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Vibrational population relaxation of carbon monoxide in the heme pocket of photolyzed carbonmonoxy myoglobin: comparison of time-resolved mid-IR absorbance experiments and molecular dynamics simulations.

The vibrational energy relaxation of carbon monoxide in the heme pocket of sperm whale myoglobin was studied by using molecular dynamics simulation and normal mode analysis methods. Molecular dynamics trajectories of solvated myoglobin were run at 300 K for both the delta- and epsilon-tautomers of the distal His-64. Vibrational population relaxation times of 335 +/- 115 ps for the delta-tautomer and 640 +/- 185 ps for the epsilon-tautomer were estimated by using the Landau-Teller model. Normal mode analysis was used to identify those protein residues that act as the primary "doorway" modes in the vibrational relaxation of the oscillator. Although the CO relaxation rates in both the epsilon- and delta-tautomers are similar in magnitude, the simulations predict that the vibrational relaxation of the CO is faster in the delta-tautomer with the distal His playing an important role in the energy relaxation mechanism. Time-resolved mid-IR absorbance measurements were performed on photolyzed carbonmonoxy hemoglobin (Hb(13)CO). From these measurements, a T(1) time of 600 +/- 150 ps was determined. The simulation and experimental estimates are compared and discussed.