| Abstract
| - The potential biological activity of vanadium analogs of AMP, ADP,ATP, 2‘,3‘-cAMP, and 3‘,5‘-cAMPstimulated 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 51V NMR spectroscopy.The study of theH+−H2VO4-−AdH−ImHsystem was performed in 0.600 M Na(Cl) medium at 25 oC inthe pHrange 2−11. In the vanadate−adenosine systemV2Ad22- and a new complex,V2Ad2-, with log β = 7.68 ±0.01and 11.89 ± 0.08, respectively (pKa = 4.21),explained all experimental observations. Although theV2Ad2--typecomplex has previously been reported in the vanadate−AMP system, theexistence of such a complex in a vanadate−nucleoside system was not previously appreciated. In thevanadate−adenosine−imidazole system a ternary mixedligand complex, VAdIm-, forms in addition to theV2Ad22- andV2Ad2- species. It existsbetween pH 5.5 and 11and has a formation constant log β = 3.04 ± 0.02. This isthe first ternary complex of this type that has beencharacterized in a qualitative (stoichiometry) and quantitative(formation constant) manner. Although the complexis fairly weak and requires a large excess of imidazole to form, it issignificantly more stable than the 1:1 complexesthat previously have been reported to form between vanadate andadenosine. Above all, it is much more stable thanthe complexes that eventually form between vanadate and imidazole.The possibilities that intramolecular imidazolestacking and/or intermolecular hydrogen bonding explain the enhancedstability in the ternary complex are discussed.Furthermore, the action of various vanadium−adenosinederivatives and the potential role ofvanadate−adenosine−imidazole complexes in biological systems is evaluated.
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