A comparison study of the Mössbauer spectra of deoxy Hb-A (low oxygen affinity) and its isolated α and β subunits (high oxygen affinity) was carried out over the temperature range 77–200°K. Within experimental error, no difference was detected between these three proteins, either in the isomeric shifts or in the quadrupole splittings. These results show that the characteristically different oxygen affinity of deoxy Hb-A and its isolated subunits is not a consequence of different electronic states for the ferrous ions in Hb-A and its isolated subunits. The electronic structure of the ferrous ion in hemoglobin was determined using a crystal field approximation. The adjustable parameters in the crystal field model were systematically searched for an electronic level configuration that would give good agreement with the experimental data of the temperature-dependent quadrupole splitting and magnetic susceptibility of deoxy Hb-A. The resulting low lying energy levels in order of increasing energy were 5B2, 1A1, 5E, and 3E. The spin and orbital degeneracy of these levels were removed by the spin-orbit interaction and the rhombic perturbation of the crystalline field. The electronic ground state obtained produces an electric field gradient at the iron nucleus with a principal component of 0.11 e<r−3> parallel to the heme plane and an asymmetry parameter η = 0.51.

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A comprehensive study of the temperature dependence of the Mössbauer spectra of oxygenated hemoglobin (oxy-Hb) and its isolated oxygenated α chains (oxy-α) and β chains (oxy-β) has been completed. A model of the iron–dioxygen bond in oxygenated hemoglobin (oxy-Hb) is proposed where the ground state of the iron ions in oxy-Hb is assumed to be a quantum mixture of ferrous and ferric states of the Mulliken’s electron-donor–acceptor type. The temperature dependence of the Mössbauer quadrupole splitting can be explained by anharmonic vibration of the oxygen molecule in the iron ion’s binding potential well. Various previously proposed models and experimental data on the electronic structure of the iron–dioxygen bond in oxy-Hb are then discussed in terms of this model.

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