SUBSTITUENT EFFECTS IN ORGANIC VANADATE ESTERS IN IMIDAZOLE-BUFFERED AQUEOUS-SOLUTIONS

The spontaneous formation of vanadate esters and diesters in imidazole-buffered aqueous solutions was observed by using V-51 NMR spectroscopy. The V-51 NMR chemical shifts of monoesters varied as follows: primary alcohols > vanadate monomer congruent-to primary alcohols substituted with electronegative substituents > secondary alcohols > tertiary alcohols. Vanadate esters formed from primary alcohols were more stable than the vanadate esters formed from secondary or tertiary alcohols. This order of stability in solution containing high ionic strength and imidazole is contrary to the order of stability observed in low ionic strength solutions containing Hepes. The role of imidazole in vanadate ester reactions was examined by using relaxation times and 1D and 2D V-51 NMR spectroscopy. The effects of imidazole on the relaxation times and the concentration of vanadate oligomers suggest that imidazole may interact directly with vanadium. The presence of imidazole or salts significantly affects the reactions of vanadate with organic molecules and may be important for the observed catalytic and biological effects of vanadate derivatives.