The energy level scheme for the ferryl heme in compound II of the peroxidase-catalase family as determined from analysis of low-temperature magnetic circular dichroism.

The expressions for temperature-dependent magnetic circular dichroism (MCD) of the ferryl heme (Fe(4+)Por, S=1), which is a model of an intermediate product of the catalytic cycle of heme enzymes (compound II), have been derived in the framework of a two-term model. Theoretical predictions for the temperature and magnetic field dependence of MCD intensity of the ferryl heme are compared with those of the high-spin and low-spin ferric heme. Analysis of reported MCD spectra of myoglobin peroxide [Foot et al., Biochem. J. 2651 (1989) 515-522] and compound II of horseradish peroxidase [Browett et al., J. Am. Chem. Soc. 110 (1987) 3633-3640] has shown the presence in the samples of approximately 1% of a low-spin ferric component, which, however, should be taken into account in simulating observed temperature dependences of MCD intensity. The values of two adjustable parameters are estimated from the fit of the observed and simulated plots of MCD intensity against the reciprocal of the absolute temperature. One of them, the energy gap between the ground and excited terms, predetermines the axial zero-field splitting. The other parameter is correlated with the energy of splitting of excited quartets arising from either the porphyrin pi-->pi* transition or the spin-allowed charge-transfer transition.