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1

Mechanistic studies of C-C bond cleavage of nitriles by dinuclear metal cryptates.  

PubMed

We previously reported that dinuclear copper(II) cryptate [Cu(2)L](4+) cleaves the C-C bond of acetonitrile at room temperature to produce a cyano-bridged dinuclear cryptate and methanol, whereby the reaction mechanism has not yet become clear. We have now systemically investigated this reaction, and four cryptates, [Cu(2)L](ClO(4))(4) (1), [Zn(2)L](ClO(4))(4) (2), [Cu(2)L(H(2)O)(2)](CF(3)SO(3))(4) (5), and [Cu(2)L(OH)(OH(2))](ClO(4))(3) (6) are reported here. Cryptates 1 and 2 can cleave the C--C bonds of acetonitrile, propionitrile, and benzonitrile at room temperature under open atmospheric conditions to give cyano-bridged cryptates [Cu(2)L(CN)](ClO(4))(3) (3) and [Zn(2)L(CN)](ClO(4))(3) (4), respectively, and the corresponding alcohol. In contrast, 5 and 6 do not show any C-C bond activation of nitriles, as the interior axial positions of Cu(II) in 5 and 6 are occupied by water/OH(-). The C-C bond cleavage of (S)-(+)-2-methylbutyronitrile by 2 produced (R)-(-)-2-butanol only; that is, the cleavage reaction proceeds through an S(N)2 pathway (Walden inversion). PMID:19810059

Yang, Li-Zi; Li, Yu; Zhuang, Xiao-Mei; Jiang, Long; Chen, Jia-Mei; Luck, Rudy L; Lu, Tong-Bu

2009-11-16

2

Photo-assisted cyanation of transition metal nitrates coupled with room temperature C-C bond cleavage of acetonitrile.  

PubMed

It is a challenge to use acetonitrile as a cyanating agent because of the difficulty in cleaving its C-CN bond. Herein, we report a mild photo-assisted route to conduct the cyanation of transition metal nitrates using acetonitrile as the cyanating agent coupled with room-temperature C-C bond cleavage. DFT calculations and experimental observations suggest a radical-involved reaction mechanism, which excludes toxicity from free cyanide ions. PMID:23282559

Zou, Shihui; Li, Renhong; Kobayashi, Hisayoshi; Liu, Juanjuan; Fan, Jie

2013-01-03

3

Cu-Catalyzed Esterification Reaction via Aerobic Oxygenation and C-C Bond Cleavage: An Approach to ?-Ketoesters.  

PubMed

The Cu-catalyzed novel aerobic oxidative esterification reaction of 1,3-diones for the synthesis of ?-ketoesters has been developed. This method combines C-C ?-bond cleavage, dioxygen activation and oxidative C-H bond functionalization, as well as provides a practical, neutral, and mild synthetic approach to ?-ketoesters which are important units in many biologically active compounds and useful precursors in a variety of functional group transformations. A plausible radical process is proposed on the basis of mechanistic studies. PMID:24032593

Zhang, Chun; Feng, Peng; Jiao, Ning

2013-09-26

4

Deamination, oxidation, and C-C bond cleavage reactivity of 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxycytosine.  

PubMed

Three new cytosine derived DNA modifications, 5-hydroxymethyl-2'-deoxycytidine (hmdC), 5-formyl-2'-deoxycytidine (fdC) and 5-carboxy-2'-deoxycytidine (cadC) were recently discovered in mammalian DNA, particularly in stem cell DNA. Their function is currently not clear, but it is assumed that in stem cells they might be intermediates of an active demethylation process. This process may involve base excision repair, C-C bond cleaving reactions or deamination of hmdC to 5-hydroxymethyl-2'-deoxyuridine (hmdU). Here we report chemical studies that enlighten the chemical reactivity of the new cytosine nucleobases. We investigated their sensitivity toward oxidation and deamination and we studied the C-C bond cleaving reactivity of hmdC, fdC, and cadC in the absence and presence of thiols as biologically relevant (organo)catalysts. We show that hmdC is in comparison to mdC rapidly oxidized to fdC already in the presence of air. In contrast, deamination reactions were found to occur only to a minor extent. The C-C bond cleavage reactions require the presence of high concentration of thiols and are acid catalyzed. While hmdC dehydroxymethylates very slowly, fdC and especially cadC react considerably faster to dC. Thiols are active site residues in many DNA modifiying enzymes indicating that such enzymes could play a role in an alternative active DNA demethylation mechanism via deformylation of fdC or decarboxylation of cadC. Quantum-chemical calculations support the catalytic influence of a thiol on the C-C bond cleavage. PMID:23980549

Schiesser, Stefan; Pfaffeneder, Toni; Sadeghian, Keyarash; Hackner, Benjamin; Steigenberger, Barbara; Schröder, Arne S; Steinbacher, Jessica; Kashiwazaki, Gengo; Höfner, Georg; Wanner, Klaus T; Ochsenfeld, Christian; Carell, Thomas

2013-09-17

5

Oxygen switch in visible-light photoredox catalysis: radical additions and cyclizations and unexpected C-C-bond cleavage reactions.  

PubMed

Visible light photoredox catalyzed inter- and intramolecular C-H functionalization reactions of tertiary amines have been developed. Oxygen was found to act as chemical switch to trigger two different reaction pathways and to obtain two different types of products from the same starting material. In the absence of oxygen, the intermolecular addition of N,N-dimethyl-anilines to electron-deficient alkenes provided ?-amino nitriles in good to high yields. In the presence of oxygen, a radical addition/cyclization reaction occurred which resulted in the formation of tetrahydroquinoline derivatives in good yields under mild reaction conditions. The intramolecular version of the radical addition led to the unexpected formation of indole-3-carboxaldehyde derivatives. Mechanistic investigations of this reaction cascade uncovered a new photoredox catalyzed C-C bond cleavage reaction. PMID:23330701

Zhu, Shaoqun; Das, Arindam; Bui, Lan; Zhou, Hanjun; Curran, Dennis P; Rueping, Magnus

2013-01-18

6

C?-C bond cleavage of the peptide backbone in MALDI in-source decay using salicylic acid derivative matrices.  

PubMed

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. PMID:21953105

Asakawa, Daiki; Takayama, Mitsuo

2011-04-19

7

Bond cleavage during isobutanol thermal decomposition and the breaking of C-C bonds in alcohols at high temperatures.  

PubMed

Isobutanol was thermally decomposed in a single pulse shock tube under conditions where chain processes were suppressed. The main reaction is the breaking of C-C bonds. Literature rate expressions, experimentally determined, are found in some cases to be in disagreement. The rate expressions for the decomposition processes at temperatures of 1090 to 1240 K and pressures of 1.5 and 6 atm are k(isobutanol ? isopropyl + hydroxymethyl) + k(isobutanol ? methyl + 1-hydroxypropyl-2) = 10(16.7±0.3) exp(-41097 ± 750) s(-1), where k(isobutanol ? isopropyl + hydroxymethyl) = 10(16.45 ±0.3) exp(-40910 ± 750/T) s(-1) and k(isobutanol ? methyl + 1-hydroxypropyl-2) = 10(16.38±0.3) exp(-41560 ± 750/T) s(-1). These values permit comparisons with recent estimates including those from ab initio calculations. A new procedure is presented that uses information on the kinetics of bond breaking reactions of alkanes and the effect of OH substitution to derive rate coefficients for similar reactions of alcohols. This leads to the following rate expression for the smaller alcohols, at temperatures of 1090 to 1240 K and pressures of 1.5 and 6 atm, k(ethanol ? methyl + hydroxymethyl) = 10(16.42±0.3) exp(-43496 ± 750 K/T) s(-1), k(isopropanol ? methyl + 1-hydroxyethyl) = 10(16.54±0.3) exp(-42495 ± 750 K/T) s(-1), k(n-propanol ? ethyl + hydroxymethyl) = 10(16.43±0.3) exp(-41696 ± 750 K/T) s(-1), and k(n-propanol ? methyl + 2-hydroxymethyl) = 10(16.53±0.3) exp(-42945 ± 750 K/T) s(-1). Extension of this approach to other alcohols is straightforward. The resulting correlations along with the data on dehydration of alcohols provide novel information of the kinetic stability of alcohols. PMID:23968459

Rosado-Reyes, Claudette; Tsang, Wing

2013-09-19

8

FeCl2-promoted cleavage of the unactivated C-C bond of alkylarenes and polystyrene: direct synthesis of arylamines.  

PubMed

Ironing it out: an efficient and convenient nitrogenation strategy involving C-C bond cleavage for the straightforward synthesis of versatile arylamines is presented. Various alkyl azides and alkylarenes, including the common industrial by-product cumene, react using this protocol. Moreover, this method provides a potential strategy for the degradation of polystyrene. PMID:22689538

Qin, Chong; Shen, Tao; Tang, Conghui; Jiao, Ning

2012-06-11

9

Decomposition of 1,1-dimethyl-1-silacyclobutane on a tungsten filament--evidence of both ring C-C and ring Si-C bond cleavages.  

PubMed

The decomposition of 1,1-dimethyl-1-silacyclobutane (DMSCB) on a heated tungsten filament has been studied using vacuum ultraviolet laser single photon ionization time-of-flight mass spectrometry. It is found that the decomposition of DMSCB on the W filament to form ethene and 1,1-dimethylsilene is a catalytic process. In addition, two other decomposition channels exist to produce methyl radicals via the Si-CH(3) bond cleavage and to form propene (or cyclopropane)/dimethylsilylene. It has been demonstrated that both the formation of ethene and that of propene are stepwise processes initiated by the cleavage of a ring C-C bond and a ring Si-C bond, respectively, to form diradical intermediates, followed by the breaking of the remaining central bonds in the diradicals. The formation of ethene via an initial cleavage of a ring C-C bond is dominant over that of propene via an initial cleavage of a ring Si-C bond. When the collision-free condition is voided, secondary reactions in the gas-phase produce various methyl-substituted 1,3-disilacyclobutane molecules. The dominant of all is found to be 1,1,3,3-tetramethyl-1,3-disilacyclobutane originated from the dimerization of 1,1-dimethylsilene. PMID:20049692

Tong, L; Shi, Y J

2010-02-01

10

Substituent effect on reactivity of triplet excited state of 2,3-diazabicyclo[2.2.1]hept-2-enes, DBH derivatives: ? C-N bond cleavage versus ? C-C bond cleavage.  

PubMed

The photoreaction of a series of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives, in which various substituents (X) were introduced at the methano bridge carbon of C(7), was investigated under direct (>290 nm) and triplet-sensitized (Ph2CO, >370 nm) irradiation conditions of the azo chromophore (?C?–C?–N?N–C?–C??). The azo compounds offered a unique opportunity to see the substituent X effect at the remote position of the azo chromophore on the reactivity of the triplet excited state of bicyclic azoalkanes. The present study led to the first observation of the unusual C?–C? bond-cleavage reaction without the ring stiffness, that is, the structural rigidity, in the triplet state of the cyclic azoalkanes. The stereoelectronic effects were found to play an important role in lowering the activation energy of the C?–C? bond-cleavage reaction. NBO analyses at the M06-2X/cc-PVDZ level of theory confirmed the stereoelectronic effect. PMID:22985271

Abe, Manabu; Watanabe, Shinji; Tamura, Hiroshi; Boinapally, Srikanth; Kanahara, Kousei; Fujiwara, Yoshihisa

2012-10-01

11

Solvent-switched benzylic methylene functionalization: addition, ring-opening, cyclization, and unexpected cleavage of C-O and C-C bonds.  

PubMed

Intermolecular benzylic methylene functionalization of exo-cyclic enol ethers has been achieved using imines as reagents and potassium tert-butoxide as the catalyst. Depending on the solvent used, the reaction proceeds by two pathways. In THF, an addition/elimination reaction of exo-cyclic enol ethers with imines provides dihydroisobenzofuran derivatives in good yield. In DMSO, an addition/ring-opening/cyclization cascade reaction occurs with unexpected cleavage of C-O and C-C bonds, affording isoquinolin-1(2H)-one products in high yield under ambient reaction conditions. PMID:23883188

Li, Deng Yuan; Shang, Xue Song; Chen, Guo Rong; Liu, Pei Nian

2013-07-24

12

Theoretical elucidation of the mechanism of cleavage of the aromatic C-C bond in quinoxaline by a tungsten-based complex [W(PMe3)4(?2-CH2PMe2)H].  

PubMed

The aromatic C-C bond cleavage by a tungsten complex reported recently by Sattler and Parkin offers fresh opportunities for the functionalization of organic molecules. The mechanism of such a process has not yet been determined, which appeals to computational assistance to understand how the unstrained C-C bond is activated at the molecular level. In this work, by performing density functional theory calculations, we studied various possible mechanisms of cleavage of the aromatic C-C bond in quinoxaline (QoxH) by the W-based complex [W(PMe(3))(4)(?(2)-CH(2)PMe(2))H]. The calculated results show that the mechanism proposed by Sattler and Parkin involves an overall barrier of as high as 42.0 kcal mol(-1) and thus does not seem to be consistent with the experimental observation. Alternatively, an improved mechanism has been presented in detail, which involves the removal and recoordination of a second PMe(3) ligand on the tungsten center. In our new mechanism, it is proposed that the C-C cleavage occurs prior to the second C-H bond addition, in contrast to Sattler and Parkin's mechanism in which the C-C bond is broken after the second C-H bond addition. We find that the rate-determining step of the reaction is the ring-opening process of the tungsten complex with an activation barrier of 28.5 kcal mol(-1) after the first PMe(3) ligand dissociation from the metal center. The mono-hydrido species is located as the global minimum on the potential-energy surface, which is in agreement with the experimental observation for this species. The present theoretical results provide new insight into the mechanism of the remarkable C-C bond cleavage. PMID:23055438

Liu, Yuxia; Zhang, Dongju; Gao, Jun; Liu, Chengbu

2012-10-10

13

Specific cleavage at peptide backbone C?-C and CO-N bonds during matrix-assisted laser desorption/ionization in-source decay mass spectrometry with 5-nitrosalicylic acid as the matrix.  

PubMed

The use of 5-nitrosalicylic acid (5-NSA) as a matrix for in-source decay (ISD) of peptides during matrix-assisted laser desorption/ionization (MALDI) is described herein. Mechanistically, the decay process is initiated by a hydrogen abstraction from a peptide backbone amide nitrogen by 5-NSA. Hydrogen abstraction results in formation of an oxidized peptide containing a radical amide nitrogen. Subsequently, the C(?)-C bond N-terminal to the peptide bond is cleaved to form an a·/x fragment pair. The C(?)-C bonds C-terminal to Gly residues were less susceptible to cleavage than were those of other residues. C(?)-C bonds N-terminal to Pro and Sar residues were not cleaved by the aforementioned mechanism; instead, after hydrogen abstraction from a Pro or Sar C(?)-H bond, the peptide bond N-terminal to the Pro was cleaved yielding b- and y-series ions. We also show that fragments produced by MALDI 5-NSA-induced ISD were formed independently of the ionization process. PMID:21793066

Asakawa, Daiki; Takayama, Mitsuo

2011-09-15

14

The C-C bond cleavage of a lignin model compound, 1,2-diarylpropane-1,3-diol, with a heme-enzyme model catalyst tetraphenylporphyrinatoiron(III)chloride in the presence of tert-butylhydroperoxide.  

PubMed

The catalytic C-C bond cleavage of a lignin model compound was investigated by use of tetraphenylporphyrinatoiron(III)chloride as a model for enzymic degradation of lignin. The C-C bond of the lignin model compound 1,2-bis(4-ethoxy-3-methoxyphenyl) propane-1,3-diol was oxidatively cleaved by catalysis of iron-porphyrins in the presence of tert-butylhydroperoxide or iodosylbenzene at a room temperature. The products formed after complete oxidation of the substrate were identified as 4-O-ethylvanillin, alpha-hydroxy-4-ethoxy-3-methoxyacetophenone, 4-O-ethylvanillic acid, 4-ethoxy-3-methoxyphenylglycol, 4-ethoxy-3-methoxy-alpha-(4-ethoxy-3-methoxyphenyl)-beta-hydroxypropi ophenone and formaldehyde. PMID:6477560

Shimada, M; Habe, T; Umezawa, T; Higuchi, T; Okamoto, T

1984-08-16

15

C=C Bond Formation  

NASA Astrophysics Data System (ADS)

The material presented in this chapter describes the general methodology used for the preparation of unsaturated sugars. The `older' methods (i. e. those being developed since at least the 1950s) which are still very useful and have general application are also presented but they are illustrated by newer examples. The direct formation of the double bond(s) is emphasized, but the methodology based on the rearrangement of unsaturated sugars into other olefinic carbohydrates is also reviewed.

Jarosz, S?awomir; Nowogródzki, Marcin

16

C-C bond formation in diiron complexes.  

PubMed

The growing effort to design new sustainable synthetic methodologies, based on readily available and environmentally friendly transition metals, has boosted research on iron complexes. This review article focuses on C-C-bond-forming reactions occurring at bridging ligands in diiron complexes, aimed at evidencing distinctive aspects and advantages associated with the presence of two adjacent iron centres. A number of diiron-mediated C-C-bond-forming reactions reported in the literature, including nucleophilic and electrophilic additions and insertion and cycloaddition reactions, have been accumulated over the years, which, together with more recent developments, indicate that diiron complexes might provide promising alternatives to precious metals in the challenging field of metal-promoted C-C bond formation. PMID:22760971

Mazzoni, Rita; Salmi, Mauro; Zanotti, Valerio

2012-07-03

17

A hydroacylation-triggered carbon--carbon triple bond cleavage in alkynes via retro-Mannich type fragmentation.  

PubMed

The carbon-carbon triple bond in alkyne is cleaved via hydroacylation followed by retro-Mannich type fragmentation in the presence of aldehyde, which triggers a successive C-C bond cleavage. PMID:12785769

Lee, Dae-Yon; Hong, Boo-Sun; Cho, Eung-Goo; Lee, Hyuk; Jun, Chul-Ho

2003-05-28

18

A C-C bonded phenoxyl radical dimer with a zero bond dissociation free energy.  

PubMed

The 2,6-di-tert-butyl-4-methoxyphenoxyl radical is shown to dimerize in solution and in the solid state. The X-ray crystal structure of the dimer, the first for a para-coupled phenoxyl radical, revealed a bond length of 1.6055(23) Å for the C4-C4a bond. This is significantly longer than typical C-C bonds. Solution equilibrium studies using both optical and IR spectroscopies showed 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 gave an exceptionally small bond dissociation enthalpy (BDE) of 6.1 ± 0.5 kcal mol(-1). To our knowledge, this is the smallest 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

2013-08-23

19

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

20

Activation of Propane C-H and C-C Bonds by Gas-Phase Pt Atom: A Theoretical Study  

PubMed Central

The reaction mechanism of the gas-phase Pt atom with C3H8 has been systematically investigated on the singlet and triplet potential energy surfaces at CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz level. Pt atom prefers the attack of primary over secondary C-H bonds in propane. For the Pt + C3H8 reaction, the major and minor reaction channels lead to PtC3H6 + H2 and PtCH2 + C2H6, respectively, whereas the possibility to form products PtC2H4 + CH4 is so small that it can be neglected. The minimal energy reaction pathway for the formation of PtC3H6 + H2, involving one spin inversion, prefers to start at the triplet state and afterward proceed along the singlet state. The optimal C-C bond cleavages are assigned to C-H bond activation as the first step, followed by cleavage of a C-C bond. The C-H insertion intermediates are kinetically favored over the C-C insertion intermediates. From C-C to C-H oxidative insertion, the lowering of activation barrier is mainly caused by the more stabilizing transition state interaction ?E?int, which is the actual interaction energy between the deformed reactants in the transition state.

Li, Fang-Ming; Yang, Hua-Qing; Ju, Ting-Yong; Li, Xiang-Yuan; Hu, Chang-Wei

2012-01-01

21

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

22

Adsorption and Bonding Properties of Cleavage Surfaces of Bismuth Telluride  

Microsoft Academic Search

No measurable adsorption of oxygen, nitrogen, or carbon monoxide was found for any of the clean surfaces produced. Water vapor had a very low sticking coefficient of the order of 10-5. The results are discussed in terms of the chemical bonding of bismuth telluride. It is concluded that the atoms on cleavage faces are in a saturated-bond condition.

D. Haneman

1960-01-01

23

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

24

Cross-coupling reactions involving metal carbene: from c?c/c-C bond formation to C-h bond functionalization.  

PubMed

Cross-coupling reactions involving metal-carbene are emerging as a new type of carbon-carbon bond-forming reaction. The aim of this JOCSynopsis is to provide an overview of the most recent development of these reactions, focusing on the use of diazo compounds (or in situ formed diazo compounds from N-tosylhydrazones) as carbene precursors. In addition, the integration of this type of reaction with C-H bond functionalization is also surveyed. PMID:24134640

Liu, Zhenxing; Wang, Jianbo

2013-09-19

25

Metal-organic cooperative catalysis in C-H and C-C bond activation and its concurrent recovery.  

PubMed

The development of an efficient catalytic activation (cleavage) system for C-H and C-C bonds is an important challenge in organic synthesis, because these bonds comprise a variety of organic molecules such as natural products, petroleum oils, and polymers on the earth. Among many elegant approaches utilizing transition metals to activate C-H and C-C bonds facilely, chelation-assisted protocols based on the coordinating ability of an organic moiety have attracted great attention, though they have often suffered from the need for an intact coordinating group in a substrate. In this Account, we describe our entire efforts to activate C-H or C-C bonds adjacent to carbonyl groups by employing a new concept of metal-organic cooperative catalysis (MOCC), which enables the temporal installation of a 2-aminopyridyl group into common aldehydes or ketones in a catalytic way. Consequently, a series of new catalytic reactions such as alcohol hydroacylation, oxo-ester synthesis, C-C triple bond cleavage, hydrative dimerization of alkynes, and skeletal rearrangements of cyclic ketones was realized through MOCC. In particular, in the quest for an optimized MOCC system composed of a Wilkinson's catalyst (Ph 3P) 3RhCl and an organic catalyst (2-amino-3-picoline), surprising efficiency enhancements could be achieved when benzoic acid and aniline were introduced as promoters for the aldimine formation process. Furthermore, a notable accomplishment of C-C bond activation has been made using 2-amino-3-picoline as a temporary chelating auxiliary in the reactions of unstrained ketones with various terminal olefins and Wilkinson's catalyst. In the case of seven-membered cyclic ketones, an interesting ring contraction to five- or six-membered ones takes place through skeletal rearrangements initiated by the C-C bond activation of MOCC. On the other hand, the fundamental advances of these catalytic systems into recyclable processes could be achieved by immobilizing both metal and organic components using a hydrogen-bonded self-assembled system as a catalyst support. This catalyst-recovery system provides a homogeneous phase at high temperature during the reaction and a heterogeneous phase at room temperature after the reaction. The product could be separated conveniently from the self-assembly support system by decanting the upper layer. The immobilized catalysts of both 2-aminopyridine and rhodium metal species sustained high catalytic activity for up to the eight catalytic reactions. In conclusion, the successful incorporation of an organocatalytic cycle into a transition metal catalyzed reaction led us to find MOCC for C-H and C-C bond activation. In addition, the hydrogen-bonded self-assembled support has been developed for an efficient and effective recovery system of homogeneous catalysts and could be successful in immobilizing both metal and organic catalysts. PMID:18247521

Park, Young Jun; Park, Jung-Woo; Jun, Chul-Ho

2008-02-02

26

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

Microsoft Academic Search

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

Donald M. Camaioni; James A. Franz

1984-01-01

27

Density Functional Theory Study of Selectivity Considerations for C–C Versus C–O Bond Scission in Glycerol Decomposition on Pt(111)  

SciTech Connect

Glycerol decomposition via a combination of dehydrogenation, C–C bond scission, and C–O bond scission reactions is examined on Pt(111) with periodic Density Functional Theory (DFT) calculations. Building upon a previous study focused on C–C bond scission in glycerol, the current work presents a ?rst analysis of the competition between C–O and C–C bond cleavage in this reaction network. The thermochemistry of various species produced from C–O bond breaking in glycerol dehydrogenation intermediates is estimated using an extension of a previously introduced empirical correlation scheme, with parameters ?t to DFT calculations. Brønsted–Evans–Polanyi (BEP) relationships are then used to estimate the kinetics of C–O bond breaking. When combined with the previous results, the thermochemical and kinetic analyses imply that, while C–O bond scission may be competitive with C–C bond scission during the early stages of glycerol dehydrogenation, the overall rates are likely to be very low. Later in the dehydrogenation process, where rates will be much higher, transition states for C–C bond scission involving decarbonylation are much lower in energy than are the corresponding transition states for C–O bond breaking, implying that the selectivity for C–C scission will be high for glycerol decomposition on smooth platinum surfaces. It is anticipated that the correlation schemes described in this work will provide an ef?cient strategy for estimating thermochemical and kinetic energetics for a variety of elementary bond breaking processes on Pt(111) and may ultimately facilitate computational catalyst design for these and related catalytic processes.

Liu, Bin; Greeley, Jeffrey P.

2012-05-01

28

Stereoselective C-C bond formation catalysed by engineered carboxymethylproline synthases  

NASA Astrophysics Data System (ADS)

The reaction of enol(ate)s with electrophiles is used extensively in organic synthesis for stereoselective C-C bond formation. Protein-based catalysts have had comparatively limited application for the stereoselective formation of C-C bonds of choice via enolate chemistry. We describe protein engineering studies on 5-carboxymethylproline synthases, members of the crotonase superfamily, aimed at enabling stereoselective C-C bond formation leading to N-heterocycles via control of trisubstituted enolate intermediates. Active site substitutions, including at the oxyanion binding site, enable the production of substituted N-heterocycles in high diastereomeric excesses via stereocontrolled enolate formation and reaction. The results reveal the potential of the ubiquitous crotonase superfamily as adaptable catalysts for the control of enolate chemistry.

Hamed, Refaat B.; Gomez-Castellanos, J. Ruben; Thalhammer, Armin; Harding, Daniel; Ducho, Christian; Claridge, Timothy D. W.; Schofield, Christopher J.

2011-05-01

29

Selective cleavage of glycyl bonds by papaya proteinase IV.  

PubMed

The specificity of papaya proteinase IV (PPIV) has been examined with small substrates and a protein. With both classes of substrate, the enzyme shows a marked selectivity for cleaving glycyl bonds. Boc-Ala-Ala-Gly-NHPhNO2 is a convenient substrate for routine assays that discriminate well against chymopapain, the most common contaminant of PPIV. Sixteen cleavage points in beta-trypsin were identified, of which 13 are glycyl bonds. Tentative suggestions are made as to the reasons for lack of cleavage of some other glycyl bonds. The structure of PPIV has been modelled on that of papain, and we suggest that the replacement of the highly conserved residues Gly-65 and Gly-23 by arginine and glutamic acid, respectively, can account for the specificity of PPIV. PMID:2404797

Buttle, D J; Ritonja, A; Pearl, L H; Turk, V; Barrett, A J

1990-01-29

30

Acetate C–C bond formation and decomposition in the anaerobic world: the structure of a central enzyme and its key active-site metal cluster  

Microsoft Academic Search

The structure of carbon monoxide dehydrogenase\\/acetyl-coenzyme A synthase (CODH\\/ACS), a central enzyme in the anaerobic metabolism of acetyl-coenzyme A (acetyl-CoA), has been solved to a resolution of 2.2Å. The active-site metal cluster responsible for catalyzing acetyl C–C bond synthesis and cleavage, designated the A center, was identified as an Fe4S4 iron sulfur cluster with one of its cysteine thiolates acting

David A. Grahame

2003-01-01

31

Microbial aldolases as C–C bonding enzymes—unknown treasures and new developments  

Microsoft Academic Search

Aldolases are a specific group of lyases that catalyze the reversible stereoselective addition of a donor compound (nucleophile) onto an acceptor compound (electrophile). Whereas most aldolases are specific for their donor compound in the aldolization reaction, they often tolerate a wide range of aldehydes as acceptor compounds. C–C bonding by aldolases creates stereocenters in the resulting aldol products. This makes

Anne K. Samland; Georg A. Sprenger

2006-01-01

32

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

NASA Astrophysics Data System (ADS)

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.

Bower, John F.; Krische, Michael J.

33

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.

Bower, John F.; Krische, Michael J.

2011-01-01

34

Endgroup-assisted siloxane bond cleavage in the gas phase.  

PubMed

Unimolecular dissociation of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) generates SiC(5)H(16)NO(+) and SiC(5)H(14)N(+). The formation of SiC(5)H(16)NO(+) involves dissociation of a Si[bond]O bond and formation of an O[bond]H bond through rearrangement. The fragmentation mechanism was investigated utilizing ab initio calculations and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in combination with hydrogen/deuterium (H/D) exchange reactions. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) studies of the fully deuterated ion D(2)N(CH(2))(3)SiOSi(CH(2))(3)ND(3)(+) provided convincing evidence for a backbiting mechanism which involves hydrogen transfer from the terminal amine group to the oxygen to form a silanol-containing species. Theoretical calculations indicated decomposition of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) through a backbiting mechanism is the lowest energy decomposition channel, compared with other alternative routes. Two mechanisms were proposed for the fragmentation process which leads to the siloxane bond cleavage and the SORI-CID results of partially deuterated precursor ions suggest both mechanisms should be operative. Rearrangement to yield a silanol-containing product ion requires end groups possessing a labile hydrogen atom. Decomposition of disiloxane ions with end groups lacking labile hydrogen atoms yielded product ions from direct bond cleavages. PMID:12954172

Chen, Huiping

2003-09-01

35

Mechanisms of photoinduced C?-C? bond breakage in protonated aromatic amino acids  

NASA Astrophysics Data System (ADS)

Photoexcitation of protonated aromatic amino acids leads to C?-C? bond breakage among other channels. There are two pathways for the C?-C? bond breakage, one is a slow process (microseconds) that occurs after hydrogen loss from the electronically excited ion, whereas the other is a fast process (nanoseconds). In this paper, a comparative study of the fragmentation of four molecules shows that the presence of the carboxylic acid group is necessary for this fast fragmentation channel to occur. We suggest a mechanism based on light-induced electron transfer from the aromatic ring to the carboxylic acid, followed by a fast internal proton transfer from the ammonium group to the negatively charged carboxylic acid group. The ion formed is a biradical since the aromatic ring is ionized and the carbon of the COOH group has an unpaired electron. Breakage of the weak C?-C? bond gives two even-electron fragments and is expected to quickly occur. The present experimental results together with the ab initio calculations support the interpretation previously proposed.

Lucas, B.; Barat, M.; Fayeton, J. A.; Perot, M.; Jouvet, C.; Grégoire, G.; Brøndsted Nielsen, S.

2008-04-01

36

C-C, C-O and C-N bond formation via rhodium(III)-catalyzed oxidative C-H activation.  

PubMed

Rhodium(III)-catalyzed direct functionalization of C-H bonds under oxidative conditions leading to C-C, C-N, and C-O bond formation is reviewed. Various arene substrates bearing nitrogen and oxygen directing groups are covered in their coupling with unsaturated partners such as alkenes and alkynes. The facile construction of C-E (E = C, N, S, or O) bonds makes Rh(III) catalysis an attractive step-economic approach to value-added molecules from readily available starting materials. Comparisons and contrasts between rhodium(III) and palladium(II)-catalyzed oxidative coupling are made. The remarkable diversity of structures accessible is demonstrated with various recent examples, with a proposed mechanism for each transformation being briefly summarized (critical review, 138 references). PMID:22377832

Song, Guoyong; Wang, Fen; Li, Xingwei

2012-02-29

37

Collision Induced Dissociation Products of Disulfide-Bonded Peptides: Ions Result from the Cleavage of More Than One Bond  

NASA Astrophysics Data System (ADS)

Disulfide bonds are a post-translational modification (PTM) that can be scrambled or shuffled to non-native bonds during recombinant expression, sample handling, or sample purification. Currently, mapping of disulfide bonds is not easy because of various sample requirements and data analysis difficulties. One step towards facilitating this difficult work is developing a better understanding of how disulfide-bonded peptides fragment during collision induced dissociation (CID). Most automated analysis algorithms function based on the assumption that the preponderance of product ions observed during the dissociation of disulfide-bonded peptides result from the cleavage of just one peptide bond, and in this report we tested that assumption by extensively analyzing the product ions generated when several disulfide-bonded peptides are subjected to CID on a quadrupole time of flight (QTOF) instrument. We found that one of the most common types of product ions generated resulted from two peptide bond cleavages, or a double cleavage. We found that for several of the disulfide-bonded peptides analyzed, the number of double cleavage product ions outnumbered those of single cleavages. The influence of charge state and precursor ion size was investigated, to determine if those parameters dictated the amount of double cleavage product ions formed. It was found in this sample set that no strong correlation existed between the charge state or peptide size and the portion of product ions assigned as double cleavages. These data show that these ions could account for many of the product ions detected in CID data of disulfide bonded peptides. We also showed the utility of double cleavage product ions on a peptide with multiple cysteines present. Double cleavage products were able to fully characterize the bonding pattern of each cysteine where typical single b/ y cleavage products could not.

Clark, Daniel F.; Go, Eden P.; Toumi, Melinda L.; Desaire, Heather

2011-03-01

38

Molecular geometry in fullerene C60: A direct determination of the bond-length difference dC-C-dC=C  

NASA Astrophysics Data System (ADS)

The molecular structure factor of fullerene C60 has recently been determined with a high accuracy using neutron scattering over a large momentum-transfer range. The two bond lengths dC-C and dC=C have been measured with a precision of 10-3 Å. As the temperature is lowered from 295 K down to 4 K, the single bond is elongated and the double bond shortened, but the radius of the sphere remains almost unchanged. Unfortunately, the covariance matrix obtained from the fit shows that the two lengths are strongly correlated; this means that any error made on the determination of one bond length is compensated by an opposite error on the other bond length without significantly increasing the ?2 and, thus, the exactness of the reported temperature effect could be argued against. We present in this paper an alternative way of processing the data which allows us to remove almost completely the coupling, using an appropriate change of variables. The temperature effect is confirmed and resolved with a better accuracy.

Damay, P.; Leclercq, F.

1994-03-01

39

a Computational Investigation of cC3H2...HX(X = F, Cl, Br) H-Bonded Complexes  

Microsoft Academic Search

Cyclopropenylidene (c-C3H2) is of significant importance in interstellar chemistry and synthetic chemistry (e.g., transition metal and organic catalysis). Because of its peculiar structure, c-C3H2 can act as a hydrogen-bond donor or acceptor. In order to gain insight into this feature, the ground-state potential energy surfaces of singlet c-C3H2 complexed with hydrogen halides HX (X = F, Cl, Br) have been

Pradeep R. Varadwaj; Arpita Varadwaj; Gilles H. Peslherbe

2011-01-01

40

Rhodium-Catalyzed C-C Bond Formation via Heteroatom-Directed C-H Bond Activation  

SciTech Connect

Once considered the 'holy grail' of organometallic chemistry, synthetically useful reactions employing C-H bond activation have increasingly been developed and applied to natural product and drug synthesis over the past decade. The ubiquity and relative low cost of hydrocarbons makes C-H bond functionalization an attractive alternative to classical C-C bond forming reactions such as cross-coupling, which require organohalides and organometallic reagents. In addition to providing an atom economical alternative to standard cross - coupling strategies, C-H bond functionalization also reduces the production of toxic by-products, thereby contributing to the growing field of reactions with decreased environmental impact. In the area of C-C bond forming reactions that proceed via a C-H activation mechanism, rhodium catalysts stand out for their functional group tolerance and wide range of synthetic utility. Over the course of the last decade, many Rh-catalyzed methods for heteroatom-directed C-H bond functionalization have been reported and will be the focus of this review. Material appearing in the literature prior to 2001 has been reviewed previously and will only be introduced as background when necessary. The synthesis of complex molecules from relatively simple precursors has long been a goal for many organic chemists. The ability to selectively functionalize a molecule with minimal pre-activation can streamline syntheses and expand the opportunities to explore the utility of complex molecules in areas ranging from the pharmaceutical industry to materials science. Indeed, the issue of selectivity is paramount in the development of all C-H bond functionalization methods. Several groups have developed elegant approaches towards achieving selectivity in molecules that possess many sterically and electronically similar C-H bonds. Many of these approaches are discussed in detail in the accompanying articles in this special issue of Chemical Reviews. One approach that has seen widespread success involves the use of a proximal heteroatom that serves as a directing group for the selective functionalization of a specific C-H bond. In a survey of examples of heteroatom-directed Rh catalysis, two mechanistically distinct reaction pathways are revealed. In one case, the heteroatom acts as a chelator to bind the Rh catalyst, facilitating reactivity at a proximal site. In this case, the formation of a five-membered metallacycle provides a favorable driving force in inducing reactivity at the desired location. In the other case, the heteroatom initially coordinates the Rh catalyst and then acts to stabilize the formation of a metal-carbon bond at a proximal site. A true test of the utility of a synthetic method is in its application to the synthesis of natural products or complex molecules. Several groups have demonstrated the applicability of C-H bond functionalization reactions towards complex molecule synthesis. Target-oriented synthesis provides a platform to test the effectiveness of a method in unique chemical and steric environments. In this respect, Rh-catalyzed methods for C-H bond functionalization stand out, with several syntheses being described in the literature that utilize C-H bond functionalization in a key step. These syntheses are highlighted following the discussion of the method they employ.

Colby, Denise; Bergman, Robert; Ellman, Jonathan

2010-05-13

41

Palladium(II)-catalyzed dehydrogenative cross-coupling between two C(sp3)-H bonds: unexpected C=C bond formation.  

PubMed

C=C bond construction: A palladium-catalyzed oxidative C(sp3)-H/C(sp3)-H cross-coupling is shown to forge C=C bonds rather than C(sp3)-C(sp3) bonds through reactions of indolin-2-ones or benzofuran-2-ones with O-benzoyl hydroxylamines in the absence of an added oxidant. PMID:23780778

Li, Gaocan; Qian, Shengyou; Wang, Chunxia; You, Jingsong

2013-06-18

42

Acetate C-C bond formation and decomposition in the anaerobic world: the structure of a central enzyme and its key active-site metal cluster.  

PubMed

The structure of carbon monoxide dehydrogenase/acetyl-coenzyme A synthase (CODH/ACS), a central enzyme in the anaerobic metabolism of acetyl-coenzyme A (acetyl-CoA), has been solved to a resolution of 2.2A. The active-site metal cluster responsible for catalyzing acetyl C-C bond synthesis and cleavage, designated the A center, was identified as an Fe(4)S(4) iron sulfur cluster with one of its cysteine thiolates acting as a bridge to an adjacent binuclear metal site. Nickel was found at one position in the binuclear site and the other metal was indicated to be copper - a surprising result, implying a previously unrecognized role for copper. Details of the A center provided new insight into the unusual organometallic mechanism of acetyl C-C bond formation and cleavage, with substantial conformational changes indicated for binding of the large methylcorrinoid protein substrate, and a unique intramolecular channel acting to contain carbon monoxide within the protein and transfer it to the site needed for acetyl-CoA synthesis. PMID:12765830

Grahame, David A

2003-05-01

43

Recent advances in organocatalytic methods for asymmetric c?c bond formation.  

PubMed

Beyond a doubt organocatalysis belongs to the most exciting and innovative chapters of organic chemistry today. Organocatalysis has emerged not only as a complement to metal-catalyzed reactions and to biocatalysis over the last decade, but also provides new asymmetric organocatalyzed reactions that cannot be accomplished by metal- or biocatalyzed reactions so far. A large number of organocatalytic processes are already well established in organic synthesis. Nevertheless, the number of publications in this field is still on the increase; new important results are produced constantly. This review gives a detailed overview of the latest developments and main streams in organocatalyzed asymmetric C?C bond formation processes of the last three years. It is intended to outline the most important current findings focused on especially new synthetic methodologies. PMID:24115407

Scheffler, Ulf; Mahrwald, Rainer

2013-09-20

44

Formation, preservation, and cleavage of the disulfide bond by vanadium.  

PubMed

Reaction of the disulfide [HpicanS](2) (HpicanS is the carboxamide based on picolinate (pic) and o-mercaptoaniline (anS); the [] brackets are used to denote disulfides) with [VOCl(2)(thf)(2)] leads to reductive scission of the disulfide bond and formation of the mixed-valence (V(IV)/V(V)) complex anion [(OVpicanS)(2)mu-O](-) (1), with the dianionic ligand coordinating through the pyridine-N atom, the deprotonated amide-N atom, and thiophenolate-S atom. Reductive cleavage of the SbondS bond is also observed as [VCl(2)(tmeda)(2)] (tmeda=tetramethylethylenediamine) is treated with the disulfides [HsalanS](2) or [HvananS](2) (HsalanS and HvananS are the Schiff bases formed between o-mercaptoaniline and salicylaldehyde (Hsal) or vanillin (Hvan), respectively), yielding the V(III) complexes [VCl(tmeda)(salanS)] (2 a), or [VCl(tmeda)(vananS)] (2 b). The disulfide bond remains intact in the aerial reaction between [HsalanS](2) and [VCl(3)(thf)(3)] to yield the V(V) complex [VOCl[salanS](2)] (3), where (salanS)(2-) coordinates through the two phenolate and one of the imine functions. The S-S bond is also preserved as [VO(van)(2)] or [VO(nap)(2)] (Hnap=2-hydroxynaphthalene-1-carbaldehyde) is treated with bis(2-aminophenyl)disulfide, [anS](2), a reaction which is accompanied by condensation of the aldehyde and the diamine, and complexation of the resulting bis(Schiff bases) [HvananS](2) or [HnapanS](2) to form the complexes [VO[vananS](2)] (4 a) or [VO[napanS](2)] (4 b). In 4 a and 4 b, the phenolate and imine functions, and presumably also one of the disulfide-S atoms, coordinate to V(IV). 2-Mercaptophenyl-2'-pyridinecarboxamide (H(2)picanS) retains its identity in the presence of V(III); reaction between [VCl(3)(thf)(3)] and H(2)picanS yields [V[picanS](2)](-) (5). The dithiophenolate 2,6-bis(mercaptophenylthio)dimethylpyridine (6 a) is oxidized, mediated by VO(2+), to the bis(disulfide) octathiadiaza-cyclo-hexaeicosane 6 b. The relevance of these reactions for the speciation of vanadium under physiological conditions is addressed. [HNEt(3)]-1.0.5 NEt(3,) 3.3 CH(2)Cl(2), [HsalanS](2), [HNEt(3)]-5, and 6 b.4 THF have been characterized by X-ray diffraction analysis. PMID:12698438

Wang, Dongren; Behrens, Axel; Farahbakhsh, Mahin; Gätjens, Jessica; Rehder, Dieter

2003-04-14

45

Cleavage at met-x bonds by cyanogen bromide.  

PubMed

INTRODUCTIONSelective cleavage of a protein by cyanogen bromide generates a small number of relatively large peptide fragments. This protocol describes protein cleavage with cyanogen bromide, using ~100-fold molar excess of cyanogen bromide over the methionine. The reaction is performed under mild acidic conditions (e.g., 70% formic acid), under nitrogen, and in the dark. All reagents are volatile and can be readily removed by lyophilization. PMID:21357041

Simpson, Richard J

2007-03-01

46

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

47

Total Synthesis of Bryostatin 7 via C-C Bond Forming Hydrogenation: Merged Redox-Construction Events for Synthetic Efficiency  

PubMed Central

The marine macrolide bryostatin 7 is prepared in 20 steps (longest linear sequence) and 36 total steps. A total of 5 C-C bonds are formed using hydrogenative methods. The present approach represents the most concise synthesis of any bryostatin reported, to date, setting the stage for practical syntheses of simplified functional analogues.

Lu, Yu; Woo, Sang Kook; Krische, Michael J.

2011-01-01

48

Unprecedented Construction of C?C Double Bonds via Ir-Catalyzed Dehydrogenative and Dehydrative Cross-Couplings.  

PubMed

Unprecedented constructions of C?C double bonds have been achieved by Ir-catalyzed intramolecular dehydrogenative and dehydrative cross-coupling of tertiary amines and ketones. The reactions are proposed to proceed via an Ir-mediated C-H activation mechanism. PMID:23621093

Nie, Shao-Zhen; Sun, Xiang; Wei, Wen-Tao; Zhang, Xue-Jing; Yan, Ming; Xiao, Jian-Liang

2013-04-26

49

Enhancement of cyanogen bromide cleavage yields for methionyl-serine and methionyl-threonine peptide bonds.  

PubMed

Cyanogen bromide (CNBr) is a common chemical used to hydrolyze peptide bonds C-terminal to methionine residues in peptides and proteins. In most cases, the efficiency of this bond cleavage is greater than 90% except in situations where a serine or threonine residue follows methionine in the amino acid sequence. We have explored the mechanism of the methionyl-serine and methionyl-threonine CNBr cleavage inefficiencies and have developed a simple methodology to more than double cleavage yields relative to standard literature conditions. This method entails increasing the concentration of water during the cleavage reaction either by reducing the formic acid concentration or by performing the cleavage in an acidic aqueous medium. This approach provides a more desirable methodology from the perspective of enhanced yields and greater ease of handling in cases of large-scale use. PMID:9887207

Kaiser, R; Metzka, L

1999-01-01

50

Concerted heavy-atom bond cleavage and proton and electron transfers illustrated by proton-assisted reductive cleavage of an O-O bond  

PubMed Central

Electron transfer may be concerted with proton transfer. It may also be concerted with the cleavage of a bond between heavy atoms. All three events may also be concerted. A model is presented to analyze the kinetics of these all-concerted reactions for homogeneous or electrochemical reduction or oxidation processes. It allows the estimation of the kinetic advantage that derives from the increase of the bond-breaking driving force resulting from the concerted proton transfer. Application of the model to the electrochemical reductive cleavage of the O–O bond of an organic peroxide in the presence of a proximal acid group illustrates the applicability of the model and provides an example demonstrating that electron transfer, heavy-atom bond breaking, and proton transfer may be all concerted. Such analyses are expected to be useful for the invention, analysis, and optimization of reactions involved in contemporary energy challenges as well as for the comprehension of major biochemical processes, a number of which involve electron and proton transfer together with cleavage of bonds between heavy atoms.

Costentin, Cyrille; Hajj, Viviane; Robert, Marc; Saveant, Jean-Michel; Tard, Cedric

2011-01-01

51

The Janus-faced role of external forces in mechanochemical disulfide bond cleavage.  

PubMed

Recent force microscopy measurements on the mechanically activated cleavage of a protein disulfide bond through reaction with hydroxide ions revealed that for forces greater than 0.5 nN, the acceleration of the reaction rate is substantially reduced. Here, using ab initio simulations, we trace this 'reactivity switch' back to a dual role played by the mechanical force, which leads to antagonistic effects. On the one hand, the force performs work on the system, and thereby accelerates the reaction. On the other hand, the force also induces a conformational distortion that involves the S-S-C-C dihedral angle, which drives the disulfide into a conformation that is shielded against nucleophilic attack because of steric hindrance. The discovery of force-induced conformational changes that steer chemical reactivity provides a new key concept that is expected to be relevant beyond this specific case, for example in understanding how 'disulfide switches' regulate protein function and for the rational design of mechanoresponsive materials. PMID:23881500

Dopieralski, Przemyslaw; Ribas-Arino, Jordi; Anjukandi, Padmesh; Krupicka, Martin; Kiss, Janos; Marx, Dominik

2013-06-16

52

Selective C(sp2)-C(sp) bond cleavage: the nitrogenation of alkynes to amides.  

PubMed

Breakthrough: A novel catalyzed direct highly selective C(sp2)-C(sp) bond functionalization of alkynes to amides has been developed. Nitrogenation is achieved by the highly selective C(sp2)-C(sp) bond cleavage of aryl-substituted alkynes. The oxidant-free and mild conditions and wide substrate scope make this method very practical. PMID:23804537

Qin, Chong; Feng, Peng; Ou, Yang; Shen, Tao; Wang, Teng; Jiao, Ning

2013-06-26

53

Efficient approach to 1,2-diazepines via formal diazomethylene insertion into the C-C bond of cyclobutenones.  

PubMed

Efficient monocyclic 1,2-diazepine formation via a tandem electrocyclization reaction of cyclobutenones with lithiodiazoacetate is demonstrated. The reaction proceeds through an oxy anion-accelerated 4?-ring opening of cyclobutene followed by an 8?-ring closure of the resultant oxy anion-substituted diazo-diene under mild conditions to furnish a 1,2-diazepine via formal diazomethylene insertion into the C-C bond of cyclobutenone. PMID:22721410

Sugimoto, Kenji; Hayashi, Rie; Nemoto, Hideo; Toyooka, Naoki; Matsuya, Yuji

2012-06-21

54

Ternary electrocatalysts for oxidizing ethanol to carbon dioxide: making ir capable of splitting C-C bond.  

PubMed

Splitting the C-C bond is the main obstacle to electrooxidation of ethanol (EOR) to CO(2). We recently demonstrated that the ternary PtRhSnO(2) electrocatalyst can accomplish that reaction at room temperature with Rh having a unique capability to split the C-C bond. In this article, we report the finding that Ir can be induced to split the C-C bond as a component of the ternary catalyst. We characterized and compared the properties of several carbon-supported nanoparticle (NP) electrocatalysts comprising a SnO(2) NP core decorated with multimetallic nanoislands (MM' = PtIr, PtRh, IrRh, PtIrRh) prepared using a seeded growth approach. An array of characterization techniques were employed to establish the composition and architecture of the synthesized MM'/SnO(2) NPs, while electrochemical and in situ infrared reflection absorption spectroscopy studies elucidated trends in activity and the nature of the reaction intermediates and products. Both EOR reactivity and selectivity toward CO(2) formation of several of these MM'/SnO(2)/C electrocatalysts are significantly higher compared to conventional Pt/C and Pt/SnO(2)/C catalysts. We demonstrate that the PtIr/SnO(2)/C catalyst with high Ir content shows outstanding catalytic properties with the most negative EOR onset potential and reasonably good selectivity toward ethanol complete oxidation to CO(2). PMID:23210450

Li, Meng; Cullen, David A; Sasaki, Kotaro; Marinkovic, Nebojsa S; More, Karren; Adzic, Radoslav R

2012-12-14

55

Predicting kinetically unstable c?c bonds from the ground-state properties of a molecule.  

PubMed

Determining the weakest link: A simple analysis pinpoints kinetically unstable carbon-carbon bonds. No prior knowledge of reaction pathways is necessary and the approach is particularly helpful for the prediction of novel molecules. krel =relaxed force constant, req =bond length. PMID:23946235

Markopoulos, Georgios; Grunenberg, Jörg

2013-08-14

56

N-CN bond cleavage of cyanamides by a transition-metal complex.  

PubMed

N-CN bond cleavage of cyanamides (R(2)N-CN) has been attained at room temperature in the reaction of R(2)N-CN with Cp(CO)(2)Fe(SiEt(3)). The mechanistic investigation revealed that silyl migration from Fe to CN nitrogen of cyanamide gave an N-silylated eta(2)-amidino iron complex, which was isolated and characterized by X-ray analysis. Catalytic N-CN bond cleavage was also attained using a methyl molybdenum complex under thermal conditions. PMID:19128169

Fukumoto, Kozo; Oya, Tsukuru; Itazaki, Masumi; Nakazawa, Hiroshi

2009-01-14

57

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.

Hassan, Abbas; Krische, Michael J.

2011-01-01

58

Switchable conductance in functionalized carbon nanotubes via reversible sidewall bond cleavage.  

