Note: This page contains sample records for the topic c-c bond cleavage from Science.gov.
While these samples are representative of the content of Science.gov,
they are not comprehensive nor are they the most current set.
We encourage you to perform a real-time search of Science.gov
to obtain the most current and comprehensive results.
Last update: August 15, 2014.
1

A general nonaqueous route to binary metal oxide nanocrystals involving a C-C bond cleavage.  

PubMed

A widely applicable solvothermal route to nanocrystalline iron, indium, gallium, and zinc oxide based on the reaction between the corresponding metal acetylacetonate as metal oxide precursor and benzylamine as solvent and reactant is presented. Detailed XRD, TEM, and Raman studies prove that, with the exception of the iron oxide system, where a mixture of the two phases magnetite and maghemite is formed, only phase pure materials are obtained, gamma-Ga(2)O(3), zincite ZnO, and cubic In(2)O(3). The particle sizes lie in the range of 15-20 nm for the iron, 10-15 nm for the indium, 2.5-3.5 nm for gallium, and around 20 nm for zinc oxide. GC-MS analysis of the final reaction solution after removal of the nanoparticles showed that the composition is rather complex consisting of more than eight different organic compounds. Based on the fact that N-isopropylidenebenzylamine, 4-benzylamino-3-penten-2-one, and N-benzylacetamide were the main species found, we propose a detailed formation mechanism encompassing solvolysis of the acetylacetonate ligand, involving C-C bond cleavage, as well as ketimine and aldol-like condensation steps. PMID:15826200

Pinna, Nicola; Garnweitner, Georg; Antonietti, Markus; Niederberger, Markus

2005-04-20

2

Cleavage of C-C and C-Si ?-Bonds and Their Intramolecular Exchange.  

PubMed

C-C and C-Si ?-bonds are cleaved to undergo bond exchange when substrates equipped with cyclobutanone and silacyclobutane moieties are treated with a palladium(0) catalyst. The skeletal exchange results in construction of silabicyclo[5.2.1]decanes in a diastereoselective manner. PMID:24720573

Ishida, Naoki; Ikemoto, Wataru; Murakami, Masahiro

2014-04-23

3

Mechanistic Examination of C?C? Bond Cleavages of Tryptophan Residues during Dissociations of Molecular Peptide Radical Cations  

SciTech Connect

In this study, we used collision-induced dissociation (CID) to examine the gas-phase fragmentations of [GnW]+ (n = 2-4) and [GXW]+ (X = C, S, L, F, Y, Q) species. The C?C? bond cleavage of a C-terminal decarboxylated tryptophan residue ([M - CO2]+) can generate [M - CO2 - 116]+, [M - CO2 - 117]+, and [1H-indole]+ (m/z 117) species as possible product ions. Competition between the formation of [M - CO2 - 116]+ and [1H-indole]+ systems implies the existence of a proton-bound dimer formed between the indole ring and peptide backbone. Formation of such a proton-bound dimer is facile via a protonation of the tryptophan ?-carbon atom as suggested by density functional theory (DFT) calculations. DFT calculations also suggested the initially formed ion 2--the decarboxylated species that is active against C?C? bond cleavage -can efficiently isomerize to form a more-stable -radical isomer (ion 9) as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. The C?C? bond cleavage of a tryptophan residue also can occur directly from peptide radical cations containing a basic residue. CID of [WGnR]+ (n = 1-3) radical cations consistently resulted in predominant formation of [M-116]+ product ions. It appears that the basic arginine residue tightly sequesters the proton and allows the charge-remote C?C? bond cleavage to prevail over the charge-directed one. DFT calculations predicted the barrier for the former is 6.2 kcal mol -1 lower than that of the latter. Furthermore, the pathway involving a salt-bridge intermediate also was accessible during such a bond cleavage event.

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

2013-02-14

4

Copper-Catalyzed Oxidative Cyclization of 1,5-Enynes with Concomitant C-C Bond Cleavage: An Unexpected Access to 3-Formyl-1-indenone Derivatives.  

PubMed

A Cu(0)/Selectfluor system-mediated oxidative cyclization of 1,5-enynes with concomitant C-C bond cleavage to access 3-formyl-1-indenone derivatives is described. Preliminary mechanistic investigations disclosed that the C-C bond cleavage involved a novel water-participated oxygen-insertion ?-carbon elimination through double oxycuprations. PMID:24807823

Zhang, Jian; Wu, Degui; Chen, Xiaoling; Liu, Yunkui; Xu, Zhenyuan

2014-06-01

5

Nitrosobenzene-Mediated C-C Bond Cleavage Reactions and Spectral Observation of an oxazetidin-4-one Ring System  

PubMed Central

While bond formation processes have traditionally garnered the attention of the chemical community, methods facilitating bond-breaking remain relatively undeveloped. We report a novel, transition metal-free oxidative C-C bond cleavage process for a broad range of ester and dicarbonyl compounds involving carbanion addition to nitrosobenzene. ReactIR experiments on the nitrosobenzene-mediated oxidative decarboxylation of esters indicate the reaction proceeds via fragmentation of a previously unobserved oxazetidin-4-one heterocycle, characterized by an intense IR stretching frequency at 1846 cm-1. These mechanistic studies have allowed further expansion of this protocol to ketone cleavage reactions of a diverse array of ?-ketoester and 1,3-diketone substrates. The conceptual and mechanistic insights offered by this study are likely to provide a platform for further development of bond-breaking methodologies.

Payette, Joshua N.; Yamamoto, Hisashi

2009-01-01

6

Nitrosobenzene-mediated C-C bond cleavage reactions and spectral observation of an oxazetidin-4-one ring system.  

PubMed

While bond formation processes have traditionally garnered the attention of the chemical community, methods facilitating bond breaking remain relatively undeveloped. We report a novel, transition-metal-free oxidative C-C bond cleavage process for a broad range of ester and dicarbonyl compounds involving carbanion addition to nitrosobenzene. ReactIR experiments on the nitrosobenzene-mediated oxidative decarboxylation of esters indicate the reaction proceeds via fragmentation of a previously unobserved oxazetidin-4-one heterocycle, characterized by an intense IR stretching frequency at 1846 cm-1. These mechanistic studies have allowed further expansion of this protocol to ketone cleavage reactions of a diverse array of beta-ketoester and 1,3-diketone substrates. The conceptual and mechanistic insights offered by this study are likely to provide a platform for further development of bond-breaking methodologies. PMID:18722431

Payette, Joshua N; Yamamoto, Hisashi

2008-09-17

7

The Benzoyl Peroxide Promoted Dual C-C Bond Formation via Dual C-H Bond Cleavage: ?-Phenanthridinylation of Ether by Isocyanide.  

PubMed

The benzoyl peroxide-promoted ?-phenanthridinylation of ether by isocyanide is developed, proceeding through dual C-H bond cleavage and dual C-C bond formation. The procedure tolerates a series of functional groups, such as methyl, fluoro, chloro, acetyl, methoxy carbonyl, cyano, and trifluoromethyl. Thus, it represents a facile pathway leading to 6-substituted phenanthridine derivatives. The addition of radical to the isonitrile followed by a radical aromatic cyclization is involved in this transformation. PMID:24684422

Wang, Lei; Sha, Wanxing; Dai, Qiang; Feng, Xiaomei; Wu, Wenting; Peng, Haibo; Chen, Bin; Cheng, Jiang

2014-04-18

8

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

PubMed

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

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

2014-07-01

9

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(16.70.3) 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(16.380.3) 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(16.420.3) exp(-43496 750 K/T) s(-1), k(isopropanol ? methyl + 1-hydroxyethyl) = 10(16.540.3) exp(-42495 750 K/T) s(-1), k(n-propanol ? ethyl + hydroxymethyl) = 10(16.430.3) exp(-41696 750 K/T) s(-1), and k(n-propanol ? methyl + 2-hydroxymethyl) = 10(16.530.3) 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

2013-10-10

10

Water promoted C-C bond cleavage: facile synthesis of 3,3-bipyrrole derivatives from dienones and tosylmethyl isocyanide (TosMIC).  

PubMed

A simple and highly efficient synthetic strategy to access 3,3-bipyrrole derivatives by the reaction of dienone derivatives with TosMIC is reported. The reaction involves a van Leusen's pyrrole synthesis followed by an unusual C-C bond cleavage in the presence of water under mild conditions. PMID:24493005

Wang, Rong; Wang, Shun-Yi; Ji, Shun-Jun

2014-03-21

11

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?NC?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

2013-03-01

12

Reactivity of Biomimetic Iron(II)-2-aminophenolate Complexes toward Dioxygen: Mechanistic Investigations on the Oxidative C-C Bond Cleavage of Substituted 2-Aminophenols.  

PubMed

The isolation and characterization of a series of iron(II)-2-aminophenolate complexes [(6-Me3-TPA)Fe(II)(X)](+) (X = 2-amino-4-nitrophenolate (4-NO2-HAP), 1; X = 2-aminophenolate (2-HAP), 2; X = 2-amino-3-methylphenolate (3-Me-HAP), 3; X = 2-amino-4-methylphenolate (4-Me-HAP), 4; X = 2-amino-5-methylphenolate (5-Me-HAP), 5; X = 2-amino-4-tert-butylphenolate (4-(t)Bu-HAP), 6 and X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-(t)Bu-HAP), 7) and an iron(III)-2-amidophenolate complex [(6-Me3-TPA)Fe(III)(4,6-di-(t)Bu-AP)](+) (7(Ox)) supported by a tripodal nitrogen ligand (6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine) are reported. Substituted 2-aminophenols were used to prepare the biomimetic iron(II) complexes to understand the effect of electronic and structural properties of aminophenolate rings on the dioxygen reactivity and on the selectivity of C-C bond cleavage reactions. Crystal structures of the cationic parts of 5ClO4 and 7BPh4 show six-coordinate iron(II) centers ligated by a neutral tetradentate ligand and a monoanionic 2-aminophenolate in a bidentate fashion. While 1BPh4 does not react with oxygen, other complexes undergo oxidative transformation in the presence of dioxygen. The reaction of 2ClO4 with dioxygen affords 2-amino-3H-phenoxazin-3-one, an auto-oxidation product of 2-aminophenol, whereas complexes 3BPh4, 4BPh4, 5ClO4 and 6ClO4 react with O2 to exhibit C-C bond cleavage of the bound aminophenolates. Complexes 7ClO4 and 7(Ox)BPh4 produce a mixture of 4,6-di-tert-butyl-2H-pyran-2-imine and 4,6-di-tert-butyl-2-picolinic acid. Labeling experiments with (18)O2 show the incorporation of one oxygen atom from dioxygen into the cleavage products. The reactivity (and stability) of the intermediate, which directs the course of aromatic ring cleavage reaction, is found to be dependent on the nature of ring substituent. The presence of two tert-butyl groups on the aminophenolate ring in 7ClO4 makes the complex slow to cleave the C-C bond of 4,6-di-(t)Bu-HAP, whereas 4BPh4 containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me3-TPA)Fe(III)(4-(t)Bu-AP)](+) (6(Ox)) to gain a mechanistic insight into the regioselective C-C bond cleavage reaction. On the basis of the experimental and computational studies, an iron(II)-2-iminobenzosemiquinonate intermediate is proposed to react with dioxygen resulting in the oxidative C-C bond cleavage of the coordinated 2-aminophenolates. PMID:24787025

Chakraborty, Biswarup; Bhunya, Sourav; Paul, Ankan; Paine, Tapan Kanti

2014-05-19

13

Oxidative C(sp(3))-H bond cleavage, C-C and C[double bond, length as m-dash]C coupling at a boron center with O2 as the oxidant mediated by platinum(ii).  

PubMed

Dimethyl- and diphenylplatinum(ii) fragments Pt(II)R2 (R = Me, Ph) enable facile and efficient oxidative C(sp(3))-H bond cleavage and stepwise C-C and C[double bond, length as m-dash]C coupling at the boron atom of a coordinated 1,5-cyclooctanediyldi(2-pyridyl)borato ligand with O2 as the sole oxidant. PMID:24287586

Pal, S; Zavalij, P Y; Vedernikov, A N

2014-05-25

14

Rh-catalyzed oxidative C-C bond formation and C-N bond cleavage: direct access to C2-olefinated free (NH)-indoles and pyrroles.  

PubMed

The rhodium-catalyzed oxidative C2-olefination of indoles and pyrroles containing N-arylcarboxamide directing groups with a range of alkenes and subsequent cleavage of directing groups is described. This method provides direct and efficient access to C2-functionalized free (NH)-heterocycles. PMID:24480981

Sharma, Satyasheel; Han, Sangil; Kim, Mirim; Mishra, Neeraj Kumar; Park, Jihye; Shin, Youngmi; Ha, Jimin; Kwak, Jong Hwan; Jung, Young Hoon; Kim, In Su

2014-03-21

15

Effects of alkoxy groups on arene rings of lignin ?-O-4 model compounds on the efficiencies of single electron transfer-promoted photochemical and enzymatic C-C Bond Cleavage Reactions.  

PubMed

To gain information about how alkoxy substitution in arene rings of ?-O-4 structural units within lignin governs the efficiencies/rates of radical cation C1-C2 bond cleavage reactions, single electron transfer (SET) photochemical and lignin peroxidase-catalyzed oxidation reactions of dimeric/tetrameric model compounds have been explored. The results show that the radical cations derived from less alkoxy-substituted dimeric ?-O-4 models undergo more rapid C1-C2 bond cleavage than those of more alkoxy-substituted analogues. These findings gained support from the results of DFT calculations, which demonstrate that C1-C2 bond dissociation energies of ?-O-4 radical cations decrease as the degree of alkoxy substitution decreases. In SET reactions of tetrameric compounds consisting of two ?-O-4 units, containing different degrees of alkoxy substitution, regioselective radical cation C-C bond cleavage was observed to occur in one case at the C1-C2 bond in the less alkoxy-substituted ?-O-4 moiety. However, regioselective C1-C2 cleavage in the more alkoxy-substituted ?-O-4 moiety was observed in another case, suggesting that other factors might participate in controlling this process. These observations show that lignins containing greater proportions of less rather than more alkoxylated rings as part of ?-O-4 units would be more efficiently cleaved by SET mechanisms. PMID:23992466

Lim, Suk Hyun; Nahm, Keepyung; Ra, Choon Sup; Cho, Dae Won; Yoon, Ung Chan; Latham, John A; Dunaway-Mariano, Debra; Mariano, Patrick S

2013-09-20

16

Hydrogenolysis and homologation of linear and branched pentenes on Ru/SiO/sub 2/ catalysts: implication in the mechanism of C-C bond formation and cleavage on metal surfaces  

SciTech Connect

Hydrogenolysis and homologation of 1-pentene to butenes and hexenes take place simultaneously and at the same rate over a Ru/SiO/sub 2/ catalysts at 110/sup 0/C, suggesting that these two reactions are mechanistically related. /sup 13/C labeling experiments indicate that C-C cleavage occurs at the double bond of 1-pentene-1-/sup 13/C leading to unlabeled 1-butene and labeled hexenes. The product distribution in the hydrogenolysis of 1-pentene, 2-pentenes, 3-methyl-1-butene, 2-methyl-2-butene, and 2-methyl-1-butene is accounted for by a carbene-olefin mechanism, which can therefore be considered as a reasonable common path for the formation and cleavage of carbon-carbon bonds on metal surfaces.

Rodriguez, E.; Leconte, M.; Basset, J.M.; Tanaka, K.; Tanaka, K.I.

1988-01-06

17

Shorter still: compressing C-C single bonds.  

PubMed

How short can a C-C single bond get? The bonding in a set of molecules that are related structurally to previously synthesized or theoretically examined systems with short C-C bonds is investigated. According to calculations, a single C-C bond could be compressed to 1.313 ! To the best of our knowledge, this is the shortest single C-C bond reported to date. This shortening is a consequence of a change in the C-C-C bond angle, ?, to minimize strain in the cages and an effort to offset the tension in the surrounding bridges. PMID:20718457

Martnez-Guajardo, Gerardo; Donald, Kelling J; Wittmaack, Bernard K; Vazquez, Miguel Angel; Merino, Gabriel

2010-09-17

18

Tandem application of C-C bond-forming reactions with reductive ozonolysis  

PubMed Central

Several variants of reductive ozonolysis, defined here as the in situ generation of aldehydes or ketones during ozonolytic cleavage of alkenes, are demonstrated to work effectively in tandem with a number of C-C bond-forming reactions. For reactions involving basic nucleophiles (1,2-addition of Grignard reagents, Wittig or Horner-Emmons olefinations, and directed Aldol reactions of lithium enolates) the one-pot process offers a rapid and high-yielding alternative to traditional two-step protocols.

Willand-Charnley, Rachel; Dussault, Patrick H.

2012-01-01

19

C-C bond-forming desulfurizations of sulfoximines.  

PubMed

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

Reggelin, M; Slavik, S; Bhle, P

2008-09-18

20

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

21

Biotic and abiotic carbon to sulfur bond cleavage  

SciTech Connect

Mechanisms for cleavage of RCH{sub 2}-S bonds catalyzed by Escherichia coli can best be categorized by whether an alcohol RCH{sub 2}OH or an aldehyde RCHO are the products of the degradation. A study of the chemical processes involved has been used to establish the best formulation of carbon to sulfur bond cleavage. 2 figs.

Frost, J.W.

1991-01-01

22

Regioselective [2 + 2] cycloaddition of a fullerene dimer with an alkyne triggered by thermolysis of an interfullerene C-C bond.  

PubMed

Heating of a singly bonded fullerene dimer in the presence of an alkyne forms a cyclobutene structure on only one of the two fullerene moieties, through a stereo- and regioselective [2 + 2] cycloaddition. Experimental and theoretical data suggest that the reaction is triggered by cleavage of the interfullerene C-C bond and formation of a monomeric fullerene radical. PMID:23611627

Xiao, Zuo; Matsuo, Yutaka; Maruyama, Masashi; Nakamura, Eiichi

2013-05-01

23

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

24

Reductive cleavage of the peptide bond  

NASA Technical Reports Server (NTRS)

In many biological research efforts, long chain organic molecules are studied by breaking large molecules into smaller components. Cleavage technique of recent interest is the use of solvated electrons. These are formed when aqueous solutions are bombarded with gamma radiation. Solvated electron is very reactive and can reduce most any species present, even to form free radicals.

Holian, J.; Garrison, W. M.

1973-01-01

25

Valence Bond Interpretation of Elastic Anisotropy in B.C.C. Transition Metals.  

National Technical Information Service (NTIS)

Valence bond theory provides a practical microscopic phenomeno-logical context for interpretation of many properties of transition metals. In body centered cubic (b.c.c.) transition metals, the partition of the total bonding hybrids into nearest-neighbour...

D. R. Hay P. D. Parikh

1969-01-01

26

NO2 bond cleavage by MoL3 complexes.  

PubMed

The cleavage of one N-O bond in NO2 by two equivalents of Mo(NRAr)3 has been shown to occur to form molybdenum oxide and nitrosyl complexes. The mechanism and electronic rearrangement of this reaction was investigated using density functional theory, using both a model Mo(NH2)3 system and the full [N((t)Bu)(3,5-dimethylphenyl)] experimental ligand. For the model ligand, several possible modes of coordination for the resulting complex were observed, along with isomerisation and bond breaking pathways. The lowest barrier for direct bond cleavage was found to be via the singlet ?(2)-N,O complex (7 kJ mol(-1)). Formation of a bimetallic species was also possible, giving an overall decrease in energy and a lower barrier for reaction (3 kJ mol(-1)). Results for the full ligand showed similar trends in energies for both isomerisation between the different isomers, and for the mononuclear bond cleavage. The lowest calculated barrier for cleavage was only 21 kJ mol(-1)via the triplet ?(1)-O isomer, with a strong thermodynamic driving force to the final products of the doublet metal oxide and a molecule of NO. Formation of the full ligand dinuclear complex was not accompanied by an equivalent decrease in energy seen with the model ligand. Direct bond cleavage via an ?(1)-O complex is thus the likely mechanism for the experimental reaction that occurs at ambient temperature and pressure. Unlike the other known reactions between MoL3 complexes and small molecules, the second equivalent of the metal does not appear to be necessary, but instead irreversibly binds to the released nitric oxide. PMID:24217121

Shaw, Miranda F; Mahdizadeh Ghohe, Narges; Ariafard, Alireza; Brookes, Nigel J; Stranger, Robert; Yates, Brian F

2014-01-28

27

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

PubMed Central

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

Huang, He; Zhou, Yu

2011-01-01

28

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

2013-10-18

29

Energy and Rate Determinations to Activate the C-C ?-BOND of Acetone by Gaseous NI^+  

NASA Astrophysics Data System (ADS)

A unique application of a custom fabricated photodissociation spectrometer permits the determination of thermodynamic properties (activation energies), reaction rates, and mechanistic details of bare metal cation mediated C-C ?-bond activation in the gas phase. Specifically, the products and rates resulting from the unimolecular decomposition of the Ni^+Acetone (Ni^+Ac) adduct are monitored after absorption of a known amount of energy. The three dissociative products which are observed in high yield are Ni^+, Ni^+CO, and CH3CO^+. The latter two fragment ions result from the activation of a C-C ?-bond. It was found that minimally 14 000 cm^{-1} of energy must be deposited into the adduct ion to induce C-C bond breakage. Preliminary results for the Ni^+ activation of the C-C ?-bond of acetone indicate that there are (at least) two low energy reaction coordinates leading to C-C bond breakage. The lower energy pathway emerges from the doublet ground state with an upper limit to the activation energy of 14 000 cm^{-1} and reaction rate ?0.14 molecules/?s. The higher energy path is assumed to be along the quartet reaction coordinate with a minimum activation energy of 18 800 cm^{-1} (relative to the ground state) and a slightly slower reaction rate.

Castleberry, Vanessa A.; Dee, S. Jason; Villarroel, Otsmar J.; Laboren, Ivanna E.; Frey, Sarah E.; Bellert, Darrin J.

2009-06-01

30

Carbonhydrogen versus carbonchalcogen bond cleavage of furan, thiophene and selenophene by ansa molybdenocene complexes  

Microsoft Academic Search

[Me2Si(C5Me4)2]MoH2 reacts with furan and benzofuran to yield products resulting from CH bond cleavage, namely [Me2Si(C5Me4)2]Mo(?1-C-C4H3O)H and [Me2Si(C5Me4)2]Mo(?1-C-C8H5O)H, whereas the corresponding reactions of selenophene and benzoselenophene yield products resulting from CSe bond cleavage, namely [Me2Si(C5Me4)2]Mo(?2-C,Se-SeC4H4) and [Me2Si(C5Me4)2]Mo(?2-C,Se-SeC8H6). On this basis, the reactivity of the selenophene derivatives is analogous to that of previously reported thiophene derivatives, while the reactivity of the

David G. Churchill; Brian M. Bridgewater; Guang Zhu; Keliang Pang; Gerard Parkin

2006-01-01

31

Catalytic C-C bond formation via capture of hydrogenation intermediates.  

PubMed

Although catalytic hydrogenation has been practiced for over a century, use of hydrogen as a terminal reductant in catalytic C-C bond formation has been restricted to processes involving migratory insertion of carbon monoxide, e.g., alkene hydroformylation and related Fischer-Tropsch-type reactions. In an effort to develop hydrogenation as a new method for catalytic cross-coupling, a catalytic system enabling capture of hydrogenation intermediates was recently developed in our lab. These results support the feasibility of developing a broad new family of hydrogen-mediated C-C bond formations. PMID:15379581

Jang, Hye-Young; Krische, Michael J

2004-09-01

32

Palladium supported on detonation nanodiamond as a highly effective catalyst of the C=C and C?C bond hydrogenation  

Microsoft Academic Search

Palladium loaded on detonation nanodiamond was used for the first time as a catalyst for the C=C and C?C bond hydrogenation. By the example of tolane hydrogenation, the Pd(0)\\/nanodiamond was found to greatly surpass in catalytic activity other Pd(0)\\/nanocarbon catalysts.

Olga V. Turova; Eugenia V. Starodubtseva; Maxim G. Vinogradov; Viacheslav I. Sokolov; Natalya V. Abramova; Alexander E. Alexenskiy

2011-01-01

33

Carbon-chalcogen bond cleavage reactions characterized for dinuclear sulfur-bridged cyclopentadienyl molybdenum complexes  

Microsoft Academic Search

The types of carbon-chalcogen bond cleavage reactions which have been characterized for dinuclear cyclopentadienylmolybdenum complexes are reviewed. The reactions include both the homolytic cleavage and the protonolysis of carbon-chalcogen bonds in neutral ?-thiolate derivatives of MoIII, nucleophilic cleavage of carbon-chalcogen bonds in certain cationic derivatives of MoIII or MoIV, and the reactions of certain dinuclear complexes with molecular hydrogen. Unique

1997-01-01

34

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

PubMed Central

The formation of CC bonds via catalytic hydrogenation and transfer hydrogenation enables carbonyl and imine addition in the absence of stoichiometric organometallic reagents. In this review, iridium-catalyzed CC 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 electrophilenucleophile pairs en route to products of hydro-hydroxyalkylation, representing a direct method for the functionalization of carbinol CH bonds.

Bower, John F.; Krische, Michael J.

2011-01-01

35

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

PubMed Central

Disulfide bonds are a posttranslational 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 difficult due, to 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 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. This data shows 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

2012-01-01

36

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

37

Intramolecular Aminocyanation of Alkenes by N?CN Bond Cleavage.  

PubMed

A metal-free, Lewis acid promoted intramolecular aminocyanation of alkenes was developed. B(C6 F5 )3 activates N-sulfonyl cyanamides, thus leading to a formal cleavage of the N?CN bonds in conjunction with vicinal addition of sulfonamide and nitrile groups across an alkene. This method enables atom-economical access to indolines and tetrahydroquinolines in excellent yields, and provides a complementary strategy for regioselective alkene difunctionalizations with sulfonamide and nitrile groups. Labeling experiments with (13) C suggest a fully intramolecular cyclization pattern due to the lack of label scrambling in double crossover experiments. Catalysis with Lewis acid is realized and the reaction can be conducted under air. PMID:24719371

Pan, Zhongda; Pound, Sarah M; Rondla, Naveen R; Douglas, Christopher J

2014-05-12

38

Cleavage of an amide bond by a ribozyme  

NASA Technical Reports Server (NTRS)

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

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

1995-01-01

39

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

PubMed Central

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

Kuhbeck, Dennis; Bijayi Dhar, Basab; Schon, Eva-Maria; Cativiela, Carlos; Gotor-Fernandez, Vicente

2013-01-01

40

Hydrocarbation of c?c bonds: quantification of the nucleophilic reactivity of ynamides.  

PubMed

Donor-substituted diarylcarbenium ions Ar2 CH(+) react with ynamides to give 1-amido-substituted allyl cations (?,?-unsaturated iminium ions). Kinetic studies show that these adducts, which correspond to the addition of a C?H bond across the C?C bond, are formed stepwise with initial formation of keteniminium ions and subsequent 1,3-hydride shifts. The linear correlations between the second-order rate constants (lg?k2 , 20?C) with the electrophilicity parameters E of the diarylcarbenium ions allow us to include ynamides in our comprehensive nucleophilicity scale and thus predict potential electrophilic reaction partners. PMID:24715471

Laub, Hans A; Evano, Gwilherm; Mayr, Herbert

2014-05-01

41

Benzaldehyde lyase-catalyzed diastereoselective C-C bond formation by simultaneous carboligation and kinetic resolution.  

PubMed

Enzymes create chiral microenvironments that may simultaneously generate several stereogenic centers in the same catalytic cycle, broadening the possibilities of biocatalysis. Benzaldehyde lyase (BAL) affords highly diastereoselective ?-hydroxy-ketones by simultaneously performing ligation and kinetic resolution of a racemic aldehyde. Thus, to the well-known enantioselective BAL-carboligation of aldehydes (C-C bond formation), another property, namely diastereoselectivity, is added in this paper for the first time. PMID:23280121

Mller, Christoph R; Prez-Snchez, Mara; Domnguez de Mara, Pablo

2013-03-28

42

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

National Technical Information Service (NTIS)

The objective of the project is to develop a biochemical pathway for the selective cleavage of C- N bonds in molecules found in petroleum. Specifically a novel biochemical pathway will be developed for the selective cleavage of C-N bonds in carbazole. The...

2006-01-01

43

Base-Catalyzed N?N Bond Cleavage of Hydrazones: Synthesis of ?-Amino Ketones.  

PubMed

An efficient Cs2 CO3 -promoted synthesis of ?-amino ketones using hydrazines, aldehydes, and ?-haloketones as starting materials through a cascade condensation/nucleophilic substitution/N?N bond cleavage route is developed. The carbonyl group plays a key role in this novel N?N bond cleavage process. PMID:24596344

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

2014-05-01

44

C-C Bond Activation and Coupling of Propene Induced by la Atom  

NASA Astrophysics Data System (ADS)

A series of La(C_nH_m) complexes with n ? 6 and m ? 12 were produced by the reactions between propene and La in a supersonic molecular beam source. Their formation and structures were investigated using mass-analyzed threshold ionization (MATI) spectroscopy in combination with theoretical calculations. Previously, we identified the formation of La(C_3H_4) and H-La(C_3H_5) through dehydrogenation and metal insertion mechanisms. In this work, we will discuss the formation of La(CH_2) and La(C_4H_6) by La induced C-C bond activation and coupling. La(CH_2) is formed by the C-C bond breakage and 1,2-hydride shift of propene and is a Schrock-type carbene complex. This complex is then coupled with the C=C bond of a second propene molecule to form La(C_4H_6) by removing two hydrogen atoms. The resultant La(C_4H_6) complex was idetified in two low-energy isomeric forms: one was a metallacycle (isomer A) and the other was lanthanum trimethylenemethane (isomer B). Both La(C_4H_6) isomers are in a doublet ground state, with isomer A in C_s point group and isomer B in C_3_v. Adiabatic ionization energies and several vibrational frequencies of the two complexes were obtained from the sharp MATI spectra.

Hewage, Dilrukshi; Tao, Hong; Silva, Ruchira; Kumari, Sudesh; Yang, Dong-Sheng

2013-06-01

45

Reversible C-C bond formation between redox-active pyridine ligands in iron complexes.  

PubMed

This manuscript describes the formally iron(I) complexes L(Me)Fe(Py-R)(2) (L(Me) = bulky ?-diketiminate; R = H, 4-tBu), in which the basal pyridine ligands preferentially accept significant unpaired spin density. Structural, spectroscopic, and computational studies on the complex with 4-tert-butylpyridine ((tBu)py) indicate that the S = 3/2 species is a resonance hybrid between descriptions as (a) high-spin iron(II) with antiferromagnetic coupling to a pyridine anion radical and (b) high-spin iron(I). When the pyridine lacks the protection of the tert-butyl group, it rapidly and reversibly undergoes radical coupling reactions that form new C-C bonds. In one reaction, the coordinated pyridine couples to triphenylmethyl radical, and in another, it dimerizes to give a pyridine-derived dianion that bridges two iron(II) ions. The rapid, reversible C-C bond formation in the dimer stores electrons from the formally reduced metal as a C-C bond in the ligands, as demonstrated by using the coupled diiron(II) complex to generate products that are known to come from iron(I) precursors. PMID:23181620

Dugan, Thomas R; Bill, Eckhard; MacLeod, K Cory; Christian, Gemma J; Cowley, Ryan E; Brennessel, William W; Ye, Shengfa; Neese, Frank; Holland, Patrick L

2012-12-19

46

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

NASA Technical Reports Server (NTRS)

The bond dissociation energies (D sub e) of C2H2, C2H4, C2H6, N2, N2H2, and N2H4 are studied at various levels of correlation treatment. The convergence of D sub e with respect to the one particle basis is studied at the single reference modified coupled-pair functional (MCPF) level. At all levels of correlation treatment, the errors in the bond dissociation energies increase with the degree of multiple bond character. The multireference configuration interaction (MRCI) D sub e values, corrected for an estimate of higher excitations, are in excellent agreement with those determined using the size extensive averaged coupled pair functional (ACPF) method. It was found that the full valence complete active space self consistent field (CASSCF)/MRCI calculations are reproduced very well by MRCI calculations based on a CASSCF calculation that includes in the active space only those electrons involved in the C-C or N-N bonds. To achieve chemical accuracy (1 kcal/mole) for the D sub e values of the doubly bonded species C2H4 and N2H2 requires one particle basis sets including up through h angular momentum functions (l = 5) and a multireference treatment of electron correlation: still higher levels of calculation are required to achieve chemical accuracy for the triply bonded species C2H2 and N2.

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

1989-01-01

47

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

2013-07-22

48

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

49

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

PubMed

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

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

2014-03-13

50

Pd-catalyzed autotandem C-C/C-C bond-forming reactions with tosylhydrazones: synthesis of spirocycles with extended ?-conjugation.  

PubMed

A new Pd-catalyzed autotandem process is presented by the reaction of tosylhydrazones of cyclic ketones and 2,2'-dibromobiphenyls and related systems. The process involves cross-coupling with tosylhydrazone followed by an intramolecular Heck reaction and gives rise to spirocyclic structures. Noteworthy, two C-CAr bonds are formed on the hydrazonic carbon during the process. Depending on the starting dibromide, an array of spirofluorenes, spirodibenzofluorenes, spiroacridines, and spiroanthracenes have been prepared. Thus, this methodology may be applied for the preparation of interesting structures useful in the development of optoelectronic materials. PMID:24708061

Barroso, Raquel; Valencia, Roco A; Cabal, Mara-Paz; Valds, Carlos

2014-04-18

51

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

PubMed Central

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

Jana, Ranjan; Tunge, Jon A.

2011-01-01

52

Bond activation with an apparently benign ethynyl dithiocarbamate Ar-C?C-S-C(S)NR2.  

PubMed

The hedgehog molecule: A simple ethynyl dithiocarbamate [Ar-C?C-S-C(S)NR(2)] is able to cleave a broad range of enthalpically strong ? bonds and to activate carbon dioxide and elemental sulfur. Depending on the substrate, the bond activation process involves either the existence of an equilibrium with the nonobservable mesoionic carbene isomer or the cooperation of the nucleophilic carbon-carbon triple bond and the electrophilic CS carbon atom. PMID:23210141

Ung, Gal; Frey, Guido D; Schoeller, Wolfgang W; Bertrand, Guy

2011-10-10

53

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

NASA Astrophysics Data System (ADS)

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.

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

2013-08-01

54

Cleavage enhancement of specific chemical bonds in DNA-Cisplatin complexes induced by X-rays  

NASA Astrophysics Data System (ADS)

The chemical bond transformation of cisplatin-DNA complexes can be probed efficiently by XPS which provides a concomitant X-ray irradiation source as well. The presence to Pt could considerably increase formation of the SE induced by X-ray and that the further interaction of these LEE with DNA leads to the enhancement of bond cleavages.

Zheng, Yi; Yao, Xiaobin; Luo, Xinglan; Fu, Xianzhi

2014-04-01

55

Photoinduced Cleavage of N-N Bonds of Aromatic Hydrazines and Hydrazides by Visible Light  

PubMed Central

A photocatalytic system involving [Ru(bpyrz)3](PF6)22H2O, visible light, and air has been developed for cleavage of the NN bonds of hydrazines and hydrazides. This catalytic system is generally effective for N,N-disubstituted hydrazine and hydrazide derivatives, including arylhydrazides, N-alkyl-N-arylhydrazines, and N,N-diarylhydrazines. The utility of this cleavage reaction has been demonstrated by synthesizing a variety of secondary aromatic amines.

Zhu, Mingzhao

2012-01-01

56

Substituent effects in X?C?C?H⋯NH 3 (or OH 2, FH) hydrogen bonding  

NASA Astrophysics Data System (ADS)

MP2/6-31g(p,d) calculations were performed on the C?H⋯N (or O, F) hydrogen bonding between 18 substituted X?C?C?H and NH3, H2O, and HF. It was found that the Hammett-type correlation was applicable to the above systems, which indicated that the field/inductive and resonance effects could both affect the X?C?C?H⋯NH 3 (or H 2O, HF) hydrogen bonding.

Chen, Rong; Zhang, Ke-Chun; Liu, Lei; Li, Xiao-Song; Guo, Qing-Xiang

2001-04-01

57

C?-C? chromophore bond dissociation in protonated tyrosine-methionine, methionine-tyrosine, tryptophan-methionine, methionine-tryptophan and their sulfoxide analogs.  

PubMed

C(?)-C(?) chromophore bond dissociation in some selected methionine-containing dipeptides induced by UV photons is investigated. In methionine containing dipeptides with tryptophan as the UV chromophore, the tryptophan side chain is ejected either as an ion or as a neutral fragment while in dipeptides with tyrosine, the tyrosine side chain is lost only as a neutral fragment. Mechanisms responsible for these fragmentations are proposed based on measured branching ratios and fragmentation times, and on the results of DFT/B3-LYP calculations. It appears that the C(?)-C(?) bond cleavage is a non-statistical dissociation for the peptides containing tyrosine, and occurs after internal conversion for those with tryptophan. The proposed mechanisms are governed by the ionization potential of the aromatic side chain compared to that of the rest of the molecule, and by the proton affinity of the aromatic side chain compared to that of the methionine side chain. In tyrosine-containing peptides, the presence of oxygen on sulfur of methionine presumably reduces the ionization potential of the peptide backbone, facilitating the loss of the side chain as a neutral fragment. In tryptophan-containing peptides, the presence of oxygen on methionyl-sulfur expedites the transfer of the proton from the side chain to the sulfoxide, which facilitates the loss of the neutral side chain. PMID:22722478

Sunil Kumar, S; Lucas, B; Soorkia, S; Barat, M; Fayeton, J A

2012-08-01

58

Cleavage of the CSi bond in Trifluoro(phenyl)silane with Aliphatic Alcohols  

Microsoft Academic Search

Trifluoro(phenyl)silane reacts with aliphatic alcohols under reflux. The reaction involves not only Si-F bond cleavage to\\u000a form ethoxyfluoro(phenyl)silanes, but also C-Si bond cleavage to form benzene and alkoxyfluoro- and tetraalkoxysilanes. The\\u000a formation of the latter products was proved by 19F and 29Si NMR spectroscopy and also by model disproportionation reactions of trifluoro(phenyl)silane with trimethoxy-(phenyl)-, tetramethoxy-,\\u000a or tetraethoxysilanes.

M. G. Voronkov; E. V. Boyarkina; A. I. Albanov; S. V. Basenko

2005-01-01

59

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

SciTech Connect

The objective of the project is to develop a biochemical pathway for the selective cleavage of C-N bonds in molecules found in petroleum. Specifically a novel biochemical pathway will be developed for the selective cleavage of C-N bonds in carbazole. 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. The selective cleavage of the second C-N bond has been challenging, and efforts to overcome that challenge have been the focus of recent research in this project. Enrichment culture experiments succeeded in isolating bacterial cultures that can metabolize 2-aminobiphenyl, but no enzyme capable of selectively cleaving the C-N bond in 2-aminobiphenyl has been identified. Aniline is very similar to the structure of 2-aminobiphenyl and aniline dioxygenase catalyzes the conversion of aniline to catechol and ammonia. For the remainder of the project the emphasis of research will be to simultaneously express the genes for carbazole dioxygenase and for aniline dioxygenase in the same bacterial host and then to select for derivative cultures capable of using carbazole as the sole source of nitrogen.

John J. Kilbane II

2005-10-01

60

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; Rmond, Emmanuelle; Chatani, Naoto

2012-01-11

61

Nickel-catalyzed amination of aryl pivalates by the cleavage of aryl C-O bonds.  

PubMed

Catalytic amination: The title reaction demonstrates the use of aryl carboxylates as suitable electrophilic coupling substrates in catalytic amination reactions. N-heterocyclic carbene ligands and NaOtBu promote the amination of aryl pivalates through the cleavage of normally unreactive aryl carbon-oxygen bonds (see scheme; cod=cyclooctadiene). PMID:20229558

Shimasaki, Toshiaki; Tobisu, Mamoru; Chatani, Naoto

2010-04-01

62

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

63

Reaction Pathways and Energetics of Etheric C?O Bond Cleavage Catalyzed by Lanthanide Triflates  

SciTech Connect

Efficient and selective cleavage of etheric C?O bonds is crucial for converting biomass into platform chemicals and liquid transportation fuels. In this contribution, computational methods at the DFT B3LYP level of theory are employed to understand the efficacy of lanthanide triflate catalysts (Ln(OTf)3, Ln = La, Ce, Sm, Gd, Yb, and Lu) in cleaving etheric C?O bonds. In agreement with experiment, the calculations indicate that the reaction pathway for C?O cleavage occurs via a C?H ? O?H proton transfer in concert with weakening of the C?O bond of the coordinated ether substrate to ultimately yield a coordinated alkenol. The activation energy for this process falls as the lanthanide ionic radius decreases, reflecting enhanced metal ion electrophilicity. Details of the reaction mechanism for Yb(OTf)3-catalyzed ring opening are explored in depth, and for 1-methyl-d3-butyl phenyl ether, the computed primary kinetic isotope effect of 2.4 is in excellent agreement with experiment (2.7), confirming that etheric ring-opening pathway involves proton transfer from the methyl group alpha to the etheric oxygen atom, which is activated by the electrophilic lanthanide ion. Calculations of the catalytic pathway using eight different ether substrates indicate that the more rapid cleavage of acyclic versus cyclic ethers is largely due to entropic effects, with the former C?O bond scission processes increasing the degrees of freedom/particles as the transition state is approached.

