The Mo¨ssbauer spectra of the azide and cyanide complexes of myoglobin and cytochromecperoxidase as well as the spectra of the synthetic analogues hemebis‐pyridine, heme cyanopyridine, and hemebis‐cyanide were recorded a 4.2 K in small applied magnetic fields. These low‐spin ferric ions exhibit well‐resolved paramagnetic hyperfine structure below 20 K and the results were analyzed by assuming that the iron ion is in a low‐symmetry crystal field. The crystal field parameters and the relative orientations and components of the magnetogyric tensor g˜, the magnetic hyperfine tensor A˜, and the quadrupole tensor V˜ were determined for each sample. Good fits to the experimental data were achieved. In particular the orientation of theg‐tensors obtained from the Mo¨ssbauer measurements on frozen solutions of the myoglobin complexes are in good agreement with published results derived from single‐crystal EPR measurements.
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