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1

Asymmetric transformations via C-C bond cleavage.  

PubMed

Catalytic asymmetric transformations operating by carbon-carbon (C-C) bonds cleavages have emerged as intriguing strategies to access transient organometallic species from different reaction pathways. The reactions and the applicable substrate range have expanded considerably over the last decade. This overview covers the main developments in this field. A major focus is placed on ?-carbon eliminations of strained tert-alcohols and related processes which have been shown to be particularly versatile in a broad range of transformations. Furthermore, exciting developments of asymmetric processes based on direct oxidative C-C bond insertion reactions, for instance into the acyl C-C bond of ketones or the C-CN bond of nitriles, are discussed. PMID:24531752

Souillart, Laetitia; Parker, Evelyne; Cramer, Nicolai

2014-01-01

2

Novel C?-C? bond cleavages of tryptophan-containing peptide radical cations.  

PubMed

In this study, we observed unprecedented cleavages of the C(?)-C(?) bonds of tryptophan residue side chains in a series of hydrogen-deficient tryptophan-containing peptide radical cations (M(•+)) during low-energy collision-induced dissociation (CID). We used CID experiments and theoretical density functional theory (DFT) calculations to study the mechanism of this bond cleavage, which forms [M - 116](+) ions. The formation of an ?-carbon radical intermediate at the tryptophan residue for the subsequent C(?)-C(?) bond cleavage is analogous to that occurring at leucine residues, producing the same product ions; this hypothesis was supported by the identical product ion spectra of [LGGGH - 43](+) and [WGGGH - 116](+), obtained from the CID of [LGGGH](•+) and [WGGGH](•+), respectively. Elimination of the neutral 116-Da radical requires inevitable dehydrogenation of the indole nitrogen atom, leaving the radical centered formally on the indole nitrogen atom ([Ind](•)-2), in agreement with the CID data for [WGGGH](•+) and [W(1-CH3)GGGH](•+); replacing the tryptophan residue with a 1-methyltryptophan residue results in a change of the base peak from that arising from a neutral radical loss (116 Da) to that arising from a molecule loss (131 Da), both originating from C(?)-C(?) bond cleavage. Hydrogen atom transfer or proton transfer to the ?-carbon atom of the tryptophan residue weakens the C(?)-C(?) bond and, therefore, decreases the dissociation energy barrier dramatically. PMID:22135037

Song, Tao; Hao, Qiang; Law, Chun-Hin; Siu, Chi-Kit; Chu, Ivan K

2012-02-01

3

NBS mediated nitriles synthesis through C=C double bond cleavage.  

PubMed

An NBS mediated nitriles synthesis through C=C double bond cleavage has been developed. TMSN3 was employed as the nitrogen source for this Cu(OAc)2 promoted nitrogenation reaction. This transformation has a relatively high regio-selectivity to form aromatic nitriles. PMID:24425249

Zong, Xiaolin; Zheng, Qing-Zhong; Jiao, Ning

2014-02-28

4

C-C or C-O bond cleavage in a phenolic lignin model compound: selectivity depends on vanadium catalyst.  

PubMed

The aerobic oxidation of a phenolic lignin model compound with a vanadium catalyst results in the oxidative cleavage of the C-C bond between the aryl ring and the adjacent hydroxy-substituted carbon atom. Labeling experiments indicate key mechanistic differences to a previously reported related C-O bond cleavage reaction. The selectivity in C-C versus C-O bond cleavage depends on the choice of the vanadium catalyst. PMID:22266711

Hanson, Susan K; Wu, Ruilian; Silks, Louis A Pete

2012-04-01

5

Extended Reaction Scope of Thiamine Diphosphate Dependent Cyclohexane-1,2-dione Hydrolase: From C?C Bond Cleavage to C?C Bond Ligation.  

PubMed

ThDP-dependent cyclohexane-1,2-dione hydrolase (CDH) catalyzes the C?C bond cleavage of cyclohexane-1,2-dione to 6-oxohexanoate, and the asymmetric benzoin condensation between benzaldehyde and pyruvate. One of the two reactivities of CDH was selectively knocked down by mutation experiments. CDH-H28A is much less able to catalyze the C?C bond formation, while the ability for C?C bond cleavage is still intact. The double variant CDH-H28A/N484A shows the opposite behavior and catalyzes the addition of pyruvate to cyclohexane-1,2-dione, resulting in the formation of a tertiary alcohol. Several acyloins of tertiary alcohols are formed with 54-94?% enantiomeric excess. In addition to pyruvate, methyl pyruvate and butane-2,3-dione are alternative donor substrates for C?C bond formation. Thus, the very rare aldehyde-ketone cross-benzoin reaction has been solved by design of an enzyme variant. PMID:25382418

Loschonsky, Sabrina; Wacker, Tobias; Waltzer, Simon; Giovannini, Pier Paolo; McLeish, Michael J; Andrade, Susana L A; Müller, Michael

2014-12-22

6

Organic chemistry. Rh-catalyzed C-C bond cleavage by transfer hydroformylation.  

PubMed

The dehydroformylation of aldehydes to generate olefins occurs during the biosynthesis of various sterols, including cholesterol in humans. Here, we implement a synthetic version that features the transfer of a formyl group and hydride from an aldehyde substrate to a strained olefin acceptor. A Rhodium (Xantphos)(benzoate) catalyst activates aldehyde carbon-hydrogen (C-H) bonds with high chemoselectivity to trigger carbon-carbon (C-C) bond cleavage and generate olefins at low loadings (0.3 to 2 mole percent) and temperatures (22° to 80°C). This mild protocol can be applied to various natural products and was used to achieve a three-step synthesis of (+)-yohimbenone. A study of the mechanism reveals that the benzoate counterion acts as a proton shuttle to enable transfer hydroformylation. PMID:25554782

Murphy, Stephen K; Park, Jung-Woo; Cruz, Faben A; Dong, Vy M

2015-01-01

7

Structure sensitivity of hydrogenolytic cleavage of endocyclic and exocyclic C-C bonds in methylcyclohexane over supported iridium particles  

SciTech Connect

Structure sensitivities, H2 pressure effects and temperature dependencies for rates and selectivities of endo- and exocyclic C–C bond cleavage in methylcyclohexane were studied over supported Ir catalysts. The rate of endocyclic C–C bond cleavage first decreased and then increased with declining Ir dispersion from 0.65 to 0.035. The ring opening (RO) product distribution remained unchanged with varying H2 pressure on small Ir particles, while further shifting to methylhexanes with increasing H2 pressure on large particles. In contrast, the rate and selectivity of exocyclic C–C bond cleavage decreased monotonically with increasing H2 pressure and decreasing Ir particle size. The distinct dependencies of endocyclic and exocyclic C–C bond cleavage pathways on Ir dispersion and H2 pressure suggest that they are mediated by surface species with different ensemble size requirements. DFT calculations were performed on an Ir50 cluster and an Ir(111) surface, with or without pre-adsorbed hydrogen atoms, to provide insight into the observed effects of particle size and H2 pressure on RO pathways. On small Ir particles, the calculated dehydrogenation enthalpies for all endocyclic bonds were similar and affected to similar extents by H2 pressure; on large particles, the selectivity to n-heptane (via substituted C-C bond cleavage) was even lower than on small particles as a result of the least favorable adsorption and dehydrogenation energetics for hindered bonds. This work was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences under Contract DE-AC05-76RL01830. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. The computing time is provided by the user project from EMSL, a national scientific user facility sponsored by the US Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

Shi, Hui; Gutierrez, Oliver Y.; Haller, Gary L.; Mei, Donghai; Rousseau, Roger J.; Lercher, Johannes A.

2013-01-02

8

Mechanistic examination of C?-C? bond cleavages of tryptophan residues during dissociations of molecular peptide radical cations.  

PubMed

In this study, we used collision-induced dissociation (CID) to examine the gas-phase fragmentations of [G(n)W](•+) (n = 2-4) and [GXW](•+) (X = C, S, L, F, Y, Q) species. The C(?)-C(?) bond cleavage of a C-terminal decarboxylated tryptophan residue ([M - CO(2)](•+)) can generate [M - CO(2) - 116](+), [M - CO(2) - 117](•+), and [1H-indole](•+) (m/z 117) species as possible product ions. Competition between the formation of [M - CO(2) - 116](+) and [1H-indole](•+) systems implies the existence of a proton-bound dimer formed between the indole ring and peptide backbone. Formation of such a proton-bound dimer is facile via a protonation of the tryptophan ?-carbon atom as suggested by density functional theory (DFT) calculations. DFT calculations also suggested the initially formed ion 2, the decarboxylated species that is active against C(?)-C(?) bond cleavage, can efficiently isomerize to form a more stable ?-radical isomer (ion 9) as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. The C(?)-C(?) bond cleavage of a tryptophan residue also can occur directly from peptide radical cations containing a basic residue. CID of [WG(n)R](•+) (n = 1-3) radical cations consistently resulted in predominant formation of [M - 116](+) product ions. It appears that the basic arginine residue tightly sequesters the proton and allows the charge-remote C(?)-C(?) bond cleavage to prevail over the charge-directed one. DFT calculations predicted that the barrier for the former is 6.2 kcal mol(-1) lower than that of the latter. Furthermore, the pathway involving a salt-bridge intermediate also was accessible during such a bond cleavage event. PMID:22697598

Song, Tao; Ma, Ching-Yung; Chu, Ivan K; Siu, Chi-Kit; Laskin, Julia

2013-02-14

9

Crystallographic snapshots of tyrosine phenol-lyase show that substrate strain plays a role in C-C bond cleavage.  

PubMed

The key step in the enzymatic reaction catalyzed by tyrosine phenol-lyase (TPL) is reversible cleavage of the C?-C? bond of L-tyrosine. Here, we present X-ray structures for two enzymatic states that form just before and after the cleavage of the carbon-carbon bond. As for most other pyridoxal 5'-phosphate-dependent enzymes, the first state, a quinonoid intermediate, is central for the catalysis. We captured this relatively unstable intermediate in the crystalline state by introducing substitutions Y71F or F448H in Citrobacter freundii TPL and briefly soaking crystals of the mutant enzymes with a substrate 3-fluoro-L-tyrosine followed by flash-cooling. The X-ray structures, determined at ~2.0 Å resolution, reveal two quinonoid geometries: "relaxed" in the open and "tense" in the closed state of the active site. The "tense" state is characterized by changes in enzyme contacts made with the substrate's phenolic moiety, which result in significantly strained conformation at C? and C? positions. We also captured, at 2.25 Å resolution, the X-ray structure for the state just after the substrate's C?-C? bond cleavage by preparing the ternary complex between TPL, alanine quinonoid and pyridine N-oxide, which mimics the ?-aminoacrylate intermediate with bound phenol. In this state, the enzyme-ligand contacts remain almost exactly the same as in the "tense" quinonoid, indicating that the strain induced by the closure of the active site facilitates elimination of phenol. Taken together, structural observations demonstrate that the enzyme serves not only to stabilize the transition state but also to destabilize the ground state. PMID:21899319

Mili?, Dalibor; Demidkina, Tatyana V; Faleev, Nicolai G; Phillips, Robert S; Matkovi?-?alogovi?, Dubravka; Antson, Alfred A

2011-10-19

10

Metal-catalyzed C-C bond cleavage in alkanes: effects of methyl substitution on transition-state structures and stability.  

PubMed

Methyl substituents at C-C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C2-C8 alkanes on Ir, Rh, Ru, and Pt catalysts. C-C cleavage transition states form via equilibrated dehydrogenation steps that replace several C-H bonds with C-metal bonds, desorb H atoms (H*) from saturated surfaces, and form ? H2(g) molecules. Activation enthalpies (?H(‡)) and entropies (?S(‡)) and ? values for (3)C-(x)C cleavage are larger than for (2)C-(2)C or (2)C-(1)C bonds, irrespective of the composition of metal clusters or the cyclic/acyclic structure of the reactants. (3)C-(x)C bonds cleave through ?,?,?- or ?,?,?,?-bound transition states, as indicated by the agreement between measured activation entropies and those estimated for such structures using statistical mechanics. In contrast, less substituted C-C bonds involve ?,?-bound species with each C atom bound to several surface atoms. These ?,? configurations weaken C-C bonds through back-donation to antibonding orbitals, but such configurations cannot form with (3)C atoms, which have one C-H bond and thus can form only one C-M bond. (3)C-(x)C cleavage involves attachment of other C atoms, which requires endothermic C-H activation and H* desorption steps that lead to larger ?H(‡) values but also larger ?S(‡) values (by forming more H2(g)) than for (2)C-(2)C and (2)C-(1)C bonds, irrespective of alkane size (C2-C8) or cyclic/acyclic structure. These data and their mechanistic interpretation indicate that low temperatures and high H2 pressures favor cleavage of less substituted C-C bonds and form more highly branched products from cyclic and acyclic alkanes. Such interpretations and catalytic consequences of substitution seem also relevant to C-X cleavage (X = S, N, O) in desulfurization, denitrogenation, and deoxygenation reactions. PMID:24961991

Flaherty, David W; Hibbitts, David D; Iglesia, Enrique

2014-07-01

11

Unexpected C-C bond cleavage: synthesis of 1,2,4-oxadiazol-5-ones from amidoximes with pentafluorophenyl or trifluoromethyl anion acting as leaving group.  

PubMed

An unexpected C-C bond cleavage has been observed on pentafluorobenzoylamidoximes under mild basic conditions. This observation has been exploited to develop a new synthesis of 1,2,4-oxadiazol-5-ones from amidoximes using pentafluorobenzoyl chloride or trifluoroacetic anhydride (TFAA) as a double acylating agent. The pentafluorophenyl anion and the trifluoromethyl anion acted as leaving groups in this transformation. PMID:22050220

Gerfaud, Thibaud; Wei, Hai-Long; Neuville, Luc; Zhu, Jieping

2011-12-01

12

Kinetics of C-C and C-h bond cleavage in phenyl alkane radical cations generated by photoinduced electron transfer.  

PubMed

Employing nanosecond laser flash photolysis, we determined the relative importance of two fragmentation modes, namely, C-C bond cleavage and deprotonation, for the radical cation of 1,1,2,2-tetraphenylethane photogenerated by electron transfer to cyanoaromatic singlet excited states in acetonitrile at room temperature. Analysis of the kinetic data for this phenyl alkane suggests that the C-C bond cleavage dominates over the deprotonation by a ratio of about 2:1. In addition, the deprotonation kinetics of diphenylmethane, 1,1-diphenylethane, triphenylmethane, and several phenyl-substituted alcohols have been investigated. To aid identification and characterization, experiments based on two laser pulses in tandem (308 and 337.1 nm) were performed to probe fluorescence and photochemistry of the transient radicals formed as products of radical ion fragmentation. The first-order rate constants for growth of transient absorptions due to fragmentation-derived radicals were measured to be ?1 × 10(6) s(-1). Activation parameters, with activation enthalpies in the range 10-18 kJ/mol and activation entropies between -60 and -91 J/(mol.K), are also reported for fragmentation kinetics of radical cations of several systems under study. PMID:25347283

Cyr, Douglas; Das, Paritosh

2014-11-26

13

C?-C? and C?-N bond cleavage in the dissociation of protonated N-benzyllactams: dissociative proton transfer and intramolecular proton-transport catalysis.  

PubMed

In mass spectrometry of protonated N-benzylbutyrolactams, the added proton is initially localized on the carbonyl oxygen, which is the thermodynamically preferred protonation site. Upon collisional activation, dissociative proton transfer takes place leading to the occurrence of fragmentation reactions. The major fragmentations observed are the cleavages of C(?)-C(?) and C(?)-N bonds on the two sides of the methylene linker, which is different to the cleavage of the amide bond itself seen in most amide cases. Theoretical calculations and isotopic labeling experiments demonstrate that the phenyl ring regulates the proton transfer reactions. The proton directly migrates to the C(?) position via a 1,5-H shift leading to the efficient loss of benzene, while it stepwise migrates to the amide nitrogen resulting in the formation of a benzyl cation. The stepwise proton transfer is achieved via intramolecular proton-transport catalysis. The C(?) position accepts the proton from the carbonyl oxygen via a 1,6-H shift, and then donates it to the amide nitrogen via a 1,4-H shift. The general 1,3-H shift from the carbonyl oxygen to the amide nitrogen can be excluded in this case due to its significant energy barrier. The substituent effects are also applied to explore the reaction mechanism, and it proves that both C(?) and C(?) are involved in the dissociative proton transfer processes. For monosubstituted N-benzylbutyrolactams, the abundance ratios of the two competing product ions are well correlated with the nature of the substituents. PMID:22120184

Chai, Yunfeng; Guo, Cheng; Jiang, Kezhi; Pan, Yuanjiang; Sun, Cuirong

2012-01-28

14

Degradation of fluorene by Brevibacterium sp. strain DPO 1361: a novel C-C bond cleavage mechanism via 1,10-dihydro-1,10-dihydroxyfluoren-9-one.  

PubMed Central

Angular dioxygenation has been established as the crucial step in dibenzofuran degradation by Brevibacterium sp. strain DPO 1361 (V. Strubel, K. H. Engesser, P. Fischer, and H.-J. Knackmuss, J. Bacteriol. 173:1932-1937, 1991). The same strain utilizes biphenyl and fluorene as sole sources of carbon and energy. The fluorene degradation sequence is proposed to be initiated by oxidation of the fluorene methylene group to 9-fluorenol. Cells grown on fluorene exhibit pronounced 9-fluorenol dehydrogenase activity. Angular dioxygenation of the 9-fluorenone thus formed yields 1,10-dihydro-1,10-dihydroxyfluoren-9-one (DDF). A mechanistic model is presented for the subsequent C-C bond cleavage by an NAD(+)-dependent DDF dehydrogenase, acting on the angular dihydrodiol. This enzyme was purified and characterized as a tetramer of four identical 40-kDa subunits. The following Km values were determined: 13 microM for DDF and 65 microM for 2,3-dihydro-2,3-dihydroxybiphenyl. The enzyme also catalyzes the production of 3-(2'-carboxyphenyl)catechol, which was isolated, and structurally characterized, in the form of the corresponding lactone, 4-hydroxydibenzo-(b,d)-pyran-6-one. Stoichiometry analysis unequivocally demonstrates that angular dioxygenation constitutes the principal pathway in Brevibacterium sp. strain DPO 1361. Images PMID:8300532

Trenz, S P; Engesser, K H; Fischer, P; Knackmuss, H J

1994-01-01

15

Density functional theory calculations on oxidative C-C bond cleavage and N-O bond formation of [Ru(II)(bpy)2(diamine)](2+) via reactive ruthenium imide intermediates.  

PubMed

DFT calculations are performed on [Ru(II)(bpy)2(tmen)](2+) (M1, tmen = 2,3-dimethyl-2,3-butanediamine) and [Ru(II)(bpy)2(heda)](2+) (M2, head = 2,5-dimethyl-2,5-hexanediamine), and on the oxidation reactions of M1 to give the C-C bond cleavage product [Ru(II)(bpy)2(NH=CMe2)2](2+) (M3) and the N-O bond formation product [Ru(II)(bpy)2(ONCMe2CMe2NO)](2+) (M4). The calculated geometrical parameters and oxidation potentials are in good agreement with the experimental data. As revealed by the DFT calculations, [Ru(II)(bpy)2(tmen)](2+) (M1) can undergo oxidative deprotonation to generate Ru-bis(imide) [Ru(bpy)2(tmen-4?H)](+) (A) or Ru-imide/amide [Ru(bpy)2(tmen-3?H)](2+) (A') intermediates. Both A and A' are prone to C-C bond cleavage, with low reaction barriers (?G(?)) of 6.8 and 2.9?kcal?mol(-1) for their doublet spin states (2)A and (2)A', respectively. The calculated reaction barrier for the nucleophilic attack of water molecules on (2)A' is relatively high (14.2?kcal?mol(-1)). These calculation results are in agreement with the formation of the Ru(II)-bis(imine) complex M3 from the electrochemical oxidation of M1 in aqueous solution. The oxidation of M1 with Ce(IV) in aqueous solution to afford the Ru(II)-dinitrosoalkane complex M4 is proposed to proceed by attack of the cerium oxidant on the ruthenium imide intermediate. The findings of ESI-MS experiments are consistent with the generation of a ruthenium imide intermediate in the course of the oxidation. PMID:25267445

Guan, Xiangguo; Law, Siu-Man; Tse, Chun-Wai; Huang, Jie-Sheng; Che, Chi-Ming

2014-11-10

16

Molecular approach to the mechanisms of C-C bond formation and cleavage on metal surfaces: Hydrogenolysis, homologation, and dimerization of ethylene over Ru/SiO sub 2 catalysts  

SciTech Connect

At temperatures above ca. 50 C, over Ru/SiO{sub 2} catalysts and in the presence of hydrogen, ethylene undergoes hydrogenation, hydrogenolysis, homologation, and dimerization reactions. The influence of contact times and reaction temperatures on conversions and selectivities has been examined. At low temperatures (C-C bond cleavage and formation; two mechanisms are proposed which involve either metallocarbene insertion-elimination reactions or formation and decomposition of dimetallacyclic intermediates. Several mechanisms are envisioned for dimerization of ethylene; experimental data seem to support a mechanism which involves formation and coupling of two ethylidene species.

Rodriguez, E.; Leconte, M.; Basset, J.M.; Tanaka, K. (Institut de Recherches sur la Catalyse, Villeurbanne (France))

1989-09-01

17

Mechanisms of C-C bond formation and cleavage on metal surfaces: Formation of butenes and hexenes from linear and branched pentenes over Ru/SiO sub 2 catalysts  

SciTech Connect

Over Ru/SiO{sub 2} catalyst, at temperatures above 100-150C and in the presence of hydrogen, linear and branched pentenes (1-pentene, cis- and trans-2-pentene, 2-methyl-2-butene, 3-methyl-1-butene, and 2-methyl-1-butene) undergo isomerization, hydrogenation, hydrogenolysis, and homologation. The main primary products of these last two reactions of C-C bond cleavage and formation are methane, butenes, and hexanes. At low temperature (100-150C), the formation of methane is reduced and the major products are C{sub 4} and C{sub 6} olefinic hydrocarbons, which are obtained in roughly comparable amounts. The distribution of the butenes isomers and of the hexenes isomers strongly depends on the structure of the starting pentene (linear or branched, terminal or internal). The results confirm that hydrogenolysis and homologation of a C{sub 5} olefinic hydrocarbon occur at comparable rates and involve: (1) cleavage of mainly a terminal C-C bond of the pentene isomer leading to C{sup 4} and C{sup 1} fragments, (2) reaction of this C{sup 1} fragment with the starting C{sup 5} to give C{sup 6} hydrocarbons, and (or) (3) hydrogenation of the C{sup 1} fragment to methane. Two mechanisms, based on concepts of organometallic chemistry, can account for the results (especially for the distribution of the C{sup 4} and C{sup 6} olefinic isomers): (1) a methylene insertion-deinsertion mechanism or (2) a mechanism that involves formation and decomposition of dimetallacyclic intermediates. Several experimental results seem to be in favor of the last proposed mechanism.

Rodriguez, E.; Leconte, M.; Basset, J. (Conventionne a l'Universite Claude-Bernard, Lyon (France))

1991-12-01

18

Catalytic C-C Bond Activation in Biphenylene and Cyclotrimerization of Alkynes: Increased Reactivity of  

E-print Network

the target bond is both thermodynamically stable and kinetically inert, the cleavage of C-C single bonds-C bonds has been observed in the pres- ence of highly reactive species, by forcing the target molecule. (b) Murakami, M.; Ito, Y. In Topics in Organometallic Chemistry; Murai, S.

Jones, William D.

19

Exocyclic bond cleavage in oxaphosphirane complexes?  

PubMed

A first computational insight into the intrinsic strength of exocyclic bonds to phosphorus in oxaphosphirane ?P-pentacarbonylmetal(0) complexes 1a-f (M=Cr, Mo) is provided using a set of P-R derivatives (R=Me, tBu, CPh(3)). Whereas homolytic cleavage of the exocyclic P-R bond was found to be always unfavored (for neutral complexes), heterolytic cleavage leading to a carbocation R(+) moiety and the oxaphosphiranide complex 2(-) constitutes the lowest-energy process, especially if R is bulky and can stabilize the positive charge, that is, triphenylmethyl (trityl), efficiently. The energies required for P-M bond cleavage are about 30 kcal mol(-1), and decrease with the increasing bulk of the R substituent (from Me to trityl) and ongoing from Cr to Mo. The reactivities of complexes 1a-f towards oxidative and reductive single electron transfer (SET) reactions were analyzed using the facile variation of bond-strength-related descriptors (VBSD) methodology, thus enabling the design of synthetically useful strategies addressing decomplexation and P-functionalization. Reductive SET reactions with sodium naphthalenides enable selective P-M bond cleavage (i.e., decomplexation) for the case of P-Me and P-tBu substitution, whereas reductive P-R bond cleavage is favored in the case of the P-trityl complexes 1c,f, and results in the formation of the (anionic) oxaphosphiranide complex 2(-), which may be regarded as a potential key intermediate for further P-functionalization. PMID:22951978

Espinosa, Arturo; Streubel, Rainer

2012-10-15

20

Carbon-carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride  

NASA Astrophysics Data System (ADS)

The cleavage of carbon-carbon (C-C) bonds by transition metals is of great interest, especially as this transformation can be used to produce fuels and other industrially important chemicals from natural resources such as petroleum and biomass. Carbon-carbon bonds are quite stable and are consequently unreactive under many reaction conditions. In the industrial naphtha hydrocracking process, the aromatic carbon skeleton of benzene can be transformed to methylcyclopentane and acyclic saturated hydrocarbons through C-C bond cleavage and rearrangement on the surfaces of solid catalysts. However, these chemical transformations usually require high temperatures and are fairly non-selective. Microorganisms can degrade aromatic compounds under ambient conditions, but the mechanistic details are not known and are difficult to mimic. Several transition metal complexes have been reported to cleave C-C bonds in a selective fashion in special circumstances, such as relief of ring strain, formation of an aromatic system, chelation-assisted cyclometallation and ?-carbon elimination. However, the cleavage of benzene by a transition metal complex has not been reported. Here we report the C-C bond cleavage and rearrangement of benzene by a trinuclear titanium polyhydride complex. The benzene ring is transformed sequentially to a methylcyclopentenyl and a 2-methylpentenyl species through the cleavage of the aromatic carbon skeleton at the multi-titanium sites. Our results suggest that multinuclear titanium hydrides could serve as a unique platform for the activation of aromatic molecules, and may facilitate the design of new catalysts for the transformation of inactive aromatics.

Hu, Shaowei; Shima, Takanori; Hou, Zhaomin

2014-08-01

21

Protocols for the selective cleavage of carbon-sulfur bonds in coal  

SciTech Connect

Removal of the organic sulfur in coal constitutes one of the major challenges facing fossil fuel scientists today. A cost--effective of desulfurizing Illinois coal is non-existent at the present time. Research in our group aims to develop a simple protocol for sulfur removal by gaining understanding of how various additives can enhance the rates of C-S bond cleavage in Illinois coal and coal model compounds, relative to fragmentation of the coal macromolecule via C-C, C-O, and C-N bond cleavage. During this funding period, we plan to carry out examinations of: (a) the effects of various reaction conditions on radical-initiated and Lewis acid-catalyzed C-S bond cleavages; (b) the effects of caustic impregnation and subsequent alcoholic reflux on C-S bond cleavage strategies; (c) the reactions of coal model compounds with electron-deficient substrates; (d) examinations of photooxidative C-S bond cleavage reactions; (e) the effects of moderate (300--400{degrees}C) temperatures and pressures as well as ultrasonic radiation on (a) - (c). Also planned are differential scanning calorimetric (DSC) examinations of selected C-S bond cleavage protocols, including those on Illinois coals that possess varying amounts of organic and inorganic sulfur.

Bausch, M.

1991-01-01

22

Formal carbene insertion into C-C bond: Rh(I)-catalyzed reaction of benzocyclobutenols with diazoesters.  

PubMed

A Rh(I)-catalyzed formal carbene insertion into C-C bond of benzocyclobutenols has been realized by employing diazoesters as carbene precursors. The product indanol derivatives were obtained in good yields and in diastereoselective manner under mild reaction conditions. All-carbon quaternary center is constructed at the carbenic carbon. This catalytic reaction involves selective cleavage of C-C bond, Rh(I) carbene insertion, and intramolecular aldol reaction. PMID:24512084

Xia, Ying; Liu, Zhenxing; Liu, Zhen; Ge, Rui; Ye, Fei; Hossain, Mohammad; Zhang, Yan; Wang, Jianbo

2014-02-26

23

Carbon-carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride.  

PubMed

The cleavage of carbon-carbon (C-C) bonds by transition metals is of great interest, especially as this transformation can be used to produce fuels and other industrially important chemicals from natural resources such as petroleum and biomass. Carbon-carbon bonds are quite stable and are consequently unreactive under many reaction conditions. In the industrial naphtha hydrocracking process, the aromatic carbon skeleton of benzene can be transformed to methylcyclopentane and acyclic saturated hydrocarbons through C-C bond cleavage and rearrangement on the surfaces of solid catalysts. However, these chemical transformations usually require high temperatures and are fairly non-selective. Microorganisms can degrade aromatic compounds under ambient conditions, but the mechanistic details are not known and are difficult to mimic. Several transition metal complexes have been reported to cleave C-C bonds in a selective fashion in special circumstances, such as relief of ring strain, formation of an aromatic system, chelation-assisted cyclometallation and ?-carbon elimination. However, the cleavage of benzene by a transition metal complex has not been reported. Here we report the C-C bond cleavage and rearrangement of benzene by a trinuclear titanium polyhydride complex. The benzene ring is transformed sequentially to a methylcyclopentenyl and a 2-methylpentenyl species through the cleavage of the aromatic carbon skeleton at the multi-titanium sites. Our results suggest that multinuclear titanium hydrides could serve as a unique platform for the activation of aromatic molecules, and may facilitate the design of new catalysts for the transformation of inactive aromatics. PMID:25164752

Hu, Shaowei; Shima, Takanori; Hou, Zhaomin

2014-08-28

24

Alkali metal mediated C-C bond coupling reaction.  

PubMed

Metal catalyzed carbon-carbon (C-C) bond formation is one of the important reactions in pharmacy and in organic chemistry. In the present study, the electron and hole capture dynamics of a lithium-benzene sandwich complex, expressed by Li(Bz)2, have been investigated by means of direct ab-initio molecular dynamics method. Following the electron capture of Li(Bz)2, the structure of [Li(Bz)2](-) was drastically changed: Bz-Bz parallel form was rapidly fluctuated as a function of time, and a new C-C single bond was formed in the C1-C1' position of Bz-Bz interaction system. In the hole capture, the intermolecular vibration between Bz-Bz rings was only enhanced. The mechanism of C-C bond formation in the electron capture was discussed on the basis of theoretical results. PMID:25681900

Tachikawa, Hiroto

2015-02-14

25

Author's personal copy Bond cleavage reactions in substituted thiophenes  

E-print Network

Author's personal copy Bond cleavage reactions in substituted thiophenes by a rhodium complex with transition metal fragments has been elucidated. The fragment [(C5Me5)Rh(PMe3)] reacts with furan to give C -S bound complexes, although no C­H or C­S cleavage was observed [6]. A few reported investiga- tions

Jones, William D.

26

C-C bond-forming desulfurizations of sulfoximines.  

PubMed

Highly substituted, enantiomerically pure azaheterocyclic ring systems play an important role in medicinal chemistry as potential peptide mimetics. Metalated 2-alkenyl sulfoximines offer an efficient entry to this class of compounds. In this paper, we describe a new means to remove the sulfonimidoyl auxiliary with concomitant formation of a C-C double bond. PMID:18712875

Reggelin, M; Slavik, S; Bühle, P

2008-09-18

27

Microbial cleavage of organic C-S bonds  

DOEpatents

A microbial process is described for selective cleavage of organic C-S bonds which may be used for reducing the sulfur content of sulfur-containing organic carbonaceous materials. Microorganisms of Rhodococcus rhodochrous and Bacillus sphaericus have been found which have the ability of selective cleavage of organic C-S bonds. Particularly preferred microorganisms are Rhodococcus rhodochrous strain ATCC 53968 and Bacillus sphaericus strain ATCC 53969 and their derivatives.

Kilbane, J.J. II.

1994-10-25

28

Microbial cleavage of organic C-S bonds  

DOEpatents

A microbial process for selective cleavage of organic C--S bonds which may be used for reducing the sulfur content of sulfur-containing organic carbonaceous materials, Microorganisms of Rhodococcus rhodochrous and Bacillus sphaericus have been found which have the ability of selective cleavage of organic C--S bonds. Particularly preferred microorganisms are Rhodococcus rhodochrous strain ATCC 53968 and Bacillus sphaericus strain ATCC 53969 and their derivatives.

Kilbane, II, John J. (Woodstock, IL)

1994-01-01

29

Protocols for the selective cleavage of carbon-sulfur bonds in coal. Quarterly report, September 1, 1991--November 30, 1991  

SciTech Connect

Removal of the organic sulfur in coal constitutes one of the major challenges facing fossil fuel scientists today. A cost--effective of desulfurizing Illinois coal is non-existent at the present time. Research in our group aims to develop a simple protocol for sulfur removal by gaining understanding of how various additives can enhance the rates of C-S bond cleavage in Illinois coal and coal model compounds, relative to fragmentation of the coal macromolecule via C-C, C-O, and C-N bond cleavage. During this funding period, we plan to carry out examinations of: (a) the effects of various reaction conditions on radical-initiated and Lewis acid-catalyzed C-S bond cleavages; (b) the effects of caustic impregnation and subsequent alcoholic reflux on C-S bond cleavage strategies; (c) the reactions of coal model compounds with electron-deficient substrates; (d) examinations of photooxidative C-S bond cleavage reactions; (e) the effects of moderate (300--400{degrees}C) temperatures and pressures as well as ultrasonic radiation on (a) - (c). Also planned are differential scanning calorimetric (DSC) examinations of selected C-S bond cleavage protocols, including those on Illinois coals that possess varying amounts of organic and inorganic sulfur.

Bausch, M.

1991-12-31

30

A C–C Bonded Phenoxyl Radical Dimer with a Zero Bond Dissociation Free Energy  

PubMed Central

The 2,6-di-tert-butyl-4-methoxy-phenoxyl radical is shown to dimerize in solution and in the solid state. The X-ray crystal structure for the dimer, the first for a para-coupled phenoxyl radical, reveals a bond length of 1.6055(23) Å for the 4-4’ C–C bond. This is significantly longer than typical C–C bonds. Solution equilibrium studies using both optical and IR spectroscopies show that the Keq for dissociation is 1.3 ± 0.2 M at 20 °C, indicating a C–C bond dissociation free energy of ?0.15 ± 0.1 kcal mol?1. Van’t Hoff analysis gives an exceptionally small bond dissociation enthalpy (BDE) of 6.1 ± 0.5 kcal mol?1. To our knowledge, this is the weakest BDE measured for a C–C bond. This very weak bond shows a large deviation from the correlation of C–C bond lengths and strengths, but the computed force constant follows Badger’s rule. PMID:23952108

Wittman, Jessica M.; Hayoun, Rebecca; Kaminsky, Werner; Coggins, Michael K.; Mayer, James M.

2014-01-01

31

Biotic and abiotic carbon to sulfur bond cleavage  

SciTech Connect

Cleavage of aliphatic organosulfonate carbon to sulfur (C-S) bonds, a critical link in the global biogeochemical sulfur cycle, has been identified in Escherichia coli K-12. Enormous quantities of inorganic sulfate are continuously converted (Scheme I) into methanesulfonic acid 1 and acylated 3-(6-sulfo-{alpha}-D-quinovopyranosyl)-L-glycerol 2. Biocatalytic desulfurization (Scheme I) of 1 and 2, which share the structural feature of an aliphatic carbon bonded to a sulfonic acid sulfur, completes the cycle, Discovery of this desulfurization in E. coli provides an invaluable paradigm for study of a biotic process which, via the biogeochemical cycle, significantly influences the atmospheric concentration of sulfur-containing molecules.

Frost, J.W.

1991-01-01

32

Carbon-hydrogen vs. carbon-carbon bond cleavage of 1,2-diarylethane radical cations in acetonitrile-water  

SciTech Connect

Radical cations of 1,2-diarylethanes and 1-phenyl-2-arylethanes (Ar = phenyl, p-tolyl, p-anisyl) were generated in acidic 70% acetonitrile-water by Cu/sup 2 +/-catalyzed peroxydisulfate oxidation. The radical cations fragment mainly by loss of benzylic protons (C-H cleavage) rather than by alkyl C-C bond cleavage. The 1,2-diarylethanol products undergo further selective oxidation to aryl aldehydes and arylmethanols via rapid equilibration of diarylethane and diarylethanol radical cations. The radical cation of 2,3-dimethyl-2,3-diphenylbutane fragments efficiently by C-C cleavage, forming cumyl radical and cumyl cation. Oxidations of bibenzyl-bicumyl mixtures show selective oxidation of bicumyl dependent on total substrate concentration, providing evidence of equilibrating radical cations and showing that bicumyl fragments faster than bibenzyl loses protons. The effects of reaction conditions and substrate structure on reactivity are discussed.

Camaioni, D.M.; Franz, J.A.

1984-05-04

33

Cleavage of thymine N3H bonds by low-energy electrons attached Magali Theodore 1  

E-print Network

barriers to cleavage because they produce very stable anionic fragments, 0301-0104/$ - see front matter �Cleavage of thymine N3­H bonds by low-energy electrons attached to base p* orbitals Magali The the effects of base p-stacking on the rates of such bond cleavages. To date, our results suggest that sugar

Simons, Jack

34

Carbon-carbon bond cleavage of 1,2-hydroxy ethers b7 vanadium(V) dipicolinate complexes  

SciTech Connect

The development of alternatives to current petroleum-based fuels and chemicals is becoming increasingly important due to concerns over climate change, growing world energy demand, and energy security issues. Using non-food derived biomass to produce renewable feedstocks for chemicals and fuels is a particularly attractive possibility. However, the majority of biomass is in the form of lignocellulose, which is often not fully utilized due to difficulties associated with breaking down both lignin and cellulose. Recently, a number of methods have been reported to transform cellulose directly into more valuable materials such as glucose, sorbitol, 5-(chloromethyl)furfural, and ethylene glycol. Less progress has been made with selective transformations of lignin, which is typically treated in paper and forest industries by kraft pulping (sodium hydroxide/sodium sulfide) or incineration. Our group has begun investigating aerobic oxidative C-C bond cleavage catalyzed by dipicolinate vanadium complexes, with the idea that a selective C-C cleavage reaction of this type could be used to produce valuable chemicals or intermediates from cellulose or lignin. Lignin is a randomized polymer containing methoxylated phenoxy propanol units. A number of different linkages occur naturally; one of the most prevalent is the {beta}-O-4 linkage shown in Figure 1, containing a C-C bond with 1,2-hydroxy ether substituents. While the oxidative C-C bond cleavage of 1,2-diols has been reported for a number of metals, including vanadium, iron, manganese, ruthenium, and polyoxometalate complexes, C-C bond cleavage of 1,2-hydroxy ethers is much less common. We report herein vanadium-mediated cleavage of C-C bonds between alcohol and ether functionalities in several lignin model complexes. In order to explore the scope and potential of vanadium complexes to effect oxidative C-C bond cleavage in 1,2-hydroxy ethers, we examined the reactivity of the lignin model complexes pinacol monomethyl ether (A), 2-phenoxyethanol (B), and 1,2-diphenyl-2-methoxyethanol (C) (Figure 1). Reaction of (dipic)V{sup V}(O)O{sup i}Pr (1a) or (dipic)V{sup v}(O)OEt (lb) with A, B, or C in acetonitrile yielded new vanadium(V) complexes where the alcohol-ether ligand was bound in a chelating fashion. From the reaction of 1b with pinacol monomethyl ether (A) in acetonitrile solution, (dipic)V{sup v}(O)(pinOMe) (2) (PinOMe = 2,3-dimethyl-3-methoxy-2-butanoxide) was isolated in 61 % yield. Reaction of 1b with 2-phenoxyethanol (B) in acetonitrile gave the new complex (dipic)V{sup v}(O)(OPE) (3) (OPE = 2-phenoxyethoxide), which was isolated in 76% yield. In a similar fashion, 1a reacted with 1,2-diphenyl-2-methoxyethanol (C) to give (dipic)V(O)(DPME) (4) (DPME = 1,2-diphenyl-2-methoxyethoxide), which was isolated in 39% yield. Complexes 2, 3, and 4 were characterized by {sup 1}H NMR and IR spectroscopy, elemental analysis, and X-ray crystallography. Compared to the previously reported vanadium(V) pinacolate complex (dipic)V(O)(pinOH) the X-ray structure of complex 2 reveals a slightly shorter V = O bond, 1.573(2) {angstrom} vs 1.588(2) {angstrom} for the pinOH structure. Complexes 3 and 4 display similar vanadium oxo bond distances of 1.568(2) {angstrom} and 1.576(2) {angstrom}, respectively. All three complexes show longer bonds to the ether-oxygen trans to the oxo (2.388(2) {angstrom} for 2, 2.547(2) {angstrom} for 3, and 2.438(2) {angstrom} for 4) than to the hydroxy-oxygen in the pinOH structure (2.252(2) {angstrom}).

Hanson, Susan K [Los Alamos National Laboratory; Gordon, John C [Los Alamos National Laboratory; Thorn, David L [Los Alamos National Laboratory; Scott, Brian L [Los Alamos National Laboratory; Baker, R Tom [Los Alamos National Laboratory

2009-01-01

35

Cleavage of an amide bond by a ribozyme  

NASA Technical Reports Server (NTRS)

A variant form of a group I ribozyme, optimized by in vitro evolution for its ability to catalyze magnesium-dependent phosphoester transfer reactions involving DNA substrates, also catalyzes the cleavage of an unactivated alkyl amide when that linkage is presented in the context of an oligodeoxynucleotide analog. Substrates containing an amide bond that joins either two DNA oligos, or a DNA oligo and a short peptide, are cleaved in a magnesium-dependent fashion to generate the expected products. The first-order rate constant, kcat, is 0.1 x 10(-5) min-1 to 1 x 10(-5) min-1 for the DNA-flanked substrates, which corresponds to a rate acceleration of more than 10(3) as compared with the uncatalyzed reaction.

Dai, X.; De Mesmaeker, A.; Joyce, G. F.; Miller, S. L. (Principal Investigator)

1995-01-01

36

Base-catalyzed N-N bond cleavage of hydrazones: synthesis of ?-amino ketones.  

PubMed

An efficient Cs2CO3-promoted synthesis of ?-amino ketones using hydrazines, aldehydes, and ?-haloketones as starting materials through a cascade condensation/nucleophilic substitution/N-N bond cleavage route is developed. The carbonyl group plays a key role in this novel N-N bond cleavage process. PMID:24596344

Tang, Hai-Tao; Zhou, Yun-Bing; Zhu, Yu; Sun, Hong-Chao; Lin, Min; Zhan, Zhuang-Ping

2014-05-01

37

Recent advances in the gold-catalyzed additions to C–C multiple bonds  

PubMed Central

Summary C–O, C–N and C–C bonds are the most widespread types of bonds in nature, and are the cornerstone of most organic compounds, ranging from pharmaceuticals and agrochemicals to advanced materials and polymers. Cationic gold acts as a soft and carbophilic Lewis acid and is considered one of the most powerful activators of C–C multiple bonds. Consequently, gold-catalysis plays an important role in the development of new strategies to form these bonds in more convenient ways. In this review, we highlight recent advances in the gold-catalyzed chemistry of addition of X–H (X = O, N, C) bonds to C–C multiple bonds, tandem reactions, and asymmetric additions. This review covers gold-catalyzed organic reactions published from 2008 to the present. PMID:21804887

Huang, He; Zhou, Yu

2011-01-01

38

CarbonCarbon Bond Cleavage and Dehydrogenation of Isobutane Over HZSM-5 at Low Pressures and Temperatures  

E-print Network

Carbon­Carbon Bond Cleavage and Dehydrogenation of Isobutane Over HZSM-5 at Low Pressures 2012 Abstract Carbon­carbon bond cleavage and dehydroge- nation are observed following low.4 kJ mol-1 (C­C bond cleavage) and 62.8 ± 1.0 kJ mol-1 (dehydrogenation). Such low energy barriers

Tesfatsion, Leigh

39

Radical induced disulfide bond cleavage within peptides via ultraviolet irradiation of an electrospray plume.  

PubMed

Radical induced disulfide bond cleavage in peptides was demonstrated by ultraviolet (UV) radiation of the electrospray ionization (ESI) plume using a low pressure mercury (LP-Hg) lamp. Tandem mass spectrometry and accurate mass measurements confirmed that the primary reaction products were due to disulfide bond cleavage to form thiol (-SH) and sulfinyl radical (-SO?). Mechanistic studies showed that the 185 nm emission from a LP-Hg lamp was responsible for UV photolysis of atmospheric O2, which further initiated secondary radical formation and subsequent disulfide bond cleavage by radical attack. The radical induced disulfide bond cleavage was found to be analytically useful in providing rich sequence information for naturally occurring peptides containing intrachain disulfide bonds. The utility of this method was also demonstrated for facile disulfide peptide identification and characterization from protein digests. PMID:23549113

Stinson, Craig A; Xia, Yu

2013-05-21

40

Mass spectrometric and theoretical studies on dissociation of the Ssbnd S bond in the allicin: Homolytic cleavage vs heterolytic cleavage  

NASA Astrophysics Data System (ADS)

On the basis of the tandem mass spectrometry (ESI-MS/MS) technique and DFT calculations, an experimental and theoretical investigation has been conducted into the gas-phase dissociation of the S1sbnd S1' bond in the allicin as well as that of the Ssbnd C (S1sbnd C2, S1'sbnd C2') bond. Meanwhile, the influence of protonation, alkali metal ion and electron transfer on the dissociation of the S1sbnd S1' bond has been taken into account. ESI-MS/MS experiments and DFT calculations show that in the neutral allicin, [allicin + Li]+ and [allicin + Na]+, the S1sbnd S1' bond favors homolytic cleavage, while in the allicin radical cation and protonated allicin, the S1sbnd S1' bond prefers heterolytic cleavage. In addition, alkali metal ions can strengthen the S1sbnd S1' bond in the allicin, while protonation or the loss of an electron will weaken the S1sbnd S1' bond.

Zhang, Xiang

2012-08-01

41

Separation of long DNA molecules through cleavage of hydrogen bonds under a stretching force  

E-print Network

Separation of long DNA molecules through cleavage of hydrogen bonds under a stretching force Lizeng fragments,5 and discovery of chromosome- length polymorphisms.6 Pulsed field gel electrophoresis7 PFGE

Wu, Jianzhong

42

Ping-pong protons: how hydrogen-bonding networks facilitate heterolytic bond cleavage in peptide radical cations.  

PubMed

Electron capture and electron transfer dissociation (ECD/ETD) tandem mass spectrometry (MS/MS) are commonly employed techniques for biomolecular analysis. The ECD/ETD process predominately cleaves N-C? peptide backbone bonds, leading to primary sequence information complementary to other mass spectrometry techniques. Despite frequent laboratory use, the mechanistic underpinnings surrounding N-C? bond cleavage remain debated. While the majority of mechanisms assume a homolytic bond rupture, we recently showed that heterolytic cleavage is also thermodynamically viable. For a cleavage of this type to be feasible, the charge separation created upon breaking of the N-C? backbone bond must be quickly annihilated. In this work, we show, using density functional computations, that specific hydrogen-bonding motifs and structural rearrangements involving proton transfers stabilize the transition state associated with heterolytic cleavage and eliminate the ensuing charge separation from the final product fragments. The movement of protons can occur either directly from the z- to c-fragment or in a more complex manner including a ping-pong-type mechanism. The nature of these diverse hydrogen-bonding motifs reveals that not only those functional groups proximate to the bond rupture site, but also the entire global chemical environment, play important roles in backbone cleavage characteristic of ECD/ETD MS/MS. For doubly charged systems, both conformation and electron localization site dictate which of the two fragments retains the final positive charge. PMID:24555737

Zhurov, Konstantin O; Wodrich, Matthew D; Corminboeuf, Clémence; Tsybin, Yury O

2014-03-13

43

Magnesium-Mediated Cleavage of Phosphorus-Oxygen Bond: A Ribozyme Reaction  

Microsoft Academic Search

The hammerhead ribozyme belongs to the class of molecules known as antisense RNAs. However, because of short extra sequences that form the so-called catalytic loop, it can act as an enzyme. Since the catalytic domain captures Mg ions and Mg ions can cleave phosphodiester bonds, hammerhead ribozymes are recognized as metalloenzymes. In general, the cleavage of phosphodiester bonds involves acid\\/base

De-Min Zhou; Li-He Zhang; Masaya Orita; Sinya Sawata; Koichi Yoshinari; Kazunari Taira

1996-01-01

44

Carbon-carbon bond cleavage in activation of the prodrug nabumetone.  

PubMed

Carbon-carbon bond cleavage reactions are catalyzed by, among others, lanosterol 14-demethylase (CYP51), cholesterol side-chain cleavage enzyme (CYP11), sterol 17?-lyase (CYP17), and aromatase (CYP19). Because of the high substrate specificities of these enzymes and the complex nature of their substrates, these reactions have been difficult to characterize. A CYP1A2-catalyzed carbon-carbon bond cleavage reaction is required for conversion of the prodrug nabumetone to its active form, 6-methoxy-2-naphthylacetic acid (6-MNA). Despite worldwide use of nabumetone as an anti-inflammatory agent, the mechanism of its carbon-carbon bond cleavage reaction remains obscure. With the help of authentic synthetic standards, we report here that the reaction involves 3-hydroxylation, carbon-carbon cleavage to the aldehyde, and oxidation of the aldehyde to the acid, all catalyzed by CYP1A2 or, less effectively, by other P450 enzymes. The data indicate that the carbon-carbon bond cleavage is mediated by the ferric peroxo anion rather than the ferryl species in the P450 catalytic cycle. CYP1A2 also catalyzes O-demethylation and alcohol to ketone transformations of nabumetone and its analogs. PMID:24584631

Varfaj, Fatbardha; Zulkifli, Siti N A; Park, Hyoung-Goo; Challinor, Victoria L; De Voss, James J; Ortiz de Montellano, Paul R

2014-05-01

45

Formation of C–C Bonds via Iridium-Catalyzed Hydrogenation and Transfer Hydrogenation  

PubMed Central

The formation of C–C bonds via catalytic hydrogenation and transfer hydrogenation enables carbonyl and imine addition in the absence of stoichiometric organometallic reagents. In this review, iridium-catalyzed C–C bond-forming hydrogenations and transfer hydrogenations are surveyed. These processes encompass selective, atom-economic methods for the vinylation and allylation of carbonyl compounds and imines. Notably, under transfer hydrogenation conditions, alcohol dehydrogenation drives reductive generation of organoiridium nucleophiles, enabling carbonyl addition from the aldehyde or alcohol oxidation level. In the latter case, hydrogen exchange between alcohols and ?-unsaturated reactants generates electrophile–nucleophile pairs en route to products of hydro-hydroxyalkylation, representing a direct method for the functionalization of carbinol C–H bonds. PMID:21822399

Bower, John F.; Krische, Michael J.

2011-01-01

46

Addition of C-C and C-H bonds by pincer-iridium complexes: a combined experimental and computational study.  

PubMed

We report that pincer-ligated iridium complexes undergo oxidative addition of the strained C-C bond of biphenylene. The sterically crowded species ((tBu)PCP)Ir ((R)PCP = ?(3)-1,3-C6H3(CH2PR2)2) initially reacts with biphenylene to selectively add the C(1)-H bond, to give a relatively stable aryl hydride complex. Upon heating at 125 °C for 24 h, full conversion to the C-C addition product, ((tBu)PCP)Ir(2,2'-biphenyl), is observed. The much less crowded ((iPr)PCP)Ir undergoes relatively rapid C-C addition at room temperature. The large difference in the apparent barriers to C-C addition is notable in view of the fact that the addition products are not particularly crowded, since the planar biphenyl unit adopts an orientation perpendicular to the plane of the (R)PCP ligands. Based on DFT calculations the large difference in the barriers to C-C addition can be explained in terms of a "tilted" transition state. In the transition state the biphenylene cyclobutadiene core is calculated to be strongly tilted (ca. 50°-60°) relative to its orientation in the product in the plane perpendicular to that of the PCP ligand; this tilt results in very short, unfavorable, non-bonding contacts with the t-butyl groups in the case of the (tBu)PCP ligand. The conclusions of the biphenylene studies are applied to interpret computational results for cleavage of the unstrained C-C bond of biphenyl by ((R)PCP)Ir. PMID:25250874

Laviska, David A; Guan, Changjian; Emge, Thomas J; Wilklow-Marnell, Miles; Brennessel, William W; Jones, William D; Krogh-Jespersen, Karsten; Goldman, Alan S

2014-11-21

47

Bronsted-Evans-Polany relationships for C-C bond forming and C-C bond breaking reactions in thiamine-catalyzed decarboxylation of 2-keto acids using density functional theory.  

SciTech Connect

The concept of generalized enzyme reactions suggests that a wide variety of substrates can undergo enzymatic transformations, including those whose biotransformation has not yet been realized. The use of quantum chemistry to evaluate kinetic feasibility is an attractive approach to identify enzymes for the proposed transformation. However, the sheer number of novel transformations that can be generated makes this impractical as a screening approach. Therefore, it is essential to develop structure/activity relationships based on quantities that are more efficient to calculate. In this work, we propose a structure/activity relationship based on the free energy of binding or reaction of non-native substrates to evaluate the catalysis relative to that of native substrates. While Broensted-Evans-Polanyi (BEP) relationships such as that proposed here have found broad application in heterogeneous catalysis, their extension to enzymatic catalysis is limited. We report here on density functional theory (DFT) studies for C-C bond formation and C-C bond cleavage associated with the decarboxylation of six 2-keto acids by a thiamine-containing enzyme (EC 1.2.7.1) and demonstrate a linear relationship between the free energy of reaction and the activation barrier. We then applied this relationship to predict the activation barriers of 17 chemically similar novel reactions. These calculations reveal that there is a clear correlation between the free energy of formation of the transition state and the free energy of the reaction, suggesting that this method can be further extended to predict the kinetics of novel reactions through our computational framework for discovery of novel biochemical transformations.

Assary, R. S.; Broadbelt, L. J.; Curtiss, L. A. (Center for Nanoscale Materials); ( MSD); (Northwestern Univ.)

2012-01-01

48

Bond cleavage, fragment modification and reassembly in enantioselective three-component reactions  

PubMed Central

Chemical bond cleavage and reconstruction are common processes in traditional rearrangement reactions. In contrast, the process that involves bond cleavage, fragment modification and then reconstruction of the modified fragment provides an efficient way to build structurally diversified molecules. Here, we report a palladium(II)/chiral phosphoric acid catalysed three-component reaction of aryldiazoacetates, enamines and imines to afford ?-amino-?-oxo pentanoic acid derivatives in good yields with excellent diastereoselectivities and high enantioselectivities. The stereoselective reaction went through a unique process that involves cleavage of a C–N bond, modification of the resulting amino fragment and selective reassembly of the modified fragment. This innovative multi-component process represents a highly efficient way to build structurally diversified polyfunctional molecules in an atom and step economic fashion. A keto-iminium is proposed as a key intermediate and a chiral palladium/phosphate complex is proposed as an active catalyst. PMID:25586817

Zhang, Dan; Zhou, Jun; Xia, Fei; Kang, Zhenghui; Hu, Wenhao

2015-01-01

49

Bond cleavage, fragment modification and reassembly in enantioselective three-component reactions.  

PubMed

Chemical bond cleavage and reconstruction are common processes in traditional rearrangement reactions. In contrast, the process that involves bond cleavage, fragment modification and then reconstruction of the modified fragment provides an efficient way to build structurally diversified molecules. Here, we report a palladium(II)/chiral phosphoric acid catalysed three-component reaction of aryldiazoacetates, enamines and imines to afford ?-amino-?-oxo pentanoic acid derivatives in good yields with excellent diastereoselectivities and high enantioselectivities. The stereoselective reaction went through a unique process that involves cleavage of a C-N bond, modification of the resulting amino fragment and selective reassembly of the modified fragment. This innovative multi-component process represents a highly efficient way to build structurally diversified polyfunctional molecules in an atom and step economic fashion. A keto-iminium is proposed as a key intermediate and a chiral palladium/phosphate complex is proposed as an active catalyst. PMID:25586817

Zhang, Dan; Zhou, Jun; Xia, Fei; Kang, Zhenghui; Hu, Wenhao

2015-01-01

50

Theoretical study on isomerization and peptide bond cleavage at aspartic residue.  

PubMed

Isomerization and peptide bond cleavage at aspartic residue (Asp) in peptide models have been reported. In this study, the mechanisms and energies concerning the isomerization and peptide bond cleavage at Asp residue were investigated by the density functional theory (DFT) at B3LYP/6-311++G(d,p). The integral equation formalism-polarizable continuum model (IEF-PCM) was utilized to calculate solvation effect by single-point calculation of the gas-phase B3LYP/6-311++G(d,p)-optimized structure. Mechanisms and energies of the dehydration in isomerization reaction of Asp residue were comparatively analyzed with the deamidation reaction of Asn residue. The results show that the succinimide intermediate was formed preferentially through the step-wise reaction via the tetrahedral intermediate. The cleavage at C-terminus is more preferential than those at N-terminus. In comparison to isomerization, peptide bond cleavage is ? 20 kcal mol(-1) and lower in activation barrier than the isomerization. So, in this case, the isomerization of Asp is inhibited by the peptide bond cleavage. PMID:23754169

Sang-aroon, Wichien; Ruangpornvisuti, Vithaya

2013-09-01

51

C-C Bond formation catalyzed by natural gelatin and collagen proteins.  

PubMed

The activity of gelatin and collagen proteins towards C-C bond formation via Henry (nitroaldol) reaction between aldehydes and nitroalkanes is demonstrated for the first time. Among other variables, protein source, physical state and chemical modification influence product yield and kinetics, affording the nitroaldol products in both aqueous and organic media under mild conditions. Significantly, the scale-up of the process between 4-nitrobenzaldehyde and nitromethane is successfully achieved at 1 g scale and in good yield. A comparative kinetic study with other biocatalysts shows an increase of the first-order rate constant in the order chitosan < gelatin < bovine serum albumin (BSA) < collagen. The results of this study indicate that simple edible gelatin can promote C-C bond forming reactions under physiological conditions, which may have important implications from a metabolic perspective. PMID:23843902

Kühbeck, Dennis; Bijayi Dhar, Basab; Schön, Eva-Maria; Cativiela, Carlos; Gotor-Fernández, Vicente; Díaz Díaz, David

2013-01-01

52

C–C Bond formation catalyzed by natural gelatin and collagen proteins  

PubMed Central

Summary The activity of gelatin and collagen proteins towards C–C bond formation via Henry (nitroaldol) reaction between aldehydes and nitroalkanes is demonstrated for the first time. Among other variables, protein source, physical state and chemical modification influence product yield and kinetics, affording the nitroaldol products in both aqueous and organic media under mild conditions. Significantly, the scale-up of the process between 4-nitrobenzaldehyde and nitromethane is successfully achieved at 1 g scale and in good yield. A comparative kinetic study with other biocatalysts shows an increase of the first-order rate constant in the order chitosan < gelatin < bovine serum albumin (BSA) < collagen. The results of this study indicate that simple edible gelatin can promote C–C bond forming reactions under physiological conditions, which may have important implications from a metabolic perspective. PMID:23843902

Kühbeck, Dennis; Bijayi Dhar, Basab; Schön, Eva-Maria; Cativiela, Carlos; Gotor-Fernández, Vicente

2013-01-01

53

Involvement of the Arg-Asp-His Catalytic Triad in Enzymatic Cleavage of the Phosphodiester Bond  

E-print Network

Involvement of the Arg-Asp-His Catalytic Triad in Enzymatic Cleavage of the Phosphodiester Bond observed in RNase A, the His32 general base, the His82 general acid, and Arg69 acting as a phosphate investigated kinetic properties of Arg69, Asp33, and His82 mutants with phosphorothioate substrate analogues

Tsai, Ming-Daw

54

Benzene from bacterial cleavage of the carbon-phosphorus bond of phenylphosphonates.  