Assary, Rajeev S.; Atesin, Abdurrahman C.; Li, Zhi; Curtiss, Larry A.; Marks, Tobin J.

2013-07-15

64

A quantum chemical topological analysis of the C-C bond formation in organic reactions involving cationic species.  

PubMed

ELF topological analysis of the ionic Diels-Alder (I-DA) reaction between the N,N-dimethyliminium cation and cyclopentadiene (Cp) has been performed in order to characterise the C-C single bond formation. The C-C bond formation begins in the short range of 2.00-1.96 via a C-to-C pseudoradical coupling between the most electrophilic center of the iminium cation and one of the two most nucleophilic centers of Cp. The electron density of the pseudoradical center generated at the most electrophilic carbon of the iminium cation comes mainly from the global charge transfer which takes place along the reaction. Analysis of the global reactivity indices indicates that the very high electrophilic character of the iminium cation is responsible for the negative activation energy found in the gas phase. On the other hand, the analysis of the radical P Parr functions of the iminium cation, and the nucleophilic Pk(-) Parr functions of Cp makes the characterisation of the most favourable two-center interaction along the formation of the C-C single bond possible. PMID:24901220

Domingo, Luis R; Prez, Patricia

2014-07-21

65

C-N bond cleavage of anilines by a (salen)ruthenium(VI) nitrido complex.  

PubMed

We report experimental and computational studies of the facile oxidative C-N bond cleavage of anilines by a (salen)ruthenium(VI) nitrido complex. We provide evidence that the initial step involves nucleophilic attack of aniline at the nitrido ligand of the ruthenium complex, which is followed by proton and electron transfer to afford a (salen)ruthenium(II) diazonium intermediate. This intermediate then undergoes unimolecular decomposition to generate benzene and N2. PMID:23537370

Man, Wai-Lun; Xie, Jianhui; Pan, Yi; Lam, William W Y; Kwong, Hoi-Ki; Ip, Kwok-Wa; Yiu, Shek-Man; Lau, Kai-Chung; Lau, Tai-Chu

2013-04-17

66

Copper-catalyzed rearrangement of (Z)-propynal hydrazones via N-N bond cleavage.  

PubMed

Propynal hydrazones are successfully converted to the corresponding 3-aminoacrylonitriles in the presence of copper catalysts in good to high yields. As an example, (Z)-N-(hex-2-ynylidene)morpholin-4-amine reacted in the presence of 10 mol % Cu(OAc)(2) in acetonitrile at 25 C to afford (E)-3-morpholinohex-2-enenitrile ((E)-2 h) in 77% yield via C-N bond formation and subsequent ?-elimination involving cleavage of N-N and C-H bonds. PMID:20718477

Nakamura, Itaru; Shiraiwa, Naozumi; Kanazawa, Ryo; Terada, Masahiro

2010-09-17

67

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

SciTech Connect

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

Frost, J.W.

1991-12-31

68

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/(molK) (relative to 7 1 cal/(molK) 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

69

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

70

Base-promoted aryl-bromine bond cleavage with cobalt(II) porphyrins via a halogen atom transfer mechanism.  

PubMed

Aryl-bromine bonds are successfully cleaved by cobalt(II) porphyrins in basic media to give Co(por)Ar (por = porphyrin) in good yields. Mechanistic studies suggested that the aryl-bromine bond is cleaved through a halogen atom transfer mechanism, which is different from the aryl-halogen bond cleavage mechanism with other group 9 metalloporphyrins. PMID:24699823

Liu, Chun Ran; Qian, Ying Ying; Chan, Kin Shing

2014-06-01

71

Oxaloacetate hydrolase, the C-C bond lyase of oxalate secreting fungi.  

PubMed

Oxalate secretion by fungi is known to be associated with fungal pathogenesis. In addition, oxalate toxicity is a concern for the commercial application of fungi in the food and drug industries. Although oxalate is generated through several different biochemical pathways, oxaloacetate acetylhydrolase (OAH)-catalyzed hydrolytic cleavage of oxaloacetate appears to be an especially important route. Below, we report the cloning of the Botrytis cinerea oahA gene and the demonstration that the disruption of this gene results in the loss of oxalate formation. In addition, through complementation we have shown that the intact B. cinerea oahA gene restores oxalate production in an Aspergillus niger mutant strain, lacking a functional oahA gene. These observations clearly indicate that oxalate production in A. niger and B. cinerea is solely dependent on the hydrolytic cleavage of oxaloacetate catalyzed by OAH. In addition, the B. cinera oahA gene was overexpressed in Escherichia coli and the purified OAH was used to define catalytic efficiency, substrate specificity, and metal ion activation. These results are reported along with the discovery of the mechanism-based, tight binding OAH inhibitor 3,3-difluorooxaloacetate (K(i) = 68 nM). Finally, we propose that cellular uptake of this inhibitor could reduce oxalate production. PMID:17244616

Han, Ying; Joosten, Henk-Jan; Niu, Weiling; Zhao, Zhiming; Mariano, Patrick S; McCalman, Melisa; van Kan, Jan; Schaap, Peter J; Dunaway-Mariano, Debra

2007-03-30

72

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

73

C?C Bond activation with selective functionalization: preparation of unsymmetrical biaryls from benzonitriles  

Microsoft Academic Search

For the first time, benzonitriles have been shown to participate in metal-catalyzed cross coupling reactions via activation of the C?CN bond. Thus, reaction of a benzonitrile with an aryl Grignard derivative in the presence of a Ni catalyst can readily provide the corresponding unsymmetrical biaryl in high yield and with high selectivity.

Joseph A. Miller

2001-01-01

74

Moving towards strong-field femtosecond control of bond cleavage and charge localization in triatomic molecules  

NASA Astrophysics Data System (ADS)

A 3-D momentum imaging technique is employed to study intense ultrafast laser-induced dissociation of triatomic molecular ions from an ion beam. Utilizing our measured kinetic energy release and angular distribution spectra along with the calculated electronic structure of these molecules, we elucidate possible dissociation pathways and anticipate and explore various laser parameters that could be used to drive transitions to specific final products. For example, we have studied N2O^+, in which we find that for typical intense IR laser pulses (30 fs, transform-limited, 800 nm, 10^15 W/cm^2 pulses), the preferred bond cleavage (i.e., breaking the N-N bond vs. the N-O bond) and charge localization patterns are those that are the most energetically favorable. We investigate laser parameters that could be used to steer this and other systems to less likely outcomes.

Jochim, Bethany; Ablikim, U.; Zohrabi, M.; Gaire, B.; Carnes, K. D.; Esry, B. D.; Ben-Itzhak, I.

2012-06-01

75

Competition between Covalent and Noncovalent Bond Cleavages in Dissociation of Phosphopeptide-Amine Complexes  

SciTech Connect

Interactions between quaternary amino or guanidino groups with anions are ubiquitous in nature. Here, we present a first study focused on quantifying such interactions using complexes of phosphorylated A3pXA3-NH2 (X=S, T, Y) peptides with decamethonium (DCM) or diaguanidinodecane (DGD) ligands as model systems. Time- and collision energy-resolved surface-induced dissociation (SID) of the singly charged complexes was examined using a specially configured Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS). Dissociation thresholds and activation energies were obtained from RRKM modeling of the experimental data that has been described and carefully characterized in our previous studies. We demonstrate that covalent bond cleavages resulting in phosphate abstraction by the cationic ligand are characterized by low dissociation thresholds and relatively tight transition states. In contrast, high dissociation barriers and large positive activation entropies were obtained for cleavages of non-covalent bonds. Dissociation parameters obtained from the modeling of the experimental data are in excellent agreement with the results of density functional theory (DFT) calculations. Comparison between the experimental data and theoretical calculations indicate that phosphate abstraction by the ligand is rather localized and mainly affected by the identity of the phosphorylated side chain. The hydrogen bonding in the peptide and ligand properties play a minor role in determining the energetics and dynamics of the phosphate abstraction channel

Laskin, Julia; Yang, Zhibo; Woods, Amina S.

2011-04-21

76

Aerobic synthesis of substituted quinoline from aldehyde and aniline: copper-catalyzed intermolecular C-H active and C-C formative cyclization.  

PubMed

An efficient method for the direct synthesis of substituted quinolines from anilines and aldehydes through C-H functionalization, C-C/C-N bond formation, and C-C bond cleavage has been developed. The method is simple and practical and employs air as an oxidant. PMID:24024912

Yan, Rulong; Liu, Xingxing; Pan, Congming; Zhou, Xiaoqiang; Li, Xiaoni; Kang, Xing; Huang, Guosheng

2013-09-20

77

Bond cleavages of adenosine 5'-triphosphate induced by monochromatic soft X-rays  

NASA Astrophysics Data System (ADS)

To investigate which type of bond is likely to be cleaved by soft X-ray exposure to an adenosine 5'-triphosphate (ATP), we observed spectral changes in X-ray absorption near edge structure (XANES) around nitrogen and oxygen K-edge of an ATP film by soft X-ray irradiation. Experiments were performed at a synchrotron soft X-ray beamline at SPring-8, Japan. The XANES spectra around the nitrogen and oxygen .K-edge slightly varied by exposure to 560 eV soft X-rays. These changes are originated from the cleavage of C-N bonds between a sugar and a nucleobase site and of C-O, P-O or O-H bond of sugar and phosphate site. From the comparison between the change in XANES intensity of ?* peak at nitrogen and that at oxygen K-edges, it is inferred that the C-O, P-O or O-H bond of sugar and phosphate is much efficiently cleaved than the C-N of N-glycoside bond by the exposure of 560 eV soft X-ray to ATP film.

Fujii, K.; Narita, A.; Yokoya, A.

2014-04-01

78

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 was focused 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 deaminase. The objective of the final phase of the project will be to develop derivative C-N bond cleaving enzymes that have broader substrate ranges and to demonstrate the use of such strains to selectively remove nitrogen from petroleum. During the first year of the project (October, 2002-September, 2003) enrichment culture experiments resulted in the isolation of microbial cultures that utilize aromatic amides as sole nitrogen sources, several amidase genes were cloned and were included in directed evolution experiments 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. During the second year of the project (October, 2003-September, 2004) enrichment culture experiments succeeded in isolating a mixed bacterial culture that can utilize 2-aminobiphenyl as a sole nitrogen source, directed evolution experiments were focused on the aniline dioxygenase enzyme that is capable of deaminating aniline, and expression vectors were constructed to enable the expression of genes encoding C-N bond cleaving enzymes in Rhodococcus hosts. 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. Aromatic compounds such as carbazole are representative of the difficult-to-treat organonitrogen compounds most commonly encountered in petroleum. There are two C-N bonds in carbazole and the construction of a metabolic pathway for the removal of nitrogen from carbazole will require enzymes capable cleaving both C-N bonds. A multi-component enzyme, carbazole dioxygenase, which can selectively cleave the first C-N bond has been identified and the genes that encode this enzyme have been cloned, sequenced, and are being expressed in Rhodococcus erythropolis, a bacterial culture that tolerates exposure to petroleum. An enzyme capable of selectively cleaving the second C-N bond in carbazole has not yet been identified, but enrichment culture experiments have recently succeeded in isolating a bacterial culture that is a likely candidate and may possess a suitable enzyme. Research in the near future will verify if a suitable enzyme for the cleavage of the second C-N bond in carbazole has indeed been found, then the genes encoding a suitable enzyme will be identified, cloned, and sequenced. Ultimately genes encoding enzymes for selective cleavage of both C-N bonds in carbazole will be assembled into a new metabolic pathway and the ability of the resulting bacterial culture to remove nitrogen from petroleum will be determined.

John J. Kilbane II

2004-10-01

79

Synthesis of hydroxydiamines and triamines via reductive cleavage of N-N bond in substituted pyrazolidines.  

PubMed

Aliphatic polyamines, being a versatile class of organic compounds, are widely used in many fields of medicine and organic chemistry. However, the general approach to the synthesis of chiral aliphatic polyamines has been still undeveloped. Here, we describe a new method for the synthesis of chiral trifunctional amino compounds, namely hydroxydiamines and triamines. The initial compounds, namely substituted hydroxy- or aminopyrazolidines and pyrazolines, are readily available using convenient stereoselective methods developed earlier by us. The proposed method allows synthesizing of chiral diaminoalcohols and triamines, which are the analogs of a well-known anti-TB drug, namely ethambutol, and cannot be obtained alternatively. The key step of the synthesis is N-N bond cleavage in substituted hydroxy- or aminopyrazolidines and pyrazolines with borane-tetrahydrofuran complex; other known methods for N-N bond cleavage turned out to be ineffective. The main advantage of the proposed method is the retention of a certain configuration of stereocenters in the course of the reaction. Six new chiral diasteomerically pure substituted hydroxydiamines and triamines and the enantiomerically pure triamine with four chiral centers were synthesized and characterized using NMR, IR and mass spectroscopy, as well as elemental analysis. PMID:22418864

Sviridova, Ludmila A; Golubeva, Galina A; Tavtorkin, Alexander N; Kochetkov, Konstantin A

2012-09-01

80

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

81

A quantum chemical study of the C-C bond fission pathways of alkoxy radicals formed following OH addition to isoprene  

SciTech Connect

Isoprene is one of the most important non-methane organic compounds in tropospheric chemistry, yet its atmospheric degradation pathways remain incompletely understood. The uncertain fates of alkoxy radicals formed in the first stages of the OH-initiated degradation of isoprene contribute significantly to ignorance. This paper examines the C-C bond fission pathways of these radicals at the B3LYP/6-311G(2df,2p) level of theory. For the four {beta}-hydroxyalkoxy radicals that are expected to be formed, C-C bond fission (decomposition) pathways exist with very low barriers (0.7--2.1 kcal/mol) that are likely to dominate the chemistry. These radicals appear to possess intramolecular hydrogen bonds which typically persist in the transition states. For the two {delta}-hydroxyalkoxy radicals formed, C-C bond fission is endothermic (16--20 kcal/mol) and this pathway is unlikely to be important.

Dibble, T.S.

1999-10-21

82

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

83

K2CO3-catalyzed synthesis of chromones and 4-quinolones through the cleavage of aromatic C-O bonds.  

PubMed

Phenol-derived electrophiles are favorable substrates because phenols are naturally abundant or can be readily prepared from other aromatic compounds. However, the cleavage of aromatic C-O bonds is a great challenge because of their high energy. K(2)CO(3)-catalyzed intramolecular cyclization of 1-(2-alkoxyphenyl)-3-akylpropane-1,3-dione and 3-(alkylimino)-1-(2-methoxyphenyl)-2-methylpropan-1-one derivatives via the selective cleavage of aromatic C-O bonds is reported. The corresponding chromone and 4-quinolone derivatives were obtained in reasonable yields. PMID:22587645

Zhao, Jie; Zhao, Yufen; Fu, Hua

2012-06-01

84

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 [(NN)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 [(NN)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

85

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

PubMed

The complex [Nb(NMe2)2{(NMe2)C=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; Antiolo, Antonio; Lpez-Solera, Isabel; Fernndez-Galn, Rafael; Villaseor, Elena

2013-10-01

86

Facile C(sp(2))/OR bond cleavage by Ru or Os.  

PubMed

Os(H)(3)ClL(2) (L = P(i)Pr(3) or P(t)Bu(2)Me) are shown to be useful "precursors" to "OsHClL(2)", which react with vinyl ethers to form first an eta(2)-olefin adduct and then isomerize to the carbenes, OsHCl[CMe(OR)]L(2). Subsequent R- and L-dependent reactions involve C(sp(2))-OR bond cleavage, to make either carbyne or vinylidene complexes. The mechanisms of these reactions are explored, and the thermodynamic disparity of Ru versus Os and the influence of the OR group and the spectator phosphine ligands are discussed based on DFT (B3PW91) calculations. PMID:11735470

Ferrando, G; Grard, H; Spivak, G J; Coalter, J N; Huffman, J C; Eisenstein, O; Caulton, K G

2001-12-17

87

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

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

88

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

PubMed

A one-step colloidal process has been adopted to prepare silver (Ag) and silver sulfide (Ag?S) nanocrystals, thus avoiding presynthesis of an organometallic precursor and the injection of a toxic phosphine agent. During the reaction, a layered intermediate compound is first formed, which then acts as a precursor, decomposing into the nanocrystals. The composition of the as-obtained products can be controlled by selective cleavage of S-C bonds or Ag-S bonds. Pure Ag?S nanocrystals can be obtained by directly heating silver acetate (Ag(OAc)) and n-dodecanethiol (DDT) at 200? C without any surfactant, and pure Ag nanocrystals can be synthesized successfully if the reaction temperature is reduced to 190? C and the amount of DDT is decreased to 1 ml in the presence of a non-coordinating organic solvent (1-octadecene, ODE). Otherwise, the mixture of Ag and Ag?S is obtained by directly heating Ag(OAc) in DDT by increasing the reaction temperature or in a mixture of DDT and ODE at 200? C. The formation mechanism has been discussed in detail in terms of selective S-C and Ag-S bond dissociation due to the nucleophilic attack of DDT and the lower bonding energy of Ag-S. Interestingly, some products can easily self-assemble into two- or three-dimensional (2D or 3D) highly ordered superlattice structures on a copper grid without any additional steps. The excess DDT plays a key role in the superlattice structure due to the bundling and interdigitation of the thiolate molecules adsorbed on the as-obtained nanocrystals. PMID:23924847

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

2013-09-01

89

Catalytic C-O bond cleavage of 2-aryloxy-1-arylethanols and its application to the depolymerization of lignin-related polymers.  

PubMed

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. PMID:20731348

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

2010-09-15

90

A new 3-(phenylseleno)allylic cation: its regioselective C-C bond formation reaction with nucleophiles.  

PubMed

Highly useful C-C bond formation using 2-ethoxy-3-(phenylseleno)prop-2-enal acetal 2 was examined with various Lewis acids. The reaction of 2 with the silyl enol ether in the presence of BF(3)*Et2O, ZnBr2, or SnBr4 regioselectively provided (Z)-3,4-diethoxy-5-(phenylseleno)pent-4-enophenone 5a in high yields. On the other hand, the reaction with other Lewis acids such as EtAlCl2 or SnCl4 gave 5-(phenylseleno)- 6 or non-selenopentane-1,4-dione 7, respectively. Novel prop-2-enal acetals 2-4 and 13-15 reacted with various nucleophiles to give pent-4-enophenones 5a,b, 10a, 12, and 16-18, S-ethyl pent-4-enoate 5b, alkylated vinylic sulfide 10b, 3-pentenenitrile 5d, and 10c. A versatile pent-4-enophenone 5a could be converted to tetrahydrofuran 20 and penta-2,4-dienophenone 19, the Diels-Alder reactions of which with dienophiles gave the adducts 24 and 25. PMID:11846647

Hibino, Masaru; Koike, Tomoko; Yoshimatsu, Mitsuhiro

2002-02-22

91

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

PubMed

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

Wu, Tao; Mu, Xin; Liu, Guosheng

2011-12-23

92

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

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. Unfortunately, several classes of reactions that lead to carbon-sulfur bond cleavage are not well understood. Planned in ``Protocols for the Selective Cleavage of Carbon-Sulfur Bonds in Coal`` are reactions in which organic sulfur-containing coal model compounds are subjected to different conditions of temperature, solvent mixtures and radiation. Summarized in this quarterly report are results of our investigations of the following topics: (a) the reactions of coal model compounds, namely, benzyl phenyl sulfide (BPS), diphenyl sulfide (hereafter referred to as phenylsulfide, PS) and dibenzothiophene (DBT) with various reagents (Lewis acid catalysts, radical initiators, electron acceptors) using different solvents and temperature in an attempt to maximize the degree of carbon-sulfur (C-S) bond cleavage; and (b) the results of photooxidation of coal model compounds under controlled conditions. Quantitative product analyses are presented in this report.

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

1992-10-01

93

Copper-catalyzed gem-difluoroolefination of diazo compounds with TMSCF3 via C-F bond cleavage.  

PubMed

A novel Cu-catalyzed gem-difluoroolefination of diazo compounds is described. This transformation starts from readily available TMSCF3 and diazo compounds, via trifluoromethylation followed by C-F bond cleavage, to afford the desired 1,1-difluoroalkene products. PMID:24171471

Hu, Mingyou; He, Zhengbiao; Gao, Bing; Li, Lingchun; Ni, Chuanfa; Hu, Jinbo

2013-11-20

94

An Unusual Carbon-Carbon Bond Cleavage Reaction During Phosphinothricin Biosynthesis  

PubMed Central

Natural products containing phosphorus-carbon bonds have found widespread use in medicine and agriculture1. One such compound, phosphinothricin tripeptide (PTT), contains the unusual amino acid phosphinothricin (PT) attached to two alanine residues (Fig. 1). Synthetic PT (glufosinate) is a component of two top-selling herbicides (Basta and Liberty), and is widely used with resistant transgenic crops including corn, cotton and canola. Recent genetic and biochemical studies showed that during PTT biosynthesis 2-hydroxyethylphosphonate (HEP) is converted to hydroxymethylphosphonate (HMP) (Fig. 1)2. Reported here are the in vitro reconstitution of this unprecedented C(sp3)-C(sp3) bond cleavage reaction and X-ray crystal structures of the enzyme. The protein is a mononuclear non-heme iron(II)-dependent dioxygenase that converts HEP to HMP and formate. In contrast to most other members of this family, the oxidative consumption of HEP does not require additional cofactors or the input of exogenous electrons. The current study expands the scope of reactions catalyzed by the 2-His-1-carboxylate mononuclear non-heme iron family of enzymes.

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

2010-01-01

95

C-H Bond activation and C-C bond formation in the reaction of 2,5-dimethylthiophene with TpMe2Ir compounds.  

PubMed

The bulky 2,5-dimethylthiophene (2,5-Me2T) reacts at 60 degrees C with TpMe2Ir(C2H4)2 to give a mixture of two TpMe2Ir(III) hydride products, 3 and 4, that contain in addition a thienyl (3) or a thienyl-derived ligand (4). For the generation of 3 only sp2 C-H activation is needed, but the formation of 4 requires also the activation of an sp3 C-H bond and the formation of a new C-C bond (between vinyl and thienyl fragments). In the presence of 2,5-Me2T, compound 4 reacts further to produce a complex thiophenic structure (5, characterized by X-ray methods) that derives formally from two molecules of 2,5-Me2T and a vinyl fragment. Compounds 3-5 can be readily protonated by [H(OEt2)2][BAr'4](Ar'= 3,5-C6H3(CF3)2), with initial generation of carbene ligands (in the case of 3 and 5) as a consequence of H+ attack at the beta-carbon of the Ir-thienyl unit. Free, substituted thiophenes, derived from the original 2,5-Me2T, may be isolated in this way. PMID:15824780

Paneque, Margarita; Poveda, Manuel L; Carmona, Ernesto; Salazar, Vernica

2005-04-21

96

Mapping disulfide bonds in insulin with the Route 66 Method: selective cleavage of S-C bonds using alkali and alkaline earth metal enolate complexes.  

PubMed

Simple and fast identification of disulfide linkages in insulin is demonstrated with a peptic digest using the Route 66 method. This is accomplished by collisional activation of singly and doubly charged cationic Na(+) and Ca(2+) complexes generated using electrospray ionization mass spectrometry (ESI-MS). Collisional activation of doubly charged metal complexes of peptides with intermolecular disulfide linkages yields two sets of singly charged paired products separated by 66 mass units resulting from selective SC bond cleavages. Highly selective elimination of 66 mass units, which corresponds to the molecular weight of hydrogen disulfide (H(2)S(2)), is observed from singly charged metal complexes of peptides with disulfide linkages. The mechanism proposed for these processes is initiated by formation of a metal-stabilized enolate at Cys, followed by cleavage of the S-C bond. Further activation of the products yields sequence information that facilitates locating the position of the disulfide linkages in the peptic digest fragments. For example, the doubly charged Ca(2+) complex of the peptic digest product GIVEQCCASVCSL/FVNQHLCGSHL yields paired products separated by 66 mass units resulting from selective SC bond cleavages at an intermolecular disulfide linkage under low-energy collision-induced dissociation. Further activation of the product comprising the A chain reveals the presence of a second disulfide bridge, an intramolecular linkage. Experimental and theoretical studies of the disulfide linked model peptides provide mechanistic details for the selective cleavage of the S-C bond. PMID:18990587

Kim, Hugh I; Beauchamp, J L

2009-01-01

97

Highly Stereoselective C--C Bond Formation by Rhodium-Catalyzed Tandem Ylide Formation/[2,3]-Sigmatropic Rearrangement Between Donor/Acceptor Carbenoids and Chiral Allylic Alcohols  

PubMed Central

The tandem ylide-formation/[2,3]-sigmatropic rearrangement between donor/acceptor rhodium-carbenoids and chiral allyl alcohols is a convergent CC bond forming process, which generates two vicinal stereogenic centers. Any of the four possible stereoisomers can be selectively synthesized by appropriate combination of the chiral catalyst Rh2(DOSP)4 and the chiral alcohol.

Li, Zhanjie; Parr, Brendan T.; Davies, Huw M. L.

2012-01-01

98

Design of leaving groups in radical c?c fragmentations: through-bond 2c-3e interactions in self-terminating radical cascades.  

PubMed

Radical cascades terminated by ?-scission of exocyclic C?C bonds allow for the formation of aromatic products. Whereas ?-scission is common for weaker bonds, achieving this reactivity for carbon-carbon bonds requires careful design of radical leaving groups. It has now been found that the energetic penalty for breaking a strong ?-bond can be compensated by the gain of aromaticity in the product and by the stabilizing two-center, three-electron "half-bond" present in the radical fragment. Furthermore, through-bond communication of a radical and a lone pair accelerates the fragmentation by selectively stabilizing the transition state. The stereoelectronic design of radical leaving groups leads to a new, convenient route to Sn-functionalized aromatics. PMID:24898770

Mondal, Sayantan; Gold, Brian; Mohamed, Rana K; Alabugin, Igor V

2014-07-01

99

A DFT study on the NHC catalysed Michael addition of enols to ?,?-unsaturated acyl-azoliums. A base catalysed C-C bond-formation step.  

PubMed

The NHC catalysed nucleophilic additions of enols to ?,?-unsaturated acyl-azolium intermediates have been investigated using DFT methods at the MPWB1K/6-31G** computational level. In the direct and the conjugate additions, formation of a hydrogen bond (HB) with the carboxyl oxygen is not sufficient to favour the C-C bond formation as a consequence of the low nucleophilic character of enols. Interestingly, when enols form a HB with the chloride counterion, the activation energies associated with the conjugate addition decrease as a consequence of the increased nucleophilic character of enols and the increased electrophilic character of the 'acyl-azolium + Cl' ion pair. Analysis of the DFT reactivity indices allows establishing a base catalysed C-C bond-formation step promoted by the chloride counterion. PMID:24343422

Domingo, Luis R; Sez, Jos A; Arn, Manuel

2014-02-14

100

Bond cleavage reactions in the tripeptide trialanine upon free electron capture  

NASA Astrophysics Data System (ADS)

In the present study we performed dissociative electron attachment (DEA) measurements with the tripeptide trialanine, C9H17N3O4, utilizing a crossed electron-molecular beam experiment with high electron energy resolution (~100 meV). Anion efficiency yields as a function of the incident electron energy are obtained for the most abundant anions up to electron energies of ~4 eV. Quantum chemical calculations are performed to determine the thermochemical thresholds for the anions observed in the measurements. There is no evidence of a molecular anion with lifetime of mass spectrometric timescales. The dehydrogenated closed shell anion (M-H)- is one of the fragment anions observed for which the calculations show that H-loss is energetically possible from carboxyl, as well as amide groups. In contrast to the dipeptide dialanine and monomer alanine the cleavage of the N-C? bond in the peptide chain is already possible by attachment of electrons at ~0 eV. Contribution to the Topical Issue "Nano-scale Insights into Ion-beam Cancer Therapy", edited by Andrey V. Solov'yov, Nigel Mason, Paulo Limo-Vieira and Malgorzata Smialek-Telega.

Puschnigg, Benjamin; Huber, Stefan E.; Scheier, Paul; Probst, Michael; Denifl, Stephan

2014-05-01

101

Cleavage of four carbon-carbon bonds during biosynthesis of the griseorhodin a spiroketal pharmacophore.  

PubMed

The rubromycins, such as gamma-rubromycin, heliquinomycin, and griseorhodin A, are a family of extensively modified aromatic polyketides that inhibit HIV reverse transcriptase and human telomerase. Telomerase inhibition crucially depends on the presence of a spiroketal moiety that is unique among aromatic polyketides. Biosynthetic incorporation of this pharmacophore into the rubromycins results in a dramatic distortion of the overall polyketide structure, but how this process is achieved by the cell has been obscure. To identify the enzymes involved in spiroketal construction, we generated 14 gene-deletion variants of the griseorhodin A biosynthetic gene cluster isolated from the tunicate-associated bacterium Streptomyces sp. JP95. Heterologous expression and metabolic analysis allowed for an assignment of most genes to various stages of griseorhodin tailoring and pharmacophore generation. The isolation of the novel advanced intermediate lenticulone, which exhibits cytotoxic, antibacterial, and elastase-inhibiting activity, provided direct evidence that the spiroketal is formed by cleavage of four carbon-carbon bonds in a pentangular polyketide precursor. This remarkable transformation is followed by an epoxidation catalyzed by an unusual cytochrome P450/NADPH:ubiquinone oxidoreductase pair that utilizes a saturated substrate. In addition, the absolute configuration of griseorhodin A was determined by quantum-chemical circular dichroism (CD) calculations in combination with experimental CD measurements. PMID:19175308

Yunt, Zeynep; Reinhardt, Kathrin; Li, Aiying; Engeser, Marianne; Dahse, Hans-Martin; Gtschow, Michael; Bruhn, Torsten; Bringmann, Gerhard; Piel, Jrn

2009-02-18

102

Antiprotozoal activity of 3'-deoxyinosine. Inverse correlation to cleavage of the glycosidic bond.  

PubMed

Two nucleosides related to the known antiprotozoal agent 1-(beta-D-ribofuranosyl)-1,5-dihydro-4H-pyrazolo-[3,4-d]pyrimidine-4-one (allopurinol riboside, 1) were prepared and evaluated against Leishmania donovani, Trypanosoma cruzi, and Trypanosoma gambiense. 3'-Deoxyinosine (2) exhibited potent antiprotozoal activity against the three protozoal pathogens with minimal toxicity for host cells. It was found to be especially effective against the Columbia strain of T. cruzi reported to be resistant to 1. The antiprotozoal activity of 2 appeared to be inversely related to the rate of cleavage of the glycosidic bond, as shown by metabolic profiles of 2 in the various pathogenic hemoflagellates and host cells. Combining the key structural elements of 1 and 2 led to the synthesis of 1-(3-deoxy-beta-D-erythro-pentofuranosyl)-1,5-dihydro-4H-pyrazolo[3,4-d] pyrimidin-4-one (3'-deoxy-allopurinol riboside, 3). which was found to be inactive as an antiprotozoal agent. PMID:1859443

Moorman, A R; LaFon, S W; Nelson, D J; Carter, H H; Marr, J J; Berens, R L

1991-07-01

103

Ruthenium-catalyzed arylation of fluorinated aromatic ketones via ortho-selective carbonfluorine bond cleavage  

Microsoft Academic Search

We report here ruthenium-catalyzed arylation of fluorinated aromatic ketones via ortho-selective carbonfluorine bond cleavage. In the presence of trimethylvinylsilane and cesium fluoride, ortho carbonfluoride bonds of aromatic ketones were phenylated by 5,5-dimethyl-2-phenyl-1,3,2-dioxaborinane using RuH2(CO)(PPh3)3 as a catalyst. Tandem CF phenylation\\/CH alkylation was observed for substrates bearing both one ortho hydrogen and one ortho fluorine atoms.

Keisuke Kawamoto; Takuya Kochi; Mitsuo Sato; Eiichiro Mizushima; Fumitoshi Kakiuchi

2011-01-01

104

HC[triple bond]P and H3C-C[triple bond]P as proton acceptors in protonated complexes containing two phosphorus bases: structures, binding energies, and spin-spin coupling constants.  

PubMed

Ab initio calculations at the MP2/aug'-cc-pVTZ level have been carried out to investigate the structures and binding energies of cationic complexes involving protonated sp, sp2, and sp3 phosphorus bases as proton donor ions and the sp-hybridized phosphorus bases H-C[triple bond]P and H3C-C[triple bond]P as proton acceptors. These proton-bound complexes exhibit a variety of structural motifs, but all are stabilized by interactions that occur through the pi cloud of the acceptor base. The binding energies of these complexes range from 6 to 15 kcal/mol. Corresponding complexes with H3C-C[triple bond]P as the proton acceptor are more stable than those with H-C[triple bond]P as the acceptor, a reflection of the greater basicity of H3C-C[triple bond]P. In most complexes with sp2- or sp3-hybridized P-H donor ions, the P-H bond lengthens and the P-H stretching frequency is red-shifted relative to the corresponding monomers. Complex formation also leads to a lengthening of the C[triple bond]P bond and a red shift of the C[triple bond]P stretching vibration. The two-bond coupling constants 2pihJ(P-P) and 2pihJ(P-C) are significantly smaller than 2hJ(P-P) and 2hJ(P-C) for complexes in which hydrogen bonding occurs through lone pairs of electrons on P or C. This reflects the absence of significant s electron density in the hydrogen-bonding regions of these pi complexes. PMID:17760429

Alkorta, Ibon; Elguero, Jos; Bene, Janet E Del

2007-10-01

105

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

PubMed

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

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

2014-01-01

106

NAD(P)H-independent asymmetric C=C bond reduction catalyzed by ene reductases by using artificial co-substrates as the hydrogen donor.  

PubMed

To develop a nicotinamide-independent single flavoenzyme system for the asymmetric bioreduction of C=C bonds, four types of hydrogen donor, encompassing more than 50 candidates, were investigated. Six highly potent, cheap, and commercially available co-substrates were identified that (under the optimized conditions) resulted in conversions and enantioselectivities comparable with, or even superior to, those obtained with traditional two-enzyme nicotinamide adenine dinucleotide phosphate (NAD(P)H)-recycling systems. PMID:24382795

Winkler, Christoph K; Clay, Dorina; Entner, Marcello; Plank, Markus; Faber, Kurt

2014-01-27

107

An approach to benzophosphole oxides through silver- or manganese-mediated dehydrogenative annulation involving C-C and C-P bond formation.  

PubMed

Benzophosphole construction was achieved through the Ag(I) -mediated dehydrogenative annulation of phenylphosphine oxides with internal alkynes in a process involving C?C and C?P bond formation. A wide range of asymmetrical phenylacetylenes could be employed and the reactions proceeded with perfect regioselectivity. Moreover, the annulation could be conducted even at room temperature when a Mn(III) promoter was used in place of Ag(I) . PMID:24127410

Unoh, Yuto; Hirano, Koji; Satoh, Tetsuya; Miura, Masahiro

2013-12-01

108

Presolvated low energy electron attachment to peptide methyl esters in aqueous solution: C-O bond cleavage at 77 K.  

PubMed

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

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

2013-03-14

109

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

PubMed

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. PMID:24050357

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

2013-09-14

110

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

PubMed Central

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

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

2013-01-01

111

Highly stable organic monolayers for reacting silicon with further functionalities: the effect of the C-C bond nearest the silicon surface.  

PubMed

Crystalline Si(111) surfaces have been alkylated in a two-step chlorination/alkylation process using various organic molecules having similar backbones but differing in their C-C bond closest to the silicon surface (i.e., C-C vs C=C vs C[triple bond]C bonds). X-ray photoelectron spectroscopic (XPS) data show that functionalization of silicon surfaces with propenyl magnesium bromide (CH3-CH=CH-MgBr) organic molecules gives nearly full coverage of the silicon atop sites, as on methyl- and propynyl-terminated silicon surfaces. Propenyl-terminated silicon surface shows less surface oxidation and is more robust against solvent attacks when compared to methyl- and propynyl-terminated silicon surfaces. We also show a secondary functionalization process of propenyl-terminated silicon surface with 4'-[3-Trifluoromethyl-3H-diazirin-3-yl]-benzoic acid N-hydroxysuccinimide ester [TDBA-OSu] cross-linker. The Si-CH=CH-CH3 surfaces thus offer a means of attaching a variety of chemical moieties to a silicon surface through a short linking group, enabling applications in molecular electronics, energy conversion, catalysis, and sensing. PMID:18803387

Puniredd, Sreenivasa Reddy; Assad, Ossama; Haick, Hossam

2008-10-15

112

Electronic effects in C-H and C-C bond activation: Reactions of excited state Cr{sup +} with propane, butane, methylpropane, and dimethylpropane  

SciTech Connect

Guided ion beam mass spectrometry is used to study the reactions of excited states of Cr{sup +} with propane, butane, methylpropane, and dimethylpropane. The effect of electronic energy as well as kinetic energy on the reactivity of atomic chromium ions is examined and reveals several interesting aspects of Cr{sup +} chemistry. The present results include the first direct evidence for reaction of the Cr{sup +}({sup 6}D) first excited state. Most interesting is the observation that the excited quartet states of Cr{sup +} react with alkanes very differently than ground-state Cr{sup +}({sup 6}S) activates only C-C bonds of alkanes, while in the present study we find that the excited Cr{sup +}({sup 6}S). Previously we found that Cr{sup +}({sup 6}S) activates only C-C bonds of alkanes, while in the present study we find that the excited Cr{sup +}({sup 4}d, {sup 4}g) states activates both C-H and C-C bonds of alkanes. The reactivity of the {sup 6}D first excited state of Cr{sup +} is similar to that of the {sup 6}S ground state. These reaction systems are currently the only examples where electronic excitation of a transition-metal ion drastically changes the products formed. 32 refs., 4 figs., 5 tabs.

Fisher, E.R.; Armentrout, P.B. [Univ. of Utah, Salt Lake City, UT (United States)

1992-03-11

113

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

National Technical Information Service (NTIS)

Robust assembly and integration technologies are critically needed for the manufacturing of heat rejection system (HRS) components for current and future space exploration missions. Active metal brazing and adhesive bonding technologies are being assessed...

M. Singh T. Shpargel J. Cerny

2006-01-01

114

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

115

Organochromium(III) macrocyclic complexes. Factors controlling homolytic vs heterolytic cleavage of the chromium-carbon bond  

Microsoft Academic Search

Kinetics studies were done of the homolytic and heterolytic cleavage reactions of the chromium-carbon bonds in the complexes RCr(L)A{sup n+} (where L = (15)aneN = 1,4,8,12-tetraazacyclopentadecane; A = HO, OH⁻). Activation parameters for homolysis of RCrL(HO){sup 2+} are as follows: 111 {plus minus} 2, 54 {plus minus} 6 (R = p-CHCHCH); 103 {plus minus} 2, 28 {plus minus} 5 (CHCH);

Shu Shi; James H. Espenson; Andreja Bakac

1990-01-01

116

Bonding and Integration of C-C Composite to Cu-Clad-Molybdenum for Thermal Management Applications  

NASA Technical Reports Server (NTRS)

Two- and three-dimensional carbon-carbon composites with either resin-derived matrix or CVI matrix were joined to Cu-clad-Mo using active Ag-Cu braze alloys for thermal management applications. The joint microstructure and composition were examined using Field-Emission Scanning Electron Microscopy and Energy-Dispersive Spectroscopy, and the joint hardness was characterized using the Knoop microhardness testing. Observations on the infiltration of the composite with molten braze, dissolution of metal substrate, and solute segregation at the C-C surface have been discussed. The thermal response of the integrated assembly is also briefly discussed.

Asthana, R.; Singh, M.; Shpargel, T.P.

2008-01-01

117

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

NASA Technical Reports Server (NTRS)

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

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

2006-01-01

118

Oxidative addition of the C?-C? bond in ?-O-4 linkage of lignin to transition metals using a relativistic pseudopotential-based ccCA-ONIOM method.  

