Perimeter Model for the Magnetic Circular Dichroism Spectrum of Deoxy Ferrous Heme in Myoglobin

The magnetic circular dichroism (MCD) spectra of deoxy heme in Sperm whale myoglobin are explained by using a theory based on the perimeter model (PM) of metalloporphyrin spectra. The perimeter model is shown to be valid by comparison with the heme of carbonmonoxy myoglobin and previous reports including both Zn protoporphyrin and ferric heme MCD spectra. The PM approach, applied to closed shell metalloporphyrins, models the highest occupied molecular orbital as L_z = ±4 and the lowest unoccupied molecular orbital as L_z = ±5. According to the PM, the allowed intense Soret band transition has L_z = ±1, while the vibronically allowed weak Q-band has L_z = ±9. Analysis of the experimental spectra based on the scaled first derivative of the absorption spectrum is demonstrated to give good agreement with calculated spectra, although the experimentally measured values of L_z are somewhat smaller than those predicted by the PM theory. Application of the PM to open shell metals, and in particular deoxy heme, is shown using a vibronic approach that accounts for mixing of charge-transfer states. A Soret excited-state split due to vibronic coupling (VC) is modeled by a porphyrin π excited state (L_z = ±5) that strongly vibronically couples with a dπ state (L_z = ±1). The vibronic coupling model has relevance not only for deoxy heme but also for species such as the heme oxo species known as compound I. The model developed for MCD spectra is consistent with recent resonant Raman spectroscopic studies of deoxy heme.