Electronic Structure and Quadrupole Splittings of Ferrous Iron in Hemoglobin

Abstract

Unexplained quadrupole splittings ${\mathrm{\Delta E}}_Q$ of some Fe porphyrin compounds, deuteroheme ${\mathrm{(P}}_y{)}_2$ , mesoheme ${\mathrm{(P}}_y{)}_2$ , HbCO, HbO₂, and Hb, measured by the Mössbauer effect, gave the impulse to evaluate within the ${\text{3d}}^6$ configuration, the eigenvalues and eigenvectors of a Hamiltonian, involving the Coulomb repulsion of $\text{3d}$ electrons and the tetragonal point symmetry of the iron cation. From these calculations it was found that the levels ${}^1{A}_1{,}^3{\mathrm{E,}}^5{B}_2$ , and ${}^{5}E$ 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 ${\mathrm{\Delta E}}_Q$ in deuteroheme ${\mathrm{(P}}_y{)}_2$ results from spin–orbit interaction between the ground‐state ${}^1{A}_1$ and the low‐lying ${}^{3}E$ level (≈ 345 cm⁻¹), which is raised to at least 1000 cm⁻¹ in mesoheme ${\mathrm{(P}}_y{)}_2$ and HbCO. In the case of HbO₂, it is supposed that an antiparallel orientation of the oxygen spin $\text{(S\hspace{0.17em}=\hspace{0.17em}1)}$ and the spin of the ${}^{3}E$ level would result in zero net spin. The calculated EFG, caused by the ${}^{3}E$ level, agrees with experiment. For all diamagnetic compounds there is evidence for $\alpha$ low‐lying high‐spin level ${}^5{B}_2$ . 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 ${\mathrm{\Delta E}}_Q\mathrm{(T)}$ values, low‐lying levels ${}^1{A}_1{,}^3{\mathrm{E,}}^{5}E$ , beside the ground‐state ${}^5{B}_2$ , have to be included in our calculations.