Mechanism of the oxidation of DL-penicillamine and glutathione by chromium(VI) in aqueous solution

Abstract

The kinetics of the oxidation of DL-penicillamine (3-sulfanyl-D-valine) and glutathione (γ-glutamylcysteinylglycine) by potassium chromate has been studied at pH 7 under pseudo-first-order conditions of an excess of thiol, at I= 0.50 mol dm^–3(NaClO₄). The glutathione reaction is biphasic, while for penicillamine it is monophasic. The first stage of the oxidation of glutathione obeys the simple expression (i). The second stage of the same reaction is slower than the first by a factor of 10⁴ and k_obs=k₁[RSH]+k_–1(i), obeys the rate expression (ii). The biphasic nature is consistent with the formation and decay of a Rate =(k₁k₂[RSH]²+k₁k₃[RSH])//_{(k–1+k3+k2[RSH])}[Cr^VI]_T(ii), chromate–glutathione adduct. Parameter k₁/k_–1 at 25.3 °C has been determined as 184 ± 4 dm³ mol^–1. The second-order rate constant, k₂, for the decomposition of the intermediate is (1.07 ± 0.04)× 10^–2 dm³ mol^–1 s^–1. The activation parameters were calculated as ΔH^‡= 90 ± 7 kJ mol^–1 and ΔS^‡= 63 ± 29 J K^–1 mol^–1. The oxidation of DL-penicillamine shows simple first-order kinetics with respect to [DL-penicillamine], and may also be represented by expression (ii). The second-order rate constant, k₁, obtained at 25.2 °C for the formation of a chromate–penicillamine intermediate is 0.23 ± 0.01 dm³ mol^–1 s^–1. The corresponding activation parameters are ΔH^‡= 72 ± 7 kJ mol^–1 and ΔS^‡=–18 ± 28 J K^–1 mol^–1. A common mechanism which is compatible with the kinetics of both reactions is proposed.