Computational study of the effect of the imidazole ring orientation on the EPR parameters for vanadyl-imidazole complexes

The vanadium hyperfine coupling constant for vanadyl-imidazole complexes depends on the orientation of the imidazole ring with respect to the vanadyl bond as illustrated by a recent electron paramagnetic resonance (EPR) study of vanadyl-imidazole model complexes (Pecoraro, et al. J. Am. Chem. Soc. 2000, 122, 767). In the study reported here, density functional theory (DFT) calculations of EPR hyperfine and quadrupole coupling constants for a model complex, [VO(imid)(H2O)(4)] (2+), were used to elucidate the orientation dependence of the vanadium and nitrogen hyperfine coupling constants for an equatorially coordinated imidazole ligand. The computational results for the orientation dependence of the vanadium hyperfine coupling constant reproduce the functional dependence (A(II)(imidazole) = A + B sin(2theta - 90)) observed in the experimental EPR data. The computational results predict similar orientation dependence for the vanadium quadrupole coupling constant and for the nitrogen hyperfine coupling constant for the coordinated nitrogen of the imidazole ligand. These results have important implications for EPR and pulsed EPR studies of vanadoproteins.