decreasing device dimension. In contrast to most VO{sub 2}-based oscillators reported to date, which are electrically triggered, current-induced Joule heating in the Pt overlayer is found to play a dominant role in the generation of oscillation in Pt/VO{sub 2} bilayers. A simple model involving thermally triggered transition of VO{sub 2} on a heat sink is able to account for the experimental observations. The results in this work provide an alternative view of the triggering mechanism in VO{sub 2}-based oscillators.

  6. Formation of droplet interface bilayers in a Teflon tube

    PubMed Central

    Walsh, Edmond; Feuerborn, Alexander; Cook, Peter R.

    2016-01-01

    Droplet-interface bilayers (DIBs) have applications in disciplines ranging from biology to computing. We present a method for forming them manually using a Teflon tube attached to a syringe pump; this method is simple enough it should be accessible to those without expertise in microfluidics. It exploits the properties of interfaces between three immiscible liquids, and uses fluid flow through the tube to pack together drops coated with lipid monolayers to create bilayers at points of contact. It is used to create functional nanopores in DIBs composed of phosphocholine using the protein α-hemolysin (αHL), to demonstrate osmotically-driven mass transfer of fluid across surfactant-based DIBs, and to create arrays of DIBs. The approach is scalable, and thousands of DIBs can be prepared using a robot in one hour; therefore, it is feasible to use it for high throughput applications. PMID:27681313

  7. Concentration fluctuations and phase transitions in coupled modulated bilayers

    NASA Astrophysics Data System (ADS)

    Hirose, Yuichi; Komura, Shigeyuki; Andelman, David

    2012-08-01

    We consider the formation of finite-size domains in lipid bilayers consisting of saturated and hybrid lipids. First, we describe a monolayer model that includes a coupling between a compositional scalar field and a two-dimensional vectorial order parameter. Such a coupling yields an effective two-dimensional microemulsion free energy for the lipid monolayer, and its characteristic length of compositional modulations can be considered as the origin of finite-size domains in biological membranes. Next, we consider a coupled bilayer composed of two modulated monolayers and discuss the static and dynamic properties of concentration fluctuations above the transition temperature. We also investigate the micro-phase separation below the transition temperature and compare the micro-phase separated structures with statics and dynamics of concentration fluctuations above the transition.

  8. Wrinkled bilayer graphene with wafer scale mechanical strain

    NASA Astrophysics Data System (ADS)

    Mikael, Solomon; Seo, Jung-Hun; Javadi, Alireza; Gong, Shaoqin; Ma, Zhenqiang

    2016-05-01

    Wafer-scale strained bilayer graphene is demonstrated by employing a silicon nitride (Si3N4) stressor layer. Different magnitudes of compressive stress up to 840 MPa were engineered by adjusting the Si3N4 deposition recipes, and different strain conditions were analyzed using Raman spectroscopy. The strained graphene displayed significant G peak shifts and G peak splitting with 16.2 cm-1 and 23.0 cm-1 of the G band and two-dimensional band shift, which corresponds to 0.26% of strain. Raman mapping of large regions of the graphene films found that the largest shifts/splitting occurred near the bilayer regions of the graphene films. The significance of our approach lies in the fact that it can be performed in a conventional microfabrication process, i.e., the plasma enhanced chemical vapor deposition system, and thus easily implemented for large scale production.

  9. AA-stacked bilayer square ice between graphene layers

    NASA Astrophysics Data System (ADS)

    Sobrino Fernandez Mario, M.; Neek-Amal, M.; Peeters, F. M.

    2015-12-01

    Water confined between two graphene layers with a separation of a few Å forms a layered two-dimensional ice structure. Using large scale molecular dynamics simulations with the adoptable ReaxFF interatomic potential we found that flat monolayer ice with a rhombic-square structure nucleates between the graphene layers which is nonpolar and nonferroelectric. We provide different energetic considerations and H-bonding results that explain the interlayer and intralayer properties of two-dimensional ice. The controversial AA stacking found experimentally [Algara-Siller et al., Nature (London) 519, 443 (2015), 10.1038/nature14295] is consistent with our minimum-energy crystal structure of bilayer ice. Furthermore, we predict that an odd number of layers of ice has the same lattice structure as monolayer ice, while an even number of ice layers exhibits the square ice AA stacking of bilayer ice.

  10. Bilayer thickness mismatch controls domain size in biomimetic membranes

    NASA Astrophysics Data System (ADS)

    Heberle, Frederick A.; Petruzielo, Robin S.; Pan, Jianjun; Drazba, Paul; Kučerka, Norbert; Standaert, Robert F.; Feigenson, Gerald W.; Katsara, John

    2013-03-01

    In order to promote functionality, cells may alter the spatial organization of membrane lipids and proteins, including separation of liquid phases into distinct domains. In model membranes, domain size and morphology depend strongly on composition and temperature, but the physicochemical mechanisms controlling them are poorly understood. Theoretical work suggests a role for interfacial energy at domain boundaries, which may be driven in part by thickness mismatch between a domain and its surrounding bilayer. However, no direct evidence linking thickness mismatch to domain size in free-standing bilayers has been reported. We describe the use of Small Angle Neutron Scattering (SANS) to detect domains in simplified lipid-only models that mimic the composition of plasma membrane. We find that domain size is controlled by the degree of acyl chain unsaturation of low-melting temperature lipids, and that this size transition is correlated to changes in the thickness mismatch between coexisting liquid phases.

  11. Chitosan-caseinate bilayer coatings for paper packaging materials.

    PubMed

    Khwaldia, Khaoula; Basta, Altaf H; Aloui, Hajer; El-Saied, Houssni

    2014-01-01

    Papers coated with caseinate and caseinate/chitosan bilayer films were developed. Caseinate, chitosan and caseinate/chitosan films were preliminary characterized by FTIR spectroscopy and thermal stability analyses. The effects of coating weight, caseinate concentration (7%, 10%, and 12%, w/w), and coating application methods (single layer and bilayer) on the physical and mechanical properties of coated papers were studied. Increasing the concentration of caseinate led to a decrease in water vapor permeability (WVP) of the resulting coated paper sheets. Chitosan significantly (p<0.05) increased the elongation at break (%E) of coated paper. However, the application of chitosan as a second layer on wet or dry caseinate films did not significantly affect (p>0.05) the tensile strength (TS) of coated paper. The greatest reduction in paper WVP is achieved by addition of a chitosan layer to the dried preformed caseinate-coated paper.

  12. Piezoelectric enhancement by surface effect in hydrofluorinated graphene bilayer

    SciTech Connect

    Kim, Hye Jung; Noor-A-Alam, Mohammad; Shin, Young-Han

    2015-04-14

    We investigated the piezoelectricity of dipolar hydrofluorinated graphene (C{sub 2}HF){sub n} multilayers with first-principles calculations. Our results reveal that the dipole moment decreases as the number of layers increases, because electron and hole carriers are induced at the top and bottom layers due to the depolarization field. These carriers make (C{sub 2}HF){sub n} multilayers more stable by decreasing the depolarization field in the material. Through the calculation of the average layer piezoelectric stress constant e{sub 31}/ℓ in ℓ-layer chair (C{sub 2}HF){sub n} multilayers, we confirmed that the piezoelectricity of the bilayer is about three times larger than that of the monolayer and bulk material. Moreover, we found that the electron and hole carriers on the top and bottom layers played a significant role in the piezoelectric enhancement of the bilayer.

  13. Supported Lipid Bilayer Technology for the Study of Cellular Interfaces

    PubMed Central

    Crites, Travis J.; Maddox, Michael; Padhan, Kartika; Muller, James; Eigsti, Calvin; Varma, Rajat

    2015-01-01

    Glass-supported lipid bilayers presenting freely diffusing proteins have served as a powerful tool for studying cell-cell interfaces, in particular, T cell–antigen presenting cell (APC) interactions, using optical microscopy. Here we expand upon existing protocols and describe the preparation of liposomes by an extrusion method, and describe how this system can be used to study immune synapse formation by Jurkat cells. We also present a method for forming such lipid bilayers on silica beads for the study of signaling responses by population methods, such as western blotting, flow cytometry, and gene-expression analysis. Finally, we describe how to design and prepare transmembrane-anchored protein-laden liposomes, following expression in suspension CHO (CHOs) cells, a mammalian expression system alternative to insect and bacterial cell lines, which do not produce mammalian glycosylation patterns. Such transmembrane-anchored proteins may have many novel applications in cell biology and immunology. PMID:26331983

  14. Formation of droplet interface bilayers in a Teflon tube

    NASA Astrophysics Data System (ADS)

    Walsh, Edmond; Feuerborn, Alexander; Cook, Peter R.

    2016-09-01

    Droplet-interface bilayers (DIBs) have applications in disciplines ranging from biology to computing. We present a method for forming them manually using a Teflon tube attached to a syringe pump; this method is simple enough it should be accessible to those without expertise in microfluidics. It exploits the properties of interfaces between three immiscible liquids, and uses fluid flow through the tube to pack together drops coated with lipid monolayers to create bilayers at points of contact. It is used to create functional nanopores in DIBs composed of phosphocholine using the protein α-hemolysin (αHL), to demonstrate osmotically-driven mass transfer of fluid across surfactant-based DIBs, and to create arrays of DIBs. The approach is scalable, and thousands of DIBs can be prepared using a robot in one hour; therefore, it is feasible to use it for high throughput applications.

  15. Ferromagnetic resonance of exchange-coupled perpendicularly magnetized bilayers

    NASA Astrophysics Data System (ADS)

    Devolder, Thibaut

    2016-04-01

    Strong ferromagnetic interlayer exchange couplings J in perpendicularly magnetized systems are becoming increasingly desirable for applications. We study whether ferromagnetic interlayer exchange couplings can be measured by a combination of broadband ferromagnetic resonance methods and magnetometry hysteresis loops. For this, we model the switching and the eigenexcitations in bilayer systems comprising a soft layer coupled to a thicker harder layer that possesses higher perpendicular magnetic anisotropy. For large J > 0, the switching fields are essentially independent of J but the frequency of the optical eigenmode of the bilayer and the linewidth of the acoustical and optical eigenmode are directly sensitive to the coupling. We derive a corpus of compact analytical expressions to analyze these frequencies, their linewidth and discuss the meaning thereof. We illustrate this corpus on a system mimicking the fixed layers of a magnetic tunnel junction meant for spin torque applications.

  16. kT-Scale interactions between supported lipid bilayers.

    PubMed

    Everett, W Neil; Bevan, Michael A

    2014-01-14

    We use total internal reflection microscopy (TIRM) and confocal laser scanning microscopy (CSLM) to study supported lipid bilayer (SLB)-modified silica colloids with various SLB compositions (e.g., PEGylated vs. non-PEGylated) that control colloidal and bilayer stability. Measured and predicted potentials accurately capture stable configurations. For unstable conditions when SLBs adhere, fuse, or spread between surfaces, SLB structures are connected to effective potentials as well as time-dependent behavior. In all cases, directly measured and inferred interactions are well described by steric interactions between PEG brushes and van der Waals weakened by substrate roughness. Our findings quantify non-specific kT-scale interactions between SLB-modified colloids and surfaces, which enables the design of such systems for use in biomedical applications and studies of biomolecular interactions.

  17. The interaction of N-oleylethanolamine with phospholipid bilayers.

    PubMed

    Epps, D E; Cardin, A D

    1987-10-16

    Long chain acylamides of ethanolamine were previously found to increase in the infarcted canine myocardium. Subsequent in vitro experiments established a number of interesting biological and physiological properties of these compounds including alteration of rabbit skeletal sarcoplasmic reticulum function and inhibition of permeability dependent calcium release from heart mitochondria. These results suggested an interaction between the N-acylethanolamines and biological membranes. In the present work we show that the most potent species in previous studies, N-oleylethanolamine, forms stable complexes with phospholipid vesicles, lowers diphenylhexatriene polarization ratios in dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine uni- and multilamellar bilayer vesicles, and also produces a concentration dependent decrease in the phase transitions of these lipid structures. In addition studies with parinaric acids also suggested that N-oleylethanolamine partitions preferentially into more fluid areas of the bilayer. The results are discussed in terms of possible effects on biological membranes.

  18. Electric field-induced concentration gradients in planar supported bilayers.

    PubMed Central

    Groves, J T; Boxer, S G

    1995-01-01

    A simple method of generating electric field-induced concentration gradients in planar supported bilayers has been developed. Gradients of charged, fluorescently labeled probes were visualized by epifluorescence microscopy and could be observed at field strengths as low as 1 V/cm. Steady-state concentration gradients can be described by a simple competition between random diffusion and electric field-induced drift. A model based on this principle has been used to determine the diffusion coefficient of the fluorescent probes. This technique achieves a degree of electrical manipulation of supported bilayers that offers a variety of possibilities for the development of new molecular architectures and the study of biological membranes. Images FIGURE 2 FIGURE 4 PMID:8580340

  19. Stacking-dependent electronic structure of bilayer silicene

    SciTech Connect

    Fu, Huixia; Zhang, Jin; Ding, Zijing; Li, Hui E-mail: smeng@iphy.ac.cn; Meng, Sheng E-mail: smeng@iphy.ac.cn

    2014-03-31

    Bilayer silicene (BLS) is a class of material that possibly holds both topological and superconducting properties; however, its structure is not fully understood. By scanning stacking modes and lattice constants using first principles calculations, several meta-stable configurations are identified, including a slightly faulted-AA packing structure, named slide-2AA. Different from the metallic properties of conventional AA and AB stacking forms, band structure of slide-2AA bilayer presents a sizeable indirect energy gap of ∼1.16 eV. A metal-semiconductor phase transition along the sliding pathway with a small energy barrier is also observed, indicating its electronic properties can be easily tuned by applying small shear force along the BLS surface plane. Such unique quantitative relationship of structure and electronic properties has profound implications in nanoelectronics and electromechanical devices.

  20. Josephson inplane and tunneling currents in bilayer quantum Hall system

    SciTech Connect

    Ezawa, Z. F.; Tsitsishvili, G.; Sawada, A.

    2013-12-04

    A Bose-Einstein condensation is formed by composite bosons in the quantum Hall state. A composite boson carries the fundamental charge (–e). We investigate Josephson tunneling of such charges in the bilayer quantum Hall system at the total filling ν = 1. We show the existence of the critical current for the tunneling current to be coherent and dissipationless in tunneling experiments with various geometries.

  1. Twisted bilayer blue phosphorene: A direct band gap semiconductor

    NASA Astrophysics Data System (ADS)

    Ospina, D. A.; Duque, C. A.; Correa, J. D.; Suárez Morell, Eric

    2016-09-01

    We report that two rotated layers of blue phosphorene behave as a direct band gap semiconductor. The optical spectrum shows absorption peaks in the visible region of the spectrum and in addition the energy of these peaks can be tuned with the rotational angle. These findings makes twisted bilayer blue phosphorene a strong candidate as a solar cell or photodetection device. Our results are based on ab initio calculations of several rotated blue phosphorene layers.

  2. Amorphization and anisotropy effects on a ferromagnetic bilayer system

    NASA Astrophysics Data System (ADS)

    Bengrine, M.; Benyoussef, A.; El Kenz, A.; Mhirech, F.; Peliti, L.

    1999-07-01

    A ferromagnetic amorphous bilayer system, consisting of two magnetic monolayers (A and B) with spins ( S A= {1}/{2} and S B= {3}/{2}) and different interaction constants coupled together with interlayer coupling, is studied by the use of effective field theory. In addition to the influence of crystal field interaction the effects of the amorphization in the monolayer B and in the interlayer coupling give some interesting phenomena in the system. Phase diagrams and magnetization are investigated in detail.

  3. Optimizing non-Pb radiation shielding materials using bilayers

    SciTech Connect

    McCaffrey, J. P.; Mainegra-Hing, E.; Shen, H.

    2009-12-15

    Purpose: The objective of this study was to demonstrate that the weight of non-Pb radiation shielding materials can be minimized by structuring the material as a bilayer composed of different metal-powder-embedded elastomer layers. Methods: Measurements and Monte Carlo (MC) calculations were performed to study the attenuation properties of several non-Pb metal bilayers over the x-ray energy range 30-150 keV. Metals for the layers were chosen on the basis of low cost, nontoxicity, and complementary photoelectric absorption characteristics. The EGSnrc user code cavity.cpp was used to calculate the resultant x-ray fluence spectra after attenuation by these metal layers. Air kerma attenuation was measured using commercially manufactured metal/elastomer test layers. These layers were irradiated using the primary standard calibration beams at the Institute for National Measurement Standards in Ottawa, Canada utilizing the six x-ray beam qualities recommended in the German Standard DIN 6857. Both the measurements and the calculations were designed to approximate surface irradiation as well as penetrating radiation at 10 mm depth in soft tissue. The MC modeling point and the position of the measurement detector for surface irradiation were both directly against the downstream face of the attenuating material, as recommended in DIN 6857. Results: The low-Z upstream/high-Z downstream ordering of the metal bilayers provided substantially more attenuation than the reverse order. Optimal percentages of each metal in each bilayer were determined for each x-ray radiation beam quality. Conclusions: Depending on the x-ray quality, appropriate choices of two complementary metal-embedded elastomer layers can decrease the weight of radiation shielding garments by up to 25% compared to Pb-based elastomer garments while providing equivalent attenuation.

  4. Molecular doping and band-gap opening of bilayer graphene.

    PubMed

    Samuels, Alexander J; Carey, J David

    2013-03-26

    The ability to induce an energy band gap in bilayer graphene is an important development in graphene science and opens up potential applications in electronics and photonics. Here we report the emergence of permanent electronic and optical band gaps in bilayer graphene upon adsorption of π electron containing molecules. Adsorption of n- or p-type dopant molecules on one layer results in an asymmetric charge distribution between the top and bottom layers and in the formation of an energy gap. The resultant band gap scales linearly with induced carrier density though a slight asymmetry is found between n-type dopants, where the band gap varies as 47 meV/10(13) cm(-2), and p-type dopants where it varies as 40 meV/10(13) cm(-2). Decamethylcobaltocene (DMC, n-type) and 3,6-difluoro-2,5,7,7,8,8-hexacyano-quinodimethane (F2-HCNQ, p-type) are found to be the best molecules at inducing the largest electronic band gaps up to 0.15 eV. Optical adsorption transitions in the 2.8-4 μm region of the spectrum can result between states that are not Pauli blocked. Comparison is made between the band gaps calculated from adsorbate-induced electric fields and from average displacement fields found in dual gate bilayer graphene devices. A key advantage of using molecular adsorption with π electron containing molecules is that the high binding energy can induce a permanent band gap and open up possible uses of bilayer graphene in mid-infrared photonic or electronic device applications.

  5. Supported lipid bilayer membranes for water purification by reverse osmosis.

    PubMed

    Kaufman, Yair; Berman, Amir; Freger, Viatcheslav

    2010-05-18

    Some biological plasma membranes pass water with a permeability and selectivity largely exceeding those of commercial membranes for water desalination using specialized trans-membrane proteins aquaporins. However, highly selective transport of water through aquaporins is usually driven by an osmotic rather mechanical pressure, which is not as attractive from the engineering point of view. The feasibility of adopting biomimetic membranes for water purification driven by a mechanical pressure, i.e., filtration is explored in this paper. Toward this goal, it is proposed to use a commercial nanofiltration (NF) membrane as a support for biomimetic lipid bilayer membranes to render them robust enough to withstand the required pressures. It is shown in this paper for the first time that by properly tuning molecular interactions supported phospholipid bilayers (SPB) can be prepared on a commercial NF membrane. The presence of SPB on the surface was verified and quantified by several spectroscopic and microscopic techniques, which showed morphology close to the desired one with very few defects. As an ultimate test it is shown that hydraulic permeability of the SPB supported on the NF membrane (NTR-7450) approaches the values deduced from the typical osmotic permeabilities of intact continuous bilayers. This permeability was unaffected by the trans-membrane flow of water and by repeatedly releasing and reapplying a 10 bar pressure. Along with a parallel demonstration that aquaporins could be incorporated in a similar bilayer on mica, this demonstrates the feasibility of the proposed approach. The prepared SPB structure may be used as a platform for preparing biomimetic filtration membranes with superior performance based on aquaporins. The concept of SPBs on permeable substrates of the present type may also be useful in the future for studying transport of various molecules through trans-membrane proteins. PMID:20099798

  6. Persistent currents and dissipation in narrow bilayer quantum Hall bars

    NASA Astrophysics Data System (ADS)

    Kyriakidis, Jordan; Radzihovsky, Leo

    2001-11-01

    Bilayer quantum Hall states support a flow of nearly dissipationless staggered current which can only decay through collective channels. We study the dominant finite-temperature dissipation mechanism which in narrow bars is driven by thermal nucleation of pseudospin solitons. We find the finite-temperature resistivity, predict the resulting staggered current-voltage characteristics, and calculate the associated zero-temperature critical staggered current and gate voltage.

