Reduction of some Pt(IV) complexes with biologically important sulfur-donor ligands

The reduction of the Pt(IV) complexes [PtCl4(bipy)], [PtCl4(dach)] and [PtCl4(en)] by glutathione (GSH), L-cysteine (L-Cys) and L-methionine (L-Met) was investigated by stopped-flow spectrophotometry at pH 2.0 (in 0.01 M perchloric acid) and at pH 7.2 (in 25 mM Hepes buffer). Kinetic measurements were performed under pseudo-first order conditions with an excess of the reducing agent. The order of the reactivity of the studied complexes was [PtCl4(bipy)] > [PtCl4(dach)] > [PtCl4(en)], and reactivity of investigated reducing agents followed the order GSH > L-Cys > L-Met. All the reactions between the selected Pt(IV) complexes and the sulfur donor biomolecules proceeded by a reductive elimination process that included nucleophilic attack by the reducing agent on one of the mutually trans-coordinated chloride ligands, which led to a two-electron transfer process. The final products of the redox reactions were the corresponding reduced Pt(II) complexes and the oxidized form of the reducing agents.