PubMed Central

An organism, identified as a strain of Klebsiella pneumoniae, was proved by direct assay to utilize ionic methyl phenylphosphonate as sole phosphorus source. One product from C-P-bond cleavage was identified as benzene by combined g.l.c.-mass spectrometry. The molar yield of benzene from the phosphonate was 89%. PMID:393257

Cook, A M; Daughton, C G; Alexander, M

1979-01-01

55

Direct enantiospecific substitution of primary ?-aminoalkylferrocenes via Lewis acid-catalyzed C-N bond cleavage.  

PubMed

Highly enantioenriched primary ?-aminoalkylferrocenes were found to undergo zinc chloride-catalyzed substitution with various carbon, nitrogen, and sulfur nucleophiles in an enantiospecific fashion through C-N bond cleavage. The reaction tolerates air and moisture and exhibits high atom-economy by releasing ammonia as the sole byproduct. PMID:25307165

Zhou, Meng-Guang; Zhang, Wen-Zhao; Tian, Shi-Kai

2014-12-01

56

Palladium-Catalyzed Synthesis of Six-Membered Benzofuzed Phosphacycles via Carbon-Phosphorus Bond Cleavage.  

PubMed

The palladium-catalyzed synthesis of dibenzofused six-membered phosphacycles via carbon-phosphorus bond cleavage is developed. This method is compatible with a range of functional groups, such as esters, amides, and carbamates, which is in sharp contrast to the limitations of the classical method using organolithium reagents. PMID:25521158

Baba, Katsuaki; Tobisu, Mamoru; Chatani, Naoto

2015-01-01

57

Kinetic Measurements on Single-molecule Disulfide Bond Cleavage  

PubMed Central

We use single-molecule force clamp spectroscopy (SMFCS) to explore the reactivity of tris(2-carboxyethyl)phosphine (TCEP), 1, 4-DL-dithiothreitol (DTT) and hydrosulfide anion (HS-) on disulfide bonds within a mechanically stretched polypeptide. The single-bond level bimolecular nucleophilic substitution (SN2) events are recorded at a series of precisely-controlled temperatures so that the Arrhenius kinetic parameters, i.e. the height of the activation energy barrier (Ea) and the attempting frequency (A) of the chemical reactions, can be determined. The values of A are typically at the order of 107 M-1s-1, which is far lower than that predicted by the transition-state theory, in which A is given by kBT/h and around 1012 M-1s-1 at room temperature. Furthermore, Ea is derived to be 30-40 kJ/mol, which can be lowered by ~6-8% with every 100 pN mechanical force applied. The correlation of the A and Ea with the molecular structures reveals that the relative magnitude of these two parameters cannot be simply judged from the size of the molecule or the nucleophilicity of the attacking atom. The comparison of the influences on the reaction rate induced by force and temperature indicates an equivalent accelerating effect by every 50 pN or 10 K increment, giving for the first time the relationship between mechanical and thermal effects on a single-molecule SN2 chemical reaction. PMID:21341766

Liang, Jian; Fernández, Julio M.

2011-01-01

58

Biotic and abiotic carbon to sulfur bond cleavage. Final report  

SciTech Connect

The microbial desulfurization of organosulfur compounds occurs by unprecedented and largely unexplored biochemical processes. A study of such biotic desulfurizations can be expected to give rise to new and useful chemistry and enzymology. The potential value of understanding and harnessing these processes is seen in relation to the need for methods for the removal of organically bound sulfur from coal and the degradation of organic sulfur-containing pollutants. This research effort has been directed towards an examination of desulfurization ability in well characterized microorganisms, the isolation of bacteria with desulfurization ability from natural sources, the characterization and mechanistic evaluation of the observed biocatalytic processes, the development of biomimetic synthetic organic chemistry based on biotic desulfurization mechanisms and the design and preparation of improved coal model compounds for use in microbial selection processes. A systematic approach to studying biodesulfurizations was undertaken in which organosulfur compounds have been broken down into classes based on the oxidation state of the sulfur atom and the structure of the rest of the organic material. Microbes have been evaluated in terms of ability to degrade organosulfur compounds with sulfur in its sulfonic acid oxidation state. These compounds are likely intermediates in coal desulfurization and are present in the environment as persistent pollutants in the form of detergents. It is known that oxygen bonded to sulfur lowers the carbon-sulfur bond energy, providing a thermodynamic basis for starting with this class of compounds.

Frost, J.W.

1994-05-01

59

C-C bond fragmentation by Grob/Eschenmoser reactions, applications in dendrimer synthesis.  

PubMed

C-C bond fragmentation of structurally diverse carbocycles has been applied to the divergent synthesis of dendrimers. The fragmentation has been paired to deprotection or thio-Michael reaction, allowing the preparation of a fourth generation dendrimer of narrow molecular weight distribution. Methodologies to increase water solubility have been examined using appended carboxylic acid or oligoether moieties. In addition, incorporation of chiral prolinol derivatives has resulted in the synthesis of dendrimers that have been shown to catalyse the ?-amination of aldehydes in good yield and modest enantioselectivity. PMID:23925375

Hierold, Judith; Lupton, David W

2013-09-28

60

An erbium-based bifuctional heterogeneous catalyst: a cooperative route towards C-C bond formation.  

PubMed

Heterogeneous bifuctional catalysts are multifunctional synthetic catalysts enabling efficient organic transformations by exploiting two opposite functionalities without mutual destruction. In this paper we report the first Er(III)-based metallorganic heterogeneous catalyst, synthesized by post-calcination MW-assisted grafting and modification of the natural aminoacid L-cysteine. The natural acid-base distance between sites was maintained to assure the cooperation. The applicability of this new bifunctional heterogeneous catalyst to C-C bond formation and the supposed mechanisms of action are discussed as well. PMID:25029070

Oliverio, Manuela; Costanzo, Paola; Macario, Anastasia; De Luca, Giuseppina; Nardi, Monica; Procopio, Antonio

2014-01-01

61

The computation of C-C and N-N bond dissociation energies for singly, doubly, and triply bonded systems  

NASA Technical Reports Server (NTRS)

The bond dissociation energies (De) of C2H2, C2H4, C2H6, N2, N2H2, and N2H4 are studied at various levels of correlation treatment. The convergence of De with respect to the one-particle basis is studied at the single-reference modified coupled-pair-functional (MCPF)level. At all levels of correlation treatment, the errors in the bond dissociation energies increase with the degree of multiple bond character. The multireference configuration-interaction (MRCI) De values, corrected for an estimate of higher excitations, are in excellent agreement with those determined using the size-extensive averaged-coupled-pair-functional (ACPF) method. The full-valence complete-active-space self-consistent-field (CASSCF)/MRCI calculations are reproduced very well by MRCI calculations based on a CASSCF calculation that includes in the active space only those electrons involved in the C-C or N-N bonds.

Langhoff, Stephen R.; Bauschlicher, Charles W., Jr.; Taylor, Peter R.

1991-01-01

62

Mercury Detoxification by Bacteria: Simulations of Transcription Activation and Mercury-Carbon Bond Cleavage  

SciTech Connect

In this chapter, we summarize recent work from our laboratory and provide new perspective on two important aspects of bacterial mercury resistance: the molecular mechanism of transcriptional regulation by MerR, and the enzymatic cleavage of the Hg-C bond in methylmercury by the organomercurial lyase, MerB. Molecular dynamics (MD) simulations of MerR reveal an opening-and-closing dynamics, which may be involved in initiating transcription of mercury resistance genes upon Hg(II) binding. Density functional theory (DFT) calculations on an active-site model of the enzyme reveal how MerB catalyzes the Hg-C bond cleavage using cysteine coordination and acid-base chemistry. These studies provide insight into the detailed mechanisms of microbial gene regulation and defense against mercury toxicity.

Guo, Hao-Bo [ORNL; Parks, Jerry M [ORNL; Johs, Alexander [ORNL; Smith, Jeremy C [ORNL

2011-01-01

63

Entropic Origin of Cobalt-Carbon Bond Cleavage Catalysis in Adenosylcobalamin-Dependent Ethanolamine Ammonia-Lyase  

PubMed Central

Adenosylcobalamin-dependent enzymes accelerate the cleavage of the cobalt-carbon (Co-C) bond of the bound coenzyme by >1011-fold. The cleavage-generated 5?-deoxyadenosyl radical initiates the catalytic cycle by abstracting a hydrogen atom from substrate. Kinetic coupling of the Co-C bond cleavage and hydrogen atom transfer steps at ambient temperatures has interfered with past experimental attempts to directly address the factors that govern Co-C bond cleavage catalysis. Here, we use time-resolved, full-spectrum electron paramagnetic resonance spectroscopy, temperature-step reaction initiation, starting from the enzyme-coenzyme-substrate ternary complex, and 2H-labeled substrate, to study radical pair generation in ethanolamine ammonia-lyase from Salmonella typhimurium at 234-248 K in a dimethylsulfoxide/water cryosolvent system. The monoexponential kinetics of formation of the 2H- and 1H-substituted substrate radicals are the same, indicating that Co-C bond cleavage rate-limits radical pair formation. Analysis of the kinetics by using a linear, three-state model allows extraction of the microscopic rate constant for Co-C bond cleavage. Eyring analysis reveals that the activation enthalpy for Co-C bond cleavage is 32 ±1 kcal/mol, which is the same as for the cleavage reaction in solution. The origin of Co-C bond cleavage catalysis in the enzyme is, therefore, the large, favorable activation entropy of 61 ±6 cal/mol/K (relative to 7 ±1 cal/mol/K in solution). This represents a paradigm shift from traditional, enthalpy-based mechanisms that have been proposed for Co-C bond breaking in B12 enzymes. The catalysis is proposed to arise from an increase in protein configurational entropy along the reaction coordinate. PMID:24028405

Wang, Miao; Warncke, Kurt

2013-01-01

64

Cleavage of Multiple Disulfide Bonds in Insulin via Gold Cationization and Collision-induced Dissociation  

PubMed Central

Intact bovine insulin, with its two chains linked via two disulfide linkages, has been used as a model system to study the incorporation of one or more gold cations as means for facilitating the cleavage of multiple disulfide bonds in a tandem mass spectrometry experiment. Gas-phase ion/ion reactions involving Au(I)Cl2? or Au(III)Cl4? were used to incorporate either one or two gold cations into multiply-protonated insulin cations, followed by ion trap collision-induced dissociation (CID) of the products. The incorporation of a single gold cation followed by CID showed little evidence for disulfide bond cleavage. Rather, the CID spectra were similar to those acquired for the same charge state with only excess protons present. However, the incorporation of two gold cations, regardless of oxidation state, resulted in efficient cleavage of the disulfide bonds connecting the two chains of insulin. Furthermore, ion trap CID of the insulin complexes containing two gold cations showed more sequence information compared to the complexes containing only one gold cation or no gold cations. The partitioning of the gold cations between the two chains upon CID proved to be largely asymmetric, as both gold cations tended to stay together. There appeared to be a slight preference for both gold cations to partition into the B-chain. However, the relatively low contribution from single chain ions with only one gold ion suggests a degree of cooperativity in the overall mechanism for separation of the two chains. PMID:22125416

Mentinova, Marija; McLuckey, Scott A.

2011-01-01

65

Biotic and abiotic carbon to sulfur bond cleavage. Technical report, July 1, 1991--September 30, 1991  

SciTech Connect

Cleavage of aliphatic organosulfonate carbon to sulfur (C-S) bonds, a critical link in the global biogeochemical sulfur cycle, has been identified in Escherichia coli K-12. Enormous quantities of inorganic sulfate are continuously converted (Scheme I) into methanesulfonic acid 1 and acylated 3-(6-sulfo-{alpha}-D-quinovopyranosyl)-L-glycerol 2. Biocatalytic desulfurization (Scheme I) of 1 and 2, which share the structural feature of an aliphatic carbon bonded to a sulfonic acid sulfur, completes the cycle, Discovery of this desulfurization in E. coli provides an invaluable paradigm for study of a biotic process which, via the biogeochemical cycle, significantly influences the atmospheric concentration of sulfur-containing molecules.

Frost, J.W.

1991-12-31

66

Bimetallic cleavage of aromatic C-h bonds by rare-Earth-metal complexes.  

PubMed

A new type of C-H bond activation mediated by rare-earth metals under reducing conditions is reported. The synergy between reductants and rare-earth-metal complexes allows the cleavage of unactivated aromatic C-H bonds. The reaction between rare-earth-metal iodides supported by a 1,1'-ferrocenediamide ligand and potassium graphite in benzene leads to the formation of a 1:1 metal molar ratio of the corresponding metal hydride and metal phenyl complex. A proposed mechanism involving an inverse sandwich arene bimetallic intermediate is supported by experimental and computational studies. PMID:25431837

Huang, Wenliang; Dulong, Florian; Khan, Saeed I; Cantat, Thibault; Diaconescu, Paula L

2014-12-17

67

Transition-metal-free C-C bond forming reactions of aryl, alkenyl and alkynylboronic acids and their derivatives.  

PubMed

Investigation of new methods for the synthesis of C-C bonds is fundamental for the development of new organic drugs and materials. Aryl-, alkenyl- and alkynylboronic acids and their derivatives constitute attractive reagents towards this end, due to their stability, low toxicity and ease of handling. However, these compounds are only moderately nucleophilic. Consequently, the most popular C-C bond forming reactions of these boronic acids, such as the Suzuki-Miyaura, Heck, and Hayashi-Miyaura reactions, or additions to C=O and C=N bonds, require catalysis by transition metals. However, due to the toxicity and cost of transition metals, some new methods for C-C bond formation using aryl-, alkenyl- and alkynylboronic acids under transition-metal-free conditions are beginning to emerge. In this tutorial review, the recent synthetic advances in this field are highlighted and discussed. PMID:25181967

Roscales, S; Csákÿ, A G

2014-12-21

68

Reversible C-C bond activation enables stereocontrol in rh-catalyzed carbonylative cycloadditions of aminocyclopropanes.  

PubMed

Upon exposure to neutral or cationic Rh(I)-catalyst systems, amino-substituted cyclopropanes undergo carbonylative cycloaddition with tethered alkenes to provide stereochemically complex N-heterocyclic scaffolds. These processes rely upon the generation and trapping of rhodacyclopentanone intermediates, which arise by regioselective, Cbz-directed insertion of Rh and CO into one of the two proximal aminocyclopropane C-C bonds. For cyclizations using cationic Rh(I)-systems, synthetic and mechanistic studies indicate that rhodacyclopentanone formation is reversible and that the alkene insertion step determines product diastereoselectivity. This regime facilitates high levels of stereocontrol with respect to substituents on the alkene tether. The option of generating rhodacyclopentanones dynamically provides a new facet to a growing area of catalysis and may find use as a (stereo)control strategy in other processes. PMID:25539136

Shaw, Megan H; McCreanor, Niall G; Whittingham, William G; Bower, John F

2015-01-14

69

A Homogeneous, Recyclable Polymer Support for Rh(I)-Catalyzed C-C Bond Formation  

PubMed Central

A robust and practical polymer-supported, homogeneous, recyclable biphephos rhodium(I) catalyst has been developed for C-C bond formation reactions. Control of polymer molecular weight allowed tuning of the polymer solubility such that the polymer-supported catalyst is soluble in nonpolar solvents and insoluble in polar solvents. Using the supported rhodium catalysts, addition of aryl and vinylboronic acids to the electrophiles such as enones, aldehydes, N-sulfonyl aldimines, and alkynes occurs smoothly to provide products in high yields. Additions of terminal alkynes to enones and industrially relevant hydroformylation reactions have also been successfully carried out. Studies show that the leaching of Rh from the polymer support is low and catalyst recycle can be achieved by simple precipitation and filtration. PMID:21895010

Jana, Ranjan; Tunge, Jon A.

2011-01-01

70

Aryl ether as a Negishi coupling partner: an approach for constructing C-C bonds under mild conditions.  

PubMed

An etheric Negishi coupling: The first cross-coupling reaction between aryl alkyl ethers and dianion-type zincate reagents to afford biaryl compounds through selective cleavage of the etheric C(sp(2))-O bond was developed. Dianion-type zincates showed excellent reactivity toward the aromatic ethers under mild conditions, with good functional group compatibility (see scheme). PMID:22374683

Wang, Chao; Ozaki, Takashi; Takita, Ryo; Uchiyama, Masanobu

2012-03-19

71

[Polycationic catalysts for phosphodiester bond cleavage on the basis of 1,4-diazabicyclo[2.2.2]octane].  

PubMed

A number of tetracationic compounds capable of phosphodiester bond cleavage within a 21 -membered ribooligonucleotide were designed and synthesized. The artificial ribonucleases represent two residues of quaternized 1,4-diazabicyclo[2.2.2]octane bearing alkyl substituents of various lengths and connected with a rigid linker. The efficiency of cleavage of phosphodiester bonds in an RNA target depends on the linker structure and the length of alkyl substituent. PMID:18050663

Burakova, E A; Kovalev, N A; Kuznetsova, I L; Zenkova, M A; Vlasov, V V; Sil'nikov, V N

2007-01-01

72

The mechanism of hydrocarbon oxygenate reforming: C-C bond scission, carbon formation, and noble-metal-free oxide catalysts.  

PubMed

Towards a molecular understanding of the mechanism behind catalytic reforming of bioderived hydrocarbon oxygenates, we explore the C-C bond scission of C2 model compounds (acetic acid, ethanol, ethylene glycol) on ceria model catalysts of different complexity, with and without platinum. Synchrotron photoelectron spectroscopy reveals that the reaction pathway depends very specifically on both the reactant molecule and the catalyst surface. Whereas C-C bond scission on Pt sites and on oxygen vacancies involves intermittent surface carbon species, the reaction occurs without any carbon formation and deposition for ethylene glycol on CeO2(111). PMID:24203922

Lykhach, Yaroslava; Neitzel, Armin; Šev?íková, Klára; Johánek, Viktor; Tsud, Nataliya; Skála, Tomáš; Prince, Kevin C; Matolín, Vladimír; Libuda, Jörg

2014-01-01

73

Concise Synthesis of the Bryostatin A-Ring via Consecutive C-C Bond Forming Transfer Hydrogenations  

PubMed Central

Under the conditions of C-C bond forming transfer hydrogenation, 1,3-propanediol 1 engages in double asymmetric carbonyl allylation to furnish the C2-symmetric diol 2. Double ozonolysis of 2 followed by TBS protection delivers aldehyde 3, which is subject to catalyst directed carbonyl prenylation via transfer hydrogenation to deliver neopentyl alcohol 4 and, ultimately, the bryostatin A-ring 7. Through use of two consecutive C-C bond forming transfer hydrogenations, the Evans’ bryostatin A-ring 7 is prepared in less than half the manipulations previously reported. PMID:19586066

Lu, Yu; Krische, Michael J.

2009-01-01

74

Inspired by nature: an exploration of biocatalyzed siloxane bond formation and cleavage.  

PubMed

The intricate siliceous architectures of diatom species have inspired our exploration of biosilicification. In vitro studies of natural systems within the area of silica biosynthesis are complicated. Previous studies, which included biomimetic approaches, often failed to recognize the chemistry of silicic acid and its analogues. To better understand the role of various proteins in the biosilicification process, recent studies have been conducted to test the ability of enzymes to catalyze the formation and cleavage of siloxane bonds. Notably, biocatalysis at silicon was observed. Further understanding of the biotransformation strategy in the design and synthesis of structurally complex materials would be beneficial. PMID:16002277

Brandstadt, Kurt F

2005-08-01

75

Central C-C Bonding Increases Optical and Chemical Stability of NIR Fluorophores  

PubMed Central

Functional near-infrared (NIR) fluorophores have played a major role in the recent advances in bioimaging. However, the optical and physicochemical stabilities of NIR fluorophores in the biological and physiological environment are still a challenge. Especially, the ether linkage on the meso carbon of heptamethine core is fragile when exposed to serum proteins or other amine-rich biomolecules. To solve such a structural limitation, a rigid carbon-carbon bond was installed onto the framework of ether-linked NIR fluorophores through the Suzuki coupling. The robust fluorophores replaced as ZW800-1C and ZW800-3C displayed enhanced optical and chemical stability in various solvents and a 100% warm serum environment (> 99%, 24 h). The biodistribution and clearance of C-C coupled ZW800 compounds were almost identical to the previously developed oxygen-substituted ZW800 compounds. When conjugated with a small molecule ligand, ZW800-1C maintained the identical stable form in warm serum (>98%, 24 h), while ZW800-1A hydrolyzed quickly after 4 h incubation (34%, 24 h). PMID:25530846

Hyun, Hoon; Owens, Eric A.; Narayana, Lakshminarayana; Wada, Hideyuki; Gravier, Julien; Bao, Kai; Frangioni, John V.; Choi, Hak Soo; Henary, Maged

2014-01-01

76

Activation of Ethane C-H and C-C Bonds by Gas Phase Th+ and U+: A Theoretical Study  

NASA Astrophysics Data System (ADS)

Two different approaches of density functional theory were used to analyze the C-H and C-C bond activation mechanisms during the reaction of bare Th+ and U+ ions with ethane. We report a complete exploration of the potential energy surfaces taking into consideration different spin states. According to B3LYP/SDD computations the double dehydrogenation of C2H6 is thermodynamically favorable only in the case of Th+. It is shown that the overall C-H and C-C bond activation processes are exothermic in the case of Th+ and endothermic for U+. In both cases, the C-C insertion transition state barrier exceeds the energy of the ground state reactants, preventing the observation of these species under thermal conditions.

di Santo, E.; Michelini, M. C.; Russo, N.

2009-09-01

77

Activation of ethane C-H and C-C bonds by gas phase Th+ and U+: a theoretical study.  

PubMed

Two different approaches of density functional theory were used to analyze the C-H and C-C bond activation mechanisms during the reaction of bare Th(+) and U(+) ions with ethane. We report a complete exploration of the potential energy surfaces taking into consideration different spin states. According to B3LYP/SDD computations the double dehydrogenation of C(2)H(6) is thermodynamically favorable only in the case of Th(+). It is shown that the overall C-H and C-C bond activation processes are exothermic in the case of Th(+) and endothermic for U(+). In both cases, the C-C insertion transition state barrier exceeds the energy of the ground state reactants, preventing the observation of these species under thermal conditions. PMID:20028168

Di Santo, E; Michelini, M C; Russo, N

2009-12-31

78

Transition-metal/Lewis acid free synthesis of acyl benzothiophenes via C-C bond forming reaction.  

PubMed

A simple and single-step synthesis of 2- and 3-acyl substituted benzothiophenes has been described via environmentally benign acylation of benzothiophene with in situ generated acyl trifluoroacetates. Both aliphatic and aromatic carboxylic acids participated in trifluoroacetic anhydride/phosphoric acid mediated C-C bond forming reactions under solvent-free conditions affording acyl benzothiophenes in good overall yields. PMID:17961232

Pal, Sarbani; Khan, Mohammad Ashrafuddin; Bindu, P; Dubey, P K

2007-01-01

79

Transition-metal/Lewis acid free synthesis of acyl benzothiophenes via C-C bond forming reaction  

PubMed Central

A simple and single-step synthesis of 2- and 3-acyl substituted benzothiophenes has been described via environmentally benign acylation of benzothiophene with in situ generated acyl trifluoroacetates. Both aliphatic and aromatic carboxylic acids participated in trifluoroacetic anhydride/phosphoric acid mediated C-C bond forming reactions under solvent-free conditions affording acyl benzothiophenes in good overall yields. PMID:17961232

Pal, Sarbani; Khan, Mohammad Ashrafuddin; Bindu, P; Dubey, PK

2007-01-01

80

P-C Bond Scission at the TRIPHOS Ligand and C-CN Bond Cleavage in 2-Methyl-3-butenenitrile with [Ni(COD)2  

E-print Network

P-C Bond Scission at the TRIPHOS Ligand and C-CN Bond Cleavage in 2-Methyl-3-butenenitrile with [Ni(COD The use of catalytic amounts of [Ni(COD)2] and TRIPHOS (bis(2-diphenylphosphinoethyl)phe- nylphosphine

Jones, William D.

81

RhI-Catalyzed Decarbonylative Spirocyclization via C–C Cleavage of Benzocyclobutenones: An Efficient Approach to Access Functionalized Spirocycles  

PubMed Central

We describe a rhodium-catalyzed all-carbon spirocenter formation through a decarbonylative coupling of trisubstituted cyclic olefins and benzocyclobutenones via C–C activation. A [Rh(CO)2Cl]2/P(C6F5)3 metal-ligand combination was found to catalyze this transformation most efficiently. A range of diverse spirocyclic rings were synthesized in good to excellent yields and many sensitive functional groups were tolerated. Mechanistic study supports the hydrogen-transfer process that occurs via a ?-H elimination/decarbonylation pathway. PMID:24446067

Xu, Tao; Savage, Nikolas A.; Dong, Guangbin

2014-01-01

82

Aerobic carbon-carbon bond cleavage of alkenes to aldehydes catalyzed by first-row transition-metal-substituted polyoxometalates in the presence of nitrogen dioxide.  

PubMed

A new aerobic carbon-carbon bond cleavage reaction of linear di-substituted alkenes, to yield the corresponding aldehydes/ketones in high selectivity under mild reaction conditions, is described using copper(II)-substituted polyoxometalates, such as {?2-Cu(L)P2W17O61}(8-) or {[(Cu(L)]2WZn(ZnW9O34)2}(12-), as catalysts, where L = NO2. A biorenewable-based substrate, methyl oleate, gave methyl 8-formyloctanoate and nonanal in >90% yield. Interestingly, cylcoalkenes yield the corresponding epoxides as products. These catalysts either can be prepared by pretreatment of the aqua-coordinated polyoxometalates (L = H2O) with NO2 or are formed in situ when the reactions are carried with nitroalkanes (for example, nitroethane) as solvents or cosolvents. Nitroethane was shown to release NO2 under reaction conditions. (31)P NMR shows that the Cu-NO2-substituted polyoxometalates act as oxygen donors to the C-C double bond, yielding a Cu-NO product that is reoxidized to Cu-NO2 under reaction conditions to complete a catalytic cycle. Stoichiometric reactions and kinetic measurements using {?2-Co(NO2)P2W17O61}(8-) as oxidant and trans-stilbene derivatives as substrates point toward a reaction mechanism for C-C bond cleavage involving two molecules of {?2-Co(NO2)P2W17O61}(8-) and one molecule of trans-stilbene that is sufficiently stable at room temperature to be observed by (31)P NMR. PMID:25020036

Rubinstein, Amir; Jiménez-Lozanao, Pablo; Carbó, Jorge J; Poblet, Josep M; Neumann, Ronny

2014-08-01

83

Phosphodiester and N-glycosidic bond cleavage in DNA induced by 4-15 eV electrons  

NASA Astrophysics Data System (ADS)

Thin molecular films of the short single strand of DNA, GCAT, were bombarded under vacuum by electrons with energies between 4 and 15 eV. Ex vacuo analysis by high-pressure liquid chromatography of the samples exposed to the electron beam revealed the formation of a multitude of products. Among these, 12 fragments of GCAT were identified by comparison with reference compounds and their yields were measured as a function of electron energy. For all energies, scission of the backbone gave nonmodified fragments containing a terminal phosphate, with negligible amounts of fragments without the phosphate group. This indicates that phosphodiester bond cleavage by 4-15 eV electrons involves cleavage of the C-O bond rather than the P-O bond. The yield functions exhibit maxima at 6 and 10-12 eV, which are interpreted as due to the formation of transient anions leading to fragmentation. Below 15 eV, these resonances dominate bond dissociation processes. All four nonmodified bases are released from the tetramer, by cleavage of the N-glycosidic bond, which occurs principally via the formation of core-excited resonances located around 6 and 10 eV. The formation of the other nonmodified products leading to cleavage of the phosphodiester bond is suggested to occur principally via two different mechanisms: (1) the formation of a core-excited resonance on the phosphate unit followed by dissociation of the transient anion and (2) dissociation of the CO bond of the phosphate group formed by resonance electron transfer from the bases. In each case, phosphodiester bond cleavage leads chiefly to the formation of stable phosphate anions and sugar radicals with minimal amounts of alkoxyl anions and phosphoryl radicals.

Zheng, Yi; Cloutier, Pierre; Hunting, Darel J.; Wagner, J. Richard; Sanche, Léon

2006-02-01

84

Mechanistic Insights into N-N Bond Cleavage in Catalytic Guanylation Reactions between 1,2-Diarylhydrazines and Carbodiimides.  

PubMed

Cleavage of the N-N bond in 1,2-diarylhydrazine was achieved through an alkyllithium-catalyzed guanylation reaction of 1,2-diarylhydrazine with carbodiimide, affording guanidine and azo compounds. This N-N bond cleavage via thermal rearrangement was driven by an intramolecular proton shift. No reductants, oxidants, bases, or external protons were needed. The proposed mechanism has been well elucidated by the isolation, characterization, and reaction studies of two important amido lithium intermediates and an ArHN-substituted guanidine. PMID:25311373

Xu, Ling; Wang, Yu-Chen; Ma, Wangyang; Zhang, Wen-Xiong; Xi, Zhenfeng

2014-12-19

85

Cleavage enhancement of specific chemical bonds in DNA by cisplatin radiosensitization.  

PubMed

X-ray photoelectron spectroscopy (XPS) is harnessed as an in situ efficient characterization technique for monitoring chemical bond transformation in DNA and cisplatin-DNA complexes under synergic X-ray irradiation. By analyzing the variation of relative peak area of core elements of DNA as a function of irradiation time, we find that the most vulnerable scission sites in DNA are those containing phosphate and glycosidic bonds. Compared to DNA, the effective rate constants of the corresponding phosphodiester and glycosidic bond cleavages for cisplatin-DNA complexes are 1.8 and 1.9 folds larger. These damages and their enhancements are similar to those induced by low energy electrons (LEE). Consistently, the magnitude of the secondary electron distribution produced by the X-rays on the cisplatin-DNA complexes is considerably increased compared to that of pristine DNA. The data suggest that DNA radiosensization by cisplatin results not only from the sensitization of DNA to the action of LEE, but also from an increase the production of LEE at the site of binding of the cisplatin. The results provide new insights into the mechanisms of cisplatin-induced sensitization of DNA under X-ray irradiation, which could be helpful in the design of new cisplatin-based antitumor drugs. PMID:23551177

Xiao, Fangxing; Luo, Xinglan; Fu, Xianzhi; Zheng, Yi

2013-05-01

86

Intramolecular C-F and C-H bond cleavage promoted by butadienyl heavy Grignard reagents.  

PubMed

Organomagnesium compounds (Grignard reagents) are among the most useful organometallic reagents and have greatly accelerated the advancement of synthetic chemistry and related sciences. Nevertheless, heavy Grignard reagents based on the metals calcium, strontium or barium are not widely used, mainly due to their rather inert heavy alkaline-earth metals and extremely high reactivity of their corresponding Grignard-type reagents. Here we report the generation and reaction chemistry of butadienyl heavy Grignard reagents whose extremely high reactivity is successfully tamed. Facile synthesis of perfluoro-?-extended pentalene and naphthalene derivatives is realized by the in situ generated heavy Grignard reagents via intramolecular C-F/C-H bond cleavage. These obtained perfluorodibenzopentalene and perfluorodinaphthopentalene derivatives show low-lying LUMO levels, with one being the lowest value so far among all pentalene derivatives. Our results set an exciting example for the meaningful synthetic application of heavy Grignard reagents. PMID:25047678

Li, Heng; Wang, Xiao-Ye; Wei, Baosheng; Xu, Ling; Zhang, Wen-Xiong; Pei, Jian; Xi, Zhenfeng

2014-01-01

87

Gold-Catalyzed Oxidation of Propargylic Ethers with Internal C-C Triple Bonds: Impressive Regioselectivity Enabled by Inductive Effect.  

PubMed

Inductive perturbations of C-C triple bonds are shown to dictate the regiochemistry of gold-catalyzed oxidation of internal C-C triple bonds in the cases of propargylic ethers, resulting in highly regioselective formation of ?-alkoxy-?,?-unsaturated ketones (up to >50/1 selectivity) via ?-oxo gold carbene intermediates. Ethers derived from primary propargylic alcohols can be reliably transformed in good yields, and various functional groups are tolerated. With substrates derived from secondary propargylic alcohols, the development of a new P,N-bidentate ligand enables the minimization of competing alkyl group migration to the gold carbene center over the desired hydride migration; the preferred migration of a phenyl group, however, results in efficient formation of a ?-phenyl-?-alkoxy-?,?-unsaturated ketone. These results further advance the surrogacy of a propargyl moiety to synthetically versatile enone function with reliable and readily predictable regioselectivity. PMID:25284890

Ji, Kegong; D'Souza, Brendan; Nelson, Jon; Zhang, Liming

2014-11-01

88

Enantioselective Allylation, Crotylation and Reverse Prenylation of Substituted Isatins via Iridium Catalyzed C-C Bond Forming Transfer Hydrogenation**  

PubMed Central

Oxindoles with a Twist Transfer hydrogenation of substituted isatins in the presence of allyl acetate, ?-methyl allyl acetate or 1,1,-dimethylallene employing an ortho-cyclometallated iridium catalyst modified by CTH-(R)-P-PHOS provides products of carbonyl allylation, crotylation and reverse prenylation, respectively, in highly enantiomerically enriched form. These studies represent the first use of activated ketones as electrophilic partners in asymmetric C-C bond forming transfer hydrogenation. PMID:19606435

Itoh, Junji; Han, Soo Bong; Krische, Michael J.

2010-01-01

89

Controllable synthesis of silver and silver sulfide nanocrystals via selective cleavage of chemical bonds  

NASA Astrophysics Data System (ADS)

A one-step colloidal process has been adopted to prepare silver (Ag) and silver sulfide (Ag2S) nanocrystals, thus avoiding presynthesis of an organometallic precursor and the injection of a toxic phosphine agent. During the reaction, a layered intermediate compound is first formed, which then acts as a precursor, decomposing into the nanocrystals. The composition of the as-obtained products can be controlled by selective cleavage of S-C bonds or Ag-S bonds. Pure Ag2S nanocrystals can be obtained by directly heating silver acetate (Ag(OAc)) and n-dodecanethiol (DDT) at 200?° C without any surfactant, and pure Ag nanocrystals can be synthesized successfully if the reaction temperature is reduced to 190?° C and the amount of DDT is decreased to 1 ml in the presence of a non-coordinating organic solvent (1-octadecene, ODE). Otherwise, the mixture of Ag and Ag2S is obtained by directly heating Ag(OAc) in DDT by increasing the reaction temperature or in a mixture of DDT and ODE at 200?° C. The formation mechanism has been discussed in detail in terms of selective S-C and Ag-S bond dissociation due to the nucleophilic attack of DDT and the lower bonding energy of Ag-S. Interestingly, some products can easily self-assemble into two- or three-dimensional (2D or 3D) highly ordered superlattice structures on a copper grid without any additional steps. The excess DDT plays a key role in the superlattice structure due to the bundling and interdigitation of the thiolate molecules adsorbed on the as-obtained nanocrystals.

Tang, Aiwei; Wang, Yu; Ye, Haihang; Zhou, Chao; Yang, Chunhe; Li, Xu; Peng, Hongshang; Zhang, Fujun; Hou, Yanbing; Teng, Feng

2013-09-01

90

{{text{C}}_{? }} - {text{C}} Bond Cleavage of the Peptide Backbone in MALDI In-Source Decay Using Salicylic Acid Derivative Matrices  

NASA Astrophysics Data System (ADS)

The use of 5-formylsalicylic acid (5-FSA) and 5-nitrosalicylic acid (5-NSA) as novel matrices for in-source decay (ISD) of peptides in matrix-assisted laser desorption/ionization (MALDI) is described. The use of 5-FSA and 5-NSA generated a- and x-series ions accompanied by oxidized peptides [M - 2 H + H]+. The preferential formation of a- and x-series ions was found to be dependent on the hydrogen-accepting ability of matrix. The hydrogen-accepting ability estimated from the ratio of signal intensity of oxidized product [M - 2 H + H]+ to that of non-oxidized protonated molecule [M + H]+ of peptide was of the order 5-NSA > 5-FSA > 5-aminosalicylic acid (5-ASA) ? 2,5-dihydroxyl benzoic acid (2,5-DHB) ? 0. The results suggest that the hydrogen transfer reaction from peptide to 5-FSA and 5-NSA occurs during the MALDI-ISD processes. The hydrogen abstraction from peptides results in the formation of oxidized peptides containing a radical site on the amide nitrogen with subsequent radical-induced cleavage at the {{{C}}_{? }} - {{C}} bond, leading to the formation of a- and x-series ions. The most significant feature of MALDI-ISD with 5-FSA and 5-NSA is the specific cleavage of the {{{C}}_{? }} - {{C}} bond of the peptide backbone without degradation of side-chain and post-translational modifications (PTM). The matrix provides a useful complementary method to conventional MALDI-ISD for amino acid sequencing and site localization of PTMs in peptides.

Asakawa, Daiki; Takayama, Mitsuo

2011-07-01

91

Palladium-catalyzed oxidative arylalkylation of activated alkenes: dual C-H bond cleavage of an arene and acetonitrile.  

PubMed

Not one but two: The title reaction proceeds through the dual C-H bond cleavage of both aniline and acetonitrile. The reaction affords a variety of cyano-bearing indolinones in excellent yield. Mechanistic studies demonstrate that this reaction involves a fast arylation of the olefin and a rate-determining C-H activation of the acetonitrile. PMID:22076660

Wu, Tao; Mu, Xin; Liu, Guosheng

2011-12-23

92

Sc(OTf)3-catalyzed transfer diazenylation of 1,3-dicarbonyls with triazenes via N-N bond cleavage.  

PubMed

A new and efficient method for diazenylation reactions was developed with a Sc(OTf)3-catalyzed nitrogen-nitrogen bond cleavage process with triazenes. The transfer diazenylation reactions accommodate a diverse range of active methylene substrates including simple ketones to give aliphatic azo compounds that are of significant potential as azo prodrugs in high yields under mild conditions. PMID:25295708

Liu, Cong; Lv, Jian; Luo, Sanzhong; Cheng, Jin-Pei

2014-10-17

93

An unusual carbon?carbon bond cleavage reaction during phosphinothricin biosynthesis  

SciTech Connect

Natural products containing phosphorus-carbon bonds have found widespread use in medicine and agriculture. One such compound, phosphinothricin tripeptide, contains the unusual amino acid phosphinothricin attached to two alanine residues. Synthetic phosphinothricin (glufosinate) is a component of two top-selling herbicides (Basta and Liberty), and is widely used with resistant transgenic crops including corn, cotton and canola. Recent genetic and biochemical studies showed that during phosphinothricin tripeptide biosynthesis 2-hydroxyethylphosphonate (HEP) is converted to hydroxymethylphosphonate (HMP). Here we report the in vitro reconstitution of this unprecedented C(sp{sup 3})-C(sp{sup 3}) bond cleavage reaction and X-ray crystal structures of the enzyme. The protein is a mononuclear non-haem iron(II)-dependent dioxygenase that converts HEP to HMP and formate. In contrast to most other members of this family, the oxidative consumption of HEP does not require additional cofactors or the input of exogenous electrons. The current study expands the scope of reactions catalysed by the 2-His-1-carboxylate mononuclear non-haem iron family of enzymes.

Cicchillo, Robert M.; Zhang, Houjin; Blodgett, Joshua A.V.; Whitteck, John T.; Li, Gongyong; Nair, Satish K.; van derDonk, Wilfred A.; Metcalf, William W.; (UIUC)

2010-01-12

94

Restatement of order parameters in biomembranes: calculation of C-C bond order parameters from C-D quadrupolar splittings.  

PubMed Central

An expression for the C-C bond order parameter, SCC, of membrane hydrocarbon chains has been derived from the observed C-D bond order parameters. It allows calculation of the probability of each of the C-C bond rotamers and, consequently, the number of gauche defects per chain as well as their projected average length onto the bilayer normal, thus affording the calculation of accurate hydrophobic bilayer thicknesses. The effect of temperature has been studied on dilauroyl-, dimyristoyl-, and dipalmitoylphosphatidylcholine (DLPC, DMPC, DPPC) membranes, as has the effect of cholesterol on DMPC. The salient results are as follows: 1) an odd-even effect is observed for the SCC versus carbon position, k, whose amplitude increases with temperature; 2) calculation of SCC, from nonequivalent deuterons on the sn-2 chain of lipids, SCC2, leads to negative values, indicating the tendency for the C1-C2 bond to be oriented parallel to the bilayer surface; this bond becomes more parallel to the surface as the temperature increases or when cholesterol is added; 3) calculation on the sn-2 chain length can be performed from C1 to Cn, where n is the number of carbon atoms in the chain, and leads to 10.4, 12.2, and 13.8 A for DLPC, DMPC, and DPPC close to the transition temperature, TC, of each of the systems and to 9.4, 10.9, and 12.6 for T-TC = 30-40 degrees C, respectively; 4) separation of intra- and intermolecular motions allows quantitation of the number of gauche defects per chain, which is equal to 1.9, 2.7, and 3.5 for DLPC, DMPC, and DPPC near TC and to 2.7, 3.5, and 4.4 at T-TC = 30-40 degrees C, respectively. Finally, the validity of our model is discussed and compared with previously published models. PMID:7612816

Douliez, J P; Léonard, A; Dufourc, E J

1995-01-01

95

Unlocking Hydrogenation for C-C Bond Formation: A Brief Overview of Enantioselective Methods  

PubMed Central

Hydrogenation of ?-unsaturated reactants in the presence of carbonyl compounds or imines promotes reductive C-C coupling, providing a byproduct-free alternative to stoichiometric organometallic reagents in an ever-increasing range of C=X (X = O, NR) additions. Under transfer hydrogenation conditions, hydrogen exchange between alcohols and ?-unsaturated reactants triggers generation of electrophile-nucleophile pairs, enabling carbonyl addition directly from the alcohol oxidation level, bypassing discrete alcohol oxidation and generation of stoichiometric byproducts. PMID:22125398

Hassan, Abbas; Krische, Michael J.

2011-01-01

96

Oxidative C-C bond formation (Scholl reaction) with DDQ as an efficient and easily recyclable oxidant.  

PubMed

DDQ in the presence of an acid is known to oxidize a variety of aromatic donors to the corresponding cation radicals. Herein, we now demonstrate that the DDQ/H(+) system can be effectively utilized for the oxidative C-C bond formations or biaryl synthesis. The efficient preparation of a variety of polyaromatic hydrocarbons including graphitic hexa-peri-hexabenzocoronenes, ease of isolation of the clean products, and ready regeneration of DDQ from easily recovered reduced DDQ-H(2) advances the use of DDQ/H(+) for Scholl reactions. PMID:19594139

Zhai, Linyi; Shukla, Ruchi; Rathore, Rajendra

2009-08-01

97

Metal-catalyzed activation of ethers via C-O bond cleavage: a new strategy for molecular diversity.  

PubMed

In 1979, the seminal work of Wenkert set the standards for the utilization of aryl and vinyl ethers as coupling partners via C-O bond-cleavage. Although the topic remained dormant for almost three decades, the last few years have witnessed a renaissance in this area of expertise, experiencing an exponential growth and becoming a significant discipline within the cross-coupling arena. The means to utilize readily accessible aryl or vinyl ethers as counterparts does not only represent a practical, powerful and straightforward alternative to organic halides, but also constitutes an excellent opportunity to improve our chemical knowledge about a relatively unexplored area of expertise. This review summarizes the most significant developments in the area of C-O bond-cleavage when employing aryl or vinyl ethers, providing a detailed overview of the current state of the art and including future aspects, when applicable. PMID:25157613

Cornella, Josep; Zarate, Cayetana; Martin, Ruben

2014-12-01

98

SmI 2\\/H 2O\\/amine promoted reductive cleavage of benzyl-heteroatom bonds: optimization and mechanism  

Microsoft Academic Search

The SmI2\\/H2O\\/pyrrolidine mediated cleavage of benzylic alcohols and benzyl groups was studied and found to be a viable alternative to the Birch reduction yielding the corresponding deoxygenated product in excellent yield. The reaction has been investigated by kinetic methods, and a mechanism involving a pre-complexation of the alcohol to SmI2 followed by an amine mediated electron transfer and subsequent bond

Tobias Ankner; Göran Hilmersson

2009-01-01

99

Total Synthesis of (+)-Roxaticin via C-C Bond Forming Transfer Hydrogenation: A Departure from Stoichiometric Chiral Reagents, Auxiliaries and Premetallated Nucleophiles in Polyketide Construction  

PubMed Central

A total synthesis of the oxo-polyene macrolide (+)-roxaticin is achieved in 20 steps from 1,3-propanediol. In this approach, nine of ten C-C bonds formed in the longest linear sequence are made via metal catalysis, including 7 C-C bonds formed via iridium catalyzed alcohol C-C coupling. Notably, the present synthesis, which represents the most concise preparation of any oxo-polyene macrolide reported to date, is achieved in the absence of chiral reagents, chiral auxiliaries with minimal use of premetallated C-nucleophiles. PMID:20961111

Han, Soo Bong; Hassan, Abbas; Kim, In Su; Krische, Michael J.

2010-01-01

100

Total synthesis of (+)-roxaticin via C-C bond forming transfer hydrogenation: a departure from stoichiometric chiral reagents, auxiliaries, and premetalated nucleophiles in polyketide construction.  

PubMed

A total synthesis of the oxo-polyene macrolide (+)-roxaticin is achieved in 20 steps from 1,3-propanediol. In this approach, 9 of 10 C-C bonds formed in the longest linear sequence are made via metal catalysis, including 7 C-C bonds formed by iridium catalyzed alcohol C-C coupling. Notably, the present synthesis, which represents the most concise preparation of any oxo-polyene macrolide reported to date, is achieved in the absence of chiral reagents and chiral auxiliaries with minimal use of premetalated C-nucleophiles. PMID:20961111

Han, Soo Bong; Hassan, Abbas; Kim, In Su; Krische, Michael J

2010-11-10

101

Presolvated Low Energy Electron Attachment to Peptide Methyl esters in Aqueous Solution: C-O Bond Cleavage at 77K  

PubMed Central

In this study, the reactions of presolvated electrons with glycine methyl ester and N-acetylalanylalanine methyl ester (N-aAAMe) are investigated by electron spin resonance (ESR) spectroscopy and DFT calculations. Electrons were produced by gamma irradiation in neutral 7.5 M LiCl-D2O aqueous glasses at low temperatures. For glycine methyl ester electron addition at 77K results in both N-terminal deamination to form a glycyl radical and C-O ester bond cleavage to form methyl radicals. For samples of N-acetylalanylalanine methyl ester electrons are found to add to the peptide bonds at 77K and cleave the carboxyl ester groups to produce methyl radicals. On annealing to 160K electron adducts at the peptide links undergo chain scission to produce alanyl radicals and further annealing to 170K ?-carbon peptide backbone radicals are produced by hydrogen abstraction. DFT calculations for electron addition to the methyl ester portion of N-aAAMe show the cleavage reaction is highly favorable (free energy equals to ?30.7 kcal/mol) with the kinetic barrier of only 9.9 kcal/mol. A substantial electron affinity of the ester link (38.0 kcal/mol) provides more than sufficient energy to overcome this small barrier. Protonated peptide bond electron adducts, also show favorable N-C chain cleavage reactions of ?12.7 to ?15.5 kcal/mol with a barrier from 7.4 to 10.0 kcal/mol. The substantial adiabatic electron affinity (AEA) of the peptide bond and ester groups provides sufficient energy for the bond dissociation. PMID:23406302

Kheir, Jeanette; Chomicz, Lidia; Engle, Alyson; Rak, Janusz; Sevilla, Michael D.

2013-01-01

102

Low-oxidation state indium-catalyzed C-C bond formation.  

PubMed

The development of innovative metal catalysis for selective bond formation is an important task in organic chemistry. The group 13 metal indium is appealing for catalysis because indium-based reagents are minimally toxic, selective, and tolerant toward various functional groups. Among elements in this group, the most stable oxidation state is typically +3, but in molecules with larger group 13 atoms, the chemistry of the +1 oxidation state is also important. The use of indium(III) compounds in organic synthesis has been well-established as Lewis acid catalysts including asymmetric versions thereof. In contrast, only sporadic examples of the use of indium(I) as a stoichiometric reagent have been reported: to the best of our knowledge, our investigations represent the first synthetic method that uses a catalytic amount of indium(I). Depending on the nature of the ligand or the counteranion to which it is coordinated, indium(I) can act as both a Lewis acid and a Lewis base because it has both vacant p orbitals and a lone pair of electrons. This potential ambiphilicity may offer unique reactivity and unusual selectivity in synthesis and may have significant implications for catalysis, particularly for dual catalytic processes. We envisioned that indium(I) could be employed as a metallic Lewis base catalyst to activate Lewis acidic boron-based pronucleophiles for selective bond formation with suitable electrophiles. Alternatively, indium(I) could serve as an ambiphilic catalyst that activates both reagents at a single center. In this Account, we describe the development of low-oxidation state indium catalysts for carbon-carbon bond formation between boron-based pronucleophiles and various electrophiles. We discovered that indium(I) iodide was an excellent catalyst for ?-selective allylations of C(sp(2)) electrophiles such as ketones and hydrazones. Using a combination of this low-oxidation state indium compound and a chiral semicorrin ligand, we developed catalytic highly enantioselective allylation, crotylation, and ?-chloroallylation reactions of hydrazones. These transformations proceeded with rare constitutional selectivities and remarkable diastereoselectivities. Furthermore, indium(I) triflate served as the most effective catalyst for allylations and propargylations of C(sp(3)) electrophiles such as O,O-acetals, N,O-aminals, and ethers, and we applied this methodology to carbohydrate chemistry. In addition, a catalyst system composed of indium(I) chloride and a chiral silver BINOL-phosphate facilitated the highly enantioselective allylation and allenylation of N,O-aminals. Overall, these discoveries demonstrate the versatility, efficiency, and sensitivity of low-oxidation state indium catalysts in organic synthesis. PMID:22626010

Schneider, Uwe; Kobayashi, Shu

2012-08-21

103

Photosensitized oxidation of alkyl phenyl sulfoxides. C-S bond cleavage in alkyl phenyl sulfoxide radical cations.  

PubMed

The 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ(+)ClO4(-))-photosensitized oxidation of phenyl alkyl sulfoxides (PhSOCR1R2R3, 1, R1 = R2 = H, R3 = Ph; 2, R1 = H, R2 = Me, R3 = Ph; 3, R1 = R2 = Ph, R3 = H; 4, R1 = R2 = Me, R3 = Ph; 5, R1 = R2 = R3 = Me) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Steady-state photolysis showed the formation of products deriving from the heterolytic C-S bond cleavage in the sulfoxide radical cations (alcohols, R1R2R3COH, and acetamides, R1R2R3CNHCOCH3) accompanied by sulfur-containing products (phenyl benzenethiosulfinate, diphenyl disulfide, and phenyl benzenethiosulfonate). By laser irradiation, the formation of 3-CN-NMQ(*) (lambda(max) = 390 nm) and sulfoxide radical cations 1(*+) , 2(*+), and 5(*+) (lambda(max) = 550 nm) was observed within the laser pulse. The radical cations decayed by first-order kinetics with a process attributable to the heterolytic C-S bond cleavage leading to the sulfinyl radical and an alkyl carbocation. The radical cations 3(*+) and 4(*+) fragment too rapidly, decaying within the laser pulse. The absorption band of the cation Ph2CH(+) (lambda(max) = 440 nm) was observed with 3 while the absorption bands of 3-CN-NMQ(*) and PhSO(*) (lambda(max) = 460 nm) were observed just after the laser pulse in the LFP experiment with 4. No competitive beta-C-H bond cleavage has been observed in the radical cations from 1-3. The C-S bond cleavage rates were measured for 1(*+), 2(*+), and 5(*+). For 3(*+) and 4(*+), only a lower limit (ca. >3 x 10(7) s(-1)) could be given. Quantum yields (Phi) and fragmentation first-order rate constants (k) appear to depend on the structure of the alkyl group and on the bond dissociation free energy (BDFE) of the C-S bond of the radical cations determined by a thermochemical cycle using the C-S BDEs for the neutral sulfoxides 1-5 obtained by DFT calculations. Namely, Phi and k increase as the C-S BDFE becomes more negative, that is in the order 1 < 5 < 2 < 3, 4, which is also the stability order of the alkyl carbocations formed in the cleavage. An estimate of the difference in the C-S bond cleavage rate between sulfoxide and sulfide radical cations was possible by comparing the fragmentation rate of 5(*+) (1.4 x 10(6) s(-1)) with the upper limit (10(4) s(-1)) given for tert-butyl phenyl sulfide radical cation (Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Org. Lett. 2006, 8, 641-644). It turns out that sulfoxide radical cations undergo C-S bond breaking at a rate at least 2 orders of magnitude faster than that of corresponding sulfide radical cations. PMID:18578497

Baciocchi, Enrico; Del Giacco, Tiziana; Lanzalunga, Osvaldo; Mencarelli, Paolo; Procacci, Barbara

2008-08-01

104

Aromatic Cations from Oxidative Carbon–Hydrogen Bond Cleavage in Bimolecular Carbon–Carbon Bond Forming Reactions  

PubMed Central

Chromenes and isochromenes react quickly with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to form persistent aromatic oxocarbenium ions through oxidative carbon–hydrogen cleavage. This process is tolerant of electron-donating and electron-withdrawing groups on the benzene ring and additional substitution on the pyran ring. A variety of nucleophiles can be added to these cations to generate a diverse set of structures. PMID:22780559

Clausen, Dane J.

2012-01-01

105

Purification and characterization of carbon-phosphorus bond-cleavage enzyme from glyphosate degrading Pseudomonas putida T5.  

PubMed

An inducible, carbon-phosphorus bond-cleavage enzyme was purified from cells of Pseudomonas putida T5 grown on N-phosphonomethyl glycine. The native enzyme had a molecular mass of approximately 70 kD and upon sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), yielded a homogeneous protein band with an apparent molecular mass of about 70 kD. Activity of purified enzyme was increased by 627-fold compared to the crude extract and showed pH and temperature optima of approximately 7 and 30°C, respectively. The purified enzyme had an apparent Km and Vmax of 3.7 mM and 6.8 mM/min, respectively, for its sole substrate N-phosphonomethyl glycine. The enzyme was inhibited by phenylmethylsulfonyl fluoride (PMSF), indicating the presence of serine at the active site. The enzyme was not inhibited by SDS, suggesting the absence of disulfide linkage in the enzyme. The enzyme was found to be inhibited by most of the metals studied except Mg(2+). Detergents studied also inhibited glyphosate acting as a carbon-phosphorus bond-cleavage enzyme. Thus initial characterization of the purified enzyme suggested that it could be used as a potential candidate for glyphosate bioremediation. PMID:24840030

Selvi, A Arul; Manonmani, H K

2015-01-01

106

A comparison of dioxygen bond-cleavage in ribonucleotide reductase (RNR) and methane monooxygenase (MMO)  

NASA Astrophysics Data System (ADS)

The O 2 cleavage by the iron dimer complex in RNR is experimentally found to be at least 400 times faster than the one in MMO, in spite of very similar iron dimers. The origin of this difference is investigated using the hybrid density functional method B3LYP with models containing about 60 atoms. The calculations support a suggestion that this difference could be due to the presence of a second sphere tryptophan (Trp48) in RNR.

Siegbahn, Per E. M.

2002-01-01

107

Light-mediated heterogeneous cross dehydrogenative coupling reactions: metal oxides as efficient, recyclable, photoredox catalysts in C-C bond-forming reactions.  

PubMed

Let there be light: A heterogeneous photocatalytic system based on easily recyclable TiO(2) or ZnO allows cross dehydrogenative coupling reactions of tertiary amines. The newly developed protocols have successfully been applied to various C-C and C-P bond-forming reactions to provide nitro amines as well as amino ketones, nitriles and phosphonates. PMID:22314870

Rueping, Magnus; Zoller, Jochen; Fabry, David C; Poscharny, Konstantin; Koenigs, René M; Weirich, Thomas E; Mayer, Joachim

2012-03-19

108

Importance of Protein Dynamics during Enzymatic C–H Bond Cleavage Catalysis  

PubMed Central

Quantum tunneling and protein dynamics have emerged as important components of enzyme function. This review focuses on soybean lipoxygenase-1, to illustrate how the properties of enzymatic C–H bond activation link protein motions to the fundamental bond making–breaking processes. PMID:23373460

Klinman, Judith P.

2014-01-01

109

Hydrogen-bonding effects on the reactivity of [X-Fe(III)-O-Fe(IV)?O] (X = OH, F) complexes toward C-H bond cleavage.  

PubMed

Complexes 1-OH and 1-F are related complexes that share similar [X-Fe(III)-O-Fe(IV)?O](3+) core structures with a total spin S of ½, which arises from antiferromagnetic coupling of an S = 5/2 Fe(III)-X site and an S = 2 Fe(IV)?O site. EXAFS analysis shows that 1-F has a nearly linear Fe(III)-O-Fe(IV) core compared to that of 1-OH, which has an Fe-O-Fe angle of ~130° due to the presence of a hydrogen bond between the hydroxo and oxo groups. Both complexes are at least 1000-fold more reactive at C-H bond cleavage than 2, a related complex with a [OH-Fe(IV)-O-Fe(IV)?O](4+) core having individual S = 1 Fe(IV) units. Interestingly, 1-F is 10-fold more reactive than 1-OH. This raises an interesting question about what gives rise to the reactivity difference. DFT calculations comparing 1-OH and 1-F strongly suggest that the H-bond in 1-OH does not significantly change the electrophilicity of the reactive Fe(IV)?O unit and that the lower reactivity of 1-OH arises from the additional activation barrier required to break its H-bond in the course of H-atom transfer by the oxoiron(IV) moiety. PMID:23496330

Xue, Genqiang; Geng, Caiyun; Ye, Shengfa; Fiedler, Adam T; Neese, Frank; Que, Lawrence

2013-04-01

110

Hydrogen bonding effects on the reactivity of [X-FeIII–O–FeIV=O] (X = OH, F) complexes towards C–H bond cleavage  

PubMed Central

Complexes 1-OH and 1-F are related complexes that share similar [X–FeIII–O–FeIV=O]3+ core structures with a total spin S of ½, which arises from antiferromagnetic coupling of an S = 5/2 FeIII-X site and an S = 2 FeIV=O site. EXAFS analysis shows that 1-F has a nearly linear FeIII–O–FeIV core compared to that of 1-OH, which has an Fe–O–Fe angle of ~130° due to the presence of a hydrogen bond between the hydroxo and oxo groups. Both complexes are at least a thousand-fold more reactive at C–H bond cleavage than 2, a related complex with a [OH–FeIV–O–FeIV=O]4+ core having individual S = 1 FeIV units. Interestingly, 1-F is tenfold more reactive than 1-OH. This raises an interesting question about what gives rise to the reactivity difference. DFT calculations comparing 1-OH and 1-F strongly suggest that the H-bond in 1-OH does not significantly change the electrophilicity of the reactive FeIV=O unit and that the lower reactivity of 1-OH arises from the additional activation barrier required to break its H-bond in the course of H-atom transfer by the oxoiron(IV) moiety. PMID:23496330

Xue, Genqiang; Geng, Caiyun; Ye, Shengfa; Fiedler, Adam T.; Neese, Frank; Que, Lawrence

2013-01-01

111

Reaction of HppE with substrate analogues: evidence for carbon-phosphorus bond cleavage by a carbocation rearrangement.  