PubMed

We propose several covalent functionalizations for carbon nanotubes that display switchable on/off conductance in metallic tubes. The switching action is achieved by reversible control of bond-cleavage chemistry in [1 + 2] cycloadditions via the sp(3) ? sp(2) rehybridization that it induces; this leads to remarkable changes of conductance even at very low degrees of functionalization. Reversible bond-cleavage chemistry is achieved by identifying addends that provide optimal compensation between the bond-preserving through-space ? orbital interactions with the tube against the bond-breaking strain energy of the cyclopropane moiety. Several strategies for real-time control, based on redox or hydrolysis reactions, cis-trans isomerization or excited-state proton transfer are proposed. Such designer functional groups would allow for the first time direct control of the electrical properties of metallic carbon nanotubes, with extensive applications in nanoscale devices. PMID:21591731

Li, Elise Y; Poilvert, Nicolas; Marzari, Nicola

2011-06-01

59

Endgroup-assisted siloxane bond cleavage in the gas phase  

Microsoft Academic Search

Unimolecular dissociation of H2N(CH2)3SiOSi(CH2)3NH3+ generates SiC5H16NO+ and SiC5H14N+. The formation of SiC5H16NO+ involves dissociation of a Si-O bond and formation of an O-H bond through rearrangement. The fragmentation mechanism was\\u000a investigated utilizing ab initio calculations and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in combination\\u000a with hydrogen\\/deuterium (H\\/D) exchange reactions. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID)\\u000a studies of the

Huiping Chen

2003-01-01

60

Ternary Electrocatalysts for Oxidizing Ethanol to Carbon Dioxide: Making Ir Capable of Splitting C-C bond  

SciTech Connect

Splitting the C-C bond is the main obstacle to electroxidation of ethanol (EOR) to CO2. We recently demonstrated that the ternary PtRhSnO2 electrocatalyst can accomplish that reaction at room temperature with Rh having a unique capability to split the C-C bond. In this article we report the finding that Ir can be induced to split the C-C bond as a component of the ternary catalyst. We synthesized, characterized and compared the properties of several ternary electrocatalysts. Carbon-supported nanoparticle (NP) electrocatalysts comprising a SnO2 NP core decorated with multi-metallic nanoislands (MM = PtIr, PtRh, IrRh, PtIrRh) were prepared using a seeded growth approach. An array of characterization techniques were employed to establish the composition and architecture of the synthesized MM /SnO2 NPs, while electrochemical and in situ infrared reflection absorption spectroscopy studies elucidated trends in activity and the nature of the reaction intermediates and products. Both EOR reactivity and selectivity towards CO2 formation of several of these MM /SnO2/C electrocatalysts are significantly higher compared to conventional Pt/C and Pt/SnO2/C catalysts. We demonstrate that the PtIr/SnO2/C catalyst with high Ir content shows outstanding catalytic property with the most negative EOR onset potential and reasonably good selectivity towards ethanol complete oxidation to CO2. PtRh/SnO2/C catalysts with a moderate Rh content exhibit the highest EOR selectivity, as deduced from infrared studies.

Li, Meng [Brookhaven National Laboratory (BNL); Cullen, David A [ORNL; Sasaki, Kotaro [Brookhaven National Laboratory (BNL); Marinkovic, N. [University of Delaware; More, Karren Leslie [ORNL; Adzic, Radoslav R. [Brookhaven National Laboratory (BNL)

2013-01-01

61

Rhodium(I)-catalyzed borylation of nitriles through the cleavage of carbon-cyano bonds.  

PubMed

The reaction of aryl cyanides with diboron in the presence of a rhodium/Xantphos catalyst and DABCO affords arylboronic esters via carbon-cyano bond cleavage. This unprecedented mode of reactivity for a borylrhodium species allows the regioselective introduction of a boryl group in a late stage of synthesis. PMID:22185487

Tobisu, Mamoru; Kinuta, Hirotaka; Kita, Yusuke; Rémond, Emmanuelle; Chatani, Naoto

2011-12-20

62

Protocols for the selective cleavage of carbon-sulfur bonds in coal. Technical report, March 1, 1992--May 30, 1992.  

National Technical Information Service (NTIS)

Chemical reactions that result in carbon-sulfur bond cleavage are an essential aspect of any protocol designed to remove organic sulfur from coal. Unfortunately, several classes of reactions that lead to carbon-sulfur bond cleavage are not well understood...

M. Bausch

1992-01-01

63

To protonate or alkylate? Stereoselective Brønsted acid catalysis of C-C bond formation using diazoalkanes.  

PubMed

A new means to activate diazoalkanes has been discovered and applied broadly over the past few years. Brønsted acids, both achiral and chiral, have been used to promote the formation of carbon-carbon and carbon-heteroatom bonds with a growing number of diazoalkane derivatives. Aside from their straightforward ability to build structural and stereochemical complexity in innovative new ways, these transformations are remarkable owing to their ability to skirt competitive diazo protonation--a reaction that has long been used to prepare esters efficiently and cleanly from carboxylic acids. In cases where achiral Brønsted acids are used, high diastereoselection can be achieved. Meanwhile, chiral Brønsted acids can deliver products with both high diastereo- and enantioselectivity. More recently, systems have emerged that combine Brønsted acids and either Lewis acids or transition metals to promote carbon-carbon bond formation from diazoalkanes. PMID:20209537

Johnston, Jeffrey N; Muchalski, Hubert; Troyer, Timothy L

2010-03-22

64

Borane B-C Bond Cleavage by a Low-Coordinate Iron Hydride Complex and N-N Bond Cleavage by the Hydridoborate Product  

PubMed Central

The iron(II) hydride dimers [LRFe(?-H)2FeLR] (LMe = 2,4-bis(2,6-diisopropylphenylimino) pent-3-yl; LtBu = 2,2,6,6-tetramethyl-3,5-bis(2,6-diisopropylphenylimino)hept-4-yl) abstract hydrocarbyl groups from BR?3 (R? = Et, Ph) to give LRFeR? and LRFe(?-H)2BR?2. Mechanistic studies with R = R? = Me are consistent with a process in which the hyride dimer opens one Fe-H bond, and subsequent B-H bond formation is concerted with dissociation of an Fe-H unit. Cleavage of boron-carbon bonds is likely to proceed at least in part from transient quaternary borate anions. In a separate bond-breaking reaction, LMeFe(?-H)2BEt2 reacts with N2H4 to eject H2 from the bridging hydrides and cleave the N-N bond in the diaminoborate complex LMeFe(?-NH2)2BEt2. These novel bond-breaking reactions are facilitated by the low coordination number at the iron(II) center.

Yu, Ying; Brennessel, William W.; Holland, Patrick L.

2008-01-01

65

Borane B-C Bond Cleavage by a Low-Coordinate Iron Hydride Complex and N-N Bond Cleavage by the Hydridoborate Product.  

PubMed

The iron(II) hydride dimers [L(R)Fe(?-H)(2)FeL(R)] (L(Me) = 2,4-bis(2,6-diisopropylphenylimino) pent-3-yl; L(tBu) = 2,2,6,6-tetramethyl-3,5-bis(2,6-diisopropylphenylimino)hept-4-yl) abstract hydrocarbyl groups from BR'(3) (R' = Et, Ph) to give L(R)FeR' and L(R)Fe(?-H)(2)BR'(2). Mechanistic studies with R = R' = Me are consistent with a process in which the hyride dimer opens one Fe-H bond, and subsequent B-H bond formation is concerted with dissociation of an Fe-H unit. Cleavage of boron-carbon bonds is likely to proceed at least in part from transient quaternary borate anions. In a separate bond-breaking reaction, L(Me)Fe(?-H)(2)BEt(2) reacts with N(2)H(4) to eject H(2) from the bridging hydrides and cleave the N-N bond in the diaminoborate complex L(Me)Fe(?-NH(2))(2)BEt(2). These novel bond-breaking reactions are facilitated by the low coordination number at the iron(II) center. PMID:18725998

Yu, Ying; Brennessel, William W; Holland, Patrick L

2007-01-01

66

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

67

Unusual photochemical C-N bond cleavage in the novel methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate.  

PubMed

The structure, preferred conformers, vibrational spectrum, and photochemical behavior of the novel azirine, methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate (MCMAC) were investigated in low-temperature matrixes and in the neat solid amorphous state by infrared spectroscopy and quantum-chemical calculations. Two conformers of the compound were observed in argon, krypton, and xenon matrixes, in agreement with the DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) theoretical calculations. Both conformers were found to exhibit the carboxylic ester group in the cis conformation, differing in the arrangement defined by the O=C-C-Cl dihedral angle (cis and trans, for Ct and Cc forms, respectively). The Ct conformer was found to be the most stable conformer in the gaseous phase as well as in both argon and krypton matrixes, whereas the more polar Cc conformer became the most stable form in the xenon matrix and in the neat solid amorphous phase. In situ broadband UV (lambda > 235 nm) excitation of matrix-isolated MCMAC led to azirine ring C-C and C-N bond cleavages, the latter process corresponding to the most efficient reaction channel. The photochemical cleavage of the C-N bond had never been previously observed in the case of aliphatic 2H-azirines. Two electron withdrawing substituents (methoxycarbonyl group and chlorine atom) are connected to the azirine ring in the novel MCMAC azirine. The simultaneous presence of these two groups accelerates intersystem crossing toward the triplet state where cleavage of the C-N bond takes place. The primary photoproducts resulting from the C-N and C-C ring-opening reactions were also found to undergo further photochemical decarbonylation or decarboxylation reactions. PMID:16805494

Gómez-Zavaglia, Andrea; Kaczor, Agnieszka; Cardoso, Ana L; Pinho e Melo, Teresa M V D; Fausto, Rui

2006-07-01

68

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

69

Bite angle effects of diphosphines in C-C and C-X bond forming cross coupling reactions.  

PubMed

Catalytic reactions of C-C and C-X bond formation are discussed in this critical review with particular emphasis on cross coupling reactions catalyzed by palladium and wide bite angle bidentate diphosphine ligands. Especially those studies have been collected that allow comparison of the ligand bite angles for the selected ligands: dppp, BINAP, dppf, DPEphos and Xantphos. Similarities with hydrocyanation and CO/ethene/MeOH reactions have been highlighted, while rhodium hydroformylation has been mentioned as a contrasting example, in which predictability is high and steric and electronic effects follow smooth trends. In palladium catalysis wide bite angles and bulkiness of the ligands facilitate generally the reductive elimination thus giving more efficient cross coupling catalysis (174 references). PMID:19421583

Birkholz, Mandy-Nicole; Freixa, Zoraida; van Leeuwen, Piet W N M

2009-02-10

70

Biodehalogenation: The kinetics and rates of the microbial cleavage of carbon-halogen bonds  

Microsoft Academic Search

Specific rate constants associated with defined molecular paths of carbon-halogen bond cleavage from a variety of alkyl halide substrates by six soil organisms are presented. Five aerobes (three pseudomonads, one methylotroph, and one flavobacterium) and one anaerobe (a methanogen) are compared. The rate constants were obtained with resting cells in phosphate buffer at pH 7.4 in the absence of nutrients

C. E. Castro

1993-01-01

71

Quantitative determination of double bond positions in unsaturated fatty acids after oxidative cleavage  

Microsoft Academic Search

The position and amt of unsaturation in fatty acids have been determined, especially in pure fractions of partially hydrogenated\\u000a fats. In developing a quantitative method for determination of ethylenic bonds in monounsaturated and polyunsaturated fatty\\u000a acids several procedures were combined. Key features include oxidative cleavage; recovery of cleaved acids as salts; and their\\u000a conversion to methyl, ethyl or butyl esters

E. P. Jones; V. L. Davison

1965-01-01

72

Unusual electrocyclic rearrangements with group 14 element compounds: reversible isomerization of a ?-aromatic digermyl complex with carbon-carbon and germanium-germanium multiple bond cleavage.  

PubMed

Reaction of a digermyne with cyclooctatetraene (cot) gave two isomeric products. A Ge(II) inverse sandwich is formed as the kinetic product, which was a result of complete Ge?Ge bond cleavage and the formation of a ?-bound cot ring. This isomerized in solution at room temperature over a period of 5 days to give the thermodynamic product, a tetracyclic diene-digermane, in which a single-bonded Ge-Ge moiety has inserted into a C?C bond of the cot carbocycle. Kinetic studies afforded an activation enthalpy (?H(‡)) and entropy (?S(‡)) of 14.9 kcal mol(-1) and -6.2 cal mol(-1) K(-1) respectively. Heating crystals of the thermodynamic product at ca. 120 °C cleanly regenerated the original inverse sandwich isomer. PMID:21155563

Summerscales, Owen T; Jiménez-Halla, J Oscar C; Merino, Gabriel; Power, Philip P

2010-12-14

73

Electrophile-induced C-C bond activation of vinylcyclopropanes for the synthesis of Z-alkylidenetetrahydrofurans.  

PubMed

We present a detailed study on the behavior of vinylcyclopropanes as masked donor-acceptor system toward the stereoselective synthesis of Z-alkylidenetetrahydrofurans. Results of bromenium catalyzed indirect activation of C-C bond of vinylcyclopropanes and concomitant cyclization to alkylidenetetrahydrofuran and other heterocycles have been discussed. The stereoselective formation of the Z-isomer is strongly controlled by the extent of destabilization of one of the gauche conformers of the vinylcyclopropane. The ring-opening/cyclization step was found to be stereospecific as in the case of DA cyclopropanes. The activation of the C-C bond leads to a tight-carbocation intermediate, which is evident from the complete retention of the stereochemistry. The retention of configuration has been established by a necessary control experiment that rules out the possibility of a double inversion pathway. The present results serve as direct stereochemical evidence in support of a tight ion-pair intermediate versus the controversial S(N)2 pathway. A 2D potential energy scan has been carried out at B3LYP/6-31G(d) level theory to obtain the relative energies of the conformers. The Z-selectivity observed has been explained on the basis of the relative population of the conformers and modeling the intermediate and transition state involved in the reaction at M06-2x/6-31+G(d) level. Energy profile for the cyclization step was modeled considering various possible pathways through which cyclization can happen. The methodology has been successfully demonstrated on vinylcyclobutanes as well. PMID:23176087

Ganesh, Venkataraman; Kundu, Taraknath; Chandrasekaran, Srinivasan

2012-12-18

74

Hydrogenolysis of cyclohexane over Ir/SiO(2) catalyst: a mechanistic study of carbon--carbon bond cleavage on metallic surfaces.  

PubMed

The hydrogenolysis of cyclohexane catalyzed by supported Ir/SiO(2) has been studied to get mechanistic information on the elementary steps of C--C bond cleavage for cyclic saturated hydrocarbons. The reaction was studied under conditions in which no dehydrogenation to benzene occurs. When a mixture of cyclohexane and H(2) flows over a Ir/SiO(2) catalyst at 200 degrees C and for a H(2)/cyclohexane ratio superior to 40, methane, ethane, propane, n-butane, n-pentane, and n-hexane are identified to be primary products. The hydrogenolysis of ethane and n-hexane has also been studied to clarify several mechanistic questions. To account for the primary products in the above reactions, a mechanism is proposed in which the key step of the carbon--carbon bond cleavage occurs via concerted electronic transfer in dimetallacyclopentane intermediate. The comparison of product distributions in the hydrogenolysis of cyclohexane and that observed for n-hexane led to conclusions about the relative ease of carbon--carbon bond cleavage with respect to surface alkyl isomerization. PMID:11456765

Locatelli, F; Candy, J P; Didillon, B; Niccolai, G P; Uzio, D; Basset, J M

2001-02-28

75

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.

Mentinova, Marija; McLuckey, Scott A.

2011-01-01

76

Metal-bridging mechanism for O-O bond cleavage in cytochrome C oxidase.  

PubMed

Density functional theory (B3LYP) has been applied to large models of the Fe(II)-Cu(I) binuclear center in cytochrome oxidase, investigating the mechanism of O-O bond cleavage in the mixed valence form of the enzyme. To comply with experimental information, the O(2) molecule is assumed to be bridging between iron and copper during the O-O bond cleavage, leading to the formation of a ferryl-oxo group and a cupric hydroxide. In accord with previous suggestions, the calculations show that it is energetically feasible to take the fourth electron needed in this reaction from the tyrosine residue that is cross-linked to one of the copper ligands, resulting in the formation of a neutral tyrosyl radical. However, the calculations indicate that simultaneous transfer of an electron and a proton from the tyrosine to dioxygen during bond cleavage leads to a barrier more than 10 kcal/mol higher than that experimentally determined. This may be overcome in two ways. If an extra proton in the binuclear center assists in the mechanism, the calculated reaction barrier agrees with experiment. Alternatively, the fourth electron might initially be supplied by a residue in the vicinity other than the tyrosine. PMID:12924894

Blomberg, Margareta R A; Siegbahn, Per E M; Wikström, Mårten

2003-08-25

77

Entropic origin of cobalt-carbon bond cleavage catalysis in adenosylcobalamin-dependent ethanolamine ammonia-lyase.  

PubMed

Adenosylcobalamin-dependent enzymes accelerate the cleavage of the cobalt-carbon (Co-C) bond of the bound coenzyme by >10(10)-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, with temperature-step reaction initiation, starting from the enzyme-coenzyme-substrate ternary complex and (2)H-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 (2)H- and (1)H-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

2013-10-01

78

Two coordination polymers created via in situ ligand synthesis involving C-N and C-C bond formation.  

PubMed

We report the synthesis and crystal structures of two transition metal-based coordination polymers comprising ligand molecules not included in the original reaction mixtures but instead formed in situ during hydrothermal treatment. Zinc meso-iminodisuccinate hydrate (I), Zn(2)(C(8)H(7)NO(8)).0.57H(2)O, formed from zinc acetate and L-aspartic acid, and tetraaquanickel(II) 5,10-dioxo-5,10-dihydro-4,9-dioxa-pyrene-2,7-dicarboxylate (II), Ni(H(2)O)(4)(C(16)H(4)O(8)), formed from nickel acetate and 5-hydroxyisophthalic acid. We show that the formation of I takes place via a fumaric acid intermediate, while the formation of II requires the formation of a new C-C bond. The structure of I consists of weakly interacting sheets, while the structure of II consists of strongly hydrogen-bonded chains. Crystal data: for I, P2(1)/n (14), a = 10.073 A, b = 9.894 A, c = 12.053 A, beta = 105.605 degrees, V = 1156.87(13) A(3), Z = 4; for II, P1 (2), a = 5.011 A, b = 6.526 A, c = 12.305 A, alpha = 76.868 degrees, beta = 84.988 degrees, gamma = 87.619 degrees, V = 390.3(4) A(3), Z = 1. PMID:17854172

Feller, Russell K; Forster, Paul M; Wudl, Fred; Cheetham, Anthony K

2007-09-14

79

Four-Electron Donor Hemilabile n3-PPh3 Ligand that Binds Through a C = C Bond Rather than an Agostic C-H Interaction, and Displacement of the C = C by Methyl Iodide or Water  

SciTech Connect

Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+ BAr'4 [Ar' = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)2(?3 PPh3)Mo]+[BAr'4] . Spectroscopic and crystallographic data indicate that one C=C of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is four-electron donor ligand. Computations (DFT/B3LYP and MP2 on [Cp(CO)2(?3 PPh3)Mo]+ and [Cp(CO)2(?3 PH2Ph)Mo]+, and DFT/B3LYP on [Cp(CO)2(?3 PHtBuPh)Mo]+ and [Cp(CO)2(?3 PH2Ph)Nb]) provide further information on the bonding, and on the preference for bonding of the metal to the C=C bond rather than an agostic C H interaction found in many related complexes. The hemilabile C=C bond is readily displaced by CH3I or H2O, and crystal structures are reported for [Cp(CO)2(PPh3)Mo(ICH3)]+ and [Cp(CO)2(PPh3)Mo(OH2)]+. The equilibrium constant for [Cp(CO)2(?3 PPh3)Mo]+ + ICH3 to give [Cp(CO)2(PPh3)Mo(ICH3)]+ is Keq = 5.2 ? 102 M 1 in CD2Cl2 at 22 °C.

Cheng, Tan-Yun; Szalda, David J.; Hanson, Jonathan C.; Muckerman, J. T.; Bullock, R. Morris

2008-08-11

80

[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

81

Metabolic Engineering to Develop a Pathway for the Selective Cleavage of Carbon-Nitrogen Bonds  

SciTech Connect

The objective of the project was to develop a biochemical pathway for the selective cleavage of C-N bonds in molecules found in petroleum. Specifically, the development of a novel biochemical pathway for the selective cleavage of C-N bonds in carbazole was the focus of research in this project. The cleavage of the first C-N bond in carbazole is accomplished by the enzyme carbazole dioxygenase, that catalyzes the conversion of carbazole to 2-aminobiphenyl-2,3-diol. The genes encoding carbazole dioxygenase were cloned from Sphingomonas sp. GTIN11 and from Pseudomonas resinovorans CA10. Obtaining an enzyme capable of selectively cleaving the C-N bond in 2-aminobiphenyl-2,3-diol was the focus of much of the research in this project, however; no suitable enzyme was found. Project accomplishments included expressing the genes for carbazole dioxygenase in Rhodococcus erythropolis and Escherichia coli, development of gene expression vectors for Rhodococcus, and isolation of a Pseudomonas sp. strain GTIN-G4 that has the novel biochemical ability to replace one of the nitrogen-associated hydrogen atoms in 2-aminobiphenyl with formaldehyde. Rhodococcus cultures are capable of metabolizing a wide range of substrates, including hydrophobic substrates like petroleum, and may find widespread use in the development of biotechnology processes in the future. The results of this project will directly benefit the development of future biotechnology processes/projects employing Rhodococcus hosts. Three manuscripts were prepared and submitted for publication based on the data obtained in this project: (1) ''Formylation of 2-aminobiphenyl by Pseudomonas sp. strain GTIN-G4''; (2) ''Screening and Analysis of DNA Fragments that Show Promoter Activities in Rhodococcus erythropolis''; and (3) ''Microbial Biocatalyst Developments to Upgrade Fossil Fuels''.

John J. Kilbane II

2006-04-01

82

Characterization of adhesive bonded lap joints of C\\/C–SiC composite and Ti–6Al–4V alloy under varying conditions  

Microsoft Academic Search

The effect of glueline thickness, glueline length, etching time, temperature, exposure time and strain rate on C\\/C–SiC composite and Ti–6Al–4V alloy adherences were examined before and after adhesive bonding. The results show that the C\\/C–SiC adherence is more sensitive to strain rate and temperature than the Ti–6Al–4V adherence because of poor oxidizing resistance and high brittleness.

V. K Srivastava

2003-01-01

83

Pd-catalyzed C-3 functionalization of indolizines via C-H bond cleavage.  

PubMed

New transition metal-catalyzed methods for the arylation of indolizines by the direct cleavage of C-H bonds have been developed. A wide range of aryltrifluoroborate salts react with indolizines in the presence of Pd(OAc)(2) catalyst and AgOAc oxidant to give the arylated indolizines in high yields. Both electron-donating and electron-withdrawing groups perform smoothly while bromide and chlorine substituents are tolerated. In addition, the indolizines display similar reactivities in the Pd-catalyzed reaction with 3-phenylpropiolic acid to afford the corresponding C-3 alkynylated indolizines. These methods allow the direct functionalization of indolizines in one step. PMID:22868382

Zhao, Baoli

2012-08-07

84

Carbon-carbon bond cleavage in the photoionization of ethanol and 1-propanol clusters.  

PubMed

Tunable VUV laser was used to initiate the ion-molecule reactions in the clusters of ethanol and 1-propanol by photoionization in the region between 10.49 to 10.08 eV. Ionic products were detected by the time-of-flight mass spectrometer. In addition to the protonated clusters from proton transfer reactions, the products corresponding to beta carbon-carbon bond cleavage were found to be one of the major products for small sizes of clusters. A comparison with photoionization of methanol clusters and the results of ab initio calculation has been made. PMID:15267833

Tsai, Shang-Ting; Jiang, Jyh-Chiang; Lin, Ming-Fu; Lee, Yuan T; Ni, Chi-Kung

2004-05-15

85

Use of a Novel Fluorinated Organosulfur Compound To Isolate Bacteria Capable of Carbon-Sulfur Bond Cleavage  

Microsoft Academic Search

The vacuum residue fraction of heavy crudes contributes to the viscosity of these oils. Specific microbial cleavage of COS bonds in alkylsulfide bridges that form linkages in this fraction may result in dramatic viscosity reduction. To date, no bacterial strains have been shown conclusively to cleave COS bonds within alkyl chains. Screening for microbes that can perform this activity was

Jonathan D. Van Hamme; Phillip M. Fedorak; Julia M. Foght; Murray R. Gray; Heather D. Dettman

2004-01-01

86

A substrate-assisted mechanism of nucleophile activation in a ser-his-asp containing C-C bond hydrolase.  

PubMed

The meta-cleavage product (MCP) hydrolases utilize a Ser-His-Asp triad to hydrolyze a carbon-carbon bond. Hydrolysis of the MCP substrate has been proposed to proceed via an enol-to-keto tautomerization followed by a nucleophilic mechanism of catalysis. Ketonization involves an intermediate, ES(red), which possesses a remarkable bathochromically shifted absorption spectrum. We investigated the catalytic mechanism of the MCP hydrolases using DxnB2 from Sphingomonas wittichii RW1. Pre-steady-state kinetic and LC ESI/MS evaluation of the DxnB2-mediated hydrolysis of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid to 2-hydroxy-2,4-pentadienoic acid and benzoate support a nucleophilic mechanism catalysis. In DxnB2, the rate of ES(red) decay and product formation showed a solvent kinetic isotope effect of 2.5, indicating that a proton transfer reaction, assigned here to substrate ketonization, limits the rate of acylation. For a series of substituted MCPs, this rate was linearly dependent on MCP pKa2 (?nuc ? 1). Structural characterization of DxnB2 S105A:MCP complexes revealed that the catalytic histidine is displaced upon substrate-binding. The results provide evidence for enzyme-catalyzed ketonization in which the catalytic His-Asp pair does not play an essential role. The data further suggest that ES(red) represents a dianionic intermediate that acts as a general base to activate the serine nucleophile. This substrate-assisted mechanism of nucleophilic catalysis distinguishes MCP hydrolases from other serine hydrolases. PMID:24067021

Ruzzini, Antonio C; Bhowmik, Shiva; Ghosh, Subhangi; Yam, Katherine C; Bolin, Jeffrey T; Eltis, Lindsay D

2013-10-09

87

Unexpected mild C-N bond cleavage mediated by guanidine coordination to a niobium iminocarbamoyl complex.  

PubMed

The complex [Nb(NMe2)2{(NMe2)C[double bond, length as m-dash]N(t)Bu}{N(2,6-(i)Pr2C6H3)}] reacts with trialkylguanidines and undergoes a room temperature C-N bond cleavage of the iminocarbamoyl moiety. This reaction affords the guanidinate complexes [Nb(NMe2)2{N(2,6-(i)Pr2C6H3)}{(N(i)Pr)2C(NH(i)Pr)}] or [Nb(NMe2)2{N(2,6-(i)Pr2C6H3)}{(N(i)Pr)2C(NH(n)Bu)}] and free isocyanide. The first crystal structure of a niobium iminocarbamoyl complex is reported. PMID:23955374

Elorriaga, David; Carrillo-Hermosilla, Fernando; Antiñolo, Antonio; López-Solera, Isabel; Fernández-Galán, Rafael; Villaseñor, Elena

2013-08-29

88

C-C coupling reactions of superstrong CF3 groups with C(sp2)-H bonds: reactivity and synthetic utility of zero-valent niobium catalyst.  

PubMed

It was found that zero-valent niobium is an efficient catalyst for the intramolecular C-C coupling reactions of o-aryl and o-alkenyl alpha,alpha,alpha-trifluorotoluene derivatives. The superstrong C-F bonds of CF3 groups and neighboring C(sp2)-H bonds were doubly activated, and fluorenes and indenes were obtained in good yields. The niobium fluorocarbenoid species is proposed to be the key intermediate. PMID:18175305

Fuchibe, Kohei; Mitomi, Ken; Suzuki, Ryo; Akiyama, Takahiko

2008-02-01

89

Correlation between shape resonance energies and C-C bond length in carbon-containing molecules: Elastic electron scattering and carbon K-shell excitation by photons.  

National Technical Information Service (NTIS)

We document the correlation of shape resonance energies resulting from (i) elastic electron scattering and (ii) carbon K-shell excitation with ic bond order (C-C bond length) for C(sub 2)H(sub 2), C(sub 2)H(sub 4), C(sub 2)H(sub 6), C(sub 3)H(sub 8), C(su...

M. Kimura

1994-01-01

90

CO bond cleavage on supported nano-gold during low temperature oxidation.  

PubMed

The oxidation of CO by Au/Fe(2)O(3) and Au/ZnO catalysts is compared in the very early stages of the reaction using a temporal analysis of products (TAP) reactor. For Au/Fe(2)O(3) pre-dosing the catalyst with (18)O labelled water gives an unexpected evolution order for the labelled CO(2) product with the C(18)O(2) emerging first, whereas no temporal differentiation is found for Au/ZnO. High pressure XPS experiments are then used to show that CO bond cleavage does occur for model catalysts consisting of Au particles deposited on iron oxide films but not when deposited on ZnO films. DFT calculations, show that this observation requires carbon monoxide to dissociate in such a way that cleavage of the CO bond occurs along with dynamically co-adsorbed oxygen so that the overall process of Au oxidation and CO dissociation is energetically favourable. Our results show that for Au/Fe(2)O(3) there is a pathway for CO oxidation that involves atomic C and O surface species which operates along side the bicarbonate mechanism that is widely discussed in the literature. However, this minor pathway is absent for Au/ZnO. PMID:21152570

Carley, Albert F; Morgan, David J; Song, Nianxue; Roberts, M Wyn; Taylor, Stuart H; Bartley, Jonathan K; Willock, David J; Howard, Kara L; Hutchings, Graham J

2010-12-10

91

METABOLIC ENGINEERING TO DEVELOP A PATHWAY FOR THE SELECTIVE CLEAVAGE OF CARBON-NITROGEN BONDS  

SciTech Connect

The objective of the project is to develop biochemical pathways for the selective cleavage of C-N bonds in molecules found in petroleum. The initial phase of the project will focus on the isolation or development of an enzyme capable of cleaving the C-N bond in aromatic amides, specifically 2-aminobiphenyl. The objective of the second phase of the research will be to construct a biochemical pathway for the selective removal of nitrogen from carbazole by combining the carA genes from Sphingomonas sp. GTIN11 with the gene(s) encoding an appropriate amidase. The objective of the final phase of the project will be to develop derivative CN bond cleaving enzymes that have broader substrate ranges and to demonstrate the use of such strains to selectively remove nitrogen from petroleum. The project is on schedule and no major difficulties have been encountered. During the first year of the project (October, 2002-September, 2003) enrichment culture experiments have resulted in the isolation of promising cultures that may be capable of cleaving C-N bonds in aromatic amides, several amidase genes have been cloned and are currently undergoing directed evolution to obtain derivatives that can cleave C-N bonds in aromatic amides, and the carA genes from Sphingomonas sp. GTIN11, and Pseudomonas resinovorans CA10 were cloned in vectors capable of replicating in Escherichia coli. Future research will address expression of these genes in Rhodococcus erythropolis. Enrichment culture experiments and directed evolution experiments continue to be a main focus of research activity and further work is required to obtain an appropriate amidase that will selectively cleave C-N bonds in aromatic substrates. Once an appropriate amidase gene is obtained it must be combined with genes encoding an enzyme capable of converting carbazole to 2'aminobiphenyl-2,3-diol: specifically carA genes. The carA genes from two sources have been cloned and are ready for construction of C-N bond cleavage pathway. The construction of a new metabolic pathway to selectively remove nitrogen from carbazole and other molecules typically found in petroleum should lead to the development of a process to improve oil refinery efficiency by reducing the poisoning, by nitrogen, of catalysts used in the hydrotreating and catalytic cracking of petroleum.

John J. Kilbane III

2003-12-01

92

Protocols for the selective cleavage of carbon-sulfur bonds in coal. Technical report, December 1, 1992--February 28, 1993  

SciTech Connect

Chemical reactions that result in carbon-sulfur bond cleavage are an essential aspect of any protocol designed to remove organic sulfur from coal. Planned in the second year of our project Protocols for the Selective Cleavage of Carbon-Sulfur Bonds in Coal are investigations of reactions in which organic sulfur-containing coal model compounds are subjected to different conditions of temperature, solvent mixtures and radiation. Other investigations that will result in analyses of the likelihood of C-S bond cleavages resulting from various oxidative processes will also be undertaken. Summarized in this quarterly report are results of our investigations of the following topics: (a) desulfurization of coal model sulfones; (b) desulfurization of coal model sulfides; (c) photooxidation of organic sulfides; and (d) photolytic desulfurization of coal.

Bausch, M. [Southern Illinois Univ., Carbondale, IL (United States); Ho, K.K. [Illinois Clean Coal Inst., Carterville, IL (United States)

1993-05-01

93

Oxidative addition of the ethane C-C bond to Pd. An ab initio benchmark and DFT validation study.  

PubMed

We have computed a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the ethane C-C bond to the palladium atom and have used this to evaluate the performance of 24 popular density functionals, covering LDA, GGA, meta-GGA, and hybrid density functionals, for describing this reaction. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods [HF, MP2, CCSD, CCSD(T)] in combination with a hierarchical series of five Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Our best estimate of kinetic and thermodynamic parameters is -10.8 (-11.3) kcal/mol for the formation of the reactant complex, 19.4 (17.1) kcal/mol for the activation energy relative to the separate reactants, and -4.5 (-6.8) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal-d-electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta-GGA, and hybrid DFT, with mean absolute errors of 1.5 to 2.5 kcal/mol and errors in activation energies ranging from -0.2 to -3.2 kcal/mol. Interestingly, the well-known BLYP functional compares very reasonably with a slight underestimation of the overall barrier by -0.9 kcal/mol. For comparison, with B3LYP we arrive at an overestimation of the overall barrier by 5.8 kcal/mol. On the other hand, B3LYP performs excellently for the central barrier (i.e., relative to the reactant complex) which it underestimates by only -0.1 kcal/mol. PMID:15880815

De Jong, G Theodoor; Geerke, Daan P; Diefenbach, Axel; Solà, Miquel; Bickelhaupt, F Matthias

2005-07-30

94

The role of alkane coordination in CH bond cleavage at a Pt(II) center  

PubMed Central

The rates of CH bond activation for various alkanes by [(N–N)Pt(Me)(TFEd3)]+ (N N = ArNC(Me)C(Me)NAr; Ar = 3,5-di-tert-butylphenyl; TFE-d3 = CF3CD2OD) were studied. Both linear and cyclic alkanes give the corresponding alkene-hydride cation [(N–N)Pt(H)(alkene)]+ via (i) rate determining alkane coordination to form a CH ? complex, (ii) oxidative cleavage of the coordinated CH bond to give a platinum(IV) alkyl-methyl-hydride intermediate, (iii) reductive coupling to generate a methane ? complex, (iv) dissociation of methane, and (v) ?-H elimination to form the observed product. Second-order rate constants for cycloalkane activation (CnH2n), are proportional to the size of the ring (k ? n). For cyclohexane, the deuterium kinetic isotope effect (kH/kD) of 1.28 (5) is consistent with the proposed rate determining alkane coordination to form a CH ? complex. Statistical scrambling of the five hydrogens of the Pt-methyl and the coordinated methylene unit, via rapid, reversible steps ii and iii, and interchange of geminal CH bonds of the methane and cyclohexane CH ? adducts, is observed before loss of methane.

Chen, George S.; Labinger, Jay A.; Bercaw, John E.

2007-01-01

95

Energetics and site specificity of the homolytic C-H bond cleavage in benzenoid hydrocarbons: An ab initio electronic structure study  

SciTech Connect

Electronic structure calculations carried out at the BLYP/6-311G{sub **} level of theory accurately predict the dissociation energy of the C-H bond in benzene. The analogous energies of the homolytic C-H bond cleavage in the other nine polycyclic aromatic hydrocarbons (PAHs) are found to be governed almost entirely by steric factors, the hydrogens from congested regions of the PAHs being removed preferentially. The removal of hydrogens is accompanied by highly regular changes in the molecular geometries, namely a widening of the ipso bond angle by ca. 6.0{degree} and a concomitant shortening of the adjacent C-C bonds by ca. 0.02 A. These observations suggest an almost complete localization of the unpaired {sigma} electrons on single carbon atoms and the separation of the local {sigma} and {pi} effects in the aryl radicals under study. This localization is confirmed by the computed charges and spin populations of atoms in the phenyl, 1-naphthalenyl, and 2-naphthalenyl radicals. In contrast with their UHF counterparts, the UBLYP electronic wave functions are only mildly spin contaminated. 20 refs., 2 figs., 3 tabs.

Cioslowski, J.; Liu, G.; Martinov, M.; Piskorz, P.; Moncrieff, D. [Florida State Univ., Tallahassee, FL (United States)

1996-06-05

96

{{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

97

Disulfide bond cleavages observed in SORI-CIS of three nonapeptides complexed with divalent transition-metal cations  

Microsoft Academic Search

Tandem MS sequencing of peptides that contain a disulfide bond is often hampered when using a slow\\u000aheating technique.We show that complexation of a transition-metal ion with a disulfide-bridge-containing\\u000anonapeptide yields very rich tandem mass spectra, including fragments that involve the cleavage of the\\u000adisulfide bond up to 56% of the total product ion intensity. On the contrary, MS\\/MS of

R. Mihalca; Y. E. M. van der Burgt; A. J. R. Heck; R. M. A. Heeren

2007-01-01

98

Raman spectroscopic determination of the length, strength, compressibility, Debye temperature, elasticity, and force constant of the C-C bond in graphene.  

PubMed

From the perspective of bond relaxation and bond vibration, we have formulated the Raman phonon relaxation of graphene, under the stimuli of the number-of-layers, the uni-axial strain, the pressure, and the temperature, in terms of the response of the length and strength of the representative bond of the entire specimen to the applied stimuli. Theoretical unification of the measurements clarifies that: (i) the opposite trends of the Raman shifts, which are due to the number-of-layers reduction, of the G-peak shift and arises from the vibration of a pair of atoms, while the D- and the 2D-peak shifts involve the z-neighbor of a specific atom; (ii) the tensile strain-induced phonon softening and phonon-band splitting arise from the asymmetric response of the C(3v) bond geometry to the C(2v) uni-axial bond elongation; (iii) the thermal softening of the phonons originates from bond expansion and weakening; and (iv) the pressure stiffening of the phonons results from bond compression and work hardening. Reproduction of the measurements has led to quantitative information about the referential frequencies from which the Raman frequencies shift as well as the length, energy, force constant, Debye temperature, compressibility and elastic modulus of the C-C bond in graphene, which is of instrumental importance in the understanding of the unusual behavior of graphene. PMID:22105904

Yang, X X; Li, J W; Zhou, Z F; Wang, Y; Yang, L W; Zheng, W T; Sun, Chang Q

2011-11-21

99

Ir(I)-catalyzed synthesis of N-substituted pyridones from 2-alkoxypyridines via C-O bond cleavage.  

PubMed

A cationic Ir(I) complex-catalyzed O-to-N-alkyl migration in 2-alkoxypyridines bearing a secondary alkyl group on the oxygen atom by C-O bond cleavage is described. The present transformation gave various N-alkylpyridones in moderate to good yields. The addition of sodium acetate played a key role in suppressing ?-hydrogen elimination. PMID:23540513

Pan, Shiguang; Ryu, Naoto; Shibata, Takanori

2013-03-29

100

Silver-catalyzed amidation of benzoylformic acids with tertiary amines via selective carbon-nitrogen bond cleavage.  

PubMed

A novel approach towards the synthesis of ?-ketoamides using tertiary amines as nitrogen group sources via C-N bond cleavage has been developed. In the presence of Ag2CO3 and K2S2O8, ?-keto acids reacted with tertiary amines to afford the corresponding ?-ketoamides in good yields. PMID:23598925

Zhang, Xiaobin; Yang, Wenchao; Wang, Lei

2013-06-14

101

C-C bond coupling between the organometallic cations CH3Ag2+, CH3Cu2+ and CH3AgCu+ and allyliodide.  

PubMed

Electrospray ionisation on a mixture of AgNO(3) (in MeOH/H(2)O/acetic acid), (CH(3)CO(2))(2)Cu (in MeOH) and acetic acid (in MeOH) yields the metal carboxylate cations CH(3)CO(2)Ag(2)(+), CH(3)CO(2)AgCu(+) and CH(3)CO(2)Cu(2)(+). Collision induced dissociation of these carboxylate cations yields the organometallic cations CH(3)Ag(2)(+), CH(3)AgCu(+) and CH(3)Cu(2)(+). The ion-molecule reactions of these organometallic cations with allyliodide were studied in a quadrupole ion trap mass spectrometer. C-C bond coupling occurred to yield the ionic products Ag(2)I(+), AgCuI(+) and Cu(2)I(+). DFT calculations were carried out on these C-C bond coupling reactions. In all cases, the reactions are highly exothermic and involve initial coordination of the allyliodide to both metal atoms, with the iodine coordinating to one atom and the alkene moiety coordinating to the other. The overall mechanism of C-C bond coupling involves oxidative addition of the allyliodide followed by reductive elimination of 1-butene. PMID:19333507

Khairallah, George N; Waters, Tom; O'Hair, Richard A J

2009-02-20

102

Catalytic C-O Bond Cleavage of 2-Aryloxy-1-arylethanols and Its Application to the Depolymerization of Lignin Related Polymers  

PubMed Central

A ruthenium-catalyzed, redox neutral C-O bond cleavage of 2-aryloxy-1-arylethanols was developed that yields cleavage products in 62-98% isolated yield. This reaction is applicable to breaking the key ethereal bond found in lignin-related polymers. The bond transformation proceeds by a tandem dehydrogenation/reductive ether cleavage. Initial mechanistic investigations indicate that the ether cleavage is most likely an organometallic C-O activation. A catalytic depolymerization of a lignin-related polymer quantitatively yields the corresponding monomer with no added reagent.

Nichols, Jason M.; Bishop, Lee M.; Bergman, Robert G.; Ellman, Jonathan A.

2010-01-01

103

Ruthenium-catalyzed hydroarylation of methylenecyclopropanes through C-H bond cleavage: scope and mechanism.  

PubMed

Intermolecular hydroarylation reactions of highly strained methylenecyclopropanes 2-phenylmethylenecyclopropane (1), 2,2-diphenylmethylenecyclopropane (2), methylenespiropentane (3), bicyclopropylidene (4), (dicyclopropylmethylene)cyclopropane (5), and benzhydrylidenecyclopropane (6) through C-H bond functionalization of 2-phenylpyridine (7?a) and other arenes with directing groups were studied. The reaction was very sensitive to the substitution on the methylenecyclopropanes. Although these transformations involved (cyclopropylcarbinyl)-metal intermediates, substrates 1 and 4 furnished anti-Markovnikov hydroarylation products with complete conservation of all cyclopropane rings in 11-93?% yield, whereas starting materials 3 and 5 were inert toward hydroarylation. Methylenecyclopropane 6 formed the products of formal hydroarylation reactions of the longest distal C-C bond in the methylenecyclopropane moiety in high yield, and hydrocarbon 2 afforded mixtures of hydroarylated products in low yields with a predominance of compounds that retained the cyclopropane unit. As byproducts, Diels-Alder cycloadducts and self-reorganization products were obtained in several cases from substrates 1-3 and 5. The structures of the most important new products have been unambiguously determined by X-ray diffraction analyses. On the basis of the results of hydroarylation experiments with isotopically labeled 7?a-[D(5)], a plausible mechanistic rationale and a catalytic cycle for these unusual ruthenium-catalyzed hydroarylation reactions have been proposed. Arene-tethered ruthenium-phosphane complex 53, either isolated from the reaction mixture or independently prepared, did not show any catalytic activity. PMID:22887713

Ackermann, Lutz; Kozhushkov, Sergei I; Yufit, Dmitry S

2012-08-09

104

Preferential cleavage of SS and CS bonds in electron detachment dissociation and infrared multiphoton dissociation of disulfide-linked peptide anions  

NASA Astrophysics Data System (ADS)

Disulfide bonds generally show only limited cleavage in positive ion mode collision activated dissociation (CAD). However, it has been demonstrated that a reverse situation exists in negative ion mode in which preferential SS and CS bond cleavage occurs. Here, we show that electron detachment dissociation (EDD) and infrared multiphoton dissociation (IRMPD) of peptide anions containing disulfide linkages also result in preferential cleavage of SS and CS bonds. Resulting products are mainly radical ions in EDD whereas IRMPD produces even-electron product ions, as expected, thereby supporting different disulfide cleavage mechanisms for these two fragmentation processes. We also show that, in EDD, the presence of tryptophan can result in abundant side chain loss (129 Da), which effectively can compete with disulfide bond cleavage.

Kalli, Anastasia; Håkansson, Kristina

2007-05-01

105

Structural effects on the C-S bond cleavage in aryl tert-butyl sulfoxide radical cations.  

PubMed

The oxidation of a series of aryl tert-butyl sulfoxides (4-X-C6H4SOC(CH3)3: 1, X = OCH3; 2, X = CH3; 3, X = H; 4, X = Br) photosensitized by 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ(+)) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Products deriving from the C-S bond cleavage in the radical cations 1(+•)-4(+•) have been observed in the steady-state photolysis experiments. By laser irradiation, the formation of 3-CN-NMQ(•) (?(max) = 390 nm) and 1(+•)-4(+•) (?(max) = 500-620 nm) was observed. A first-order decay of the sulfoxide radical cations, attributable to C-S bond cleavage, was observed with fragmentation rate constants (k(f)) that decrease by increasing the electron donating power of the arylsulfinyl substituent from 1.8 × 10(6) s(-1) (4(+•)) to 2.3 × 10(5) s(-1) (1(+•)). DFT calculations showed that a significant fraction of the charge is delocalized in the tert-butyl group of the radical cations, thus explaining the small substituent effect on the C-S bond cleavage rate constants. Via application of the Marcus equation to the kinetic data, a very large value for the reorganization energy (? = 62 kcal mol(-1)) has been calculated for the C-S bond scission reaction in 1(+•)-4(+•). PMID:23581281

Cavattoni, Tullio; Del Giacco, Tiziana; Lanzalunga, Osvaldo; Mazzonna, Marco; Mencarelli, Paolo

2013-04-25

106

Pt-catalyzed C-C activation induced by C-H activation.  

PubMed

The catalytic cleavage of two C-C single bonds is achieved by treatment of the hydrocarbon substrate spiro[bicyclo[2.2.1]hept-2-ene-7,1'-cyclopropane] with Pt(II) catalysts such as (Me2bpy)PtPh(NTf2) (Me2bpy = 4,4'-dimethyl-2,2'-bipyridine, NTf2(-) = N(SO2CF3)2(-)). The surprising rearrangement product 1,2,4,7,7a-pentahydroindene is generated in good yield. The mechanism of C-C bond activation is investigated using NMR spectroscopy, electrospray ionization mass spectrometry, and deuterium labeling, along with density functional theory calculations. These studies support an unusual catalytic mechanism in which an initial masked C-H bond activation initiates successive C-C bond cleavage events. PMID:23962182

Bowring, Miriam A; Bergman, Robert G; Tilley, T Don

2013-08-20

107

Cu-catalyzed cross-dehydrogenative coupling: A versatile strategy for C-C bond formations via the oxidative activation of sp3 C-H bonds  

PubMed Central

Cu-catalyzed cross-dehydrogenative coupling (CDC) methodologies were developed based on the oxidative activation of sp3 C–H bonds adjacent to a nitrogen atom. Various sp, sp2, and sp3 C–H bonds of pronucleophiles were used in the Cu-catalyzed CDC reactions. Based on these results, the mechanisms of the CDC reactions also are discussed.

Li, Zhiping; Bohle, D. Scott; Li, Chao-Jun

2006-01-01

108

Trimethyl phosphite as a trap for alkoxy radicals formed from the ring opening of oxiranylcarbinyl radicals. Conversion to alkenes. Mechanistic applications to the study of C-C versus C-O ring cleavage.  

PubMed

Trimethyl phosphite, (MeO)(3)P, is introduced as an efficient and selective trap in oxiranylcarbinyl radical (2) systems, formed from haloepoxides 8-13 under thermal AIBN/n-Bu(3)SnH conditions at about 80 degrees C. Initially, the transformations of 8-13, in the absence of phosphite, to allyl alcohol 7 and/or vinyl ether 5 were measured quantitatively (Table 1). Structural variations in the intermediate oxiranylcarbinyl (2), allyloxy (3), and vinyloxycarbinyl (4) radicals involve influences of the thermodynamics and kinetics of the C-O (2 --> 3, k(1)) and C-C (2 --> 4, k(2)) radical scission processes and readily account for the changes in the amounts of product vinyl ether (5) and allyl alcohol (7) formed. Added (MeO)(3)P is inert to vinyloxycarbinyl radical 4 and selectively and rapidly traps allyloxy radical 3, diverting it to trimethyl phosphate and allyl radical 6. Allyl radicals (6) dimerize or are trapped by n-Bu(3)SnH to give alkenes, formed from haloepoxides 8, 9, and 13 in 69-95% yields. Intermediate vinyloxycarbinyl radicals (4), in the presence or absence of (MeO)(3)P, are trapped by n-Bu(3)SnH to give vinyl ethers (5). The concentrations of (MeO)(3)P and n-Bu(3)SnH were varied independently, and the amounts of phosphate, vinyl ether (5), and/or alkene from haloepoxides 10, 11, and 13 were carefully monitored. The results reflect readily understood influences of changes in the structures of radicals 2-4, particularly as they influence the C-O (k(1)) and C-C (k(2)) cleavages of intermediate oxiranylcarbinyl radical 2 and their reverse (k(-1), k(-2)). Diversion by (MeO)(3)P of allyloxy radicals (3) from haloepoxides 11 and 12 fulfills a prior prediction that under conditions closer to kinetic control, products of C-O scission, not just those of C-C scission, may result. Thus, for oxiranylcarbinyl radicals from haloepoxides 11, 12, and 13, C-O scission (k(1), 2 --> 3) competes readily with C-C cleavage (k(2), 2 --> 4), even though C-C scission is favored thermodynamically. PMID:12630880

Ding, Bangwei; Bentrude, Wesley G

2003-03-19

109

Electron-pair distributions in the C=C double bond. Effects of a push-pull substitution  

SciTech Connect

By application of a recently developed method, the populations of ionic and spin-alternant electron-pair distributions are calculated in ethylene, aminoethylene, nitroethylene, and 2-nitroethenamine, one of the simplest push-pull ethylenes. Occupation numbers, giving a measure of the existence of an ionic electron pair in one carbon atom and of a spin-alternant electron pair in the two carbon atoms of the ethylenic double bond, are investigated from ab initio MO wave functions. The effects of either an electron-donating or an electron-withdrawing group and the cooperative push-pull substitution are discussed.

Marcos, E.S.; Sanz, J.F. (Univ. of Sevilla (Spain)); Karafiloglou, P. (Aristotelian Univ. of Thessaloniki (Greece))

1990-04-05

110

Correlation between shape resonance energies and C-C bond length in carbon-containing molecules: Elastic electron scattering and carbon K-shell excitation by photons  

SciTech Connect

We document the correlation of shape resonance energies resulting from (i) elastic electron scattering and (ii) carbon K-shell excitation with ic bond order (C-C bond length) for C{sub 2}H{sub 2}, C{sub 2}H{sub 4}, C{sub 2}H{sub 6}, C{sub 3}H{sub 8}, C{sub 4}H{sub 10}, and C{sub 6}H{sub 6}. A relationship between K-shell {sigma} resonances and bond length was experimentally pointed out previously. These correlations are qualitatively interpreted to indicate that as molecular size increases (or as bond length increases), the configuration space available for valence electrons increases, reducing energy levels rather uniformly and mowing these correlations to emerge. The similarity of shape resonances in electron scattering and photoexcitation occurs because major events in the resonances take place slightly outside the molecular field and receive little influence from the inner structure of the molecule.

Kimura, Mineo [Argonne National Lab., IL (United States)]|[Rice Univ., Houston, TX (United States). Dept. of Physics

1994-06-01

111

Pathways and kinetics of methane and ethane C-H bond cleavage on PdO(101)  

NASA Astrophysics Data System (ADS)

We used conventional density functional theory (DFT) and dispersion-corrected DFT (DFT-D3) calculations to investigate C-H bond activation pathways for methane and ethane ?-complexes adsorbed on the PdO(101) surface. The DFT-D3 calculations predict lower and more physically realistic values of the apparent C-H bond cleavage barriers, which are defined relative to the gas-phase energy level, while giving nearly the same energy differences between stationary states as predicted by conventional DFT for a given reaction pathway. For the stable CH4 ?2 complex on PdO(101), DFT-D3 predicts that the C-H bond cleavage barriers are 55.2 and 16.1 kJ/mol relative to the initial molecularly adsorbed and gaseous states, respectively. We also predict that dehydrogenation of the resulting CH3 groups and conversion to CH3O species are significantly more energetically demanding than the initial C-H bond activation of CH4 on PdO(101). Using DFT-D3, we find that an ?2 and an ?1 ethane complex can undergo C-H bond cleavage on PdO(101) with intrinsic energy barriers that are similar to that of the methane complex, but with apparent barriers that are close to zero. We also investigated the dissociation kinetics of methane and ethane on PdO(101) using microkinetic models, with parameters derived from the DFT-D3 relaxed structures. We find that a so-called 3N - 2 model, in which two frustrated adsorbate motions are treated as free motions, predicts desorption pre-factors and alkane dissociation probabilities that agree well with estimates obtained from the literature. The microkinetic simulations demonstrate the importance of accurately describing entropic contributions in kinetic simulations of alkane dissociative chemisorption.

Antony, Abbin; Asthagiri, Aravind; Weaver, Jason F.

