Kinetics of the interaction of sulfate and hydrogen phosphate radicals with small peptides of glycine, alanine, tyrosine and tryptophan

Photochemical & Photobiological Sciences Pub Date: 2005-08-16 DOI: 10.1039/B507856C

Abstract

The kinetics and mechanism of the oxidation of Glycine (Gly), Alanine (Ala), Tyrosine (Tyr), Tryptophan (Trp) and some di-(Gly-Gly, Ala-Ala, Gly-Ala, Gly-Trp, Trp-Gly, Gly-Tyr, Tyr-Gly), tri-(Gly-Gly-Gly, Ala-Gly-Gly) and tetrapeptides (Gly-Gly-Gly-Gly) mediated by sulfate (SO4˙?) and hydrogen phosphate (HPO4˙?) radicals was studied, employing the flash-photolysis technique. The substrates were found to react with sulfate radicals (SO4˙?, produced by photolysis of the S2O82?) faster than with hydrogen phosphate radicals (HPO4˙?, generated by photolysis of P2O84? at pH = 7.1). The reactions of the zwitterions of the aliphatic amino acids and peptides with SO4˙? radicals take place by electron transfer from the carboxylate moiety to the inorganic radical, whereas those of the HPO4˙? proceed by H-abstraction from the α carbon atom. The phenoxyl radical of Tyr-Gly and Gly-Tyr are formed as intermediate species of the oxidation of these peptides by the inorganic radicals. The radical cations of Gly-Trp and Trp-Gly (at pH = 4.2) and their corresponding deprotonated forms (at pH = 7) were detected as intermediates species of the oxidation of these peptides with SO4˙? and HPO4˙?. Reaction mechanisms which account for the observed intermediates are proposed.

Graphical abstract: Kinetics of the interaction of sulfate and hydrogen phosphate radicals with small peptides of glycine, alanine, tyrosine and tryptophan
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