MIDINFRARED VIBRATIONAL-SPECTRUM OF CO AFTER PHOTODISSOCIATION FROM HEME EVIDENCE FOR A LIGAND DOCKING SITE IN THE HEME POCKET OF HEMOGLOBIN AND MYOGLOBIN

Time-resolved mid-ir absorption spectra of CO at 283 K have been measured 100 ps after photodissociation from human hemoglobin A, horse myoglobin, and sperm whale myoglobin. The spectra reveal two vibrational features that are narrower than any reported for CO in the condensed phase near room temperature, indicating that CO becomes localized in a rotationally constrained environment. The integrated absorbance under these narrow features is 0.53 ± 0.05 times that found for sperm whale myoglobin in low temperature glasses. A model is developed that relates this reduction of integrated absorbance to molecular motion in a rotationally constrained environment. From this model, the barrier to CO rotation is found to be 1.5 ± 0.25 kcal/mol. The two vibrational features are tentatively assigned to CO oriented oppositely in the same site within the heme pocket. Evidently, the residues circumscribing the heme pocket in hemoglobin and myoglobin fashion a cavity near the binding site that accommodates the dissociated CO and restricts its rotational motion. This "docking" site mediates ligand transport to and from the active binding site and may be important to the function of ligand-binding heme proteins. © 1995 American Institute of Physics.