Structure of carboxymyoglobin in crystals and in solution.

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Abstract
The configuration of the heme-carbonyl group upon binding of CO to sperm whale myoglobin (Mb) in crystals is evaluated on the basis of IR spectroscopic methods. Multiplets of the totally symmetric C-O stretching mode are observed for the heme-bound ligand near 1933, 1944, and 1967 cm-1, corresponding to 3 different heme-carbonyl conformers. Variations in the relative proportions of these conformers can be induced by incorporation of small fractions of metMb or deoxyMb into MbCO crystals. The configuration of the Fe-carbonyl with respect to the immediate coordination environment of the heme Fe is assigned for each .nu.(CO) streching frequency on the basis of a detailed comparison of the 3-dimensional structures of the heme environments of MbCO, metMb, and deoxyMb defined by crystallographic methods. The structures of the 3 heme-carbonyl conformers account for the .nu.(CO) IR absorption bands that can be observed for MbCO in solution.