Showing papers on "Bond cleavage published in 1980"
TL;DR: It is proposed that strand scission at a dimer site by the M. luteus enzyme requires two activities, a pyrimidine dimer DNA-glycosylase and an apyrimidinic/apurinic endonuclease.
Abstract: Pyrimidine dimer formation in response to UV radiation is governed by the thymine content of the potential dimer and the two flanking nucleotides. An enzymatic activity can be purified from Micrococcus luteus that cleaves the N-glycosyl bond between the 5′ pyrimidine of a dimer and the corresponding sugar without rupture of a phosphodiester bond. We propose that strand scission at a dimer site by the M. luteus enzyme requires two activities, a pyrimidine dimer DNA-glycosylase and an apyrimidinic/apurinic endonuclease.
279 citations
120 citations
TL;DR: It is concluded that the carbon-hydrogen bond cleavage step does not contribute significantly to Vmax and the contribution of the Carbon-Hydrogen bond Cleavage step to V max is not detectably increased through changes in the cytochrome P-450/NADPH-cytochromeP-450 reductase mole ratio.
81 citations
TL;DR: Trialkylaluminium-mediated alkylation of enol phosphates under the CO bond cleavage is performed stereospecifically in the presence of a catalytic amount of Pd(PPh3)4.
73 citations
TL;DR: In this paper, even-electron ions of lifetimes ≥ 10−5s were correlated with electron and chemical ionization and by collisional activation, and various reaction classifications were proposed, such as cleavage of one bond with charge migration, cleavage through cyclization-displacement, cyclic ion with charge retention, and rearrangement and charge retention.
Abstract: Fragmentations of even-electron ions of lifetimes ≥10−5s caused by electron and chemical ionization and by collisional activation have been correlated. Proposed reaction classifications include: cleavage of one bond with charge migration; cleavage of one bond through cyclization-displacement; cleavage of two bonds in a cyclic ion with charge retention; and cleavage of two bonds in an acyclic ion with rearrangement and charge retention. Such reactions are compared with those of odd-electron ions; despite a higher tendency for rearrangement, the decompositions of a positive even-electron ion, in particular as displayed in its collisional activation spectrum, have substantial utility for characterizing its structure.
59 citations
TL;DR: In this article, the authors investigated the reaction of 5-C5H5Fe(CO)2R (R = alkyl and aryl) with CF3CO2H to give a given FeR bond in organic solvents, mostly at 25°C.
42 citations
TL;DR: It is shown that dihydrolipoic acid (2,3-thioctic acid) acylates the distal oxygen of ferrous oxygenated Pseudomonas cytochrome P-450, forming a transient acyl peroxide intermediate that facilitates oxygen-oxygen bond cleavage.
Abstract: Using isotopic tracer methods, we have shown that dihydrolipoic acid (2,3-thioctic acid) acylates the distal oxygen of ferrous oxygenated Pseudomonas cytochrome P-450, forming a transient acyl peroxide intermediate that facilitates oxygen-oxygen bond cleavage. Single-turnover studies with 18O2 indicate one oxygen-18 atom incorporated into the carboxylate group of lipoic acid for each oxygen-18 inserted into the substrate, camphor, forming the product, exo-5-hydroxycamphor. Such a branching ratio for label indicates that water is initially released from an unlageled position and illustrates that the general P-450 mixed-function oxidase stoichiometry generates H218O from 18O2 only after multiple-turnover equilibration with the acylating carboxylate oxygen. Formation of an acyl peroxide state is a natural intermediate in peracid, "oxene", or radical mechanisms for methylene carbone oxygenation.
40 citations
38 citations
TL;DR: Nickelacyclobutane complexes decompose when heated and undergo competitive carbon-carbon bond cleavage to give isobutene and ethylene, with reductive elimination affording 1,1-dimethylcyclopropane and skeletal isomeri-zation of the metallacyclic ring yielding 3-methyl-1-butene.
32 citations
TL;DR: Aqueous solutions of the antitumor alkaloid camptothecin, its sodium salt, or a number of derivatives, when irradiated with 360 nm light in the presence of covalently closed circular DNA, produced single strand breaks in the DNA.
