Stabilisation of superoxide ion in aqueous media by tryptophan

Abstract

Air saturated solutions of tryptophan (Trp) produce a fundamental harmonic ac polarogram around 0 V vs. SCE which disappears when the solution is perfectly deaerated. The peak width at 1 2 I p, the peak potential U p and its linear variation with log[O 2], giving a one-electron slope, and the linear variation of I p with [O 2] indicate that this polarogram is due to the reduction of oxygen in the presence of Trp to form the superoxide ion, O 2 •−. An optimum concentration of 10 −5 M of Trp is required for the maximum peak current. Evidence for the electrogeneration of superoxide ion has been obtained from cyclic voltammograms and from the instantaneous generation of a deep blue TMPD •+ from TMPD ( N,N,N′,N′-tetramethyl- p-phenylenediamine) added to the electrolysed solution. Trp is oxidised chemically by O 2 •− as is observed from bulk electrolysis, which yielded kynurenine,quinoline, 4-hydroxy-quinoline and kynurenic acid as major products. A free radical mechanism has been proposed for the oxidative degradation of Trp. Detection of the single electron transfer to oxygen, forming O 2 •− in pure supporting electrolytes by cyclic voltammetry, is reported.