Application of DFT Methods in the Study of (VO2+)-O-IV-Peptide Interactions

(VO2+)-O-IV complexes formed by histidylglycylglycine (HisGly-GlyH), glycylglycylhistidine (GlyGlyHisH), glycylglycylcysteine (GlyGlyCysH(2)), N-glycyl-bis(imidazol-2-yl)methylamine (Gly-BIMA), N-glycyl-bis(pyridin-2-yl)methylamine (Gly-BPMA), salicylglycyl-L-alanine (SalGly-L-AlaH(2)) and 1,2-bis(2-hydroxybenzamido)benzene (H(2)hybeb) in their fully deprotonated form were studied by density functional theory (DFT) methods. They are characterised by different total electric charges, total equatorial charges and number of V-N-amide(-) bonds. DFT calculations enable structural features, like V-donor bond lengths, and spectroscopic features, like electron paramagnetic resonance (EPR) and electronic absorption parameters, to be calculated. The results suggest that an amide group coordinates vanadium in the "amide-" rather than the "imine-like" form with the nitrogen atom negatively charged and with a double bond between the carbon and oxygen atoms of the carbonyl group, and that the equatorial charge is delocalised among all the donors bound to vanadium (O-oxido included). The analysis of the molecular orbital composition reveals that the d(xy) orbital is the vanadium orbital at lower energy, that it can participate in a pi bond with the nitrogen p(z) orbital of the amide groups, that the vanadium d(xz) and d(yz) orbitals are involved in a large pi interaction with the oxido p(x) and p(y) orbitals and that differences in the donor strengths of the ligands and deviations from the ideal square-pyramidal symmetry can result in the separation of the energies of the vanadium d(xz) and d(yz) orbitals.