Electron paramagnetic resonance studies of highly anisotropic low-spin states of ferrimyoglobin derivatives

The effects of addition of nitrogenous bases, which gave low-spin ferric porphyrin complexes with highly anisotropic g values, were investigated for ferrimyoglobin by low-temperature EPR measurements. Concomitant denaturation of myoglobin upon addition of the exogenous bases was also of interest. By addition of pyridine-type bases under regulated pH, Mb(Fe3+) complexes showing EPR spectra with highly anisotropic g values were formed. These complexes have the electronic states close to the spin-crossover point but not so close as that of the ferric porphyrin highly anisotropic low-spin (HALS) complexes previously reported. Several types of low-spin species, LSi, LSa and LSb, were produced by the denaturation of myoglobin caused by addition of some exogenous ligands. The LSi was assigned to a complex with histidine-E7 coordinated on the sixth position and LSa to the one with OH- and histidine-F8[Im0].