Showing papers on "Bond cleavage published in 1975"

Kinetics of action of pepsin on fluorescent peptide substrates

Abstract: Oligopeptide substrates of porcine pepsin (E) of the type A-Phe-Phe-B (S) that are cleaved solely at the Phe-Phe bond under the conditions of these studies, and bearing an amino-terminal fluorescent probe group (mansyl or dansyl), have been used for stopped-flow measurements of the rate of formation of the A-Phe product. These experiments were conducted under conditions of [E] greater than [S], and the kinetic data were compared with those obtained under conditions of [S] greater than [E] for the formation of the Phe-B product (the same in all cases). The results for substrates with A = mansyl-Gly, mansyl-Gly-Gly, and dansyl-Gly-Gly support the conclusion that the rate-limiting step in the over-all catalytic process is associated with the scission of the Phe-Phe bond in the first detectables ES complex. Although the rate of this step varies widely with the nature of the A portion of A-Phe-Phe-B, the magnitude of the dissociation constant of ES is relatively invariant. This supports the view that, in the cleavage of oligopeptide substrates by pepsin, secondary enzyme--substrate interactions may cause conformational changes at the catalytic site, and that a portion of the total binding energy may be used for the attainment of the transition state in the bond-breaking step. With substrates that are hydrolyzed extremely rapidly (A = dansyl-Gly-Ala, dansyl-Ala-Ala), the rate of formation of the A-Phe product appears to be faster than the steady-state rate, suggesting that an additional step has become kinetically significant in the over-all process. This step may be associated with the return of the conformation of the active site to its original state.

Conformational properties of bovine plasma albumin with a cleaved internal peptide bond.

Abstract: As shown previously, proteinases frequently associated with plasma albumin samples catalyze a very limited and specific cleavage of the albumin molecule when it exists in the F conformational state near pH 3.7. The primary proteolytic product, BPA, has a molecular weight similar to or identical with that of the parent protein but yields two large fragments of molecular weight approximately 46000 and 23000 on reduction. Evidence is presented here that cleavage occurs within the disulfide loop between Cys390 and Cys434 with no detectable loss of small peptides, the amino acid composition of BPA being identical with that of the parent protein within experimental error. Cleavage exposes a new amino-terminal phenylalanine residue and may occur at the Glx392-Phe393 bond although the possibility exists that it occurs at another X-Phe bond in the unsequenced region of residues 400-402. The damaged protein has a somewhat altered secondary structure as judged from optical rotatory dispersion and circular dichroism measurements, probably an approximate 15% loss in helicity. The hydrodynamic volume is increased by approximately 20%. However, various physical studies indicate the tertiary structure to be strikingly similar to that of the native protein. Of most significance is the fact that the protein still undergoes the N-F and N-B transitions, although in both cases they occur at somewhat more moderate pH than in the parent protein. Moreover a sensitivity of the N-B transition to Ca2+ is still seen and binding behavior toward the dye 8-anilino-1-naphthalenesulfonic acid is essentially unaltered. The results are best understood in terms of the concept of a multidomain structure which has been suggested frequently for plasma ablumin. Bond cleavage damages one domain but leaves the overall structure essentially unaltered except for some weakening of the interaction between domains.

Radiation Chemistry of DNA-Model Compounds, VIII Dephosphorylation Products from Reactions of OH Radicals with Ribose-5-phosphate in Aqueous Solution. The Effect of Oxygen

Abstract: The reaction of hydroxyl radicals with ribose-5-phosphate in oxygenated aqueous solution leads to the following phosphate free products (G-values): Erythrodialdose (0.4), formic acid (0.4) and ribodialdose (0.1). In addition, inorganic phosphate (0.6) and some minor phosphate free products (total G≈ 0.08) are formed.Within the limits of experimental error material balance between dephosphorylation products and inorganic phosphate is obtained. A mechanism is proposed which can account for the main products. The primary steps of the dephosphorylation are H atom abstraction from C-5 of ribose-5-phosphate by OH radicals and subsequent addition of O₂. The peroxy radicals formed decompose to ~80% by C-C bond cleavage followed by elimination of inorganic phosphate and only to ~20% by direct dephosphorylation. The relevance of these results with respect to the radiolysis of DNA is discussed.