PubMed

A multi-level multi-layer QM/QM method, the relativistic pseudopotential correlation-consistent composite approach within an ONIOM framework (rp-ccCA-ONIOM), was applied to study the oxidative addition of the C(?)-C(?) bond in an archetypal arylglycerol ?-aryl ether (?-O-4 linkage) substructure of lignin to Ni, Cu, Pd and Pt transition metal atoms. The chemically active high-level layer is treated using the relativistic pseudopotential correlation-consistent composite approach (rp-ccCA), an efficient methodology designed to reproduce an accuracy that would be obtained using the more computationally demanding CCSD(T)/aug-cc-pCV?Z-PP, albeit at a significantly reduced computational cost, while the low-level layer is computed using B3LYP/cc-pVTZ. The thermodynamic and kinetic feasibilities of the model reactions are reported in terms of enthalpies of reactions at 298 K (?H(298)) and activation energies (?H-act). The results obtained from the rp-ccCA:B3LYP hybrid method are compared to the corresponding values using CCSD(T) and several density functionals including B3LYP, M06, M06 L, B2PLYP, mPWPLYP and B2GP-PLYP. The energetics of the oxidative addition of C?C bond in ethane to Ni, Cu, Pd and Pt atoms are also reported to demonstrate that the rp-ccCA method effectively reproduces the accuracy of the CCSD(T)/aug-cc-pCV?Z method. Our results show that in the catalytic activation of the C(?)-C(?) bond of ?-O-4, the use of platinum metal catalysts will lead to the most thermodynamically favored reaction with the lowest activation barrier. PMID:22144374

Oyedepo, Gbenga A; Wilson, Angela K

2011-12-01

119

Photodissociation Dynamics of 2-BROMOETHYLNITRITE at 351 NM and C-C Bond Fission in the ? - Radical Product  

NASA Astrophysics Data System (ADS)

We used a crossed laser-molecular beam scattering experiment to investigate the primary photodissociation channels of bromoethylnitrite at 351 nm. Only the O-NO bond fission channel forming the ? -bromoethoxy radical and NO, no HBr photoelimination, was detected upon 351 nm photoexcitation,. The subsequent decomposition of the highly vibrational excited ? -bromoethoxy radical to formaldehyde + CH{_2}Br was also investigated.

Wang, Lei; Chhantyal-Pun, Rabi; Brynteson, Matt D.; Miller, Terry A.; Butler, Laurie J.

2013-06-01

120

Computational Study of Low-Temperature Catalytic C-C Bond Activation of Alkanes for Portable Power.  

National Technical Information Service (NTIS)

The development of a room temperature (<50 C) fuel cell that would use a generally available fuel such as JP8 would be most valuable. However there are no known catalysts that can selectively activate the CC bonds of such fuels at such temperature. The go...

2013-01-01

121

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

PubMed

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

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

2014-06-11

122

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

2011-09-01

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

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

2013-05-01

125

The low-energy unimolecular reaction rate constants for the gas phase, Ni+-mediated dissociation of the C-C sigma bond in acetone.  

PubMed

The time dependence of the gaseous unimolecular decomposition of the jet-cooled adduct ion, Ni+-OC(CH3)2, was monitored through selective detection of the Ni+CO fragment ion. Various resolved amounts of energy in the range 15600-18800 cm(-1) were supplied to initiate the dissociation reaction through absorption of laser photons by the title molecular complex. First-order rate constants, k(E), ranged from 113000 to 55000 s(-1) and decreased with decreasing amounts of internal excitation. The energy used to initiate the reaction is well below that required to fragment C-C sigma bonds and indicates the necessity of the Ni+ cation to induce bond activation and fragmentation. These measurements are carried out in a unique apparatus and represent the first direct kinetic study of such catalytic type reactions. PMID:19725574

Castleberry, Vanessa A; Dee, S Jason; Villarroel, Otsmar J; Laboren, Ivanna E; Frey, Sarah E; Bellert, Darrin J

2009-10-01

126

Structural and mechanistic studies of Pd-catalyzed C C bond formation: The case of carbonylation and Heck reaction  

Microsoft Academic Search

The extraordinary catalytic activity of palladium in CC bond forming processes is reviewed in respect to Heck and carbonylation reactions. Examples of catalytic systems based on palladium complexes with phosphorus ligands and phosphine-free systems and based on the Pd(0) colloid are presented. The two key steps of both reaction mechanisms: first, reduction of Pd(II) to Pd(0) complexes or Pd(0) colloids

Anna M. Trzeciak; Jzef J. Zi?kowski

2005-01-01

127

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

PubMed Central

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

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

2013-01-01

128

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

PubMed Central

The synthesis and characterization of four low-spin (carbonyl)iron(II) tetraphenylporphyrinates, [Fe(TPP)(CO)(L)], where L = 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated) and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of ?CO in toluene solution (196972 cm?1) but a large range of CO stretching frequencies in the solid-state (19261968 cm?1). The large solid-state variation results from CO interactions in the solid-state as shown by an examination of the crystal structures of the four complexes. The high precision of the four structures obtained allows us to make a number of structural and spectroscopic correlations that describe the FeCO and NImFeCO units. The values of ?CO and the FeC and CO bond distances are strongly correlated and provide a structural as well as a spectroscopic correlation of the ? back-bonding model. The interactions of CO described are closely related to the large range of CO stretching frequencies observed in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO).

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

2006-01-01

129

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 CS 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 CS 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 CS 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 CS 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 CS 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 CS 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

130

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

131

Million-fold activation of the [Fe2(?-O)2] diamond core for C-H bond cleavage  

PubMed Central

In biological systems, the cleavage of strong CH bonds is often carried out by iron centers such as the methane monooxygenase in methane hydroxylation through dioxygen activation mechanisms. High valent species with [Fe2(?-O)2] diamond cores are thought to act as the oxidizing moieties, but the synthesis of complexes that cleave strong CH bonds efficiently has remained a challenge. We report here the conversion of a synthetic complex with a valence-delocalized [Fe3.5(?-O)2Fe3.5]3+ diamond core (1) into a complex with a valence-localized [HO-FeIII-O-FeIV=O]2+ open core (4), which cleaves CH bonds over million-fold faster. This activity enhancement results from three factors: the formation of a terminal oxoiron(IV) moiety, the conversion of the low-spin (S = 1) FeIV=O center to a high-spin (S = 2) center, and the concentration of the oxidizing capability to the active terminal oxoiron(IV) moiety. This suggests that similar isomerization strategies might be employed by nonheme diiron enzymes.

Xue, Genqiang; De Hont, Raymond; Munck, Eckard; Que, Lawrence

2010-01-01

132

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 Brnsted 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

Bojarov, 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

133

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

PubMed Central

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

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

2013-01-01

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

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

PubMed Central

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

2014-01-01

137

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

PubMed

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

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

2014-05-20

138

Microwave and quantum chemical study of propa-1,2-dienyl thiocyanate (H2C=C=CHSC triple bond N).  

PubMed

The microwave spectrum of propa-1,2-dienyl thiocyanate (H2C=C=CHSC triple bond N) has been investigated in the 24-40 and 50-80 GHz spectral regions. The spectrum of one conformer was assigned. This rotamer, which has a C-C-S-C dihedral angle of about 134 degrees from synperiplanar, is at least 2 kJ/mol more stable than any other form. Two vibrationally excited states assumed to belong to the first excited state of the C-S torsional vibration and to a low bending mode were assigned. Their frequencies were determined to be 62(20) and 155(30) cm-1, respectively. The microwave work has been augmented by ab initio calculations at the MP2/aug-cc-pVTZ and density functional theory calculations at the B3LYP/aug-cc-pVTZ level of theory. The B3LYP calculations are generally in better agreement with the observations than the MP2 calculations. PMID:17388347

Mllendal, Harald; Cole, George C; Guillemin, Jean-Claude

2007-04-01

139

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

SciTech Connect

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

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

1996-10-16

140

Selective bond cleavage in potassium collisions with pyrimidine bases of DNA.  

PubMed

Electron transfer in alkali-molecule collisions to gas phase thymine and uracil yielding H- formation is selectively controlled in the energy range between 5.3 and 66.1 eV. By tuning the collision energy, electron transfer from the alkali to partly deuterated thymine, methylated thymine at the N1 and methylated uracil at the N3 positions, H- loss proceeds not only through the breaking of the (C-H) against (N-H) bonds but also through N1 against N3 sites. Such selectivity, as far as bond and site are concerned, is here reported for the first time by electron transfer induced dissociation experiments in alkali-molecule collisions. PMID:23383904

Almeida, Diogo; Ferreira da Silva, Filipe; Garca, Gustavo; Limo-Vieira, Paulo

2013-01-11

141

Selective Bond Cleavage in Potassium Collisions with Pyrimidine Bases of DNA  

NASA Astrophysics Data System (ADS)

Electron transfer in alkali-molecule collisions to gas phase thymine and uracil yielding H- formation is selectively controlled in the energy range between 5.3 and 66.1 eV. By tuning the collision energy, electron transfer from the alkali to partly deuterated thymine, methylated thymine at the N1 and methylated uracil at the N3 positions, H- loss proceeds not only through the breaking of the (C-H) against (N-H) bonds but also through N1 against N3 sites. Such selectivity, as far as bond and site are concerned, is here reported for the first time by electron transfer induced dissociation experiments in alkali-molecule collisions.

Almeida, Diogo; Ferreira da Silva, Filipe; Garca, Gustavo; Limo-Vieira, Paulo

2013-01-01

142

CuI-Catalyzed C-N Bond Formation and Cleavage for the Synthesis of Benzimidazo[1,2-a]quinazoline Derivatives.  

PubMed

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

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

2014-06-20

143

Single-Pulse Shock-Tube Studies on C-NO2 Bond Cleavage during the Decomposition of Some Nitro Aromatic Compounds.  

National Technical Information Service (NTIS)

Nitrobenzene (NB), o-nitrotoluene (o-NT), and p-nitrotoluene (p-NT) have been decomposed in single-pulse shock-tube experiments. The rate expressions for C-NO2 bond cleavage have been found. These lead to a recombination rate of phenyl + NO2 in the 400,00...

W. Tsang D. Robaugh W. G. Mallard

1986-01-01

144

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

145

Characterization of carbon-sulfur bond cleavage by axenic and mixed cultures of Rhodococcus rhodochrous IGTS8  

SciTech Connect

Growth assays reveal that Rhodococcus rhodochrous IGTS8 can utilize a wide range of organosulfur compounds as the sole source of sulfur. Compounds that are utilized include thiophenes, sulfides, disulfides, mercaptans, sulfoxides, and sulfones. None of the organosulfur compounds tested can serve as a carbon source. A convenient spectrophotometric assay (Gibbs assay) based on the chromogenic reaction of 2,6-dichloroquinone-4-chloroimide with aromatic hydroxyl groups was developed and used in conjunction with GC/MS analysis to examine the kinetics of carbon-sulfur bond cleavage by axenic and mixed cell cultures of Rhodococcus rhodochrous IGTS8. The desulfurization trait is expressed at uniform levels during the mid-exponential phase, reaches a maximum during idiophase, and then declines in stationary-phase cells. Desulfurization rates for dibenzothiophene (DBT) range from 8 to 15 {mu}M of DBT/10{sup 12} cells/hour. Mixtures of genetically marked Rhodococcus rhodochrous IGTS8 and an organisms incapable of cleaning carbon-sulfur bonds in relevant test compounds, Enterobacter cloacae, were prepared in ratios that varied over six orders of magnitude. Growth studies revealed that Enterobacter cloacae was able to gain access to sulfur liberated from organosulfur compounds by IGTS8; however, cell-to-cell contact was required. These data also indicate that the desulfurization activity of IGTS8 cells in mixed cultures may be as much as 200-fold higher than in axenic cultures.

Kayser, K.J.; Bielaga, B.A.; Jackowski, K.; Oduson, O.; Kilbane, J. II

1992-12-31

146

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

2013-07-01

147

Terminal titanium-ligand multiple bonds. Cleavages of C=O and C=S double bonds with Ti imido complexes.  

PubMed

Treatment of (t-)BuN=TiCl(2)Py(3) with 2 equiv lithium ketiminate compound, Li[OCMeCHCMeN(Ar)] (where Ar = 2,6-diisopropylphenyl), in toluene at room temperature gave (t-)BuN=Ti[OCMeCHCMeN(Ar)](2) (1) in high yield. The reaction of 1 with phenyl isocyanate at room-temperature resulted in imido ligand exchange producing PhN=Ti[OCMeCHCMeN(Ar)](2) (2). Compound 1 decomposed at 90 degrees C to form a terminal titanium oxo compound O=Ti[OCMeCHCMeN(Ar)](2) (3) and (t-)BuNHCMeCHCMeNAr (4). Also, the compound 3 could be obtained by reacting 1 with CO(2) under mild condition. Similarly, while 1 reacts with an excess of carbon disulfide, a novel terminal titanium sulfido compound S=Ti[OCMeCHCMeN(Ar)](2) (5) was formed via a C=S bond breaking reaction. A novel titanium isocyanate compound Ti[OCMeCHCMeN(Ar)](2)(NCO)(OEt) (6) was formed on heating 1 with 1 equiv of urethane, H(2)NCOOEt. Compounds 1-6 have been characterized by (1)H and (13)C NMR spectroscopies. The molecular structures of 1, 3, 5, and 6 were determined by single-crystal X-ray diffraction. A theoretical calculation predicted that the cleavage of the C-S double bonds for carbon disulfide with the Ti=N bond of compound 1 was estimated at ca. 21.8 kcal.mol(-1) exothermic. PMID:15476379

Hsu, Shih-Hsien; Chang, Jr-Chiuan; Lai, Chun-Liang; Hu, Ching-Han; Lee, Hon Man; Lee, Gene-Hsiang; Peng, Shie-Ming; Huang, Jui-Hsien

2004-10-18

148

Photoinduced cleavage of the carbon-indium bond in ethylindium(III) tetraphenylporphyrin  

SciTech Connect

The photochemistry of ethylindium(III) tetraphenylporphyrin, C/sub 2/H/sub 5/In/sup III/TPP, in benzene solutions was studied by steady light and laser flash photolysis. From the analysis of the products, the C/sub 2/H/sub 5/-In bond in C/sub 2/H/sub 5/In/sup III/TPP was confirmed to be photodissociated. The quantum yields for the photodecomposition of C/sub 2/H/sub 5/In/sup III/TPP were found to be diminished by the addition of ferrocene. However, 2,4,7-trinitro-9-fluorenone, a strong electron acceptor, was found to increase the quantum yields. The laser photolysis studies revealed that the photoreaction of C/sub 7/H/sub 5/In/sup III/TPP occurs via the triplet state originating from the porphyrin ligand.

Hoshino, M.; Ida, H.; Yasufuku, K.; Tanaka, K.

1986-08-14

149

Regioselective functionalization of iminophosphoranes through Pd-mediated C-H bond activation: C-C and C-X bond formation.  

PubMed

The orthopalladation of iminophosphoranes [R(3)P=N-C(10)H(7)-1] (R(3) = Ph(3) 1, p-Tol(3) 2, PhMe(2) 3, Ph(2)Me 4, N-C(10)H(7)-1 = 1-naphthyl) has been studied. It occurs regioselectively at the aryl ring bonded to the P atom in 1 and 2, giving endo-[Pd(?-Cl)(C(6)H(4)-(PPh(2=N-1-C(10)H(7))-2)-?-C,N](2) (5) or endo-[Pd(?-Cl)(C(6)H(3)-(P(p-Tol)(2)=N-C(10)H(7)-1)-2-Me-5)-?-C,N](2) (6), while in 3 the 1-naphthyl group is metallated instead, giving exo-[Pd(?-Cl)(C(10)H(6)-(N=PPhMe(2))-8)-?-C,N](2) (7). In the case of 4, orthopalladation at room temperature affords the kinetic exo isomer [Pd(?-Cl)(C(10)H(6)-(N=PPh(2)Me)-8)-?-C,N](2) (11exo), while a mixture of 11exo and the thermodynamic endo isomer [Pd(?-Cl)(C(6)H(4)-(PPhMe=N-C(10)H(7)-1)-2)-?-C,N](2) (11endo) is obtained in refluxing toluene. The heating in toluene of the acetate bridge dimer [Pd(?-OAc)(C(10)H(6)-(N=PPh(2)Me)-8)-?-C,N](2) (13exo) promotes the facile transformation of the exo isomer into the endo isomer [Pd(?-OAc)(C(6)H(4)-(PPhMe=N-C(10)H(7)-1)-2)-?-C,N](2) (13endo), confirming that the exo isomers are formed under kinetic control. Reactions of the orthometallated complexes have led to functionalized molecules. The stoichiometric reactions of the orthometallated complexes [Pd(?-Cl)(C(10)H(6)-(N=PPhMe(2))-8)-?-C,N](2) (7), [Pd(?-Cl)(C(6)H(4)-(PPh(2)[=NPh)-2)](2) (17) and [Pd(?-Cl)(C(6)H(3)-(C(O)N=PPh(3))-2-OMe-4)](2) (18) with I(2) or with CO results in the synthesis of the ortho-halogenated compounds [PhMe(2)P=N-C(10)H(6)-I-8] (19), [I-C(6)H(4)-(PPh(2)=NPh)-2] (21) and [Ph(3)P=NC(O)C(6)H(3)-I-2-OMe-5] (23) or the heterocycles [C(10)H(6)-(N=PPhMe(2))-1-(C(O))-8]Cl (20), [C(6)H(5)-(N=PPh(2)-C(6)H(4)-C(O)-2]ClO(4) (22) and [C(6)H(3)-(C(O)-1,2-N-PPh(3))-OMe-4]Cl (24). PMID:20927428

Aguilar, David; Navarro, Rafael; Soler, Tatiana; Urriolabeitia, Esteban P

2010-11-21

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

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

2013-08-20

152

Mild and Highly Selective Ultrasound-Promoted Zinc/Acetic Acid Reduction of C = C Bonds in alpha, Beta-Unsaturated gamma-Dicarbonyl Compounds. (Reannouncement with New Availability Information).  

National Technical Information Service (NTIS)

Sonication of various, alpha, Beta-unsaturated Gamma-diketones, quinones, alpha-Beta-unsaturated Gamma-diacids or Gamma-diesters with powdered zinc and acetic acid at room temperature results in selective reduction of the C=C bond in each case within ca. ...

A. P. Marchand M. Reddy

1991-01-01

153

Phosphine nickel complex as catalyst in reactions of organic bromides RBr with {alpha},{beta}-unsaturated ketones, nitriles, and esters. Conjugate addition of R group and H atom across the C=C bond  

SciTech Connect

Reactions of organic bromides, particularly of secondary and tertiary alkyl bromides, with {alpha},{beta}-unsaturated carbonyl compounds and acrylonitrile in the presence of the catalytic system nickel complex-zinc. The products correspond to the conjugate addition of the organic moiety of the bromide and a hydrogen atom across the C=C double bond of the unsaturated substrate. 9 refs., 1 tab.

Lebedev, S.A.; Lopatina, V.S.; Petrov, E.S.

1995-09-10

154

Indium(III) chloride-catalyzed oxidative cleavage of carboncarbon multiple bonds by tert-butyl hydroperoxide in watera safer alternative to ozonolysis  

Microsoft Academic Search

An efficient and general method for the oxidative cleavage of alkenes and alkynes using tert-butyl hydroperoxide and indium(III) chloride as catalyst in water to give the corresponding carboxylic acids or ketones has been achieved. The reaction conditions are compatible with sensitive moieties such as peptide bonds, tert-butyl carboxylic esters and N-Boc-protected tryptophan. The catalyst could be recycled.

Brindaban C. Ranu; Sukalyan Bhadra; Laksmikanta Adak

2008-01-01

155

Sensitivity of a Strained C-C Single Bond to Charge Transfer: Redox Activity in Mononuclear and Dinuclear Ruthenium Complexes of Bis(arylimino)acenaphthene (BIAN) Ligands.  

PubMed

The new compounds [Ru(acac)2(BIAN)], BIAN = bis(arylimino)acenaphthene (aryl = Ph (1a), 4-MeC6H4 (2a), 4-OMeC6H4 (3a), 4-ClC6H4 (4a), 4-NO2C6H4 (5a)), were synthesized and structurally, electrochemically, spectroscopically, and computationally characterized. The ?-diimine sections of the compounds exhibit intrachelate ring bond lengths 1.304 < d(CN) < 1.334 and 1.425 < d(CC) < 1.449 , which indicate considerable metal-to-ligand charge transfer in the ground state, approaching a Ru(III)(BIAN(-)) oxidation state formulation. The particular structural sensitivity of the strained peri-connecting C-C bond in the BIAN ligands toward metal-to-ligand charge transfer is discussed. Oxidation of [Ru(acac)2(BIAN)] produces electron paramagnetic resonance (EPR) and UV-vis-NIR (NIR = near infrared) spectroelectrochemically detectable Ru(III) species, while the reduction yields predominantly BIAN-based spin, in agreement with density functional theory (DFT) spin-density calculations. Variation of the substituents from CH3 to NO2 has little effect on the spin distribution but affects the absorption spectra. The dinuclear compounds {(?-tppz)[Ru(Cl)(BIAN)]2}(ClO4)2, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine; aryl (BIAN) = Ph ([1b](ClO4)2), 4-MeC6H4 ([2b](ClO4)2), 4-OMeC6H4 ([3b](ClO4)2), 4-ClC6H4 ([4b](ClO4)2), were also obtained and investigated. The structure determination of [2b](ClO4)2 and [3b](ClO4)2 reveals trans configuration of the chloride ligands and unreduced BIAN ligands. The DFT and spectroelectrochemical results (UV-vis-NIR, EPR) indicate oxidation to a weakly coupled Ru(III)Ru(II) mixed-valent species but reduction to a tppz-centered radical state. The effect of the ? electron-accepting BIAN ancillary ligands is to diminish the metal-metal interaction due to competition with the acceptor bridge tppz. PMID:24983788

Mondal, Prasenjit; Agarwala, Hemlata; Jana, Rahul Dev; Plebst, Sebastian; Grupp, Anita; Ehret, Fabian; Mobin, Shaikh M; Kaim, Wolfgang; Lahiri, Goutam Kumar

2014-07-21

156

Low-temperature N-O bond cleavage of nitrogen monoxide in heterometallic carbonyl complexes. An experimental and theoretical study.  

PubMed

The reaction of Na[RuCp(CO) 2] with [MnCp'(CO) 2(NO)]BF 4 gives the corresponding heterometallic derivative [MnRuCpCp'(mu-CO) 2(CO)(NO)] (Cp = eta (5)-C 5H 5; Cp' = eta (5)-C 5H 4Me). In contrast, the group 6 metal carbonyl anions [MCp(CO) 2L] (-) (M = Mo, W; L = CO, P(OMe) 3, PPh 3) react with the Mn and Re complexes [M'Cp'(CO) 2(NO)]BF 4 to give the heterometallic derivatives [MM'CpCp'(mu-N)(CO) 3L] having a nitride ligand linearly bridging the metal centers (W-N = 1.81(3) A, N-Re = 1.97(3) A, W-N-Re = 179(1) (o), in [WReCpCp'(mu-N)(CO) 3{P(OMe) 3}]). Density-functional theory calculations on the reactions of [WCp(CO) 3] (-) and [RuCp(CO) 2] (-) with [MnCp(CO) 2(NO)] (+) revealed a comparable qualitative behavior. Thus, two similar and thermodynamically allowed reaction pathways were found in each case, one implying the displacement of CO from the cation and formation of a metal-metal bond, the other implying the cleavage of the N-O bond of the nitrosyl ligand and release of a carbonyl from the anion as CO 2. The second pathway is more exoergonic and is initiated through an orbitally controlled attack of the anion on the N atom of the NO ligand in the cation. In contrast, the first pathway is initiated through a charge-controlled attack of the anion to the C atom of a CO ligand in the cation. The CO 2-elimination pathway requires at the intermediate stages a close approach of the NO and CO ligands, which is more difficult for the Ru compound because of its lower coordination number (compared to W). This effect, when combined with a stronger stabilization of the initial intermediate in the Ru reaction, makes the CO 2-elimination pathway slower in that case. PMID:18928276

Garca, M Esther; Meln, Sonia; Ruiz, Miguel A; Lpez, Ramn; Sordo, Toms; Marchi, Luciano; Tiripicchio, Antonio

2008-11-17

157

Unified view of oxidative C-H bond cleavage and sulfoxidation by a nonheme iron(IV)-oxo complex via Lewis acid-promoted electron transfer.  

PubMed

Oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by a nonheme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), were remarkably enhanced by the presence of triflic acid (HOTf) and Sc(OTf)3 in acetonitrile at 298 K. All the logarithms of the observed second-order rate constants of both the oxidative C-H bond cleavage and sulfoxidation reactions exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes between PCET and MCET were taken into account, respectively. Thus, the mechanisms of both the oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by [(N4Py)Fe(IV)(O)](2+) in the presence of HOTf and Sc(OTf)3 have been unified as the rate-determining electron transfer, which is coupled with binding of [(N4Py)Fe(IV)(O)](2+) by proton (PCET) and Sc(OTf)3 (MCET). There was no deuterium kinetic isotope effect (KIE) on the oxidative C-H bond cleavage of toluene via the PCET pathway, whereas a large KIE value was observed with Sc(OTf)3, which exhibited no acceleration of the oxidative C-H bond cleavage of toluene. When HOTf was replaced by DOTf, an inverse KIE (0.4) was observed for PCET from both toluene and [Ru(II)(bpy)3](2+) (bpy =2,2'-bipyridine) to [(N4Py)Fe(IV)(O)](2+). The PCET and MCET reactivities of [(N4Py)Fe(IV)(O)](2+) with Brnsted acids and various metal triflates have also been unified as a single correlation with a quantitative measure of the Lewis acidity. PMID:24605985

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

2014-04-01

158

KO(t)Bu-mediated synthesis of dimethylisoindolin-1-ones and dimethyl-5-phenylisoindolin-1-ones: selective C-C coupling of an unreactive tertiary sp3 C-H bond.  

PubMed

A new reaction for the synthesis of dimethylisoindolinones has been presented from 2-halo-N-isopropyl-N-alkylbenzamide substrates and KO(t)Bu by the selective C-C coupling of an unreactive tertiary sp(3) C-H bond. The reaction manifested an excellent selectivity toward a tertiary sp(3) C-H bond over primary or sec C-H bond. Moreover, biaryl C-C coupling along with alkyl-aryl C-C coupling can be achieved in one pot using dihalobenzamides for the synthesis of biaryl 5-phenylisoindolin-1-ones. It seems that the reaction proceeds via a radical pathway in which the aryl radical translocates via 1,5-hydrogen atom transfer (HAT), forming a tertiary alkyl carbon-centered radical. The generated tertiary alkyl radical could attack the benzamide ring in a 5-exo/endo-trig manner followed by the release of an electron and a proton, leading to a five-membered isoindolinone ring. HAT seems to be responsible for the selective functionalization of the tertiary alkyl group over primary and secondary C-H bonds. PMID:24597670

Bhakuni, Bhagat Singh; Yadav, Abhimanyu; Kumar, Shailesh; Patel, Saket; Sharma, Shubham; Kumar, Sangit

2014-04-01

159

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

160

Discovery and synthetic applications of novel silicon-carbon bond cleavage reactions based on the coordination number change of organosilicon compounds  

PubMed Central

Some synthetically useful transformations of organosilicon compounds have been developed since the mid 1970s, based on the new concept that the silicon-carbon bonds are activated toward electrophilic cleavage via the formation of penta- and hexa-coordinate species. This review mainly consists of the following aspects: (1) a general concept for the activation of the silicon-carbon bond via penta- and hexa-coordinate species, (2) synthetic application of hexa-coordinate organopentafluorosilicates, and (3) development of the H2O2 oxidation of the silicon-carbon bond and its synthetic applications via the intramolecular hydrosilylation, silicon-tethered intramolecular radical cyclization and Diels-Alder reaction, and some silicon-containing organometallic reagents for nucleophilic hydroxymethylation and hydroxyallylation synthons.

TAMAO, Kohei

2008-01-01

161

Assessment of the Intermediacy of Arylpalladium Carboxylate Complexes in the Direct Arylation of Benzene: Evidence for C-H Bond Cleavage by "Ligandless" Species  

PubMed Central

Palladium-catalyzed direct arylations of benzene have been proposed to occur by the generation of a phosphine-ligated arylpalladium pivalate complex LPd(Ar)(OPiv) and reaction of this complex with benzene. We have isolated an example of the proposed intermediate and evaluated whether this complex reacts with benzene to form the biaryl products of the catalytic process. In contrast to the proposed mechanism, no biaryl product was formed from cleavage of the benzene C-H bond by LPd(Ar)(OPiv). However, reactions of LPd(Ar)OPiv with benzene and additives that displace or consume the phosphine ligand formed the arylated products in good yield, suggesting that a ligandless arylpalladium(II) carboxylate complexes undergoes the C-H cleavage step. Consistent with this conclusion, we found that reactions catalyzed by Pd(OAc)2 without ligand occur faster than, and with comparable selectivities to, reactions catalyzed by Pd(OAc)2 and phosphine ligand.

Tan, Yichen; Hartwig, John F.

2011-01-01

162

Electrochiroptical response of 2,2'-(2,2-diarylethenyl)binaphthyl-type electron donors that undergo reversible C-C bond formation/breaking upon two-electron transfer.  

PubMed

2,2'-[2,2-Bis(4-dimethylaminophenyl)ethenyl]biphenyl (1) is a strong electron donor that undergoes oxidative C-C bond formation to give a stable dication rac-2(2+), the 9,10-dihydrophenanthrene derivative substituted with two bis(4-dimethylaminophenyl)methylium chromophores. This dication salt regenerates the starting diolefin 1 by reductive C-C bond breaking, thus realizing a new electrochromic system with high electrochemical bistability and a vivid change in color from yellow to deep blue. Similarly, the binaphthylic diolefin rac-3 and the helicene-type dication rac-4(2+) are interconvertible upon two-electron transfer. Both the UV-vis and CD spectra changed drastically upon electrochemical transformation between optically pure 3 and 4(2+), which represents a new electrochiroptical system. PMID:12919023

Higuchi, Hiroki; Ohta, Eisuke; Kawai, Hidetoshi; Fujiwara, Kenshu; Tsuji, Takashi; Suzuki, Takanori

2003-08-22

163

Heterolytic cleavage of hydrogen by an iron hydrogenase model: an Fe-H???H-N dihydrogen bond characterized by neutron diffraction.  

PubMed

Hydrogenase enzymes in nature use hydrogen as a fuel, but the heterolytic cleavage of H-H bonds cannot be readily observed in enzymes. Here we show that an iron complex with pendant amines in the diphosphine ligand cleaves hydrogen heterolytically. The product has a strong Fe-H???H-N dihydrogen bond. The structure was determined by single-crystal neutron diffraction, and has a remarkably short H???H distance of 1.489(10)? between the protic N-H(?+) and hydridic Fe-H(?-) part. The structural data for [Cp(C5F4N)FeH(P(tBu)2N(tBu)2H)](+) provide a glimpse of how the H-H bond is oxidized or generated in hydrogenase enzymes. These results now provide a full picture for the first time, illustrating structures and reactivity of the dihydrogen complex and the product of the heterolytic cleavage of H2 in a functional model of the active site of the [FeFe] hydrogenase enzyme. PMID:24757087

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

2014-05-19

164

Silver(I) double and multiple salts containing the 1,3-butadiynediide dianion: coordination diversity and assembly with the supramolecular synthon Ag4[cap]C[triple bond]C-C[triple bond]C[cap]Ag4.  

PubMed

A series of 13 silver(I) double and multiple salts containing 1,3-butadiynediide, C4(2-), were synthesized by dissolving the silver carbide Ag2C4 in a concentrated aqueous solution of one or more of the silver salts AgNO3, AgCF3CO2, AgC2F5CO2, AgF, AgBF4, and AgPF6. The 1,3-butadiynediide anion invariably adopts a mu4,mu4 coordination mode in these compounds, which indicates that the Ag4[cap]C[triple bond]C-C[triple bond]C[cap]Ag4 moiety can be used as a new type of metalloligand supramolecular synthon for the construction of coordination networks. Fine-tuning with various ancillary anionic ligands caused the Ag4 aggregate at each ethynide terminus to adopt a butterfly-shaped, planar, or barblike configuration, within which the silver-ethynide interactions can be classified into three types: sigma, pi, and mixed (sigma,pi). The effect of coexisting nitrile ligands and quaternary ammonium salts on supramolecular assembly with the above synthon was also explored. The hydrolysis of PF6(-) and BF4(-) led to the formation of the quadruple salt Ag2C4 x 4 AgNO3 x AgPF2O2 x Ag3PO4 and a novel (F)2(H2O)18 hydrogen-bonded tape in the triple salt Ag2C4 x 2 AgF x 10 AgC2F5CO2 x CH3CN x 12 H2O, respectively. The largest silver-ethynide cluster aggregate described to date, (C4)3@Ag18, occurs in 3 Ag2C4 x 12 AgC2F5CO2 x 5 [(BnMe3N)C2F5CO2] x 4 H2O (Bn = benzyl). PMID:17768727

Zhao, Liang; Du, Miao; Mak, Thomas C W

2007-10-01

165

Direct amination of olefins through sequential triazolinedione ene reaction and carbanion-assisted cleavage of the N-N urazole bond  

PubMed

[formula: see text] Allylic amines 5 are obtained in 30-55% overall yields by the base-catalyzed hydrolysis of trialkylated allylic urazoles 3; the latter are prepared by the TAD ene reaction of the appropriate olefin and further N-alkylation with alpha-bromoacetophenone. The proposed mechanism for this novel urazole rupture is based on the generation of a carbanion adjacent to the hydrazide functionality, which induces urazole ring-opening by cleavage of the N-N bond. PMID:10810731

Adam; Pastor; Wirth

2000-05-01

166

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

167

Kinetics, thermodynamics, and effect of BPh3 on competitive C-C and C-H bond activation reactions in the interconversion of allyl cyanide by [Ni(dippe)].  

PubMed

Reaction of [(dippe)Ni(micro-H)](2) with allyl cyanide at low temperature quantitatively generates the eta(2)-olefin complex (dippe)Ni(CH(2)=CHCH(2)CN) (1). At ambient temperature or above, the olefin complex is converted to a mixture of C-CN cleavage product (dippe)Ni(eta(3)-allyl)(CN) (3) and the olefin-isomerization products (dippe)Ni(eta(2)-crotonitrile) (cis- and trans-2), which form via C-H activation. The latter are the exclusive products at longer reaction times, indicating that C-CN cleavage is reversible and the crotononitrile complexes 2 are more thermodynamically stable than eta(3)-allyl species 3. The kinetics of this reaction have been followed as a function of temperature, and rate constants have been extracted by modeling of the reaction. The rate constants for C-CN bond formation (the reverse of C-CN cleavage) show a stronger temperature dependence than those for C-CN and C-H activation, making the observed distribution of C-H versus C-CN cleavage products strongly temperature-dependent. The activation parameters for the C-CN formation step are also quite distinct from those of the C-CN and C-H cleavage steps (larger DeltaH(++) and positive DeltaS(++)). Addition of the Lewis acid BPh(3) to 1 at low temperature yields exclusively the C-CN activation product (dippe)Ni(eta(3)-allyl)(CNBPh(3)) (4). Independently prepared (dippe)Ni(crotononitrile-BPh(3)) (cis- and trans-7) does not interconvert with 4, indicating that 4 is the kinetic product of the BPh(3)-mediated reaction. On standing in solution at ambient temperature, 4 decomposes slowly to complex 5, with structure [(dippe)Ni(eta(3)-allyl)(N triple bond C-BPh(3)), while addition of a second equivalent of BPh(3) immediately produces [(dippe)Ni(eta(3)-allyl)](+)[Ph(3)BC triple bond NBPh(3)](-) (6). Comparison of the barriers to pi-sigma allyl interconversion (determined via dynamic (1)H NMR spectroscopy) for all of the eta(3)-allyl complexes reveals that axial cyanide ligands facilitate pi-sigma interconversion by moving into the P(2)Ni square plane when the allyl group is sigma-bound. PMID:15025492

Brunkan, Nicole M; Brestensky, Donna M; Jones, William D

2004-03-24

168

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

PubMed Central

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

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

2014-01-01

169

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

170

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

171

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

172

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

SciTech Connect

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

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

1981-11-12

173

Cleavage of the Nb=O bond of oxoniobium(V) porphyrins. Synthesis and characterization of novel niobium(V) porphyrins with two distinct catechols  

SciTech Connect

A novel catecholato complex, Nb{sup v}(tpp)(cat)(Hcat), where cat and Hcat are two distinct catechol ligands (a bidentate catecholate dianion and a monodentate catecholate monoanion, respectively) and tpp is 5, 10, 15, 20-tetaphenylporphyrin dianion, has been isolated in the reaction of Nb{sup v} (tpp)(O)(AcO) with catechol, where AcO is an acetatoligand. Its molecular structure has been determined by X-ray crystallography. Crystal data: monoclinic, space group P2{sub 1}/c, Z = 4, a = 14.592(3) {Angstrom}, b = 23.46(1) {Angstrom}, c = 14.415(4) {Angstrom}, {beta} = 100.95(2){degrees}, R = 0.079. The heptacoordinate niobium atom is displaced by 1.02 {Angstrom} from the mean plane of the four nitrogen atoms. The structure of the complex in solution and the mechanism of the Nb=O cleavage were investigated by means of {sup 1}H-NMR spectroscopy. The bidentate catechol is oriented in C{sub s} symmetry with respect to the porphyrin plane, and the monodentate catechol is located perpendicularly to both the bidentate catechol and the porphyrin plane. Two intermediates with the bidentate catechol were observed after addition of 2 equiv of catechol to Nb(tmp)(O)(AcO) at -30 {degrees}C, where tmp denotes the 5,10,15,20-tetramesitylporphyrin dianion. These intermediates were determined to be Nb(tmp)(cat)(OH) and Nb(tmp)(cat)(AcO). Thus, the Nb=O bond of Nb(tmp)(O)(AcO) was easily cleaved to create the two intermediates. The authors propose a unique route to the Nb=O cleavage that involves an intramolecular electron transfer from the catechol ligand coordinated at the first stage through a ligand exchange with AcO. Both protonation and electron transfer to the Nb=O moiety play important roles in the Nb=O cleavage.

Kurihara, Masato; Kotoh, Noriyuki; Kojima, Takahiko [Kyushu Univ., Fukuoka (Japan)] [and others

1995-09-13

174

CeCl37H2O catalyzed C-C and C-N bond-forming cascade cyclization with subsequent side-chain functionalization and rearrangement: a domino approach to pentasubstituted pyrrole analogues.  

PubMed

CeCl(3)7H(2)O is found as an efficient catalyst for new intermolecular domino reactions of three-, four- and seven-component assemblies of common precursors under benign reaction conditions. Generation of enaminioesters from ?-keto esters and primary amines, activation of their allylic sp(3) C-H, vinylic sp(2) C-H and N-H bonds, multi C-C and C-N bond-forming cascade cyclization with 1,2-diketones and subsequent side-chain alkylation have been developed to construct functionalized pentasubstituted pyrroles and their chiral analogues. The scope of the domino reaction is successfully explored toward synthesis of highly aryl-substituted pyrroles, pentasubstituted pyrroles bearing C2-olefinic side-chain and spiro-2-pyrrolinones and their chiral analogues via unusual side-chain amination, elimination and ring contraction. The new domino reaction is operationally simple, robust, substrate specific, selective and high yielding. PMID:23113545

Dhara, Dipankar; Gayen, Krishnanka S; Khamarui, Saikat; Pandit, Palash; Ghosh, Sukla; Maiti, Dilip K

2012-11-16

175

Synthesis of ()-aphanorphine: a new approach to tricyclic 3-benzazepine scaffold using two radical CC bond-forming reactions  

Microsoft Academic Search

An expeditious approach to ()-aphanorphine has been established using readily available starting materials. The present synthesis relies on the direct assembly between N-methylpyrrolidone (NMP) and 2-bromoanisaldehyde, which takes place through Et3B\\/air-mediated transformation of the ?-nitrogen-substituted sp3CH bond, and features a new design concept for the synthesis of the tricyclic 3-benzazepine skeleton.

