A kinetic study of the complexation of cysteine and related compounds with aqueous vanadium(II) and vanadium(III) at approximately neutral pH; the mediating role of sulphur compounds in electron transfer

Abstract

Vanadium(II) and vanadium(III) form with cysteine (cys) and other sulphhydryl compounds intensely yellow complexes soluble in neutral and weakly alkaline solutions. Some of the vanadium(II) complexes are powerful reductants. Thus, VII–cys reduces water to dihydrogen under mild conditions. The formation of the reducing species, which is [VII(cysOS)3]4–[cysOS = cysteinate(2–)] proceeds in two stages, i.e. a stage corresponding to a jump in the absorbance at zero time and a second stage, which was followed kinetically in a stopped-flow instrument. The complexation of cysteine with vanadium(III) also proceeds in two stages, but leads to the formation of a species containing two cysteines instead of three (at pH values around neutral). The observed activation energy (Ea) and pre-exponential factor (A) for the stages that were followed kinetically are as follows: Ea= 41 ± 4 kJ mol–1, A= 1.4 × 107(pH 8.2) for VII and Ea= 41 ± 4 kJ mol–1, A= 1.4 × 1010(pH 8.8)for VIII. The reduction of water to dihydrogen by VII–cys proceeds with a rate first order in [VII]total and in the pH range 7.5–8.5 it is independent of hydrogen-ion concentration. The activation parameters are: Ea= 54 ± 2 kJ mol–1, A= 5 × 106. Dihydrogen is also obtained with VII–cys (cysa = cysteamine) and VII–cysme (cysme = cysteine methyl ester). The corresponding reaction of VII–ser (ser = serine) is ca. one thousand times slower compared to the reaction of VII–cys, even though the polarographic half-wave potentials have comparable values.