A comparison study of the Mouml;ssbauer spectra of deoxy Hbhyphen;A (low oxygen affinity) and its isolated agr; and bgr; subunits (high oxygen affinity) was carried out over the temperature range 77ndash;200deg;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 Hbhyphen;A and its isolated subunits is not a consequence of different electronic states for the ferrous ions in Hbhyphen;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 temperaturehyphen;dependent quadrupole splitting and magnetic susceptibility of deoxy Hbhyphen;A. The resulting low lying energy levels in order of increasing energy were5B2,1A1,5E, and3E. The spin and orbital degeneracy of these levels were removed by the spinhyphen;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 0.11erminus;3 parallel to the heme plane and an asymmetry parameter eegr; equals; 0.51.
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