Takehiko Yoshimitsu; Chie Atsumi; Emiko Iimori; Hiroto Nagaoka; Tetsuaki Tanaka

2008-01-01

176

The Catalytic Conversion of C1-Cn Hydrocarbons to Olefins and Hydrogen: Microwave-Assisted C-C and C-H Bond Activation  

SciTech Connect

The gas phase carbon catalyzed microwave promoted conversion of methane to ethylene, ethane and acetylene and hydrogen is reported. A selection of C1-C4 hydrocarbons, hexadecane, and a cyclic hydrocarbon, cyclodecane, were also subjected to microwave conversion, resulting primarily in a-olefins, ethylene and hydrogen. For methane conversion, the products are reminiscent of those found in methane pyrolysis. Microwave induced cleavage of the liquid hydrocarbons provides conditions for the stabilization, by rapid thermal quenching in ambient temperature liquid reagent, of products such as terminal olefins that would be labile under conventional (thermal bath) pyrolysis reaction conditions. The reactions of long chain acyclic and cyclic hydrocarbons involve high temperatures in the region of the spark leading to a cascade of unimolecular scission reactions from initially formed biradicals from cycloalkanes or radical pairs from linear alkanes, largely to the exclusion of intermolecular radical-radical and radical-molecule reactions. The observed products are discussed in terms of the thermochemistry and dynamics of high temperature unimolecular biradical and radical decomposition reactions, and mechanisms involving reactive surface metal sites. The reaction rates of alkanes were found to increase with the molecular weight of the reactants. Mechanistic pathways consistent with these results are discussed.

Tanner, Dennis D.; Kandanarachchi, Pramod; Ding, Qizhu; Shao, Huawu; Vizitiu, Despina; Franz, James A.

2001-01-17

177

Reactions of a tungsten-germylyne complex with ?,?-unsaturated ketones: complete cleavage of the W?Ge bond and formation of two types of ?3-germoxyallyl tungsten complexes.  

PubMed

Germylyne complex Cp*(CO)2W?Ge{C(SiMe3)3} (1) reacted with two molecules of RC(O)CH?CH2 (R = Me, Et) to give ?(3)-allyl complexes, in which an oxagermacyclopentene framework was bound to an ?(3)-allyl ligand through an oxygen atom. In the reaction with ?-Me-substituted MeC(O)C(Me)?CH2, 1 reacted with only one molecule of the substrate to give another type of ?(3)-allyl complex, in which a five-membered oxagermacyclopentenyl ring was coordinated to the W center in an ?(3) fashion. Both reactions resulted in unprecedented complete cleavage of a W?Ge triple bond. PMID:24328309

Fukuda, Tetsuya; Hashimoto, Hisako; Tobita, Hiromi

2014-01-01

178

The Mechanism of Homogeneous CO2 Reduction by Ni(cyclam): Product Selectivity, Concerted Proton-Electron Transfer and C-O Bond Cleavage.  

PubMed

Homogeneous CO2 reduction catalyzed by [Ni(I)(cyclam)](+) (cyclam = 1,4,8,11-tetraazacyclotetradecane) exhibits high efficiency and selectivity yielding CO only at a relatively low overpotential. In this work, a density functional theory study of the reaction mechanism is presented. Earlier experiments have revealed that the same reaction occurring on mercury surfaces generates a mixture of CO and formate. According to the proposed mechanism, an ?(1)-CO2 adduct is the precursor for CO evolution, whereas formate is obtained from an ?(1)-OCO adduct. Our calculations show that generation of the ?(1)-CO2 adduct is energetically favored by ?14.0 kcal/mol relative to that of the ?(1)-OCO complex, thus rationalizing the product selectivity observed experimentally. Binding of ?(1)-CO2 to Ni(I) only leads to partial electron transfer from the metal center to CO2. Hence, further CO2 functionalization likely proceeds via an outer-sphere electron-transfer mechanism, for which concerted proton coupled electron transfer (PCET) is calculated to be the most feasible route. Final C-O bond cleavage involves rather low barriers in the presence of H3O(+) and H2CO3 and is therefore essentially concerted with the preceding PCET. As a result, the entire reaction mechanism can be described as concerted proton-electron transfer and C-O bond cleavage. On the basis of the theoretical results, the limitations of the catalytic activity of Ni(cyclam) are discussed, which sheds light on future design of more efficient catalysts. PMID:24957425

Song, Jinshuai; Klein, Eric L; Neese, Frank; Ye, Shengfa

2014-07-21

179

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

180

Nickel-catalyzed reductive and borylative cleavage of aromatic carbon-nitrogen bonds in N-aryl amides and carbamates.  

PubMed

The nickel-catalyzed reaction of N-aryl amides with hydroborane or diboron reagents resulted in the formation of the corresponding reduction or borylation products, respectively. Mechanistic studies revealed that these reactions proceeded via the activation of the C(aryl)-N bonds of simple, electronically neutral substrates and did not require the presence of an ortho directing group. PMID:24684671

Tobisu, Mamoru; Nakamura, Keisuke; Chatani, Naoto

2014-04-16

181

Highly selective fluorescence turn-on sensing of gold ions by a nanoparticle generation/C-I bond cleavage sequence.  

PubMed

We have developed a quickly responsive, and specific fluorescent assay for the detection of Au(III) on the basis of the formation of gold nanoparticles in the presence of HEPES, which cleave the C-I bond of I-BODIPY 1 to yield the highly fluorescent H-BODIPY 2. PMID:22866326

Park, Jihye; Choi, Sohee; Kim, Tae-Il; Kim, Youngmi

2012-10-01

182

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

PubMed Central

Attempted syntheses of (smif)2Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) based on metatheses of TiClnLm (n = 24) with M(smif) (M = Li, Na), in the presence of a reducing agent (Na/Hg) when necessary, failed, but several apparent Ti(II) species were identified by X-ray crystallography and multidimensional NMR spectroscopy: (smif){Li(smif-smif)}Ti (1, X-ray), [(smif)Ti]2(?-?3,?3-N,N(py)2-smif,smif) (2), (smif)Ti(?3-N,N(py)2-smif,(smif)H) (3), and (smif)Ti(dpma) (4). NMR spectroscopy and K-edge XAS showed that each compound possesses ligands that are redox non-innnocent, such that d1 Ti(III) centers AF-couple to ligand radicals: (smif){Li(smif-smif)2?}TiIII (1), [(smif2?)TiIII]2(?-?3,?3-N,N(py)2-smif,smif) (2), [(smif2?)TiIII](?3-N,N(py)2-smif,(smif)H) (3), and (smif2?)TiIII(dpma) (4). The instability of the (smif)2Ti relative to its C-C coupled dimer, 2, is rationalized via the complementary nature of the amide and smif radical anion ligands, which are also common to 3 and 4. Calculations support this contention.

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

2012-01-01

183

Glycosidic Bond Cleavage is Not Required for Phytosteryl Glycoside-Induced Reduction of Cholesterol Absorption in Mice  

PubMed Central

Phytosteryl glycosides occur in natural foods but little is known about their metabolism and bioactivity. Purified acylated steryl glycosides (ASG) were compared with phytosteryl esters (PSE) in mice. Animals on a phytosterol-free diet received ASG or PSE by gavage in purified soybean oil along with tracers cholesterol-d7 and sitostanol-d4. In a three-day fecal recovery study, ASG reduced cholesterol absorption efficiency by 45 6% compared with 40 6% observed with PSE. Four hours after gavage, plasma and liver cholesterol-d7 levels were reduced 86% or more when ASG was present. Liver total phytosterols were unchanged after ASG administration but were significantly increased after PSE. After ASG treatment both ASG and deacylated steryl glycosides (SG) were found in the gut mucosa and lumen. ASG was quantitatively recovered from stool samples as SG. These results demonstrate that ASG reduces cholesterol absorption in mice as efficiently as PSE while having little systemic absorption itself. Cleavage of the glycosidic linkage is not required for biological activity of ASG. Phytosteryl glycosides should be included in measurements of bioactive phytosterols.

Lin, Xiaobo; Ma, Lina; Moreau, Robert A.

2012-01-01

184

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

PubMed Central

Electron paramagnetic resonance (EPR) and online UVvisible absorption microspectrophotometry with X-ray crystallography have been used in a complementary manner to follow X-ray-induced disulfide-bond cleavage. Online UVvisible spectroscopy showed that upon X-irradiation, disulfide radicalization appeared to saturate at an absorbed dose of approximately 0.50.8?MGy, in contrast to the saturating dose of ?0.2?MGy observed using EPR at much lower dose rates. The observations suggest that a multi-track model involving product formation owing to the interaction of two separate tracks is a valid model for radiation damage in protein crystals. The saturation levels are remarkably consistent given the widely different experimental parameters and the range of total absorbed doses studied. The results indicate that even at the lowest doses used for structural investigations disulfide bonds are already radicalized. Multi-track considerations offer the first step in a comprehensive model of radiation damage that could potentially lead to a combined computational and experimental approach to identifying when damage is likely to be present, to quantitate it and to provide the ability to recover the native unperturbed structure.

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

2013-01-01

185

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI.  

PubMed

It is well known that ultraviolet photoexcitation of halomethanes results in halogen-carbon bond cleavage. Each halogen-carbon bond has a dominant ultraviolet (UV) absorption that promotes an electron from a nonbonding halogen orbital (nX) to a carbon-halogen antibonding orbital (?*C-X). UV absorption into specific transitions in the gas phase results primarily in selective cleavage of the corresponding carbon-halogen bond. In the present work, broadband ultrafast UV-visible transient absorption studies of CH2BrI reveal a more complex photochemistry in solution. Transient absorption spectra are reported spanning the range from 275 nm to 750 nm and 300 fs to 3?ns following excitation of CH2BrI at 266 nm in acetonitrile, 2-butanol, and cyclohexane. Channels involving formation of CH2Br + I radical pairs, iso-CH2Br-I, and iso-CH2I-Br are identified. The solvent environment has a significant influence on the branching ratios, and on the formation and stability of iso-CH2Br-I. Both iso-CH2Br-I and iso-CH2I-Br are observed in cyclohexane with a ratio of ~2.8:1. In acetonitrile this ratio is 7:1 or larger. The observation of formation of iso-CH2I-Br photoproduct as well as iso-CH2Br-I following 266 nm excitation is a novel result that suggests complexity in the dissociation mechanism. We also report a solvent and concentration dependent lifetime of iso-CH2Br-I. At low concentrations the lifetime is >4 ns in acetonitrile, 1.9 ns in 2-butanol and ~1.4 ns in cyclohexane. These lifetimes decrease with higher initial concentrations of CH2BrI. The concentration dependence highlights the role that intermolecular interactions can play in the quenching of unstable isomers of dihalomethanes. PMID:24320326

Anderson, Christopher P; Spears, Kenneth G; Wilson, Kaitlynn R; Sension, Roseanne J

2013-11-21

186

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

SciTech Connect

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

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

2013-11-21

187

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

PubMed Central

Certain side-on peroxo dicopper(II) species with particularly low ?OO (710730 cm?1) have been found in equilibrium with their bis-?-oxo dicopper(III) isomer. An issue is whether such side-on peroxo bridges are further activated for OO cleavage. In a previous study (Liang, H.-C., et al., J. Am. Chem. Soc. 2002, 124, 41704171), we showed that oxygenation of the three-coordinate complex [CuI(MeAN)]+ (MeAN=N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) leads to a low-temperature stable [{CuII(MeAN)}2(?-?2:?2-O22?)]2+ peroxo species with low ?OO (721 cm?1), as characterized by UV-Vis absorption and resonance Raman (rR) spectroscopies. Here, this complex has been crystallized as its SbF6? salt and an X-ray structure indicates the presence of an unusually long OO bond (1.540(5) ) consistent with the low ?OO. EXAFS and rR spectroscopic and reactivity studies indicate the exclusive formation of [{CuII(MeAN)}2(?-?2:?2-O22?)]2+ without any bis-?-oxo-dicopper(III) isomer present. This is the first structure of a side-on peroxo dicopper(II) species with a significantly long and weak OO bond. DFT calculations show that the weak OO bond results from strong ? donation from the MeAN ligand to Cu that is compensated by a decrease in the extent of peroxo to Cu charge transfer. Importantly, the weak OO bond does not reflect an increase in backbonding into the ?* orbital of the peroxide. Thus, although the OO bond is unusually weak, this structure is not further activated for reductive cleavage to form a reactive bis-?-oxo-dicopper(III) species. These results highlight the necessity of understanding electronic structure changes associated with spectral changes for correlations to reactivity.

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

2012-01-01

188

Ether complexes of tungsten with two different binding modes: An O-bound ether and an {eta}{sup 2}-(C=C) vinyl ether. Evidence for C-H...O hydrogen bonding of vinylic C-H groups  

SciTech Connect

The reaction of PhCH(OCH{sup 3}){sup 2} with Cp(CO){sup 3}WH and HOTf gives [Cp(CO){sup 3}W(PhCH{sup 2}OCH{sup 3})]{sup +}OTf{sup -}. The structure of this benzyl methyl ether complex was determined by single crystal X-ray diffraction and was shown to have the ether bonded to tungsten through the oxygen. This compound was isolated as a kinetic product of the reaction; it decomposes in solution by releasing free PhCH{sup 2}OCH{sup 3} and forming Cp(CO){sup 3}WOTf. An analog with the BAr`{sup 4}{sup -} counterion [Ar` = 3, 5-bis(trifluoromethyl)phenyl] is more stable. The reaction of the vinyl acetal CH{sup 2}=CHCH(OEt){sup 2} with Cp(CO){sup 3}WH and HOTf produces [Cp(CO){sup 3}W({eta}{sup 2}-EtOCH=CHCH{sup 3})]{sup +} OTf{sup -}, in which the ether is bonded to tungsten through the C=C bond of the vinyl ether. The crystal structure of this compound shows that the W-C(OEt) distance (2.69(3) A) is significantly longer than the W-C(CH{sup 3}) distance (2.37(3) A). There are weak C-H...O hydrogen bonds between both vinyl CH`s and oxygens of the triflate counterions. Evidence is presented that some of these weak hydrogen bonds are maintained in CD{sup 2}Cl{sup 2} solution but not in CD{sup 3}CN. 44 refs., 4 figs., 3 tabs.

Song, J.S.; Szalda, D.J.; Bullock, R.M. [Brookhaven National Lab., Upton, NY (United States)] [Brookhaven National Lab., Upton, NY (United States)

1996-11-13

189

Do amines react with protonated peptides in the gas phase via transacylation reactions to induce peptide bond cleavage?  

PubMed

The proposal that protonated peptides react with NH(3) in the gas phase via transacylation reactions (Tabet et al., Spectros. Int. J. 5: 253 1987) has been investigated by studying the reactions of the fixed charge derivatives [RC(O)NMe(2)CH(2)CO(2)H](+) (R=Me and Ph) with pyridine and triethylamine and the reactions of protonated glycine oligomers and leucine enkenphalin with butylamine. Under the near thermal conditions of the quadrupole ion trap, both the fixed charge derivatives as well as the protonated peptides react with the amines via either proton transfer or proton bound dimer formation. Collision induced dissociation of protonated peptides in the presence of butylamine yields b(n) and y(n) sequence ions as well as [b(n) + BuNH(2)](+) and [y(n) + BuNH(2)](+) ions. MS(3) experiments reveal that a major route to these [b(n) + BuNH(2)](+) and [y(n) + BuNH(2)](+) ions involves ion-molecule reactions between the b(n) and y(n) sequence ions and butylamine. MS(4) experiments, carried out to determine the nature of the [b(n) + BuNH(2)](+) ions, reveal that they correspond to a mixture of hydrogen bonded (i.e. proton bound dimer) and covalent amide bond structures. PMID:10962495

O'Hair, R A; Androutsopoulos, N K; Reid, G E

2000-01-01

190

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

PubMed Central

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

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

2013-01-01

191

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

192

Strong Lewis acid air-stable cationic titanocene perfluoroalkyl(aryl)sulfonate complexes as highly efficient and recyclable catalysts for C-C bond forming reactions.  

PubMed

A series of strong Lewis acid air-stable titanocene perfluoroalkyl(aryl)sulfonate complexes Cp2Ti(OH2)2(OSO2X)2THF (X = C8F17, 1THF; X = C4F9, 2H2OTHF; X = C6F5, 3) were successfully synthesized by the treatment of Cp2TiCl2 with C8F17SO3Ag, C4F9SO3Ag and C6F5SO3Ag, respectively. In contrast to well-known titanocene bis(triflate), these complexes showed no change in open air over three months. TG-DSC analysis showed that 1THF, 2H2OTHF and 3 were thermally stable at 230 C, 220 C and 280 C, respectively. Conductivity measurements showed that these complexes underwent ionic dissociation in CH3CN solution. X-ray analysis results confirmed that 2H2OTHF and 3 were cationic. ESR spectra showed that the Lewis acidity of 1THF (1.06 eV) was higher than that of Sc(3+) (1.00 eV) and Y(3+) (0.85 eV). UV/Vis spectra showed a significant red shift due to the strong complex formation between 10-methylacridone and 2H2OTHF. Fluorescence spectra showed that the Lewis acidity of 2 (?em = 477 nm) was higher than that of Sc(3+) (?em = 474 nm). These complexes showed high catalytic ability in various carbon-carbon bond forming reactions. Moreover, they show good reusability. Compared with 1THF, 2H2OTHF and 3 exhibit higher solubility and better catalytic activity, and will find broad applications in organic synthesis. PMID:24950799

Li, Ningbo; Wang, Jinying; Zhang, Xiaohong; Qiu, Renhua; Wang, Xie; Chen, Jinyang; Yin, Shuang-Feng; Xu, Xinhua

2014-08-14

193

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

194

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 Et2CHOH to CpW(CO)2(IMes)+B(C6F5)4- gives the alcohol complex [CpM(CO)2(IMes)(Et2CHOH)]+[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 Et2CHOH 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

195

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

NASA Astrophysics Data System (ADS)

This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H2COCH)CH2Cl. The three dominant photoproduct channels analyzed are c-(H2COCH)CH2+Cl, c-(H2COCH)+CH2Cl, and C3H4O+HCl. In the second channel, the c-(H2COCH) photofission product is a higher energy intermediate on C2H3O 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, H2CCO. The final primary photodissociation pathway HCl+C3H4O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H2COC)=CH2; the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C3H5O 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(3P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C2H4 and H2CO+C2H3 product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C2H4 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 H2CO+C2H3 product channel of the O(3P)+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+H3CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C3H5O 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+C2H4, and H2CO+C2H3 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+C2H4 and H2CO+C2H3 product channels. Using the C2H4 bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C2H4 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.; Lee, Shih-Huang; Lin, Jim-Min, Jr.

2010-09-01

196

Cleavage of peptide bonds bearing ionizable amino acids at P(1) by serine proteases with hydrophobic S(1) pocket.  

PubMed

Enzymatic hydrolysis of the synthetic substrate succinyl-Ala-Ala-Pro-Xxx-pNA (where Xxx=Leu, Asp or Lys) catalyzed by bovine chymotrypsin (CHYM) or Streptomyces griseus protease B (SGPB) has been studied at different pH values in the pH range 3-11. The pH optima for substrates having Leu, Asp, and Lys have been found to be 7.5-8.0, 5.5-6.0, and ?10, respectively. At the normally reported pH optimum (pH 7-8) of CHYM and SGPB, the substrate with Leu at the reactive site is more than 25,000-fold more reactive than that with Asp. However, when fully protonated, Asp is nearly as good a substrate as Leu. The pK values of the side chains of Asp and Lys in the hydrophobic S(1) pocket of CHYM and SGPB have been calculated from pH-dependent hydrolysis data and have been found to be about 9 for Asp and 7.4 and 9.7 for Lys for CHYM and SGPB, respectively. The results presented in this communication suggest a possible application of CHYM like enzymes in cleaving peptide bonds contributed by acidic amino acids between pH 5 and 6. PMID:20800580

Qasim, Mohammad A; Song, Jikui; Markley, John L; Laskowski, Michael

2010-10-01

197

Cleavage of Peptide Bonds bearing Ionizable Amino Acids at P1 by Serine Proteases with Hydrophobic S1 Pocket  

PubMed Central

Enzymatic hydrolysis of the synthetic substrate succinyl-Ala-Ala-Pro-Xxx-pNA (where Xxx = Leu, Asp or Lys) catalyzed by bovine chymotrypsin (CHYM) or Streptomyces griseus protease B (SGPB) has been studied at different pH values in the pH range 3 to 11. The pH optima for substrates having Leu, Asp, and Lys have been found to be 7.58.0, 5.56.0 and ~ 10, respectively. At the normally reported pH optimum (pH 7 to 8) of CHYM and SGPB, the substrate with Leu at the reactive site is more than 25,000-fold more reactive than that with Asp. However, when fully protonated, Asp is nearly as good a substrate as Leu. The pK values of the side chains of Asp and Lys in the hydrophobic S1 pocket of CHYM and SGPB have been calculated from pH-dependent hydrolysis data and have been found to be about 9 for Asp and 7.4 and 9.7 for Lys for CHYM and SGPB respectively. The results presented in this communication suggest a possible application of CHYM like enzymes in cleaving peptide bonds contributed by acidic amino acids between pH 5 and 6.

Qasim, Mohammad A.; Song, Jikui; Markley, John L.; Laskowski, Michael

2011-01-01

198

Direct evidence for preferential {beta} C-H bond cleavage resulting from 248 nm photolysis of the n-propyl radical using selectively-deuterated 1-bromopropane precursors  

SciTech Connect

A series of selectively deuterated 1-bromopropane precursors have been used to study site-specific photolysis in the n-propyl radical. A two-color photolysis approach (222 nm followed by 248 nm radiation) is used to create an intermediate photofragment and produce an H or a D atom, which is detected by 1 + 1 resonance ionization through Lyman-{alpha}. Target precursors are BrCH{sub 2}CD{sub 2}CD{sub 3}, BrCD{sub 2}CH{sub 2}CD{sub 3}, and BrCD{sub 2}CD{sub 2}CH{sub 3} as well as BrCD{sub 2}CH{sub 2}CH{sub 3}, BrCH{sub 2}CD{sub 2}CH{sub 3}, and BrCH{sub 2}CH{sub 2}CD{sub 3}. The `enhanced` H (or D) atom signals clearly demonstrate that C-H (or C-D) bond cleavage at the {beta} position is strongly favored. The net enhancement process undoubtedly involves photolysis of an intermediate, almost certainly the n-propyl radical. A comparison with systems involving ethyl and isopropyl radical photolysis is also presented. 14 refs., 4 figs.

Wang, Z.; Mathews, M.G.; Koplitz, B. [Tulane Univ., New Orleans, LA (United States)

1995-05-04

199

Radical Reactions with Double Memory of Chirality (2MOC) for the Enantiospecific Synthesis of Adjacent Stereogenic Quaternary Centers in Solution: Cleavage and Bonding Faster than Radical Rotation  

PubMed Central

The solution photochemistry of bis(phenylpyrrolidinonyl)ketones (R,R)-1b and (S,S)-1b exhibited a remarkably high memory of chirality. Stereospecific decarbonylation to products (R,R)-3b and (S,S)-3b, respectively, occurred with an ee of ca. 80%. The reaction is thought to occur along the single state manifold by sequential Norrish type-I ?-cleavage decarbonylation, and radicalradical combination in a time scale that is comparable to that required for the radical intermediate to expose its other enantiotopic face by rotation about an axis perpendicular to that of the p orbital (ca. 37 ps). The absolute configuration of a key intermediate and that of ketone (R,R)-1b were determined by single-crystal X-ray diffraction and the ee values of the photochemical products with the help of chiral shift reagent (+)-Eu(tfc)3 and chiral LC-MS/ MS. On the basis of the ee and de values at 25 C, it could be determined that ca. 70% of the bond forming events occur with double memory of chirality, ca. 21% occur after rotation of one radical to form the meso product (R,S)-3b, and only 9% occur after double rotation to form the opposite enantiomer. This report represents the first example of a doubly enantiospecific Norrish type-I and decarbonylation reaction in solution and illustrates potentially efficient ways to obtain compounds with adjacent stereogenic quaternary centers.

Resendiz, Marino J. E.; Family, Farnosh; Fuller, Kerrian; Campos, Luis M.; Khan, Saeed I.; Lebedeva, Natalia V.; Forbes, Malcolm D. E.

2014-01-01

200

Rate-determining water-assisted O-O bond cleavage of an Fe(III)-OOH intermediate in a bio-inspired nonheme iron-catalyzed oxidation.  

PubMed

Hydrocarbon oxidations by bio-inspired nonheme iron catalysts and H2O2 have been proposed to involve an Fe(III)-OOH intermediate that decays via a water-assisted mechanism to form an Fe(V)(O)(OH) oxidant. Herein we report kinetic evidence for this pathway in the oxidation of 1-octene catalyzed by [Fe(II)(TPA)(NCCH3)](2+) (1, TPA = tris(2-pyridylmethyl)amine). The (TPA)Fe(III)(OOH) intermediate 2 can be observed at -40 C and is found to undergo first-order decay, which is accelerated by water. Interestingly, the decay rate of 2 is comparable to that of product formation, indicating that the decay of 2 results in olefin oxidation. Furthermore, the Eyring activation parameters for the decay of 2 and product formation are identical, and both processes are associated with an H2O/D2O KIE of 2.5. Taken together with previous (18)O-labeling data, these results point to a water-assisted heterolytic O-O bond cleavage of 2 as the rate-limiting step in olefin oxidation. PMID:23594282

Oloo, Williamson N; Fielding, Andrew J; Que, Lawrence

2013-05-01

201

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

202

Facile C-H bond cleavage via a proton-coupled electron transfer involving a C-H...Cu(II) interaction.  

PubMed

The present study provides mechanistic details of a mild aromatic C-H activation effected by a copper(II) center ligated in a triazamacrocylic ligand, affording equimolar amounts of a Cu(III)-aryl species and Cu(I) species as reaction products. At low temperatures the Cu(II) complex 1 forms a three-center, three-electron C-H...Cu(II) interaction, identified by pulse electron paramagnetic resonance spectroscopy and supported by density functional theory calculations. C-H bond cleavage is coupled with copper oxidation, as a Cu(III)-aryl product 2 is formed. This reaction proceeds to completion at 273 K within minutes through either a copper disproportionation reaction or, alternatively, even faster with 1 equiv of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), quantitatively yielding 2. Kinetic studies of both reactions strongly implicate a rate-limiting proton-coupled electron transfer as the key C-H activation step, a mechanism that does not conform to the C-H activation mechanism in a Ni(II) analogue or to any previously proposed C-H activation mechanisms. PMID:20712320

Ribas, Xavi; Calle, Carlos; Poater, Albert; Casitas, Alicia; Gmez, Laura; Xifra, Ral; Parella, Teodor; Benet-Buchholz, Jordi; Schweiger, Arthur; Mitrikas, George; Sol, Miquel; Llobet, Antoni; Stack, T Daniel P

2010-09-01

203

Bonding  

NSDL National Science Digital Library

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

Cutting, Mrs.

2008-03-08

204

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

205

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

206

Ion beam studies of organometallic chemistry. High energy sampling of reaction intermediates involved in carbon-carbon bond cleavage by transition metals  

SciTech Connect

Chemical transformation often involves reactive intermediates which correspond to local minima on a complex potential energy surface. In this study of the reactions of cobalt ions with alkanes, the intermediate is provided more energy than needed for reaction. The quantitative assessment of the fragmentation of reaction intermediates provided thermodynamic data. These investigations involved organo transition metal reactions using ion beam technqiues. Results for the reaction of Co/sup +/ with 2-methylpropane and 2,2-dimethylpropane are included. The observation that the cross section for production of CoCH/sub 3//sup +/ is always greater than that of C/sub 3/H/sub 7/+ implies that IP(C/sub 0/CH/sub 3/) < IP(iso-C/sub 3/H/sub 7/) = 7.36 eV. A similar analysis of the 2,2-dimethylpropane system establishes IP(CoCH/sub 3/) > IP(tert-C/sub 4/H/sub 9/) = 6.70 eV. Measurement of the thresholds for the endothermic processes observed at high energies provides band energies of the products. Preliminary data for the endothermic reactions of Co/sup +/ with hydrogen and ethane indicate that D/sup 0/(Co/sup +/-H) = 52 +- 4 and D/sup 0/(Co -- CH/sub 3/) = 61 +- 4 kcal/mol. Combined with the ionization potentials, IP(CoH) = 7.3 +- 0.1 and IP(CoCH/sub 3/) = 7.0 +- 0.3 EV; we find the neutral band dissociation energies, D/sup 0/(CoH) = 39 +- 6 and D/sup 0/(CoCH/sub 3/) = 41 +- 10 kcal/mol. The results provide substantial evidence for the viability of a mechanism for carbon-carbon bond cleavage of alkanes which involves direct insertion of a metal into the carbon-carbon band as a first step. Thermochemical data indicating strong metal-carbon bonds corroborate this hypothesis. 2 figures. (DP)

Armentrout, P.B.; Beauchamp, J.L.

1980-02-27

207

Nonaqueous and halide-free route to crystalline BaTiO3, SrTiO3, and (Ba,Sr)TiO3 nanoparticles via a mechanism involving C-C bond formation.  

PubMed

A novel nonaqueous route for the preparation of nanocrystalline BaTiO(3), SrTiO(3), and (Ba,Sr)TiO(3) has been developed. In a simple one-pot reaction process, the elemental alkaline earth metals are directly dissolved in benzyl alcohol at slightly elevated temperatures. After the addition of Ti(O(i)Pr)(4), the reaction mixture is heated to 200 degrees C, resulting in the formation of a white precipitate. XRD measurements prove the exclusive presence of the perovskite phase without any other crystalline byproducts such as BaCO(3) or TiO(2). TEM investigations reveal that the BaTiO(3) nanoparticles are nearly spherical in shape with diameters ranging from 4 to 5 nm. The SrTiO(3) particles display less uniform particle shapes, and the size varies between 5 and 10 nm. Lattice fringes observed in HRTEM measurements further prove the high crystallinity of the nanoparticles. Surprisingly, GC-MS analysis of the reaction solution after hydrothermal treatment shows that hardly any ether formation occurs during the BaTiO(3) synthesis. Instead, the presence of 4-phenyl-2-butanol in stoichiometric amounts gives evidence that the formation mechanism proceeds mainly via a novel pathway involving C-C bond formation between benzyl alcohol and the isopropanolate ligand. PMID:15264847

Niederberger, Markus; Garnweitner, Georg; Pinna, Nicola; Antonietti, Markus

2004-07-28

208

Hydrogen-mediated reductive coupling of conjugated alkynes with ethyl (N-Sulfinyl)iminoacetates: synthesis of unnatural alpha-amino acids via rhodium-catalyzed C-C bond forming hydrogenation.  

PubMed

Rhodium-catalyzed hydrogenation of 1,3-enynes 1a-8a and 1,3-diynes 9a-13a at ambient temperature and pressure in the presence of ethyl (N-tert-butanesulfinyl)iminoacetate and ethyl (N-2,4,6-triisopropylbenzenesulfinyl)iminoacetates, respectively, results in reductive coupling to afford unsaturated alpha-amino acid esters 1b-13b in good to excellent yields with exceptional levels of regio- and stereocontrol. Further hydrogenation of the diene containing alpha-amino acid esters 1b-8b using Wilkinson's catalyst at ambient temperature and pressure results in regioselective reduction to afford the beta,gamma-unsaturated alpha-amino acid esters 1c-8c in good to excellent yields. Exhaustive hydrogenation of the unsaturated side chains of the Boc- and Fmoc-protected derivatives of enyne and diyne coupling products 14b-16b occurs in excellent yield using Crabtree's catalyst at ambient temperature and pressure providing the alpha-amino acid esters 14d-16d, which possess saturated side chains. Finally, cross-metathesis of the Boc-protected reductive coupling product 14b with cis-1,4-diacetoxy-2-butene proceeds readily to afford the allylic acetate 14e. Isotopic labeling studies that involve reductive coupling of enyne 1a and diyne 9a under an atmosphere of elemental deuterium corroborate a catalytic mechanism in which oxidative coupling of the alkyne and imine residues is followed by hydrogenolytic cleavage of the resulting metallacycle. A stereochemical model accounting for the observed sense of asymmetric induction is provided. These studies represent the first use of imines as electrophilic partners in hydrogen-mediated reductive carbon-carbon bond formation. PMID:16089454

Kong, Jong-Rock; Cho, Chang-Woo; Krische, Michael J

2005-08-17

209

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

210

Sequence-Specific Ultrasonic Cleavage of DNA  

PubMed Central

We investigated the phenomenon of ultrasonic cleavage of DNA by analyzing a large set of cleavage patterns of DNA restriction fragments using polyacrylamide gel electrophoresis. The cleavage intensity of individual phosphodiester bonds was found to depend on the nucleotide sequence and the position of the bond with respect to the ends of the fragment. Therelative intensities of cleavage of the central phosphodiester bond in 16 dinucleotides and 256 tetranucleotides were determined by multivariate statistical analysis. We observed a remarkable enhancement of the mean values of the relative intensities of cleavage (cleavage rates) in phosphodiester bonds following deoxycytidine, which diminished in the row of dinucleotides: d(CpG)> d(CpA) > d(CpT) >> d(CpC). The cleavage rates for all pairs of complementary dinucleotides were significantly different from each other. The effect of flanking nucleotides in tetranucleotides on cleavage rates of all 16 types of central dinucleotides was also statistically significant. The sequence-dependent ultrasonic cleavage rates of dinucleotides are consistent with reported data on the intensity of the conformational motion of their 5?-deoxyribose. As a measure of local conformational dynamics, cleavage rates may be useful for characterizing functional regions of the genome.

Grokhovsky, Sergei L.; Il'icheva, Irina A.; Nechipurenko, Dmitry Yu.; Golovkin, Michail V.; Panchenko, Larisa A.; Polozov, Robert V.; Nechipurenko, Yury D.

2011-01-01

211

Oxidative one-carbon cleavage of the octyl side chain of olanexidine, a novel antimicrobial agent, in dog liver microsomes.  

PubMed

1. The oxidative one-carbon cleavage reaction in the octyl side chain of olanexidine [1-(3,4-dichlorobenzyl)-5-octylbiguanide], a new potent biguanide antiseptic, was characterized in dog liver microsomes. 2. Olanexidine was initially biotransformed to a monohydroxylated metabolite, 8-[5-(3,4-dichlorobenzyl)-1-biguanidino]-2-octanol (DM-215), and DM-215 was subsequently oxidized to the diol derivative, 8-[5-(3,4-dichlorobenzyl)-1-biguanidino]-1,2-octandiol (DM-220). DM-220 was further biotransformed to 2-hydroxy aldehyde derivative, 2-hydroxy carboxylic acid derivative, and an oxidative C-1-C-2 bond cleavage metabolite, 7-[5-(3,4-dichlorobenzyl)-1-biguanidino] heptanoic acid [DM-223 (C7), a seven-carbon chain derivative], after incubation with dog liver microsomes. 3. DM-223 formation required NADPH as a cofactor and was inhibited by quinidine and quinine, relatively selective inhibitors of CYP2D subfamilies in dogs. 4. The results suggest that the one-carbon fragment of the octyl side chain of olanexidine could be removed by the oxidative C-C bond cleavage with the possible involvement of cytochrome P450 systems such as CYP2D subfamily. This oxidative C-C bond cleavage reaction by cytochrome P450s could play an important role in the removal of one-carbon fragment of other drugs or endogenous compounds containing aliphatic chains. PMID:14742137

Umehara, K; Shimokawa, Y; Koga, T; Ohtani, T; Miyamoto, G

2004-01-01

212

Design of electrochemical detection of thiols based on the cleavage of the disulfide bond coupled with thionine modified gold nanoparticle-assisted amplification.  

PubMed

A new strategy for the electrochemical detection of thiols based on the disulfide cleavage combined with gold nanoparticle (AuNP) assisted signal amplification and an AuNP and graphene (GR) and ionic liquid (IL) modified carbon paste electrode (AuNP/GR/CILE). A superior detection limit of 0.4 pM toward glutathione could be achieved. PMID:24018771

Hun, Xu; Sun, Wei; Zhu, Huanhuan; Du, Feng; Liu, Fang; Xu, Yaqiong; He, Yunhua

2013-10-25

213

CS, CH, and NH bond cleavage of heterocycles by a zero-valent iron complex, Fe(N 2)(depe) 2 [depe=1,2-bis(diethylphosphino)ethane  

Microsoft Academic Search

Treatment of Fe(N2)(depe)2 [depe=1,2-bis(diethylphosphino)ethane] (1) with benzo[b]thiophene at room temperature results in the regioselective CS and CH bond cleavages giving Fe(SC6H4CH?CH)(depe)2 (2a) and trans-FeH(C?CHC6H4S)(depe)2 (3a) in 72 and 19% yields, respectively. Complex 1 also reacts with thiophene, 2- and 3-acetylthiophenes and 2- and 3-methylthiophenes to give both CS and CH bond oxidative addition products: Fe(SCH?CHCH?CH)(depe)2 (2b) and trans-FeH(C?CHCH?CHS)(depe)2 (3b), Fe[SC(COMe)?CHCH?CH](depe)2

Takashi Morikita; Masafumi Hirano; Akito Sasaki; Sanshiro Komiya

1999-01-01

214

Bipodal surface organometallic complexes with surface N-donor ligands and application to the catalytic cleavage of C-H and C-C bonds in n-butane.  

PubMed

We present a new generation of "true vicinal" functions well-distributed on the inner surface of SBA15: [(?Si-NH2)(?Si-OH)] (1) and [(?Si-NH2)2] (2). From these amine-modified SBA15s, two new well-defined surface organometallic species [(?Si-NH-)(?Si-O-)]Zr(CH2tBu)2 (3) and [(?Si-NH-)2]Zr(CH2tBu)2 (4) have been obtained by reaction with Zr(CH2tBu)4. The surfaces were characterized with 2D multiple-quantum (1)H-(1)H NMR and infrared spectroscopies. Energy-filtered transmission electron microscopy (EFTEM), mass balance, and elemental analysis unambiguously proved that Zr(CH2tBu)4 reacts with these vicinal amine-modified surfaces to give mainly bipodal bis(neopentyl)zirconium complexes (3) and (4), uniformly distributed in the channels of SBA15. (3) and (4) react with hydrogen to give the homologous hydrides (5) and (6). Hydrogenolysis of n-butane catalyzed by these hydrides was carried out at low temperature (100 C) and low pressure (1 atm). While (6) exhibits a bis(silylamido)zirconium bishydride, [(?Si-NH-)2]Zr(H)2 (6a) (60%), and a bis(silylamido)silyloxozirconium monohydride, [(?Si-NH-)2(?Si-O-)]ZrH (6b) (40%), (5) displays a new surface organometallic complex characterized by an (1)H NMR signal at 14.46 ppm. The latter is assigned to a (silylimido)(silyloxo)zirconium monohydride, [(?Si-N?)(?Si-O-)]ZrH (5b) (30%), coexistent with a (silylamido)(silyloxo)zirconium bishydride, [(?Si-NH-)(?Si-O-)]Zr(H)2 (5a) (45%), and a silylamidobis(silyloxo)zirconium monohydride, [(?Si-NH-)(?Si-O-)2]ZrH (5c) (25%). Surprisingly, nitrogen surface ligands possess catalytic properties already encountered with silicon oxide surfaces, but interestingly, catalyst (5) with chelating [N,O] shows better activity than (6) with chelating [N,N]. PMID:24195760

Bendjeriou-Sedjerari, Anissa; Azzi, Joachim M; Abou-Hamad, Edy; Anjum, Dalaver H; Pasha, Fahran A; Huang, Kuo-Wei; Emsley, Lyndon; Basset, Jean-Marie

2013-11-27

215

Cleavage of resveratrol in fungi: characterization of the enzyme Rco1 from Ustilago maydis.  

PubMed

Ustilago maydis, the causative agent of corn smut disease, contains two genes encoding members of the carotenoid cleavage oxygenase family, a group of enzymes that cleave double bonds in different substrates. One of them, Cco1, was formerly identified as a ?-carotene cleaving enzyme. Here we elucidate the function of the protein encoded by the second gene, termed here as Ustilago maydis Resveratrol cleavage oxygenase 1 (Um Rco1). In vitro incubations of heterologously expressed and purified UM Rco1 with different carotenoid and stilbene substrates demonstrate that it cleaves the interphenyl C?-C? double bond of the phytoalexin resveratrol and its derivative piceatannol. Um Rco1 exhibits a high degree of substrate specificity, as suggested by the lack of activity on carotenoids and the other resveratrol-related compounds tested. The activity of Um Rco1 was confirmed by incubation of U. maydis rco1 deletion and over-expression strains with resveratrol. Furthermore, treatment with resveratrol resulted in striking alterations of cell morphology. However, pathogenicity assays indicated that Um rco1 is largely dispensable for biotrophic development. Our work reveals Um Rco1 as the first eukaryotic resveratrol cleavage enzyme identified so far. Moreover, Um Rco1 represents a subfamily of fungal enzymes likely involved in the degradation of stilbene compounds, as suggested by the cleavage of resveratrol by homologs from Aspergillus fumigatus, Chaetomium globosum and Botryotinia fuckeliana. PMID:21073977

Brefort, Thomas; Scherzinger, Daniel; Limn, M Carmen; Estrada, Alejandro F; Trautmann, Danika; Mengel, Carina; Avalos, Javier; Al-Babili, Salim

2011-02-01

216

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

PubMed Central

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

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

2009-01-01

217

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

PubMed

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

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

2014-02-24

218

N-benzylideneaniline and N-benzylaniline are potent inhibitors of lignostilbene-alpha,beta-dioxygenase, a key enzyme in oxidative cleavage of the central double bond of lignostilbene.  

