NMR, CD AND MCD STUDIES OF VANADATE NUCLEOSIDE COMPLEXES

Complex formation between tetraoxovanadate(V) and each of the nucleosides adenosine, guanosine, cytidine and uridine has been studied in a constant salt medium at pH 7. C-13 and V-51 NMR studies show that only complexes with the formula V2L2 (V = vanadate, L = nucleoside) are formed, and their formation constants have been determined. They have V-51 NMR resonances around -523 ppm relative to VOCI3 and they exhibit no CD in the spectral region of the charge-transfer transitions. MCD spectra were also measured, and all experiments are in accord with a molecular structure composed by two edge-sharing VO6 octahedra forming an O4V(mu-O)2VO4 skeleton with each of the nucleoside ligands bridging the two vanadium centres through the ribose 2',3'-oxygens, which are the oxygens outside the V2O6 plane. Admixture of imidazole-HCl buffer at pH 7 gives rise to additional complexes of 1:1 stoichiometry. They have been characterized by V-51 NMR and CD, and their formation constants are reported. Vanadate(V) and the deoxynucleosides deoxyadenosine, deoxyguanosine, deoxycytidine and thymidine form very weak complexes which cannot be detected by V-51 NMR or CD under conditions for which vanadate and the nucleosides form complexes.