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

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 CuI. It is concluded that OH˙ reacts with the amino acid, whether free or coordinated with CuII, to give a radical that is oxidised by CuII, forming ligand decomposition products and CuI. Under conditions where the complex CuII(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 CuIII species (possibly OH˙ bonded to CuII). The fate of the CuIII species depends on the prevailing CuI concentration. With and excess of CuI, CuIII is reduced to CuII by CuI, without ligand decomposition: the kinetics of the decay of CuIII, observed by pulse radiolysis, is consistent with this reaction, and gives a rate constant value k(CuIII+CuI)=(1.0±0.4)× 109 dm3 mol–1 s–1. If CuI is absent, or complexed with DMP, CuIII can react to oxidise the amino-acid ligand.