2013-09-01

112

Organolanthanide-induced C-C bond formation. Preparation and properties of monomeric lanthanide aldolates and enolates  

SciTech Connect

The reactivity of early-lanthanide carbyls Cp* {sub 2}LnCH(SiMe{sub 3}){sub 2} (1, Ln = La; 2, Ln = Ce) toward ketones has been studied. No reaction was observed with the bulky di-tert-butyl ketone. The corresponding lanthanum hydride [Cp* {sub 2}LaH]{sub 2} (3) is more reactive and afforded the addition product Cp*{sub 2}LaOCH(t-Bu){sub 2}{sup center-dot}O=C(t-Bu){sub 2} (4). Hydrogen transfer and formation of lanthanide aldolates Cp* {sub 2}LnOCMe{sub 2}C(=O)Me (5, Ln = Ce; 6, Ln =La) together with the protonated carbyl CH{sub 2}(SiMe{sub 3}){sub 2} were observed when 1 or 2 was treated with acetone. The molecular structure of the cerium analogue 5 was determined by X-ray diffraction and was found to be a monomeric bent metallocene compound with a {eta}{sup 2}-coordinated aldol fragment. Addition of 3-pentanone to the carbyls 1 and 2 did not result in C-C coupling but provided enolate-ketone adducts Cp*{sub 2}LnOC(Et)=C(H)MeO=CEt{sub 2} (7, Ln = La; 8, Ln = Ce). Reactions of the carbyls 1 and 2 with 4-methyl-5-hydroxy-5ethyl-3-heptanone yielded the enolate-ketone adducts as well and suggest that the differences in reactivity between acetone and 3-pentanone are thermodynamic and not kinetic in origin. 32 refs., 2 figs., 5 tabs.

Heeres, H.J.; Maters, M.; Teuben, J.H. [Univ. of Groningen (Netherlands)] [and others

1992-01-01

113

Collision Induced Dissociation Products of Disulfide-Bonded Peptides: Ions Result from the Cleavage of More Than One Bond  

Microsoft Academic Search

Disulfide bonds are a post-translational modification (PTM) that can be scrambled or shuffled to non-native bonds during recombinant\\u000a expression, sample handling, or sample purification. Currently, mapping of disulfide bonds is not easy because of various\\u000a sample requirements and data analysis difficulties. One step towards facilitating this difficult work is developing a better\\u000a understanding of how disulfide-bonded peptides fragment during collision

Daniel F. Clark; Eden P. Go; Melinda L. Toumi; Heather Desaire

2011-01-01

114

Rh(I)-catalyzed decarbonylation of diynones via C-C activation: orthogonal synthesis of conjugated diynes.  

PubMed

Utilization of C-C bond activation as a unique mode of reactivity for constructing C-C bonds provides new strategies for preparing important organic molecules. Development of a Rh(I)-catalyzed C-C activation of diynones to synthesize symmetrical and unsymmetrical conjugated diynes through decarbonylation is reported. This C-C cleavage strategy takes advantage of the innate reactivity of conjugated ynones without relying on any ring strain or auxiliary directing group. This alkynation method also has orthogonal properties compared to typical cross-coupling reactions. PMID:23586742

Dermenci, Alpay; Whittaker, Rachel E; Dong, Guangbin

2013-04-15

115

Disulfide bond cleavages observed in SORI-CID of three nonapeptides complexed with divalent transition-metal cations.  

PubMed

Tandem MS sequencing of peptides that contain a disulfide bond is often hampered when using a slow heating technique. We show that complexation of a transition-metal ion with a disulfide-bridge-containing nonapeptide yields very rich tandem mass spectra, including fragments that involve the cleavage of the disulfide bond up to 56% of the total product ion intensity. On the contrary, MS/MS of the corresponding protonated nonapeptides results predominantly in fragments from the region that is not involved in the disulfide bond. Eleven different combinations of three nonapeptides and three metal ions were measured using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) combined with sustained off-resonance irradiation collision induced dissociation (SORI-CID). All observed fragments are discussed with respect to four different types of product ions: neutral losses, b/y-fragmentation with and without the disulfide bond cleavage, and losses of internal amino acids without rupture of the disulfide bridge. Furthermore, it is shown that the observed complementary fragment pairs obtained from peptide-metal complexes can be used to determine the region of the binding site of the metal ion. This approach offers an efficient way to cleave disulfide-bridged structures using low energy MS/MS, which leads to increased sequence coverage and more confidence in peptide or protein assignments. PMID:17295413

Mihalca, Romulus; van der Burgt, Yuri E M; Heck, Albert J R; Heeren, Ron M A

2007-04-01

116

Transition metal-catalyzed carbon-carbon bond activation.  

PubMed

This tutorial review deals with recent developments in the activation of C-C bonds in organic molecules that have been catalyzed by transition metal complexes. Many chemists have devised a variety of strategies for C-C bond activation and significant progress has been made in this field over the past few decades. However, there remain only a few examples of the catalytic activation of C-C bonds, in spite of the potential use in organic synthesis, and most of the previously published reviews have dwelt mainly on the stoichiometric reactions. Consequently, this review will focus mainly on the catalytic reaction of C-C bond cleavage by homogeneous transition metal catalysts. The contents include cleavage of C-C bonds in strained and unstrained molecules, and cleavage of multiple C-C bonds such as C[triple bond]C triple bonds in alkynes. Multiple bond metathesis and heterogeneous systems are beyond the scope of this review, though they are also fascinating areas of C-C bond activation. In this review, the strategies and tactics for C-C bond activation will be explained. PMID:15592626

Jun, Chul-Ho

2004-11-04

117

A two-state reactivity model explains unusual kinetic isotope effect patterns in C-H bond cleavage by nonheme oxoiron(IV) complexes.  

PubMed

It's in the bond: The cleavage of C-H bonds by two related oxoiron(IV) complexes shows a range of kinetic isotope effect (KIE) values that exhibit an unusual dependence on the C-H bond strength. Large nonclassical KIEs are observed for bond strengths below 93 kcal mol(-1), while semiclassical values are found above this value (see graph, DHA = 9,10-dihydroanthracene). This nonintuitive behavior can be rationalized by invoking a two-state reactivity model. PMID:19130517

Klinker, Eric J; Shaik, Sason; Hirao, Hajime; Que, Lawrence

2009-01-01

118

Unique Reactivity of a Diphenyldisilane Unit Incorporated into the Bicyclic Ring System: Generation of a Disilanyllithium via Silicon-Phenyl Bond Cleavage with Lithium  

Microsoft Academic Search

A bicyclic diphenyldisilane bearing two tetramethylene tethers reacts with lithium to form predominantly the disilanyllithium as a result of Si-CPh bond fission, while the pentamethylene homolog undergoes ordinary Si-Si bond cleavage to afford the expected phenylsilyllithium. The compressed Si-Si bond incorporated in the bicyclic ring system may be kinetically stabilized (“compression effect”), resulting in the unusual Si-C bond fission. When

H. Tsuji; A. Toshimitsu; K. Tamao

2001-01-01

119

Protocols for the selective cleavage of carbon-sulfur bonds in coal. Technical report, December 1, 1991--February 29, 1992  

SciTech Connect

Planned in this project ``Protocols for the Selective Cleavage of Carbon-Sulfur Bonds in Coal`` are reactions in which organic free radicals and/or organic anions are allowed to react with physically cleaned Illinois coal and sulfur-containing coal model compounds. This quarterly report contains the results of photooxidation of coal model compounds, namely, benzyl phenyl sulfide, phenylsulfide, dibenzothiophene, benzothiophene and thiophene, in the presence of 9,10-dicyanoanthracene or anthracene. Quantitative product analysis for the matrix of reactions whereby the coal model compounds are subjected to various solvents and temperature changes is presented in this quarterly report. Further quantitative analyses of the products are being undertaken.

Bausch, M.

1992-08-01

120

Net heterolytic cleavage of B-H and B-B bonds across the N-Pd bond in a cationic (PNP)Pd fragment.  

PubMed

The use of weakly coordinating anions BAr(F)(4) (where Ar(F) = 3,5-(CF(3))(2)C(6)H(3)) and CB(11)H(12) allows one to access clean reactions of the [(PNP)Pd](+) fragment (PNP = bis(2-(i)Pr(2)P4-Me-phenyl)amido) with the B-H bond in catecholborane (CatBH) and catecholdiboron (CatBBCat). In both cases, a net heterolytic cleavage of B-H or B-B takes place, with the nitrogen atom of PNP being a recipient of a boryl fragment. The resultant products [(PN(BCat)P)PdH](+) (2) and [(PN(BCat)P)PdBCat](+) (3) were isolated as either BAr(F)(4) or CB(11)H(12) salts and fully characterized. They are susceptible to hydrolysis, with the B-N bond hydrolyzing selectively and rapidly at RT to give [(PN(H)P)PdH](+) (1) and [(PN(H)P)PdBCat](+) (4). Notably, 4 and 2 are isomers, but they do not interconvert even under thermolysis at 90 °C. The Pd-B bond in 4 can be further hydrolyzed more slowly, to give 1. On the other hand, a Pd-B bond was formed from the Pd-H bond in 2 by reaction with excess CatBH (and evolution of H(2)), producing 3. PMID:21819075

Zhu, Yanjun; Chen, Chun-Hsing; Fafard, Claudia M; Foxman, Bruce M; Ozerov, Oleg V

2011-08-05

121

Transition-metal-free oxidative coupling reactions for the formation of C-C and C-N bonds mediated by TEMPO and its derivatives.  

PubMed

The application of nitroxides for the development of new synthetic methods and their implementation in polymer chemistry, material science and beyond is one of the major research topics in our laboratory in the institute of organic chemistry at the WWU Münster. This short review focuses on our recent progress towards nitroxide-based transition-metal-free oxidative coupling reactions. The demand for organic surrogates for transition metals in such transformations is in our eyes unquestionable, since environmental and economic issues have become progressively more important in recent years. For this purpose, the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO) is shown to be a highly efficient oxidant for the homo- and cross-coupling of Grignard reagents. This powerful C-C bond forming strategy allows the generation of conjugated polymers from bifunctional Grignard reagents. Moreover, cross-coupling of alkynyl Grignard compounds and nitrones can be accomplished under aerobic atmosphere with catalytic amounts of TEMPO. It is also shown that TEMPO-derived N-oxoammonium salts can act as suitable oxidants for formation of C-N bonds between non-preactivated benzoxazoles and secondary amines under metal-free conditions. PMID:22871285

Murarka, Sandip; Wertz, Sebastian; Studer, Armido

2012-01-01

122

Protocols for the selective cleavage of carbon-sulfur bonds in coal. Final technical report, September 1, 1992--December 31, 1993.  

National Technical Information Service (NTIS)

Results of research pertaining to chemical reactions that aim to selectively cleave C-S bonds in model compounds as well as Illinois coal are summarized. Chemical reactions that result in carbon-sulfur bond cleavage are an essential aspect of any protocol...

M. Bausch

1993-01-01

123

Protocols for the selective cleavage of carbon-sulfur bonds in coal. [Quarterly] technical report, March 1, 1993--May 31, 1993  

SciTech Connect

Chemical reactions that result in carbon-sulfur bond cleavage are an essential aspect of any protocol designed to remove organic sulfur from coal. Planned in the second year of our project ``Protocols for the Selective Cleavage of Carbon-Sulfur Bonds in Coal`` are investigations of reactions in which organic sulfur-containing coal model compounds are subjected to different conditions of temperature, solvent mixtures and radiation. other investigations that will result in analyses of the likelihood of C-S bond cleavages resulting from various oxidative processes will also be undertaken. Summarized in this quarterly report are results of our investigations of the following topics: (a) desulfurization of coal model sulfones and sulfides; (b) photolytic desulfurization of coal; (c) differential scanning calorimetric experiments on photooxidized coal; and (d) discussions on C-S bond strengths in radical cations.

Bausch, M. [Southern Illinois Univ., Carbondale, IL (United States)

1993-09-01

124

P3-P3' Residues Flanking Scissile Bonds in Factor VIII Modulate Rates of Substrate Cleavage and Procofactor Activation by Thrombin†  

PubMed Central

Thrombin-catalyzed activation of factor VIII (FVIII) occurs through proteolysis at three P1 Arg residues: Arg372 and Arg740 in the FVIII heavy chain and Arg1689 in the FVIII light chain. Cleavage at the latter two sites is relatively fast compared with cleavage at Arg372, which appears rate limiting. Examination of the P3-P3? residues flanking each P1 site revealed that those sequences at Arg740 and Arg1689 are more optimal for thrombin cleavage than at Arg372, suggesting these sequences may impact reaction rates. Recombinant FVIII variants were prepared with mutations swapping scissile bond flanking sequences in the heavy chain individually and in combination with a second swap or with a P1 point mutation. Rates of generation of A1 and A3-C1-C2 subunits were determined by Western blotting and correlated with rates of cleavage at Arg372 and Arg1689, respectively. Rates of thrombin cleavage at Arg372 were increased ~10-fold and ~3-fold compared with WT FVIII when replaced with P3-P3? residues flanking Arg740 or Arg1689, respectively, and these values paralleled increased rates of A2 subunit generation and procofactor activation. Positioning of more optimal residues flanking Arg372 abrogated the need for initial cleavage at Arg740 to facilitate this step. These results show marked changes in cleavage rates correlate with the extent of cleavage-optimal residues flanking the scissile bond and modulate the mechanism for procofactor activation.

Newell-Caito, Jennifer L.; Griffiths, Amy E.; Fay, Philip J.

2012-01-01

125

Electron-capture induced dissociation of doubly charged dipeptides: on the neutral losses and N-C? bond cleavages.  

PubMed

Electron capture by doubly charged peptide cations leads to neutral losses in addition to N-C(?) bond cleavages that give c and z fragments. In this work we discuss the influence of amino acid sequence on hydrogen versus ammonia loss and the propensity for subsequent partial side-chain cleavage after ammonia loss to give w fragment ions. Experiments were done on two series of doubly protonated dipeptides, [XK+2H](2+) and [XR+2H](2+), where X is one of the twenty common amino acid residues, excluding aspartic acid (D), and K and R are lysine and arginine, respectively. While it was previously established that NH(3) is lost exclusively from the N-terminal ammonium group and not from side-chain ammonium groups, we find here that ammonia can be lost from guanidinium radicals as well. The ratio between H loss and NH(3) loss reveals some information on internal ionic hydrogen bonds and peptide conformation since proton sharing between the N-terminal ammonium group and a basic side chain decreases the probability for NH(3) loss due to a lower recombination energy and as a result reduced capture probability. The abundance of w ions was found to correlate with the reaction energy for their formation; highest yield was found for CK and lowest for AK and HK. The survival rate of charge-reduced species was higher for XR than for XK, which is likely linked to the formation of long-lived C(?) radicals in the latter case. The probability for N-C(?) bond cleavage is smaller on average for XR than for XK which indicates that hydrogen transfer from the ?-ammonium radical to the amide group triggers some of the cleavages, or is a result of the different distances between the amide group and the charges in XR and XK. Finally, our data support the previous concept that charge partitioning between c and z fragments can be explained by competition between the two fragments for the proton. PMID:21826349

Jensen, Camilla Skinnerup; Wyer, Jean Ann; Houmøller, Jørgen; Hvelplund, Preben; Nielsen, Steen Brøndsted

2011-08-08

126

Cyanogen bromide cleavage at methionine residues of polypeptides containing disulfide bonds.  

PubMed

A method for cleaving polypeptides at their methionine residues without affecting intramolecular disulfide bonds is described. This method may be applied for cleaving recombinant heterologous hybrid polypeptides with release of the interesting peptide. The method may also be applied to assign the correct positions of disulfide bonds in protein molecules. PMID:2662814

Villa, S; De Fazio, G; Canosi, U

1989-02-15

127

Use of a Novel Fluorinated Organosulfur Compound To Isolate Bacteria Capable of Carbon-Sulfur Bond Cleavage  

PubMed Central

The vacuum residue fraction of heavy crudes contributes to the viscosity of these oils. Specific microbial cleavage of C—S bonds in alkylsulfide bridges that form linkages in this fraction may result in dramatic viscosity reduction. To date, no bacterial strains have been shown conclusively to cleave C—S bonds within alkyl chains. Screening for microbes that can perform this activity was greatly facilitated by the use of a newly synthesized compound, bis-(3-pentafluorophenylpropyl)-sulfide (PFPS), as a novel sulfur source. The terminal pentafluorinated aromatic rings of PFPS preclude growth of aromatic ring-degrading bacteria but allow for selective enrichment of strains capable of cleaving C—S bonds. A unique bacterial strain, Rhodococcus sp. strain JVH1, that used PFPS as a sole sulfur source was isolated from an oil-contaminated environment. Gas chromatography-mass spectrometry analysis revealed that JVH1 oxidized PFPS to a sulfoxide and then a sulfone prior to cleaving the C—S bond to form an alcohol and, presumably, a sulfinate from which sulfur could be extracted for growth. Four known dibenzothiophene-desulfurizing strains, including Rhodococcus sp. strain IGTS8, were all unable to cleave the C—S bond in PFPS but could oxidize PFPS to the sulfone via the sulfoxide. Conversely, JVH1 was unable to oxidize dibenzothiophene but was able to use a variety of alkyl sulfides, in addition to PFPS, as sole sulfur sources. Overall, PFPS is an excellent tool for isolating bacteria capable of cleaving subterminal C—S bonds within alkyl chains. The type of desulfurization displayed by JVH1 differs significantly from previously described reaction results.

Van Hamme, Jonathan D.; Fedorak, Phillip M.; Foght, Julia M.; Gray, Murray R.; Dettman, Heather D.

2004-01-01

128

Atomic force microscopy imaging of TiO{sub 2} surfaces active for C-C bond formation reactions in ultrahigh vacuum  

SciTech Connect

TiO{sub 2}(001) single crystal surfaces active for a variety of different chemistries were examined using atomic force microscopy (AFM). C-C bond forming reactions previously identified on these surfaces include carboxylic acid ketonization, aldol condensation, reductive carbonyl coupling, and alkyne cyclotrimerization. The surfaces were prepared in ultrahigh vacuum (UHV) and examined by AFM in air. Surfaces examined included the (011)-faceted surface, (114)-faceted surface, and argon-ion-bombarded surfaces, as well as the mechanically polished single-crystal surface prior to treatment in UHV. The one unifying feature of all the images was their extreme flatness. Root-mean-square roughnesses were routinely less than 10 {Angstrom} in 500 x 500 nm scans. These same scans showed the surfaces to have surface areas exceeding that of an ideal flat surface by no more than 1.2%. Images of the polished surface revealed a variety of surface features, including polishing scratches and particle-like features. The argon-ion-bombarded surface and the faceted surfaces were composed of large flat plateaus ranging in size from 21 to 75 nm. The size of the plateaus was essentially the same for the ion bombarded surface and the (011)-faceted surface. The (114)-faceted surfaced exhibited slightly smaller plateau regions than the other surfaces. The images indicate that argon-ion bombardment, while disordering the surface and causing significant composition changes, does not lead to observable morphological changes on this scale. The relative uniformity of the surfaces examined is consistent with the selectivity of carbon-carbon bond-forming reactions that have been shown to take place on these surfaces. The images also reveal how the surface topography on the scale of the plateau structures observed is only slightly changed during the transformation of the surface unit cell structure from the (011)- to the (114)-faceted surface. 29 refs., 6 figs., 1 tab.

Watson, B.A.; Barteau, M.A. [Univ. of Delaware, Newark, DE (United States)

1994-06-01

129

Rapid cleavage of carbon-carbon and carbon-oxygen bonds in coal structures  

SciTech Connect

The cited illustrate that: (1) 1,2-Diarylethanes (which are probable sub-units in real coals) have central bonds sufficiently weakened to undergo homolysis at rates consistent with the dissolation of coals at 400C. (2) The presence of phenolic groups on diarylmethanes can make structures containing no weak carbon-carbon or carbon-oxygen bonds moderately reactive by way of a mechanism involving ionic tautomerization to the keto isomer followed by central bond homolysis. (3) The presence of additional hydroxy groups, and/or homogeneous or hetergeneous catalysts can amplify these effects such that strongly bonded diaryl methane structures can have half-lives of several minutes or less at 400C. The resulting rates are well into the region where these structural features could contribute to the rapid dissolution of coals at 400C. Thus, a combination of homogeneous ionic, free radical, and/or heterogeneously catalyzed reactions can account for rapid fragmentation of certain strongly bonded coal structures. It is not neccessary, at least on the basis of rate, to invoke exclusively the break-up of hydrogen-bonded structures or other non-radical depolymerization processes to explain rapid dissolution of coals. 8 refs., 1 fig., 1 tab. tab.

McMillen, D.F.

1981-01-01

130

Partial non-cleavage by cyanogen bromide of a methionine--cystine bond from human serum albumin and bovine alpha-lactalbumin.  

PubMed Central

When human albumin was treated with CNBr, a fragment designated D was obtained and attributed to the absence from some of the albumin molecules of methionine at position 123 [Lapresle & Doyen (1975) Biochem. J. 151, 637-643]. The present study shows that methionine-123 is converted into homoserine without cleavage of the subsequent methionine-cystine bond. With bovine alpha-lactalbumin, a further example of non-cleavage of a methionine-cystine bond with conversion of methionine into homoserine is reported. Images Fig. 1.

Doyen, N; Lapresle, C

1979-01-01

131

Direct evidence for ArO-S bond cleavage upon inactivation of Pseudomonas aeruginosa arylsulfatase by aryl sulfamates.  

PubMed

Pseudomonas aeruginosa arylsulfatase catalyses the cleavage of aryl sulfates and is an excellent model for human estrone sulfatase, which is implicated in hormone-dependent breast cancer. Aryl sulfamates are inactivators of sulfatases; however, little is known about their mechanism. We studied the inactivation of Pseudomonas aeruginosa arylsulfatase A by a range of aryl sulfamates, including the clinical agent 667COUMATE (STX64) used to inactivate estrone sulfatase. Inactivation was time dependent, irreversible, and active-site directed, consistent with a covalent modification at the active site. In terms of the kinetic parameters of inactivation k(inact) and K(i), K(i) values are in the micromolar to nanomolar range, and the inactivation half-life is less than 30 s. A Brønsted plot of k(inact)/K(i) has a steep slope (beta(lg) = -1.1), which implies that the transition state for the first irreversible chemical step of inactivation involves a high degree of charge transfer and cleavage of the ArO-S bond. Detection of the released phenol and titration of the residual activity showed the stoichiometry of inactivation to be in the range 3-6, with the greatest values found for the most effective inactivators. Thus, multiple sulfamoylation events appear to occur during the inactivation process. These data provide valuable insight into the mechanism of sulfatase inactivation by sulfamates. PMID:18288656

Bojarová, Pavla; Denehy, Emma; Walker, Ian; Loft, Karen; De Souza, David P; Woo, L W Lawrence; Potter, Barry V L; McConville, Malcolm J; Williams, Spencer J

2008-03-01

132

Ruthenium-Catalyzed Direct Arylations Through C–H Bond Cleavages  

Microsoft Academic Search

\\u000a Stoichiometric cycloruthenation reactions of substrates containing Lewis-basic functionalities set the stage for efficient\\u000a ruthenium-catalyzed C–H bond functionalization reactions. Thereby, selective addition reactions of C–H bonds across alkenes\\u000a or alkynes enabled atom-economical synthesis of substituted arenes. More recently, ruthenium-catalyzed direct arylation reactions\\u000a were examined, which display an unparalleled scope and, hence, represent economically and environmentally benign alternatives\\u000a to traditional cross-coupling chemistry.

Lutz Ackermann; Rubén Vicente

2010-01-01

133

Long live vinylidene! A new view of the H(2)C=C: --> HC triple bond CH rearrangement from ab initio molecular dynamics.  

PubMed

We present complete active space self-consistent field (CASSCF) ab initio molecular dynamics (AIMD) simulations of the preparation of the metastable species vinylidene, and its subsequent, highly exothermic isomerization to acetylene, via electron removal from vinylidene anion (D(2)C=C(-) --> D(2)C=C: --> DC triple bond CD). After equilibrating vinylidene anion-d(2) at either 600 +/- 300 K (slightly below the isomerization barrier) or 1440 K +/- 720 K (just above the isomerization barrier), we remove an electron to form a vibrationally excited singlet vinylidene-d(2) and follow its dynamical evolution for 1.0 ps. Remarkably, we find that none of the vinylidenes equilibrated at 600 K and only 20% of the vinylidenes equilibrated at 1440 K isomerized, suggesting average lifetimes >1 ps for vibrationally excited vinylidene-d(2). Since the anion and neutral vinylidene are structurally similar, and yet extremely different geometrically from the isomerization transition state (TS), neutral vinylidene is not formed near the TS so that it must live until it has sufficient instantaneous kinetic energy in the correct vibrational mode(s). The origin of the delay is explained via both orbital rearrangement and intramolecular vibrational energy redistribution (IVR) effects. Unique signatures of the isomerization dynamics are revealed in the anharmonic vibrational frequencies extracted from the AIMD, which should be observable by ultrafast vibrational spectroscopy and in fact are consistent with currently available experimental spectra. Most interestingly, of those trajectories that did isomerize, every one of them violated conventional transition-state theory by recrossing back to vinylidene multiple times, against conventional notions that expect highly exothermic reactions to be irreversible. The dynamical motion responsible for the multiple barrier recrossings involves strong mode-coupling between the vinylidene CD(2) rock and a local acetylene DCC bend mode that has been recently observed experimentally. The multiple barrier recrossings can be used, via a generalized definition of lifetime, to reconcile extremely disparate experimental estimates of vinylidene's lifetime (differing by at least 6 orders of magnitude). Last, a caveat: These results are constrained by the approximations inherent in the simulation (classical nuclear motion, neglect of rotation-vibration coupling, and restriction to C(s) symmetry); refinement of these predictions may be necessary when more exact simulations someday become feasible. PMID:11456576

Hayes, R L; Fattal, E; Govind, N; Carter, E A

2001-01-31

134

Protocols for the selective cleavage of carbon-sulfur bonds in coal. Final technical report, September 1, 1992--December 31, 1993  

SciTech Connect

Results of research pertaining to chemical reactions that aim to selectively cleave C-S bonds in model compounds as well as Illinois coal are summarized. Chemical reactions that result in carbon-sulfur bond cleavage are an essential aspect of any protocol designed to remove organic sulfur from coal. In the second year of the project ``Protocols for the Selective Cleavage of Carbon-Sulfur Bonds in Coal`` investigations of reactions in which organic sulfur-containing coal model compounds are subjected to different conditions of temperature, solvent mixtures, reagents, and radiation have been completed. A series of reactions have been undertaken in which physically cleaned Illinois coal has been subjected to many of the same reaction conditions that were shown, via the use of model sulfides, to result in substantial C-S bond cleavage and or sulfur oxidation. Therefore, summarized in this final report are results of the investigations of the photooxidation reactions of coal model sulfones and sulfides; the photolytic desulfurization of coal; and various other topics, including a summary of endeavors aimed at initiating C-S bond cleavage reactions using oxidation/chlorination/desulfurization protocols, and various tellurium reagents.

Bausch, M.

1993-12-31

135

Protocols for the selective cleavage of carbon-sulfur bonds in coal. Interim final technical report, September 1, 1992--August 31, 1993  

SciTech Connect

This report presents results of research pertaining to chemical reactions that aim to selectively cleave C-S bonds in model compounds as well as Illinois coal. Chemical reactions that result in carbon-sulfur bond cleavage are an essential aspect of any protocol designed to remove organic sulfur from coal. In the second year of the project {open_quotes}Protocols for the Selective Cleavage of Carbon-Sulfur Bonds in Coal, the author has completed investigations of reactions in which organic sulfur-containing coal model compounds are subjected to different conditions of temperature, solvent mixtures, reagents, and radiation. He has also undertaken a series of reactions in which physically cleaned Illinois coal has been subjected to many of the same reaction conditions that were shown, via the use of model sulfides, to result in substantial C-S bond cleavage and or sulfur oxidation. Therefore, summarized in this interim final report are results of the investigations of the photooxidation reactions of coal model sulfones and sulfides; the photolytic desulfurization of coal; and various other topics, including a summary of the endeavors aimed at initiating C-S bond cleavage reactions using oxidation/chlorination/desulfurization protocols, and various tellurium reagents. Important experiments remain to be completed on this project; therefore, efforts in these areas will continue through the end of calendar year 1993.

Bausch, M. [Southern Illinois Univ., Carbondale, IL (United States)

1993-12-31

136

Four-Electron-Donor Hemilabile N3-PPh3 Ligand that Binds through a C=C Bond Rather than an Agostic C-H Interaction, and Displaceement of the C=C by Methyl Iodide or Water  

SciTech Connect

Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+BAr'4- [Ar' = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)2(3-PPh3)Mo]+[BAr'4]-. Spectroscopic and crystallographic data indicate that one C{double_bond}C of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is a four-electron-donor ligand. Computations (DFT/B3LYP and MP2 on [Cp(CO)2(3-PPh3)Mo]+ and [Cp(CO)2(3-PH2Ph)Mo]+, and DFT/B3LYP on [Cp(CO)2(3-PHtBuPh)Mo]+ and [Cp(CO)2(3-PH2Ph)Nb]) provide further information on the bonding and on the preference for bonding of the metal to the C{double_bond}C bond rather than an agostic C-H interaction found in many related complexes. The hemilabile C{double_bond}C bond is readily displaced by CH3I or H2O, and crystal structures are reported for [Cp(CO)2(PPh3)Mo(ICH3)]+ and [Cp(CO)2(PPh3)Mo(OH2)]+. The equilibrium constant for [Cp(CO)2(3-PPh3)Mo]+ + ICH3 to give [Cp(CO)2(PPh3)Mo(ICH3)]+ is Keq = 5.2 x 102 M-1 in CD2Cl2 at 22 C.

Cheng,T.; Szalda, D.; Hanson, J.; Muckerman, J.; Bullock, R.

2008-01-01

137

Cleavage of metal-carbon bonds in hydride-bridged platinum and palladium A-frame complexes  

SciTech Connect

The hydride-bridged A-frame complexes [RPt({mu}-H)({mu} -dppm){sub 2}PtR`]PF{sub 6} and [RPt({mu}-H)({mu}-dppm){sub 2} PdR`]PF{sub 6} react with HCL (generated by reaction of acetyl chloride with water) in acetone-d{sub 6} solution by cleavage of one of the metal-carbon bonds. In the diplatinum systems the relative rates of Pt-C bond cleavage decrease in the order Me > Et > Ph, resulting in a single product in each case. For the heterometallic complexes reaction is faster at palladium, but mixtures of [RPt({mu}-H)({mu}-dppm){sub 2}PdCl]PF{sub 6} and [ClPt({mu}-H)({mu}-dppm){sub 2}PdR`]PF{sub 6} result in several instances. Further reaction of these with HCl and/or CCl{sub 4} generates the chloride-bridged species [RPt({mu}-Cl)({mu}-dppm){sub 2}PdCl]PF{sub 6}, formed, in some cases, by migration of the organic group from palladium to platinum. These complexes have been prepared also by reaction of [PtR(dppm-PP)(dppm-P)]PF{sub 6} with [PdCl{sub 2}(cod)]. The [RPt({mu}-Cl)({mu}-dppm){sub 2}PdCl]PF{sub 6} complexes are fluxional at 298 K, although the motion may be arrested slightly below ambient temperature. A mechanism involving reversible bridge opening is proposed to account for the fluxional behavior. 22 refs., 2 tabs.

Xu, C.; Anderson, G.K. [Univ. of Missouri, St. Louis, MO (United States)

1996-04-02

138

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

PubMed

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 proteolysis as well as of use in the design of peptide substrates and inhibitor lead compounds. Typically the determination of peptide bond cleavage specificity of snake venom serine proteinases (SVSPs) and snake venom metalloproteinases (SVMPs) has been performed using limited sets of peptides or small oligopeptides as experimental substrates. Although this approach has yielded valuable data it is generally limited in scope due to the relatively small sets of substrates used to generate the consensus specificity sequences for these proteinases. In this study we use a large, plasma based, proteome-derived peptide library as substrates along with mass spectrometry to explore the peptide bond specificity of three PI SVMPs and one PIII SVMP to determine their individual peptide cleavage consensus sequences. All of the proteinases assayed displayed a clear preference for a leucine residue in the P1' site. Careful analysis of the specificity profiles of the SVMPs examined showed interesting differences in the preferences at the other P and P' sites suggesting functional differences between these proteinases. The PI SVMPs, leucurolysin-a, atrolysin C, and BaP1, showed preferences across the full P4 to P4' range whereas the PIII SVMP bothropasin showed a narrower range of preferences across the sites. In silico docking experiments with the experimentally derived consensus sequences as well as with comparison of the results to those in the literature regarding peptide bond specificity based on both peptide and protein substrates give rise to a fresh understanding of the specificity of these SVMPS and may serve as a foundation for future experiments to better elucidate their mechanism of action in the complex pathophysiology of snakebite envenomation. PMID:21156218

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

2010-12-13

139

Ligand cleavage put into reverse: P--C bond breaking and remaking in an alkylphosphane iron complex.  

PubMed

Complex formation between FeX(2)6 H(2)O (X=BF(4) or ClO(4)) and the pyridine-derived tetrapodal tetraphosphane C(5)H(3)N[CMe(CH(2)PMe(2))(2)](2) (1) in methanol proceeds with solvent-induced cleavage of one PMe(2) group. Depending on the reaction temperature and the nature of the counterion, iron(II) is coordinated, in distorted square-pyramidal fashion, by the anionic remainder of the chelating ligand, C(5)H(3)N[CMe(CH(2)PMe(2))(2)][CMe(CH(2)PMe(2))(CH(2) (-))] (NP(3)C(-) donor set: X=BF(4), -50 degrees C: 2; X=ClO(4), RT: 4) or its protonated form C(5)H(3)N[CMe(CH(2)PMe(2))(2)][CMe(CH(2)PMe(2))(CH(3))], in which the methyl group is in agostic interaction with the metal centre (X=BF(4), RT: 3; X=ClO(4), +50 degrees C: 5). A monodentate phosphinite ligand Me(2)POMe, formed from the cleaved PMe(2) group and methanol, completes the coordination octahedron in both cases. Working in CD(3)OD (X=BF(4), RT) gives the deuterium-substituted analogue of 3, with ligands L(CH(2)D) (L=residual chelating ligand) and Me(2)POCD(3). A mechanism for the observed phosphorus-carbon bond cleavage is suggested. Complex 2, when isolated at -50 degrees C, is stable in the solid state even at room temperature. The reaction of 2 in methanol with carbon monoxide (10.5 bar) at elevated temperature forms, in addition to as yet unidentified side products, the carbonyl complex [(1)Fe(CO)](BF(4))(2) (7), in which the previous P--C bond cleavage has been reversed, reforming the original tetrapodal pentadentate NP(4) ligand 1. All compounds have been fully characterised, including X-ray structure analyses in most cases. PMID:16528780

Kohl, Stephan W; Heinemann, Frank W; Hummert, Markus; Bauer, Walter; Grohmann, Andreas

2006-05-24

140

Selective cleavage of the two CS bonds in asymmetrically alkylated dibenzothiophenes by Rhodococcus erythropolis KA2-5-1.  

PubMed

The Rhodococcus erythropolis strain KA2-5-1 was characterized by its ability to cleave carbon-sulfur bonds in the dibenzothiophene (DBT) ring by asymmetrically alkyl substitution, such as C2-DBTs (e.g., dimethyl and ethyl DBTs) and C3-DBTs (e.g., trimethyl and propyl DBTs), which are known to remain in hydrodesulfurization-treated diesel fuels. After treatment by solid-phase extraction (SPE) of solvents from microbial reactions of alkylated DBTs (Cx-DBTs), we used gas chromatography (GC), GC-atomic emission detection, GC-mass spectrometry and 1H nuclear magnetic resonance spectroscopy to identify and quantitatively evaluate the Cx-DBT metabolites. Molar ratios of metabolic isomers of the desulfurization products suggested that resting-cell reactions of KA2-5-1 against these Cx-DBTs occurrs through specific carbon-sulfur-bond-targeted cleavages, yielding alkylated hydroxybiphenyls, and that the manner of the attack on the DBT skeleton is affected not only by the position but also by the number and length of the alkyl substituents. PMID:16233063

Onaka, T; Kobayashi, M; Ishii, Y; Konishi, J; Maruhashi, K

2001-01-01

141

Competitive Radiative Decay and Metal-Metal Bond Cleavage from the Lowest Excited State of Triphenyltin- and Triphenylgermanium-Tricarbonyl (1,10-phenanthroline)Rhenium.  

National Technical Information Service (NTIS)

The complexes Ph3ERe(CO)3( ,10-phenanthroline) (E = Sn, Ge) exhibit optical emission (approx. 750 nm) from the lowest excited state in CH2Cl2/CCl4 solutions at 298deg. K and also give E-Re bond cleavage (Phi approx. 0.25) upon photoexcitation under the sa...

J. C. Luong R. A. Faltynek M. S. Wrighton

1979-01-01

142

Four-Electron Donor Hemilabile n3PPh3 Ligand that Binds Through a C = C Bond Rather than an Agostic CH Interaction, and Displacement of the C = C by Methyl Iodide or Water  

Microsoft Academic Search

Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+ BAr'4 [Ar' = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)2(3 PPh3)Mo]+[BAr'4] . Spectroscopic and crystallographic data indicate that one C=C of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is four-electron donor ligand. Computations (DFT\\/B3LYP and MP2 on [Cp(CO)2(3 PPh3)Mo]+ and [Cp(CO)2(3 PH2Ph)Mo]+, and DFT\\/B3LYP on [Cp(CO)2(3 PHtBuPh)Mo]+ and [Cp(CO)2(3 PH2Ph)Nb]) provide

Tan-Yun Cheng; David J. Szalda; Jonathan C. Hanson; J. T. Muckerman; R. Morris Bullock

2008-01-01

143

Four-Electron-Donor Hemilabile N3PPh3 Ligand that Binds through a C=C Bond Rather than an Agostic CH Interaction, and Displaceement of the C=C by Methyl Iodide or Water  

Microsoft Academic Search

Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+BAr'4- [Ar' = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)2(3-PPh3)Mo]+[BAr'4]-. Spectroscopic and crystallographic data indicate that one C{double_bond}C of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is a four-electron-donor ligand. Computations (DFT\\/B3LYP and MP2 on [Cp(CO)2(3-PPh3)Mo]+ and [Cp(CO)2(3-PH2Ph)Mo]+, and DFT\\/B3LYP on [Cp(CO)2(3-PHtBuPh)Mo]+ and [Cp(CO)2(3-PH2Ph)Nb]) provide further information on the bonding and on

T. Cheng; D. Szalda; J. Hanson; J. Muckerman; R. Bullock

2008-01-01

144

Cleavage of the interchain disulfide bonds in rituximab increases its affinity for Fc?RIIIA.  

PubMed

The Fc region of human IgG1 mediates effector function via binding to Fc? receptors and complement activation. The H and L chains of IgG1 antibodies are joined by four interchain disulfide bonds. In this study, these bonds within the therapeutic IgG1 rituximab (RTX) were cleaved either by mild reduction followed by alkylation or by mild S-sulfonation; consequently, two modified RTXs - A-RTX (alkylated) and S-RTX (S-sulfonated) - were formed, and both were almost as potent as unmodified RTX when binding CD20 antigen. Unexpectedly, each modified RTX had a higher binding affinity for Fc?RIIIA (CD16A) than did unmodified RTX. However, S-RTX and A-RTX were each less potent than RTX in an assay of antibody-dependent cellular cytotoxicity (ADCC). In this ADCC assay, each modified RTX showed decreased secretion of granzyme B, but no change in perforin secretion, from effector cells. These results provide significant information on the structures within IgG1 that are involved in binding Fc?RIIIA, and they may be useful in the development of therapeutic antagonists for Fc?RIIIA. PMID:23764395

Suzuki, Mami; Yamanoi, Ayaka; Machino, Yusuke; Kobayashi, Eiji; Fukuchi, Kaori; Tsukimoto, Mitsutoshi; Kojima, Shuji; Kohroki, Junya; Akimoto, Kazunori; Masuho, Yasuhiko

2013-06-10

145

New titanium complexes with symmetric or asymmetric cis-9,10-dihydrophenanthrenediamide ligands formed through sequential intramolecular C-C bond-forming reactions.  

PubMed

A series of new titanium(IV) complexes with symmetric or asymmetric cis-9,10-dihydrophenanthrenediamide ligands, cis-9,10-PhenH(2)(NR)(2)Ti(O(i)Pr)(2) [PhenH(2) = 9,10-dihydrophenanthrene, R = 2,6-(i)Pr(2)C(6)H(3) (2a), 2,6-Et(2)C(6)H(3) (2b), 2,6-Me(2)C(6)H(3) (2c)], cis-9,10-PhenH(2)(NR(1))(NR(2))Ti(O(i)Pr)(2) [R(1) = 2,6-(i)Pr(2)C(6)H(3), R(2) = 2,6-Et(2)C(6)H(3) (2d); R(1) = 2,6-(i)Pr(2)C(6)H(3), R(2) = 2,6-Me(2)C(6)H(3) (2e)], and [cis-9,10-PhenH(2)(NR(1))(2)][o-C(6)H(4)(CH=NR(2))]TiO(i)Pr [R(1) = 2,6-(i)Pr(2)C(6)H(3), R(2) = 2,6-Et(2)C(6)H(3) (3a); R(1) = 2,6-(i)Pr(2)C(6)H(3), 2,6-Me(2)C(6)H(3) (3b)], have been synthesized from the reactions of TiCl(2)(O(i)Pr)(2) with o-C(6)H(4)(CH=NR)Li [R = 2,6-(i)Pr(2)C(6)H(3), 2,6-Et(2)C(6)H(3), 2,6-Me(2)C(6)H(3)]. The symmetric complexes 2a-2c were obtained from the reactions of TiCl(2)(O(i)Pr)(2) with 2 equiv of the corresponding o-C(6)H(4)(CH=NR)Li followed by intramolecular C-C bond-forming reductive elimination and oxidative coupling processes, while the asymmetric complexes 2d-2e were formed from the reaction of TiCl(2)(O(i)Pr)(2) with two different types of o-C(6)H(4)(CH=NR)Li sequentially. The complexes 3a and 3b were also isolated from the reactions for complexes 2d and 2e. All complexes were characterized by (1)H and (13)C NMR spectroscopy, and the molecular structures of 2a, 2b, 2e, and 3a were determined by X-ray crystallography. PMID:21114325

Zhao, Dapeng; Gao, Bo; Gao, Wei; Luo, Xuyang; Tang, Duihai; Mu, Ying; Ye, Ling

2010-11-29

146

A theoretical study of c-C 5H 8 adsorption on Ge (0 0 1)-2 × 1 and on dimer vacancies on the surface: Electronic structure and bonding  

NASA Astrophysics Data System (ADS)

In this work we analyzed the geometry and the chemical interactions for c-C 5H 8 adsorption on Ge (0 0 1), using density functional theory calculations (DFT). We examined the changes in the atomic interactions using a slab model. We considered two cases, the cyclopentene adsorption on Ge(0 0 1) and on dimer vacancies on the surface. We found an average distance H-Ge, -C-Ge and dbnd C-Ge of 1.50, 1.70 and 1.65 Å, respectively, on dimer vacancies; and an average dbnd C-Ge distance of 2.05 Å on Ge-Ge dimer. We also computed the density of states (DOS) and the DOS weighted overlap populations (OPDOS) corresponding to C-C, C-Ge, C-H, and Ge-Ge bonds. During adsorption the main contribution are the C dbnd C double bond in both cases, and the next C and the H's belonging to this bonds in the case of adsorption on dimer vacancies. The orbital contribution includes participation of the 2p y and 2p z orbitals corresponding to unsaturated C atoms, 2p z corresponding to side saturated C, and the 4p orbitals of Ge for the adsorption on dimer vacancies; 2s and 2p z orbitals corresponding to double bond C atoms, 4s and 4p z orbitals of Ge for the adsorption on Ge(0 0 1).

Germán, E.; López-Corral, I.; Juan, A.; Brizuela, G.

2010-08-01

147

Five-coordinate aluminum bromides: synthesis, structure, cation formation, and cleavage of phosphate ester bonds.  

PubMed

The alkane elimination reaction between Salen((t)Bu)H(2) ligands and diethylaluminum bromide was used to prepare three Salen aluminum bromide compounds salen((t)Bu)AlBr (1) (salen = N,N'-ethylenebis(3,5-di-tert-butylsalicylideneimine)), salpen((t)Bu)AlBr (2) (salpen = N,N'-propylenebis(3,5-di-tert-butylsalicylideneimine)), and salophen((t)Bu)AlBr (3) (salophen = N,N'-o-phenylenenebis(3,5-di-tert-butylsalicylideneimine)). The compounds contain five-coordinate aluminum either in a distorted square pyramidal or a trigonal bipyramidal environment. The bromide group in these compounds could be displaced by triphenylphosphine oxide or triphenyl phosphate to produce the six-coordinate cationic aluminum compounds [salen((t)Bu)Al(Ph(3)PO)(2)]Br (4), [salpen((t)Bu)Al(Ph(3)PO)(2)]Br (5), [salophen((t)Bu)Al(Ph(3)PO)(2)]Br (6), and [salophen((t)Bu)Al[(PhO)(3)PO)](2)]Br (7). All the compounds were characterized by (1)H, (13)C, (27)Al, and (31)P NMR, IR, mass spectrometry, and melting point. Furthermore, compounds 1-3 and 5-7 were structurally characterized by single-crystal X-ray diffraction. Compounds 1-3 dealkylated a series of organophosphates in stoichiometric reactions by breaking the ester C-O bond. Also, they were catalytic in the dealkylation reaction between trimethyl phosphate and added boron tribromide. PMID:16433530

Mitra, Amitabha; Depue, Lauren J; Parkin, Sean; Atwood, David A

2006-02-01

148

New Ru(II)–DMSO complexes of ON\\/SN chelates: Synthesis, behavior of Schiff bases towards hydrolytic cleavage of C N bond, electrochemistry and biological activities  

Microsoft Academic Search

The reaction of cis-[RuCl2(DMSO)4] with salicylaldehyde semicarbazone in ethanol resulted in the chemoselective cleavage of the CN bond of the Schiff base, forming a complex in which the semicarbazide remains coordinated to the metal, as observed previously with trans-[RuCl2(DMSO)4]. In another set of reactions of cis-[RuCl2(DMSO)4] with 4-aminoantipyrine derivatives of salicylaldehyde, 2-hydroxy-1-naphthaldehyde and o-vanillin, CN cleavage was observed in all

Viswanathan Mahalingam; Nataraj Chitrapriya; Frank R. Fronczek; Karuppannan Natarajan

2010-01-01

149

Cryovoltammetrically probing functional group reductive cleavage: alkyl-sulfur versus aryl-sulfur bond cleavage in an alkyl naphthyl thioether under single electron-transfer is temperature switchable.  

PubMed

A series of single electron-transfer (SET) reactions on a naphthyl thioether have shown that the reductive cleavage mechanism changes at low temperatures and this selectivity is proved using an electrochemical analysis that mimics the SET reaction conditions. PMID:16896475

Paddon, Christopher A; Bhatti, Farrah L; Donohoe, Timothy J; Compton, Richard G

2006-07-10

150

Room temperature C-N bond cleavage of anionic guanidinate ligand in rare-earth metal complexes.  

PubMed

The dissociation of the anionic guanidinate ligand N=C(NMe2)2 promoted by rare-earth metal complexes at room temperature is described. Treatment of CpLnCl2(THF)3 with two equiv. of Li[N=C(NMe2)2] in THF at room temperature affords [Cp2Ln(?-?(1):?(2)-L2)]2 (Ln = Y; L = N=C(NMe2)N=C(NMe2)2) and CpLn[N=C(NMe2)2](?-?(1):?(2)-L)2LnCp2 (Ln = Dy) in moderated yields, respectively. YCl3 reacts with three equiv. of Li[N=C(NMe2)2] under the same conditions to give a trinuclear yttrium guanidinate [(Me2N)2C=N]5Y3[?-N=C(NMe2)2]2(?-?(1):?(2)-L)2 in 63% yield. These reactions show that rare-earth metals can promote a C-N bond cleavage of the guanidine anion [N=C(NMe2)2](-) at room temperature. All new complexes were characterized by elemental analysis and spectroscopic properties, and their solid-state structures were determined through single-crystal X-ray diffraction analysis. PMID:23456152

Zhang, Jie; Yi, Weiyin; Chen, Zhenxia; Zhou, Xigeng

2013-04-28

151

Synthesis of selenium compounds by free radical addition based on visible-light-activated se-se bond cleavage.  

PubMed

Upon irradiation with near-UV or visible light, organic diselenides undergo homolytic cleavage of their selenium-selenium linkage to generate the corresponding seleno radicals, which can add to alkynes, allenes, and related unsaturated compounds. In the case of alkynes, vicinally diselenated alkenes are synthesized successfully. Photoinduced bisselenation of allenes takes place selectively at the terminal double bond of allenes. In sharp contrast, photoinduced addition of organic diselenides to alkenes is an inefficient process. However, combination of diselenides and disulfides under photoirradiation conditions results in highly regioselective thioselenation of alkenes based on the higher reactivity of thio radicals toward alkenes and the higher carbon radical capturing ability of diselenides. Similar conditions can be employed with a variety of unsaturated compounds such as alkynes, allenes, conjugated dienes, vinylcyclopropanes, and isocyanides. This protocol can also be applied to selenotelluration, selenophosphination, and perfluoroalkylselenation of unsaturated compounds. The excellent carbon radical capturing ability of diselenides makes it possible to attain sequential addition of diselenides to several unsaturated compounds by suppression of polymerization of unsaturated compounds. When the sequential addition takes place intramolecularly under photoirradiation conditions, cyclic products are obtained successfully via a radical cyclization process. In addition, novel photoinduced electrocyclic reaction of o-alkynylaryl isocyanides with diselenides efficiently affords diselenated quinoline derivatives. PMID:23544462

Nomoto, Akihiro; Higuchi, Yoshihiro; Kobiki, Yohsuke; Ogawa, Akiya

2013-05-01

152

C-C (alkynylation) vs C-O (ether) bond formation under Pd/C-Cu catalysis: synthesis and pharmacological evaluation of 4-alkynylthieno[2,3-d]pyrimidines.  

PubMed

The Pd/C-CuI-PPh(3) catalytic system facilitated C-C bond formation between 4-chlorothieno[2,3-d]pyrimidines and terminal alkynes in methanol with high selectivity without generating any significant side products arising from C-O bond formation between the chloro compounds and methanol. A variety of novel 4-alkynylthieno[2,3- d]pyrimidines were prepared via alkynylation of 4-chlorothieno[2,3-d]pyrimidines in good to excellent yields. Some of the compounds synthesized were tested for cytotoxic activity in vitro. PMID:21512602

Gorja, Dhilli Rao; Kumar, K Shiva; Mukkanti, K; Pal, Manojit

2011-03-21

153

Direct, Redox Neutral Prenylation and Geranylation of Secondary Carbinol C-H Bonds: C4 Regioselectivity in Ruthenium Catalyzed C-C Couplings of Dienes to ?-Hydroxy Esters  

PubMed Central

The ruthenium catalyst generated in situ from Ru3(CO)12 and tricyclohexylphosphine, PCy3, promotes the redox-neutral C-C coupling of aryl substituted ?-hydroxy esters to isoprene and myrcene at the diene C4-position, resulting in direct carbinol C-H prenylation and geranylation, respectively. This process enables direct conversion of secondary to tertiary alcohols in the absence of stoichiometric byproducts or premetallated reagents, and is the first example of C4-regioselectivity in catalytic C-C couplings of 2-substituted dienes to carbonyl partners. Mechanistic studies corroborate a catalytic cycle involving diene-carbonyl oxidative coupling.

Leung, Joyce C.; Geary, Laina M.; Chen, Te-Yu; Zbieg, Jason R.

2012-01-01

154

The Central Bond C = C Isotope Effect for Superconductivity in the High-Tc ?*-(ET)2l3 Phase and its Implications Regarding the Superconducting Pairing Mechanism in TTF-Based Organic Superconductors  

Microsoft Academic Search

We report a new study, based on magnetization measurements, of the isotope effect for C substitution in the electronically active central double-bonded carbon atoms (C=C) of the TTF moiety of ET [bis(ethylenedithio)tetrathiafulvalene] in ? -(ET)2I3 [also denoted as ?H-(ET)2[3], which is the crystallographically ordered form of ?-(ET)2I3 produced by application of pressure. A recent report by Merzhanov et al. [C.

