Kinetics and mechanism of chromium(VI) oxidation of L-methionine

The kinetics and mechanism of chromium(VI) oxidation of L-methionine in acidic medium have been studied spectrophotometrically. The reaction proceeds via the formation of a transient intermediate (λmax = 410–420 nm) which decomposes by a proton catalyzed pathway. The rate law is: \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$\frac{{ - {\text{d[Cr}}^{{\text{VI}}} {\text{]}}}}{{{\text{d}}t}} = \frac{{k_3 k_1 [{\text{MSH}}][{\text{H}}^{\text{ + }} ][{\text{Cr}}^{{\text{VI}}} ]}}{{[{\text{H}}]^ + + KK_1 [{\text{MSH}}]}}([{\text{MSH}}] - {\text{protonated L - methionine}})$$ \end{document} The activation enthalpy and activation entropy for the reaction have been calculated to be ΔH* = 43.85 kJ mol−1, ΔS* = −286.87 JK−1 mol−1. Also values of k1, k−1 and k3 were determined: 27.2 × 10−3 M−1 S−1, 1.97 × 10−3 S−1, 7.2 × 10−3 s−1, respectively. The results are compared with those of related studies for reduction of chromate by amino acids.