  7. Penetration of Cell Membranes and Synthetic Lipid Bilayers by Nanoprobes

    PubMed Central

    Angle, Matthew R.; Wang, Andrew; Thomas, Aman; Schaefer, Andreas T.; Melosh, Nicholas A.

    2014-01-01

    Nanoscale devices have been proposed as tools for measuring and controlling intracellular activity by providing electrical and/or chemical access to the cytosol. Unfortunately, nanostructures with diameters of 50–500 nm do not readily penetrate the cell membrane, and rationally optimizing nanoprobes for cell penetration requires real-time characterization methods that are capable of following the process of membrane penetration with nanometer resolution. Although extensive work has examined the rupture of supported synthetic lipid bilayers, little is known about the applicability of these model systems to living cell membranes with complex lipid compositions, cytoskeletal attachment, and membrane proteins. Here, we describe atomic force microscopy (AFM) membrane penetration experiments in two parallel systems: live HEK293 cells and stacks of synthetic lipid bilayers. By using the same probes in both systems, we were able to clearly identify membrane penetration in synthetic bilayers and compare these events with putative membrane penetration events in cells. We examined membrane penetration forces for three tip geometries and 18 chemical modifications of the probe surface, and in all cases the median forces required to penetrate cellular and synthetic lipid bilayers with nanoprobes were greater than 1 nN. The penetration force was sensitive to the probe's sharpness, but not its surface chemistry, and the force did not depend on cell surface or cytoskeletal properties, with cells and lipid stacks yielding similar forces. This systematic assessment of penetration under various mechanical and chemical conditions provides insights into nanoprobe-cell interactions and informs the design of future intracellular nanoprobes. PMID:25418094

  8. Sponge mesoporous silica formation using disordered phospholipid bilayers as template.

    PubMed

    Galarneau, Anne; Sartori, Federica; Cangiotti, Michela; Mineva, Tzonka; Di Renzo, Francesco; Ottaviani, M Francesca

    2010-02-18

    Lecithin/dodecylamine/lactose mixtures in ethanol/aqueous media led to the formation of sponge mesoporous silica (SMS) materials by means of tetraethoxysilane (TEOS) as silica source. SMS materials show a "sponge-mesoporous" porosity with a pore diameter of about 5-6 nm, in accordance to the length of a lecithin bilayer. SMS synthesis was developed to create a new class of powerful biocatalysts able to efficiently encapsulate enzymes by adding a porosity control to the classical sol-gel synthesis and by using phospholipids and lactose as protecting agents for the enzymes. In the present study, the formation of SMS was investigated by using electron paramagnetic resonance (EPR) probes inserted inside phospholipid bilayers. The influence of progressive addition of each component (ethanol, dodecylamine, lactose, TEOS) on phospholipid bilayers was first examined; then, the time evolution of EPR spectra during SMS synthesis was studied. Parameters informative of mobility, structure, order, and polarity around the probes were extracted by computer analysis of the EPR line shape. The results were discussed on the basis of solids characterization by X-ray diffraction, nitrogen isotherm, transmission electron microscopy, and scanning electron microscopy. The results, together with the well-known ability of ethanol to promote membrane hemifusion, suggested that the templating structure is a bicontinuous phospholipid bilayer phase, shaped as a gyroid, resulting of multiple membrane hemifusions induced by the high alcohol content used in SMS synthesis. SMS synthesis was compared to hexagonal mesoporous silica (HMS) synthesis accomplished by adding TEOS to a dodecylamine/EtOH/water mixture. EPR evidenced the difference between HMS and SMS synthesis; the latter uses an already organized but slowly growing mesophase of phospholipids, never observed before, whereas the former shows a progressive elongation of micelles into wormlike structures. SMS-type materials represent a new

  9. Supported lipid bilayer membranes for water purification by reverse osmosis.

    PubMed

    Kaufman, Yair; Berman, Amir; Freger, Viatcheslav

    2010-05-18

    Some biological plasma membranes pass water with a permeability and selectivity largely exceeding those of commercial membranes for water desalination using specialized trans-membrane proteins aquaporins. However, highly selective transport of water through aquaporins is usually driven by an osmotic rather mechanical pressure, which is not as attractive from the engineering point of view. The feasibility of adopting biomimetic membranes for water purification driven by a mechanical pressure, i.e., filtration is explored in this paper. Toward this goal, it is proposed to use a commercial nanofiltration (NF) membrane as a support for biomimetic lipid bilayer membranes to render them robust enough to withstand the required pressures. It is shown in this paper for the first time that by properly tuning molecular interactions supported phospholipid bilayers (SPB) can be prepared on a commercial NF membrane. The presence of SPB on the surface was verified and quantified by several spectroscopic and microscopic techniques, which showed morphology close to the desired one with very few defects. As an ultimate test it is shown that hydraulic permeability of the SPB supported on the NF membrane (NTR-7450) approaches the values deduced from the typical osmotic permeabilities of intact continuous bilayers. This permeability was unaffected by the trans-membrane flow of water and by repeatedly releasing and reapplying a 10 bar pressure. Along with a parallel demonstration that aquaporins could be incorporated in a similar bilayer on mica, this demonstrates the feasibility of the proposed approach. The prepared SPB structure may be used as a platform for preparing biomimetic filtration membranes with superior performance based on aquaporins. The concept of SPBs on permeable substrates of the present type may also be useful in the future for studying transport of various molecules through trans-membrane proteins.

  10. Collective Modes in Strongly Coupled Electronic Bilayer Liquids

    SciTech Connect

    Kalman, G.; Valtchinov, V.; Valtchinov, V.; Golden, K.I.

    1999-04-01

    We present the first reliable calculation of the collective mode structure of a strongly coupled electronic bilayer. The calculation is based on a classical model through the 3rd frequency-moment-sum rule preserving quasi-localized-charge approximation, using the recently calculated hypernetted-chain pair correlation functions. The out-of-phase spectrum shows an energy gap at k=0 and the absence of a previously conjectured dynamical instability. {copyright} {ital 1999} {ital The American Physical Society}

  11. Comparison of thermal fluctuations in foam films and bilayer structures

    NASA Astrophysics Data System (ADS)

    Ivanova, N. G.; Tsekov, R.

    2013-12-01

    In the frames of the DLVO theory the root mean square amplitude and correlation length of capillary waves in thin liquid films are calculated. Their dependencies on some important physical parameters are studied. Two models are considered: films with classical interfaces and films between lipid bilayers. The performed numerical analysis demonstrates essential difference in their behavior, which is due to the different elastic properties of the film surfaces in the models.

  12. Quantum Hall effect in bilayer system with array of antidots

    NASA Astrophysics Data System (ADS)

    Pagnossin, I. R.; Gusev, G. M.; Sotomayor, N. M.; Seabra, A. C.; Quivy, A. A.; Lamas, T. E.; Portal, J. C.

    2007-04-01

    We have studied the Quantum Hall effect in a bilayer system modulated by gate-controlled antidot lattice potential. The Hall resistance shows plateaus which are quantized to anomalous multiplies of h/e2. We suggest that this complex behavior is due to the nature of the edge-states in double quantum well (DQW) structures coupled to an array of antidots: these plateaus may be originated from the coexistence of normal and counter-rotating edge-states in different layers.

  13. Electrochemical characterization of bilayer lipid membrane-semiconductor junctions

    SciTech Connect

    Zhao, Xiao Kang; Baral, S.; Fendler, J.H. )

    1990-03-08

    Three different systems of glyceryl monooleate (GMO), bilayer lipid membrane (BLM) supported semiconductor particles have been prepared and characterized. A single composition of particulate semiconductor deposited only on one side of the BLM constituted system A, two different compositions of particulate semiconductors sequentially deposited on the same side of the BLM represented system B, and two different compositions of particulate semiconductors deposited on the opposite sides of the BLM made up system C.

  14. Exciton Transport in a Bilayer Quantum Hall Superfluid

    NASA Astrophysics Data System (ADS)

    Eisenstein, J. P.; Finck, A. D. K.; Nandi, D.; Pfeiffer, L. N.; West, K. W.

    2013-08-01

    Bilayer quantum Hall systems at vT = 1 support an excitonic ground state. In addition to the usual charged quasiparticles, this system possesses a condensate degree of freedom: exciton transport. Detection of this neutral transport mode is facilitated by the use of the Corbino multiply-connected geometry in which charge transport is suppressed. We here summarize our recent experiments on Corbino devices which directly demonstrate exciton transport across the bulk of the incompressible vT = 1 quantum Hall state.

  15. Thermoelectric Power in Bilayer Graphene Device with Ionic Liquid Gating.

    PubMed

    Chien, Yung-Yu; Yuan, Hongtao; Wang, Chang-Ran; Lee, Wei-Li

    2016-01-01

    The quest for materials showing large thermoelectric power has long been one of the important subjects in material science and technology. Such materials have great potential for thermoelectric cooling and also high figure of merit ZT thermoelectric applications. We have fabricated bilayer graphene devices with ionic-liquid gating in order to tune its band gap via application of a perpendicular electric field on a bilayer graphene. By keeping the Fermi level at charge neutral point during the cool-down, we found that the charge puddles effect can be greatly reduced and thus largely improve the transport properties at low T in graphene-based devices using ionic liquid gating. At (Vig, Vbg) = (-1 V, +23 V), a band gap of about 36.6 ± 3 meV forms, and a nearly 40% enhancement of thermoelectric power at T = 120 K is clearly observed. Our works demonstrate the feasibility of band gap tuning in a bilayer graphene using ionic liquid gating. We also remark on the significant influence of the charge puddles effect in ionic-liquid-based devices.

  16. Self-consistent perturbation theory for two dimensional twisted bilayers

    NASA Astrophysics Data System (ADS)

    Shirodkar, Sharmila N.; Tritsaris, Georgios A.; Kaxiras, Efthimios

    Theoretical modeling and ab-initio simulations of two dimensional heterostructures with arbitrary angles of rotation between layers involve unrealistically large and expensive calculations. To overcome this shortcoming, we develop a methodology for weakly interacting heterostructures that treats the effect of one layer on the other as perturbation, and restricts the calculations to their primitive cells. Thus, avoiding computationally expensive supercells. We start by approximating the interaction potential between the twisted bilayers to that of a hypothetical configuration (viz. ideally stacked untwisted layers), which produces band structures in reasonable agreement with full-scale ab-initio calculations for commensurate and twisted bilayers of graphene (Gr) and Gr/hexagonal boron nitride (h-BN) heterostructures. We then self-consistently calculate the charge density and hence, interaction potential of the heterostructures. In this work, we test our model for bilayers of various combinations of Gr, h-BN and transition metal dichalcogenides, and discuss the advantages and shortcomings of the self-consistently calculated interaction potential. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.

  17. Finite element analysis of the cyclic indentation of bilayer enamel

    NASA Astrophysics Data System (ADS)

    Jia, Yunfei; Xuan, Fu-zhen; Chen, Xiaoping; Yang, Fuqian

    2014-04-01

    Tooth enamel is often subjected to repeated contact and often experiences contact deformation in daily life. The mechanical strength of the enamel determines the biofunctionality of the tooth. Considering the variation of the rod arrangement in outer and inner enamel, we approximate enamel as a bilayer structure and perform finite element analysis of the cyclic indentation of the bilayer structure, to mimic the repeated contact of enamel during mastication. The dynamic deformation behaviour of both the inner enamel and the bilayer enamel is examined. The material parameters of the inner and outer enamel used in the analysis are obtained by fitting the finite element results with the experimental nanoindentation results. The penetration depth per cycle at the quasi-steady state is used to describe the depth propagation speed, which exhibits a two-stage power-law dependence on the maximum indentation load and the amplitude of the cyclic load, respectively. The continuous penetration of the indenter reflects the propagation of the plastic zone during cyclic indentation, which is related to the energy dissipation. The outer enamel serves as a protective layer due to its great resistance to contact deformation in comparison to the inner enamel. The larger equivalent plastic strain and lower stresses in the inner enamel during cyclic indentation, as calculated from the finite element analysis, indicate better crack/fracture resistance of the inner enamel.

  18. Gravimetric antigen detection utilizing antibody-modified lipid bilayers.

    PubMed

    Larsson, Charlotte; Bramfeldt, Hanna; Wingren, Christer; Borrebaeck, Carl; Höök, Fredrik

    2005-10-01

    Lipid bilayers containing 5% nitrilotriacetic acid (NTA) lipids supported on SiO2 have been used as a template for immobilization of oligohistidine-tagged single-chained antibody fragments (scFvs) directed against cholera toxin. It was demonstrated that histidine-tagged scFvs could be equally efficiently coupled to an NTA-Ni2+-containing lipid bilayer from a purified sample as from an expression supernatant, thereby providing a coupling method that eliminates time-consuming protein prepurification steps. Irrespective of whether the coupling was made from the unpurified or purified antibody preparation, the template proved to be efficient for antigen (cholera toxin) detection, verified using quartz crystal microbalance with dissipation monitoring. In addition, via a secondary amplification step using lipid vesicles containing GM1 (the natural membrane receptor for cholera toxin), the detection limit of cholera toxin was less than 750 pM. To further strengthen the coupling of scFvs to the lipid bilayer, scFvs containing two histidine tags, instead of just one tag, were also evaluated. The increased coupling strength provided via the bivalent anchoring significantly reduced scFv displacement in complex solutions containing large amounts of histidine-containing proteins, verified via cholera toxin detection in serum.

  19. Hybrid bilayer plasmonic metasurface efficiently manipulates visible light

    PubMed Central

    Qin, Fei; Ding, Lu; Zhang, Lei; Monticone, Francesco; Chum, Chan Choy; Deng, Jie; Mei, Shengtao; Li, Ying; Teng, Jinghua; Hong, Minghui; Zhang, Shuang; Alù, Andrea; Qiu, Cheng-Wei

    2016-01-01

    Metasurfaces operating in the cross-polarization scheme have shown an interesting degree of control over the wavefront of transmitted light. Nevertheless, their inherently low efficiency in visible light raises certain concerns for practical applications. Without sacrificing the ultrathin flat design, we propose a bilayer plasmonic metasurface operating at visible frequencies, obtained by coupling a nanoantenna-based metasurface with its complementary Babinet-inverted copy. By breaking the radiation symmetry because of the finite, yet small, thickness of the proposed structure and benefitting from properly tailored intra- and interlayer couplings, such coupled bilayer metasurface experimentally yields a conversion efficiency of 17%, significantly larger than that of earlier single-layer designs, as well as an extinction ratio larger than 0 dB, meaning that anomalous refraction dominates the transmission response. Our finding shows that metallic metasurface can counterintuitively manipulate the visible light as efficiently as dielectric metasurface (~20% in conversion efficiency in Lin et al.’s study), although the metal’s ohmic loss is much higher than dielectrics. Our hybrid bilayer design, still being ultrathin (~λ/6), is found to obey generalized Snell’s law even in the presence of strong couplings. It is capable of efficiently manipulating visible light over a broad bandwidth and can be realized with a facile one-step nanofabrication process. PMID:26767195

  20. Aggregatibacter actinomycetemcomitans leukotoxin cytotoxicity occurs through bilayer destabilization

    PubMed Central

    Brown, Angela C.; Boesze-Battaglia, Kathleen; Du, Yurong; Stefano, Frank P.; Kieba, Irene R.; Epand, Raquel F.; Kakalis, Lazaros; Yeagle, Philip L.; Epand, Richard M.; Lally, Edward T.

    2012-01-01

    Summary The Gram-negative bacterium, Aggregatibacter actinomycetemcomitans, is a common inhabitant of the human upper aerodigestive tract. The organism produces an RTX (Repeats in ToXin) toxin (LtxA) that kills human white blood cells. LtxA is believed to be a membrane-damaging toxin, but details of the cell surface interaction for this and several other RTX toxins have yet to be elucidated. Initial morphological studies suggested that LtxA was bending the target cell membrane. Because the ability of a membrane to bend is a function of its lipid composition, we assessed the proficiency of LtxA to release of a fluorescent dye from a panel of liposomes composed of various lipids. Liposomes composed of lipids that form nonlamellar phases were susceptible to LtxA-induced damage while liposomes composed of lipids that do not form non-bilayer structures were not. Differential scanning calorimetry demonstrated that the toxin decreased the temperature at which the lipid transitions from a bilayer to a nonlamellar phase, while 31P nuclear magnetic resonance studies showed that the LtxA-induced transition from a bilayer to an inverted hexagonal phase occurs through the formation of an isotropic intermediate phase. These results indicate that LtxA cytotoxicity occurs through a process of membrane destabilization. PMID:22309134

  1. Growth from below: graphene bilayers on Ir(111).

    PubMed

    Nie, Shu; Walter, Andrew L; Bartelt, Norman C; Starodub, Elena; Bostwick, Aaron; Rotenberg, Eli; McCarty, Kevin F

    2011-03-22

    We elucidate how graphene bilayers form on Ir(111). Low-energy electron diffraction (LEED) reveals that the two graphene layers are not always rotationally aligned. Monitoring this misalignment during growth shows that second-layer islands nucleate between the existing layer and the substrate. This mechanism occurs both when C segregates from the Ir and when elemental C is deposited from above. Low-energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES) show that second-layer nucleation occurs preferentially under the first-layer rotational variants that are more weakly bound to the substrate. New-layer nucleation tends to occur inhomogeneously at substrate defects. Thus new-layer nucleation should be rapid on substrates that weakly bind graphene, making growth unstable toward mound formation initiated at substrate defects. In contrast, stronger binding permits layer-by-layer growth, as for Ru(0001). ARPES shows that bilayer graphene has two slightly p-doped π-bands. The work function of bilayer graphene is dominated by the orientation of the bottom layer. PMID:21322532

  2. Amphotericin B induced interdigitation of apolipoprotein stabilized nanodisk bilayers

    SciTech Connect

    Nguyen, T; Weers, P M; Sulchek, T; Hoeprich, P D; Ryan, R O

    2006-12-07

    Amphotericin B nanodisks (AMB-ND) are ternary complexes of AMB, phospholipid (PL) and apolipoprotein organized as discrete nanometer scale disk-shaped bilayers. In gel filtration chromatography experiments, empty ND lacking AMB elute as a single population of particles with a molecular weight in the range of 200 kDa. AMB-ND formulated at a 4:1 PL:AMB weight ratio, separated into two peaks. Peak 1 eluted at the position of control ND lacking AMB while the second peak, containing all of the AMB present in the original sample, eluted in the void volume. When ND prepared with increased AMB (1:1 phospholipid:AMB molar ratio) were subjected to gel filtration chromatography, an increased proportion of phospholipid and apolipoprotein were recovered in the void volume with the AMB. Prior to gel filtration the AMB-ND sample could be passed through a 0.22 {micro}m filter without loss of AMB while the voided material was lost. Native gel electrophoresis studies corroborated the gel permeation chromatography data. Far UV circular dichroism analyses revealed that apoA-I associated with AMB-ND denatures at a lower guanidine HCl concentration than apoA-I associated with ND lacking AMB. Atomic force microscopy revealed that AMB induces compression of the ND bilayer thickness consistent with bilayer interdigitation, a phenomenon that is likely related to the ability of AMB to induce pore formation in susceptible membranes.

  3. Optical rectification at visible frequency in biased bilayer graphene

    NASA Astrophysics Data System (ADS)

    Hipolito, F.; Pereira, Vitor M.

