Oxidation of Glutathione, Cysteinesulfinic Acid and N,N-dimethylhydroxylamine Hydrochloride by Transition Metals

Abstract

The aqueous oxidations of glutathione (GSH) by the substitutionally inert outer-sphere oxidants [IrCl6]2–, [Fe(bpy)2(CN)2]+ and [Fe(bpy)(CN)4]– have been investigated at 25 °C and ionic strength (μ) = 0.1 M between pH 1 and 11. All these reactions are catalyzed by trace amounts of Cu2+ ions. Such catalysis can be effectively prevented with addition of 1 mM dipic for the reduction of [IrCl6]2– and [Fe(bpy)2(CN)2]+ and is completely impeded with 5 mM EDTA for the reduction of [Fe(bpy)(CN)4]–. The kinetic reactions with FeIII are mildly inhibited by the product FeII at low pH and are controlled by the use of spin trap PBN (N-tert-butyl-αphenylnitrone). [IrCl6]2– oxidizes GSH yielding [IrCl6]3– and GSO3- with some GSSG in presence of O2; in the absence of O2, the FeIII compounds are reduced to their corresponding FeII products with almost quantitative formation of GSSG. All three reactions have common rate laws that are first order in [oxidant] and [reductant]. The general rate law is –d[Mox]/dt = kobs[Mox] with kobs = k[GSH]t = k1[HGSH+] + k2[GSH0] + k3[GSH-H–] + k4[GSH-2H2–] + k5[GSH-3H3–]. The rate laws show a general increase in rate with increasing pH. The pH resolved rate constants are described with a mechanism having rate-limiting outer-sphere electron transfer from the various thiolate forms of GSH. An investigation of oxidation of cysteinesulfinic acid (CysSO2H) by [IrCl6]2– is described at 25 °C and ionic strength (μ) = 0.1 M between pH 3 and 5.6 adjusted with acetate buffers. This reaction is unaffected by the trace metal ions Cu2+ and Fe2+, as observed with thiols. 1H-NMR analysis shows that CysSO2H is quantitatively oxidized to the corresponding sulfonic acid and UV-vis data indicate the reduction of [IrCl6]2– to [IrCl6]3–. The rate law is first order in [CysSO2H] and second order in [IrCl6]2– with simple pH independent kinetics and an inverse dependence on [IrCl6]3–. The oxidation of N,N-dimethylhydroxylamine by [IrCl6]2– has been examined within the pH range 3.6-7.8 applying suitable buffers and keeping ionic strength at 0.1 M (NaClO4). These reactions are catalyzed by Cu2+ and Fe2+ ions and the catalysis can be inhibited with 1 mM [C2O42-]. The pH resolved rate constants are k1 = (5.19 ± 0.15) × 102 M-1 s-1 and k’ = (4.45 ± 0.28) × 10-5 s-1.