Oxovanadium(IV) binding to ligands containing donor sites of biological relevance

Abstract

The V IVO complex formation by a number of sugar ligands and their phosphate derivatives has been studied by the combined application of pH-potentiometric and spectroscopic (EPR and electronic absorption) techniques. The study allows a comparison of the relative effectiveness of phosphate and sugar residues in V IVO binding. Quinic acid (Quin) behaves similarly as a sugar molecule provided with chelating sets consisting of carboxyl and either deprotonated or undissociated hydroxy groups, or of couples of adjacent alkoxo groups. Glucuronic and galacturonic acids (Glu-Ac and Gal-Ac, respectively) are carboxylic sugars. The position of the OH(4) group, cis or trans with respect to the carboxylic function, is critical for the complexing behaviour. In both cases the closure of a six-membered chelated ring is possible and bis chelated species are formed but only with galacturonic acid. Both these ligands co-ordinate via the alkoxo groups in basic solution. Uridine 5′-diphosphoglucose (UDP-Glu) and uridine 5′-diphosphoglucuronic acid (UDP-Glu-Ac) act as diphosphate ligands at low pH. The glucuronic residue permits the carboxylate site to compete effectively with the phosphate moiety in the acidic pH range and to form a transient species with (CO 2 − , O −) donor set. However, hydroxo species predominate in the neutral and basic pH range, whereas the sugar co-ordination is observed only in very basic solution. The chelation ability of carboxylic sugars is discussed.