PubMed

(S)-2-hydroxypropylphosphonic acid ((S)-2-HPP) epoxidase (HppE) is an unusual mononuclear non-heme iron enzyme that catalyzes the oxidative epoxidation of (S)-2-HPP in the biosynthesis of the antibiotic fosfomycin. Recently, HppE has been shown to accept (R)-1-hydroxypropylphosphonic acid as a substrate and convert it to an aldehyde product in a reaction involving a biologically unprecedented 1,2-phosphono migration. In this study, a series of substrate analogues were designed, synthesized, and used as mechanistic probes to study this novel enzymatic transformation. The resulting data, together with insights obtained from density functional theory calculations, are consistent with a mechanism of HppE-catalyzed phosphono group migration that involves the formation of a carbocation intermediate. As such, this reaction represents a new paradigm for biological C-P bond cleavage. PMID:23672451

Chang, Wei-chen; Mansoorabadi, Steven O; Liu, Hung-wen

2013-06-01

112

The Reaction of HppE with Substrate Analogues: Evidence for Carbon-Phosphorus Bond Cleavage by a Carbocation Rearrangement  

PubMed Central

(S)-2-Hydroxypropylphosphonic acid ((S)-2-HPP) epoxidase (HppE) is an unusual mononuclear non-heme iron enzyme that catalyzes the oxidative epoxidation of (S)-2-HPP in the biosynthesis of the antibiotic fosfomycin. Recently, HppE has been shown to accept (R)-1-hydroxypropylphosphonic acid ((R)-1-HPP) as a substrate and convert it to an aldehyde product in a reaction involving a biologically unprecedented 1,2-phosphono migration. In this study, a series of substrate analogues were designed, synthesized, and used as mechanistic probes to study this novel enzymatic transformation. The resulting data, together with insights obtained from density functional theory calculations, are consistent with a mechanism of HppE-catalyzed phosphono group migration that involves the formation of a carbocation intermediate. As such, this reaction represents a new paradigm for biological C-P bond cleavage. PMID:23672451

Chang, Wei-chen; Mansoorabadi, Steven O.; Liu, Hung-wen

2013-01-01

113

Effect of water on hydrolytic cleavage of non-terminal ?-glycosidic bonds in cyclodextrins to generate monosaccharides and their derivatives in a dimethyl sulfoxide-water mixture.  

PubMed

Hydrolytic cleavage of the non-terminal ?-1,4-glycosidic bonds in ?-, ?-, and ?-cyclodextrins and the anomeric-terminal one in d-maltose was investigated to examine how the cleavage rate for ?-, ?-, and ?-cyclodextrins is slower than that for d-maltose. Effects of water and temperature were studied by applying in situ (13)C NMR spectroscopy and using a dimethyl sulfoxide (DMSO)-water mixture over a wide range of water mole fraction, xw = 0.004-1, at temperatures of 120-180 °C. The cleavage rate constant for the non-anomeric glycosidic bond was smaller by a factor of 6-10 than that of the anomeric-terminal one. The glycosidic-bond cleavage is significantly accelerated through the keto-enol tautomerization of the anomeric-terminal d-glucose unit into the d-fructose one. The smaller the size of the cyclodextrin, the easier the bond cleavage due to the ring strain. The remarkable enhancement in the cleavage rate with decreasing water content was observed for the cyclodextrins and d-maltose as well as d-cellobiose. This shows the important effect of the solitary water whose hydrogen bonding to other water molecules is prohibited by the presence of the organic dipolar aprotic solvent, DMSO, and which has more naked partial charges and higher reactivity. A high 5-hydroxymethyl-2-furaldehyde (5-HMF) yield of 64% was attained in a non-catalytic conversion by tuning the water content to xw = 0.30, at which the undesired polymerization by-paths can be most effectively suppressed. This study provides a step toward designing a new optimal, earth-benign generation process of 5-HMF starting from biomass. PMID:24527672

Kimura, Hiroshi; Hirayama, Masaki; Yoshida, Ken; Uosaki, Yasuhiro; Nakahara, Masaru

2014-02-27

114

Luminescent Pincer Platinum(II) Complexes with Emission Quantum Yields up to Almost Unity: Photophysics, Photoreductive C?C Bond Formation, and Materials Applications.  

PubMed

Luminescent pincer-type Pt(II) ?complexes supported by C-deprotonated ?-extended tridentate R?C^N^N?R' ligands and pentafluorophenylacetylide ligands show emission quantum yields up to almost unity. Femtosecond time-resolved fluorescence measurements and time-dependent DFT calculations together reveal the dependence of excited-state structural distortions of [Pt(R?C^N^N?R')(C?C-C6 F5 )] on the positional isomers of the tridentate ligand. Pt?complexes [Pt(R-C^N^N?R')(C?C-Ar)] are efficient photocatalysts for visible-light-induced reductive C?C bond formation. The [Pt(R-C^N^N?R')(C?C-C6 F5 )] complexes perform strongly as phosphorescent dopants for green- and red-emitting organic light-emitting diodes (OLEDs) with external quantum efficiency values over 22.1?%. These complexes are also applied in two-photon cellular imaging when incorporated into mesoporous silica nanoparticles (MSNs). PMID:25581564

Chow, Pui-Keong; Cheng, Gang; Tong, Glenna So Ming; To, Wai-Pong; Kwong, Wai-Lun; Low, Kam-Hung; Kwok, Chi-Chung; Ma, Chensheng; Che, Chi-Ming

2015-02-01

115

CuI-catalyzed C-N bond formation and cleavage for the synthesis of benzimidazo[1,2-a]quinazoline derivatives.  

PubMed

A copper(I)-catalyzed domino reaction of N-(2-benzimidazolyl)-2-aminobenzamide and 2-halogenated benzaldehyde has been studied. The procedure is based on a sequential CuI-catalyzed Ullmann reaction (C-N bond formation) and two bond cleavage reactions and provides an efficient strategy for the synthesis of benzimidazo[1,2-a]quinazolines catalyzed by CuI/L-proline. PMID:24885515

Li, Chao; Zhang, Wen-Ting; Wang, Xiang-Shan

2014-06-20

116

Mechanistic Investigation of Catalytic Carbon-Carbon Bond Activation and Formation by Platinum and Palladium Phosphine  

E-print Network

Mechanistic Investigation of Catalytic Carbon-Carbon Bond Activation and Formation by Platinum impede their development. First, the metal- carbon bond resulting from C-C insertion has been calculated often results in thermal decomposition of the metal complex prior to C-C cleavage. To date, most

Jones, William D.

117

Substituent-controlled selective synthesis of N-acyl 2-aminothiazoles by intramolecular zwitterion-mediated C-N bond cleavage.  

PubMed

The cleavage of C-N bonds is an interesting and challenging subject in modern organic synthesis. We have achieved the first zwitterion-controlled C-N bond cleavage in the MCR reaction among lithium alkynethiolates, bulky carbodiimides, and acid chlorides to construct N-acyl 2-aminothiazoles. This is a simple, highly efficient, and general method for the preparation of N-acyl 2-aminothiazoles with a broad range of substituents. The selective synthesis of N-acyl 2-aminothiazoles significantly depends on the steric hindrance of carbodiimides. The result is in striking contrast with our previous convergent reaction giving 5-acyl-2-iminothiazolines via 1,5-acyl migration. It is indeed interesting that the slight change of the substituents on the carbodiimides can completely switch the product structure. Experimental and theoretical results demonstrate the reason why the C-N bond cleavage in the present system is prior to the acyl migration. The intramolecular hydrogen relay via unprecedented Hofmann-type elimination is essential for this totally new zwitterion-controlled C-N bond cleavage. PMID:25361067

Wang, Yang; Zhao, Fei; Chi, Yue; Zhang, Wen-Xiong; Xi, Zhenfeng

2014-11-21

118

Mechanism-based inhibition of HsaD: a C-C bond hydrolase essential for survival of Mycobacterium tuberculosis in macrophage.  

PubMed

Mycobacterium tuberculosis remains the leading cause of death by a bacterial pathogen worldwide. Increasing prevalence of multidrug-resistant organisms means prioritizing identification of targets for antituberculars. 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate hydrolase (HsaD), part of the cholesterol metabolism operon, is vital for survival within macrophage. The C-C bond hydrolase, HsaD, has a serine protease-like catalytic triad. We tested a range of serine protease and esterase inhibitors for their effects on HsaD activity. As well as providing a potential starting point for drug development, the data provides evidence for the mechanism of C-C bond hydrolysis. This screen also provides a route to initiate development of fragment-based inhibitors. PMID:24164668

Ryan, Ali; Keany, Sebastian; Eleftheriadou, Olga; Ballet, Romain; Cheng, Hung-Yuan; Sim, Edith

2014-01-01

119

A high speed parallel synthesis of 1,2-diaryl-1-ethanones via a clean-chemistry C–C bond formation reaction  

Microsoft Academic Search

In this report, we describe the parallel as well as conventional synthesis of 1,2-diaryl-1-ethanones via environmentally benign acylation of arenes with in situ generated arylacetyl trifluoroacetates. A wide variety of arylacetic acids I participated in trifluoroacetic anhydride\\/ phosphoric acid mediated C–C bond formation reaction when reacted with arenes of type II to give 1,2-diaryl-1-ethanones III in good to excellent yield.

Venugopal Rao Veeramaneni; Manojit Pal; Koteswar Rao Yeleswarapu

2003-01-01

120

Transition-metal-free oxidative carboazidation of acrylamides via cascade C-N and C-C bond-forming reactions.  

PubMed

A novel transition-metal-free oxidative carboazidation of acrylamides using inexpensive NaN3 and K2S2O8 was achieved, which not only provided an efficient method to prepare various N3-substituted oxindoles, but also represented a novel strategy for C-N and C-C bond formation via a free-radical cascade process. This transformation exhibits excellent functional group tolerance, affording the desired oxindoles in good to excellent yields. PMID:24854242

Qiu, Jun; Zhang, Ronghua

2014-07-01

121

Design and synthesis of chiral Zn2+ complexes mimicking natural aldolases for catalytic C-C bond forming reactions in aqueous solution.  

PubMed

Extending carbon frameworks via a series of C-C bond forming reactions is essential for the synthesis of natural products, pharmaceutically active compounds, active agrochemical ingredients, and a variety of functional materials. The application of stereoselective C-C bond forming reactions to the one-pot synthesis of biorelevant compounds is now emerging as a challenging and powerful strategy for improving the efficiency of a chemical reaction, in which some of the reactants are subjected to successive chemical reactions in just one reactor. However, organic reactions are generally conducted in organic solvents, as many organic molecules, reagents, and intermediates are not stable or soluble in water. In contrast, enzymatic reactions in living systems proceed in aqueous solvents, as most of enzymes generally function only within a narrow range of temperature and pH and are not so stable in less polar organic environments, which makes it difficult to conduct chemoenzymatic reactions in organic solvents. In this review, we describe the design and synthesis of chiral metal complexes with Zn2+ ions as a catalytic factor that mimic aldolases in stereoselective C-C bond forming reactions, especially for enantioselective aldol reactions. Their application to chemoenzymatic reactions in aqueous solution is also presented. PMID:24481060

Itoh, Susumu; Sonoike, Shotaro; Kitamura, Masanori; Aoki, Shin

2014-01-01

122

Design and Synthesis of Chiral Zn2+ Complexes Mimicking Natural Aldolases for Catalytic C–C Bond Forming Reactions in Aqueous Solution  

PubMed Central

Extending carbon frameworks via a series of C–C bond forming reactions is essential for the synthesis of natural products, pharmaceutically active compounds, active agrochemical ingredients, and a variety of functional materials. The application of stereoselective C–C bond forming reactions to the one-pot synthesis of biorelevant compounds is now emerging as a challenging and powerful strategy for improving the efficiency of a chemical reaction, in which some of the reactants are subjected to successive chemical reactions in just one reactor. However, organic reactions are generally conducted in organic solvents, as many organic molecules, reagents, and intermediates are not stable or soluble in water. In contrast, enzymatic reactions in living systems proceed in aqueous solvents, as most of enzymes generally function only within a narrow range of temperature and pH and are not so stable in less polar organic environments, which makes it difficult to conduct chemoenzymatic reactions in organic solvents. In this review, we describe the design and synthesis of chiral metal complexes with Zn2+ ions as a catalytic factor that mimic aldolases in stereoselective C–C bond forming reactions, especially for enantioselective aldol reactions. Their application to chemoenzymatic reactions in aqueous solution is also presented. PMID:24481060

Itoh, Susumu; Sonoike, Shotaro; Kitamura, Masanori; Aoki, Shin

2014-01-01

123

Promotion of exocyclic bond cleavages in the decomposition of 1,3-disilacyclobutane in the presence of a metal filament.  

PubMed

The primary decomposition of 1,3-disilacyclobutane (DSCB) on a tungsten filament and its secondary gas-phase reactions in a hot-wire chemical vapor deposition (CVD) reactor have been studied using laser ionization mass spectrometry. Under the collision-free conditions, DSCB decomposes on the W filament to produce H2 molecules with an activation energy of 43.6 ± 4.1 kJ·mol(-1). With the help of the isotope labeling and chemical trapping methods, the mechanistic details in the secondary gas-phase reactions important in the hot-wire CVD reactor setup have been examined. The dominant pathway has been demonstrated to be the insertion of the cyclic 1,3-disilacyclobut-1-ylidene, generated by exocyclic Si-H bond rupture, into the Si-H bond in DSCB to form 1,1'-bis(1,3-disilacyclobutane) (174 amu). The successful trapping of 1,3-disilacyclobut-1-ylidene by both 1,3-butadiene and trimethylsilane provides compelling evidence for the existence of this cyclic silylene species in the hot-wire CVD reactor with DSCB. Other reactions operating in the reactor include the DSCB cycloreversion to form silene and the ring opening of DSCB via 1,2-H shift to produce silene/methylsilylene and 1-methylsilene/silylene. The introduction of an additional Si atom in the four-membered ring monosilacyclobutane molecule has caused two major changes in the reaction chemistry assumed by DSCB: (1) The endocyclic cycloreversion reactions that dominate in the decomposition of monosilacyclobutane molecules only play a much less important role in the dissociation of DSCB; and (2) the exocyclic bond cleavages are promoted in DSCB due to the ring stabilization caused by the introduction of one additional Si atom. PMID:25560235

Badran, I; Shi, Y J

2015-01-29

124

Substrate-triggered activation of a synthetic [Fe2(?-O)2] diamond core for C-H bond cleavage.  

PubMed

An [Fe(IV)(2)(?-O)(2)] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C-H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Because of the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe(2)(?-O)(2)] cores. We report here that water or alcohol substrates can activate synthetic [Fe(III)Fe(IV)(?-O)(2)] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C-H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [Fe(III)Fe(IV)(?-O)(2)] core, resulting in the formation of a more reactive species with a [X-Fe(III)-O-Fe(IV)?O] ring-opened structure (1-X, 2-X, X = OH(-) or OR(-)). Treatment of 2 with methoxide at -80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 Fe(III)/S = 2 Fe(IV)] pair. Even at this low temperature, the complex undergoes facile intramolecular C-H bond cleavage to generate formaldehyde, showing that the terminal high-spin Fe(IV)?O unit is capable of oxidizing a C-H bond as strong as 96 kcal mol(-1). This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C-H bond (D(C-H) 81 kcal mol(-1)). The activation of the [Fe(III)Fe(IV)(?-O)(2)] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second-order rate constant that is 3.6 × 10(7)-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 Fe(IV)?O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [Fe(IV)(2)(?-O)(2)] diamond core of sMMO-Q to selectively attack the strong C-H bond of methane in the presence of weaker C-H bonds of amino acid residues that define the diiron active site pocket. PMID:21899336

Xue, Genqiang; Pokutsa, Alexander; Que, Lawrence

2011-10-19

125

Efficient C-C bond splitting on Pt monolayer and sub-monolayer catalysts during ethanol electro-oxidation: Pt layer strain and morphology effects.  

PubMed

Efficient catalytic C-C bond splitting coupled with complete 12-electron oxidation of the ethanol molecule to CO2 is reported on nanoscale electrocatalysts comprised of a Pt monolayer (ML) and sub-monolayer (sML) deposited on Au nanoparticles (Au@Pt ML/sML). The Au@Pt electrocatalysts were synthesized using surface limited redox replacement (SLRR) of an underpotentially deposited (UPD) Cu monolayer in an electrochemical cell reactor. Au@Pt ML showed improved catalytic activity for ethanol oxidation reaction (EOR) and, unlike their Pt bulk and Pt sML counterparts, was able to generate CO2 at very low electrode potentials owing to efficient C-C bond splitting. To explain this, we explore the hypothesis that competing strain effects due to the Pt layer coverage/morphology (compressive) and the Pt-Au lattice mismatch (tensile) control surface chemisorption and overall activity. Control experiments on well-defined model Pt monolayer systems are carried out involving a wide array of methods such as high-energy X-ray diffraction, pair-distribution function (PDF) analysis, in situ electrochemical FTIR spectroscopy, and in situ scanning tunneling microscopy. The vibrational fingerprints of adsorbed CO provide compelling evidence on the relation between surface bond strength, layer strain and morphology, and catalytic activity. PMID:25081353

Loukrakpam, Rameshwori; Yuan, Qiuyi; Petkov, Valeri; Gan, Lin; Rudi, Stefan; Yang, Ruizhi; Huang, Yunhui; Brankovic, Stanko R; Strasser, Peter

2014-09-21

126

Sulfur Dioxide Activation: A Theoretical Investigation into Dual S?O Bond Cleavage by Three-Coordinate Molybdenum(III) Complexes.  

PubMed

Cummins et al. have observed that 3 equiv of Mo(N[R]Ar)3 (R = C(CD3)2CH3, Ar = 3,5-C6H3Me2) are required for dual S?O bond cleavage within a SO2 molecule. Using density functional theory calculations, this theoretical study investigates a mechanism for this SO2 cleavage reaction that is mediated by MoL3, where L = NH2 or N[(t)Bu]Ph. Our results indicate that an electron transfers into the SO2 ligand, which leads to Mo oxidation and initiates SO2 coordination along the quartet surface. The antiferromagnetic (AF) nature of the (NH2)3Mo-SO2 adduct accelerates intersystem crossing onto the doublet surface. The first S?O bond cleavage occurs from the resulting doublet adduct and leads to formation of L3Mo?O and SO. Afterward, the released SO molecule is cleaved by the two remaining MoL3, resulting in formation of L3Mo?S and an additional L3Mo?O. This mononuclear mechanism is calculated to be strongly exothermic and proceeds via a small activation barrier, which is in accordance with experimental results. An additional investigation into a binuclear process for this SO2 cleavage reaction was also evaluated. Our results show that the binuclear mechanism is less favorable than that of the mononuclear mechanism. PMID:25559336

Robinson, Robert; Abbasi, Kiana Khadem; Ariafard, Alireza; Stranger, Robert; Yates, Brian F

2015-01-20

127

Reactions in 1,1,1-trifluoroacetone triggered by low energy electrons (0-10 eV): from simple bond cleavages to complex unimolecular reactions.  

PubMed

The impact of low energy electrons (0-10 eV) to 1,1,1-trifluoroacetone yields a variety of fragment anions which are formed via dissociative electron attachment (DEA) through three pronounced resonances located at 0.8 eV, near 4 eV, and in the energy range 8-9 eV. The fragment ions arise from different reactions ranging from the direct cleavage of one single or double bond (formation of F(-), CF3(-), O(-), (M-H)(-), and M-F)(-)) to remarkably complex unimolecular reactions associated with substantial geometric and electronic rearrangement in the transitory intermediate (formation of OH(-), FHF(-), (M-HF)(-), CCH(-), and HCCO(-). The ion CCH(-), for example, is formed by an excision of unit from the target molecule through the concerted cleavage of four bonds and recombination to H2O within the neutral component of the reaction. PMID:24828451

Illenberger, Eugen; Meinke, Martina C

2014-08-21

128

Synthesis of 1-amino-2-aroyl/acetylnaphthalenes through a base mediated one pot inter and intramolecular C-C bond formation strategy.  

PubMed

A new precursor 2-(1-cyano-2,2-bis(methylthio)vinyl)benzonitrile has been synthesized by the reaction of 2-cyanomethylbenzonitrile, carbon disulfide and methyl iodide under basic conditions. The reaction of 2-(1-cyano-2,2-bis(methylthio)vinyl)benzonitrile with various functionalized aryl/heteroaryl methyl ketones or acetone under basic conditions afforded 4-amino-3-aroyl/heteroaroyl/acetyl-2-methylsulfanylnaphthalene-1-carbonitriles in good yields through a (5C + 1C) annulation strategy; this involves sequential intermolecular, followed by intramolecular, C-C bond formation reactions. The structure of the product was confirmed by single crystal X-ray crystallography. PMID:24871917

Singh, Surjeet; Yadav, Pratik; Sahu, Satya Narayan; Althagafi, Ismail; Kumar, Abhinav; Kumar, Brijesh; Ram, Vishnu Ji; Pratap, Ramendra

2014-07-14

129

Naphthoquinone-directed C-H annulation and C(sp³)-H bond cleavage: one-pot synthesis of tetracyclic naphthoxazoles.  

PubMed

One-pot synthesis of tetracyclic naphthoxazole derivatives from electron-deficient naphthoquinones and alkynes was achieved via Rh(III)-catalyzed C-H activation and C(sp(3))-H bond cleavage for the first time. This approach proceeds through a tandem cascade process involving substrate tautomerization, C-H activation, oxidative addition, cyclization, and aromatization. In addition, broad substrate scope, simple starting materials, and steric tolerance make this strategy of great practicality. PMID:24746121

Wang, Meining; Zhang, Chi; Sun, Li-Ping; Ding, Chunyong; Zhang, Ao

2014-05-16

130

Metal-Free Reductive Cleavage of C–N and S–N Bonds by Photoactivated Electron Transfer from a Neutral Organic Donor**  

PubMed Central

A photoactivated neutral organic super electron donor cleaves challenging arenesulfonamides derived from dialkylamines at room temperature. It also cleaves a) ArC–NR and b) ArN–C bonds. This study also highlights the assistance given to these cleavage reactions by the groups attached to N in (a) and to C in (b), by lowering LUMO energies and by stabilizing the products of fragmentation. PMID:24311295

O'Sullivan, Steven; Doni, Eswararao; Tuttle, Tell; Murphy, John A

2014-01-01

131

Copper-catalyzed aerobic oxidation and cleavage/formation of C-S bond: a novel synthesis of aryl methyl sulfones from aryl halides and DMSO.  

PubMed

With atmospheric oxygen as the oxidant, a novel copper(I)-catalyzed synthesis of aryl methyl sulfones from aryl halides and widely available DMSO is described. The procedure tolerates aryl halides with various functional groups (such as methoxy, acetyl, chloro, fluoro and nitro groups), which could afford aryl methyl sulfones in moderate to high yields. The copper-catalyzed aerobic oxidation and the cleavage/formation of C-S bond are the key steps for this transformation. PMID:22728918

Yuan, Gaoqing; Zheng, Junhua; Gao, Xiaofang; Li, Xianwei; Huang, Liangbin; Chen, Huoji; Jiang, Huanfeng

2012-08-01

132

Analysis of C–F bond cleavages in methylfluorobenzoates—Fragmentation and dimerization of anion radicals using convolution potential sweep voltammetry  

Microsoft Academic Search

The electrochemical reduction of methylfluorobenzoates at glassy carbon electrodes is analyzed using the convolution potential sweep voltammetry (CPSV). The stabilization of the radical anion due to the electron-withdrawing group is shown to lead to intra-molecular stepwise dissociative electron transfer. While methyl 2-fluorobenzoate (ortho isomer) follows EC mechanism, the methyl 4-fluorobenzoate (para-isomer) undergoes electro-dimerization prior to C–F bond cleavage. The first

A. Muthukrishnan; M. V. Sangaranarayanan

2010-01-01

133

Rhodium-catalyzed acyl-transfer reaction between benzyl ketones and thioesters: synthesis of unsymmetric ketones by ketone CO-C bond cleavage and intermolecular rearrangement.  

PubMed

In the presence of catalytic amounts of RhH(CO)(PPh3)3 and 1,2-bis(diphenylphosphino)benzene (dppBz), acyl groups were transferred between benzyl ketones and thioesters/aryl esters. The rhodium complex catalyzed the cleavage of ketone CO-C bonds and intermolecular rearrangement giving unsymmetric ketones. The acyl-transfer reaction also occurred with 1-(p-chlorophenyl)-3-(p-cyanophenyl)propane-2-one giving unsymmetric ketones. PMID:22780710

Arisawa, Mieko; Kuwajima, Manabu; Toriyama, Fumihiko; Li, Guangzhe; Yamaguchi, Masahiko

2012-07-20

134

Demonstration of the heterolytic O-O bond cleavage of putative nonheme iron(II)-OOH(R) complexes for Fenton and enzymatic reactions.  

PubMed

One-electron reduction of mononuclear nonheme iron(III) hydroperoxo (Fe(III)-OOH) and iron(III) alkylperoxo (Fe(III)-OOR) complexes by ferrocene (Fc) derivatives resulted in the formation of the corresponding iron(IV) oxo complexes. The conversion rates were dependent on the concentration and oxidation potentials of the electron donors, thus indicating that the reduction of the iron(III) (hydro/alkyl)peroxo complexes to their one-electron reduced iron(II) (hydro/alkyl)peroxo species is the rate-determining step, followed by the heterolytic O-O bond cleavage of the putative iron(II) (hydro/alkyl)peroxo species to give the iron(IV) oxo complexes. Product analysis supported the heterolytic O-O bond-cleavage mechanism. The present results provide the first example showing the one-electron reduction of iron(III) (hydro/alkyl)peroxo complexes and the heterolytic O-O bond cleavage of iron(II) (hydro/alkyl)peroxo species to form iron(IV) oxo intermediates which occur in nonheme iron enzymatic and Fenton reactions. PMID:24916304

Bang, Suhee; Park, Sora; Lee, Yong-Min; Hong, Seungwoo; Cho, Kyung-Bin; Nam, Wonwoo

2014-07-21

135

Cobryketone derived from vitamin B12 via palladium-catalyzed cleavage of the sp3-sp3 carbon-carbon bond.  

PubMed

Heptamethyl cobyrinate was transformed into hexamethyl 8-nor-cobyrinate. The crucial step involved the synthesis of new, vitamin B12 derived cobryketone via palladium-catalyzed cleavage of the sp(3)-sp(3) carbon-carbon bond with the liberation of the ketone. The replacement of sp(3) carbon atom with sp(2) (C?O) at the 8-position produces a bathochromic shift of all absorption bands and makes ? and ? bands equal as a consequence of the expansion of the existing conjugated system of double bonds. PMID:23544362

Kurco?, Sylwester; Proinsias, Keith ó; Gryko, Dorota

2013-04-19

136

Active Metal Brazing and Adhesive Bonding of Titanium to C/C Composites for Heat Rejection System  

NASA Technical Reports Server (NTRS)

Robust assembly and integration technologies are critically needed for the manufacturing of heat rejection system (HRS) components for current and future space exploration missions. Active metal brazing and adhesive bonding technologies are being assessed for the bonding of titanium to high conductivity Carbon-Carbon composite sub components in various shapes and sizes. Currently a number of different silver and copper based active metal brazes and adhesive compositions are being evaluated. The joint microstructures were examined using optical microscopy, and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS). Several mechanical tests have been employed to ascertain the effectiveness of different brazing and adhesive approaches in tension and in shear that are both simple and representative of the actual system and relatively straightforward in analysis. The results of these mechanical tests along with the fractographic analysis will be discussed. In addition, advantages, technical issues and concerns in using different bonding approaches will also be presented.

Singh, M.; Shpargel, Tarah; Cerny, Jennifer

2006-01-01

137

Formation of C-C Bonds via Ruthenium Catalyzed Transfer Hydrogenation: Carbonyl Addition from the Alcohol or Aldehyde Oxidation Level  

PubMed Central

Under the conditions of ruthenium catalyzed transfer hydrogenation employing isopropanol as terminal reductant, ?-unsaturated compounds (1,3-dienes, allenes, 1,3-enynes and alkynes) reductively couple to aldehydes to furnish products of carbonyl addition. In the absence of isopropanol, ?-unsaturated compounds couple directly from the alcohol oxidation level to form identical products of carbonyl addition. Such “alcohol-unsaturate C-C couplings” enable carbonyl allylation, propargylation and vinylation from the alcohol oxidation level in the absence of stoichiometric organometallic reagents or metallic reductants. Thus, direct catalytic C-H functionalization of alcohols at the carbinol carbon is achieved. PMID:21927534

Shibahara, Fumitoshi; Krische, Michael J.

2011-01-01

138

Competition between N-H and N-D bond cleavage in thephotodissociation of NH2D and ND2H  

SciTech Connect

The adiabatic dissociation dynamics of NH{sub 2}D({tilde A}) and ND{sub 2}H({tilde A}) have been probed by time-resolved Fourier transform infrared emission spectroscopy. A product-state spectral pattern recognition technique is employed to separate out the emission features arising from the different photofragmentation channels following the simultaneous excitation of mixtures of the four parent molecules NH{sub 3}, NH{sub 2}D, ND{sub 2}H, and ND{sub 3} at 193.3 nm. The rotational energy partitioning about the primary a-axis of the fragments NH{sub 2}({tilde A}, {nu}{sub 2}{prime} = 0) and ND{sub 2}({tilde A}, {nu}{sub 2}{prime} = 0) from NH{sub 2}D({tilde A}) and ND{sub 2}H({tilde A}), respectively, is bimodal. We suggest that the origin of this excitation reflects the competition between two distinct dissociation mechanisms that sample two different geometries during the bond cleavage. A larger quantum yield for producing ND{sub 2}({tilde A}, {nu}{sub 2}{prime} = 0) from the photodissociation of ND{sub 2}H than ND{sub 3} is attributed to the lower dissociation energy of the N-H as compared with the N-D bond and to the enhanced tunneling efficiency of H atoms over D atoms through the barrier to dissociation. Similarly, the quantum yield for producing the NH{sub 2}({tilde A}, {nu}{sub 2}{prime} = 0) fragment is lower when an N-D bond must be cleaved in comparison to an N-H bond. Photodissociation of ND{sub 2}H by cleavage of an N-H bond leads to an ND{sub 2}({tilde A}) fragment with a much larger degree of vibrational excitation ({nu}{sub 2}{prime} = 1,2), accompanied by substantial rotation about the minor b/c-axes, than when an N-D bond is cleaved in the photodissociation of ND{sub 3}. The quantum yield for producing NHD({tilde A}) is larger for cleavage of an N-H bond from NH{sub 2}D than by cleavage of an N-D from ND{sub 2}H.

Reid, Jonathan P.; Loomis, Richard A.; Leone, Stephen R.

2000-03-20

139

Understanding the mechanisms of unusually fast H-H, C-H, and C-C bond reductive eliminations from gold(III) complexes.  

PubMed

Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid C?C reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25?%) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for H?H, C?H, and C?C bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52?°C, about 82?% of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs. PMID:25224135

Nijamudheen, A; Karmakar, Sharmistha; Datta, Ayan

2014-11-01

140

Selective Scission of C-O and C-C Bonds in Ethanol Using Bimetal Catalysts for the Preferential Growth of Semiconducting SWNT Arrays.  

PubMed

For the application of single-walled carbon nanotubes (SWNTs) to electronic and optoelectronic devices, techniques to obtain semiconducting SWNT (s-SWNT) arrays are still in their infancy. We have developed herein a rational approach for the preferential growth of horizontally aligned s-SWNT arrays on a ST-cut quartz surface through the selective scission of C-O and C-C bonds of ethanol using bimetal catalysts, such as Cu/Ru, Cu/Pd, and Au/Pd. For a common carbon source, ethanol, a reforming reaction occurs on Cu or Au upon C-C bond breakage and produces Cads and CO, while a deoxygenating reaction occurs on Ru or Pd through C-O bond breaking resulting in the production of Oads and C2H4. The produced C2H4 by Ru or Pd can weaken the oxidative environment through decomposition and the neutralization of Oads. When the bimetal catalysts with an appropriate ratio were used, the produced Cads and C2H4 can be used as carbon source for SWNT growth, and Oads promotes a suitable and durable oxidative environment to inhibit the formation of metallic SWNTs (m-SWNTs). Finally, we successfully obtained horizontally aligned SWNTs on a ST-cut quartz surface with a density of 4-8 tubes/?m and an s-SWNT ratio of about 93% using an Au/Pd (1:1) catalyst. The synergistic effects in bimetallic catalysts provide a new mechanism to control the growth of s-SWNTs. PMID:25585016

Zhang, Shuchen; Hu, Yue; Wu, Juanxia; Liu, Dan; Kang, Lixing; Zhao, Qiuchen; Zhang, Jin

2015-01-28

141

Carbon-carbon bond activation of cyclobutenones enabled by the addition of chiral organocatalyst to ketone.  

PubMed

The activation of carbon-carbon (C-C) bonds is an effective strategy in building functional molecules. The C-C bond activation is typically accomplished via metal catalysis, with which high levels of enantioselectivity are difficult to achieve due to high reactivity of metal catalysts and the metal-bound intermediates. It remains largely unexplored to use organocatalysis for C-C bond activation. Here we describe an organocatalytic activation of C-C bonds through the addition of an NHC to a ketone moiety that initiates a C-C single bond cleavage as a key step to generate an NHC-bound intermediate for chemo- and stereo-selective reactions. This reaction constitutes an asymmetric functionalization of cyclobutenones using organocatalysts via a C-C bond activation process. Structurally diverse and multicyclic compounds could be obtained with high optical purities via an atom and redox economic process. PMID:25652912

Li, Bao-Sheng; Wang, Yuhuang; Jin, Zhichao; Zheng, Pengcheng; Ganguly, Rakesh; Chi, Yonggui Robin

2015-01-01

142

Heme Carbonyls: Environmental Effects on ?C–O and Fe–C/C–O Bond Length Correlations  

PubMed Central

The synthesis and characterization of four low-spin (carbonyl)iron(II) tetraphenylporphyrinates, [Fe(TPP)(CO)(L)], where L = 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated) and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of ?C–O in toluene solution (1969–72 cm?1) but a large range of CO stretching frequencies in the solid-state (1926–1968 cm?1). The large solid-state variation results from CO interactions in the solid-state as shown by an examination of the crystal structures of the four complexes. The high precision of the four structures obtained allows us to make a number of structural and spectroscopic correlations that describe the Fe–C–O and NIm–Fe–CO units. The values of ?C–O and the Fe–C and C–O bond distances are strongly correlated and provide a structural as well as a spectroscopic correlation of the ? back-bonding model. The interactions of CO described are closely related to the large range of CO stretching frequencies observed in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO). PMID:16218637

Silvernail, Nathan J.; Roth, Arne; Noll, Bruce C.; Scheidt, W. Robert; Schulz, Charles E.

2006-01-01

143

Bis(trifluoromethyl)methylene Addition to Vinyl-Terminated SAMs: A Gas-Phase C–C Bond-Forming Reaction on a Surface  

PubMed Central

Vinyl-terminated self-assembled monolayers (SAMs) on silicon oxide substrates were chemically modified by the addition of a bis(trifluoromethyl)methylene group in a rare gas-phase C–C bond-forming reaction to directly generate films carrying terminal CF3 groups. The vinyl-terminated films were treated with hexafluoroacetone azine (HFAA) for modification. The films were characterized with ellipsometry, contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). In this study, we find that for optimized conditions clean reactions occur on a surface between SAMs with terminal olefins and HFAA, and the product is consistent with bis(trifluoromethyl)cyclopropanation formation after nitrogen extrusion. PMID:24806554

2014-01-01

144

Bis(trifluoromethyl)methylene addition to vinyl-terminated SAMs: a gas-phase C-C bond-forming reaction on a surface.  

PubMed

Vinyl-terminated self-assembled monolayers (SAMs) on silicon oxide substrates were chemically modified by the addition of a bis(trifluoromethyl)methylene group in a rare gas-phase C-C bond-forming reaction to directly generate films carrying terminal CF3 groups. The vinyl-terminated films were treated with hexafluoroacetone azine (HFAA) for modification. The films were characterized with ellipsometry, contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). In this study, we find that for optimized conditions clean reactions occur on a surface between SAMs with terminal olefins and HFAA, and the product is consistent with bis(trifluoromethyl)cyclopropanation formation after nitrogen extrusion. PMID:24806554

Adamkiewicz, Malgorzata; O'Hagan, David; Hähner, Georg

2014-05-20

145

Tailored synthesis of various nanomaterials by using a graphene-oxide-based gel as a nanoreactor and nanohybrid-catalyzed c?c bond formation.  

PubMed

New graphene oxide (GO)-based hydrogels that contain vitamin?B2 /B12 and vitamin?C (ascorbic acid) have been synthesized in water (at neutral pH value). These gel-based soft materials have been used to synthesize various metal nanoparticles, including Au, Ag, and Pd nanoparticles, as well as nanoparticle-containing reduced graphene oxide (RGO)-based nanohybrid systems. This result indicates that GO-based gels can be used as versatile reactors for the synthesis of different nanomaterials and hybrid systems on the nanoscale. Moreover, the RGO-based nanohybrid hydrogel with Pd nanoparticles was used as an efficient catalyst for C?C bond-formation reactions with good yields and showed high recyclability in Suzuki-Miyaura coupling reactions. PMID:25224859

Biswas, Abhijit; Banerjee, Arindam

2014-12-01

146

Characterization of Protein Contributions to Cobalt-Carbon Bond Cleavage Catalysis in Adenosylcobalamin-Dependent Ethanolamine Ammonia-Lyase by using Photolysis in the Ternary Complex†  

PubMed Central

Protein contributions to the substrate-triggered cleavage of the cobalt-carbon (Co-C) bond and formation of the cob(II)alamin-5?-deoxyadenosyl radical pair in the adenosylcobalamin (AdoCbl)-dependent ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium have been studied by using pulsed-laser photolysis of AdoCbl in the EAL-AdoCbl-substrate ternary complex, and time-resolved probing of the photoproduct dynamics by using ultraviolet-visible absorption spectroscopy on the 10?7 ? 10?1 s time scale. Experiments were performed in a fluid dimethylsulfoxide/water cryosolvent system at 240 K, under conditions of kinetic competence for thermal cleavage of the Co-C bond in the ternary complex. The static ultraviolet-visible absorption spectra of holo-EAL and ternary complex are comparable, indicating that the binding of substrate does not labilize the cofactor cobalt-carbon (Co-C) bond by significantly distorting the equilibrium AdoCbl structure. Photolysis of AdoCbl in EAL at 240 K leads to cob(II)alamin-5?-deoxyadenosyl radical pair quantum yields of <0.01 at 10?6 s in both holo-EAL and ternary complex. Three photoproduct states are populated following a saturating laser pulse, and labeled, Pf, Ps, and Pc. The relative amplitudes and first-order recombination rate constants of Pf (0.4-0.6; 40-50 s?1), Ps, (0.3-0.4; 4 s?1) and Pc (0.1-0.2; 0) are comparable in holo-EAL and in the ternary complex. Time-resolved, full-spectrum electron paramagnetic resonance (EPR) spectroscopy shows that visible irradiation alters neither the kinetics of thermal cob(II)alamin-substrate radical pair formation, nor the equilibrium between ternary complex and cob(II)alamin-substrate radical pair, at 246 K. The results indicate that substrate binding to holo-EAL does not “switch” the protein to a new structural state, which promptly stabilizes the cob(II)alamin-5?-deoxyadenosyl radical pair photoproduct, either through an increased barrier to recombination, a decreased barrier to further radical pair separation, or lowering of the radical pair state free energy, or a combination of these effects. Therefore, we conclude that such a change in protein structure, which is independent of changes in the AdoCbl structure, and specifically the Co-C bond length, is not a basis of Co-C bond cleavage catalysis. The results suggest that, following the substrate trigger, the protein interacts with the cofactor to contiguously guide the cleavage of the Co-C bond, at every step along the cleavage coordinate, starting from the equilibrium configuration of the ternary complex. The cleavage is thus represented by a diagonal trajectory across a free energy surface, that is defined by chemical (Co-C separation) and protein configuration coordinates. PMID:21491908

Robertson, Wesley D.; Wang, Miao; Warncke, Kurt

2011-01-01

147

Gas-phase reactivity of lanthanide cations with fluorocarbons: C-F versus C-H and C-C bond activation  

SciTech Connect

The gas-phase reactivity of the fluorinated hydrocarbons CF{sub 4}, CHF{sub 3}, CH{sub 3}F, C{sub 2}F{sub 6}, 1,1-C{sub 2}H{sub 4}F{sub 2}, and C{sub 6}F{sub 6} with the lanthanide cations Ce{sup +}, Pr{sup +}, Sm{sup +}, Ho{sup +}, Tm{sup +}, and Yb{sup +} and the reactivity of C{sub 6}H{sub 5}F with all lanthanide cations Ln{sup +} (Ln = La-Lu, with the exception of Pm{sup +}) have been examined by Fourier-transform ion cyclotron resonance mass spectrometry. The perfluorinated compounds tetrafluoromethane and hexafluoroethane as well as trifluoromethane do not react with any lanthanide cation. Selective activation of the strong C-F bonds in fluoromethane, 1,1-difluoroethane, hexafluorobenzene, and fluorobenzene appears as a general reaction scheme along the 4f row. Experimental evidence is given for a `harpoon`-like mechanism for the F atom abstraction process which operates via an initial electron transfer from the lanthanide cation to the fluorinated substrate in the encounter complex Ln{sup +}RF. The most reactive lanthanides La{sup +}, Ce{sup +}, Gd{sup +}, and Tb{sup +} and also the formal closed-shell species Lu{sup +} exhibit additional C-H and C-C bond activation pathways in the reaction with fluorobenzene, namely dehydrohalogenation as well as loss of a neutral acetylene molecule. In the case of Tm{sup +} and Yb{sup +} the formation of neutral LnF{sub 3} is observed in a multistep process via C-C coupling and charge transfer. 17 refs., 2 figs., 2 tabs.

Cornehl, H.H.; Hornung, G.; Schwarz, H. [Technischen Unibersitaet Berlin (Germany)] [Technischen Unibersitaet Berlin (Germany)

1996-10-16

148

Regioselective CC bond cleavage in arylhydrazones of 4,4,4-trifluoro-1-(thiophen-2-yl)butane-1,3-diones  

NASA Astrophysics Data System (ADS)

New (Z)-2-(2-(para-substituted phenyl)hydrazono)-4,4,4-trifluoro-1-(thiophen-2-yl)butane-1,3-diones with chloro (1), bromo (2) and carboxy (3) substituents were synthesized and characterized by ESI-MS, IR, 1H and 13C NMR spectroscopies and elemental analysis. The regioselective carbon-carbon bond cleavage with the formation (E)-2-(2-(4-substituted phenyl) hydrazono)-1-(thiophen-2-yl)ethanones was achieved upon heating (80 °C) of 1-3 in methanol-water.

Solhnejad, Reza; Aliyeva, Farqana S.; Maharramov, Abel M.; Aliyeva, Rafiga A.; Chyragov, Famil M.; Gurbanov, Atash V.; Mahmudov, Kamran T.; Kopylovich, Maximilian N.

2013-10-01

149

Tomato carotenoid cleavage dioxygenases 1A and 1B: Relaxed double bond specificity leads to a plenitude of dialdehydes, mono-apocarotenoids and isoprenoid volatiles  

PubMed Central

The biosynthetic processes leading to many of the isoprenoid volatiles released by tomato fruits are still unknown, though previous reports suggested a clear correlation with the carotenoids contained within the fruit. In this study, we investigated the activity of the tomato (Solanum lycopersicum) carotenoid cleavage dioxygenase (SlCCD1B), which is highly expressed in fruits, and of its homolog SlCCD1A. Using in vitro assays performed with purified recombinant enzymes and by analyzing products formed by the two enzymes in carotene-accumulating Escherichia coli strains, we demonstrate that SlCCD1A and, to a larger extent, SlCCD1B, have a very relaxed specificity for both substrate and cleavage site, mediating the oxidative cleavage of cis- and all-trans-carotenoids as well as of different apocarotenoids at many more double bonds than previously reported. This activity gives rise to a plenitude of volatiles, mono-apocarotenoids and dialdehyde products, including cis-pseudoionone, neral, geranial, and farnesylacetone. Our results provide a direct evidence for a carotenoid origin of these compounds and point to CCD1s as the enzymes catalyzing the formation of the vast majority of tomato isoprenoid volatiles, many of which are aroma constituents. PMID:25057464

Ilg, Andrea; Bruno, Mark; Beyer, Peter; Al-Babili, Salim

2014-01-01

150

Kinetic determination of alkaline phosphatase activity based on hydrolytic cleavage of the P-F bond in monofluorophosphate and fluoride ion-selective electrode.  

PubMed

Alkaline phosphatase catalyzes the hydrolytic cleavage of the P-F bond in monofluorophosphate with the subsequent release of fluoride ions. A kinetic potentiometric method is described in which a fluoride ion-selective electrode is used for the sensitive and selective measurement of the released F- for the determination of alkaline phosphatase activity. It is shown that monofluorophosphate can be used as an alternative substrate for alkaline phosphatase. The reaction demonstrates a well-defined correlation with the hydrolysis of the P-O bond in 4-nitrophenyl phosphate. The serum alkaline phosphatase was determined in human serum samples by the potentiometric technique, and the results obtained compared well with a standard spectrophotometric method. PMID:2077934

Venetz, W P; Mangan, C; Siddiqi, I W

1990-11-15

151

Effect of thermal denaturation, inhibition, and cleavage of disulfide bonds on the low-frequency Raman and FTIR spectra of chymotrypsin and albumin.  

PubMed

The analysis of the structure-function relationship is extremely important in the study of proteins. The importance of function-related motions of large parts or subglobules of protein molecules stimulates the spectroscopic study in the low-frequency (terahertz) domain. However, only tentative assignments are available and the spectroscopic data are insufficiently discussed in terms of structural changes. This work is aimed at the analysis of regularities of changes in the low-frequency (100 to 600 cm(-1)) FTIR and Raman spectra of proteins related to their structural modifications. We study the spectra of two proteins with substantially different structures (albumin and chymotrypsin) and the spectra of samples in which the structures of protein molecules are modified using inhibition, thermal denaturation, and cleavage of disulfide bonds. The results indicate that the low-frequency spectral interval can be used to characterize protein conformations. Correlated variations in the intensities of several low-frequency bands are revealed in the spectra of the modified proteins. The strongest spectral changes are caused by thermal denaturation of proteins, and the effect of cleavage of disulfide bonds is generally weaker. It is demonstrated that the inhibitor binding in the active site causes spectral changes that can be compared to the changes induced by thermal denaturation. PMID:25478913

Brandt, Nikolay N; Chikishev, Andrey Yu; Mankova, Anna A; Sakodynskaya, Inna K

2015-05-01

152

Identification and cleavage of breakable single bonds by selective oxidation, reduction, and hydrolysis. Annual report, October 1, 1980-September 30, 1981  

SciTech Connect

The objective of this project is to determine the structure of bituminous coal by determining the proportions of the various kinds of connecting bonds and how they can best be broken. Results obtained during the past quarter are presented for the following tasks: (1) extractions and fractionations of coal products which covers pyridine extraction, fractionation of TIPS fractions, EDA extraction of Illinois No. 6 coal and swelling ratios of coal samples; (2) experiments on breakable single bonds which cover reactions of ethylenediamine and model ethers, reaction of pyridine-extracted coal with Me/sub 3/SiI, Baeyer-Villiger oxidations, reaction to diphenylmethane with 15% HNO/sub 3/, cleavage of TIPS with ZnI/sub 2/, and cleavage of black acids; and (3) oxygen oxidation No. 18. Some of the highlights of these studies are: (1) some model ethers are not cleaved by EDA under extraction conditions; (2) oxidation of diaryl ketones with m-chloroperbenzoic acid and saponification of the resulting esters in promising for identifying ketones, (3) treatment of a black acid with pyridine hydroiodide reduced the acid's molecular weight and increased its solubility in pyridine, but treatment with ZnI/sub 2/ was ineffective; (4) in comparison with 0.1 M K/sub 2/S/sub 2/O/sub 8/, 0.01 M persulfate is relatively ineffective in accelerating oxidation of BnNH/sub 2/-extracted coal in water suspension. 2 figures, 3 tables.

Hirschon, A.S.; Zevely, J.; Mayo, F.R.

1981-11-12

153

Formation of C-C and C-O bonds and oxygen removal in reactions of alkanediols, alkanols, and alkanals on copper catalysts.  

PubMed

This study reports evidence for catalytic deoxygenation of alkanols, alkanals, and alkanediols on dispersed Cu clusters with minimal use of external H(2) and with the concurrent formation of new C-C and C-O bonds. These catalysts selectively remove O-atoms from these oxygenates as CO or CO(2) through decarbonylation or decarboxylation routes, respectively, that use C-atoms present within reactants or as H(2)O using H(2) added or formed in situ from CO/H(2)O mixtures via water-gas shift. Cu catalysts fully convert 1,3-propanediol to equilibrated propanol-propanal intermediates that subsequently form larger oxygenates via aldol-type condensation and esterification routes without detectable involvement of the oxide supports. Propanal-propanol-H(2) equilibration is mediated by their chemisorption and interconversion at surfaces via C-H and O-H activation and propoxide intermediates. The kinetic effects of H(2), propanal, and propanol pressures on turnover rates, taken together with measured selectivities and the established chemical events for base-catalyzed condensation and esterification reactions, indicate that both reactions involve kinetically relevant bimolecular steps in which propoxide species, acting as the base, abstract the ?-hydrogen in adsorbed propanal (condensation) or attack the electrophilic C-atom at its carbonyl group (esterification). These weakly held basic alkoxides render Cu surfaces able to mediate C-C and C-O formation reactions typically catalyzed by basic sites inherent in the catalyst, instead of provided by coadsorbed organic moieties. Turnover rates for condensation and esterification reactions decrease with increasing Cu dispersion, because low-coordination corner and edge atoms prevalent on small clusters stabilize adsorbed intermediates and increase the activation barriers for the bimolecular kinetically relevant steps required for both reactions. PMID:22023723

Sad, María E; Neurock, Matthew; Iglesia, Enrique

2011-12-21

154

Discovery and Mechanistic Studies of Facile N-Terminal C?–C Bond Cleavages in the Dissociation of Tyrosine-Containing Peptide Radical Cations  

SciTech Connect

Gas phase fragmentations of protein and peptide (M) ions in a mass spectrometer—induced by, for example, electron-capture dissociation1-2 and electron-transfer dissociation3-422 —form the foundation for top-down amino acid sequencing approaches for the rapid identification of protein components in complex biological samples. During these processes, protonated protein and peptide radicals ([M + nH]•(n – 1)+)5–8 are generated; their fragmentations are governed largely by the properties of the unpaired electron. Because of their importance in modern bioanalytical chemistry, considerable attention has been drawn recently toward understanding the radical cation chemistry behind the fragmentations of these odd-electron biomolecular ions in the gas phase.

Mu, Xiaoyan; Song, Tao; Xu, Minjie; Lai, Cheuk-Kuen; Siu, Chi-Kit; Laskin, Julia; Chu, Ivan K.

2014-03-28

155

Resonance Raman Spectroscopy Reveals pH-Dependent Active Site Structural Changes of Lactoperoxidase Compound 0 and Its Ferryl Heme O-O Bond Cleavage Products.  

PubMed

The first step in the enzymatic cycle of mammalian peroxidases, including lactoperoxidase (LPO), is binding of hydrogen peroxide to the ferric resting state to form a ferric-hydroperoxo intermediate designated as Compound 0, the residual proton temporarily associating with the distal pocket His109 residue. Upon delivery of this "stored" proton to the hydroperoxo fragment, it rapidly undergoes O-O bond cleavage, thereby thwarting efforts to trap it using rapid mixing methods. Fortunately, as shown herein, both the peroxo and the hydroperoxo (Compound 0) forms of LPO can be trapped by cryoradiolysis, with acquisition of their resonance Raman (rR) spectra now permitting structural characterization of their key Fe-O-O fragments. Studies were conducted under both acidic and alkaline conditions, revealing pH-dependent differences in relative populations of these intermediates. Furthermore, upon annealing, the low pH samples convert to two forms of a ferryl heme O-O bond-cleavage product, whose ?(Fe?O) frequencies reflect substantially different Fe?O bond strengths. In the process of conducting these studies, rR structural characterization of the dioxygen adduct of LPO, commonly called Compound III, has also been completed, demonstrating a substantial difference in the strengths of the Fe-O linkage of the Fe-O-O fragment under acidic and alkaline conditions, an effect most reasonably attributed to a corresponding weakening of the trans-axial histidyl imidazole linkage at lower pH. Collectively, these new results provide important insight into the impact of pH on the disposition of the key Fe-O-O and Fe?O fragments of intermediates that arise in the enzymatic cycles of LPO, other mammalian peroxidases, and related proteins. PMID:25506715

Mak, Piotr J; Thammawichai, Warut; Wiedenhoeft, Dennis; Kincaid, James R

2015-01-14

156

Facile reductive cleavage of the C=O bond of acyl halides and the C identical with N bond of nitriles by hydrogen in the presence of sulfido-bridged molybdenum complexes  

SciTech Connect

The authors have recently reported that cationic dinuclear molybdenum complexes of the type ((CpMo)/sub 2/(S/sub 2/CH/sub 2/)(..mu..-S)(..mu..-SR))/sup +/ undergo unusual reactions with molecular hydrogen to form protons and types of molybdenum and organic products which vary depending on the nature of R. They report here extensions of this reactivity which result in the reductive cleavage of the C=O and C identical with N bonds in acyl halides and nitriles, respectively, under mild homogeneous conditions.

Coons, D.E.; Laurie, J.C.V.; Haltiwanger, R.C.; DuBois, M.R.

1987-01-07

157

Radical ions in photochemistry. Carbon-carbon bond cleavage of radical cations in solution: Theory and application  

SciTech Connect

The cleavage of radical cations of two series of alkanes, 1,1,2-triaryl- and 1,1,2,2-tetraarylalkanes, generated by photoinduced single electron transfer in acetonitrile-methanol, occurs with formation of radical and carbocation fragments. The radical cations of some unsymmetrically substituted alkanes cleave to give all four of the possible products, two hydrocarbons emanating from the radicals and two methyl ethers from the carbocations, in proportion to the oxidation potentials of the two possible radical fragments. There is an excellent linear correlation between the logarithm of the observed ratio of products and that calculated from the reported electrochemically determined oxidation potentials (r = 0.998, 5 points). The proportionality constant (1.27) for this relationship is close to unity which indicates that the product ratio is determined by the relative rates of cleavage in the two possible modes or by equilibration of the radicals and carbocations before separation of the geminate radical carbocation pair and not by equilibration upon reencounter of freely solvated radical and carbocation fragments.

Popielarz, R.; Arnold, D.R. (Dalhousie Univ., Halifax, Nova Scotia (Canada))

1990-04-11

158

Model studies of methyl CoM reductase: methane formation via CH3-S bond cleavage of Ni(I) tetraazacyclic complexes having intramolecular methyl sulfide pendants.  

PubMed

The Ni(I) tetraazacycles [Ni(dmmtc)](+) and [Ni(mtc)](+), which have methylthioethyl pendants, were synthesized as models of the reduced state of the active site of methyl coenzyme M reductase (MCR), and their structures and redox properties were elucidated (dmmtc, 1,8-dimethyl-4,11-bis{(2-methylthio)ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane; mtc, 1,8-{bis(2-methylthio)ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane). The intramolecular CH(3)-S bond of the thioether pendant of [Ni(I)(dmmtc)](OTf) was cleaved in THF at 75 °C in the presence of the bulky thiol DmpSH, which acts as a proton source, and methane was formed in 31% yield and a Ni(II) thiolate complex was concomitantly obtained (Dmp = 2,6-dimesityphenyl). The CH(3)-S bond cleavage of [Ni(I)(mtc)](+) also proceeded similarly, but under milder conditions probably due to the lower potential of the [Ni(I)(mtc)](+) complex. These results indicate that the robust CH(3)-S bond can be homolytically cleaved by the Ni(I) center when they are properly arranged, which highlights the significance of the F430 Ni environment in the active site of the MCR protein. PMID:22439643

Nishigaki, Jun-ichi; Matsumoto, Tsuyoshi; Tatsumi, Kazuyuki

2012-05-01

159

Identification and cleavage of breakable single bonds by selective oxidation, reduction, and hydrolysis. Quarterly report No. 12, June 1-September 30, 1981  

SciTech Connect

We assume that bituminous coal consists mostly of an aggregate of condensed aromatic and aliphatic rings, connected and made insoluble (but swellable) by crosslinks containing single bonds. The objective of this project is to determine the proportions of the various kinds of connecting links and how they can best be broken - in other words, to determine the structure of bituminous coal, with emphasis on the crosslinks and breakable single bonds. The program began with an investigation of the structure of the TIPS fraction of Illinois No. 6 coal, that is, the two-thirds of the 16% extracted by pyridine that is toluene-insoluble, pyridine-soluble, mostly through changes in molecular weight during cleavage reactions in pyridine solution. The most promising of these cleavage reactions are now being applied to the 84% of coal that is insoluble in pyridine and presents the main problem in coal liquefaction, following the progress of the reactions by formation of soluble material and swelling of the insoluble portion. We found that benzylamine (BnNH/sub 2/) would extract an additional 14% (of the original weight of coal) of material from pyridine-extracted coal, and later that an ethylenediamine/dimethyl sulfoxide (EDA/DMSO) mixture would dissolve another 21% of the original coal. The BnNH/sub 2/ extract is soluble in pyridine. Our best present guess is that the BnNH/sub 2/ extract cleaves most of the ester groups in coal and that EDA/DMSO cleaves the remaining ester and most of the ether groups.

Hirschon, A.S.; Zevely, J.; Mayo, F.R.

1981-11-12

160

Significant evidence of C···O and C···C long-range contacts in several heterodimeric complexes of CO with CH3-X, should one refer to them as carbon and dicarbon bonds!  