K. Douglas Carlson; Jack M. Williams; Urs Geiser; Aravinda M. Kim; H. Hau Wang; Richard A. Klemm; S. Kalyan Kumar; John A. Schlueter; John. R. Ferraro; Keith R. Lykke; Peter Wurz; Deborah Holmes Parker; Jason D. B. Sutin; James E. Schirber; Eugene L. Venturini; Phil Stout

1993-01-01

155

An ab initio Study of Decay Mechanism of Adenine: the Facile Path of the Amino NH Bond Cleavage  

SciTech Connect

A comprehensive study of the radiationless decay processes of the lowest excited singlet states in the isolated 9H-Adenine has been performed at the CASPT2//CASSCF level. The minimum energy paths of the L{sub a}, L{sub b} and n{pi}* singlet states along different skeletal distortions have been computed and the Conical Intersections (CIs) involving these states have been determined. The fast deactivation path of L{sub a} along a skeletal deformation, which leads to a S{sub 0}/L{sub a} CI, as previously discussed, is confirmed. Moreover, low-lying CIs between S{sub 0} and {pi}{sigma}* singlet states have been characterized, where {sigma}* is the antibonding orbital localized on a N-H bond of the amino ({pi}{sigma}{sub NH2*}) or of the azine group ({pi}{sigma}{sub N9H*}). We have found that the repulsive {pi}{sigma}{sub NH2*} state associated with an amino N-H bond can be populated through a barrierless way. Therefore, the decay path shows a bifurcation leading to two possible ways of radiationless deactivation: on one hand a non-photochemical decay through the S{sub 0}/L{sub a} or S{sub 0}/n{pi}* CIs and on the other hand a photochemical process via the possible access to the S{sub 0}/{pi}{sigma}{sub NH2*} CI that produces N-H cleavage. In this way, we can explain the H atom loss found upon UV excitation. We have considered also the decay of higher energy bright states. We have found that these states can decay also by converting to the repulsive {pi}{sigma}{sub N9H*} state associated with the azine NH bond. This new channel suggests an increase of H-atom photoproduction yield by excitating Adenine with lower wavelength radiations. The study of the decay processes of an Adenine molecule in the double strand d(A)10{center_dot}d(T)10 in water solvent is currently underway: Adenine is treated by the Quantum Mechanical (QM) approach and the remaining molecules are described at the Molecular Mechanics (MM) level. We use the COBRAMM program that is a tunable QM/MM approach to complex molecular architectures developed by our research group.

Conti, Irene; Garavelli, Marco; Orlandi, Giorgio [Dipartimento di Chimica 'G. Ciamician', Universita di Bologna, Via F. Selmi, 2, 40126 Bologna (Italy)

2007-12-26

156

Processive Degradation of Unstructured Protein by Escherichia coli Lon Occurs via the Slow, Sequential Delivery of Multiple Scissile Sites Followed by Rapid and Synchronized Peptide Bond Cleavage Events.  

PubMed

Processive protein degradation is a common feature found in ATP-dependent proteases. This study utilized a physiological substrate of Escherichia coli Lon protease known as the lambda N protein (?N) to initiate the first kinetic analysis of the proteolytic mechanism of this enzyme. To this end, experiments were designed to determine the timing of three selected scissile sites in ?N approaching the proteolytic site of ELon and their subsequent cleavages to gain insight into the mechanism by which ATP-dependent proteases attain processivity in protein degradation. The kinetic profile of peptide bond cleavage at different regions of ?N was first detected by the iTRAQ/mass spectrometry technique. Fluorogenic ?N constructs were then generated as reporter substrates for transient kinetic characterization of the ATP- versus AMPPNP-dependent peptide bond cleavage and the delivery of the scissile sites near the amino- versus carboxyl-terminal of the ?N protein to the proteolytic site of ELon. Collectively, our results support a mechanism by which the cleavage of multiple peptide bonds awaits the "almost complete" delivery of all the scissile sites in ?N to the proteolytic site in an ATP-dependent manner. Comparing the time courses of delivery to the active site of the selected scissile sites further implicates the existence of a preferred directionality in the final stage of substrate delivery, which begins at the carboxyl-terminal. The subsequent cleavage of the scissile sites in ?N, however, appears to lack a specific directionality and occurs at a much faster rate than the substrate delivery step. PMID:23822859

Mikita, Natalie; Cheng, Iteen; Fishovitz, Jennifer; Huang, Jonathan; Lee, Irene

2013-07-24

157

Biodesulfurization of dibenzothiophene and its derivatives through the selective cleavage of carbon-sulfur bonds by a moderately thermophilic bacterim Bacillus subtilis WU-S2B  

Microsoft Academic Search

Heterocyclic organosulfur compounds such as dibenzothiophene (DBT) in petroleum cannot be completely removed by hydrodesulfurization using chemical catalysts. A moderately thermophilic bacterium Bacillus subtilis WU-S2B, which could desulfurize DBT at 50°C through the selective cleavage of carbon-sulfur (C?S) bonds, was newly isolated. At 50°C, growing cells of WU-S2B could degrade 0.54 mM DBT within 120 h to produce 2-hydroxybiphenyl, and

Kohtaro Kirimura; Toshiki Furuya; Yasuhiro Nishii; Yoshitaka Ishii; Kuniki Kino; Shoji Usami

2001-01-01

158

Facile N-H bond cleavage of ammonia by an iridium complex bearing a noninnocent PNP-pincer type phosphaalkene ligand.  

PubMed

A novel PNP-pincer type phosphaalkene complex of iridium bearing a dearomatized pyridine unit (3) has been prepared. Complex 3 rapidly reacts with ammonia at room temperature to afford a parent amido complex in high yield. DFT calculations indicate that the phosphaalkene unit with a strong ?-accepting property effectively facilitates the N-H bond cleavage of ammonia via metal-ligand cooperation. PMID:23895386

Chang, Yung-Hung; Nakajima, Yumiko; Tanaka, Hiromasa; Yoshizawa, Kazunari; Ozawa, Fumiyuki

2013-07-31

159

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

2013-04-10

160

Electroreduction of a series of alkylcobalamins: mechanism of stepwise reductive cleavage of the Co-C bond.  

PubMed

The electrochemical (EC) reduction mechanism of methylcobalamin (Me-Cbl) in a mixed DMF/MeOH solvent in 0.2 M tetrabutylammonium fluoroborate electrolyte was studied as a function of temperature and solvent ratio vs a nonaqueous Ag/AgCl/Cl(-) reference electrode. Double-potential-step chronoamperometry allowed the rate constant of the subsequent homogeneous reaction to be measured over the temperature range from 0 to -80 degrees C in 40:60 and 50:50 DMF:MeOH ratios. Activation enthalpies are 5.8 +/- 0.5 and 7.6 +/- 0.3 kcal/mol in the 40:60 and 50:50 mixtures of DMF/MeOH, respectively. Digital simulation and curve-fitting for an EC mechanism using a predetermined homogeneous rate constant of 5.5 x 10(3) s(-1) give E degrees' = -1.466 V, k degrees = 0.016 cm/s, and alpha = 0.77 at 20 degrees C for a quasi-reversible electrode process. Digital simulation of the results of Lexa and Savéant (J. Am. Chem. Soc. 1978, 100, 3220-3222) shows that the mechanism is a series of stepwise homogeneous equilibrium processes with an irreversible step following the initial electron transfer (ET) and allows estimation of the equilibrium and rate constants of these reactions. An electron coupling matrix element of H(kA) = (4.7 +/- 1.1) x 10(-4) eV ( approximately 46 J/mol) is calculated for the nonadiabatic ET step for reduction to the radical anion. A reversible bond dissociation enthalpy for homolytic cleavage of Me-Cbl is calculated as 31 +/- 2 kcal/mol. The voltammetry of the ethyl-, n-propyl-, n-butyl-, isobutyl-, and adenosyl-substituted cobalamin was studied, and estimated reversible redox potentials were correlated with Co-C bond distances as determined by DFT (B3LYP/ LANL2DZ) calculations. PMID:16464094

Birke, Ronald L; Huang, Qingdong; Spataru, Tudor; Gosser, David K

2006-02-15

161

Protocols for the selective cleavage of carbon-sulfur bonds in coal. Final technical report, September 1, 1991--August 31, 1992  

SciTech Connect

Summarized in the final technical report for our project ``Protocols for the Selective Cleavage of Carbon-Sulfur Bonds in Coal`` are results of research pertaining to chemical reactions that aim to selectively cleave C-S bonds in model compounds as well as Illinois coal. Removal of the organic sulfur in coal constitutes one of the major challenges facing fossil fuel scientists today. A cost-effective means of desulfurizing Illinois coal is, at present, non-existent. Research in our group aims to develop a simple protocol for sulfur removal by gaining understanding of how various additives and reaction conditions, including solvents, bases, added reagents, catalysts, oxidizing agents, electron acceptors, temperature, pressure, and light energy, can enhance the rates of C-S bond cleavage in Illinois coal and coal model compounds. These experiments have been at the focus of our research effort for the past twelve months. Previous quarterly reports described research results in which simple aromatic and aliphatic sulfides were allowed to react with (a) Lewis Acids such as zinc chloride and tin chloride; (b) electron accepting substrates such as 9-fluorenone and benzoquinone; (c) strong bases such as NaOH and KOH; (d) radical initiators such as AIBN; (e) neat solvents at reflux temperatures and higher temperatures; (f) molecular oxygen in the presence of dyes or sensitizers such as anthracene. In this final report, we report on additional experiments involving the photooxidation of organic sulfides, as well as some experiments aimed at evaluating and comparing the reactivities of simple organic sulfones with their sulfidyl analogues. Also contained in this final report is a brief summary of the research described in the previous three quarterly reports for ``Protocols for the Selective Cleavage of Carbon-Sulfur Bonds in Coal.``

Bausch, M. [Southern Illinois Univ., Carbondale, IL (United States)

1992-12-31

162

Hydrogen migration and C-C bond breaking in 1,3-butadiene in intense laser fields studied by coincidence momentum imaging  

NASA Astrophysics Data System (ADS)

Two-body dissociation processes of H 2C dbnd CH-CH dbnd CH22+ induced by an intense laser field were investigated by the coincidence momentum imaging method. Four dissociation pathways, CH62+ ? CH 2+ + CH4+, CH62+ ? CH 3+ + CH3+, CH62+ ? CH3+ + CH3+ and CH62+ ? CH2+ + CH4+, were identified. The existence of the second and fourth pathways can be regarded as evidences of the chemical bond rearrangement processes associated with hydrogen migration in the intense laser field. It was found that the hydrogen atom bonded originally to one of the two central carbon atoms migrates preferentially to its neighboring terminal carbon atom site.

Xu, Huailiang; Okino, Tomoya; Nakai, Katsunori; Yamanouchi, Kaoru; Roither, Stefan; Xie, Xinhua; Kartashov, Daniil; Schöffler, Markus; Baltuska, Andrius; Kitzler, Markus

2010-01-01

163

Copper(II)-catalyzed aerobic oxidative synthesis of substituted 1,2,3- and 1,2,4-triazoles from bisarylhydrazones via C-H functionalization/C-C/N-N/C-N bonds formation.  

PubMed

An unprecedented copper(II)-catalyzed aerobic oxidative synthesis of 2,4,5-triaryl-1,2,3-triazoles and 1,3,5-triaryl-1,2,4-triazoles from bisarylhydrazones as the common starting precursor has been achieved via cascade C-H functionalization/C-C/N-N/C-N bonds formation under mild reaction conditions. One of the enthralling outcomes of this strategy is the copper(II)-catalyzed room temperature C-H functionalization/C-N bond formation in presence of air, which has been accomplished during the synthesis of substituted 1,2,4-triazoles. This new class of compounds could give prospective luminescence as an iconic component in the area of pharmaceutical and biological sciences. The intermediates for both the processes have been isolated to elucidate the mechanistic scenario. PMID:22571669

Guru, Murali Mohan; Punniyamurthy, Tharmalingam

2012-05-11

164

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

165

Location of intrachain disulfide bonds in the VP5* and VP8* trypsin cleavage fragments of the rhesus rotavirus spike protein VP4.  

PubMed Central

Because the rotavirus spike protein VP4 contains conserved Cys residues at positions 216, 318, 380, and 774 and, for many animal rotaviruses, also at position 203, we sought to determine whether disulfide bonds were structural elements of VP4. Electrophoretic analysis of untreated and trypsin-treated rhesus rotavirus (RRV) and simain rotavirus SA11 in the presence and absence of the reducing agent dithioerythritol revealed that VP4 and its cleavage fragments VP5* and VP8* possessed intrachain disulfide bonds. Given that the VP8* fragments of RRV and SA11 contain only two Cys residues, those at positions 203 and 216, these data indicated that these two residues were covalently linked. Electrophoretic examination of truncated species of VP4 and VP4 containing Cys-->Ser mutations synthesized in reticulocyte lysates provided additional evidence that Cys-203 and Cys-216 in VP8* of RRV were linked by a disulfide bridge. VP5* expressed in vitro was able to form a disulfide bond analogous to that in the VP5* fragment of trypsin-treated RRV. Analysis of a Cys-774-->Ser mutant of VP5* showed that, while it was able to form a disulfide bond, a Cys-318-->Ser mutant of VP5* was not. These results indicated that the VP4 component of all rotaviruses, except B223, contains a disulfide bond that links Cys-318 and Cys-380 in the VP5* region of the protein. This bond is located between the trypsin cleavage site and the putative fusion domain of VP4. Because human rotaviruses lack Cys-203 and, hence, unlike many animal rotaviruses cannot possess a disulfide bond in VP8*, it is apparent that VP4 is structurally variable in nature, with human rotaviruses generally containing one disulfide linkage and animal rotaviruses generally containing two such linkages. Considered with the results of anti-VP4 antibody mapping studies, the data suggest that the disulfide bond in VP5* exists within the 2G4 epitope and may be located at the distal end of the VP4 spike on rotavirus particles. Images

Patton, J T; Hua, J; Mansell, E A

1993-01-01

166

Methanol conversion to hydrocarbons over zeolite H-ZSM-5: Investigation of the role of CO and ketene in the formation of the initial C-C bond  

SciTech Connect

Mechanistic studies concerning the formation of the initial carbon-carbon bond in the methanol conversion reaction over zeolite H-ZSM-5 are described and discussed. In particular, the possible roles of CO as a reaction intermediate or as a catalyst, via the formation of ketene, are evaluated. Experiments using [sup 13]CH[sub 3]OH/[sup 12]CO reactant mixtures demonstrate that no CO is incorporated into ethene, the primary product of this reaction. In addition, CO is found to have no significant effect on the induction period for this reaction. Model experiments for the methylation of ketene by reaction with Me[sub 2]SO[sub 4] and Me[sub 3]O[sup +]SbCl[sub 6][sup [minus

Hutchings, G.J.; Johnston, P. (Univ. of Liverpool (United Kingdom)); Hunter, R. (Univ. of Cape Town, Rondesbosch (South Africa)); Van Rensburg, L.J. (Univ. of Witwatersrand (South Africa))

1993-08-01

167

Cleavage of the glycosidic C-O-C bond on size-selected subnanometer cobalt catalysts and on ALD-cobalt coated nanoporous membranes.  

SciTech Connect

The cleavage of the C-O-C bond was studied under oxidizing conditions on nanostructured membrane supported cobalt-based catalysts using a cellulose model surrogate, 1-methoxy-2-methyl-2-propanol. The cobalt catalysts were found to break the C-O-C bond, producing alcohols and/or ketones by further oxidation. The size-selected sub-nanometer size cobalt clusters exhibited a per metal activity of up to 5 orders of magnitude higher than the with atomic layer deposition uniformly coated membranes. The large difference in activity is attributed to the high fraction of the surface atoms of the subnanometer clusters. The positioning of the clusters at the entrance vs. exit of the catalytic membrane allows for a control of the contact time and consequently of the selectivity of the catalyst.

Deng, W.; Lee, S.; Libera, J. A.; Elam, J. W.; Vajda, S.; Marshall, C. L. (Center for Nanoscale Materials); ( CSE); ( ES)

2011-02-15

168

Cleavage of the C-O-C bond on size-selected subnanometer cobalt catalysts and on ALD-cobalt coated nanoporous membranes.  

SciTech Connect

The cleavage of the C-O-C bond was studied under oxidizing conditions on nanostructured membrane supported cobalt-based catalysts using a cellulose model surrogate, 1-methoxy-2-methyl-2-propanol. The cobalt catalysts were found to break the C-O-C bond, producing alcohols and/or ketones by further oxidation. The size-selected sub-nanometer size cobalt clusters exhibited a per metal activity of up to 5 orders of magnitude higher than the with atomic layer deposition uniformly coated membranes. The large difference in activity is attributed to the high fraction of the surface atoms of the subnanometer clusters. The positioning of the clusters at the entrance vs. exit of the catalytic membrane allows for a control of the contact time and consequently of the selectivity of the catalyst.

Deng, W.; Lee, S.; Libera, J. A.; Elam, J. W.; Vajda, S.; Marshall, C. L.; Yale Univ.

2011-02-15

169

Photoreactions of Ir/sub 8/(CO)/sub 22//sup 2/minus//. Homolytic cleavage of an unsupported metal-metal bond linking two tetrahedral iridium carbonyl clusters  

SciTech Connect

The photolysis of an anion, Ir/sub 8/(CO)/sub 22//sup 2/minus//, where the metal-metal bond links two polyhedral metal clusters is discussed herein. The photolytic reaction apparently leads to homolytic bond cleavage to yield the reactive anion Ir/sub 4/(CO)/sub 11//sup /minus//. The photolysis of a complex incorporating the named anion was studied in dichloromethane under N/sub 2/ and in tetrahydrofuran under CO. The relative reactivity of the intermediate, Ir/sub 4/(CO)/sub 11//sup /minus// is illustrated by its ability to extract a chlorine atom from dichloromethane. The results of the photolysis study indicated that the tetrairidium radical may be sufficiently reactive to abstract a hydrogen atom from tetrahydrofuran. 13 references, 1 figure.

Vandenberg, D.M.; Friedman, A.E.; Ford, P.C.

1988-02-24

170

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

2012-03-01

171

Biodesulfurization of dibenzothiophene and its derivatives through the selective cleavage of carbon-sulfur bonds by a moderately thermophilic bacterium Bacillus subtilis WU-S2B.  

PubMed

Heterocyclic organosulfur compounds such as dibenzothiophene (DBT) in petroleum cannot be completely removed by hydrodesulfurization using chemical catalysts. A moderately thermophilic bacterium Bacillus subtilis WU-S2B, which could desulfurize DBT at 50 degrees C through the selective cleavage of carbon-sulfur (CS) bonds, was newly isolated. At 50 degrees C, growing cells of WU-S2B could degrade 0.54 mM DBT within 120 h to produce 2-hydroxybiphenyl, and the resting cells could also degrade 0.81 mM DBT within 12 h. The DBT-desulfurizing ability of WU-S2B is high over a wide temperature range from 30 to 50 degrees C, and highest at 50 degrees C for both the growing and resting cells, and this is an extremely advantageous property for the practical biodesulfurization. In addition, WU-S2B could also desulfurize DBT derivatives such as 2,8-dimethylDBT, 4,6-dimethylDBT and 3,4-benzoDBT. Therefore, B. subtilis WU-S2B is considered to have more beneficial properties than other desulfurizing bacteria such as Rhodococcus strains previously reported, particularly from the viewpoint of its capacity for thermophilic desulfurization through the CS bond cleavage. PMID:16232986

Kirimura, K; Furuya, T; Nishii, Y; Ishii, Y; Kino, K; Usami, S

2001-01-01

172

Mechanism of ribonucleotide reductase from Herpes simplex virus type 1. Evidence for 3' carbon-hydrogen bond cleavage and inactivation by nucleotide analogs  

SciTech Connect

Isotope effects of 2.5, 2.1, and 1.0 were measured on the conversion of (3'-3H)ADP, (3'-H)UDP, and (5-3H) UDP to the corresponding 2'-deoxynucleotides by herpes simplex virus type 1 ribonucleotide reductase. These results indicate that the reduction of either purine or pyrimidine nucleotides requires cleavage of the 3' carbon-hydrogen bond of the substrate. The substrate analogs 2'-chloro-2'-deoxyuridine 5'-diphosphate (ClUDP), 2'-deoxy-2'-fluorouridine 5'-diphosphate, and 2'-azido-2'-deoxyuridine 5'-diphosphate were time-dependent inactivators of the herpes simplex virus type 1 ribonucleotide reductase. Incubation of (3'-3H)ClUDP with the enzyme was accompanied by time-dependent release of 3H to the solvent. Reaction of (beta-32P)ClUDP with the reductase resulted in the production of inorganic pyrophosphate. These results are consistent with the enzyme-mediated cleavage of the 3' carbon-hydrogen bond of ClUDP and the subsequent conversion of the nucleotide to 2-methylene-3(2H)furanone, as previously reported with the Escherichia coli ribonucleotide reductase.

Ator, M.A.; Stubbe, J.; Spector, T.

1986-03-15

173

Copper-catalyzed aerobic oxidative C-H and C-C functionalization of 1-[2-(arylamino)aryl]ethanones leading to acridone derivatives.  

PubMed

Efficient copper-catalyzed aerobic oxidative C-H and C-C functionalization of 1-[2-(arylamino)aryl]ethanones leading to acridones has been developed. The procedure involves cleavage of aromatic C-H and acetyl C-C bonds with intramolecular formation of a diarylketone bond. The protocol uses inexpensive Cu(O2CCF3)2 as catalyst, pyridine as additive, and economical and environmentally friendly oxygen as the oxidant, and the corresponding acridones with various functional groups were obtained in moderate to good yields. PMID:23401330

Yu, Jipan; Yang, Haijun; Jiang, Yuyang; Fu, Hua

2013-02-10

174

Determination of effective potentials for the stretching of C? ? C? virtual bonds in polypeptide chains for coarse-grained simulations of proteins from ab initio energy surfaces of N-methylacetamide and N-acetylpyrrolidine  

PubMed Central

The potentials of mean force (PMF’s) for the deformation of the C? ? C? virtual bonds in polypeptide chains were determined from the diabatic energy surfaces of N-methylacetamide (modeling regular peptide groups) and N-acetylpyrrolidine (modeling the peptide groups preceding proline), calculated at the Møller-Plesset (MP2) ab initio level of theory with the 6-31G(d,p) basis set. The energy surfaces were expressed in the C? ? C? virtual-bond length (d) and the H-N-C? ? C? improper dihedral angle (?) that describes the pyramidicity of the amide nitrogen, or in the C?-C?(O)-N-C? dihedral angle (?) and the angle ?. For each grid point, the potential energy was minimized with respect to all remaining degrees of freedom. The PMF’s obtained from the (d, ?) energy surfaces produced realistic free-energy barriers to the trans-cis transition (10 kcal/mol and 13 kcal/mol for the regular and proline peptide groups, respectively, compared to 12.6 – 13.9 kcal/mol and 17.3 – 19.6 kcal/mol determined experimentally for glycylglycine and N-acylprolines, respectively), while those obtained from the (?, ?) energy maps produced either low-quality PMF curves when direct Boltzmann summation was implemented to compute the PMF’s or too-flat curves with too-low free-energy barriers to the trans-cis transition if harmonic extrapolation was used to estimate the contributions to the partition function. An analytical bimodal logarithmic-Gaussian expression was fitted to the PMF’s, and the potentials were implemented in the UNRES force field. Test Langevin-dynamics simulations were carried out for the Gly-Gly and Gly-Pro dipeptides, which showed a 106-fold increase of the simulated rate of the trans-cis isomerization with respect to that measured experimentally; effectively the same result was obtained with the analytical Kramers theory of reaction rate applied to the UNRES representation of the peptide groups. Application of Kramers’ theory to compute the rate constants from the all-atom ab initio energy surfaces of the model compounds studied resulted in isomerization rates close to the experimental values, which demonstrates that the increase of the isomerization rate in UNRES simulations results solely from averaging out the secondary degrees of freedom.

Sieradzan, Adam K.; Scheraga, Harold A.; Liwo, Adam

2012-01-01

175

Determination of effective potentials for the stretching of C(?) ? C(?) virtual bonds in polypeptide chains for coarse-grained simulations of proteins from ab initio energy surfaces of N-methylacetamide and N-acetylpyrrolidine.  

PubMed

The potentials of mean force (PMF's) for the deformation of the C(?) ? C(?) virtual bonds in polypeptide chains were determined from the diabatic energy surfaces of N-methylacetamide (modeling regular peptide groups) and N-acetylpyrrolidine (modeling the peptide groups preceding proline), calculated at the Møller-Plesset (MP2) ab initio level of theory with the 6-31G(d,p) basis set. The energy surfaces were expressed in the C(?) ? C(?) virtual-bond length (d) and the H-N-C(?) ? C' improper dihedral angle (?) that describes the pyramidicity of the amide nitrogen, or in the C(?)-C'(O)-N-C(?) dihedral angle (?) and the angle ?. For each grid point, the potential energy was minimized with respect to all remaining degrees of freedom. The PMF's obtained from the (d, ?) energy surfaces produced realistic free-energy barriers to the trans-cis transition (10 kcal/mol and 13 kcal/mol for the regular and proline peptide groups, respectively, compared to 12.6 - 13.9 kcal/mol and 17.3 - 19.6 kcal/mol determined experimentally for glycylglycine and N-acylprolines, respectively), while those obtained from the (?, ?) energy maps produced either low-quality PMF curves when direct Boltzmann summation was implemented to compute the PMF's or too-flat curves with too-low free-energy barriers to the trans-cis transition if harmonic extrapolation was used to estimate the contributions to the partition function. An analytical bimodal logarithmic-Gaussian expression was fitted to the PMF's, and the potentials were implemented in the UNRES force field. Test Langevin-dynamics simulations were carried out for the Gly-Gly and Gly-Pro dipeptides, which showed a 10(6)-fold increase of the simulated rate of the trans-cis isomerization with respect to that measured experimentally; effectively the same result was obtained with the analytical Kramers theory of reaction rate applied to the UNRES representation of the peptide groups. Application of Kramers' theory to compute the rate constants from the all-atom ab initio energy surfaces of the model compounds studied resulted in isomerization rates close to the experimental values, which demonstrates that the increase of the isomerization rate in UNRES simulations results solely from averaging out the secondary degrees of freedom. PMID:23087598

Sieradzan, Adam K; Scheraga, Harold A; Liwo, Adam

2012-02-24

176

Is Se-Se bond cleavage the most favourable process in electron attachment to diselenides? The importance of asymmetry.  

PubMed

Accurate ab initio calculations reveal that, in contrast with what has been commonly assumed up to now, electron attachment to diselenides does not always result in a Se-Se bond fission and, in general, the mechanism behind the bond fission may involve the crossing of several states. PMID:21350756

Gámez, José A; Yáñez, Manuel

2011-02-24

177

Versatile synthesis of isocoumarins and ?-pyrones by ruthenium-catalyzed oxidative C-H/O-H bond cleavages.  

PubMed

An inexpensive cationic ruthenium(II) catalyst enabled the expedient synthesis of isocoumarins through oxidative annulations of alkynes by benzoic acids. This C-H/O-H bond functionalization process also proved applicable to the preparation of ?-pyrones and was shown to proceed by rate-limiting C-H bond ruthenation. PMID:22273364

Ackermann, Lutz; Pospech, Jola; Graczyk, Karolina; Rauch, Karsten

2012-01-24

178

Chelation-assisted carbon-hydrogen and carbon-carbon bond activation by transition metal catalysts.  

PubMed

Herein we describe the chelation-assisted C-H and C-C bond activation of carbonyl compounds by Rh1 catalysts. Hydroacylation of olefins was accomplished by utilizing 2-amino-3-picoline as a chelation auxiliary. The same strategy was employed for the C-C bond activation of unstrained ketones. Allylamine 24 was devised as a synthon of formaldehyde. Hydroiminoacylation of alkynes with allylamine 24 was applied to the alkyne cleavage by the aid of cyclohexylamine. PMID:12180320

Jun, Chul-Ho; Moon, Choong Woon; Lee, Dae-Yon

2002-06-01

179

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

180

Primary photodissociation pathways of epichlorohydrin and analysis of the C-C bond fission channels from an O((3)P)+allyl radical intermediate.  

PubMed

This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H(2)COCH)CH(2)Cl. The three dominant photoproduct channels analyzed are c-(H(2)COCH)CH(2)+Cl, c-(H(2)COCH)+CH(2)Cl, and C(3)H(4)O+HCl. In the second channel, the c-(H(2)COCH) photofission product is a higher energy intermediate on C(2)H(3)O global potential energy surface and has a small isomerization barrier to vinoxy. The resulting highly vibrationally excited vinoxy radicals likely dissociate to give the observed signal at the mass corresponding to ketene, H(2)CCO. The final primary photodissociation pathway HCl+C(3)H(4)O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H(2)COC)=CH(2); the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C(3)H(5)O radical produced from C-Cl bond photofission. When the epoxide radical photoproduct undergoes facile ring opening, it is the radical intermediate formed in the O((3)P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C(2)H(4) and H(2)CO+C(2)H(3) product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C(2)H(4) products at m/e=29 and 28 shows that the dissociation of the radical intermediate imparts a high relative kinetic energy, peaking near 20 kcal/mol, between the products. Similarly, the energy imparted to relative kinetic energy in the H(2)CO+C(2)H(3) product channel of the O((3)P)+allyl radical intermediate also peaks at high-recoil kinetic energies, near 18 kcal/mol. The strongly forward-backward peaked angular distributions and the high kinetic energy release result from tangential recoil during the dissociation of highly rotationally excited nascent radicals formed photolytically in this experiment. The data also reveal substantial branching to an HCCH+H(3)CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C(3)H(5)O potential energy surface calculated with coupled cluster theory, accounting for the rotational and vibrational energy imparted to the radical intermediate and the resulting competition between the H+acrolein, HCO+C(2)H(4), and H(2)CO+C(2)H(3) product channels. We compare the results of the theoretical prediction with our measured branching ratios. We also report photoionization efficiency (PIE) curves extending from 9.25 to 12.75 eV for the signal from the HCO+C(2)H(4) and H(2)CO+C(2)H(3) product channels. Using the C(2)H(4) bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C(2)H(4) and HCO yields a value of 11.6+1/-3?Mb for the photoionization cross section of HCO at 11.27 eV. This determination puts the PIE curve of HCO measured here on an absolute scale, allowing us to report the absolute photoionization efficiency of HCO over the entire range of photoionization energies. PMID:20831317

Fitzpatrick, Benjamin L; Alligood, Bridget W; Butler, Laurie J; Lee, Shih-Huang; Lin, Jim Jr-Min

2010-09-01

181

A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Toward Catalyst Deactivation During Ketone Hydrogenation  

SciTech Connect

The initial reaction observed between N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO)2(PPh3)H (M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H]+[CpM(CO)2(PPh3)]– . At longer reaction times and higher temperatures, the reaction of IMes with CpM(CO)2(PR3)H (M = Mo, W; R = Me, Ph) produces CpM(CO)2(IMes)H. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+B(C6F5)4- gives CpW(CO)2(IMes)+B(C6F5)4- which was crystallographically characterized using x-ray radiation from a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a C=C bond of one mesityl ring. The weakly bound C=C ligand is hemilabile, being readily displaced by H2, THF, ketones or alcohols. Reaction of CpW(CO)2(IMes)+ with H2 gives the dihydride complex [CpW(CO)2(IMes)(H)2]+. Addition of Et2CH–OH to CpW(CO)2(IMes)+B(C6F5)4- gives the alcohol complex [CpM(CO)2(IMes)(Et2CH–OH)]+[B(C6F5)4]– which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OH group. Addition of H2 to the ketone complex [CpW(CO)2(IMes)(Et2C=O)]+[B(C6F5)4]– produces an equilibrium with the dihydride [CpW(CO)2(IMes)(H)2]+ (Keq = 1.1 ? 103 at 25 °C). The tungsten ketone complex [CpW(CO)2(IMes)(Et2C=O)]+[B(C6F5)4]– serves as a modest catalyst for hydrogenation of Et2C=O to Et2CH–OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)]+B(C6F5)4-, indicating that these catalysts with N-heterocyclic carbenes ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.

Wu, Fan; Dioumaev, Vladimir K.; Szalda, David J.; Hanson, Jonathan; Bullock, R. Morris

2007-09-24

182

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

2013-02-28

183

Primary photodissociation pathways of epichlorohydrin and analysis of the C-C bond fission channels from an O({sup 3}P)+allyl radical intermediate  

SciTech Connect

This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H{sub 2}COCH)CH{sub 2}Cl. The three dominant photoproduct channels analyzed are c-(H{sub 2}COCH)CH{sub 2}+Cl, c-(H{sub 2}COCH)+CH{sub 2}Cl, and C{sub 3}H{sub 4}O+HCl. In the second channel, the c-(H{sub 2}COCH) photofission product is a higher energy intermediate on C{sub 2}H{sub 3}O global potential energy surface and has a small isomerization barrier to vinoxy. The resulting highly vibrationally excited vinoxy radicals likely dissociate to give the observed signal at the mass corresponding to ketene, H{sub 2}CCO. The final primary photodissociation pathway HCl+C{sub 3}H{sub 4}O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H{sub 2}COC)=CH{sub 2}; the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C{sub 3}H{sub 5}O radical produced from C-Cl bond photofission. When the epoxide radical photoproduct undergoes facile ring opening, it is the radical intermediate formed in the O({sup 3}P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C{sub 2}H{sub 4} and H{sub 2}CO+C{sub 2}H{sub 3} product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C{sub 2}H{sub 4} products at m/e=29 and 28 shows that the dissociation of the radical intermediate imparts a high relative kinetic energy, peaking near 20 kcal/mol, between the products. Similarly, the energy imparted to relative kinetic energy in the H{sub 2}CO+C{sub 2}H{sub 3} product channel of the O({sup 3}P)+allyl radical intermediate also peaks at high-recoil kinetic energies, near 18 kcal/mol. The strongly forward-backward peaked angular distributions and the high kinetic energy release result from tangential recoil during the dissociation of highly rotationally excited nascent radicals formed photolytically in this experiment. The data also reveal substantial branching to an HCCH+H{sub 3}CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C{sub 3}H{sub 5}O potential energy surface calculated with coupled cluster theory, accounting for the rotational and vibrational energy imparted to the radical intermediate and the resulting competition between the H+acrolein, HCO+C{sub 2}H{sub 4}, and H{sub 2}CO+C{sub 2}H{sub 3} product channels. We compare the results of the theoretical prediction with our measured branching ratios. We also report photoionization efficiency (PIE) curves extending from 9.25 to 12.75 eV for the signal from the HCO+C{sub 2}H{sub 4} and H{sub 2}CO+C{sub 2}H{sub 3} product channels. Using the C{sub 2}H{sub 4} bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C{sub 2}H{sub 4} and HCO yields a value of 11.6+1/-3 Mb for the photoionization cross section of HCO at 11.27 eV. This determination puts the PIE curve of HCO measured here on an absolute scale, allowing us to report the absolute photoionization efficiency of HCO over the entire range of photoionization energies.

FitzPatrick, Benjamin L.; Alligood, Bridget W.; Butler, Laurie J. [Department of Chemistry and James Franck Institute, University of Chicago, Chicago, Illinois 60637 (United States); Lee, Shih-Huang [National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan (China); Lin, Jim Jr-Min [Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 10617 (China)

2010-09-07

184

Rhodium-catalyzed cascade oxidative annulation leading to substituted naphtho[1,8-bc]pyrans by sequential cleavage of C(sp2)-H/C(sp3)-H and C(sp2)-H/O-H bonds.  

PubMed

The cascade oxidative annulation reactions of benzoylacetonitrile with internal alkynes proceed efficiently in the presence of a rhodium catalyst and a copper oxidant to give substituted naphtho[1,8-bc]pyrans by sequential cleavage of C(sp(2))-H/C(sp(3))-H and C(sp(2))-H/O-H bonds. These cascade reactions are highly regioselective with unsymmetrical alkynes. Experiments reveal that the first-step reaction proceeds by sequential cleavage of C(sp(2))-H/C(sp(3))-H bonds and annulation with alkynes, leading to 1-naphthols as the intermediate products. Subsequently, 1-naphthols react with alkynes by cleavage of C(sp(2))-H/O-H bonds, affording the 1:2 coupling products. Moreover, some of the naphtho[1,8-bc]pyran products exhibit intense fluorescence in the solid state. PMID:22989331

Tan, Xing; Liu, Bingxian; Li, Xiangyu; Li, Bin; Xu, Shansheng; Song, Haibin; Wang, Baiquan

2012-09-25

185

Thermochemistry of organic and heteroorganic species. Part XIII. Structural aspects and thermochemical approach to isomerization and fragmentation of negative ions. Simple bond cleavage and hydrogen rearrangement processes  

NASA Astrophysics Data System (ADS)

Many hundreds of isomerization and fragmentation processes for negatively charged ions obtained in electron attachment dissociation, collision induced dissociation and negative chemical ionization have been analyzed to find out certain regularities in gas-phase monomolecular chemistry of negative ions like it was earlier performed for positive ions [Eur. J. Mass Spectrom., 8 (2002) 409]. This analysis combined with systematic application of the data on energies of the processes (appearance energies, isolation of stable resonance states on energetical scale in the range 0 15 eV, enthalpies of formation of even negative ions and electron affinities of corresponding free radicals) allowed to come to quite definite conclusion of validity of Eyring's quasi-equilibrium theory (QET) to mass spectrometry of negative ions. However, it is valid only in the limits of a stable resonance state rather than in a whole range of energies being applied. The validity of QET was illustrated by many diverse simple bond cleavage and rearrangement processes. Those hundreds of known processes for negative ions dissociation were reduced to a few isomerization and fragmentation types for odd- and even-charged negative ions—8 types for simple bond cleavage and 7—for H-migration, all being in certain aspect common with those suggested earlier for positive ions. The isomerization and fragmentation types were assigned to about 370 processes of simple bond cleavage and H-migration with presentation of possible structures for isomeric molecular and fragment ions. Possibility of such specification of the processes for negative ions, partial thermochemical approach for their description, general and specific application of QET make unnecessary the appeal to participation of high-energetical electron excitation, nonergodic fragmentation, to any specifity in charge-remote processes—all these used in literature to explain occurrence of some unusual processes. The latter could be described by the introduced reaction types or their combination, by formation of many (instead of a single one) neutrals in one-step process, H-wandering over ionized benzene ring, ring opening as a routine event (e.g. to explain the appearance of molecular ions in several domains of energies for aromatic compounds.).

Ponomarev, Dmitry; Takhistov, Viatcheslav

2006-02-01

186

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

2012-03-22

187

Catalysis and co-catalysis of bond cleavages in coal and coal analogs. Final report, August 1, 1990--January 31, 1994  

SciTech Connect

In work prior to the inception of this project, the authors observed that mixtures of phenolic materials and polyalkoxyaromatic molecules were appreciably more effective in catalyzing the decompositions of di-2-naphthyl ether and of di-1-naphthyl sulfide in tetralin solutions at 450{degrees}C than were the phenols by themselves, even though the polyalkoxyaromatic molecules, in the absence of phenolic co- catalysts, show essentially no catalytic activity. This was of appreciable interest in coal research because dinapthyl ether and dinapthyl sulfide have been employed as model compounds for coals in studies aimed at cleaving ether and sulfide bonds similar to those in coals. The authors proposed (R. K. Sharma, K. P. Raman, and B. Miller) that the mixed catalysts used in these studies catalyze cleavages of ether and sulfide bonds by means of a mechanism involving electron transfer from the polyalkoxyaromatics to the substrates, which are activated as electron acceptors by hydrogen bonding to phenols. Since phenols themselves are electron donors, they also proposed that the well known effects of phenols in catalyzing the conversion of coals are due to similar electron transfer mechanisms.

Miller, B.

1994-05-01

188

Iron-oxidation-state-dependent O-O bond cleavage of meta-chloroperbenzoic acid to form an iron(IV)-oxo complex  

PubMed Central

The mechanism of formation of [FeIV(O)(N4Py)]2+ (2, N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) from the reaction of [FeII(N4Py)(CH3CN)]2+ (1) with m-chloroperbenzoic acid (mCPBA) in CH2Cl2 at ?30 °C has been studied on the basis of the visible spectral changes observed and the reaction stoichiometry. It is shown that the conversion of 1 to 2 in 90% yield requires 1.5 equiv peracid and takes place in two successive one-electron steps via an [FeIII(N4Py)OH]2+(3) intermediate. The first oxidation step uses 0.5 equiv peracid and produces 0.5 equiv 3-chlorobenzoic acid, while the second step uses 1 equiv peracid and affords byproducts derived from chlorophenyl radical. We conclude that the FeII(N4Py) center promotes O-O bond heterolysis, while the FeIII(N4Py) center favors O-O bond homolysis, so the nature of O-O bond cleavage is dependent on the iron oxidation state.

Ray, Kallol; Lee, Sang Mok; Que, Lawrence

2008-01-01

189

Iron-oxidation-state-dependent O-O bond cleavage of meta-chloroperbenzoic acid to form an iron(IV)-oxo complex.  

PubMed

The mechanism of formation of [Fe(IV)(O)(N4Py)](2+) (2, N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) from the reaction of [Fe(II)(N4Py)(CH(3)CN)](2+) (1) with m-chloroperbenzoic acid (mCPBA) in CH(2)Cl(2) at -30 °C has been studied on the basis of the visible spectral changes observed and the reaction stoichiometry. It is shown that the conversion of 1 to 2 in 90% yield requires 1.5 equiv peracid and takes place in two successive one-electron steps via an [Fe(III)(N4Py)OH](2+)(3) intermediate. The first oxidation step uses 0.5 equiv peracid and produces 0.5 equiv 3-chlorobenzoic acid, while the second step uses 1 equiv peracid and affords byproducts derived from chlorophenyl radical. We conclude that the Fe(II)(N4Py) center promotes O-O bond heterolysis, while the Fe(III)(N4Py) center favors O-O bond homolysis, so the nature of O-O bond cleavage is dependent on the iron oxidation state. PMID:18443654

Ray, Kallol; Lee, Sang Mok; Que, Lawrence

2008-03-01

190

Selective activation of C-H and C-C bonds on metal carbides: A comparison of reactions of n-Butane and 1,3-Butadiene on vanadium carbide films on V(110)  

SciTech Connect

The authors have investigated the adsorption and decomposition of n-butane and 1,3-butadiene on clean and carbide-modified vanadium (110) surfaces. By using high-resolution electron energy loss spectroscopy and thermal desorption spectrometry, The authors observe that the formation of carbide significantly modifies the reactivities of vanadium. The 1,3-butadiene molecules interact strongly with clean V(110) via the interaction between the d-band of vanadium and the {pi} orbitals of the adsorbates; the interaction is much weaker on the carbide-modified surfaces. On the other hand, n-butane interacts very weakly and reversibly with the clean surface; the reactivity is enhanced on carbide-modified surfaces. These experimental results are compared to the existing theories on the activation of C-H bonds of alkanes and C-C bonds of unsaturated hydrocarbons on transition metals. Such a comparison indicates that, although the reactivities of clean vanadium agree very well with the theoretical predictions for early transition metals, the properties of carbide-modified surfaces are more similar to those of Group VIIIB metals. 49 refs., 10 figs., 2 tabs.

Chen, J.G. [Exxon Research and Engineering Company, Annandale, NJ (United States)

1995-06-01

191

Carbon-oxygen bond cleavage reactions by electron transfer. 1. Electrochemical studies on the formation and subsequent reaction pathways of cyanoanisole radical anions  

SciTech Connect

The radical anions of three isomers of cyanoanisole have been electrochemically generated and subsequently shown to react by at least three different reaction pathways in dry N,N-dimethylformamide (DMF). The o-cyanoanisole radical anion (E/sub pc/ = -2.3 V vs. SCE) dimerizes (k/sub 2/ = 3.2 X 10/sup 2/ M/sup -1/s/sup -1/) to form an intermediate dimeric dianion. The dianion, which can be oxidized (E/sub pa = 1.1 V vs. SCE), undergoes a slow intramolecular disproportionation reaction to form o-cyanophenoxide ion, methide ion, and unreduced substrate (k/sub 3/ = 1.9 X 10/sup -2/s/sup -1/). Subsequent protonation results in the formation of methane and p-cyanophenol in an overall two-electron process. The m-cyanoanisole radical anion (E/sub pc/ = -2.3 V vs. SCE) is very stable (t/sub 1/2/ > 10/sup 3/ s) under anhydrous DMF conditions. Overall, slow carbon-carbon bond cleavage with loss of cyanide occurs competitively with ..beta.. carbon-oxygen bond cleavage to produce anisole and m-cyanophenol, respectively. The final products of the reduction of p-cyanoanisole are p-cyanophenol and methane; however, the radical anion of p-cyanoanisole (E/sub pc/ = -2.5 V vs. SCE) undergoes a relatively rapid unimolecular fragmentation reaction (k/sub 1/ = 7 s/sup -1/). The initial products of the fragmentation are p-cyanophenoxide ion and the methyl radical, which is reduced further to methide ion. Hydrogen atom abstraction reactions by the methyl radical can also occur in the bulk solution to produce methane.

Koppang, M.D.; Woolsey, N.F.; Bartak, D.E.

1984-05-16

192

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

193

Gold-catalyzed glycosidations: unusual cleavage of the interglycosidic bond while studying the armed\\/disarmed effect of propargyl glycosides  

Microsoft Academic Search

Armed\\/disarmed effect of propargyl glycosides in the presence of AuBr3 is studied. Observed that oxophilic AuBr3 cleaves interglycosidic bond of an armed disaccharide resulting in the formation of a disaccharide and a 1,6-anhydro sugar. Trisaccharides were obtained after fine tuning the reactivity of the glycosyl donor with different protecting groups.

Abhijeet K. Kayastha; Srinivas Hotha

2010-01-01

194

Facile synthesis of rhodium and iridium complexes bearing a [PEP]-type ligand (E = Ge or Sn) via E-C bond cleavage.  

PubMed

Rhodium and iridium complexes bearing a tridentate [PEP] type ligand ([PEP] = {o-(Ph(2)P)C(6)H(4)}(2)E(Me); E = Ge or Sn) were synthesized through the phosphine exchange reaction accompanied by selective E-C bond cleavage. The ligand precursors {o-(Ph(2)P)C(6)H(4)}(2)EMe(2) (E = Ge or Sn) were readily obtained in excellent yields by treating {o-(Ph(2)P)C(6)H(4)}(2)Li with 0.5 equivalents of Me(2)ECl(2). Tris(triphenylphosphine)rhodium(I) carbonyl hydride M(H)(CO)(PPh(3))(3) (M = Rh, Ir) cleaved one of the E-Me bonds of {o-(Ph(2)P)C(6)H(4)}(2)EMe(2) exclusively to afford the trigonal bipyramidal (TBP) complexes, [PEP]M(CO)(PPh(3)). Square-planar rhodium complexes [PEP]Rh(PPh(3)) were also prepared from the reactions of tetrakis(triphenylphosphine)rhodium(I) hydride Rh(H)(PPh(3))(4) with {o-(Ph(2)P)C(6)H(4)}(2)EMe(2). Further, the trans influence of group 14 elements E (E = Si, Ge, Sn) in [PEP]Rh(PPh(3)) is discussed in terms of the (1)J(Rh-P) coupling constants, indicating that E exhibited a stronger trans labilizing effect in the order Sn < Ge < Si. PMID:22885800

Kameo, Hajime; Ishii, Sho; Nakazawa, Hiroshi

2012-08-13

195

Diversification of Function in the Haloacid Dehalogenase Enzyme Superfamily: The Role of the Cap Domain in Hydrolytic Phosphorus-Carbon Bond Cleavage. †,‡  

PubMed Central

Phosphonatase functions in the 2-aminoethylphosphonate (AEP) degradation pathway of bacteria, catalyzing the hydrolysis of the C-P bond in phosphonoacetaldehyde (Pald) via formation of a bi-covalent Lys53ethylenamine/Asp12 aspartylphosphate intermediate. Because phosphonatase is a member of the haloacid dehalogenase superfamily, a family predominantly comprised of phosphatases, the question arises as to how this new catalytic activity evolved. The source of general acid-base catalysis for Schiff-base formation and aspartylphosphate hydrolysis was probed using pH-rate profile analysis of active-site mutants and X-ray crystallographic analysis of modified forms of the enzyme. The 2.9 Å X-ray crystal structure of the mutant Lys53Arg complexed with Mg+2 and phosphate shows that the equilibrium between the open and the closed conformation is disrupted, favoring the open conformation. Thus, proton dissociation from the cap domain Lys53 is required for cap domain-core domain closure. The likely recipient of the Lys53 proton is a water-His56 pair that serves to relay the proton to the carbonyl oxygen of the phosphonoacetaldehyde (Pald) substrate upon addition of the Lys53. The pH-rate profile analysis of active-site mutants was carried out to test this proposal. The proximal core domain residues Cys22 and Tyr128 were ruled out, and the role of cap domain His56 was supported by the results. The X-ray crystallographic structure of wild-type phosphonatase reduced with NaBH4 in the presence of Pald was determined at 2.4 Å resolution to reveal N?-ethyl-Lys53 juxtaposed with a sulfate ligand bound in the phosphate site. The position of the C(2) of the N-ethyl group in this structure is consistent with the hypothesis that the cap domain N?-ethylenamine-Lys53 functions as a general base in the hydrolysis of the aspartylphosphate bi-covalent enzyme intermediate. Because the enzyme residues proposed to play a key role in P-C bond cleavage are localized on the cap domain, this domain appears to have evolved to support the diversification of the HAD phosphatase core domain for catalysis of hydrolytic P-C bond cleavage.

Lahiri, Sushmita D.; Zhang, Guofeng; Dunaway-Mariano, Debra; Allen, Karen N.

2007-01-01

196

Solvent-mediated hydrogenolysis in coal liquefaction: Quarterly report No. 4 for the period September 16, 1985 to July 15, 1986. [Modeling of bond cleavage of dinaphthylmethane in anthracene/dihydroanthracene and phenanthrene/dihydrophenanthrene  

SciTech Connect

The objective of the present study is to explore the relevance of solvent-mediated hydrogenolysis to coal liquefaction. To this end we have investigated the cleavage rates of suitable model compounds in a variety of solvent systems with and without added coals, and assessed the relative importance of hydrogenolysis via free H-atoms and via radical hydrogen-transfer (RHT). We have been able to differentiate between hydrogenolysis via free H-atoms from that via RHT by examining the selectivity of cleavage of 1,2'-dinaphthylmethane. To further aid our understanding of the competition in cleavage of strong bonds by various modes of H-transfer, we have also conducted some mechanistic modeling studies. Finally, we are also trying to understand the impact of mass transport limitations that are likely to be important in actual coal liquefactions because of the heterogeneous nature of coal. Our approach is to bond model structures onto silica gel and study their cleavage in aromatic solvents in the presence of coal. In these experiments, the model structures, whose cleavage can be precisely monitored, cannot directly migrate to sites in coal where hydrogen becomes available, but must rely on the shuttling ability of the aromatic solvent. In this report we discuss our results of modeling cleavage of strong bonds in aromatic/hydroaromatic mixtures. During this reporting period we have optimized parameters for the RHT reactions, extended the model to the phenanthrene system, and rationalized some of the observed trends. The report also includes a brief description of our attempts to bond organic structures to silica. These materials will be used in liquefaction experiments to test the shuttling ability of different solvents. 12 refs., 3 figs., 1 tab.

McMillen, D.F.; Malhotra, R.; Nigenda, S.E.

1987-03-12

197

A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Towards Catalyst Deactivation During Ketone Hydrogenation  

SciTech Connect

The initial reaction observed between the N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO){sub 2}(PPh{sub 3})H (M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H]{sup +}[CpM(CO){sub 2}(PPh{sub 3})]{sup -}. At longer reaction times and higher temperatures, the reaction of IMes with CpM(CO){sub 2}(PR{sub 3})H (M = Mo, W; R = Me, Ph) produces CpM(CO){sub 2}(IMes)H. Hydride transfer from CpW(CO)2(IMes)H to Ph{sub 3}C{sub +}B(C{sub 6}F{sub 5}){sub 4}{sup -} gives CpW(CO){sub 2}(IMes){sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -}, which was crystallographically characterized using X-ray radiation from a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a C=C bond of one mesityl ring. The weakly bound C=C ligand is hemilabile, being readily displaced by H{sub 2}, THF, ketones, or alcohols. Reaction of CpW(CO){sub 2}(IMes){sup +} with H{sub 2} gives the dihydride complex [CpW(CO){sub 2}(IMes)(H){sub 2}]{sup +}. Addition of Et{sub 2}CH-OH to CpW(CO){sub 2}(IMes){sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -} gives the alcohol complex [CpW(CO){sub 2}(IMes)(Et{sub 2}CH-OH)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -}, which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OH group. Addition of H{sub 2} to the ketone complex [CpW(CO){sub 2}(IMes)(Et{sub 2}C=O)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -} produces an equilibrium with the dihydride [CpW(CO){sub 2}(IMes)(H){sub 2}]{sup +} (K{sub eq} = 1.1 x 10{sup 3} at 25 {sup o}C). The tungsten ketone complex [CpW(CO){sub 2}(IMes)(Et{sub 2}C=O)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -}- serves as a modest catalyst for hydrogenation of Et{sub 2}C=O to Et{sub 2}CH-OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)]{sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -}, indicating that these catalysts with N-heterocyclic carbene ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.

