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 (H2dip), 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 [VIII(dip)(MeOH)(phen)]Cl. Aerial oxidation/hydrolysis of the vanadium(III) species gives their oxovanadium(IV) analogues of the general formula [VIVO(dip)(phen)]. X-ray crystallographic characterization of the [VIVO(dip)(phen)] compounds (where dip2–=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 dip2– ligand at the Namine atom, the deprotonated Npeptide atom and one of the Ocarboxylate atoms, as well as an oxo group and two phenanthroline nitrogen atoms. Circular dichroism characterization of the VIII/VIVO2+-dipeptide compounds revealed a strong signal for the VIVO2+ species in the visible range of the spectrum, with a characteristic pattern which may be exploited to identify the Nam, Npep and Ocar ligation of a peptide or a protein to VIVO2+ center, and a weak Cotton effect of opposite sign to their vanadium(III) analogues. The visible spectra of the VIII-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 VIVO2+-dipeptide compounds at ~460 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 VIII/VIVO2+-dipeptide complexes were characterized by EPR and IR spectroscopies as well as conductivity and magnetic susceptibility measurements.