Speciation in vanadium bioinorganic systems .4. Interactions between vanadate, adenosine, and imidazole - An aqueous potentiometric and V-51 NMR study

The potential biological activity of vanadium analogs of AMP, ADP, ATP, 2',3'-cAMP, and 3',5'-cAMP stimulated the full speciation study of the vanadate-adenosine(AdH) and vanadate-adenosine-imidazole(ImH) systems in aqueous solution, using a combination of potentiometry (glass electrode) and V-51 NMR spectroscopy. The study of the H+-H2VO4--AdH-ImH system was performed in 0.600 M Na(Cl) medium at 25 degrees C in the pH range 2-11. In the vanadate-adenosine system V(2)Ad(2)(2-) and a new complex, V(2)Ad(2)(-), With log beta = 7.68 +/- 0.01 and 11.89 +/- 0.08, respectively (pK(a) = 4.21), explained all experimental observations. Although the V(2)Ad(2)(-)-type complex has previously been reported in the vanadate-AMP system, the existence of such a complex in a vanadate-nucleoside system was not previously appreciated. In the vanadate-adenosine-imidazole system a ternary mixed ligand complex, VAdIm(-), forms in addition to the V(2)Ad(2)(2-) and V(2)Ad(2)(-) species. It exists between pH 5.5 and 11 and has a formation constant log beta = 3.04 +/- 0.02. This is the first ternary complex of this type that has been characterized in a qualitative (stoichiometry) and quantitative (formation constant) manner. Although the complex is fairly weak and requires a large excess of imidazole to form, it is significantly more stable than the 1:1 complexes that previously have been reported to form between vanadate and adenosine. Above all, it is much more stable than the complexes that eventually form between vanadate and imidazole. The possibilities that intramolecular imidazole stacking and/or intermolecular hydrogen bonding explain the enhanced stability in the ternary complex are discussed. Furthermore, the action of various vanadium-adenosine derivatives and the potential role of vanadate-adenosine-imidazole complexes in biological systems is evaluated.