Abstract
Heme a 3+ isolated from bovine cardiac muscle cytochrome oxidase has been converted to the bis-imidazole species and studied by magnetic circular dichroism (MCD) spectroscopy. Spectra have been recorded down to 1.5°K, enabling the MCD magnetization curves to be measured at a number of wavelengths in the visible and near infrared regions. The experimentally determined curves show excellent correlation to a curve using the g-values determined by electron paramagnetic resonance spectroscopy to be g z = 2.96, g y = 2.29, and g x = 1.73. The data show that the bis-imidazole derivative of extracted heme a 3+ is an excellent model of cytochrome a in the enzyme, confirming the presence of two histidine residues in the protein as the fifth and sixth ligands. The spectral features of heme a 3+ bis-imidazole in the near infrared region are consistent with transitions of the porphyrin ( a 1u, a 2u,) to ferric ( e g ) charge transfer type.
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