DETERMINATION OF THE REDOX ACTIVE FORMS OF OXINDOLYLALANINE AND PEPTIDES OF KYNURENINE ON THE BASIS OF ELECTROCHEMICAL STUDIES AT MERCURY-ELECTRODES IN AQUEOUS-MEDIA

Oxidation of trytophan is a biologically important process which produces a number of compounds including oxindolylalanine, dioxindolylalanine, 2-carboxy-3a-hydroxy-1,2,3,3a,8,8a-hexahydropyrrolo-(2,3-b)-indole and kynurenine. In the case of tryptophan and kynurenine it is known that they are electroactive at mercury electrodes to give characteristic processes involving the formation of a mercury-tryptophan complex (mercury electrode oxidation) and gamma-(o-aminophenyl)-homoserine (reduction of gamma-carbonyl functional group) respectively. Consequently, electrochemical studies at mercury electrodes provide considerable information on the structures of the redox active forms of oxidation products of tryptophan. The electrochemistry of oxindolylalanine in alkaline media at mercury electrode gives rise to a process consistent with the 2-hydroxytryptophan structure being important in aqueous solution thereby establishing the importance of the equilibrium. [GRAPHICS] In contrast dioxindolylalanine and 2-carboxy-3a-hydroxy-1,2,3,3a,8,8a-hexahydropyrrolo-(2,3-b)-indole are electroinactive at mercury electrodes consistent with the fact that they have no forms in aqueous solution containing either tryptophan or kynurenine. However, peptides of kynurenine, depicted below, give 2 or 3 pH dependent reduction processes at mercury electrode. In acid media, but not in basic media reactant adsorption accompanies electron transfer and except in very alkaline media the processes are irreversible two electron, two proton reduction steps involving reduction at the gamma-carbonyl group to give the gamma-(o-aminophenyl)-homoserine peptide. That is, kynurenine type behaviour is retained by the kynurenine peptide.