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 degrees C in the pH range 2-11, using a combination of potentiometry and V-51 NMR spectroscopy. In the H+-H2VO4--UrH(2) system, two dimeric complexes form : V(2)Ur(2)(2-) with log beta = 7.66 +/- 0.02 and V(2)Ur(2)(3-) with log beta = -1.09 +/- 0.04. The errors given are 3 sigma. Both species give rise to NMR resonances at -523 ppm. A minor V-51 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 V-51 MMR shift superimposed on the vanadate monomer. In the H+-H2VO4--UrH(2)-ImH system, two monomeric, mixed ligand species are formed : VUrIm(-) with log beta = 3.12 +/- 0.03 and VUrIm(2-) with log beta = -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 O-17 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, C-13 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.