The kinetics and mechanism of the oxidation of s-methyl-L-cysteine, L-cystine and L-cysteine by potassium ferrate

The kinetics of the reactions of s-methyl-L-cysteine, L-cystine and L-cyste ine with potassium ferrate were all investigated under pseudo and non pseud o first-order conditions. Methyl cysteine was oxidised to the sulfoxide and cystine was oxidised to the thiosulfonate within 300 s, with kinetics that were first-order in the concentration of the hydrogen ions, the reductant and the ferrate ions above a pH of approximately 8.4, below this pH the kin etics were independent of the hydrogen ion concentration. Under pseudo firs t-order conditions cysteine was oxidised within 300 ms to form the sulfinic acid with subsequent formation of a ferric-cysteine complex in excess cyst eine. The kinetics involved two terms, one being first-order in the hydroge n ion, the cysteine and the ferrate ion concentrations above a pH of 8.2, a nd independent of the hydrogen ion concentration below this pH, and the oth er being first-order in only cysteine and the ferrate ion concentrations. F or all three reductants the proposed mechanism involves a rate-determining step between the reductant and the protonated ferrate ion: for cysteine the re is also a rate-determining step between cysteine and the unprotonated fe rrate. The results are consistent with those obtained for other organosulfu r and thiol reductants. (C) 2000 Elsevier Science S.A. All rights reserved.