PubMed

Noncovalent interactions in 18 weakly bound binary complexes formed between either of the two end-on orientations of the CO molecule and the methylated carbon positive ?-hole associated with the hydrophobic part of the CH3-X molecules are exploited using the density functional theory to examine the physical chemistry of the recently introduced 'carbon bonds' (Phys. Chem. Chem. Phys., 2013, 15, 14377), where X = -NO2, -CN, -F, -Cl, -Br, -OH, -CF3, -CCl3, and -NH2. The two important types of interactions are identified as C···O and C···C, the latter has probably never studied before, and are found to be stabilized by charge-transfer delocalizations between the electron-acceptor and -donor natural bond orbitals of the interacting partners involved, unveiled using natural bond orbital analysis. Application of atoms in molecular theory revealed preferable quantum mechanical exchange-correlation energy channels and (3, -1) bond critical points (bcps) between the atoms of noncovalently bonded pairs in these complexes, in excellent agreement with the results of the noncovalent-interaction reduced-density-gradient (NCI-RDG) theory that revealed expected isosurfaces and troughs in the low density region of the RDG vs. sign(?2)? plots. The dependencies of the C···O and C···C bcp charge densities on their corresponding local energy densities, as well as on their corresponding bond electron delocalization indices are found to exhibit nontrivial roles of these topological descriptors to explain the stabilities of the investigated binary complexes. Moreover, the vibrational red- and blue-shifts in the CO bond stretching frequencies, and concomitant elongations and contractions of the corresponding bond lengths, both with respect to the monomer values, are observed upon the formation of the C···O- and C···C-bonded complexes, respectively. The increase and decrease in the complex dipole moments, relative to the sum of their respective monomer values, are found to be a characteristic that separates the aforementioned red- and blue-shifted interactions. In analogy with dihydrogen bonding, as well as that with the charge and electrostatic surface potential model descriptions, we suggest the C···C interactions to be referred to as dicarbon bonds. PMID:25017184

Varadwaj, Pradeep R; Varadwaj, Arpita; Jin, Bih-Yaw

2014-08-28

161

Mechanistic investigation of phosphate ester bond cleavages of glycylphosphoserinyltryptophan radical cations under low-energy collision-induced dissociation.  

PubMed

Under the conditions of low-energy collision-induced dissociation (CID), the canonical glycylphosphoserinyltryptophan radical cation having its radical located on the side chain of the tryptophan residue ([G(p)SW](•+)) fragments differently from its tautomer with the radical initially generated on the ?-carbon atom of the glycine residue ([G(•)(p)SW](+)). The dissociation of [G(•)(p)SW](+) is dominated by the neutral loss of H3PO4 (98 Da), with backbone cleavage forming the [b2 - H](•+)/y1(+) pair as the minor products. In contrast, for [G(p)SW](•+), competitive cleavages along the peptide backbone, such as the formation of [G(p)SW - CO2](•+) and the [c2 + 2H](+)/[z(1) - H](•+) pair, significantly suppress the loss of neutral H3PO4. In this study, we used density functional theory (DFT) to examine the mechanisms for the tautomerizations of [G(•)(p)SW](+) and [G(p)SW](•+) and their dissociation pathways. Our results suggest that the dissociation reactions of these two peptide radical cations are more efficient than their tautomerizations, as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. We also propose that the loss of H3PO4 from both of these two radical cationic tautomers is preferentially charge-driven, similar to the analogous dissociations of even-electron protonated peptides. The distonic radical cationic character of [G(•)(p)SW](+) results in its charge being more mobile, thereby favoring charge-driven loss of H3PO4; in contrast, radical-driven pathways are more competitive during the CID of [G(p)SW](•+). PMID:23516067

Quan, Quan; Hao, Qiang; Song, Tao; Siu, Chi-Kit; Chu, Ivan K

2013-04-01

162

Mechanistic Investigation of Phosphate Ester Bond Cleavages of Glycylphosphoserinyltryptophan Radical Cations under Low-Energy Collision-Induced Dissociation  

NASA Astrophysics Data System (ADS)

Under the conditions of low-energy collision-induced dissociation (CID), the canonical glycylphosphoserinyltryptophan radical cation having its radical located on the side chain of the tryptophan residue ([G p SW]•+) fragments differently from its tautomer with the radical initially generated on the ?-carbon atom of the glycine residue ([G• p SW]+). The dissociation of [G• p SW]+ is dominated by the neutral loss of H3PO4 (98 Da), with backbone cleavage forming the [b2 - H]•+/y1 + pair as the minor products. In contrast, for [G p SW]•+, competitive cleavages along the peptide backbone, such as the formation of [G p SW - CO2]•+ and the [c2 + 2H]+/[z1 - H]•+ pair, significantly suppress the loss of neutral H3PO4. In this study, we used density functional theory (DFT) to examine the mechanisms for the tautomerizations of [G• p SW]+ and [G p SW]•+ and their dissociation pathways. Our results suggest that the dissociation reactions of these two peptide radical cations are more efficient than their tautomerizations, as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. We also propose that the loss of H3PO4 from both of these two radical cationic tautomers is preferentially charge-driven, similar to the analogous dissociations of even-electron protonated peptides. The distonic radical cationic character of [G• p SW]+ results in its charge being more mobile, thereby favoring charge-driven loss of H3PO4; in contrast, radical-driven pathways are more competitive during the CID of [G p SW]•+.

Quan, Quan; Hao, Qiang; Song, Tao; Siu, Chi-Kit; Chu, Ivan K.

2013-04-01

163

Anandamide Hydrolysis in FAAH Reveals a Dual Strategy for Efficient Enzyme-Assisted Amide Bond Cleavage via Nitrogen Inversion.  

PubMed

Herein, we combined classical molecular dynamics (MD) and quantum mechanical/molecular mechanics (QM/MM) simulations to unravel the whole catalytic cycle of fatty acid amide hydrolase (FAAH) in complex with anandamide, the main neurotransmitters involved in the control of pain. While microsecond MD simulations of FAAH in a realistic membrane/water environment provided a solid model for the reactant state of the enzymatic complex (Palermo et al. J. Chem. Theory Comput. 2013, 9, 1202-1213.), QM/MM simulations depict now a highly concerted two-step catalytic mechanism characterized by (1) acyl-enzyme formation after hydrolysis of the substrate amide bond and (2) deacylation reaction with restoration of the catalytic machinery. We found that a crucial event for anandamide hydrolysis is the inversion of the reactive nitrogen of the scissile amide bond, which occurs during the acylation rate-limiting step. We show that FAAH uses an exquisite catalytic strategy to induce amide bond distortion, reactive nitrogen inversion, and amide bond hydrolysis, promoting catalysis to completion. This new strategy is likely to be of general applicability to other amidases/peptidases that show similar catalytic site architectures, providing crucial insights for de novo enzyme design or drug discovery efforts. PMID:25205244

Palermo, Giulia; Campomanes, Pablo; Cavalli, Andrea; Rothlisberger, Ursula; De Vivo, Marco

2015-01-22

164

High resolution analysis of snake venom metalloproteinase (SVMP) peptide bond cleavage specificity using proteome based peptide libraries and mass spectrometry  

Microsoft Academic Search

Both serine and metalloproteinases have been shown to play the role of toxins in the venoms of many snakes. Determination of the natural protein substrates of these toxins is an important feature in the toxinological characterization of these proteinases. Furthermore, characterization of their peptide bond specificity is of value for understanding active site preference of the proteinase associated with effective

Adriana F. Paes Leme; Teresa Escalante; Jose G. C. Pereira; Ana K. Oliveira; Eladio F. Sanchez; José M. Gutiérrez; Solange M. T. Serrano; Jay W. Fox

2011-01-01

165

Insights into the mechanism of X-ray-induced disulfide-bond cleavage in lysozyme crystals based on EPR, optical absorption and X-ray diffraction studies  

PubMed Central

Electron paramagnetic resonance (EPR) and online UV–visible absorption microspectrophotometry with X-ray crystallography have been used in a complementary manner to follow X-ray-induced disulfide-bond cleavage. Online UV–visible spectroscopy showed that upon X-irradiation, disulfide radicalization appeared to saturate at an absorbed dose of approximately 0.5–0.8?MGy, in contrast to the saturating dose of ?0.2?MGy observed using EPR at much lower dose rates. The observations suggest that a multi-track model involving product formation owing to the interaction of two separate tracks is a valid model for radiation damage in protein crystals. The saturation levels are remarkably consistent given the widely different experimental parameters and the range of total absorbed doses studied. The results indicate that even at the lowest doses used for structural investigations disulfide bonds are already radicalized. Multi-track considerations offer the first step in a comprehensive model of radiation damage that could potentially lead to a combined computational and experimental approach to identifying when damage is likely to be present, to quantitate it and to provide the ability to recover the native unperturbed structure. PMID:24311579

Sutton, Kristin A.; Black, Paul J.; Mercer, Kermit R.; Garman, Elspeth F.; Owen, Robin L.; Snell, Edward H.; Bernhard, William A.

2013-01-01

166

Electronic Structure of the Peroxy Intermediate and Its Correlation to the Native Intermediate in the Multicopper Oxidases: Insights into the Reductive Cleavage of the O-O Bond  

PubMed Central

The multicopper oxidases (MCOs) utilize a blue type 1 (T1) copper site and a trinuclear Cu cluster comprised of a type 2 (T2) and a binuclear type 3 (T3) site that together catalyze the four-electron reduction of O2 to H2O. Reaction of the fully reduced enzyme with O2 proceeds via two sequential two-electron steps generating the peroxy intermediate (PI) and the native intermediate (NI). While a detailed description of the geometric and electronic structure of NI has been developed, this has been more elusive for PI largely due to the diamagnetic nature of its ground state. Density functional theory (DFT) calculations have been used to correlate to spectroscopic data to generate a description of the geometric and electronic structure of PI. A highly conserved carboxylate residue near the T2 site is found to play a critical role in stabilizing the PI structure, which induces oxidation of the T2 and one T3 Cu center and strong superexchange stabilization via the peroxide bridge, allowing irreversible binding of O2 at the trinuclear Cu site. Correlation of PI to NI is achieved using a two-dimensional potential energy surface generated to describe the catalytic two-electron reduction of the peroxide O-O bond by the MCOs. It is found that the reaction is thermodynamically driven by the relative stability of NI and the involvement of the simultaneous two-electron transfer process. A low activation barrier (calculated ~5–6 kcal/mol and experimental ~3–5 kcal/mol) is produced by the triangular topology of the trinuclear Cu cluster site, as this symmetry provides good donor-acceptor frontier molecular orbital (FMO) overlap. Finally, the O-O bond cleavage in the trinuclear Cu cluster can be achieved via either a proton-assisted or a proton-unassisted process, allowing the MCOs to function over a wide range of pH. It is found that while the proton helps to stabilize the acceptor O22? ?* orbital in the proton-assisted process for better donor-acceptor FMO overlap, the third oxidized Cu center in the trinuclear site assumes the role as a Lewis acid in the proton-unassisted process for similarly efficient O-O bond cleavage. PMID:17918839

Yoon, Jungjoo; Solomon, Edward I.

2008-01-01

167

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH{sub 2}BrI  

SciTech Connect

It is well known that ultraviolet photoexcitation of halomethanes results in halogen-carbon bond cleavage. Each halogen-carbon bond has a dominant ultraviolet (UV) absorption that promotes an electron from a nonbonding halogen orbital (n{sub X}) to a carbon-halogen antibonding orbital (?*{sub C-X}). UV absorption into specific transitions in the gas phase results primarily in selective cleavage of the corresponding carbon-halogen bond. In the present work, broadband ultrafast UV-visible transient absorption studies of CH{sub 2}BrI reveal a more complex photochemistry in solution. Transient absorption spectra are reported spanning the range from 275 nm to 750 nm and 300 fs to 3 ns following excitation of CH{sub 2}BrI at 266 nm in acetonitrile, 2-butanol, and cyclohexane. Channels involving formation of CH{sub 2}Br + I radical pairs, iso-CH{sub 2}Br-I, and iso-CH{sub 2}I-Br are identified. The solvent environment has a significant influence on the branching ratios, and on the formation and stability of iso-CH{sub 2}Br-I. Both iso-CH{sub 2}Br-I and iso-CH{sub 2}I-Br are observed in cyclohexane with a ratio of ?2.8:1. In acetonitrile this ratio is 7:1 or larger. The observation of formation of iso-CH{sub 2}I-Br photoproduct as well as iso-CH{sub 2}Br-I following 266 nm excitation is a novel result that suggests complexity in the dissociation mechanism. We also report a solvent and concentration dependent lifetime of iso-CH{sub 2}Br-I. At low concentrations the lifetime is >4 ns in acetonitrile, 1.9 ns in 2-butanol and ?1.4 ns in cyclohexane. These lifetimes decrease with higher initial concentrations of CH{sub 2}BrI. The concentration dependence highlights the role that intermolecular interactions can play in the quenching of unstable isomers of dihalomethanes.

Anderson, Christopher P.; Spears, Kenneth G.; Wilson, Kaitlynn R.; Sension, Roseanne J. [Department of Chemistry and Department of Physics, University of Michigan, Ann Arbor, Michigan 48109 (United States)] [Department of Chemistry and Department of Physics, University of Michigan, Ann Arbor, Michigan 48109 (United States)

2013-11-21

168

Heterolytic Cleavage of Hydrogen by an Iron Hydrogenase Model: An Fe-H - - - H-N Dihydorgen Bond Characterized by Neutron Diffraction  

SciTech Connect

Use of hydrogen as a fuel by [FeFe]-hydrogenase enzymes in nature requires heterolytic cleavage of the H-H bond into a proton (H+) and hydride (H-), a reaction that is also a critical step in homogeneous catalysts for hydrogenation of C=O and C=N bonds. An understanding of the catalytic oxidation of H2 by hydrogenases provides insights into the design of synthetic catalysts that are sought as cost-effective alternatives to the use of the precious metal platinum in fuel cells. Crystallographic studies on the [FeFe]-hydrogenase enzyme were critical to understanding of its reactivity, but the key H-H cleavage step is not readily observed experimentally in natural hydrogenases. Synthetic biomimics have provided evidence for H2 cleavage leading to hydride transfer to the metal and proton transfer to an amine. Limitations on the precise location of hydrogen atoms by x-ray diffraction can be overcome by use of neutron diffraction, though its use is severely limited by the difficulty of obtaining suitable crystals and by the scarcity of neutron sources. Here we show that an iron complex with a pendant amine in the diphosphine ligand cleaves hydrogen heterolytically under mild conditions, leading to [CpC5F4NFeH(PtBu2NtBu2H)]+BArF4-, [PtBu2NtBu2 = 1,5-di(tert-butyl)-3,7-di(tert-butyl)-1,5-diaza-3,7-diphosphacyclooctane; ArF = 3,5-bis(trifluoromethyl)phenyl]. The Fe-H- - - H-N moiety has a strong dihydrogen bond, with a remarkably short H • • • H distance of 1.489(10) Å between the protic N-H?+ and hydridic Fe-H?-. The structural data for [CpC5F4NFeH(PtBu2NtBu2H)]+ provide a glimpse of how the H-H bond is oxidized or generated in hydrogenase enzymes, with the pendant amine playing a key role as a proton relay. The iron complex [CpC5F4NFeH(PtBu2NtBu2H)]+BArF4- is an electrocatalyst for oxidation of H2 (1 atm) at 22 °C, so the structural data are obtained on a complex that is a functional model for catalysis by [FeFe]-hydrogenase enzymes. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

Liu, Tianbiao L.; Wang, Xiaoping; Hoffmann, Christina; DuBois, Daniel L.; Bullock, R. Morris

2014-05-19

169

Geometric and Electronic Structure of [{Cu(MeAN)}2(?-?2:?2(O22?))]2+ with an Unusually Long O–O Bond: O–O Bond Weakening vs Activation for Reductive Cleavage  

PubMed Central

Certain side-on peroxo dicopper(II) species with particularly low ?O–O (710–730 cm?1) have been found in equilibrium with their bis-?-oxo dicopper(III) isomer. An issue is whether such side-on peroxo bridges are further activated for O–O cleavage. In a previous study (Liang, H.-C., et al., J. Am. Chem. Soc. 2002, 124, 4170–4171), we showed that oxygenation of the three-coordinate complex [CuI(MeAN)]+ (MeAN=N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) leads to a low-temperature stable [{CuII(MeAN)}2(?-?2:?2-O22?)]2+ peroxo species with low ?O–O (721 cm?1), as characterized by UV-Vis absorption and resonance Raman (rR) spectroscopies. Here, this complex has been crystallized as its SbF6? salt and an X-ray structure indicates the presence of an unusually long O–O bond (1.540(5) Å) consistent with the low ?O–O. EXAFS and rR spectroscopic and reactivity studies indicate the exclusive formation of [{CuII(MeAN)}2(?-?2:?2-O22?)]2+ without any bis-?-oxo-dicopper(III) isomer present. This is the first structure of a side-on peroxo dicopper(II) species with a significantly long and weak O–O bond. DFT calculations show that the weak O–O bond results from strong ? donation from the MeAN ligand to Cu that is compensated by a decrease in the extent of peroxo to Cu charge transfer. Importantly, the weak O–O bond does not reflect an increase in backbonding into the ?* orbital of the peroxide. Thus, although the O–O bond is unusually weak, this structure is not further activated for reductive cleavage to form a reactive bis-?-oxo-dicopper(III) species. These results highlight the necessity of understanding electronic structure changes associated with spectral changes for correlations to reactivity. PMID:22571744

Park, Ga Young; Qayyum, Munzarin F.; Woertink, Julia; Hodgson, Keith O.; Hedman, Britt; Narducci Sarjeant, Amy A.; Solomon, Edward I.; Karlin, Kenneth D.

2012-01-01

170

Molecular Basis of 1,6-Anhydro Bond Cleavage and Phosphoryl Transfer by Pseudomonas aeruginosa 1,6-Anhydro-N-acetylmuramic Acid Kinase*  

PubMed Central

Anhydro-N-acetylmuramic acid kinase (AnmK) catalyzes the ATP-dependent conversion of the Gram-negative peptidoglycan (PG) recycling intermediate 1,6-anhydro-N-acetylmuramic acid (anhMurNAc) to N-acetylmuramic acid-6-phosphate (MurNAc-6-P). Here we present crystal structures of Pseudomonas aeruginosa AnmK in complex with its natural substrate, anhMurNAc, and a product of the reaction, ADP. AnmK is homodimeric, with each subunit comprised of two subdomains that are separated by a deep active site cleft, which bears similarity to the ATPase core of proteins belonging to the hexokinase-hsp70-actin superfamily of proteins. The conversion of anhMurNAc to MurNAc-6-P involves both cleavage of the 1,6-anhydro ring of anhMurNAc along with addition of a phosphoryl group to O6 of the sugar, and thus represents an unusual enzymatic mechanism involving the formal addition of H3PO4 to anhMurNAc. The structural complexes and NMR analysis of the reaction suggest that a water molecule, activated by Asp-182, attacks the anomeric carbon of anhMurNAc, aiding cleavage of the 1,6-anhydro bond and facilitating the capture of the ? phosphate of ATP by O6 via an in-line phosphoryl transfer. AnmK is active only against anhMurNAc and not the metabolically related 1,6-anhydro-N-acetylmuramyl peptides, suggesting that the cytosolic N-acetyl-anhydromuramyl-l-alanine amidase AmpD must first remove the stem peptide from these PG muropeptide catabolites before anhMurNAc can be acted upon by AnmK. Our studies provide the foundation for a mechanistic model for the dual activities of AnmK as a hydrolase and a kinase of an unusual heterocyclic monosaccharide. PMID:21288904

Bacik, John-Paul; Whitworth, Garrett E.; Stubbs, Keith A.; Yadav, Anuj K.; Martin, Dylan R.; Bailey-Elkin, Ben A.; Vocadlo, David J.; Mark, Brian L.

2011-01-01

171

Organization of the BcgI restriction-modification protein for the cleavage of eight phosphodiester bonds in DNA  

PubMed Central

Type IIB restriction-modification systems, such as BcgI, feature a single protein with both endonuclease and methyltransferase activities. Type IIB nucleases require two recognition sites and cut both strands on both sides of their unmodified sites. BcgI cuts all eight target phosphodiester bonds before dissociation. The BcgI protein contains A and B polypeptides in a 2:1 ratio: A has one catalytic centre for each activity; B recognizes the DNA. We show here that BcgI is organized as A2B protomers, with B at its centre, but that these protomers self-associate to assemblies containing several A2B units. Moreover, like the well known FokI nuclease, BcgI bound to its site has to recruit additional protomers before it can cut DNA. DNA-bound BcgI can alternatively be activated by excess A subunits, much like the activation of FokI by its catalytic domain. Eight A subunits, each with one centre for nuclease activity, are presumably needed to cut the eight bonds cleaved by BcgI. Its nuclease reaction may thus involve two A2B units, each bound to a recognition site, with two more A2B units bridging the complexes by protein–protein interactions between the nuclease domains. PMID:23147005

Smith, Rachel M.; Marshall, Jacqueline J. T.; Jacklin, Alistair J.; Retter, Susan E.; Halford, Stephen E.; Sobott, Frank

2013-01-01

172

Efficient catalytic addition of aromatic carbon-hydrogen bonds to olefins  

Microsoft Academic Search

The selective cleavage of carbon-hydrogen bonds in organic compounds is a critical step in many organic syntheses, and is particularly important in the conversion of hydrocarbons to useful organic compounds. An organometallic ruthenium complex can cleave C-H bonds in a variety of aromatic systems, leading to addition to alkenes by C-C bond formation. The catalyst operates with a degree of

Shinji Murai; Fumitoshi Kakiuchi; Shinya Sekine; Yasuo Tanaka; Asayuki Kamatani; Motohiro Sonoda; Naoto Chatani

1993-01-01

173

The Antibiotic Dehydrophos Is Converted to a Toxic Pyruvate Analog by Peptide Bond Cleavage in Salmonella enterica?  

PubMed Central

The metabolic processing of dehydrophos, a broad-spectrum peptide antibiotic containing an unusual vinyl-phosphonate moiety, was examined by using a panel of Salmonella enterica mutants deficient in peptide uptake and catabolism. Dehydrophos bioactivity is lost in opp tpp double mutants, demonstrating a requirement for uptake via nonspecific oligopeptide permeases. Dehydrophos bioactivity is also abolished in a quadruple Salmonella mutant lacking the genes encoding peptidases A, B, D, and N, showing that hydrolysis of the peptide bond is required for activity. 31P nuclear magnetic resonance spectroscopy was used to assess the fate of dehydrophos following in vitro digestion of the antibiotic with purified PepA. The results suggest that the initial product of peptidase processing is 1-aminovinyl-phosphonate O-methyl ester. This phosphonate analogue of dehydroalanine undergoes rearrangement to the more stable imine, followed by spontaneous hydrolysis to yield O-methyl-acetylphosphonate, a structural analogue of pyruvate. This compound is a known inhibitor of pyruvate dehydrogenase and pyruvate oxidase and is probably the active species responsible for dehydrophos bioactivity. PMID:21537024

Circello, Benjamin T.; Miller, Charles G.; Lee, Jin-Hee; van der Donk, Wilfred A.; Metcalf, William W.

2011-01-01

174

Recognition and cleavage of single-stranded DNA containing hairpin structures by oligonucleotides forming both Watson-Crick and Hoogsteen hydrogen bonds.  

PubMed

A new approach is described to design antisense oligonucleotides targeted against single-stranded nucleic acids containing hairpin structures by use of both Watson-Crick and Hoogsteen hydrogen bond interactions for recognition. The oligonucleotide has two different domains, one allowing double helix formation involving Watson-Crick base pairs and the other one forming a triple helix involving Hoogsteen-type base triplets in the major groove of a hairpin stem. Spectroscopic and gel retardation experiments provided evidence for such Watson-Crick/Hoogsteen (WC/H) recognition of hairpin structures in single-stranded DNA. An antisense oligonucleotide designed to form only Watson-Crick base pairs was unable to disrupt the stable stem structure of the target under conditions where the oligonucleotide designed with the Watson-Crick/Hoogsteen interactions could bind efficiently to the hairpin-containing target. The addition of one nucleotide to the oligonucleotide at the junction between the double helix and triple helix regions in WC/H complexes had an effect on stability which was dependent on the relative orientation of the Watson-Crick and Hoogsteen domains in the target. An oligodeoxynucleotide-phenanthroline conjugate targeted against such a hairpin-containing DNA fragment induced specific cleavage in the double-stranded stem. This WC/H approach may be useful in designing artificial regulators of gene expression. PMID:7819224

François, J C; Hélène, C

1995-01-10

175

C-O bond cleavage of dimethyl ether by transition metal ions: a systematic study on catalytic properties of metals and performance of DFT functionals.  

PubMed

Studies were focused on late 3d and 4d transition metal ion (Fe, Co, Ni, Cu, Ru, Rh, Pd, and Ag) mediated activation of dimethyl ether, to investigate the intrinsic catalytic properties of metals on C-O bond cleavage. A set of density functional (DFT) methods (BLYP, B3LYP, M06, M06-L, B97-1, B97-D, TPSS, and PBE0) with aug-cc-pVTZ were utilized, and the results were calibrated with CCSD(T)/CBS. The utility of CCSD(T)/CBS calculations for these systems was validated by MRCI/aug-cc-pVTZ calculations. Calculations showed an interesting energetic trend as a function of metal; earlier transition metals tend to give smaller reaction barriers and more exergonic reactions than later metals. This applies to both 3d and 4d systems. For the performance of DFT functionals, PBE0 gave the lowest root mean squared deviations (RMSDs) in terms of both reaction energies and barriers for both 3d and 4d systems, compared to the other functionals. Our studies found that the percentage of Hartree-Fock (HF) exchange plays an important role in the accuracy of DFT methods for these systems, and 26% HF exchange for 3d systems and 34% HF exchange for 4d systems gave the lowest RMSDs. PMID:23650902

Liu, Cong; Peterson, Charles; Wilson, Angela K

2013-06-20

176

Intramolecular C?H/O?H Bond Cleavage with Water and Alcohol Using a Phosphine-Free Ruthenium Carbene NCN Pincer Complex.  

PubMed

Transition metal complexes that exhibit metal-ligand cooperative reactivity could be suitable candidates for applications in water splitting. Ideally, the ligands around the metal should not contain oxidizable donor atoms, such as phosphines. With this goal in mind, we report new phosphine-free ruthenium NCN pincer complexes with a central N-heterocyclic carbene donor and methylpyridyl N-donors. Reaction with base generates a neutral, dearomatized alkoxo-amido complex, which has been structurally and spectroscopically characterized. The tert-butoxide ligand facilitates regioselective, intramolecular proton transfer through a C?H/O?H bond cleavage process occurring at room temperature. Kinetic and thermodynamic data have been obtained by VT NMR experiments; DFT calculations support the observed behavior. Isolation and structural characterization of a doubly dearomatized phosphine complex also strongly supports our mechanistic proposal. The alkoxo-amido complex reacts with water to form a dearomatized ruthenium hydroxide complex, a first step towards phosphine-free metal-ligand cooperative water splitting. PMID:25266279

Prokopchuk, Demyan E; Tsui, Brian T H; Lough, Alan J; Morris, Robert H

2014-12-15

177

Mechanistic Elucidation of the Stepwise Formation of a Tetranuclear Manganese Pinned Butterfly Cluster via N-N Bond Cleavage, Hydrogen Atom Transfer, and Cluster Rearrangement.  

PubMed

A mechanistic pathway for the formation of the structurally characterized manganese-amide-hydrazide pinned butterfly complex, Mn4(?3-PhN-NPh-?(3)N,N')2(?-PhN-NPh-?(2)-N,N')(?-NHPh)2L4 (L = THF, py), is proposed and supported by the use of labeling studies, kinetic measurements, kinetic competition experiments, kinetic isotope effects, and hydrogen atom transfer reagent substitution, and via the isolation and characterization of intermediates using X-ray diffraction and electron paramagnetic resonance spectroscopy. The data support a formation mechanism whereby bis[bis(trimethylsilyl)amido]manganese(II) (Mn(NR2)2, where R = SiMe3) reacts with N,N'-diphenylhydrazine (PhNHNHPh) via initial proton transfer, followed by reductive N-N bond cleavage to form a long-lived Mn(IV) imido multinuclear complex. Coordinating solvents activate this cluster for abstraction of hydrogen atoms from an additional equivalent of PhNHNHPh resulting in a Mn(II)phenylamido dimer, Mn2(?-NHPh)2(NR2)2L2. This dimeric complex further assembles in fast steps with two additional equivalents of PhNHNHPh replacing the terminal silylamido ligands with ?(1)-hydrazine ligands to give a dimeric Mn2(?-NHPh)2(PhN-NHPh)2L4 intermediate, and finally, the addition of two additional equivalents of Mn(NR2)2 and PhNHNHPh gives the pinned butterfly cluster. PMID:25424971

Hamilton, Clifton R; Gau, Michael R; Baglia, Regina A; McWilliams, Sean F; Zdilla, Michael J

2014-12-31

178

Reactivity of triruthenium thiophyne and furyne clusters: competitive S-C and P-C bond cleavage reactions and the generation of highly unsymmetrical alkyne ligands.  

PubMed

The synthesis and reactivity of the thiophyne and furyne clusters [Ru3(CO)7(mu-dppm)(mu3-eta2-C4H2E)(mu-P(C4H3E)2)(mu-H)] (E = S, O) is reported. Addition of P(C4H3E)3 to [Ru3(CO)10(mu-dppm)] (1) at room temperature in the presence of Me3NO gives simple substitution products [Ru3(CO)9(mu-dppm)(P(C4H3E)3)] (E = S, 2; E = O, 3). Mild thermolysis in the presence of further Me3NO affords the thiophyne and furyne complexes [Ru3(CO)7(mu-dppm)(mu3-eta2-C4H2E)(mu-P(C4H3E)2)(mu-H)] (E = S, 4; E = O, 6) resulting from both carbon-hydrogen and carbon-phosphorus bond activation. In each the C4H2E (E = S, O) ligand donates 4-electrons to the cluster and the rings are tilted with respect to the mu-dppm and the phosphido-bridged open triruthenium unit. Heating 4 at 80 degrees C leads to the formation of the ring-opened cluster [Ru3(CO)5(mu-CO)(mu-dppm)(mu3-eta3-SC4H3)(mu-P(C4H3S)2)] (5) resulting from carbon-sulfur bond scission and carbon-hydrogen bond formation and containing a ring-opened mu3-eta3-1-thia-1,3-butadiene ligand. In contrast, a similar thermolysis of 3 affords the phosphinidene cluster [Ru3(CO)7(mu-dppm)(mu3-eta2-C4H2O)(mu3-P(C4H3O))] (7) resulting from a second phosphorus-carbon bond cleavage and (presumably) elimination of furan. Treatment of 4 and 6 with PPh3 affords the simple phosphine-substituted products [Ru3(CO)6(PPh3)(mu-dppm)(mu3-eta2-C4H2E)(mu-P(C4H3E)2)(mu-H)] (E = S, 8; E = O, 9). Both thiophyne and furyne clusters 4 and 6 readily react with hydrogen bromide to give [Ru3(CO)6Br(mu-Br)(mu-dppm)(mu3-eta2-eta1-C4H2E)(mu-P(C4H3E)2)(mu-H)] (E = S, 10; E = O, 11) containing both terminal and bridging bromides. Here the alkynes bind in a highly unsymmetrical manner with one carbon acting as a bridging alkylidene and the second as a terminally bonded Fisher carbene. As far as we are aware, this binding mode has only previously been noted in ynamine complexes or those with metals in different oxidation states. The crystal structures of seven of these new triruthenium clusters have been carried out, allowing a detailed analysis of the relative orientations of coordinated ligands. PMID:18985255

Uddin, Md Nazim; Begum, Noorjahan; Hassan, Mohammad R; Hogarth, Graeme; Kabir, Shariff E; Miah, Md Arzu; Nordlander, Ebbe; Tocher, Derek A

2008-11-28

179

Transition metal-mediated C?O and C?C bond-forming reactions: a regioselective strategy for the synthesis of imidazo[1,2-a]pyridines and imidazo[1,2-a]pyrazines.  

PubMed

A novel and convenient transformation for the regiospecific synthesis of functionalized imidazo[1,2-a]pyridine aldehydes/ketones and 3-vinyl imidazo[1,2-a]pyridines has been developed via copper(I)- and palladium(II)-catalyzed cyclization. The one-pot reaction proceeds smoothly with commercially available catalysts and affords the products in moderate to good yields. It represents an efficient approach for the formation of C-N, C?O, and C?C bonds under mild conditions. PMID:25369461

Cao, Hua; Liu, Xiaohang; Liao, Jinqiang; Huang, Jianping; Qiu, Huifang; Chen, Qinlin; Chen, Yaoyi

2014-11-21

180

Rapid carbon-carbon bond formation and cleavage revealed by carbon isotope exchange between the carboxyl carbon and inorganic carbon in hydrothermal fluids  

NASA Astrophysics Data System (ADS)

The carbon isotopic composition of organic compounds in water-rock systems (e.g., hydrothermal vents, sedimentary basins, and carbonaceous meteorites) is generally interpreted in terms of the isotopic composition of the sources of such molecules, and the kinetic isotope effects of metabolic or abiotic reactions that generate or transform such molecules. This hinges on the expectation that the carbon isotopic composition of many organic compounds is conserved under geochemical conditions. This expectation is reasonable in light of the strength of carbon-carbon bonds (ca. 81 kcal/mol); in general, environmental conditions conducive to carbon-carbon bond cleavage typically lead to transformations of organic molecules (decarboxylation is a notable example). Geochemically relevant reactions that involve isotopic exchange between carbon atoms in organic molecules and inorganic forms of carbon with no change in molecular structure appear to be rare. Notwithstanding such rarity, there have been preliminary reports of relatively rapid carbon isotope exchange between the carboxyl group in carboxylic acids and carbon dioxide in hot water [1,2]. We have performed laboratory hydrothermal experiments to gain insights into the mechanism of this surprising reaction, using phenylacetate as a model structure. By mass spectrometry, we confirm that the carboxyl carbon undergoes facile isotopic exchange with 13C-labeled bicarbonate at moderate temperatures (i.e., 230 C). Detailed kinetic analysis reveals that the reaction rate is proportional to the concentrations of both reactants. Further experiments demonstrate that the exchange reaction only occurs if the carbon atom adjacent to the carboxyl carbon is bonded to a hydrogen atom. As an example, no carbon isotope exchange was observed for benzoate in experiments lasting up to one month. The requirement of an alpha C-H bond suggests that enolization (i.e., deprotonation of the H) is a critical step in the mechanism of the exchange reaction. We confirmed this by determining the exchange kinetics for various ring-substituted phenylacetates, and we find that the observed rate constants are consistent with the amount of negative charge that would be expected on the alpha carbon of the enolate. The emerging picture of the reaction mechanism is that enolization allows fixation of labeled carbon dioxide as a beta-carboxy group, where subsequent heterolytic decarboxylation of the unlabeled carboxy group completes the exchange process. Solvent stabilization of ionic reaction intermediates would lead to faster than expected rates of exchange, and ultimately to an isotopic equilibrium over even short geologic timescales. For enolizable carboxylic acids in natural systems, one has to consider this exchange mechanism when interpreting the carbon isotopic composition. [1] Dias R.F. (2000) Ph.D. Dissertation, Penn. State Univ. [2] Seewald J.S. & Boekelheide N. (2005) Goldschmidt Conf., A558.

Glein, C. R.; Cody, G. D.

2013-12-01

181

Formation of a dinuclear copper(II) complex through the cleavage of CN bond of 1-benzoyl-3-(pyridin-2-yl)-1H-pyrazole  

NASA Astrophysics Data System (ADS)

A simple mononuclear octahedral copper(II) complex was attempted from the reaction of three moles of 1-benzoyl-3-(pyridin-2-yl)-1H-pyrazole and one mole of copper(II) perchlorate hexahydrate in methanol. However, the product of the reaction was confirmed to be a dinuclear copper(II) complex with ?-{3-(pyridin-2-yl)-pyrazolato} and 3-(pyridin-2-yl)-1H-pyrazole ligands attached to each of the Cu(II) centre atom. The copper(II) ion assisted the cleavage of the CbenzoylN bond afforded a 3-(pyridin-2-yl)-1H-pyrazole molecule. Deprotonation of the 3-(pyridin-2-yl)-1H-pyrazole gave a 3-(pyridin-2-yl)-pyrazolato, which subsequently reacted with the Cu(II) ion to give the {3-(pyridin-2-yl)-pyrazolato}{3-(pyridin-2-yl)-1H-pyrazole}Cu(II) product moiety. The structure of the dinuclear complex was confirmed by x-ray crystallography. The complex crystallized in a monoclinic crystal system with P2(1)/n space group and cell dimensions of a = 12.2029(8) Å, b = 11.4010(7) Å, c = 14.4052(9) Å and ? = 102.414(2)°. The compound was further characterized by mass spectrometry, CHN elemental analysis, infrared and UV-visible spectroscopy and the results concurred with the x-ray structure. The presence of d-d transition at 671 nm (? = 116 dm3 mol-1 cm-1) supports the presence of Cu(II) centres.

Shardin, Rosidah; Pui, Law Kung; Yamin, Bohari M.; Kassim, Mohammad B.

2014-09-01

182

Bonding  

NSDL National Science Digital Library

Bonding Purpose: To review the 3 Types of Bonds and the 4 Intermolecular Forces. Assignment: Answer the following questions on a separate piece of paper: 1) List the 3 types of bonding, give descriptions and one example of each 2) List the 4 intermolecular forces and draw examples of each. Be ...

Cutting, Mrs.

2008-03-08

183

Activation of C-O and C-C bonds and formation of novel HAlOH-ether complexes: an EPR study of the reaction of ground-state Al atoms with methylethyl ether and diethyl ether.  

PubMed

Reaction mixtures, containing Al atoms and methylethyl ether (MEE) or diethyl ether (DEE) in an adamantane matrix, were prepared with the aid of a metal-atom reactor known as a rotating cryostat. The EPR spectra of the resulting products were recorded from 77-260 K, at 10 K intervals. Al atoms were found to insert into methyl-O, ethyl-O, and C-C bonds to form CH(3)AlOCH(2)CH(3), CH(3)OAlCH(2)CH(3), and CH(3)OCH(2)AlCH(3), respectively, in the case of MEE while DEE produced CH(3)CH(2)AlOCH(2)CH(3) and CH(3)AlCH(2)OCH(2)CH(3), respectively. From the intensity of the transition lines attributed to the Al atom C-O insertion products of MEE, insertion into the methyl-O bond is preferred. The Al hyperfine interaction (hfi) extracted from the EPR spectra of the C-O insertion products was greater than that of the C-C insertion products, that is, 5.4% greater for the DEE system and 7% greater for the MEE system. The increase in Al hfi is thought to arise from the increased electron-withdrawing ability of the substituents bonded to Al. Besides HAlOH, resulting from the reaction of Al atoms with adventitious water, novel mixed HAlOH:MEE and HAlOH:DEE complexes were identified with the aid of isotopic studies involving H(2)(17)O and D(2)O. The Al and H hfi of HAlOH were found to decrease upon complex formation. These findings are consistent with the nuclear hfi calculated using a density functional theory (DFT) method with close agreement between theory and experiment occurring at the B3LYP level using a 6-311+G(2df,p) basis set. PMID:22299675

Brunet, François D; Feola, Julie C; Joly, Helen A

2012-03-15

184

A comparative theoretical investigation into the strength of the trigger-bond in the Na?, Mg²? and HF complexes involving the nitro group of R-NO? (R = -CH?, -NH? and -OCH?) or the C = C bond of (E)-O?N-CH = CH-NO?.  

PubMed

A comparative theoretical investigation into the change in strength of the trigger-bond upon formation of the Na(+), Mg(2+) and HF complexes involving the nitro group of RNO? (R = -CH?, -NH?, -OCH?) or the C = C bond of (E)-O?N-CH = CH-NO? was carried out using the B3LYP and MP2(full) methods with the 6-311++G**, 6-311++G(2df,2p) and aug-cc-pVTZ basis sets. Except for the Mg(2+)?? system with (E)-O2N-CH = CH-NO? (i.e., C?H?N?O??Mg(2+)), the strength of the trigger-bond X-NO? (X = C, N or O) was enhanced upon complex formation. Furthermore, the increment of bond dissociation energy of the X-NO? bond in the Na(+) complex was far greater than that in the corresponding HF system. Thus, the explosive sensitivity in the former might be lower than that in the latter. For C?H?N?O??Mg(2+), the explosive sensitivity might also be reduced. Therefore, it is possible that introducing cations into the structure of explosives might be more efficacious at reducing explosive sensitivity than the formation of an intermolecular hydrogen-bonded complex. AIM, NBO and electron density shifts analyses showed that the electron density shifted toward the X-NO? bond upon complex formation, leading to a strengthened X-NO? bond and possibly reduced explosive sensitivity. PMID:23446440

Zhang, Lin; Ren, Fu-de; Cao, Duan-lin; Wang, Jian-long; Gao, Jian-feng

2013-06-01

185

Reactivity of Tp(Me2) -supported yttrium alkyl complexes toward aromatic N-heterocycles: ring-opening or C-C bond formation directed by C-H activation.  

PubMed

Unusual chemical transformations such as three-component combination and ring-opening of N-heterocycles or formation of a carbon-carbon double bond through multiple C-H activation were observed in the reactions of Tp(Me2) -supported yttrium alkyl complexes with aromatic N-heterocycles. The scorpionate-anchored yttrium dialkyl complex [Tp(Me2) Y(CH2 Ph)2 (THF)] reacted with 1-methylimidazole in 1:2 molar ratio to give a rare hexanuclear 24-membered rare-earth metallomacrocyclic compound [Tp(Me2) Y(?-N,C-Im)(?(2) -N,C-Im)]6 (1; Im=1-methylimidazolyl) through two kinds of C-H activations at the C2- and C5-positions of the imidazole ring. However, [Tp(Me2) Y(CH2 Ph)2 (THF)] reacted with two equivalents of 1-methylbenzimidazole to afford a C-C coupling/ring-opening/C-C coupling product [Tp(Me2) Y{?(3) -(N,N,N)-N(CH3 )C6 H4 NHCH?C(Ph)CN(CH3 )C6 H4 NH}] (2). Further investigations indicated that [Tp(Me2) Y(CH2 Ph)2 (THF)] reacted with benzothiazole in 1:1 or 1:2 molar ratio to produce a C-C coupling/ring-opening product {(Tp(Me2) )Y[?-?(2) :?(1) -SC6 H4 N(CH?CHPh)](THF)}2 (3). Moreover, the mixed Tp(Me2) /Cp yttrium monoalkyl complex [(Tp(Me2) )CpYCH2 Ph(THF)] reacted with two equivalents of 1-methylimidazole in THF at room temperature to afford a trinuclear yttrium complex [Tp(Me2) CpY(?-N,C-Im)]3 (5), whereas when the above reaction was carried out at 55?°C for two days, two structurally characterized metal complexes [Tp(Me2) Y(Im-Tp(Me2) )] (7; Im-Tp(Me2) =1-methyl-imidazolyl-Tp(Me2) ) and [Cp3 Y(HIm)] (8; HIm=1-methylimidazole) were obtained in 26 and 17?% isolated yields, respectively, accompanied by some unidentified materials. The formation of 7 reveals an uncommon example of construction of a C?C bond through multiple C-H activations. PMID:24339020

Yi, Weiyin; Zhang, Jie; Huang, Shujian; Weng, Linhong; Zhou, Xigeng

2014-01-13

186

A novel chelate-assisted C?C bond formation on a Cd(II) complex of an asymmetric heptadentate(N 7) tripodal Schiff base ligand  

Microsoft Academic Search

Cadmium(II) complexes of some potentially heptadentate tripodal Schiff base ligands with the general formula N{(CH2)nNCHC5H4N}2{(CH2)mNCHC5H4N}) (where n=m=2, L222; n=2, m=3, L322; n=3, m=2, L332) have been studied. These ligands are products of the full condensation of a number of tripodal tetraamines with 2-pyridinecarboxaldehyde. An unusual reaction of an additional 2-pyridine-carboxaldehyde with a methylene group adjacent to the imine bond only

Sadegh Salehzadeh; Michael D. Ward; Harry Adams

2009-01-01

187

The reaction mechanism of the enantioselective Tsuji allylation: inner-sphere and outer-sphere pathways, internal rearrangements, and asymmetric C–C bond formation  

PubMed Central

We use first principles quantum mechanics (density functional theory) to report a detailed reaction mechanism of the asymmetric Tsuji allylation involving prochiral nucleophiles and non-prochiral allyl fragments, which is consistent with experimental findings. The observed enantioselectivity is best explained with an inner-sphere mechanism involving the formation of a 5-coordinate Pd species that undergoes a ligand rearrangement, which is selective with regard to the prochiral faces of the intermediate enolate. Subsequent reductive elimination generates the product and a Pd0 complex. The reductive elimination occurs via an unconventional seven-centered transition state that contrasts dramatically with the standard three-centered C–C reductive elimination mechanism. Although limitations in the present theory prevent the conclusive identification of the enantioselective step, we note that three different computational schemes using different levels of theory all find that inner-sphere pathways are lower in energy than outer-sphere pathways. This result qualitatively contrasts with established allylation reaction mechanisms involving prochiral nucleophiles and prochiral allyl fragments. Energetic profiles of all reaction pathways are presented in detail. PMID:23102088

Keith, John A.; Behenna, Douglas C.; Sherden, Nathaniel; Mohr, Justin T.; Ma, Sandy; Marinescu, Smaranda C.; Nielsen, Robert J.; Oxgaard, Jonas

2012-01-01

188

Synthetic Approaches to (smif)2Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) Reveal Redox Non-Innocence and C-C Bond-Formation  

PubMed Central

Attempted syntheses of (smif)2Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) based on metatheses of TiClnLm (n = 2–4) with M(smif) (M = Li, Na), in the presence of a reducing agent (Na/Hg) when necessary, failed, but several apparent Ti(II) species were identified by X-ray crystallography and multidimensional NMR spectroscopy: (smif){Li(smif-smif)}Ti (1, X-ray), [(smif)Ti]2(?-?3,?3-N,N(py)2-smif,smif) (2), (smif)Ti(?3-N,N(py)2-smif,(smif)H) (3), and (smif)Ti(dpma) (4). NMR spectroscopy and K-edge XAS showed that each compound possesses ligands that are redox non-innnocent, such that d1 Ti(III) centers AF-couple to ligand radicals: (smif){Li(smif-smif)2?}TiIII (1), [(smif2?)TiIII]2(?-?3,?3-N,N(py)2-smif,smif) (2), [(smif2?)TiIII](?3-N,N(py)2-smif,(smif)H) (3), and (smif2?)TiIII(dpma) (4). The instability of the (smif)2Ti relative to its C-C coupled dimer, 2, is rationalized via the complementary nature of the amide and smif radical anion ligands, which are also common to 3 and 4. Calculations support this contention. PMID:22830452

Frazier, Brenda A.; Wolczanski, Peter T.; Keresztes, Ivan; DeBeer, Serena; Lobkovsky, Emil B.; Pierpont, Aaron W.; Cundari, Thomas R.

2012-01-01

189

In vitro characterization of a carotenoid cleavage dioxygenase from Nostoc sp. PCC 7120 reveals a novel cleavage pattern, cytosolic localization and induction by highlight.  

PubMed

Carotenoid oxygenases catalyse the cleavage of C-C double bonds forming apocarotenoids, a diverse group of compounds, including retinoids and the precursors of some phytohormones. Some apocarotenoids, like beta-ionone (C(13)), are ecologically important volatiles released by plants and cyanobacteria. In this work, we elucidated the activity of the Nostoccarotenoid cleavage dioxygenase (NosCCD, previously named NSC1) using synthetic and cyanobacterial substrates. NosCCD converted bicyclic and monocyclic xanthophylls, including myxoxanthophylls, glycosylated carotenoids that are essential for thylakoid and cell wall structure. The products identified revealed two different cleavage patterns. The first is observed with bicyclic xanthophylls and is identical with that of plant orthologues, while the second is novel and occurs upon cleavage of monocyclic substrates at the C9-C10 and C7'-C8' double bonds. These properties enable the enzyme to produce a plenitude of different C(10) and C(13) apocarotenoids. Expression analyses indicated a role of NosCCD in response to highlight stress. Western blot analyses of Nostoc cells revealed NosCCD as a soluble enzyme in the cytosol, which also accomodates NosCCD substrates. Incubation of the corresponding fraction with synthetic substrates revealed the activity of the native enzyme and confirmed its induction by highlight. PMID:18485074

Scherzinger, Daniel; Al-Babili, Salim

2008-07-01

190

Preparation of phosphinated polymer-incarcerated palladium and its application to C-N and C-C bond-forming reactions.  

PubMed

Phosphinated polymer-incarcerated (PI) Pd catalysts were prepared by immobilization of palladium with phosphinated polymers by using the PI method. The phosphinated PI Pd catalysts showed good catalytic activity without externally added phosphine ligands in the amination of aryl halides for C-N bond-forming reactions, as well as in Suzuki-Miyaura and Sonogashira coupling. No leaching of palladium from the immobilized Pd was observed by fluorescence X-ray analysis. Furthermore, it was found that immobilization of Pd by the PI process facilitated the suppression of poisoning of the metal by amines. These effects can be ascribed to stabilization of the catalyst by both the phosphine moieties and the benzene rings in the swollen polymer support. The phosphinated PI Pd catalysts could also be recovered by simple filtration and reused several times without leaching of palladium in both the amination and Suzuki-Miyaura coupling reactions. PMID:17591724

Nishio, Ryo; Sugiura, Masaharu; Kobayashi, Shu

2007-08-01

191

C-C and C-Heteroatom Bond Dissociation Energies in CH3R?C(OH)2: Energetics for Photocatalytic Processes of Organic Diolates on TiO2 Surfaces  

SciTech Connect

The bond energies of a range of gem-diols, CH3R?C(OH)2 (R? = H, F, Cl, Br, CN, NO2, CF3, CH3CH2, CH3CH2CH2, CH3CH2CH2CH2, ((CH3)2)CH, (CH3)3C, ((CH3)2CH)CH2, (CH3CH2)(CH3)CH, C6H5 (CH3CH2)(CH3)CH) which serve as models for binding to a surface have been studied with density functional theory (DFT) and the molecular orbital G3(MP2) methods to provide thermodynamic data for the analysis of the photochemistry of ketones on TiO2. The ultraviolet (UV) photon-induced photodecomposition of adsorbed acetone and 3,3-dimethylbutanone on the rutile TiO2 (110) surface have been investigated with photon stimulated desorption (PSD) and temperature programmed desorption (TPD). The C-CH3 and C-C(R?) bond dissociation energies in CH3R?C(OH)2 were predicted, and our calculated bond dissociation energies are in excellent agreement with the available experimental values. We used a series of isodemic reactions to provide small corrections to the various bond dissociation energies. The calculated bond dissociation energies are in agreement with the observed photodissociation processes except for R? = CF3, suggesting that these processes are under thermodynamic control. For R? = CF3, reaction dynamics also play a role in determining the photodissociation mechanism. The gas phase Brönsted acidities of the gem-diols were calculated. For three molecules, R? = Cl, Br, and NO2, loss of a proton leads to the formation of a complex of acetic acid with the anion Cl-, Br-, and NO2-. The acidities of these three species are very high with the former two having acidities comparable to CF3SO3H. The ketones (R?RC(=O)) are weak Lewis acids except where addition of OH- leads to the dissociation of the complex to form an anion bonded to acetic acid, R' = NO2, Cl, and Br. The X-C bond dissociation energies for a number of X-CO2- species were calculated and these should be useful in correlating with photochemical reactivity studies.

Wang, Tsang-Hsiu; Dixon, David A.; Henderson, Michael A.

2010-08-26

192

Elementary Steps of Iron Catalysis: Exploring the Links between Iron Alkyl and Iron Olefin Complexes for their Relevance in C?H Activation and C?C Bond Formation.  

PubMed

The alkylation of complexes 2 and 7 with Grignard reagents containing ?-hydrogen atoms is a process of considerable relevance for the understanding of C-H activation as well as C-C bond formation mediated by low-valent iron species. Specifically, reaction of 2 with EtMgBr under an ethylene atmosphere affords the bis-ethylene complex 1 which is an active precatalyst for prototype [2+2+2] cycloaddition reactions and a valuable probe for mechanistic studies. This aspect is illustrated by its conversion into the bis-alkyne complex 6 as an unprecedented representation of a cycloaddition catalyst loaded with two substrates molecules. On the other hand, alkylation of 2 with 1?equivalent of cyclohexylmagnesium bromide furnished the unique iron alkyl species 11 with a 14-electron count, which has no less than four ?-H atoms but is nevertheless stable at low temperature against ?-hydride elimination. In contrast, the exhaustive alkylation of 1 with cyclohexylmagnesium bromide triggers two consecutive C-H activation reactions mediated by a single iron center. The resulting complex has a diene dihydride character in solution (15), whereas its structure in the solid state is more consistent with an ?(3) -allyl iron hydride rendition featuring an additional agostic interaction (14). Finally, the preparation of the cyclopentadienyl iron complex 25 illustrates how an iron-mediated C-H activation cascade can be coaxed to induce a stereoselective C?C bond formation. The structures of all relevant new iron complexes in the solid state are presented. PMID:25504935

Casitas, Alicia; Krause, Helga; Goddard, Richard; Fürstner, Alois

2015-01-26

193

Reactivity studies of a pseudo three-coordinate vanadium(II) complex: Synthesis of terminal oxo and sulfido complexes of vanadium(IV) and S?S and Se?Se reductive bond cleavage reactions  

SciTech Connect

Terminal oxo and sulfido complexes in the form of (nacnac)V=E(Ntol{sub 2}) (nacnac = [ArNC(CH{sub 3})]{sub 2}CH{sup -}, Ar = 2,6-(CHMe{sub 2}){sub 2}C{sub 6}H{sub 3}, Ntol{sub 2} = {sup -}N(C{sub 6}H{sub 4}-4-Me), E = O (1), S (2)) were isolated from treatment of the masked three-coordinate vanadium(II) complex, (nacnac)V(Ntol{sub 2}), with C{sub 5}H{sub 5}NO and S{sub 8}, respectively. Both vanadium(IV) species, 1 and 2, have been characterized by room temperature X-band EPR spectroscopic studies, and in the case of complex 1, a single crystal molecular structure confirmed the presence of a terminal oxo moiety. Moreover, reaction of (nacnac)V(Ntol{sub 2}) with diphenyl-disulfide and diphenyl-diselenide results in the reductive cleavage of these compounds to produce the vanadium(III) complexes (nacnac)V(XPh)(Ntol{sub 2}) (X = S, (3), Se (4)). A molecular structure of the phenylsulfide complex, 3, confirmed formation of the d{sup 2} complex resulting from reductive cleavage of the S-S bond.

Tran, Ba L.; Chen, Chun-Hsing; Mindiola, Daniel J. (Indiana)

2012-02-07

194

Palladium-catalyzed one-pot three- or four-component coupling of aryl iodides, alkynes, and amines through C-N bond cleavage: efficient synthesis of indole derivatives.  

PubMed

An efficient synthesis of N-substituted indole derivatives was realized by combining the Pd-catalyzed one-pot multicomponent coupling approach with cleavage of the C(sp(3))-N bonds. Three or four components of aryl iodides, alkynes, and amines were involved in this coupling process. The cyclopentadiene-phosphine ligand showed high efficiency. A variety of aryl iodides, including cyclic and acyclic tertiary amino aryl iodides, and substituted 1-bromo-2-iodobenzene derivatives could be used. Both symmetric and unsymmetric alkynes substituted with alkyl, aryl, or trimethylsilyl groups could be applied. Cyclic secondary amines such as piperidine, morpholine, 4-methylpiperidine, 1-methylpiperazine, 2-methylpiperidine, and acyclic amines including secondary and primary amines all showed good reactivity. Further application of the resulting indole derivatives was demonstrated by the synthesis of benzosilolo[2,3-b]indole. PMID:24449487

Hao, Wei; Geng, Weizhi; Zhang, Wen-Xiong; Xi, Zhenfeng

2014-02-24

195

Oxidative Coupling of Dichalcogenides with Sodium Sulfinates via Copper-Catalyzed Cleavage of S-S and Se-Se Bonds.  

PubMed

A copper-catalyzed sulfonylation of disulfides was achieved using sodium sulfinates in air. The reaction formed various sulfur-sulfone bonds efficiently and afforded thiosulfonates in good yields. Selenosulfonates could also be prepared with this procedure. Furthermore, both chalcogenide groups on the dichalcogenides were available in these reactions. PMID:25562106

Taniguchi, Nobukazu

2015-02-01

196

The Specific Cleavage of Lactone Linkage to Open-Loop in Cyclic Lipopeptide during Negative ESI Tandem Mass Spectrometry: The Hydrogen Bond Interaction Effect of 4-Ethyl Guaiacol  

PubMed Central

Mass spectrometry is a valuable tool for the analysis and identification of chemical compounds, particularly proteins and peptides. Lichenysins G, the major cyclic lipopeptide of lichenysin, and the non-covalent complex of lichenysins G and 4-ethylguaiacol were investigated with negative ion ESI tandem mass spectrometry. The different fragmentation mechanisms for these compounds were investigated. Our study shows the 4-ethylguaiacol hydrogen bond with the carbonyl oxygen of the ester group in the loop of lichenysins G. With the help of this hydrogen bond interaction, the ring structure preferentially opens in lactone linkage rather than O-C bond of the ester-group to produce alcohol and ketene. Isothermal titration 1H-NMR analysis verified the hydrogen bond and determined the proportion of subject and ligand in the non-covalent complex to be 1?1. Theoretical calculations also suggest that the addition of the ligand can affect the energy of the transition structures (TS) during loop opening. PMID:25144459

Guo, Mengzhe; Pan, Youlu; Zhang, Rong; Cao, Yang; Chen, Jianzhong; Pan, Yuanjiang

2014-01-01

197

Transition metal catalyzed manipulation of non-polar carbon–hydrogen bonds for synthetic purpose  

PubMed Central

The direct addition of ortho C–H bonds in various aromatic compounds such as ketones, esters, imines, imidates, nitriles, and aldehydes to olefins and acetylenes can be achieved with the aid of transition metal catalysts. The ruthenium catalyzed reaction is usually highly efficient and useful as a general synthetic method. The coordination to the metal center by a heteroatom in a directing group such as carbonyl and imino groups in aromatic compounds is the key step in this process. Mechanistically, the reductive elimination to form a C–C bond is the rate-determining step, while the C–H bond cleavage step is not. PMID:21558759

MURAI, Shinji

2011-01-01

198

Silicon-Carbon bond cleavage reactions of Ansa tungstenocene compounds: the [Me2Si] bridge as a site for metallocene functionalization.  

PubMed

[Me(2)Si(Cp(Me(2)))(2)]W(H)Cl is obtained via reaction of WCl(6) with a mixture of [Me(2)Si(Cp(Me(2)))(2)]Li(2) and NaBH(4), from which the dichloride [Me(2)Si(Cp(Me(2)))(2)]WCl(2) is obtained via treatment with CHCl(3). [Me(2)Si(Cp(Me(2)))(2)]WCl(2) provides a means to access other ansa tungstenocene compounds, such as [Me(2)Si(Cp(Me(2)))(2)]WH(2), [Me(2)Si(Cp(Me(2)))(2)]WMe(2), and [Me(2)Si(Cp(Me(2)))(2)]WCO. Of most interest, the reactions of [Me(2)Si(Cp(Me(2)))(2)]W(H)Cl with organolithium reagents do not yield simple ansa tungstenocene derivatives. Specifically, the reactions of [Me(2)Si(Cp(Me(2)))(2)]W(H)Cl with MeLi, Bu(n)Li, or PhLi result in the formation of mixed-ring tungstenocene compounds resulting from C-Si cleavage and functionalization of the ansa bridge, namely (Cp(Me(2)))(eta(5),kappa(1)-C(5)H(2)Me(2)SiMe(2)CH(2))WH, (Cp(Me(2)))[eta(5),kappa(1)-C(5)H(2)Me(2)Si(Me)(Bu(n))CH(2)]WH, and (Cp(Me(2)))[eta(5),kappa(1)-C(5)H(2)Me(2)SiMe(2)(C(6)H(4))]WH, respectively. In contrast to the C-Si cleavage achieved by MeLi, Bu(n)Li, and PhLi, the ansa bridge of [Me(2)Si(Cp(Me(2)))(2)]W(H)Cl is inert to Bu(t)Li and the product obtained is the fulvene ("tuck-in") complex [Me(2)Si(Cp(Me(2)))(eta(6)-C(5)MeH(2)CH(2))]WH derived from dehydrohalogenation. PMID:18635687

Zachmanoglou, Cary E; Lee, Hyosun; Jang, Seung Ho; Pang, Keliang; Parkin, Gerard

2008-08-12

199

Dihydrogen Catalysis of the Reversible Formation and Cleavage of C?H and N?H Bonds of Aminopyridinate Ligands Bound to (?(5) -C5 Me5 )Ir(III.).  

