Directed energy "Funneling" mechanism for heme cooling following ligand photolysis or direct excitation in solvated carbonmonoxy myoglobin

The kinetic energy relaxation of photolyzed heme in myoglobin was investiga ted using molecular dynamics simulations. Following photolysis, the heme wa s found to lose most of its excess kinetic energy within 10 ps. The kinetic energy decay was found to be a single exponential with a time constant of 5.9 ps in agreement with the experimental observations of Lim, Jackson and Anfinrud [J. Phys. Chem. 100, 12 043 (1996)]. The flow of kinetic energy wa s found to occur primarily through nonbonded contacts. The heme doming moti on causes collisions with nearby residues and large scale collective motion in the protein. However, the strong electrostatic interaction of the isopr oprionate side chains, and the solvating water appears to be the single mos t important "doorway" for dissipation of excess kinetic energy in the heme. Those water molecules in close contact with the heme side chains were foun d to "warm" in less than 1.0 ps. Direct energy transfer from the heme to th e protein is found to occur by "through projectile" (ligand collisions with the distal heme pocket residue), "through bond" (heme bond to proximal his tidine), and "through space" (nonbonded collisional) channels. These result s provide strong evidence for a spatially directed "funneling" of kinetic e nergy through the heme side chains to the surrounding solvent suggested by Hochstrasser and co-workers.