A study of vibrational relaxation of B-state carbon monoxide in the heme pocket of photolyzed carboxymyoglobin

The vibrational energy relaxation of dissociated carbon monoxide in the hem e pocket of sperm whale myoglobin has been studied using equilibrium molecu lar dynamics simulation and normal mode analysis methods. Molecular dynamic s trajectories of solvated myoglobin were run at 300 K for both the delta- and epsilon-tautomers of the distal histidine, His(64) Vibrational populati on relaxation times were estimated using the Landau-Teller model. For carbo n monoxide (CO) in the myoglobin E-tautomer, for a frequency of omega(0) = 2131 cm(-1) corresponding to the B-1 state, T-1(epsilon)(B-1) = 640 +/- 185 ps, and for a frequency of omega(0) = 2119 cm(-1) corresponding to the B-2 state, T-1(epsilon)(B-2) = 590 +/- 175 ps. Although the CO relaxation rate s in both the epsilon- and delta-tautomers are similar in magnitude, the si mulations predict that the vibrational relaxation of the CO is faster in th e delta-tautomer. For CO in the myoglobin delta-tautomer, it was found that the relaxation times were identical within error for the two CO substate f requencies, T-1(delta)(B-1) = 335 +/- 115 ps and T-1(delta)(B-2) = 330 +/- 145 ps. These simulation results are in reasonable agreement with experimen tal results of Anfinrud and coworkers (unpublished results). Normal mode ca lculations were used to identify the dominant coupling between the protein and CO molecules. The calculations suggest that the residues of the myoglob in pocket, acting as a first solvation shell to the CO molecule, contribute the primary "doorway" modes in the vibrational relaxation of the oscillato r.