Wu,F.; Dioumaev, V.; Szalda, D.; Hanson, J.; Bullock, R.

2007-01-01

198

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

PubMed

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 A(2)B protomers, with B at its centre, but that these protomers self-associate to assemblies containing several A(2)B 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 A(2)B units, each bound to a recognition site, with two more A(2)B 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

2012-11-11

199

Molecular basis of 1,6-anhydro bond cleavage and phosphoryl transfer by Pseudomonas aeruginosa 1,6-anhydro-N-acetylmuramic acid kinase.  

PubMed

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

2011-02-02

200

Analysis of positional isotope exchange in ATP by cleavage of the. beta. P-O. gamma. P bond. Demonstration of negligible positional isotope exchange by myosin  

SciTech Connect

A method for analysis of positional isotope exchange (PIX) during ATP in equilibrium HOH oxygen exchange is presented that uses a two-step degradation of ATP resulting in cleavage of the ..beta..P-O..gamma..P bond. This cleavage yields P/sub i/ derived from the ..gamma..-phosphoryl of ATP that contains all four of the ..gamma.. oxygens. Both PIX between the ..beta.., ..gamma..-bridge and ..beta..-nonbridge positions and washout of the ..gamma..-nonbridge oxygens can be simultaneously followed by using ATP labeled with /sup 17/O at the ..beta..-nonbridge positions and /sup 18/O at the ..beta..,..gamma..-bridge and ..gamma..-nonbridge positions. Application of this method to ATP in equilibrium HOH exchange during single turnovers of myosin indicates that the bulk of the ATP undergoes rapid washout of ..gamma..-nonbridge oxygens in the virtual absence of PIX. At 25/sup 0/C with subfragment 1 the scrambling rate is at the limit of detectability of approximately 0.001 s/sup -1/, which is 50-fold slower than the steady-state rate. This corresponds to a probability of scrambling for the ..beta..-oxygens of bound ADP of 1 in 10,000 for each cycle of reversible hydrolysis of bound ATP. A fraction of the ATP, however, does not undergo rapid washout. With myosin and stoichiometric ATP at 0/sup 0/C, this fraction correspond to 10% of the ATP remaining at 36 s, or 2% of the initial ATP, and an equivalent level of ATP is found that does not bind irreversibly to myosin in a cold chase experiment. A significant level of apparent PIX is observed with subfragment 1 in the fraction that resists washout, and this apparent PIX is shown to be due to contaminant adenylate kinase activity. This apparent PIX due to adenylate kinase provides a possible explanation for the PIX observed by Geeves et al. with subfragment 1.

Dale, M.P.; Hackney, D.D.

1987-12-15

201

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.

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

202

Mechanisms of hydrogen transfer and associated bond cleavage reactions: a rationalization of the effectiveness of pyrene in coal liquefaction solvents  

SciTech Connect

Model compound studies and kinetic analyses have suggested that the effectiveness of pyrene as a hydrogen-transfer agent in coal liquefaction is due to its increased effectiveness in solvent mediated hydrogenolyses which is due in turn to: (1) its relatively high stability among 4-ring aromatics, and (2) the inability of the most predominant radical in the system to terminate as readily as those in other systems. This explanation will be tested by extending to the pyrene system our studies of bond scission promoted by hydrogen-transfer (solvent-mediated hydrogenolysis), and by ''spiking'' coal liquefaction mixtures with model coal linkages subject to such hydrogenolysis to determine if, under actual coal liquefaction conditions, the scission of such structures is accelerated in pyrene. 17 refs., 1 fig., 2 tabs.

McMillen, D.F.; Ogier, W.C.; Chang, S.J.; Fleming, R.H.; Malhotra, R.

1983-01-01

203

Glycosidic bond cleavage in deoxynucleotides: effects of solvent and the DNA phosphate backbone in the computational model.  

PubMed

Density functional theory (B3LYP) was employed to examine the hydrolysis of the canonical 2'-deoxynucleotides in varied environments (gas phase or water) using different computational models for the sugar residue (methyl or phosphate group at C5') and nucleophile (water activated through full or partial proton abstraction). Regardless of the degree of nucleophile activation, our results show that key geometrical parameters along the reaction pathway are notably altered upon direct inclusion of solvent effects in the optimization routine, which leads to significant changes in the reaction energetics and better agreement with experiment. Therefore, despite the wide use of gas-phase calculations in the literature, small model computational work, as well as large-scale enzyme models, that strive to understand nucleotide deglycosylation must adequately describe the environment. Alternatively, although inclusion of the phosphate group at C5' also affects the geometries of important stationary points, the effects cancel to yield unchanged deglycosylation barriers, and therefore smaller computational models can be used to estimate the energy associated with nucleotide deglycosylation, with the 5' phosphate group included if full (geometric) details of the reaction are desired. Hydrogen-bonding interactions with the nucleobase can significantly reduce the barrier to deglycosylation, which supports suggestions that discrete hydrogen-bonding interactions with active-site amino acid residues can play a significant role in enzyme-catalyzed nucleobase excision. Taken together with previous studies, the present work provides vital clues about the components that must be included in future studies of the deglycosylation of isolated noncanonical nucleotides, as well as the corresponding enzyme-catalyzed reactions. PMID:23167947

Lenz, Stefan A P; Kellie, Jennifer L; Wetmore, Stacey D

2012-11-30

204

Role of aromatic structure in pathways of hydrogen transfer and bond cleavage in coal liquefaction: Theoretical studies  

SciTech Connect

The mechanisms by which strong carbon-carbon bonds between aromatic rings and side chains are cleaved under hydropyrolysis conditions remain a subject of wide interest to fuel science. Recently, the authors have studied in detail an alternate pathway for hydrogen atom transfer to {pi}-systems, radical hydrogen transfer (RHT). RHT is the direct, bimolecular transfer of hydrogen from the {beta}-position of an organic radical to the target {pi}-system. In the initial theoretical study, they examined the reaction ethyl radical + ethylene = ethylene + ethyl at the spin-projected UMP2/6-31G** level of theory. Recently, they have used a calibrated ROHF-MNDO-PM3 method to predict thermoneutral RHT barriers for hydrogen transfer between hydroaryl radicals and the corresponding arene. Because of the inherent limitations of semiempirical methods such as ROHF-MNDO-PM3, they have extended the initial work with the ethyl + ethylene study to examine this reaction at the ROHF-MBPT[2]-6-31G** and ROHF-CCSD[T]-6-31G** levels of ab initio theory. The primary objective was to determine how intrinsic RHT barriers change with conjugative stabilization of the radicals. The spin-restricted ROHF approach has been applied to study several RHT reactions, and they present completed ROHF results for the ethyl + ethylene system and preliminary results for the methallyl + butadiene system. The methallyl + butadiene system serves as a model for highly stabilized hydroaryl radicals: the methallyl radical exhibits a C-H bond strength of 46.5 kcal/mol compared to 9-hydroanthracenyl, 43.1 kcal/mol.

Franz, J.A.; Autrey, T.; Camaioni, D.M. [Pacific Northwest Lab., Richland, WA (United States); Watts, J.D.; Bartlett, R.J. [Univ. of Florida, Gainesville, FL (United States). Quantum Theory Project

1995-09-01

205

Cobalt(II) and cobalt(III) complexes of thioether-containing hexadentate pyrazine amide ligands: C-S bond cleavage and cyclometallation reaction.  

PubMed

Anaerobic reaction of Co(O2CMe)2.4H2O with the thioether-containing acyclic pyrazine amide hexadentate ligand 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-dithiobutane (H2L1) (-CH2CH2- spacer between the two pyrazine amide tridentate coordination units) furnishes [CoII(L1)].MeOH (1a) having CoN2(pyrazine)N'2(amide)S2(thioether) coordination. It exhibits an eight-line EPR spectrum, attesting to a low-spin (S = 1/2) state of CoII. A similar reaction in air, however, furnishes [CoIII(L3a)(L3b)].2MeOH (2a) (S = 0), resulting from a C-S bond cleavage reaction triggered by an acetate ion as a base, having CoN2(pyrazine)N'2(amide)S(thioether)S'(thiolate) coordination. On the other hand, the reaction of Co(O2CMe)2.4H2O with 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,5-dithiopentane (H2) (-CH2CH2CH2- spacer between the two pyrazine amide tridentate coordination units) in air affords a cobalt(II) complex [CoII(L2)].MeOH (1b.MeOH) (S = 1/2); its structurally characterized variety has the composition 1b.C6H6. Interestingly, 1b.MeOH undergoes facile metal-centred oxidation by aerial O2-H2O2-[Fe(eta5-C5H5)2][PF6], which led to the isolation of the corresponding cobalt(iii) complex [CoIII(L2)][ClO4] (2b). When treated with methanolic KOH, 2b affords a low-spin (S = 0) organocobalt(III) complex [Co(III)((L2')] (3). Structures of all complexes, except 1a, have been authenticated by X-ray crystallography. A five-membered chelate-ring forming ligand L1(2-) effects C-S bond cleavage and a six-membered chelate-ring forming ligand L2(2-) gives rise to Co-C bond formation, in cobalt(III)-coordinated thioether functions due to alpha C-H bond activation by the base. A rationale has been provided for the observed difference in the reactivity properties. The spectroscopic properties of the complexes have also been investigated. Cyclic voltammetry experiments in MeCN-CH2Cl2 reveal facile metal-centred reversible-to-quasireversible CoIV-CoIII (or a ligand-centred redox process; 2a), CoIII-CoII (1a, 1b.MeOH, 2a, 2b and 3), CoII-CoI (1a, 1b.MeOH, 2aand 2b), and CoI-Co0 (1a, 1b.MeOH and 2b) redox processes. PMID:18097493

Singh, Akhilesh Kumar; Mukherjee, Rabindranath

2007-10-22

206

Identification and cleavage of breakable single bonds by selective oxidation, reduction, and hydrolysis. Quarterly report No. 9, October 1-December 31, 1980  

SciTech Connect

Objective is to determine the structure of bituminous coal with emphasis on the crosslinks and breakable single bonds. Some of the highlights of this quarter are: (1) large-scale extraction of benzylamine-extracted coal with ethylene diamine-dimethyl sulfoxide (EDA-DMSO) led to 50.4% recovery of soluble material and 40.5% recovery of undissolved coal; (2) EDA-DMSO extraction appears to have removed essentially all of the ether and ester links in both fractions, leaving only hydrocarbon links; (3) extraction of whole coal by a mixture of N-methylpyrrolidinone (NMP) and EDA is no better than EDA-DMSO, even though NMP alone is a better solvent than DMSO alone; (4) investigators have no evidence that NaOCl will oxidize sulfide links to sulfuric acid and NaOCl appears to be unsuitable for determining sulfide links in coal; (5) black acid fraction from several NaOCl oxidations precipitated between pH 4.98 and 4.66 was characterized; (6) measurements of hydroxyl and carboxyl contents on pyridine and benzylamine extracts and extracted coal show increases in carboxyl content on reactions of these fractions with alcoholic KOH, and an increase in hydroxyl content on reactions of the pyridine-extracted coal with benzylamine, consistent with ester cleavage. 6 tables.

Pavelka, L.A.; Mayo, F.R.; Zevely, J.

1981-03-24

207

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

208

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

209

Combined experimental and theoretical study on the reductive cleavage of inert C-O bonds with silanes: ruling out a classical Ni(0)/Ni(II) catalytic couple and evidence for Ni(I) intermediates.  

PubMed

A mechanistic and computational study on the reductive cleavage of C-OMe bonds catalyzed by Ni(COD)(2)/PCy(3) with silanes as reducing agents is reported herein. Specifically, we demonstrate that the mechanism for this transformation does not proceed via oxidative addition of the Ni(0) precatalyst into the C-OMe bond. In the absence of an external reducing agent, the in-situ-generated oxidative addition complexes rapidly undergo ?-hydride elimination at room temperature, ultimately leading to either Ni(0)-carbonyl- or Ni(0)-aldehyde-bound complexes. Characterization of these complexes by X-ray crystallography unambiguously suggested a different mechanistic scenario when silanes are present in the reaction media. Isotopic-labeling experiments, kinetic isotope effects, and computational studies clearly reinforced this perception. Additionally, we also found that water has a deleterious effect by deactivating the Ni catalyst via formation of a new Ni-bridged hydroxo species that was characterized by X-ray crystallography. The order in each component was determined by plotting the initial rates of the C-OMe bond cleavage at varying concentrations. These data together with the in-situ-monitoring experiments by (1)H NMR, EPR, IR spectroscopy, and theoretical calculations provided a mechanistic picture that involves Ni(I) as the key reaction intermediates, which are generated via comproportionation of initially formed Ni(II) species. This study strongly supports that a classical Ni(0)/Ni(II) for C-OMe bond cleavage is not operating, thus opening up new perspectives to be implemented in other related C-O bond-cleavage reactions. PMID:23316793

Cornella, Josep; Gómez-Bengoa, Enrique; Martin, Ruben

2013-01-23

210

Phenol near Ni(111), Ni(110), and Ni(221) surfaces in a vertical ring geometry: A density functional study of the oxygen-surface bonding and O-H cleavage  

NASA Astrophysics Data System (ADS)

Existing experimental studies of phenol on Ni(111) surfaces show no direct evidence for the formation of an oxygen-surface bond and O-H cleavage. Such processes are known for other transition metals [e.g., Pt(111) and Rh(111)] or other surface structures such as Ni(110). For this reason, we investigate the structure and energetics of a possible oxygen-surface bond and O-H cleavage for a phenol molecule interacting with a (close-packed) Ni(111) surface. In order to clarify the surface-molecule interaction, we make a comparison with different Ni surfaces, namely, the (110) and the (221)-step surfaces. By using a density functional theory approach, we find interesting agreement with experiments.

Ghiringhelli, Luca M.; Caputo, Riccarda; Site, Luigi Delle

2007-03-01

211

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

212

Biodesulfurization of Naphthothiophene and Benzothiophene through Selective Cleavage of Carbon-Sulfur Bonds by Rhodococcus sp. Strain WU-K2R  

PubMed Central

Naphtho[2,1-b]thiophene (NTH) is an asymmetric structural isomer of dibenzothiophene (DBT), and in addition to DBT derivatives, NTH derivatives can also be detected in diesel oil following hydrodesulfurization treatment. Rhodococcus sp. strain WU-K2R was newly isolated from soil for its ability to grow in a medium with NTH as the sole source of sulfur, and growing cells of WU-K2R degraded 0.27 mM NTH within 7 days. WU-K2R could also grow in the medium with NTH sulfone, benzothiophene (BTH), 3-methyl-BTH, or 5-methyl-BTH as the sole source of sulfur but could not utilize DBT, DBT sulfone, or 4,6-dimethyl-DBT. On the other hand, WU-K2R did not utilize NTH or BTH as the sole source of carbon. By gas chromatography-mass spectrometry analysis, desulfurized NTH metabolites were identified as NTH sulfone, 2?-hydroxynaphthylethene, and naphtho[2,1-b]furan. Moreover, since desulfurized BTH metabolites were identified as BTH sulfone, benzo[c][1,2]oxathiin S-oxide, benzo[c][1,2]oxathiin S,S-dioxide, o-hydroxystyrene, 2-(2?-hydroxyphenyl)ethan-1-al, and benzofuran, it was concluded that WU-K2R desulfurized NTH and BTH through the sulfur-specific degradation pathways with the selective cleavage of carbon-sulfur bonds. Therefore, Rhodococcus sp. strain WU-K2R, which could preferentially desulfurize asymmetric heterocyclic sulfur compounds such as NTH and BTH through the sulfur-specific degradation pathways, is a unique desulfurizing biocatalyst showing properties different from those of DBT-desulfurizing bacteria.

Kirimura, Kohtaro; Furuya, Toshiki; Sato, Rika; Ishii, Yoshitaka; Kino, Kuniki; Usami, Shoji

2002-01-01

213

Direct observation of the primary and secondary C-Br bond cleavages from the 1,2-dibromopropane photodissociation at 234 and 265 nm using the velocity map ion imaging technique.  

PubMed

Photodissociation dynamics of 1,2-dibromopropane has been investigated at 234 and 265 nm by using the velocity map ion imaging method. At both pump energies, a single Gaussian-shaped speed distribution is observed for the Br*((2)P(1/2)) fragment, whereas at least three velocity components are found to be existent for the Br((2)P(3/2)) product. The secondary C-Br bond cleavage of the bromopropyl radical which is energized from the ultrafast primary C-Br bond rupture should be responsible for the multicomponent translational energy distribution at the low kinetic energy region of Br((2)P(3/2)). The recoil anisotropy parameter (beta) of the fragment from the primary C-Br bond dissociation is measured to be 0.53 (0.49) and 1.26 (1.73) for Br((2)P(3/2)) and Br*((2)P(1/2)), respectively, at 234 (265) nm. The beta value of Br((2)P(3/2)) from the secondary C-Br bond dissociation event at 265 nm is found to be 0.87, reflecting the fact that the corresponding Br((2)P(3/2)) fragment carried the initial vector component of the bromopropyl radical produced from the primary bond dissociation event. Density functional theory has been used to calculate energetics involved both in the primary and in the secondary C-Br bond dissociation dynamics. PMID:18665578

Lee, Kyoung-Seok; Yeon, Ki Young; Jung, Kyung-Hoon; Kim, Sang Kyu

2008-07-30

214

Pyramidalization of the glycosidic nitrogen provides the way for efficient cleavage of the N-glycosidic bond of 8-OxoG with the hOGG1 DNA repair protein.  

PubMed

A mechanistic pathway for cleavage of the N-glycosidic bond of 8-oxo-2'-deoxyguanosine (oxoG) catalyzed with the human 8-oxoguanine glycosylase 1 DNA repair protein (hOGG1) is proposed in this theoretical study. The reaction scheme suggests direct proton addition to the glycosidic nitrogen N9 of oxoG from the N?-ammonium of Lys249 residue of hOGG1 that is enabled owing to the N9 pyramidal geometry. The N9-pyramidalization of oxoG is induced within hOGG1 active site. The coordination of N9 nitrogen to the N?-ammonium of Lys249 unveiled by available crystal structures enables concerted, synchronous substitution of the N9-C1' bond by the N9-H bond. The reaction is compared with other pathways already proposed by means of calculated activation energies. The ?G(#) energy for the newly proposed reaction mechanism calculated with the B3LYP/6-31G(d,p) method 17.0 kcal mol(-1) is significantly lower than ?G(#) energies for other reactions employing attack of the N?-amino group to the anomeric carbon C1' of oxoG and attack of the N?-ammonium to the N3 nitrogen of oxoG base. Moreover, activation energy for the oxoG cleavage proceeding via N9-pyramidalization is lower than energy calculated for normal G because the electronic state of the five-membered aromatic ring of oxoG is better suited for the reaction. The modification of aromatic character introduced by oxidation to the nucleobase thus seems to be the factor that is checked by hOGG1 to achieve base-specific cleavage. PMID:22989268

Šebera, Jakub; Trantírek, Lukáš; Tanaka, Yoshiyuki; Sychrovský, Vladimír

2012-10-09

215

Effects of Protein S and Factor Xa on Peptide Bond Cleavages during Inactivation of Factor Va and Factor VaR506Q by Activated Protein C  

Microsoft Academic Search

Inactivation of membrane-bound factor Va by acti- vated protein C (APC) proceeds via a biphasic reaction that consists of a rapid and a slow phase, which are associated with cleavages at Arg506 and Arg306 of the heavy chain of factor Va, respectively. We have investi- gated the effects of protein S and factor Xa on APC- catalyzed factor Va inactivation.

Jan Rosing; Lico Hoekema; Gerry A. F. Nicolaes; M. Christella; L. G. D. Thomassen; H. Coenraad Hemker; Hans P. Schwarz; Guido Tans

216

Catalytic oxidative carbon?carbon bond cleavage of ketones with dioxygen: assessment of some metal complexes. Some alternatives for preparing ?, ?-dicarboxylic acids  

Microsoft Academic Search

This account considers the catalytic oxidative cleavage of ketones with dioxygen. It can be brought about by Keggin-type heteropolyanion complexes containing molybdenum and vanadium as well as by several transition metal catalysts (V(IV, V), Cu(I, II), Fe(III), Ce(IV), etc.), instead of using stoichiometric oxidation with periodate, lead tetra-acetate, etc. Benzylic ketones, ArCH2C(O)R are oxidised to ArCHO and the corresponding carboxylic

Jean-Marie Brégeault; Franck Launay; Ahmed Atlamsani

2001-01-01

217

Aliphatic C-H to C-C Conversion: Synthesis of (-)-Cameroonan-7?-ol  

PubMed Central

In the course of a synthesis of the tricyclic sesquiterpene (?)-cameroonan-7?-ol from the acyclic (+)-citronellal, seven aliphatic C-H bonds were converted to C-C bonds, and three rings and four new stereogenic centers were established.

Taber, Douglass F.; Nelson, Christopher G.

2011-01-01

218

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.

MURAI, Shinji

2011-01-01

219

Near-Stoichiometric Conversion of H2O2 to FeIV=O at a Non-Heme Iron(II) Center. Insights Into the O-O Bond Cleavage Step  

PubMed Central

Near-quantitative formation of an oxoiron(IV) intermediate [FeIV(O)(TMC)(CH3CN)]2+ (2) from stoichiometric H2O2 was achieved with [FeII(TMC)]2+ (1) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane). This important outcome is best rationalized by invoking a direct reaction between 1 and H2O2 followed by a heterolytic O–O bond cleavage facilitated by an acid-base catalyst (2,6-lutidine in our case). A sizable H/D KIE of 3.7 was observed for the formation of 2, emphasizing the importance of proton transfer in the cleavage step. Pyridines with different pKa values were also investigated, and less basic pyridines were found to function less effectively than 2,6-lutidine. This study demonstrates that the reaction of Fe(II) with H2O2 to form Fe(IV)=O can be quite facile. Two factors promote the near-stoichiometric conversion of H2O2 to Fe(IV)=O in this case: a) the low reactivity between 1 and 2 and b) the poor H-atom abstracting ability of 2, which inhibits subsequent reaction with residual H2O2. Both factors inhibit formation of the Fe(III) byproduct commonly found in reactions of Fe(II) complexes with H2O2. These results may shed light into the nature of the O–O bond cleaving step in the activation of dioxygen by nonheme iron enzymes and in the first step of the Fenton reaction.

Li, Feifei; England, Jason

2010-01-01

220

Near-stoichiometric conversion of H(2)O(2) to Fe(IV)=O at a nonheme iron(II) center. Insights into the O-O bond cleavage step.  

PubMed

Near-quantitative formation of an oxoiron(IV) intermediate [Fe(IV)(O)(TMC)(CH(3)CN)](2+) (2) from stoichiometric H(2)O(2) was achieved with [Fe(II)(TMC)](2+) (1) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane). This important outcome is best rationalized by invoking a direct reaction between 1 and H(2)O(2) followed by a heterolytic O-O bond cleavage facilitated by an acid-base catalyst (2,6-lutidine in our case). A sizable H/D KIE of 3.7 was observed for the formation of 2, emphasizing the importance of proton transfer in the cleavage step. Pyridines with different pK(a) values were also investigated, and less basic pyridines were found to function less effectively than 2,6-lutidine. This study demonstrates that the reaction of Fe(II) with H(2)O(2) to form Fe(IV)= O can be quite facile. Two factors promote the near-stoichiometric conversion of H(2)O(2) to Fe(IV)=O in this case: (a) the low reactivity between 1 and 2 and (b) the poor H-atom abstracting ability of 2, which inhibits subsequent reaction with residual H(2)O(2). Both factors inhibit formation of the Fe(III) byproduct commonly found in reactions of Fe(II) complexes with H(2)O(2). These results may shed light into the nature of the O-O bond cleaving step in the activation of dioxygen by nonheme iron enzymes and in the first step of the Fenton reaction. PMID:20121136

Li, Feifei; England, Jason; Que, Lawrence

2010-02-24

221

Models for Deep Hydrodesulfurization of Alkylated Benzothiophenes. Reductive Cleavage of C-S Bonds Mediated by Precoordination of Manganese Tricarbonyl to the Carbocyclic Ring  

SciTech Connect

The reduction of (eta6-benzothiophene)Mn(CO)3 + with cobaltocene under CO leads to insertion of the Mn(CO)4 - fragment into the C(aryl)-S bond to afford a neutral bimetallic complex. When there are methyl substituents at the 2,3-, 2,7-, or 3,7-positions, the regioselectivity and product distribution in the reductive insertion reactions are significantly affected, and several new types of complexes are formed.

Huazhi, Li; Yu, Kunquan; Watson, Eric J.; Virkaitis, Kurtis L.; D'Acchioli, Jason S.; Carpenter, Gene B.; Sweigart, D A.; Czech, Paul T.; Overly, Kenneth R.; Coughlin, Fred

2002-03-18

222

Hydrodenitrogenation chemistry. I. Cleavage of alkylcarbon-nitrogen bonds, methane and ammonia formation in the HDN reaction of 1,2,3,4-tetrahydroquinoline with a nickel oxide catalyst supported on silica/alumina  

SciTech Connect

The hydrodenitrogenation reaction (HDN) is one of the most important industrial processes used in the refining of petroleum feedstocks and involves the removal of the nitrogen atom, as ammonia, from polynuclear heteroaromatic nitrogen compounds at high temperatures and high pressures of hydrogen gas (350-500/sup 0/C and 2000 psi). It is interesting to note that most of the reported heterogeneous catalysts require the complete hydrogenation of both the nitrogen heterocyclic ring and the aromatic ring before carbon-nitrogen bond cleavage can occur. A major breakthrough in the technical and economic aspects of the HDN reaction would take place if, in fact a catalyst could be found that would selectively cleave the C-N bond din the saturated nitrogen ring and subsequently produce ammonia, without substantial reduction of the aromatic rings, at lower temperatures as well as lower pressures of hydrogen gas. In this note, the authors report on a highly loaded nickel oxide catalyst (50% by weight Ni), supported on silica/alumina, that will effectively provide some of the criteria for an ideal HDN catalyst. 8 references.

Fish, R.H.; Thormodsen, A.D.; Moore, R.S.; Perry, D.L.; Heinemann, H.

1986-11-01

223

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.  

PubMed

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. PMID:23697416

Ramazani, Shapour

2013-05-21

224

Substrate specificity and kinetics for VP14, a carotenoid cleavage dioxygenase in the ABA biosynthetic pathway  

Microsoft Academic Search

The plant growth regulator, abscisic acid (ABA), is synthesized via the oxidative cleavage of an epoxy-carotenoid. Specifically, a double bond is cleaved by molecular oxygen and an aldehyde is formed at the site of cleavage in both products. The Vp14 gene from maize encodes an oxidative cleavage enzyme for ABA biosynthesis and the recombinant VP14 protein catalyzes the cleavage reaction

Steven H. Schwartz; Bao C. Tan; Donald R. McCarty; William Welch; Jan A. D. Zeevaart

2003-01-01

225

Enzymatic enantioselective C-C-bond formation in microreactors  

Microsoft Academic Search

We have demonstrated that multiple crude enzyme lysates containing a hydroxynitrile lyase can be used for the enantioselective synthesis of cyanohydrins from aldehydes in microchannels. Using a microreactor setup, two important parameters were efficiently screened consuming only minute amounts of reagents. More impor- tantly, results from the continuous flow reaction were fully consistent with results obtained from larger batchwise processes

K. Koch; R. J. F. van den Berg; P. J. Nieuwland; R. Wijtmans; H. E. Schoemaker; J. C. M. van Hest; F. P. J. T. Rutjes

2008-01-01

226

Large kinetic isotope effects in methane oxidation catalyzed by methane monooxygenase: evidence for C-H bond cleavage in a reaction cycle intermediate.  

PubMed

The reduced hydroxylase component (MMOH) of soluble methane monooxygenase (MMO) from Methylosinus trichosporium OB3b reacts with O2 and CH4 to produce CH3OH and H2O in a single-turnover reaction. Transient kinetic analysis of this reaction has revealed at least five and probably six intermediates during the turnover [Lee, S.-K., Nesheim, J. C., & Lipscomb, J. D. (1993) J. Biol. Chem. 268, 21569-21577; Liu, Y., Nesheim, J. C., Lee, S.-K., & Lipscomb, J. D. (1995) J. Biol. Chem. 270, 24662-24665]. One intermediate, termed compound Q, reacts with CH4 to yield enzyme-bound product. It is shown here that the deuterium kinetic isotope effect (KIE) for the reaction of compound Q with CH4 is 50-100, which is one of the largest effects observed to date. The rate constants for the reactions of the deuterated homologs of methane decrease monotonically as the deuterium content increases, suggesting that a large primary isotope effect dominates. The KIEs determined by analyzing the products after a single turnover have the following values: 1:1 CH4:CD4 (19); CD3H (12); CD2H2 (9); and CH3D (4). The KIE values determined by directly observing the reactive intermediate and by monitoring product ratios are all large, consistent with complete C-H bond breaking in the oxygenation step of the reaction. However, the differences in the KIE values determined by these two methods suggest that the reaction is more complex than currently proposed. A modified mechanism introducing the possibility of hydrogen-atom reabstraction by an intermediate methyl radical is proposed. PMID:8756490

Nesheim, J C; Lipscomb, J D

1996-08-01

227

Protonation of a Peroxodiiron(III) Complex and Conversion to a Diiron(III/IV) Intermediate: Implications for Proton-assisted O-O Bond Cleavage in Nonheme Diiron Enzymes  

PubMed Central

Oxygenation of a diiron(II) complex,[FeII2(?-OH)2(BnBQA)2(NCMe)2]2+ (2) (where BnBQA is N-benzyl-N,N-bis(2-quinolinylmethyl)amine) results in the formation of a metastable peroxodiferric intermediate (3). Treatment of 3 with strong acid affords its conjugate acid 4 in which the (?-oxo)(?-1,2-peroxo)diiron(III) core of 3 is protonated at the oxo bridge. The core structures of 3 and 4 are characterized in detail by UV-vis, Mössbauer, resonance Raman, and X-ray absorption spectroscopies. Complex 4 is shorter lived than 3 and decays to generate in 20–25% yield a diiron(III/IV) species (5) that can be identified by EPR and Mössbauer spectroscopy. This reaction sequence demonstrates for the first time that protonation of the oxo bridge of a (?-oxo)(?-1,2-peroxo)diiron(III) complex leads to the cleavage of the peroxo O–O bond and formation of a high-valent diiron complex, thereby mimicking the steps involved in the formation of intermediate X in the activation cycle of ribonucleotide reductase.

Cranswick, Matthew A.; Meier, Katlyn K.; Shan, Xiaopeng; Stubna, Audria; Kaizer, Joszef; Mehn, Mark P.; Munck, Eckard; Que, Lawrence

2012-01-01

228

Catalyzed intramolecular olefin insertion into a carbon-carbon single bond.  

PubMed

Intramolecular insertion of a C-C double bond into a C-C single bond was achieved by treatment of cyclobutanone bearing an o-styryl group at the 3-position with a catalytic amount of a cationic rhodium(I)-dppp complex. Initially, rhodium is inserted between the carbonyl carbon and the alpha-carbon of the cyclobutanone. Intramolecular coordination of the vinyl group results in its migratory insertion into the C-Rh linkage. Reductive elimination affords benzobicyclo[3.2.1]octan-3-one. Notably, a ring-opened alpha,beta-unsaturated ketone was obtained when dppe was used instead of dppp. In this reaction, rhodium cleaved the bond between the alpha sp3 carbon and the beta sp3 carbon of the cyclobutanone. The coordinating vinyl group directs this new regioselectivity of cleavage observed with the dppe ligand. PMID:12440879

Murakami, Masahiro; Itahashi, Tamon; Ito, Yoshihiko

2002-11-27

229

A DFT study on the interaction between adsorbed silver on C?? and disulfide bond.  

PubMed

Adsorption of a silver atom on the surface of Buckyball (C??) was investigated using density functional theory (DFT). The Ag atom tends to occupy the bridge site over C--C bond in pentagon-hexagon ring junction with the binding energy of -38.33 kcal mol?¹. The capability of destroying S--S bond by both a single silver atom and the silver atom adsorbed on C?? was also investigated by DFT calculations using dimethyl disulfide as the molecular model. The results of the natural bond orbital (NBO) and population analysis indicate that the cleavage of the S--S bond effectively occurs by the silver atom adsorbed on C??. Since denaturation of disulfide bonds of envelope glycoprotein (gp) 120 is a key step in the prevention of the spread of HIV-1, the development of the proposed study is promised to HIV-1 research field. PMID:23085174

Azizi, Khaled; Sohrabinia, Ali

2012-06-26

230

Facilitating room-temperature Suzuki coupling reaction with light: Mott-Schottky photocatalyst for C-C-coupling  

NASA Astrophysics Data System (ADS)

The Suzuki coupling reaction is one of the most practiced classes of catalytic C-C bond formation. The development of new means of activating molecules and bonds over old catalysts for C-C bond formation is a fundamental objective for chemists. Here, we report the room-temperature C-C bond formation over heterogeneous Pd catalysts by light-mediated catalyst activation. We employ stimulated electron transfer at the metal-semiconductor interface from optically active mesoporous carbon nitride nanorods to Pd nanoparticles. This photocatalytic pathway is highly efficient for coupling aryl halides with various coupling partners with high activity and selectivity under photo irradiation and very mild conditions.

Li, Xin-Hao; Baar, Moritz; Blechert, Siegfried; Antonietti, Markus

2013-04-01

231

Facilitating room-temperature Suzuki coupling reaction with light: Mott-Schottky photocatalyst for C-C-coupling  

PubMed Central

The Suzuki coupling reaction is one of the most practiced classes of catalytic C-C bond formation. The development of new means of activating molecules and bonds over old catalysts for C-C bond formation is a fundamental objective for chemists. Here, we report the room-temperature C-C bond formation over heterogeneous Pd catalysts by light-mediated catalyst activation. We employ stimulated electron transfer at the metal-semiconductor interface from optically active mesoporous carbon nitride nanorods to Pd nanoparticles. This photocatalytic pathway is highly efficient for coupling aryl halides with various coupling partners with high activity and selectivity under photo irradiation and very mild conditions.

Li, Xin-Hao; Baar, Moritz; Blechert, Siegfried; Antonietti, Markus

2013-01-01

232

The Amino Acid Sequence of Bovine Carboxypeptidase A. Iii. Specificity of Peptide-Bond Cleavage by Thermolysin and the Complete Sequence of the Cyanogen Bromide Fragment F(III).  

National Technical Information Service (NTIS)

The 81-residue fragment (F(III)) obtained from bovine carboxypeptidase A after cleavage with cyanogen bromide has been digested by thermolysin and the resulting peptides were isolated. Three of the peptides have been completely structured by Edman degrada...

R. A. Bradshaw

1969-01-01

233

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

2013-03-25

234

The Oxygenase CAO-1 of Neurospora crassa Is a Resveratrol Cleavage Enzyme.  

PubMed

The genome of the ascomycete Neurospora crassa encodes CAO-1 and CAO-2, two members of the carotenoid cleavage oxygenase family that target double bonds in different substrates. Previous studies demonstrated the role of CAO-2 in cleaving the C40 carotene torulene, a key step in the synthesis of the C35 apocarotenoid pigment neurosporaxanthin. In this work, we investigated the activity of CAO-1, assuming that it may provide retinal, the chromophore of the NOP-1 rhodopsin, by cleaving ?-carotene. For this purpose, we tested CAO-1 activity with carotenoid substrates that were, however, not converted. In contrast and consistent with its sequence similarity to family members that act on stilbenes, CAO-1 cleaved the interphenyl C?-C? double bond of resveratrol and its derivative piceatannol. CAO-1 did not convert five other similar stilbenes, indicating a requirement for a minimal number of unmodified hydroxyl groups in the stilbene background. Confirming its biological function in converting stilbenes, adding resveratrol led to a pronounced increase in cao-1 mRNA levels, while light, a key regulator of carotenoid metabolism, did not alter them. Targeted ?cao-1 mutants were not impaired by the presence of resveratrol, a phytoalexin active against different fungi, which did not significantly affect the growth and development of wild-type Neurospora. However, under partial sorbose toxicity, the ?cao-1 colonies exhibited faster radial growth than control strains in the presence of resveratrol, suggesting a moderate toxic effect of resveratrol cleavage products. PMID:23893079

Díaz-Sánchez, Violeta; F Estrada, Alejandro; Limón, M Carmen; Al-Babili, Salim; Avalos, Javier

2013-07-26

235

Tensile, shear and cleavage bond strengths of alginate adhesive 1 This paper is based on a dissertation for partial fulfilment of the degree of Master of Dental Surgery of the University of Hong Kong and was presented at the 43rd Annual Conference of the British Society for the Study of Prosthetic Dentistry, Belfast, April 1996. The presentation was awarded the Schottlanders' Poster Prize. 1  

Microsoft Academic Search

Objectives. This study determined the effect of alginate adhesive on various bond strengths of alginate to stainless steel.Methods. Three test assemblies were designed and machined in stainless steel for tension, shear and cleavage tests. Alginate adhesive (Fix™) was applied thinly and dried for 5min. Alginate (Blueprint™) was then loaded and allowed to set for 5min before testing. The force at

K. C. M. Leung; T. W. Chow; C. W. Woo; R. K. F. Clark

1998-01-01

236

Vibrational overtone spectroscopy, energy levels, and intensities of (CH3)3C-C?C-H.  

PubMed

The vibrational overtone spectra of the acetylenic (?? = 4, 5) and methyl (?? = 5, 6) C-H stretch transitions of tert-butyl acetylene [(CH(3))(3)C-C?C-H] were obtained using the phase shift cavity ring down (PS-CRD) technique at 295 K. The C-H stretch fundamental and overtone absorptions of the acetylenic (?? = 2 and 3) and methyl (?? = 2-4) C-H bonds have been obtained using a Fourier transform infrared and near-infrared spectrophotometer. Harmonic frequency ?(?(1)) and anharmonicities x(?(1)) and x(?(1), ?(24)) are reported for the acetylenic C-H bond. Molecular orbital calculations of geometry and vibrational frequencies were performed. A harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions of methyl C-H bonds. Band strength values were obtained experimentally and compared with intensities calculated in terms of the HCAO model where only the C-H modes are considered. No adjustable parameters were used to get order of magnitude agreement with experimental intensities for all pure local mode C-H transitions. PMID:22263573

Perez-Delgado, Yasnahir; Barroso, Jenny Z; Garofalo, Lauren A; Manzanares, Carlos E

2012-02-22

237

Activation and cleavage of the N-N bond in side-on bound [L?M-NN-ML?] (L = NH?, NMe?, N(i)Pr?, C?H?, C?Me?H) dinitrogen complexes of transition metals from groups 4 through 9.  

PubMed

The activation and cleavage of the N-N bond in side-on bound [L?M-NN-ML?] (L = NH?, NMe?, N(i)Pr?, C?H?, C?Me?H) dinitrogen complexes of transition metals in groups 4 through 9 have been investigated using density functional theory. Emphasis has been placed on Ti, Zr, and Hf (group 4) complexes due to their experimental relevance. Calculations on these species have shown that for cases when the structural configuration corresponds to the terminal [ML?] fragments adopting a perpendicular orientation with respect to the central [N-N] unit, a considerably higher degree of N-N activation is predicted relative to that observed in the experimentally characterized cyclopentadienyl analogues and in related systems involving end-on dinitrogen coordination. An examination of the orbital interactions between the metal-based fragments and the dinitrogen unit shows that both ? and ? bonding are important in the side-on binding mode, in contrast to the end-on mode where metal-nitrogen ? interactions are dominant. This analysis also reveals that the model amide systems possess the orbital properties identified as necessary for successful N-N hydrogenation. A significant result obtained for the amide complexes containing metals from groups 5 (V, Nb, Ta), 6 (Cr, Mo, W), and 7 (Mn, Tc, Re), is the presence of metal-metal bonding in configurations that are considerably distorted from planarity. As a consequence, these complexes exhibit strongly enhanced stability relative to species where metal-metal bonding is absent. In contrast, the d² metal-based configurations in the group 4 complexes of Ti, Zr, and Hf are unable to provide the six electrons required for complete reductive cleavage of the dinitrogen unit which is necessary to allow the metal centres to approach one another sufficiently for metal-metal bond formation. PMID:20372747

Cavigliasso, Germán; Wilson, Laurence; McAlpine, Sarlae; Attar, Mariam; Stranger, Robert; Yates, Brian F

2010-05-21

238

XPS study of hydrogen permeation effect on SiC-C films  

NASA Astrophysics Data System (ADS)

70%SiC-C films were deposited with r.f. magnetron sputtering on stainless steel substrates followed by ion beam mixing. These films were permeated by hydrogen gas under the pressure of 3.23×107 Pa for 3 h at 500 K. X-ray photoelectron spectroscopy was used to analyze the chemical bonding states of C and Si in these 70%SiC-C films before and after hydrogen gas permeation. In addition, chemical states of contaminated oxygen were also checked. The effect of hydrogen permeation on those states in the 70%SiC-C films is discussed in this paper.

Huang, N. K.; Wang, D. Z.; Xiong, Q.; Yang, B.

2003-08-01

239

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

240

Symmetry and cleavage fracture  

SciTech Connect

The role of the symmetry of the interaction of shock waves with free and supported surfaces in the formation of cleavage fractures is studied. A qualitative analysis is made of the schemes of two limiting cases of the development of relief processes in a plate compressed by a shock wave. The results of the analysis are compared with our experiments and those of other researchers. The data obtained indicate that the process of cleavage fractures can be controlled by changing the loading symmetry.

Lebedev, M.A.; Litvinov, B.V.

1995-11-01

241

Anomalous side chain cleavage in alkylaromatic pyrolysis  

SciTech Connect

In reactions of hydrocarbons, alkylaromatic side chain cleavage occurs as the result of thermal or acid catalyzed cracking chemistry. In general, acid catalysis leads to direct bond cleavage at the aromatic ring whereas thermally promoted bond breaking gives primarily substituted aromatics. The formation of a core aromatic by a thermal process is expected only at higher temperatures and in low yield due to the need for hydrogen atoms. Recently, for the alkylpyrene system, extensive direct side chain cleavage has been observed for liquid phase, low temperature (375-425{degree}C) pyrolysis (43% yield of pyrene at 400{degree}C and 180 min.). Pyrene formation was reported to be the result of autocatalysis. This yield of core aromatic is much greater than that found for single ring aromatics. In this paper, the authors with to report the results of a combined theoretical and experimental study which they believe does provide experimental confirmation of a direct cleavage path. In support of this conclusion, they present an analysis of the possible reaction pathways.

Freund, H.; Matturro, M.G.; Olmstead, W.N.; Reynolds, R.P.; Upton, T.H. (Exxon Research and Engineering Co., Annandale, NJ (USA))

1990-01-01

242

Reactivity of pyridine on Mo(110): C-H and C-N bond activation  

SciTech Connect

Pyridine undergoes competing desorption (30 {plus minus} 10%) and decomposition to surface carbon, nitrogen, and gaseous dihydrogen on Mo(110). Pyridine is shown to molecularly desorb, since no reversible C-H bond activation occurs in isotopic exchange experiments. The temperature at which total decomposition involving the cleavage of C-H, C-C, and C-N bonds occurs was found to increase with increasing coverage of pyridine and related surface fragments. Some C-H bond breaking occurs at temperatures less than 325 K as evidenced by isotopic labeling results. X-ray photoelectron spectroscopy results show that C-C bond activation occurs below 500 K. Notably, at least one of the C-N bonds in pyridine is intact at temperatures up to 500 K at high coverages on the basis of X-ray photoelectron spectroscopy results. An isotope effect in dihydrogen formation was observed in the reaction of a mixture of C{sub 5}D{sub 5}N and C{sub 5}H{sub 5}N. At high coverage and low temperature there is more than one species on the surface, one of which is assigned as molecular pyridine bound through nitrogen. The presence of surface hydrogen increases the amount of molecular desorption and, therefore, decreases the total amount of irreversible decomposition by {approx} 40%. As a result of the lower coverage of species on the surface after molecular desorption, total decomposition, and in particular C-N bond cleavage, occurs at lower temperature than on the initially clean surface.

Serafin, J.G.; Friend, C.M. (Harvard Univ., Cambridge, MA (USA))

1989-03-09

243

Addition, cycloaddition, and metathesis reactions of the cationic carbyne complexes [Cp(CO)[sub 2]Mn[triple bond]CCH[sub 2]R][sup +] and neutral vinylidene complexes Cp(CO)[sub 2]M=C=C(H)R (M = Mn, Re)  

SciTech Connect

The cationic alkylidyne complexes [Cp(CO)[sub 2]M=VCCH[sub 2]R][sup +] (M = Re, R = H; M = Mn, R = H, Me, Ph) undergo facile deprotonation to give the corresponding neutral vinylidene complexes Cp(CO)[sub 2]M=C=C(H)R. For [Cp(CO)[sub 2]Re=VCCH[sub 3

Terry, M.R.; Mercando, L.A.; Kelley, C.; Geoffroy, G.L. (Pennsylvania State Univ., University Park, PA (United States)); Nombel, P.; Lugan, N.; Mathieu, R. (Laboratoire de Chimie de Coordination du CNRS, Toulouse (France)); Ostrander, R.L.; Owens-Waltermire, B.E.; Rheingold, A.L. (Univ. of Delaware, Newark, DE (United States))

1994-03-01

244

CH Bond Functionalization in Complex Organic Synthesis  

Microsoft Academic Search

Direct and selective replacement of carbon-hydrogen bonds with new bonds (such as C-C, C-O, and C-N) represents an important and long-standing goal in chemistry. These transformations have broad potential in synthesis because C-H bonds are ubiquitous in organic substances. At the same time, achieving selectivity among many different C-H bonds remains a challenge. Here, we focus on the functionalization of

Kamil Godula; Dalibor Sames

2006-01-01

245

Hydrogen Induced C-C, C-N, & C-S Bond Activation on Pt & Ni Surfaces  

SciTech Connect

The primary reactions investigated were chosen based on their importance in fuel and chemical production as well as in environmental remediation, and include reactions for hydrodesulfurization (HDS), hydrodenitrogenation (HDN), carbon-carbon hydrogenolysis, and hydrocarbon oxidation.

Gland, J. L.

2004-07-29

246

I. Spectroscopic and theoretical studies of heterolytic and homolytic N-H and C-H bond cleavage in heterocycles. II. Photooxidation of sulfur-containing coal model compounds  

SciTech Connect

Theoretical estimation of proton affinities for the two types of urazole conjugate bases failed to predict which of the protons present in urazole (hydrazyl vs imide) is more acidic. Analogous calculations performed for hydantoin and thiourazole gave results that are in good agreement with the experimental pK[sub a] measurements. A comparison of [sup 13]C NMR spectra for the urazoles and related nitrogen acids with those obtained for the conjugate bases derived from these compounds confirm previous estimates of the high acidity of the hydrazyl protons. An excellent linear correlation is observed for the [sup 13]C[double bond]O and [sup 13]C[double bond]S NMR signals in a hydantoin-thiohydantoin series. [sup 13]C NMR results for the urazole-thiourazole series appear to correlate with [delta][sub C[double bond]O]/[delta][sub C[double bond]S] data for hydantoin-thiohydantoin anions. Ab initio molecular orbital calculations have been conducted for syn and anti conformers of methyl acetate, and corresponding radical, to evaluate the magnitude of cyclization effect on the homolytic C-H bond strength in the Meldrum's acid. In analogous calculations, conducted for anions and radicals, the results obtained for butanone conformers are in agreement with the experimental data, but fail for propionamide. Two methods of photooxidation in a series of coal model sulfides have been examined. Singlet oxygen reactions generally do not produce sulfoxides and sulfones with satisfactory yields. Superoxide oxygenations have been found to be more efficient. These reactions produce polymeric products for thiophene and benzothiophene, but for dibenzothiophene result in selective formation of the corresponding sulfoxide and sulfone with good yields. Efficient photooxidation of dibenzothiophene is also observed in the anthracene-sensitized reaction, which is of importance for coal photochemistry.

Dobrowolski, P.

1993-01-01

247

Tyrosine Deprotonation Yields Abundant and Selective Backbone Cleavage in Peptide Anions upon Negative Electron transfer Dissociation and Ultraviolet Photodissociation  

PubMed Central

Tyrosine deprotonation in peptides yields preferential electron detachment upon NETD or UVPD, resulting in prominent N – C? bond cleavage N-terminal to the tyrosine residue. UVPD of iodo-tyrosine modified peptides was used to generate localized radicals on neutral tyrosine side chains by homolytic cleavage of the C – I bond. Subsequent collisional activation of the radical species yielded the same preferential cleavage of the adjacent N-terminal N – C? bond. LC-MS/MS analysis of a tryptic digest of BSA demonstrated that these cleavages are regularly observed for peptides when using high pH mobile phases.

Shaw, Jared B.; Ledvina, Aaron R.; Zhang, Xing; Julian, Ryan R.; Brodbelt, Jennifer S.

2012-01-01

248

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

249

Study of B¯??c?¯c- and B¯??c+?¯c-K¯ decays at BABAR  

NASA Astrophysics Data System (ADS)

We report measurements of B-meson decays into two- and three-body final states containing two charmed baryons using a sample of 230×106 ?(4S)?BB¯ decays. We find significant signals in two modes, measuring branching fractions B(B-??c+?¯c-K-)=(1.14±0.15±0.17±0.60)×10-3 and B(B-??c0?¯c-)×B(?c0??-?+)=(2.08±0.65±0.29±0.54)×10-5, where the uncertainties are statistical, systematic, and from the branching fraction B(?c+?pK-?+), respectively. We also set upper limits at the 90% confidence level on two other modes: B(B¯0??c+?¯c-)×B(?c+??-?+?+)<5.6×10-5 and B(B¯0??c+?¯c-K¯0)<1.5×10-3. We observe structure centered at an invariant mass of 2.93GeV/c2 in the ?c+K- mass distribution of the decay B-??c+?¯c-K-.

