Oxidation of methionines by oxochromium(V) cations: A kinetic and spectral study

Spectroscopic, kinetic, and theoretical analyses of oxidation of dl-ethionine by Pt(IV) anticancer model compounds

Ethionine is an S-ethyl analog of methionine (Met) having a small change in structure. But it is a chemical carcinogen and an antagonist of Met, thus displaying a disparate biological profile. The oxidations of ethionine by biologically important oxidants have not been exploited. Oxidations of dl-ethionine by Pt(IV) anticancer model complexes trans-[PtX₂(CN₄)]^2- (X = Cl or Br) were thus analyzed by time-resolved and stopped-flow spectral techniques. Overall second-order kinetics was established, being first-order in [Pt(IV)] and [Ethionine]_tot (the total concentration of ethionine); the observed second-order rate constant k' versus pH profiles were obtained. A stoichiometry of Δ[Pt(IV)]:Δ[Ethionine]_tot = 1:1 was unraveled, indicating that ethionine was oxidized to ethionine-sulfoxide which was confirmed by NMR spectroscopic and high-resolution mass spectral analyses. In the proposed reaction mechanism which is similar to that for the oxidation of Met by the same Pt(IV) compounds, the rate-determining steps are rationalized in terms of a bridge formation between one of the coordinated halides in [PtX₂(CN₄)]^2- and the sulfur atom in ethionine, followed by an X⁺ transfer. Moreover, a large rate enhancement for the reaction of ethionine with [PtBr₂(CN₄)]^2- compared with [PtCl₂(CN₄)]^2- strongly supports an X⁺ transfer mechanism. Furthermore, a combined quantum-mechanical/molecular-mechanical (QM/MM) method was utilized to simulate a Cl⁺ transfer mechanism from trans-[PtCl₂(CN)₄]^2- to ethionine. The simulations unraveled the energetically stable structures of reactants and products, which favor the Cl⁺ transfer process. Rate constants of the rate-determining steps have been derived. Ratios of k (ethionine)/k (Met) are between 2.2 and 2.6 obtained for the three protolytic species of ethionine and Met; the enhanced reactivity might be partially responsible for the disparate biological profiles.