Reactions of the hydroxyl free radical with copper(II)–amino-acid complexes in aqueous solution

Abstract

Reactions of the hydroxyl radical (OH^˙) in amino acid (glycine and α-alanine)–copper(II) aqueous systems have been investigated by measurement of the γ-radiation yields of the products: ammonia, formaldehyde and glyoxylic acid (from glycine), acetaldehyde and pyruvic acid (from analine) and Cu^I. It is concluded that OH^˙ reacts with the amino acid, whether free or coordinated with Cu^II, to give a radical that is oxidised by Cu^II, forming ligand decomposition products and Cu^I. Under conditions where the complex Cu^II(amino acid)₂ is the main OH^˙ scavenger, the reaction results in ligand decomposition for only ca. 65% of the OH^˙ radicals. The results are interpreted in terms of a mechanism that assumes that OH^˙ reacts with both the attached amino acid and at the metal centre. The latter reaction gives a Cu^III species (possibly OH^˙ bonded to Cu^II). The fate of the Cu^III species depends on the prevailing Cu^I concentration. With and excess of Cu^I, Cu^III is reduced to Cu^II by Cu^I, without ligand decomposition: the kinetics of the decay of Cu^III, observed by pulse radiolysis, is consistent with this reaction, and gives a rate constant value k(Cu^III+Cu^I)=(1.0±0.4)× 10⁹ dm³ mol^–1 s^–1. If Cu^I is absent, or complexed with DMP, Cu^III can react to oxidise the amino-acid ligand.