Perimeter Model for the Magnetic Circular Dichroism Spectrum of Deoxy Ferrous Heme in
Myoglobin
FranzenStefan
2002
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 <i>L</i><i><sub>z</sub></i> = ±4 and the lowest unoccupied molecular orbital as <i>L</i><i><sub>z</sub></i>
= ±5. According to the PM, the allowed intense Soret band transition has <i>L</i><i><sub>z</sub></i> = ±1, while the vibronically
allowed weak Q-band has <i>L</i><i><sub>z</sub></i> = ±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 <i>L</i><i><sub>z</sub></i> 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 (<i>L</i><i><sub>z</sub></i> = ±5) that strongly vibronically couples with a dπ state (<i>L</i><i><sub>z</sub></i> = ±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.