Speciation in the aqueous peroxovanadate–maltol and (peroxo)vanadate–uridine systems

Abstract

The speciation in the aqueous H⁺/H₂VO₄⁻/H₂O₂/maltol (Ma), H⁺/H₂VO₄⁻/uridine (Ur) and H⁺/H₂VO₄⁻/H₂O₂/Ur systems was determined in the physiological medium of 0.150 M Na(Cl) at 25 °C. A combination of quantitative ⁵¹V NMR (Bruker AMX500) and potentiometric data (glass electrode) was collected and treated simultaneously by the computer program LAKE. In the quaternary maltol system, the two species VXMa₂⁻ and VX₂Ma²⁻ (where X denotes the peroxo ligand) were found in the pH region 5–10, in addition to all binary and ternary complexes. Their formation was fast. In the ternary uridine (H⁺/H₂VO₄⁻/Ur) subsystem, altogether three vanadate-uridine (V–Ur) species were found in the pH region 4–10, with compositions VUr²⁻, V₂Ur₂²⁻ and V₂Ur₂³⁻. Equilibrium was fast except in weakly acidic solutions when slowly decomposing decavanadates formed. In the quaternary H⁺/H₂VO₄⁻/H₂O₂/Ur system, five additional species were found at pH > 7. They were of VXUr and VX₂Ur compositions. Their formation was fast. Formation constants, compositions and ⁵¹V NMR chemical shifts are given for all the species found in the systems, and equilibrium conditions are illustrated in distribution diagrams as well as the fit of the model to the experimental data. Biological and medical relevance of the species (in the treatment of diabetes) are also discussed, with pseudo-physiological conditions modelled.