Peptide radiolysis

In all amino acid and peptide radiolysis, sulfur dioxide and carbonyl sulfide were found in varying amounts, but at present no explanation for the formation of these compounds can be offered. 

Model system studies show that radiochemical yields of the major products of peptide radiolysis are drastically altered when irradiations are done at liquid nitrogen temperatures rather than at 20°C. 

Ni(II) by strong oxidants, such as OH, Br and (SCN), produced by pulse radiolysis and flash photolysis. Rate constants are 10 M" s for oxidation by OH and Brf and = 10 M s for (SCN)f Ref. 259. The most popular means of production in both aqueous and nonaqueous solution is electrolytic, jjjg ligands which stabilize Ni(III) are cyanide, deprotonated peptides, amines and aminocarboxylates, a-diimines and tetraaza macrocycles, including porphyrins. Low spin d Ni(III) resembles low spin Co(II). The kinetics of the following types of reactions have been studied ... 

Several peroxidative reactions initiated by OH radical that may be generated by either y- and X-radiolysis of aqueous solutions or by transition metal-catalyzed reduction of H2O2 have been identified in free amino acids and short peptides. In this respect we may distinguish oxidizing reactions that involve the polypeptide backbone on the one hand... 

Re-examination of the radiolysis of aqueous solutions of alanine (absence of oxygen) shows that electrons react rapidly with the cationic form, less rapidly with the zwitterion, and much less rapidly with the anionic form. These conclusions have been confirmed by pulse radiolysis. Rate constants for amino acids, peptides, proteins, and numerous other substances have been obtained. Critical evaluation of these and correlation with molecular properties is now well under way. In living systems the reactions of the hydrated electron vary with the part of the cell concerned, with the developmental stage of the cell, and possibly with the nature of any experimentally added substances. 

Interaction of the solute with radicals from the water is the first of a sequence of reactions which finally leads to stable products. Kinetic studies of the type cited give valuable information about the primary radical species and their relative reaction rates with molecules of different types. When sufficient data have been accumulated, it should be possible to predict the course of radiolysis in complex molecules. From the nature and yields of the products and by observing the effects on them of various factors such as concentration, pH, 02, and specific radical scavengers, it is often possible to speculate about the mechanisms by which products are formed. More often than not, this is a difficult problem because the products, even from relatively simple compounds, prove to be complex. Furthermore, it is often possible to produce more than one mechanism to fit the experimental data. The proteins are particularly difficult because of their complex structures. They contain approximately 20 different amino acids with an average of more than three carbon atoms in the side chains, which vary considerably in their structure hence, the possible number of products is large. For this reason, model compounds such as peptides and polyamino acids have been studied because they contain the peptide linkage but are free from the complications which arise from the diversity of the amino acid residues in a protein. A further practical difficulty which applies to chem-... 

Mieden OJ, von Sonntag C (1989) Peptide free-radicals the reactions of OH radicals with glycine anhydride and its methyl derivatives sarcosine and alanine anhydride. A pulse radiolysis and product study. Z Naturforsch 44b 959-974... 

Mieden OJ, Schuchmann MN, von Sonntag C (1993) Peptide peroxyl radicals base-induced O2 elimination versus bimolecular decay. A pulse radiolysis and product study. J Phys Chem 97 3783-3790... 

The absolute rate constants for these reversible H-atom transfer reactions [reaction (17)] in three model peptides (N-Ac-Cys-Glyg, N-Ac-Cys-Glyj-Asp-Glyj and N-Ac-Cys-Alaj-Asp-Alaj) were measured by means of pulse radiolysis. ... 

The effect of N-acetyl substitution in methionine on the nature of transients formed after one-electron oxidation was studied as a function of pH and NAM concentration. The observed absorption bands with X = 290 nm, 360 nm, and 490 nm were respectively assigned to a-(alkylthio)alkyl, hydroxysulfuranyl and dimeric radical cations with intermolecular three-electron bond between sulfur atoms. N-acetylmethionine amide (NAMA) (Chart 7) represents a simple chemical model for the methionine residue incorporated in a peptide. Pulse radiolysis studies coupled to time-resolved UV-Vis spectroscopy and conductivity detection of N-acetyl methionine amide delivered the first experimental evidence that a sulfur radical cation can associate with the oxygen of an amide function vide infra). ... 

The first experimental evidence that Met in j8-APl-42 is more easily oxidized than in other peptides and proteins comes from one-electron oxidation of /3AP1-40 using azide radicals (Nj) produced by pulse radiolysis.Thermodynamic considerations indicate that Nj should not oxidize Met residues unless the one-electron reduction potential of Met is lowered because of favorable environment. It was shown that Met is the target in /3-AP(l-40) oxidation. Conversely the oxidation of /3-AP(40-l) with a reversed sequence of amino acids has shown that Tyr is the target of Nj radicals. These observations are the first experimental evidences that (i) Met in AP(l-40) is more easily oxidized than in other peptides or proteins, and (ii) a change in a primary sequence drastically affects the one-electron reduction potential of Met, even in a small peptides. 

In this regard, pulse radiolysis studies with UV/Vis spectro-photometrical and conductometric detection were performed in model peptides containing a single methionine residue, N-Ac-Gly-(Gly) -Met-(Gly) They show for the first time that ... 

The most important recent results obtained by means of time-resolved techniques (pulse radiolysis and laser flash photolysis) on model peptides containing single or multiple Met-residues and in selected naturally occurring peptides (Met-enkephalin and amyloid peptide) and proteins (thioredoxin and calmodulin) have been recently reviewed. ... 

The reactions of superoxide radical anions (O2") with sulfide radical cation complexes might represent an important and efficient reaction pathway for the formation of sulfoxides in peptides and proteins containing methionine residues. Absolute rate constants for two sulfide radical cation complexes (S..S) from 1,5-dithia-3-hydroxycyclooctane and (S..N) from Met-Gly dipeptide with Of were measured using pulse radiolysis. The rate constant for the... 

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