Kinetics and mechanism of the oxidation of l-methionine by aqueous solution of chromium(VI)

Article abstract of DOI:10.1016/S0277-5387(00)81175-4

The kinetics of the reduction of Cr(V1) by l-methionine (represented by RSCH₃) have been studied in aqueous perchloric acid. The data are consistent with the rate law: -d[Cr(VI)] dt = (k₁ + k₂[H⁺]²[RSCH₃])[Cr(VI)] with k₁ = (3.1 ± 0.5) x 10^-4 s^-1; k₂ = 12.3 ± 1.1 mol^-3 dm⁹ s^-1 at I = 1.0 mol dm^-3 (NaClO₄), [H⁺] = 40.04-0.10 mol dm^-3 and T 25.1°C. The corresponding activation parameters are ΔH₁^* = 26.2 ± 0.4 kJ mol^-1; ΔS₁^* = -224± 1 JK^-1 mol^-1; ΔH₁^* = 33.9±0.01 KJ mol_-1; ΔS₁^* -111±1 JK^-1 mol^-1. The above rate is very much slower than those of other related reactions. This is interpreted in terms of a rate-determining CrS bonded intermediate which undergoes hydrolysis after electron transfer to yield Cr(III) and methionine sulphoxide. The significance of pathways involving second-order substrate dependence is also discussed.