A systematic study of the physiologically interesting vanadium-maltol (V-MaH) system has been performed in 0.150 M Na(Cl) at 25 degrees C, using NMR, ESR, and potentiometric techniques. Complexation occurs within a wide pH range, from around 1 up to 10.5. However, a pH-, concentration-, and time-dependent spontaneous reduction of vanadium(V) to vanadium(IV) occurs. From ESR spectra the conditions for this reduction are evaluated and discussed. From potentiometric (glass electrode) and quantitative (51)V NMR measurements, the full speciation in the H(+)-H(2)VO(4)(-)-MaH system was determined in the pH range 5-10.5. Data were evaluated with the computer program LAKE, which is able to treat combined emf and NMR data. The pK(a) value for MaH was determined to be 8.437 +/- 0.005. In the ternary system, three complexes are formed: VMa(2)(-), VMa(-), and VMa(2)(-), having log beta(0,1,2) = 7.02 +/- 0.03, log beta(0,1,1) = 2.66 +/- 0.05, and log beta(-)(1,1,1) = -7.37 +/- 0.21. The errors given are 3sigma. The VMa(2)(-) complex appears as the main species in a pH range from 4.5 to 8.5, whereas both mononuclear monoligand species are minor. Equilibrium conditions are illustrated in distribution diagrams, and the structures of the complexes formed are proposed.

No abstract

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 51V NMR spectroscopy. The study of the H+−H2VO4 -−AdH−ImH system was performed in 0.600 M Na(Cl) medium at 25 oC in the pH range 2−11. In the vanadate−adenosine system V2Ad2 2- and a new complex, V2Ad2 -, with log β = 7.68 ± 0.01 and 11.89 ± 0.08, respectively (pK a = 4.21), explained all experimental observations. Although the V2Ad2 --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 V2Ad2 2- and V2Ad2 - species. It exists between pH 5.5 and 11 and has a formation constant log β = 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.

The full speciation has been determined in the aqueous vanadate-uridine (UrH 2 ) and vanadate-uridine-imidazole (ImH) systems. The studies performed allow direct comparison with the corresponding adenosine systems, and the role of the pyrimidine and purine bases on various complex properties could be evaluated. The studies also provide insight into possible biological mechanism of action. Formation constants have been determined in 0.600 M Na(Cl) medium at 25°C in the pH range 2-11, using a combination of potentiometry and 51 V NMR spectroscopy. In the H + -H 2 VO 4 --UrH 2 system, two dimeric complexes form: V 2 Ur 2 2-with log ) 7.66 ( 0.02 and V 2 Ur 2 3-with log ) -1.09 ( 0.04. The errors given are 3σ. Both species give rise to NMR resonances at -523 ppm. A minor 51 V NMR resonance at -507 ppm is probably originating from a VUr 2-species. No evidence was found for a 1:1 species at -523 ppm or a 1:1 species with 51 V NMR shift superimposed on the vanadate monomer. In the H + -H 2 VO 4 --UrH 2 -ImH system, two monomeric, mixed ligand species are formed: VUrIm -with log ) 3.12 ( 0.04 and VUrIm 2-with log ) -6.26 ( 0.20. The pK a values for the V 2 Ur 2 2-and VUrIm -species, 8.75 and 9.38, are both close to that of uridine (9.02) and are consistent with the interpretation that the deprotonation site is located on the base part of the nucleoside. For both the V 2 Ur 2 n-and VUrIm n-species, isomers have been identified. Equilibrium conditions are illustrated in distribution diagrams. A quantitative comparison with the analogous vanadate-adenosine and vanadate-adenosine-imidazole systems is presented. Furthermore, the solution structure of V 2 Ur 2 2-was examined using 17 O NMR spectroscopy. The complex gives rise to a broad resonance at approximately -1010 ppm, which at higher temperature sharpened into two signals of equal intensities at -1007 and -1032 ppm. This spectrum is consistent only with a solution structure containing pentacoordinate vanadium in an arrangement in which one hydroxyl group of the uridine forms an ester with one vanadium atom, and the other hydroxyl group bridges the two vanadium atoms. In addition, 13 C NMR studies revealed that the dinuclear vanadate-uridine species exchange while no evidence for intermolecular exchange between the complexes and free uridine was observed. The similarity between the vanadate-uridine and vanadate-adenosine complexes suggests that the role of the base in these complexes is very limited with respect to structural features and thermodynamic and kinetic properties.