PubMed

Lignostilbene-alpha,beta-dioxygenase (LSD, EC 1.13.11.43) is involved in oxidative cleavage of the central double bond of lignostilbene to form the corresponding aldehydes by a mechanism similar to those of 9-cis-epoxycarotenoid dioxygenase and beta-carotene 15,15'-dioxygenase, key enzymes in abscisic acid biosynthesis and vitamin A biosynthesis, respectively. In this study, several N-benzylideneanilines and amine were synthesized and examined for their efficacy as inhibitors of LSD. N-(4-Hydroxybenzylidene)-3-methoxyaniline was found to be a potent inhibitor with IC50 = 0.3 microM and N-(4-hydroxybenzyl)-3-methoxyaniline was also active with IC50 = 10 microM. The information obtained from the structure-activity relationships study here can aid in discovering inhibitors of both abscisic acid and vitamin A biosynthesis. PMID:14506920

Han, Sun-Young; Inoue, Hiroki; Terada, Tamami; Kamoda, Shigehiro; Saburi, Yoshimasa; Sekimata, Katsuhiko; Saito, Tamio; Kobayashi, Masatomo; Shinozaki, Kazuo; Yoshida, Shigeo; Asami, Tadao

2003-06-01

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 OO 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 OO 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

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

Microsoft Academic Search

The objective of this project is to determine the structure of bituminous coal by determining the proportions of the various kinds of connecting bonds and how they can best be broken. Results obtained during the past quarter are presented for the following tasks: (1) extractions and fractionations of coal products which covers pyridine extraction, fractionation of TIPS fractions, EDA extraction

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

1981-01-01

221

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

PubMed Central

The direct addition of ortho CH 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 CC bond is the rate-determining step, while the CH bond cleavage step is not.

MURAI, Shinji

2011-01-01

222

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

PubMed Central

[Me2Si(CpMe2)2]W(H)Cl is obtained via reaction of WCl6 with a mixture of [Me2Si(CpMe2)2]Li2 and NaBH4, from which the dichloride [Me2Si(CpMe2)2]WCl2 is obtained via treatment with CHCl3. [Me2Si(CpMe2)2]WCl2 provides a means to access other ansa tungstenocene compounds, such as [Me2Si(CpMe2)2]WH2, [Me2Si(CpMe2)2]WMe2, and [Me2Si(CpMe2)2]WCO. Of most interest, the reactions of [Me2Si(CpMe2)2]W(H)Cl with organolithium reagents do not yield simple ansa tungstenocene derivatives. Specifically, the reactions of [Me2Si(CpMe2)2]W(H)Cl with MeLi, BunLi, or PhLi result in the formation of mixed-ring tungstenocene compounds resulting from CSi cleavage and functionalization of the ansa bridge, namely (CpMe2)(?5,?1C5H2Me2SiMe2CH2)WH, (CpMe2)[?5,?1C5H2Me2Si(Me)(Bun)CH2]WH, and (CpMe2)[?5,?1C5H2Me2SiMe2(C6H4)]WH, respectively. In contrast to the CSi cleavage achieved by MeLi, BunLi, and PhLi, the ansa bridge of [Me2Si(CpMe2)2]W(H)Cl is inert to ButLi and the product obtained is the fulvene (tuck-in) complex [Me2Si(CpMe2)(?6C5MeH2CH2)]WH derived from dehydrohalogenation.

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

2008-01-01

223

Unusual carbon-sulfur bond cleavage in the reaction of a new type of bulky hexathioether with a zerovalent palladium complex.  

PubMed

The reaction of a bulky hexathioether, TbtS(o-Phen)S(o-Phen)SS(o-Phen)S(o-Phen)STbt (o-Phen = o-phenylene, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) (1), with 3 molar amounts of Pd(PPh3)4 afforded trinuclear palladium complex bridged by two benzenedithiolato ligands via a three-step palladium insertion reaction into one sulfur-sulfur and two carbon-sulfur bonds of 1. PMID:16372097

Shimizu, Daisuke; Takeda, Nobuhiro; Tokitoh, Norihiro

2006-01-14

224

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

PubMed

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

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

2014-01-17

225

Cleavage of ether, ester, and tosylate C(sp3)-O bonds by an iridium complex, initiated by oxidative addition of C-H bonds. Experimental and computational studies.  

PubMed

A pincer-ligated iridium complex, (PCP)Ir (PCP = ?(3)-C6H3-2,6-[CH2P(t-Bu)2]2), is found to undergo oxidative addition of C(sp(3))-O bonds of methyl esters (CH3-O2CR'), methyl tosylate (CH3-OTs), and certain electron-poor methyl aryl ethers (CH3-OAr). DFT calculations and mechanistic studies indicate that the reactions proceed via oxidative addition of C-H bonds followed by oxygenate migration, rather than by direct C-O addition. Thus, methyl aryl ethers react via addition of the methoxy C-H bond, followed by ?-aryloxide migration to give cis-(PCP)Ir(H)(CH2)(OAr), followed by iridium-to-methylidene hydride migration to give (PCP)Ir(CH3)(OAr). Methyl acetate undergoes C-H bond addition at the carbomethoxy group to give (PCP)Ir(H)[?(2)-CH2OC(O)Me] which then affords (PCP-CH2)Ir(H)(?(2)-O2CMe) (6-Me) in which the methoxy C-O bond has been cleaved, and the methylene derived from the methoxy group has migrated into the PCP Cipso-Ir bond. Thermolysis of 6-Me ultimately gives (PCP)Ir(CH3)(?(2)-O2CR), the net product of methoxy group C-O oxidative addition. Reaction of (PCP)Ir with species of the type ROAr, RO2CMe or ROTs, where R possesses ?-C-H bonds (e.g., R = ethyl or isopropyl), results in formation of (PCP)Ir(H)(OAr), (PCP)Ir(H)(O2CMe), or (PCP)Ir(H)(OTs), respectively, along with the corresponding olefin or (PCP)Ir(olefin) complex. Like the C-O bond oxidative additions, these reactions also proceed via initial activation of a C-H bond; in this case, C-H addition at the ?-position is followed by ?-migration of the aryloxide, carboxylate, or tosylate group. Calculations indicate that the ?-migration of the carboxylate group proceeds via an unusual six-membered cyclic transition state in which the alkoxy C-O bond is cleaved with no direct participation by the iridium center. PMID:23469859

Kundu, Sabuj; Choi, Jongwook; Wang, David Y; Choliy, Yuriy; Emge, Thomas J; Krogh-Jespersen, Karsten; Goldman, Alan S

2013-04-01

226

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

PubMed

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

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

2014-04-11

227

Elucidation of the Double-Bond Position of Long-Chain Unsaturated Fatty Acids by Multiple-Stage Linear Ion-Trap Mass Spectrometry with Electrospray Ionization  

PubMed Central

Linear ion-trap (LIT) MS2 mass spectrometric approach toward locating the position of double bond(s) of unsaturated long-chain fatty acids and toward discerning among isomeric unsaturated fatty acids as dilithiated adduct ([M ? H + 2Li]+) ions are described in this report. Upon resonance excitation in a LIT instrument, charge-remote fragmentation that involves ?-cleavage with ?-H shift (McLafferty rearrangement) is the predominant fragmentation pathway seen for the [M ? H + 2Li]+ ions of monoenoic long-chain fatty acids. The fragmentation process results in a dilithiated product ion of terminally unsaturated fatty acid, which undergoes consecutive McLafferty rearrangement to eliminate a propylene residue, and gives rise to another dilithiated adduct ion of terminally unsaturated fatty acid. In addition to the above-cited fragmentation process, the [M ? H + 2Li]+ ions of homoconjugated dienoic long-chain fatty acids also undergo ?-cleavage(s) with shift of the allylic hydrogen situated between the homoconjugated double bonds to the unsaturated site. These fragmentation pathways lead to two types of CC bond cleavages that are allylic (?-cleavage) or vinylic, respectively, to the proximal CC double bond, resulting in two distinct sets of ion series, in which each ion series is separated by a CH2CH=CH (40 Da) residue. These latter fragmentations are the predominant processes seen for the polyunsaturated long-chain fatty acids. The spectrum feature dependent on the position of unsaturated double bond(s) affords unambiguous assignment of the position of double bond(s) of long-chain unsaturated fatty acids.

Hsu, Fong-Fu; Turk, John

2008-01-01

228

Effect of substituents on the preferred modes of one-electron reductive cleavage of N-Cl and N-Br bonds.  

PubMed

In this study, we investigate the effect of substituents in determining the modes of one-electron reductive cleavage of X-NRR' (X = Cl and Br) molecules. We achieve this through comparison of the calculated gas-phase electron affinities (EAs) and aqueous-phase one-electron reduction potentials (E's) for a range of nitrogen-centered radicals (()NRR') with the corresponding EA and E values for ()Cl and ()Br. The gas-phase EAs have been obtained using the benchmark-quality W1w thermochemical protocol, whereas E values have been obtained by additionally applying free energy of solvation corrections, obtained using the conductor-like polarizable continuum (CPCM) model. We find that the N-halogenated derivatives of amines and amides should generally cleave in such a way as to afford ()NRR' and X(-). For the N-halogenated imides, on the other hand, the N-brominated derivatives are predicted to produce ()Br in solution, whereas the N-chlorinated derivatives again would give Cl(-). Importantly, we predict that N-bromouracil is likely to afford ()Br. This may have important implications in terms of inflammatory-related diseases, because ()Br may damage biomolecules such as proteins and DNA. To assist in the determination of the gas-phase EAs of larger ()NRR' radicals, not amenable to investigation using W1w, we have evaluated the performance of a wide range of lower-cost theoretical methods. Of the standard density functional theory (DFT) procedures, M06-2X, ?-HCTHh, and B3-LYP show good performance, with mean absolute deviations (MADs) from W1w of 4.8-6.8 kJ mol(-1), whereas ROB2-PLYP and B2-PLYP emerge as the best of the double-hybrid DFTs (DHDFTs), with MADs of 2.5 and 3.0 kJ mol(-1), respectively. Of the Gn-type procedures, G3X and G4 show very good performance (MADs = 2.4 and 2.6 kJ mol(-1), respectively). The G4(MP2)-6X+ procedure performs comparably, with an MAD of 2.7 kJ mol(-1), with the added advantage of significantly reduced computational expense. PMID:23282473

O'Reilly, Robert J; Karton, Amir; Radom, Leo

2013-01-17

229

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

230

Si-H and Si-C Bond Cleavage Reactions of Silane and Phenylsilanes with Mo(PMe3)6: Silyl, Hypervalent Silyl, Silane, and Disilane Complexes.  

PubMed

Mo(PMe3)6 cleaves the Si-H bonds of SiH4, PhSiH3, and Ph2SiH2 to afford a variety of novel silyl, hypervalent silyl, silane, and disilane complexes, as respectively illustrated by Mo(PMe3)4(SiH3)2H2, Mo(PMe3)4(?(2)-H2-H2SiPh2H)H, Mo(PMe3)3(?-HSiHPh2)H4, and Mo(PMe3)3(?(2)-H2-H2Si2Ph4)H2. Mo(PMe3)4(?(2)-H2-H2SiPh2H)H and Mo(PMe3)3(?(2)-H2-H2Si2Ph4)H2 are respectively the first examples of complexes that feature a hypervalent ?(2)-H2-H2SiPh2H silyl ligand and a chelating disilane ligand, and both compounds convert to the diphenylsilane adduct, Mo(PMe3)3(?-HSiHPh2)H4, in the presence of H2. Mo(PMe3)4(SiH3)2H2 undergoes isotope exchange with SiD4, and NMR spectroscopic analysis of the SiHxD4-x isotopologues released indicates that the reaction does not occur via initial reductive elimination of SiH4, but rather by a metathesis pathway. PMID:24874576

Zuzek, Ashley A; Parkin, Gerard

2014-06-11

231

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

SciTech Connect

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

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

2010-10-01

232

Solvent and Primary Deuterium Isotope Effects Show That Lactate CH and OH Bond Cleavages Are Concerted in Y254F Flavocytochrome b2, Consistent with a Hydride Transfer Mechanism  

PubMed Central

Yeast flavocytochrome b2 catalyzes the oxidation of lactate to pyruvate; because of the wealth of structural and mechanistic information available, this enzyme has served as the model for the family of flavoproteins catalyzing oxidation of ?-hydroxy acids. Primary deuterium and solvent isotope effects have now been used to analyze the effects of mutating the active site residue Tyr254 to phenylalanine. Both the Vmax and the V/Klactate values decrease about 40-fold in the mutant enzyme. The primary deuterium isotope effects on the Vmax and the V/Klactate values increase to 5.0, equivalent to the intrinsic isotope effect for the wild-type enzyme. In addition, both the Vmax and the V/Klactate values exhibit solvent isotope effects of 1.5. Measurement of the solvent isotope effect with deuterated lactate establishes that the primary and solvent isotope effects arise from the same chemical step, consistent with concerted cleavage of the lactate OH and CH bonds. The pH dependence of the mutant enzyme is not significantly different from that of the wild-type enzyme; this is most consistent with a requirement that the side chain of Tyr254 be uncharged for catalysis. The results support a hydride transfer mechanism for the mutant protein and, by extension, wild-type flavocytochrome b2 and the other flavoproteins catalyzing oxidation of ?-hydroxy acids.

Sobrado, Pablo; Fitzpatrick, Paul F.

2006-01-01

233

Electronic effect in C-H and C-C bond activation: state-specific reactions of Fe/sup +/(/sup 6/D, /sup 4/F) with methane, ethane, and propane  

SciTech Connect

Reactions of atomic iron ions with methane, ethane, and propane are studied with guided ion beam mass spectrometry. By using different ion sources different electronic states of the ion can be prepared and studied in detail. The first excited state, Fe/sup +/(/sup 4/F), is more reactive than the ground state, Fe/sup +/(/sup 6/D), for all endothermic reactions in all three systems. This result is similar to recent observations of the reactions of these states with H/sub 2/. The different reactivities are explained by using simple molecular orbital arguments. In contrast, Fe/sup +/(/sup 4/F) reacts less efficiently than Fe/sup +/(/sup 6/D) in the exothermic reactions of ethane and propane below 0.5 eV but more efficiently at higher energies. This behavior is explained by a potential energy surface crossing that is avoided at low kinetic energies due to spin-orbit interactions and is permitted at higher energies. Finally, analysis of the threshold behavior of the endothermic reactions provides the bond dissociation energies, D/sup 0/(Fe/sup +/-CH/sub 3/) = 2.51 +/- 0.10 eV (57.9 +/- 2.4 kcal/mol) and D/sup 0/(FeH) = 1.98 +/- 0.13 eV (45.7 +/- 3.0 kcal/mol).

Schultz, R.H.; Elkind, J.L.; Armentrout, P.B.

1988-01-20

234

C-F or C-H bond activation and C-C coupling reactions of fluorinated pyridines at rhodium: synthesis, structure and reactivity of a variety of tetrafluoropyridyl complexes.  

PubMed

Reactions of [RhH(PEt3)3] (1) or [RhH(PEt3)4] (2) with pentafluoropyridine or 2,3,5,6-tetrafluoropyridine afford the activation product [Rh(4-C5NF4)(PEt3)3] (3). Treatment of 3 with CO, 13CO or CNtBu effects the formation of trans-[Rh(4-C5NF4)(CO)(PEt3)2] (4a), trans-[Rh(4-C5NF4)(13CO)(PEt3)2] (4b) and trans-[Rh(4-C5NF4)(CNtBu)(PEt3)2] (5). The rhodium(III) compounds trans-[RhI(CH3)(4-C5NF4)(PEt3)2] (6a) and trans-[RhI(13CH3)(4-C5NF4)(PEt3)2] (6b) are accessible on reaction of 3 with CH3I or 13CH3I. In the presence of CO or 13CO these complexes convert into trans-[RhI(CH3)(4-C5NF4)(CO)(PEt3)2] (7a), trans-[RhI(13CH3)(4-C5NF4)(CO)(PEt3)2] (7b) and trans-[RhI(13CH3)(4-C5NF4)(13CO)(PEt3)2] (7c). The trans arrangement of the carbonyl and methyl ligand in 7a-7c has been confirmed by the 13C-13C coupling constant in the 13C NMR spectrum of 7c. A reaction of 4a or 4b with CH3I or 13CH3I yields the acyl compounds trans-[RhI(COCH3)(4-C5NF4)(PEt3)2] (8a) and trans-[RhI(13CO13CH3)(4-C5NF4)(PEt3)2] (8b), respectively. Complex 8a slowly reacts with more CH3I to give [PEt3Me][Rh(I)2(COCH3)(4-C5NF4)(PEt3)](9). On heating a solution of 7a, the complex trans-[RhI(CO)(PEt3)2] (10) and the C-C coupled product 4-methyltetrafluoropyridine (11) have been obtained. Complex 8a also forms 10 at elevated temperatures in the presence of CO together with the new ketone 4-acetyltetrafluoropyridine (12). The structures of the complexes 3, 4a, 5, 6a, 8a and 9 have been determined by X-ray crystallography. 19F-1H HMQC NMR solution spectra of 6a and 8a reveal a close contact of the methyl groups in the phosphine to the methyl or acyl ligand bound at rhodium. PMID:15573161

Noveski, Daniel; Braun, Thomas; Neumann, Beate; Stammler, Anja; Stammler, Hans-Georg

2004-12-21

235

C. C. Jones: Scientific Photographs  

NSDL National Science Digital Library

This image gallery, by C. C. Jones of Union College, contains many examples of physical optics and also x-ray diffraction. Several images of diffraction show how the pattern changes when the wavelength, the distance between light sources, and the aperture is varied. Also included are pinhole photos and comparisons of emission spectra of various elements and the absorption spectrum of the sun. Captions provide background information.

2007-04-25

236

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

NASA Astrophysics Data System (ADS)

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

Ramazani, Shapour

2013-05-01

237

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?. 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

2012-09-01

238

Steric and electronic effects influencing ?-aryl elimination in the Pd-catalyzed carbon-carbon single bond activation of triarylmethanols.  

PubMed

An analysis of the palladium-catalyzed activation of carbon-carbon single bonds within triarylmethanols has led to a greater understanding of factors influencing the ?-aryl elimination process responsible for C-C bond cleavage. A series of competition reactions were utilized to determine that ?-aryl elimination of aryl substituents containing ortho-substitution proceeds with significant preference to unsubstituted phenyl rings. Further experiments indicate that substrates containing either strongly donating or withdrawing substituents are cleaved from triarylmethanols more readily than relatively neutral species. PMID:23346914

Bour, James R; Green, Jacob C; Winton, Valerie J; Johnson, Jeffrey B

2013-02-15

239

Selective cleavage of pepsin by molybdenum metallopeptidase.  

PubMed

In this study, the cleavage of protein by molybdenum cluster is reported for the first time. The protein target used is porcine pepsin. The data presented in this study show that pepsin is cleaved to at least three fragments with molecular weights of ?23, ?19 and ?16 kDa when the mixture of the protein and ammonium heptamolybdate tetrahydrate ((NH(4))(6)Mo(7)O(24)4H(2)O) was incubated at 37C for 24h. No self cleavage of pepsin occurs at 37 C, 24h indicating that the reaction is mediated by the metal ions. N-terminal sequencing of the peptide fragments indicated three cleavage sites of pepsin between Leu 112-Tyr 113, Leu 166-Leu 167 and Leu 178-Asn 179. The cleavage reaction occurs after incubation of the mixture of pepsin and (NH(4))(6)Mo(7)O(24)4H(2)O) only for 2h. However, the specificity of the cleavage decreases when incubation time is longer than 48 h. The mechanism for cleavage of pepsin is expected to be hydrolytic chemistry of the amide bonds in the protein backbone. PMID:22330807

Yenjai, Sudarat; Malaikaew, Pinpinat; Liwporncharoenvong, Teerayuth; Buranaprapuk, Apinya

2012-03-01

240

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

PubMed Central

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.

Diaz-Sanchez, Violeta; F. Estrada, Alejandro; Limon, M. Carmen; Al-Babili, Salim

2013-01-01

241

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

Daz-Snchez, Violeta; Estrada, Alejandro F; Limn, M Carmen; Al-Babili, Salim; Avalos, Javier

2013-09-01

242

Electron stimulated C F bond breaking kinetics in fluorine-containing organic thin films  

NASA Astrophysics Data System (ADS)

By monitoring the variation in the fluorine content and the distribution of CFx (x=0-3) species in a semifluorinated self-assembled monolayer (CF-SAM) upon prolonged X-ray irradiation, the electron stimulated C-F bond breaking kinetics in fluorine-containing organic films has been determined. At short irradiation times, X-ray irradiation induced changes in the film's chemical composition are consistent with the presence of C-F, C-C and S-Au bond cleavage events. In contrast, C-F bond breaking is identified as the dominant kinetic process for longer X-ray exposures. The kinetics of X-ray induced defluorination are consistent with a first-order decay process mediated by a series of consecutive C-F bond breaking events (e.g., CF?C) whose rate constants are in excellent agreement with a stochastic model of defluorination.

Perry, C. C.; Wagner, Anthony J.; Howard Fairbrother, D.

2002-06-01

243

Cleavage of nucleic acids  

DOEpatents

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

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

2000-01-01

244

Cleavage of nucleic acids  

DOEpatents

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

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

2010-11-09

245

Cleavage of nucleic acids  

DOEpatents

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

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

2007-12-11

246

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

PubMed

Methyliminopropadienone MeN=C=C=C=O 1a was generated by flash vacuum thermolysis from four different precursors and isolated in solid argon. The matrix-isolation infrared spectrum is dominated by unusually strong anharmonic effects resulting in complex fine structure of the absorptions due to the NCCCO moiety in the 2200 cm(-1) region. Doubling and tripling of the corresponding absorption bands are observed for phenyliminopropadienone PhN=C=C=C=O 1b and bis(phenylimino)propadiene PhN=C=C=C=NPh 9, respectively. Anharmonic vibrational frequency calculations allow the identification of a number of overtones and combination bands as the cause of the splittings for each molecule. This method constitutes an important tool for the characterization of reactive intermediates and unusual molecules by matrix-isolation infrared spectroscopy. PMID:24182036

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

2013-10-28

247

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

NASA Astrophysics Data System (ADS)

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

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

2013-10-01

248

Localization of double bonds in wax esters by high-performance liquid chromatography/atmospheric pressure chemical ionization mass spectrometry utilizing the fragmentation of acetonitrile-related adducts.  

PubMed

Unsaturated wax esters (WEs) provided molecular adducts with C(3)H(5)N ([M + 55](+)) in APCI sources in the presence of acetonitrile. CID MS/MS of [M + 55](+) yielded fragments allowing the localization of double bond(s) in the hydrocarbon chains of the WEs. These fragments were formed by a cleavage on each side of the double bond. In methylene-interrupted polyunsaturated WEs, diagnostic fragments related to each double bond were detected; the most abundant were those corresponding to the cleavage of the C-C bond next to the first and the last double bond. To differentiate between those fragments differing in their structure or origin, a simple nomenclature based on ? and ? ions has been introduced. Fragmentation of the ?-type ions (fragments containing an ester bond) provided information on the occurrence of a double bond in the acid or alcohol part of the WEs. While no significant differences between the spectra of the WEs differing by cis/trans isomerism were found, the isomers were separated chromatographically. A data-dependent HPLC/APCI-MS(2) method for the comprehensive characterization of WEs in their complex mixtures has been developed and applied to natural mixtures of WEs isolated from jojoba oil and beeswax. More than 50 WE molecular species were completely identified, including the information on the acid and alcohol chain length and the position of the double bonds. PMID:21428309

Vrkoslav, Vladimr; Hkov, Martina; Peckov, Karolina; Urbanov, Klra; Cva?ka, Josef

2011-04-15

249

Effect of phosphorus on cleavage fracture in ? -carbide  

NASA Astrophysics Data System (ADS)

To understand the origin of cleavage fracture which dominates in Fe(Mn)-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 ? -carbide. We show that strong anisotropy of the Fe-P bonds in Fe3(Al,P)C under tensile stress, leads to the appearance of large structural voids and may facilitate crack nucleation.

Medvedeva, N. I.; Howell, R. A.; van Aken, D. C.; Medvedeva, J. E.

2010-01-01

250

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

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

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

251

Low-energy reaction rate constants for the Ni+-assisted decomposition of acetaldehyde: observation of C-H and C-C activation.  

PubMed

Rate constants for the low-energy Ni(+)-assisted dissociative reaction of acetaldehyde have been measured under jet-cooled conditions in the gas phase. The rate constants are acquired through monitoring the time dependence of fragment Ni(+)CO formation. The decomposition of the precursor Ni(+)-acetaldehyde cluster ion proceeds via consecutive, parallel reaction coordinates that originate with the Ni(+)-assisted cleavage of either a C-C or an aldehyde C-H bond. The energies used to initiate these reactions are well below that required to cleave sigma-bonds in the isolated acetaldehyde molecule. Direct measurement of the reaction kinetics over a range of energies indicates that the rate-limiting step in the dissociative mechanism changes at cluster ion internal energies = 17,200 +/- 400 cm(-1). Arguments are presented that this energy marks the closure of the dissociative coordinate that initiates with C-H sigma-bond activation and thus provides a measure of the activation energy of this dissociative pathway. PMID:20050634

Dee, S Jason; Castleberry, Vanessa A; Villarroel, Otsmar J; Laboren, Ivanna E; Bellert, Darrin J

2010-02-01

252

Cleavage and Fracture  

NSDL National Science Digital Library

In this activity, students will learn the properties of cleavage and fracture (how minerals break when struck with a hammer), and will use these properties as a diagnostic test for identifying minerals. Working in small groups, they will break a selection of specimens with a hammer, observe the results, and record their observations. A student worksheet and discussion questions are provided.

253

Synthesis and cleavage reactions of metal-metal-bonded [Mo(2)(S(2)CNR(2))(6)](OTf)(2), a source of the Tris(dithiocarbamato)molybdenum(IV) fragment.  

PubMed

Halide abstraction from the chlorotris(dialkyldithiocarbamato)molybdenum(IV) complexes MoCl(S(2)CNR(2))(3) (R = Et, Me) with silver triflate produces the diamagnetic dimeric complexes [Mo(2)(S(2)CNR(2))(6)](OTf)(2) in good yield. The crystallographically determined structure of the diethyldithiocarbamato complex indicates that the dimer consists of two pentagonal bipyramids sharing an axial edge, with a Mo-Mo separation (2.8462(8) A) indicative of a metal-metal bond. A qualitative analysis of the bonding indicates that this bond is of order 2 and consists of one normal sigma bond and one relatively weak "skewed pi" interaction. The dimers [Mo(2)(S(2)CNR(2))(6)](OTf)(2) react with a variety of reagents to give monomeric seven-coordinate complexes, including the new cationic molybdenum(IV) complex [Mo(PMe(2)Ph)(S(2)CNEt(2))(3)](OTf), which has been structurally characterized. Kinetic studies of the reaction of [Mo(2)(S(2)CNEt(2))(6)](OTf)(2) with halides indicate the presence of competing dissociative and associative substitution pathways, although neutral donors may react by different mechanisms. PMID:11735478

Seymore, S B; Brown, S N

2001-12-17

254

Ion beam studies of the reactions of atomic cobalt ions with alkanes: determination of metal-hydrogen and metal-carbon bond energies and an examination of the mechanism by which transition metals cleave carbon-carbon bonds  

SciTech Connect

An ion beam apparatus is employed to study the reactions of singly charged cobalt positive ions with hydrogen and 17 alkanes. Reaction cross sections and product distributions as a function of kinetic energy are determined. Exothermic carbon-carbon bond cleavage reactions are observed for all alkanes but methane and ethane. A mechanism involving oxidative addition of C-C and C-H bonds to cobalt as a first step is demonstrated to account for all major reactions at all energies. Interpretation of several endothermic processes allows the extraction of thermochemical data. The bond dissociation energies obtained are D/sup 0/(Co/sup +/-H) = 52 +- 4 kcal/mol, D/sup 0/(Co-H) = 39 +- 6 kcal/mol, D/sup 0/(Co/sup +/-CH/sub 3/) = 61 +- 4 kcal/mol, and D/sup 0/(Co-CH/sub 3/) = 41 +- 10 kcal/mol.

Armentrout, P.B.; Beauchamp, J.L.

1981-02-25

255

Columbus Schema for C\\/C++ Preprocessing  

Microsoft Academic Search

File inclusion, conditional compilation and macro pro- cessing has made the C\\/C++ preprocessor a powerful tool for programmers. However, program code with lots of di- rectives often causes difculties in program understanding and maintenance. The main source of the problem is the dif- ference between the code that the programmer sees and the preprocessed code that the compiler gets. To

Lszl Vidcs; rpd Beszdes; Rudolf Ferenc

2004-01-01

256

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

257

Asymmetric catalytic oxidative cleavage of polycyclic systems: the synthesis of atropisomeric diazonanes and diazecanes.  

PubMed

Oxidative cleavage of internal double bonds in polycyclic systems can give access to compounds containing medium- to large-sized rings. In this example, the nine- and ten-membered ring containing compounds that resulted from the mCPBA-mediated (mCPBA=meta-chloroperoxybenzoic acid) oxidative cleavage reaction were shown to exhibit atropisomerism. The reaction of the polycyclic system with catalytic amounts of ruthenium tetraoxide followed by diol cleavage achieved the same synthetic goal. Use of the Nishiyama-Beller ruthenium-based catalysts enabled the synthesis of optically-enriched samples, providing the first example of an atropselective oxidative cleavage reaction. PMID:21469229

Jones, Alan M; Liu, Gu; Lorion, Magali M; Patterson, Stephen; Lebl, Tomas; Slawin, Alexandra M Z; Westwood, Nicholas J

2011-05-01

258

C\\/C composite oxidation model  

Microsoft Academic Search

A model of the oxidation behaviour of C\\/C composites was developed. The purpose of the model is to predict the carbon loss that occurs at the outer surface and within the porosity of the composite. It takes into account the transport of species in the material, chemical gasgas and gassolid reactions, the interaction of both transport phenomena and chemistry, the

M.-P Bacos; J.-M Dorvaux; O Lavigne; Y Renollet

2000-01-01

259

C-H bond activation as the initial step in the Co sup + -mediated demethanation of propane: The critical role of angular momentum at the rate-limiting transition state  

SciTech Connect

Exothermic reactions of transition-metal ions with alkanes have in many instances been shown to be facile in the gas phase. Reactions mainly result in the loss of molecular hydrogen and small alkanes to yield metal ion-olefin complexes. A variety of experimental techniques have provided thermochemical, kinetic, dynamic, and mechanistic information for these reactions, with an important focus being the identification of the initial activation step. The question of C-H versus C-C bond activation as the initial step in the formation of C-C bond cleavage products has yet to be resolved. The authors have measured reaction cross sections and kinetic energy release distributions for the exothermic reactions of Co{sup +} with propane, propane-2-d{sub 1}, propane-2,2-d{sub 2}, propane-1,1,1-d{sub 3}, propane-1,1,1,3,3,3-d{sub 6}, and propane-d{sub 8}.

van Koppen, P.A.M.; Brodbelt-Lustig, J.; Bowers, M.T. (Univ. of California, Santa Barbara (USA)); Dearden, D.V.; Beauchamp, J.L. (California Institute of Technology, Pasadena (USA)); Fisher, E.R.; Armentrout, P.B. (Univ. of Utah, Salt Lake City (USA))

1990-07-04

260

Reactions of the allenylidenes trans-[IrCl[=C=C=C(ph)R](PiPr3)2] with electrophiles: generation of butatriene-, carbene-, and carbyne- iridium complexes.  

PubMed

The allenylidenciridium(I) complexes trans-[IrX(=C=C-CPh2)(PiPr3)2] (X = Cl: 1; X = I: 2) react with excess methyl iodide by C-C coupling and elimination of HI to give the eta2-butatriene compounds trans-[IrX-(eta2-CH2=C=C=CPh2)(PiPr3)2] (3, 4), of which 3 (X = Cl) was characterized by X-ray crystallography. Treatment of 1 and 5 (containing C=C=C(Ph)tBu as the allenylidene ligand) with HCI leads to the formation of the six-coordinate hydridoiridium(III) complexes [IrHCl2[= C=C=C(Ph)R](PiPr3)2] (6, 7) by oxidative addition at the metal center. In contrast, the reactions of 1 and 5 with both CF3CO2H and CF3SO3H afford the four-coordinate vinylcarbene compounds trans-[IrCl[=C(X)-CH=C(Ph)R[(PiPr3)2] (8-10). For X= CF3CO2, in nitromethane a dissociation of the C-X bond occurs and the cationic iridium carbynes trans-[IrCl[=C-CH=C(Ph)R](PiPr3)2]+ are generated. Upon addition of NaBPh4, the stable carbyne complexes 11b (R= Ph) and 12b (R = tBu) with BPh4 as the counterion were isolated in almost quantitative yields. The X-ray crystal structure analysis of 6 reveals that the chloro ligands are cis and the phosphane ligands trans disposed. PMID:11757655

Ilg, K; Werner, H

2001-11-01

261

Invasive cleavage of nucleic acids  

DOEpatents

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

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

2002-01-01

262

Invasive cleavage of nucleic acids  

DOEpatents

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

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

1999-01-01

263

Mutations in the interglobular domain of aggrecan alter matrix metalloproteinase and aggrecanase cleavage patterns. Evidence that matrix metalloproteinase cleavage interferes with aggrecanase activity.  

PubMed

We have expressed G1-G2 mutants with amino acid changes at the DIPEN(341) downward arrow(342)FFGVG and ITEGE(373) downward arrow(374)ARGSV cleavage sites, in order to investigate the relationship between matrix metalloproteinase (MMP) and aggrecanase activities in the interglobular domain (IGD) of aggrecan. The mutation DIPEN(341) to DIGSA(341) partially blocked cleavage by MMP-13 and MMP-8 at the MMP site, while the mutation (342)FFGVG to (342)GTRVG completely blocked cleavage at this site by MMP-1, -2, -3, -7, -8, -9, -13, -14. Each of the MMP cleavage site mutants, including a four-amino acid deletion mutant lacking residues ENFF(343), were efficiently cleaved by aggrecanase, suggesting that the primary sequence at the MMP site had no effect on aggrecanase activity in the IGD. The mutation (374)ARGSV to (374)NVYSV completely blocked cleavage at the aggrecanase site by aggrecanase, MMP-8 and atrolysin C but had no effect on the ability of MMP-8 and MMP-13 to cleave at the Asn(341) downward arrowPhe bond. Susceptibility to atrolysin C cleavage at the MMP site was conferred in the DIGSA(341) mutant but absent in the wild-type, (342)GTRVG, (374)NVYSV, and deletion mutants. To further explore the relationship between MMP and aggrecanase activities, sequential digest experiments were done in which MMP degradation products were subsequently digested with aggrecanase and vice versa. Aggrecanase-derived G1 domains with ITEGE(373) C termini were viable substrates for MMPs; however, MMP-derived G2 fragments were resistant to cleavage by aggrecanase. A 10-mer peptide FVDIPENFFG, which is a substrate analogue for the MMP cleavage site, inhibited aggrecanase cleavage at the Glu(373) downward arrowAla bond. This study demonstrates that MMPs and aggrecanase have unique substrate recognition in the IGD of aggrecan and suggests that sequences at the C terminus of the DIPEN(341) G1 domain may be important for regulating aggrecanase cleavage. PMID:11032846

Mercuri, F A; Maciewicz, R A; Tart, J; Last, K; Fosang, A J

2000-10-20

264

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

265

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

NASA Astrophysics Data System (ADS)

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

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

2014-01-01

266

Observation of B+??c+?c-K+ and B0??c+?c-K0 Decays  

NASA Astrophysics Data System (ADS)

We report the first measurements of the doubly charmed baryonic B decays B??c+?c-K. The B+??c+?c-K+ decay is observed with a branching fraction of (6.5-0.9+1.01.13.4)10-4 and a statistical significance of 15.4?. The B0??c+?c-K0 decay is observed with a branching fraction of (7.9-2.3+2.91.24.1)10-4 and a statistical significance of 6.6?. The branching fraction errors are statistical, systematic, and the error resulting from the uncertainty of the ?c+?pK-?+ decay branching fraction. The analysis is based on 357fb-1 of data accumulated at the ?(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider.

Gabyshev, N.; Abe, K.; Abe, K.; Adachi, I.; Aihara, H.; Asano, Y.; Aulchenko, V.; Aushev, T.; Bahinipati, S.; Bakich, A. M.; Balagura, V.; Barberio, E.; Bartel, W.; Bay, A.; Bedny, I.; Bitenc, U.; Bizjak, I.; Bondar, A.; Bozek, A.; Bra?ko, M.; Browder, T. E.; Chen, A.; Chen, W. T.; Cheon, B. G.; Chistov, R.; Choi, Y.; Chuvikov, A.; Cole, S.; Dalseno, J.; Danilov, M.; Dash, M.; Drutskoy, A.; Eidelman, S.; Garmash, A.; Gershon, T.; Gokhroo, G.; Golob, B.; Haba, J.; Hayasaka, K.; Hayashii, H.; Hazumi, M.; Hokuue, T.; Hoshi, Y.; Hou, S.; Hou, W.-S.; Hsiung, Y. B.; Ikado, K.; Imoto, A.; Inami, K.; Itoh, R.; Iwasaki, M.; Iwasaki, Y.; Kang, J. H.; Kawasaki, T.; Khan, H. R.; Kichimi, H.; Kim, S. M.; Korpar, S.; Krokovny, P.; Kulasiri, R.; Kuo, C. C.; Kuzmin, A.; Kwon, Y.-J.; Leder, G.; Lesiak, T.; Lin, S.-W.; Liventsev, D.; Majumder, G.; Matsumoto, T.; Mitaroff, W.; Miyabayashi, K.; Miyata, H.; Miyazaki, Y.; Mizuk, R.; Nakano, E.; Nakao, M.; Natkaniec, Z.; Nishida, S.; Ogawa, S.; Ohshima, T.; Okabe, T.; Okuno, S.; Olsen, S. L.; Ozaki, H.; Palka, H.; Park, C. W.; Park, K. S.; Pestotnik, R.; Piilonen, L. E.; Sakai, Y.; Sato, N.; Satoyama, N.; Schietinger, T.; Schneider, O.; Schwanda, C.; Seidl, R.; Senyo, K.; Sevior, M. E.; Shapkin, M.; Shibuya, H.; Somov, A.; Soni, N.; Stamen, R.; Stani?, S.; Stari?, M.; Sumiyoshi, T.; Tamai, K.; Tamura, N.; Tanaka, M.; Taylor, G. N.; Teramoto, Y.; Tian, X. C.; Tsukamoto, T.; Uehara, S.; Uglov, T.; Ueno, K.; Uno, S.; Urquijo, P.; Varner, G.; Varvell, K. E.; Villa, S.; Wang, C. C.; Wang, C. H.; Watanabe, Y.; Won, E.; Xie, Q. L.; Yamaguchi, A.; Yamauchi, M.; Ying, J.; Zhang, Z. P.

2006-11-01

267

Cleavage of Prolactin by Its Target Organs and the Possible Significance of this Process  

Microsoft Academic Search

An enzymatically cleaved form of rat prolactin (rPRL) was first described in 1980. This cleavage produces a molecule that consists of two chains of amino acids linked by a disulfide bond between two Cys residues. Reduction of that bond produces two fragments of 6 and 16 Kd. A considerable amount of information has accrued in recent years about the cleaved

Charles S. Nicoll

1997-01-01

268

Clinical value of early cleavage embryo  

Microsoft Academic Search

Objective: This study was undertaken to see if embryo transfer containing early cleavage embryos resulted in better clinical pregnancy rate. Method: The treatment outcomes of IVF-ET were retrospectively reviewed. Out of 258 transfer cycles, 160 cycles contained no early cleavage embryos (Group I) and 98 cycles contained at least one early cleavage embryo (Group II). The definition of early cleavage

Y. C Tsai; M. T Chung; Y. H Sung; T. F Tsai; Y. T Tsai; L. Y Lin

2002-01-01

269

Mechanisms for cleavage and intergranular embrittlement in Fe  

SciTech Connect

The mechanism of cleavage in bcc Fe has been investigated with electronic structure calculations and compared to similar calculations modeling cleavage in other bcc materials (Li, Nb, Mo). It has been found in the BCC materials we have studied that those known to undergo (100) cleavage show enhancement of the density of states (DOS) on the virtual surface layer as the material is strained to failure. Using total energy calculations it is shown that there is a point of inflection (corresponding to the point of maximum force versus displacement) in the total energy at nearly the same point as the maximum enhancement in the DOS. We conjecture that this point represents the transition state for the fracture process. Extending these ideas, the electronic structure near a Fe(111) grain boundary has been calculated both with and without segregated P. We find clear evidence for the formation of a P band of states, suggesting the existence of P-P interactions within the segregated layer. A chemical model of strain-induced bond failure is suggested in which bonds parallel to the fracture surface compete with the cohesive Fe-Fe bonds normal to the surface. 7 refs., 5 figs.