    2015-03-01

    The second order response of the electrical current to an electromagnetic field is analyzed within the framework of non-equilibrium many-body perturbation theory for the case of a two-dimensional electronic system such as graphene and its bilayer. The absence of inversion symmetry in a biased graphene bilayer allows a finite DC response in second order to an AC electromagnetic wave. The induced DC current is evaluated for biased bilayer at finite temperature, and its tunability is analyzed as a function of electron density, which can be experimentally varied by means of a global gate voltage applied to the sample. Both intrinsic and photon drag microscopic processes are considered, as they contribute on similar footing to the photocurrent in general. However, the dependencies of these two contributions on the polarization state of the incident light are different, which allows the manipulation of the relative contribution of intrinsic versus photon drag contributions by tuning the experimental parameters. For example, the photocurrent emerging from circularly polarized light stems entirely from photon drag, as the circular photogalvanic effect is forbidden by the C3 rotation symmetry of the honeycomb lattice.

  4. Soliton-dependent plasmon reflection at bilayer graphene domain walls

    NASA Astrophysics Data System (ADS)

    Jiang, Lili; Shi, Zhiwen; Zeng, Bo; Wang, Sheng; Kang, Ji-Hun; Joshi, Trinity; Jin, Chenhao; Ju, Long; Kim, Jonghwan; Lyu, Tairu; Shen, Yuen-Ron; Crommie, Michael; Gao, Hong-Jun; Wang, Feng

    2016-08-01

    Layer-stacking domain walls in bilayer graphene are emerging as a fascinating one-dimensional system that features stacking solitons structurally and quantum valley Hall boundary states electronically. The interactions between electrons in the 2D graphene domains and the one-dimensional domain-wall solitons can lead to further new quantum phenomena. Domain-wall solitons of varied local structures exist along different crystallographic orientations, which can exhibit distinct electrical, mechanical and optical properties. Here we report soliton-dependent 2D graphene plasmon reflection at different 1D domain-wall solitons in bilayer graphene using near-field infrared nanoscopy. We observe various domain-wall structures in mechanically exfoliated graphene bilayers, including network-forming triangular lattices, individual straight or bent lines, and even closed circles. The near-field infrared contrast of domain-wall solitons arises from plasmon reflection at domain walls, and exhibits markedly different behaviours at the tensile- and shear-type domain-wall solitons. In addition, the plasmon reflection at domain walls exhibits a peculiar dependence on electrostatic gating. Our study demonstrates the unusual and tunable coupling between 2D graphene plasmons and domain-wall solitons.

  5. Quantum Monte Carlo study of magnetic impurity in bilayer grephene

    NASA Astrophysics Data System (ADS)

    Sun, J. H.; Hu, F. M.; Tang, H. K.; Lin, Hai-Qing

    2012-02-01

    It is expected to observe many different properties in bilayer graphene when compared with single layer graphene due to the differences in crystal structure. Additionally, bilayer system offers a freedom of inducing a gap in the energy band by applying a shift in the electrochemical potential to two graphene layers. In this work, we study the magnetic properties of an Anderson magnetic adatom in Bernal stacking bilayer graphene and compare the results with those of single layer counterpart. Several different cases such as different adatom position and different potential bias of two layers are studied using the quantum Monte Carlo method. In all the cases, we find that the impurity local magnetic moment can be switched between relatively large and small values by tuning the chemical potential. We apply MaxEnT method to compute impurity spectral density and find its behavior to differ from that of an impurity in a single layer graphene. We also calculate various correlation functions and make comparisons.

  6. Multivalent Ligand-Receptor Binding on Supported Lipid Bilayers

    PubMed Central

    Jung, Hyunsook; Robison, Aaron D.; Cremer, Paul S.

    2009-01-01

    Fluid supported lipid bilayers provide an excellent platform for studying multivalent protein-ligand interactions because the two-dimensional fluidity of the membrane allows for lateral rearrangement of ligands in order to optimize binding. Our laboratory has combined supported lipid bilayer-coated microfluidic platforms with total internal reflection fluorescence microscopy (TIRFM) to obtain equilibrium dissociation constant (KD) data for these systems. This high throughput, on-chip approach provides highly accurate thermodynamic information about multivalent binding events while requiring only very small sample volumes. Herein, we review some of the most salient findings from these studies. In particular, increasing ligand density on the membrane surface can provide a modest enhancement or attenuation of ligand-receptor binding depending upon whether the surface ligands interact strongly with each other. Such effects, however, lead to little more than one order of magnitude change in the apparent KD values. On the other hand, the lipophilicity and presentation of lipid bilayer-conjugated ligands can have a much greater impact. Indeed, changing the way a particular ligand is conjugated to the membrane can alter the apparent KD value by at least three orders of magnitude. Such a result speaks strongly to the role of ligand availability for multivalent ligand-receptor binding. PMID:19508894

  7. Supported lipid bilayer microarrays created by non-contact printing.

    PubMed

    Kaufmann, Stefan; Sobek, Jens; Textor, Marcus; Reimhult, Erik

    2011-07-21

    Arrays of supported lipid bilayers (SLBs) provide great potential for future drug development and multiplexed biological research, but are difficult to prepare due to the sensitivity of both the lipid and protein structural arrangement to air exposure. A novel way to produce arrays of SLBs is presented based on non-contact dispensing of vesicles to a substrate through a thin surface confined water film. The approach presents many degrees of freedom since it is not limited to a specific substrate, lipid composition, linker or controlled environment. The method allows adjustment of spot size (180-360 μm) by repeated dispensing as well as control over the composition of the spots and subsequent analytes. SLB formation by vesicle adsorption and rupture allows for incorporation of membrane proteins through pre-formed proteoliposomes. Dispensing through a dip-and-rinse water film avoids contamination, disruptive drying and the need for complex buffer compositions. Furthermore, no humidity control is necessary which simplifies the production step and prolongs the life-time of the spotting system. We characterize the method with respect to control over spot size, bilayer mobility and the formation process as well as demonstrate the possibility to fuse bilayer spots with subsequently added vesicles. Since complex lipid compositions and multiple spotting nozzles can be used, this novel technique is expected to be a promising platform for future applications, e.g. patterning to monitor peptide/protein-lipid interactions, for glycomics using glycolipids or lipopolysaccharides, and to study mixing of spatially confined lipid membranes.

  8. Viscoelastic changes measured in partially suspended single bilayer membranes.

    PubMed

    Hasan, Imad Younus; Mechler, Adam

    2015-07-21

    For studies involving biomimetic phospholipid membrane systems, such as membrane-protein interactions, it is crucial that the supported membrane is biomimetic in its physical properties as well as in its composition. Two often overlooked aspects of biomimicry are the need for unrestrained lipid mobility, reflected in the viscoelastic properties of the membrane, and sufficient space between the membrane and the support for the insertion of transmembrane proteins. Here we show for a series of DMPC-based membranes that a partially suspended single bilayer membrane can be formed on functionalized gold surface without tethering. These membranes exhibit sufficient freedom of motion to represent the viscoelastic properties of a free lamellar bilayer membrane as demonstrated by determining the phase transition temperatures of these single bilayer membranes from the viscosity change upon chain melting using the dissipation signal of a quartz crystal microbalance (QCM-D). Atomic force microscopy imaging confirmed confluent, smooth membrane coverage of the QCM-D sensor that completely obscured the roughness of the sputtered gold surface. High-force AFM imaging was able to push membrane patches into the valleys of the gold morphology, confirming the inherently suspended nature of the MPA supported membrane. We show that the correlation between frequency and dissipation changes in the QCM-D sensograms is a sensitive indicator of the morphology of the membrane. PMID:26073288

  9. Amphotericin B induces interdigitation of apolipoprotein stabilized nanodisk bilayers.

    PubMed

    Nguyen, Thanh-Son; Weers, Paul M M; Raussens, Vincent; Wang, Zhen; Ren, Gang; Sulchek, Todd; Hoeprich, Paul D; Ryan, Robert O

    2008-01-01

    Amphotericin B nanodisks (AMB-ND) are ternary complexes of AMB, phospholipid and apolipoprotein organized as discrete nanometer scale disk-shaped bilayers. In gel filtration chromatography experiments, empty ND lacking AMB elute as a single population of particles with a molecular weight in the range of 200 kDa. AMB-ND formulated at a 4:1 phospholipid:AMB weight ratio separated into two peaks. One peak eluted at the position of control ND lacking AMB while the second peak, containing all of the AMB present in the original sample, eluted in the void volume. When ND prepared with increased AMB were subjected to gel filtration chromatography an increased proportion of phospholipid and apolipoprotein was recovered in the void volume with AMB. Native gradient gel electrophoresis corroborated the gel filtration chromatography data and electron microscopy studies revealed an AMB concentration-dependent heterogeneity in ND particle size. Stability studies revealed that introduction of AMB into ND decreases the ability of apoA-I to resist denaturation. Atomic force microscopy experiments showed that AMB induces compression of ND bilayer thickness while infrared spectroscopy analysis revealed that the presence of AMB does not induce extreme lipid disorder or alter the mean angle of the molecular axis along fatty acyl chains of ND phospholipids. Taken together the results are consistent with AMB-induced bilayer interdigitation, a phenomenon that likely contributes to AMB-dependent pore formation in susceptible membranes.

  10. TARP phosphorylation regulates synaptic AMPA receptors through lipid bilayers

    PubMed Central

    Sumioka, Akio; Yan, Dan; Tomita, Susumu

    2010-01-01

    Summary Neurons use neurotransmitters to communicate across synapses, constructing neural circuits in the brain. AMPA-type glutamate receptors are the predominant excitatory neurotransmitter receptors mediating fast synaptic transmission. AMPA receptors localize at synapses by forming protein complexes with transmembrane AMPA receptor regulatory proteins (TARPs) and PSD-95-like MAGUKs. Among the three classes of ionotropic glutamate receptors (AMPA-, NMDA, kainate-type), AMPA receptor activity is most regulatable by neuronal activity to adjust synaptic strength. Here, we mutated the prototypical TARP, stargazin, and found that TARP phosphorylation regulates synaptic AMPA receptor activity in vivo. We also found that stargazin interacts with negatively-charged lipid bilayers in its phosphorylation dependent manner, and that the lipid interaction inhibited stargazin binding to PSD-95. Cationic lipids dissociated stargazin from lipid bilayers and enhanced synaptic AMPA receptor activity in a stargazin phosphorylation-dependent manner. Thus, TARP phosphorylation plays a critical role in regulating AMPA receptor-mediated synaptic transmission via a lipid bilayer interaction. PMID:20547132

  11. 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. PMID:27046262

  12. 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.

  13. Enzyme modification of platinum microelectrodes for detection of cholesterol in vesicle lipid bilayer membranes.

    PubMed

    Devadoss, Anando; Palencsár, M Simona; Jiang, Dechen; Honkonen, Michael L; Burgess, James D

    2005-11-15

    Platinum microelectrodes are modified with a lipid bilayer membrane incorporating cholesterol oxidase. Details for electrode surface modification are presented along with characterization studies of electrode response to cholesterol solution and to cholesterol contained in the lipid bilayer membrane of vesicles. Ferrocyanide voltammetric experiments are used to track deposition of a submonolayer of a thiol-functionalized lipid on the platinum electrode surface, vesicle fusion for bilayer formation on the thiolipid-modified surface, and incorporation of cholesterol oxidase in the electrode-supported thiolipid/lipid bilayer membrane. The data are consistent with formation of a lipid bilayer structure on the electrode surface that contains defects. Experiments for detection of cholesterol solubilized in cyclodextrin solution show steady-state current responses that correlate with cholesterol concentration. Direct contact between the electrode and a vesicle lipid bilayer membrane shows a response that correlates with vesicle membrane cholesterol content. PMID:16285691

  14. 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.

  15. Time-resolved photoresponse of nanometer-thick Nb/NiCu bilayers

    NASA Astrophysics Data System (ADS)

    Parlato, L.; Pepe, G. P.; Latempa, R.; De Lisio, C.; Altucci, C.; D'Acunto, P.; Peluso, G.; Barone, A.; Taneda, T.; Sobolewski, R.

    2005-07-01

    We present femtosecond optical time-resolved pump-probe investigations of superconducting hybrids structures consisting of Nb/NiCu bilayers with various thickness. Measurements performed on pure Nb and NiCu films are also given. The photoresponse experiments provide the quasiparticle relaxation times in bilayers of different thickness ratios. The study of the photoresponse as a function of the temperature reveals the spatial evolution of the superconductor order parameter across the bilayers.

  16. Electrowetting on dielectric-based microfluidics for integrated lipid bilayer formation and measurement

    NASA Astrophysics Data System (ADS)

    Poulos, Jason L.; Nelson, Wyatt C.; Jeon, Tae-Joon; Kim, Chang-Jin ``Cj''; Schmidt, Jacob J.

    2009-07-01

    We present a microfluidic platform for the formation and electrical measurement of lipid bilayer membranes. Using electrowetting on dielectric (EWOD), two or more aqueous droplets surrounded by a lipid-containing organic phase were manipulated into contact to form a lipid bilayer at their interface. Thin-film Ag/AgCl electrodes integrated into the device enabled electrical measurement of membrane formation and the incorporation of gramicidin channels of two bilayers in parallel.

  17. The assembly and use of tethered bilayer lipid membranes (tBLMs).

    PubMed

    Cranfield, Charles; Carne, Sonia; Martinac, Boris; Cornell, Bruce

    2015-01-01

    Because they are firmly held in place, tethered bilayer lipid membranes (tBLMs) are considerably more robust than supported lipid bilayers such as black lipid membranes (BLMs) (Cornell et al. Nature 387(6633): 580-583, 1997). Here we describe the procedures required to assemble and test tethered lipid bilayers that can incorporate various lipid species, peptides, and ion channel proteins. PMID:25331126

  18. 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.

  19. 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

  20. Positively charged supported lipid bilayer formation on gold surfaces for neuronal cell culture.

    PubMed

    Choi, Sung-Eun; Greben, Kyrylo; Wördenweber, Roger; Offenhäusser, Andreas

    2016-06-06

    Supported lipid bilayers are widely used as cell membrane models and sensor platforms, but the usage on gold surface needs additional surface modification or optimized experimental conditions. In this work, the authors show lipid bilayer formation on plasma activated gold surfaces in physiological conditions without any other modification if at least 30% positively charged lipids are present. Details of bilayer formation from small unilamellar vesicles were monitored using quartz crystal microbalance with dissipation in both basic and acidic environment. The authors also confirmed that this positively charged bilayer system can sustain primary cortical neuron growth and lipid transfer. This method will provide simple means to construct biomimetic interface on gold electrodes.

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

    SciTech Connect

    Richter, Andrew; Dergunov, Sergey; Ganus, Bill; Thomas, Zachary; Pingali, Sai Venkatesh; Urban, Volker S; Liu, Yun; Porcar, Lionel; Pinkhassik, Eugene

    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 scattering (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.

  2. Scattering Studies of Hydrophobic Monomers in Liposomal Bilayers: An Expanding shell Model of Monomer Distribution

    SciTech Connect

    Richter, Andrew G; Dergunov, Sergey; Ganus, Bill; Thomas, Zachary P; Pingali, Sai Venkatesh; Urban, Volker S; Liu, Yun; Porcar, Lionel; Pinkhassik, Eugene

    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 scattering (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.

  3. Formation of "solvent-free" black lipid bilayer membranes from glyceryl monooleate dispersed in squalene.

    PubMed

    White, S H

    1978-09-01

    A simple technique for forming "black" lipid bilayer membranes containing negligible amounts of alkyl solvent is described. The membranes are formed by the method of Mueller et al (Circulation. 1962. 26:1167.) from glyceryl monooleate (GMO) dispersed in squalene. The squalene forms an annulus to satisfy the boundary conditions of the planar bilayer but does not appear to dissolve noticeably in the bilayer itself. The specific geometric capacitance (Cg) of the membranes at 20 degrees C formed by this technique is 0.7771 +/- 0.0048 muF/cm2. Theoretical estimates of Cg for solvent-free bilayers range from 0.75 to 0.81 muF/cm2. Alkane-free GMO bilayers formed from n-octadecane by the solvent freeze-out method of White (Biochim. Biophys. Acta. 1974. 356:8) have values of Cg = 0.7903 +/- 0.0013 muF/cm2 at 20.5 degrees C. The agreement between the various values of Cg strongly suggests that the bilayers are free of squalene. DC potentials applied to the bilayers have no detectable effect on the value of Cg, as expected for solvent-free films. The ability to form bilayers essentially free of the solvent used in the forming solution makes it possible to determine the area per molecule of the surface active lipid in the bilayer. The area per molecule of GMO at 20 degrees C is estimated to be 37.9 +/- 0.2 A2.

  4. Surface tension effects on the phase transition of a DPPC bilayer with and without protein: a molecular dynamics simulation.

    PubMed

    Kong, Xian; Qin, Shanshan; Lu, Diannan; Liu, Zheng

    2014-05-14

    While the surface tension of a cell membrane, or a plasma membrane, regulates cell functions, little is known about its effect on the conformational changes of the lipid bilayer and hence the resulting changes in the cell membrane. To obtain some insights into the phase transition of the lipid bilayer as a function of surface tension, we used a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer as a model lipid bilayer and aquaporin (AqpZ), a transmembrane channel protein for water, as a model embedded protein. A coarse-grained molecular dynamics simulation was applied to illustrate the phase transition behavior of the pure DPPC bilayer and aquaporin-embedded DPPC bilayer under different surface tensions. It was shown that an increased surface tension reduced the phase transition temperature of the DPPC bilayer. As for the DPPC bilayer in gel form, no significant changes occurred in the structure of the bilayer in response to the surface tension. Once in a liquid crystal state, both the structure and properties of the DPPC bilayer, such as area per lipid, lipid order parameters, bilayer thickness and lateral diffusion coefficients, were responsive to the magnitude of surface tension in a linear way. The presence of aquaporin attenuated the compact alignment of the lipid bilayer, hindered the parallel movement, and thus made the DPPC bilayer less sensitive to the surface tension.

