Directed Energy “Funneling" Mechanism for Heme Cooling Following Ligand Photolysis or Direct Excitation in Solvated Carbonmonoxy Myoglobin

Abstract

The kinetic energy relaxation of photolyzed heme in myoglobin was investigated using molecular dynamics simulations. Following photolysis, the heme was 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 was found to occur primarily through nonbonded contacts. The heme doming motion causes collisions with nearby residues and large scale collective motion in the protein. However, the strong electrostatic interaction of the isoproprionate side chains, and the solvating water appears to be the single most important “doorway” for dissipation of excess kinetic energy in the heme. Those water molecules in close contact with the heme side chains were found to “warm” in less than 1.0 ps. Direct energy transfer from the heme to the protein is found to occur by “through projectile” (ligand collisions with the distal heme pocket residue), “through bond” (heme bond to proximal histidine), and “through space” (nonbonded collisional) channels. These results provide strong evidence for a spatially directed “funneling” of kinetic energy through the heme side chains to the surrounding solvent suggested by Hochstrasser and co-workers.