Antioxidant chemistry: Oxidation of L-cysteine and its metabolites by chlorite and chlorine dioxide

The oxidation of L-cysteine and its metabolites cystine and L-cysteinesulfinic acid by chlorite and chlorine dioxide has been studied in unbuffered neutral and slightly acidic media. The stoichiometry of the oxidation of L-cysteine was deduced to be 3ClO ₂ ^- + 2H ₂NCH(COOH)CH ₂SH → 3Cl ^- + 2H ₂NCH(COOH)CH ₂SO ₃H with the final product as cysteic acid. The stoichiometry of the chlorite-cysteinesulfinic acid gave a ratio of 1:2, ClO ₂ ^- + 2H ₂NCH(COOH)CH ₂SO ₂H → Cl ^- + 2H ₂NCH(COOH) CH ₂SO ₃H. There was no further oxidation past cysteic acid, and there was no evidence of sulfate formation which would have indicated the cleavage of the carbon-sulfur bond. The reaction is oligooscillatory in chlorine dioxide formation. In conditions of excess oxidant, the reaction is characterized by a short induction period followed by a rapid and autocatalytic formation of chlorine dioxide. Chlorine dioxide is formed by the reaction of intermediate HOCl with the excess chlorite: 2ClO ₂ ^- + 2HOCl + H ^- → 2ClO ₂(aq) + Cl ^- + H ₂O. Oligooscillations observed in chlorine dioxide formation result from the competition between this pure oxyhalogen reaction and reactions that consume chlorine dioxide. The rate of the reaction of chlorine dioxide with cysteine and its metabolites is fast and is of comparable magnitude with the reactions that form chlorine dioxide. The reaction of chlorine dioxide with L-cysteine is first order in both oxidant and substrate, retarded by acid, and has a lower-limit bimolecular rate constant of 405 ± 50 M ^-1 s ^-1, while for the reaction with L-cysteinesulfinic acid the rate constant is 210 ± 15 M ^-1 s ^-1. It would appear that the existence of a zwitterion on the asymmetric carbon atom precludes the formation of N-chloramines as has been observed with taurine and aminomethanesulfonic acid. The mechanism for the reaction is satisfactorily described by a network of 28 elementary reactions which include autocatalysis by HOCl.