  5. Kinematics, material symmetry, and energy densities for lipid bilayers with spontaneous curvature

    PubMed Central

    Maleki, Mohsen; Seguin, Brian; Fried, Eliot

    2013-01-01

    Continuum mechanical tools are used to describe the deformation, energy density, and material symmetry of a lipid bilayer with spontaneous curvature. In contrast to conventional approaches in which lipid bilayers are modeled by material surfaces, here we rely on a three-dimensional approach in which a lipid bilayer is modeling by a shell-like body with finite thickness. In this setting, the interface between the leaflets of a lipid bilayer is assumed to coincide with the mid-surface of the corresponding shell-like body. The three-dimensional deformation gradient is found to involve the curvature tensors of the mid-surface in the spontaneous and the deformed states, the deformation gradient of the mid-surface, and the transverse deformation. Attention is also given to the coherency of the leaflets and to the area compatibility of closed lipid bilayers (i.e., vesicles). A hyperelastic constitutive theory for lipid bilayers in the liquid phase is developed. In combination, the requirements of frame-indifference and material symmetry yield a representation for the energy density of a lipid bilayer. This representation shows that three scalar invariants suffice to describe the constitutive response of a lipid bilayer exhibiting in-plane fluidity and transverse isotropy. In addition to exploring the geometrical and physical properties of these invariants, fundamental constitutively-associated kinematical quantities are emphasized. On this basis, the effect on the energy density of assuming that the lipid bilayer is incompressible is considered. Lastly, a dimension reduction argument is used to extract an areal energy density per unit area from the three-dimensional energy density. This step explains the origin of spontaneous curvature in the areal energy density. Importantly, along with a standard contribution associated with the natural curvature of lipid bilayer, our analysis indicates that constitutive asymmetry between the leaflets of the lipid bilayer gives rise to

  6. Stress Transfer and Structural Failure of Bilayered Material Systems

    NASA Astrophysics Data System (ADS)

    Prieto-Munoz, Pablo Arthur

    Bilayered material systems are common in naturally formed or artificially engineered structures. Understanding how loads transfer within these structural systems is necessary to predict failure and develop effective designs. Existing methods for evaluating the stress transfer in bilayered materials are limited to overly simplified models or require experimental calibration. As a result, these methods have failed to accurately account for such structural failures as the creep induced roofing panel collapse of Boston's I-90 connector tunnel, which was supported by adhesive anchors. The one-dimensional stress analyses currently used for adhesive anchor design cannot account for viscoelastic creep failure, and consequently results in dangerously under-designed structural systems. In this dissertation, a method for determining the two-dimensional stress and displacement fields for a generalized bilayered material system is developed, and proposes a closed-form analytical solution. A general linear-elastic solution is first proposed by decoupling the elastic governing equations from one another through the so-called plane assumption. Based on this general solution, an axisymmetric problem and a plane strain problem are formulated. These are applied to common bilayered material systems such as: (1) concrete adhesive anchors, (2) material coatings, (3) asphalt pavements, and (4) layered sedimentary rocks. The stress and displacement fields determined by this analytical analysis are validated through the use of finite element models. Through the correspondence principle, the linear-elastic solution is extended to consider time-dependent viscoelastic material properties, thus facilitating the analysis of adhesive anchors and asphalt pavements while incorporating their viscoelastic material behavior. Furthermore, the elastic stress analysis can explain the fracturing phenomenon of material coatings, pavements, and layered rocks, successfully predicting their fracture

  7. Dielectric properties, screening and compressibility for bilayer systems

    NASA Astrophysics Data System (ADS)

    Kyrkos, Stamatios

    We examine the dielectric properties, screening and compressibility for charged particle bilayer systems. In the process, we first review the screening problem of a charged impurity in a one component plasma in 3D, 2D and bilayer systems within the framework of the linear response theory, both for weak and strong coupling. In the weakly coupled domain, the approach is based on the RPA response matrix. The behavior that emerges is marked by an unexpected nonmonotonic dependence of the screened charges and potential on the layer separation. In the strong coupling domain we use the link provided by the classical fluctuation dissipation theorem between the response matrix and the structure function matrix that was independently calculated by W. L. Slattery, et al. and S. Chakravatry et al . What emerges is a screening scenario where one layer exhibits an overscreening and the other an antiscreening behavior. To go beyond the linear response approximation, we also examine, within the RPA approach, higher order (quadratic) contributions to the screening both in 2D and 3D, based on the quadratic density response previously calculated by J. M. Rommel. These contributions are expected to play an important role at higher coupling or higher Z values. Finally, since the response function and the screening behavior are intimately linked to the isothermal compressibility of the system, we examine the inverse compressibility matrix and its properties for a bilayer system, both within the Debye approximation and for strongly coupled systems. The later as based on previously obtained hypernetted chain (HNC) and molecular dynamics results by G. D. Doolen and Z. Donko.

  8. Interaction of curcumin with lipid monolayers and liposomal bilayers.

    PubMed

    Karewicz, Anna; Bielska, Dorota; Gzyl-Malcher, Barbara; Kepczynski, Mariusz; Lach, Radosław; Nowakowska, Maria

    2011-11-01

    Curcumin shows huge potential as an anticancer and anti-inflammatory agent. However, to achieve a satisfactory bioavailability and stability of this compound, its liposomal form is preferable. Our detailed studies on the curcumin interaction with lipid membranes are aimed to obtain better understanding of the mechanism and eventually to improve the efficiency of curcumin delivery to cells. Egg yolk phosphatidylcholine (EYPC) one-component monolayers and bilayers, as well as mixed systems containing additionally dihexadecyl phosphate (DHP) and cholesterol, were studied. Curcumin binding constant to EYPC liposomes was determined based on two different methods: UV/Vis absorption and fluorescence measurements to be 4.26×10(4)M(-1) and 3.79×10(4)M(-1), respectively. The fluorescence quenching experiment revealed that curcumin locates in the hydrophobic region of EYPC liposomal bilayer. It was shown that curcumin impacts the size and stability of the liposomal carriers significantly. Loaded into the EYPC/DPH/cholesterol liposomal bilayer curcumin stabilizes the system proportionally to its content, while the EYPC/DPH system is destabilized upon drug loading. The three-component lipid composition of the liposome seems to be the most promising system for curcumin delivery. An interaction of free and liposomal curcumin with EYPC and mixed monolayers was also studied using Langmuir balance measurements. Monolayer systems were treated as a simple model of cell membrane. Condensing effect of curcumin on EYPC and EYPC/DHP monolayers and loosening influence on EYPC/DHP/chol ones were observed. It was also demonstrated that curcumin-loaded EYPC liposomes are more stable upon interaction with the model lipid membrane than the unloaded ones.

  9. Assembly of RNA nanostructures on supported lipid bilayers.

    PubMed

    Dabkowska, Aleksandra P; Michanek, Agnes; Jaeger, Luc; Rabe, Michael; Chworos, Arkadiusz; Höök, Fredrik; Nylander, Tommy; Sparr, Emma

    2015-01-14

    The assembly of nucleic acid nanostructures with controlled size and shape has large impact in the fields of nanotechnology, nanomedicine and synthetic biology. The directed arrangement of nano-structures at interfaces is important for many applications. In spite of this, the use of laterally mobile lipid bilayers to control RNA three-dimensional nanostructure formation on surfaces remains largely unexplored. Here, we direct the self-assembly of RNA building blocks into three-dimensional structures of RNA on fluid lipid bilayers composed of cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or mixtures of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) and cationic sphingosine. We demonstrate the stepwise supramolecular assembly of discrete building blocks through specific and selective RNA-RNA interactions, based on results from quartz crystal microbalance with dissipation (QCM-D), ellipsometry, fluorescence recovery after photobleaching (FRAP) and total internal reflection fluorescence microscopy (TIRF) experiments. The assembly can be controlled to give a densely packed single layer of RNA polyhedrons at the fluid lipid bilayer surface. We show that assembly of the 3D structure can be modulated by sequence specific interactions, surface charge and changes in the salt composition and concentration. In addition, the tertiary structure of the RNA polyhedron can be controllably switched from an extended structure to one that is dense and compact. The versatile approach to building up three-dimensional structures of RNA does not require modification of the surface or the RNA molecules, and can be used as a bottom-up means of nanofabrication of functionalized bio-mimicking surfaces.

  10. Assembly of RNA nanostructures on supported lipid bilayers

    PubMed Central

    Dabkowska, Aleksandra P.; Michanek, Agnes; Jaeger, Luc; Rabe, Michael; Chworos, Arkadiusz; Höök, Fredrik; Nylander, Tommy; Sparr, Emma

    2014-01-01

    The assembly of nucleic acid nanostructures with controlled size and shape has large impact in the fields of nanotechnology, nanomedicine and synthetic biology. The directed arrangement of nanostructures at interfaces is important for many applications. In spite of this, the use of laterally mobile lipid bilayers to control RNA three-dimensional nanostructure formation on surfaces remains largely unexplored. Here, we direct the self-assembly of RNA building blocks into three-dimensional structures of RNA on fluid lipid bilayers composed of cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or mixtures of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) and cationic sphingosine. We demonstrate the stepwise supramolecular assembly of discrete building blocks through specific and selective RNA-RNA interactions, based on results from quartz crystal microbalance with dissipation (QCM-D), ellipsometry, fluorescence recovery after photobleaching (FRAP) and total internal reflection fluorescence microscopy (TIRF) experiments. The assembly can be controlled to give a densely packed single layer of RNA polyhedrons at the fluid lipid bilayer surface. We show that assembly of the 3D structure can be modulated by sequence specific interactions, surface charge and changes in the salt composition and concentration. In addition, the tertiary structure of the RNA polyhedron can be controllably switched from an extended structure to one that is dense and compact. The versatile approach to building up three-dimensional structures of RNA does not require modification of the surface or the RNA molecules, and can be used as a bottom-up means of nanofabrication of functionalized bio-mimicking surfaces. PMID:25417592

  11. Giant Frictional Drag in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Lee, Kayoung; Xue, Jiamin; Dillen, David C.; Watanabe, Kenji; Taniguchi, Takashi; Tutuc, Emanuel

    2016-07-01

    We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2-5 nm thick hexagonal boron nitride dielectric. At temperatures (T ) lower than ˜10 K , we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing T , and becomes comparable to the layer resistivity at the lowest T =1.5 K . At low T the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag.

  12. Giant Frictional Drag in Double Bilayer Graphene Heterostructures.

    PubMed

    Lee, Kayoung; Xue, Jiamin; Dillen, David C; Watanabe, Kenji; Taniguchi, Takashi; Tutuc, Emanuel

    2016-07-22

    We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2-5 nm thick hexagonal boron nitride dielectric. At temperatures (T) lower than ∼10  K, we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing T, and becomes comparable to the layer resistivity at the lowest T=1.5  K. At low T the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag. PMID:27494492

  13. Superfluidity and density order in a bilayer extended Hubbard model

    NASA Astrophysics Data System (ADS)

    Vanhala, Tuomas I.; Baarsma, Jildou E.; Heikkinen, Miikka O. J.; Troyer, Matthias; Harju, Ari; Törmä, Päivi

    2015-04-01

    We use cellular dynamical mean-field theory to study the phase diagram of the square lattice bilayer Hubbard model with an interlayer interaction. The layers are populated by two-component fermions, and the densities in both layers and the strength of the interactions are varied. We find that an attractive interlayer interaction can induce a checkerboard density-ordered phase and superfluid phases, with either interlayer or intralayer pairing. Remarkably, the latter phase does not require an intralayer interaction to be present: it can be attributed to an induced attractive interaction caused by density fluctuations in the other layer.

  14. Emerging activity in bilayered dispersions with wake-mediated interactions

    NASA Astrophysics Data System (ADS)

    Bartnick, Jörg; Kaiser, Andreas; Löwen, Hartmut; Ivlev, Alexei V.

    2016-06-01

    In a bilayered system of particles with wake-mediated interactions, the action-reaction symmetry for the effective forces between particles of different layers is broken. Under quite general conditions we show that, if the interaction nonreciprocity exceeds a certain threshold, this creates an active dispersion of self-propelled clusters of Brownian particles. The emerging activity promotes unusual melting scenarios and an enormous diffusivity in the dense fluid. Our results are obtained by computer simulation and analytical theory and can be verified in experiments with colloidal dispersions and complex plasmas.

  15. Directional photoelectric current across the bilayer graphene junction.

    PubMed

    Shafranjuk, S E

    2009-01-01

    A directional photon-assisted resonant chiral tunneling through a bilayer graphene barrier is considered. An external electromagnetic field applied to the barrier switches the transparency T in the longitudinal direction from its steady state value T = 0 to the ideal T = 1 at no energy costs. The switch happens because the ac field affects the phase correlation between the electrons and holes inside the graphene barrier, changing the whole angular dependence of the chiral tunneling (directional photoelectric effect). The suggested phenomena can be implemented in relevant experiments and in various sub-millimeter and far-infrared optical electronic devices.

  16. Valley filtering using electrostatic potentials in bilayer graphene

    NASA Astrophysics Data System (ADS)

    da Costa, D. R.; Chaves, Andrey; Sena, S. H. R.; Farias, G. A.; Peeters, F. M.

    2015-07-01

    Propagation of an electron wave packet through a quantum point contact (QPC) defined by electrostatic gates in bilayer graphene is investigated. The gates provide a bias between the layers, in order to produce an energy gap. If the gates on both sides of the contact produce the same bias, steps in the electron transmission probability are observed, as in the usual QPC. However, if the bias is inverted on one of the sides of the QPC, only electrons belonging to one of the Dirac valleys are allowed to pass, which provides a very efficient valley filtering.

  17. Giant Frictional Drag in Double Bilayer Graphene Heterostructures.

    PubMed

    Lee, Kayoung; Xue, Jiamin; Dillen, David C; Watanabe, Kenji; Taniguchi, Takashi; Tutuc, Emanuel

    2016-07-22

    We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2-5 nm thick hexagonal boron nitride dielectric. At temperatures (T) lower than ∼10  K, we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing T, and becomes comparable to the layer resistivity at the lowest T=1.5  K. At low T the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag.

  18. Negative terahertz conductivity in remotely doped graphene bilayer heterostructures

    SciTech Connect

    Ryzhii, V.; Ryzhii, M.; Mitin, V.; Shur, M. S.; Otsuji, T.

    2015-11-14

    Injection or optical generation of electrons and holes in graphene bilayers (GBLs) can result in the interband population inversion enabling the terahertz (THz) radiation lasing. The intraband radiative processes compete with the interband transitions. We demonstrate that remote doping enhances the indirect interband generation of photons in the proposed GBL heterostructures. Therefore, such remote doping helps to surpass the intraband (Drude) absorption, and results in large absolute values of the negative dynamic THz conductivity in a wide range of frequencies at elevated (including room) temperatures. The remotely doped GBL heterostructure THz lasers are expected to achieve higher THz gain compared with previously proposed GBL-based THz lasers.

  19. Lipid Bilayer Vesicle Dynamics in AC Electric Fields

    NASA Astrophysics Data System (ADS)

    McConnell, Lane; Vlahovska, Petia; Miksis, Michael

    2014-11-01

    Vesicles are closed, fluid-filled lipid bilayers which are mechanically similar to biological cells and which undergo shape transitions in the presence of electric fields. Here we model the vesicle membrane as an infinitely thin, capacitive, area-incompressible interface with the surrounding fluids acting as charge-advecting leaky dielectrics. We then implement the boundary integral method to numerically investigate the dynamics of a vesicle in various AC electric field profiles. Our numerical results are then compared with recent small deformation theory and experimental data. We also note our observation of a new theoretical vesicle behavior that has yet to be observed experimentally.

  20. Electronic Structure and Optical Properties of Twisted Bilayer Black Phosphorus

    NASA Astrophysics Data System (ADS)

    Cao, Ting; Li, Zhenglu; Qiu, Diana Y.; Louie, Steven G.

    Using first-principles calculations, we find that the electronic structure and optical properties of bilayer black phosphorus can be modified significantly through changing the interlayer twist angle. We demonstrate the origin of these twist angle dependent effects, and connect our predicted results to experimental measurements. This work was supported by NSF Grant No. DMR15-1508412, and the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at Lawrence Berkeley National Laboratory's NERSC facility.

  1. Band Structure Asymmetry of Bilayer Graphene Revealed by Infrared Spectroscopy

    SciTech Connect

    Li, Z.Q.; Henriksen, E.A.; Jiang, Z.; Hao, Zhao; Martin, Michael C.; Kim, P.; Stormer, H.L.; Basov, Dimitri N.

    2008-12-10

    We report on infrared spectroscopy of bilayer graphene integrated in gated structures. We observe a significant asymmetry in the optical conductivity upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer and the next-nearest-neighbor interlayer coupling. From the conductivity data, the energy difference of the two sublattices and the interlayer coupling energy are directly determined.

  2. The Coulomb excitations of Bernal bilayer graphene under external fields

    SciTech Connect

    Wu, Jhao-Ying; Lin, Ming-Fa

    2014-03-31

    We study the field effects on the Coulomb excitation spectrum of Bernal bilayer graphene by using the tight-binding model and the random-phase approximation. The electric field opens the band gap and creates the saddle points, the latter brings about a prominent interband plasmon. On the other hand, the magnetic field induces the dispersionless Landau levels (LLs) that causes the inter-LL plasmons. The two kinds of field-induced plasmon modes can be further tuned by the magnitude of momentum transfer and the field strength. The predicted results may be further validated by the inelastic light-scattering or high-resolution electron-energy-loss spectroscopy (HREELLS)

  3. Theory of interlayer tunneling in bilayer quantum Hall ferromagnets.

    PubMed

    Stern, A; Girvin, S M; MacDonald, A H; Ma, N

    2001-02-26

    Spielman et al. [Phys. Rev. Lett. 84, 5808 (2000] recently observed a large and sharp Josephson-like zero-bias peak in the tunnel conductance of a bilayer system in a quantum Hall ferromagnet state. We argue that disorder-induced topological defects in the pseudospin order parameter limit the peak size and destroy the predicted Josephson effect. We predict that the peak would be split and shifted by an in-plane magnetic field in a way that maps the dispersion relation of the ferromagnet's Goldstone mode. We also predict resonant structures in the dc I-V characteristic under bias by an ac electric field.

  4. Superior thermal conductivity in suspended bilayer hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Wang, Chengru; Guo, Jie; Dong, Lan; Aiyiti, Adili; Xu, Xiangfan; Li, Baowen

    2016-05-01

    We reported the basal-plane thermal conductivity in exfoliated bilayer hexagonal boron nitride h-BN that was measured using suspended prepatterned microstructures. The h-BN sample suitable for thermal measurements was fabricated by dry-transfer method, whose sample quality, due to less polymer residues on surfaces, is believed to be superior to that of PMMA-mediated samples. The measured room temperature thermal conductivity is around 484 Wm‑1K‑1(+141 Wm‑1K‑1/ ‑24 Wm‑1K‑1) which exceeds that in bulk h-BN, providing experimental observation of the thickness-dependent thermal conductivity in suspended few-layer h-BN.

  5. Inverse Proximity Effect in Superconductor-ferromagnet Bilayer Structures

    SciTech Connect

    Xia, Jing

    2010-04-05

    Measurements of the polar Kerr effect using a zero-area-loop Sagnac magnetometer on Pb/Ni and Al/(Co-Pd) proximity-effect bilayers show unambiguous evidence for the 'inverse proximity effect,' in which the ferromagnet (F) induces a finite magnetization in the superconducting (S) layer. To avoid probing the magnetic effects in the ferromagnet, the superconducting layer was prepared much thicker than the light's optical penetration depth. The sign and size of the effect, as well as its temperature dependence agree with recent predictions by Bergeret et al.[1].

  6. Influence of defects on the electronic structures of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kishimoto, Ken; Okada, Susumu

    2016-02-01

    Based on first-principles total-energy calculation, we investigate the electronic structures of bilayer graphene, one of which layers possesses atomic or topological defects, to explore the possibility of band gap engineering of graphene by means of physisorption of defective graphene. Our calculations show that the pristine graphene layer possesses a finite energy gap between bonding and antibonding π states because of the potential undulation caused by the other graphene layers with defects. We also found that the gap values strongly depend on the defect species and their mutual arrangement with respect to the pristine layer.

  7. The interaction of bee melittin with lipid bilayer membranes.

    PubMed

    Dawson, C R; Drake, A F; Helliwell, J; Hider, R C

    1978-06-16

    The influence of melittin and the related 8-26 peptide on the stability and electrical properties of bilayer lipid membranes is reported. Melittin, unlike the 8-26 peptide, has a dramatic influence on lipid membranes, causing rupture at dilute concentrations. The circular dichroism of melittin demonstrated that under physiological conditions, in water, melittin is in extended conformation, which is enhanced in aqueous ethanol. However in 'membrane-like' conditions it is essentially alpha-helical. Secondary structure predictions were used to locate possible alpha-helical nucleation centres and a model of melittin was built according to these predictions. It is postulated that melittin causes a wedge effect in membranes.

  8. Lipid bilayer array for simultaneous recording of ion channel activities

    NASA Astrophysics Data System (ADS)

    Hirano-Iwata, Ayumi; Nasu, Tomohiro; Oshima, Azusa; Kimura, Yasuo; Niwano, Michio

    2012-07-01

    This paper describes an array of stable and reduced-solvent bilayer lipid membranes (BLMs) formed in microfabricated silicon chips. BLMs were first vertically formed simultaneously and then turned 90° in order to realize a horizontal BLM array. Since the present BLMs are mechanically stable and robust, the BLMs survive this relatively tough process. Typically, a ˜60% yield in simultaneous BLM formation over 9 sites was obtained. Parallel recordings of gramicidin channel activities from different BLMs were demonstrated. The present system has great potential as a platform of BLM-based high throughput drug screening for ion channel proteins.

