CONDENSATION-REACTIONS BETWEEN VANADATE AND SMALL FUNCTIONALIZED PEPTIDES IN AQUEOUS-SOLUTION

The role that histidyl, seryl, tyrosyl, and other side chains of small peptides play in the formation of vanadate complexes has been investi gated using H-1, C-13, and V-51 nuclear magnetic resonance spectroscop y. Formation constants of product complexes in aqueous solution have b een determined and the influence of pH on the equilibria established. The results revealed that, in histidine-containing compounds, the prot onation state of the imidazole ring of the free ligand strongly influe nces product formation. The results unequivocally showed that no vanad ium-to-imidazole bond was formed for any of the histidine-containing c omplexes studied. However, imidazole and other aromatic side-chain res idues have an effect on product stability via their influence on the p rotonation state (pK(a)) of the products. Aliphatic side-chains, such as those in asparagine or glutamate, do not appear to play a significa nt role in the formation of products. This is not true, however, of se rine or threonine where the side-chains readily generate vanadium-oxyg en bonds.