Structural study of the interaction of vanadate with the ligand 1,2-dimethyl-3-hydroxy-4-pyridinone (Hdmpp) in aqueous solution

The interaction of vanadate with the ligand 1,2-dimethyl-3-hydroxy-4-pyridi none (Hdmpp) was studied in aqueous solution using a combination of multinu clear NMR and EPR spectroscopies, as well as potentiometry and cyclic volta mmetry. The different species in solution were identified and characterized , and their pK(a) values and stability constants determined. The vanadium c omplexes formed in solution are strongly dependent on media composition (io nic strength, presence of buffer), pH and metal-to-ligand ratio (M:L). Two major species - V(V)/dmpp and V(V)/(dmpp)(2) - are formed in a 140 mM NaCl solution within the pH range 4.5 to 9.0, when M:L = 1:2. In the presence of excess ligand (M:L less than or equal to 1:5), only the 1:2 complex is pre sent, and at pH < 4 paramagnetic species are detected by EPR in solution, t hus indicating a reducing capacity of the ligand. Cyclic voltammetry shows that redox processes in solution are not just electron transfer, but are ac companied by chemical reactions. The pK(a) values and stability constants w ere determined both by V-51 NMR spectroscopy and potentiometry. The present results have a particular interest in the understanding of the aqueous sol ution chemistry in aerobic conditions of bis(1,2-dimethyl-3-hydroxy-4-pyrid inonato) oxovanadium(IV) complex, VO(dmpp)(2), a vanadium compound with pot ential insulin-mimetic properties. (C) 2000 Elsevier Science Inc. All right s reserved.