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-P₂V^VW₁₇O₆₂]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 V^IVW₁₇O₆₂]⁹⁻ as an example. The rate constant measured by chronocoulometry for this system compares favourably with that extracted from stopped flow experiments.Dedicated in honor of Professor Michael T. Pope on the occasion of his retirement.