  9. Molecular dynamics simulation of oleic acid/oleate bilayers: an atomistic model for a ufasome membrane.

    PubMed

    Han, Sanghwa

    2013-01-01

    When oleic acid and oleate coexist in comparable amounts they form unilamellar vesicles called ufasomes in aqueous phase. Intrinsic pH sensitivity of ufasomes makes it an attractive vehicle for drug delivery. Physical properties of ufasomes have been studied by using spectroscopic techniques but an atomistic model for a ufasome has not been proposed. In this study molecular dynamics simulation was performed on oleic acid/oleate bilayers with the oleate concentration varying from 40 to 70 mol%. All the bilayers reached an equilibrium and stayed stable during a 40 ns simulation. Area per lipid increased with mol% of oleate probably due to charge repulsion between anionic oleate molecules. Oleate was pulled out toward the aqueous phase so that the carboxyl groups of oleic acid and oleate were separated by 0.392 nm in the bilayer of oleic acid/oleate 1:1. Water concentration at the depth of carboxyl group of oleate was five times as high as that of oleic acid. Number of hydrogen bonds between oleic acid and oleate was small in contrast to a proposal that it is an important factor for the bilayer stability. However there was an extensive array of hydrogen bonds between the lipids and water molecules. Acyl chain order was within a normal range for a lipid bilayer but lateral diffusion was an order of magnitude faster in oleic acid/oleate bilayer than in dioleoylphosphatidylcholine bilayer. Cholesterol increased the bilayer thickness and order parameter and decreased the rate of lateral diffusion.

  10. 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

  11. 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.

  12. 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.

  13. 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.

  14. Screening for small molecules' bilayer-modifying potential using a gramicidin-based fluorescence assay.

    PubMed

    Ingólfsson, Helgi I; Andersen, Olaf S

    2010-08-01

    Many drugs and other small molecules used to modulate biological function are amphiphiles that adsorb at the bilayer/solution interface and thereby alter lipid bilayer properties. This is important because membrane proteins are energetically coupled to their host bilayer by hydrophobic interactions. Changes in bilayer properties thus alter membrane protein function, which provides a possible mechanism for "off-target" drug effects. We have previously shown that channels formed by the linear gramicidins are suitable probes for changes in lipid bilayer properties, as experienced by bilayer-spanning proteins. We now report a gramicidin-based fluorescence assay for changes in bilayer properties. The assay is based on measuring the time course of fluorescence quenching in fluorophore-loaded large unilamellar vesicles, due to entry of a gramicidin channel-permeable quencher. The method is scalable and suitable for both mechanistic studies and high-throughput screening for bilayer-perturbing, potential off-target effects, which we illustrate using capsaicin (Cap) and other compounds.

  15. 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. PMID:27267089

  16. Quantum Spin Fluctuations and magnons in antiferromagnetically coupled bilayers with tuneable intra-bilayer exchange - the case of Cr2W(Te)O6

    NASA Astrophysics Data System (ADS)

    Majumdar, Kingshuk; Mahanti, S. D.

    Recent neutron diffraction studies have shown that in Cr2(W,Te)O6 systems, which consist of bilayers with strong antiferromagnetic inter-bilayer coupling between Cr moments, the intra-bilayer coupling between the Cr moments can be tuned from ferro (for W) to antiferro (for Te). Ab initio density functional calculations provide a microscopic understanding of the magnetic structure but cannot explain the magnitude of the ordered Cr3+ moments. In order to understand the reduction of the ordered moment (ROM) caused by quantum spin fluctuations we have studied the magnon dispersion and ROM using a two parameter quantum Heisenberg spin Hamiltonian with tunable intra-(j) and antiferromagnetic inter- (J) bilayer couplings. The magnon dispersion and sublattice magnetization have been calculated using non-linear spin wave theory up to second-order corrections in spin S. We acknowledge the use of HPC cluster at GVSU, supported by the National Science Foundation Grant No. CNS-1228291.

  17. Structure of sphingomyelin bilayers and complexes with cholesterol forming membrane rafts.

    PubMed

    Quinn, Peter J

    2013-07-30

    Sphingomyelin and cholesterol are of interest to biologists because they interact to form condensed structures said to be responsible for a variety of functions that membranes perform. Synchrotron X-ray diffraction methods have been used to investigate the structure of bilayers of D-erythro palmitoyl-sphingomyelin and complexes formed by palmitoyl- and egg-sphingomyelin with cholesterol in aqueous multibilayer dispersions. D-erythro palmitoyl sphingomyelin bilayers exist in two conformers that are distinguished by their lamellar repeat spacing, bilayer thickness, and polar group hydration. The distinction is attributed to hydrogen bonding to water or to intermolecular hydrogen bonds that are disrupted by the formation of ripple structure. The coexisting bilayer structures of pure palmitoyl sphingomyelin are observed in the presence of cholesterol-rich bilayers that are characterized by different bilayer parameters. The presence of cholesterol preferentially affects the conformer of D-erythro sphingomyelin with thicker, more hydrated bilayers. Coexisting bilayers of sphingomyelin and complexes with cholesterol are in register and remain coupled at temperatures at least up to 50 °C. Cholesterol forms a complex of 1.8 mols of sphingomyelin per cholesterol at 37 °C that coexists with bilayers of pure sphingomyelin up to 50 °C. Redistribution of the two lipids takes place on cooling below the fluid- to gel-phase transition temperature, resulting in the withdrawal of sphingomyelin into gel phase and the formation of coexisting bilayers of equimolar proportions of the two lipids. Cholesterol-rich bilayers fit a stripe model at temperatures less than 37 °C characterized by alternating rows of sphingomyelin and cholesterol molecules. A quasicrystalline array models the arrangement at higher temperatures in which each cholesterol molecule is surrounded by seven hydrocarbon chains, each of which is in contact with two cholesterol molecules. The thickness of bilayer

  18. 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.

  19. Filament networks attached to membranes: cytoskeletal pressure and local bilayer deformation

    NASA Astrophysics Data System (ADS)

    Auth, Thorsten; Safran, S. A.; Gov, Nir S.

    2007-11-01

    Several cell types, among them red blood cells, have a cortical, two-dimensional (2D) network of filaments sparsely attached to their lipid bilayer. In many mammalian cells, this 2D polymer network is connected to an underlying 3D, more rigid cytoskeleton. In this paper, we consider the pressure exerted by the thermally fluctuating, cortical network of filaments on the bilayer and predict the bilayer deformations that are induced by this pressure. We treat the filaments as flexible polymers and calculate the pressure that a network of such linear chains exerts on the bilayer; we then minimize the bilayer shape in order to predict the resulting local deformations. We compare our predictions with membrane deformations observed in electron micrographs of red blood cells. The polymer pressure along with the resulting membrane deformation can lead to compartmentalization, regulate in-plane diffusion and may influence protein sorting as well as transmit signals to the polymerization of the underlying 3D cytoskeleton.

  20. Removal of the mechanoprotective influence of the cytoskeleton reveals PIEZO1 is gated by bilayer tension

    PubMed Central

    Cox, Charles D.; Bae, Chilman; Ziegler, Lynn; Hartley, Silas; Nikolova-Krstevski, Vesna; Rohde, Paul R.; Ng, Chai-Ann; Sachs, Frederick; Gottlieb, Philip A.; Martinac, Boris

    2016-01-01

    Mechanosensitive ion channels are force-transducing enzymes that couple mechanical stimuli to ion flux. Understanding the gating mechanism of mechanosensitive channels is challenging because the stimulus seen by the channel reflects forces shared between the membrane, cytoskeleton and extracellular matrix. Here we examine whether the mechanosensitive channel PIEZO1 is activated by force-transmission through the bilayer. To achieve this, we generate HEK293 cell membrane blebs largely free of cytoskeleton. Using the bacterial channel MscL, we calibrate the bilayer tension demonstrating that activation of MscL in blebs is identical to that in reconstituted bilayers. Utilizing a novel PIEZO1–GFP fusion, we then show PIEZO1 is activated by bilayer tension in bleb membranes, gating at lower pressures indicative of removal of the cortical cytoskeleton and the mechanoprotection it provides. Thus, PIEZO1 channels must sense force directly transmitted through the bilayer. PMID:26785635

  1. Quantum anomalous Hall and quantum spin-Hall phases in flattened Bi and Sb bilayers

    PubMed Central

    Jin, Kyung-Hwan; Jhi, Seung-Hoon

    2015-01-01

    Discovery of two-dimensional topological insulator such as Bi bilayer initiates challenges in exploring exotic quantum states in low dimensions. We demonstrate a promising way to realize the Kane-Mele-type quantum spin Hall (QSH) phase and the quantum anomalous Hall (QAH) phase in chemically-modified Bi and Sb bilayers using first-principles calculations. We show that single Bi and Sb bilayers exhibit topological phase transitions from the band-inverted QSH phase or the normal insulator phase to Kane-Mele-type QSH phase upon chemical functionalization. We also predict that the QAH effect can be induced in Bi or Sb bilayers upon nitrogen deposition as checked from calculated Berry curvature and the Chern number. We explicitly demonstrate the spin-chiral edge states to appear in nitrogenated Bi-bilayer nanoribbons. PMID:25672932

  2. Specific heat of twisted bilayer graphene: Engineering phonons by atomic plane rotations

    SciTech Connect

    Nika, Denis L.; Cocemasov, Alexandr I.; Balandin, Alexander A.

    2014-07-21

    We have studied the phonon specific heat in single-layer, bilayer, and twisted bilayer graphene. The calculations were performed using the Born-von Karman model of lattice dynamics for intralayer atomic interactions and spherically symmetric interatomic potential for interlayer interactions. We found that at temperature T < 15 K, specific heat varies with temperature as T{sup n}, where n = 1 for graphene, n = 1.6 for bilayer graphene, and n = 1.3 for the twisted bilayer graphene. The phonon specific heat reveals an intriguing dependence on the twist angle in bilayer graphene, which is particularly pronounced at low temperature. The results suggest a possibility of phonon engineering of thermal properties of layered materials by twisting the atomic planes.

  3. Removal of the mechanoprotective influence of the cytoskeleton reveals PIEZO1 is gated by bilayer tension

    NASA Astrophysics Data System (ADS)

    Cox, Charles D.; Bae, Chilman; Ziegler, Lynn; Hartley, Silas; Nikolova-Krstevski, Vesna; Rohde, Paul R.; Ng, Chai-Ann; Sachs, Frederick; Gottlieb, Philip A.; Martinac, Boris

    2016-01-01

    Mechanosensitive ion channels are force-transducing enzymes that couple mechanical stimuli to ion flux. Understanding the gating mechanism of mechanosensitive channels is challenging because the stimulus seen by the channel reflects forces shared between the membrane, cytoskeleton and extracellular matrix. Here we examine whether the mechanosensitive channel PIEZO1 is activated by force-transmission through the bilayer. To achieve this, we generate HEK293 cell membrane blebs largely free of cytoskeleton. Using the bacterial channel MscL, we calibrate the bilayer tension demonstrating that activation of MscL in blebs is identical to that in reconstituted bilayers. Utilizing a novel PIEZO1-GFP fusion, we then show PIEZO1 is activated by bilayer tension in bleb membranes, gating at lower pressures indicative of removal of the cortical cytoskeleton and the mechanoprotection it provides. Thus, PIEZO1 channels must sense force directly transmitted through the bilayer.

  4. The formation and electronic properties of hydrogenated bilayer silicene from first-principles

    SciTech Connect

    Rui, Wang; Shaofeng, Wang; Xiaozhi, Wu

    2014-07-14

    Density-functional calculations of the formation and electronic properties of bilayer silicene and hydrogenated bilayer silicene are present. The structure optimization and phonon calculations are carried out to demonstrate that bilayer silicene has stable structure with AB stacking and preserves the linear energy dispersions near the K and K′ points in the hexagonal Brillouin zone, and our results agree well with the previous calculations. Based on the symmetry of bilayer silicene, we study the stability of four hydrogenated bilayer silicene by the phonon-mode analysis. The configurations of semihydrogenated one-layer and semihydrogenated double-layer are confirmed as dynamically stable with no imaginary phonon frequencies. The case of semihydrogenated one-layer shows ferromagnetic ground state due to the unpaired 3p electrons, while that of semihydrogenated double-layer is predicted to be a indirect band gap semiconductor with nonmagnetic ground state.

  5. Gramicidin-based fluorescence assay; for determining small molecules potential for modifying lipid bilayer properties.

    PubMed

    Ingólfsson, Helgi I; Sanford, R Lea; Kapoor, Ruchi; Andersen, Olaf S

    2010-10-13

    Many drugs and other small molecules used to modulate biological function are amphiphiles that adsorb at the bilayer/solution interface and thereby alter lipid bilayer properties. This is important because membrane proteins are energetically coupled to their host bilayer by hydrophobic interactions. Changes in bilayer properties thus alter membrane protein function, which provides an indirect way for amphiphiles to modulate protein function and a possible mechanism for "off-target" drug effects. We have previously developed an electrophysiological assay for detecting changes in lipid bilayer properties using linear gramicidin channels as probes. Gramicidin channels are mini-proteins formed by the transbilayer dimerization of two non-conducting subunits. They are sensitive to changes in their membrane environment, which makes them powerful probes for monitoring changes in lipid bilayer properties as sensed by bilayer spanning proteins. We now demonstrate a fluorescence assay for detecting changes in bilayer properties using the same channels as probes. The assay is based on measuring the time-course of fluorescence quenching from fluorophore-loaded large unilamellar vesicles due to the entry of a quencher through the gramicidin channels. We use the fluorescence indicator/quencher pair 8-aminonaphthalene-1,3,6-trisulfonate (ANTS)/Tl(+) that has been successfully used in other fluorescence quenching assays. Tl(+) permeates the lipid bilayer slowly but passes readily through conducting gramicidin channels. The method is scalable and suitable for both mechanistic studies and high-throughput screening of small molecules for bilayer-perturbing, and potential "off-target", effects. We find that results using this method are in good agreement with previous electrophysiological results.

  6. Membrane fusion promoters and inhibitors have contrasting effects on lipid bilayer structure and undulations.

    PubMed

    McIntosh, T J; Kulkarni, K G; Simon, S A

    1999-04-01

    It has been established that the fusion of both biological membranes and phospholipid bilayers can be modulated by altering their lipid composition (Chernomordik et al., 1995 .J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine (MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic acid (OA), and arachidonic acid (AA) promote fusion. This present study uses x-ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic pressure. The addition of 10 and 30 mol% MMPC to egg phosphatidylcholine (EPC) bilayers maintains the bilayer structure, even when the interbilayer fluid spacing is reduced to approximately 3 A, and increases the repulsive pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 A, respectively. Thus MMPC increases the undulation pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. In contrast, the addition of GMO has minor effects on the undulation pressure; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 A, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units approximately 50 A in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as apposing bilayers are brought into contact, which could contribute to their role in promoting membrane fusion.

  7. Periodic barrier structure in AA-stacked bilayer graphene

    NASA Astrophysics Data System (ADS)

    Redouani, Ilham; Jellal, Ahmed

    2016-06-01

    We study the charge carriers transport in an AA-stacked bilayer graphene modulated by a lateral one-dimensional multibarrier structure. We investigate the band structures of our system, that is made up of two shifted Dirac cones, for finite and zero gap. We use the boundary conditions to explicitly determine the transmission probability of each individual cone (τ =+/- 1) for single, double and finite periodic barrier structure. We find that the Klein tunneling is only possible when the band structure is gapless and can occur at normal incidence as a result of the Dirac nature of the quasiparticles. We observe that the band structure of the barriers can have more than one Dirac points for finite periodic barrier. The resonance peaks appear in the transmission probability, which correspond to the positions of new cones index like associated with τ =+/- 1. Two conductance channels through different cones (τ =+/- 1) are found where the total conductance has been studied and compared to the cases of single layer and AB-stacked bilayer graphene.

  8. van Hove Singularity Enhanced Photochemical Reactivity of Twisted Bilayer Graphene.

    PubMed

    Liao, Lei; Wang, Huan; Peng, Han; Yin, Jianbo; Koh, Ai Leen; Chen, Yulin; Xie, Qin; Peng, Hailin; Liu, Zhongfan

    2015-08-12

    Twisted bilayer graphene (tBLG) exhibits van Hove singularities (VHSs) in the density of states that can be tuned by changing the twist angle (θ), sparking various novel physical phenomena. Much effort has been devoted to investigate the θ-dependent physical properties of tBLG. Yet, the chemical properties of tBLG with VHSs, especially the chemical reactivity, remain unexplored. Here we report the first systematic study on the chemistry of tBLG through the photochemical reaction between graphene and benzoyl peroxide. Twisted bilayer graphene exhibits θ-dependent reactivity, and remarkably enhanced reactivity is obtained when the energy of incident laser matches with the energy interval of the VHSs of tBLG. This work provides an insight on the chemistry of tBLG, and the successful enhancement of chemical reactivity derived from VHS is highly beneficial for the controllable chemical modification of tBLG as well as the development of tBLG based devices.

  9. Droplet-interface-bilayer assays in microfluidic passive networks

    PubMed Central

    Schlicht, Bárbara; Zagnoni, Michele

    2015-01-01

    Basic biophysical studies and pharmacological processes can be investigated by mimicking the intracellular and extracellular environments across an artificial cell membrane construct. The ability to reproduce in vitro simplified scenarios found in live cell membranes in an automated manner has great potential for a variety of synthetic biology and compound screening applications. Here, we present a fully integrated microfluidic system for the production of artificial lipid bilayers based on the miniaturisation of droplet-interface-bilayer (DIB) techniques. The platform uses a microfluidic design that enables the controlled positioning and storage of phospholipid-stabilized water-in-oil droplets, leading successfully to the scalable and automated formation of arrays of DIBs to mimic cell membrane processes. To ensure robustness of operation, we have investigated how lipid concentration, immiscible phase flow velocities and the device geometrical parameters affect the system performance. Finally, we produced proof-of-concept data showing that diffusive transport of molecules and ions across on-chip DIBs can be studied and quantified using fluorescence-based assays. PMID:25909686

  10. Enhanced configurational entropy in high-density nanoconfined bilayer ice

    NASA Astrophysics Data System (ADS)

    Corsetti, Fabiano; Zubeltzu, Jon; Artacho, Emilio

    Understanding the structural tendencies of nanoconfined water is of great interest for nanoscience and biology, where nano/micro-sized objects may be separated by very few layers of water. We present a study of water confined to a 2D geometry by a featureless, chemically neutral potential, in order to characterize its intrinsic behaviour. We use molecular dynamics simulations with the TIP4P/2005 potential, combined with density-functional theory calculations with a non-local van der Waals density functional and an ab initio random structure search procedure. We propose a novel kind of crystal order in high-density nanoconfined bilayer ice. A first-order transition is observed between a low-temperature proton-ordered solid and a high-temperature proton-disordered solid. The latter is shown to possess crystalline order for the oxygen positions, arranged on a close-packed triangular lattice with AA stacking. Uniquely amongst the ice phases, the triangular bilayer is characterized by two levels of disorder (for the bonding network and for the protons) which results in a configurational entropy twice that of bulk ice.

  11. Dynamic and mechanical properties of supported lipid bilayers

    NASA Astrophysics Data System (ADS)

    Wu, Hsing-Lun; Tsao, Heng-Kwong; Sheng, Yu-Jane

    2016-04-01

    Supported lipid bilayers (SLBs) offer an excellent model system for investigating the physico-chemical properties of the cell membrane. In this work, dynamic and mechanical properties of SLBs are explored by dissipative particle dynamics simulations for lipids with different architectures (chain length, kink, and asymmetry associated with lipid tails). It is found that the lateral diffusivity (Dx) and flip-flop rate (FF) grow with increasing temperature in both gel and liquid phases and can be described by an Arrhenius-like expression. Three regimes can be clearly identified for symmetric and asymmetric saturated lipids but only two regimes are observed for kinked lipids. Both Dx and FF grow with decreasing tail length and increasing number of kinks. The stretching (KA) and apparent bending (KB) moduli exhibit concave upward curves with temperature and the minima are attained at Tm. In general, the minima of KA and KB decrease with the chain length and increase with number of kinks. The typical relation among the bending modulus, area stretching modulus, and bilayer thickness is still followed, KB = βKAh2 and β is much smaller in the gel phase. The dynamic and mechanical properties of lipids with asymmetric tails are found to situate between their symmetric counterparts.

  12. Melittin-induced cholesterol reorganization in lipid bilayer membranes

    DOE PAGES

    Qian, Shuo; Heller, William T.

