Single-Crystal Spectra of Ferrimyoglobin Complexes in Polarized Light

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Abstract
The polarized absorption spectra of ferrimyoglobin cyanide, ammonia, azide, fluoride, formate, and water (acid ferrimyoglobin) complexes in single crystals are described. Special emphasis is placed on the polarization-ratio (PR) spectrum—the spectrum of the intensity ratio for light absorbed along and perpendicular to the crystal symmetry axis. It is shown that the PR spectrum magnifies spectral effects that are otherwise obscured by wide absorption bands, and it is shown to be a most useful parameter for studying the spectra of those systems that are intrinsically broad and whose spectra do not show any sharpening at low temperature. The results confirm many general features of porphyrin spectra such as the polarization of the B and Q states, the vibronic nature of the Qv state, and the intensity imbalance in the Q0 state. The substances studied span a variety of spectral types, from low-spin, through mixed-spin, to high-spin iron complexes. The high-spin spectra show, in addition to porphyrin bands, charge-transfer bands that are associated with iron dxz and dyz orbitals. The spectra manifest the splitting of these iron dπ orbitals, and some reasons for this are discussed. The Soret (B) bands are also shown to be split. The PR spectrum of the low-spin azide complex uncovered a new excited state, transitions to which were shown to be polarized out of plane. Possible mechanisms for this transition are discussed. The absolute intensity of myoglobin crystal absorption spectra are shown to be unchanged from the solution values, indicating that the heme environment is comparable in the two media. The heme orientation is shown to be identical, within the precision of the subtle variations of the PR spectrum, for all the derivatives studied, with the possible exception of the azide.

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