ULTRAFAST ROTATION AND TRAPPING OF CARBON-MONOXIDE DISSOCIATED FROM MYOGLOBIN

The nature of ligand motion within proteins has been investigated by m easuring femtosecond time-resolved infrared (IR) spectra of CO photodi ssociated from the haem of myoglobin. Upon dissociation, the CO rotate s approximately 90 degrees and becomes trapped within a ligand docking site located near the binding site. Two trajectories, distinguished s pectroscopically and kinetically with time constants of 0.20+/-0.05 ps and 0.52+/-0.10 ps, lead to CO located within the docking site with o pposite orientations. The protein reorganizes about the 'docked' CO wi th a time constant of 1.6+/-0.3 ps and quickly establishes an energeti c barrier that inhibits the reverse rebinding process.