Mössbauer studies of low-symmetry crystal fields in low-spin ferric heme complexes

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Abstract
The Mössbauer spectra of the azide and cyanide complexes of myoglobin and cytochrome c peroxidase as well as the spectra of the synthetic analogues heme b i s‐pyridine, heme cyanopyridine, and heme b i s‐cyanide were recorded a 4.2 K in small applied magnetic fields. These low‐spin ferric ions exhibit well‐resolved paramagnetichyperfine 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 g̃, the magnetic hyperfinetensor Ã, and the quadrupoletensor Ṽ were determined for each sample. Good fits to the experimental data were achieved. In particular the orientation of the g‐tensors obtained from the Mössbauer measurements on frozen solutions of the myoglobin complexes are in good agreement with published results derived from single‐crystal EPR measurements.