30 citations
TL;DR: The chapter explains the best overall procedure for mapping of the binding region of a polysaccharide depolymerase, which includes the energetic of binding of the productive positional isomers and on the respective hydrolytic rate coefficients.
Abstract: Publisher Summary This chapter discusses subsite mapping, the term applied to the experimental determination of the number of subsites, comprising the binding region, the energetics of interaction of each subsite with a monomer residue, and the hydrolytic rate coefficients The value of the generated subsite map lies in its ability to correctly predict the action pattern of the enzyme This development is restricted to subsite mapping of polysaccharide depolymerases; however, the general principles developed here are also applicable, within certain limitations, to other polymer-acting enzymes The interactions of maltotetraose with the binding region of a hypothetical five-subsite amylase are discussed in the chapter If the substrate binds to expose a susceptible bond to the catalytic amino acids on the enzyme, the complex is productive and bond cleavage can occur The remaining positional isomers are nonproductive in the sense that they cannot lead to bond scission The rates of bond scission of the substrate depend on the energetic of binding of the productive positional isomers and on the respective hydrolytic rate coefficients The chapter explains the best overall procedure for mapping of the binding region of a polysaccharide depolymerase These are as follows: (1) test for complicating reactions, (2) use bond cleavage frequencies to establish the binding region topography and to measure the apparent subsite binding energies, and (3) use K m and V measured as a function of substrate chain length to complete the subsite map
TL;DR: Acid phosphatase activities have been partially purified from an aqueous extract of an acetone powder from orange flavedo with the use of a gel filtration step with an ionic gradient allowing a dissociation of proteins from pigments, thus facilitating purification and stabilization of the enzymes.
TL;DR: In this paper, the 1,2 aryl migration and fragmentation reactions of 1-indanylmethyl (1), 2-tetralyl (2), 2-(o-vinylphenyl)ethyl radical were studied by flash vacuum pyrolysis of the tert-butyl perester precursors at 327 to 627/sup 0/C and 10/sup -2/ torr.
Abstract: The 1,2 aryl migration and fragmentation reactions of 1-indanylmethyl (1), 2-tetralyl (2), 2-indanylmethyl (3), 1-tetralyl (4), 2-methyl-1-indanyl (5), and 1-methyl-2-indanyl (6) radicals were studied by flash vacuum pyrolysis of the tert-butyl perester precursors at 327 to 627/sup 0/C and 10/sup -2/ torr. Radicals 1 and 2 are interconverted via a 1,2 aryl migration which is readily reversible at all temperatures. This equilibrium is depleted by ..beta.. scission of 1 and recyclization to 4 and by ..beta.. scission of 2 followed by recyclization to 2 or to 3 in modest yields. The reverse neophyl-like rearrangement of 2 to 1 occurs with a lower activation barrier than ..beta.. scission of 1 to form a 2-(o-vinylphenyl)ethyl radical. Enthalpies, entropies, and free energies of reaction were calculated for the above reactions from group additivity parameters, and activation energies were estimated from values reported for simple alkyl radicals. It is shown that the ..beta.. scission of 4 and recyclization to 1 is important only at very high temperatures (> 500/sup 0/C) as a mechanism for the isomerization of tetralin and related hydroaromatic structures to alkylindans and that the reverse neophyl-like rearrangement of 2 to 1 is the favored pathway for isomerizations observed during dissolution ofmore » coal in hydroaromatic media at elevated temperatures.« less
TL;DR: A study conducted in which optical and circular dichroism spectroscopy were used to explore the interaction between ethanoloamine ammonia-lyase and a series of adenosylcobalamin analogs found that at least one element of the force which weakens the C-Co bond of the enzyme-bound cofactor is the relief of conformational strain which occurs when C- Co bond cleavage occurs.
TL;DR: In this article, a cis-trans equilibrium is attained at 20 −30°C with the trans isomer being thermodynamically more stable in nearly all cases, while the bis-phosphine isomerise intramolecularly without bond cleavage.
TL;DR: Results indicate that the phosphodiesterase-catalyzed hydrolysis of cGMP proceeds by nucleophilic substitution at phosphorus resulting in P-O bond cleavage and the stoichiometry of 18O incorporation into 5'-GMP indicates that the reaction proceeds without phosphate-water oxygen exchange.
