OXOVANADIUM(IV) COMPLEXATION BY ADENOSINE 5'-DI-PHOSPHATE AND 5'-TRI-PHOSPHATE AND NUCLEOTIDE BUILDING-BLOCKS

Complex formation between oxovanadium(IV) and the bioligands adenosine 5'-diphosphate (ADP), adenosine 5'-triphosphate (ATP), adenine, adenosine and 2'-deoxyadenosine 5'-diphosphate (dADP) was investigated in aqueous solution by EPR, electronic absorption spectroscopy, and pH-potentiometry. The reaction scheme implies that, in acidic media, nucleotides chelate the metal ion through unshared oxygen atoms of the polyphosphate chain, whereas vicinal cis-oriented hydroxyls of the ribose moiety are the binding set in alkaline solution. Species with mixed co-ordination were also detected. Dihydroxo-bridged dimers can be formed by ATP and ADP molecules co-ordinating through both the phosphate and ribose residues; however, formation of dihydroxo-bridged dimers with mixed chelation is the most favoured. Owing to the poor affinity of VO2+ towards N-donors, adenine is not able to bind VO2+ within the whole measurable Ph range. In adenosine the ribose unit provides the ligand with pairs of vicinal cis-oriented hydroxyls, the right arrangement to form stable five-membered chelated rings. Lack of a C(2)OH group means that dADP does not possess a pair of cis hydroxyls and only diphosphate chelation is observed with this ligand.