An analysis of the electron spin resonance of low spin ferric heme compounds

The electron spin resonance spectra of low spin ferric heme proteins are calculated and compared with experimental spectra recently obtained for a large number of heme proteins. On the basis of observed g values, these heme proteins have been categorized into five prototypes. Since electron spin resonance spectra are quite sensitive to small changes in the local environment about the paramagnetic ion, we have explored in some detail, the possibility that the g value variation in these different types of proteins is due to systematic environmental changes. To this end, we have calculated the g values as a function of tetragonal and rhombic distortions and compared our results with observed behavior. In addition we have also calculated the energy intervals between the low spin states in zero magnetic field and the temperature dependent magnetic moments and studied the effect on these properties also of small symmetry changes.