Oxovanadium(IV) complexes of phosphates of biological relevance: NAD(+), NADP(+) and thiamine mono- and diphosphate

Complex formation between VO(IV) and organic phosphates, NAD(+) (beta-nicotinamide adenine dinucleotide), NADP(+) (beta-nicotinamide adenine dinucleotide phosphate), TDP (thiamine diphosphate) and TMP (thiamine monophosphate), has been studied by combined application of pH-potentiometric and spectroscopic (EPR and electronic absorption) techniques. The species formed in aqueous solution have been examined as a function of pH and ligand-to-metal molar ratio. Different behaviour was found for the ligands. NAD(+) acts as a diphosphate chelator in the acidic pH range, whereas in basic media the binding of the metal ion at the deprotonated hydroxyls of the ribose moieties is strongly favoured. The presence of a basic monophosphate group on the ribose: residue of NADP(+) affects the speciation processes and the ligand behaves as a simple monophosphate in the acidic pH range. Also different is the behaviour of TDP and TMP. While the former behaves as a simple diphosphate ligand, the latter gives rise to monophosphate complexes. In particular. TMP yields stable dinuclear species in which the ligand acts as a bridge between two VO(IV) ions by coordinating through both phosphate and N( 1') donor groups. (C) 1999 Elsevier Science Inc. All rights reserved.