PubMed

This study focuses on a series of cationic complexes of iridium that contain aminopyridinate (Ap) ligands bound to an (?(5) -C5 Me5 )Ir(III) fragment. The new complexes have the chemical composition [Ir(Ap)(?(5) -C5 Me5 )](+) , exist in the form of two isomers (1(+) and 2(+) ) and were isolated as salts of the BArF (-) anion (BArF =B[3,5-(CF3 )2 C6 H3 ]4 ). Four Ap ligands that differ in the nature of their bulky aryl substituents at the amido nitrogen atom and pyridinic ring were employed. In the presence of H2 , the electrophilicity of the Ir(III) centre of these complexes allows for a reversible prototropic rearrangement that changes the nature and coordination mode of the aminopyridinate ligand between the well-known ?(2) -N,N'-bidentate binding in 1(+) and the unprecedented ?-N,?(3) -pseudo-allyl-coordination mode in isomers 2(+) through activation of a benzylic C?H bond and formal proton transfer to the amido nitrogen atom. Experimental and computational studies evidence that the overall rearrangement, which entails reversible formation and cleavage of H?H, C?H and N?H bonds, is catalysed by dihydrogen under homogeneous conditions. PMID:25504864

Zamorano, Ana; Rendón, Nuria; López-Serrano, Joaquín; Valpuesta, José E V; Álvarez, Eleuterio; Carmona, Ernesto

2015-02-01

200

Characterization of Co-C bonding in dichlorovinylcobaloxime complexes.  

PubMed

This study combines theory and experiment in an examination of Co-C bonding and reductive Co-C cleavage in cobalt dichlorovinyl complexes. It is motivated by the role of dichlorovinyl complexes as intermediates in the dechlorination of trichloroethylene by cobalamin and cobalamin model complexes. A series of seven cis-1,2-dichlorovinyl(L)cobaloxime complexes were prepared (L = m- and p-substituted pyridines; cobaloxime = bis(dimethylglyoximato)cobalt). The complexes were characterized using 1H NMR, 13C NMR, cyclic voltammetry, and X-ray crystallography. Examination of the metrical parameters of the Co-C=C unit across the series shows very little change in the C=C bond length and a slight increase in the Co-C bond length with increasing electron-donating ability of the pyridine ligand. These structural changes along with electronic structure calculations indicate that Co-C pi-bonding is not important in these complexes. The stronger Co-C bonds of vinylcobaloximes compared to those of alkylcobaloximes are best explained by the higher s character at C. Changes in the reduction potential across the series indicate that the pyridine-bound form is the primary electrochemically active species. Theoretical examination of the Co-C cleavage following reduction supports the direct formation of the cis-1,2-dichlorovinyl anion and not the cis-1,2-dichlorovinyl radical. PMID:17286398

Follett, Angela D; McNabb, Katherine A; Peterson, Alicia A; Scanlon, Joseph D; Cramer, Christopher J; McNeill, Kristopher

2007-03-01

201

C-C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach  

SciTech Connect

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the {sigma}/{pi} hyperconjugation in EtFC and the {pi}-conjugation in VFC and EFC.

Boccia, A.; Lanzilotto, V.; Marrani, A. G.; Zanoni, R. [Dipartimento di Chimica, Universita degli Studi di Roma ''La Sapienza'', piazzale Aldo Moro 5, I-00185 Rome (Italy); Stranges, S. [Dipartimento di Chimica, Universita degli Studi di Roma ''La Sapienza'', piazzale Aldo Moro 5, I-00185 Rome (Italy); IOM-CNR, Laboratorio TASC, I-34149 Basovizza, Trieste (Italy); Alagia, M. [IOM-CNR, Laboratorio TASC, I-34149 Basovizza, Trieste (Italy); Fronzoni, G.; Decleva, P. [Dipartimento di Scienze Chimiche, Universita di Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy and IOM-CNR Democritos, Trieste (Italy)

2012-04-07

202

C-C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach  

NASA Astrophysics Data System (ADS)

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the ?/? hyperconjugation in EtFC and the ?-conjugation in VFC and EFC.

Boccia, A.; Lanzilotto, V.; Marrani, A. G.; Stranges, S.; Zanoni, R.; Alagia, M.; Fronzoni, G.; Decleva, P.

2012-04-01

203

Incorporation of the fluoride induced Si-O bond cleavage and functionalized gold nanoparticle aggregation into one colorimetric probe for highly specific and sensitive detection of fluoride.  

PubMed

A highly selective and sensitive probe was developed for the field test of F(-) in environmental waters. The probe was fabricated by anchoring 4-mercaptopyridine (MPD) on AuNPs via Au-S interaction to form MPD-AuNPs, and further assembling 3-aminopropyltrimethoxysilane (APTMS) on the surface of MPD-AuNPs. The hydrolysis and cross-link of APTMS resulted in a thin monolayer of Si-O-Si protecting layer to encapsulated MPD-AuNPs. In the assay, F(-) reacted with Si-O bond and thus destroyed the outer protecting layer of the probe, and further triggered the aggregation of internal MPD-AuNPs by forming N-H-F hydrogen bond. The F(-) induced aggregation of functionalized AuNPs gave rise to significant solution color switch from red to blue, which facilitated visual assay of F(-) in the range of 1.0-7.0 ?g mL(-1) by naked eyes. The probe is able to discriminate F(-) from a wide range of environmentally dominant ions, thus it can be applied to detect F(-) in drinkable water with satisfactory results that is agreed well with that of using ion chromatography. PMID:24745747

Sun, Jie-Fang; Liu, Rui; Zhang, Zhong-Mian; Liu, Jing-Fu

2014-04-11

204

Elastin Degradation by Matrix Metalloproteinases CLEAVAGE SITE SPECIFICITY AND MECHANISMS OF ELASTOLYSIS*  

E-print Network

Elastin Degradation by Matrix Metalloproteinases CLEAVAGE SITE SPECIFICITY AND MECHANISMS. For both the serine and matrix met- alloproteinases, catalysis of peptide bond cleavage in insoluble being readily accessible to enzymatic attack. Elastin is the extracellular matrix protein that imparts

Mecham, Robert

205

The Tautomeric Half-reaction of BphD, a C-C Bond Hydrolase Kinetic and Structural Evidence Supporting a Key Role for Histidine 265 of the Catalytic triad  

SciTech Connect

BphD of Burkholderia xenovorans LB400 catalyzes an unusual C-C bond hydrolysis of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) to afford benzoic acid and 2-hydroxy-2,4-pentadienoic acid (HPD). An enol-keto tautomerization has been proposed to precede hydrolysis via a gem-diol intermediate. The role of the canonical catalytic triad (Ser-112, His-265, Asp-237) in mediating these two half-reactions remains unclear. We previously reported that the BphD-catalyzed hydrolysis of HOPDA ({lambda}{sub max} is 434 nm for the free enolate) proceeds via an unidentified intermediate with a red-shifted absorption spectrum ({lambda}{sub max} is 492 nm) (Horsman, G. P., Ke, J., Dai, S., Seah, S. Y. K., Bolin, J. T., and Eltis, L. D. (2006) Biochemistry 45, 11071-11086). Here we demonstrate that the S112A variant generates and traps a similar intermediate ({lambda}{sub max} is 506 nm) with a similar rate, 1/{tau} {approx} 500 s{sup -1}. The crystal structure of the S112A:HOPDA complex at 1.8-{angstrom} resolution identified this intermediate as the keto tautomer, (E)-2,6-dioxo-6-phenyl-hex-3-enoate. This keto tautomer did not accumulate in either the H265A or the S112A/H265A double variants, indicating that His-265 catalyzes tautomerization. Consistent with this role, the wild type and S112A enzymes catalyzed tautomerization of the product HPD, whereas H265A variants did not. This study thus identifies a keto intermediate, and demonstrates that the catalytic triad histidine catalyzes the tautomerization half-reaction, expanding the role of this residue from its purely hydrolytic function in other serine hydrolases. Finally, the S112A:HOPDA crystal structure is more consistent with hydrolysis occurring via an acyl-enzyme intermediate than a gem-diol intermediate as solvent molecules have poor access to C6, and the closest ordered water is 7{angstrom} away.

Horsman, Geoff P.; Bhowmik, Shiva; Seah, Stephen Y.K.; Kumar, Pravindra; Bolin, Jeffrey T.; Eltis, Lindsay D. (Purdue); (UBC)

2010-01-07

206

Reactivity of nickel(II) and copper(II) complexes of a ?-aminohydrazone ligand with pyridine-2-aldehyde: macrocyclization vs unprecedented pyrazole ring synthesis via C-C bond-forming reaction.  

PubMed

The synthesis and characterization of a mononuclear nickel(II) complex [Ni(L(2))](ClO(4))(2) (1) and an analogous mononuclear copper(II) complex [Cu(L(2))](ClO(4))(2) (2) of a 15-membered azamacrocycle (L(2) = 3-(2-pyridyl)-6,8,8,13,13,15-hexamethyl-1,2,4,5,9,12-hexaazacyclopentadeca-5,15-diene) are reported. The macrocyclic ligand is formed during the reaction of 4,4,9,9-tetramethyl-5,8-diazadodecane-2,11-dione dihydrazone (L(1)) with pyridine-2-aldehyde (PyCHO) templated by metal ions. The X-ray crystal structure of 1 exhibits a distorted square-pyramidal coordination geometry, where the metal ion sits in the macrocyclic cavity and the pendant pyridine group of L(2) occupies the axial position. While 1 is stable in the presence of an excess of PyCHO, 2 reacts further with copper(II) salt and PyCHO to form a mononuclear copper(I) complex, [Cu(H(2)L(3))](ClO(4))(3) (3). The structure of the complex cation of 3 reveals a distorted tetrahedral coordination geometry at the copper center with a pseudo 2-fold screw axis. A two-dimensional (2D) polymeric copper(II) complex, {[Cu(2)(L(4))(2)](ClO(4))(2)}(n) (4) is obtained by reacting complex 2 (or [Ni(L(1))](ClO(4))(2)) with copper(II) perchlorate and pyridine-2-aldehyde in a methanol-water solvent mixture. Complex 4 is also obtained by treating 3 with copper(II) perchlorate and pyridine-2-aldehyde in the presence of a base. The X-ray structural analysis of 4 confirms the formation of a pyrazolate bridged dimeric copper(II) complex. The extended structure in the solid state of 4 revealed the formation of a 2D coordination polymer with the dimeric core as the repeating unit. The ligand (HL(4)) in 4 is a 3,4,5-trisubstituted pyrazole ring formed in situ via C-C bond formation and represents an unprecedented transformation reaction. PMID:21805958

Das, Oindrila; Zangrando, Ennio; Paine, Tapan Kanti

2011-09-01

207

Cyclopalladated complexes containing 2-C6R4PPh2 ligands (R = H, F): one-electron electrochemical reduction leading to metal-carbon ?-bond cleavage via palladium(i).  

PubMed

Three new ortho-metallated palladium complexes, [Pd(O,O'-hfacac)(?(2)-2-C6F4PPh2)] (), [Pd2(O,O'-hfacac)2(?-2-C6F4PPh2)2] () and [Pd(O,O'-hfacac)(?C-2-C6F4PPh2)(PPh3)] () (hfacac = hexafluoroacetylacetonate), have been prepared and fully characterised. The electrochemical reductions of complexes , together with those of other cyclopalladated complexes containing 2-C6R4PPh2 ligands (R = H, F) were studied by cyclic, rotating disk and microelectrode voltammetry. Evidence for the one-electron reduction of [PdI(?(2)-2-C6F4PPh2)(PPh2Fc)] () was obtained from coulometric analysis, although the product is unstable and undergoes further chemical processes. Preparative electro-reduction of [Pd2(?-Br)2(?(2)-2-C6F4PPh2)2] () in CH2Cl2 causes reductive cleavage of its Pd-C ?-bonds and formation of the complex [PdBr2{PPh2(2-C6F4H)}2] (); possible mechanisms are discussed. PMID:25600503

Kar, Gopa; Privér, Steven H; Jones, Lathe A; Guo, Si-Xuan; Torriero, Angel A J; Bond, Alan M; Bennett, Martin A; Bhargava, Suresh K

2015-02-01

208

Cleavage of peptide bonds bearing ionizable amino acids at P{sub 1} by serine proteases with hydrophobic S{sub 1} pocket  

SciTech Connect

Research highlights: {yields} Large pK shifts in ionizable groups when buried in the protein interior. {yields} Substrate dependent shifts in pH optimum for serine proteases. {yields} Lys side chain is a stronger acid in serine protease S{sub 1} pocket than Asp side chain. -- Abstract: Enzymatic hydrolysis of the synthetic substrate succinyl-Ala-Ala-Pro-Xxx-pNA (where Xxx = Leu, Asp or Lys) catalyzed by bovine chymotrypsin (CHYM) or Streptomyces griseus protease B (SGPB) has been studied at different pH values in the pH range 3-11. The pH optima for substrates having Leu, Asp, and Lys have been found to be 7.5-8.0, 5.5-6.0, and {approx}10, respectively. At the normally reported pH optimum (pH 7-8) of CHYM and SGPB, the substrate with Leu at the reactive site is more than 25,000-fold more reactive than that with Asp. However, when fully protonated, Asp is nearly as good a substrate as Leu. The pK values of the side chains of Asp and Lys in the hydrophobic S{sub 1} pocket of CHYM and SGPB have been calculated from pH-dependent hydrolysis data and have been found to be about 9 for Asp and 7.4 and 9.7 for Lys for CHYM and SGPB, respectively. The results presented in this communication suggest a possible application of CHYM like enzymes in cleaving peptide bonds contributed by acidic amino acids between pH 5 and 6.

Qasim, Mohammad A., E-mail: qasimm@ipfw.edu [Department of Chemistry, Indiana University Purdue University Fort Wayne, 2101 E. Coliseum Blvd., Fort Wayne, IN 46805 (United States); Song, Jikui; Markley, John L. [Department of Biochemistry, University of Wisconsin-Madison, WI 53706 (United States)] [Department of Biochemistry, University of Wisconsin-Madison, WI 53706 (United States); Laskowski, Michael [Department of Chemistry, Purdue University, West Lafayette, IN 47907 (United States)] [Department of Chemistry, Purdue University, West Lafayette, IN 47907 (United States)

2010-10-01

209

Metallacyclocumulenes: a theoretical perspective on the structure, bonding, and reactivity.  

PubMed

Conspectus Transition metals help to stabilize highly strained organic fragments. Metallacycles, especially unsaturated ones, provide much variety in this area. We had a sustained interest in understanding new C-C bond formation reactions affected by binuclear transition metal fragments Cp2M. One such study led to the exploration of the bimetallic C-C cleavage and coupled complexes, where the acetylide ligands bridge two metal atoms. The underlying M-C interaction in these complexes inspired the synthesis of a five-membered cyclocumulene complex, which opened a new phase in organometallic chemistry. The metallacyclocumulene produces a variety of C-C cleavage and coupled products including a radialene complex. Group 4 metallocenes have thus unlocked a fascinating chemistry by stabilizing strained unsaturated C4 organic fragments in the form of five-membered metallacyclocumulenes, metallacyclopentynes, and metallacycloallenes. Over the years, we have carried out a comprehensive theoretical study to understand the unusual stability and reactivity of these metallacycles. The unique (M-C?) interaction of the internal carbon atoms with the metal atom is the reason for unusual stability of the metallacycles. We have also shown that there is a definite dependence of the C-C coupling and cleavage reactions on the metal of metallacyclocumulenes. It demonstrates unexpected reaction pathways for these reactions. Based on this understanding, we have predicted and unraveled the stabilization factors of an unusual four-membered metallacycloallene complex. Indeed, our prediction about a four-membered heterometallacycle has led to an interesting bonding situation, which is experimentally realized. This type of M-C bonding is intriguing from a fundamental perspective and has great relevance in synthesizing unusual structures with interesting properties. In this Account, we first give a short prologue of what led to the present study and describe the salient features of the structure and bonding of the metallacyclocumulenes. The unusual reaction pathway of this metallacycle is explored next. Similar features of the metallacyclopentynes and metallacycloallenes are briefly mentioned. Then, we discuss the exploitation of the unique M-C bonding to design some exotic molecules such as a four-membered metallacycloallene complex. Our efforts to build a conceptual framework to understand these metallacycles and to exploit their chemistry continue. PMID:25171518

Roy, Subhendu; Rosenthal, Uwe; Jemmis, Eluvathingal D

2014-10-21

210

Cleavage of nucleic acids  

DOEpatents

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The structure-specific nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof.

Prudent, James R. (Madison, WI); Hall, Jeff G. (Waunakee, WI); Lyamichev, Victor I. (Madison, WI); Brow; Mary Ann D. (Madison, WI); Dahlberg, James E. (Madison, WI)

2010-11-09

211

Cleavage of nucleic acids  

DOEpatents

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The structure-specific nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof.

Prudent, James R. (Madison, WI); Hall, Jeff G. (Madison, WI); Lyamichev, Victor L. (Madison, WI); Brow, Mary Ann D. (Madison, WI); Dahlberg, James E. (Madison, WI)

2007-12-11

212

Cleavage of nucleic acids  

DOEpatents

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The structure-specific nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof.

Prudent, James R. (Madison, WI); Hall, Jeff G. (Madison, WI); Lyamichev, Victor I. (Madison, WI); Brow, Mary Ann D. (Madison, WI); Dahlberg, James E. (Madison, WI)

2000-01-01

213

Brønsted acid-promoted C-H bond cleavage via electron transfer from toluene derivatives to a protonated nonheme iron(IV)-oxo complex with no kinetic isotope effect.  

PubMed

The reactivity of a nonheme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), was markedly enhanced by perchloric acid (70% HClO4) in the oxidation of toluene derivatives. Toluene, which has a high one-electron oxidation potential (Eox = 2.20 V vs SCE), was oxidized by [(N4Py)Fe(IV)(O)](2+) in the presence of HClO4 in acetonitrile (MeCN) to yield a stoichiometric amount of benzyl alcohol, in which [(N4Py)Fe(IV)(O)](2+) was reduced to [(N4Py)Fe(III)(OH2)](3+). The second-order rate constant (kobs) of the oxidation of toluene derivatives by [(N4Py)Fe(IV)(O)](2+) increased with increasing concentration of HClO4, showing the first-order dependence on [HClO4]. A significant kinetic isotope effect (KIE) was observed when mesitylene was replaced by mesitylene-d12 in the oxidation with [(N4Py)Fe(IV)(O)](2+) in the absence of HClO4 in MeCN at 298 K. The KIE value drastically decreased from KIE = 31 in the absence of HClO4 to KIE = 1.0 with increasing concentration of HClO4, accompanied by the large acceleration of the oxidation rate. The absence of KIE suggests that electron transfer from a toluene derivative to the protonated iron(IV)-oxo complex ([(N4Py)Fe(IV)(OH)](3+)) is the rate-determining step in the acid-promoted oxidation reaction. The detailed kinetic analysis in light of the Marcus theory of electron transfer has revealed that the acid-promoted C-H bond cleavage proceeds via the rate-determining electron transfer from toluene derivatives to [(N4Py)Fe(IV)(OH)](3+) through formation of strong precursor complexes between toluene derivatives and [(N4Py)Fe(IV)(OH)](3+). PMID:23528016

Park, Jiyun; Lee, Yong-Min; Nam, Wonwoo; Fukuzumi, Shunichi

2013-04-01

214

Selectivity of the alpha and [beta] bond fissions for bromoacetyl chloride upon n[right arrow][pi]* excitation: A combined complete-active-space self-consistent field and multireference configuration interaction study  

NASA Astrophysics Data System (ADS)

The potential energy surfaces for the BrCH2COCl dissociations into Br+CH2COCl, BrCH2CO+Cl, and BrCH2+COCl in the S0, S1, and T1 states have been investigated at the complete-active-space self-consistent field, density functional theory, and multireference configuration interaction levels with the 6-31G* and cc-pVDZ basis sets, which provide some new insights into the mechanism of the BrCH2COCl photodissociation at 248 nm. It is found that the most probable pathway is the S1 C-Cl alpha and C-Br beta bond fissions, which are a pair of competitive dissociation channels with some preference of the alpha C-Cl bond cleavage. The C-C alpha bond fission can take place along the S1 pathway upon photoexcitation at 248 nm, but it is not in competition with the C-Cl alpha bond cleavage. These results are consistent with the experimental findings. The relative strength of the C-C and C-Cl alpha bonds is one of the factors that influences the selectivity of the alpha bond fissions. However, the selectivity is mainly determined by the mechanism of dissociation upon n[right arrow]pi* excitation. The preference of the C-Cl alpha bond fission over the C-Br beta bond cleavage was attributed to the nonadiabatic recrossing in previous studies. The present calculations predict that the distance dependence of intramolecular energy relaxation is another important factor that influences the selectivity of the alpha and beta bond cleavages.

Ding, Wan-Jian; Fang, Wei-Hai; Liu, Ruo-Zhuang; Fang, De-Cai

2002-11-01

215

Catalytic Carbonyl Allylation, Propargylation and Vinylation from the Alcohol or Aldehyde Oxidation Level via C-C Bond Forming Hydrogenation and Transfer Hydrogenation: A Departure from Preformed Organometallic Reagents**  

PubMed Central

Classical protocols for carbonyl allylation, propargylation and vinylation typically rely upon the use of preformed allyl metal, allenyl metal and vinyl metal reagents, respectively, mandating stoichiometric generation of metallic byproducts. Through transfer hydrogenative C-C coupling, carbonyl addition may be achieved from the aldehyde or alcohol oxidation level in the absence of stoichiometric organometallic reagents or metallic reductants. Here, we review transfer hydrogenative methods for carbonyl addition, which encompass the first cataltyic protocols enabling direct C–H functionalization of alcohols. PMID:19040235

Bower, John F.; Kim, In Su; Patman, Ryan L.; Krische, Michael J.

2009-01-01

216

Samarium(III)-catalyzed C(sp3)-H bond activation: synthesis of indolizines via C-C and C-N coupling between 2-alkylazaarenes and propargylic alcohols.  

PubMed

A new rare earth metal and samarium-catalyzed C(sp(3))-H bond activation is reported in which 2-alkylazaarenes and propargylic alcohols were converted to indolizines. This process operates under mild conditions and solvent-free conditions. A broad scope of coupling partners has been established, and a likely mechanism has also been suggested. PMID:24359214

Wang, Xu; Li, Shen-yan; Pan, Ying-ming; Wang, Heng-shan; Liang, Hong; Chen, Zhen-feng; Qin, Xiao-huan

2014-01-17

217

Detection of temperature rise during cleavage of silicon  

NASA Astrophysics Data System (ADS)

We have measured a rise in temperature upon cleavage of Si wafers, which is consistent with the energy of bond rupture appearing as heat on the surface. The temperature of the surface is high for a significant period, requiring consideration in cleaved-surface structure calculations. Cleavage in vacuum gives a shorter temporal profile than in air, suggesting a more efficient surface-to-bulk phonon coupling for clean surfaces.

Zhao, Dong; Haneman, D.; McAlpine, N. S.

1996-12-01

218

Direct-dynamics VTST study of hydrogen or deuterium abstraction and C-C bond formation or dissociation in the reactions of CH3 + CH4, CH3 + CD4, CH3D + CD3, CH3CH3 + H, and CH3CD3 + D  

NASA Astrophysics Data System (ADS)

Direct-dynamics variational transition-state theory calculations are studied at the MPWB1K/6-311++G(d,p) level for the four parts of reactions. The first part is hydrogen or deuterium abstraction in the reactions of CH3 + CH4, CH3 + CD4, and CH3D + CH3. The second part involves C-C bond formation in these reactions. The third one is the reactions of CH3CH3 + H and CH3CD3 + D to form of H2, HD, and D2. The last one is the dissociation of C-C bonds in the last group of reactions. The ground-state vibrational adiabatic potential is plotted for all channels. We have carried out direct-dynamics calculations of the rate constants, including multidimensional tunneling in the temperature range T = 200-2200 K. The results of CVT/?OMT rate constants were in good agreement with the experimental data which were available for some reactions. Small-curvature tunneling and Large-curvature tunneling with the LCG4 version were used to include the quantum effects in calculation of the rate constants. To try to find the region of formation and dissociation of bounds we have also reported the variations of harmonic vibrational frequencies along the reaction path. The thermally averaged transmission probability (P(E)exp (-?E/RT)) and representative tunneling energy at 298 K are reported for the reactions in which tunneling is important. We have calculated kinetic isotope effect which shows tunneling and vibrational contributions are noticeable to determine the rate constant. Nonlinear least-squares fitting is used to calculate rate constant expressions in the temperature range 200-2200 K. These expressions revealed that pre-exponential factor includes two parts; the first part is a constant number which is important at low temperatures while the second part is temperature dependent which is significant at high temperatures.

Ramazani, Shapour

2013-05-01

219

Photoinduced C-C reactions on insulators toward photolithography of graphene nanoarchitectures.  

PubMed

On-surface chemistry for atomically precise sp(2) macromolecules requires top-down lithographic methods on insulating surfaces in order to pattern the long-range complex architectures needed by the semiconductor industry. Here, we fabricate sp(2)-carbon nanometer-thin films on insulators and under ultrahigh vacuum (UHV) conditions from photocoupled brominated precursors. We reveal that covalent coupling is initiated by C-Br bond cleavage through photon energies exceeding 4.4 eV, as monitored by laser desorption ionization (LDI) mass spectrometry (MS) and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) gives insight into the mechanisms of C-Br scission and C-C coupling processes. Further, unreacted material can be sublimed and the coupled sp(2)-carbon precursors can be graphitized by e-beam treatment at 500 °C, demonstrating promising applications in photolithography of graphene nanoarchitectures. Our results present UV-induced reactions on insulators for the formation of all sp(2)-carbon architectures, thereby converging top-down lithography and bottom-up on-surface chemistry into technology. PMID:24524804

Palma, Carlos-Andres; Diller, Katharina; Berger, Reinhard; Welle, Alexander; Björk, Jonas; Cabellos, Jose Luis; Mowbray, Duncan J; Papageorgiou, Anthoula C; Ivleva, Natalia P; Matich, Sonja; Margapoti, Emanuela; Niessner, Reinhard; Menges, Bernhard; Reichert, Joachim; Feng, Xinliang; Räder, Hans Joachim; Klappenberger, Florian; Rubio, Angel; Müllen, Klaus; Barth, Johannes V

2014-03-26

220

Mechanisms of Catalytic Cleavage of Benzyl Phenyl Ether in Aqueous and Apolar Phases  

SciTech Connect

Catalytic pathways for the cleavage of the ether bonds in benzyl phenyl ether (BPE) in the condensed liquid phases using Ni and zeolite based catalysts are explored. In absence of catalysts, the C?O bond is selectively cleaved in water by hydrolysis forming phenol and benzyl alcohol as intermediates, followed by C?C bond alkylation. The hydronium ions catalyzing the reactions are provided by the dissociation of water at the high temperature (523 K). Upon addition of a solid acid (HZSM-5), rates of hydrolysis and alkylation are markedly increased in proportion to the acid concentrations. In the presence of a metal (Ni/SiO2), the selective hydrogenolysis dominates for cleaving the Caliphatic?O bond. Catalyzed by the dual-functional Ni/HZSM-5, hydrogenolysis occurs as the major route rather than hydrolysis (minor). In the apolar undecane, the non-catalytic thermal pyrolysis route dominates. Hydrogenolysis of BPE appears to be the major reaction pathway in undecane in presence of Ni/SiO2 or Ni/HZSM-5, almost suppressing the radical reactions completely. The density functional theory (DFT) calculations perfectly support the proposed C?O bond cleavage mechanisms on BPE in the aqueous and apolar phases. DFT calculations show that BPE is initially protonated and subsequently hydrolyzed in the aqueous phase. The radical reaction plays a significant role for generating primary benzyl and phenoxy radicals in undecane evidenced by DFT calculation, which leads to heavier condensation products without the aid of metals for providing dissociated hydrogen radicals. J.H., L.L., and C.Z. gratefully acknowledge support from the graduate school (Faculty Graduate Center of Chemistry) of the Technische Universität München and the Elitenetzwerk Bayern (graduate school NanoCat). D.M. and J.A.L. thank the support from the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. Computing time was granted by the grand challenge of computational catalysis of the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) and by the National Energy Research Scientific Computing Center (NERSC). EMSL is a national scientific user facility located at Pacific Northwest National Laboratory (PNNL) and sponsored by DOE’s Office of Biological and Environmental Research.

He, Jiayue; Lu, Lu; Zhao, Chen; Mei, Donghai; Lercher, Johannes A.

2014-03-03

221

Palladium-catalyzed C-C, C-N and C-O bond formation  

E-print Network

New methods for Pd-catalyzed cross-coupling reactions of aryl halides or arenesulfonates are described. Key to the success of these transformations is the proper choice of ligand and reaction conditions. Palladium catalysts ...

Huang, Xiaohua, 1973-

2003-01-01

222

Mild P-P bond cleavage in the methyldiphosphenyl complex [Mo2Cp2(?-PCy2)(?-?(2):?(2)-P2Me)(CO)2] to give novel phosphide-bridged trinuclear derivatives.  

PubMed

Reactions of the title diphosphenyl complex with [Fe2(CO)9] and [W(CO)4(THF)2] gave the trinuclear species [Mo2FeCp2(?3-P)(?-PCy2)(?3-PMe)(CO)5] and [Mo2WCp2(?3-P)(?-PCy2)(?3-PMe)(CO)6] following from formal insertion of the 14-electron fragments Fe(CO)3 and W(CO)4, respectively, in the P-P bond of the diphosphenyl ligand and formation of a new heterometallic bond [Mo-Fe = 2.9294(6) Å and Mo-W = 3.146(1) Å]. Reactions of the diphosphenyl complex with the tetrahydrofuran adducts [MLn(THF)] (MLn = MnCp'(CO)2, W(CO)5) led instead to trinuclear diphosphenyl complexes [Mo2MCp2(?-PCy2)(?3-?(2):?(2):?(1)-P2Me)(CO)2Ln] following from coordination in each case of the corresponding 16-electron fragment MLn to the lone-pair-bearing P atom of the P2Me ligand. However, these diphosphenyl complexes were unstable and decomposed at room temperature or under mild heating by the release of methylphosphinidene (PMe), to give the corresponding derivatives [Mo2MCp2(?3-P)(?-PCy2)(CO)2Ln] displaying trigonal-planar phosphide ligands, giving rise to strongly deshielded (31)P NMR resonances (?P ca. 1100 ppm), while being involved in strong ? bonding with the unsaturated Mo2 center of these molecules [Mo-Mo = 2.749(1) Å and Mo-P = ca. 2.30 Å when M = W]. An isolobal analogy could be established between the P?MLn fragments in these products and a carbyne ligand (CR), supported by density functional theory calculations on the tungsten compound, which also enabled an easy interpretation and prediction of their chemical behavior. Thus, the manganese complex could be reversibly carbonylated (pCO = ca. 3 atm, 293 K) to give the corresponding electron-precise pentacarbonyl [MnMo2Cp2Cp'(?3-P)(?-PCy2)(CO)5] [Mo-Mo = 3.1318(7) Å], a process also involving a trans-to-cis rearrangement of the Mo2Cp2 subunit. On the other hand, decarbonylation of the tungsten complex was accomplished in a refluxing toluene solution to give the hexacarbonyl [Mo2WCp2(?3-P)(?-PCy2)(?-CO)(CO)5], a derivative containing an unsaturated 30-electron dimolybdenum center with an intermetallic triple bond. PMID:25300937

Alvarez, M Angeles; García, M Esther; García-Vivó, Daniel; Lozano, Raquel; Ramos, Alberto; Ruiz, Miguel A

2014-10-20

223

Invasive cleavage of nucleic acids  

DOEpatents

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The structure-specific nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof.

Prudent, James R. (Madison, WI); Hall, Jeff G. (Madison, WI); Lyamichev, Victor I. (Madison, WI); Brow, Mary Ann D. (Madison, WI); Dahlberg, James E. (Madison, WI)

1999-01-01

224

Invasive cleavage of nucleic acids  

DOEpatents

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The structure-specific nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof.

Prudent, James R. (Madison, WI); Hall, Jeff G. (Madison, WI); Lyamichev, Victor I. (Madison, WI); Brow, Mary Ann D. (Madison, WI); Dahlberg, James E. (Madison, WI)

2002-01-01

225

Rh-Catalyzed Decarbonylative Coupling with Alkynes via C-C Activation of Isatins.  

PubMed

Herein we report a [5 + 2 - 1] transformation though catalytic decarbonylative coupling between isatins and alkynes, which provides a unique way to synthesize 2-quinolinone derivatives. A broad range of alkynes can be coupled efficiently with high regioselectivity. This reaction is proposed to go through C-C activation of isatins, followed by decarbonylation and alkyne insertion. Directing group (DG) plays a critical role in this transformation. Assisted by the DG, the C-C cleavage of isatins occurs at room temperature. PMID:25569352

Zeng, Rong; Dong, Guangbin

2015-02-01

226

Palladium-catalyzed allylic alkylation via decarboxylative and retro-Claisen C-C cleavage methods  

E-print Network

Presented herein is the development of new methods for Pd-catalyzed allylic alkylation with a central focus on reactions that generate molecular complexity rapidly with little waste byproduct. With this simply stated, yet ...

Grenning, Alexander James

2012-05-31

227

A comparative study of semiempirical bond dissociation energy calculations  

SciTech Connect

Anderson's modified version of the extended Hueckel molecular orbital method and the MNDO and AM1 methods of Dewar are used to study bond cleavage in molecular fragments of interest in coal liquefaction. Geometric conformations, molecular orbital coefficients for the HOMO's and LUMO's, and bond dissociation energies are computed and compared for the three methods. Qualitative agreement with experiment for bond cleavage is observed for the three methods for the neutral molecule. However the ASED-MO method appears to best describe the bond cleavage in the presence of an electron-accepting catalyst.

Ades, H.F.; Companion, A.L.; Subbaswamy, K.R. (Univ. of Kentucky, Lexington (United States))

1991-08-22

228

Reductive cleavage of nitrite to form terminal uranium mono-oxo complexes.  

PubMed

Uranium terminal mono-oxo complexes are prepared with a unique activation of nitrite following reductive cleavage of an N-O bond with loss of nitric oxide. The thermodynamic driving force of U?O bond formation differentiates this reactivity from known mechanisms of nitrite reduction, which are typically mediated by proton transfer. Mechanistic details are explored by DFT supporting a simple homolytic cleavage pathway from a ?(1)-ONO bound intermediate. Complexes of the formula U(VI)OX[N(SiMe(3))(2)](3) are formed providing a trigonal bipyramidal framework into which ligands trans to the U?O bond may be installed. PMID:23215157

Lewis, Andrew J; Carroll, Patrick J; Schelter, Eric J

2013-01-01

229

New Insight into the Cleavage Reaction of Nostoc sp. Strain PCC 7120 Carotenoid Cleavage Dioxygenase in Natural and Nonnatural Carotenoids  

PubMed Central

Carotenoid cleavage dioxygenases (CCDs) are enzymes that catalyze the oxidative cleavage of carotenoids at a specific double bond to generate apocarotenoids. In this study, we investigated the activity and substrate preferences of NSC3, a CCD of Nostoc sp. strain PCC 7120, in vivo and in vitro using natural and nonnatural carotenoid structures. NSC3 cleaved ?-apo-8?-carotenal at 3 positions, C-13C-14, C-15C-15?, and C-13?C-14?, revealing a unique cleavage pattern. NSC3 cleaves the natural structure of carotenoids 4,4?-diaponeurosporene, 4,4?-diaponeurosporen-4?-al, 4,4?-diaponeurosporen-4?-oic acid, 4,4?-diapotorulene, and 4,4?-diapotorulen-4?-al to generate novel cleavage products (apo-14?-diaponeurosporenal, apo-13?-diaponeurosporenal, apo-10?-diaponeurosporenal, apo-14?-diapotorulenal, and apo-10?-diapotorulenal, respectively). The study of carotenoids with natural or nonnatural structures produced by using synthetic modules could provide information valuable for understanding the cleavage reactions or substrate preferences of other CCDs in vivo and in vitro. PMID:23524669

Heo, Jinsol; Kim, Se Hyeuk

2013-01-01

230

In vivo and in vitro studies on the carotenoid cleavage oxygenases from Sphingopyxis alaskensis RB2256 and Plesiocystis pacifica SIR-1 revealed their substrate specificities and non-retinal-forming cleavage activities.  

PubMed

Carotenoid cleavage oxygenases are nonheme iron enzymes that specifically cleave carbon-carbon double bonds of carotenoids. Their apocarotenoid cleavage products serve as important signaling molecules that are involved in various biological processes. A database search revealed the presence of putative carotenoid cleavage oxygenase genes in the genomes of Sphingopyxis alaskensis RB2256 and Plesiocystis pacifica SIR-1. The four genes sala_1698, sala_1008, ppsir1_15490 and ppsir1_17230 were cloned and heterologously expressed in carotenoid-producing Escherichia coli JM109 strains. Two of the four encoded proteins exhibited carotenoid cleavage activity. S. alaskensis RB2256 carotenoid cleavage oxygenase (SaCCO), which is encoded by sala_1698, was shown to cleave acyclic and monocyclic substrates. Coexpression of sala_1698 in carotenoid-producing E. coli JM109 strains revealed cleavage activity for lycopene, hydroxylycopene, and dihydroxylycopene. The monocyclic substrate apo-8'-carotenal was cleaved in vitro by purified SaCCO at the 9'/10' and 11'/12' double bonds. The second enzyme, P. pacifica SIR-1 carotenoid cleavage oxygenase (PpCCO), is encoded by ppsir1_15490. PpCCO-mediated carotenoid cleavage requires the presence of either hydroxy or keto groups. PpCCO cleaved zeaxanthin, hydroxylycopene, and dihydroxylycopene, and also the C(50) carotenoids decaprenoxanthin, sarprenoxanthin and sarcinaxanthin, in carotenoid-producing E. coli JM109 strains. Whole cells of E. coli JM109 overexpressing ppsir1_15490mut, a mutant of ppsir1_15490 with enhanced gene expression, were applied for the conversion of carotenoids. Analysis of the carotenoid cleavage products revealed a single cleavage site at the 13'/14' double bond for astaxanthin, and two cleavage sites at the 11'/12' or 13'/14' double bond for zeaxanthin, nostoxanthin, and canthaxanthin. PMID:22901074

Hoffmann, Jana; Bóna-Lovász, Judit; Beuttler, Holger; Altenbuchner, Josef

2012-10-01

231

Iminopropadienones RN=C=C=C=O and bisiminopropadienes RN=C=C=C=NR: Matrix infrared spectra and anharmonic frequency calculations  

NASA Astrophysics Data System (ADS)

Methyliminopropadienone MeN=C=C=C=O 1a was generated by flash vacuum thermolysis from four different precursors and isolated in solid argon. The matrix-isolation infrared spectrum is dominated by unusually strong anharmonic effects resulting in complex fine structure of the absorptions due to the NCCCO moiety in the 2200 cm-1 region. Doubling and tripling of the corresponding absorption bands are observed for phenyliminopropadienone PhN=C=C=C=O 1b and bis(phenylimino)propadiene PhN=C=C=C=NPh 9, respectively. Anharmonic vibrational frequency calculations allow the identification of a number of overtones and combination bands as the cause of the splittings for each molecule. This method constitutes an important tool for the characterization of reactive intermediates and unusual molecules by matrix-isolation infrared spectroscopy.

Bégué, Didier; Baraille, Isabelle; Andersen, Heidi Gade; Wentrup, Curt

2013-10-01

232

?-Cleavage of cellular prion protein  

PubMed Central

The cellular prion protein (PrPC) is subjected to various processing under physiological and pathological conditions, of which the ?-cleavage within the central hydrophobic domain not only disrupts a region critical for both PrP toxicity and PrPC to PrPSc conversion but also produces the N1 fragment that is neuroprotective and the C1 fragment that enhances the pro-apoptotic effect of staurosporine in one report and inhibits prion in another. The proteases responsible for the ?-cleavage of PrPC are controversial. The effect of ADAM10, ADAM17, and ADAM9 on N1 secretion clearly indicates their involvement in the ?-cleavage of PrPC, but there has been no report of direct PrPC ?-cleavage activity with any of the three ADAMs in a purified protein form. We demonstrated that, in muscle cells, ADAM8 is the primary protease for the ?-cleavage of PrPC, but another unidentified protease(s) must also play a minor role. We also found that PrPC regulates ADAM8 expression, suggesting that a close examination on the relationships between PrPC and its processing enzymes may reveal novel roles and underlying mechanisms for PrPC in non-prion diseases such as asthma and cancer. PMID:23052041

Liang, Jingjing; Kong, Qingzhong

2012-01-01

233

Cleavage of DNA fragments induced by UV nanosecond laser excitation at 193 nm  

Microsoft Academic Search

The cleavage of dsDNA fragments in aqueous solution after irradiation with UV laser pulses at 193 nm has been studied. Samples\\u000a were investigated using polyacrylamide gel electrophoresis. The intensity of damage of particular phosphodiester bond after\\u000a hot alkali treatment was shown to depend on the base pair sequence. It was established that the probability of cleavage is\\u000a twice higher for

N. N. Vtyurina; S. L. Grokhovski; I. V. Filimonov; O. I. Medvedkov; D. Yu. Nechipurenko; S. A. Vasiliev; Yu. D. Nechipurenko

2011-01-01

234

N–O Cleavage reactions of heterobicycloalkene-fused 2-isoxazolines  

PubMed Central

Summary Transition metal-mediated N–O bond cleavage reactions of heterobicycloalkene-fused 3-methyl-2-isoxazolines were investigated. Optimal cleavage conditions were found with Raney nickel/AlCl3 mediation in aqueous methanol. The reaction provided a diverse collection of novel heterobicycle-fused ?-hydroxyketones with good to excellent yields (66–95%) and without the need for chromatographic purification. PMID:25246978

Nagireddy, Jaipal R; Tranmer, Geoffrey K; Carlson, Emily

2014-01-01

235

Nucleosome phasing and micrococcal nuclease cleavage of African green monkey component alpha DNA.  

PubMed Central

The micrococcal nuclease cleavage of intact nuclear chromatin from African green monkey cells and of the completely deproteinized sequences was studied by using high-resolution analytical and DNA sequencing gels and secondary restriction enzyme analysis. When deproteinized component alpha DNA was used as substrate, not all phosphodiester bonds in the 172-base-pair repeat units were cleaved with equal frequency by the nuclease. A distinct preference for the cleavage of A-T rather than G-C bonds was observed; however, A + T-richness in itself did not confer susceptibility to cleavage by micrococcal nuclease. The results suggested that, in deproteinized DNA, nuclease cleavage at particular dinucleotides may be influenced more by the effect of adjacent sequences than by the composition of the dinucleotide. In contrast to complex cleavage patterns of the deproteinized component alpha DNA which arose because of multiple cleavage sites in the repeat unit, micrococcal nuclease cleaved component alpha nuclear chromatin at one site per nucleosome repeat, near position 126 in the nucleotide sequence. This simple chromatin cleavage pattern is consistent with the discrete nucleosomal structure of component alpha in chromatin and a direct phase relationship between the component alpha DNA sequence repeats and the nucleosome protein structural repeats. Images PMID:6275381

Musich, P R; Brown, F L; Maio, J J

1982-01-01

236

98. Catalog HHistory 1, C.C.C., 19 Tree Planting, Negative No. ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

98. Catalog H-History 1, C.C.C., 19 Tree Planting, Negative No. P 474c (Photographer and date unknown) TRANSPLANTING TREE. - Skyline Drive, From Front Royal, VA to Rockfish Gap, VA , Luray, Page County, VA

237

100. Catalog HHistory 1, C.C.C., 34 Landscaping, Negative No. P ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

100. Catalog H-History 1, C.C.C., 34 Landscaping, Negative No. P 733c (Photographer and date unknown) SLOPE MAINTENANCE WORK BY CCC. - Skyline Drive, From Front Royal, VA to Rockfish Gap, VA , Luray, Page County, VA

238

101. Catalog HHistory 1, C.C.C., 34 Landscaping, Negative No. 1340 ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

101. Catalog H-History 1, C.C.C., 34 Landscaping, Negative No. 1340 (Photographer and date unknown) BANK BLENDING WORK BY CCC. - Skyline Drive, From Front Royal, VA to Rockfish Gap, VA , Luray, Page County, VA

239

99. Catalog HHistory 1, C.C.C., 23 Guard Rail Construction, Negative ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

99. Catalog H-History 1, C.C.C., 23 Guard Rail Construction, Negative No. P455e (Photographer and date unknown) GUARD RAIL INSTALLATION. - Skyline Drive, From Front Royal, VA to Rockfish Gap, VA , Luray, Page County, VA

240

Serine-selective aerobic cleavage of peptides and a protein using a water-soluble copper-organoradical conjugate.  

PubMed

The site-specific cleavage of peptide bonds is an important chemical modification of biologically relevant macromolecules. The reaction is not only used for routine structural determination of peptides, but is also a potential artificial modulator of protein function. Realizing the substrate scope beyond the conventional chemical or enzymatic cleavage of peptide bonds is, however, a formidable challenge. Here we report a serine-selective peptide-cleavage protocol that proceeds at room temperature and near neutral pH?value, through mild aerobic oxidation promoted by a water-soluble copper-organoradical conjugate. The method is applicable to the site-selective cleavage of polypeptides that possess various functional groups. Peptides comprising D-amino acids or sensitive disulfide pairs are competent substrates. The system is extendable to the site-selective cleavage of a native protein, ubiquitin, which comprises more than 70 amino acid residues. PMID:24826926

Seki, Yohei; Tanabe, Kana; Sasaki, Daisuke; Sohma, Youhei; Oisaki, Kounosuke; Kanai, Motomu

2014-06-16

241

Mineral Cleavage: a practical experiment  

NSDL National Science Digital Library

In this geology activity, students investigate the physical property of mineral cleavage by physically trying to break down a block of halite and describing the results. This lab addresses many misunderstandings non-majors have about the physical properties of minerals and includes a brief write up of their conclusions.

Browning, Sharon

242

Photodegradation of human growth hormone: a novel backbone cleavage between Glu-88 and Pro-89.  

PubMed

The exposure of protein pharmaceuticals to light can cause loss of potency, oxidation, structural changes and aggregation. To elucidate the chemical pathways of photodegradation, we irradiated human growth hormone (hGH) at ? = 254 nm, ? ? 265-340 nm, and ? ? 295-340 nm (using the spectral cutoff of borosilicate glass) and analyzed the products by mass spectrometry. By means of LC-MS/MS analysis, we observed an unusual peptide backbone cleavage between Glu-88 and Pro-89. The crystal structure of hGH indicates that these residues are in proximity to Trp-86, which likely mediates this backbone cleavage. The two cleavage fragments observed by MS/MS analysis indicate the loss of CO from the amide bond and replacement of the Glu-C(? O)Pro bond with a Glu-H bond, accompanied by double bond formation on proline. The reaction is oxygen-independent and likely involves hydrogen transfer to the C? of Glu-88. To probe the influence of the protein fold, we irradiated hGH in its unfolded state, in 1:1 (v/v) acetonitrile/water, and also the isolated tryptic peptide Ile-78-Arg-90, which contains the Glu-88-Pro-89 sequence. In both cases, the cleavage between Glu-88 and Pro-89 was largely suppressed, while other cleavage pathways became dominant, notably between Gln-84 and Ser-85, as well as Ser-85 and Trp-86. PMID:23721578

Steinmann, Daniel; Ji, J Andrea; Wang, Y John; Schöneich, Christian

2013-07-01

243

Cleavage and creep fracture of rock salt  

Microsoft Academic Search

The dominant failure mechanism in rock salt at ambient temperature is either cleavage or creep fracture. Since the transition of creep fracture to cleavage in a compressive stress field is not well understood, failure of rock salt by cleavage and creep fracture is analyzed in this paper to elucidate the effect of stress state on the competition between these two

K. S. Chan; D. E. Munson; S. R. Bodner; A. F. Fossum

1996-01-01

244

Copper-catalyzed, C-C coupling-based one-pot tandem reactions for the synthesis of benzofurans using o-iodophenols, acyl chlorides, and phosphorus ylides.  

PubMed

One-pot reactions involving acyl chlorides, phosphorus ylides, and o-iodophenols with copper catalysis have been established for the rapid synthesis of functionalized benzofurans. With all of these easily available and stable reactants, the construction of the target products has been accomplished via tandem transformations involving a key C-C coupling, leading to the formation of one C(sp(2))-C bond, one C(sp(2))-O bond, and one C ? C bond. PMID:25279735

Liu, Yunyun; Wang, Hang; Wan, Jie-Ping

2014-11-01

245

A discrete lattice plane analysis of coherent f.c.c.\\/B1 interfacial energy  

Microsoft Academic Search

A discrete lattice plane, nearest neighbor, broken bond model with constant bond energies, which had been used to calculate the energy of coherent interphase boundaries in substitutional alloys, was extended to a ternary substitutional–interstitial system to study the chemical interfacial energy between a f.c.c. solid solution and a B1(NaCl) compound. When the regular solution interaction coefficient of substitutional (metal) atoms

Z.-G. Yang; M. Enomoto

1999-01-01

246

Carbon-Selenium Bond Cleavage by a Rhodium Complex  

E-print Network

, 1997X Summary: The complex Cp*Rh(PMe3)PhH (Cp* ) C5- Me5) reacts thermally with selenophene to give a C butadiene structure. 77Se NMR studies display an upfield resonance consistent with a non-aromatic met as well as identified the 77Se chemical shifts of the metal-selenophene species formed. 77Se NMR data

Jones, William D.

247

Dinitrogen cleavage and hydrogenation by a trinuclear titanium polyhydride complex.  

PubMed

Both the Haber-Bosch and biological ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong N?N triple bond and forming an N-H bond. This has spurred investigations of the reactivity of molecular multimetallic hydrides with dinitrogen. We report here the reaction of a trinuclear titanium polyhydride complex with dinitrogen, which induces dinitrogen cleavage and partial hydrogenation at ambient temperature and pressure. By (1)H and (15)N nuclear magnetic resonance, x-ray crystallographic, and computational studies of some key reaction steps and products, we have determined that the dinitrogen (N2) reduction proceeds sequentially through scission of a N2 molecule bonded to three Ti atoms in a ?-?(1):?(2):?(2)-end-on-side-on fashion to give a ?2-N/?3-N dinitrido species, followed by intramolecular hydrogen migration from Ti to the ?2-N nitrido unit. PMID:23812710

Shima, Takanori; Hu, Shaowei; Luo, Gen; Kang, Xiaohui; Luo, Yi; Hou, Zhaomin

2013-06-28

248

Hyperconjugation involving strained carbon-carbon bonds. structural analysis of ester and ether derivatives and one-bond 13C-13C coupling constants of ?- and ?-nopinol.  

PubMed

?(C-C)-?*(C-O) interactions involving the strained carbon-carbon bonds of ?- and ?-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C-OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C-C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the (13)C-(13)C one-bond coupling constants. For the donor C-C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C-C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases. PMID:23215360

Yeoh, Shinn Dee; Skene, Colin E; White, Jonathan M

2013-01-18

249

Determination of the orientation of OH bond axes in layer silicates by infrared absorption  

USGS Publications Warehouse

It is observed that, among the micas and related crystallizations, trioctahedral compositions exhibit an OH bond axis normal to the cleavage flake, with an infrared absorption frequency near 3700 cm.-1, but that dioctahedral compositions exhibit OH bond axes near the plane of the cleavage flake and of lesser absorption frequencies.

Serratosa, J.M.; Bradley, W.F.

1958-01-01

250

Characterization of CF bonds with multiple-bond character: bond lengths, stretching force constants, and bond dissociation energies.  

PubMed

Isoelectronic C=F(+) and C=O bonds contained in fluoro-substituted carbenium ions, aldehydes, and ketones are investigated with regard to their bond properties by utilizing the vibrational spectra of these molecules. It is demonstrated that bond dissociation energies (BDEs), bond lengths, vibrational stretching frequencies, and bond densities are not reliable descriptors of the bond strength. The latter is related to the intrinsic BDE, which corresponds to nonrelaxed dissociation products retaining the electronic structure and geometry they have in the molecule. It is shown that the harmonic stretching force constants k(a) of the localized internal coordinate vibrations (adiabatic vibrational modes) reflect trends in the intrinsic BDEs. The k(a) values of both CO and CF bonds are related to the bond lengths through a single exponential function. This observation is used to derive a common bond order n for 46 CO- and CF-containing molecules that reliably describes differences in bonding. CF bonds in fluorinated carbenium ions possess bond orders between 1.3 and 1.7 as a result of significant pi back-bonding from F to C, which is sensitive to electronic effects caused by substituents at the carbenium center. Therefore, the strength of the C=F(+) bond can be used as a sensor for (hyper)conjugation and other electronic effects influencing the stability of the carbenium ion. The diatomic C=F(+) ion has a true double bond due to pi donation from the F atom. The characterization of CF bonds with the help of adiabatic stretching modes is also applied to fluoronium ions (n = 0.3-0.6) and transition states involving CF cleavage and HF elimination (n = 0.7-0.8). PMID:19152353

Kraka, Elfi; Cremer, Dieter

2009-03-01

251

Novel carbon–carbon bond formations for biocatalysis  

PubMed Central

Carbon–carbon bond formation is the key transformation in organic synthesis to set up the carbon backbone of organic molecules. However, only a limited number of enzymatic C–C bond forming reactions have been applied in biocatalytic organic synthesis. Recently, further name reactions have been accomplished for the first time employing enzymes on a preparative scale, for instance the Stetter and Pictet–Spengler reaction or oxidative C–C bond formation. Furthermore, novel enzymatic C–C bond forming reactions have been identified like benzylation of aromatics, intermolecular Diels-Alder or reductive coupling of carbon monoxide. PMID:21354781

Resch, Verena; Schrittwieser, Joerg H; Siirola, Elina; Kroutil, Wolfgang

2011-01-01

252

A tandem in situ peptide cyclization through trifluoroacetic acid cleavage.  

PubMed

We present a new approach for peptide cyclization during solid phase synthesis under highly acidic conditions. Our approach involves simultaneous in?situ deprotection, cyclization and trifluoroacetic acid (TFA) cleavage of the peptide, which is achieved by forming an amide bond between a lysine side chain and a succinic acid linker at the peptide N-terminus. The reaction proceeds via a highly active succinimide intermediate, which was isolated and characterized. The structure of a model cyclic peptide was solved by NMR spectroscopy. Theoretical calculations support the proposed mechanism of cyclization. Our new methodology is applicable for the formation of macrocycles in solid-phase synthesis of peptides and organic molecules. PMID:24827640

Chandra, Koushik; Roy, Tapta Kanchan; Shalev, Deborah E; Loyter, Abraham; Gilon, Chaim; Gerber, R Benny; Friedler, Assaf

2014-09-01

253

Rhodium-catalyzed C-h activation of phenacyl ammonium salts assisted by an oxidizing C-N bond: a combination of experimental and theoretical studies.  

PubMed

Rh(III)-catalyzed C-H activation assisted by an oxidizing directing group has evolved to a mild and redox-economic strategy for the construction of heterocycles. Despite the success, these coupling systems are currently limited to cleavage of an oxidizing N-O or N-N bond. Cleavage of an oxidizing C-N bond, which allows for complementary carbocycle synthesis, is unprecedented. In this article, ?-ammonium acetophenones with an oxidizing C-N bond have been designed as substrates for Rh(III)-catalyzed C-H activation under redox-neutral conditions. The coupling with ?-diazo esters afforded benzocyclopentanones, and the coupling with unactivated alkenes such as styrenes and aliphatic olefins gave ortho-olefinated acetophenoes. In both systems the reactions proceeded with a broad scope, high efficiency, and functional group tolerance. Moreover, efficient one-pot coupling of diazo esters has been realized starting from ?-bromoacetophenones and triethylamine. The reaction mechanism for the coupling with diazo esters has been studied by a combination of experimental and theoretical methods. In particular, three distinct mechanistic pathways have been scrutinized by DFT studies, which revealed that the C-H activation occurs via a C-bound enolate-assisted concerted metalation-deprotonation mechanism and is rate-limiting. In subsequent C-C formation steps, the lowest energy pathway involves two rhodium carbene species as key intermediates. PMID:25569022

Yu, Songjie; Liu, Song; Lan, Yu; Wan, Boshun; Li, Xingwei

2015-02-01

254

Bent Bonds and Multiple Bonds.  

ERIC Educational Resources Information Center

Considers carbon-carbon multiple bonds in terms of Pauling's bent bond model, which allows direct calculation of double and triple bonds from the length of a CC single bond. Lengths of these multiple bonds are estimated from direct measurements on "bent-bond" models constructed of plastic tubing and standard kits. (CS)

Robinson, Edward A.; Gillespie, Ronald J.

1980-01-01

255

103. Catalog HHistory 1, C.C.C., 58 Landscaping, Negative No. 870 ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

103. Catalog H-History 1, C.C.C., 58 Landscaping, Negative No. 870 10 ca. 1936 PROPAGATION AND PLANTING. ROOTED PLANTS TRANSPLANTED FROM HOT BEDS TO CANS TO SHADED BEDS IN PREPARATION FOR PLANTING ON ROAD SLOPES. NURSERY AT NORTH ENTRANCE. - Skyline Drive, From Front Royal, VA to Rockfish Gap, VA , Luray, Page County, VA

256

104. Catalog HHistory 1, C.C.C., 73 Picnic Furniture Construction, Negative ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

104. Catalog H-History 1, C.C.C., 73 Picnic Furniture Construction, Negative No. 8821 ca. 1936 WOOD UTILIZATION. COMPLETED RUSTIC BENCH MADE BY CCC ENROLLEES AT CAMP NP-3 FOR USE AT PARKING OVERLOOKS AND PICNIC GROUNDS. NOTE SAW IN BACKGROUND USED FOR HALVING CHESTNUT. - Skyline Drive, From Front Royal, VA to Rockfish Gap, VA , Luray, Page County, VA

257

102. Catalog HHistory 1, C.C.C., 34 Landscaping, Negative No. 6040a ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

102. Catalog H-History 1, C.C.C., 34 Landscaping, Negative No. 6040a (Photographer and date unknown) BEAUTIFICATION PROGRAM STARTED AS SOON AS GRADING ALONG THE DRIVE WAS COMPLETED. CCC CAMP 3 SHOWN PLANTING LAUREL. - Skyline Drive, From Front Royal, VA to Rockfish Gap, VA , Luray, Page County, VA

258

Hardware synthesis from C\\/C++ models  

Microsoft Academic Search

Software programming languages, such as C\\/C++, have been used as means for specifying hardware for quite a while. Different design methodologies have exploited the advantages of flexibility and fast simulation of models de- scribed with programming languages. At the same time, the mismatch (of software languages) in expressing power (for hardware systems) has caused several difficulties. In the recent past,

Giovanni De Micheli

1999-01-01

259

Evaluation by Rocket Combustor of C/C Composite Cooled Structure Using Metallic Cooling Tubes  

NASA Astrophysics Data System (ADS)

In this study, the cooling performance of a C/C composite material structure with metallic cooling tubes fixed by elastic force without chemical bonding was evaluated experimentally using combustion gas in a rocket combustor. The C/C composite chamber was covered by a stainless steel outer shell to maintain its airtightness. Gaseous hydrogen as a fuel and gaseous oxygen as an oxidizer were used for the heating test. The surface of these C/C composites was maintained below 1500 K when the combustion gas temperature was about 2800 K and the heat flux to the combustion chamber wall was about 9 MW/m2. No thermal damage was observed on the stainless steel tubes that were in contact with the C/C composite materials. The results of the heating test showed that such a metallic tube-cooled C/C composite structure is able to control the surface temperature as a cooling structure (also as a heat exchanger) as well as indicated the possibility of reducing the amount of coolant even if the thermal load to the engine is high. Thus, application of this metallic tube-cooled C/C composite structure to reusable engines such as a rocket-ramjet combined-cycle engine is expected.

