Structural characterization of the myoglobin active site using infrared crystallography

We use polarized IR absorption on single crystals to determine the orientation of carbon monoxide bound at the active site of myoglobin, and conclude that the C-O bond lies approximately 7 degrees from the normal to the mean plane of the heme. This result disagrees with much larger angular displacements reported in structural models derived from X-ray and neutron diffraction measurements. The insensitivity of the IR-derived orientation to changes in pH or crystal packing contrasts with the wide variations in CO orientation among diffraction-based models and suggests that the latter are in error. The small energies required to displace the C-O bond 7 degrees from its energetically preferred upright geometry suggest that distortion of the surrounding protein, rather than the relatively undeformable Fe-C-O unit, is the main steric mechanism inhibiting CO binding to myoglobin.