Aubert, B.; Bona, M.; Boutigny, D.; Karyotakis, Y.; Lees, J. P.; Poireau, V.; Prudent, X.; Tisserand, V.; Zghiche, A.; Tico, J. Garra; Grauges, E.; Lopez, L.; Palano, A.; Pappagallo, M.; Eigen, G.; Stugu, B.; Sun, L.; Abrams, G. S.; Battaglia, M.; Brown, D. N.; Button-Shafer, J.; Cahn, R. N.; Groysman, Y.; Jacobsen, R. G.; Kadyk, J. A.; Kerth, L. T.; Kolomensky, Yu. G.; Kukartsev, G.; Pegna, D. Lopes; Lynch, G.; Mir, L. M.; Orimoto, T. J.; Osipenkov, I. L.; Ronan, M. T.; Tackmann, K.; Tanabe, T.; Wenzel, W. A.; Del Amo Sanchez, P.; Hawkes, C. M.; Watson, A. T.; Koch, H.; Schroeder, T.; Walker, D.; Asgeirsson, D. J.; Cuhadar-Donszelmann, T.; Fulsom, B. G.; Hearty, C.; Mattison, T. S.; McKenna, J. A.; Barrett, M.; Khan, A.; Saleem, M.; Teodorescu, L.; Blinov, V. E.; Bukin, A. D.; Druzhinin, V. P.; Golubev, V. B.; Onuchin, A. P.; Serednyakov, S. I.; Skovpen, Yu. I.; Solodov, E. P.; Todyshev, K. Yu.; Bondioli, M.; Curry, S.; Eschrich, I.; Kirkby, D.; Lankford, A. J.; Lund, P.; Mandelkern, M.; Martin, E. C.; Stoker, D. P.; Abachi, S.; Buchanan, C.; Foulkes, S. D.; Gary, J. W.; Liu, F.; Long, O.; Shen, B. C.; Vitug, G. M.; Zhang, L.; Paar, H. P.; Rahatlou, S.; Sharma, V.; Berryhill, J. W.; Campagnari, C.; Cunha, A.; Dahmes, B.; Hong, T. M.; Kovalskyi, D.; Richman, J. D.; Beck, T. W.; Eisner, A. M.; Flacco, C. J.; Heusch, C. A.; Kroseberg, J.; Lockman, W. S.; Schalk, T.; Schumm, B. A.; Seiden, A.; Wilson, M. G.; Winstrom, L. O.; Chen, E.; Cheng, C. H.; Fang, F.; Hitlin, D. G.; Narsky, I.; Piatenko, T.; Porter, F. C.; Andreassen, R.; Mancinelli, G.; Meadows, B. T.; Mishra, K.; Sokoloff, M. D.; Blanc, F.; Bloom, P. C.; Chen, S.; Ford, W. T.; Hirschauer, J. F.; Kreisel, A.; Nagel, M.; Nauenberg, U.; Olivas, A.; Smith, J. G.; Ulmer, K. A.; Wagner, S. R.; Zhang, J.; Gabareen, A. M.; Soffer, A.; Toki, W. H.; Wilson, R. J.; Winklmeier, F.; Altenburg, D. D.; Feltresi, E.; Hauke, A.; Jasper, H.; Merkel, J.; Petzold, A.; Spaan, B.; Wacker, K.; Klose, V.; Kobel, M. J.; Lacker, H. M.; Mader, W. F.; Nogowski, R.; Schubert, J.; Schubert, K. R.; Schwierz, R.; Sundermann, J. E.; Volk, A.; Bernard, D.; Bonneaud, G. R.; Latour, E.; Lombardo, V.; Thiebaux, Ch.; Verderi, M.; Clark, P. J.; Gradl, W.; Muheim, F.; Playfer, S.; Robertson, A. I.; Watson, J. E.; Xie, Y.; Andreotti, M.; Bettoni, D.; Bozzi, C.; Calabrese, R.; Cecchi, A.; Cibinetto, G.; Franchini, P.; Luppi, E.; Negrini, M.; Petrella, A.; Piemontese, L.; Prencipe, E.; Santoro, V.; Anulli, F.; Baldini-Ferroli, R.; Calcaterra, A.; de Sangro, R.; Finocchiaro, G.; Pacetti, S.; Patteri, P.; Peruzzi, I. M.; Piccolo, M.; Rama, M.; Zallo, A.; Buzzo, A.; Contri, R.; Lo Vetere, M.; Macri, M. M.; Monge, M. R.; Passaggio, S.; Patrignani, C.; Robutti, E.; Santroni, A.; Tosi, S.; Chaisanguanthum, K. S.; Morii, M.; Wu, J.; Dubitzky, R. S.; Marks, J.; Schenk, S.; Uwer, U.; Bard, D. J.; Dauncey, P. D.; Flack, R. L.; Nash, J. A.; Vazquez, W. Panduro; Tibbetts, M.; Behera, P. K.; Chai, X.; Charles, M. J.; Mallik, U.; Cochran, J.; Crawley, H. B.; Dong, L.; Eyges, V.; Meyer, W. T.; Prell, S.; Rosenberg, E. I.; Rubin, A. E.; Gao, Y. Y.; Gritsan, A. V.; Guo, Z. J.; Lae, C. K.; Denig, A. G.; Fritsch, M.; Schott, G.; Arnaud, N.; Béquilleux, J.; D'Orazio, A.; Davier, M.; Grosdidier, G.; Höcker, A.; Lepeltier, V.; Le Diberder, F.; Lutz, A. M.; Pruvot, S.; Rodier, S.; Roudeau, P.; Schune, M. H.; Serrano, J.; Sordini, V.; Stocchi, A.; Wang, W. F.; Wormser, G.; Lange, D. J.; Wright, D. M.; Bingham, I.; Burke, J. P.; Chavez, C. A.; Fry, J. R.; Gabathuler, E.; Gamet, R.; Hutchcroft, D. E.; Payne, D. J.; Schofield, K. C.; Touramanis, C.; Bevan, A. J.; George, K. A.; di Lodovico, F.; Sacco, R.; Cowan, G.; Flaecher, H. U.; Hopkins, D. A.; Paramesvaran, S.; Salvatore, F.; Wren, A. C.; Brown, D. N.; Davis, C. L.; Allison, J.; Bailey, D.; Barlow, N. R.; Barlow, R. J.; Chia, Y. M.; Edgar, C. L.; Lafferty, G. D.; West, T. J.; Yi, J. I.; Anderson, J.; Chen, C.; Jawahery, A.; Roberts, D. A.; Simi, G.; Tuggle, J. M.; Blaylock, G.; Dallapiccola, C.; Hertzbach, S. S.; Li, X.; Moore, T. B.; Salvati, E.; Saremi, S.; Cowan, R.; Dujmic, D.; Fisher, P. H.; Koeneke, K.; Sciolla, G.; Spitznagel, M.; Taylor, F.; Yamamoto, R. K.; Zhao, M.; Zheng, Y.; McLachlin, S. E.; Patel, P. M.; Robertson, S. H.; Lazzaro, A.; Palombo, F.; Bauer, J. M.; Cremaldi, L.; Eschenburg, V.; Godang, R.; Kroeger, R.; Sanders, D. A.; Summers, D. J.; Zhao, H. W.; Brunet, S.; Côté, D.; Simard, M.; Taras, P.; Viaud, F. B.; Nicholson, H.; de Nardo, G.; Fabozzi, F.; Lista, L.; Monorchio, D.; Sciacca, C.; Baak, M. A.; Raven, G.; Snoek, H. L.; Jessop, C. P.; Knoepfel, K. J.; Losecco, J. M.; Benelli, G.; Corwin, L. A.; Honscheid, K.; Kagan, H.; Kass, R.; Morris, J. P.; Rahimi, A. M.; Regensburger, J. J.; Sekula, S. J.; Wong, Q. K.

2008-02-01

250

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

2011-08-04

251

[Me2NN]Co(eta6-toluene): O=O, N=N, and O=N bond cleavage provides beta-diketiminato cobalt mu-oxo and imido complexes.  

PubMed

The synthesis and structure of a novel beta-diketiminato Co(I) arene adduct [Me2NN]Co(eta6-toluene) (2) are described, that serves as a synthon to the reactive, "naked" 12-electron [Me2NN]Co fragment via loss of toluene in its reactions with dioxygen, organoazides, and a nitrosobenzene. Exposure of 2 to dioxygen in ether leads to {[Me2NN]Co}2(mu-O)2 (3), a rare example of a cobalt-oxo complex thermally stable at room temperature. The X-ray structure of 3 reveals a short Co-Co separation of 2.716(4) A and exhibits positional disorder for the bridging oxo groups; the predominant configuration contains oxygen atoms in square-planar sites with short Co-O distances (1.784(3) and 1.793(4) A). Reaction of 2 with organoazides N3R (R = 3,5-Me2C6H3 (Ar) or 1-adamantyl (Ad)) results in the formation of imido complexes whose structure depends on the nature of the azido substituent. The synthesis and structures of both {Me2NN]Co}2(mu-NAr)2 (4) with arylimido groups in tetrahedral bridging sites or the three-coordinate, 16-electron [Me2NN]CoNAd (5) are described. The X-ray structure of terminal imide 5 reveals a short Co-N bond distance (1.624(4) A) and only somewhat bent imido linkage (Co-N-C = 161.5(3) degrees ) consistent with a significant degree of multiple bond character. Complex 2 cleaves the O=N bond of the nitrosobenzene O=NAr (Ar = 3,5-Me2C6H3) to form the binuclear oxo-imido complex {[Me2NN]Co}2(mu-O)(mu-NAr) (6) that possesses a structure intermediate between square-planar 3 and tetrahedral 4 in which the [Me2NN]Co fragments are mutually orthogonal. PMID:15080682

Dai, Xuliang; Kapoor, Pooja; Warren, Timothy H

2004-04-21

252

Design and synthesis of quasi-diastereomeric molecules with unchanging central, regenerating axial and switchable helical chirality via cleavage and formation of Ni(II)-O and Ni(II)-N coordination bonds  

PubMed Central

Summary We describe herein the design and synthesis of asymmetric, pentadentate ligands, which are able to coordinate to Ni(II) cations leading to quasi-diastereomeric complexes displaying two new elements of chirality: stereogenic axis and helix along with configurational stabilization of the stereogenic center on the nitrogen. Due to the stereocongested structural characteristics of the corresponding Ni(II) complexes, the formation of quasi-diastereomeric products is highly stereoselective providing formation of only two, (R a*,M h*,R c*) and (R a*,P h*,R c*), out of the four possible stereochemical combinations. The reversible quasi-diastereomeric transformation between the products (R a*,M h*,R c*) and (R a*,P h*,R c*) occurs by intramolecular trans-coordination of Ni–NH and Ni–O bonds providing a basis for a chiral switch model.

Acena, Jose Luis; Ueki, Hisanori; Han, Jianlin

2012-01-01

253

Synthesis and decarbonylation reactions of the triiron phosphinidene complex [Fe3Cp3(?-H)(?3-PPh)(CO)4]: easy cleavage and formation of P-H and Fe-Fe bonds.  

PubMed

The binuclear phosphine complex [Fe(2)Cp(2)(?-CO)(2)(CO)(PH(2)Ph)] (Cp = ?(5)-C(5)H(5)) reacted with the acetonitrile adduct [Fe(2)Cp(2)(?-CO)(2)(CO)(NCMe)] in dichloromethane at 293 K to give the trinuclear hydride-phosphinidene derivative [Fe(3)Cp(3)(?-H)(?(3)-PPh)(CO)(4)] as a mixture of cis,anti and trans isomers (Fe-Fe = 2.7217(6) Å for the cis,anti isomer). In contrast, photochemical treatment of the phosphine complex with [Fe(2)Cp(2)(CO)(4)] gave the phosphide-bridged complex trans-[Fe(3)Cp(3)(?-PHPh)(?-CO)(2)(CO)(3)] as the major product, along with small amounts of the binuclear hydride-phosphide complexes [Fe(2)Cp(2)(?-H)(?-PHPh)(CO)(2)] (cis and trans isomers), which are more selectively prepared from [Fe(2)Cp(2)(CO)(4)] and PH(2)Ph at 388 K. The photochemical decarbonylation of either of the mentioned triiron compounds led reversibly to three different products depending on the reaction conditions: (a) the 48-electron phosphinidene cluster [Fe(3)Cp(3)(?-H)(?(3)-PPh)(?-CO)(2)] (Fe-Fe = 2.592(2)-2.718(2) Å); (b) the 50-electron complex [Fe(3)Cp(3)(?-H)(?(3)-PPh)(?-CO)(CO)(2)], also having carbonyl- and hydride-bridged metal-metal bonds (Fe-Fe = 2.6177(3) and 2.7611(4) Å, respectively); and (c) the 48-electron phosphide cluster [Fe(3)Cp(3)(?-PHPh)(?(3)-CO)(?-CO)(2)], an isomer of the latter phosphinidene complex now having three intermetallic bonds (Fe-Fe = 2.5332(8)-2.6158(8) Å). PMID:21981036

Alvarez, Celedonio M; Alvarez, M Angeles; García, M Esther; González, Rocío; Ramos, Alberto; Ruiz, Miguel A

2011-10-07

254

Cleavage of Ge–S and C–H bonds in the reaction of electron-deficient [Os 3(CO) 8(?-H)(? 3Ph 2PCH 2P(Ph)C 6H 4)] with Ph 3GeSPh: Generation of thiophenol derivatives [Os 3(CO) 8(?-H)(?-SPh)(?-dppm)] and [Os 3(CO) 7(?-H)(?-SPh)(? 3SC 6H 4)(?-dppm)  

Microsoft Academic Search

Heating the electron-deficient [Os3(CO)8(?-H)(?3-Ph2PCH2P(Ph)C6H4)] (1) and Ph3GeSPh in benzene at 80°C led to the thiolato bridged compounds, [Os3(CO)8(?-H)(?-SPh)(?-dppm)] (2) and [Os3(CO)7(?-H)(?-SPh)(?3-SC6H4)(?-dppm)] (3), formed by cleavage of Ge–S and C–S bonds of the ligand, in 40% and 17% yields, respectively. Both compounds 2 and 3 have been characterized by a combination of elemental analysis, infrared and 1H NMR spectroscopic data together

Arun K. Raha; Shishir Ghosh; Shariff E. Kabir; Brian K. Nicholson; Derek A. Tocher

2009-01-01

255

Peptide substrate cleavage specificity of the human cytomegalovirus protease.  

PubMed

The human cytomegalovirus UL80 gene encodes an 80-kDa precursor polyprotein whose N-terminal 256-amino acid domain is a protease. This enzyme cleaves a specific peptide bond that results in its own release from the precursor, as well as a peptide bond near the C terminus of the viral assembly protein. The latter cleavage is apparently required for encapsidation of the viral genomic DNA and maturation of the viral capsid. A series of peptide substrates, representing the assembly protein cleavage site, was used to study the enzyme's substrate requirements and specificity. It was found that efficient cleavage minimally required the amino acid residues spanning the P4 to P4' positions. Substitution at any of these residues adversely affected the reaction. Conservation of the hydrophobic residues at P3 and P4 was essential. In addition, cleavage of a peptide representing the protease domain release site was reduced almost 100-fold relative to cleavage of the assembly protein maturation site peptide substrate. PMID:8182034

Sardana, V V; Wolfgang, J A; Veloski, C A; Long, W J; LeGrow, K; Wolanski, B; Emini, E A; LaFemina, R L

1994-05-20

256

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

257

Molybdenum complexes of 1,2-bis(diphenylphosphino)benzene. Mononuclear molybdenum(II) species formed by facile metal-metal bond cleavage of the (Mo-/sup 4/Mo)/sup 4+/ core  

SciTech Connect

The quadruply bonded dimolybdenum(II) complexes K/sub 4/Mo/sub 2/Cl/sub 8/, (NH/sub 4/)/sub 5/MoCl/sub 9/ /times/ H/sub 2/O, and (NH/sub 4/)/sub 4/Mo/sub 2/Br/sub 8/ react with 1,2-C/sub 6/H/sub 4/(PPh/sub 2/)/sub 2/(dppbe) in methanol at room temperature to afford /alpha/-Mo/sub 2/X/sub 4/(dppbe)/sub 2/ complexes (X = Cl, Br), which do not isomerize to the /beta/ isomers. Under more forcing reactions conditions (refluxing 1-propanol), these same reactions give mononuclear trans-MoX/sub 2/(dppbe)/sub 2/ in good yield (ca. 50%) together with some (MoOX(dppbe)/sub 2/)X /times/ nH/sub 2/O. An alternative synthetic strategy for the preparation of /alpha/-Mo/sub 2/X/sub 4/(dppbe)/sub 2/ involves the reaction of Mo/sub 2/(O/sub 2/CCH/sub 3/)/sub 4/ with dppbe and Me/sub 3/SiX in THF. The compound MoCl/sub 2/(dppbe)/sub 2/ forms crystals in space group P2/sub 1//n, with the following unit cell parameters: a = 10.884 (2) /angstrom/, b = 12.753 (2) /angstrom/, c = 18.141 (4) /angstrom/, /beta/ = 91.43 (2)/degree/, V = 2517 (2) /angstrom//sup 3/, and Z = 2. The centrosymmetric trans molecule has Mo-Cl = 2.410 (1) /angstrom/, Mo-P = 2.481 (1), 2.511 (1) /angstrom/, and P-Mo-P(intra-ring) = 78.68 (4)/degree/. 25 refs., 1 fig., 4 tabs.

Bakir, M.; Cotton, F.A.; Cudahy, M.M.; Simpson, C.Q.; Smith, T.J.; Vogel, E.F.; Walton, R.A.

1988-07-27

258

Facile bismuth-oxygen bond cleavage, C-H activation, and formation of a monodentate carbon-bound oxyaryl dianion, (C?H?(t)Bu?-3,5-O-4)²?.  

PubMed

The Bi(3+) (N,C,N)-pincer complex Ar'BiCl(2) (1) [Ar' = 2,6-(Me(2)NCH(2))(2)C(6)H(3)], reacts with 2 equiv of KOC(6)H(3)Me(2)-2,6 and KOC(6)H(3)(i)Pr(2)-2,6 by ionic metathesis to form the anticipated bis(aryloxide) complexes Ar'Bi(OC(6)H(3)Me(2)-2,6)(2) (2) and Ar'Bi(OC(6)H(3)(i)Pr(2)-2,6)(2) (3), respectively. However, the analogous reaction with 2 equiv of KOC(6)H(3)(t)Bu(2)-2,6 forms HOC(6)H(3)(t)Bu(2)-2,6 and a dark-orange complex containing only one aryloxide-derived ligand bound via a Bi-C and not a Bi-O linkage. This complex is formulated as Ar'Bi(C(6)H(2)(t)Bu(2)-3,5-O-4) (4), a product of para C-H bond activation. Structural, spectroscopic, and DFT studies and a comparison with the protonated analogue [Ar'Bi(C(6)H(2)(t)Bu(2)-3,5-OH-4)][BPh(4)] (5), which was obtained by treatment of 4 with [HNEt(3)][BPh(4)], suggest that 4 contains an oxyaryl dianion. Complex 4 represents a fully characterizable product of a bismuth-mediated C-H activation and rearrangement of the type postulated in catalytic SOHIO processes. PMID:21417273

Casely, Ian J; Ziller, Joseph W; Fang, Ming; Furche, Filipp; Evans, William J

2011-03-18

259

Disulfide bonds as switches for protein function  

Microsoft Academic Search

The prevailing view is that disulfide bonds have been added during evolution to enhance the stability of proteins that function in a fluctuating cellular environment. However, recent evidence indicates that disulfide bonds can be more than inert structural motifs. The function of some secreted soluble proteins and cell-surface receptors is controlled by cleavage of one or more of their disulfide

Philip J. Hogg

2003-01-01

260

Atomistic simulation of an f.c.c./b.c.c. interface in Ni-Cr alloys  

SciTech Connect

The embedded atom method is applied to study the atomic structure and energy of an f.c.c./b.c.c. interface in Ni-Cr. The two phases are oriented in a Kurdjumov-Sachs orientation relationship, and the interface considered is the (1 {bar 2} 1){sub f} habit plane adopted by precipitate laths of the b.c.c. phase. The interfacial energy and coherent strain energy at 0 K are calculated for boundaries between an f.c.c. Ni-Cr solid solution and b.c.c. Cr. The calculated interfacial energy varies from 216 mJ/m{sup 2} when the f.c.c. phase is pure Ni to 200 mJ/m{sup 2} when the f.c.c. phase is Ni-50 at.% Cr. Atomic relaxations appear limited to atoms in contact with the interphase boundary. Most of the interfacial energy is attributed to the structural difference across the f.c.c./b.c.c. boundary, and the chemical contribution to the energy is estimated to be less than 20% of the total energy. The values of the calculated energies and the widespread occurrence of the (1{bar 2}1){sub f} habit plane in a variety of alloy systems indicate this boundary orientation has a relatively low interfacial energy.

Chen, J.K. [Exxon Research and Engineering Co., Clinton Township, NJ (United States). Corporate Research Lab.; Reynolds, W.T. Jr. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Materials Science and Engineering Dept.

1997-11-01

261

Modelling of c-C2H4O formation on grain surfaces  

NASA Astrophysics Data System (ADS)

Despite its potential reactivity due to ring strain, ethylene oxide (c-C2H4O) is a complex molecule that seems to be stable under the physical conditions of an interstellar dense core; indeed, it has been detected towards several high-mass star-forming regions with a column density of the order of 1013 cm-2. To date, its observational abundances cannot be reproduced by chemical models and this may be due to the significant contribution played by its chemistry on grain surfaces. Recently, Ward & Price have performed experiments in order to investigate the surface formation of ethylene oxide starting with oxygen atoms and ethylene ice as reactants. We present a chemical model which includes the most recent experimental results from Ward & Price on the formation of c-C2H4O. We study the influence of the physical parameters of dense cores on the abundances of c-C2H4O. We verify that ethylene oxide can indeed be formed during the cold phase (when the interstellar medium dense cores are formed), via addition of an oxygen atom across the C=C double bond of the ethylene molecule, and released by thermal desorption during the hot core phase. A qualitative comparison between our theoretical results and those from the observations shows that we are able to reproduce the abundances of ethylene oxide towards high-mass star-forming regions.

Occhiogrosso, A.; Viti, S.; Ward, M. D.; Price, S. D.

2012-12-01

262

Interaction of EcoRII restriction and modification enzymes with synthetic DNA fragments. V. Study of single-strand cleavages.  

PubMed Central

Concatemer DNA duplexes which contain at the EcoRII restriction endonuclease cleavage sites (formula; see text) phosphodiester, phosphoamide or pyrophosphate internucleotide bonds have been synthesized. It has been shown that this enzyme did not cleave the substrate at phosphoamide bond. EcoRII endonuclease catalyzes single-strand cleavages both in dA- and dT-containing strands of the recognition site if the cleavage of the other strand has been blocked by modification of scissile bond or if the other strand has been cleaved. This enzyme interacts with both strands of the DNA recognition site, each of them being cleaved independently on the cleavage of another one. Nucleotide sequences flanking the EcoRII site on both sides are necessary for effective cleavage of the substrate. Images

Yolov, A A; Gromova, E S; Kubareva, E A; Potapov, V K; Shabarova, Z A

1985-01-01

263

Cleavage and identification of proteins: a modified aspartyl-prolyl cleavage.  

PubMed

We have developed a method for rapidly cleaving and identifying proteins electroblotted onto poly(vinylidene difluoride) membranes. Cleavage is performed with 10% acetic acid in 7 M guanidine chloride at pH 2.5 for 1 h at 90 degrees C, resulting in fragmentation primarily at aspartyl-prolyl bonds. Peptides resulting from non-Asp-Pro cleavage are N-terminally blocked by reaction with orthophthalaldehyde (OPA) prior to automated Edman degradation. Reaction with OPA after cleavage blocks all amino acids containing primary amino groups. Only peptides containing an N-terminal amino acid with a secondary amino group (proline) will be available for reaction with the Edman reagent. The sequences obtained are used for protein database searching. Using this approach, proteins that are found to be N-terminally blocked can be removed from the sequencer, cleaved with acetic acid, blocked with OPA, and reapplied to the sequencer. The protein can then be identified from a database search using the sequence mixture obtained. PMID:11080897

Kishiyama, A; Zhang, Z; Henzel, W J

2000-11-01

264

Fracture toughness of f. c. c. nickel and strain aging b. c. c. iron in the temperature range 77-773 K  

SciTech Connect

Ductile initiation fracture toughness J[sub IC] of b.c.c. Armco iron and f.c.c. nickel has been measured in the temperature range 77-773 K. Armco iron exhibits dynamics strain aging (DSA) in the temperature range 383-573 K while nickel of the purity used does not evince DSA. Load vs load line displacement (LLD) plots during fracture toughness testing of Armco iron show serrations in the temperature range 383-573 K similar to those observed in the tensile stress-strain curves. DSA is found to have a beneficial effect on the fracture toughness J[sub IC]. A marked increase in tensile strength and fracture toughness occurs in Armco iron in the DSA regime. The strain hardening exponent, known to have a bearing on the plastic zone size and the void growth rate, seems a clear parameter in terms of which the observed J[sub IC] variation with temperature can be understood. Remarkably, the variation of n with temperature is found to closely follow the observed trend in J[sub IC]. However, the slope of the J-R curve, dJ/da, decreases in the DSA regime with a minimum at 423 K. The decrease has been related to the fracture propagation process which is shown to occur by an alternate fast fracture and the ductile dimpled mode in the DSA regime. In the case of nickel, free from DSA, J[sub IC] or dJ/da are found to be largely unaffected by the test temperature. At room temperature f.c.c. nickel, at closely matching strength levels, possesses higher fracture toughness as compared to b.c.c. Armco iron. The crystal structure effect is more pronounced at temperatures below the ambient. At 77 K, the fracture toughness of iron is drastically reduced due to the onset of cleavage while nickel, not prone to a change in the fracture mode, maintains the same level of J[sub IC], as at the ambient.

Srinivas, M.; Malakondaiah, G.; Rama Rao, P. (Defence Metallurgical Research Lab., Hyderabad (India))

1993-04-01

265

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.

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

2011-01-01

266

Steady-state and laser flash photolysis study of the carbon-carbon bond fragmentation reactions of 2-arylsulfanyl alcohol radical cations.  

PubMed

The N-methylquinolinium tetrafluoroborate (NMQ(+))-sensitized photolysis of the erythro-1,2-diphenyl-2-arylsulfanylethanols 1-3 (1, aryl = phenyl; 2, aryl = 4-methylphenyl; 3, aryl = 3-chlorophenyl) has been investigated in MeCN, under laser flash and steady-state photolysis. Under laser irradiation, the formation of sulfide radical cations of 1-3, in the monomeric (lambda(max) = 520-540 nm) and dimeric form (lambda(max) = 720-->800 nm), was observed within the laser pulse. The radical cations decayed by first-order kinetics, and under nitrogen, the formation of ArSCH(*)Ph (lambda(max) = 350-360 nm) was clearly observed. This indicates that the decay of the radical cation is due to a fragmentation process involving the heterolytic C-C bond cleavage, a conclusion fully confirmed by steady-state photolysis experiments (formation of benzaldehyde and the dimer of the alpha-arylsulfanyl carbon radical). Whereas the fragmentation rate decreases as the C-C bond dissociation energy (BDE) increases, no rate change was observed by the replacement of OH by OD in the sulfide radical cation (k(OH)/k(OD) = 1). This suggests a transition state structure with partial C-C bond cleavage where the main effect of the OH group is the stabilization of the transition state by hydrogen bonding with the solvent. The fragmentation rate of 2-hydroxy sulfanyl radical cations turned out to be significantly slower than that of nitrogen analogues of comparable reduction potential, probably due to a more efficient overlap between the SOMO in the heteroatom and the C-C bond sigma-orbital in the second case. The fragmentation rates of 1(+*)-3(+*) were found to increase by addition of a pyridine, and plots of k(base) against base strength were linear, allowing calculation of the beta Bronsted values, which were found to increase as the reduction potential of the radical cation decreases, beta = 0.21 (3(+*)), 0.34 (1(+*)), and 0.48 (2(+*)). The reactions of 1(+*) exhibit a deuterium kinetic isotope effect with values that increase as the base strength increases: k(OH)/k(OD) = 1.3 (pyridine), 1.9 (4-ethylpyridine), and 2.3 (4-methoxypyridine). This finding and the observation that with the above three bases the rate decreases in the order 3(+*) > 1(+*) > 2(+*), i.e., as the C-C BDE increases, suggest that C-C and O-H bond cleavages are concerted but not synchronous, with the role of OH bond breaking increasing as the base becomes stronger (variable transition state). It is probable that, with the much stronger base, 4-(dimethylamino)pyridine, a change to a stepwise mechanism may occur where the slow step is the formation of a radical zwitterion that then rapidly fragmentates to products. PMID:15549803

Baciocchi, Enrico; Giacco, Tiziana Del; Elisei, Fausto; Gerini, Maria Francesca; Lapi, Andrea; Liberali, Prisca; Uzzoli, Barbara

2004-11-26

267

Enhanced RNA cleavage within bulge-loops by an artificial ribonuclease  

PubMed Central

Cleavage of phosphodiester bonds by small ribonuclease mimics within different bulge-loops of RNA was investigated. Bulge-loops of different size (1–7 nt) and sequence composition were formed in a 3? terminal fragment of influenza virus M2 RNA (96 nt) by hybridization of complementary oligodeoxynucleotides. Small bulges (up to 4 nt) were readily formed upon oligonucleotide hybridization, whereas hybridization of the RNA to the oligonucleotides designed to produce larger bulges resulted in formation of several alternative structures. A synthetic ribonuclease mimic displaying Pyr–Pu cleavage specificity cleaved CpA motifs located within bulges faster than similar motifs within the rest of the RNA. In the presence of 10 mM MgCl2, 75% of the cleavage products resulted from the attack of this motif. Thus, selective RNA cleavage at a single target phosphodiester bond was achieved by using bulge forming oligonucleotides and a small ribonuclease A mimic.

Kuznetsova, Irina L.; Zenkova, Marina A.; Gross, Hans J.; Vlassov, Valentin V.

2005-01-01

268

Enhanced RNA cleavage within bulge-loops by an artificial ribonuclease.  

PubMed

Cleavage of phosphodiester bonds by small ribonuclease mimics within different bulge-loops of RNA was investigated. Bulge-loops of different size (1-7 nt) and sequence composition were formed in a 3' terminal fragment of influenza virus M2 RNA (96 nt) by hybridization of complementary oligodeoxynucleotides. Small bulges (up to 4 nt) were readily formed upon oligonucleotide hybridization, whereas hybridization of the RNA to the oligonucleotides designed to produce larger bulges resulted in formation of several alternative structures. A synthetic ribonuclease mimic displaying Pyr-Pu cleavage specificity cleaved CpA motifs located within bulges faster than similar motifs within the rest of the RNA. In the presence of 10 mM MgCl2, 75% of the cleavage products resulted from the attack of this motif. Thus, selective RNA cleavage at a single target phosphodiester bond was achieved by using bulge forming oligonucleotides and a small ribonuclease A mimic. PMID:15731340

Kuznetsova, Irina L; Zenkova, Marina A; Gross, Hans J; Vlassov, Valentin V

2005-02-24

269

7. Historic American Buildings Survey, C. C. Adams, Photographer August ...  

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

7. Historic American Buildings Survey, C. C. Adams, Photographer August 1931, SEED PACKING ROOM, Gift of New York State Department of Education. - Shaker North Family Washhouse (first), Shaker Road, New Lebanon, Columbia County, NY

270

Potential energy surfaces for CH bond cleavage reactions  

SciTech Connect

Ab initio, multi-reference, configuration interaction calculations are reported for CH{sub 4}{leftrightarrow}CH{sub 3}+H, CH{sub 3}F{leftrightarrow}CH{sub 2}F+H, CH{sub 2}F{sub 2}{leftrightarrow}CHF{sub 2}+H, and CHF{sub 3}{leftrightarrow}CF{sub 3}+H. Two equivalent, barrier-less paths are found for the CH{sub 3}+H recombination, two inequivalent, barrier-less paths are found for the CH{sub 2}F+H and CHF{sub 2}+H recombinations (depending on which side of the radical the H atom approaches), and only one barrier-less path is found for the CF{sub 3}+H recombination. Minimum energy path for H atom approaching CF{sub 3} from the concave side is predicted to have a barrier of 27 kcal/mole. Both minimum energy path energies and transitional frequencies as function of R{sub CH} for all 4 reactions are predicted to be similar.

Harding, L.B.

1996-12-31

271

?-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.

Liang, Jingjing; Kong, Qingzhong

2012-01-01

272

Functional characterization of the cleavage specificity of the sapovirus chymotrypsin-like protease.  

PubMed

Sapovirus is a positive-stranded RNA virus with a translational strategy based on processing of a polyprotein precursor by a chymotrypsin-like protease. So far, the molecular mechanisms regulating cleavage specificity of the viral protease are poorly understood. In this study, the catalytic activities and substrate specificities of the predicted forms of the viral protease, the 3C-like protease (NS6) and the 3CD-like protease-polymerase (NS6-7), were examined in vitro. The purified NS6 and NS6-7 were able to cleave synthetic peptides (15 to 17 residues) displaying the cleavage sites of the sapovirus polyprotein, both NS6 and NS6-7 proteins being active forms of the viral protease. High-performance liquid chromatography and subsequent mass spectrometry analysis of digested products showed a specific trans cleavage of peptides bearing Gln-Gly, Gln-Ala, Glu-Gly, Glu-Pro, or Glu-Lys at the scissile bond. In contrast, peptides bearing Glu-Ala or Gln-Asp at the scissile bond (NS4-NS5 and NS5-NS6, or NS6-NS7 junctions, respectively) were resistant to trans cleavage by NS6 or NS6-7 proteins, whereas cis cleavage of the Glu-Ala scissile bond of the NS5-NS6 junction was evidenced. Interestingly, the presence of a Phe at position P4 overruled the resistance to trans cleavage of the Glu-Ala junction (NS5-NS6), whereas substitutions at the P1 and P2' positions altered the cleavage efficiency. The differential cleavage observed is supported by a model of the substrate-binding site of the sapovirus protease, indicating that the P4, P1, and P2' positions in the substrate modulate the cleavage specificity and efficiency of the sapovirus chymotrypsin-like protease. PMID:18550673

Robel, Ivonne; Gebhardt, Julia; Mesters, Jeroen R; Gorbalenya, Alexander; Coutard, Bruno; Canard, Bruno; Hilgenfeld, Rolf; Rohayem, Jacques

2008-06-11

273

Functional Characterization of the Cleavage Specificity of the Sapovirus Chymotrypsin-Like Protease? †  

PubMed Central

Sapovirus is a positive-stranded RNA virus with a translational strategy based on processing of a polyprotein precursor by a chymotrypsin-like protease. So far, the molecular mechanisms regulating cleavage specificity of the viral protease are poorly understood. In this study, the catalytic activities and substrate specificities of the predicted forms of the viral protease, the 3C-like protease (NS6) and the 3CD-like protease-polymerase (NS6-7), were examined in vitro. The purified NS6 and NS6-7 were able to cleave synthetic peptides (15 to 17 residues) displaying the cleavage sites of the sapovirus polyprotein, both NS6 and NS6-7 proteins being active forms of the viral protease. High-performance liquid chromatography and subsequent mass spectrometry analysis of digested products showed a specific trans cleavage of peptides bearing Gln-Gly, Gln-Ala, Glu-Gly, Glu-Pro, or Glu-Lys at the scissile bond. In contrast, peptides bearing Glu-Ala or Gln-Asp at the scissile bond (NS4-NS5 and NS5-NS6, or NS6-NS7 junctions, respectively) were resistant to trans cleavage by NS6 or NS6-7 proteins, whereas cis cleavage of the Glu-Ala scissile bond of the NS5-NS6 junction was evidenced. Interestingly, the presence of a Phe at position P4 overruled the resistance to trans cleavage of the Glu-Ala junction (NS5-NS6), whereas substitutions at the P1 and P2? positions altered the cleavage efficiency. The differential cleavage observed is supported by a model of the substrate-binding site of the sapovirus protease, indicating that the P4, P1, and P2? positions in the substrate modulate the cleavage specificity and efficiency of the sapovirus chymotrypsin-like protease.

Robel, Ivonne; Gebhardt, Julia; Mesters, Jeroen R.; Gorbalenya, Alexander; Coutard, Bruno; Canard, Bruno; Hilgenfeld, Rolf; Rohayem, Jacques

2008-01-01

274

Slip and cleavage systems in the new crystal Li 6 YB 3 O 9  

Microsoft Academic Search

The structure of single crystals of the double lithium-yttrium borate Li6YB3O9 is investigated. It is shown that the cleavage planes are parallel to the layers that are located at the largest distance\\u000a from each other and characterized by the weakest electrostatic interaction. Thus, cleavage of a crystal occurs through the\\u000a longest Li-O bonds in the lithium five-vertex polyhedra and the

E. F. Dolzhenkova; V. N. Baumer; A. V. Tolmachev

2005-01-01

275

C,C-diacetylenic phosphaalkenes in palladium-catalyzed cross-coupling reactions.  

PubMed

The reactivity of bis-TMS-substituted C,C-diacetylenic phosphaalkene (A(2)PA) 1 in Sonogashira-Hagihara cross-coupling reactions has been examined. The selective and successive deprotection of the two silyl groups in 1 is enabled by the steric bulk of the Mes* group which renders the acetylene trans to Mes* more reactive and thereby facilitates selective and consecutive couplings with iodoarenes. In situ transformation of the TMS-protected acetylenes into Cu(i)acetylides is the key step in the synthetic sequence and enables the preparation of the first dimeric A(2)PA linked by a phenylene spacer. cis-trans Isomerization across the P[double bond, length as m-dash]C bond is triggered by a tertiary amine and exclusively observed in the case of nitrophenyl-substituted A(2)PAs. The introduced aryl groups are integral parts of the entire ?-system as evidenced by spectroscopic and electrochemical studies. PMID:21792460

Oberg, Elisabet; Geng, Xue-Li; Santoni, Marie-Pierre; Ott, Sascha

2011-07-27

276

On the influence of the phenyl group of propenylbenzene on the rate of hydrogenation of a double bond conjugated with it  

Microsoft Academic Search

1.On skeletal Ni, the disubstituted ethylenic bond of propenylbenzene is hydrogenated at approximately the same rate as the monosubstituted C=C bond of allylbenzene and allylcyclohexane. Under conditions in which the double bond of allylbenzene is displaced into theß-position, the C=C bond of allylcyclohexane does not migrate. These facts are evidence of activation of the C=C bond of propenylbenzene by the

L. Kh. Freidlin; N. M. Nazarova; M. A. Abduraimova

1967-01-01

277

Femtosecond dynamics of cyclopropenylidene, c-C3H2.  

PubMed

The photophysics of the B (1)B(1) state of isolated cyclopropenylidene, c-C(3)H(2), has been studied by femtosecond time-resolved photoionisation and photoelectron spectroscopy. The carbene was produced by flash pyrolysis of 3-chlorocycloprop-1-ene. The bands at 266.9 nm and 264.6 nm have been investigated. The excited state deactivates in a two step process. The first time constant of less than 50 fs corresponds most likely to a nonradiative transition to the A-state, the second one on the order of 200 fs describes the internal conversion to the electronic ground state. The data are compared to those measured for the chlorinated carbene c-C(3)HCl. In the photoelectron spectrum of c-C(3)H(2) resonances were observed which can be assigned to members of a Rydberg d-series. PMID:22173743

Hemberger, Patrick; Köhler, Juliane; Fischer, Ingo; Piani, Giovanni; Poisson, Lionel; Mestdagh, Jean-Michel

2011-12-16

278

Interstellar Detection of c-C3D2  

NASA Astrophysics Data System (ADS)

We report the first interstellar detection of c-C3D2. Doubly deuterated cyclopropenylidene, a carbene, has been detected toward the starless cores TMC-1C and L1544 using the IRAM 30 m telescope. The J_{K_a,K_c} = 3_{0,3} {--} 2_{1,2}, 31, 3-20, 2, and 22, 1-11, 0 transitions of this species have been observed at 3 mm in both sources. The expected 1:2 intensity ratio has been found in the 30, 3-21, 2 and 31, 3-20, 2 lines, belonging to the para and ortho species, respectively. We also observed lines of the main species, c-C3H2, singly deuterated c-C3HD, and the species with one 13C off of the principal axis of the molecule, c-H13CC2H. The lines of c-C3D2 have been observed with high signal-to-noise ratio, better than 7.5? in TMC-1C and 9? in L1544. The abundance of doubly deuterated cyclopropenylidene with respect to the normal species is found to be 0.4%-0.8% in TMC-1C and 1.2%-2.1% in L1544. The deuteration of this small hydrocarbon ring is analyzed with a comprehensive gas-grain model, the first including doubly deuterated species. The observed abundances of c-C3D2 can be explained solely by gas-phase processes, supporting the idea that c-C3H2 is a good indicator of gas-phase deuteration. Based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

Spezzano, S.; Brünken, S.; Schilke, P.; Caselli, P.; Menten, K. M.; McCarthy, M. C.; Bizzocchi, L.; Trevinõ-Morales, S. P.; Aikawa, Y.; Schlemmer, S.

2013-06-01

279

DNA photoreacts by nucleobase ring cleavage to form labile isocyanates.  

PubMed

Differential infrared absorption spectroscopy was used to study the formation of isocyanates and further photo-products in the oligonucleotides dG10, dC10 and dT10 and in their mononucleosides by ultraviolet light at 266 nm. We find that ?-cleavage takes place in oligonucleotides and mononucleosides both in films and in solution. The very intense and spectrally isolated isocyanate (N[double bond, length as m-dash]C[double bond, length as m-dash]O) asymmetric stretch vibration at 2277 cm(-1) is used as a spectroscopic marker for detection of the photo-product. The band disappears upon reaction with small amounts of water vapour as expected for isocyanates. Quantum yields for isocyanate formation by nucleobase ring cleavage in the ?-position to the carbonyl group are ?5 × 10(-5) in the mononucleosides and up to 5 × 10(-4) in the oligonucleotides. In the mixed oligonucleotides dG10/dC10 and dA10/dT10 the quantum yield of ?-cleavage drops by a factor of 10 compared to the single oligonucleotides. Implications for DNA repair and photo-induced DNA-protein cross-linking via isocyanate reaction with NH2 groups of amino acids are discussed. PMID:23860445

Buschhaus, Laura; Rolf, Josefin; Kleinermanns, Karl

2013-10-01

280

Ni-catalyzed cleavage of aryl ethers in the aqueous phase.  

PubMed

A novel Ni/SiO(2)-catalyzed route for selective cleavage of ether bonds of (lignin-derived) aromatic ethers and hydrogenation of the oxygen-containing intermediates at 120 °C in presence of 6 bar H(2) in the aqueous phase is reported. The C-O bonds of ?-O-4 and ?-O-4 linkages are cleaved by hydrogenolysis on Ni, while the C-O bond of the 4-O-5 linkage is cleaved via parallel hydrogenolysis and hydrolysis. The difference is attributed to the fact that the C(aliphatic)-OH fragments generated from hydrolysis of ?-O-4 and ?-O-4 linkages can undergo further hydrogenolysis, while phenol (produced by hydrolysis of the 4-O-5 linkage) is hydrogenated to produce cyclohexanol under conditions investigated. The apparent activation energies, E(a)(?-O-4) < E(a)(?-O-4) < E(a)(4-O-5), vary proportionally with the bond dissociation energies. In the conversion of ?-O-4 and 4-O-5 ether bonds, C-O bond cleavage is the rate-determining step, with the reactants competing with hydrogen for active sites, leading to a maximum reaction rate as a function of the H(2) pressure. For the very fast C-O bond cleavage of the ?-O-4 linkage, increasing the H(2) pressure increases the rate-determining product desorption under the conditions tested. PMID:23190332

He, Jiayue; Zhao, Chen; Lercher, Johannes A

2012-12-12

281

Phosphino imidazoles and imidazolium salts for Suzuki C-C coupling reactions.  

PubMed

The consecutive syntheses of imidazoles 1-(4-X-C(6)H(4))-4,5-R(2)-(c)C(3)HN(2) (3a, X = Br, R = H; 3b, X = I, R = Me; 3c, X = H, R = Me; 5, X = Fc, R = H; 7, X = C?CFc, R = H; 9, X = C(6)H(5), R = Me; Fc = Fe(?(5)-C(5)H(4))(?(5)-C(5)H(5))), phosphino imidazoles 1-(4-X-C(6)H(4))-2-PR'(2)-4,5-R(2)-(c)C(3)N(2) (11a-k; X = Br, I, Fc, FcC?C, Ph; R = H, Me; R' = Ph, (c)C(6)H(11), (c)C(4)H(3)O), imidazolium salts [1-(4-X-C(6)H(4))-3-R''-4,5-R(2)-(c)C(3)HN(2)]I (16a; X = Br, R = H, R'' = n-Bu; 16b, X = Br, R = H, R'' = n-C(8)H(17); 16c, X = I, R = Me, R'' = n-C(8)H(17), 16d, X = H, R = Me, R'' = n-C(8)H(17)) and phosphino imidazolium salts [1-C(6)H(5)-2-PR'(2)-3-n-C(8)H(17)-4,5-Me(2)-(c)C(3)N(2)]PF(6) (17a, R' = C(6)H(5); 17b, R' = (c)C(6)H(11)) or [1-(4-P(C(6)H(5))(2)-C(6)H(4))-3-n-C(8)H(17)-4,5-Me(2)-(c)C(3)HN(2)]PF(6), (20) and their selenium derivatives 1-(4-X-C(6)H(4))-2-P([double bond, length as m-dash]Se)R'(2)-4,5-R(2)-(c)C(3)N(2) (11a-Se-f-Se; X = Br, I; R = H, Me; R' = C(6)H(5), (c)C(6)H(11), (c)C(4)H(3)O) are reported. The structures of 11a-Se and [(1-(4-Br-C(6)H(4))-(c)C(3)H(2)N(2)-3-n-Bu)(2)PdI(2)] (19) in the solid state were determined. Cyclovoltammetric measurements were performed with the ferrocenyl-containing molecules 5 and 7 showing reversible redox events at E(0) = 0.108 V (?E(p) = 0.114 V) (5) and E(0) = 0.183 V (?E(p) = 0.102 V) (7) indicating that 7 is more difficult to oxidise. Imidazole oxidation does not occur up to 1.3 V in dichloromethane using [(n-Bu)(4)N][B(C(6)F(5))(4)] as supporting electrolyte, whereas an irreversible reduction is observed between -1.2 - -1.5 V. The phosphino imidazoles 11a-k and the imidazolium salts 17a,b and 20, respectively, were applied in the Suzuki C-C cross-coupling of 2-bromo toluene with phenylboronic acid applying [Pd(OAc)(2)] as palladium source. Depending on the electronic character of 11a-k, 17a,b and 20 the catalytic performance of the in situ generated catalytic active species can be predicted. As assumed, more electron-rich phosphines with their higher donor capability show higher activity and productivity. Additionally, 11e was applied in the coupling of 4-chloro toluene with phenylboronic acid showing an excellent catalytic performance when compared to catalysts used by Fu, Beller and Buchwald. Furthermore, 11e is eligible for the synthesis of sterically hindered biaryls under mild reaction conditions. C-C Coupling reactions with the phosphino imidazolium salts 17b and 20 in ionic liquids [BMIM][PF(6)] and [BDMIM][BF(4)] were performed, showing less activity than in common organic solvents. PMID:22450875

Milde, Bianca; Schaarschmidt, Dieter; Rüffer, Tobias; Lang, Heinrich

2012-03-27

282

Relaxation and dissociation following photoexcitation of the (?-N2)[Mo(N[t-Bu]Ar)3]2 dinitrogen cleavage intermediate.  

PubMed

Frequency resolved pump-probe spectroscopy was performed on isolated (?-N(2))[Mo(N[t-Bu]Ar)(3)](2) (Ar = 3,5-C(6)H(3)Me(2)), an intermediate formed in the reaction of Mo(N[t-Bu]Ar)(3) to bind and cleave dinitrogen. Evidence is presented for 300 fs internal conversion followed by subpicosecond vibrational cooling on the ground electronic state in competition with bond dissociation. Fast cooling following photoexcitation leads to a relatively low overall dissociation yield of 5%, in quantitative agreement with previous work [Curley, J. J.; Cooke, T. R.; Reece, S. Y.; Mueller, P.; Cummins, C. C. J. Am. Chem. Soc. 2008, 130, 9394]. Coupling of vibrational modes to the excitation and internal conversion results in a nonthermal distribution of energy following conversion, and this provides sufficient bias to allow the nitrogen cleavage reaction to compete with breaking of the Mo-NN bond despite a higher energetic barrier on the ground state. PMID:23249096

Huss, Adam S; Curley, John J; Cummins, Christopher C; Blank, David A

2013-01-28

283

Chemical Bonds  

NSDL National Science Digital Library

Electrons are key to forming the two broad categories of chemical bonds: covalent and ionic. Atoms, which have a nucleus surrounded by electrons, are represented in several different ways. In the Chemical Bonds activity, students explore the different kinds of chemical bonds that can form, ranging from non-polar covalent to ionic. In the model depicted above students adjust the electronegativity of two atoms and see the effect it has on electron distribution and bond type.

Consortium, The C.

2011-12-11

284

Sticker Bonding.  

ERIC Educational Resources Information Center

|Introduces a science activity on the bonding of chemical compounds. Assigns students the role of either a cation or anion and asks them to write the ions they may bond with. Assesses students' understanding of charge, bonding, and other concepts. (YDS)|

Frazier, Laura Corbin

2000-01-01

285

Sticker Bonding.  

ERIC Educational Resources Information Center

Introduces a science activity on the bonding of chemical compounds. Assigns students the role of either a cation or anion and asks them to write the ions they may bond with. Assesses students' understanding of charge, bonding, and other concepts. (YDS)

Frazier, Laura Corbin

2000-01-01

286

C.C.D. Readout of a Picosecond Streak Camera with an Intensified C.C.D.  

National Technical Information Service (NTIS)

This paper deals with a digital streak camera readout device. The device consists in a low light level television camera made of a solid state C.C.D. array coupled to an image intensifier associated to a video-digitizer coupled to a micro-computer system....

M. Lemonier J. C. Richard C. Cavailler A. Mens G. Raze

1984-01-01

287

A theoretical study of cyclohexyne addition to carbonyl-C? bonds: allowed and forbidden electrocyclic and nonpericyclic ring-openings of strained cyclobutenes.  

PubMed

The mechanism of cyclohexyne insertion into a C(O)-C(?) bond of cyclic ketones, explored experimentally by the Carreira group, has been investigated using density functional theory. B3LYP and M06-2X calculations were performed in both gas phase and THF (CPCM, UAKS radii). The reaction proceeds through a stepwise [2 + 2] cycloaddition of cyclohexyne to the enolate, followed by three disparate ring-opening possibilities of the cyclobutene alkoxide to give the product: (1) thermally allowed conrotatory electrocyclic ring-opening, (2) thermally forbidden disrotatory electrocyclic ring-opening, or (3) nonpericyclic C-C bond cleavage. Our computational results for the model alkoxide and potassium alkoxide systems show that the thermally allowed electrocyclic ring-opening pathway is favored by less than 1 kcal/mol. In more complex systems containing a potassium alkoxide (e-f), the barrier of the allowed conrotatory ring-opening is disfavored by 4-8 kcal/mol. This suggests that the thermodynamically more stable disrotatory product can be formed directly through a "forbidden" pathway. Analysis of geometrical parameters and atomic charges throughout the ring-opening pathways provides evidence for a nonpericyclic C-C bond cleavage, rather than a thermally forbidden disrotatory ring-opening. A true forbidden disrotatory ring-opening transition structure was computed for the cyclobutene alcohol; however, it was 19 kcal/mol higher in energy than the allowed conrotatory transition structure. An alternate mechanism in which the disrotatory product forms via isomerization of the conrotatory product was also explored for the alkoxide and potassium alkoxide systems. PMID:22537557

Sader, C Avery; Houk, K N

2012-05-11

288

Rhodium-Catalyzed CC Bond Formation via Heteroatom-Directed CH Bond Activation  

Microsoft Academic Search

Once considered the 'holy grail' of organometallic chemistry, synthetically useful reactions employing C-H bond activation have increasingly been developed and applied to natural product and drug synthesis over the past decade. The ubiquity and relative low cost of hydrocarbons makes C-H bond functionalization an attractive alternative to classical C-C bond forming reactions such as cross-coupling, which require organohalides and organometallic

Denise A. Colby; Robert G. Bergman; Jonathan A. Ellman

2010-01-01

289

Cathepsin B cleavage of the trypsinogen activation peptide  

PubMed Central

Background Cathepsin B is thought to play a central role in intrapancreatic trypsinogen activation and the onset of pancreatitis. A recent investigation of the cathepsin B mediated activability of wildtype trypsinogen and their mutations N29I, N29T and R122H, which are associated to hereditary pancreatitis, revealed no differences. This action seems to be restricted to the K23-I24 peptide bond, which is the trypsinogen activation bond. Here we investigated the influence of the mutations D22G and K23R of the trypsinogen activation peptide on the cleavability by cathepsin B. Methods To investigate the functional impact of the TAP mutations on cathepsin B mediated cleavage of the trypsinogen activating K23-I24 bond, the corresponding peptides pWT, APFDDDDKIVGG; pD22G, APFDDDGKIVGG; and pK23R, APFDDDDRIVGG were digested with cathepsin B for 30 min at pH 3.8 and 5.0, and the fragments were analysed by high-performance liquid chromatography. Results Without cathepsin B, less than 1 % of the peptides were hydrolysed. After a 30-minute digestion with cathepsin B at pH 5, 96% of pWT, 48% of pK23R, but only 2.4% of pD22G were hydrolysed. At pH 3.8, the cathepsin B cleavage of pWT and pK23R was less than at pH 5, whereas the cleavage of pD22G was completely inhibited Conclusions Cathepsin B mediated trypsinogen activation seems not to be a crucial pathogenic step in hereditary pancreatitis patients with the trypsinogen mutations D22G and K23R.

Teich, Niels; Bodeker, Hans; Keim, Volker

2002-01-01

290

Using 3'-Bridging Phosphorothiolates to Isolate the Forward DNA Cleavage Reaction of Human Topoisomerase II?†  

PubMed Central

The ability to cleave DNA is critical to the cellular and pharmacological functions of human type II topoisomerases. However, the low level of cleavage at equilibrium and the tight coupling of the cleavage and ligation reactions make it difficult to characterize the mechanism by which these enzymes cut DNA. Therefore, to establish a system that isolates topoisomerase II-mediated DNA scission from ligation, oligonucleotide substrates were developed that contained a 3’-bridging phosphorothiolate at the scissile bond. Scission of these substrates generates a 3’-terminal –SH moiety that is a poor nucleophile relative to the normal 3’-terminal –OH. Consequently, topoisomerase II cannot efficiently ligate phosphorothiolate substrates once they are cleaved. The characteristics of topoisomerase II?-mediated cleavage of phosphorothiolate oligonucleotides were identical to those seen with wild-type substrates, except that no ligation was observed. This unidirectional accumulation of cleavage complexes provided critical information regarding coordination of the protomer subunits of topoisomerase II? and the mechanism of action of topoisomerase II poisons. Results indicate that the two enzyme subunits are partially coordinated and that cleavage at one scissile bond increases cleavage at the other. Furthermore, anticancer drugs such as etoposide and amsacrine that strongly inhibit topoisomerase II-mediated DNA ligation have little effect on the forward scission reaction. In contrast, abasic sites that increase levels of cleavage complexes without affecting ligation stimulate the forward rate of scission. Phosphorothiolate substrates provide significant advantages over traditional “suicide substrates” and should be valuable for future studies on DNA scission and the topoisomerase II-DNA cleavage complex.

