Redox behaviour of cysteine in the presence of ammonium trioxovanadate(V)

The interaction of ammonium trioxovanadate(V) with cysteine in aqueous solution was studied by cyclic voltammetry and absorption spectroscopy techniques. In the absence of cysteine, the cyclic voltammogram (CV) of ammonium trioxovanadate(V) solution in 0.1 M phosphate buffer (pH 7) gave two peaks at −0.130 V (reversible) and −0.400 V (irreversible). These peaks (−0.130 V, −0.400 V) can be attributed to V(V)/V(IV) and V(IV)/V(III) redox processes, respectively. In the presence of cysteine at low scan rate (40 mV/s), the peak at −0.780 V, which is assigned to the irreversible reduction of free cystine, was observed. In addition, the reduction peak of the disulfidic anion S22− was seen at −0.650 V. Under aerobic conditions, the peaks of the disulfidic anion S22− and free cystine are well separated. From electronic spectra of ammonium trioxovanadate(V) and cysteine mixtures, LMCT transition associated with V(V)–cyteine complex was obtained at 743 nm. The stoichiometry (ML2) and stability constant (log β1:2=6.67) of V(V)–cysteine complex were determined by means of mole ratio method.