Orientational distribution of CO before and after photolysis of MbCO and HbCO: a determination using time-resolved polarized Mid-IR spectroscopy.

The technique of time-resolved polarized mid-IR spectroscopy was used to probe the orientational distribution of carbon monoxide (CO) bound to and docked within horse myoglobin, sperm whale myoglobin, and human hemoglobin A in neutral pH solution at 283 K. An accurate determination of the orientation required that the experimentally measured polarization anisotropy be corrected for the effects of fractional photolysis in an optically thick sample. The experimental method measures the direction of the transition dipole, which is parallel to the CO bond axis when docked and nearly parallel when bound to the heme. The polarization anisotropy of bound CO is virtually the same for all protein systems investigated and is unchanging across its inhomogeneously broadened mid-IR absorption spectrum. From these results, it was concluded that the transition dipole moment of bound CO is oriented