Accumulation and Reactions of H2O2 during the copper ion Catalyzed Autoxidation of Cysteine in Alkaline Medium

Abstract

The stoichiometry of the copper catalysed alkaline autoxidation of cysteine has been investigated. Products of this oxidation reaction are cystine, H{2}O{2} and H{2}O. No oxygen containing sulfur acids are produced as long as cysteine is present in the reaction liquid.The H{2}O{2} generated reacts with cysteine selectively to form cystine and H{2}O with r{H}{2}{O}{2} = k{H}{2}{O}{2} [CyS][H{2}O{2}] (k{H}{2}{O}{2} = 0.17 1 mol}-{}1{ s}-{}1{).The reaction between H{2}O{2} and cysteine is not catalysed by the copper complex as present during the catalytic autoxidation reaction. On the other hand, experiments carried out in the absence of oxygen show a marked catalytic effect ascribed to the presence of a Cu(I) complex.Accumulation curves of H{2}O{2} have been measured during the copper catalysed autoxidation of cysteine. Information about the rate of production of H{2}O{2} at the catalytic site has been obtained by making use of these accumulation curves together with the kinetic data obtained for the reaction of H{2}O{2} with cysteine. It is concluded that apart from a two electron reduction of dioxygen to H{2}O{2} a four electron reduction to H{2}O should also be taken into account.The selectivity of the oxidation reaction justified a reconsideration of the free thiyl radical mechanism proposed in the literature to occur during the copper catalysed autoxidation of cysteine.