Reevaluation of the additivity relationship for vanadyl-imidazole complexes: Correlation of the EPR hyperfine constant with ring orientation

Five new vanadyl-imidazole complexes with the imidazole ring in different o rientations to the vanadyl unit have been synthesized and characterized by X-ray crystallography and electron paramagnetic resonance (EPR). The hyperf ine coupling constants (A(parallel to)) from the EPR spectra of these compl exes were analyzed using the additivity relationship. There is some discuss ion as to what contribution an "aromatic imine" such as imidazole bound in the equatorial plane of a vanadyl complex makes to the hyperfine coupling. This analysis, applied to the new complexes as well as those vanadyl-imidaz ole complexes previously published, reveals a trend in the contribution to the hyperfine coupling related to the orientation of the imidazole ring rel ative to the vanadyl unit; an imidazole parallel to the vanadium-oxygen dou ble bond was found to have a contribution of 40 x 10(-4) cm(-1), while one perpendicular gave a contribution of 45 x 10(-4) cm(-1). A similar trend fo r imine values was also observed in the study of these complexes. These fin dings will significantly influence EPR studies of vanadium in biological sy stems, whether naturally-occurring or substituted in as a spectroscopic pro be. These data are also used to draw structural conclusions on molecules co ntaining vanadyl ion whose are not published, including the reduced vanadiu m bromoperoxidase (VBrPO).