Reactions of V-Substituted Polyoxometalates with L-Cysteine

Three polyoxometalates (POMs), (X=P or As) and , in their oxidized and reduced forms, were selected for direct reaction or electrocatalytic reaction with L-cysteine, because they have the most negative formal potentials among those POMs active for the desired reaction. The good linearity of the UV–Visible calibration curve obtained for the reaction of α2-[P2VVW17O62]7− with L-cysteine indicates both a simple 1:1 stoichiometry for the process and the possibility to select a wavelength domain in which the one-electron reduced forms of this POM is the only strongly absorbing species in the mixture. Another general result among the three selected POMs is the existence, in each example, of a sharp isosbestic point during the recording of individual spectrakinetics using a photodiode array system. The kinetics could be fitted accuretely to a mono-exponential rate equation and the rate constants were determined. Electrocatalysis of the oxidation of L-cysteine was carried out in the presence of α2-[H4P VIVW17O62]9− as an example. The rate constant measured by chronocoulometry for this system compares favourably with that extracted from stopped flow experiments.