Low‐temperature EPR and near‐infrared MCD studies of highly anisotropic low‐spin ferrihaem complexes

Abstract

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.