Vibrational Energy Relaxation of “Tailored” Hemes in Myoglobin Following Ligand Photolysis Supports Energy Funneling Mechanism of Heme “Cooling”

In a previous molecular dynamics simulation study, the kinetic energy relaxation of photolyzed heme in solvated carbonmonoxymyoglobin was found to be a single exponential decay process with the relaxation time constant 5.9 ps [Sagnella, D. E.; Straub, J. E. J. Phys. Chem. B 2001, 105, 7057]. The strong electrostatic interaction of the isopropionate side chains and the solvating water molecules was shown to be the single most important “doorway” for dissipation of excess kinetic energy in the heme. In this work, the results of a molecular dynamics simulation study of heme “cooling” in two modified myoglobins, in which (1) the two isopropionate side chains in the heme are replaced by hydrogen or (2) the proximal histidine is replaced by glycine, His93Gly, are presented. For each “tailored” protein, the relaxation of the heme’s excess kinetic energy is found to be a single exponential decay process. For the His93Gly mutant protein, the relaxation time is found to be 5.9 ps, in agreement with the relaxation time in native wild-type myoglobin. For myoglobin with the modified heme lacking isopropionate side chains, the relaxation time was found to be 8.8 pssa decrease by 50% compared to that for native myoglobin. These results lend strong support to the proposal that the predominant channel for fast kinetic energy relaxation of the heme in native myoglobin is directed energy “funneling” through the heme side chains to the surrounding solvent.