Deweese, Joseph E.; Burgin, Alex B.; Osheroff, Neil

2009-01-01

291

The mechanism of brittle fracture in a microalloyed steel: Part 1. Inclusion-induced cleavage  

SciTech Connect

The cleavage resistance of two microalloyed steels (steels A and B) was studied using several tests, including the instrumented precracked Charpy and Charpy V-notch (CVN) techniques. Ductile-to-brittle transition temperatures were measured for the base-metal and simulated heat-affected zone (HAZ) microstructures. Steel B showed inferior cleavage resistance to steel A, and this could not be explained by differences in gross microstructure. Scanning electron fractography revealed that TiN inclusions were responsible for cleavage initiation in steel B. These inclusions were well bonded to the ferritic matrix. It is believed that a strong inclusion-matrix bond is a key factor in why TiN inclusions are potent cleavage initiators in steel. Strong bonding allows high stresses in a crack/notch-tip plastic zone to act on the inclusions without debonding the interface. Once an inclusion cleaves, the strong bond allows for transfer of the TiN crack into the ferritic matrix. It was estimated that only 0.0016 wt pct Ti was tied up in the offending inclusion in steel B. This indicates that extended times at high temperatures during the casting of such steels could produce TiN-related toughness deterioration at even modest Ti contents.

Fairchild, D.P.; Howden, D.G.; Clark, W.A.T.

2000-03-01

292

N-terminal cysteinyl proteins can be prepared using thrombin cleavage  

Microsoft Academic Search

Expressed protein ligation – which allows native proteins to be selectively linked together by a normal peptide bond in an aqueous environment – has emerged as a powerful technique. The technique requires the formation of a C-terminal ?-thioester and an N-terminal Cys. An N-terminal Cys can be formed by enzymatic cleavage, commonly using the Factor Xa and TEV proteases. We

Dongsheng Liu; Rong Xu; Kaushik Dutta; David Cowburn

2008-01-01

293

Staggering in the cleavage pattern of E. coli ABC-excinuclease  

SciTech Connect

E. coli ABC excinuclease is a complex of three proteins encoded by the uvrA, uvrB, and uvrC genes. The enzyme repairs DNA mono and diadducts by the single strand cleavage of DNA eight phosphodiester bond 5' and four or five phosphodiester bonds 3' to a DNA lesion and facilitates the removal of the resulting twelve or thirteen nucleotide fragment. In this study, the authors have investigated the excision pattern for ultraviolet (UV) induced diadducts, i.e. cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) photoproducts. Terminally (5' or 3') labeled DNA was irradiated with 254nm UV and treated with ABC excinuclease before and after photoreactivation of cyclobutane dimers by E. coli DNA photolyase. In this way, the authors were able to differentiate between the cleavage pattern of pyrimidine dimers and of (6-4) photoproducts. Their results show that certain TT cyclobutane dimers and rare TT (6-4) photoproducts are excised by cleavage seven and, less frequently, six phosphodiester bonds to the 5' side of the DNA lesion in addition to the primary cutting site at the eight 5' phosphodiester bond. The 3' cleavage sites are maintained at the fourth and fifth phosphodiester bonds for the these UV induced lesions. These data indicate that the cleavage pattern of the ABC excinuclease may be dependent upon both the type of DNA lesion as well as it surrounding nucleotide sequence. In addition, the authors analysis shows that (6-4) photoproducts are much better substrates for ABC excinuclease than are pyrimidine dimers.

Myles, G.M.; Van Houten, B.; Sancar, A.

1986-05-01

294

Reconstruction of the cleavage faces of tetrahedrally coordinated compound semiconductors  

NASA Astrophysics Data System (ADS)

Tetrahedrally coordinated compound semiconductors occur in two crystallographic allotropes: zincblende and wurtzite. Zincblende materials exhibit a single cleavage face: The (110) surface consisting of equal numbers of anions and cations which form zig-zag chains directed along <110> directions in the surface. Wurtzite materials exhibit two cleavage faces, both consisting of equal numbers of anion and cation species. The (10overline 1 0) cleavage surfaces consist of isolated anion-cation dimers back bonded to the layer beneath whereas the (10overline 2 0) surfaces consist of anion-cation chains, analogous to those on zincblende (110) but with four rather than two inequivalent atoms per surface unit cell. All three surfaces exhibit reconstructions which do not alter the symmetry of the surface unit cell but which lead to large (? 1 Å) deviations of the positions of the atomic species in the uppermost layer(s) from those in the truncated bulk solid. These reconstructed surface geometries have been determined quantitatively for the (110) surfaces of zincblende structure AlP, AlAs, GaP, GaAs, GaSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe and CdTe; the (10overline 1 0) surfaces of wurtzite structure ZnO and CdSe; and the (11overline 2 0) surfaces of CdSe. Theoretical predictions of these reconstructed geometries have been given which are in either quantitative or semiquantitative correspondence with the experimentally determined structures. Analysis of the trends exhibited by the members of each class of cleavage surface and comparison thereof with theoretical predictions permit the extraction from these results of generalizations characteristic of novel types of surface chemical bonding. The most important of these is the notion that for each class of surface the atomic geometries are approximately "universal" when their coordinates are properly scaled with the bulk lattice constant. A quantitative description of this result is presented which reveals that extensions of the concepts of inorganic molecular coordination chemistry are required to predict the cleavage-surface atomic geometries and electronic structures of binary tetrahedrally coordinated compound semiconductors.

Duke, C. B.

295

Cleavage fracture. George R. Irwin symposium: Proceedings  

SciTech Connect

The symposium proceedings is a collection of twenty nine papers and ten abstracts on the subjects of fracture mechanics, dislocation theory of fracture, atomistic fracture, micromechanical and microstructural modeling of cleavage, cleavage mechanisms, fractographic characterization, experimental techniques, and cleavage studies in metals, welds, intermetallics, ceramics, and minerals. The diverse topics reflect the wide range of disciplines that Professor Irwin has impacted through his pioneer work on fracture mechanics. Papers are arranged under the following topical sections: Historical reviews; Fracture mechanics; Dislocation and atomistic theories; Micromechanical and microstructural modeling; Fracture mechanisms; and Cleavage fracture in aluminides and ceramics. Papers have been processed separately for inclusion on the data base.

Chan, K.S. [ed.] [Southwest Research Inst., San Antonio, TX (United States)

1997-12-31

296

Structure and electronic spectra of the C C6- anion  

NASA Astrophysics Data System (ADS)

Ab initio calculations are employed for determining structures, spectroscopic parameters and transitions to the excited electronic states of the astrophysical relevant C C6- anion, which is a potential intermediate of chemical processes involving carbon chains and polycyclic aromatic hydrocarbons. Calculations confirm the prominent stability of linear carbon chain anions which guarantees their formation. The lowest doublet potential energy surface of C? displays six isomers, whereas in the lowest quartet potential energy surface, a unique minimum energy geometry is found. The most stable form is the linear anion l-C? (X2?u) followed by the c-C?(X2A1')at 9291 cm-1 above in energy, which presents a distorted D3h structure. The other isomers are also located at more than 9000 cm-1 above l-C? (X2?u). For each isomer, a set of spectroscopic parameters including their equilibrium structures, rotational constants at equilibrium, harmonic wavenumbers, dipole moments and electron affinities are predicted. For linear C?, eight electronic states lying below the electron detachment threshold, are computed. Four doublet electronic states, which play important roles in the C? reactivity, lie below 2 eV. Spin-orbit constants for the upper electronic states of l-C? are predicted.

Hammoutene, D.; Hochlaf, M.; Senent, M. L.

2012-08-01

297

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

2012-04-05

298

Cleavage in the chick embryo.  

PubMed

Chick embryos ranging from the stage of first cleavage to that of about 700 cells were removed from the oviduct and examined by transmission electron microscopy. Beneath the cell membrane is yolk-free cortical region containing microfilaments. Beneath this lies cytoplasm which contains yolk spheres which are graded in size, the dorsal ones being smaller than the ventral ones. The subgerminal periblast possesses a greater proportion of yolk to cytoplasm than do the cells proper, but it merges with the cytoplasm at the incomplete borders of the 'open' cells. Specialized accumulations of membranes lie in the marginal periblast, and it is suggested that they play a role in cell membrane formation. PMID:564938

Bellairs, R; Lorenz, F W; Dunlap, T

1978-02-01

299

Histidine Residue Mediates Radical-induced Hinge Cleavage of Human IgG1  

PubMed Central

Hydroxyl radicals induce hinge cleavage in a human IgG1 molecule via initial radical formation at the first hinge Cys231 followed by electron transfer to the upper hinge residues. To enable engineering of a stable monoclonal antibody hinge, we investigated the role of the hinge His229 residue using structure modeling and site-directed mutagenesis. Direct involvement of His229 in the reaction mechanism is suggested by a 75–85% reduction of the hinge cleavage for variants in which His229 was substituted with either Gln, Ser, or Ala. In contrast, mutation of Lys227 to Gln, Ser, or Ala increased hinge cleavage. However, the H229S/K227S double mutant shows hinge cleavage levels similar to that of the single H229S variant, further revealing the importance of His229. Examination of the hinge structure shows that His229 is capable of forming hydrogen bonds with surrounding residues. These observations led us to hypothesize that the imidazole ring of His229 may function to facilitate the cleavage by forming a transient radical center that is capable of extracting a proton from neighboring residues. The work presented here suggests the feasibility of engineering a new generation of monoclonal antibodies capable of resisting hinge cleavage to improve product stability and efficacy.

Yates, Zac; Gunasekaran, Kannan; Zhou, Hongxing; Hu, Zhonghua; Liu, Zhi; Ketchem, Randal R.; Yan, Boxu

2010-01-01

300

The effect of peptide length on the cleavage kinetics of 2-chlorotrityl resin-bound ethers.  

PubMed

Different characteristics of cleavage kinetics of resin-bound amino alcohols and their peptide derivatives were observed in acid containing protic and aprotic solvent mixtures. The hydrolysis reactions are hindered by steric crowding around the cleaving C--O bond and accelerated by the special solvation effect of CF(3)CH(2)OH on the peptide chain as well as the increase of the strength and concentration of the acid. In trifluoroacetic acid containing mixtures, trifluoroacetylation of the peptide alcohols was detected. The appearance of O-trifluoroacetyl serine and threonine derivatives is detected in cleavage mixtures containing trifluoroacetic acid in anhydrous solvent. PMID:16485312

Kocsis, László; Ruff, Ferenc; Orosz, György

2006-06-01

301

The role of the plasma membrane and a non-lysosomal compartment in the disulfide cleavage of endocytosed macromolecules  

SciTech Connect

The cleavage of disulfide bonds in endocytosed macromolecules was investigated using new disulfide containing macromolecular conjugates. A conjugate, in which ({sup 125}I-tyr) was linked to the nondegradable macromolecular carrier poly D-lysine (PDL) through a disulfide spacer ({sup 125}I-tyr-SS-PDL), was used to monitor disulfide cleavage in adsorptive endocytosis in Chinese hamster ovary cells. Reductive cleavage of this probe released 3-thiopropionyl-{sup 125} {sup 125}I-tyramine, measurable as acid soluble radioactivity. In pulse experiments, reductive cleavage of {sup 125}I-tyr-SS-PDL differed in its kinetics from the proteolysis of {sup 125}I-labeled Poly L-lysine. Proteolytic degradation began after a 15 to 30 min lag, i.e. the time required for transport of poly(lysine) to heavy lysosomes, while reductive cleavage increased linearly between 0 and 15 min. In the first hour of chase, proteolytic and reductive cleavage amounted to 30% and 7% of the total cell bound radioactivity, respectively. The reductive cleavage observed during the first 30 min of chase was inhibited by 80-90% with cell impermeant sulfhydryl reagents (dithiobis-(2-nitrobenzoic acid) and p-chloromercuriphenyl-sulfonate), which indicated that cleavage occurred at the cell surface. In contrast, disulfide cleavage observed after 1 hr chase was not significantly inhibited by these reagents and, therefore, resulted from an intracellular process. Subcellular fractionation demonstrated that lysosomes could be excluded as a site of disulfide cleavage, but that a subcellular fraction characterized by a buoyant density of 1.03g/ml was associated with the cleavage of {sup 125}I-tyr-SS-PDL. Of the relevant structures which constitute this subcellular fraction, early endosomes and plasma membrane could be excluded as the reducing structures on the basis of kinetic considerations.

Feener, E.P.

1990-01-01

302

Formation of annealing twins in f.c.c. crystals  

SciTech Connect

A microscopic model for the formation of annealing twins in f.c.c. crystals is proposed. It is argued that Shockley partial loops nucleate on consecutive {l_brace}111{r_brace} planes by growth accidents occurring on migrating {l_brace}111{r_brace} steps associated with a moving grain boundary. The higher the velocity of the boundary, the higher the twin density. The absence of twins in high stacking fault energy materials and the influence of temperature on twin density has been developed by examining the influence of deformation damage on the incidence of twinning in copper and the effect of boron in reducing twin density in annealed nickel.

Mahajan, S. [Carnegie Mellon Univ., Pittsburgh, PA (United States). Dept. of Materials Science and Engineering; Pande, C.S.; Imam, M.A.; Rath, B.B. [Naval Research Lab., Washington, DC (United States)

1997-06-01

303

Electrochemical oxidation and cleavage of tyrosine- and tryptophan-containing tripeptides.  

PubMed

Electrochemical oxidation of peptides and proteins has been shown to lead to specific cleavage next to tyrosine (Tyr) and tryptophan (Trp) residues which makes the coupling of electrochemistry to mass spectrometry (EC-MS) a potential instrumental alternative to chemical and enzymatic cleavage. A set of Tyr and Trp-containing tripeptides has been studied to investigate the mechanistic aspects of electrochemical oxidation and the subsequent chemical reactions including peptide bond cleavage, making this the first detailed study of the electrochemistry of Trp-containing peptides. The effect of adjacent amino acids was studied leading to the conclusion that the ratios of oxidation and cleavage products are peptide-dependent and that the adjacent amino acid can influence the secondary chemical reactions occurring after the initial oxidation step. The effect of parameters such as potential and solvent conditions showed that control of the oxidation potential is crucial to avoid dimer formation for Tyr and an increasing number of oxygen insertions (hydroxylations) for Trp, which occur above 1000 mV (vs Pd/H(2)). While the formation of reactive intermediates after the first oxidation step is not strongly dependent on experimental conditions, an acidic pH is required for good cleavage yields. Working under strongly acidic conditions (pH 1.9-3.1) led to optimal cleavage yields (40-80%), whereas no or little cleavage occurred under basic conditions. Online EC-MS allowed determining the optimal potential for maximum cleavage yields, whereas EC-LC-MS/MS revealed the nature and distribution of the reaction products. PMID:20726506

Roeser, Julien; Permentier, Hjalmar P; Bruins, Andries P; Bischoff, Rainer

2010-09-15

304

[([superscript t]Bu[subscript 2]PCH[subscript 2]SiMe[subscript 2])[subscript 2]N]Rh[superscript I]? Rapidly Reversible H-C(sp[superscript 3]) and H?C(sp[superscript 2]) Bond Cleavage by Rhodium(I)  

SciTech Connect

The product of the reaction of (tBu{sub 2}PCH{sub 2}SiMe{sub 2}){sub 2}N{sup -} (MgCl{sup +} salt) with [RhCl(cyclooctene){sub 2}]{sub 2} is a Rh{sup III} complex where one {sup t}Bu methyl C-H bond has oxidatively added to Rh: (PNP*)RhH. This is in rapid exchange among all 9 x 4 C-H bonds of the four {sup t}Bu groups. (PNP*)RhH undergoes oxidative addition equilibrium with the C-H bonds of benzene at {approx}10{sup 3} s{sup -1} at 25 C and oxidatively adds the ring C-H of other arenes. (PNP*)RhH forms {eta}{sup 2}-olefin complexes with several olefins and dehydrogenates allylic C-H bonds to form (PNP)Rh(H){sub 2}.

Verat, Alexander Y.; Pink, Maren; Fan, Hongjun; Tomaszewski, John; Caulton, Kenneth G. (Indiana)

2008-10-03

305

Effect of pyrocarbon content in C\\/C preforms on microstructure and mechanical properties of the C\\/C–SiC composites  

Microsoft Academic Search

Carbon fiber reinforced silicon carbide matrix (C\\/C–SiC) composites were prepared by liquid silicon infiltration (LSI) process, using four kinds of quasi-three-dimensional C\\/C preforms with the same fiber volume fraction (32%) but different pyrocarbon content (from ?20% to ?50%, in volume) filled by chemical vapor infiltration (CVI). The microstructure and mechanical properties of C\\/C–SiC composites have been investigated. X-ray diffractmeter (XRD)

Yonghui Zhang; Zhichao Xiao; Jiping Wang; Jianfeng Yang; Zhihao Jin

2009-01-01

306

Cleavage site selection within a folded substrate by the ATP-dependent lon protease.  

PubMed

Mechanistic studies of ATP-dependent proteolysis demonstrate that substrate unfolding is a prerequisite for processive peptide bond hydrolysis. We show that mitochondrial Lon also degrades folded proteins and initiates substrate cleavage non-processively. Two mitochondrial substrates with known or homology-derived three-dimensional structures were used: the mitochondrial processing peptidase alpha-subunit (MPPalpha) and the steroidogenic acute regulatory protein (StAR). Peptides generated during a time course of Lon-mediated proteolysis were identified and mapped within the primary, secondary, and tertiary structure of the substrate. Initiating cleavages occurred preferentially between hydrophobic amino acids located within highly charged environments at the surface of the folded protein. Subsequent cleavages proceeded sequentially along the primary polypeptide sequence. We propose that Lon recognizes specific surface determinants or folds, initiates proteolysis at solvent-accessible sites, and generates unfolded polypeptides that are then processively degraded. PMID:15870080

Ondrovicová, Gabriela; Liu, Tong; Singh, Kamalendra; Tian, Bin; Li, Hong; Gakh, Oleksandr; Perecko, Dusan; Janata, Jirí; Granot, Zvi; Orly, Joseph; Kutejová, Eva; Suzuki, Carolyn K

2005-05-03

307

Enzyme based cleavage strategy of Bacillus lentus BI377 in response to metabolism of azoic recalcitrant.  

PubMed

Bacillus lentus BI377 (B. lentus BI377) an alkaliphilic strain has accomplished the discriminate color removal strategy for Reactive Red sulfonated azoic recalcitrant irrespective of their molecular structure. During the decolorization experiment, it was observed that the diazo dye first followed chromophoric cleavage by azoreductase via typical azoreduction whereas, in case of monoazo dye, cleavage took place by peroxidase via successive electron transfers to oxide surface resulting in the asymmetric cleavage of the azo bond. Dismutation of oxidative stress by reactive metabolites has confirmed by superoxide dismutase activity. Carbon monoxide (CO) binding spectra, the content of cytochrome P450 and spectroscopy analysis by GCMS, FTIR and (1)H NMR of intermediate metabolites indicated the differentiate pattern of diazo and monoazo dye decolorization fuse to central metabolic pathway. Declined percentage of TOC and the cytotoxicity (MTT) study confirmed that environmentally benign intermediates may lead to mineralization. PMID:23313681

Oturkar, Chetan C; Patole, Milind S; R Gawai, Kachru; Madamwar, Datta

2012-12-14

308

Solar System planetary tests of {dot c/c}  

NASA Astrophysics Data System (ADS)

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 {?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 classical/relativistic dynamical force models of the EPM epehemerides, we obtained {dot c/c =(0.5± 2)× 10^{-7} yr^{-1}} . Moreover, the possibility that {dot c/cneq 0} over the last century is ruled out at 3-12 ? level by taking the ratios of the perihelia for different pairs of planets. Our results are independent of any measurement of the variations of other fundamental constants which may be explained by a variation of c itself (with the meaning of electromagnetic constant c EM). It will be important to repeat such tests if and when other teams of astronomers will estimate their own corrections to the standard Newtonian/Einsteinian planetary perihelion precessions with different ephemerides.

Iorio, Lorenzo

2010-01-01

309

Unexpected relationship between interlayer distances and surface/cleavage energies in ?-TiAl: density functional study  

NASA Astrophysics Data System (ADS)

Density functional calculations were performed to study the ?-TiAl (001), (100), (110) and (111) surfaces. The (100) surface is the most stable under Ti-rich conditions, while the Al-termination (110) surface becomes the most stable with the increase of Al chemical potential. We calculate that in ?-TiAl intermetallic compound the larger the interlayer distance, the larger the surface energy and cleavage energy. This is different from the situation in a pure metal. This phenomenon can be explained by the analysis of the bonding characteristics in ?-TiAl. In particular there are both metallic and covalent bonds in ?-TiAl, and the strongest covalent bonds mainly focus on the center of three Ti-Al-Ti atoms. It is the covalent bonds that affect greatly the cleavage energy, the surface energy and the surface stability.

Wang, Lu; Shang, Jia-Xiang; Wang, Fu-He; Zhang, Yue; Chroneos, Alexander

2011-07-01

310

Unexpected relationship between interlayer distances and surface/cleavage energies in ?-TiAl: density functional study.  

PubMed

Density functional calculations were performed to study the ?-TiAl (001), (100), (110) and (111) surfaces. The (100) surface is the most stable under Ti-rich conditions, while the Al-termination (110) surface becomes the most stable with the increase of Al chemical potential. We calculate that in ?-TiAl intermetallic compound the larger the interlayer distance, the larger the surface energy and cleavage energy. This is different from the situation in a pure metal. This phenomenon can be explained by the analysis of the bonding characteristics in ?-TiAl. In particular there are both metallic and covalent bonds in ?-TiAl, and the strongest covalent bonds mainly focus on the center of three Ti-Al-Ti atoms. It is the covalent bonds that affect greatly the cleavage energy, the surface energy and the surface stability. PMID:21673399

Wang, Lu; Shang, Jia-Xiang; Wang, Fu-He; Zhang, Yue; Chroneos, Alexander

2011-06-15

311

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

312

Microstatistical Model for Carbide Induced Cleavage Fracture.  

National Technical Information Service (NTIS)

Carbide induced cleavage fracture initiation process is shown to be conveniently represented by a three dimensional statistical micromechanistic model. The model is applicable to ferritic and bainitic steels, and can be used to predict the temperature dep...

K. Wallin T. Saario K. Toerroenen J. Forsten

1983-01-01

313

Enediyne-mediated cleavage of RNA.  

PubMed

RNA cleavage by enediyne anticancer antibiotics was shown to occur with no apparent sequence selectivity, but RNA structure appears to be important in those substrates where cleavage was observed. Neocarzinostatin (NCS) cleaved a wider variety of RNA substrates than either esperamicin (ESP) or calicheamicin (CAL), and dynemicin (DYN) has yet to cleave any RNA substrate tried. NCS, ESP, and CAL were all observed to cleave RNA substrates near the 5'-end, and all three compounds exhibited cleavage in single-stranded loop regions of the RNA substrates. NCS required no thiol for activation and subsequent cleavage, but ESP and CAL required addition of thiol, as expected, for cleavage to occur. An RNA hairpin substrate containing a UCCU sequence, equivalent to the TCCT sequence preferred by CAL in double-stranded DNA substrates, was cleaved by CAL, but no retention of selectivity for the UCCU site was retained by CAL in this RNA substrate. This study confirms an earlier observation that RNA is a substrate for enediyne cleavage, and indicates that nucleic acid cleaving compounds such as the enediynes could be useful probes of RNA three-dimensional structure. PMID:7582960

Battigello, J M; Cui, M; Roshong, S; Carter, B J

1995-06-01

314

Ring cleavage reactions of methyl ?-D-allopyranoside derivatives with phenylboron dichloride and triethylsilane.  

PubMed

In the course of our studies on the regioselective carbon-oxygen bond cleavage of the benzylidene acetal group of hexopyranosides with a reducing agent, we found that a combination of a Lewis acid and a reducing agent triggered a ring-opening reaction of the pyranose ring of methyl ?-D-allopyranosides. The formation of an acyclic boronate ester by the attachment of a hydride ion at C-1 indicated that the unexpected endocyclic cleavage of the bond between the anomeric carbon atom and the pyranose ring oxygen atom proceeded via an oxacarbenium ion intermediate produced by the chelation between O5/O6 of the pyranoside and the Lewis acid, followed by nucleophile substitution with a hydride ion at C1. PMID:22158683

Kojima, Masaru; Nakamura, Yutaka; Ito, Yuusuke; Takeuchi, Seiji

2011-12-13

315

Cleavage of deoxyoxanosine-containing oligodeoxyribonucleotides by bacterial endonuclease V  

PubMed Central

Oxanine (O) is a deamination product derived from guanine with the nitrogen at the N1 position substituted by oxygen. Cytosine, thymine, adenine, guanine as well as oxanine itself can be incorporated by Klenow Fragment to pair with oxanine in a DNA template with similar efficiency, indicating that oxanine in DNA may cause various mutations. As a nucleotide, deoxyoxanosine may substitute for deoxyguanosine to complete a primer extension reaction. Endonuclease V, an enzyme known for its enzymatic activity on uridine-, inosine- and xanthosine-containing DNA, can cleave oxanosine-containing DNA at the second phosphodiester bond 3? to the lesion. Mg2+ or Mn2+, and to a small extent Co2+ or Ni2+, support the oxanosine-containing DNA cleavage activity. All four oxanosine-containing base pairs (A/O, T/O, C/O and G/O) were cleaved with similar efficiency. The cleavage of double-stranded oxanosine-containing DNA was ?6-fold less efficient than that of double-stranded inosine-containing DNA. Single-stranded oxanosine-containing DNA was cleaved with a lower efficiency as compared with double-stranded oxanosine-containing DNA. A metal ion enhances the binding of endonuclease V to double-stranded and single-stranded oxanosine-containing DNA 6- and 4-fold, respectively. Hypothetic models of oxanine-containing base pairs and deaminated base recognition mechanism are presented.

Hitchcock, Thomas M.; Gao, Honghai; Cao, Weiguo

2004-01-01

316

Hydrogen chemisorption on cleavage faces of III-V compounds  

NASA Astrophysics Data System (ADS)

In this paper a number of experimental results dealing with the study of the electronic properties of H-exposed III-V semiconductor cleavage surfaces will be reviewed. The principal aim of the paper is to show how surface sensitive spectroscopies like electron energy loss spectroscopy (EELS) and ultraviolet photoemission spectroscopy (UPS) can be used to investigate these systems. The paper is focused on GaAs(1 1 0):H. GaAs(1 1 0) is one of the most widely investigated and better known semiconductor surfaces: this system can be taken as a case study for the interaction of H with III-V semiconductor cleavage surfaces. The paper is divided into two parts. In the first part EELS and UPS are used and combined to get information about the surface valence states. In the second part core level (CL) photoemission results are presented and discussed. Combining theory with the above results, a unified picture of the H interaction with the (1 1 0) surface of GaAs comes out. H atoms chemisorb at the early stages along the dangling bond directions of the ideal not relaxed surface, removing the surface relaxation and introducing at the same time defects at the surface. At the same time the interaction with H produces a Ga enrichment at the surface ascribed to As desorption. On leave from: Istituto di Acustica ``O.M. Corbino'' del C.N.R., Via Cassia 1216, 00189 Roma, Italy.

Nannarone, S.; Sorba, L.

1991-04-01

317

Iron-Dependent Oxidative Inactivation with Affinity Cleavage of Pyruvate Kinase  

Microsoft Academic Search

Treatment of rabbit muscle pyruvate kinase with iron\\/ascorbate caused an inactivation with the cleavage of peptide bond. The\\u000a 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\\u000a suggests that hydroxyl radical produced through the ferrous ion-dependent reduction

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

2009-01-01

318

Enhanced RNA cleavage within bulge-loops by an artificial ribonuclease  

Microsoft Academic Search

Cleavage of phosphodiester bonds by small ribonuc- lease mimics within different bulge-loops of RNA was investigated. Bulge-loops of different size (1-7 nt) and sequence composition were formed in a 30 terminal fragment of influenza virus M2 RNA (96 nt) by hybrid- ization of complementary oligodeoxynucleotides. Small bulges (up to 4 nt) were readily formed upon oligonucleotide hybridization, whereas hybridization of

Irina L. Kuznetsova; Marina A. Zenkova; Hans J. Gross; Valentin V. Vlassov

2005-01-01

319

Altered cleavage site preference of a proteolytic antibody light chain induced by denaturation  

Microsoft Academic Search

A recombinant antibody light chain (Lchain) maintained under non-denaturing conditions displayed preferential cleavage of synthetic peptides conjugated to methylcoumarinamide (MCA) on the C-terminal side of Arg and Lys residues. The same Lchain renatured from a denaturing solvent (guanidine hydrochloride) acquired the capability of cleaving Tyr–MCA and Leu–MCA bonds, and its ability to cleave MCA linked to basic residues was decreased.

Mei Sun; Sudhir Paul

1997-01-01

320

DNA and RNA Cleavage Mediated by Phenanthroline-Cuprous Oligonucleotides: From Properties to Applications  

Microsoft Academic Search

\\u000a Several metallic ions when bound to a ligand bringing them in close proximity to nucleic acids are able to cleave phosphodiester\\u000a bonds, in the presence of oxygen. Redox-active iron and copper ions induced DNA cleavage in the presence of bleomycin, a well-known\\u000a antitumoral antibiotic interacting with DNA double helix. Small molecules such as ethylene-diamine tetraacetic acid (EDTA),\\u000a porphyrin, and 1,10-phenanthroline

J.-C. Francois; M. Faria; D. Perrin; C. Giovannangeli

321

Sequences Flanking Arg336 in Factor VIIIa Modulate Factor Xa-catalyzed Cleavage Rates at this Site and Cofactor Function*  

PubMed Central

Factor (F)VIII can be activated to FVIIIa by FXa following cleavages at Arg372, Arg740, and Arg1689. FXa also cleaves FVIII/FVIIIa at Arg336 and Arg562 resulting in inactivation of the cofactor. These inactivating cleavages occur on a slower time scale than the activating ones. We assessed the contributions to cleavage rate and cofactor function of residues flanking Arg336, the primary site yielding FVIII(a) inactivation, following replacement of these residues with those flanking the faster-reacting Arg740 and Arg372 sites and the slower-reacting Arg562 site. Replacing P4-P3? residues flanking Arg336 with those from Arg372 or Arg740 resulted in ?4–6-fold increases in rates of FXa-catalyzed inactivation of FVIIIa, which paralleled the rates of proteolysis at Arg336. Examination of partial sequence replacements showed a predominant contribution of prime residues flanking the scissile bonds to the enhanced rates. Conversely, replacement of this sequence with residues flanking the slow-reacting Arg562 site yielded inactivation and cleavage rates that were ?40% that of the WT values. The capacity for FXa to activate FVIII variants where cleavage at Arg336 was accelerated due to flanking sequence replacement showed marked reductions in peak activity, whereas reducing the cleavage rate at this site enhanced peak activity. Furthermore, plasma-based thrombin generation assays employing the variants revealed significant reductions in multiple parameter values with acceleration of Arg336 cleavage suggesting increased down-regulation of FXase. Overall, these results are consistent with a model of competition for activating and inactivating cleavages catalyzed by FXa that is modulated in large part by sequences flanking the scissile bonds.

DeAngelis, Jennifer P.; Wakabayashi, Hironao; Fay, Philip J.

2012-01-01

322

Water-stabilized three- and four-atom palladium clusters as highly active catalytic species in ligand-free c?c cross-coupling reactions.  

PubMed

Elite cliques: Palladium clusters with three and four atoms were found to be the catalytically active species for ligand-free palladium-catalyzed C?C bond-forming reactions. These palladium cluster species could be stabilized in water and stored for long periods of time for use on demand with no loss of activity. High yields of products and turnover frequencies (TOFs) of up to 10(5) ?h(-1) were observed. PMID:24038914

Leyva-Pérez, Antonio; Oliver-Meseguer, Judit; Rubio-Marqués, Paula; Corma, Avelino

2013-09-13

323

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

324

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

325

Bond energy analysis revisited and designed toward a rigorous methodology  

NASA Astrophysics Data System (ADS)

The present study theoretically revisits and numerically assesses two-body energy decomposition schemes including a newly proposed one. The new decomposition scheme is designed to make the equilibrium bond distance equivalent with the minimum point of bond energies. Although the other decomposition schemes generally predict the wrong order of the C-C bond strengths of C2H2, C2H4, and C2H6, the new decomposition scheme is capable of reproducing the C-C bond strengths. Numerical assessment on a training set of molecules demonstrates that the present scheme exhibits a stronger correlation with bond dissociation energies than the other decomposition schemes do, which suggests that the new decomposition scheme is a reliable and powerful analysis methodology.

Nakai, Hiromi; Ohashi, Hideaki; Imamura, Yutaka; Kikuchi, Yasuaki

2011-09-01

326

Diffusion bonding  

DOEpatents

1. A method for joining beryllium to beryllium by diffusion bonding, comprising the steps of coating at least one surface portion of at least two beryllium pieces with nickel, positioning a coated surface portion in a contiguous relationship with an other surface portion, subjecting the contiguously disposed surface portions to an environment having an atmosphere at a pressure lower than ambient pressure, applying a force upon the beryllium pieces for causing the contiguous surface portions to abut against each other, heating the contiguous surface portions to a maximum temperature less than the melting temperature of the beryllium, substantially uniformly decreasing the applied force while increasing the temperature after attaining a temperature substantially above room temperature, and maintaining a portion of the applied force at a temperature corresponding to about maximum temperature for a duration sufficient to effect the diffusion bond between the contiguous surface portions.

Anderson, Robert C. (Oak Ridge, TN)

1976-06-22

327

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

328

Changing cleavage structure in new democracies: An empirical analysis of political cleavages in Korea  

Microsoft Academic Search

Cleavage structure cuts across the members of a state and affects the electorate's choice of parties and candidates. Social cleavage structure can be stable or go through changes depending on on-going social change as well as political parties' electoral strategies. With the collapse of communism and the advent of the third wave of democracy, many countries in Eastern Europe, Latin

HeeMin Kim; Jun Young Choi; Jinman Cho

2008-01-01

329

TMP21 Transmembrane Domain Regulates ?-Secretase Cleavage*  

PubMed Central

TMP21 has been shown to be associated with the ?-secretase complex and can specifically regulate ?-cleavage without affecting ?-mediated proteolysis. To explore the basis of this activity, TMP21 modulation of ?-secretase activity was investigated independent of ?-cleavage using an amyloid-? precursor protein? (APP?) construct which lacks the amyloid intracellular domain domain. The APP? construct behaves similarly to the full-length precursor protein with respect to ?- and ?-cleavages and is able to undergo normal ?-processing. Co-expression of APP? and TMP21 resulted in the accumulation of membrane-embedded higher molecular weight A?-positive fragments, consistent with an inhibition of ?-secretase cleavage. The APP? system was used to examine the functional domains of TMP21 through the investigation of a series of TMP21-p24a chimera proteins. It was found that chimeras containing the transmembrane domain bound to the ?-secretase complex and could decrease ?-secretase proteolytic processing. This was confirmed though investigation of a synthetic peptide corresponding to the TMP21 transmembrane helix. The isolated TMP21 TM peptide but not the homologous p24a domain was able to reduce A? production in a dose-dependent fashion. These observations suggest that the TMP21 transmembrane domain promotes its association with the presenilin complex that results in decreased ?-cleavage activity.

Pardossi-Piquard, Raphaelle; Bohm, Christopher; Chen, Fusheng; Kanemoto, Soshi; Checler, Frederic; Schmitt-Ulms, Gerold; St. George-Hyslop, Peter; Fraser, Paul E.

2009-01-01

330

A Study of the Interactions between the Double Bonds in Unsaturated Ketones.  

National Technical Information Service (NTIS)

The interactions between C=C and C-O double bonds in several unsaturated ketones have been studied by comparing MINDO/2 calculations with ionisation potentials determined by photoelectron spectroscopy (PES). With one exception (norbornadienone) the direct...

N. Bodor M. J. S. Dewar Z. B. Maksic

1975-01-01

331

Mechanochemistry: One Bond at a Time  

PubMed Central

Single-molecule force clamp spectroscopy offers a novel platform for mechanically denaturing proteins by applying a constant force to a polyprotein. A powerful emerging application of the technique is that, by introducing a disulfide bond in each protein module, the chemical kinetics of disulfide bond cleavage under different stretching forces can be probed at the single-bond level. Even at forces much lower than that can rupture the chemical bond, the breaking of the S-S bond at the presence of various chemical reducing agents is significantly accelerated. Our previous work demonstrated that the rate of thiol/disulfide exchange reaction is force-dependent, and well described by an Arrhenius term of the form: r = A(exp((F?xr-Ea)/kBT)[nucleophile]). From Arrhenius fits to the force dependency of the reduction rate we measured the bond elongation parameter, ?xr, along the reaction coordinate to the transition state of the SN2 reaction cleaved by different nucleophiles and enzymes, never before observed by any other technique. For S-S cleavage by various reducing agents, obtaining the ?xr value can help depicting the energy landscapes and elucidating the mechanisms of the reactions at the single-molecule level. Small nucleophiles, such as 1, 4-DL-dithiothreitol (DTT), tris(2-carboxyethyl)phosphine (TCEP) and L-cysteine, react with the S-S bond with monotonically increasing rates under the applied force; while thioredoxin enzymes exhibit both stretching-favored and —resistant reaction-rate regimes. These measurements demonstrate the power of single-molecule force clamp spectroscopy approach in providing unprecedented access to chemical reactions.

Liang, Jian; Fernandez, Julio M.

2009-01-01

332

Computational prediction of cleavage using proteasomal in vitro digestion and MHC I ligand data.  

PubMed

Proteasomes are responsible for the production of the majority of cytotoxic T lymphocyte (CTL) epitopes. Hence, it is important to identify correctly which peptides will be generated by proteasomes from an unknown protein. However, the pool of proteasome cleavage data used in the prediction algorithms, whether from major histocompatibility complex (MHC) I ligand or in vitro digestion data, is not identical to in vivo proteasomal digestion products. Therefore, the accuracy and reliability of these models still need to be improved. In this paper, three types of proteasomal cleavage data, constitutive proteasome (cCP), immunoproteasome (iCP) in vitro cleavage, and MHC I ligand data, were used for training cleave-site predictive methods based on the kernel-function stabilized matrix method (KSMM). The predictive accuracies of the KSMM+pair coefficients were 75.0%, 72.3%, and 83.1% for cCP, iCP, and MHC I ligand data, respectively, which were comparable to the results from support vector machine (SVM). The three proteasomal cleavage methods were combined in turn with MHC I-peptide binding predictions to model MHC I-peptide processing and the presentation pathway. These integrations markedly improved MHC I peptide identification, increasing area under the receiver operator characteristics (ROC) curve (AUC) values from 0.82 to 0.91. The results suggested that both MHC I ligand and proteasomal in vitro degradation data can give an exact simulation of in vivo processed digestion. The information extracted from cCP and iCP in vitro cleavage data demonstrated that both cCP and iCP are selective in their usage of peptide bonds for cleavage. PMID:24009202

Lu, Yu-feng; Sheng, Hao; Zhang, Yi; Li, Zhi-yang

2013-09-01

333

More sustainable formation of C-N and C-C bonds for the synthesis of N-heterocycles.  

PubMed

Heterocycles made green: New methodologies for the synthesis of pyrroles were recently developed based on domino Ir- and Ru-catalyzed amination and alkylations of alcohols. The concept provides a greener approach to interesting N-heterocyclic compounds. PMID:23775609

Schranck, Johannes; Tlili, Anis; Beller, Matthias

2013-06-17

334

Fluorescence spectroscopy as a novel method for on-line analysis of biocatalytic C–C bond formations  

Microsoft Academic Search

On-line analysis of bioprocesses is of increasing interest avoiding the time delay for off-line sample preparation and the following analyses via chromatographic methods. Moreover, continuous monitoring of the reaction components during chemo- or biocatalytic transformations provides a direct control of the process. Since productivity of the processes can be controlled simultaneously, on-line monitoring of the processes is attractive for industrial

Selin Kara; Fabienne Anton; Dörte Solle; Markus Neumann; Bernd Hitzmann; Thomas Scheper; Andreas Liese

2010-01-01

335

C-C bond formation via copper-catalyzed conjugate addition reactions to enones in water at room temperature.  

PubMed

Conjugate addition reactions to enones can now be done in water at room temperature with in situ generated organocopper reagents. Mixing an enone, zinc powder, TMEDA, and an alkyl halide in a micellar environemnt containing catalytic amounts of Cu(I), Ag(I), and Au(III) leads to 1,4-adducts in good isolated yields: no organometallic precursor need be formed. PMID:23190029

Lipshutz, Bruce H; Huang, Shenlin; Leong, Wendy Wen Yi; Zhong, Guofu; Isley, Nicholas A

2012-11-28

336

A heterogeneous layered bifunctional catalyst for the integration of aerobic oxidation and asymmetric C-C bond formation.  

PubMed

The design and synthesis of a heterogeneous bifunctional chiral catalyst for the sequential aerobic oxidation-asymmetric Michael reactions between primary allylic alcohols and dibenzyl malonate are described. Interestingly, we found that layering bimetallic nanoparticles over the organocatalyst, within the chiral composite material, is crucial for catalytic activity. PMID:24036576

Miyamura, Hiroyuki; Choo, Gerald C Y; Yasukawa, Tomohiro; Yoo, Woo-Jin; Kobayashi, Shu

2013-10-01

337

Acetylacetone-cleaving enzyme Dke1: a novel C-C-bond-cleaving enzyme from Acinetobacter johnsonii.  

PubMed Central

The toxicity of acetylacetone has been demonstrated in various studies. Little is known, however, about metabolic pathways for its detoxification or mineralization. Data presented here describe for the first time the microbial degradation of acetylacetone and the characterization of a novel enzyme that initiates the metabolic pathway. From an Acinetobacter johnsonii strain that grew with acetylacetone as the sole carbon source, an inducible acetylacetone-cleaving enzyme was purified to homogeneity. The corresponding gene, coding for a 153 amino acid sequence that does not show any significant relationship to other known protein sequences, was cloned and overexpressed in Escherichia coli and gave high yields of active enzyme. The enzyme cleaves acetylacetone to equimolar amounts of methylglyoxal and acetate, consuming one equivalent of molecular oxygen. No exogenous cofactor is required, but Fe(2+) is bound to the active protein and essential for its catalytic activity. The enzyme has a high affinity for acetylacetone with a K (m) of 9.1 microM and a k(cat) of 8.5 s(-1). A metabolic pathway for acetylacetone degradation and the putative relationship of this novel enzyme to previously described dioxygenases are discussed.

Straganz, Grit D; Glieder, Anton; Brecker, Lothar; Ribbons, Douglas W; Steiner, Walter

2003-01-01

338

Synthesis and reactions of group 6 metal half-sandwich complexes of 2,2-dicyanoethylene-1,1-dichalcogenolates [(Cp*)M[E(2)C=C(CN)(2)](2)]-(M = Mo, W; E = S, Se).  

PubMed

A series of group 6 transition metal half-sandwich complexes with 1,1-dichalcogenide ligands have been prepared by the reactions of Cp*MCl(4)(Cp* = eta(5)-C(5)Me(5); M = Mo, W) with the potassium salt of 2,2-dicyanoethylene-1,1-dithiolate, (KS)(2)C=C(CN)(2) (K(2)-i-mnt), or the analogous seleno compound, (KSe)(2)C=C(CN)(2) (K(2)-i-mns). The reaction of Cp*MCl(4) with (KS)(2)C=C(CN)(2) in a 1:3 molar ratio in CH(3)CN gave rise to K[Cp*M(S(2)C=C(CN)(2))(2)] (M = Mo, 1a, 74%; M = W, 2a, 46%). Under the same conditions, the reaction of Cp*MoCl(4) with 3 equiv of (KSe)(2)C=C(CN)(2) afforded K[Cp*Mo(Se(2)C=C(CN)(2))(2)] (3a) and K[Cp*Mo(Se(2)C=C(CN)(2))(Se(Se(2))C=C(CN)(2))] (4) in respective yields of 45% and 25%. Cation exchange reactions of 1a, 2a, and 3a with Et(4)NBr resulted in isolation of (Et(4)N)[Cp*Mo(S(2)C=C(CN)(2))(2)] (1b), (Et(4)N)[Cp*W(S(2)C=C(CN)(2))(2)] (2b), and (Et(4)N)[Cp*Mo(Se(2)C=C(CN)(2))(2)] (3b), respectively. Complex 4 crystallized with one THF and one CH(3)CN molecule as a three-dimensional network structure. Inspection of the reaction of Cp*WCl(4) with (KSe)(2)C=C(CN)(2) by ESI-MS revealed the existence of three species in CH(3)CN, [Cp*W(Se(2)C=C(CN)(2))(2)]-, [Cp*W(Se(2)C=C(CN)(2))(Se(Se(2))C=C(CN)(2))]-, and [Cp*W(Se(Se(2))C=C(CN)(2))(2)]-, of which [Cp*W(Se(2)C=C(CN)(2))(Se(Se(2))C=C(CN)(2))]-(5) was isolated as the main product. Treatment of 2a with 1/4 equiv of S(8) in refluxing THF resulted in sulfur insertion and gave rise to K[Cp*W(S(2)C=C(CN)(2))(S(S(2))C=C(CN)(2))](6), which crystallized with two THF molecules forming a three-dimensional network structure. 6 can also be prepared by refluxing 2a with 1/4 equiv of S(8) in THF. 3a readily added one Se atom upon treatment with 1 mol of Se powder in THF to give 4 in high yield, while the treatment of 3a or 4 with 2 equiv of Na(2)Se in THF led to formation of a dinuclear complex [(Cp*Mo)(2)(mu-Se)(mu-Se(Se(3))C=C(CN)(2))] (7). The structure of 7 consists of two Cp*Mo units bridged by a Se(2-) and a [Se(Se(3))C=C(CN)(2)](2-) ligand in which the triselenido group is arranged in a nearly linear way (163 degrees). The reaction of 2a with 2 equiv of CuBr in CH(3)CN yielded a trinuclear complex [Cp*WCu(2)(mu-Br)(mu(3)-S(2)C=C(CN)(2))(2)] (8), which crystallized with one CH(3)CN and generated a one-dimensional chain polymer through bonding of Cu to the N of the cyano groups. PMID:12206711

Hong, Maochun; Cao, Rong; Kawaguchi, Hiroyuki; Tatsumi, Kazuyuki

2002-09-01

339

Theoretical study on aluminum carbide endohedral fullerene-Al4C@C80.  

PubMed

The possibility of a new endohedral fullerene with a trapped aluminum carbide cluster, Al(4)C@C(80)-I( h ), was theoretical investigated. The geometries and electronic properties of it were investigated using density functional theory methods. The Al(4)C unit formally transfers six electrons to the C(80) cage which induces stabilization of Al(4)C@C(80). A favorable binding energy, relatively large HOMO-LUMO gap, electron affinities and ionization potentials suggested the Al(4)C@C(80) is rather stable. The analysis of vertical ionization potential and vertical electron affinity indicate Al(4)C@C(80) is a good electron acceptor. PMID:23160935

Lu, Qi Liang; Song, Wen Jun; Meng, Jun Wei; Wan, Jian Guo

2012-11-17

340

Basics of Fidelity Bonding.  

ERIC Educational Resources Information Center

|Fidelity bonds are important for an agency to hold to protect itself against any financial loss that can result from dishonest acts by its employees. Three types of fidelity bonds are available to an agency: (1) public official bonds; (2) dishonesty bonds; and (3) faithful performance bonds. Public official bonds are required by state law to be…

Kahn, Steven P.

341

Centrosomes: CNN's broadcast reaches the cleavage furrow.  

PubMed

Centrosomin (CNN), a core Drosophila centrosome protein, interacts with the newly identified protein Centrocortin to promote cleavage furrow formation in the early embryo. Significantly, this activity is distinct from CNN's well-established role in centrosome-based microtubule organization. PMID:19602412

Sullivan, William

2009-07-14

342

Regime Change and Ethnic Cleavages in Africa  

Microsoft Academic Search

This article explores a hitherto overlooked consequence of regime change in Africa. It shows how the shift from one-party to multiparty rule in the region altered the kinds of ethnic cleavages that structure political competition and conflict. The article demonstrates how the different strategic logics of political competition in one-party and multiparty settings create incentives for political actors to emphasize

Daniel N. Posner

2007-01-01

343

Tuning the redox potential in molecular monolayers covalently bound to H Si(100) electrodes via distinct C C tethering arms  

NASA Astrophysics Data System (ADS)

The spatial self-organization of molecular species on an Si oriented surface can be less ideal than that of an SAM on a metal, likely affecting the electronic structure of the resulting hybrids and their electrochemical response as electrodes in solution. In order to investigate such effects, a series of molecular precursors was investigated, consisting of three substituted ferrocenes with a lateral C C group fully saturated (ethylferrocene) or with a single (vinylferrocene) or double unsaturation (ethynylferrocene). The corresponding functionalized Si(100) wafers were produced following new or literature recipes, starting from hydrogenated Si surfaces. The relationship between the degree of unsaturation in the anchored arm of each adduct and its electronic structure and electrochemical behaviour was investigated by comparing experimental (XPS, electrochemical) and ab initio results of the redox potentials in the series. Density functional theory (DFT) was applied, with inclusion of solute solvent interactions. Different bond arrangements of the C C arm with Si surface dimer atoms have been produced theoretically within the series of ferrocenes. Distinct values of redox potentials were displayed by the hybrids, which can be consistently related to the structural differences presented. In fact, measured and computed potentials showed a very satisfactory match only for specific adduct isomers, providing strong indications that the carbon carbon unsaturation initially present in the anchoring arm is preserved upon addition reaction, an unprecedented result. The demonstrated tunability of a well-defined switching potential in these molecules on silicon can be beneficial to the development of Si-based memory devices.

Zanoni, R.; Cossi, M.; Iozzi, M. F.; Cattaruzza, F.; Dalchiele, E. A.; Decker, F.; Marrani, A. G.; Valori, M.

2008-10-01

344

Screening for mutations by enzyme mismatch cleavage with T4 endonuclease VII.  

PubMed Central

Each of four possible sets of mismatches (G.A/C.T, C.C/G.G, A.A/T.T, and C.A/G.T) containing the 8 possible single-base-pair mismatches derived from isolated mutations were examined to test the ability of T4 endonuclease VII to consistently detect mismatches in heteroduplexes. At least two examples of each set of mismatches were studied for cleavage in the complementary pairs of heteroduplexes formed between normal and mutant DNA. Four deletion mutations were also included in this study. The various PCR-derived products used in the formation of heteroduplexes ranged from 133 to 1502 bp. At least one example of each set showed cleavage of at least one strand containing a mismatch. Cleavage of at least one strand of the pairs of heteroduplexes occurred in 17 of the 18 known single-base-pair mutations tested, with an A.A/T.T set not being cleaved in any mismatched strand. We propose that this method may be effective in detecting and positioning almost all mutational changes when DNA is screened for mutations. Images Fig. 1 Fig. 2 Fig. 3 Fig. 4

Youil, R; Kemper, B W; Cotton, R G

1995-01-01

345

Regulated cleavage-secretion of the membrane-bound angiotensin-converting enzyme.  

