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-cysteine with potassium ferrate were all investigated under pseudo and non pseudo 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 kinetics were independent of the hydrogen ion concentration. Under pseudo first-order conditions cysteine was oxidised within 300 ms to form the sulfinic acid with subsequent formation of a ferric–cysteine complex in excess cysteine. The kinetics involved two terms, one being first-order in the hydrogen ion, the cysteine and the ferrate ion concentrations above a pH of 8.2, and independent of the hydrogen ion concentration below this pH, and the other being first-order in only cysteine and the ferrate ion concentrations. For all three reductants the proposed mechanism involves a rate-determining step between the reductant and the protonated ferrate ion; for cysteine there is also a rate-determining step between cysteine and the unprotonated ferrate. The results are consistent with those obtained for other organosulfur and thiol reductants.