Speciation in Vanadium Bioinorganic Systems. 5. Interactions between Vanadate, Uridine, and ImidazoleAn Aqueous Potentiometric, ⁵¹V, ¹⁷O, and ¹³C NMR Study

Abstract

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.

Speciation in aqueous vanadate–ligand and peroxovanadate–ligand systems

“…It has to be noted that Table 1a does not list any isomers in the V-Ur system, because we were unable to determine their formation constants in the physiological medium. We know, however, from previous NMR studies in 0.600 M Na(Cl) medium that isomers of the dominant V 2 Ur 2À 2 species do exist [52]. This can also be seen by the asymmetrical shape of the V 2 Ur 2 resonance in the physiological medium.…”

Fractionation of vanadium complexes in serum, packed cells and tissues of Wistar rats by means of gel filtration and anion-exchange chromatography

“…This is planned for the near future. Another interesting option will be the use of NMR or ESR spectroscopy on the different chromatographic fractions to try to identify the different ligands and to establish the oxidation state of vanadium in the different complexes [36,37].…”

Kinetic study of the dissolution of vanadyl sulfate and vanadium pentoxide in sulfuric acid aqueous solution

“…Moreover, the aqueous chemistry of vanadium (V), studied by several authors (Crans et al, 2004;Elvingson et al, 1998), inferred the presence of many species as a function of the solution's pH. It was suggested that pervanadyl or dioxovanadium cation (VO 2 + ) is the main species formed in the lower pH range (0.5 1.3) (Baes and Mesmer, 1976).…”