Dynamics of conformational relaxation in myoglobin

The picosecond evolution of the tertiary conformation of myoglobin following photodissociation of carbonmonoxymyoglobin was investigated at room temperature by probing band III, a weak ironporphyrin charge-transfer transition near 13110 cm^-1 (763 nm) whose position is sensitive to the outof-plane displacement of the iron. Upon photolysis, the iron moves out of the plane of the porphyrin causing a blue shift of band III and a concomitant change in the protein conformation. The conformational relaxation reveals a viscosity dependence even at early times (< 2 ps), indicating that the primary motion of the protein involves a displacement of the surrounding solvent. This motion likely corresponds to a displacement of the F-helix. The ensuing relaxation is highly nonexponential, in agreement with recent molecular dynamics simulations. The conformational changes occurring near the heme likely affect the height of the barrier to ligand rebinding and may explain nonexponential rebinding of ligands at ambient temperatures.