Oxidation of L-cystine by 12-tungstocobaltate(III) in aqueous perchlorate medium: A kinetic approach

Abstract

The oxidation reaction of L-cystine with 12-tungstocobaltate(III) (CoˡˡˡW₁₂O₄₀⁵⁻ or CoˡˡˡW⁻⁵) has been studied spectrophotometrically over the range 1.15 ≤ 10³[H₄L²⁺]T. ≤ 3.5; 2.0 ≤ pH ≤ 5.0; 25ºC ≤ t ≤ 40ºC and I = 0.3 mol dm⁻³ (NaClO₄). Both the conjugate bases of L-cystine participate in the electron transfer reaction. The rate of the reaction has been found to increase with the increase in pH and [L-cystine]T. The reaction shows first order dependence both on [CoˡˡˡW⁻⁵] and [L-cystine]T. The ΔH (kJ mol⁻¹) and ΔS (J K⁻¹ mol⁻¹) for k₁ and k₂ paths are 16.9±2.0, 16.1±0.5 and –205±6, –204±1.5, respectively. Negative activation entropy is indicative of ordered transition state for the reaction. The product of the reaction has been found to be the cystinesulphoxide.