STEREOCHEMICAL REQUIREMENTS FOR THE FORMATION OF VANADATE COMPLEXES WITH PEPTIDES

The condensation reactions occurring between vanadate and a number of amino acids and simple peptides have been studied by V-51 nuclear magnetic resonance. Vanadate and ligand stoichiometry has been established for the complexes formed and their formation constants determined. The amino acids were all found to undergo rather weak interactions with vanadate and in general provided two products with V-51 chemical shifts near -545 and -555 ppm. The peptides also gave minor products with NMR signals occurring with that of vanadate itself. However, in this case, the major products occur at approximately -510 ppm. These latter complexes are monovanadate, monoligand complexes which require the terminal amino, the carboxylate groups and an unsubstituted nitrogen in the peptide linkage in order for product formation to occur. Sidechains promote product formation, apparently by favouring a more readily complexed peptide conformation. Formation constants vary from about 20 to 50 M-1 depending on the sidechain. The carboxylate group can be replaced by a hydroxymethyl. This, however, leads to a significant decrease in product formation at pH 7. Hydrogen ion concentration studies showed that the complexes, when considered to be formed from VO4H2- and neutral peptide, did not require or give off protons. However, when the carboxylate was replaced by the alcohol functionality a proton was released. On the basis of this study the coordination of the vanadate/peptide complexes has been assigned.