PubMed

Angiotensin-converting enzyme (ACE) is an ectoprotein anchored in the plasma membrane through a hydrophobic domain near its carboxyl-terminal region. Mouse epithelial cells transfected with rabbit testicular ACE cDNA, synthesize, glycosylate, and secrete ACE by cleavage processing of its membrane-anchoring carboxyl-terminal region. Because the cleavage-secretion process is slow, the enzyme accumulates on the cell surface. We show that this process can be enhanced by treatment of cells with tumor-promoting phorbol esters leading to depletion of the cell surface enzyme. The cleavage processing occurs only after the protein has reached the cell surface and is not affected by disruption of the Golgi apparatus or the lysosomal compartments. The exact peptide bond cleaved has been identified by sequencing the amino-terminal residues of the purified COOH-terminal tail left in the cells after ACE is secreted and the carboxyl-terminal residues of secreted ACE. The cleavage occurs at a monobasic site between Arg-663 and Ser-664 generating the soluble enzyme and leaving a cell-bound protein of 74 residues. These results demonstrate the existence of cellular mechanisms that regulate the conversion of cell-bound ACE to a soluble enzyme. PMID:8294466

Ramchandran, R; Sen, G C; Misono, K; Sen, I

1994-01-21

346

One-Electron-Transfer Reactions of Polychlorinated Ethylenes: Concerted and Stepwise Cleavages  

SciTech Connect

Reaction barriers were calculated by using ab initio electronic structure methods for the reductive dechlorination of the polychlorinated ethylenes: C2CL4, C2Cl4, C2HCl3, trans-1,2-C2H2Cl2, cis-1,2-C2H2Cl2, 1,1-C2H2Cl2, and C2HCl3. Concerted and stepwise cleavages of R-Cl bonds were considered. Stepwise cleavages yielded lower activation barriers than concerted cleavages for the reduction of C2Cl4, C2HCl3, and trans-1,2-C2H2Cl2 via strong reducing agents. However, for typical ranges of reducing strength concerted cleavages were found to be favored. Both gas-phase and aqueous-phase calculations predicted C2Cl4 to have the lowest reaction barrier. Additionally, the reduction of C2HCl3 was predicted to have a significant amount of selectivity of cis-1,2-C2HCl2 over the corresponding reactions leading to the trans-1,2-C2HCl2, and 1,1-C2HCl2 radicals. These results illustrate how ab initio electronic structure methods, by providing experimentally inaccessible thermodynamics properties and activation energies, are able to sort out possible reactions mechanisms of reactions that have broad relevance in environmental chemistry.

Bylaska, Eric J.; Dupuis, Michel; Tratnyek, Paul G.

2008-04-24

347

Mechanism of cleavage and ligation by FLP recombinase: classification of mutations in FLP protein by in vitro complementation analysis.  

PubMed

The FLP recombinase of the 2 microns plasmid of Saccharomyces cerevisiae is a member of the integrase family of site-specific recombinases. Recombination catalyzed by members of this family proceeds via the ordered cleavage and religation of four strands of DNA. Although the amino acid sequences of integrase family members are quite different, each recombinase maintains an absolutely conserved tetrad of amino acids (R-191, H-305, R-308, Y-343; numbers are those of the FLP protein). This tetrad is presumed to reflect a common chemical mechanism for cleavage and ligation that has evolved among all family members. The tyrosine is the nucleophile that causes phosphodiester bond cleavage and covalently attaches to the 3'-PO4 terminus, whereas the other three residues have been implicated in ligation of strands. It has recently been shown that cleavage by FLP takes place in trans; that is, a FLP molecule binds adjacent to the site of cleavage but receives the nucleophilic tyrosine from a molecule of FLP that is bound to another FLP-binding element (J.-W. Chen, J. Lee, and M. Jayaram, Cell 69:647-658, 1992). These studies led us to examine whether the ligation step of the FLP reaction is performed by the FLP molecule bound adjacent to the cleavage site (ligation in cis). We have found that FLP promotes ligation in cis. Furthermore, using in vitro complementation analysis, we have classified several mutant FLP proteins into one of two groups: those proteins that are cleavage competent but ligation deficient (group I) and those that are ligation competent but cleavage defective (group II). This observation suggests that the active site of FLP is composed of several amino acid residues from each of two FLP molecules. PMID:8497247

Pan, G; Luetke, K; Sadowski, P D

1993-06-01

348

Coincidence of cleavage sites of intron endonuclease I-TevI and critical sequences of the host thymidylate synthase gene.  

PubMed

To maximize spread of their host intron or intein, many homing endonucleases recognize nucleotides that code for important and conserved amino acid residues of the target gene. Here, we examine the cleavage requirements for I-TevI, which binds a stretch of thymidylate synthase (TS) DNA that codes for functionally critical residues in the TS active site. Using an in vitro selection scheme, we identified two base-pairs in the I-TevI cleavage site region as important for cleavage efficiency. These were confirmed by comparison of I-TevI cleavage efficiencies on mutant and on wild-type substrates. We also showed that nicking of the bottom strand by I-TevI is not affected by mutation of residues surrounding the bottom-strand cleavage site, unlike other homing endonucleases. One of these two base-pairs is universally conserved in all TS sequences, and is identical with a previously identified cleavage determinant of I-BmoI, a related GIY-YIG endonuclease that binds a homologous stretch of TS-encoding DNA. The other base-pair is conserved only in a subset of TS genes that includes the I-TevI, but not the I-BmoI, target sequence. Both the I-TevI and I-BmoI cleavage site requirements correspond to functionally critical residues involved in an extensive hydrogen bond network within the TS active site. Remarkably, these cleavage requirements correlate with TS phylogeny in bacteria, suggesting that each endonuclease has individually adapted to efficiently cleave distinct TS substrates. PMID:15491609

Edgell, David R; Stanger, Matthew J; Belfort, Marlene

2004-11-01

349

Detecting type errors and secure coding in C\\/C++ applications  

Microsoft Academic Search

The programming languages such as C\\/C++ suffer from memory management and security of code especially when their codes are used in critical systems. Therefore, we need an efficient mechanism to detect memory and type errors. Some researches have been done and many tools have been developed to detect these errors and to secure C\\/C++ code. However, theses tools have some

Hamid Mcheick; Heni Dhiab; Mohamad Dbouk; Rakan Mcheik

2010-01-01

350

Cleavage of carbon dioxide by an iridium-supported Fischer carbene. A DFT investigation.  

PubMed

The reaction of CO(2), OCS, and PhNCO with an iridium-supported Fischer alkoxycarbene has been investigated with density functional theory. We have confirmed the mechanism for the important CO(2) reaction and successfully rationalized the selective cleavage of the CS and CN bonds in OCS and PhNCO. Armed with this information we have used our model to predict that the same iridium system will preferentially cleave the CS bond in methyl thiocyanate (MeNCS) rather than the CN bond. The formation of the iridium-supported carbene itself has also been investigated and a fascinating autocatalytic mechanism has been discovered which nicely fits the observed experimental behavior. PMID:19338328

Brookes, Nigel J; Ariafard, Alireza; Stranger, Robert; Yates, Brian F

2009-04-29

351

Oxidative addition of methane and benzene C–H bonds to rhodium center: A DFT study  

Microsoft Academic Search

A density functional theory study on mechanisms of the oxidative addition of methane and benzene C–H bonds to the rhodium center containing Cp and PMe3 ligands has been performed. Our calculated results confirm that the C–H bond cleavage from a sigma complex to a hydride alkyl complex is the rate-determining step. Compared with the case of methane C–H bond, the

Siwei Bi; Zhenwei Zhang; Shufen Zhu

2006-01-01

352

Contribution of disulfide bonds to the conformational stability and catalytic activity of ribonuclease A  

Microsoft Academic Search

Disulfide bonds between the side chains of cysteine residues are the only common crosslinks in proteins. Bovine pancreatic ribonuclease A (RNase A) is a 124-residue enzyme that contains four interweaving disulfide bonds (Cys26-Cys84, Cys40-Cys95, Cys58-Cys110, and Cys65-Cys72) and catalyzes the cleavage of RNA. The contribution of each disulfide bond to the conformational stability and catalytic activity of RNase A has

Tony A. Klink; Kenneth J. Woycechowsky; Kimberly M. Taylor; Ronald T. Raines

2000-01-01

353

Cleavage fracture stress of pearlitic steels  

NASA Astrophysics Data System (ADS)

Traditionally, pearlitic steels have been treated as homogeneous materials with properties that reflect their assumed homogeneity. In reality, pearlitic steels are two phase, lamellar microstructures containing ferrite, which is commonly considered to be ductile, and cementite, which is commonly considered to be brittle. Because of this microstructure, the properties such as the tensile strength and the cleavage fracture stress are dependent on the dimensional characteristics of the pearlite. Therefore, improvement of the mechanical performance of pearlitic steels resides with understanding the relationship between the microstructural characteristics and the mechanical properties. The primary objective of this study was to define the microstructural and cleavage fracture stress relationship which exists for hypo-, hyper-, and eutectoid steel compositions with cementite contents ranging from 7 to 15 volume percent. By controlling the quench conditions, predominately pearlitic microstructures were achieved in plain carbon steels with carbon concentrations ranging from 0.45 to 1.0 weight percent. Utilizing continuous cooling procedures, mechanical specimens were fabricated from AISI 1045, 1065, 1080, and 10100 grade steels. Mechanical testing to determine bend fracture strengths and tensile properties was performed at temperatures ranging from 125 to 250°C. From the nominal bend fracture strength and the yield strength, a determination of the cleavage fracture stress was made. SEM analysis of polished and picral etched specimens allowed for the measurement of the pearlite dimensional characteristics including the spacing and cementite thickness. These parameters were then linked to the mechanical performance. Fractography on the bend specimens was utilized to identify the fracture initiation sites, and link the experimental and theoretical fracture behaviors. The results of the mechanical testing and the correlation to the pearlite dimensional characteristics revealed that the cleavage fracture stresses of the five different pearlitic microstructures were inversely proportional to the square root of the pearlite spacings. The inverse square root relationship between spacing and cleavage fracture stress was linked to the increase in the effective surface energy for cleavage fracture which accompanies refinement of the pearlite spacing. Effective surface energies of 60 and 100 J/m 2 were determined for pearlite spacings of 0.11 and 0.08 micrometers, respectively.

Nixon, Thomas Dwayne

354

Identification of succinimide sites in proteins by N-terminal sequence analysis after alkaline hydroxylamine cleavage.  

PubMed Central

Under favorable conditions, Asp or Asn residues can undergo rearrangement to a succinimide (cyclic imide), which may also serve as an intermediate for deamidation and/or isoaspartate formation. Direct identification of such succinimides by peptide mapping is hampered by their lability at neutral and alkaline pH. We determined that incubation in 2 M hydroxylamine, 0.2 M Tris buffer, pH 9, for 2 h at 45 degrees C will specifically cleave on the C-terminal side of succinimides without cleavage at Asn-Gly bonds; yields are typically approximately 50%. N-terminal sequence analysis can then be used to identify an internal sequence generated by cleavage of the succinimide, hence identifying the succinimide site.

Kwong, M. Y.; Harris, R. J.

1994-01-01

355

Recognition and cleavage of a nonstructured CRISPR RNA by its processing endoribonuclease Cas6.  

PubMed

Clustered regularly interspaced short palindromic repeats (CRISPRs) confer adaptive immunity to prokaryotes through a small RNA-mediated mechanism. Specific endoribonucleases are required by all CRISPR-bearing organisms to process CRISPR RNAs into small RNA that serve as guides for defensive effector complexes. The molecular mechanism of how the endoribonucleases process the class of CRISPR RNA containing no predicted secondary structural features remains largely elusive. Here, we report cocrystal structures of a processing endoribonuclease bound with a noncleavable RNA substrate and its product-like fragment derived from a nonpalindramic repeat. The enzyme stabilizes a short RNA stem-loop structure near the cleavage site and cleaves the phosphodiester bond using an active site comprised of arginine and lysine residues. The distinct RNA binding and cleavage mechanisms underline the diversity in CRISPR RNA processing. PMID:23454186

Shao, Yaming; Li, Hong

2013-03-01

356

Semiconductor wafer bonding  

Microsoft Academic Search

When mirror-polished, flat, and clean wafers are brought into contact, they are locally attracted to each other and adhere or bond. This phenomenon is known as semiconductor wafer bonding. Different adhesion forces (van der Waals forces, hydrogen bonding) are the reason for the bonding effect at room temperature. The different bonding mechanisms acting in dependence on the surface conditions (hydrophilic,

M. Reiche

2006-01-01

357

Mutations affecting cleavage at the p10-capsid protease cleavage site block Rous sarcoma virus replication  

PubMed Central

A series of amino acid substitutions (M239F, M239G, P240F, V241G) were placed in the p10-CA protease cleavage site (VVAM*PVVI) to change the rate of cleavage of the junction. The effects of these substitutions on p10-CA cleavage by RSV PR were confirmed by measuring the kinetics of cleavage of model peptide substrates containing the wild type and mutant p10-CA sites. The effects of these substitutions on processing of the Gag polyprotein were determined by labeling Gag transfected COS-1 cells with 35S-Met and -Cys, and immunoprecipitation of Gag and its cleavage products from the media and lysate fractions. All substitutions except M239F caused decreases in detectable Gag processing and subsequent release from cells. Several of the mutants also caused defects in production of the three CA proteins. The p10-CA mutations were subcloned into an RSV proviral vector (RCAN) and introduced into a chick embryo fibroblast cell line (DF-1). All of the mutations except M239F blocked RSV replication. In addition, the effects of the M239F and M239G substitutions on the morphology of released virus particles were examined by electron microscopy. While the M239F particles appeared similar to wild type particles, M239G particles contained cores that were large and misshapen. These results suggest that mutations affecting cleavage at the p10-CA protease cleavage site block RSV replication and can have a negative impact on virus particle morphology.

Vana, Marcy L; Chen, Aiping; Boross, Peter; Weber, Irene; Colman, Dalbinder; Barklis, Eric; Leis, Jonathan

2005-01-01

358

Cleavage Structures and Voter Alignments within Nations  

Microsoft Academic Search

Despite its general popularity the cleavage theory has only rarely been used to explain regional electoral outcome. In this\\u000a paper we examine whether a region's social structure and its electoral results are systematically linked. To analyse the political\\u000a preferences of an entire regional electorate in a single step, electoral results are translated into a left-right position\\u000a for each region using

Raphael Magin; Markus Freitag; Adrian Vatter

2009-01-01

359

Transcriptional activity regulates alternative cleavage and polyadenylation  

Microsoft Academic Search

Genes containing multiple pre-mRNA cleavage and polyadenylation sites, or polyA sites, express mRNA isoforms with variable 3? untranslated regions (UTRs). By systematic analysis of human and mouse transcriptomes, we found that short 3?UTR isoforms are relatively more abundant when genes are highly expressed whereas long 3?UTR isoforms are relatively more abundant when genes are lowly expressed. Reporter assays indicated that

Zhe Ji; Wenting Luo; Wencheng Li; Mainul Hoque; Zhenhua Pan; Yun Zhao; Bin Tian

2011-01-01

360

Cleavage map of colicin E1 plasmid  

Microsoft Academic Search

COLICIN E1 plasmid (colE1) is a closed circular DNA molecule with a molecular weight of 4.2 × 106 (ref. 1). ColE1 DNA has extensively been used as a molecular vehicle for cloning and amplification of DNA in genetic engineering2,3. In order to expand such investigations, it is useful to make a cleavage map ordering colEl DNA pieces produced by bacterial

Atsuhiro Oka; Mituru Takanami

1976-01-01

361

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

362

Chemical Bonding Technology.  

National Technical Information Service (NTIS)

Primers employed in bonding together the various material interfaces in a photovoltaic module are being developed. The approach develops interfacial adhesion by generating actual chemical bonds between the various materials bonded together. The current st...

E. Plueddemann

1986-01-01

363

Catalytic asymmetric carbon-carbon bond formation via allylic alkylations with organolithium compounds  

Microsoft Academic Search

Carbon-carbon bond formation is the basis for the biogenesis of nature's essential molecules. Consequently, it lies at the heart of the chemical sciences. Chiral catalysts have been developed for asymmetric C-C bond formation to yield single enantiomers from several organometallic reagents. Remarkably, for extremely reactive organolithium compounds, which are among the most broadly used reagents in chemical synthesis, a general

Manuel Pérez; Martín Fañanás-Mastral; Pieter H. Bos; Alena Rudolph; Syuzanna R. Harutyunyan; Ben L. Feringa

2011-01-01

364

Insights into avian influenza virus pathogenicity: the hemagglutinin precursor HA0 of subtype H16 has an alpha-helix structure in its cleavage site with inefficient HA1/HA2 cleavage.  

PubMed

With a new serotype (H17) of hemagglutinin (HA) recently being discovered, there are now 17 serotypes (H1 to H17) of influenza A viruses in total. It is believed that HA is initially expressed as a precursor of HA0 and then cleaved into HA1 and HA2, forming a disulfide bond-linked complex, for its full function. Structural data show that a loop structure exists in the cleavage site between HA1 and HA2, and this flexible loop is crucial for the efficient cleavage of HA0. Here, the crystal structures of H16 (a low-pathogenicity avian influenza virus) in their HA0 form (H16HA0) have been solved at 1.7-? and 2.0-? resolutions. To our surprise, an ?-helix element in the cleavage site which inserts into the negatively charged cavity with the key residue R329 hidden behind the helix was observed. In vitro trypsin cleavage experiments demonstrated inefficient cleavage of H16HA0 under both neutral and low-pH conditions. The results provide new insights into influenza A virus pathogenicity; both the relatively stable ?-helix structure in the flexible cleavage loop and inaccessibility of the cleavage site likely contribute to the low pathogenicity of avian influenza A virus. Furthermore, compared to all of the HAs whose structures have been solved, H16 is a good reference for assigning the HA subtypes into two groups on the basis of the three-dimensional structure, which is consistent with the phylogenetic grouping. We conclude that in light of the current H16HA0 structure, the natural ?-helix element might provide a new opportunity for influenza virus inhibitor design. PMID:22993148

Lu, Xishan; Shi, Yi; Gao, Feng; Xiao, Haixia; Wang, Ming; Qi, Jianxun; Gao, George F

2012-09-19

365

Modulation of ascorbic acid-induced DNA cleavage by polyamide: cleavage manner, kinetics and mechanism.  

PubMed

Manipulation of DNA presents a great interest in biotechnology and therapeutics. The molecules that damage DNA selectively offer new prospects for controlled manipulation of DNA. The conjugations of DNA-code reading molecules such as polyamides to reagents that induce DNA damages provide an approach to reach this goal. In this work, a new compound which contained polyamide and ascorbic acid conjugated by flexible linker (polyamide-Vc), was successfully synthesized, characterized, and evaluated as DNA cleavage agent, compared with that by using ascorbic acid molecule. The kinetics data showed that polyamide-Vc successfully promoted the cleavage of plasmid DNA, with k(max) of 0.314 h(-1) and K(d) of 0.105 mM. The evaluation of DNA linearization elicited that the activity of cleaving double-strand in the supercoiled pUC18 plasmid DNA by polyamide-Vc was enhanced remarkably, achieving n1/n2 ratio of 13.9 at 1.2 mM for 1 h. The introduction of polyamide to Vc could also partially weaken the inhibition of hydrogen radical to double-strand cleavage process because of its good binding activity to DNA. We anticipate that this work could provide a method for improving the efficiency of double-strand cleavage, especially to oxidative cleavage agents. PMID:22214457

Li, C; Duan, S; Xu, J; Qiao, R; Xu, P; Zhao, Y

2012-01-01

366

Small molecule activators of pre-mRNA 3? cleavage  

PubMed Central

3? Cleavage and polyadenylation are obligatory steps in the biogenesis of most mammalian pre-mRNAs. In vitro reconstitution of the 3? cleavage reaction from human cleavage factors requires high concentrations of creatine phosphate (CP), though how CP activates cleavage is not known. Previously, we proposed that CP might work by competitively inhibiting a cleavage-suppressing serine/threonine (S/T) phosphatase. Here we show that fluoride/EDTA, a general S/T phosphatase inhibitor, activates in vitro cleavage in place of CP. Subsequent testing of inhibitors specific for different S/T phosphatases showed that inhibitors of the PPM family of S/T phosphatases, which includes PP2C, but not the PPP family, which includes PP1, PP2A, and PP2B, activated 3? cleavage in vitro. In particular, NCI 83633, an inhibitor of PP2C, activated extensive 3? cleavage at a concentration 50-fold below that required by fluoride or CP. The testing of structural analogs led to the identification of a more potent compound that activated 3? cleavage at 200 ?M. While testing CP analogs to understand the origin of its cleavage activation effect, we found phosphocholine to be a more effective activator than CP. The minimal structural determinants of 3? cleavage activation by phosphocholine were identified. Our results describe a much improved small molecule activator of in vitro pre-mRNA cleavage, identify the molecular determinants of cleavage activation by phosphoamines such as phosphocholine, and suggest that a PPM family phosphatase is involved in the negative regulation of mammalian pre-mRNA 3? cleavage.

Ryan, Kevin; Khleborodova, Asya; Pan, Jingyi; Ryan, Xiaozhou P.

2009-01-01

367

A novel methodology for assignment of disulfide bond pairings in proteins.  

PubMed Central

A novel methodology is described for the assignment of disulfide bonds in proteins of known sequence. The denatured protein is subjected to limited reduction by tris(2-carboxyethyl)phosphine (TCEP) in pH 3.0 citrate buffer to produce a mixture of partially reduced protein isomers; the nascent sulfhydryls are immediately cyanylated by 1-cyano-4-dimethylamino-pyridinium tetrafluoroborate (CDAP) under the same buffered conditions. The cyanylated protein isomers, separated by and collected from reversed-phase HPLC, are subjected to cleavage of the peptide bonds on the N-terminal side of cyanylated cysteines in aqueous ammonia to form truncated peptides that are still linked by residual disulfide bonds. The remaining disulfide bonds are then completely reduced to give a mixture of peptides that can be mass mapped by MALDI-MS. The masses of the resulting peptide fragments are related to the location of the paired cysteines that had undergone reduction, cyanylation, and cleavage. A side reaction, beta-elimination, often accompanies cleavage and produces overlapped peptides that provide complementary confirmation for the assignment. This strategy minimizes disulfide bond scrambling and is simple, fast, and sensitive. The feasibility of the new approach is demonstrated in the analysis of model proteins that contain various disulfide bond linkages, including adjacent cysteines. Experimental conditions are optimized for protein partial reduction, sulfhydryl cyanylation, and chemical cleavage reactions.

Wu, J.; Watson, J. T.

1997-01-01

368

21. Historic American Buildings Survey Copyright C.C. Pierce Original: ...  

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

21. Historic American Buildings Survey Copyright - C.C. Pierce Original: About 1902 Re-photo: April 1940 QUANDRANGLE (view from north) - Mission Nuestra Senora de la Soledad, Soledad, Monterey County, CA

369

Twinning and multiaxial cyclic plasticity of a low stacking-fault-energy F. C. C. alloy  

Microsoft Academic Search

Out-of-phase cyclic tension and torsion tests followed by metallographic measurements are carried out on Co33Ni specimens. It appears that, even for small strain ranges, nonproportional loadings induce profuse mechanical twinning and a strong kinematic hardening in this low stacking-fault-energy f.c.c. alloy. These results are analyzed and an attempt to incorporate twinning into micromechanical simulations of f.c.c. polycrystals elastoplastic behaviour is

V DOQUET

1993-01-01

370

Thermal shock resistance of SiC compositionally graded C\\/C composites  

Microsoft Academic Search

Thermal shock resistance of SiC compositionally graded carbon fiber reinforced carbon composite (CFC) materials that are composed of a CVD SiC layer, a SiC\\/C graded layer and a CFC substrate was studied by thermal shock tests in air. Two types of CFCs were used: 1-D continuous fiber reinforced and 2-D felt reinforced composites. It was found that the SiC\\/C graded

Kimio Fujii; Reiji Yamada

1998-01-01

371

Cyanogen bromide cleavage and partial sequence of the heavy chain of a pathological immunoglobulin G  

PubMed Central

The heavy chain of a pathological immunoglobulin G (Daw) of type L, subclass ?2b (We) and Gm(a+)(f?), has been cleaved with cyanogen bromide. The five fragments resulting from the cleavage have been isolated and analysed. The papain-digest fragments, Fab and Fc, have also been cleaved with cyanogen bromide and the products analysed and compared with those from the heavy chain. The order of the five fragments in the heavy chain has been established and the location of some of the inter-chain and inter-fragment disulphide bonds has been determined. The sequence of the N-terminal fragment consisting of 34 residues is reported.

Piggot, P. J.; Press, E. M.

1967-01-01

372

Exosite Interactions Contribute to Tension-Induced Cleavage of von Willebrand Factor by the Antithrombotic ADAMTS13 Metalloprotease  

Microsoft Academic Search

Von Willebrand factor (VWF) is a multimeric protein that mediates platelet adhesion at sites of vascular injury, and ADAMTS13 (a disintegrin and metalloprotease with thrombospondin)is a multidomain metalloprotease that limits platelet adhesion by a feedback mechanism in which fluid shear stress induces proteolysis of VWF and prevents disseminated microvascular thrombosis. Cleavage of the $Tyr^{1605}-Met^{1606}$ scissile bond in the VWF A2

Weiqiang Gao; Patricia J. Anderson; Elaine M. Majerus; Elodee A. Tuley; J. Evan Sadler

2006-01-01

373

Reductive Cleavage of Demeton-S-Methyl by Corynebacterium glutamicum in Cometabolism on More Readily Metabolizable Substrates  

PubMed Central

Corynebacterium glutamicum is able to biotransform demeton-S-methyl, an organophosphorus compound, during cometabolism with more readily metabolizable substrates. Among the cosubstrates used, fructose is the growth substrate that is most favorable for demeton-S-methyl biotransformation. The reaction mechanism of demeton-S-methyl biotransformation involves reductive cleavage of an S-C bond, which leads to accumulation of dimethyl thiophosphate in the culture medium.

Girbal, Laurence; Hilaire, Didier; Leduc, Sebastien; Delery, Laure; Rols, Jean-Luc; Lindley, Nicholas D.

2000-01-01

374

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

375

Detection and Formation of Interstellar c-C_3D_2  

NASA Astrophysics Data System (ADS)

Multiply deuterated molecules are unique observational probes for the earliest stages of star formation. Cyclopropenylidene, c-C_3H_2, is an ideal probe for deuterium chemistry. It is one of the most widespread molecules in our Galaxy and it has the possibility of double deuteration. Furthermore, since c-C_3H_2 is an "early-type" molecule, it is a particular useful tool to investigate early stages of a molecular cloud. This makes observations of its deuterated forms particularly important to test time-dependent chemical codes which include deuteration processes. The centimeter and millimeter wavelength spectra of doubly deuterated cyclopropenylidene have recently been measured in the laboratory, allowing for the first time a search for c-C_3D_2 in space. We report the detection of c-C_3D_2 in two starless cores, L1544 and TMC-1C. The deuteration of this small hydrocarbon ring is analysed with a comprehensive gas-grain model, the first including doubly deuterated species. The observed abundances of c-C_3D_2 can be explained solely by gas-phase processes, supporting the idea that c-C_3H_2 is a good indicator of gas-phase deuteration. S. Spezzano, F. Tamassia, S. Thorwirth, P. Thaddeus, C. A. Gottlieb, and M. C. McCarthy Astroph. J. Supp. Series {200}(1), 2012.

Spezzano, Silvia; Brunken, Sandra; Schilke, Peter; Menten, Karl M.; Caselli, Paola; McCarthy, Michael C.; Bizzocchi, Luca; Trevino, Sandra; Aikawa, Yuri; Schlemmer, Stephan

2013-06-01

376

Acid-mediated cleavage of oligonucleotide P3' --> N5' phosphoramidates triggered by sequence-specific triplex formation.  

PubMed

The P-N bond in oligonucleotide P3' --> N5' phosphoramidates (5'-amino-DNA) is known to be chemoselectively cleaved under mild acidic conditions. We prepared homopyrimidine oligonucleotides containing 5'-amino-5'-deoxythymidine (5'-amino-DNA thymine monomer) or its conformationally locked congener, 5'-amino-2',4'-BNA thymine monomer, at midpoint of the sequence. The effect of triplex formation with homopurineohomopyrimidine dsDNA targets on acid-mediated hydrolysis of the P3' --> N5' phosphoramidate linkage was evaluated. Very interestingly, it was found that the triplex formation significantly accelerates the P-N bond cleavage. PMID:18058503

Obika, Satoshi; Tomizu, Masaharu; Negoro, Yoshinori; Osaki, Tomohisa; Orita, Ayako; Ueyama, Yuji; Nakagawa, Osamu; Imanishi, Takeshi

2007-01-01

377

Single-strand DNA cleavages by eukaryotic topoisomerase II.  

PubMed

A new purification method for eukaryotic type II DNA topoisomerase (EC 5.99.1.3) is described, and the avian enzyme has been purified and characterized. An analysis of the cleavage reaction has revealed that topoisomerase II can be trapped as a DNA-enzyme covalent complex containing DNA with double-stranded and single-stranded breaks. The data indicate that DNA cleavage by topoisomerase II proceeds by two asymmetric single-stranded cleavage and resealing steps on opposite strands (separated by 4 bp) with independent probabilities of being trapped upon addition of a protein denaturant. Single-strand cleavages were directly demonstrated at both strong and weak topoisomerase II sites. Thus, a match to the vertebrate topoisomerase II consensus sequence (sequence; see text) (N is any base, and cleavage occurs between -1 and +1) [Spitzner, J.R., & Muller, M.T. (1988) Nucleic Acids Res. 16, 5533-5556)] does not predict whether a cleavage site will be single stranded or double stranded; however, sites cleaved by topoisomerase II that contain two conserved consensus bases (G residue at +2 and T at +4) generally yield double-strand cleavage whereas recognition sites lacking these two consensus elements yield single-strand cleavages. Finally, single-strand cleavages with topoisomerase II do not appear to be an artifact caused by damaged enzyme molecules since topoisomerase II in freshly prepared, crude extracts also shows the property of single-strand cleavages. PMID:2853964

Muller, M T; Spitzner, J R; DiDonato, J A; Mehta, V B; Tsutsui, K; Tsutsui, K

1988-11-01

378

Bond Paths Are Not Chemical Bonds  

NASA Astrophysics Data System (ADS)

This account takes to task papers that criticize the definition of a bond path as a criterion for the bonding between the atoms it links by mistakenly identifying it with a chemical bond. It is argued that the notion of a chemical bond is too restrictive to account for the physics underlying the broad spectrum of interactions between atoms and molecules that determine the properties of matter. A bond path on the other hand, as well as being accessible to experimental verification and subject to the theorems of quantum mechanics, is applicable to any and all of the interactions that account for the properties of matter. It is shown that one may define a bond path operator as a Dirac observable, making the bond path the measurable expectation value of a quantum mechanical operator. Particular attention is given to van der Waals interactions that traditionally are assumed to represent attractive interactions that are distinct from chemical bonding. They are assumed by some to act in concert with Pauli repulsions to account for the existence of condensed states of molecules. It is such dichotomies of interpretation that are resolved by the experimental detection of bond paths and the delineation of their properties in molecular crystals. Specific criticisms of the stabilization afforded by the presence of bond paths derived from spectroscopic measurements performed on dideuteriophenanthrene are shown to be physically unsound. The concept of a bond path as a "bridge of density" linking bonded atoms was introduced by London in 1928 following the definition of the electron density by Schrödinger in 1926. These papers marked the beginning of the theory of atoms in molecules linked by bond paths.

Bader, Richard F. W.

2009-09-01

379

Proteolytic cleavage of Notch: "HIT and RUN".  

PubMed

The Notch pathway is a highly conserved signaling pathway in multicellular eukaryotes essential in controlling spatial patterning, morphogenesis and homeostasis in embryonic and adult tissues. Notch proteins coordinate cell-cell communication through receptor-ligand interactions between adjacent cells. Notch signaling is frequently deregulated by oncogenic mutation or overexpression in many cancer types. Notch activity is controlled by three sequential cleavage steps leading to ectodomain shedding and transcriptional activation. Here we review the key regulatory steps in the activation of Notch, from receptor maturation to receptor activation (HIT) via a rate-limiting proteolytic cascade (RUN) in the context of species-specific differences. PMID:21506924

van Tetering, G; Vooijs, M

2011-06-01

380

Femtosecond quantum control of molecular bond formation  

PubMed Central

Ultrafast lasers are versatile tools used in many scientific areas, from welding to eye surgery. They are also used to coherently manipulate light–matter interactions such as chemical reactions, but so far control experiments have concentrated on cleavage or rearrangement of existing molecular bonds. Here we demonstrate the synthesis of several molecular species starting from small reactant molecules in laser-induced catalytic surface reactions, and even the increase of the relative reaction efficiency by feedback-optimized laser pulses. We show that the control mechanism is nontrivial and sensitive to the relative proportion of the reactants. The control experiments open up a pathway towards photocatalysis and are relevant for research in physics, chemistry, and biology where light-induced bond formation is important.

Nuernberger, Patrick; Wolpert, Daniel; Weiss, Horst; Gerber, Gustav

2010-01-01

381

Semiconductor wafer bonding and ion-cut layer transfer  

NASA Astrophysics Data System (ADS)

This dissertation describes a semiconductor layer transfer process using wafer bonding and hydrogen-induced semiconductor cleavage. In this process, hydrogen is implanted into a wafer that has the layer to be transferred. The implanted hydrogen ions form a highly damaged region around the hydrogen stopping range. The implanted wafer is then bonded to another wafer using low-temperature direct bonding. With appropriate heat or mechanical treatment, the bonded wafer pair separates along the highly damaged region, resulting in the transfer of the layer from one substrate to the other. With this technique, we have been able to fabricate silicon-on-insulator (SOI) structures by transferring single- and poly-crystalline silicon layers, especially using hydrogen plasma implantation, oxygen plasma-activated wafer bonding, and thermal cleavage and mechanical cleavage methods. We have also formed silicon, SOI, and oxide membranes on buried cavities and channels, which can be applied for use in pressure transducers, micro-fluidic systems, and radio frequency filters and resonators. In these demonstrations, we have observed good thickness uniformity (<1%) across a 100 mm wafer and surface microroughness (<10 nm) of the transferred layers. For the transfer of pre-fabricated electronic device layers, gate oxide damage was first evaluated after high-dose and high-energy hydrogen implantation through metal-oxide-silicon (MOS) transistors. The results showed that stress-induced leakage current (SILO) through the gate oxide increased as hydrogen dose increased for the 5 nm-thick oxide. For the 1.8 nm-thick gate oxide, no SILC was observed, showing that the implantation damage is not significant for the ultra-thin (<2 nm) oxides. To protect the thicker (>3 nm) oxides from damage during the hydrogen implantation, we have proposed layer transfer with patterned implantation of hydrogen. In this process, active device regions were masked during the implantation. This experiment showed that the hydrogen induced silicon layer cleavage is feasible even without a continuous hydrogen implantation of the entire wafer, and that the silicon cleavage can propagate across at least 16 microns of non-implanted area from a 4 micron-wide implanted region each side. Furthermore, it has shown that the mechanical cleaving can overcome some non-implantation area limitations imposed by the thermal cleavage process.

Yun, Changhan

382

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.

383

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

384

Sequence-specific cleavage of RNA by Type II restriction enzymes.  

PubMed

The ability of 223 Type II restriction endonucleases to hydrolyze RNA-DNA heteroduplex oligonucleotide substrates was assessed. Despite the significant topological and sequence asymmetry introduced when one strand of a DNA duplex is substituted by RNA we find that six restriction enzymes (AvaII, AvrII, BanI, HaeIII, HinfI and TaqI), exclusively of the Type IIP class that recognize palindromic or interrupted-palindromic DNA sequences, catalyze robust and specific cleavage of both RNA and DNA strands of such a substrate. Time-course analyses indicate that some endonucleases hydrolyze phosphodiester bonds in both strands simultaneously whereas others appear to catalyze sequential reactions in which either the DNA or RNA product accumulates more rapidly. Such strand-specific variation in cleavage susceptibility is both significant (up to orders of magnitude difference) and somewhat sequence dependent, notably in relation to the presence or absence of uracil residues in the RNA strand. Hybridization to DNA oligonucleotides that contain endonuclease recognition sites can be used to achieve targeted hydrolysis of extended RNA substrates produced by in vitro transcription. The ability to 'restrict' an RNA-DNA hybrid, albeit with a limited number of restriction endonucleases, provides a method whereby individual RNA molecules can be targeted for site-specific cleavage in vitro. PMID:20702422

Murray, Iain A; Stickel, Shawn K; Roberts, Richard J

2010-08-11

385

Ir-Catalyzed Functionalization of C-H Bonds  

NASA Astrophysics Data System (ADS)

The ability to selectively functionalize C-H bonds holds enormous potential value in virtually every sphere of organic chemistry, from fuels to pharmaceuticals. Transition metal complexes have shown great promise in this context. Iridium provided the first examples of oxidative addition of C-H bonds; this addition is key to iridium's leading role in alkane dehydrogenation and related reactions. Catalysts based on iridium have also proven highly effective for valuable borylations of C-H bonds and, to a lesser extent, for C-Si coupling. Compared with other platinum group metals, iridium chemistry has not been developed as extensively for the elaboration of C-C bonds from C-H bonds, but significant promise is indicated, particularly for coupling with simple hydrocarbons which lack functionalities that can act as directing groups.

Choi, Jongwook; Goldman, Alan S.

386

Effect of phosphorus on cleavage fracture in ? -carbide  

Microsoft Academic Search

To understand the origin of cleavage fracture which dominates in FeMn-Al-C alloys at a high phosphorus concentration, we performed first-principles study of the phosphorus effect on ideal cleavage energy and critical stress in -carbide, Fe3AlC, a precipitate in the austenitic alloys. We find that phosphorus has higher solubility in Fe3AlC than in -Fe and sharply reduces the cleavage characteristics of

N. I. Medvedeva; R. A. Howell; D. C. Van Aken; J. E. Medvedeva

2010-01-01

387

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.

388

Autocatalytic Cleavage within Classical Swine Fever Virus NS3 Leads to a Functional Separation of Protease and Helicase.  

PubMed

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; Rümenapf, Till

2013-08-28

389

Strong-bonding calcium phosphate coatings on carbon/carbon composites by ultrasound-assisted anodic oxidation treatment and electrochemical deposition  

NASA Astrophysics Data System (ADS)

The current work describes the ultrasound-assisted anodic oxidation treatment of carbon fiber-reinforced carbon (C/C) composites with H3PO4. The aim was to create chemical bonds between the surface-treated C/C composites and subsequently deposited calcium phosphate (CaP) coatings. The surface energy of C/C obviously increased after undergoing ultrasound-assisted anodic oxidation treatment at a current density of 50 mA/cm2 in aqueous 0.5 M H3PO4 solution (USAT-C/C-50). A large number of O-containing groups and some P-containing groups were found on the treated C/C substrate. The formation of O-containing groups may have been promoted by the energy input of the ultrasound and electric field. P-containing species could be bonded to C/C by CPO or COP bonding. Among the species formed, the formation of (H2PO4)- on C/C was promoted by ultrasound. When USAT-C/C-50 was used as the substrate for CaP coatings deposited by ultrasound-assisted electrochemical deposition (USECD), the surface of USAT-C/C-50 acquired strong CaP induction ability, and large amounts of CaP were deposited on it during the initial deposition process. After USECD, very strong-bonding CaP coatings formed on USAT-C/C-50. The average shear bond strength of the coatings obtained on the USAT-C/C-50 substrate [(12.14 ± 1.30) MPa] was about twice that of the coatings obtained on untreated C/C [(6.78 ± 1.06) MPa].

Zhao, Xueni; Li, Hejun; Chen, Mengdi; Li, Kezhi; Wang, Bin; Xu, Zhanwei; Cao, Sheng; Zhang, Leilei; Deng, Hailiang; Lu, Jinhua

2012-04-01

390

Effects of Anti-Oxidant Migration on Friction and Wear of C/C Aircraft Brakes  

NASA Astrophysics Data System (ADS)

The surfaces of carbon-carbon (C/C) aircraft brakes are usually coated with anti-oxidant to protect them from oxidation. These surfaces do not include the friction surfaces since it is known that when anti-oxidant get onto the friction surface, the friction coefficient decreases. The anti-oxidant migration (AOM), however, happens during processing, heat treatment and application. In this study, phosphorus based anti-oxidants inhibited 3-D C/C aircraft brake system was investigated. The effects of their migration on friction and wear in the 3-D C/C brakes were revealed by sub-scale dynamometer tests and microscopic analysis. Dynamometer results showed that when AOM occurred, both landing and taxi coefficients decreased in humid environment and the wear was slightly lowered. Microscopic study showed that under high humidity conditions there was no formation of the friction film.

Don, Jarlen; Wang, Zhe

2009-04-01

391

Disulphide bond assignment in human tissue inhibitor of metalloproteinases (TIMP).  

PubMed

Disulphide bonds in human recombinant tissue inhibitor of metalloproteinases (TIMP) were assigned by resolving proteolytic digests of TIMP on reverse-phase h.p.l.c. and sequencing those peaks judged to contain disulphide bonds by virtue of a change in retention time on reduction. This procedure allowed the direct assignment of Cys-145-Cys-166 and the isolation of two other peptides containing two disulphide bonds each. Further peptide cleavage in conjunction with fast-atom-bombardment m.s. analysis permitted the assignments Cys-1-Cys-70, Cys-3-Cys-99, Cys-13-Cys-124 and Cys-127-Cys-174 from these peptides. The sixth bond Cys-132-Cys-137 was assigned by inference, as the native protein has no detectable free thiol groups. PMID:2163605

Williamson, R A; Marston, F A; Angal, S; Koklitis, P; Panico, M; Morris, H R; Carne, A F; Smith, B J; Harris, T J; Freedman, R B

1990-06-01

392

Disulphide bond assignment in human tissue inhibitor of metalloproteinases (TIMP).  

PubMed Central

Disulphide bonds in human recombinant tissue inhibitor of metalloproteinases (TIMP) were assigned by resolving proteolytic digests of TIMP on reverse-phase h.p.l.c. and sequencing those peaks judged to contain disulphide bonds by virtue of a change in retention time on reduction. This procedure allowed the direct assignment of Cys-145-Cys-166 and the isolation of two other peptides containing two disulphide bonds each. Further peptide cleavage in conjunction with fast-atom-bombardment m.s. analysis permitted the assignments Cys-1-Cys-70, Cys-3-Cys-99, Cys-13-Cys-124 and Cys-127-Cys-174 from these peptides. The sixth bond Cys-132-Cys-137 was assigned by inference, as the native protein has no detectable free thiol groups. Images Fig. 1.

Williamson, R A; Marston, F A; Angal, S; Koklitis, P; Panico, M; Morris, H R; Carne, A F; Smith, B J; Harris, T J; Freedman, R B

1990-01-01

393

Case for induced bond scission during coal pyrolysis  

SciTech Connect

Most coal pyrolysis models invoke spontaneous thermal scission of inherently weak bonds as the sole pyrolytic reaction leading to depolymerization of coal structures. In this view, bond scission is dependent only on temperature; the only additional chemical factors that impact the net cleavage are scavenging and crosslinking of thermally generated radicals, which can be affected primarily by varying heating rates and other factors that affect heat and mass transport. In contrast to this traditional picture, data obtained by various researches (partly in the context of coal liquefaction), when taken together, provide a strong argument that induced scission of strong bonds plays a significant role during coal pyrolysis. In this paper we attempt to summarize this argument. In the following paragraphs, we outline some of the evidence that induced bond scission can take place under pyrolysis conditions, and show how including them helps explain certain coal conversion phenomena.

McMillen, D.F.; Malhotra, R.; Nigenda, S.E.

1988-01-01

394

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.

1998-01-01

395

Investigation of TiC C Eutectic and WC C Peritectic High-Temperature Fixed Points  

NASA Astrophysics Data System (ADS)

TiC C eutectic (2,761°C) and WC C peritectic (2,749°C) fixed points were investigated to compare their potential as high-temperature thermometric reference points. Two TiC C and three WC C fixed-point cells were constructed, and the melting and freezing plateaux were evaluated by means of radiation thermometry. The repeatability of the TiC C eutectic within a day was 60 mK with a melting range roughly 200 mK. The repeatability of the melting temperature of the WC C peritectic within 1 day was 17 mK with a melting range of ˜70 mK. The repeatability of the freezing temperature of the WC C peritectic was 21 mK with a freezing range less than 20 mK. One of the TiC C cells was constructed from a TiC and graphite powder mixture. The filling showed the reaction with the graphite crucible was suppressed and the ingot contained less voids, although the lack of high-purity TiC powder poses a problem. The WC C cells were easily constructed, like metal carbon eutectic cells, without any evident reaction with the crucible. From these results, it is concluded that the WC C peritectic has more potential than the TiC C eutectic as a high-temperature reference point. The investigation of the purification of the TiC C cell during filling and the plateau observation are also reported.

Sasajima, Naohiko; Yamada, Yoshiro

2008-06-01

396

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.

Rodrigo, Maria J.; Alquezar, Berta; Al-Babili, Salim

2013-01-01

397

A novel carotenoid cleavage activity involved in the biosynthesis of Citrus fruit-specific apocarotenoid pigments.  

PubMed

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; Alós, Enriqueta; Medina, Víctor; Carmona, Lourdes; Bruno, Mark; Al-Babili, Salim; Zacarías, Lorenzo

2013-09-04

398

Human IgG1 Hinge Fragmentation as the Result of H2O2-mediated Radical Cleavage  

PubMed Central

Hinge cleavage of a recombinant human IgG1 antibody, generated during production in a Chinese hamster ovary cell culture, was observed in the purified material. The cleavage products could be reproduced by incubation of the antibody with H2O2 and featured complementary ladders of the C- and N-terminal residues (Asp226–Lys227–Thr228–His229–Thr230) in the heavy chain of the Fab domain and the upper hinge of one of the Fc domains, respectively. Two adducts of +45 and +71 Da were also observed at the N-terminal residues of some Fc fragments and were identified as isocyanate and ?-ketoacyl derivatives generated by radical cleavage at the ?-carbon position through the diamide and ?-amidation pathways. We determined that the hinge cleavage was initiated by radical-induced breakage of the disulfide bond between the two hinge cysteines at position 231 (Cys231-Pro-Pro-Cys-Pro), followed by the formation of a thiyl radical (Cys231-S•) on one cysteine and sulfenic acid (Cys231-SOH) on the other. The location of the initial radical attack and the critical role of Cys231 were demonstrated by the observation that 5,5-dimethyl-1-pyrroline N-oxide only reacted with the Cys231 radical and completely blocked hinge cleavage, suggesting the necessity of an electron/radical transfer from the Cys231 radical to the hinge residues where cleavage was observed. As a precursor of hydroxyl radicals, H2O2 is widely produced in healthy cells and tissues and therefore could be the source for the radical-induced fragmentation of human IgG1 antibodies in vivo.

Yan, Boxu; Yates, Zac; Balland, Alain; Kleemann, Gerd R.

2009-01-01

399

The N-terminal Nucleophile Serine of Cephalosporin Acylase Executes the Second Autoproteolytic Cleavage and Acylpeptide Hydrolysis*  

PubMed Central

Cephalosporin acylase (CA) precursor is translated as a single polypeptide chain and folds into a self-activating pre-protein. Activation requires two peptide bond cleavages that excise an internal spacer to form the mature ?? heterodimer. Using Q-TOF LC-MS, we located the second cleavage site between Glu159 and Gly160, and detected the corresponding 10-aa spacer 160GDPPDLADQG169 of CA mutants. The site of the second cleavage depended on Glu159: moving Glu into the spacer or removing 5–10 residues from the spacer sequence resulted in shorter spacers with the cleavage at the carboxylic side of Glu. The mutant E159D was cleaved more slowly than the wild-type, as were mutants G160A and G160L. This allowed kinetic measurements showing that the second cleavage reaction was a first-order, intra-molecular process. Glutaryl-7-aminocephalosporanic acid is the classic substrate of CA, in which the N-terminal Ser170 of the ?-subunit, is the nucleophile. Glu and Asp resemble glutaryl, suggesting that CA might also remove N-terminal Glu or Asp from peptides. This was indeed the case, suggesting that the N-terminal nucleophile also performed the second proteolytic cleavage. We also found that CA is an acylpeptide hydrolase rather than a previously expected acylamino acid acylase. It only exhibited exopeptidase activity for the hydrolysis of an externally added peptide, supporting the intra-molecular interaction. We propose that the final CA activation is an intra-molecular process performed by an N-terminal nucleophile, during which large conformational changes in the ?-subunit C-terminal region are required to bridge the gap between Glu159 and Ser170.

Yin, Jun; Deng, Zixin; Zhao, Guoping; Huang, Xi

2011-01-01

400

Hot rolling textures of f.c.c. metals. Part 2: Numerical simulations  

SciTech Connect

The development of hot rolling textures in f.c.c. metals has been simulated numerically using a viscoplastic crystal plasticity model assuming different combinations of octahedral and non-octahedral slip systems for several grain-matrix interaction schemes. A combination of quasi-pencil glide and the Lebensohn and Tome self-consistent model gives a satisfactory simulation of typical hot rolling f.c.c. texture components, i.e. well balanced {beta}-fiber components and a small amount of ``Cube.``

Maurice, C.; Driver, J.H. [Ecole des Mines de Saint Etienne (France). Microstructures and Processing Lab.

1997-11-01

401

Long-term tropospheric trend of octafluorocyclobutane (cC4F8 or PFC318)  

Microsoft Academic Search

Air samples collected at Cape Grim, Tasmania\\u000abetween 1978 and 2008 and during a series of more recent\\u000aaircraft sampling programmes have been analysed to determine\\u000athe atmospheric abundance and trend of octafluorocyclobutane\\u000a(c-C4F8 or PFC-318). c-C4F8 has an atmospheric\\u000alifetime in excess of 3000 yr and a global warming potential\\u000a(GWP) of 10 300 (100 yr time horizon), making

D. E. Oram; F. S. Mani; J. C. Laube; M. J. Newland; C. E. Reeves; W. T. Sturges; S. A. Penkett; C. A. M. Brenninkmeijer; T. Röckmann; P. J. Fraser

2012-01-01

402

Brittle to ductile transition in cleavage fracture  

SciTech Connect

The problem of interpretation of fracture transition from brittle to ductile or vice versa is the subject of study. An instrumented tapered double cantilever beam (TDCB) has been developed as a definitive tool in the study of the intrinsic mechanism in single crystalline samples. In this experiment, the crack velocity is directly proportional to actuator velocity. In experiments performed on TDCB shaped Si single crystals, oriented for cleavage on either [l brace]111[r brace] or [l brace]110[r brace] planes, a number of troubling features of jerky carck extension were encountered. Evidence suggests that nucleation of dislocation loops from crack tip is easier than moving these dislocations away from crack tip. 14 refs, 1 fig.

Argon, A.S.; Berg, Q.

1992-09-30

403

The role of disulfide bond in hyperthermophilic endocellulase.  

PubMed

The hyperthermophilic endocellulase, EGPh (glycosyl hydrolase family 5) from Pyrococcus horikoshii possesses 4 cysteine residues forming 2 disulfide bonds, as identified by structural analysis. One of the disulfide bonds is located at the proximal region of the active site in EGPh, which exhibits a distinct pattern from that of the thermophilic endocellulase EGAc (glycosyl hydrolase family 5) of Acidothermus cellulolyticus despite the structural similarity between the two endocellulases. The structural similarity between EGPh and EGAc suggests that EGPh possesses a structure suitable for changing the position of the disulfide bond corresponding to that in EGAc. Introduction of this alternative disulfide bond in EGPh, while removing the original disulfide bond, did not result in a loss of enzymatic activity but the EGPh was no longer hyperthermostable. These results suggest that the contribution of disulfide bond to hyperthermostability at temperature higher than 100 °C is restrictive, and that its impact is dependent on the specific structural environment of the hyperthermophilic proteins. The data suggest that the structural position and environment of the disulfide bond has a greater effect on high-temperature thermostability of the enzyme than on the potential energy of the dihedral angle that contributes to disulfide bond cleavage. PMID:23624891

Kim, Han-Woo; Ishikawa, Kazuhiko

2013-04-27

404

Regulatory properties of araC(c) mutants in the L-arabinose operon of escherichia coliB/r.  

PubMed Central

Merodiploids containing a high-constitutive and a low-constitutive araC(c) allele were assayed for constitutive expression of the ara operon. Low-constitutive araC(c) alleles either were unable to repress the constitutive rate of ara operon expression exhibited by by high-constitutive araC(c) alleles or achieved a partial repression of the high-constitutive rate of operon expression. Either mutation to a low-constitutive araC(c) mutant resulted in a partial or complete loss of repressor function, or subunit mixing between the two araC(c) mutant proteins resulted in a partial or complete dominance of the high-constitutive araC(c) allele. Five of the six araC(c) alleles tested allowed a partial induction of the ara operon in cya crp background. In general, a higher level of ara operon induction was achieved in the cya crp background by high araC(c) alleles than by low araC(c) alleles. Furthermore, several araC(c) mutants exhibited decreased sensitivity to catabolite repression, particularly in the presence of inducer. The results suggest a model in which certain araC(c) gene products can achieve ara operon induction in the presence of either arabinose (inducer) or catabolite activator protein-cyclic adenosine monophosphate, whereas the wild-type araC gene product requires the presence of both of these factors for operon expression.

MacInnes, K R; Sheppard, D E; Falgout, B

1978-01-01

405

Political Cleavages in a Media-Driven Environment  

Microsoft Academic Search

In this article we argue that the cleavage model of Lipset and Rokkan still merits the attention of students of contemporary elec- toral behavior. In particular, we argue that the normative and institutional elements inherent in the model may be more relevant than the social base of cleavages. However, while the increasing influence of the mass media affects the relationship

BERNT AARDAL; RAGNAR WALDAHL

406