The electronic structure and coordination geometry of the oxyheme complex in myoglobin

Abstract

The polarized single crystal electronic absorption spectra of oxymyoglobin, carboxymyoglobin, and cyanomyoglobin are presented. Four regions of the oxymyoglobin spectrum in which charge-transfer states are detected are defined as probes of oxyheme orbital structure and coordination geometry. The z-polarized absorption intensities near 15 500, 21 000, and 31 500 cm−1 is assigned to porphyrin–iron charge-transfer transitions, while the broad x,y-polarized band at 10 800 cm−1 is assigned to porphyrin(π) →O2(πg) promotions. Correlation of the observed transition energies and polarizations of the charge-transfer states with predictions based on extended Hückel studies of oxyheme complexes [Kirchner and Loew, J. Am. Chem. Soc. 99, 4639 (1977)] indicates that the electronic absorption spectrum of sperm whale oxymyoglobin is diagnostic of a bent (Fe–O–O) oxyheme coordination geometry with totally spin paired, ground state, electronic configurations of the iron and the dioxygen ligand.