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

Abstract

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 (λ_max = 410–420 nm) which decomposes by a proton catalyzed pathway. The rate law is:

$$\frac{{ - {\text{d[Cr}}^{{\text{VI}}} {\text{]}}}}{{{\text{d}}t}} = \frac{{k_3 k_1 [{\text{MSH}}][{\text{H}}^{\text{ + }} ][{\text{Cr}}^{{\text{VI}}} ]}}{{[{\text{H}}]^ + + KK_1 [{\text{MSH}}]}}([{\text{MSH}}] - {\text{protonated L - methionine}})$$

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 k ₁, k ₋₁ and k ₃ 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.