Eberhart, M.E.; MacLaren, J.M. (Massachusetts Inst. of Tech., Cambridge, MA (USA). Dept. of Materials Science and Engineering; Los Alamos National Lab., NM (USA))

1987-01-01

270

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

NASA Astrophysics Data System (ADS)

Using a sample of 1.06108 ?(3686) events collected with the BESIII detector, we present the first observation of the decays of ?(3686)???+?-+c.c. and ?(3686)???-?++c.c. The branching fractions are measured to be B(?(3686)???+?-+c.c.)=(1.400.030.13)10-4 and B(?(3686)???-?++c.c.)=(1.540.040.13)10-4, where the first errors are statistical and the second ones systematic.

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

2013-12-01

271

Selective Metal-Assisted Oxidative Cleavage of a C-N Bond: Synthesis and Characterization of the Mononuclear Iron(III) [Fe(BPG)Cl(2)] Complex and Its Two [Fe(BPA)Cl(3)] and [Fe(BPE)Cl(3)] Derivatives.  

PubMed

A new example of a site-selective metal-assisted oxidative cleavage of a C-N bond is reported. This phenomenon occurs in the nitrogen-centered tetradentate tripodal BPG ligand (BPG = (bis(2-pyridylmethyl)amino)acetate), which combines one carboxylate and two pyridines as pendant groups, when coordinated to iron(III). The corresponding iron(III) [Fe(BPG)Cl(2)] complex (1) is transformed to the iron(III) [Fe(BPA)Cl(3)] complex (2) with the tridentate BPA ligand (BPA = bis(2-pyridylmethyl)amine) retaining only two pyridine pendant groups, while the initial BPG carboxylate group is transformed to glyoxylic acid. The [Fe(BPA)Cl(3)] complex (2) has been fully characterized as well as another, [Fe(BPE)Cl(3)] (3), formed with the tridentate BPE ligand (BPE = methyl (bis(2-pyridylmethyl)amino)acetate) which, in addition to two pyridines, presents an ester group. The crystal structures of these two complexes have been resolved. The asymmetric unit of complex 2 has been characterized by two mononuclear neutral molecules linked by two hydrogen bonds. (Crystal data for 2: orthorhombic, Pna2(1), a = 15.603(8) , b = 8.485(4) , c = 22.752(11) , alpha = beta = gamma = 90 degrees, V = 3012(3) (3), Z = 8, R = 0.0582 [2813 reflections with I > 2sigma(I)], and R(w) = 0.1430.) Complex 3 is symmetric with the iron center, the amine nitrogen, one choride and the three atoms of the carboxylate group in special positions in the mirror plane. (Crystal data for 3: orthorhombic, Pmn2(1), a = 10.418(4) , b = 12.952(5) , c = 7.353(3) , alpha = beta = gamma = 90 degrees, V = 992.2(7) (3), Z = 2, R = 0.0500 [513 reflections with I > 2sigma(I)], and R(w) = 0.1231.) Redox potentials corroborate the structural features (E degrees = -38 mV for 1, -19 mV for 2, and +145 mV for 3 vs SCE in acetonitrile). Preliminary studies of the [Fe(BPG)Cl(2)] complex have shown that it reacts in its reduced form with dioxygen. PMID:11670033

Rodriguez, Marie-Carmen; Lambert, Franois; Morgenstern-Badarau, Irne; Cesario, Michle; Guilhem, Jean; Keita, Bineta; Nadjo, Louis

1997-07-30

272

Nondisulfide crosslinking and chemical cleavage of tubulin subunits: pH and temperature dependence.  

PubMed

Tubulin is known to be extremely unstable. The denaturation process partially involves irreversible aggregation, mediated by disulfide crosslinking. In addition, tubulin is known to undergo chemical cleavage during boiling in sodium dodecyl sulfate (SDS), a process that generates small peptides that have been mistaken for low molecular weight MAPs. Similar peptide cleavage has now been observed during two-dimensional denaturing isoelectric focusing-SDS-polyacrylamide gel electrophoresis. This phenomenon has complicated interpretation of limited proteolysis studies of tubulin by subtilisin. In an effort to avoid this problem we have undertaken a detailed study of the solution conditions that promote chemical cleavage of tubulin. The cleavage reaction is found to be strongly pH, time, and temperature dependent. Nondenatured and denatured tubulin is susceptible to peptide cleavage, suggesting that primary structure is more important than secondary structure in selection of susceptible bonds. After transfer of cleavage products from an SDS gel to a polyvinylidene difluoride membrane, amino acid sequencing has confirmed cleavage at Asp-Pro bonds, at position 306 in alpha-tubulin and at position 304 in beta-tubulin. We also infer cleavage at the only additional Asp-Pro peptide bond located at position 31 in beta-tubulin. Heat-induced cleavage at Asp-Pro accounts for 5 of the 13 bands observed on SDS gels. In addition, a minor alpha-tubulin band has been sequenced from a two-dimensional gel, corresponds to cleavage at Asp-Cys located at alpha-tubulin position 200, and accounts for two additional bands observed on SDS gels. Under nondenaturing and nonpolymerizing conditions tubulin undergoes extensive intermolecular, disulfide crosslinking. At elevated temperatures and high pH, a small fraction of the crosslinking is not reduced by beta-mercaptoethanol. Disulfide-crosslinked aggregates are not suspected because carboxymethylation of tubulin does not prevent their formation. Lysinoalanine has been found by amino acid analysis and thus covalent lysine-dehydroalanine crosslinks are suspected. Dehydroalanine is formed by beta-elimination at serine and thus the presence of lysinoalanine is consistent with cleavage at Gly-Ser peptides, the most unstable serine peptide bond, and accounts for most of the remaining cleavage data. PMID:8424642

Correia, J J; Lipscomb, L D; Lobert, S

1993-01-01

273

Bent Bonds and Multiple Bonds.  

ERIC Educational Resources Information Center

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

Robinson, Edward A.; Gillespie, Ronald J.

1980-01-01

274

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

PubMed

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

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

1982-01-01

275

Pd(II)-Catalyzed C-H Activation/C-C Cross-Coupling Reactions: Versatility and Practicality  

PubMed Central

In the past decade, palladium-catalyzed CH activation/CC bond forming reactions have emerged as promising new catalytic transformations; however, development in this field is still at an early stage compared to the state of the art in cross-coupling reactions using aryl and alkyl halides. This Review begins with a brief introduction of four extensively investigated modes of catalysis for forming CC bonds from CH bonds: Pd(II)/Pd(0), Pd(II)/Pd(IV), Pd(0)/Pd(II)/Pd(IV) and Pd(0)/Pd(II) catalysis. More detailed discussion is then directed towards the recent development of Pd(II)-catalyzed coupling of CH bonds with organometallic reagents through a Pd(II)/Pd(0) catalytic cycle. Despite much progress made to date, improving the versatility and practicality of this new reaction remains a tremendous challenge.

Chen, Xiao; Engle, Keary M.; Wang, Dong-Hui; Yu, Jin-Quan

2009-01-01

276

Identification of the cleavage site and determinants required for poliovirus 3CPro-catalyzed cleavage of human TATA-binding transcription factor TBP.  

PubMed Central

Host cell RNA polymerase II-mediated transcription is inhibited by poliovirus infection. We have shown previously that the human TATA-binding protein (TBP), a general transcription factor required for transcription of all RNA polymerase II genes, is directly cleaved both in vitro and in vivo by the virus-coded protease 3CPro. 3CPro specifically cleaves glutamine-glycine bonds in the viral polyprotein. Cellular transcription factor TBP contains three glutamine-glycine sites, at amino acids 12, 18, and 108. By using site-directed mutagenesis, we determined that the glutamine-glycine bond at amino acid 18, but not that at amino acid 12 or 108, is cleaved by the viral protease. Both the glutamine and the glycine appear to be important for the cleavage. Further mutations around the glutamine-glycine site at position 18 suggest that determinants other than the glutamine-glycine bond in TBP are also required for 3CPro-induced cleavage. An alanine at position P4 and a proline at position P2, proximal to the scissile glutamine-glycine pair, appear to be important for 3CPro-mediated cleavage of TBP. Our results suggest that the cleavage specificity of 3CPro for a cellular transcription factor is very similar to its mode of cleavage of viral polyproteins. Images

Das, S; Dasgupta, A

1993-01-01

277

Does Electron Capture Dissociation Cleave Protein Disulfide Bonds?  

PubMed Central

Peptide and protein characterization by mass spectrometry (MS) relies on their dissociation in the gas phase into specific fragments whose mass values can be aligned as mass ladders to provide sequence information and to localize possible posttranslational modifications. The most common dissociation method involves slow heating of even-electron (M+n H)n+ ions from electrospray ionization by energetic collisions with inert gas, and cleavage of amide backbone bonds. More recently, dissociation methods based on electron capture or transfer were found to provide far more extensive sequence coverage through unselective cleavage of backbone NC? bonds. As another important feature of electron capture dissociation (ECD) and electron transfer dissociation (ETD), their unique unimolecular radical ion chemistry generally preserves labile posttranslational modifications such as glycosylation and phosphorylation. Moreover, it was postulated that disulfide bond cleavage is preferred over backbone cleavage, and that capture of a single electron can break both a backbone and a disulfide bond, or even two disulfide bonds between two peptide chains. However, the proposal of preferential disulfide bond cleavage in ECD or ETD has recently been debated. The experimental data presented here reveal that the mechanism of protein disulfide bond cleavage is much more intricate than previously anticipated.

Ganisl, Barbara; Breuker, Kathrin

2012-01-01

278

Mineral Cleavage: a practical experiment  

NSDL National Science Digital Library

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

Browning, Sharon

279

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

PubMed

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

Kraka, Elfi; Cremer, Dieter

2009-03-01

280

Novel carbon-carbon bond formations for biocatalysis  

PubMed Central

Carboncarbon 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 CC 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 PictetSpengler reaction or oxidative CC bond formation. Furthermore, novel enzymatic CC 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

281

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

PubMed

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

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

2013-07-01

282

Cleavage of Plasmid DNA by Eukaryotic Topoisomerase II  

PubMed Central

Topoisomerase II is an essential enzyme that is required for a number of critical nuclear processes. All of the catalytic functions of topoisomerase II require the enzyme to generate a transient double-stranded break in the backbone of the double helix. To maintain genomic integrity during the cleavage event, topoisomerase II forms covalent bonds between active site tyrosyl residues and the newly generated 5?-DNA termini. In addition to the critical cellular functions of the type II enzyme, several important anticancer drugs kill cells by increasing levels of covalent topoisomerase II-DNA cleavage complexes. Due to the physiological importance of topoisomerase II and its role in cancer chemotherapy, several methods have been developed to monitor the in vitro DNA cleavage activity of the type II enzyme. The plasmid-based system described in this chapter quantifies enzyme-mediated double-stranded DNA cleavage by monitoring the conversion of covalently-closed supercoiled DNA to linear molecules. The assay is simple, straightforward, and does not require the use of radiolabeled substrates.

Bandele, Omari J.; Osheroff, Neil

2010-01-01

283

Linear ketenimines. Variable structures of C,C-dicyanoketenimines and C,C-bis-sulfonylketenimines.  

PubMed

C,C-dicyanoketenimines 10a-c were generated by flash vacuum thermolysis of ketene N,S-acetals 9a-c or by thermal or photochemical decomposition of alpha-azido-beta-cyanocinnamonitrile 11. In the latter reaction, 3,3-dicyano-2-phenyl-1-azirine 12 is also formed. IR spectroscopy of the keteniminines isolated in Ar matrixes or as neat films, NMR spectroscopy of 10c, and theoretical calculations (B3LYP/6-31G) demonstrate that these ketenimines have variable geometry, being essentially linear along the CCN-R framework in polar media (neat films and solution), but in the gas phase or Ar matrix they are bent, as is usual for ketenimines. Experiments and calculations agree that a single CN substituent as in 13 is not enough to enforce linearity, and sulfonyl groups are less effective that cyano groups in causing linearity. C,C-bis(methylsulfonyl)ketenimines 4-5 and a C-cyano-C-(methylsulfonyl)ketenimine 15 are not linear. The compound p-O2NC6H4N=C=C(COOMe)2 previously reported in the literature is probably somewhat linearized along the CCNR moiety. A computational survey (B3LYP/6-31G) of the inversion barrier at nitrogen indicates that electronegative C-substituents dramatically lower the barrier; this is also true of N-acyl substituents. Increasing polarity causes lower barriers. Although N-alkylbis(methylsulfonyl)ketenimines are not calculated to be linear, the barriers are so low that crystal lattice forces can induce planarity in N-methylbis(methylsulfonyl)ketenimine 3. PMID:11846648

Finnerty, Justin; Mitschke, Ullrich; Wentrup, Curt

2002-02-22

284

Controllable protein cleavages through intein fragment complementation.  

PubMed

Intein-based protein cleavages, if carried out in a controllable way, can be useful tools of recombinant protein purification, ligation, and cyclization. However, existing methods using contiguous inteins were often complicated by spontaneous cleavages, which could severely reduce the yield of the desired protein product. Here we demonstrate a new method of controllable cleavages without any spontaneous cleavage, using an artificial S1 split-intein consisting of an 11-aa N-intein (I(N)) and a 144-aa C-intein (I(C)). In a C-cleavage design, the I(C) sequence was embedded in a recombinant precursor protein, and the small I(N) was used as a synthetic peptide to trigger a cleavage at the C-terminus of I(C). In an N-cleavage design, the short I(N) sequence was embedded in a recombinant precursor protein, and the separately produced I(C) protein was used to catalyze a cleavage at the N-terminus of I(N). These N- and C-cleavages showed >95% efficiency, and both successfully avoided any spontaneous cleavage during expression and purification of the precursor proteins. The N-cleavage design also revealed an unexpected and interesting structural flexibility of the I(C) protein. These findings significantly expand the effectiveness of intein-based protein cleavages, and they also reveal important insights of intein structural flexibility and fragment complementation. PMID:19768808

Volkmann, Gerrit; Sun, Wenchang; Liu, Xiang-Qin

2009-11-01

285

Investigating CN- cleavage by three-coordinate M[N(R)Ar]3 complexes.  

PubMed

Three-coordinate Mo[N((t)Bu)Ar]3 binds cyanide to form the intermediate [Ar((t)Bu)N]3Mo-CN-Mo[N((t)Bu)Ar]3 but, unlike its N2 analogue which spontaneously cleaves dinitrogen, the C-N bond remains intact. DFT calculations on the model [NH2]3Mo/CN- system show that while the overall reaction is significantly exothermic, the final cleavage step is endothermic by at least 90 kJ mol(-1), accounting for why C-N bond cleavage is not observed experimentally. The situation is improved for the [H2N]3W/CN- system where the intermediate and products are closer in energy but not enough for CN- cleavage to be facile at room temperature. Additional calculations were undertaken on the mixed-metal [H2N]3Re+/CN- /W[NH2]3 and [H2N]3Re+/CN-/Ta[NH2]3 systems in which the metals ions were chosen to maximise the stability of the products on the basis of an earlier bond energy study. Although the reaction energetics for the [H2N]3Re+/CN /W[NH2]3 system are more favourable than those for the [H2N]3W/CN- system, the final C-N cleavage step is still endothermic by 32 kJ mol(-1) when symmetry constraints are relaxed. The resistance of these systems to C-N cleavage was examined by a bond decomposition analysis of [H2N]M-L1[triple bond]L2-M[NH2]3 intermediates for L1[triple bond]L2 = N2, CO and CN which showed that backbonding from the metal into the L1[triple bond]L2 pi* orbitals is significantly less for CN than for N2 or CO due to the negative charge on CN- which results in a large energy gap between the metal d(pi), and the pi* orbitals of CN-. This, combined with the very strong M-CN- interaction which stabilises the CN intermediate, makes C-N bond cleavage in these systems unfavourable even though the C[triple bond]N triple bond is not as strong as the bond in N2 or CO. PMID:18411843

Christian, Gemma; Stranger, Robert; Yates, Brian F; Cummins, Christopher C

2008-01-21

286

Photo-Fenton reaction of graphene oxide: a new strategy to prepare graphene quantum dots for DNA cleavage.  

PubMed

Graphene quantum dots (GQDs) are great promising in various applications owing to the quantum confinement and edge effects in addition to their intrinsic properties of graphene, but the preparation of the GQDs in bulk scale is challenging. We demonstrated in this work that the micrometer sized graphene oxide (GO) sheets could react with Fenton reagent (Fe(2+)/Fe(3+)/H(2)O(2)) efficiently under an UV irradiation, and, as a result, the GQDs with periphery carboxylic groups could be generated with mass scale production. Through a variety of techniques including atomic force microscopy, X-ray photoelectron spectroscopy, gas chromatography, ultraperformance liquid chromatography-mass spectrometry, and total organic carbon measurement, the mechanism of the photo-Fenton reaction of GO was elucidated. The photo-Fenton reaction of GO was initiated at the carbon atoms connected with the oxygen containing groups, and C-C bonds were broken subsequently, therefore, the reaction rate depends strongly on the oxidization extent of the GO. Given the simple and efficient nature of the photo-Fenton reaction of GO, this method should provide a new strategy to prepare GQDs in mass scale. As a proof-of-concept experiment, the novel DNA cleavage system using as-generated GQDs was constructed. PMID:22813062

Zhou, Xuejiao; Zhang, Yan; Wang, Chong; Wu, Xiaochen; Yang, Yongqiang; Zheng, Bin; Wu, Haixia; Guo, Shouwu; Zhang, Jingyan

2012-08-28

287

Slaty cleavage development and magnetic anisotropy fabrics  

NASA Astrophysics Data System (ADS)

The shale-to-slate transition preserved in the Ordovician Martinsburg Formation at the Lehigh Water Gap, Pennsylvania, provides an opportunity to study the relationship between magnetic anisotropy fabrics and the development of slaty cleavage. Our previous work has indicated that anisotropy of magnetic susceptibility (AMS) does not record changes in finite strain associated with cleavage development in these rocks but instead measures the degree of dissolution and new growth of chlorite. Additional AMS data presented in this paper lend further support to this conclusion. Conversely, anhysteretic remanent magnetization anisotropy (ARMA), which is not affected by paramagnetic chlorite, accurately reflects the strain-induced rock fabrics associated with cleavage formation. ARMA results show that magnetite dimensional orientations vary from bedding-parallel in shale samples to cleavage-parallel in samples with well-developed slaty cleavage. Samples with weak and pencil cleavage display scattered ARMA orientations which lie in between bedding and cleavage. These intermediate orientations may be due to either passive rotation of magnetite from bedding-parallel to cleavage-parallel or (re)crystallization of magnetite. If rotation occurred, grain rotation was highly heterogeneous in the samples with incipient cleavage. The intermediate ARMA orientations may also reflect the varying contribution of two magnetite preferred orientations, a depositional orientation parallel to bedding and a new growth orientation parallel to cleavage.

Housen, Bernard A.; van der Pluijm, Ben A.

1991-06-01

288

Electron Transfer Dissociation (ETD) of Peptides Containing Intrachain Disulfide Bonds  

NASA Astrophysics Data System (ADS)

The fragmentation chemistry of peptides containing intrachain disulfide bonds was investigated under electron transfer dissociation (ETD) conditions. Fragments within the cyclic region of the peptide backbone due to intrachain disulfide bond formation were observed, including: c (odd electron), z (even electron), c-33 Da, z + 33 Da, c + 32 Da, and z-32 Da types of ions. The presence of these ions indicated cleavages both at the disulfide bond and the N-C? backbone from a single electron transfer event. Mechanistic studies supported a mechanism whereby the N-C? bond was cleaved first, and radical-driven reactions caused cleavage at either an S-S bond or an S-C bond within cysteinyl residues. Direct ETD at the disulfide linkage was also observed, correlating with signature loss of 33 Da (SH) from the charge-reduced peptide ions. Initial ETD cleavage at the disulfide bond was found to be promoted amongst peptides ions of lower charge states, while backbone fragmentation was more abundant for higher charge states. The capability of inducing both backbone and disulfide bond cleavages from ETD could be particularly useful for sequencing peptides containing intact intrachain disulfide bonds. ETD of the 13 peptides studied herein all showed substantial sequence coverage, accounting for 75%-100% of possible backbone fragmentation.

Cole, Scott R.; Ma, Xiaoxiao; Zhang, Xinrong; Xia, Yu

2012-02-01

289

Cleavage of resveratrol in fungi: Characterization of the enzyme Rco1 from Ustilago maydis  

Microsoft Academic Search

Ustilago maydis, the causative agent of corn smut disease, contains two genes encoding members of the carotenoid cleavage oxygenase family, a group of enzymes that cleave double bonds in different substrates. One of them, Cco1, was formerly identified as a ?-carotene cleaving enzyme. Here we elucidate the function of the protein encoded by the second gene, termed here as Ustilago

Thomas Brefort; Daniel Scherzinger; M. Carmen Limn; Alejandro F. Estrada; Danika Trautmann; Carina Mengel; Javier Avalos; Salim Al-Babili

2011-01-01

290

A photolabile linker for the mild and selective cleavage of enriched biomolecules from solid support.  

PubMed

Selective release of enriched biomolecules from solid support is a desirable goal in proteomic and metabolomic studies. Here we demonstrate that photocleavage of a light-sensitive phenacyl ester bond is a suitable alternative cleavage strategy for the selective release of enriched biomolecules form avidin beads circumventing the disadvantages of conventional heat denaturation procedures. PMID:19821566

Orth, Ronald; Sieber, Stephan A

2009-11-01

291

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

PubMed Central

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

RajanBabu, T. V.

2009-01-01

292

Thin Bonded P.C.C. Resurfacing: Interim Report Number 1.  

National Technical Information Service (NTIS)

The purpose of this study was to evaluate the construction techniques and performance characteristics of the La. DOTD's first portland cement concrete resurfacing project which was constructed over a short section of an existing 9-inch concrete pavement l...

W. H. Temple M. Rasoulian

1982-01-01

293

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

294

Detecting malware's failover C&C strategies with squeeze  

Microsoft Academic Search

The ability to remote-control infected PCs is a fundamental component of modern malware campaigns. At the same time, the command and control (C&C) infrastructure that provides this capability is an attractive target for mitigation. In recent years, more or less successful takedown operations have been conducted against botnets employing both client-server and peer-to-peer C&C architectures. To improve their robustness against

Matthias Neugschwandtner; Paolo Milani Comparetti; Christian Platzer

2011-01-01

295

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

296

Characterization of Brazed Joints of C-C Composite to Cu-clad-Molybdenum  

NASA Technical Reports Server (NTRS)

Carbon-carbon composites with either pitch+CVI matrix or resin-derived matrix were joined to copper-clad molybdenum using two active braze alloys, Cusil-ABA (1.75% Ti) and Ticusil (4.5% Ti). The brazed joints revealed good interfacial bonding, preferential precipitation of Ti at the composite/braze interface, and a tendency toward de-lamination in resin-derived C-C composite due to its low inter-laminar shear strength. Extensive braze penetration of the inter-fiber channels in the pitch+CVI C-C composites was observed. The relatively low brazing temperatures (<950 C) precluded melting of the clad layer and restricted the redistribution of alloying elements but led to metallurgically sound composite joints. The Knoop microhardness (HK) distribution across the joint interfaces revealed sharp gradients at the Cu-clad-Mo/braze interface and higher hardness in Ticusil (approx.85-250 HK) than in Cusil-ABA (approx.50-150 HK). These C-C/Cu-clad-Mo joints with relatively low thermal resistance may be promising for thermal management applications.

Singh, M.; Asthana, R.

2008-01-01

297

Structural basis of carotenoid cleavage: from bacteria to mammals.  

PubMed

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

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

2013-11-15

298

The strong aromatic hydrogen bonding in crystalline propargylammonium tetraphenylborate  

NASA Astrophysics Data System (ADS)

Crystalline propargylammonium tetraphenylborate contains strong N +?H and moderate C?C?H hydrogen bond donors, but no conventional acceptors. Therefore, the donors can only form ?-type hydrogen bonds with the phenyl groups of the anion, leading to a dense system of aromatic hydrogen bonds which involves also the ethynyl groups. For the latter, this is the first documented case of C?C?HPh hydrogen bonds in an ionic compound; the infrared absorption spectrum shows very similar characteristics as previously observed for C?C?HPh interactions in uncharged systems. The distances of the donor H-atoms to the aromatic centroids are in the range 2.08-2.37 for N +?H and 2.55-2.77 for C?C?H donors.

Steiner, Thomas; Schreurs, Antoine M. M.; Kanters, Jan A.; Kroon, Jan; van der Maas, John; Lutz, Bert

1997-12-01

299

An Unbiased Proteomic Screen Reveals Caspase Cleavage Is Positively and Negatively Regulated by Substrate Phosphorylation*  

PubMed Central

Post-translational modifications of proteins regulate diverse cellular functions, with mounting evidence suggesting that hierarchical cross-talk between distinct modifications may fine-tune cellular responses. For example, in apoptosis, caspases promote cell death via cleavage of key structural and enzymatic proteins that in some instances is inhibited by phosphorylation near the scissile bond. In this study, we systematically investigated how protein phosphorylation affects susceptibility to caspase cleavage using an N-terminomic strategy, namely, a modified terminal amino isotopic labeling of substrates (TAILS) workflow, to identify proteins for which caspase-catalyzed cleavage is modulated by phosphatase treatment. We validated the effects of phosphorylation on three of the identified proteins and found that Yap1 and Golgin-160 exhibit decreased cleavage when phosphorylated, whereas cleavage of MST3 was promoted by phosphorylation. Furthermore, using synthetic peptides we systematically examined the influence of phosphoserine throughout the entirety of caspase-3, -7, and -8 recognition motifs and observed a general inhibitory effect of phosphorylation even at residues considered outside the classical consensus motif. Overall, our work demonstrates a role for phosphorylation in controlling caspase-mediated cleavage and shows that N-terminomic strategies can be tailored to study cross-talk between phosphorylation and proteolysis.

Turowec, Jacob P.; Zukowski, Stephanie A.; Knight, James D. R.; Smalley, David M.; Graves, Lee M.; Johnson, Gary L.; Li, Shawn S. C.; Lajoie, Gilles A.; Litchfield, David W.

2014-01-01

300

Transition-state stabilization in Escherichia coli ribonuclease P RNA-mediated cleavage of model substrates  

PubMed Central

We have used model substrates carrying modified nucleotides at the site immediately 5? of the canonical RNase P cleavage site, the ?1 position, to study Escherichia coli RNase P RNA-mediated cleavage. We show that the nucleobase at ?1 is not essential but its presence and identity contribute to efficiency, fidelity of cleavage and stabilization of the transition state. When U or C is present at ?1, the carbonyl oxygen at C2 on the nucleobase contributes to transition-state stabilization, and thus acts as a positive determinant. For substrates with purines at ?1, an exocyclic amine at C2 on the nucleobase promotes cleavage at an alternative site and it has a negative impact on cleavage at the canonical site. We also provide new insights into the interaction between E. coli RNase P RNA and the ?1 residue in the substrate. Our findings will be discussed using a model where bacterial RNase P cleavage proceeds through a conformational-assisted mechanism that positions the metal(II)-activated H2O for an in-line attack on the phosphorous atom that leads to breakage of the phosphodiester bond.

Wu, Shiying; Chen, Yu; Mao, Guanzhong; Trobro, Stefan; Kwiatkowski, Marek; Kirsebom, Leif A.

2014-01-01

301

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 7585% 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

302

CC bond formation at polynuclear metal centers  

Microsoft Academic Search

Studies on C-C bond formation between simple hydrocarbon species such as CH2, C=CH2, CH=CH2, CH2=CH2, CH2=C=CH2 and CH=CH at a diruthenium center suggest that the process is promoted when the dimetal center can readily compensate for the two electrons lost in the formation of the new C-C bond. Thus, whereas -CH2 and ethene combine only under forcing conditions, the combination

Selby A. R. Knox

1992-01-01

303

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, Lszl; Ruff, Ferenc; Orosz, Gyrgy

2006-06-01

304

Facile scission of isonitrile carbon-nitrogen triple bond using a diborane(4) reagent  

PubMed Central

Transition metal reagents and catalysts are generally effective to cleave all three bonds (one ? and two ?) in a triple bond despite its high bonding energy. Recently, chemistry of single-bond cleavage by using main-group element compounds is rapidly being developed in the absence of transition metals. However, the cleavage of a triple bond using non-transition-metal compounds is less explored. Here we report that an unsymmetrical diborane(4) compound could react with carbon monoxide and tert-butyl isonitrile at room temperature. In the latter case, the carbonnitrogen triple bond was completely cleaved in the absence of transition metal as confirmed by X-ray crystallographic analysis, 13C NMR spectroscopy with 13C labelling and DFT calculations. The DFT calculations also revealed the detailed reaction mechanism and indicated that the key for the carbonnitrogen triple-bond cleavage could be attributed to the presence of nucleophilic nitrogen atom in one of the intermediates.

Asakawa, Hiroki; Lee, Ka-Ho; Lin, Zhenyang; Yamashita, Makoto

2014-01-01

305

Cleavage speed and implantation potential of early-cleavage embryos in IVF or ICSI cycles.  

PubMed

We examined whether there is a correlation among early embryo cleavage, speed of cleavage, and implantation potential for in-vitro fertilization (IVF) treatment and intracytoplasmic sperm injection (ICSI). This retrospective study examined 112 cycles of IVF and 82 cycles of ICSI in patients less than 40 years of age. Early cleavage was defined as embryonic mitosis occurring 25-27 h after insemination. These day-3 embryos were then grouped according to cleavage speed (rapid, normal, and slow) and morphological quality (good or poor). A larger proportion of early-cleavage embryos developed normally compared to non-early-cleavage embryos (IVF: 69.1 % vs. 47.1 %, respectively; ICSI: 63.0 % vs. 45.6 %, respectively). The early-cleavage embryos also produced more good quality embryos than the non-early-cleavage embryos (IVF: 80.2 % vs. 56.4 %, respectively; ICSI: 73.4 % vs. 59.4 %). The implantation rate was significantly higher with early-cleavage embryos in both IVF (42.9 % vs. 19.7 %) and ICSI (48.1 % vs. 24 %). These results indicate that early-cleavage embryos have a higher rate of normal development and develop into better quality embryos on day 3, resulting in more and higher quality embryos to choose from for day-3 embryo transfer. Thus, early cleavage may be a useful criterion when selecting embryos for IVF or ICSI. PMID:22825967

Lee, Meng-Ju; Lee, Robert Kuo-Kuang; Lin, Ming-Huei; Hwu, Yuh-Ming

2012-08-01

306

Modelling ab initio non-bonded interactions for sigma bond, pi bond, and lone pair orbitals  

NASA Astrophysics Data System (ADS)

Ab initio eigenvalue splittings between symmetric and anti-symmetric combinations of energetically degenerate and environmentally equivalent molecular orbitals (MOs) are presented for pairs of ethane molecules and pairs of formaldehyde molecules in various geometrical configurations and as a function of separation. These weak non-bonded orbital interactions are computed for the MO of C?C sigma bond character in ethane, and for MOs of C?O pi bond character and those of axial and perpendicular oxygen lone pair character in formaldehyde. The relationship between simple orbital overlaps and the HartreeFock splittings in this beyond-van-der-Waals distance range is examined using overlaps based on valence bond wavefunctions constructed from single zeta Slater type orbitals. The role of the hydrogen atoms interposed in the region between the interacting MOs is examined. The suitability of the computed interactions as calibrations of effective transfer integrals needed in macromolecular electronic analyses is discussed.

Gruschus, James M.; Kuki, Atsuo

1992-05-01

307

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

PubMed

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

Rawlings, Neil D

2009-01-01

308

Combining preprocessor slicing with C\\/C++ language slicing  

Microsoft Academic Search

Of the very few practical implementations of program slicing algorithms, the majority deal with C\\/C++ programs. Yet, preprocessor-related issues have been marginally addressed by these slicers, despite the fact that ignoring (or only partially handling) these constructs may lead to serious inaccuracies in the slicing results and hence in the program analysis task being performed. Recently, an accurate slicing method

Lszl Vidcs; rpd Beszdes; Tibor Gyimthy

2009-01-01

309

Thermodynamic properties of carbon in b.c.c. and f.c.c. iron-silicon-carbon solid solutions.  

NASA Technical Reports Server (NTRS)

The equilibrium between hydrogen-methane gas mixtures and Fe-Si-C solid solutions has been investigated both as a function of temperature and carburizing gas composition. The thermodynamic properties of the carbon atoms in both b.c.c. and f.c.c. solid solution have been derived from the equilibrium measurements. The results found have been compared with those of earlier investigations and with the predictions of recent theoretical models on ternary solid solutions containing both substitutional and interstitial solute atoms.

Chraska, P.; Mclellan, R. B.

1971-01-01

310

Catalytic and regiospecific extradiol cleavage of catechol by a biomimetic iron complex.  

PubMed

An iron(III)-catecholate complex of a facial tridentate ligand reacts with dioxygen in the presence of ammonium acetate-acetic acid buffer to cleave the aromatic C-C bond of 3,5-di-tert-butylcatechol regiospecifically resulting in the formation of an extradiol product with multiple turnovers. PMID:24061230

Chatterjee, Sayanti; Sheet, Debobrata; Paine, Tapan Kanti

2013-11-11

311

Selective Inhibition of Carotenoid Cleavage Dioxygenases  

PubMed Central

Members of the carotenoid cleavage dioxygenase family catalyze the oxidative cleavage of carotenoids at various chain positions, leading to the formation of a wide range of apocarotenoid signaling molecules. To explore the functions of this diverse enzyme family, we have used a chemical genetic approach to design selective inhibitors for different classes of carotenoid cleavage dioxygenase. A set of 18 arylalkyl-hydroxamic acids was synthesized in which the distance between an iron-chelating hydroxamic acid and an aromatic ring was varied; these compounds were screened as inhibitors of four different enzyme classes, either in vitro or in vivo. Potent inhibitors were found that selectively inhibited enzymes that cleave carotenoids at the 9,10 position; 50% inhibition was achieved at submicromolar concentrations. Application of certain inhibitors at 100 ?m to Arabidopsis node explants or whole plants led to increased shoot branching, consistent with inhibition of 9,10-cleavage.

Sergeant, Martin J.; Li, Jian-Jun; Fox, Christine; Brookbank, Nicola; Rea, Dean; Bugg, Timothy D. H.; Thompson, Andrew J.

2009-01-01

312

On the Ultrasonic Cleavage of Cyclohexanol.  

National Technical Information Service (NTIS)

The ultrasonic cleavage of cyclohexanol in aqueous suspension produces acetylene. The rate of the reaction is independent of the concentration of cyclohexanol and is unaffected by the addition of the solid radical scavengers, acylamide and allylthiourea, ...

D. L. Currell S. S. Nagy

1970-01-01

313

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

314

Forming Bonds.  

ERIC Educational Resources Information Center

Explains how one North Carolina school district was able to pass high-dollar bond issues for school construction. Discusses the planning and bidding processes, and the district's philosophy towards incorporating innovative changes in design and equipment. (GR)

Spoor, Dana L.

1998-01-01

315

Site-specific cleavage of duplex DNA by a semisynthetic nuclease via triple-helix formation.  

PubMed Central

A Lys-84----Cys mutant staphylococcal nuclease was selectively linked to the 5' and/or 3' terminus of a thiol-containing polypyrimidine oligonucleotide via a disulfide bond. The oligonucleotide-staphylococcal nuclease adduct is capable of binding to a homopurine-homopyrimidine region of Watson-Crick duplex DNA by the formation of a triple-helical structure. Upon the addition of Ca2+, the nuclease cleaves DNA at sites adjacent to the homopurine tract. Specific double-strand cleavage occurred predominantly at A + T-rich sites to the 5' side of the homopurine tract for both the 5'-derivatized and the 5',3'-diderivatized nucleases; the 3'-derivatized nuclease gave no cleavage. The cleavage pattern is asymmetric and consists of multiple cleavage sites shifted to the 5' side on each strand, centered at the terminal base pair of the binding site. Microgram amounts of plasmid pDP20 DNA (4433 base pairs) containing a homopurine-homopyrimidine tract were selectively cleaved by a semisynthetic nuclease with greater than 75% efficiency at room temperature within 1 hr. Cleavage reaction conditions were optimized with respect to pH, temperature, reaction times, and reaction components. Semisynthetic nucleases of this type should provide a powerful tool in chromosomal DNA manipulations. Images

Pei, D; Corey, D R; Schultz, P G

1990-01-01

316

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

SciTech Connect

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

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

1993-02-12

317

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 ?46-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

318

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

319

Evaluation Of Single-Stranded Oligonucleotide Cleavage Function Of Seryl-Histidine Dipeptide By Electrospray Ionization Mass Spectrometry  

Microsoft Academic Search

The cleavage action of Ser-His on a series of single-stranded oligonucleotides was investigated by Fourier-transform ion cyclotron resonance mass spectrometry with electrospray ionization. It was found that polyadenine was selectively hydrolyzed at the terminal phosphodiester bond.

Pei-yan Chen; Yan Liu; Xiang Gao; Niu-sheng Xu; Jun Niu; Shu-ying Liu; Yufen Zhao

2011-01-01

320

Selective reductive cleavage of 2,3-epoxybromides by the InCl 3NaBH 4 reagent system  

Microsoft Academic Search

A combination of sodium borohydride and a catalytic amount of indium(III) chloride in acetonitrile reduces 2,3-epoxybromides to the corresponding allylic alcohols in good yields involving reduction of the bromo moiety followed by selective CO bond cleavage through a radical process. Several aromatic, cyclic and open-chain bromoepoxides successfully participated in this reaction.

Brindaban C. Ranu; Subhash Banerjee; Arijit Das

2004-01-01

321

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

322

Neutral and ionic metal-hydrogen and metal-carbon bond energies: Reactions of Co sup + , Ni sup + , and Cu sup + with ethane, propane, methylpropane, and dimethylpropane  

SciTech Connect

The reactions of Co{sup +}, Ni{sup +}, and Cu{sup +} with a series of alkanes are examined by guided ion beam mass spectrometry. The emphasis of this study is on C-H and C-C bond cleavage channels from which bond dissociation energies for M-H, M-CH{sub 3}, and M{sup +}-CH{sub 3} are derived from the endothermic reaction thresholds. For these three bond energies, we find values (in kilocalories per mole) of 46 {plus minus} 3, 46 {plus minus} 3, and 49.1 {plus minus} 3.5, respectively, for M = Co; 58 {plus minus} 3, 55 {plus minus} 3, and 45.0 {plus minus} 2.4, respectively, for M = Ni; 61 {plus minus} 4, 58 {plus minus} 2, 29.7 {plus minus} 1.7, respectively, for M = Cu. Trends in the thermochemistry of these species and M{sup +}-H are briefly discussed. The reactivity of Cu{sup +}, which has not been previously studied, is compared with that for other transition-metal ions. Also, unusual features in the reactivity of Co{sup +} and Ni{sup +} that have not previously been commented on are discussed.

Georgiadis, R. (Univ. of California, Berkeley (USA)); Fisher, E.R.; Armentrout, P.B. (Univ. of Utah, Salt Lake City (USA))

1989-06-07

323

A Simple Test to Determine the Effectiveness of Different Braze Compositions for Joining Ti-Tubes to C/C Composite Plates  

NASA Technical Reports Server (NTRS)

A simple tube-plate joint tensile test was implemented to compare the effectiveness of commercial brazes, namely, TiCuNi, TiCuSil, and Cu-ABA, used for bonding Ti-tubes joined to C-C composite plates. The different braze systems yielded different; yet, repeatable results. The Cu-ABA system proved to have about twice the load-carrying ability of the other two systems due to the fact that the bonded area between the braze material and the C-C plate was largest for this system. The orientation of the surface fiber tows also had a significant effect on load-carrying ability with tows oriented perpendicular to the tube axis displaying the highest failure loads. Increasing the process load and modifying the surface of the C-C plate by grooving out channels for the Ti-Tube to nest in resulted in increased load-carrying ability for the TiCuSil and Cu-ABA systems due to increased bonded area and better penetration of the braze material into the C-C composite.