TL;DR: In this article, E.s.r. spectroscopy has been employed to characterize radical pathways in the thermal and photolytic decomposition of a variety of diaryl sulphoxides and aryl arenethiolsulphonates.
Abstract: E.s.r. spectroscopy has been employed to characterize radical pathways in the thermal and photolytic decomposition of a variety of diaryl sulphoxides and aryl arenethiolsulphonates (ArSO2SAr). For the sulphoxides, S–C bond cleavage leads to the formation of the delocalized and relatively unreactive sulphinyl radicals (ArSO·); their subsequent reaction evidently involves disproportionation to sulphonyl (ArSO2·) and thiyl (ArS·) radicals. This pair of radicals is also formed directly in the homolysis of the thiolsulphonates, and the subsequent formation of sulphinyl radicals has been investigated. The results are analysed in terms of the products of the reactions and the efficacy of some of the parent substrates as antioxidants.
TL;DR: In this paper, the oxime ethers (7) and (8) undergo base-catalysed elimination to benzonitriles (3) in water-dioxan (4 : 1) at 25°.
Abstract: The oxime ethers (7) and (8)(R = alkyl or aryl) undergo base-catalysed elimination to benzonitriles (3) in water–dioxan (4 : 1) at 25°. The Z-isomer (8Z) reacts via hydroxide-catalysed antiperiplanar elimination 70-fold more rapidly than the corresponding E-isomer. Electronic effects on syn-elimination (from 7E) show that electronwithdrawing groups aid elimination in both Ar and in the leaving group –OR; no intramolecular assistance is observed in these substrates. These results are interpreted in terms of a central E2 elimination, with appreciable C–H and N–O– bond cleavage in the transition state.
TL;DR: No direct evidence was obtained for nicking occurring through phosphotriester hydrolysis, but the kinetics of nicking were characteristic of a multistep reaction such as depurination/depyrimidination strand scission and the detection of apurinic sites in BaP-diol epoxide alkylated DNA strongly supported this mechanism.
Abstract: Approximately 1% of (+-) -7..beta.., 8..cap alpha..-dihydroxy-9..cap alpha..,10..cap alpha..-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (BaP-diol epoxide) DNA alkylation sites rearrange with strand scission at neutral pH. Phosphotriester hydrolysis and depurination/depyrimidination strand scission were critically examined as possible mechanisms for this phenomenon. No direct evidence was obtained for nicking occurring through phosphotriester hydrolysis. Studies with model substrates, including dibutyl phosphate, DNA homopolymers, and TMV RNA, indicated that if BaP-diol epoxide forms phosphotriesters in DNA or RNA, they do not hydrolyze with strand scission. Besides apurinic/apyrimidinic sites, a second alkali-sensitive rearrangement product was present in BaP-diol epoxide modified DNA. These latter sites accumulated with time and after 24 h accounted for as much as 4% of the initial alkylation events.
TL;DR: In this article, the IR and NMR data (1H, 13C, 29Si, 119Sn, 207Pb) of the new heterometal-diazoalkanes are reported and discussed in comparison to relevant compounds of the organometallic diazo-alkane series.
TL;DR: Results show the existence of an identical cleavage site and a similar NH2- and COOH-terminal amino acid sequence in both the polypeptides, and indicate that the difference in the molecular weights of p27 and p27(0) results from an insertion of amino acids in the COOh- terminal peptide rather than a shift in the scission site of the precursor molecule.
TL;DR: Hparan sulphate isolated from monolayers of human lung fibroblasts and subjected to degradations revealed that iduronic acid may be located in sequences such as glucosamine-N-sulphate-->iduronic Acid-->N-acetylglucosamine, and periodate-insensitive uronic acid residues were recovered in saccharides of the general structure.
