Reactions of V-substituted polyoxometalates with L-cysteine

Three polyoxometalates (POMs), alpha(2)-[H4XVW17O62](8- or 9-) (X-P or As) and [alpha(2)-P2VW17O62](7- or 8-) 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 alpha(2)-[(P2VW17O62)-W-V](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 alpha(2)-[H4P (VW17O62)-W-IV](9-) as an example. The rate constant measured by chronocoulometry for this system compares favourably with that extracted from stopped flow experiments.