Binding of Oxovanadium(IV) to Tripeptides Containing Histidine and Cysteine Residues and Its Biological Implication in the Transport of Vanadium and Insulin-Mimetic Compounds

The complexation of VIVO ion with three tripeptides of biological importance containing L-histidine or L-cysteine (His- GlyGly, GlyGlyHis and GlyGlyCys) has been studied. This study was performed in aqueous solution by the combined application of potentiometric and spectroscopic (electronic absorption and EPR) techniques. The results indicate that these oligopeptides, if a ligand-to-metal molar ratio of 10 or 15 is used, can keep VIVO ion in solution until the deprotonation of the amide group with the donor set (NH2, CO, Nim ax) for HisGlyGly or (COO–, CO) for GlyGlyHis and GlyGlyCys. In all the systems, at pH values around neutrality, a VOLH–2 species is formed with an (NH2, N–, N–, COO–) donor set for HisGlyGly, (NH2, N–, N–, Nim) for GlyGlyHis and (NH2, N–, N–, S–) for GlyGlyCys. These species, and those with one deprotonated amide group coordinated to the VIVO ion, can be detected by EPR spectroscopy. The N–(amide) contribution to the hyperfine coupling constant along the z axis, Az, depends on the total charge of the donor atoms in the equatorial plane. The participation of albumin in the transport of vanadium and insulin-mimetic VIVO compounds is reconsidered based on these results.