Theoretical determination of the electronic structure and the spatial arrangement of ferrous iron in deoxygenated sperm whale myoglobin and human hemoglobin from Mössbauer experiments

Abstract

The electronic term scheme of ferrous iron in sperm whale myoglobin (Mb) and human hemoglobin (HbA) is evaluated by a Hamiltonian which involves the Coulomb repulsion of the 3d electrons, their interaction with the tetragonally arranged ligands, a small rhombic perturbation of the C_4v point symmetry, and spin–orbit coupling. The temperature dependence of the quadrupole splitting in both compounds was measured and the adjustable parameters of the theory were determined by a least squares fit to the experimental results. It was found that the five lowest singlets mainly descend from the rhombic‐split ⁵E term. The low‐lying ¹A₁, ⁵B₂, and ³E levels contribute to the low‐energy spectrum too. The least squares fit yields a different term scheme for Mb and HbA. In both compounds, however, the spin–orbit coupling constant λ and the covalency factor α² of the quadrupole interaction prove to be λ?69 cm⁻¹ and α²?0.89. The calculated electric field gradient agrees with the single crystal experiment on Mb at 77 K. Owing to these results, we discuss within the crystalline field theory the structural properties of the iron cation and its neighboring ligands: In Mb the iron is found to lie 0.4±0.1 Å above the heme plane, while in HbA the out of plane distance is about 0.1 Å greater. The bond length with the imidazole nitrogen is smallest in HbA, indicating a strong interaction with the proximal histidin; calculations give 2.00±0.03 and 2.12±0.05 Å for HbA and Mb, respectively.