Mössbauer effect: studies of the electronic structures of the heme

On the basis of experimental Mössbauer evidence, the electronic structures of heme iron are discussed for Mb and Hb. In the deoxygenated state, it is concluded from the electric field gradient tensor and the temperature dependence of the quadrupole splitting that the ferrous iron is not in the tetragonal ligand field with the symmetry axis perpendicular to the heme. The experimental results are well explained solely by rhombic perturbation. The electronic ground state is a component of the 5E term, and the principal axis of the anisotrophy is parallel to the heme plane. In the oxygenated state, there is no decisive conclusion on the bonding structure of the O2 molecule to the heme. Still, the two models of Pauling and Griffith are consistent with experimental results. Considering a relaxation between two possible conformational states, the peculiar temperature dependence of the quadrupole splitting is resolved in Fe-O2 model compounds.