Antioxidant Chemistry:  Oxidation of l-Cysteine and Its Metabolites by Chlorite and Chlorine Dioxide

Abstract

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 2 NCH(COOH)CH 2 SH f 3Cl -+ 2H 2 NCH(COOH)CH 2 SO 3 H with the final product as cysteic acid. The stoichiometry of the chlorite-cysteinesulfinic acid gave a ratio of 1:2, ClO 2 -+ 2H 2 NCH(COOH)CH 2 SO 2 H f Cl -+ 2H 2 NCH(COOH)CH 2 SO 3 H. 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 + f 2ClO 2 (aq) + Cl -+ H 2 O. 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.

Kinetics and mechanism of oxidation of l-cystine by manganese(III) in sulfuric acid medium

“…L-cystine is a covalently linked dimeric nonessential amino acid formed by the oxidation of L-cysteine. A literature survey reveals that kinetic studies on the oxidation of L-cystine are somewhat limited, using oxidants like iodine [1], alkaline permanganate [2], potassium ferrate [3], chlorite and chlorine dioxide [4], and hypochlorous acid [5]. In most of these studies cysteic acid was found to be the product of oxidation.…”

Inhibitiory effects of gold(III) ions on ribonuclease and deoxyribonuclease

“…Since Au(III) ions can oxidize amino acid residues [41], the irreversible oxidation of amino acid residues in a RNase molecule could be another reason for the incomplete recovery of RNase activity. In particular, Met and disulfide bonds are likely to be oxidized [24,42,43]. However, 90% of RNase activity was recovered by the addition of thiourea, suggesting that the majority of the Au-binding sites in RNase A would not be irreversibly oxidized by Au(III) ions.…”

Kinetics and Mechanism of Oxidation of L‐Cystine by Hexacyanoferrate(III) in Alkaline Medium

“…In fact a precise understanding of the mechanism of such biological redox reactions is important as it helps to know the toxic effects of the metals and also to synthesise specific reaction products. The literature survey reveals that kinetic studies on the oxidation of L-cystine were limited, using oxidants like iodine 1 , alkaline permanganate 2 , potassium ferrate 3 , chlorite and chlorine dioxide 4 and hypochlorous acid 5 . To have a further insight into it, we carried out kinetic investigations on the oxidation of L-cystine.…”