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

Abstract

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 Lz = ±4 and the lowest unoccupied molecular orbital as Lz = ±5. According to the PM, the allowed intense Soret band transition has Lz = ±1, while the vibronically allowed weak Q-band has Lz = ±9.1 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 Lz are somewhat smaller than those predicted by the PM theory. Application of the PM to open shell metals, and in particular deoxy heme, is show...