Insulin Mimesis of Vanadium Derivatives. Oxidation of Cysteine by V(V) oxo diperoxo complexes

Kinetics of the oxidation of cysteine to cystine by four V(V) oxo diperoxo complexes [VO(O2)2L] possessing insulin mimetic activity, where L=oxalate(oxa), picolinate (pic), bipyridil (bipy), phenanthroline(phen), were performed in water at 10 °C by the UV or stopped-flow technique. 51V NMR spectra indicate that oxa undergoes a total ligand dissociation differently from pic, bipy and phen which hold their ligands also in solution. The observed reactivity is deeply affected by the identity of the ligand. The process seems to require coordination of the cysteine to the metal, followed by oxidation within the coordination sphere. In this respect phen and bipy make the coordination of cysteine much easier than oxa and pic. It is suggested, also on the basis of some preliminary observations concerning the oxidation of C6H5CH2SH, that the oxidation process is triggered by an electron transfer step. The rate of this step would be higher for oxa and pic than for phen and bipy. The observation that the oxidative ability of these vanadium peroxo complexes is dependent upon the nature of the ligands might match the analogous finding that their insulin mimetic activity is also modulated by the ligand identities.