Takegoshi, Masao; Ono, Fumiei; Ueda, Shuichi; Saito, Toshihito; Hayasaka, Osamu

260

Densification Behavior and Performances of C/C Composites Derived from Various Carbon Matrix Precursors  

NASA Astrophysics Data System (ADS)

Three types of carbon/carbon (C/C) composites were manufactured by densifying the needled carbon fiber preform through resin and pitch impregnation/carbonization repeatedly, as well as propylene pyrolysis by chemical vapor infiltration plus carbonization after the resin impregnation/carbonization. The densification behavior and performances (involving electric, thermal, and mechanical properties, as well as impurity) of the C/C composites were investigated systematically. The results show that besides the processing and testing conditions, the electric resistivity, thermal conductivity (TC), coefficient of thermal expansion (CTE), strength, and fracture, as well as impurity content and composition of the C/C composites were closely related to the fiber orientation, interfacial bonding between carbon fiber and carbon matrix, material characteristics of the three precursors and the resulting matrix carbons. In particular, the resin-carbon matrix C/C (RC/C) composites had the highest electric resistivity, tensile, and flexural strength, as well as impurity content. Meanwhile, the pitch-carbon matrix C/C (PC/C) composites possessed the highest TC and CTE in the parallel and vertical direction. And most of the performances of pyro-carbon/resin carbon matrix C/C composites were between those of the RC/C and PC/C composites except the impurity content.

Shao, H. C.; Xia, H. Y.; Liu, G. W.; Qiao, G. J.; Xiao, Z. C.; Su, J. M.; Zhang, X. H.; Li, Y. J.

2014-01-01

261

Rapid, Reversible Heterolytic Cleavage of Bound H2  

SciTech Connect

Heterolytic cleavage of the H-H bond of H2 into a proton and hydride is a fundamentally important step in reactions of hydrogenase enzymes, oxidation of hydrogen in fuel cells, and catalytic hydrogenation of organic compounds. Incorporation of a pendant amine as a proton acceptor has been shown to facilitate reactions of the [FeFe]-hydrogenase enzyme as well as synthetic catalysts for oxidation of H2. We report the facile, reversible heterolytic cleavage of H2 in a Mn complex bearing a pendant amine, leading to an Mn-H and N-H bond, with an estimated rate of >107 s-1 at 25 ºC. We thank the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, for support of the initial parts of this work. Current work is supported by the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

Hulley, Elliott B.; Welch, Kevin D.; Appel, Aaron M.; DuBois, Daniel L.; Bullock, R. Morris

2013-08-14

262

Structural basis of carotenoid cleavage: from bacteria to mammals  

PubMed Central

Carotenoids and their metabolic derivatives serve critical functions in both prokaryotic and eukaryotic cells, including pigmentation, photoprotection and photosynthesis as well as cell signaling. These organic compounds are also important for visual function in vertebrate and non-vertebrate organisms. Enzymatic transformations of carotenoids to various apocarotenoid products are catalyzed by a family of evolutionarily conserved, non-heme iron-containing enzymes named carotenoid cleavage oxygenases (CCOs). Studies have revealed that CCOs are critically involved in carotenoid homeostasis and essential for the health of organisms including humans. These enzymes typically display a high degree of regio- and stereo-selectivity, acting on specific positions of the polyene backbone located in their substrates. By oxidatively cleaving or isomerizing specific double bonds, CCOs generate a variety of apocarotenoid isomer products. Recent structural studies have helped illuminate the mechanisms by which CCOs mobilize their lipophilic substrates from biological membranes to perform their characteristic double bond cleavage and/or isomerization reactions. In this review, we aim to integrate structural and biochemical information about CCOs to provide insights into their catalytic mechanisms. PMID:23827316

Sui, Xuewu; Kiser, Philip D.; von Lintig, Johannes; Palczewski, Krzysztof

2013-01-01

263

International Journal of Mass Spectrometry 265 (2007) 197212 Possible mechanisms for protecting N C bonds in helical  

E-print Network

attachment to such a sample, bond cleavage may occur and fragment ions can form and be detected. One simulations are carried out on small alanine-based peptide fragments with an excess electron added is to determine to what extent and by what means helix-involved N C bonds are "protected" against cleavage

Simons, Jack

264

Microstructure and cleavage in lath martensitic steels  

NASA Astrophysics Data System (ADS)

In this paper we discuss the microstructure of lath martensitic steels and the mechanisms by which it controls cleavage fracture. The specific experimental example is a 9Ni (9 wt% Ni) steel annealed to have a large prior austenite grain size, then examined and tested in the as-quenched condition to produce a relatively coarse lath martensite. The microstructure is shown to approximate the recently identified ‘classic’ lath martensite structure: prior austenite grains are divided into packets, packets are subdivided into blocks, and blocks contain interleaved laths whose variants are the two Kurjumov-Sachs relations that share the same Bain axis of the transformation. When the steel is fractured in brittle cleavage, the laths in the block share {100} cleavage planes and cleave as a unit. However, cleavage cracks deflect or blunt at the boundaries between blocks with different Bain axes. It follows that, as predicted, the block size governs the effective grain size for cleavage.

Morris, John W., Jr.; Kinney, Chris; Pytlewski, Ken; Adachi, Y.

2013-02-01

265

The Extended Cleavage Specificity of Human Thrombin  

PubMed Central

Thrombin is one of the most extensively studied of all proteases. Its central role in the coagulation cascade as well as several other areas has been thoroughly documented. Despite this, its consensus cleavage site has never been determined in detail. Here we have determined its extended substrate recognition profile using phage-display technology. The consensus recognition sequence was identified as, P2-Pro, P1-Arg, P1?-Ser/Ala/Gly/Thr, P2?-not acidic and P3?-Arg. Our analysis also identifies an important role for a P3?-arginine in thrombin substrates lacking a P2-proline. In order to study kinetics of this cooperative or additive effect we developed a system for insertion of various pre-selected cleavable sequences in a linker region between two thioredoxin molecules. Using this system we show that mutations of P2-Pro and P3?-Arg lead to an approximate 20-fold and 14-fold reduction, respectively in the rate of cleavage. Mutating both Pro and Arg results in a drop in cleavage of 200–400 times, which highlights the importance of these two positions for maximal substrate cleavage. Interestingly, no natural substrates display the obtained consensus sequence but represent sequences that show only 1–30% of the optimal cleavage rate for thrombin. This clearly indicates that maximal cleavage, excluding the help of exosite interactions, is not always desired, which may instead cause problems with dysregulated coagulation. It is likely exosite cooperativity has a central role in determining the specificity and rate of cleavage of many of these in vivo substrates. Major effects on cleavage efficiency were also observed for residues as far away as 4 amino acids from the cleavage site. Insertion of an aspartic acid in position P4 resulted in a drop in cleavage by a factor of almost 20 times. PMID:22384068

Gallwitz, Maike; Enoksson, Mattias; Thorpe, Michael; Hellman, Lars

2012-01-01

266

A large and accurate collection of peptidase cleavages in the MEROPS database  

PubMed Central

Peptidases are enzymes that hydrolyse peptide bonds in proteins and peptides. Peptidases are important in pathological conditions such as Alzheimer's disease, tumour and parasite invasion, and for processing viral polyproteins. The MEROPS database is an Internet resource containing information on peptidases, their substrates and inhibitors. The database now includes details of cleavage positions in substrates, both physiological and non-physiological, natural and synthetic. There are 39 118 cleavages in the collection; including 34 606 from a total of 10 513 different proteins and 2677 cleavages in synthetic substrates. The number of cleavages designated as ‘physiological’ is 13 307. The data are derived from 6095 publications. At least one substrate cleavage is known for 45% of the 2415 different peptidases recognized in the MEROPS database. The website now has three new displays: two showing peptidase specificity as a logo and a frequency matrix, the third showing a dynamically generated alignment between each protein substrate and its most closely related homologues. Many of the proteins described in the literature as peptidase substrates have been studied only in vitro. On the assumption that a physiologically relevant cleavage site would be conserved between species, the conservation of every site in terms of peptidase preference has been examined and a number have been identified that are not conserved. There are a number of cogent reasons why a site might not be conserved. Each poorly conserved site has been examined and a reason postulated. Some sites are identified that are very poorly conserved where cleavage is more likely to be fortuitous than of physiological relevance. This data-set is freely available via the Internet and is a useful training set for algorithms to predict substrates for peptidases and cleavage positions within those substrates. The data may also be useful for the design of inhibitors and for engineering novel specificities into peptidases. Database URL: http://merops.sanger.ac.uk PMID:20157488

2009-01-01

267

A comparison of C C rotational barrier in [2]staffane, [2]tetrahedrane and ethane  

NASA Astrophysics Data System (ADS)

Analysis of internal rotation about C-C single bond in [2]staffane, [2]tetrahedrane, and ethane has been carried out using HF, B3LYP, B3PW91, MPW1PW91, MP2, QCISD and QCISD(T) methods combined with 6-31G ??, 6-311++G ?? and AUG-cc-PVTZ basis sets. The calculated rotational barriers ? E and the trend in them depend on basis set and method. Both hyperconjugation interaction, which stabilizes the staggered conformation more than eclipsed one, and steric effect, which is negligible in ethane and is comparable with hyperconjugation interaction in [2]tetrahedrane and [2]staffane, cause the hindered rotation. In addition, the ? E values are in good agreement with electron charge densities calculated at central C-C bond critical point.

Ebrahimi, A.; Habibi, S. M.; Sanati, A.; Mohammadi, M.

2008-11-01

268

Gold for C-C coupling reactions: a Swiss-Army-knife catalyst?  

PubMed

For organic chemists, the construction of C-C bonds is the most essential aspect of the assembly of molecules. Transition-metal-catalyzed coupling reactions have evolved as one of the key tools for this task. Lately, gold has also emerged as a catalyst for this kind of transformation. Gold, with its special properties as a mild carbophilic ? Lewis acid, its ability to insert into C-H bonds, and, as discovered recently, its ability to undergo redox transformations, offers the opportunity to apply all this potent proficiency for the construction of compounds in an efficient and economical way. This Minireview critically presents the C-C coupling reactions enabled by gold catalysts to encourage further research activities in this promising area of oxidation/reduction gold catalysts. PMID:21818831

Wegner, Hermann A; Auzias, Mathieu

2011-08-29

269

[Recent knowledge about intestinal absorption and cleavage of carotenoids].  

PubMed

Our knowledge about intestinal absorption and cleavage of carotenoids has rapidly grown during the last years. New facts about carotenoid absorption have emerged while some controversies about cleavage are close to end. The knowledge of the absorption and conversion processes is indispensable to understand and interpret the perturbations that can occur in the metabolism of carotenoids and vitamin A. Recently, it has been shown that the absorption of certain carotenoids is not passive - as believed for a long time - but is a facilitated process that requires, at least for lutein, the class B-type 1 scavenger receptor (SR-B1). Various epidemiological and clinical studies have shown wide variations in carotenoid absorption from one subject to another, such differences are now explained by the structure of the concerned carotenoid, by the nature of the food that is absorbed with the carotenoid, by diverse exogenous factors like the intake of medicines or interfering components, by diet factors, by genetic factors, and by the nutritional status of the subject. Recently, the precise mechanism of beta-carotene cleavage by betabeta-carotene 15,15' monooxygenase (EC 1.14.99.36) - formerly called beta-carotene 15,15' dioxygenase (ex EC 1.13.11.21) - has been discovered, and a second enzyme which cleaves asymmetrically the beta-carotene molecule has been found. beta-carotene 15,15' monooxygenase only acts on the 15,15' bond, thus forming two molecules of retinal from one molecule of beta-carotene by central cleavage. Even though the betabeta-carotene 15,15' monooxygenase is much more active on the beta-carotene molecule, a study has shown that it can act on all carotenoids. Searchers now agree that other enzymes that can catalyse an eccentric cleavage of carotenoids probably exist, but under physiological conditions the betabeta-carotene 15,15' monooxygenase is by far the most active, and it is mainly effective in the small bowel mucosa and in the liver. However the conversion of provitamin A carotenoids into vitamin A is only partial, and requires a satisfactory protein status. PMID:15771974

Borel, P; Drai, J; Faure, H; Fayol, V; Galabert, C; Laromiguière, M; Le Moël, G

2005-01-01

270

Laser emission from unprocessed cleavage microchip  

Microsoft Academic Search

A method utilizing an unprocessed cleavage microchip as the solid-state laser gain medium is proposed. Pumped by a Ti:sapphire laser at 871 nm, 1060 nm continuous-wave laser emission with slope efficiency of 23% has been achieved in an unprocessed microchip directly obtained from a cleavage Nd3+:LaB3O6 crystal. Based on this method, it is possible to avoid the difficulty of the

Y. J. Chen; Y. D. Huang; X. Q. Lin; Q. G. Tan; Z. D. Luo; Y. F. Lin

2005-01-01

271

Antisense oligonucleotide containing an internal, non-nucleotide-based linker promote site-specific cleavage of RNA.  

PubMed Central

We have designed and synthesized a series of novel antisense methylphosphonate oligonucleotide (MPO) cleaving agents that promote site-specific cleavage on a complementary RNA target. These MPOs contain a non- nucleotide-based linking moiety near the middle of the sequence in place of one of the nucleotide bases. The region surrounding the unpaired base on the RNA strand (i.e. the one directly opposite the non-nucleotide-linker) is sensitive to hydrolytic cleavage catalyzed by ethylenediamine hydrochloride. Furthermore, the regions of the RNA comprising hydrogen bonded domains are resistant to cleavage compared with single-stranded RNA alone. Several catalytic moieties capable of supporting acid/base hydrolysis were coupled to the non-nucleotide-based linker via simple aqueous coupling chemistries. When tethered to the MPO in this manner these moieties are shown to catalyze site-specific cleavage on the RNA target without any additional catalyst. PMID:8604321

Reynolds, M A; Beck, T A; Say, P B; Schwartz, D A; Dwyer, B P; Daily, W J; Vaghefi, M M; Metzler, M D; Klem, R E; Arnold, L J

1996-01-01

272

Iron-dependent oxidative inactivation with affinity cleavage of pyruvate kinase.  

PubMed

Treatment of rabbit muscle pyruvate kinase with iron/ascorbate caused an inactivation with the cleavage of peptide bond. The inactivation or fragmentation of the enzyme was prevented by addition of Mg2+, catalase, and mannitol, but ADP and PEP the substrates did not show any effect. Protective effect of catalase and mannitol suggests that hydroxyl radical produced through the ferrous ion-dependent reduction of oxygen is responsible for the inactivation/fragmentation of the enzyme. SDS-PAGE and TOF-MS analysis confirmed five pairs of fragments, which were determined to result from the cleavage of the Lys114-Gly115, Glu117-Ile118, Asp177-Gly178, Gly207-Val208, and Phe243-Ile244 bonds of the enzyme by amino-terminal sequencing analysis. Protection of the enzyme by Mg2+ implies the identical binding sites of Fe2+ and Mg2+, but the cleavage sites were discriminated from the cofactor Mg2+-binding sites. Considering amino acid residues interacting with metal ions and tertiary structure, Fe2+ ion may bind to Asp177 neighboring to Gly207 and Glu117 neighboring to Lys114 and Phe243, causing the peptide cleavage by hydroxyl radical. Iron-dependent oxidative inactivation/fragmentation of pyruvate kinase can explain the decreased glycolytic flux under aerobic conditions. Intracellular free Mg2+ concentrations are responsible for the control of cellular respiration and glycolysis. PMID:19169653

Murakami, Keiko; Tsubouchi, Ryoko; Fukayama, Minoru; Qiao, Shanlou; Yoshino, Masataka

2009-07-01

273

Observation of the decay ?(3686)???¯±??+c.c.  

NASA Astrophysics Data System (ADS)

Using a sample of 1.06×108 ?(3686) events collected with the BESIII detector, we present the first observation of the decays of ?(3686)???¯+?-+c.c. and ?(3686)???¯-?++c.c. The branching fractions are measured to be B(?(3686)???¯+?-+c.c.)=(1.40±0.03±0.13)×10-4 and B(?(3686)???¯-?++c.c.)=(1.54±0.04±0.13)×10-4, where the first errors are statistical and the second ones systematic.

Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Ambrose, D. J.; An, F. F.; An, Q.; Bai, J. Z.; Baldini Ferroli, R.; Ban, Y.; Bennett, J. V.; Bertani, M.; Bian, J. M.; Boger, E.; Bondarenko, O.; Boyko, I.; Braun, S.; Briere, R. A.; Cai, H.; Cai, X.; Cakir, O.; Calcaterra, A.; Cao, G. F.; Cetin, S. A.; Chang, J. F.; Chelkov, G.; Chen, G.; Chen, H. S.; Chen, J. C.; Chen, M. L.; Chen, S. J.; Chen, X.; Chen, X. R.; Chen, Y. B.; Cheng, H. P.; Chu, X. K.; Chu, Y. P.; Cronin-Hennessy, D.; Dai, H. L.; Dai, J. P.; Dedovich, D.; Deng, Z. Y.; Denig, A.; Denysenko, I.; Destefanis, M.; Ding, W. M.; Ding, Y.; Dong, C.; Dong, J.; Dong, L. Y.; Dong, M. Y.; Du, S. X.; Fang, J.; Fang, S. S.; Fang, Y.; Fava, L.; Feng, C. Q.; Fu, C. D.; Fu, J. L.; Fuks, O.; Gao, Q.; Gao, Y.; Geng, C.; Goetzen, K.; Gong, W. X.; Gradl, W.; Greco, M.; Gu, M. H.; Gu, Y. T.; Guan, Y. H.; Guo, A. Q.; Guo, L. B.; Guo, T.; Guo, Y. P.; Guo, Y. P.; Han, Y. L.; Harris, F. A.; He, K. L.; He, M.; He, Z. Y.; Held, T.; Heng, Y. K.; Hou, Z. L.; Hu, C.; Hu, H. M.; Hu, J. F.; Hu, T.; Huang, G. M.; Huang, G. S.; Huang, J. S.; Huang, L.; Huang, X. T.; Hussain, T.; Ji, C. S.; Ji, Q.; Ji, Q. P.; Ji, X. B.; Ji, X. L.; Jiang, L. L.; Jiang, X. S.; Jiao, J. B.; Jiao, Z.; Jin, D. P.; Jin, S.; Jing, F. F.; Johansson, T.; Kalantar-Nayestanaki, N.; Kang, X. L.; Kavatsyuk, M.; Kloss, B.; Kopf, B.; Kornicer, M.; Kuehn, W.; Kupsc, A.; Lai, W.; Lange, J. S.; Lara, M.; Larin, P.; Leyhe, M.; Li, C. H.; Li, Cheng; Li, Cui; Li, D.; Li, D. M.; Li, F.; Li, G.; Li, H. B.; Li, J. C.; Li, K.; Li, K.; Li, Lei; Li, P. R.; Li, Q. J.; Li, T.; Li, W. D.; Li, W. G.; Li, X. L.; Li, X. N.; Li, X. Q.; Li, X. R.; Li, Z. B.; Liang, H.; Liang, Y. F.; Liang, Y. T.; Liao, G. R.; Lin, D. X.; Liu, B. J.; Liu, C. L.; Liu, C. X.; Liu, F. H.; Liu, Fang; Liu, Feng; Liu, H. B.; Liu, H. H.; Liu, H. M.; Liu, J.; Liu, J. P.; Liu, K.; Liu, K. Y.; Liu, P. L.; Liu, Q.; Liu, S. B.; Liu, X.; Liu, Y. B.; Liu, Z. A.; Liu, Zhiqiang; Liu, Zhiqing; Loehner, H.; Lou, X. C.; Lu, G. R.; Lu, H. J.; Lu, H. L.; Lu, J. G.; Lu, X. R.; Lu, Y.; Lu, Y. P.; Luo, C. L.; Luo, M. X.; Luo, T.; Luo, X. L.; Lv, M.; Ma, F. C.; Ma, H. L.; Ma, Q. M.; Ma, S.; Ma, T.; Ma, X. Y.; Maas, F. E.; Maggiora, M.; Malik, Q. A.; Mao, Y. J.; Mao, Z. P.; Messchendorp, J. G.; Min, J.; Min, T. J.; Mitchell, R. E.; Mo, X. H.; Moeini, H.; Morales Morales, C.; Moriya, K.; Muchnoi, N. Yu.; Nefedov, Y.; Nikolaev, I. B.; Ning, Z.; Nisar, S.; Niu, X. Y.; Olsen, S. L.; Ouyang, Q.; Pacetti, S.; Pelizaeus, M.; Peng, H. P.; Peters, K.; Ping, J. L.; Ping, R. G.; Poling, R.; Prencipe, E.; Qi, M.; Qian, S.; Qiao, C. F.; Qin, L. Q.; Qin, X. S.; Qin, Y.; Qin, Z. H.; Qiu, J. F.; Rashid, K. H.; Redmer, C. F.; Ripka, M.; Rong, G.; Ruan, X. D.; Sarantsev, A.; Schoenning, K.; Schumann, S.; Shan, W.; Shao, M.; Shen, C. P.; Shen, X. Y.; Sheng, H. Y.; Shepherd, M. R.; Song, W. M.; Song, X. Y.; Spataro, S.; Spruck, B.; Sun, G. X.; Sun, J. F.; Sun, S. S.; Sun, Y. J.; Sun, Y. Z.; Sun, Z. J.; Sun, Z. T.; Tang, C. J.; Tang, X.; Tapan, I.; Thorndike, E. H.; Toth, D.; Ullrich, M.; Uman, I.; Varner, G. S.; Wang, B.; Wang, D.; Wang, D. Y.; Wang, K.; Wang, L. L.; Wang, L. S.; Wang, M.; Wang, P.; Wang, P. L.; Wang, Q. J.; Wang, S. G.; Wang, W.; Wang, X. F.; Wang, Y. D.; Wang, Y. F.; Wang, Y. Q.; Wang, Z.; Wang, Z. G.; Wang, Z. H.; Wang, Z. Y.; Wei, D. H.; Wei, J. B.; Weidenkaff, P.; Wen, S. P.; Werner, M.; Wiedner, U.; Wolke, M.; Wu, G. G.; Wu, L. H.; Wu, N.; Wu, W.; Wu, Z.; Xia, L. G.; Xia, Y.; Xiao, D.; Xiao, Z. J.; Xie, Y. G.; Xiu, Q. L.; Xu, G. F.; Xu, L.; Xu, Q. J.; Xu, Q. N.; Xu, X. P.; Xue, Z.; Yan, L.; Yan, W. B.; Yan, W. C.; Yan, Y. H.; Yang, H. X.; Yang, Y.; Yang, Y. X.; Ye, H.; Ye, M.; Ye, M. H.; Yu, B. X.; Yu, C. X.; Yu, H. W.; Yu, J. S.; Yu, S. P.; Yuan, C. Z.; Yuan, W. L.; Yuan, Y.; Zafar, A. A.; Zallo, A.; Zang, S. L.; Zeng, Y.; Zhang, B. X.; Zhang, B. Y.; Zhang, C.; Zhang, C. B.; Zhang, C. C.; Zhang, D. H.; Zhang, H. H.; Zhang, H. Y.; Zhang, J. J.; Zhang, J. L.; Zhang, J. Q.; Zhang, J. W.; Zhang, J. Y.; Zhang, J. Z.; Zhang, S. H.; Zhang, X. J.; Zhang, X. Y.; Zhang, Y.; Zhang, Y. H.; Zhang, Z. H.; Zhang, Z. P.; Zhang, Z. Y.; Zhao, G.; Zhao, J. W.; Zhao, Lei; Zhao, Ling; Zhao, M. G.; Zhao, Q.; Zhao, Q. W.; Zhao, S. J.; Zhao, T. C.; Zhao, X. H.; Zhao, Y. B.; Zhao, Z. G.; Zhemchugov, A.; Zheng, B.; Zheng, J. P.; Zheng, Y. H.; Zhong, B.; Zhou, L.; Zhou, Li; Zhou, X.; Zhou, X. K.; Zhou, X. R.; Zhou, X. Y.; Zhu, K.; Zhu, K. J.; Zhu, X. L.; Zhu, Y. C.; Zhu, Y. S.; Zhu, Z. A.; Zhuang, J.; Zou, B. S.; Zou, J. H.

2013-12-01

274

Cleavage crystallography of liquid metal embrittled aluminum alloys  

Microsoft Academic Search

Failure in aluminum alloys subjected to liquid metal embrittlement (LME) typically occurs by either intergranular (IG) fracture or transgranular (TG) cleavage. In this study, the crystallography of liquid metal-induced TG cleavage in six aluminum alloys having a variety of microstructures has been determined via Laue X-ray back reflection. The cleavage crystallography was independent of alloy microstructure, and the cleavage plane

A. P. Reynolds; G. E. Stoner

1991-01-01

275

Specific DNA cleavage mediated by [SalenMn(III)][sup +  

SciTech Connect

The combination of [SalenMn(III)][sup +] and a terminal oxidant affords efficient and specific cleavage of right-handed double-helical DNA in regions rich in A:T base pairs. Metal complexes of the tetradentate chelating ligands Salen (Salen = N,N[prime]-ethylenebis(salicylideneaminato)) have been part of the inorganic chemistry literature for several decades. The cationic manganese(III) complex [SalenMn(III)][sup +] (1) is an efficient catalyst for the epoxidation of olefins with terminal oxidants such as iodosylbenzene. 1 also catalyzes oxidative C-H bond activation. The flat, crescent shape of 1, its aromatic and cationic nature, and its ability to catalyze hydrocarbon oxidation are features shared in whole or in part by metal complexes which bind to DNA and cleave it via oxidative processes. These similarities prompted the authors to evaluate the DNA-cleaving properties of 1, and they now report that 1 mediates specific cleavage of right-handed double-helical DNA in a reaction requiring a terminal oxidant. 20 refs., 3 figs., 1 tab.

Gravert, D.J.; Griffin, J.H. (Stanford Univ., CA (United States))

1993-02-12

276

SUBJECT NAME 30033 1 Evolutionary Computation C C R  

E-print Network

SUBJECT NAME 30033 1 Evolutionary Computation C C R 30034 1 Model Driven Formal Software Design C in the Web R C R 30050 1 Simulation and Formal Analysis of Complex Systems R C 30051 1 Collaborative Systems-Computer Interaction C 30876 1 Time series R C R 30871 1 Digital Control Systems C 30885 1 Biometric Recognition C C C

Autonoma de Madrid, Universidad

277

DEPARTMENT OF CELL BIOLOGY L V DJ C C C3 C1 C2b C2aC C  

E-print Network

DEPARTMENT OF CELL BIOLOGY 2011-2012 3'5' L V DJ C C C3 C1 C2b C2aC C 3'EnhEnh intronic **** 1. 2 the plasma membrane of infected host cells. #12;Department of Cell Biology Welcome to the Albert Einstein College of Medicine and the Department of Cell Biology. Our department is focused on molecular

Jenny, Andreas

278

OH-induced oxidative cleavage of dimethyl disulfide in the presence of NO.  

PubMed

We report the results of the theoretical study of (•)OH-induced oxidative cleavage of dimethyl disulfide (DMDS) and the experimental study of the CH3SSCH3 + (•)OH reaction in the presence of (•)NO. Infrared low temperature argon matrix studies combined with ab initio calculations allowed us to identify cis-CH3SONO, which evidences the formation of the CH3SO(•) and CH3SH molecules in the course of the CH3SSCH3 + (•)OH reaction. Ab initio/quantum chemical topology calculations revealed details of the oxidative cleavage of dimethyl disulfide, which is a complex multistep process involving an alteration of S-O and S-S covalent bonds as well as a hydrogen atom transfer. The ability of delocalization of the unpaired electron density by sulfur atoms and a formation of a hydrogen bond by CH3SO(•) and CH3SH are the factors which seem to explain antiradical properties of DMDS. PMID:23947660

Bil, Andrzej; Grzechnik, Katarzyna; Mierzwicki, Krzysztof; Mielke, Zofia

2013-08-29

279

Class 5, fertilization and onset of cleavage September 23, 2009 Fertilization and Cleavage: the start of making a new organism  

E-print Network

Class 5, fertilization and onset of cleavage September 23, 2009 1 Fertilization and Cleavage, fertilization and onset of cleavage September 23, 2009 2 7.4 Structure of the sea urchin egg at fertilization, fertilization and onset of cleavage September 23, 2009 3 7.11 The acrosome reaction in sea urchin sperm 7

Devoto, Stephen H.

280

An unusal case of facile non-degenerate P-C bond making and breaking.  

PubMed

Oxidation of Li/X phosphinidenoid complex 2, obtained via selective deprotonation from the P-H precursor 1, with [Ph(3)C]BF(4) led to the formation of two P-F substituted diorganophosphane complexes 6,7; the latter tautomer 7 formed via H-shift from 6. In contrast, oxidation of 2 with [(p-Tol)(3)C]BF(4) led to three major and one minor intermediates at low temperature, which we tentatively assign to two pairs of P-C atropisomers 10?a,a' and 10?c,c' and which differ by the relative orientations of their CH(SiMe(3))(2) and W(CO)(5) groups. Conversion of all isomers led finally to complex 11 having a ligand with a long P-C bond to the central trityl* carbon atom, firmly established by single-crystal X-ray analysis. DFT calculations at the B3LYP/def2-TZVPP//BP86/def2-TZVP level of theory on real molecular entities revealed the structures of the in situ formed combined singlet diradicals (4+5 and 5+9) and the nature of intermediates on the way to the final product, complex 11. Remarkable is that all isomers of 11 possess relative energies in the narrow energy regime of about 20?kcal ?mol(-1). A preliminary study revealed that complex 11 undergoes selective P-C bond cleavage at 75?°C in toluene solution. PMID:22488865

Nesterov, Vitaly; Özbolat-Schön, Aysel; Schnakenburg, Gregor; Shi, Lili; Cangönül, Asli; van Gastel, Maurice; Neese, Frank; Streubel, Rainer

2012-06-01

281

Reaction pattern of Photosystem II: oxidative water cleavage and protein flexibility  

Microsoft Academic Search

This short communication addresses three topics of photosynthetic water cleavage in Photosystem II (PS II): (a) effect of protonation in the acidic range on the extent of the ‘fast’ ns kinetics of P680+· reduction by YZ, (b) mechanism of O–O bond formation and (c) role of protein flexibility in the functional integrity of PS II. Based on measurements of light-induced

Philipp Kühn; Jörg Pieper; Olga Kaminskaya; Hann-Jörg Eckert; Ruep E. Lechner; Vladimir Shuvalov; Gernot Renger

2005-01-01

282

Understanding regioselective cleavage in peptide hydrolysis by a palladium(II) aqua complex: a theoretical point of view.  

PubMed

Hydrolytic cleavage of the oligopeptides Ace-Ala-Lys-Tyr-Gly approximately Gly-Met-Ala-Ala-Arg-Ala and Ace-Lys-Gly-Gly-Ala-Gly approximately Pro-Met-Ala-Ala-Arg-Gly by [Pd(H(2)O)(4)](2+) was theoretically investigated by using molecular dynamics simulations and quantum mechanical calculations. The Pd anchorage to the peptide sequence is crucial to provoke the cleavage of the second bond upstream from the anchored methionine. For both cases, the most favorable reaction mechanism is a three-step route. The first step coincides with the experimental suggestion found for the Gly approximately Pro-Met sequence on a cleavage caused by an external attack of a water molecule to a complex in trans conformation of the scissile Gly approximately Gly and Gly approximately Pro peptide bonds. However, our results uncover the important role played by the presence of a Pd-coordinated water molecule, which simultaneously interacts with the carbonyl oxygen atom of the Gly amino acid in the Gly approximately Gly and Gly approximately Pro bonds. In accordance with experimental facts, the rise of the hydrolysis reaction rate when the Pro amino acid is located in the scissile peptide bond was also corroborated. The findings obtained at a molecular level from the present computations not only are relevant to rationalize the previously reported experiments but also could be of importance in designing new Pd(II) complexes for the regioselective cleavage of peptides and proteins. PMID:20527949

Yeguas, Violeta; Campomanes, Pablo; López, Ramón; Díaz, Natalia; Suárez, Dimas

2010-07-01

283

THE BIOCHEMICAL CHARACTERIZATION OF FERRET CAROTENE-9', 10'-MONOOXYGENASE CATALYZING CLEAVAGE OF CAROTENOIDS IN VITRO AND IN VIVO  

Technology Transfer Automated Retrieval System (TEKTRAN)

Previous studies have shown that beta -carotene 15,15'-monooxygenase (CMO1) catalyzes the cleavage of beta -carotene at the central carbon 15, 15’-double bond, but cleaves lycopene with much lower activity. However, expressing the mouse carotene-9’,10’-monooxygenase (CMO2) in beta-carotene/lycopene...

284

Does Cleavage Work at Work? Men, but Not Women, Falsely Believe Cleavage Sells a Weak Product  

ERIC Educational Resources Information Center

We examined whether men, but not women, would be distracted by a female sales representative's exposed cleavage, leading to greater perceived efficacy for a weak, but not for a strong product. A community sample of 88 men and 97 women viewed a video of a female pharmaceutical sales representative who (a) had exposed cleavage or dressed modestly…

Glick, Peter; Chrislock, Karyna; Petersik, Korinne; Vijay, Madhuri; Turek, Aleksandra

2008-01-01

285

Nanomechanical cleavage of molybdenum disulphide atomic layers  

NASA Astrophysics Data System (ADS)

The discovery of two-dimensional materials became possible due to the mechanical cleavage technique. Despite its simplicity, the as-cleaved materials demonstrated surprising macro-continuity, high crystalline quality and extraordinary mechanical and electrical properties that triggered global research interest. Here such cleavage processes and associated mechanical behaviours are investigated by a direct in situ transmission electron microscopy probing technique, using atomically thin molybdenum disulphide layers as a model material. Our technique demonstrates layer number selective cleavage, from a monolayer to double layer and up to 23 atomic layers. In situ observations combined with molecular dynamics simulations reveal unique layer-dependent bending behaviours, from spontaneous rippling (<5 atomic layers) to homogeneous curving (~ 10 layers) and finally to kinking (20 or more layers), depending on the competition of strain energy and interfacial energy.

Tang, Dai-Ming; Kvashnin, Dmitry G.; Najmaei, Sina; Bando, Yoshio; Kimoto, Koji; Koskinen, Pekka; Ajayan, Pulickel M.; Yakobson, Boris I.; Sorokin, Pavel B.; Lou, Jun; Golberg, Dmitri

2014-04-01

286

Rotational spectrum of ethyl cyanoacetylene (C2H5C?C-C?N), a compound of potential astrochemical interest  

NASA Astrophysics Data System (ADS)

Context. New radiotelescopes, such as the very sensitive ALMA, will enable the detection of interstellar molecules in much lower concentrations than previously possible. A successful identification of an interstellar molecule requires that laboratory microwave and millimeter-wave spectra are investigated. Several cyanopolyynes and alkynylcarbonitriles have already been detected in the interstellar medium (ISM). Cyanoacetylene (HC?C-C?N) is abundant in the ISM and its methyl derivative, 2-butynenitrile (CH3C?C-C?N), is also present. The next derivative, ethyl cyanoacetylene, (2-pentynenitrile C2H5C?C-C?N) may also be present in interstellar space. Aims: We report the rotational spectrum of the ethyl cyanoacetylene (C2H5C?C-C?N). This is hoped to facilitate identifying gaseous ethyl cyanoacetylene in the ISM. Methods: We studied the rotational spectrum of C2H5C?C-C?N between 13 and 116 GHz with the microwave spectrometer of the University of Oslo. The spectroscopic study was augmented by high-level quantum-chemical calculations at B3LYP/cc-pVTZ and CCSD/cc-pVTZ levels of theory. Results: We present for the first time the rotational spectrum of the ethyl cyanoacetylene (C2H5C?C-C?N). We assigned 342 transitions of the vibrational ground state, accurate values were obtained for rotational and centrifugal distortion constants, and the dipole moment was determined as well.

Carles, S.; Møllendal, H.; Guillemin, J.-C.

2013-10-01

287

Identification of an Acyl-Enzyme Intermediate in a meta-Cleavage Product Hydrolase Reveals the Versatility of the Catalytic Triad  

SciTech Connect

Meta-cleavage product (MCP) hydrolases are members of the {alpha}/{beta}-hydrolase superfamily that utilize a Ser-His-Asp triad to catalyze the hydrolysis of a C-C bond. BphD, the MCP hydrolase from the biphenyl degradation pathway, hydrolyzes 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) to 2-hydroxypenta-2,4-dienoic acid (HPD) and benzoate. A 1.6 {angstrom} resolution crystal structure of BphD H265Q incubated with HOPDA revealed that the enzyme's catalytic serine was benzoylated. The acyl-enzyme is stabilized by hydrogen bonding from the amide backbone of 'oxyanion hole' residues, consistent with formation of a tetrahedral oxyanion during nucleophilic attack by Ser112. Chemical quench and mass spectrometry studies substantiated the formation and decay of a Ser112-benzoyl species in wild-type BphD on a time scale consistent with turnover and incorporation of a single equivalent of {sup 18}O into the benzoate produced during hydrolysis in H{sub 2}{sup 18}O. Rapid-scanning kinetic studies indicated that the catalytic histidine contributes to the rate of acylation by only an order of magnitude, but affects the rate of deacylation by over 5 orders of magnitude. The orange-colored catalytic intermediate, ES{sup red}, previously detected in the wild-type enzyme and proposed herein to be a carbanion, was not observed during hydrolysis by H265Q. In the newly proposed mechanism, the carbanion abstracts a proton from Ser112, thereby completing tautomerization and generating a serinate for nucleophilic attack on the C6-carbonyl. Finally, quantification of an observed pre-steady-state kinetic burst suggests that BphD is a half-site reactive enzyme. While the updated catalytic mechanism shares features with the serine proteases, MCP hydrolase-specific chemistry highlights the versatility of the Ser-His-Asp triad.

Ruzzini, Antonio C.; Ghosh, Subhangi; Horsman, Geoff P.; Foster, Leonard J.; Bolin, Jeffrey T.; Eltis, Lindsay D. (Purdue); (UBC)

2014-10-02

288

The dual role of oxygen functions in coal pretreatment and liquefaction: Crosslinking and cleavage reactions. Seventh quarterly report, September 30, 1992--December 31, 1992  

SciTech Connect

The work during the past quarter under Task 2 has focused on the investigation of FT-IR methods for measuring carboxyl and phenolic functions. Fourier transform infrared (FT-IR) spectra of coal contain a wealth of information that can be utilized in the development of quantitative analysis routines based on least squares curvefitting. Because of the importance of the carboxylate groups in retrogressive reactions, recent efforts have focused on the C=O stretching region. Raw and modified coal samples (acid washed, demineralized, and cation exchanged) were analyzed in order to validate the proposed band assignments in the C=O stretching region. This parameter set differentiates free carbonyl (B2) and hydrogen-bonded carbonyl (B4) from carboxylic acid carbonyl (B3) and carboxylate (B7). One test of these assignments, which are based on literature data, is to plot B3 versus B7. This should be linear, assuming that the sum of the free carboxyl and carboxylate groups is constant and that the intensity of the overlapped aromatic ring band in B7 is also constant. This relationship was found to hold for a set of raw, acid washed, and acid washed/cation-exchanged Zap coals. The work under Task 3 has involved (1) completion of the synthesis of the -C-C-0- linked, methoxy substituted lignin-network polymer, -- C{sub 6}H{sub 3}(o-OMe)-O-CH{sub 2}CH{sub 2}]{sub {eta}} -- polymer, (2) Analysis of the polymer via depolymerization under pyrolysis-FIMS (Py-FIMS) conditions, and (3) testing of several routes to selective cleavage of the O-Methyl bond so that the relative crosslinking tendencies of the methylated and unmethylated versions of the polymer could be determined.

Serio, M.A.; Kroo, E.; Charpenay, S.; Solomon, P.R.

1992-12-31

289

Lignol cleavage by Pd/C under mild conditions and without hydrogen: a role for benzylic C-H activation?  

PubMed

The cleavage of C-O bonds in lignin model compounds without hydrogen was developed using the commercially available Pd/C. Hydrogen donor solvents are helpful for this reaction through transfer hydrogenation, but not necessary. A redox neutral process that utilizes the internal hydrogen source for the cleavage is also possible. An initial mechanistic study indicates that the ?-benzylic-H atom in the substrate plays a critical role and that the present system undergoes a process different from previous reports. PMID:24692272

Zhou, Xiaoyuan; Mitra, Joyee; Rauchfuss, Thomas B

2014-06-01

290

Cleavage Specificity Analysis of Six Type II Transmembrane Serine Proteases (TTSPs) Using PICS with Proteome-Derived Peptide Libraries  

PubMed Central

Background Type II transmembrane serine proteases (TTSPs) are a family of cell membrane tethered serine proteases with unclear roles as their cleavage site specificities and substrate degradomes have not been fully elucidated. Indeed just 52 cleavage sites are annotated in MEROPS, the database of proteases, their substrates and inhibitors. Methodology/Principal Finding To profile the active site specificities of the TTSPs, we applied Proteomic Identification of protease Cleavage Sites (PICS). Human proteome-derived database searchable peptide libraries were assayed with six human TTSPs (matriptase, matriptase-2, matriptase-3, HAT, DESC and hepsin) to simultaneously determine sequence preferences on the N-terminal non-prime (P) and C-terminal prime (P’) sides of the scissile bond. Prime-side cleavage products were isolated following biotinylation and identified by tandem mass spectrometry. The corresponding non-prime side sequences were derived from human proteome databases using bioinformatics. Sequencing of 2,405 individual cleaved peptides allowed for the development of the family consensus protease cleavage site specificity revealing a strong specificity for arginine in the P1 position and surprisingly a lysine in P1? position. TTSP cleavage between R?K was confirmed using synthetic peptides. By parsing through known substrates and known structures of TTSP catalytic domains, and by modeling the remainder, structural explanations for this strong specificity were derived. Conclusions Degradomics analysis of 2,405 cleavage sites revealed a similar and characteristic TTSP family specificity at the P1 and P1? positions for arginine and lysine in unfolded peptides. The prime side is important for cleavage specificity, thus making these proteases unusual within the tryptic-enzyme class that generally has overriding non-prime side specificity. PMID:25211023

Béliveau, François; Leduc, Richard; Overall, Christopher M.

2014-01-01

291

Dissociation Channels of c-C4F8 to CF2 Radical in Reactive Plasma  

NASA Astrophysics Data System (ADS)

It has been generally assumed that octafluorocyclobutane (c-C4F8) is mainly decomposed to CF2 via C2F4 in etching process plasma. However, the detailed mechanism for the dissociations is yet ambiguous. In this paper we have calculated the probable dissociation pathways by using ab initio molecular orbital method. The results show that c-C4F8 is dissociated via the first triplet excited state T1(3A2), the fourth triplet excited state T4(32E) and the fourth singlet excited state S4(12E). One of the degenerate excited states of T4 and S4 is constituted by antibonding combination of two ? bonding orbital of C2F4. T1 state is constituted by antibonding combination of b1u antibonding ? orbital of C2F4. Therefore, in the case of the dissociation via S4 and T4 excited states c-C4F8 may dissociate to two C2F4, and in the case of the dissociation via T1 excited state c-C4F8 may dissociate to four CF2 radicals. It is also found that C3F5+ ion observed as the main peak in c-C4F8 process plasma is produced by electron collision with the slightly larger energy than the ionization threshold value. The main dissociation path of C2F4 is a vertical electron attachment. However, it is also found that dissociation pathways via 1B2g, 3B1u, and 3B2g excited states are very important and should not be ignored.

Hayashi, Toshio; Ishikawa, Kenji; Sekine, Makoto; Hori, Masaru; Kono, Akihiro; Suu, Koukou

2011-03-01

292

Cleavage crystallography of liquid metal embrittled aluminum alloys  

Microsoft Academic Search

Failure in aluminum alloys subjected to liquid metal embrittlement (LME) typically occurs by either intergranular (IG) fracture\\u000a or transgranular (TG) cleavage. In this study, the crystallography of liquid metal-induced TG cleavage in six aluminum alloys\\u000a having a variety of microstructures has been determinedvia Laue X-ray back reflection. The cleavage crystallography was independent of alloy microstructure, and the cleavage plane\\u000a was

A. P. Reynolds; G. E. Stoner

1991-01-01

293

Copper-dependent inhibition and oxidative inactivation with affinity cleavage of yeast glutathione reductase.  

PubMed

Effects of copper on the activity and oxidative inactivation of yeast glutathione reductase were analyzed. Glutathione reductase from yeast was inhibited by cupric ion and more potently by cuprous ion. Copper ion inhibited the enzyme noncompetitively with respect to the substrate GSSG and NADPH. The Ki values of the enzyme for Cu(2+) and Cu(+) ion were determined to be 1 and 0.35 ?M, respectively. Copper-dependent inactivation of glutathione reductase was also analyzed. Hydrogen peroxide and copper/ascorbate also caused an inactivation with the cleavage of peptide bond of the enzyme. The inactivation/fragmentation of the enzyme was prevented by addition of catalase, suggesting that hydroxyl radical produced through the cuprous ion-dependent reduction of oxygen is responsible for the inactivation/fragmentation of the enzyme. SDS-PAGE and TOF-MS analysis confirmed eight fragments, which were further determined to result from the cleavage of the Met17-Ser18, Asn20-Thr21, Glu251-Gly252, Ser420-Pro421, Pro421-Thr422 bonds of the enzyme by amino-terminal sequencing analysis. Based on the kinetic analysis and no protective effect of the substrates, GSSG and NADPH on the copper-mediated inactivation/fragmentation of the enzyme, copper binds to the sites apart from the substrate-sites, causing the peptide cleavage by hydroxyl radical. Copper-dependent oxidative inactivation/fragmentation of glutathione reductase can explain the prooxidant properties of copper under the in vivo conditions. PMID:24671306

Murakami, Keiko; Tsubouchi, Ryoko; Fukayama, Minoru; Yoshino, Masataka

2014-06-01

294

On the Relative Merits of Non-Orthogonal and Orthogonal Valence Bond Methods Illustrated on the Hydrogen Molecule  

ERIC Educational Resources Information Center

Valence bond (VB) is one of the cornerstone theories of quantum chemistry. Even if in practical applications the molecular orbital (MO) approach has obtained more attention, some basic chemical concepts (such as the nature of the chemical bond and the failure of the single determinant-based MO methods in describing the bond cleavage) are normally…

Angeli, Celestino; Cimiraglia, Renzo; Malrieu, Jean-Paul

2008-01-01

295

Role of posttranslational cleavage in glycinin assembly.  

PubMed Central

Glycinin, like other 11S seed storage proteins, undergoes a complex series of posttranslational events between the time proglycinin precursors are synthesized in endoplasmic reticulum and the mature glycinin subunits are deposited in vacuolar protein bodies. According to the current understanding of this process, proglycinin subunits aggregate into trimers in endoplasmic reticulum, and then the trimers move to the vacuolar protein bodies where a protease cleaves them into acidic and basic polypeptide chains. Stable glycinin hexamers, rather than trimers, are isolated from mature seeds. We used a re-assembly assay in this study to demonstrate that proteolytic cleavage of the proglycinin subunits is required for in vitro assembly of glycinin oligomers beyond the trimer stage. The possibility that the cleavage is a regulatory step and that it triggers the deposition of 11S seed storage proteins as insoluble aggregates in vivo is considered. PMID:2562565

Dickinson, C D; Hussein, E H; Nielsen, N C

1989-01-01

296

Calpain Cleavage Prediction Using Multiple Kernel Learning  

PubMed Central

Calpain, an intracellular -dependent cysteine protease, is known to play a role in a wide range of metabolic pathways through limited proteolysis of its substrates. However, only a limited number of these substrates are currently known, with the exact mechanism of substrate recognition and cleavage by calpain still largely unknown. While previous research has successfully applied standard machine-learning algorithms to accurately predict substrate cleavage by other similar types of proteases, their approach does not extend well to calpain, possibly due to its particular mode of proteolytic action and limited amount of experimental data. Through the use of Multiple Kernel Learning, a recent extension to the classic Support Vector Machine framework, we were able to train complex models based on rich, heterogeneous feature sets, leading to significantly improved prediction quality (6% over highest AUC score produced by state-of-the-art methods). In addition to producing a stronger machine-learning model for the prediction of calpain cleavage, we were able to highlight the importance and role of each feature of substrate sequences in defining specificity: primary sequence, secondary structure and solvent accessibility. Most notably, we showed there existed significant specificity differences across calpain sub-types, despite previous assumption to the contrary. Prediction accuracy was further successfully validated using, as an unbiased test set, mutated sequences of calpastatin (endogenous inhibitor of calpain) modified to no longer block calpain's proteolytic action. An online implementation of our prediction tool is available at http://calpain.org. PMID:21559271

duVerle, David A.; Ono, Yasuko; Sorimachi, Hiroyuki; Mamitsuka, Hiroshi

2011-01-01

297

Nonspecific cleavage of proteins using graphene oxide.  

PubMed

In this article, we report the intrinsic catalytic activity of graphene oxide (GO) for the nonspecific cleavage of proteins. We used bovine serum albumin (BSA) and a recombinant esterase (rEstKp) from the cold-adapted bacterium Pseudomonas mandelii as test proteins. Cleavage of BSA and rEstKp was nonspecific regarding amino acid sequence, but it exhibited dependence on temperature, time, and the amount of GO. However, cleavage of the proteins did not result in complete hydrolysis into their constituent amino acids. GO also invoked hydrolysis of p-nitrophenyl esters at moderate temperatures lower than those required for peptide hydrolysis regardless of chain length of the fatty acyl esters. Based on the results, the functional groups of GO, including alcohols, phenols, and carboxylates, can be considered as crucial roles in the GO-mediated hydrolysis of peptides and esters via general acid-base catalysis. Our findings provide novel insights into the role of GO as a carbocatalyst with nonspecific endopeptidase activity in biochemical reactions. PMID:24508487

Lee, Heeyoung; Tran, Minh-Hai; Jeong, Hae Kyung; Han, Jinwoo; Jang, Sei-Heon; Lee, ChangWoo

2014-04-15

298

Prediction of proprotein convertase cleavage sites.  

PubMed

Many secretory proteins and peptides are synthesized as inactive precursors that in addition to signal peptide cleavage undergo post-translational processing to become biologically active polypeptides. Precursors are usually cleaved at sites composed of single or paired basic amino acid residues by members of the subtilisin/kexin-like proprotein convertase (PC) family. In mammals, seven members have been identified, with furin being the one first discovered and best characterized. Recently, the involvement of furin in diseases ranging from Alzheimer's disease and cancer to anthrax and Ebola fever has created additional focus on proprotein processing. We have developed a method for prediction of cleavage sites for PCs based on artificial neural networks. Two different types of neural networks have been constructed: a furin-specific network based on experimental results derived from the literature, and a general PC-specific network trained on data from the Swiss-Prot protein database. The method predicts cleavage sites in independent sequences with a sensitivity of 95% for the furin neural network and 62% for the general PC network. The ProP method is made publicly available at http://www.cbs.dtu.dk/services/ProP. PMID:14985543

Duckert, Peter; Brunak, Søren; Blom, Nikolaj

2004-01-01

299

Cleavage of [4Fe-4S]-Type Clusters: Breaking the Symmetry  

PubMed Central

The cleavage of [4Fe-4S]-type clusters is thought to be important in proteins such as Fe-S scaffold proteins and nitrogenase. However, most [4Fe-4S]2+ clusters in proteins have two antiferromagnetically coupled high-spin layers in which a minority spin is delocalized in each layer, thus forming a symmetric Fe2.5+-Fe2.5+ pair, and how cleavage occurs between the irons is puzzling because of the shared electron. Previously we proposed a novel mechanism for the fission of a [4Fe-4S] core into two [2Fe-2S] cores in which the minority spin localizes on one iron, thus breaking the symmetry and creating a transition state with two Fe3+-Fe2+ pairs. Cleavage first through the weak Fe2+—S bonds lowers the activation energy. Here, we propose a test of this mechanism: break the symmetry of the cluster by changing the ligands to promote spin localization, which should enhance reactivity. The cleavage reactions for the homo-ligand [Fe4S4L4]2? (L = SCH3, Cl, H) and hetero-ligand [Fe4S4(SCH3)2L2]2? (L = Cl, H) clusters in the gas phase were examined via broken-symmetry density functional theory calculations. In the hetero-ligand clusters, the minority spin localized on the iron coordinated by the weaker electron donor ligand and the reaction energy and activation barrier of the cleavage were lowered, which is in accord with our proposed mechanism and consistent with photoelectron spectroscopy and collision-induced dissociation experiments. These studies suggest that proteins requiring facile fission of their [4Fe-4S] cluster in their biological function might have spin-localized [4Fe-4S] clusters. PMID:19378967

Niu, Shuqiang; Ichiye, Toshiko

2009-01-01

300

Cleavage crystallography of liquid metal embrittled aluminum alloys  

NASA Technical Reports Server (NTRS)

The crystallography of liquid metal-induced transgranular cleavage in six aluminum alloys having a variety of microstructures has been determined via Laue X-ray back reflection. The cleavage crystallography was independent of alloy microstructure, and the cleavage plane was 100-plane oriented in all cases. It was further determined that the cleavage crystallography was not influenced by alloy texture. Examination of the fracture surface indicated that there was not a unique direction of crack propagation. In addition, the existence of 100-plane cleavage on alloy 2024 fracture surfaces was inferred by comparison of secondary cleavage crack intersection geometry on the 2024 surfaces with the geometry of secondary cleavage crack intersections on the test alloys.

Reynolds, A. P.; Stoner, G. E.

1991-01-01

301

902 SULFAPYRIDINE Fig. 2. Packing diagram of SP IV, showing the hydrogen bonds. For  

E-print Network

902 SULFAPYRIDINE C C A Fig. 2. Packing diagram of SP IV, showing the hydrogen bonds. For clarity, thus leading to the proliferation of polymorphic structures. The hydrogen-bonding motifs of polymorphs distinguish the current structure is a pair of hydrogen bonds of the type N(3)-H(N3)...N(1) forming a cyclic

Paris-Sud XI, Université de

302

In Pursuit of an Ideal C-C Bond-Forming Reaction  

PubMed Central

Attempts to introduce the highly versatile vinyl group into other organic molecules in a chemo-, regio- and stereoselective fashion via catalytic activation of ethylene provided challenging opportunities to explore new ligand and salt effects in homogeneous catalysis. This review provides a personal account of the development of enantioselective reactions involving ethylene. PMID:19606231

RajanBabu, T. V.

2009-01-01

303

Total Synthesis of 6-Deoxyerythronolide B via C-C Bond-Forming Transfer Hydrogenation  

PubMed Central

The 14-membered macrolide 6-deoxyerythronolide B is prepared in 14 steps (longest linear sequence) and 20 total steps. Two different methods for alcohol CH-crotylation via transfer hydrogenation are deployed for the first time in target-oriented synthesis. Enyne metathesis is used to form the 14-membered ring. The present approach represents the most concise construction of any erythronolide reported, to date. PMID:23464668

Gao, Xin; Woo, Sang Kook; Krische, Michael J.

2013-01-01

304

C–C bond formation through olefin–thiocarbyne coupling in diiron complexes  

Microsoft Academic Search

The bridging diiron thiocarbyne complex [Fe2{?-CS(Me)}(?-CO)(CO)2(Cp)2][SO3CF3] (1) reacts with activated olefins (methyl acrylate, acrylonitrile, styrene, diethyl maleate), in the presence of Me3NO and NaH, to give the corresponding ?-allylidene complexes [Fe2{?-?1:?3-C?(SMe)C?(R?)C?(H)(R?)} (?-CO)(CO)(Cp)2] (R?=CO2Me, R?=H, 3a; R? = CN, R?=H, 3b; R?=C6H5, R?=H, 3c; R?=R?=CO2Et, 3d). The coupling reaction of olefin with thiocarbyne is regio- and stereospecific, leading to the formation

Luigi Busetto; Fabio Marchetti; Mauro Salmi; Stefano Zacchini; Valerio Zanotti

2007-01-01

305

Aminocatalytic Cross-Coupling Approach via Iminium Ions to Different C?C Bonds.  

PubMed

Given the attractive ability of iminium ions to functionalize molecules directly at ostensibly unreactive positions, the reactivity of iminium ions, in which an ? CH2 group is replaced by C?O was explored. Background studies on the ability of such iminium cations to promote reactions via an iminium-catalyzed or iminium-equivalent pathway are apparently unavailable. Previously, tandem cross-coupling reactions were reported, in which an iminium ion undergoes nucleophilic 1,2-addition to give a putative three-component intermediate that abstracts a proton in situ and undergoes self-deamination followed by unprecedented DMSO/aerobic oxidation to generate ?-ketoamides. However, later it was observed that iminium ions can generate valuable ?-ketoamides through simple aerobic oxidation. In all reactions, iminium ions were generated in situ by reaction of 2-oxoaldehydes with secondary amines. PMID:25524296

Mupparapu, Nagaraju; Battini, Narsaiah; Battula, Satyanarayana; Khan, Shahnawaz; Vishwakarma, Ram A; Ahmed, Qazi Naveed

2015-02-01

306

Strong Bond Activation with Late Transition-Metal Pincer Complexes as a Foundation for Potential Catalysis  

E-print Network

cleavage of B-H and B-B bonds across the N-Pd bond in a cationic (PNP)Pd fragment, the C-H oxidative addition to a (PNP)Ir center and the recent results on the C-H and C-O oxidative addition in reactions of aryl carboxylates with the (PNP)Rh fragment...

Zhu, Yanjun

2012-07-16

307

Carbon-Carbon Bond Activation in Pt(0)-Diphenylacetylene Complexes Bearing  

E-print Network

Carbon-Carbon Bond Activation in Pt(0)-Diphenylacetylene Complexes Bearing Chelating P,N- and P complexes bearing chelating P,N- or P,P-ligands and on the cleavage of the C(sp2)-C(sp) bond complexes bearing chelat- ing P,P-ligands. Thus, the reaction of 1 with 1 equiv bis- (diisopropylphosphino

Jones, William D.

308

Structure-based cleavage mechanism of Thermus thermophilus Argonaute DNA guide strand-mediated DNA target cleavage.  