Morscher, Gregory N.; Singh, Mrityunjay; Shpargel, Tarah; Asthana, Rajiv

2006-01-01

324

Molecular beam pulsed-discharge Fourier transform microwave spectra of CH 3-C?C-F, CH 3-(C?C) 2-F, and CH 3-(C?C) 3-F  

NASA Astrophysics Data System (ADS)

The methylfluoroacetylenes CH 3-(C?C) 2-F and CH 3-(C?C) 3-F have been generated for the first time in a pulsed-discharge nozzle and characterized by molecular beam Fourier transform microwave spectroscopy in the 5-26 GHz frequency range. The spectroscopic constants B=1086.44824(13) MHz, DJ=0.02044(70) kHz, and DJK=7.083(91) kHz of CH 3-(C?C) 2-F and B=478.908444(34) MHz, DJ=0.003060(98) kHz, and DJK=1.899(22) kHz of CH 3-(C?C) 3-F have been determined. In addition, the 13C isotopic species of CH 3-C?C-F in their natural abundances have been measured, and a partial substitution structure of CH 3-C?C-F has been derived and compared with those of related fluorine derivatives.

Blanco, Susana; Sanz, M. Eugenia; Mata, Santiago; Lesarri, Alberto; Lpez, Juan C.; Dreizler, Helmut; Alonso, Jos L.

2003-07-01

325

Osteopontin is highly susceptible to cleavage in bovine milk and the proteolytic fragments bind the ?V??-integrin receptor.  

PubMed

Site-specific and partial proteolysis of milk proteins can both alter and increase their biological activity. The milk protein osteopontin (OPN) is a highly phosphorylated integrin-binding molecule present in most tissues and body fluids. Osteopontin is a biological substrate for matrix metalloproteinases, thrombin, plasmin, and cathepsin D. These proteases cleave OPN at several positions near the integrin-binding sequence Arg-Gly-Asp(138). This cleavage can either increase or reduce the ability of OPN to bind integrins and illustrates that small differences in the cleavage pattern can have a substantial effect on the functionality of OPN. Bovine milk OPN (bOPN) exists in both intact full-length and cleaved forms, and in this study, 6 N-terminal bOPN fragments originating from proteolytic cleavage were purified and characterized by mass spectrometry. These fragments were generated by cleavage at the Lys(145)-Ser(146), Arg(147)-Ser(148), Lys(149)-Lys(150), Phe(151)-Arg(152), Arg(152)-Arg(153), and Arg(153)-Ser(154) peptide bonds. The principal protease in milk, plasmin, appeared to cleave 3 of these sites. However, the major cleavage site was observed to be at the Phe(151)-Arg(152) bond, which does not match the specificity of plasmin. The bOPN fragments were shown to interact with the integrin receptor ?V?? as efficiently as the well-characterized and highly active OPN fragment Ile(1)-Arg(152), generated by thrombin cleavage of human milk OPN. These data show that OPN in milk is highly susceptible to proteolytic cleavage in the region containing the integrin-binding motifs, and the generated fragments are highly capable of binding cells via the ?V??-integrin. PMID:24268404

Christensen, B; Srensen, E S

2014-01-01

326

Bond Dissociation Free Energies (BDFEs) of the Acidic H-A Bonds in HA(*)(-) Radical Anions by Three Different Pathways.  

PubMed

Cleavage of radical anions, HA(*)(-), have been considered to give either H(*) + A(-) (path a) or H(-) + A(*) (path b), and factors determining the preferred mode of cleavage have been discussed. It is conceivable that cleavage to give a proton and a radical dianion, HA(*)(-) right harpoon over left harpoon H(+) + A(*)(2)(-) (path c), might also be feasible. A method, based on a thermodynamic cycle, to estimate the bond dissociation free energy (BDFE) by path c has been devised. Comparison of the BDFEs for cleavage of the radical anions derived from 24 nitroaromatic OH, SH, NH, and CH acids by paths a, b, c has shown that path c is favored thermodynamically. PMID:11667531

Zhao, Yongyu; Bordwell, Frederick G.

1996-09-20

327

Structure of a hyper-cleavable monomeric fragment of phage lambda repressor containing the cleavage site region.  

PubMed

The key event in the switch from lysogenic to lytic growth of phage lambda is the self-cleavage of lambda repressor, which is induced by the formation of a RecA-ssDNA-ATP filament at a site of DNA damage. Lambda repressor cleaves itself at the peptide bond between Ala111 and Gly112, but only when bound as a monomer to the RecA-ssDNA-ATP filament. Here we have designed a hyper-cleavable fragment of lambda repressor containing the hinge and C-terminal domain (residues 101-229), in which the monomer-monomer interface is disrupted by two point mutations and a deletion of seven residues at the C terminus. This fragment crystallizes as a monomer and its structure has been determined to 1.8 A resolution. The hinge region, which bears the cleavage site, is folded over the active site of the C-terminal oligomerization domain (CTD) but with the cleavage site flipped out and exposed to solvent. Thus, the structure represents a non-cleavable conformation of the repressor, but one that is poised for cleavage after modest rearrangements that are presumably stabilized by binding to RecA. The structure provides a unique snapshot of lambda repressor in a conformation that sheds light on how its self-cleavage is tempered in the absence of RecA, as well as a framework for interpreting previous genetic and biochemical data concerning the RecA-mediated cleavage reaction. PMID:16934834

Ndjonka, Dieudonn; Bell, Charles E

2006-09-22

328

Extension of microwave-accelerated residue-specific acid cleavage to proteins with carbohydrate side chains and disulfide linkages  

NASA Astrophysics Data System (ADS)

This laboratory has introduced a chemical method for residue-specific protein cleavage and has provided a preliminary assessment of the suitability of microwave-accelerated acid cleavage as a proteomic tool. This report is a continuing assessment of the fate of common protein modifications in microwave-accelerated acid cleavage. We have examined the cleavage of ribonuclease A and the related N-linked glycoprotein ribonuclease B, and the O-linked glycoprotein alpha crystallin A chain, using MALDI-TOF and LC-ESI-MS to identify the peptide products. RNase A and B each contains four disulfide bonds, and the addition of a reducing reagent, such as dithiothreitol, was found to be required to achieve efficient acidic proteolysis. The linkage of the glycosidic group to the asparagine side chain in ribonuclease B was found not to be cleaved by brief microwave treatment in 12.5% acetic acid. The distribution of the heterogeneous carbohydrate side chain in the glycopeptide products of acid cleavage was compared to that of the glycopeptide products of tryptic digestion. Hydrolysis within the carbohydrate chain itself is minimal under the conditions used. The O-linked side chain on alpha crystalline A was found to be cleaved during acid cleavage of the protein.

Li, Jinxi; Shefcheck, Kevin; Callahan, John; Fenselau, Catherine

2008-12-01

329

Etch Pits on Calcite Cleavage Faces  

Microsoft Academic Search

THIS communication describes some of the effects observed on calcite cleavage faces after etching with dilute acid. Puchegger1 has reported etch pits which formed a rib-like pattern on either side of scratch marks made on calcite. More striking features are described here.

H. Watts

1959-01-01

330

Cleavage and inactivation of ATM during apoptosis.  

PubMed

The activation of the cysteine proteases with aspartate specificity, termed caspases, is of fundamental importance for the execution of programmed cell death. These proteases are highly specific in their action and activate or inhibit a variety of key protein molecules in the cell. Here, we study the effect of apoptosis on the integrity of two proteins that have critical roles in DNA damage signalling, cell cycle checkpoint controls, and genome maintenance-the product of the gene defective in ataxia telangiectasia, ATM, and the related protein ATR. We find that ATM but not ATR is specifically cleaved in cells induced to undergo apoptosis by a variety of stimuli. We establish that ATM cleavage in vivo is dependent on caspases, reveal that ATM is an efficient substrate for caspase 3 but not caspase 6 in vitro, and show that the in vitro caspase 3 cleavage pattern mirrors that in cells undergoing apoptosis. Strikingly, apoptotic cleavage of ATM in vivo abrogates its protein kinase activity against p53 but has no apparent effect on the DNA binding properties of ATM. These data suggest that the cleavage of ATM during apoptosis generates a kinase-inactive protein that acts, through its DNA binding ability, in a trans-dominant-negative fashion to prevent DNA repair and DNA damage signalling. PMID:10454555

Smith, G C; d'Adda di Fagagna, F; Lakin, N D; Jackson, S P

1999-09-01

331

Granzyme B cleavage of autoantigens in autoimmunity.  

PubMed

The systemic autoimmune diseases are a complex group of disorders characterized by elaboration of high titer autoantibodies and immune-mediated damage of tissues. Two striking features of autoimmune rheumatic diseases are their self-sustaining nature and capacity for autoamplification, exemplified by disease flares. These features suggest the presence of a feed-forward cycle in disease propagation, in which immune effector pathways drive the generation/release of autoantigens, which in turn fuel the immune response. There is a growing awareness that structural modification during cytotoxic granule-induced cell death is a frequent and striking feature of autoantigens, and may be an important principle driving disease. This review focuses on granzyme B (GrB)-mediated cleavage of autoantigens including (i) features of GrB cleavage sites within autoantigens, (ii) co-location of cleavage sites with autoimmune epitopes, and (iii) GrB sensitivity of autoantigens in disease-relevant target tissue. The mechanisms whereby GrB-induced changes in autoantigen structure may contribute to the initiation and propagation of autoimmunity are reviewed and reveal that GrB has the potential to create or destroy autoimmune epitopes. As there remains no direct evidence showing a causal function for GrB cleavage of antigens in the generation of autoimmunity, this review highlights important outstanding questions about the function of GrB in autoantigen selection. PMID:20075942

Darrah, E; Rosen, A

2010-04-01

332

Transcript assisted phosphodiester bond hydrolysis by eukaryotic RNA polymerase II  

PubMed Central

Hydrolysis of the phosphodiester bonds of the transcript by bacterial RNA polymerase is assisted by 3?NMP of the RNA. Here we provide evidence that this mechanism is also involved in RNA cleavage by eukaryotic RNA polymerase II, suggesting that transcript assisted hydrolysis has emerged before divergence of bacteria and archaea/eukaryotes.

Nielsen, Soren; Zenkin, Nikolay

2013-01-01

333

Hydrocarbon formation in the reductive cleavage of hydroperoxides by cytochrome P-450.  

PubMed Central

Evidence is presented that cytochrome P-450 catalyzes the reductive cleavage of hydroperoxides. For example, in a reconstituted system containing rabbit liver microsomal P-450 form 2, NADPH-cytochrome P-450 reductase, and NADPH, cumyl hydroperoxide yields acetophenone and methane, but no cumyl alcohol is formed. The stoichiometry of the reaction and similar results with alpha-methylbenzyl, benzyl, and t-butyl hydroperoxides are in accord with the following general equation, in which X represents an alkyl group and R and R' are either alkyl groups or hydrogen atoms in the starting peroxide: XRR'C-OOH + NADPH + H+----XRCO + R'H + H2O + NADP+. Because 13-hydroperoxy-9,11-octadecadienoic acid yields pentane under these conditions, we propose that the known formation of alkanes and aldehydes in membrane lipid peroxidation involves reductive cleavage by P-450 to give the products predicted by the above equation. The cleavage reaction is thought to involve stepwise one-electron transfer, resulting in homolysis of the peroxide oxygen-oxygen bond and generation of an alkoxy radical, with beta-scission of the latter followed by reduction of the secondary radical to the hydrocarbon. In accordance with this scheme, when the cleavage reaction with cumyl hydroperoxide was done in 2H2O, deuteromethane was formed.

Vaz, A D; Coon, M J

1987-01-01

334

DNA cleavage induced by antitumor antibiotic leinamycin and its biological consequences  

PubMed Central

The natural product leinamycin has been found to produce abasic sites in duplex DNA through the hydrolysis of the glycosidic bond of guanine residues modified by this drug. In the present study, using a synthetic oligonucleotide duplex, we demonstrate spontaneous DNA strand cleavage at leinamycin-induced abasic sites through a -elimination reaction. However, methoxyamine modification of leinamycin-induced abasic sites was found to be refractory to the spontaneous -elimination reaction. Furthermore, this complex was even resistant to the -elimination reaction with hot piperidine treatment. Bleomycin and methyl methanesulfonate also induced strand cleavage in a synthetic oligonucleotide duplex even without thermal treatment. However, methoxyamine has a negligible effect on DNA strand cleavage induced by both drugs, suggesting that the mechanism of DNA cleavage induced by leinamycin might be different from those induced by bleomycin or methyl methanesulfonate. In this study, we also assessed the cytotoxicity of leinamycin against a collection of mammalian cell lines defective in various repair pathways. The mammalian cell line defective in the nucleotide excision repair (NER) or base excision repair (BER) pathways was about 3 to 5 times more sensitive to leinamycin as compared to the parental cell line. In contrast, the radiosensitive mutant xrs-5 cell line deficient in V(D)J recombination showed similar sensitivity towards leinamycin compared to the parental cell line. Collectively, our findings suggest that both NER and BER pathways play an important role in the repair of DNA damage caused by leinamycin.

Viswesh, Velliyur; Hays, Allison M.; Gates, Kent; Sun, Daekyu

2012-01-01

335

Defect structures in deformed F.C.C. metals  

SciTech Connect

A high density of small defect clusters, similar to those observed in irradiated or quenched metals, has been observed in the deformed f.c.c. metals Cu, Au and Ni. The preliminary results show that the defect clusters are predominantly stacking fault tetrahedral (SFT). The SFT number density, rather than the size distribution, is deformation dependent. The defect cluster density is greater in the vicinities of dislocation tangles and grain boundaries. Their size distribution is wider than that produced by irradiation with an important number of larger clusters being formed. It is argued that these deformation-produced clusters may play a role in determining the flow stress and work hardening at low deformations.

Dai, Y.; Victoria, M. [Ecole Polytechnique Federale de Lausanne, Villigen PSI (Switzerland). CRPP-Fusion Technology Div.] [Ecole Polytechnique Federale de Lausanne, Villigen PSI (Switzerland). CRPP-Fusion Technology Div.

1997-08-01

336

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

337

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

PubMed

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

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

2014-06-01

338

Cleavage delamination in impact tested warm-rolled steel  

Microsoft Academic Search

Cleavage delaminations or separations have been observed on fracture surfaces of impact specimens made from rolled steel pipe\\u000a and plate products. These delaminations appear as cleavage cracks lying parallel to the plane of rolling. Depending primarily\\u000a on the test temperature and extent of rolling, four fracture modes have been reported: ductile, transgranular, ductile with\\u000a cleavage delaminations, and transgranular with cleavage

D. L. Bourell

1983-01-01

339

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

340

Preparation and characterization of SiC\\/C and C\\/SiC\\/C composites using pulse chemical vapor infiltration process  

Microsoft Academic Search

A porous carbon plate and carbon fiber\\/carbon composite (C\\/C) preforms were infiltrated with SiC using a pulse chemical vapor infiltration (Pulse-CVI) process, and the Pulse-CVI conditions for obtaining the densely infiltrated SiC\\/C or C\\/SiC\\/C composites were examined. The optimum reaction temperatures, at which the pores were uniformly filled with SiC deposits throughout the thickness, were 1173 K for the carbon

Akihito Sakai; Jun Gotoh; Seiji Motojima

1996-01-01

341

Chalcogen bond: a sister noncovalent bond to halogen bond.  

PubMed

A sister noncovalent bond to halogen bond, termed chalcogen bond, is defined in this article. By selecting the complexes H(2)CS...Cl(-), F(2)CS...Cl(-), OCS...Cl(-), and SCS...Cl(-) as models, the bond-length change, interaction energy, topological property of the electron charge density and its Laplacian, and the charge transfer of the chalcogen bond have been investigated in detail theoretically. It was found that the similar misshaped electron clouds of the chalcogen atom and the halogen atom result in the similar properties of the chalcogen bond and the halogen bond. Experimental results are in good agreement with the theoretical predictions. PMID:19537765

Wang, Weizhou; Ji, Baoming; Zhang, Yu

2009-07-16

342

A Dynamic Pathway for Stone-Wales Bond Rotation on Carbon Nanotubes through Diamond-Like Bonds  

NASA Technical Reports Server (NTRS)

A new lower energy barrier with a two-step pathway of Stone-Wales (SW) ,ond rotation on carbon nanotubes (CNTs) is found through molecular dynamics (MD) simulations of CNTs under tension. The first step involves going over to a stable sp3-like metastable configuration with half rotated and partially tilted C-C bond. The second step involves going over to the fully rotated C-C bond with the formation of a SW defect in the nanotube. The energy barrier for this two-step dynamic pathway is significantly lower than the previously known static barrier for in-plane rotation of the C-C bond on a tensile strained (> 4%) CNT.

Wei, Chen-Yu; Srivastava, Deepak; Cho, Kyeong-Jae; Menon, Madhu

2003-01-01

343

Pi Bond Orders and Bond Lengths  

ERIC Educational Resources Information Center

Discusses three methods of correlating bond orders and bond lengths in unsaturated hydrocarbons: the Pauling theory, the Huckel molecular orbital technique, and self-consistent-field techniques. (MLH)

Herndon, William C.; Parkanyi, Cyril

1976-01-01

344

Analysis of the cleavage site of the human immunodeficiency virus type 1 glycoprotein: requirement of precursor cleavage for glycoprotein incorporation.  

PubMed

Endoproteolytic cleavage of the glycoprotein precursor to the mature SU and TM proteins is an essential step in the maturation of retroviral glycoproteins. Cleavage of the precursor polyprotein occurs at a conserved, basic tetrapeptide sequence and is carried out by a cellular protease. The glycoprotein of the human immunodeficiency virus type 1 contains two potential cleavage sequences immediately preceding the N terminus of the TM protein. To determine the functional significance of these two potential cleavage sites, a series of mutations has been constructed in each site individually, as well as in combinations that altered both sites simultaneously. A majority of the mutations in either potential cleavage site continued to allow efficient cleavage when present alone but abrogated cleavage of the precursor when combined. Despite being transported efficiently to the cell surface, these cleavage-defective glycoproteins were unable to initiate cell-cell fusion and viruses containing them were not infectious. Viruses that contained glycoproteins with a single mutation, and that retained the ability to be processed, were capable of mediating a productive infection, although infectivity was impaired in several of these mutants. Protein analyses indicated that uncleaved glycoprotein precursors were inefficiently incorporated into virions, suggesting that cleavage of the glycoprotein may be a prerequisite to incorporation into virions. The substitution of a glutamic acid residue for a highly conserved lysine residue in the primary cleavage site (residue 510) had no effect on glycoprotein cleavage or function, even though it removed the only dibasic amino acid pair in this site. Peptide sequencing of the N terminus of gp41 produced from this mutant glycoprotein demonstrated that cleavage continued to take place at this site. These results, demonstrating that normal cleavage of the human immunodeficiency virus type 1 glycoprotein can occur when no dibasic sequence is present at the cleavage site, raise questions about the specificity of the cellular protease that mediates this cleavage and suggest that cleavage of the glycoprotein is required for efficient incorporation of the glycoprotein into virions. PMID:7609032

Dubay, J W; Dubay, S R; Shin, H J; Hunter, E

1995-08-01

345

Solar energy assisted starch-stabilized palladium nanoparticles and their application in C-C coupling reactions.  

PubMed

Present work reports a novel one step, greener protocol for the synthesis of starch-stabilized palladium nanoparticles (PdNPs) with an average particle diameter of 30-40 nm. These particles were stable and uniform in size. In present protocol, the concentrated solar energy mediated reduction of palladium chloride was achieved by using citric acid as a reducing agent and starch as a capping agent. UV-Visible spectroscopy, Transmission Electron Microscopy, Field Emission Gun-Scanning Electron Microscopy, Selected Area Electron Diffraction and Electron dispersive X-ray Spectral analysis techniques were used to characterize this starch capped PdNPs. Herein; we are reporting such combination of starch and citric acid in the synthesis of PdNPs for the first time. The catalytic activity of synthesized nanoparticles has been checked for Suzuki and Heck cross coupling reactions. The product yield was confirmed by GC. The products were confirmed using GC-MS analysis and also using GC with the help of authentic standards. Solar energy assisted starch stabilized PdNPs showed excellent activity in the C-C bond formation between aryl halides (I, Br) with phenyl boronic acid and its derivatives. In addition, the catalyst showed good activity in the Heck coupling reaction of C-C bond formation of aryl halides with aromatic alkene. The use of starch, citric acid, water and solar energy makes present protocol greener. PMID:23901531

Patil, Aniruddha B; Bhanage, Bhalchandra M

2013-07-01

346

[Gas chromatographic analysis of reduction products of paraquat, diquat and the related compounds: reductive cleavage in the pyridine ring on N-alkylpyridinium derivatives with NaBH4-NiCl2 reduction system, and inhibition of the cleavage].  

PubMed

When N-alkylpyridinium derivatives were reduced with sodium borohydride-nickel (II) chloride reduction system, reductive cleavage occurred at the C-N bond in the pyridine ring of N-alkylpyridinium derivatives to give a small amount of reductive cleavage product along with the major perhydrogenated product. It was presumed in the previous report that this reductive cleavage in the pyridine ring proceeded through a complex of nickel ion and 1,2,3,6-tetrahydropyridine derivatives produced with NaBH4 alone reduction. The abundances of these reductive cleavage products arising from N-alkylpyridinium derivatives, i.e., paraquat, diquat and so on, are capable of giving a bad effect on the accuracy of gas chromatographic analysis. For the purpose of inhibition of the reductive cleavage in this reduction system, a suitable catalyst was examined. In addition, we pursued whether borane-1,2,3,6-tetrahydropyridine derivative complexes arose from N-alkylpyridinium derivatives by NaBH4 alone reduction or not, and whether these borane-amine complexes were the precursors of reductive cleavage products or not. N-Alkyl-1,2,3,6-tetrahydropyridine derivatives (III-I, IV-I, VI-I, VII-I and VIII-I) and the corresponding borane-amine complexes (III-II, IV-II, VI-II, VII-II and VIII-II) were synthesized by NaBH4 reduction in aqueous solution of N-alkylpyridinium salts, i.e. I, II, 1,4-dimethylpyridinium iodide (III), 1-dodecylpyridinium chloride (IV), 1,1'-diethyl-4,4'-dipyridinium dichloride (V), 1-methyl-4-phenylpyridinium iodide (VI), 1-n-propylpyridinium iodide (VII) and 1-n-butylpyridinium iodide (VIII). The structure of the borane-amine complexes were proved by the Mass spectrometry and 1H- and 13C-NMR analysis. The NiCl2-NaBH4 reduction of the borane-amine complexes gave the perhydrogenated products alone, but not reductive cleavage products. In conclusion, it was recognized that the precursors of reductive cleavage products were not borane-amine complexes, but 1,2,3,6-tetrahydropyridine. Furthermore, it was found the reductive cleavage at the C-N bond in the pyridine ring of these 1,2,3,6-tetrahydropyridine derivatives was hindered by applying Amberlite-Ni2B, NaBH4 reduction system. PMID:8587033

Kanno, S; Takekoshi, Y; Kawase, S; Kiho, T; Shimizu, H; Ukai, S

1995-12-01

347

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 Smria; Abhijit Ghosh

2000-01-01

348

A new approach to monitoring proteolysis phenomena using antibodies specifically directed against the enzyme cleavage site on its substrate.  

PubMed

Proteolysis is a generic biochemical process that is central in all biological activities. A new strategy for monitoring this biochemical process is proposed here. This approach is based on the production of rabbit polyclonal anti-peptide antibodies directly against the cleavage site on the substrate of the enzyme responsible for proteolysis. So long as the molecule's cleavage site is intact, the antibody will bind to the protein. However, after cleavage of the peptide bond by the protease, the antibody will no longer be able to recognize the substrate. Thus, the development of an ELISA that uses this specific antibody allows hydrolysis of the substrate protein to be monitored. Hydrolysis of beta-casein by plasmin, the main indigenous protease of milk, during the ripening of Swiss-type cheese, has been chosen as a model for this study. PMID:12758263

Dupont, Didier; Rolet-Repecaud, Odile; Senocq, Daniel

2003-06-15

349

Specific Cleavage Analysis of Mammalian Mitochondrial DNA  

Microsoft Academic Search

Mitochondrial DNA from several mammalian species has been digested with a site-specific restriction endonuclease (HaeIII) from Haemophilus aegyptius. A quantitative analysis of the resulting specific fragments indicates that the mtDNA of any individual mammal is predominantly a single molecular clone. Gel analysis of specific cleavage products has proven quite sensitive in detecting differences in mtDNA: mtDNAs from the more distantly

S. Steven Potter; John E. Newbold; Clyde A. Hutchison; Marshall H. Edgell

1975-01-01

350

Engineering Designer Nucleases with Customized Cleavage Specificities  

PubMed Central

Engineered designer nucleases can be used to efficiently modify genomic sequence in a wide variety of model organisms and cell types. Zinc finger nucleases (ZFNs), consisting of an engineered zinc finger array fused to a non-specific cleavage domain, have been extensively used to modify a broad range of endogenous genes. Here we describe protocols for engineering ZFNs targeted to specific gene sequences of interest using the Context-Dependent Assembly (CoDA) method.

Sander, Jeffry D.; Maeder, Morgan L.; Joung, J. Keith

2012-01-01

351

Site specific enzymatic cleavage of RNA.  

PubMed Central

The hybridization of a DNA oligonucleotide a specific tetramer or longer) will direct a cleavage by RNase H (EC 3.1.4.34) to a specific site in RNA. The resulting fragments can then be labeled at their 5' or 3' ends, purified, and sequenced directly. This procedure is demonstrated with two RNA molecules of known sequence: 5.8S rRNA from yeast (158 nucleotides) and satellite tobacco necrosis virus (STNV) RNA (1240 nucleotides). Images

Donis-Keller, H

1979-01-01

352

Indium-mediated intermolecular alkyl radical addition to electron-deficient C [double bond] N bond and C [double bond] C bond in water.  

PubMed

[reaction: see text] The intermolecular alkyl radical addition to imine derivatives was studied in aqueous media by using indium as a single-electron-transfer radical initiator. The one-pot reaction based on radical addition to glyoxylic hydrazone provided a convenient method for preparing the alpha-amino acids. The indium-mediated radical addition to an electron-deficient C=C bond also proceeded effectively to provide the new carbon-carbon bond-forming method in aqueous media. PMID:11772108

Miyabe, Hideto; Ueda, Masafumi; Nishimura, Azusa; Naito, Takeaki

2002-01-10

353

Gold-catalyzed C(sp(3))-H bond functionalization.  

PubMed

C-H bonds are ubiquitous in organic molecules. Homogenous gold-catalyzed direct functionalization of unsaturated C-H bonds has emerged as a powerful method in our synthetic toolbox. However, Csp(3)-H bonds have larger dissociation energy and lower proton acidity, and thus the efficient and exquisitely selective cleavage of this kind of chemical bonds for the formation of new carbon-carbon and carbon-heteroatom bonds is still a great challenge. In this tutorial review, we will highlight the recent achievements of gold-catalyzed oxidative and redox-neutral Csp(3)-H bond functionalization, which opens new avenues for economical and sustainable construction of fine chemicals. PMID:24853478

Xie, Jin; Pan, Changduo; Abdukader, Ablimit; Zhu, Chengjian

2014-07-01

354

Effect of C.C.C. [2-(chloroethyl)] trimethyl ammonium chloride on growth and sporulation in Fusarium oxysporum f. udum (butl.) Sn. et H.  

PubMed

In Fusarium oxysporum f. udum sporulation of macroconidia was completely inhibited due to the influence of C.C.C. [2-(Chloroethyl)] trimethyl ammonium chloride). Microconidial proliferation, on the other hand, was adversely affected and with increase in concentration of this compound a parallel and proportional decrease in the amount of microconidia was found. Size of microconidia, too, showed a concomitant decline. Although the size of the chlamydospores was also reduced, a tremendous increase in its population was evident, showing a positive response in favour of C.C.C. addition. Fungal mat accumulation, another important aspect of growth, did not evince any inhibitory pattern, compared to the progressive increase in C.C.C. amoung in younger cultures. In older cultures (15-day old), an initial depressing shock of C.C.C. addition could be seen. However, after stepwise and gradual adjustment to the new environment the same positive response of mycelial growth was discernible, although in a less degree than that of the young cultures. PMID:664938

Prasad, M; Chaudhary, S K

1978-01-01

355

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

356

Highly functionalized pyridines synthesis from N-sulfonyl ketimines and alkynes using the N-S bond as an internal oxidant.  

PubMed

The N-S bond-based internal oxidant offers a distinct approach for the synthesis of highly functionalized pyridines. A novel Rh(III)-catalyzed one-pot process undergoes an efficient C-C/C-N bond formation along with desulfonylation under very mild conditions. The method is quite simple, general, and efficient. PMID:24597609

Zhang, Qian-Ru; Huang, Ji-Rong; Zhang, Wei; Dong, Lin

2014-03-21

357

Cleavage duplexes in the Marcellus Shale of the Appalachian foreland  

NASA Astrophysics Data System (ADS)

Cleavage duplexes are zones of platy, spaced cleavage, that either parallel bedding or ramp through shale sections. Examples are 2-60 m thick. The cleavage within the zones is generally subperpendicular to the zone boundaries but sigmoidally dragged against floor, roof and internal thrusts. Between the well-developed floor thrust and the more obscure roof thrusts of the duplexes the shale has been shortened >30% perpendicular to cleavage. The thrusts are sharp strain discontinuities because both overlying and underlying shale is uncleaved and less deformed (<10% layer parallel shortening). Cleavage in the duplexes was initiated perpendicular to bedding by pure shear in front of a propagating tip line. Cleavage halos at the ends of associated stiff carbonate concretions are small scale models illustrating initiation of cleavage. The cleavage is a primary crenulation cleavage with dissolution of limbs indicated by clay carbon partings. Transfer of thrusting from the floor to the roof of the cleavage duplex proceeded incrementally toward the foreland, imposing the simple shear that led to sigmoidal cleavage traces as the cleavage continuously evolved. Environmental conditions for formation of cleavage duplexes during the Alleghanian Orogeny are estimated as temperature 200-250C, and pressure 1.1-1.3 kb, based upon conodont coloration (CA1 4), fluid inclusions and restored stratigraphic thicknesses. Cleavage duplexes or similar fold duplexes have been recognized in suitable black shales of Ordovician to Carboniferous age. They are manifestations of the progressive transfer of slip from floor to roof through a disturbed zone that serves as a shear boundary between large, more internally passive, thrust sheets.

Nickelsen, Richard P.

358

Dual functionalities of hydrogen-bonding self-assembled catalysts in chelation-assisted hydroacylation.  

PubMed

A recyclable catalyst for chelation-assisted hydroacylation of an olefin with primary alcohol was developed using hydrogen-bonding self-assembled catalysts consisting of 2,6-diaminopyridine and barbiturate phosphine-rhodium(I) complex. Upon heating, these two catalysts act as homogeneous catalysts due to cleavage of the hydrogen bond, and these associate to form supramolecular assemblies via hydrogen bonding that can be separated from immiscible product phase upon cooling after the reaction. PMID:18570474

Park, Jung-Woo; Park, Ji-Hye; Jun, Chul-Ho

2008-07-18

359

Ablation properties of C/C-SiC composites tested on an arc heater  

NASA Astrophysics Data System (ADS)

Carbon fiber-reinforced carbon and silicon carbide (C/C-SiC) composites were fabricated by a combination of chemical vapor infiltration and liquid silicon infiltration. Ablation properties of C/C-SiC composites and C/C composites with similar technique were tested on a high-pressure arc heater. The results show that ablation properties of C/C-SiC composites are more severe than those of C/C composites. Ablation of C/C-SiC composites includes oxidation, sublimation of SiC (Si), and mechanical denudation. Oxidation and sublimation of SiC (Si) lead to the enlarged ablation rates between carbon fibers and matrices, which finally cause serious ablation of C/C-SiC composites.

Yin, Jian; Zhang, Hongbo; Xiong, Xiang; Zuo, Jinglv; Tao, Huijin

2011-11-01

360

Selective activation of carbon-carbon bonds next to a carbonyl group  

NASA Astrophysics Data System (ADS)

ORGANOMETALLIC complexes are used to effect a wide range of catalytic transformations in organic synthesis, such as the activation of C-H bonds1,2. Carbon-carbon bonds, however, are generally unreactive towards transition metals under homogeneous conditions. C-C bond activation by a process of oxidative addition to soluble transition-metal complexes has been limited mostly to stoichiometric (not catalytic) reactions1,3-7,18, to highly strained substrates such as cyclopropane and cubane1,8-11 or to chelating ketones19. Here we present a synthetically useful process of selective C-C bond activation in which the C-C bond adjacent to a carbonyl group is opened by insertion of a soluble rhodium(I) complex. The resulting organometallic intermediate can be transformed to a variety of products in a way that regenerates the rhodium complex. We anticipate that this catalytic scheme will have considerable utility in organic synthesis.

Murakami, Masahiro; Amii, Hideki; Ito, Yoshihiko

1994-08-01

361

Proteolytic Cleavage of Notch: "HIT and RUN"  

PubMed Central

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.

van Tetering, G.; Vooijs, M.

2014-01-01

362

Conjugated Polymer Patterning through Photooxidative Backbone Cleavage.  

PubMed

Photolithographic patterning of a xanthate precursor to poly(3,4-diphenyl-2,5-thienylene vinylene) is described. Unlike xanthate precursors to poly(p-phenylene vinylene), the thienylene vinylene analogue patterns as a positive tone resist. Characterization of irradiated films reveals photooxidative cleavage of the vinylene linker decreases the molecular weight of the polymer (increasing the solubility of the UV-exposed areas). As a result of the mechanism, the developed pattern sees no UV light exposure, which is a significant advantage compared with negative-tone-conjugated polymer resists. Single micron resolution of a low-bandgap polymer is achieved in an efficient and scalable process. PMID:24740899

Johnson, Ross S; Haworth, Jacob J; Finnegan, Patrick S; Wheeler, David R; Dirk, Shawn M

2014-06-01

363

DFT studies on the mechanism of palladium-catalyzed carbon-silicon cleavage for the synthesis of benzosilole derivatives.  

PubMed

DFT calculations have been carried out to study the detailed mechanism of Pd-catalyzed intermolecular coupling reactions of 2-silylaryl bromides with alkynes via selective cleavage of C(sp(3))-Si bonds. Through our calculations, we found that, starting from the alkenylpalladium intermediate derived from oxidative addition of the substrate C-Br bond followed by alkyne insertion, there are two possible pathways leading to the formation of the benzosilole product. Furthermore, these two pathways were found to be competitive. In this paper, we will present the detailed mechanistic study and analyze the results we have obtained. PMID:24573107

Chen, Wen-Jie; Lin, Zhenyang

2014-08-01

364

Reversible Sigma C-C Bond Formation Between Phenanthroline Ligands Activated by (C5Me5)2Yb.  

PubMed

The electronic structure and associated magnetic properties of the 1,10-phenanthroline adducts of Cp*2Yb are dramatically different from those of the 2,2'-bipyridine adducts. The monomeric phenanthroline adducts are ground state triplets that are based upon trivalent Yb(III), f(13), and (phen(-) ) that are only weakly exchange coupled, which is in contrast to the bipyridine adducts whose ground states are multiconfigurational, open-shell singlets in which ytterbium is intermediate valent ( J. Am. Chem. Soc 2009 , 131 , 6480 ; J. Am. Chem. Soc 2010 , 132 , 17537 ). The origin of these different physical properties is traced to the number and symmetry of the LUMO and LUMO+1 of the heterocyclic diimine ligands. The bipy(-) has only one ?*1 orbital of b1 symmetry of accessible energy, but phen(-) has two ?* orbitals of b1 and a2 symmetry that are energetically accessible. The carbon p?-orbitals have different nodal properties and coefficients and their energies, and therefore their populations change depending on the position and number of methyl substitutions on the ring. A chemical ramification of the change in electronic structure is that Cp*2Yb(phen) is a dimer when crystallized from toluene solution, but a monomer when sublimed at 180-190 C. When 3,8-Me2phenanthroline is used, the adduct Cp*2Yb(3,8-Me2phen) exists in the solution in a dimer-monomer equilibrium in which ?G is near zero. The adducts with 3-Me, 4-Me, 5-Me, 3,8-Me2, and 5,6-Me2-phenanthroline are isolated and characterized by solid state X-ray crystallography, magnetic susceptibility and LIII-edge XANES spectroscopy as a function of temperature and variable-temperature (1)H NMR spectroscopy. PMID:24852897

Nocton, Grgory; Lukens, Wayne W; Booth, Corwin H; Rozenel, Sergio S; Medling, Scott A; Maron, Laurent; Andersen, Richard A

2014-06-18

365

Stereochemistry of the intramolecular shift of the C-C bond in the reactions of cyclopropene compounds with electrophilic reagents  

SciTech Connect

The characteristic direction in the reactions of cyclopropene compounds with electrophilic reagents leads to the formation of acyclic products, which can in principle have the E and Z configurations. Earlier it was shown that the E isomer is formed preferentially in the reaction of 1-methylcyclopropene with bromine and almost exclusively in the reaction with N-bromosuccinimide. In the present communication the authors give stereochemical data on the reaction of 1-methylcyclopropene with deuterium chloride in CH/sub 3/COOD.

Kartashov, V.R.; Afanas'ev, P.S.; Skorobogatova, E.V.; Chertkov, V.A.; Ermolaeva, V.N.; Sergeev, N.M.; Zefirov, N.S.

1986-09-20

366

Aneuploidy in the human cleavage stage embryo.  

PubMed

The cleavage stage embryo (days 1-3) stands out due to the high level of chromosomal anomalies, especially mosaicism that arises prior to global embryonic genome activation. Molecular cytogenetic studies show that an average of 60% of in vitro derived embryos have at least one aneuploid cell by the time they are 3 days old. However, comprehensive studies of the chromosome content of individual cells have revealed that 25% of these embryos have no aneuploid cells, a fact that sits well with the knowledge that at most 1 in 5 have the capacity to implant. The evidence is that extensive mosaicism, affecting several chromosomes, interferes with development to a greater extent than does uniform aneuploidy. Follow-up studies on embryos after pre-implantation genetic aneuploidy screening indicate that the frequency of meiotic errors varies according to the referral reason, with the highest frequency being recorded for the recurrent miscarriage category and the lowest in the repeated implantation failure group where younger women have a good response to ovarian stimulation. The exceptionally high incidence of pre- and post-zygotic chromosomal anomalies seen in early human embryos is thus the product of several mechanisms. Firstly, the error-prone cell cycle during the embryonic cleavage stage and secondly, parental susceptibility to meiotic and mitotic chromosomal instability together with their general genetic background. PMID:21293113

Mantzouratou, A; Delhanty, J D A

2011-01-01

367

Resistance of actin to cleavage during apoptosis.  

PubMed

A small number of cellular proteins present in the nucleus, cytosol, and membrane fraction are specifically cleaved by the interleukin-1beta-converting enzyme (ICE)-like family of proteases during apoptosis. Previous results have demonstrated that one of these, the cytoskeletal protein actin, is degraded in rat PC12 pheochromocytoma cells upon serum withdrawal. Extracts from etoposide-treated U937 cells are also capable of cleaving actin. It was assumed that cleavage of actin represented a general phenomenon, and a mechanism coordinating proteolytic, endonucleolytic, and morphological aspects of apoptosis was proposed. We demonstrate here that actin is resistant to degradation in several different human cells induced to undergo apoptosis in response to a variety of stimuli, including Fas ligation, serum withdrawal, cytotoxic T-cell killing, and DNA damage. On the other hand, cell-free extracts from these cells and the ICE-like protease CPP32 were capable of cleaving actin in vitro. We conclude that while actin contains cleavage sites for ICE-like proteases, it is not degraded in vivo in human cells either because of lack of access of these proteases to actin or due to the presence of other factors that prevent degradation. PMID:8990178

Song, Q; Wei, T; Lees-Miller, S; Alnemri, E; Watters, D; Lavin, M F

1997-01-01

368

The Structural Basis of Hammerhead Ribozyme Self-Cleavage  

Microsoft Academic Search

We have captured an 8.7 conformational change that takes place in the cleavage site of the hammerhead ribozyme during self-cleavage, using X-ray crystallography combined with physical and chemical trapping techniques. This rearrangement brings the hammerhead ribozyme from the ground state into a conformation that is poised to form the transition state geometry required for hammerhead RNA self-cleavage. Use of

James B. Murray; Daniel P. Terwey; Lara Maloney; Alexander Karpeisky; Nassim Usman; Leonid Beigelman; William G. Scott

1998-01-01

369

DNA Recognition, Strand Selectivity, and Cleavage Mode during Integrase Family Site-specific Recombination*  

PubMed Central

We have probed the association of Flp recombinase with its DNA target using protein footprinting assays. The results are consistent with the domain organization of the Flp protein and with the general features of the protein-DNA interactions revealed by the crystal structures of the recombination intermediates formed by Cre, the Flp-related recombinase. The similarity in the organization of the Flp and Cre target sites and in their recognition by the respective recombinases implies that the overall DNA-protein geometry during strand cleavage in the two systems must also be similar. Within the functional recombinase dimer, it is the interaction between two recombinase monomers bound on either side of the strand exchange region (or spacer) that provides the allosteric activation of a single active site. Whereas Cre utilizes the cleavage nucleophile (the active site tyrosine) in cis, Flp utilizes it in trans (one monomer donating the tyrosine to its partner). By using synthetic Cre and Flp DNA substrates that are geometrically restricted in similar ways, we have mapped the positioning of the active and inactive tyrosine residues during cis and trans cleavage events. We find that, for a fixed substrate geometry, Flp and Cre cleave the labile phosphodiester bond at the same spacer end, not at opposite ends. Our results provide a model that accommodates local heterogeneities in peptide orientations in the two systems while preserving the global functional architecture of the reaction complex.

