Model Investigations for vanadium-protein interactions: vanadium(III) compounds with dipeptides and their oxovanadium(IV) analogues

The reaction of VCl3 with 1,10-phenanthroline and a series of dipeptides (H(2)dip), having aliphatic as well as aromatic side chains, in methyl alcohol and in the presence of triethylamine affords vanadium(III) compounds of the general formula [V-III(dip)(MeOH) (phen)]Cl. Aerial oxidation/hydrolysis of the vanadium(III) species gives their oxovanadium(IV) analogues of the general formula [(VO)-O-IV(dip)(phen)] X-ray crystallographic characterization of the [(VO)-O-IV(dip)(phen)] compounds (where dip(2-) = Gly-L-Ala, Gly-L-Val and Gly-L-Phe) revealed that the vanadium atom possesses a severely distorted octahedral coordination and is ligated to a tridentate dip(2-) ligand at the N-amine atom, the deprotonated N-peptide atom and one of the O-carboxylate atoms, as well as an oxo group and two phenanthroline nitrogen atoms. Circular dichroism characterization of the V-III/(VO2+)-O-IV-dipeptide compounds revealed a strong signal for the (VO2+)-O-IV species in the visible range of the spectrum, with a characteristic pattern which may be exploited to identify the N-am, N-pep and O-car ligation of a peptide or a protein to (VO2+)-O-IV center, and a weak Cotton effect of opposite sign to their vanadium(III) analogues. The visible spectra of the V-III-dipeptide compounds revealed two d-d bands with high intensity, thus indicating that the covalency of the metal-donor atoms is significant, i.e. the vanadium d orbitals are significantly mixed with the ligand orbitals, and this is confirmed by the low values of their Racah B parameters. The high-intensity band of the (VO2+)-O-IV-dipeptide compounds at similar to460 nm implies also a strong covalency of the metal with the equatorial donor atoms and this was supported by the EPR spectra of these compounds. Moreover, the V-III/(VO2+)-O-IV-dipeptide complexes were characterized by EPR and IR spectroscopies as well as conductivity and magnetic susceptibility measurements.