PubMed

We report on crystal structures of ternary Thermus thermophilus Argonaute (TtAgo) complexes with 5'-phosphorylated guide DNA and a series of DNA targets. These ternary complex structures of cleavage-incompatible, cleavage-compatible, and postcleavage states solved at improved resolution up to 2.2 Å have provided molecular insights into the orchestrated positioning of catalytic residues, a pair of Mg(2+) cations, and the putative water nucleophile positioned for in-line attack on the cleavable phosphate for TtAgo-mediated target cleavage by a RNase H-type mechanism. In addition, these ternary complex structures have provided insights into protein and DNA conformational changes that facilitate transition between cleavage-incompatible and cleavage-compatible states, including the role of a Glu finger in generating a cleavage-competent catalytic Asp-Glu-Asp-Asp tetrad. Following cleavage, the seed segment forms a stable duplex with the complementary segment of the target strand. PMID:24374628

Sheng, Gang; Zhao, Hongtu; Wang, Jiuyu; Rao, Yu; Tian, Wenwen; Swarts, Daan C; van der Oost, John; Patel, Dinshaw J; Wang, Yanli

2014-01-14

309

Stereochemical outcome and kinetic effects of Rp- and Sp-phosphorothioate substitutions at the cleavage site of vaccinia type I DNA topoisomerase.  

PubMed

To probe the mechanism of the reversible DNA phosphodiester bond cleavage and religation mechanism of the type I topoisomerase from vaccinia virus, we have synthesized DNA substrates carrying a single nonbridging Rp- or Sp-phosphorothioate (Ps) modification at the scissile phosphodiester (Pd) bond. Analysis of the stereochemical outcome of the net cleavage and rejoining reaction established that the reaction proceeds with retention of configuration, as expected for a double-displacement mechanism. Single-turnover kinetic studies on irreversible strand cleavage using 18/24 mer suicide substrates showed thio effects (k(Pd)/k(Ps)) of 340- and 30-fold for the Rp-Ps and Sp-Ps stereoisomers, respectively, but approximately 10-fold smaller thio effects for the reverse single-turnover religation reaction (Rp-Ps = 30 and Sp-Ps = 3). As compared to the smaller suicide cleavage substrates, approach-to-equilibrium cleavage studies using 32/32 mer substrates showed 7-9-fold smaller thio effects on cleavage, similar effects on religation, and the same ratio of the Rp to Sp thio effect as the suicide cleavage reaction ( approximately 10). In general, thio effects of 2.4-7.2-fold on the cleavage equilibrium are observed for the wild-type and H265A enzymes, suggesting differences in the interactions of the enzyme with the nonbridging sulfur in the noncovalent and covalent complexes. Studies of the cleavage, religation, and approach-to-equilibrium reactions catalyzed by the H265A active site mutant revealed a stereoselective, 11-fold decrease in the Rp-thio effect on cleavage and religation as compared to the wild-type enzyme. This result suggests that His-265 interacts with the nonbridging pro-Rp oxygen in the transition state for cleavage and religation, consistent with the arrangement of this conserved residue in the crystal structure of the human topoisomerase-DNA complex. In general, the greatest effect of thio substitution and the H265A mutation is to destabilize the transition state, with smaller effects on substrate binding. The interaction of His-265 with the pro-Rp nonbridging oxygen is inconsistent with the proposal that this conserved residue acts as a general acid in the strand cleavage reaction. PMID:10820030

Stivers, J T; Jagadeesh, G J; Nawrot, B; Stec, W J; Shuman, S

2000-05-01

310

Cleavage and formation of molecular dinitrogen in a single system assisted by molybdenum complexes bearing ferrocenyldiphosphine.  

PubMed

The N?N bond of molecular dinitrogen bridging two molybdenum atoms in the pentamethylcyclopentadienyl molybdenum complexes that bear ferrocenyldiphosphine as an auxiliary ligand is homolytically cleaved under visible light irradiation at room temperature to afford two molar molybdenum nitride complexes. Conversely, the bridging molecular dinitrogen is reformed by the oxidation of the molybdenum nitride complex at room temperature. This result provides a successful example of the cleavage and formation of molecular dinitrogen induced by a pair of two different external stimuli using a single system assisted by molybdenum complexes bearing ferrocenyldiphosphine under ambient conditions. PMID:25214300

Miyazaki, Takamasa; Tanaka, Hiromasa; Tanabe, Yoshiaki; Yuki, Masahiro; Nakajima, Kazunari; Yoshizawa, Kazunari; Nishibayashi, Yoshiaki

2014-10-20

311

Crystallographic Evidence for Water-assisted Photo-induced Peptide Cleavage in the Stony Coral Fluorescent Protein Kaede  

Microsoft Academic Search

A coral fluorescent protein from Trachyphyllia geoffroyi, Kaede, possesses a tripeptide of His62-Tyr63-Gly64, which forms a chromophore with green fluorescence. This chromophore's fluorescence turns red following UV light irradiation. We have previously shown that such photoconversion is achieved by a formal ?-elimination reaction, which results in a cleavage of the peptide bond found between the amide nitrogen and the ?-carbon

Ikuko Hayashi; Hideaki Mizuno; Kit I. Tong; Toshiaki Furuta; Fujie Tanaka; Masato Yoshimura; Atsushi Miyawaki; Mitsuhiko Ikura

2007-01-01

312

Testing of DLR C/C-SiC and C/C for HIFiRE 8 Scramjet Combustor  

NASA Technical Reports Server (NTRS)

Ceramic Matrix Composites (CMCs) have been proposed for use as lightweight hot structures in scramjet combustors. Previous studies have calculated significant weight savings by utilizing CMCs (active and passive) versus actively cooled metallic scramjet structures. Both a carbon/carbon (C/C) and a carbon/carbon-silicon carbide (C/C-SiC) material fabricated by DLR (Stuttgart, Germany) are being considered for use in a passively cooled combustor design for Hypersonic International Flight Research Experimentation (HIFiRE) 8, a joint Australia / Air Force Research Laboratory hypersonic flight program, expected to fly at Mach 7 for approximately 30 sec, at a dynamic pressure of 55 kilopascals. Flat panels of the DLR C/C and C/C-SiC materials were installed downstream of a hydrogen-fueled, dual-mode scramjet combustor and tested for several minutes at conditions simulating flight at Mach 5 and Mach 6. Gaseous hydrogen fuel was used to fuel the scramjet combustor. The test panels were instrumented with embedded Type K and Type S thermocouples. Zirconia felt insulation was used during some of the tests to reduce heat loss from the back surface and thus increase the heated surface temperature of the C/C-SiC panel approximately 177 C (350 F). The final C/C-SiC panel was tested for three cycles totaling over 135 sec at Mach 6 enthalpy. Slightly more erosion was observed on the C/C panel than the C/C-SiC panels, but both material systems demonstrated acceptable recession performance for the HIFiRE 8 flight.

Glass, David E.; Capriotti, Diego P.; Reimer, Thomas; Kutemeyer, Marius; Smart, Michael K.

2014-01-01

313

Early cleavage in Phoronis muelleri (Phoronida) displays spiral features.  

PubMed

The view that early cleavage in Phoronida follows a radial pattern is widely accepted. However, data supporting this characterization are ambiguous. Studies have been repeatedly reporting variation between individual embryos, and the occurrence of embryos exhibiting oblique divisions or nonradial cell arrangements. Such embryos were often considered to represent variation within radial cleavage, or artificial appearances. Cleavage in Phoronis muelleri was previously characterized as "derived radial," but also oblique spindles and cell elongations, and shifted cell arrangements were observed. We studied the early cleavage in P. muelleri applying 4D microscopy, fluorescent staining, and confocal laser scanning microscopy. To deal with the problem of variation we provide statistical evaluations of our data. These show that oblique divisions do not represent variational abnormalities. In fact, they reveal that most cells divide obliquely from the third cleavage onwards. What is more, in almost all cells the axis of the third cleavage is inclined dextrally. The fourth cleavage is even stronger sinistrally pronounced. Subsequently, the pattern of alternating cleavage orientation is largely restricted to animal and vegetal blastomeres. As a result of the obliqueness of divisions, four cells encircle the poles in most embryos. Cross furrows are occasionally present. We found no indications for radial cleavage in P. muelleri. In contrast, the observed cleavage displays several characters consistent with the pattern of spiral cleavage. A close relation of phoronid and spiralian cleavage is also suggested by molecular phylogenies, allying both groups in the Lophotrochozoa. We suggest our findings to represent morphological support for this lophotrochozoan/spiralian affinity of Phoronida. PMID:23134207

Pennerstorfer, Markus; Scholtz, Gerhard

2012-01-01

314

Investigation of the Allergenicity of ?-Lactoglobulin and Its Cleavage Fragments  

Microsoft Academic Search

Fragments of ?-lactoglobulin were produced by proteolytic cleavage with trypsin, and chemical cleavage with cyanogen bromide, followed by gel filtration on Sephadex G-50. The antigenicity and allergenicity of the products were studied, before and after reductive cleavage by treatment with 2-mercaptoethanol. The former was determined by inhibition ELISA using IgG anti-?-lactoglobulin raised in rabbits, whilst the latter was determined by

Qingtian Huang; John W. Coleman; Denis R. Stanworth

1985-01-01

315

ARTEMIS nuclease facilitates apoptotic chromatin cleavage.  

PubMed

One hallmark of apoptosis is DNA degradation that first appears as high molecular weight fragments followed by extensive internucleosomal fragmentation. During apoptosis, the DNA-dependent protein kinase (DNA-PK) is activated. DNA-PK is involved in the repair of DNA double-strand breaks (DSB) and its catalytic subunit is associated with the nuclease ARTEMIS. Here, we report that, on initiation of apoptosis in human cells by agents causing DNA DSB or by staurosporine or other agents, ARTEMIS binds to apoptotic chromatin together with DNA-PK and other DSB repair proteins. ARTEMIS recruitment to chromatin showed a time and dose dependency. It required DNA-PK protein kinase activity and was blocked by antagonizing the onset of apoptosis with a pan-caspase inhibitor or on overexpression of the antiapoptotic BCL2 protein. In the absence of ARTEMIS, no defect in caspase-3, poly(ADP-ribose) polymerase-1, and XRCC4 cleavage or in H2AX phosphorylation was observed and DNA-PK catalytic subunit was still phosphorylated on S2056 in response to staurosporine. However, DNA fragmentation including high molecular weight fragmentation was delayed in ARTEMIS-deficient cells compared with cells expressing ARTEMIS. In addition, ARTEMIS enhanced the kinetics of MLL gene cleavage at a breakage cluster breakpoint that is frequently translocated in acute or therapy-related leukemias. These results show a facilitating role for ARTEMIS at least in early, site-specific chromosome breakage during apoptosis. PMID:19808974

Britton, Sébastien; Frit, Philippe; Biard, Denis; Salles, Bernard; Calsou, Patrick

2009-10-15

316

Resistance of Actin to Cleavage during Apoptosis  

NASA Astrophysics Data System (ADS)

A small number of cellular proteins present in the nucleus, cytosol, and membrane fraction are specifically cleaved by the interleukin-1? -converting enzyme (ICE)-like family of proteases during apoptosis. Previous results have demonstrated that one of these, the cytoskeletal protein actin, is degraded in rat PC12 pheochromocytoma cells upon serum withdrawal. Extracts from etoposide-treated U937 cells are also capable of cleaving actin. It was assumed that cleavage of actin represented a general phenomenon, and a mechanism coordinating proteolytic, endonucleolytic, and morphological aspects of apoptosis was proposed. We demonstrate here that actin is resistant to degradation in several different human cells induced to undergo apoptosis in response to a variety of stimuli, including Fas ligation, serum withdrawal, cytotoxic T-cell killing, and DNA damage. On the other hand, cell-free extracts from these cells and the ICE-like protease CPP32 were capable of cleaving actin in vitro. We conclude that while actin contains cleavage sites for ICE-like proteases, it is not degraded in vivo in human cells either because of lack of access of these proteases to actin or due to the presence of other factors that prevent degradation.

Song, Qizhong; Wei, Tie; Lees-Miller, Susan; Alnemri, Emad; Watters, Dianne; Lavin, Martin F.

1997-01-01

317

Structural Basis for Accelerated Cleavage of Bovine Pancreatic Trypsin Inhibitor (BPTI) by Human Mesotrypsin  

SciTech Connect

Human mesotrypsin is an isoform of trypsin that displays unusual resistance to polypeptide trypsin inhibitors and has been observed to cleave several such inhibitors as substrates. Whereas substitution of arginine for the highly conserved glycine 193 in the trypsin active site has been implicated as a critical factor in the inhibitor resistance of mesotrypsin, how this substitution leads to accelerated inhibitor cleavage is not clear. Bovine pancreatic trypsin inhibitor (BPTI) forms an extremely stable and cleavage-resistant complex with trypsin, and thus provides a rigorous challenge of mesotrypsin catalytic activity toward polypeptide inhibitors. Here, we report kinetic constants for mesotrypsin and the highly homologous (but inhibitor sensitive) human cationic trypsin, describing inhibition by, and cleavage of BPTI, as well as crystal structures of the mesotrypsin-BPTI and human cationic trypsin-BPTI complexes. We find that mesotrypsin cleaves BPTI with a rate constant accelerated 350-fold over that of human cationic trypsin and 150,000-fold over that of bovine trypsin. From the crystal structures, we see that small conformational adjustments limited to several side chains enable mesotrypsin-BPTI complex formation, surmounting the predicted steric clash introduced by Arg-193. Our results show that the mesotrypsin-BPTI interface favors catalysis through (a) electrostatic repulsion between the closely spaced mesotrypsin Arg-193 and BPTI Arg-17, and (b) elimination of two hydrogen bonds between the enzyme and the amine leaving group portion of BPTI. Our model predicts that these deleterious interactions accelerate leaving group dissociation and deacylation.

Salameh,M.; Soares, A.; Hockla, A.; Radisky, E.

2008-01-01

318

Specific inhibition of ectodomain shedding of glycoprotein Ib? by targeting its juxtamembrane shedding cleavage site  

PubMed Central

Summary Background Ectodomain shedding of GPIb?, a proteolytic event in which metalloprotease ADAM17 cleaves the Gly464-Val465 bond and releases glycocalicin to the plasma, is considered a critical step in mediating clearance of stored platelets. Supporting evidence has largely come from studies using ADAM17 inhibitors. However, the definitive proof is lacking due to the broad substrate specificity of ADAM17. Objectives To achieve substrate-specific inhibition of GPIb? shedding. Methods Development of monoclonal antibodies that directly bind the sequence around the GPIb? shedding cleavage site and inhibit GPIb? shedding by blocking ADAM17 access to the cleavage site. Results Six anti-GPIb? monoclonal antibodies with varying binding affinities were obtained. The prototypic clone, designated 5G6, and its monomeric Fab fragment, bind specifically purified GPIb-IX complex, human platelets, and transgenic murine platelets expressing human GPIb?. 5G6 showed similar inhibitory potency as a widely used shedding inhibitor GM6001 in both constitutive and induced GPIb? shedding in human platelets. It does not recognize mouse GPIb?. Nor does it inhibit shedding of other platelet receptors. Finally, 5G6 binding displays no detectable effect on platelet activation and aggregation. Conclusion 5G6 specifically inhibits GPIb? shedding with no detectable effect on platelet functions. The method of substrate-specific shedding inhibition by macromolecular binding of the shedding cleavage site can be applicable to many other transmembrane receptors undergoing ectodomain shedding. PMID:24119228

Liang, Xin; Russell, Susan R.; Estelle, Sandra; Jones, Limei H.; Cho, Sungyun; Kahn, Mark L.; Berndt, Michael C.; Bunting, Silvia T.; Ware, Jerry; Li, Renhao

2013-01-01

319

A porphyrin with a C?C unit at its center.  

PubMed

The molecule (C?C)TTP (TTP = tetra-p-tolylporphyrin) and the triflate salt of its dication, [(C?C)TTP][OTf](2), have been synthesized and characterized. NMR spectroscopy, nucleus-independent chemical shift calculations, and the crystal structure of (C?C)TTP indicate that (C?C)TTP is antiaromatic and (C?C)TTP(2+) is aromatic. PMID:21910439

Vaid, Thomas P

2011-10-12

320

Trajectory studies of model H–C–C?H+C = C dissociation. II. Angular momenta and energy partitioning and their relation to non-RRKM dynamics  

Microsoft Academic Search

The model alkyl dissociation reaction H–C–C?H+C = C has been studied on a potential energy surface derived from an analytic potential energy function for ethyl radical dissociation. Nonrandom excitation of H–C–C is simulated by the chemical activation reaction H+C = C?H–C–C, and different initial relative translational, rotational, and vibrational energies are investigated. Comparisons are made between the unimolecular dynamics of

William L. Hase; Ralph J. Wolf

1981-01-01

321

Internal friction and gas desorption of {C}/{C} composites  

NASA Astrophysics Data System (ADS)

{C}/{C} composites are the most promising candidates as high heat flux component materials, where temperature dependence of mechanical properties and gas desorption behavior at elevated temperature are important properties. At the beginning, the newly developed internal friction measurement apparatus, which enables the accurate measurement of dynamic elastic properties up to 1373 K along with the measurement of gas desorption behavior, was used. The materials studied were unidirectional (UD) {C}/{C} composites reinforced with mesophase pitch-based carbon fibers, which were heat treated at temperatures ranging from 1473 to 2773 K which produced a variety of graphitized microstructures. Two-dimensional (2D) {C}/{C} composites reinfored with flat woven fabrics of PAN type carbon fibers were also studied. These materials were heat treated at 1873 K. From the temperature spectrum of internal friction of 2D {C}/{C} composites, these internal friction peaks were detected and were related to gas desorption. Also the temperature dependence of Young's modulus of UD {C}/{C} composites, negative and positive dependence of Young's modulus were observed reflecting microstructure changes resulting from the heat treatments.

Serizawa, H.; Sato, S.; Kohyama, A.

1994-09-01

322

A discrete lattice plane analysis of coherent F.C.C./B1 interfacial energy[Face Centered Cubic  

SciTech Connect

A discrete lattice plane, nearest neighbor, broken bond model with constant bond energies, which had been used to calculate the energy of coherent interphase boundaries in substitutional alloys, was extended to a ternary substitutional-interstitial system to study the chemical interfacial energy between a f.c.c. solid solution and a Bl(NaCl) compound. When the regular solution interaction coefficient of substitutional (metal) atoms is positive, interstitial (non-metallic) atoms which have different bond energies with the two metal atoms tend to increase both the composition difference and the interfacial energy. Even when the interaction coefficient of metal atoms is negative, a miscibility gap and a gradual composition change across the interface occur. The anisotropy of the interfacial energy varies widely according to the magnitude of interaction between the metal and the non-metallic atoms.

Yang, Z.G.; Enomoto, M.

1999-12-10

323

Autocatalytic Cleavage within Classical Swine Fever Virus NS3 Leads to a Functional Separation of Protease and Helicase  

PubMed Central

Classical swine fever virus (CSFV) is a positive-stranded RNA virus belonging to the genus Pestivirus within the Flaviviridae family. Pivotal for processing of a large portion of the viral polyprotein is a serine protease activity within nonstructural protein 3 (NS3) that also harbors helicase and NTPase activities essential for RNA replication. In CSFV-infected cells, NS3 appears as two forms, a fully processed NS3 of 80 kDa and the precursor molecule NS2-3 of 120 kDa. Here we report the identification and mapping of additional autocatalytic intramolecular cleavages. One cleavable peptide bond occurs between Leu1781 and Met1782, giving rise to a helicase subunit of 55 kDa and, depending on the substrate, a NS2-3 fragment of 78 kDa (NS2-3p) or a NS3 protease subunit of 26 kDa (NS3p). In trans-cleavage assays using NS4-5 as a substrate, NS3p acts as a fully functional protease that is able to process the polyprotein. NS3p comprises the minimal essential protease, as deletion of Leu1781 results in inactivation. A second intramolecular cleavage was mapped to the Leu1748/Lys1749 peptide bond that yields a proteolytically inactive NS3 fragment. Deletion of either of the cleavage site residues resulted in a loss of RNA infectivity, indicating the functional importance of amino acid identity at the respective positions. Our data suggest that internal cleavage within the NS3 moiety is a common process that further extends the functional repertoires of the multifunctional NS2-3 or NS3 and represents another level of the complex polyprotein processing of Flaviviridae. PMID:23986594

Lamp, Benjamin; Riedel, Christiane; Wentz, Eveline; Tortorici, Maria-Alejandra

2013-01-01

324

Palladium-catalyzed oxidative insertion of carbon monoxide to N-sulfonyl-2-aminobiaryls through C-H bond activation: access to bioactive phenanthridinone derivatives in one pot.  

PubMed

Palladium-catalyzed oxidative carbonylation of N-sulfonyl-2-aminobiaryls through C-H bond activation and C-C, C-N bond formation under TFA-free and milder conditions has been developed. The reaction tolerates a variety of substrates and provides biologically important phenanthridinone derivatives in yields up to 94%. PMID:23477600

Rajeshkumar, Venkatachalam; Lee, Tai-Hua; Chuang, Shih-Ching

2013-04-01

325

Intramembrane proteolytic cleavage of a membrane-tethered transcription factor by a metalloprotease depends on ATP  

PubMed Central

Regulated intramembrane proteolysis (RIP) involves cleavage of a transmembrane segment of a protein. RIP governs diverse processes in a wide variety of organisms and is carried out by different types of intramembrane proteases (IPs), including a large family of metalloproteases. The Bacillus subtilis SpoIVFB protein is a putative metalloprotease that cleaves membrane-tethered Pro-?K, releasing ?K to direct transcription of genes necessary for spore formation. By attaching an extra transmembrane segment to the N terminus of SpoIVFB, expression in E. coli was improved more than 100-fold, facilitating purification and demonstration of metalloprotease activity, which accurately cleaved purified Pro-?K. Uniquely for IPs examined so far, SpoIVFB activity requires ATP, which binds to the C-terminal cystathionine-?-synthase (CBS) domain of SpoIVFB. Deleting just 10 residues from the C-terminal end of SpoIVFB nearly eliminated cleavage of coexpressed Pro-?K in E. coli. The CBS domain of SpoIVFB was shown to interact with Pro-?K and ATP changed the interaction, suggesting that ATP regulates substrate access to the active site and renders cleavage sensitive to the cellular energy level, which may be a general feature of CBS-domain-containing IPs. Incorporation of SpoIVFB into preformed liposomes stimulated its ability to cleave Pro-?K. Cleavage depended on ATP and the correct peptide bond was shown to be cleaved using a rapid and sensitive mass spectrometry assay. A system for biochemical studies of RIP by a metalloprotease in a membrane environment has been established using methods that might be applicable to other IPs. PMID:19805276

Zhou, Ruanbao; Cusumano, Christina; Sui, Dexin; Garavito, R. Michael; Kroos, Lee

2009-01-01

326

Selective and Nonselective Cleavages in Positive and Negative CID of the Fragments Generated from In-Source Decay of Intact Proteins in MALDI-MS  

NASA Astrophysics Data System (ADS)

Selective and nonselective cleavages in ion trap low-energy collision-induced dissociation (CID) experiments of the fragments generated from in-source decay (ISD) with matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) of intact proteins are described in both positive and negative ion modes. The MALDI-ISD spectra of the proteins demonstrate common, discontinuous, abundant c- and z'-ions originating from cleavage at the N-C? bond of Xxx-Asp/Asn and Gly-Xxx residues in both positive- and negative-ion modes. The positive ion CID of the c- and z'-ions resulted in product ions originating from selective cleavage at Asp-Xxx, Glu-Xxx and Cys-Xxx residues. Nonselective cleavage product ions rationalized by the mechanism of a "mobile proton" are also observed in positive ion CID spectra. Negative ion CID of the ISD fragments results in complex product ions accompanied by the loss of neutrals from b-, c-, and y-ions. The most characteristic feature of negative ion CID is selective cleavage of the peptide bonds of acidic residues, Xxx-Asp/Glu/Cys. A definite influence of ?-helix on the CID product ions was not obtained. However, the results from positive ion and negative ion CID of the MALDI-ISD fragments that may have long ?-helical domains suggest that acidic residues in helix-free regions tend to degrade more than those in helical regions.

Takayama, Mitsuo; Sekiya, Sadanori; Iimuro, Ryunosuke; Iwamoto, Shinichi; Tanaka, Koichi

2014-01-01

327

Alkali Metal Control over N-N Cleavage in Iron Complexes.  

PubMed

Though N2 cleavage on K-promoted Fe surfaces is important in the large-scale Haber-Bosch process, there is still ambiguity about the number of Fe atoms involved during the N-N cleaving step and the interactions responsible for the promoting ability of K. This work explores a molecular Fe system for N2 reduction, particularly focusing on the differences in the results obtained using different alkali metals as reductants (Na, K, Rb, Cs). The products of these reactions feature new types of Fe-N2 and Fe-nitride cores. Surprisingly, adding more equivalents of reductant to the system gives a product in which the N-N bond is not cleaved, indicating that the reducing power is not the most important factor that determines the extent of N2 activation. On the other hand, the results suggest that the size of the alkali metal cation can control the number of Fe atoms that can approach N2, which in turn controls the ability to achieve N2 cleavage. The accumulated results indicate that cleaving the triple N-N bond to nitrides is facilitated by simultaneous approach of least three low-valent Fe atoms to a single molecule of N2. PMID:25412468

Grubel, Katarzyna; Brennessel, William W; Mercado, Brandon Q; Holland, Patrick L

2014-12-01

328

A novel carotenoid cleavage activity involved in the biosynthesis of Citrus fruit-specific apocarotenoid pigments  

PubMed Central

Citrus is the first tree crop in terms of fruit production. The colour of Citrus fruit is one of the main quality attributes, caused by the accumulation of carotenoids and their derivative C30 apocarotenoids, mainly ?-citraurin (3-hydroxy-?-apo-8?-carotenal), which provide an attractive orange-reddish tint to the peel of oranges and mandarins. Though carotenoid biosynthesis and its regulation have been extensively studied in Citrus fruits, little is known about the formation of C30 apocarotenoids. The aim of this study was to the identify carotenoid cleavage enzyme(s) [CCD(s)] involved in the peel-specific C30 apocarotenoids. In silico data mining revealed a new family of five CCD4-type genes in Citrus. One gene of this family, CCD4b1, was expressed in reproductive and vegetative tissues of different Citrus species in a pattern correlating with the accumulation of C30 apocarotenoids. Moreover, developmental processes and treatments which alter Citrus fruit peel pigmentation led to changes of ?-citraurin content and CCD4b1 transcript levels. These results point to the involvement of CCD4b1 in ?-citraurin formation and indicate that the accumulation of this compound is determined by the availability of the presumed precursors zeaxanthin and ?-cryptoxanthin. Functional analysis of CCD4b1 by in vitro assays unequivocally demonstrated the asymmetric cleavage activity at the 7?,8? double bond in zeaxanthin and ?-cryptoxanthin, confirming its role in C30 apocarotenoid biosynthesis. Thus, a novel plant carotenoid cleavage activity targeting the 7?,8? double bond of cyclic C40 carotenoids has been identified. These results suggest that the presented enzyme is responsible for the biosynthesis of C30 apocarotenoids in Citrus which are key pigments in fruit coloration. PMID:24006419

Rodrigo, María J.; Alquézar, Berta; Al-Babili, Salim

2013-01-01

329

Structural basis for activation of the complement system by component C4 cleavage  

PubMed Central

An essential aspect of innate immunity is recognition of molecular patterns on the surface of pathogens or altered self through the lectin and classical pathways, two of the three well-established activation pathways of the complement system. This recognition causes activation of the MASP-2 or the C1s serine proteases followed by cleavage of the protein C4. Here we present the crystal structures of the 203-kDa human C4 and the 245-kDa C4?MASP-2 substrate?enzyme complex. When C4 binds to MASP-2, substantial conformational changes in C4 are induced, and its scissile bond region becomes ordered and inserted into the protease catalytic site in a manner canonical to serine proteases. In MASP-2, an exosite located within the CCP domains recognizes the C4 C345C domain 60 Å from the scissile bond. Mutations in C4 and MASP-2 residues at the C345C–CCP interface inhibit the intermolecular interaction and C4 cleavage. The possible assembly of the huge in vivo enzyme–substrate complex consisting of glycan-bound mannan-binding lectin, MASP-2, and C4 is discussed. Our own and prior functional data suggest that C1s in the classical pathway of complement activated by, e.g., antigen–antibody complexes, also recognizes the C4 C345C domain through a CCP exosite. Our results provide a unified structural framework for understanding the early and essential step of C4 cleavage in the elimination of pathogens and altered self through two major pathways of complement activation. PMID:22949645

Kidmose, Rune T.; Laursen, Nick S.; Dobó, József; Kjaer, Troels R.; Sirotkina, Sofia; Yatime, Laure; Sottrup-Jensen, Lars; Thiel, Steffen; Gál, Péter; Andersen, Gregers R.

2012-01-01

330

Structural basis for activation of the complement system by component C4 cleavage.  

PubMed

An essential aspect of innate immunity is recognition of molecular patterns on the surface of pathogens or altered self through the lectin and classical pathways, two of the three well-established activation pathways of the complement system. This recognition causes activation of the MASP-2 or the C1s serine proteases followed by cleavage of the protein C4. Here we present the crystal structures of the 203-kDa human C4 and the 245-kDa C4·MASP-2 substrate·enzyme complex. When C4 binds to MASP-2, substantial conformational changes in C4 are induced, and its scissile bond region becomes ordered and inserted into the protease catalytic site in a manner canonical to serine proteases. In MASP-2, an exosite located within the CCP domains recognizes the C4 C345C domain 60 Å from the scissile bond. Mutations in C4 and MASP-2 residues at the C345C-CCP interface inhibit the intermolecular interaction and C4 cleavage. The possible assembly of the huge in vivo enzyme-substrate complex consisting of glycan-bound mannan-binding lectin, MASP-2, and C4 is discussed. Our own and prior functional data suggest that C1s in the classical pathway of complement activated by, e.g., antigen-antibody complexes, also recognizes the C4 C345C domain through a CCP exosite. Our results provide a unified structural framework for understanding the early and essential step of C4 cleavage in the elimination of pathogens and altered self through two major pathways of complement activation. PMID:22949645

Kidmose, Rune T; Laursen, Nick S; Dobó, József; Kjaer, Troels R; Sirotkina, Sofia; Yatime, Laure; Sottrup-Jensen, Lars; Thiel, Steffen; Gál, Péter; Andersen, Gregers R

2012-09-18

331

Specific cleavage analysis of mammalian mitochondrial DNA.  

PubMed

Mitochondrial DNA from several mammalian species has been digested with a site-specific restriction endonuclease (HaeIII) from Haemophilus aegyptius. A quantitative analysis of the resulting specific fragments indicates that the mtDNA of any individual mammal is predominantly a single molecular clone. Gel analysis of specific cleavage products has proven quite sensitive in detecting differences in mtDNA: mtDNAs from the more distantly related mammals studied (e.g., donkey and dog) are found to have few bands in common and very closely related mammals (e.g., donkey and horse) share only about 50% of their bands. This procedure has detected several intraspecies mtDNA differences. Six distinct human patterns have been found, with one pattern usually differing from another in two or three bands. mtDNAs from different organs of single individuals have also been analyzed, and no differences have been found. PMID:1060130

Potter, S S; Newbold, J E; Hutchison, C A; Edgell, M H

1975-11-01

332

LC-MS based cleavage site profiling of the proteases ADAM10 and ADAM17 using proteome-derived peptide libraries.  

PubMed

A Disintegrin and Metalloproteinase 10 (ADAM10) and ADAM17 catalyze ectodomain shedding of a number of cell surface proteins important for embryonic development and tissue homeostasis. Changes in the expression levels or dysregulated proteolytic activity of ADAM10 and ADAM17 have been shown to play important roles in multiple diseases such as inflammation, cancer, and neurodegenerative disorders. Despite the well documented substrate repertoire of ADAM10 and ADAM17, little is known about their cleavage site specificity. We optimized Q-PICS (Quantitative Proteomics for the Identification of Cleavage Sites) to elucidate the cleavage site specificity of recombinant murine ADAM10 and ADAM17. Two different yeast proteome-derived peptide libraries were used and samples were analyzed by LC-MALDI and LC-ESI MS in parallel. We show that the largest difference in the cleavage site specificities of ADAM10 and ADAM17 is at the P1' site: while both enzymes cleave N-terminal of leucine, only ADAM10 shows additional preference toward aromatic amino acids, whereas ADAM17 exhibits the highest preference for valine. Together with further amino acid preferences more adjacent to the scissile bond, our data is in good agreement with ADAM10/17 cleavage sites previously identified in native substrates. Overall, the precise identification of ADAM10 and ADAM17 cleavage site specificity provides the basis for better substrate identification in vivo and the generation of specific inhibitors or activity based probes. PMID:24635658

Tucher, Joanna; Linke, Dennis; Koudelka, Tomas; Cassidy, Liam; Tredup, Claudia; Wichert, Rielana; Pietrzik, Claus; Becker-Pauly, Christoph; Tholey, Andreas

2014-04-01

333

Surface Charge Heterogeneity in Amphibole Cleavage Fragments and Asbestos Fibers  

Microsoft Academic Search

Aspect ratio and electrophoretic mobility data for amphibole particles reveal that short fibers and blocky cleavage fragments have a smaller net charge than highly elongated particles. Asbestos fibers and cleavage fragments of the same dimensions exhibit the same net negative surface charge but positively charged ends and negatively charged lateral surfaces.

Joseph E. Schiller; Sequoyah L. Payne; Sanaa E. Khalafalla

1980-01-01

334

Surface Charge Heterogeneity in Amphibole Cleavage Fragments and Asbestos Fibers  

NASA Astrophysics Data System (ADS)

Aspect ratio and electrophoretic mobility data for amphibole particles reveal that short fibers and blocky cleavage fragments have a smaller net charge than highly elongated particles. Asbestos fibers and cleavage fragments of the same dimensions exhibit the same net negative surface charge but positively charged ends and negatively charged lateral surfaces.

Schiller, Joseph E.; Payne, Sequoyah L.; Khalafalla, Sanaa E.

1980-09-01

335

Surface charge heterogeneity in amphibole cleavage fragments and asbestos fibers.  

PubMed

Aspect ratio and electrophoretic mobility data for amphibole particles reveal that short fibers and blocky cleavage fragments have a smaller net charge than highly elongated particles. Asbestos fibers and cleavage fragments of the same dimensions exhibit the same net negative surface charge but positively charged ends and negatively charged lateral surfaces. PMID:17745966

Schiller, J E; Payne, S L; Khalafalla, S E

1980-09-26

336

Vicariance patterns in the Mediterranean Sea: eastwest cleavage and low  

E-print Network

ORIGINAL ARTICLE Vicariance patterns in the Mediterranean Sea: east­west cleavage and low dispersal oceanica is a clonal angiosperm endemic to the Mediterranean Sea. Previous studies have suggested of the Mediterranean Sea as a whole, a strong east­west cleavage was detected (amova). These results are in line

Teixeira, Sara

337

Bonded Lubricants  

NASA Technical Reports Server (NTRS)

Another spinoff to the food processing industry involves a dry lubricant developed by General Magnaplate Corp. of Linden, N.J. Used in such spacecraft as Apollo, Skylab and Viking, the lubricant is a coating bonded to metal surfaces providing permanent lubrication and corrosion resistance. The coating lengthens equipment life and permits machinery to be operated at greater speed, thus increasing productivity and reducing costs. Bonded lubricants are used in scores of commercia1 applications. They have proved particularly valuable to food processing firms because, while increasing production efficiency, they also help meet the stringent USDA sanitation codes for food-handling equipment. For example, a cookie manufacturer plagued production interruptions because sticky batter was clogging the cookie molds had the brass molds coated to solve the problem. Similarly, a pasta producer faced USDA action on a sanitation violation because dough was clinging to an automatic ravioli-forming machine; use of the anti-stick coating on the steel forming plates solved the dual problem of sanitation deficiency and production line downtime.

1977-01-01

338

In vitro analysis of cleavage and polyadenylation in Arabidopsis.  

PubMed

In eukaryotes, pre-messenger RNA (pre-mRNA) cleavage and polyadenylation is one of the necessary processing steps that produce a mature and functional mRNA. Regulation on pre-mRNA cleavage and polyadenylation affects other processes such as mRNA translocation, stability, and translation. The process of pre-mRNA cleavage and polyadenylation, and its relationship with RNA splicing and translation, have been extensively studied due to its importance in vivo. A successful in vitro system has provided enormous amount of information to the study of cleavage and polyadenylation in the mammalian and yeast systems. Here, we describe an in vitro pre-mRNA cleavage system that faithfully cleaves pre-mRNA substrate using Arabidopsis cell/tissue cultures. PMID:25487206

Zhao, Hongwei; Li, Qingshun Quinn

2015-01-01

339

Quantification of C?C and C?O Surface Carbons in Detonation Nanodiamond by NMR  

SciTech Connect

The ability of solid-state 13C NMR to detect and quantify small amounts of sp2-hybridized carbon on the surface of ?5 nm diameter nanodiamond particles is demonstrated. The C?C carbon fraction is only 1.1 ± 0.4% in pristine purified detonation nanodiamond, while a full single-layer graphitic or “bucky diamond” shell would contain ca. 25% of all C in a 5 nm diameter particle. Instead of large aromatic patches repeatedly proposed in the recent literature, sp3-hybridized CH and COH carbons cover most of the nanodiamond particle surface, accounting for ?5% each. C?O and COO groups also seen in X-ray absorption near-edge structure spectroscopy (XANES) but not detected in previous NMR studies make up ca. 1.5% of all C. They are removed by heat treatment at 800 °C, which increases the aromatic fraction. 13C{1H} NMR demonstrates that the various sp2-hybridized carbons are mostly not protonated, but cross-polarization shows that they are separated from 1H by only a few bond lengths, which proves that they are near the protonated surface. Together, the observed C–H, C–OH, C?O, and C?C groups account for 12–14% of all C, which matches the surface fraction expected for bulk-terminated 5 nm diameter diamond particles.

Cui, J.-F.; Fang, X.-W.; Schmidt-Rohr, K.

2014-05-08

340

Thermodynamic properties of carbon in b.c.c. and f.c.c. iron-silicon-carbon solid solutions.  

NASA Technical Reports Server (NTRS)

The equilibrium between hydrogen-methane gas mixtures and Fe-Si-C solid solutions has been investigated both as a function of temperature and carburizing gas composition. The thermodynamic properties of the carbon atoms in both b.c.c. and f.c.c. solid solution have been derived from the equilibrium measurements. The results found have been compared with those of earlier investigations and with the predictions of recent theoretical models on ternary solid solutions containing both substitutional and interstitial solute atoms.

Chraska, P.; Mclellan, R. B.

1971-01-01

341

A Simple Test to Determine the Effectiveness of Different Braze Compositions for Joining Ti-Tubes to C/C Composite Plates  

NASA Technical Reports Server (NTRS)

A simple tube-plate joint tensile test was implemented to compare the effectiveness of commercial brazes, namely, TiCuNi, TiCuSil, and Cu-ABA, used for bonding Ti-tubes joined to C-C composite plates. The different braze systems yielded different; yet, repeatable results. The Cu-ABA system proved to have about twice the load-carrying ability of the other two systems due to the fact that the bonded area between the braze material and the C-C plate was largest for this system. The orientation of the surface fiber tows also had a significant effect on load-carrying ability with tows oriented perpendicular to the tube axis displaying the highest failure loads. Increasing the process load and modifying the surface of the C-C plate by grooving out channels for the Ti-Tube to nest in resulted in increased load-carrying ability for the TiCuSil and Cu-ABA systems due to increased bonded area and better penetration of the braze material into the C-C composite.

Morscher, Gregory N.; Singh, Mrityunjay; Shpargel, Tarah; Asthana, Rajiv

2006-01-01

342

Site specificity of DSP-PP cleavage by BMP1.  

PubMed

Bone morphogenic protein 1 (BMP1), a metalloproteinase, is known to cleave a wide variety of extracellular matrix proteins, suggesting that a consensus substrate cleavage amino acid sequence might exist. However, while such a consensus sequence has been proposed based on P4 to P4' (i.e. the four amino acids flanking either side of the BMP1 cleavage site; P4P3P2P1|P1'P2'P3'P4') sequence homologies between two BMP1 substrates, dentin matrix protein 1 and dentin sialoprotein phosphophoryn (DSP-PP) (i.e. xMQx|DDP), no direct testing has so far been attempted. Using an Sf9 cell expression system, we have been able to produce large amounts of uncleaved DSP-PP. Point mutations introduced into this recombinant DSP-PP were then tested for their effects on DSP-PP cleavage by either Sf9 endogenous tolloid-related protein 1 (TLR-1) or by its human homolog, BMP1. Here, we have measured DSP-PP cleavage efficiencies after modifications based on P4-P4' sequence comparisons with dentin matrix protein 1, as well as for prolysyl oxidase and chordin, two other BMP1 substrates. Our results demonstrate that any mutations within or outside of the DSP-PP P4 to P4' cleavage site can block, impair or accelerate DSP-PP cleavage, and suggest that its BMP1 cleavage site is highly conserved in order to regulate its cleavage efficiency, possibly with additional assistance from its conserved exosites. Thus, BMP1 cleavage cannot be based on a consensus substrate cleavage site. PMID:25158199

Yang, Robert T; Lim, Glendale L; Yee, Colin T; Fuller, Robert S; Ritchie, Helena H

2014-08-01

343

FROM EUCLID TO ENTROPY C. C. Rodr'iguez  

E-print Network

FROM EUCLID TO ENTROPY C. C. Rodr'iguez Department of Mathematics and Statistics SUNY at Albany of geometry goes something like this... Once upon a time there was a greek man named Euclid who organized the mathematical knowledge in his time in six books known as The Elements. Euclid's work was so influential

Rodriguez, Carlos

344

Photonics Integration for THz Generation , C.C. Renaud  

E-print Network

Photonics Integration for THz Generation F. Pozzi , C.C. Renaud , D.C. Rogers , I.F. Lealman. The concept of a photonic THz generator is introduced, focusing on the optical part of the phase locking section of the photonic system. Different approaches to the integration of this element of the THz source

Haddadi, Hamed

345

Crystal structure of a wild-type Cre recombinase–loxP synapse reveals a novel spacer conformation suggesting an alternative mechanism for DNA cleavage activation  

PubMed Central

Escherichia coli phage P1 Cre recombinase catalyzes the site-specific recombination of DNA containing loxP sites. We report here two crystal structures of a wild-type Cre recombinase–loxP synaptic complex corresponding to two distinct reaction states: an initial pre-cleavage complex, trapped using a phosphorothioate modification at the cleavable scissile bond that prevents the recombination reaction, and a 3?-phosphotyrosine protein–DNA intermediate resulting from the first strand cleavage. In contrast to previously determined Cre complexes, both structures contain a full tetrameric complex in the asymmetric unit, unequivocally showing that the anti-parallel arrangement of the loxP sites is an intrinsic property of the Cre–loxP recombination synapse. The conformation of the spacer is different to the one observed for the symmetrized loxS site: a kink next to the scissile phosphate in the top strand of the pre-cleavage complex leads to unstacking of the TpG step and a widening of the minor groove. This side of the spacer is interacting with a ‘cleavage-competent’ Cre subunit, suggesting that the first cleavage occurs at the ApT step in the top strand. This is further confirmed by the structure of the 3?-phosphotyrosine intermediate, where the DNA is cleaved in the top strands and covalently linked to the ‘cleavage-competent’ subunits. The cleavage is followed by a movement of the C-terminal part containing the attacking Y324 and the helix N interacting with the ‘non-cleaving’ subunit. This rearrangement could be responsible for the interconversion of Cre subunits. Our results also suggest that the Cre-induced kink next to the scissile phosphodiester activates the DNA for cleavage at this position and facilitates strand transfer. PMID:12954782

Ennifar, Eric; Meyer, Joachim E. W.; Buchholz, Frank; Stewart, A. Francis; Suck, Dietrich

2003-01-01

346

Familial Alzheimer’s mutations within APPTM increase A?42 production by enhancing accessibility of ?-cleavage site  

NASA Astrophysics Data System (ADS)

The high A?42/A?40 production ratio is a hallmark of familial Alzheimer’s disease, which can be caused by mutations in the amyloid precursor protein (APP). The C-terminus of A? is generated by ?-secretase cleavage within the transmembrane domain of APP (APPTM), a process that is primed by an initial ?-cleavage at either T48 or L49, resulting in subsequent production of A?42 or A?40, respectively. Here we solve the dimer structures of wild-type APPTM (AAPTM WT) and mutant APPTM (FAD mutants V44M) with solution NMR. The right-handed APPTM helical dimer is mediated by GXXXA motif. From the NMR structural and dynamic data, we show that the V44M and V44A mutations can selectively expose the T48 site by weakening helical hydrogen bonds and increasing hydrogen-deuterium exchange rate (kex). We propose a structural model in which FAD mutations (V44M and V44A) can open the T48 site ?-secretase for the initial ?-cleavage, and consequently shift cleavage preference towards A?42.

Chen, Wen; Gamache, Eric; Rosenman, David J.; Xie, Jian; Lopez, Maria M.; Li, Yue-Ming; Wang, Chunyu

2014-01-01

347

ESR spectra and structures of radical cations of some branched alkanes:. beta. -proton couplings in C--C sigma cations  

SciTech Connect

The systematic ESR studies of radical cations of five kinds of methyl substituted butanes show that the unpaired electron is rather confined to one of the C--C sigma bonds in contrast to the linear alkane cations, in which the unpaired electron delocalizes over the entire sigma molecular chain. Analyses of the origin of the trans C--H/sub ..beta../ proton couplings indicate that spin transfer due to hyperconjugation plays an important role in these branched alkane cations. It is also shown that the hyperconjugative effect in these branched alkane cations is two times higher than that in the neutral alkyl ..pi.. radicals. The large difference of the front and back lobe interactions in the ..beta..-proton couplings in these C--C sigma radicals is attributable to the bent structure of the radical carbon atom. The front lobe interaction is about one-half of that of the back lobe interaction as is the case of vinyl radicals.

Nunome, K.; Toriyama, K.; Iwasaki, M.

1983-09-15

348

Use of Cleavage as an Aid in the Optical Determination of Minerals.  

ERIC Educational Resources Information Center

Described is the use of cleavage as an aid to microscopic determination of unknown minerals by immersion methods. Cleavages are examined in relation to fragment shapes, types of extinction, and cleavage-optical relationships. (Author/DS)

Ehlers, Ernest G.

1980-01-01

349

Bundled slaty cleavage in laminated argillite, north-central minnesota  

USGS Publications Warehouse

Exceptional bundled slaty cleavage (defined herein) has been found in drill cores of laminated, folded, weakly metamorphosed argillite at several localities in the early Proterozoic Animikie basin of north-central Minnesota. The cleavage domains are more closely spaced within the cleavage bundles than outside them, the mean tectosilicate grain size of siltstone layers, measured normal to cleavage, is less in the cleavage bundles than outside them, and the cleavage bundles are enriched in opaque phases and phyllosilicates relative to extra-bundle segments. These facts suggest that pressure solution was a major factor in bundle development. If it is assumed that opaque phases have been conserved during pressure solution, the modal differences in composition between intra-bundle and extra-bundle segments of beds provide a means for estimating bulk material shortening normal to cleavage. Argillite samples from the central part of the Animikie basin have been shortened a minimum of about 22%, as estimated by this method. These estimates are similar to the shortening values derived from other strain markers in other rock types interbedded with the argillite, and are also consistent with the regional pattern of deformation. ?? 1987.

Southwick, D.L.

1987-01-01

350

Rubber oxygenase and latex clearing protein cleave rubber to different products and use different cleavage mechanisms.  

PubMed

Two types of enzyme for oxidative cleavage of poly(cis-1,4-isoprene) are known. One is rubber oxygenase (RoxA) that is secreted by Xanthomonas sp. strain 35Y and a few other Gram-negative rubber-degrading bacteria during growth on polyisoprene. RoxA was studied in the past, and the recently solved structure showed a structural relationship to bacterial cytochrome c peroxidases (J. Seidel et al., Proc. Natl. Acad. Sci. U. S. A. 110:13833-13838, 2013, http://dx.doi.org/10.1073/pnas.1305560110). The other enzyme is latex-clearing protein (Lcp) that is secreted by rubber-degrading actinomycetes, but Lcp has not yet been purified. Here, we expressed Lcp of Streptomyces sp. strain K30 in a ?roxA background of Xanthomonas sp. strain 35Y and purified native (untagged) Lcp. The specific activities of Lcp and RoxA were 0.70 and 0.48 U/mg, respectively. Lcp differed from RoxA in the absence of heme groups and other characteristics. Notably, Lcp degraded polyisoprene via endo-type cleavage to tetra-C20 and higher oligo-isoprenoids with aldehyde and keto end groups, whereas RoxA used an exo-type cleavage mechanism to give the main end product 12-oxo-4,8-dimethyltrideca-4,8-diene-1-al (ODTD). RoxA was able to cleave isolated Lcp-derived oligo-isoprenoid molecules to ODTD. Inhibitor studies, spectroscopic investigations and metal analysis gave no indication for the presence of iron, other metals, or cofactors in Lcp. Our results suggest that Lcp could be a member of the growing group of cofactor-independent oxygenases and differs in the cleavage mechanism from heme-dependent RoxA. In conclusion, RoxA and Lcp represent two different answers to the same biochemical problem, the cleavage of polyisoprene, a polymer that has carbon-carbon double bonds as the only functional groups for enzymatic attack. PMID:24907333

Birke, Jakob; Jendrossek, Dieter

2014-08-01

351

Detection of nucleic acid sequences by invader-directed cleavage  

SciTech Connect

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The 5{prime} nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof. The present invention further relates to methods and devices for the separation of nucleic acid molecules based by charge.

Brow, M.A.D.; Hall, J.S.G.; Lyamichev, V.; Olive, D.M.; Prudent, J.R.

1999-12-14

352

Detection of nucleic acid sequences by invader-directed cleavage  

DOEpatents

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The 5' nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof. The present invention further relates to methods and devices for the separation of nucleic acid molecules based by charge.

Brow, Mary Ann D. (Madison, WI); Hall, Jeff Steven Grotelueschen (Madison, WI); Lyamichev, Victor (Madison, WI); Olive, David Michael (Madison, WI); Prudent, James Robert (Madison, WI)

1999-01-01

353

IBM XL Fortran and IBM XL C/C++ for AIX Interoperability of IBM XL Fortran, IBM XL C/C++ and Java  

E-print Network

IBM XL Fortran and IBM XL C/C++ for AIX Interoperability of IBM XL Fortran, IBM XL C/C++ and Java By: David Forster Level: Introductory May 2012 #12;Interoperability of IBM XL Fortran, IBM XL C XL Fortran and IBM XL C/C++ for AIX

354

C-C fragmentation: origins and recent applications.  

PubMed

It has been 60 years since Eschenmoser and Frey disclosed the archetypal C?C fragmentation reaction. New fragmentations and several variants of the original quickly followed. Many of these variations, which include the Beckmann, Grob, Wharton, Marshall, and Eschenmoser-Tanabe fragmentations, have been reviewed over the intervening years. A close examination of the origins of fragmentation has not been described. Recently, useful new methods have flourished, particularly fragmentations that give alkynes and allenes, and such reactions have been applied to a range of complex motifs and natural products. This Review traces the origins of fragmentation reactions and provides a summary of the methods, applications, and new insights of heterolytic C?C fragmentation reactions advanced over the last 20 years. PMID:24115282

Drahl, Michael A; Manpadi, Madhuri; Williams, Lawrence J

2013-10-18

355

Solar System planetary tests of \\dot c/c  

E-print Network

Analytical and numerical calculations show that a putative temporal variation of the speed of light c, with the meaning of space-time structure constant c_ST, assumed to be linear over timescales of about one century, would induce a secular precession of the longitude of the pericenter \\varpi of a test particle orbiting a spherically symmetric body. By comparing such a predicted effect to the corrections \\Delta\\dot\\varpi to the usual Newtonian/Einsteinian perihelion precessions of the inner planets of the Solar System, recently estimated by E.V. Pitjeva by fitting about one century of modern astronomical observations with the standard dynamical force models of the EPM epehemerides, we obtained \\dot c/c =(0.5 +/- 2)\\times 10^-7 yr^-1. Moreover, the possibility that \\dot c/c\

Lorenzo Iorio

2009-01-20

356

Density Functional Theory Calculations on Fe-O and O-O Cleavage of Ferric Hydroperoxide Species: Role of axial ligand and spin state  

PubMed Central

Density Functional Theory (DFT) calculations are performed on thiolate bound hydroperoxide complexes. O-O and Fe-O cleavage reaction coordinates, relevant to the active sites of Cytocrome P450 and Superoxide Reductase enzymes, were investigated for both high and low spin states and for cis and trans orientations of the thiolate ligand with respect to the hydroperoxide ligand. The results indicate that the presence of a thiolate ligand produces significant elongation of the Fe-O bond and reduction of Fe-O vibrational frequency. While the fate of the O-O cleavage reaction is not significantly altered, the presence of a thiolate induces a heterolytic Fe-O cleavage irrespective of the spin state and orientation which is very different from results obtained with a trans ammine ligand. PMID:21057606

Dey, Abhishek; Solomon, Edward I.

2010-01-01

357

One-step synthesis of diazaspiro[4.5]decane scaffolds with exocyclic double bonds.  

PubMed

Unactivated yne-en-ynes reacted with a range of substituted aryl halides in the presence of Pd(OAc)2-PPh3 to afford diazaspiro[4.5]decane with exocyclic double bonds. Three carbon-carbon bonds are formed in this domino reaction, which involves highly regioselective C-C coupling and spiro scaffold steps. PMID:24956492

Li, Lidong; Hu, Qiong; Zhou, Pingping; Xie, Haifeng; Zhang, Xiaorong; Zhang, Hao; Hu, Yadong; Yin, Fei; Hu, Yimin

2014-08-01

358

Dislocation dissociation in some f.c.c. metals  

NASA Technical Reports Server (NTRS)

The dissociation of a perfect screw dislocation into a stacking fault in an f.c.c. lattice is modeled by the modified lattice statics. The interatomic potentials are obtained from the work of Esterling and Swaroop and differ substantially from those empirical potentials usually employed in defect simulations. The calculated stacking fault widths for aluminum, copper, and silver are in good agreement with weak beam microscopy results.

Esterling, D. M.

1980-01-01

359

The Turbulent Alfvenic Aurora C. C. Chaston,1  

E-print Network

The Turbulent Alfve´nic Aurora C. C. Chaston,1 C. Salem,1 J. W. Bonnell,1 C. W. Carlson,1 R. E) It is demonstrated from observations that the Alfve´nic aurora may be powered by a turbulent cascade transverse acceleration of electrons from near-Earth space to form the aurora. We find that regions of Alfve´n wave

California at Berkeley, University of

360

Mutational analysis of the membrane-proximal cleavage site of L- selectin: relaxed sequence specificity surrounding the cleavage site  

PubMed Central

L-selectin expression is regulated in part by membrane-proximal cleavage from the cell surface of leukocytes and L-selectin-transfected cells. The downregulation of L-selectin from the surface of neutrophils is speculated to be a process involved in the adhesion cascade leading to neutrophil recruitment to sites of inflammation. We previously reported that L-selectin is cleaved between Lys321 and Ser322 in a region that links the second short consensus repeat (SCR) and the transmembrane domain. We demonstrate that replacing this cleavage domain of L-selectin with the corresponding region of E-selectin prevents L-selectin shedding, as judged by inhibiting the generation of the 68-kD soluble and 6-kD transmembrane cleavage products of L- selectin. Unexpectedly, we found that point mutations of the cleavage site, as well as mutations of multiple conserved amino acids within the cleavage domain, do not significantly affect L-selectin shedding. However, short deletions of four or five amino acids in the L-selectin cleavage domain inhibit L-selectin downregulation. Mutations that appeared to inhibit L-selectin shedding resulted in higher levels of cell surface expression, consistent with a lack of apparent proteolysis from the cell membrane. One deletion mutant, I327 delta N332, retains the native cleavage site yet inhibits L-selectin proteolysis as well. Restoring the amino acids deleted between I327 and N332 with five alanine residues restores L-selectin proteolysis. Thus, the proteolytic processing of L-selectin appears to have a relaxed sequence specificity at the cleavage site, and it may depend on the physical length or other secondary structural characteristics of the cleavage domain. PMID:7543142

1995-01-01

361

Reactions of parylenes with double bonds: An ab initio study  

NASA Astrophysics Data System (ADS)

The reactions of the hydrogen atom transfer from various compounds (containing the same ethylene fragment in their framework) to the neutral di-para-xylylene dimer were examined at the B3LYP/6-31G level to estimate the corresponding kinetic barriers. Also, an alternative path (involving the ?-bond cleavage) for such reactions were considered. It was found that the certain derivatives should easily react with the dimer at room temperature since the overall reactions are thermodynamically favorable and kinetically possible.

Smuczy?ska, Sylwia; Skurski, Piotr; Bobrowski, Maciej

2012-12-01

362

Bonded half planes containing an arbitrarily oriented crack  

NASA Technical Reports Server (NTRS)

The plane elastostatic problem for two bonded half planes containing an arbitrarily oriented crack in the neighborhood of the interface is considered. Using Mellin transforms, the problem is formulated as a system of singular integral equations. The equations are solved for various crack orientations, material combinations, and external loads. The numerical results given include the stress intensity factors, tHe strain energy release rates, and tHe probable cleavage angles giving the direction of crack propagation.

Erdogan, F.; Aksogan, O.

1973-01-01

363

Facile scission of isonitrile carbon–nitrogen triple bond using a diborane(4) reagent  

PubMed Central

Transition metal reagents and catalysts are generally effective to cleave all three bonds (one ? and two ?) in a triple bond despite its high bonding energy. Recently, chemistry of single-bond cleavage by using main-group element compounds is rapidly being developed in the absence of transition metals. However, the cleavage of a triple bond using non-transition-metal compounds is less explored. Here we report that an unsymmetrical diborane(4) compound could react with carbon monoxide and tert-butyl isonitrile at room temperature. In the latter case, the carbon–nitrogen triple bond was completely cleaved in the absence of transition metal as confirmed by X-ray crystallographic analysis, 13C NMR spectroscopy with 13C labelling and DFT calculations. The DFT calculations also revealed the detailed reaction mechanism and indicated that the key for the carbon–nitrogen triple-bond cleavage could be attributed to the presence of nucleophilic nitrogen atom in one of the intermediates. PMID:24967910

Asakawa, Hiroki; Lee, Ka-Ho; Lin, Zhenyang; Yamashita, Makoto

2014-01-01

364

Facile scission of isonitrile carbon-nitrogen triple bond using a diborane(4) reagent.  

PubMed

Transition metal reagents and catalysts are generally effective to cleave all three bonds (one ? and two ?) in a triple bond despite its high bonding energy. Recently, chemistry of single-bond cleavage by using main-group element compounds is rapidly being developed in the absence of transition metals. However, the cleavage of a triple bond using non-transition-metal compounds is less explored. Here we report that an unsymmetrical diborane(4) compound could react with carbon monoxide and tert-butyl isonitrile at room temperature. In the latter case, the carbon-nitrogen triple bond was completely cleaved in the absence of transition metal as confirmed by X-ray crystallographic analysis, (13)C NMR spectroscopy with (13)C labelling and DFT calculations. The DFT calculations also revealed the detailed reaction mechanism and indicated that the key for the carbon-nitrogen triple-bond cleavage could be attributed to the presence of nucleophilic nitrogen atom in one of the intermediates. PMID:24967910

Asakawa, Hiroki; Lee, Ka-Ho; Lin, Zhenyang; Yamashita, Makoto

2014-01-01

365

Detection of nucleic acids by multiple sequential invasive cleavages 02  

DOEpatents

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The structure-specific nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof. The present invention further relates to methods and devices for the separation of nucleic acid molecules based on charge. The present invention also provides methods for the detection of non-target cleavage products via the formation of a complete and activated protein binding region. The invention further provides sensitive and specific methods for the detection of human cytomegalovirus nucleic acid in a sample.

