The low-temperature EPR spectra and near-infrared magnetic circular dichroism (MCD) spectra are reported of the bis complexes of imidazole, 1-methylimidazole, 4-methylimidazole, 1,2-dimethylimidazole and 2-methylimidazole with Fe(III) octaethylporphyrin (OEP) in the mixed organic solvent dichloromethane/ diethyl ether. It is shown that the latter two have highly anisotropic EPR spectra characteristic of the lowspin ferric state. The optical charge-transfer bands have an unusually high MCD intensity, with a narrow linewidth. It is proposed that this feature is typical of bis-histidine-ligated haem in cytochromes with sterically strained coordination and may be used as diagnostic of such a conformation. The EPR and near-infra-red MCD spectra of the bis-butylamine complex of Fe(III(OEP are also reported. It is shown that bisamine-ligated haem may be clearly distinguished from sterically hindered bis-imidazole (bis-histidine) by near-infrared MCD spectroscopy whereas the assignment is uncertain using EPR spectroscopy alone.
