The near-infrared (700–1900 nm) magnetic circular dichroism (MCD) spectra of a horse-heart metmyoglobin—imidazole complex have been measured as a function of pD between 9.1 and 12.2 at room temperature. Two low-spin ferric haem complexes with MCD peaks at 1600 and 1350 nm, respectively, interconvert with an apparent pK a of just above 11.0. Since this process has been identified with the deprotonation of the added imidazole ligand at N-1 we identify the species showing its main peak at 1600 nm as the histidine—imidazole complex and that at 1350 nm as the histidine—imidazolate form of the haem. This study points to the value of near infrared MCD in discriminating clearly between these two species.