    2015-06-12

    The peptide melittin, a 26 amino acid, cationic peptide from honey bee (Apis mellifera) venom, disrupts lipid bilayer membranes in a concentration-dependent manner. Rather than interacting with a specific receptor, the peptide interacts directly with the lipid matrix of the membrane in a manner dependent on the lipid composition. Here, a small-angle neutron scattering study of the interaction of melittin with lipid bilayers made of mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol (Chol) is presented. Through the use of deuterium-labeled DMPC, changes in the distribution of the lipid and cholesterol in unilamellar vesicles were observed for peptide concentrations below those thatmore » cause pores to form. In addition to disrupting the in-plane organization of Chol, melittin produces vesicles having inner and outer leaflet compositions that depend on the lipid–Chol molar ratio and on the peptide concentration. The changes seen at high cholesterol and low peptide concentration are similar to those produced by alamethicin (Qian, S. et al., J. Phys. Chem. B 2014, 118, 11200–11208), which points to an underlying physical mechanism driving the redistribution of Chol, but melittin displays an additional effect not seen with alamethicin. Furthermore, a model for how the peptide drives the redistribution of Chol is proposed. The results suggest that redistribution of the lipids in a target cell membrane by membrane active peptides takes places as a prelude to the lysis of the cell.« less

  13. Melittin-induced cholesterol reorganization in lipid bilayer membranes

    SciTech Connect

    Qian, Shuo; Heller, William T.

    2015-06-12

    The peptide melittin, a 26 amino acid, cationic peptide from honey bee (Apis mellifera) venom, disrupts lipid bilayer membranes in a concentration-dependent manner. Rather than interacting with a specific receptor, the peptide interacts directly with the lipid matrix of the membrane in a manner dependent on the lipid composition. Here, a small-angle neutron scattering study of the interaction of melittin with lipid bilayers made of mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol (Chol) is presented. Through the use of deuterium-labeled DMPC, changes in the distribution of the lipid and cholesterol in unilamellar vesicles were observed for peptide concentrations below those that cause pores to form. In addition to disrupting the in-plane organization of Chol, melittin produces vesicles having inner and outer leaflet compositions that depend on the lipid–Chol molar ratio and on the peptide concentration. The changes seen at high cholesterol and low peptide concentration are similar to those produced by alamethicin (Qian, S. et al., J. Phys. Chem. B 2014, 118, 11200–11208), which points to an underlying physical mechanism driving the redistribution of Chol, but melittin displays an additional effect not seen with alamethicin. Furthermore, a model for how the peptide drives the redistribution of Chol is proposed. The results suggest that redistribution of the lipids in a target cell membrane by membrane active peptides takes places as a prelude to the lysis of the cell.

  14. One-dimensional Topological Edge States of Bismuth Bilayers

    NASA Astrophysics Data System (ADS)

    Drozdov, Ilya; Alexandradinata, Aris; Jeon, Sangjun; Nadj-Perge, Stevan; Ji, Huiwen; Cava, Robert; Bernevig, B. Andrei; Yazdani, Ali

    2014-03-01

    The hallmark of a time-reversal symmetry protected topologically insulating state of matter in two-dimensions (2D) is the existence of chiral edge modes propagating along the perimeter of the sample. Bilayers of bismuth (Bi), an elemental system theoretically predicted to be a Quantum Spin Hall (QSH) insulator1, has been studied with Scanning Tunneling Microscopy (STM) and the electronic structure of its bulk and edge modes has been experimentally investigated. Spectroscopic mapping with STM reveals the presence of the state bound to the edges of the Bi-bilayer. By visualizing quantum interference of the edge state quasi-particles in confined geometries we characterize their dispersion and demonstrate that their properties are consistent with the absence of backscattering. Hybridization of the edge modes to the underlying substrate will be discussed. [1] Shuichi Murakami, Phys. Rev. Lett. 97, 236805 (2006). The work at Princeton and the Princeton Nanoscale Microscopy Laboratory was supported by ARO MURI program W911NF-12-1-0461, DARPA-SPWAR Meso program N6601-11-1-4110, NSF-DMR1104612, and NSF-MRSEC programs through the Princeton Center for Complex Materials (DMR-0819860)

  15. Raman spectroscopy of suspended mono and bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kitt, Alexander; Feldman, Benjamin; Remi, Sebastian; Martin, Jens; Swan, Anna; Yacoby, Amir; Goldberg, Bennett

    2010-03-01

    Suspended mono and bilayer graphene flakes have been shown to have higher mobility and lower disorder than their supported counterparts^1. The geometry which decouples the flake from the substrate also causes an as yet uncharacterized backgate specific strain due to an electrostatic attraction between the graphene and the back gated substrate. We study this strain using spatially resolved Raman spectroscopy with a diffraction limited spot size. Upon application of uni-axial strain the unit cell is stretched reducing the symmetry of the system and breaking the double degeneracy of the G band causing a split in the peak. Additionally the Raman modes show a linear softening as a function of strain in the case of supported graphene. Suspended flakes provide an ideal system to study back gate tunable strain while avoiding complications due to substrates including the determination of the Poisson ratio and sample slippage^2. Here we present preliminary results of our observations. 1: B Feldman, J Martin, A Yacoby, ``Broken-symmetry states and divergent resistance in suspended bilayer graphene'', Nature Physics, doi:10.1038/nphys1406 2: C Metzger et al, ``Biaxial strain in graphene adhered to shallow depressions'', Accepted for publication in Nano Letters

  16. Nonlocal Drag of Magnons in a Ferromagnetic Bilayer.

    PubMed

    Liu, Tianyu; Vignale, G; Flatté, Michael E

    2016-06-10

    Quantized spin waves, or magnons, in a magnetic insulator are assumed to interact weakly with the surroundings, and to flow with little dissipation or drag, producing exceptionally long diffusion lengths and relaxation times. In analogy to Coulomb drag in bilayer two-dimensional electron gases, in which the contribution of the Coulomb interaction to the electric resistivity is studied by measuring the interlayer resistivity (transresistivity), we predict a nonlocal drag of magnons in a ferromagnetic bilayer structure based on semiclassical Boltzmann equations. Nonlocal magnon drag depends on magnetic dipolar interactions between the layers and manifests in the magnon current transresistivity and the magnon thermal transresistivity, whereby a magnon current in one layer induces a chemical potential gradient and/or a temperature gradient in the other layer. The largest drag effect occurs when the magnon current flows parallel to the magnetization; however, for oblique magnon currents a large transverse current of magnons emerges. We examine the effect for practical parameters, and find that the predicted induced temperature gradient is readily observable. PMID:27341254

  17. Surface and interfacial creases in a bilayer tubular soft tissue.

    PubMed

    Razavi, Mir Jalil; Pidaparti, Ramana; Wang, Xianqiao

    2016-08-01

    Surface and interfacial creases induced by biological growth are common types of instability in soft biological tissues. This study focuses on the criteria for the onset of surface and interfacial creases as well as their morphological evolution in a growing bilayer soft tube within a confined environment. Critical growth ratios for triggering surface and interfacial creases are investigated both analytically and numerically. Analytical interpretations provide preliminary insights into critical stretches and growth ratios for the onset of instability and formation of both surface and interfacial creases. However, the analytical approach cannot predict the evolution pattern of the model after instability; therefore nonlinear finite element simulations are carried out to replicate the poststability morphological patterns of the structure. Analytical and computational simulation results demonstrate that the initial geometry, growth ratio, and shear modulus ratio of the layers are the most influential factors to control surface and interfacial crease formation in this soft tubular bilayer. The competition between the stretch ratios in the free and interfacial surfaces is one of the key driving factors to determine the location of the first crease initiation. These findings may provide some fundamental understanding in the growth modeling of tubular biological tissues such as esophagi and airways as well as offering useful clues into normal and pathological functions of these tissues. PMID:27627333

  18. Dynamical conductivity of AA-stacked bilayer graphene

    NASA Astrophysics Data System (ADS)

    Tabert, C. J.; Nicol, E. J.

    2012-08-01

    We calculate the dynamical conductivity of AA-stacked bilayer graphene as a function of frequency and in the presence of a finite chemical potential due to charging. Unlike the monolayer, we find a Drude absorption at charge neutrality in addition to an interband absorption with onset of twice the interlayer hopping energy. At finite doping, the interband absorption exhibits two edges, which depend on both chemical potential and interlayer hopping energy. We study the behavior as a function of varying chemical potential relative to the interlayer hopping energy scale and compute the partial optical sum. The results are contrasted with the previously published case of AB stacking. While we focus on in-plane conductivity, we also provide the perpendicular conductivity for both AB and AA stacking. We also examine conductivity for other variations with AA stacking, such as AAA-stacked trilayer. Based on proposed models for topological insulators discussed in the literature, we also consider the effect of spin-orbit coupling on the optical properties of an AA-stacked bilayer, which illustrates the effect of an energy gap opening at points in the band structure.

  19. Nonlocal Drag of Magnons in a Ferromagnetic Bilayer.

    PubMed

    Liu, Tianyu; Vignale, G; Flatté, Michael E

    2016-06-10

    Quantized spin waves, or magnons, in a magnetic insulator are assumed to interact weakly with the surroundings, and to flow with little dissipation or drag, producing exceptionally long diffusion lengths and relaxation times. In analogy to Coulomb drag in bilayer two-dimensional electron gases, in which the contribution of the Coulomb interaction to the electric resistivity is studied by measuring the interlayer resistivity (transresistivity), we predict a nonlocal drag of magnons in a ferromagnetic bilayer structure based on semiclassical Boltzmann equations. Nonlocal magnon drag depends on magnetic dipolar interactions between the layers and manifests in the magnon current transresistivity and the magnon thermal transresistivity, whereby a magnon current in one layer induces a chemical potential gradient and/or a temperature gradient in the other layer. The largest drag effect occurs when the magnon current flows parallel to the magnetization; however, for oblique magnon currents a large transverse current of magnons emerges. We examine the effect for practical parameters, and find that the predicted induced temperature gradient is readily observable.

  20. Protein-induced bilayer perturbations: Lipid ordering and hydrophobic coupling.

    PubMed

    Petersen, Frederic N R; Laursen, Ib; Bohr, Henrik; Nielsen, Claus Hélix

    2009-10-01

    The host lipid bilayer is increasingly being recognized as an important non-specific regulator of membrane protein function. Despite considerable progress the interplay between hydrophobic coupling and lipid ordering is still elusive. We use electron spin resonance (ESR) to study the interaction between the model protein gramicidin and lipid bilayers of varying thickness. The free energy of the interaction is up to -6kJ/mol; thus not strongly favored over lipid-lipid interactions. Incorporation of gramicidin results in increased order parameters with increased protein concentration and hydrophobic mismatch. Our findings also show that at high protein:lipid ratios the lipids are motionally restricted but not completely immobilized. Both exchange on and off rate values for the lipid<-->gramicidin interaction are lowest at optimal hydrophobic matching. Hydrophobic mismatch of few A results in up to 10-fold increased exchange rates as compared to the 'optimal' match situation pointing to the regulatory role of hydrophobic coupling in lipid-protein interactions.

  1. Permeability of small nonelectrolytes through lipid bilayer membranes

    SciTech Connect

    Walter, A.; Gutknecht, J.

    1986-01-01

    Diffusion of small nonelectrolytes through planar lipid bilayer membranes (egg phosphatidylcholine-decane) was examined by correlating the permeability coefficients of 22 solutes with their partition coefficients between water and four organic solvents. High correlations were observed with hexadecane and olive oil (r = 0.95 and 0.93), but not octanol and ether (r = 0.75 and 0.74). Permeabilities of the seven smallest molecules (mol wt less than 50) (water, hydrofluoric acid, hydrochloric acid, ammonia, methylamine, formic acid and formamide) were 2- to 15-fold higher than the values predicted by the permeabilities of the larger molecules (50 less than mol wt less than 300). The extra permeabilities of the seven smallest molecules were not correlated with partition coefficients but were inversely correlated with molecular volumes. The larger solute permeabilities also decreased with increasing molecular volume, but the relationship was neither steep nor significant. The permeability pattern cannot be explained by the molecular volume dependence of partitioning into the bilayer or by the existence of transient aqueous pores. The molecular volume dependence of solute permeability suggests that the membrane barrier behaves more like a polymer than a liquid hydrocarbon. All the data are consistent with the solubility-diffusion model, which can explain both the hydrophobicity dependence and the molecular volume dependence of nonelectrolyte permeability.

  2. Lipid monolayer states and their relationships to bilayers.

    PubMed Central

    MacDonald, R C; Simon, S A

    1987-01-01

    Uncommon methods of formation and analysis of lipid monolayers have enabled the recognition of several monolayer states and the identification of that in which molecular organization corresponds closely to that of the bilayer. Monolayers were formed by continuously adding a solution of phospholipid [dimyristoyl phosphatidylcholine in hexane/ethanol, 9:1 (vol/vol)] to the air/water interface of a constant-area trough. This procedure generates unconventional surface pressure (pi)-surface concentration (gamma) isotherms, which for liquid-crystalline monolayers consist of straight lines with three prominent intersections, two of which are not apparent in conventional pi-A isotherms. The regions of linear change of pi are explicable in terms of the area dependence of alkyl chain entropy. The two breaks at lower pi delimit states in which both chains lie parallel to the surface. The third occurs at collapse, which corresponds to a true equilibrium for unstressed liposomes. Mechanical and thermodynamic properties of bilayers, particularly phase-transition parameters, correspond closely to those of monolayers with which they are in equilibrium. Images PMID:3473494

  3. Embedding Ba Monolayers and Bilayers in Boron Carbide Nanowires.

    PubMed

    Yu, Zhiyang; Luo, Jian; Shi, Baiou; Zhao, Jiong; Harmer, Martin P; Zhu, Jing

    2015-11-26

    Aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was employed to study the distribution of barium atoms on the surfaces and in the interiors of boron carbide based nanowires. Barium based dopants, which were used to control the crystal growth, adsorbed to the surfaces of the boron-rich crystals in the form of nanometer-thick surficial films (a type of surface complexion). During the crystal growth, these dopant-based surface complexions became embedded inside the single crystalline segments of fivefold boron-rich nanowires collectively, where they were converted to more ordered monolayer and bilayer modified complexions. Another form of bilayer complexion stabilized at stacking faults has also been identified. Numerous previous works suggested that dopants/impurities tended to segregate at the stacking faults or twinned boundaries. In contrast, our study revealed the previously-unrecognized possibility of incorporating dopants and impurities inside an otherwise perfect crystal without the association to any twin boundary or stacking fault. Moreover, we revealed the amount of barium dopants incorporated was non-equilibrium and far beyond the bulk solubility, which might lead to unique properties.

  4. Embedding Ba Monolayers and Bilayers in Boron Carbide Nanowires

    NASA Astrophysics Data System (ADS)

    Yu, Zhiyang; Luo, Jian; Shi, Baiou; Zhao, Jiong; Harmer, Martin P.; Zhu, Jing

    2015-11-01

    Aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was employed to study the distribution of barium atoms on the surfaces and in the interiors of boron carbide based nanowires. Barium based dopants, which were used to control the crystal growth, adsorbed to the surfaces of the boron-rich crystals in the form of nanometer-thick surficial films (a type of surface complexion). During the crystal growth, these dopant-based surface complexions became embedded inside the single crystalline segments of fivefold boron-rich nanowires collectively, where they were converted to more ordered monolayer and bilayer modified complexions. Another form of bilayer complexion stabilized at stacking faults has also been identified. Numerous previous works suggested that dopants/impurities tended to segregate at the stacking faults or twinned boundaries. In contrast, our study revealed the previously-unrecognized possibility of incorporating dopants and impurities inside an otherwise perfect crystal without the association to any twin boundary or stacking fault. Moreover, we revealed the amount of barium dopants incorporated was non-equilibrium and far beyond the bulk solubility, which might lead to unique properties.

  5. Embedding Ba Monolayers and Bilayers in Boron Carbide Nanowires

    PubMed Central

    Yu, Zhiyang; Luo, Jian; Shi, Baiou; Zhao, Jiong; Harmer, Martin P.; Zhu, Jing

    2015-01-01

    Aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was employed to study the distribution of barium atoms on the surfaces and in the interiors of boron carbide based nanowires. Barium based dopants, which were used to control the crystal growth, adsorbed to the surfaces of the boron-rich crystals in the form of nanometer-thick surficial films (a type of surface complexion). During the crystal growth, these dopant-based surface complexions became embedded inside the single crystalline segments of fivefold boron-rich nanowires collectively, where they were converted to more ordered monolayer and bilayer modified complexions. Another form of bilayer complexion stabilized at stacking faults has also been identified. Numerous previous works suggested that dopants/impurities tended to segregate at the stacking faults or twinned boundaries. In contrast, our study revealed the previously-unrecognized possibility of incorporating dopants and impurities inside an otherwise perfect crystal without the association to any twin boundary or stacking fault. Moreover, we revealed the amount of barium dopants incorporated was non-equilibrium and far beyond the bulk solubility, which might lead to unique properties. PMID:26607754

  6. Entrenched metal lift-off using a novel bilayer process

    NASA Astrophysics Data System (ADS)

    Dubois, Thomas D.; Tranjan, Farid M.; Jones, Susan K.; Bobbio, Stephen M.; Kellam, Mark D.; Frieser, Rudolph G.; Jones, A. D.

    1990-01-01

    This paper will discuss the preparation and characterization of a modified photoresist and describe its use in a novel bilayer process. The modified photoresist solutions are prepared by dissolving enough cyclic phosphonitrilic chloride trimer, PNCT, in commercially available photoresist solutions to achieve phosphorus concentrations of 10 to 12 weight percent in the resulting films. FTNMR and FTIR data will be presented which demonstrate that the cyclic phosphonitrilic chloride trimer does not undergo chemical reaction with the components of the photoresist in the photoresist solutions or photoresist films. The exposure threshold of the PNCT modified photoresist films is 1.5 times greater than that of the unmodified films. Experimental data will be discussed which suggests the decrease in exposure threshold is the result of a relatively lower concentration of photoactive compound in the PNCT modified films. The PNCT modified films will be shown to provide resolution comparable to that of the unmodified photoresist films and yield a process window of better than 20%. A mechanism for the formation of the 02/N2 plasma resistant etch barrier formed during 02/N2 plasma etching will be discussed. Finally, a bilayer process which uses the PNCT modified photoresist in generating metal features entrenched in polyimide will be presented.

  7. Nonlocal Drag of Magnons in a Ferromagnetic Bilayer

    NASA Astrophysics Data System (ADS)

    Liu, Tianyu; Vignale, G.; Flatté, Michael E.

    2016-06-01

    Quantized spin waves, or magnons, in a magnetic insulator are assumed to interact weakly with the surroundings, and to flow with little dissipation or drag, producing exceptionally long diffusion lengths and relaxation times. In analogy to Coulomb drag in bilayer two-dimensional electron gases, in which the contribution of the Coulomb interaction to the electric resistivity is studied by measuring the interlayer resistivity (transresistivity), we predict a nonlocal drag of magnons in a ferromagnetic bilayer structure based on semiclassical Boltzmann equations. Nonlocal magnon drag depends on magnetic dipolar interactions between the layers and manifests in the magnon current transresistivity and the magnon thermal transresistivity, whereby a magnon current in one layer induces a chemical potential gradient and/or a temperature gradient in the other layer. The largest drag effect occurs when the magnon current flows parallel to the magnetization; however, for oblique magnon currents a large transverse current of magnons emerges. We examine the effect for practical parameters, and find that the predicted induced temperature gradient is readily observable.

  8. Magnetoelectric interactions in single crystal ferrite-piezoelectric bilayers

    NASA Astrophysics Data System (ADS)

    Srinivasan, G.; Petrov, V. M.; Zhai, J.

    2005-03-01

    The nature of low-frequency (10-2 - 10^4 Hz) magnetoelectric (ME) coupling has been investigated in bilayers of single crystal Ni-Zn ferrites and polycrystalline lead zirconate titanate or single crystal lead magnesium niobate-lead titanate. Important observations are as follows. (i) The ME coupling in the bilayers is found to be stronger than in polycrystalline multilayers [1]. (ii) Zn substitution in ferrite is found to enhance the strength of ME interactions. (iii) ME voltage coefficients show significant variation with the orientation of the bias magnetic field. (iv) Data analysis using our model reveals that superior magneto-mechanical coupling in the ferrites is the cause of strong ME interactions [2]. 1. G. Srinivasan, E. T. Rasmussen, and R. Hayes Phys. Rev. B. 67, 014418 (2003). 2. M. I. Bichurin, V. M. Petrov and G. Srinivasan. Phys. Rev. B. 68, 054402 (2003). - supported by grants from the the National Science Foundation (DMR-0302254), Russian Ministry of Education (Å02-3.4-278), and the Universities of Russia Foundation (UNR 01.01.026).