Tribble, Gena; Ahn, Yong-Tae; Lee, Jehee; Dandekar, Thomas; Jayaram, Makkuni

2013-01-01

370

Chymase Cleavage of Stem Cell Factor Yields a Bioactive, Soluble Product  

NASA Astrophysics Data System (ADS)

Stem cell factor (SCF) is produced by stromal cells as a membrane-bound molecule, which may be proteolytically cleaved at a site close to the membrane to produce a soluble bioactive form. The proteases producing this cleavage are unknown. In this study, we demonstrate that human mast cell chymase, a chymotrypsin-like protease, cleaves SCF at a novel site. Cleavage is at the peptide bond between Phe-158 and Met-159, which are encoded by exon 6 of the SCF gene. This cleavage results in a soluble bioactive product that is 7 amino acids shorter at the C terminus than previously identified soluble SCF. This research shows the identification of a physiologically relevant enzyme that specifically cleaves SCF. Because mast cells express the KIT protein, the receptor for SCF, and respond to SCF by proliferation and degranulation, this observation identifies a possible feedback loop in which chymase released from mast cell secretory granules may solubilize SCF bound to the membrane of surrounding stromal cells. The liberated soluble SCF may in turn stimulate mast cell proliferation and differentiated functions; this loop could contribute to abnormal accumulations of mast cells in the skin and hyperpigmentation at sites of chronic cutaneous inflammation.

Longley, B. Jack; Tyrrell, Lynda; Ma, Yongsheng; Williams, David A.; Halaban, Ruth; Langley, Keith; Lu, Hsieng S.; Schechter, Norman M.

1997-08-01

371

Specific cleavage of agrin by neurotrypsin, a synaptic protease linked to mental retardation.  

PubMed

The synaptic serine protease neurotrypsin is thought to be important for adaptive synaptic processes required for cognitive functions, because humans deficient in neurotrypsin suffer from severe mental retardation. In the present study, we describe the biochemical characterization of neurotrypsin and its so far unique substrate agrin. In cell culture experiment as well as in neurotrypsin-deficient mice, we showed that agrin cleavage depends on neurotrypsin and occurs at two conserved sites. Neurotrypsin and agrin were expressed recombinantly, purified, and assayed in vitro. A catalytic efficiency of 1.3 x 10(4) M(-1) x s(-1) was determined. Neurotrypsin activity was shown to depend on calcium with an optimal activity in the pH range of 7-8.5. Mutagenesis analysis of the amino acids flanking the scissile bonds showed that cleavage is highly specific due to the unique substrate recognition pocket of neurotrypsin at the active site. The C-terminal agrin fragment released after cleavage has recently been identified as an inactivating ligand of the Na+/K+-ATPase at CNS synapses, and its binding has been demonstrated to regulate presynaptic excitability. Therefore, dysregulation of agrin processing is a good candidate for a pathogenetic mechanism underlying mental retardation. In turn, these results may also shed light on mechanisms involved in cognitive functions. PMID:17586728

Reif, Raymond; Sales, Susanne; Hettwer, Stefan; Dreier, Birgit; Gisler, Claudio; Wlfel, Jens; Lscher, Daniel; Zurlinden, Andreas; Stephan, Alexander; Ahmed, Shaheen; Baici, Antonio; Ledermann, Birgit; Kunz, Beat; Sonderegger, Peter

2007-11-01

372

Redox-Triggered C-C Coupling of Alcohols and Vinyl Epoxides: Diastereo- and Enantioselective Formation of All-Carbon Quaternary Centers via tert-(Hydroxy)-Prenylation  

PubMed Central

Iridium catalyzed primary alcohol oxidation triggers reductive C-O bond cleavage of isoprene oxide to form aldehyde-allyliridium pairs that combine to form products of tert-(hydroxy)-prenylation, a motif found in >2000 terpenoid natural products. Curtin-Hammett effects are exploited to enforce high levels of anti-diastereo- and enantioselectivity in the formation of an all-carbon quaternary center. The present redox-triggered carbonyl additions occur in the absence of stoichiometric byproducts, premetallated reagents and discrete alcohol-to-aldehyde redox manipulations.

Feng, Jiajie; Garza, Victoria J.; Krische, Michael J.

2014-01-01

373

Unusual nitrile-nitrile and nitrile-alkyne coupling of Fc-C?N and FC-C?C-C?N.  

PubMed

The reactions of the Group?4 metallocene alkyne complexes, [Cp*2M(?2-Me3SiC2SiMe3)] (1?a: M=Ti, 1?b: M=Zr, Cp*=?5-pentamethylcyclopentadienyl), with the ferrocenyl nitriles, Fc-CN and Fc-C?C-C?N (Fc=Fe(?5-C5H5)(?5-C5H4)), is described. In case of Fc-C?N an unusual nitrilenitrile C-C homocoupling was observed and 1-metalla-2,5-diaza-cyclopenta-2,4-dienes (3?a, b) were obtained. As the first step of the reaction with 1?b, the nitrile was coordinated to give [Cp*2Zr(?2-Me3SiC2SiMe3)(N?C-Fc)] (2?b). The reactions with the 3-ferrocenyl-2-propyne-nitrile FcC?C-C?N lead to an alkynenitrile C-C coupling of two substrates and the formation of 1-metalla-2-aza-cyclopenta-2,4-dienes (4?a, b). For M=Zr, the compound is stabilized by dimerization as evidenced by single-crystal X-ray structure analysis. The electrochemical behavior of 3?a, b and 4?a, b was investigated, showing decomposition after oxidation, leading to different redox-active products. PMID:24615841

Becker, Lisanne; Strehler, Frank; Korb, Marcus; Arndt, Perdita; Spannenberg, Anke; Baumann, Wolfgang; Lang, Heinrich; Rosenthal, Uwe

2014-03-10

374

Modular functionalized polyphosphines for supported materials: previously unobserved (31)P-NMR through-space ABCD spin systems and heterogeneous palladium-catalysed C-C and C-H arylation.  

PubMed

The modular design of polyphosphines, diversely functionalized for facile immobilization on virtually any kind of support, is reported. Previously unobserved ABCD (31)P NMR spin-spin systems evidence the control exercised on the polyphosphines conformation. We illustrate the catalytic performance at low Pd loading of the recyclable immobilized polyphosphines in C-C bond formation reactions. PMID:25008866

Beauprin, Matthieu; Smaliy, Radomyr; Cattey, Hlne; Meunier, Philippe; Ou, Jun; Toy, Patrick H; Hierso, Jean-Cyrille

2014-07-24

375

Research on Frictional Wear Properties of C\\/C Composite with Electric Current  

Microsoft Academic Search

\\u000a Using C\\/C composite and chrome bronze as a friction couple, the frictional wear properties of C\\/C composite with electric\\u000a current is studied in this paper. The results have shown that current, velocity and load are important factors to affect the\\u000a frictional wear properties of C\\/C composite with electric current. The coefficient of friction and the wear rate increase\\u000a with the

L. X. Jia; Y. Z. Zhang; J. Li; L. M. Sun; Y. W. Zhao

376

Specific cleavage of beta-amyloid peptides by a metallopeptidase from Xenopus laevis skin secretions.  

PubMed

Dactylysin (EC 3.5.24.60) is a metalloendopeptidase first isolated from the skin granular gland secretions of Xenopus laevis. This peptidase hydrolyzes bonds on the amino-terminus of singlets and between doublets of hydrophobic amino acids and was considered to play a role in the in vivo inactivation of biologically active regulatory peptides. Here, we show that dactylysin has also the ability to cleave human beta[1-40]-amyloid peptide and related peptides. Cleavage of the wild type beta[1-40]-amyloid peptide form, and to a lesser extent Flemish and Dutch mutants, occurred predominantly at the His14-Glu15 bond. We demonstrate that frog skin exudate contains a full-length amyloid protein precursor detected by immunochemical cross-reactivity with monoclonal antibody against C-terminal human amyloid protein precursor. The possibility that dactylysin, might be involved in normal catabolism of beta amyloid peptide of Xenopus laevis is discussed. PMID:12128061

Clamagirand, Christine; Joulie, Catherine; Panchal, Ma; Sekhri, Redha; Hanquez, Chantal; Cohen, Paul; Rholam, Mohamed

2002-08-01

377

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; Rmenapf, Till

2013-11-01

378

Trypanosome U-deletional RNA editing involves guide RNA-directed endonuclease cleavage, terminal U exonuclease, and RNA ligase activities.  

PubMed Central

We have studied the mechanism of accurate in vitro RNA editing of Trypanosoma brucei ATPase 6 mRNA, using four mRNA-guide RNA (gRNA) pairs that specify deletion of 2, 3, or 4 U residues at editing site 1 and mitochondrial extract. This extract not only catalyzes deletion of the specified number of U residues but also exhibits a novel endonuclease activity that cleaves the input pre-mRNA in a gRNA-directed manner, precisely at the phosphodiester bond predicted in a simple enzymatic model of RNA editing. This cleavage site is inconsistent with a chimera-based editing mechanism. The U residues to be deleted, present at the 3' end of the upstream cleavage product, are then removed evidently by a 3' U-specific exonuclease and not by a reverse reaction of terminal U transferase. RNA ligase can then join the mRNA halves through their newly formed 5' P and 3' OH termini, generating mRNA faithfully edited at the first editing site. This resultant, partially edited mRNA can then undergo accurate, gRNA-directed cleavage at editing site 2, again precisely as predicted by the enzymatic editing model. All of these enzymatic activities cofractionate with the U-deletion activity and may reside in a single complex. The data imply that each round of editing is a four-step process, involving (i) gRNA-directed cleavage of the pre-mRNA at the bond immediately 5' of the region base paired to the gRNA, (ii) U deletion from or U addition to the 3' OH of the upstream mRNA half, (iii) ligation of the mRNA halves, and (iv) formation of additional base pairing between the correctly edited site and the gRNA that directs subsequent nuclease cleavage at the next editing site. Images Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6

Cruz-Reyes, J; Sollner-Webb, B

1996-01-01

379

Autocatalytic Cleavage within Classical Swine Fever Virus NS3 Leads to a Functional Separation of Protease and Helicase  

PubMed Central

Classical swine fever virus (CSFV) is a positive-stranded RNA virus belonging to the genus Pestivirus within the Flaviviridae family. Pivotal for processing of a large portion of the viral polyprotein is a serine protease activity within nonstructural protein 3 (NS3) that also harbors helicase and NTPase activities essential for RNA replication. In CSFV-infected cells, NS3 appears as two forms, a fully processed NS3 of 80 kDa and the precursor molecule NS2-3 of 120 kDa. Here we report the identification and mapping of additional autocatalytic intramolecular cleavages. One cleavable peptide bond occurs between Leu1781 and Met1782, giving rise to a helicase subunit of 55 kDa and, depending on the substrate, a NS2-3 fragment of 78 kDa (NS2-3p) or a NS3 protease subunit of 26 kDa (NS3p). In trans-cleavage assays using NS4-5 as a substrate, NS3p acts as a fully functional protease that is able to process the polyprotein. NS3p comprises the minimal essential protease, as deletion of Leu1781 results in inactivation. A second intramolecular cleavage was mapped to the Leu1748/Lys1749 peptide bond that yields a proteolytically inactive NS3 fragment. Deletion of either of the cleavage site residues resulted in a loss of RNA infectivity, indicating the functional importance of amino acid identity at the respective positions. Our data suggest that internal cleavage within the NS3 moiety is a common process that further extends the functional repertoires of the multifunctional NS2-3 or NS3 and represents another level of the complex polyprotein processing of Flaviviridae.

Lamp, Benjamin; Riedel, Christiane; Wentz, Eveline; Tortorici, Maria-Alejandra

2013-01-01

380

One-step synthesis of diazaspiro[4.5]decane scaffolds with exocyclic double bonds.  

PubMed

Unactivated yne-en-ynes reacted with a range of substituted aryl halides in the presence of Pd(OAc)2-PPh3 to afford diazaspiro[4.5]decane with exocyclic double bonds. Three carbon-carbon bonds are formed in this domino reaction, which involves highly regioselective C-C coupling and spiro scaffold steps. PMID:24956492

Li, Lidong; Hu, Qiong; Zhou, Pingping; Xie, Haifeng; Zhang, Xiaorong; Zhang, Hao; Hu, Yadong; Yin, Fei; Hu, Yimin

2014-07-01

381

Selective and Nonselective Cleavages in Positive and Negative CID of the Fragments Generated from In-Source Decay of Intact Proteins in MALDI-MS  

NASA Astrophysics Data System (ADS)

Selective and nonselective cleavages in ion trap low-energy collision-induced dissociation (CID) experiments of the fragments generated from in-source decay (ISD) with matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) of intact proteins are described in both positive and negative ion modes. The MALDI-ISD spectra of the proteins demonstrate common, discontinuous, abundant c- and z'-ions originating from cleavage at the N-C? bond of Xxx-Asp/Asn and Gly-Xxx residues in both positive- and negative-ion modes. The positive ion CID of the c- and z'-ions resulted in product ions originating from selective cleavage at Asp-Xxx, Glu-Xxx and Cys-Xxx residues. Nonselective cleavage product ions rationalized by the mechanism of a "mobile proton" are also observed in positive ion CID spectra. Negative ion CID of the ISD fragments results in complex product ions accompanied by the loss of neutrals from b-, c-, and y-ions. The most characteristic feature of negative ion CID is selective cleavage of the peptide bonds of acidic residues, Xxx-Asp/Glu/Cys. A definite influence of ?-helix on the CID product ions was not obtained. However, the results from positive ion and negative ion CID of the MALDI-ISD fragments that may have long ?-helical domains suggest that acidic residues in helix-free regions tend to degrade more than those in helical regions.

Takayama, Mitsuo; Sekiya, Sadanori; Iimuro, Ryunosuke; Iwamoto, Shinichi; Tanaka, Koichi

2014-01-01

382

Selective and nonselective cleavages in positive and negative CID of the fragments generated from in-source decay of intact proteins in MALDI-MS.  

PubMed

Selective and nonselective cleavages in ion trap low-energy collision-induced dissociation (CID) experiments of the fragments generated from in-source decay (ISD) with matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) of intact proteins are described in both positive and negative ion modes. The MALDI-ISD spectra of the proteins demonstrate common, discontinuous, abundant c- and z'-ions originating from cleavage at the N-C? bond of Xxx-Asp/Asn and Gly-Xxx residues in both positive- and negative-ion modes. The positive ion CID of the c- and z'-ions resulted in product ions originating from selective cleavage at Asp-Xxx, Glu-Xxx and Cys-Xxx residues. Nonselective cleavage product ions rationalized by the mechanism of a "mobile proton" are also observed in positive ion CID spectra. Negative ion CID of the ISD fragments results in complex product ions accompanied by the loss of neutrals from b-, c-, and y-ions. The most characteristic feature of negative ion CID is selective cleavage of the peptide bonds of acidic residues, Xxx-Asp/Glu/Cys. A definite influence of ?-helix on the CID product ions was not obtained. However, the results from positive ion and negative ion CID of the MALDI-ISD fragments that may have long ?-helical domains suggest that acidic residues in helix-free regions tend to degrade more than those in helical regions. PMID:24135807

Takayama, Mitsuo; Sekiya, Sadanori; Iimuro, Ryunosuke; Iwamoto, Shinichi; Tanaka, Koichi

2014-01-01

383

29 CFR 2580.412-20 - Use of existing bonds, separate bonds and additional bonding.  

Code of Federal Regulations, 2010 CFR

...bonds, separate bonds and additional bonding. 2580.412-20 Section 2580...DEPARTMENT OF LABOR TEMPORARY BONDING RULES UNDER THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974 TEMPORARY BONDING RULES General Bond Rules §...

2009-07-01

384

29 CFR 2580.412-20 - Use of existing bonds, separate bonds and additional bonding.  

Code of Federal Regulations, 2010 CFR

...bonds, separate bonds and additional bonding. 2580.412-20 Section 2580...DEPARTMENT OF LABOR TEMPORARY BONDING RULES UNDER THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974 TEMPORARY BONDING RULES General Bond Rules §...

2010-07-01

385

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 lightmatter 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

386

Methylene blue photosensitised strand cleavage of DNA: effects of dye binding and oxygen.  

PubMed Central

It is shown that methylene blue (MB+) photosensitises DNA in either aerated or deaerated solutions, causing direct cleavage of phosphodiester bonds and rendering additional bonds labile to alkali. Evidence from unwinding and fluorimetric studies indicates that MB+ binds to DNA in at least 2 ways. Intercalation, which optimally induces helical unwinding of 24 degrees +/- 2 degrees per MB+, is markedly reduced upon neutralisation by Mg2+ of the DNA phosphates, while significant non-intercalative binding persists as shown by substantial fluorescence quenching at Mg2+ concentrations where there is little unwinding. MB+ induces photolysis at both low and high Mg2+ concentration - intercalation is apparently not required for photolysis. The quantum yield for strand breakage varies from 1-3 X 10(-7) under different conditions and is oxygen enhanced. The DNA cleavage is guanine specific. The 3' termini of the primary MB+-induced DNA photoproducts, unlike those generated by chemical sequencing retain an alkali labile adduct on the terminal phosphate. Images

OhUigin, C; McConnell, D J; Kelly, J M; van der Putten, W J

1987-01-01

387

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

388

Topological properties of hydrogen bonds and covalent bonds from charge densities obtained by the maximum entropy method (MEM)  

PubMed Central

Charge densities have been determined by the Maximum Entropy Method (MEM) from the high-resolution, low-temperature (T ? 20?K) X-ray diffraction data of six different crystals of amino acids and peptides. A comparison of dynamic deformation densities of the MEM with static and dynamic deformation densities of multipole models shows that the MEM may lead to a better description of the electron density in hydrogen bonds in cases where the multipole model has been restricted to isotropic displacement parameters and low-order multipoles (l max = 1) for the H atoms. Topological properties at bond critical points (BCPs) are found to depend systematically on the bond length, but with different functions for covalent CC, CN and CO bonds, and for hydrogen bonds together with covalent CH and NH bonds. Similar dependencies are known for AIM properties derived from static multipole densities. The ratio of potential and kinetic energy densities |V(BCP)|/G(BCP) is successfully used for a classification of hydrogen bonds according to their distance d(H?O) between the H atom and the acceptor atom. The classification based on MEM densities coincides with the usual classification of hydrogen bonds as strong, intermediate and weak [Jeffrey (1997) ?. An Introduction to Hydrogen Bonding. Oxford University Press]. MEM and procrystal densities lead to similar values of the densities at the BCPs of hydrogen bonds, but differences are shown to prevail, such that it is found that only the true charge density, represented by MEM densities, the multipole model or some other method can lead to the correct characterization of chemical bonding. Our results do not confirm suggestions in the literature that the promolecule density might be sufficient for a characterization of hydrogen bonds.

Netzel, Jeanette; van Smaalen, Sander

2009-01-01

389

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, Mara J; Alquzar, Berta; Als, Enriqueta; Medina, Vctor; Carmona, Lourdes; Bruno, Mark; Al-Babili, Salim; Zacaras, Lorenzo

2013-11-01

390

Structural basis for activation of the complement system by component C4 cleavage  

PubMed Central

An essential aspect of innate immunity is recognition of molecular patterns on the surface of pathogens or altered self through the lectin and classical pathways, two of the three well-established activation pathways of the complement system. This recognition causes activation of the MASP-2 or the C1s serine proteases followed by cleavage of the protein C4. Here we present the crystal structures of the 203-kDa human C4 and the 245-kDa C4?MASP-2 substrate?enzyme complex. When C4 binds to MASP-2, substantial conformational changes in C4 are induced, and its scissile bond region becomes ordered and inserted into the protease catalytic site in a manner canonical to serine proteases. In MASP-2, an exosite located within the CCP domains recognizes the C4 C345C domain 60 from the scissile bond. Mutations in C4 and MASP-2 residues at the C345CCCP interface inhibit the intermolecular interaction and C4 cleavage. The possible assembly of the huge in vivo enzymesubstrate complex consisting of glycan-bound mannan-binding lectin, MASP-2, and C4 is discussed. Our own and prior functional data suggest that C1s in the classical pathway of complement activated by, e.g., antigenantibody complexes, also recognizes the C4 C345C domain through a CCP exosite. Our results provide a unified structural framework for understanding the early and essential step of C4 cleavage in the elimination of pathogens and altered self through two major pathways of complement activation.

Kidmose, Rune T.; Laursen, Nick S.; Dobo, Jozsef; Kjaer, Troels R.; Sirotkina, Sofia; Yatime, Laure; Sottrup-Jensen, Lars; Thiel, Steffen; Gal, Peter; Andersen, Gregers R.

2012-01-01

391

Reinterpretation of the vibrational spectroscopy of the medicinal bioinorganic synthon c,c,t-[Pt(NH3) 2Cl 2(OH) 2].  

PubMed

The Pt(IV) complex c,c,t-[Pt(NH3)2Cl2(OH)2] is an important intermediate in the synthesis of Pt(IV) anticancer prodrugs and has been investigated as an anticancer agent in its own right. An analysis of the vibrational spectroscopy of this molecule was previously reported (Faggiani et al., Can. J. Chem. 60:529, 1982), in which crystallographic determination of the structure of the complex permitted a site group approach. The space group, however, was incorrectly assigned. In the present study we have redetermined at high resolution crystal structures of c,c,t-[Pt(NH3)2Cl2(OH)2] and c,c,t-[Pt(NH3)2Cl2(OH)2]H2O2, which makes possible discussion of the effect of hydrogen bonding on the N-H and O-H vibrational bands. The correct crystallographic site symmetry of the platinum complex in the c,c,t-[Pt(NH3)2Cl2(OH)2] structure is used to conduct a new vibrational analysis using both group-theoretical and modern density functional theory methods. This analysis reveals the nature and symmetry of the "missing band" described in the original publication and suggests a possible explanation for its disappearance. PMID:24515615

Johnstone, Timothy C; Lippard, Stephen J

2014-06-01

392

Copper-catalyzed aerobic oxidative n?s bond functionalization for c?s bond formation: regio- and stereoselective synthesis of sulfones and thioethers.  

PubMed

A regio- and stereoselective synthesis of sulfones and thioethers by means of Cu(I) -catalyzed aerobic oxidative N?S bond cleavage of sulfonyl hydrazides, followed by cross-coupling reactions with alkenes and aromatic compounds to form the C?sp?2?S bond, is described herein. N2 and H2 O are the byproducts of this transformation, thus offering an environmentally benign process with a wide range of potential applications in organic synthesis and medicinal chemistry. PMID:24860978

Li, Xianwei; Xu, Yanli; Wu, Wanqing; Jiang, Chang; Qi, Chaorong; Jiang, Huanfeng

2014-06-23

393

Surface charge heterogeneity in amphibole cleavage fragments and asbestos fibers.  

PubMed

Aspect ratio and electrophoretic mobility data for amphibole particles reveal that short fibers and blocky cleavage fragments have a smaller net charge than highly elongated particles. Asbestos fibers and cleavage fragments of the same dimensions exhibit the same net negative surface charge but positively charged ends and negatively charged lateral surfaces. PMID:17745966

Schiller, J E; Payne, S L; Khalafalla, S E

1980-09-26

394

Surface Charge Heterogeneity in Amphibole Cleavage Fragments and Asbestos Fibers  

NASA Astrophysics Data System (ADS)

Aspect ratio and electrophoretic mobility data for amphibole particles reveal that short fibers and blocky cleavage fragments have a smaller net charge than highly elongated particles. Asbestos fibers and cleavage fragments of the same dimensions exhibit the same net negative surface charge but positively charged ends and negatively charged lateral surfaces.

Schiller, Joseph E.; Payne, Sequoyah L.; Khalafalla, Sanaa E.

1980-09-01

395

Surface Charge Heterogeneity in Amphibole Cleavage Fragments and Asbestos Fibers  

Microsoft Academic Search

Aspect ratio and electrophoretic mobility data for amphibole particles reveal that short fibers and blocky cleavage fragments have a smaller net charge than highly elongated particles. Asbestos fibers and cleavage fragments of the same dimensions exhibit the same net negative surface charge but positively charged ends and negatively charged lateral surfaces.

Joseph E. Schiller; Sequoyah L. Payne; Sanaa E. Khalafalla

1980-01-01

396

Bond strengths of two dental bonding systems.  

PubMed

A new bonding system named Kevloc has been introduced. It is based on acrylization of the metal surface with the goal of preventing the occurrence of a marginal gap between the metal and the resin. The purpose of this investigation was to determine the values of the shear bond strength achieved using the Kevloc technique on Ag-Pd (Auropal SE) and Co-Cr (Basil S) alloys and to compare them with those obtained with the OVS technique. The shear bond strengths were measured with the Smitz-Schulmayer shear test in a universal testing machine for polymer materials. A microscope image analyzer was used to measure the thickness of bonding layers and to reveal the possible occurrence of the marginal gap with both techniques. No marginal gap was detected with either technique. Kevloc provided better results than OVS only in a group of specimens tested after polymerization. Immersion in water and thermocycling reduced the initially high bond strength values of Kevloc specimens, whereas the bond strength values of OVS specimens remained unchanged regardless of which aging treatment was used. Microscopic examination did not reveal the existence of the marginal gap for either bonding system. According to the results obtained, it can be concluded that the Kevloc bonding system does not provide better shearing bond strength than the OVS bonding system. PMID:10920659

Vojvodic, D; Jerolimov, V; Zabarovic, D; Loncar, A

2000-07-01

397

Control of the mitotic cleavage plane by local epithelial topology  

PubMed Central

SUMMARY For nearly 150 years, it has been recognized that cell shape strongly influences the orientation of the mitotic cleavage plane (e.g. Hofmeister, 1863). However, we still understand little about the complex interplay between cell shape and cleavage plane orientation in epithelia, where polygonal cell geometries emerge from multiple factors, including cell packing, cell growth, and cell division itself. Here, using mechanical simulations, we show that the polygonal shapes of individual cells can systematically bias the long axis orientations of their adjacent mitotic neighbors. Strikingly, analysis of both animal epithelia and plant epidermis confirm a robust and nearly identical correlation between local cell topology and cleavage plane orientation in vivo. Using simple mathematics, we show that this effect derives from fundamental packing constraints. Our results suggest that local epithelial topology is a key determinant of cleavage plane orientation, and that cleavage plane bias may be a widespread property of polygonal cell sheets in plants and animals.

Gibson, William T.; Veldhuis, James H.; Rubinstein, Boris; Cartwright, Heather N.; Perrimon, Norbert; Brodland, G. Wayne; Nagpal, Radhika; Gibson, Matthew C.

2012-01-01

398

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

NASA Technical Reports Server (NTRS)

Ceramic Matrix Composites (CMCs) have been proposed for hot structures in scramjet combustors. Previous studies have calculated significant weight savings by utilizing CMCs (active and passive) versus actively cooled metallic scramjet structures. Both a C/C and a C/C-SiC material system fabricated by DLR (Stuttgart, Germany) are being considered for use in a passively cooled combustor design for HIFiRE 8, a joint Australia / AFRL hypersonic flight program, expected to fly at Mach 7 for approximately 30 sec, at a dynamic pressure of 55 kPa. Flat panels of the DLR C/C and the C/C-SiC were tested in the NASA Langley Direct Connect Rig (DCR) at Mach 5 and Mach 6 enthalpy for several minutes. Gaseous hydrogen fuel was used to fuel the scramjet combustor. The test panels were instrumented with embedded Type K and Type S thermocouples. Zirconia felt insulation was used in some of the tests to increase the surface temperature of the C/C-SiC panel for approximately 350degF. The final C/C-SiC panel was tested for 3 cycles totaling over 135 sec at Mach 6 enthalpy. Slightly more erosion was observed on the C/C panel than the C/C-SiC panels, but both material systems demonstrated acceptable recession performance for the HIFiRE 8 flight.

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

2013-01-01

399

The covered bond market  

Microsoft Academic Search

The covered bond market offers investors an alternative to developed country government securities. The valuation of covered bonds is complex. While there is some evidence of differences in the pricing of these bonds by nationality of issuer, these appear to be only weakly related to differences in the respective legislative frameworks. Recent cases show the pricing of covered bonds to

Frank Packer; Ryan Stever; Christian Upper

2007-01-01

400

Wire Bond Temperature Sensor  

Microsoft Academic Search

This work reports the first demonstration of a bond pad test structure with embedded thermopile sensors for the measurement of the transient temperature response during the wire bonding operation. This paper will present the design and operation of the bond pad test structure and show and discuss temperature measurements during the wire bonding operation. The test structure can be used

Shivesh Suman; Michael Gaitan; Yogendra Joshi; George Harman

401

Cleavage of synthetic substrates containing non-nucleotide inserts by restriction endonucleases. Change in the cleavage specificity of endonuclease SsoII.  

PubMed Central

A study was made of the interaction between restriction endonucleases recognizing CCNGG (SsoII and ScrFI) or CCA/TGG (MvaI and EcoRII) DNA sequences and a set of synthetic substrates containing 1,3-propanediol, 1,2-dideoxy-D-ribofuranose or 9-[1'-hydroxy-2'-(hydroxymethyl)ethoxy] methylguanine (gIG) residues replacing either one of the central nucleosides or dG residues in the recognition site. The non-nucleotide inserts (except for gIG) introduced into the recognition site both increase the efficiency of SsoII and change its specificity. A cleavage at the noncanonical position takes place, in some cases in addition to the correct ones. Noncanonical hydrolysis by SsoII occurs at the phosphodiester bond adjacent to the point of modification towards the 5'-end. With the guanine base returned (the substrate with gIG), the correct cleavage position is restored. ScrFI specifically cleaves all the modified substrates. DNA duplexes with non-nucleotide inserts (except for the gIG-containing duplex) are resistant to hydrolysis by MvaI and EcoRII. Prompted by the data obtained we discuss the peculiarities of recognition by restriction endonucleases of 5-membered DNA sequences which have completely or partially degenerated central base pairs. It is suggested that SsoII forms a complex with DNA in an 'open' form. Images

Kubareva, E A; Petrauskene, O V; Karyagina, A S; Tashlitsky, V N; Nikolskaya, I I; Gromova, E S

1992-01-01

402

Gas-phase cleavage of the novel iminium ions [R1CH+NCHR2 ? R1CHN+CHR2]: An experimental and computational study  

NASA Astrophysics Data System (ADS)

The preparation of the novel iminium ions [R1CH+NCHR2 ? R1CHN+CHR2] and mechanistic study of their gas-phase reactions have been carried out by mass spectrometry and density functional theory. DFT calculations show that this kind of iminium ion is similar to the allene in structure. Two main reaction pathways have been assumed: one is the simple cleavage, including homolytic and heterolytic cleavages of the CC bond; the other might be the intramolecular cyclization, which is supported by the loss of HCN.

Zhang, Xiang

2014-01-01

403

H-H, C-H, and C-C NMR spin-spin coupling constants calculated by the FP-INDO method for aromatic hydrocarbons  

NASA Technical Reports Server (NTRS)

The FP-INDO (finite perturbation-intermediate neglect of differential overlap) method is used to calculate the H-H, C-H, and C-C coupling constants in hertz for molecules of six different benzenoid hydrocarbons: benzene, naphthalene, biphenyl, anthracene, phenanthrene, and pyrene. The calculations are based on both the actual and the average molecular geometries. It is found that only the actual molecular geometries can always yield the correct relative order of values for the H-H coupling constants. For the calculated C-C coupling constants, as for the calculated C-H coupling constants, the signs are positive (negative) for an odd (even) number of bonds connecting the two nuclei. Agreements between the calculated and experimental values of the coupling constants for all six molecules are comparable to those reported previously for other molecules.

Long, S. A. T.; Memory, J. D.

1978-01-01

404

Facile scission of isonitrile carbon-nitrogen triple bond using a diborane(4) reagent.  

PubMed

Transition metal reagents and catalysts are generally effective to cleave all three bonds (one ? and two ?) in a triple bond despite its high bonding energy. Recently, chemistry of single-bond cleavage by using main-group element compounds is rapidly being developed in the absence of transition metals. However, the cleavage of a triple bond using non-transition-metal compounds is less explored. Here we report that an unsymmetrical diborane(4) compound could react with carbon monoxide and tert-butyl isonitrile at room temperature. In the latter case, the carbon-nitrogen triple bond was completely cleaved in the absence of transition metal as confirmed by X-ray crystallographic analysis, (13)C NMR spectroscopy with (13)C labelling and DFT calculations. The DFT calculations also revealed the detailed reaction mechanism and indicated that the key for the carbon-nitrogen triple-bond cleavage could be attributed to the presence of nucleophilic nitrogen atom in one of the intermediates. PMID:24967910

Asakawa, Hiroki; Lee, Ka-Ho; Lin, Zhenyang; Yamashita, Makoto

2014-01-01

405

Determination of the proteolytic cleavage sites of the amyloid precursor-like protein 2 by the proteases ADAM10, BACE1 and ?-secretase.  

PubMed

Regulated intramembrane proteolysis of the amyloid precursor protein (APP) by the protease activities ?-, ?- and ?-secretase controls the generation of the neurotoxic amyloid ? peptide. APLP2, the amyloid precursor-like protein 2, is a homolog of APP, which shows functional overlap with APP, but lacks an amyloid ? domain. Compared to APP, less is known about the proteolytic processing of APLP2, in particular in neurons, and the cleavage sites have not yet been determined. APLP2 is cleaved by the ?-secretase BACE1 and additionally by an ?-secretase activity. The two metalloproteases ADAM10 and ADAM17 have been suggested as candidate APLP2 ?-secretases in cell lines. Here, we used RNA interference and found that ADAM10, but not ADAM17, is required for the constitutive ?-secretase cleavage of APLP2 in HEK293 and SH-SY5Y cells. Likewise, in primary murine neurons knock-down of ADAM10 suppressed APLP2 ?-secretase cleavage. Using mass spectrometry we determined the proteolytic cleavage sites in the APLP2 sequence. ADAM10 was found to cleave APLP2 after arginine 670, whereas BACE1 cleaves after leucine 659. Both cleavage sites are located in close proximity to the membrane. ?-secretase cleavage was found to occur at different peptide bonds between alanine 694 and valine 700, which is close to the N-terminus of the predicted APLP2 transmembrane domain. Determination of the APLP2 cleavage sites enables functional studies of the different APLP2 ectodomain fragments and the production of cleavage-site specific antibodies for APLP2, which may be used for biomarker development. PMID:21695060

Hogl, Sebastian; Kuhn, Peer-Hendrik; Colombo, Alessio; Lichtenthaler, Stefan F

2011-01-01

406

Determination of the Proteolytic Cleavage Sites of the Amyloid Precursor-Like Protein 2 by the Proteases ADAM10, BACE1 and ?-Secretase  

PubMed Central

Regulated intramembrane proteolysis of the amyloid precursor protein (APP) by the protease activities ?-, ?- and ?-secretase controls the generation of the neurotoxic amyloid ? peptide. APLP2, the amyloid precursor-like protein 2, is a homolog of APP, which shows functional overlap with APP, but lacks an amyloid ? domain. Compared to APP, less is known about the proteolytic processing of APLP2, in particular in neurons, and the cleavage sites have not yet been determined. APLP2 is cleaved by the ?-secretase BACE1 and additionally by an ?-secretase activity. The two metalloproteases ADAM10 and ADAM17 have been suggested as candidate APLP2 ?-secretases in cell lines. Here, we used RNA interference and found that ADAM10, but not ADAM17, is required for the constitutive ?-secretase cleavage of APLP2 in HEK293 and SH-SY5Y cells. Likewise, in primary murine neurons knock-down of ADAM10 suppressed APLP2 ?-secretase cleavage. Using mass spectrometry we determined the proteolytic cleavage sites in the APLP2 sequence. ADAM10 was found to cleave APLP2 after arginine 670, whereas BACE1 cleaves after leucine 659. Both cleavage sites are located in close proximity to the membrane. ?-secretase cleavage was found to occur at different peptide bonds between alanine 694 and valine 700, which is close to the N-terminus of the predicted APLP2 transmembrane domain. Determination of the APLP2 cleavage sites enables functional studies of the different APLP2 ectodomain fragments and the production of cleavage-site specific antibodies for APLP2, which may be used for biomarker development.

Hogl, Sebastian; Kuhn, Peer-Hendrik; Colombo, Alessio; Lichtenthaler, Stefan F.

2011-01-01

407

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.

408

Direct bonding and beyond  

Microsoft Academic Search

We present a number of recent evaluations of direct bonding, a glueless bonding technology, performed under ambient conditions. If combined with bond-strengthening, this geometry-conserving technology is well suited for an application in far ultraviolet immersion lithography. Our term beyond direct bonding refers to taking at least one additional technological step beyond direct bonding, involving chemical interface engineering, advanced silicon-on-insulator (SOI)

Jan Haisma; Nico Hattu; Esther Steding; Jan C. G. Vervest

2007-01-01

409

Lessons from Nature: Unraveling Biological C--H Bond Activation  

PubMed Central

The cleavage of unactivated CH bonds is one of the most challenging reactions in chemical biology. Metalloenzymes have evolved that efficiently perform these transformations with exquisite control of selectivity; however, a proposed requirement is the generation of highly reactive intermediates that could be lethal. A thermodynamic argument involving the putative reactive species is outlined, whereby the interplay between two tunable parameters, redox potential and pKa, may be the key to sustainable function. In addition, factors that control these parameters are also described, including hydrogen-bonding networks found within protein active sites. Synthetic examples are used to corroborate these ideas.

Stone, Kari L.; Borovik, A. S.

2009-01-01

410

Oxygen-promoted c?h bond activation at palladium.  

PubMed

[Pd(P(Ar)(tBu)2 )2 ] (1, Ar=naphthyl) reacts with molecular oxygen to form Pd(II) hydroxide dimers in which the naphthyl ring is cyclometalated and one equivalent of phosphine per palladium atom is released. This reaction involves the cleavage of both C?H and O?O bonds, two transformations central to catalytic aerobic oxidizations of hydrocarbons. Observations at low temperature suggest the initial formation of a superoxo complex, which then generates a peroxo complex prior to the C?H activation step. A transition state for energetically viable C?H activation across a Pd?peroxo bond was located computationally. PMID:24817523

Scheuermann, Margaret L; Boyce, David W; Grice, Kyle A; Kaminsky, Werner; Stoll, Stefan; Tolman, William B; Swang, Ole; Goldberg, Karen I

2014-06-16

411

LET'S BOND! A Chemical Bonding Webquest  

NSDL National Science Digital Library

Today we are going to use the internet to explore chemical bonding! Even though there are just a few questions for each website, you need to read the entire content. Don't worry about understanding all of it, but make sure that you are familiar with it! Stay on task and have fun! Let's start with some basics. Click on the link below and answer the questions on your worksheet under "Bonding Basics". Bonding Basics Good job! Lets move on and talk about ions. Ions are a big part of bonding, so make sure you get this section down pat! Click on the ...

Hicken, Mrs.

2009-04-08

412

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.

413

The VHSE-based prediction of proteasomal cleavage sites.  

PubMed

Prediction of proteasomal cleavage sites has been a focus of computational biology. Up to date, the predictive methods are mostly based on nonlinear classifiers and variables with little physicochemical meanings. In this paper, the physicochemical properties of 14 residues both upstream and downstream of a cleavage site are cha