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

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 bis-pyridine, heme cyanopyridine, and heme bis-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 g̃, the magnetic hyperfine tensor Ã, and the quadrupole tensor 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.