Hall, Jeff G. (Madison, WI); Lyamichev, Victor I. (Madison, WI); Mast, Andrea L. (Madison, WI); Brow, Mary Ann D. (Madison, WI)

2002-01-01

366

Detection of nucleic acids by multiple sequential invasive cleavages  

DOEpatents

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The structure-specific nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof. The present invention further relates to methods and devices for the separation of nucleic acid molecules based on charge. The present invention also provides methods for the detection of non-target cleavage products via the formation of a complete and activated protein binding region. The invention further provides sensitive and specific methods for the detection of human cytomegalovirus nucleic acid in a sample.

Hall, Jeff G; Lyamichev, Victor I; Mast, Andrea L; Brow, Mary Ann D

2012-10-16

367

Detection of nucleic acids by multiple sequential invasive cleavages  

DOEpatents

The present invention relates to means for the detection and characterization of nucleic acid sequences, as well as variations in nucleic acid sequences. The present invention also relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The structure-specific nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof. The present invention further relates to methods and devices for the separation of nucleic acid molecules based on charge. The present invention also provides methods for the detection of non-target cleavage products via the formation of a complete and activated protein binding region. The invention further provides sensitive and specific methods for the detection of human cytomegalovirus nucleic acid in a sample.

Hall, Jeff G. (Madison, WI); Lyamichev, Victor I. (Madison, WI); Mast, Andrea L. (Madison, WI); Brow, Mary Ann D. (Madison, WI)

1999-01-01

368

Bonding with Your Baby  

MedlinePLUS

... the child's social and cognitive development. Why Is Bonding Important? Bonding is essential for a baby. Studies ... as biological parents and their children. Back Continue Bonding With Daddy Men these days spend more time ...

369

Influence of substrate structure on cleavage by hammerhead ribozyme  

Microsoft Academic Search

We compared the cleavage by a hammerhead ribozyme of a wild-type precursor tRNA (pre-tRNA3leu) and a structurally altered mutant form. We also analyzed the cleavage reactions of these tRNAs catalyzed by a ribozyme variant that was designed to complement the mutant precursor tRNA. Kinetic analyses reveal that the kcat values are nearly the same for the wild-type and the mutant

Daniela Scarabino; Glauco P. Tocchini-Valentini

1996-01-01

370

What Determines Bond Costs. Municipal Bonds Series.  

ERIC Educational Resources Information Center

Public officials in small towns who participate infrequently in the bond market need information about bond financing. This publication, one in a series of booklets published by the Western Rural Development Center using research gathered between 1967-77, discusses factors influencing the marketability and cost of bond financing for towns and…

Young, Douglas; And Others

371

29 CFR 2580.412-20 - Use of existing bonds, separate bonds and additional bonding.  

Code of Federal Regulations, 2013 CFR

...bonds, separate bonds and additional bonding. 2580.412-20 Section 2580...DEPARTMENT OF LABOR TEMPORARY BONDING RULES UNDER THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974 TEMPORARY BONDING RULES General Bond Rules §...

2013-07-01

372

29 CFR 2580.412-20 - Use of existing bonds, separate bonds and additional bonding.  

Code of Federal Regulations, 2010 CFR

...bonds, separate bonds and additional bonding. 2580.412-20 Section 2580...DEPARTMENT OF LABOR TEMPORARY BONDING RULES UNDER THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974 TEMPORARY BONDING RULES General Bond Rules §...

2010-07-01

373

29 CFR 2580.412-20 - Use of existing bonds, separate bonds and additional bonding.  

Code of Federal Regulations, 2012 CFR

...bonds, separate bonds and additional bonding. 2580.412-20 Section 2580...DEPARTMENT OF LABOR TEMPORARY BONDING RULES UNDER THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974 TEMPORARY BONDING RULES General Bond Rules §...

2012-07-01

374

29 CFR 2580.412-20 - Use of existing bonds, separate bonds and additional bonding.  

Code of Federal Regulations, 2011 CFR

...bonds, separate bonds and additional bonding. 2580.412-20 Section 2580...DEPARTMENT OF LABOR TEMPORARY BONDING RULES UNDER THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974 TEMPORARY BONDING RULES General Bond Rules §...

2011-07-01

375

29 CFR 2580.412-20 - Use of existing bonds, separate bonds and additional bonding.  

...bonds, separate bonds and additional bonding. 2580.412-20 Section 2580...DEPARTMENT OF LABOR TEMPORARY BONDING RULES UNDER THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974 TEMPORARY BONDING RULES General Bond Rules §...

2014-07-01

376

Catalytic mechanism of RNA backbone cleavage by ribonuclease H from quantum mechanics/molecular mechanics simulations.  

PubMed

We use quantum mechanics/molecular mechanics simulations to study the cleavage of the ribonucleic acid (RNA) backbone catalyzed by ribonuclease H. This protein is a prototypical member of a large family of enzymes that use two-metal catalysis to process nucleic acids. By combining Hamiltonian replica exchange with a finite-temperature string method, we calculate the free energy surface underlying the RNA-cleavage reaction and characterize its mechanism. We find that the reaction proceeds in two steps. In a first step, catalyzed primarily by magnesium ion A and its ligands, a water molecule attacks the scissile phosphate. Consistent with thiol-substitution experiments, a water proton is transferred to the downstream phosphate group. The transient phosphorane formed as a result of this nucleophilic attack decays by breaking the bond between the phosphate and the ribose oxygen. In the resulting intermediate, the dissociated but unprotonated leaving group forms an alkoxide coordinated to magnesium ion B. In a second step, the reaction is completed by protonation of the leaving group, with a neutral Asp132 as a likely proton donor. The overall reaction barrier of ?15 kcal mol(-1), encountered in the first step, together with the cost of protonating Asp132, is consistent with the slow measured rate of ?1-100/min. The two-step mechanism is also consistent with the bell-shaped pH dependence of the reaction rate. The nonmonotonic relative motion of the magnesium ions along the reaction pathway agrees with X-ray crystal structures. Proton-transfer reactions and changes in the metal ion coordination emerge as central factors in the RNA-cleavage reaction. PMID:21539371

Rosta, Edina; Nowotny, Marcin; Yang, Wei; Hummer, Gerhard

2011-06-15

377

Novel carotenoid cleavage dioxygenase catalyzes the first dedicated step in saffron crocin biosynthesis.  

PubMed

Crocus sativus stigmas are the source of the saffron spice and accumulate the apocarotenoids crocetin, crocins, picrocrocin, and safranal, responsible for its color, taste, and aroma. Through deep transcriptome sequencing, we identified a novel dioxygenase, carotenoid cleavage dioxygenase 2 (CCD2), expressed early during stigma development and closely related to, but distinct from, the CCD1 dioxygenase family. CCD2 is the only identified member of a novel CCD clade, presents the structural features of a bona fide CCD, and is able to cleave zeaxanthin, the presumed precursor of saffron apocarotenoids, both in Escherichia coli and in maize endosperm. The cleavage products, identified through high-resolution mass spectrometry and comigration with authentic standards, are crocetin dialdehyde and crocetin, respectively. In vitro assays show that CCD2 cleaves sequentially the 7,8 and 7',8' double bonds adjacent to a 3-OH-?-ionone ring and that the conversion of zeaxanthin to crocetin dialdehyde proceeds via the C30 intermediate 3-OH-?-apo-8'-carotenal. In contrast, zeaxanthin cleavage dioxygenase (ZCD), an enzyme previously claimed to mediate crocetin formation, did not cleave zeaxanthin or 3-OH-?-apo-8'-carotenal in the test systems used. Sequence comparison and structure prediction suggest that ZCD is an N-truncated CCD4 form, lacking one blade of the ?-propeller structure conserved in all CCDs. These results constitute strong evidence that CCD2 catalyzes the first dedicated step in crocin biosynthesis. Similar to CCD1, CCD2 has a cytoplasmic localization, suggesting that it may cleave carotenoids localized in the chromoplast outer envelope. PMID:25097262

Frusciante, Sarah; Diretto, Gianfranco; Bruno, Mark; Ferrante, Paola; Pietrella, Marco; Prado-Cabrero, Alfonso; Rubio-Moraga, Angela; Beyer, Peter; Gomez-Gomez, Lourdes; Al-Babili, Salim; Giuliano, Giovanni

2014-08-19

378

Characterization of a second cleavage site and demonstration of activity in trans by the papain-like proteinase of the murine coronavirus mouse hepatitis virus strain A59.  

PubMed Central

The 21.7-kb replicase locus of mouse hepatitis virus strain A59 (MHV-A59) encodes several putative functional domains, including three proteinase domains. Encoded closest to the 5' terminus of this locus is the first papain-like proteinase (PLP-1) (S. C. Baker et al., J. Virol. 67:6056-6063, 1993; H.-J. Lee et al., Virology 180:567-582, 1991). This cysteine proteinase is responsible for the in vitro cleavage of p28, a polypeptide that is also present in MHV-A59-infected cells. Cleavage at a second site was recently reported for this proteinase (P. J. Bonilla et al., Virology 209:489-497, 1995). This new cleavage site maps to the same region as the predicted site of the C terminus of p65, a viral polypeptide detected in infected cells. In this study, microsequencing analysis of the radiolabeled downstream cleavage product and deletion mutagenesis analysis were used to identify the scissile bond of the second cleavage site to between Ala832 and Gly833. The effects of mutations between the P5 and P2' positions on the processing at the second cleavage site were analyzed. Most substitutions at the P4, P3, P2, and P2' positions were permissive for cleavage. With the exceptions of a conservative P1 mutation, Ala832Gly, and a conservative P5 mutation, Arg828Lys, substitutions at the P5, P1, and P1' positions severely diminished second-site proteolysis. Mutants in which the p28 cleavage site (Gly247 / Val248) was replaced by the Ala832 / Gly833 cleavage site and vice versa were found to retain processing activity. Contrary to previous reports, we determined that the PLP-1 has the ability to process in trans at either the p28 site or both cleavage sites, depending on the choice of substrate. The results from this study suggest a greater role by the PLP-1 in the processing of the replicase locus in vivo. PMID:8995606

Bonilla, P J; Hughes, S A; Weiss, S R

1997-01-01

379

Carbon-Sulfur Bond Cleavage of Methyl-Substituted Thiophenes with Iridium(III)  

E-print Network

, Rochester, New York 14627 ReceiVed February 12, 2009 Reaction of [Cp*IrHCl]2 (Cp* ) 5 -C5Me5) with 2,5-dimethylthiophene is [Cp*IrCl]2(µ-H)(µ-S- 2-hexyl). This product has been characterized and is present accomplished by 1 H and 13 C NMR spectroscopies, as well as by ESI-MS and elemental analysis. Reaction with 2

Jones, William D.

380

Wavepacket Splitting in the First 100 fs Determines the Products from the Bond Cleavage of Diphenylmethylchloride  

NASA Astrophysics Data System (ADS)

An elementary chemical reaction proceeds through two distinct conical intersections. Benzhydryl radicals are formed with a delay of 80 fs, cations after 125 fs. The optical signal increases more slowly due to planarization and solvation.

Sailer, C. F.; Krebs, N.; Fingerhut, B. P.; de Vivie-Riedle, R.; Riedle, E.

2013-03-01

381

Diiodoindium(III) cation, InI??, a potent yneophile. Generation and application to cationic cyclization by selective ?-activation of C?C.  

PubMed

The removal of the iodide ion from indium triiodide by means of reactive Ag(I) salts leads to the formation of the highly reactive ligandless cation InI2(+), which is unusual in having two vacant low-lying p-orbitals. This bivalent Lewis acidity leads to an especially high affinity for the two orthogonal ?-bonds of carbon-carbon triple bonds. Consequently, the double-coordinating InI2(+) is an especially effective reagent for the selective activation of C?C and the catalytic initiation of cationic cyclization processes. A number of such reactions are described to demonstrate synthetic utility. PMID:25095905

Surendra, Karavadhi; Corey, E J

2014-08-01

382

Identification and expression pattern of a new carotenoid cleavage dioxygenase gene member from Bixa orellana  

PubMed Central

Carotenoid cleavage dioxygenases (CCDs) are a class of enzymes involved in the biosynthesis of a broad diversity of secondary metabolites known as apocarotenoids. In plants, CCDs are part of a genetic family with members which cleave specific double bonds of carotenoid molecules. CCDs are involved in the production of diverse and important metabolites such as vitamin A and abscisic acid (ABA). Bixa orellana L. is the main source of the natural pigment annatto or bixin, an apocarotenoid accumulated in large quantities in its seeds. Bixin biosynthesis has been studied and the involvement of a CCD has been confirmed in vitro. However, the CCD genes involved in the biosynthesis of the wide variety of apocarotenoids found in this plant have not been well documented. In this study, a new CCD1 gene member (BoCCD1) was identified and its expression was charaterized in different plant tissues of B. orellana plantlets and adult plants. The BoCCD1 sequence showed high homology with plant CCD1s involved mainly in the cleavage of carotenoids in several sites to generate multiple apocarotenoid products. Here, the expression profiles of the BoCCD1 gene were analysed and discussed in relation to total carotenoids and other important apocarotenoids such as bixin. PMID:21813796

Rodríguez-Ávila, N. L.; Narváez-Zapata, J. A.; Ramírez-Benítez, J. E.; Aguilar-Espinosa, M. L.; Rivera-Madrid, R.

2011-01-01

383

Identification and expression pattern of a new carotenoid cleavage dioxygenase gene member from Bixa orellana.  

PubMed

Carotenoid cleavage dioxygenases (CCDs) are a class of enzymes involved in the biosynthesis of a broad diversity of secondary metabolites known as apocarotenoids. In plants, CCDs are part of a genetic family with members which cleave specific double bonds of carotenoid molecules. CCDs are involved in the production of diverse and important metabolites such as vitamin A and abscisic acid (ABA). Bixa orellana L. is the main source of the natural pigment annatto or bixin, an apocarotenoid accumulated in large quantities in its seeds. Bixin biosynthesis has been studied and the involvement of a CCD has been confirmed in vitro. However, the CCD genes involved in the biosynthesis of the wide variety of apocarotenoids found in this plant have not been well documented. In this study, a new CCD1 gene member (BoCCD1) was identified and its expression was charaterized in different plant tissues of B. orellana plantlets and adult plants. The BoCCD1 sequence showed high homology with plant CCD1s involved mainly in the cleavage of carotenoids in several sites to generate multiple apocarotenoid products. Here, the expression profiles of the BoCCD1 gene were analysed and discussed in relation to total carotenoids and other important apocarotenoids such as bixin. PMID:21813796

Rodríguez-Ávila, N L; Narváez-Zapata, J A; Ramírez-Benítez, J E; Aguilar-Espinosa, M L; Rivera-Madrid, R

2011-11-01

384

Acetylation and cleavage of purine nucleosides. Synthesis of 6-azauridine, 5-fluorouridine, and 5-methyluridine.  

PubMed Central

Inosine (I) when acetylated with acetic anhydride in the presence of acetyl chloride in acetic acid solution (the so called "acid acetylation"), affords an acetylated nucleoside III (75%) along with cleavage products of the nucleoside (hypoxanthine, 19%). The reaction of I with acetyl chloride (7 days) results in the formation of hypoxanthine (95%) and triacetylribofuranosyl chloride (IV) isolated in the form of tetraacetylribofuranose (47%). The acetylated purine nucleoside affords a similar result by reaction with acetyl chloride or acetyl bromide. 2'-Deoxyuridine gives a diacetyl derivative (80%) by reaction with acetyl bromide. On treatment with acetyl bromide, the nucleoside bond of purine nucleosides is quantitatively cleavaged (4 h, 20 degrees C) with the formation of tri-O-acetyl-D-ribofuranosyl bromide (X). The halogenose X affords pure beta-anomers, namely, 1,2,3,5-tetra-O-acetyl-beta-D-ribofuranose (75%), the triacetyl derivatives of 5-methyluridine (XVIIa; 75%, referred to guanosine), 6-azauridine (XVIII; 71%), and 5-fluorouridine (XIXa; 75%). PMID:940772

Beránek, J; Hrebabecký, H

1976-01-01

385

Tailoring structure–function properties of L-asparaginase: engineering resistance to trypsin cleavage  

PubMed Central

Bacterial L-ASNases (L-asparaginases) catalyse the conversion of L-asparagine into L-aspartate and ammonia, and are widely used for the treatment of ALL (acute lymphoblastic leukaemia). In the present paper, we describe an efficient approach, based on protein chemistry and protein engineering studies, for the construction of trypsin-resistant PEGylated L-ASNase from Erwinia carotovora (EcaL-ASNase). Limited proteolysis of EcaL-ASNase with trypsin was found to be associated with a first cleavage of the peptide bond between Lys53 and Gly54, and then a second cleavage at Arg206-Ser207 of the C-terminal fragment, peptide 54–327, showing that the initial recognition sites for trypsin are Lys53 and Arg206. Site-directed mutagenesis of Arg206 to histidine followed by covalent coupling of mPEG-SNHS [methoxypoly(ethylene glycol) succinate N-hydroxysuccinimide ester] to the mutant enzyme resulted in an improved modified form of EcaL-ASNase that retains 82% of the original catalytic activity, exhibits enhanced resistance to trypsin degradation, and has higher thermal stability compared with the wild-type enzyme. PMID:17313368

Kotzia, Georgia A.; Lappa, Katerina; Labrou, Nikolaos E.

2007-01-01

386

Molecular Basis of Catalytic Chamber-assisted Unfolding and Cleavage of Human Insulin by Human Insulin-degrading Enzyme  

SciTech Connect

Insulin is a hormone vital for glucose homeostasis, and insulin-degrading enzyme (IDE) plays a key role in its clearance. IDE exhibits a remarkable specificity to degrade insulin without breaking the disulfide bonds that hold the insulin A and B chains together. Using Fourier transform ion cyclotron resonance (FTICR) mass spectrometry to obtain high mass accuracy, and electron capture dissociation (ECD) to selectively break the disulfide bonds in gas phase fragmentation, we determined the cleavage sites and composition of human insulin fragments generated by human IDE. Our time-dependent analysis of IDE-digested insulin fragments reveals that IDE is highly processive in its initial cleavage at the middle of both the insulin A and B chains. This ensures that IDE effectively splits insulin into inactive N- and C-terminal halves without breaking the disulfide bonds. To understand the molecular basis of the recognition and unfolding of insulin by IDE, we determined a 2.6-A resolution insulin-bound IDE structure. Our structure reveals that IDE forms an enclosed catalytic chamber that completely engulfs and intimately interacts with a partially unfolded insulin molecule. This structure also highlights how the unique size, shape, charge distribution, and exosite of the IDE catalytic chamber contribute to its high affinity ( approximately 100 nm) for insulin. In addition, this structure shows how IDE utilizes the interaction of its exosite with the N terminus of the insulin A chain as well as other properties of the catalytic chamber to guide the unfolding of insulin and allowing for the processive cleavages.

Manolopoulou, Marika; Guo, Qing; Malito, Enrico; Schilling, Alexander B.; Tang, Wei-Jen; (UC); (UIC)

2009-06-02

387

Bypassing a highly unstable frustrated Lewis pair: dihydrogen cleavage by a thermally robust silylium-phosphine adduct.  

PubMed

The thermally robust silylium complex [iPr3Si-PtBu3](+)[B(C6F5)4](-) (1) activates H2/D2 at 90 °C (PhCl); no evidence for dissociation into the separated Lewis pair is found. DFT calculations show H2 cleavage proceeds via Si-P bond elongation to form an encounter complex directly from the adduct, thus avoiding the non-isolable iPr3Si(+)-PtBu3 FLP. PMID:25204297

Herrington, Thomas J; Ward, Bryan J; Doyle, Laurence R; McDermott, Joe; White, Andrew J P; Hunt, Patricia A; Ashley, Andrew E

2014-10-28

388

Mechanisms for ribotoxin-induced ribosomal RNA cleavage.  

PubMed

The Type B trichothecene deoxynivalenol (DON), a ribotoxic mycotoxin known to contaminate cereal-based foods, induces ribosomal RNA (rRNA) cleavage in the macrophage via p38-directed activation of caspases. Here we employed the RAW 264.7 murine macrophage model to test the hypothesis that this rRNA cleavage pathway is similarly induced by other ribotoxins. Capillary electrophoresis confirmed that the antibiotic anisomycin (?25ng/ml), the macrocylic trichothecene satratoxin G (SG) (?10ng/ml) and ribosome-inactivating protein ricin (?300ng/ml) induced 18s and 28s rRNA fragmentation patterns identical to that observed for DON. Also, as found for DON, inhibition of p38, double-stranded RNA-activated kinase (PKR) and hematopoietic cell kinase (Hck) suppressed MAPK anisomycin-induced rRNA cleavage, while, in contrast, their inhibition did not affect SG- and ricin-induced rRNA fragmentation. The p53 inhibitor pifithrin-? and pan caspase inhibitor Z-VAD-FMK suppressed rRNA cleavage induced by anisomycin, SG and ricin, indicating that these ribotoxins shared with DON a conserved downstream pathway. Activation of caspases 8, 9 and 3 concurrently with apoptosis further suggested that rRNA cleavage occurred in parallel with both extrinsic and intrinsic pathways of programmed cell death. When specific inhibitors of cathepsins L and B (lysosomal cysteine cathepsins active at cytosolic neutral pH) were tested, only the former impaired anisomycin-, SG-, ricin- and DON-induced rRNA cleavage. Taken together, the data suggest that (1) all four ribotoxins induced p53-dependent rRNA cleavage via activation of cathepsin L and caspase 3, and (2) activation of p53 by DON and anisomycin involved p38 whereas SG and ricin activated p53 by an alternative mechanism. PMID:23022514

He, Kaiyu; Zhou, Hui-Ren; Pestka, James J

2012-11-15

389

Preparation of isobenzofurandiones by flash vacuum pyrolysis involving retro-Diels–Alder expulsion of ethylene and concomitant C–O cleavage of methoxy or ethylenedioxy groups  

Microsoft Academic Search

Flash vacuum pyrolysis (fvp) of a number of substrates, prepared by hydrogenating adducts derived from dimethoxy- or ethylenedioxy-substituted benzynes and furan, affords isobenzofurandiones through retro-Diels–Alder expulsion of ethylene and C–O bond cleavage of the methoxy or ethylenedioxy groups. The parent isobenzofuran-4,5-dione is reactive and undergoes two-fold conjugate addition of water to afford 3,4-dihydroxybenzene-1,2-dicarboxaldehyde.

Riskiono Slamet; Dieter Wege

2007-01-01

390

Biosynthetic labeling of hypusine in mammalian cells. Carbon-hydrogen bond fissions revealed by dual labeling  

SciTech Connect

Using a dual-label technique in which /sup 3/H- and /sup 14/C-labeled forms of putrescine and of spermidine were employed as biosynthetic precursors of hypusine, two -C-H bond cleavages were detected during production of this unique amino acid in Chinese hamster ovary cells. One of these cleavages occurs at C-1 of the 4-aminobutyl group during its transfer from the secondary amine nitrogen of spermidine to the nitrogen at the epsilon-position of a specific lysine residue in the polypeptide precursor of eukaryotic initiation factor 4D. Breakage of the other -C-H bond takes place at C-2 in this aminobutyl segment after it has been coupled to lysine to form the intermediate deoxyhypusine residue. Hydroxylation at this carbon atom, which constitutes the last step in hypusine biosynthesis, is the cause of bond cleavage. The data obtained are consistent with a notion that no additional -C-H bond fissions occur during hypusine biosynthesis. Our findings permit suggestion of a mechanism for enzymic aminobutyl group transfer in which 4-aminobutyraldehyde produced by oxidative cleavage of spermidine is coupled with the epsilon-amino group of a specific lysine residue to form an enzyme-bound imine intermediate.

Park, M.H.; Folk, J.E.

1986-10-25

391

Solvent dependent reductive defluorination of aliphatic C-F bonds employing Sm(HMDS)2.  

PubMed

Sm(HMDS)(2) in n-hexane mediates fast cleavage of primary, secondary and tertiary alkyl fluorides in good to excellent yields. In n-hexane Sm(HMDS)(2) exhibits uniquely enhanced reductive ability towards the C-F bond compared to when using THF as solvent. PMID:23358653

Janjetovic, Mario; Träff, Annika M; Ankner, Tobias; Wettergren, Jenny; Hilmersson, Göran

2013-03-01

392

Bond University Queensland, Australia  

E-print Network

Bond University Queensland, Australia About: Bond University is a private university located institutions, Bond has a strong component of international students who hail from some 80 countries worldwide. Bond's small size, internationalized-student body, 14-week trimesters, and fabulous Gold Coast location

Duchowski, Andrew T.

393

Analysis of RNA cleavage by MALDI-TOF mass spectrometry.  

PubMed

A method of analysis is presented that utilizes matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) to monitor the kinetics and products of RNA cleavage, by use of a program designed to mass-match observed MS peaks with predicted RNA cleavage products. The method is illustrated through application to the study of targeted oxidation of RNA stem loops from HIV-1 Rev Response Element mRNA (RRE RNA) and ribosomal 16S A-site RNA (16S RNA) by metallonucleases. Following incubation of each RNA with catalysts and/or redox co-reactants, reaction mixtures were desalted, and MALDI-TOF MS was used to monitor both time-resolved formation of cleavage products and disappearance of full-length RNA. For each RNA, a unique list was generated that contained the predicted masses of both the full-length, and all of the possible RNA cleavage fragments that resulted from the combination of all possible cleavage sites and each of the six expected overhangs formed at nascent termini adjacent to the cleavage sites. The overhangs corresponded to 2',3'-cyclic phosphate, 3'-phosphate, 3'-phosphoglycolate, 5'- hydroxyl and 5'- phosphate, which corresponded to differing oxidative, hydrolytic, and/or 2'-OH-mediated-endonucleolytic modes of scission. Each mass spectrum was compared with a corresponding list of predicted masses, and peaks were rapidly assigned by use of a Perl script, with a mass-matching tolerance of 200 ppm. Both time-dependent cleavage mediated by metallonucleases and MALDI-TOF-induced fragmentation were observed, and these were distinguished by time-dependent experiments. The resulting data allowed a semi-quantitative assessment of the rate of formation of each overhang at each nucleotide position. Limitations included artifactual skewing of quantification by mass bias, a limited mass range for quantification, and a lack of detection of secondary cleavage products. Nevertheless, the method presented herein provides a rapid, accurate, highly-detailed and semi-quantitative analysis of RNA cleavage that should be widely applicable. PMID:22941655

Joyner, Jeff C; Keuper, Kevin D; Cowan, J A

2013-01-01

394

Savings Bonds Value Calculator  

NSDL National Science Digital Library

From the Federal Reserve Bank of New York, this site computes the redemption value of users's US savings bonds. Easy-to-use pull-down menus allow visitors to enter information such as the date of issue and face value of their Series E bonds, Series EE bonds, and Series S bonds. After entering the information, the Calculator will then show a chart of issue dates and denominations and actual worth of the bonds, if cashed within a set period of time.

395

Cleavage entropy as quantitative measure of protease specificity.  

PubMed

A purely information theory-guided approach to quantitatively characterize protease specificity is established. We calculate an entropy value for each protease subpocket based on sequences of cleaved substrates extracted from the MEROPS database. We compare our results with known subpocket specificity profiles for individual proteases and protease groups (e.g. serine proteases, metallo proteases) and reflect them quantitatively. Summation of subpocket-wise cleavage entropy contributions yields a measure for overall protease substrate specificity. This total cleavage entropy allows ranking of different proteases with respect to their specificity, separating unspecific digestive enzymes showing high total cleavage entropy from specific proteases involved in signaling cascades. The development of a quantitative cleavage entropy score allows an unbiased comparison of subpocket-wise and overall protease specificity. Thus, it enables assessment of relative importance of physicochemical and structural descriptors in protease recognition. We present an exemplary application of cleavage entropy in tracing substrate specificity in protease evolution. This highlights the wide range of substrate promiscuity within homologue proteases and hence the heavy impact of a limited number of mutations on individual substrate specificity. PMID:23637583

Fuchs, Julian E; von Grafenstein, Susanne; Huber, Roland G; Margreiter, Michael A; Spitzer, Gudrun M; Wallnoefer, Hannes G; Liedl, Klaus R

2013-04-01

396

Morphological aspects of in vivo cleavage in Myocastor coypus (coypu).  

PubMed

The objective of the present work was to characterize the in vivo cleavage stage of Myocastor coypus embryos. For this purpose a colpocytological follow-up and controlled mating of 18 females were performed. Specimens from the beginning of the first cleavage to the acquisition of a morula appearance were considered to be in cleavage stage. Embryos in cleavage were collected between days 3 and 6 post-coitus. Of the collected embryos, 80% presented an even number of blastomeres and the remaining 20% an odd number. Embryos from 3 to 7 cells were blastomere associations in a spherical disposition within the zona pellucida. Blastomeres were spherical or ovoid, presenting slight flattening in areas contacting with other blastomeres. Embryos of 8 and 9 cells were as a group of blastomeres slightly elongated, surrounded by a spherical zona pellucida. The percentage of peri-vitelline space occupied by the embryonic mass ranged from 74.1 to 95.8% for all the substages. The cleavage pattern, developed in the oviduct, was of a rotational holoblastic type and asynchronic. PMID:15027947

Felipe, A E; Teruel, M T; Cabodevila, J A; Callejas, S S

2004-04-01

397

Hypoxic X-irradiation as an external stimulus for conformational change of oligodeoxynucleotides that possess disulfide bond and regulation of DNAzyme function.  

PubMed

We achieved a conformational change of oligodeoxynucleotides and the regulation of DNAzyme function by means of a radiolytic strand exchange reaction of disulfide bond. We designed a system in which the DNAzyme function of RNA cleavage was suppressed by the hybridization of an inhibitor strand that possessed disulfide bond with an active DNAzyme. Hypoxic X-irradiation led to the recovery of RNA cleavage because the strand exchange reaction at the disulfide bond in inhibitor strand resulted in a release of inhibitor strand. This strategy may be applicable to gene regulation by hypoxic X-irradiation. PMID:25479773

Tanabe, Kazuhito; Okada, Kana; Sugiura, Masaaki; Ito, Takeo; Nishimoto, Sei-Ichi

2015-01-15

398

Investing in Bonds.com  

NSDL National Science Digital Library

Investing in Bonds was created by the Bond Market Association to educate investors about the benefits of bonds investing. The Investor's Guide to Bond Basics educates investors about the types of bonds available, criteria for evaluating a bond, a guide to buying bonds, bond investment strategies and a glossary of bond market terms. The Bond Market section provides an overview of the U.S. bond market while the Investor's Checklist section takes the investor step-by-step through the bond investment decision process. Investors will also find sections with information on municipal bonds, corporate bonds, mortgage securities and U.S. Inflation-Indexed Securities.

399

Transition Metal Catalyzed Hydroarylation of Multiple Bonds: Exploration of Second Generation Ruthenium Catalysts and Extension to Copper Systems  

SciTech Connect

Catalysts provide foundational technology for the development of new materials and can enhance the efficiency of routes to known materials. New catalyst technologies offer the possibility of reducing energy and raw material consumption as well as enabling chemical processes with a lower environmental impact. The rising demand and expense of fossil resources has strained national and global economies and has increased the importance of accessing more efficient catalytic processes for the conversion of hydrocarbons to useful products. The goals of the research are to develop and understand single-site homogeneous catalysts for the conversion of readily available hydrocarbons into useful materials. A detailed understanding of these catalytic reactions could lead to the development of catalysts with improved activity, longevity and selectivity. Such transformations could reduce the environmental impact of hydrocarbon functionalization, conserve energy and valuable fossil resources and provide new technologies for the production of liquid fuels. This project is a collaborative effort that incorporates both experimental and computational studies to understand the details of transition metal catalyzed C-H activation and C-C bond forming reactions with olefins. Accomplishments of the current funding period include: (1) We have completed and published studies of C-H activation and catalytic olefin hydroarylation by TpRu{l_brace}P(pyr){sub 3}{r_brace}(NCMe)R (pyr = N-pyrrolyl) complexes. While these systems efficiently initiate stoichiometric benzene C-H activation, catalytic olefin hydroarylation is hindered by inhibition of olefin coordination, which is a result of the steric bulk of the P(pyr){sub 3} ligand. (2) We have extended our studies of catalytic olefin hydroarylation by TpRu(L)(NCMe)Ph systems to L = P(OCH{sub 2}){sub 3}CEt. Thus, we have now completed detailed mechanistic studies of four systems with L = CO, PMe{sub 3}, P(pyr){sub 3} and P(OCH{sub 2}){sub 3}CEt, which has provided a comprehensive understanding of the impact of steric and electronic parameters of 'L' on the catalytic hydroarylation of olefins. (3) We have completed and published a detailed mechanistic study of stoichiometric aromatic C-H activation by TpRu(L)(NCMe)Ph (L = CO or PMe{sub 3}). These efforts have probed the impact of functionality para to the site of C-H activation for benzene substrates and have allowed us to develop a detailed model of the transition state for the C-H activation process. These results have led us to conclude that the C-H bond cleavage occurs by a {sigma}-bond metathesis process in which the C-H transfer is best viewed as an intramolecular proton transfer. (4) We have completed studies of Ru complexes possessing the N-heterocyclic carbene IMes (IMes = 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene). One of these systems is a unique four-coordinate Ru(II) complex that catalyzes the oxidative hydrophenylation of ethylene (in low yields) to produce styrene and ethane (utilizing ethylene as the hydrogen acceptor) as well as the hydrogenation of olefins, aldehydes and ketones. These results provide a map for the preparation of catalysts that are selective for oxidative olefin hydroarylation. (5) The ability of TpRu(PMe{sub 3})(NCMe)R systems to activate sp{sup 3} C-H bonds has been demonstrated including extension to subsequent C-C bond forming steps. These results open the door to the development of catalysts for the functionalization of more inert C-H bonds. (6) We have discovered that Pt(II) complexes supported by simple nitrogen-based ligands serve as catalysts for the hydroarylation of olefins. Given the extensive studies of Pt-based catalytic C-H activation, we believe these results will provide an entry point into an array of possible catalysts for hydrocarbon functionalization.

T. Brent Gunnoe

2011-02-17

400

LET'S BOND! A Chemical Bonding Webquest  

NSDL National Science Digital Library

Today we are going to use the internet to explore chemical bonding! Even though there are just a few questions for each website, you need to read the entire content. Don't worry about understanding all of it, but make sure that you are familiar with it! Stay on task and have fun! Let's start with some basics. Click on the link below and answer the questions on your worksheet under "Bonding Basics". Bonding Basics Good job! Lets move on and talk about ions. Ions are a big part of bonding, so make sure you get this section down pat! Click on the ...

Hicken, Mrs.

2009-04-08

401

Redox-Triggered C-C Coupling of Alcohols and Vinyl Epoxides: Diastereo- and Enantioselective Formation of All-Carbon Quaternary Centers via tert-(Hydroxy)-Prenylation  

PubMed Central

Iridium catalyzed primary alcohol oxidation triggers reductive C-O bond cleavage of isoprene oxide to form aldehyde-allyliridium pairs that combine to form products of tert-(hydroxy)-prenylation, a motif found in >2000 terpenoid natural products. Curtin-Hammett effects are exploited to enforce high levels of anti-diastereo- and enantioselectivity in the formation of an all-carbon quaternary center. The present redox-triggered carbonyl additions occur in the absence of stoichiometric byproducts, premetallated reagents and discrete alcohol-to-aldehyde redox manipulations. PMID:24915473

Feng, Jiajie; Garza, Victoria J.; Krische, Michael J.

2014-01-01

402

Methodology for hardware\\/software co-verification in C\\/C++  

Microsoft Academic Search

In this paper we present our C\\/C++-based design environment for hardware\\/software co-verification. Our approach is to use C\\/C++ to describe both hardware and software throughout the design flow. Our methodology supports the efficient mapping of C\\/C++ functional descriptions directly into hardware and software. The advantages of a C\\/C++-based flow from the verification point of view are presented. The use of

Luc Séméria; Abhijit Ghosh

2000-01-01

403

Hydroxide-catalyzed bonding  

NASA Technical Reports Server (NTRS)

A method of bonding substrates by hydroxide-catalyzed hydration/dehydration involves applying a bonding material to at least one surface to be bonded, and placing the at least one surface sufficiently close to another surface such that a bonding interface is formed between them. A bonding material of the invention comprises a source of hydroxide ions, and may optionally include a silicate component, a particulate filling material, and a property-modifying component. Bonding methods of the invention reliably and reproducibly provide bonds which are strong and precise, and which may be tailored according to a wide range of possible applications. Possible applications for bonding materials of the invention include: forming composite materials, coating substrates, forming laminate structures, assembly of precision optical components, and preparing objects of defined geometry and composition. Bonding materials and methods of preparing the same are also disclosed.

Gwo, Dz-Hung (Inventor)

2003-01-01

404

Defect structures in deformed F.C.C. metals  

SciTech Connect

A high density of small defect clusters, similar to those observed in irradiated or quenched metals, has been observed in the deformed f.c.c. metals Cu, Au and Ni. The preliminary results show that the defect clusters are predominantly stacking fault tetrahedral (SFT). The SFT number density, rather than the size distribution, is deformation dependent. The defect cluster density is greater in the vicinities of dislocation tangles and grain boundaries. Their size distribution is wider than that produced by irradiation with an important number of larger clusters being formed. It is argued that these deformation-produced clusters may play a role in determining the flow stress and work hardening at low deformations.

Dai, Y.; Victoria, M. [Ecole Polytechnique Federale de Lausanne, Villigen PSI (Switzerland). CRPP-Fusion Technology Div.] [Ecole Polytechnique Federale de Lausanne, Villigen PSI (Switzerland). CRPP-Fusion Technology Div.

1997-08-01

405

SiC/C nanocomposites with inverse opal structure.  

PubMed

The synthesis, morphology, structural and optical characteristics of SiC/C nanocomposites with an inverse opal lattice have been investigated. The samples were prepared by thermochemical treatment of opal matrices filled with carbon compounds which was followed by silicon dioxide dissolution. The samples were studied by electron microscopy, x-ray diffraction, photoluminescence, IR and Raman scattering spectroscopy. The electron microscopy data revealed a highly porous periodic structure which was a three-dimensional replica of the voids of the initial opal lattice. The hexagonal silicon carbide was found to be non-uniformly distributed throughout the volume, its greater part located in the surface layer up to 50 µm deep. The data of x-ray diffraction, IR and Raman scattering spectroscopy enabled us to assume that the composite had hexagonal diamond fragments. The photoluminescence and optical reflection spectra of the composites have been measured. PMID:21030770

Emelchenko, G A; Zhokhov, A A; Masalov, V M; Maximuk, M Yu; Fursova, T N; Bazhenov, A V; Zverkova, I I; Khasanov, S S; Steinman, E A; Tereshenko, A N

2010-11-26

406

Channel Bonding Notes 03/25/2004 Channel Bonding Notes  

E-print Network

Channel Bonding Notes 03/25/2004 Channel Bonding Notes Luke Hindman Department of Computer Science by attempting to use the Linux kernel bonding driver (bonding.o) to implement channel bonding on the master node. However, each time I would bring up the bonded interface (bond0) I would loose communication

Jain, Amit

407

The Mycobacterium tuberculosis ORF Rv0654 encodes a carotenoid oxygenase mediating central and excentric cleavage of conventional and aromatic carotenoids.  

PubMed

Mycobacterium tuberculosis, the causative agent of tuberculosis, is assumed to lack carotenoids, which are widespread pigments fulfilling important functions as radical scavengers and as a source of apocarotenoids. In mammals, the synthesis of apocarotenoids, including retinoic acid, is initiated by the ?-carotene cleavage oxygenases I and II catalyzing either a central or an excentric cleavage of ?-carotene, respectively. The M. tuberculosis ORF Rv0654 codes for a putative carotenoid oxygenase conserved in other mycobacteria. In the present study, we investigated the corresponding enzyme, here named M. tuberculosis carotenoid cleavage oxygenase (MtCCO). Using heterologously expressed and purified protein, we show that MtCCO converts several carotenoids and apocarotenoids in vitro. Moreover, the identification of the products suggests that, in contrast to other carotenoid oxygenases, MtCCO cleaves the central C15-C15' and an excentric double bond at the C13-C14 position, leading to retinal (C(20)), ?-apo-14'-carotenal (C(22)) and ?-apo-13-carotenone (C(18)) from ?-carotene, as well as the corresponding hydroxylated products from zeaxanthin and lutein. Moreover, the enzyme cleaves also 3,3'-dihydroxy-isorenieratene representing aromatic carotenoids synthesized by other mycobacteria. Quantification of the products from different substrates indicates that the preference for each of the cleavage positions is determined by the hydroxylation and the nature of the ionone ring. The data obtained in the present study reveal MtCCO to be a novel carotenoid oxygenase and indicate that M. tuberculosis may utilize carotenoids from host cells and interfere with their retinoid metabolism. PMID:20929460

Scherzinger, Daniel; Scheffer, Erdmann; Bär, Cornelia; Ernst, Hansgeorg; Al-Babili, Salim

2010-11-01

408

The Bond Market Association  

NSDL National Science Digital Library

The Bond Market Association "represents securities firms and banks that underwrite, trade and sell debt securities." Their web site is divided into several sections. The Research Desk presents the results of research conducted by the association, including forecasts of economic growth and analysis of bond market trends. The Legislative Issues and Regulatory sections provide a summary of legislative and regulatory developments of interest to the bond market community. Market Practices contains guidelines and procedures to be followed in the bond market. The site also provides information to investors on how to invest in corporate bonds, tax-exempt municipal bonds, and mortgage securities.

409

Functionalized olefin cross-coupling to construct carbon–carbon bonds  

NASA Astrophysics Data System (ADS)

Carbon-carbon (C-C) bonds form the backbone of many important molecules, including polymers, dyes and pharmaceutical agents. The development of new methods to create these essential connections in a rapid and practical fashion has been the focus of numerous organic chemists. This endeavour relies heavily on the ability to form C-C bonds in the presence of sensitive functional groups and congested structural environments. Here we report a chemical transformation that allows the facile construction of highly substituted and uniquely functionalized C-C bonds. Using a simple iron catalyst, an inexpensive silane and a benign solvent under ambient atmosphere, heteroatom-substituted olefins are easily reacted with electron-deficient olefins to create molecular architectures that were previously difficult or impossible to access. More than 60 examples are presented with a wide array of substrates, demonstrating the chemoselectivity and mildness of this simple reaction.

Lo, Julian C.; Gui, Jinghan; Yabe, Yuki; Pan, Chung-Mao; Baran, Phil S.

2014-12-01

410

Functionalized olefin cross-coupling to construct carbon-carbon bonds.  

PubMed

Carbon-carbon (C-C) bonds form the backbone of many important molecules, including polymers, dyes and pharmaceutical agents. The development of new methods to create these essential connections in a rapid and practical fashion has been the focus of numerous organic chemists. This endeavour relies heavily on the ability to form C-C bonds in the presence of sensitive functional groups and congested structural environments. Here we report a chemical transformation that allows the facile construction of highly substituted and uniquely functionalized C-C bonds. Using a simple iron catalyst, an inexpensive silane and a benign solvent under ambient atmosphere, heteroatom-substituted olefins are easily reacted with electron-deficient olefins to create molecular architectures that were previously difficult or impossible to access. More than 60 examples are presented with a wide array of substrates, demonstrating the chemoselectivity and mildness of this simple reaction. PMID:25519131

Lo, Julian C; Gui, Jinghan; Yabe, Yuki; Pan, Chung-Mao; Baran, Phil S

2014-12-18

411

Acetic acid promoted metal-free aerobic carbon-carbon bond forming reactions at ?-position of tertiary amines.  

PubMed

The oxidative functionalization of the benzylic C-H bonds in tetrahydroisoquinolines and tetrahydro-?-carboline derivatives was investigated. C-C bond forming reactions proceeded with a range of nucleophiles (nitroalkane, enol silyl ether, indole, allylstannane, and tetrabutylammonium cyanide) under metal-free conditions and an oxygen atmosphere. Acetic acid caused a significant acceleration effect. PMID:25062493

Ueda, Hirofumi; Yoshida, Kei; Tokuyama, Hidetoshi

2014-08-15

412

To jump or not to jump? C? hydrogen atom transfer in post-cleavage radical-cation complexes.  

PubMed

Conventionally, electron capture or transfer to a polyprotonated peptide ion produces an initial radical-cation intermediate which dissociates "directly" to generate complementary c(n)' and z(m)(•) sequence ions (or ions and neutrals). Alternatively, or in addition, the initial radical-cation intermediate can undergo H(•) migration to produce c(n)(•) (or c(n) - H(•)) and z(m)' (or z(m)(•) + H(•)) species prior to complex separation ("nondirect"). This reaction significantly complicates spectral interpretation, creates ambiguity in peak assignment, impairs effective algorithmic processing (reduction of the spectrum to solely (12)C m/z values), and reduces sequence ion signal-to-noise. Experimental evidence indicates that the products of hydrogen atom transfer reactions are substantially less prevalent for higher charge state precursors. This effect is generally rationalized on the basis of decreased complex lifetime. Here, we present a theoretical study of these reactions in post N-C(?) bond cleavage radical-cation complexes as a function of size and precursor charge state. This approach provides a computational estimate of the barriers associated with these processes for highly charged peptides with little charge solvation. The data indicate that the H(•) migration is an exothermic process and that the barrier governing this reaction rises steeply with precursor ion charge state. There is also some evidence for immediate product separation following N-C(?) bond cleavage at higher charge state. PMID:22809411

Bythell, Benjamin J

2013-02-14

413

Abnormal Early Cleavage Events Predict Early Embryo Demise: Sperm Oxidative Stress and Early Abnormal Cleavage  

PubMed Central

Human embryos resulting from abnormal early cleavage can result in aneuploidy and failure to develop normally to the blastocyst stage. The nature of paternal influence on early embryo development has not been directly demonstrated although many studies have suggested effects from spermatozoal chromatin packaging, DNA damage, centriolar and mitotic spindle integrity, and plasma membrane integrity. The goal of this study was to determine whether early developmental events were affected by oxidative damage to the fertilizing sperm. Survival analysis was used to compare patterns of blastocyst formation based on P2 duration. Kaplan-Meier survival curves demonstrate that relatively few embryos with short (<1?hr) P2 times reached blastocysts, and the two curves diverged beginning on day 4, with nearly all of the embryos with longer P2 times reaching blastocysts by day 6 (p < .01). We determined that duration of the 2nd to 3rd mitoses were sensitive periods in the presence of spermatozoal oxidative stress. Embryos that displayed either too long or too short cytokineses demonstrated an increased failure to reach blastocyst stage and therefore survive for further development. Although paternal-derived gene expression occurs later in development, this study suggests a specific role in early mitosis that is highly influenced by paternal factors. PMID:25307782

Burruel, Victoria; Klooster, Katie; Barker, Christopher M.; Pera, Renee Reijo; Meyers, Stuart

2014-01-01

414

Analysis of carotenoid isomerase activity in a prototypical carotenoid cleavage enzyme, apocarotenoid oxygenase (ACO).  

PubMed

Carotenoid cleavage enzymes (CCEs) constitute a group of evolutionarily related proteins that metabolize a variety of carotenoid and non-carotenoid substrates. Typically, these enzymes utilize a non-heme iron center to oxidatively cleave a carbon-carbon double bond of a carotenoid substrate. Some members also isomerize specific double bonds in their substrates to yield cis-apocarotenoid products. The apocarotenoid oxygenase from Synechocystis has been hypothesized to represent one such member of this latter category of CCEs. Here, we developed a novel expression and purification protocol that enabled production of soluble, native ACO in quantities sufficient for high resolution structural and spectroscopic investigation of its catalytic mechanism. High performance liquid chromatography and Raman spectroscopy revealed that ACO exclusively formed all-trans products. We also found that linear polyoxyethylene detergents previously used for ACO crystallization strongly inhibited the apocarotenoid oxygenase activity of the enzyme. We crystallized the native enzyme in the absence of apocarotenoid substrate and found electron density in the active site that was similar in appearance to the density previously attributed to a di-cis-apocarotenoid intermediate. Our results clearly demonstrated that ACO is in fact a non-isomerizing member of the CCE family. These results indicate that careful selection of detergent is critical for the success of structural studies aimed at elucidating structures of CCE-carotenoid/retinoid complexes. PMID:24648526

Sui, Xuewu; Kiser, Philip D; Che, Tao; Carey, Paul R; Golczak, Marcin; Shi, Wuxian; von Lintig, Johannes; Palczewski, Krzysztof

2014-05-01

415

Essays on corporate bonds  

E-print Network

This thesis consists of three empirical essays on corporate bonds, examining the role of both credit risk and liquidity. In the first chapter, I test the ability of structural models of default to price corporate bonds in ...

Bao, Jack (Jack C.)

2009-01-01

416

Bonding silicones with epoxies  

SciTech Connect

It is shown that silicones, both room temperature vulcanizing (RTV) and millable rubber (press cured) can be successfully bonded to other materials using plasma treatment and epoxy adhesives. The plasma treatment using dry air atmosphere increases the surface energy of the silicone and thus provides a lower water contact angle. This phenomenon allows the epoxy adhesive to wet the silicone surface and ultimately bond. Bond strengths are sufficiently high to result in failures in the silicone materials rather than the adhesive bond.

Tira, J.S.

1980-01-01

417

Perceiving Social Cleavages and Inequalities: The Case of Israeli Adolescents.  

ERIC Educational Resources Information Center

An analysis of perceptions of social cleavage and inequality among approximately 9000 Israeli eighth and ninth graders showed students accurately comprehended a multifaceted society with major social divisions. A social map with inequality was revealed in which ethnicity played the least prominent role. Personal and social traits influenced…

Dar, Yechezkel; Erhard, Rachel; Resh, Nura

1998-01-01

418

Modeling Radial Holoblastic Cleavage: A Laboratory Activity for Developmental Biology.  

ERIC Educational Resources Information Center

Introduces a laboratory activity designed for an undergraduate developmental biology course. Uses Play-Doh (plastic modeling clay) to build a multicellular embryo in order to provide a 3-D demonstration of cleavage. Includes notes for the instructor and student directions. (YDS)

Ellis, Linda K.

2000-01-01

419

Scientific Correspondence Cleavage of Bipartite Substrates by Rice and Maize  

E-print Network

Scientific Correspondence Cleavage of Bipartite Substrates by Rice and Maize Ribonuclease P with rice (Oryza sativa) and maize (Zea mays) nuclear RNase P. By successively employing ion from rice (Pusa Basmati-1) and maize (Black Mexican Sweet) calli. Details of the purification

Gopalan, Venkat

420

Concept for support and cleavage of brittle crystals.  

PubMed

We report on sample holders for crystals to be cleaved for the preparation of surfaces with large atomically flat terraces. The concept for mounting sample crystals is based on a vicelike clamping mechanism to securely hold the crystal in position while reducing the risk of fragmentation. Sample holders based on this concept and made of suitable materials allow preparation and cleavage of crystals in the ultrahigh vacuum at high or low temperatures. To cleave the crystal, we employ a scalpel blade mounted on a wobble stick to generate a highly localized stress field initiating the cleavage process. The sample holders are used for experiments of highest resolution scanning force microscopy, however, the concept can be transferred to any other system where cleavage faces of crystals are of interest. Exemplarily, scanning force microscopy results demonstrate that (111) cleavage faces of CaF2 crystals can be prepared with steps only a few F-Ca-F triple-layers high and atomically flat terraces extending over areas of several microm2. PMID:19566206

Tröger, L; Schütte, J; Ostendorf, F; Kühnle, A; Reichling, M

2009-06-01

421

CLEAVAGE FRACTURE MICROMECHANISMS RELATED TO WPS EFFECT IN RPV STEEL  

E-print Network

temperature, the formation of high compressive stresses ahead of the crack tip on elastic unloading by warm pre-stress (WPS) on the cleavage fracture micromechanisms of a 18MND5 (A533B) reactor pressure. One unveiled characteristic of the WPS effect is the deactivation of particles at high temperature

Boyer, Edmond

422

Mitochondria Localize to the Cleavage Furrow in Mammalian Cytokinesis  

PubMed Central

Mitochondria are dynamic organelles with multiple cellular functions, including ATP production, calcium buffering, and lipid biosynthesis. Several studies have shown that mitochondrial positioning is regulated by the cytoskeleton during cell division in several eukaryotic systems. However, the distribution of mitochondria during mammalian cytokinesis and whether the distribution is regulated by the cytoskeleton has not been examined. Using live spinning disk confocal microscopy and quantitative analysis of mitochondrial fluorescence intensity, we demonstrate that mitochondria are recruited to the cleavage furrow during cytokinesis in HeLa cells. After anaphase onset, the mitochondria are recruited towards the site of cleavage furrow formation, where they remain enriched as the furrow ingresses and until cytokinesis completion. Furthermore, we show that recruitment of mitochondria to the furrow occurs in multiple mammalian cells lines as well as in monopolar, bipolar, and multipolar divisions, suggesting that the mechanism of recruitment is conserved and robust. Using inhibitors of cytoskeleton dynamics, we show that the microtubule cytoskeleton, but not actin, is required to transport mitochondria to the cleavage furrow. Thus, mitochondria are specifically recruited to the cleavage furrow in a microtubule-dependent manner during mammalian cytokinesis. Two possible reasons for this could be to localize mitochondrial function to the furrow to facilitate cytokinesis and / or ensure accurate mitochondrial inheritance. PMID:23991162

Lawrence, Elizabeth J.; Mandato, Craig A.

2013-01-01

423

The pattern of DNA cleavage intensity around indels  

PubMed Central

Indels (insertions and deletions) are the second most common form of genetic variations in the eukaryotic genomes and are responsible for a multitude of genetic diseases. Despite its significance, detailed molecular mechanisms for indel generation are still unclear. Here we examined 2,656,597 small human and mouse germline indels, 16,742 human somatic indels, 10,599 large human insertions, and 5,822 large chimpanzee insertions and systematically analyzed the patterns of DNA cleavage intensities in the 200 base pair regions surrounding these indels. Our results show that DNA cleavage intensities close to the start and end points of indels are significantly lower than other regions, for both small human germline and somatic indels and also for mouse small indels. Compared to small indels, the patterns of DNA cleavage intensity around large indels are more complex, and there are two low intensity regions near each end of the indels that are approximately 13?bp apart from each other. Detailed analyses of a subset of indels show that there is slight difference in cleavage intensity distribution between insertion indels and deletion indels that could be contributed by their respective enrichment of different repetitive elements. These results will provide new insight into indel generation mechanisms. PMID:25660536

Chen, Wei; Zhang, Liqing

2015-01-01

424

The pattern of DNA cleavage intensity around indels.  

PubMed

Indels (insertions and deletions) are the second most common form of genetic variations in the eukaryotic genomes and are responsible for a multitude of genetic diseases. Despite its significance, detailed molecular mechanisms for indel generation are still unclear. Here we examined 2,656,597 small human and mouse germline indels, 16,742 human somatic indels, 10,599 large human insertions, and 5,822 large chimpanzee insertions and systematically analyzed the patterns of DNA cleavage intensities in the 200 base pair regions surrounding these indels. Our results show that DNA cleavage intensities close to the start and end points of indels are significantly lower than other regions, for both small human germline and somatic indels and also for mouse small indels. Compared to small indels, the patterns of DNA cleavage intensity around large indels are more complex, and there are two low intensity regions near each end of the indels that are approximately 13?bp apart from each other. Detailed analyses of a subset of indels show that there is slight difference in cleavage intensity distribution between insertion indels and deletion indels that could be contributed by their respective enrichment of different repetitive elements. These results will provide new insight into indel generation mechanisms. PMID:25660536

Chen, Wei; Zhang, Liqing

2015-01-01

425

Oxidative DNA Cleavage by the Antitumor Antibiotic Leinamycin and Simple  

E-print Network

leinamycin analogs (2-4) prompted us to investigate whether 1 is capable of mediating oxidative DNA damageOxidative DNA Cleavage by the Antitumor Antibiotic Leinamycin and Simple 1,2-Dithiolan-3-one 1-Oxides: Evidence for Thiol-Dependent Conversion of Molecular Oxygen to DNA-Cleaving Oxygen Radicals

Gates, Kent. S.

426

Transferability of cleavage fracture parameters between notched and cracked geometries  

E-print Network

and specimen geometry dependence of cleavage fracture micromechanisms of a French pressure vessel steel (A508 (iii) the Weibull parameters. The fractographic investigations showed that Manganese sulfide clusters of steels is one of the fields where local approach has brought a lot of understanding (e.g. effect

Boyer, Edmond

427

C?H Bond Activation by f-Block Complexes.  

PubMed

Most homogeneous catalysis relies on the design of metal complexes to trap and convert substrates or small molecules to value-added products. Organometallic lanthanide compounds first gave a tantalizing glimpse of their potential for catalytic C?H bond transformations with the selective cleavage of one C?H bond in methane by bis(permethylcyclopentadienyl)lanthanide methyl [(?(5) -C5 Me5 )2 Ln(CH3 )] complexes some 25 years ago. Since then, numerous metal complexes from across the periodic table have been shown to selectively activate hydrocarbon C?H bonds, but the challenges of closing catalytic cycles still remain; many f-block complexes show great potential in this important area of chemistry. PMID:25384554

Arnold, Polly L; McMullon, Max W; Rieb, Julia; Kühn, Fritz E

2014-11-10

428

Rapid adhesive bonding concepts  

NASA Technical Reports Server (NTRS)

Adhesive bonding in the aerospace industry typically utilizes autoclaves or presses which have considerable thermal mass. As a consequence, the rates of heatup and cooldown of the bonded parts are limited and the total time and cost of the bonding process is often relatively high. Many of the adhesives themselves do not inherently require long processing times. Bonding could be performed rapidly if the heat was concentrated in the bond lines or at least in the adherends. Rapid adhesive bonding concepts were developed to utilize induction heating techniques to provide heat directly to the bond line and/or adherends without heating the entire structure, supports, and fixtures of a bonding assembly. Bonding times for specimens are cut by a factor of 10 to 100 compared to standard press bonding. The development of rapid adhesive bonding for lap shear specimens (per ASTM D1003 and D3163), for aerospace panel bonding, and for field repair needs of metallic and advanced fiber reinforced polymeric matrix composite structures are reviewed.

Stein, B. A.; Tyeryar, J. R.; Hodges, W. T.

1984-01-01

429

Local Currency Bond Markets  

Microsoft Academic Search

This paper analyzes the development of 49 local bond markets. The main finding is that policies and laws matter: countries with stable inflation rates and strong creditor rights have more developed local bond markets and rely less on foreign- currency-denominated bonds. The results suggest that \\

FRANCIS E. WARNOCK

2006-01-01

430

Acrylic mechanical bond tests  

SciTech Connect

The tensile strength of bonded acrylic is tested as a function of bond joint thickness. 0.125 in. thick bond joints were found to posses the maximum strength while the acceptable range of joints varied from 0.063 in. to almost 0.25 in. Such joints are used in the Sudbury Neutrino Observatory.

Wouters, J.M.; Doe, P.J.

1991-02-01