Linear Electric Field-induced Shifts in Electron Paramagnetic Resonance: a New Method for Study of the Ligands of Cytochrome P-450

The linear electric field shift in paramagnetic resonance has, for the first time, been observed in frozen solutions. The magnitude of the shift parameters, measured at g(max) of the electron paramagnetic resonance absorption envelope, has been used to characterize lowspin ferric heme mercaptide complexes where the nonmercaptide ligand was varied. The magnitude of the shift provides a measure of the difference between the crystal field contribution of the mercaptide and nonmercaptide ligands. In low-spin ferric cytochrome P-450 of rat liver, the presence of an electric field shift indicates a deviation from inversion symmetry and proves that the axial ligands to the heme are not the same. From the magnitude of the shift and its dependence upon the angle between applied electric and magnetic fields, it is suggested that the non-axial ligand to the heme has a crystal field contribution greater than imidazole and smaller than guanidine.