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

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.