Abstract: 1. 3H- and 35S-labelled heparan sulphate was isolated from monolayers of human lung fibroblasts and subjected to degradations by (a) deaminative cleavage and (b) periodate oxidation/alkaline elimination. Fragments were resolved by gel- and ion-exchange-chromatography. 2. Deaminative cleavage of the radioactive glycan afforded mainly disaccharides with a low content of ester-sulphate and free sulphate, indicating that a large part (approx. 80%) of the repeating units consisted of uronosyl-glucosamine-N-sulphate. Blocks of non-sulphated [glucuronosyl-N-acetyl glucosamine] repeats (3–4 consecutive units) accounted for the remainder of the chains. 3. By selective oxidation of glucuronic acid residues associated with N-acetylglucosamine, followed by scission in alkali, the radioactive glycan was degraded into a series of fragments. The glucuronosyl-N-acetylglucosamine-containing block regions yielded a compound N-acetylglucosamine–R, where R is the remnant of an oxidized and degraded glucuronic acid. Periodate-insensitive uronic acid residues were recovered in saccharides of the general structure glucosamine–(uronic acid–glucosamine)n–R. 4. Further degradations of these saccharides via deaminative cleavage and re-oxidations with periodate revealed that iduronic acid may be located in sequences such as glucosamine-N-sulphate→iduronic acid→N-acetylglucosamine. Occasionally the iduronic acid was sulphated. Blocks of iduronic acid-containing repeats may contain up to five consecutive units. Alternating arrangements of iduronic acid- and glucuronic acid-containing repeats were also observed. 5. 3H- and 35S-labelled heparan sulphates from sequential extracts of fibroblasts (medium, EDTA, trypsin digest, dithiothreitol extract, cell-soluble and cell-insoluble material) afforded similar profiles after both periodate oxidation/alkaline elimination and deaminative cleavage.
TL;DR: In this paper, reactions in the presence of rhodium or palladium catalysts involving CX (X = Cl, Br, I) bond breaking, single and double insertion of unsaturated substrates, and finally H-transfer are reported.
TL;DR: In this article, the synthesis of gem-difluorocyclopropyl ketones (3a,c,d) and their reactions with nucleophiles are described.
TL;DR: In this paper, it was shown that the allyldimethylsilyl cation in the ion cyclotron resonance spectrometer produces adducts which decompose by loss of C3H6 to yield the ion.
Abstract: Diallyldimethylsilane provides a source of the allyldimethylsilyl cation in the ion cyclotron resonance spectrometer; reaction of this cation with alcohols (ROH) produces adducts which decompose by loss of C3H6 to yield the ion . This elimination is thought to occur by a 1,5-hydrogen shift, together with either stepwise or concerted silicon-carbon bond cleavage. The corresponding aducts from ethers ROR1 (R1⩾R, R1⩾Et) first lose (R1H˙) and then undergo the elimination described above.
TL;DR: Hemin dimethylester and cobalt(II/III) octaethylporphyrins have been found to catalyze the autoxidation of squalene, C30H50.
TL;DR: In this article, a carbon anode was found to be effective for N,N-coupling of cyclic and open-chain diamines with the use of platinum, silver, nickel, and carbon anodes.
Abstract: Intramolecular N,N-coupling by the electrochemical oxidation of cyclic and open-chain diamines such as 3,3,7,7-tetraethylperhydro-1,5-diazocine (1) and N,N-dimethyl-2,2-diethyl-1,3-diaminopropane (2) was investigated with the use of platinum, silver, nickel, and carbon anodes. In the case of 1, cyclic hydrazine derivatives, 3,3,7,7-tetraethyl-1,5-diazabicyclo[3.3.0]octane and 1-(2-ethyl-2-formylbutyl)-4,4-diethyl-4,5-dihydropyrazole, were formed in high current efficiency. In the case of 2, the corresponding hydrazines, 1,2-dimethyl-4,4-diethylpyrazolidine and 1-methyl-3,3-diethyl-4,5-dihydropyrazole, were obtained in low current efficiency, because C–N bond cleavage took place before N,N-coupling. A carbon anode was found to be effective for N,N-coupling of such diamines. Chemical oxidative N,N-coupling of 1 and 2 was studied by using sodium peroxodisulfate and lead tetraacetate.
TL;DR: In weakly coordinating solvents, a parallel rearrangement of I occurs through CO bond cleavage of the allylic ether by Fe2(CO)9 yielding an unsaturated ketone (VI) bonded to two Fe(CO)-3 groups through a trimethylenemethane and a 1,3-diene system, respectively as discussed by the authors.