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, V^II–cys reduces water to dihydrogen under mild conditions. The formation of the reducing species, which is [V^II(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 (E_a) and pre-exponential factor (A) for the stages that were followed kinetically are as follows: E_a= 41 ± 4 kJ mol^–1, A= 1.4 × 10⁷(pH 8.2) for V^II and E_a= 41 ± 4 kJ mol^–1, A= 1.4 × 10¹⁰(pH 8.8)for V^III. The reduction of water to dihydrogen by V^II–cys proceeds with a rate first order in [V^II]_total and in the pH range 7.5–8.5 it is independent of hydrogen-ion concentration. The activation parameters are: E_a= 54 ± 2 kJ mol^–1, A= 5 × 10⁶. Dihydrogen is also obtained with V^II–cys (cysa = cysteamine) and V^II–cysme (cysme = cysteine methyl ester). The corresponding reaction of V^II–ser (ser = serine) is ca. one thousand times slower compared to the reaction of V^II–cys, even though the polarographic half-wave potentials have comparable values.