Mössbauer and susceptibility experiments on different compounds of Fe³⁺-myoglobin

The static magnetic susceptibilities of different ferric high spin and low spin compounds of myoglobin (Mb(H₂O), Mb(H₂O) frozen under high pressure, MbF, MbCN) were measured in the temperature region between 4.2 K and 130 K. Mössbauer absorption experiments on Mb(H₂O) and MbF were perormed at different temperatures between 4.2 K and 180 K and in small magnetizing fields H≤1 kOe. The evaluation of our experimental data was performed with a Hamiltonian describing the 3 d⁵-configuration of the ferric iron by taking into account the Coulomb repulsion of the five electrons within the 3 d-shell, the crystal electric field of {Mathematical expression}-symmetry, and the spin-orbit coupling. The Hamiltonian contains the splitting energies of the five antibonding d-orbitals (d_xy, d_xz, d_yz, d_x2_-y2, d_z2) as parameters. The values of these energies were obtained by a least squares fitting procedure using our magnetic susceptibility data together with the g-factors taken from the literature. In the case of MbF the energy difference between the two lowest Kramers doublets was also determined from present Mössbauer data. The results of the susceptibility and the Mössbauer data are in good agreement. The splitting energies of the 3 d-orbitals can be correlated to the distances between the iron and its nearest neighbours. The different positions of the iron in the compounds investigated are discussed.