Ultrafast rotation and trapping of carbon monoxide dissociated from myoglobin

The nature of ligand motion within proteins has been investigated by measuring femtosecond time-resolved infrared (IR) spectra of CO photodissociated from the haem of myoglobin. Upon dissociation, the CO rotates approximately 90 degrees and becomes trapped within a ligand docking site located near the binding site. Two trajectories, distinguished spectroscopically 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 opposite orientations. The protein reorganizes about the 'docked' CO with a time constant of 1.6+/-0.3 ps and quickly establishes an energetic barrier that inhibits the reverse rebinding process.