  9. Dynamic and mechanical properties of supported lipid bilayers.

    PubMed

    Wu, Hsing-Lun; Tsao, Heng-Kwong; Sheng, Yu-Jane

    2016-04-21

    Supported lipid bilayers (SLBs) offer an excellent model system for investigating the physico-chemical properties of the cell membrane. In this work, dynamic and mechanical properties of SLBs are explored by dissipative particle dynamics simulations for lipids with different architectures (chain length, kink, and asymmetry associated with lipid tails). It is found that the lateral diffusivity (Dx) and flip-flop rate (FF) grow with increasing temperature in both gel and liquid phases and can be described by an Arrhenius-like expression. Three regimes can be clearly identified for symmetric and asymmetric saturated lipids but only two regimes are observed for kinked lipids. Both Dx and FF grow with decreasing tail length and increasing number of kinks. The stretching (KA) and apparent bending (KB) moduli exhibit concave upward curves with temperature and the minima are attained at Tm. In general, the minima of KA and KB decrease with the chain length and increase with number of kinks. The typical relation among the bending modulus, area stretching modulus, and bilayer thickness is still followed, KB = βKAh(2) and β is much smaller in the gel phase. The dynamic and mechanical properties of lipids with asymmetric tails are found to situate between their symmetric counterparts. PMID:27389237

  10. Modeling the Elastic Properties of Lipid Bilayer Membranes

    NASA Astrophysics Data System (ADS)

    Barry, Edward; Gibaud, Thomas; Zakhary, Mark; Dogic, Zvonimir

    2011-03-01

    Model membranes such as lipid bilayers have been indispensable tools for our understanding of the elastic properties of biological membranes. In this talk, I will introduce a colloidal model for membranes and demonstrate that the physical properties of these colloidal membranes are identical to lipid bilayers. The model system is unique in that the constituent molecules are homogenous and non-amphiphilic, yet their self-assembly into membranes and other hierarchical assemblages, such as a lamellar type phases and chiral ribbons, proceeds spontaneously in solution. Owing to the large size of the constituent molecules, individual molecules can be directly visualized and simultaneous observations at the continuum and molecular lengthscales are used to characterize the behavior of model membranes with unprecedented detail. Moreover, once assembled in solution, molecular interactions can be controlled in situ. In particular, the strength of chiral interactions can be varied, leading to fascinating transitions in behavior that resembles the formation of starfish vesicles. These observations point towards the important role of line tension, and have potential implications for phase separated lipid mixtures or lipid rafts.

  11. Surface and interfacial creases in a bilayer tubular soft tissue

    NASA Astrophysics Data System (ADS)

    Razavi, Mir Jalil; Pidaparti, Ramana; Wang, Xianqiao

    2016-08-01

    Surface and interfacial creases induced by biological growth are common types of instability in soft biological tissues. This study focuses on the criteria for the onset of surface and interfacial creases as well as their morphological evolution in a growing bilayer soft tube within a confined environment. Critical growth ratios for triggering surface and interfacial creases are investigated both analytically and numerically. Analytical interpretations provide preliminary insights into critical stretches and growth ratios for the onset of instability and formation of both surface and interfacial creases. However, the analytical approach cannot predict the evolution pattern of the model after instability; therefore nonlinear finite element simulations are carried out to replicate the poststability morphological patterns of the structure. Analytical and computational simulation results demonstrate that the initial geometry, growth ratio, and shear modulus ratio of the layers are the most influential factors to control surface and interfacial crease formation in this soft tubular bilayer. The competition between the stretch ratios in the free and interfacial surfaces is one of the key driving factors to determine the location of the first crease initiation. These findings may provide some fundamental understanding in the growth modeling of tubular biological tissues such as esophagi and airways as well as offering useful clues into normal and pathological functions of these tissues.

  12. Nanometer Scale Hard/Soft Bilayer Magnetic Antidots.

    PubMed

    Béron, Fanny; Kaidatzis, Andreas; Velo, Murilo F; Arzuza, Luis C C; Palmero, Ester M; Del Real, Rafael P; Niarchos, Dimitrios; Pirota, Kleber R; García-Martín, José Miguel

    2016-12-01

    The effect of arrays of nanometer scale pores on the magnetic properties of thin films has been analyzed. Particularly, we investigated the influence of the out-of-plane magnetization component created by the nanopores on the in-plane magnetic behavior of patterned hard/soft magnetic thin films in antidot morphology. Its influence on the coupling in Co/Py bilayers of few tens of nanometer thick is compared for disordered and ordered antidots of 35-nm diameter. The combination of magneto-optical Kerr effect (MOKE) and first-order reversal curve (FORC) technique allows probing the effects of the induced perpendicular magnetization component on the bilayer magnetic behavior, while magnetic force microscopy (MFM) is used to image it. We found that ordered antidots yield a stronger out-of-plane component than disordered ones, influencing in a similar manner the hard layer global in-plane magnetic behavior if with a thin or without soft layer. However, its influence changes with a thicker soft layer, which may be an indication of a weaker coupling.

  13. Fibonacci anyons from Abelian bilayer quantum Hall states.

    PubMed

    Vaezi, Abolhassan; Barkeshli, Maissam

    2014-12-01

    The possibility of realizing non-Abelian statistics and utilizing it for topological quantum computation (TQC) has generated widespread interest. However, the non-Abelian statistics that can be realized in most accessible proposals is not powerful enough for universal TQC. In this Letter, we consider a simple bilayer fractional quantum Hall system with the 1/3 Laughlin state in each layer. We show that interlayer tunneling can drive a transition to an exotic non-Abelian state that contains the famous "Fibonacci" anyon, whose non-Abelian statistics is powerful enough for universal TQC. Our analysis rests on startling agreements from a variety of distinct methods, including thin torus limits, effective field theories, and coupled wire constructions. We provide evidence that the transition can be continuous, at which point the charge gap remains open while the neutral gap closes. This raises the question of whether these exotic phases may have already been realized at ν=2/3 in bilayers, as past experiments may not have definitively ruled them out. PMID:25526149

  14. The pathological prion protein forms ionic conductance in lipid bilayer.

    PubMed

    Paulis, Daniele; Maras, Bruno; Schininà, M Eugenia; di Francesco, Laura; Principe, Serena; Galeno, Roberta; Abdel-Haq, Hanin; Cardone, Franco; Florio, Tullio; Pocchiari, Maurizio; Mazzanti, Michele

    2011-08-01

    Transmissible spongiform encephalopathies (TSEs) are neurodegenerative pathologies characterized by the accumulation of amyloid fibrils mainly composed of the pathological isoform of the prion protein (PrP(TSE)). PrP(TSE) pre-amyloid fibrils are supposed to induce neurodegenerative lesions possibly through the alteration of membrane permeability. The effect of PrP(TSE) on cellular membranes has been modeled in vitro by synthetic peptides that are, however, only partially representative of PrP(TSE) isoforms found in vivo. In the present work we show that a synthetic membrane exposed to PrP27-30 extracted from TSE-infected hamster brains changes its permeability because of the formation of molecular pores that alter the conductance of the synthetic lipid bilayer. Synthetic membrane challenged with the recombinant prion peptide PrP90-231 shows a much lower conductance. Elevation of calcium ion concentration not only increases the current amplitude due to the action of both PrP27-30 and PrP90-231 on the membrane, but also amplifies the interaction of PrP90-231 with the lipid bilayer.

  15. Electromagnetic coupling of spins and pseudospins in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Winkler, R.; Zülicke, U.

    2015-03-01

    We present a theoretical study of bilayer-graphene's electronic properties in the presence of electric and magnetic fields. In contrast to known materials, including single-layer graphene, any possible coupling of physical quantities to components of the electric field has a counterpart where the analogous component of the magnetic field couples to exactly the same quantities. For example, a purely electric spin splitting appears as the magneto-electric analogue of the magnetic Zeeman spin splitting. The measurable thermodynamic response induced by magnetic and electric fields is thus completely symmetric. The Pauli magnetization induced by a magnetic field takes exactly the same functional form as the polarization induced by an electric field. Although they seem counterintuitive, our findings are consistent with fundamental principles such as time reversal symmetry. For example, only a magnetic field can give rise to a macroscopic spin polarization, whereas only a perpendicular electric field can induce a macroscopic polarization of the sublattice-related pseudospin in bilayer graphene. These rules enforced by symmetry for the matter-field interactions clarify the nature of spins versus pseudospins. We have obtained numerical values of prefactors for relevant terms. NSF Grant DMR-1310199 and Marsden Fund Contract No. VUW0719.

  16. Assembly of RNA nanostructures on supported lipid bilayers

    NASA Astrophysics Data System (ADS)

    Dabkowska, Aleksandra P.; Michanek, Agnes; Jaeger, Luc; Rabe, Michael; Chworos, Arkadiusz; Höök, Fredrik; Nylander, Tommy; Sparr, Emma

    2014-12-01

    The assembly of nucleic acid nanostructures with controlled size and shape has large impact in the fields of nanotechnology, nanomedicine and synthetic biology. The directed arrangement of nano-structures at interfaces is important for many applications. In spite of this, the use of laterally mobile lipid bilayers to control RNA three-dimensional nanostructure formation on surfaces remains largely unexplored. Here, we direct the self-assembly of RNA building blocks into three-dimensional structures of RNA on fluid lipid bilayers composed of cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or mixtures of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) and cationic sphingosine. We demonstrate the stepwise supramolecular assembly of discrete building blocks through specific and selective RNA-RNA interactions, based on results from quartz crystal microbalance with dissipation (QCM-D), ellipsometry, fluorescence recovery after photobleaching (FRAP) and total internal reflection fluorescence microscopy (TIRF) experiments. The assembly can be controlled to give a densely packed single layer of RNA polyhedrons at the fluid lipid bilayer surface. We show that assembly of the 3D structure can be modulated by sequence specific interactions, surface charge and changes in the salt composition and concentration. In addition, the tertiary structure of the RNA polyhedron can be controllably switched from an extended structure to one that is dense and compact. The versatile approach to building up three-dimensional structures of RNA does not require modification of the surface or the RNA molecules, and can be used as a bottom-up means of nanofabrication of functionalized bio-mimicking surfaces.The assembly of nucleic acid nanostructures with controlled size and shape has large impact in the fields of nanotechnology, nanomedicine and synthetic biology. The directed arrangement of nano-structures at interfaces is important for many applications. In spite of

  17. Studies of exciton condensation and transport in quantum Hall bilayers

    NASA Astrophysics Data System (ADS)

    Finck, Aaron David Kiyoshi

    This thesis is a report of the transport properties of bilayer two-dimensional electron systems found in GaAs/AlGaAs double quantum well semiconductor heterostructures. When a strong perpendicular magnetic field is applied so that the total Landau filling factor is equal to one and if the two layers are close enough together, a novel quantum Hall (QH) state with strong interlayer correlations can form. This QH state is often described as an excitonic condensate, in which electrons in one layer pair with holes in the other. As neutral particles, excitons feel no Lorentz force and are not confined to the edges of the bilayer system like charged quasiparticles are. Instead, excitons are expected to be able to move freely through the bulk and even flow without any dissipation under proper conditions (i.e.,~excitonic superfluidity). Counterflow studies that directly probe the bulk verify this exciton transport in the electrically insulating interior. We also report on studies of the phase boundary between the correlated and uncorrelated phases at total Landau filling factor one as the effective interlayer separation is tuned. When both phases are fully spin polarized at high Zeeman energy, the phase transition is much broader than when the uncorrelated phase is incompletely polarized at low Zeeman energy. This suggests a possible change in the nature of the phase transition in the regime of complete spin polarization.

  18. Structure of the ripple phase in lecithin bilayers.

    PubMed Central

    Sun, W J; Tristram-Nagle, S; Suter, R M; Nagle, J F

    1996-01-01

    The phases of the x-ray form factors are derived for the ripple (Pbeta') thermodynamic phase in the lecithin bilayer system. By combining these phases with experimental intensity data, the electron density map of the ripple phase of dimyristoyl-phosphatidylcholine is constructed. The phases are derived by fitting the intensity data to two-dimensional electron density models, which are created by convolving an asymmetric triangular ripple profile with a transbilayer electron density profile. The robustness of the model method is indicated by the result that many different models of the transbilayer profile yield essentially the same phases, except for the weaker, purely ripple (0,k) peaks. Even with this residual ambiguity, the ripple profile is well determined, resulting in 19 angstroms for the ripple amplitude and 10 degrees and 26 degrees for the slopes of the major and the minor sides, respectively. Estimates for the bilayer head-head spacings show that the major side of the ripple is consistent with gel-like structure, and the minor side appears to be thinner with lower electron density. Images Fig. 1 Fig. 2 PMID:8692934

  19. Surface and interfacial creases in a bilayer tubular soft tissue.

    PubMed

    Razavi, Mir Jalil; Pidaparti, Ramana; Wang, Xianqiao

    2016-08-01

    Surface and interfacial creases induced by biological growth are common types of instability in soft biological tissues. This study focuses on the criteria for the onset of surface and interfacial creases as well as their morphological evolution in a growing bilayer soft tube within a confined environment. Critical growth ratios for triggering surface and interfacial creases are investigated both analytically and numerically. Analytical interpretations provide preliminary insights into critical stretches and growth ratios for the onset of instability and formation of both surface and interfacial creases. However, the analytical approach cannot predict the evolution pattern of the model after instability; therefore nonlinear finite element simulations are carried out to replicate the poststability morphological patterns of the structure. Analytical and computational simulation results demonstrate that the initial geometry, growth ratio, and shear modulus ratio of the layers are the most influential factors to control surface and interfacial crease formation in this soft tubular bilayer. The competition between the stretch ratios in the free and interfacial surfaces is one of the key driving factors to determine the location of the first crease initiation. These findings may provide some fundamental understanding in the growth modeling of tubular biological tissues such as esophagi and airways as well as offering useful clues into normal and pathological functions of these tissues.

  20. Dynamic Screening and Spectral Functions in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Sensarma, Rajdeep; Hwang, Euyheon; Das Sarma, Sankar

    2011-03-01

    We study the dynamic screening of Coulomb interactions in a bilayer graphene system within Random phase approximation. We derive an analytic expression for the dielectric function of the system and study the dispersion and damping of low energy plasmon modes. The quadratic dispersion and chirality of bilayer graphene systems lead to a plasmon dispersion which is distinct both from 2D electron gas and monolayer graphene plasmons. We also look at the effects of dynamic screening on the single particle spectral function of the system. We determine the quasiparticle weight, the effective mass and the damping of quasiparticles, which give a complete description of the low energy spectral function of the system.The compressibility of the system is also obtained from the self-energy renormalization of the chemical potential. We find that the presence of the second band leads to a well screened effective interaction, leading to much smaller renormalization of the Fermi liquid parameters in comparison to a 2D electron gas. However, the dynamic nature of the screening is very important in obtaining the single particle properties of this system. The authors acknowledge support from CNAM, Univ. of Maryland, and US-ONR MURI.

  1. Protein-induced bilayer perturbations: Lipid ordering and hydrophobic coupling

    SciTech Connect

    Petersen, Frederic N.R.; Laursen, Ib; Bohr, Henrik; Nielsen, Claus Helix

    2009-10-02

    The host lipid bilayer is increasingly being recognized as an important non-specific regulator of membrane protein function. Despite considerable progress the interplay between hydrophobic coupling and lipid ordering is still elusive. We use electron spin resonance (ESR) to study the interaction between the model protein gramicidin and lipid bilayers of varying thickness. The free energy of the interaction is up to -6 kJ/mol; thus not strongly favored over lipid-lipid interactions. Incorporation of gramicidin results in increased order parameters with increased protein concentration and hydrophobic mismatch. Our findings also show that at high protein:lipid ratios the lipids are motionally restricted but not completely immobilized. Both exchange on and off rate values for the lipid {r_reversible} gramicidin interaction are lowest at optimal hydrophobic matching. Hydrophobic mismatch of few A results in up to 10-fold increased exchange rates as compared to the 'optimal' match situation pointing to the regulatory role of hydrophobic coupling in lipid-protein interactions.

  2. Nanometer Scale Hard/Soft Bilayer Magnetic Antidots.

    PubMed

    Béron, Fanny; Kaidatzis, Andreas; Velo, Murilo F; Arzuza, Luis C C; Palmero, Ester M; Del Real, Rafael P; Niarchos, Dimitrios; Pirota, Kleber R; García-Martín, José Miguel

    2016-12-01

    The effect of arrays of nanometer scale pores on the magnetic properties of thin films has been analyzed. Particularly, we investigated the influence of the out-of-plane magnetization component created by the nanopores on the in-plane magnetic behavior of patterned hard/soft magnetic thin films in antidot morphology. Its influence on the coupling in Co/Py bilayers of few tens of nanometer thick is compared for disordered and ordered antidots of 35-nm diameter. The combination of magneto-optical Kerr effect (MOKE) and first-order reversal curve (FORC) technique allows probing the effects of the induced perpendicular magnetization component on the bilayer magnetic behavior, while magnetic force microscopy (MFM) is used to image it. We found that ordered antidots yield a stronger out-of-plane component than disordered ones, influencing in a similar manner the hard layer global in-plane magnetic behavior if with a thin or without soft layer. However, its influence changes with a thicker soft layer, which may be an indication of a weaker coupling. PMID:26873261

  3. 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

    2014-09-01

    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

  4. Interaction of β(3) /β(2) -peptides, consisting of Val-Ala-Leu segments, with POPC giant unilamellar vesicles (GUVs) and white blood cancer cells (U937)--a new type of cell-penetrating peptides, and a surprising chain-length dependence of their vesicle- and cell-lysing activity.

    PubMed

    Kolesinska, Beata; Eyer, Klaus; Robinson, Tom; Dittrich, Petra S; Beck, Albert K; Seebach, Dieter; Walde, Peter

    2015-05-01

    Many years ago, β(2) /β(3) -peptides, consisting of alternatively arranged β(2) - and β(3) h-amino-acid residues, have been found to undergo folding to a unique type of helix, the 10/12-helix, and to exhibit non-polar, lipophilic properties (Helv. Chim. Acta 1997, 80, 2033). We have now synthesized such 'mixed' hexa-, nona-, dodeca-, and octadecapeptides, consisting of Val-Ala-Leu triads, with N-terminal fluorescein (FAM) labels, i.e., 1-4, and studied their interactions with POPC (=1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) giant unilamellar vesicles (GUVs) and with human white blood cancer cells U937. The methods used were microfluidic technology, fluorescence correlation spectroscopy (FCS), a flow-cytometry assay, a membrane-toxicity assay with the dehydrogenase G6PDH as enzymatic reporter, and visual microscopy observations. All β(3) /β(2) -peptide derivatives penetrate the GUVs and/or the cells. As shown with the isomeric β(3) /β(2) -, β(3) -, and β(2) -nonamers, 2, 5, and 6, respectively, the derivatives 5 and 6 consisting exclusively of β(3) - or β(2) -amino-acid residues, respectively, interact neither with the vesicles nor with the cells. Depending on the method of investigation and on the pretreatment of the cells, the β(3) /β(2) -nonamer and/or the β(3) /β(2) -dodecamer derivative, 2 and/or 3, respectively, cause a surprising disintegration or lysis of the GUVs and cells, comparable with the action of tensides, viral fusion peptides, and host-defense antimicrobial peptides. Possible sources of the chain-length-dependent destructive potential of the β(3) /β(2) -nona- and β(3) /β(2) -dodecapeptide derivatives, and a possible relationship with the phosphate-to-phosphate and hydrocarbon thicknesses of GUVs, and eukaryotic cells are discussed. Further investigations with other types of GUVs and of eukaryotic or prokaryotic cells will be necessary to elucidate the mechanism(s) of interaction of 'mixed' β(3) /β(2) -peptides with

  5. Interaction of β(3) /β(2) -peptides, consisting of Val-Ala-Leu segments, with POPC giant unilamellar vesicles (GUVs) and white blood cancer cells (U937)--a new type of cell-penetrating peptides, and a surprising chain-length dependence of their vesicle- and cell-lysing activity.

