Magnetic circular dichroism studies of pyridine-heme complexes in aqueous media

Abstract

The magnetic circular dichroism (MCD) spectra for Fe 3+ and Fe 2+ protoporphyrins (hemin and heme) with and without pyridine have revealed the sensitivity of the electronic structures of the complexes formed to the solvent system: (1) The hemin in an aqueous ethylene glycol solution gave the practically same Q and Soret MCD as that in an alkaline aqueous solution, indicating that there is no fundamental differences in the electronic states between the monomeric and the dimeric hemins, and that they are both in high spin states. (2) The hemin-pyridine complex in an aqueous solution showed an MCD essentially similar to that for the hemin described above in (1), indicating that this complex is in the high spin state. (3) The addition of pyridine to the monomeric hemin in an aqueous ethylene glycol solution induced the typical spin change from high to low like myoglobin derivatives. (4) The heme in an aqueous solution showed the MCD fairly different from that in an aqueous ethylene glycol solution, the latter showing the MCD of monomeric heme of deoxy-myoglobin. This indicates the existence of the heme-heme interaction in the aggregated or dimeric high-spin heme. (5) The heme-pyridine complexes showed the MCD characteristic of Fe 2+ low-spin complex irrespective of the solvent system.