Electronic structure and quadrupole splittings of ferrous iron in hemoglobin.

Abstract

Unexplained quadrupole splittings ΔEQ of some Fe porphyrin compounds, deuteroheme (Py)2, mesoheme (Py)2, HbCO, HbO2, and Hb, measured by the Mössbauer effect, gave the impulse to evaluate within the 3d6 configuration, the eigenvalues and eigenvectors of a Hamiltonian, involving the Coulomb repulsion of 3d electrons and the tetragonal point symmetry of the iron cation. From these calculations it was found that the levels A11,E3,B25, and E5 lay lowest in energy. The resulting EFG tensors were compared with experiment in order to get information about the energy term scheme of these low-lying levels. The relatively large value of ΔEQ in deuteroheme (Py)2 results from spin–orbit interaction between the ground-state A11 and the low-lying E3 level (≈ 345 cm−1), which is raised to at least 1000 cm−1 in mesoheme (Py)2 and HbCO. In the case of HbO2, it is supposed that an antiparallel orientation of the oxygen spin (S = 1) and the spin of the E3 level would result in zero net spin. The calculated EFG, caused by the E3 level, agrees with experiment. For all diamagnetic compounds there is evidence for α low-lying high-spin level B25. In the case of ferrous high-spin iron in Hb, spin–orbit interaction was proved to be most important, too. In order to fit the measured ΔEQ(T) values, low-lying levels A11,E3,E5, beside the ground-state B25, have to be included in our calculations.