    PubMed

    Kolesinska, Beata; Eyer, Klaus; Robinson, Tom; Dittrich, Petra S; Beck, Albert K; Seebach, Dieter; Walde, Peter

    2015-05-01

    Many years ago, β(2) /β(3) -peptides, consisting of alternatively arranged β(2) - and β(3) h-amino-acid residues, have been found to undergo folding to a unique type of helix, the 10/12-helix, and to exhibit non-polar, lipophilic properties (Helv. Chim. Acta 1997, 80, 2033). We have now synthesized such 'mixed' hexa-, nona-, dodeca-, and octadecapeptides, consisting of Val-Ala-Leu triads, with N-terminal fluorescein (FAM) labels, i.e., 1-4, and studied their interactions with POPC (=1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) giant unilamellar vesicles (GUVs) and with human white blood cancer cells U937. The methods used were microfluidic technology, fluorescence correlation spectroscopy (FCS), a flow-cytometry assay, a membrane-toxicity assay with the dehydrogenase G6PDH as enzymatic reporter, and visual microscopy observations. All β(3) /β(2) -peptide derivatives penetrate the GUVs and/or the cells. As shown with the isomeric β(3) /β(2) -, β(3) -, and β(2) -nonamers, 2, 5, and 6, respectively, the derivatives 5 and 6 consisting exclusively of β(3) - or β(2) -amino-acid residues, respectively, interact neither with the vesicles nor with the cells. Depending on the method of investigation and on the pretreatment of the cells, the β(3) /β(2) -nonamer and/or the β(3) /β(2) -dodecamer derivative, 2 and/or 3, respectively, cause a surprising disintegration or lysis of the GUVs and cells, comparable with the action of tensides, viral fusion peptides, and host-defense antimicrobial peptides. Possible sources of the chain-length-dependent destructive potential of the β(3) /β(2) -nona- and β(3) /β(2) -dodecapeptide derivatives, and a possible relationship with the phosphate-to-phosphate and hydrocarbon thicknesses of GUVs, and eukaryotic cells are discussed. Further investigations with other types of GUVs and of eukaryotic or prokaryotic cells will be necessary to elucidate the mechanism(s) of interaction of 'mixed' β(3) /β(2) -peptides with

  6. Quantum many body physics in single and bilayer graphene

    NASA Astrophysics Data System (ADS)

    Nandkishore, Rahul

    Two dimensional electron systems (2DES) provide a uniquely promising avenue for investigation of many body physics. Graphene constitutes a new and unusual 2DES, which may give rise to unexpected collective phenomena. However, the vanishing density of states in charge neutral single layer graphene suppresses many body effects, and one has to alter the system to observe strongly ordered states. We consider three ways of accessing quantum many body physics using graphene. First, we consider doping single layer graphene to a Van Hove singularity in the density of states. We show that there are strong instabilities to several strongly ordered states, with the leading instability being to a d-wave superconducting state. The superconducting state realizes chiral superconductivity, an exotic form of superconductivity wherein the phase of the order parameter winds by 47r as we go around the Fermi surface. We also discuss the nature of the spin density wave state which is the principal competitor to superconductivity in doped graphene. Next, we study bilayer graphene (BLG), which has a non-vanishing density of states even at charge neutrality. We show that Coulomb interactions give rise to a zero bias anomaly in the tunneling density of states for BLG, which manifests itself at high energy scales. We also show that the quadratic band crossing in BLG is unstable to arbitrarily weak interactions, and estimate the energy scale for formation of strongly ordered states. We show that gapped states in BLG have topological properties, and we classify the various possible gapped and gapless states in terms of symmetries. We study the competition between various ordered states, and discuss how the nature of the ground state may be deduced experimentally. We also discuss recent experimental observations of strongly ordered states in bilayer graphene. Finally, we study bilayer graphene in a transverse magnetic field, focusing on the properties of the quantum Hall ferromagnet (QHF) state

  7. Small angle x-ray scattering studies of magnetically oriented lipid bilayers.

    PubMed Central

    Hare, B J; Prestegard, J H; Engelman, D M

    1995-01-01

    Magnetically oriented lipid/detergent bilayers are potentially useful for studies of membrane-associated molecules and complexes using x-ray scattering and nuclear magnetic resonance (NMR). To establish whether the system is a reasonable model of a phospholipid bilayer, we have studied the system using x-ray solution scattering to determine the bilayer thickness, interparticle spacing, and orientational parameters for magnetically oriented lipid bilayers. The magnetically orientable samples contain the phospholipid L-alpha-dilauroylphosphatidylcholine (DLPC) and the bile salt analog 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO) in a 3:1 molar ratio in 70% water (w/v) and are similar to magnetically orientable samples used as NMR media for structural studies of membrane-associated molecules. A bilayer thickness of 30 A was determined for the DLPC/CHAPSO particles, which is the same as the bilayer thickness of pure DLPC vesicles, suggesting that the CHAPSO is not greatly perturbing the lipid bilayer. These data, as well as NMR data on molecules incorporated in the oriented lipid particles, are consistent with the sample consisting of reasonably homogeneous and well dispersed lipid particles. Finally, the orientational energy of the sample suggests that the size of the cooperatively orienting unit in the samples is 2 x 10(7) phospholipid molecules. Images FIGURE 1 PMID:8580332

  8. Effect of monoglyceride structure and cholesterol content on water permeability of the droplet bilayer.

    PubMed

    Michalak, Zuzanna; Muzzio, Michelle; Milianta, Peter J; Giacomini, Rosario; Lee, Sunghee

    2013-12-23

    The process of water permeation across lipid membranes has significant implications for cellular physiology and homeostasis, and its study may lead to a greater understanding of the relationship between the structure of lipid bilayer and the role that lipid structure plays in water permeation. In this study, we formed a droplet interface bilayer (DIB) by contacting two aqueous droplets together in an immiscible solvent (squalane) containing bilayer-forming surfactant (monoglycerides). Using the DIB model, we present our results on osmotic water permeabilities and activation energy for water permeation of an associated series of unsaturated monoglycerides as the principal component of droplet bilayers, each having the same chain length but differing in the position and number of double bonds, in the absence and presence of a varying concentration of cholesterol. Our findings suggest that the tailgroup structure in a series of monoglyceride bilayers is seen to affect the permeability and activation energy for the water permeation process. Moreover, we have also established the insertion of cholesterol into the droplet bilayer, and have detected its presence via its effect on water permeability. The effect of cholesterol differs depending on the type of monoglyceride. We demonstrate that the DIB can be employed as a convenient model membrane to rapidly explore subtle structural effects on bilayer water permeability.

  9. Revealing the preferred interlayer orientations and stackings of two-dimensional bilayer gallium selenide crystals

    SciTech Connect

    Li, Xufan; Basile Carrasco, Leonardo A.; Yoon, Mina; Ma, Cheng; Puretzky, Alexander A.; Lee, Jaekwang; Idrobo Tapia, Juan Carlos; Chi, Miaofang; Rouleau, Christopher M.; Geohegan, David B.; Xiao, Kai

    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 and 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.

  10. Role of substrate induced electron-phonon interactions in biased graphitic bilayers

    NASA Astrophysics Data System (ADS)

    Davenport, A. R.; Hague, J. P.

    2016-08-01

    Bilayers of graphitic materials have potential applications in field effect transistors (FETs). A potential difference applied between certain ionic bilayers made from insulating graphitic materials such as BN, ZnO and AlN could reduce gap sizes, turning them into useful semiconductors. On the other hand, opening of a small semiconducting gap occurs in graphene bilayers under applied field. The aim here is to investigate to what extent substrate induced electron-phonon interactions (EPIs) modify this gap change. We examine EPIs in several lattice configurations of graphitic bilayers, using a perturbative approach. The typical effect of EPIs on the ionic bilayers is an undesirable gap widening. The size of this gap change varies considerably with lattice structure and the magnitude of the bias. When bias is larger than the non-interacting gap size, EPIs have the smallest effect on the bandgap, especially in configurations with A{{A}\\prime} and AB structures. Thus careful selection of substrate, lattice configuration and bias strength to minimise the effects of EPIs could be important for optimising the properties of electronic devices. We use parameters related to BN in this article. In practice, the results presented here are broadly applicable to other graphitic bilayers, and are likely to be qualitatively similar in metal dichalcogenide bilayers such as MoS2, which are already of high interest for their use in FETs.

  11. Doping, strain engineering, and interlayer interaction in bilayer hexagonal boron nitride sheets

    NASA Astrophysics Data System (ADS)

    Saito, Susumu; Fujimoto, Yoshitaka

    We study electronic properties of bilayer hexagonal boron nitride (h-BN) sheets with different stacking sequences in the framework of the density-functional theory. The bulk h-BN material usually takes the so-called AA (or AA') stacking, corresponding to the ''non-polar'' bilayer h-BN sheet. On the other hand, the rhombohedral BN takes the ABC stacking, and the corresponding bilayer sheet has ''upper'' and ''lower'' layers which are not equivalent with each other. Interestingly, the energetics of stacking sequences for bilayer h-BN sheets is found to be different from that for bulk h-BN materials. We report that strain engineering for bilayer h-BN sheets can possess much wider possibilities than that for monolayer h-BN due to the modification of the interlayer interaction. We also study the substitutional C doping into bilayer h-BN sheets, and report the energetics and the strain effect for these C-doped sheets. Finally we discuss the similarities and differences between bilayer h-BN sheets and double-wall h-BN nanotubes. This work was partly supported by the MEXT Elements Strategy Initiative to Form Core Research Center, Grant in Aid for Scientific Research, MEXT Japan, ``Science of Atomic Layers'', and JSPS KAKENHI Grant No. 26390062.

  12. Multi-Stacked Supported Lipid Bilayer Micropatterning through Polymer Stencil Lift-Off.

    PubMed

    Zhu, Yujie; Negmi, Ahmed; Moran-Mirabal, Jose

    2015-01-01

    Complex multi-lamellar structures play a critical role in biological systems, where they are present as lamellar bodies, and as part of biological assemblies that control energy transduction processes. Multi-lamellar lipid layers not only provide interesting systems for fundamental research on membrane structure and bilayer-associated polypeptides, but can also serve as components in bioinspired materials or devices. Although the ability to pattern stacked lipid bilayers at the micron scale is of importance for these purposes, limited work has been done in developing such patterning techniques. Here, we present a simple and direct approach to pattern stacked supported lipid bilayers (SLBs) using polymer stencil lift-off and the electrostatic interactions between cationic and anionic lipids. Both homogeneous and phase-segregated stacked SLB patterns were produced, demonstrating that the stacked lipid bilayers retain lateral diffusivity. We demonstrate patterned SLB stacks of up to four bilayers, where fluorescence resonance energy transfer (FRET) and quenching was used to probe the interactions between lipid bilayers. Furthermore, the study of lipid phase behaviour showed that gel phase domains align between adjacent layers. The proposed stacked SLB pattern platform provides a robust model for studying lipid behaviour with a controlled number of bilayers, and an attractive means towards building functional bioinspired materials or devices. PMID:26343733

  13. Multi-Stacked Supported Lipid Bilayer Micropatterning through Polymer Stencil Lift-Off

    PubMed Central

    Zhu, Yujie; Negmi, Ahmed; Moran-Mirabal, Jose

    2015-01-01

    Complex multi-lamellar structures play a critical role in biological systems, where they are present as lamellar bodies, and as part of biological assemblies that control energy transduction processes. Multi-lamellar lipid layers not only provide interesting systems for fundamental research on membrane structure and bilayer-associated polypeptides, but can also serve as components in bioinspired materials or devices. Although the ability to pattern stacked lipid bilayers at the micron scale is of importance for these purposes, limited work has been done in developing such patterning techniques. Here, we present a simple and direct approach to pattern stacked supported lipid bilayers (SLBs) using polymer stencil lift-off and the electrostatic interactions between cationic and anionic lipids. Both homogeneous and phase-segregated stacked SLB patterns were produced, demonstrating that the stacked lipid bilayers retain lateral diffusivity. We demonstrate patterned SLB stacks of up to four bilayers, where fluorescence resonance energy transfer (FRET) and quenching was used to probe the interactions between lipid bilayers. Furthermore, the study of lipid phase behaviour showed that gel phase domains align between adjacent layers. The proposed stacked SLB pattern platform provides a robust model for studying lipid behaviour with a controlled number of bilayers, and an attractive means towards building functional bioinspired materials or devices. PMID:26343733

  14. The effect of temperature on supported dipalmitoylphosphatidylcholine (DPPC) bilayers: structure and lubrication performance.

    PubMed

    Wang, Min; Zander, Thomas; Liu, Xiaoyan; Liu, Chao; Raj, Akanksha; Wieland, D C Florian; Garamus, Vasil M; Willumeit-Römer, Regine; Claesson, Per Martin; Dėdinaitė, Andra

    2015-05-01

    Phospholipids fulfill an important role in joint lubrication. They, together with hyaluronan and glycoproteins, are the biolubricants that sustain low friction between cartilage surfaces bathed in synovial fluid. In this work we have investigated how the friction force and load bearing capacity of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers on silica surfaces are affected by temperature, covering the temperature range 25-52°C. Friction forces have been determined utilizing the AFM colloidal probe technique, which showed that DPPC bilayers are able to provide low friction forces over the whole temperature interval. However, the load bearing capacity is improved at higher temperatures. We interpret this finding as being a consequence of lower rigidity and higher self-healing capacity of the DPPC bilayer in the liquid disordered state compared to the gel state. The corresponding structure of solid supported DPPC bilayers at the silica-liquid interface has been followed using X-ray reflectivity measurements, which suggests that the DPPC bilayer is in the gel phase at 25°C and 39°C and in the liquid disordered state at 55°C. Well-defined bilayer structures were observed for both phases. The deposited DPPC bilayers were also imaged using AFM PeakForce Tapping mode, and these measurements indicated a less homogeneous layer at temperatures below 37°C.

  15. Chain elongation of diacylphosphatidylcholine induces fully bilayer interdigitation under atmospheric pressure.

    PubMed

    Goto, Masaki; Wilk, Agnieszka; Kazama, Akira; Chodankar, Shirish; Kohlbrecher, Joachim; Matsuki, Hitoshi

    2011-05-01

    The phase transitions of dibehenoylphosphatidylcholine (C22PC) bilayer membrane were observed by differential scanning calorimetry under atmospheric pressure and light-transmittance measurements under high pressure. The constructed temperature-pressure phase diagram suggests that the gel phase at low temperatures is the interdigitated gel phase. To confirm the phase state, we performed small-angle neutron scattering and fluorescence measurements using a polarity-sensitive probe Prodan for the C22PC bilayer membrane under atmospheric pressure. The peaks obtained in both measurements clearly showed the characteristic patterns of the fully interdigitated gel phase. Taking into account of previous studies on the gel phase for long-chain PC bilayers under atmospheric pressure and our studies on the pressure-induced bilayer interdigitaion of diacyl-PCs, it turned out that the interdigitation of diacyl-PC bilayer membranes occurs when the carbon number of acyl chain reaches at least 22. The present study revealed that the interdigitation of PC bilayer membranes occurs not only by weakening the attractive force of polar head groups but also by strengthening the cohesive force of acyl chains. When dominating the force of acyl chains, the interdigitation can be induced even in a diacyl-PC bilayer membrane by only hydration under atmospheric pressure.

  16. Coexistence of a Two-States Organization for a Cell-Penetrating Peptide in Lipid Bilayer

    PubMed Central

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

    2005-01-01

    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(α), a primary amphipathic cell-penetrating peptide which remains α-helical whatever the environment, with dipalmitoylphosphatidylcholine (DPPC) bilayers. Addition of P(α) 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(α) 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(α). The simultaneous two-states organization of P(α), at the membrane surface and deeply embedded in the bilayer, may be involved in the transmembrane carrier function of this primary amphipathic peptide. PMID:16199494

  17. Revealing the preferred interlayer orientations and stackings of two-dimensional bilayer gallium selenide crystals

    DOE PAGES

    Li, Xufan; Basile Carrasco, Leonardo A.; Yoon, Mina; Ma, Cheng; Puretzky, Alexander A.; Lee, Jaekwang; Idrobo Tapia, Juan Carlos; Chi, Miaofang; Rouleau, Christopher M.; Geohegan, David B.; et al

    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

  18. Role of substrate induced electron–phonon interactions in biased graphitic bilayers

    NASA Astrophysics Data System (ADS)

    Davenport, A. R.; Hague, J. P.

    2016-08-01

    Bilayers of graphitic materials have potential applications in field effect transistors (FETs). A potential difference applied between certain ionic bilayers made from insulating graphitic materials such as BN, ZnO and AlN could reduce gap sizes, turning them into useful semiconductors. On the other hand, opening of a small semiconducting gap occurs in graphene bilayers under applied field. The aim here is to investigate to what extent substrate induced electron–phonon interactions (EPIs) modify this gap change. We examine EPIs in several lattice configurations of graphitic bilayers, using a perturbative approach. The typical effect of EPIs on the ionic bilayers is an undesirable gap widening. The size of this gap change varies considerably with lattice structure and the magnitude of the bias. When bias is larger than the non-interacting gap size, EPIs have the smallest effect on the bandgap, especially in configurations with A{{A}\\prime} and AB structures. Thus careful selection of substrate, lattice configuration and bias strength to minimise the effects of EPIs could be important for optimising the properties of electronic devices. We use parameters related to BN in this article. In practice, the results presented here are broadly applicable to other graphitic bilayers, and are likely to be qualitatively similar in metal dichalcogenide bilayers such as MoS2, which are already of high interest for their use in FETs.

  19. Assessing smectic liquid-crystal continuum models for elastic bilayer deformations

    PubMed Central

    Lee, Kyu; Pastor, Richard W.; Andersen, Olaf S.; Im, Wonpil

    2013-01-01

    For four decades, since W. Helfrich’s pioneering study of smectic A liquid crystals in 1973, continuum elastic models (CEMs) have been employed as tools to understand the energetics of protein-induced lipid bilayer deformations. Among the assumptions underlying this use is that all relevant protein–lipid interactions can be included in the continuum representation of the protein–bilayer interactions through the physical parameters determined for protein-free bilayers and the choice of boundary conditions at the protein/bilayer interface. To better understand this assumption, we review the general structure of CEMs, examine how different choices of boundary conditions and physical moduli profiles alter the predicted bilayer thickness profiles around gramicidin A (gA) and mitochondrial voltage-dependent anion channels (VDAC), respectively, and compare these profiles with those obtained from all-atom molecular dynamics simulations. We find that the profiles differ qualitatively in the first lipid shell around the channels, indicating that the CEMs do not capture accurately the consequences of the protein-induced local changes in lipid bilayer dynamics. Therefore, one needs to be careful when interpreting the results of CEM-based analyses of lipid bilayer-membrane protein interactions. PMID:23348553

  20. Stabilising lamellar stacks of lipid bilayers with soft confinement and steric effects.

    PubMed

    Bougis, K; Leite Rubim, R; Ziane, N; Peyencet, J; Bentaleb, A; Février, A; Oliveira, C L P; Andreoli de Oliveira, E; Navailles, L; Nallet, F

    2015-07-01

    Structure and interactions stabilising the lamellar stack of mixed lipid bilayers in their fluid state are investigated by means of small-angle X-ray scattering. The (electrically neutral) bilayers are composed of a mixtures of lecithin, a zwitterionic phospholipid, and Simulsol, a non-ionic cosurfactant with an ethoxylated polar head. The soft confinement of the bilayer hydrophilic components is varied by changing hydration and bilayer composition, as well as the length of the cosurfactant polar head. Structural transitions are observed at low hydration, in the stacking order for the longer cosurfactant, and in the mixed bilayers for the shorter one. At higher hydration, the swelling of the lamellar stacks occurs with a significant, but continuous evolution in the mixed bilayer structure. The bilayer structural changes are discussed in analogy with the so-called "brush-to-mushroom" transition induced by lateral confinement, relevant for long linear polymers grafted onto rigid surfaces, taking also into account the role of vertical confinement.