Oxidation of Cyclopropylamines and Aziridines with Lead Tetraacetate

Abstract: The title reaction of 2-phenylcyclopropylamine gives cinnamaldehyde, while that of the 1-phenyl isomer produces benzonitrile and ethylene. The similar treatment of 2-phenylaziridine provides benzaldehyde as the only isolable product. These reactions are explained by assuming the common intermediacy of nitrenium ions such as (Remark: Graphics omitted.) respectively. The assumption is confirmed by the oxidation of the corresponding amines with sodium hypochlorite. Entention of the reaction to 2,3-polymethyleneaziridines with R group on C-2 afforded R–CO–(CH2)n–C≡N type products (R=H, alkyl, phenyl) in preparative yields. This provides a novel method of C=C bond cleavage of cycloalkenes.

Phenyl participation as a mechanistic probe in the electrophilic cleavage of the iron–carbon bond

Abstract: Electrophilic cleavage of the Fe–C bond of (3) by halogens involves the formation of phenonium ion (4), while cleavage by HgX2 and HX does not, nor does halogen cleavage of the Co–C bond in (5).

The effect of temperature and light on the carbon-13 nuclear magnetic resonance spectra of alkylcorrinoids, selectively enriched with carbon-13

Abstract: 13 C N.m.r. spectra of aqueous (D2O) solutions of methyl-, ethyl-, and adenoxyl-cobalamin, and methylaquo-cobinamide, selectively enriched with carbon-13 in the alkyl ligands attached to cobalt have been recorded at 25.2 MHz and over the temperature range 5–90 °C. The methyl resonances of [13C]methylcobalamin and [13C]methylaquocobinamide, as well as the methylene resonances of [1,2-l3C2]ethylcobalamin and [5′-13C]adenosylcobalamin, exhibit small upfield shifts with increasing temperature. These small differences in chemical shift can best be accounted for by invoking a temperature-dependent conformational change involving substituents on the corrin ring, rather than a change in the co-ordination number of cobalt. 13C N.m.r. spectroscopy has also been used to study homolytic cleavage of the carbon–cobalt bond of the alkylcobalamins. Ethylcobalamin undergoes thermal decomposition even at 60 °C, while methyl- and adenosyl-cobalamin are stable in aqueous solution at 94 °C for periods of up to 5 h. Methylcobalamin is very resistant to photolytic cleavage in the absence of oxygen, whereas ethyl- and adenosyl-cobalamin are photolyzed to cob(II)alamin under the same conditions.

The oxazolinone intermediate in the hydrolysis and aminolysis of N-benzoylglycine derivatives

Abstract: The Bronsted type relationship with the pKa of the departing hydroxy-group for the alkaline hydrolysis (kOH) of N-benzoylglycine esters exhibits a break consistent with a change in mechanism. Substituted phenyl esters hydrolyse via an oxazolinone intermediate and the high Bronsted selectivity indicates a rate-determining step with considerable C–OAr bond cleavage in its transition state. The formation of the intermediate occurs via a pathway where ionisation of the N-benzoylglycine NH, intramolecular attack, and decomposition of the tetrahedral intermediate are discrete steps. Aminolysis proceeds largely through the ester rather than via the oxazolinone intermediate.

Reduction of aryltrimethylsilanes as a synthetic method. Part II. Birch reduction

Abstract: Sixteen aryltrimethylsilanes have been reduced by a modification of the Birch reduction (with lithium–ethanol; lithium added last). Along with the expected cyclohexa-1,4-dienes, products of Si–C bond cleavage are also formed, especially when an allylic or benzylic Si–C bond is involved; further reduction to substituted cyclohexenes or cyclohexanes may take place if the primary product contains vinylic trimethylsilyl groups.

The C–O Bond Cleavage of Allyl Ethers by Rhodium Compounds

Abstract: Allyl ethers were cleaved by rhodium compounds under mild conditions to give aldehydes and hydrocarbons This reaction can be extended to allyl esters

The oxidative cyclization of formazans to tetrazolium salts

Abstract: Bromine oxidizes 1,3,5-triarylformazans (1) quantitatively to the corresponding 2,3,5-triaryltetrazolium salts (2) in methanol or aqueous acetic acid. The reaction is ionic and is markedly aided by electron donation in the aryl rings. In methanol containing 0·1M-sodium bromide (at 25°) substituents in the C-aryl ring give a ρ value of –1·40, while those at N-1 and -5 give ρ–1·10 and –2·42. A mechanism of oxidation is proposed involving electrophilic attack by bromine at carbon and formation of a covalent intermediate, followed by unimolecular C–Br bond cleavage to yield the tetrazolium salt.