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The spin transition of Fe in the active center of a myoglobin molecule, stimulated by a temperature variation, was studied by the theoretical multiple scattering analysis of experimental X-ray absorption data. The spin transition was followed by the movement of the Fe ion out of the plane of the heme without substantial changes in the Fe-N distance. It was shown that the X-ray absorption fine structure above the Fe K-edge is sensitive to both local geometry changes near the Fe ion in the active site of the protein and the spin state of the Fe ion itself. The change in the symmetry of Fe coordination lead to modifications of the spectrum shape in the entire interval up to 40 eV above the main edge. It was found that the spin state effects mostly the rising edge, and at energies above 15 eV becomes negligibly small.

The fine structure of X-ray absorption spectrum of Fe in rubredoxin was interpreted on the basis of the multiple scattering theory and the results of calculations of the self-consistent potential. For biological molecules, such calculations were made for the first time. It was found that the Fe-S interaction is the main factor, which determines the electronic structure of the protein active center. The changes in spectrum shape are mostly due to the spin configuration of 3d-electrons. It was shown that the dipole transition element significantly changes near the absorption edge; therefore, it is impossible to determine the distribution of unoccupied electronic p-states directly from experiment. However, the results of calculations obtained in this work are consistent with the corresponding experimental data, indicating the adequacy of the calculated densities of free electronic states.