Electron spin resonance studies of the solution structure of vanadyl amino acid complexes and mixed ligand complexes of oxalate

Esr and electronic spectra of complexes of the general composition VO(AA)₂ and VO(ox)(AA) have been characterized; AA = gly, his, cys, pro, val, met, asp amino acids. Spectra of the formulation VO(ox)(LL) (with LL = imidazole plus monodentate oxalate, histamine plus monodentate oxalate, histidine, cysteine, 4-imidazolepropionic acid, mercaptopropionic acid, ethylenediamine and ethanolamine) have been used to deduce a self-consistent assignment of A_L, a ligand donor additivity constant contribution to the observed hyperfine splitting, A_iso. Values of A_L are sensitive to inductive effects in the ligand structure. The solution structures and likely coordination geometries of VO(his)₂, VO(gly)₂, and VO(cys)₂²⁻ are discussed. The role of the imidazole moiety as a σ donor and sulfhydryl sulfur as a π acceptor is observed in VO(AA)₂ and VO(ox)(AA) complexes.