A Study of Vibrational Relaxation of B-State Carbon Monoxide in the Heme Pocket of Photolyzed Carboxymyoglobin

Abstract

The vibrational energy relaxation of dissociated carbon monoxide in the heme pocket of sperm whale myoglobin has been studied using equilibrium molecular dynamics simulation and normal mode analysis methods. Molecular dynamics trajectories of solvated myoglobin were run at 300K for both the δ- and ∈-tautomers of the distal histidine, His64. Vibrational population relaxation times were estimated using the Landau–Teller model. For carbon monoxide (CO) in the myoglobin ∈-tautomer, for a frequency of ω0=2131cm−1 corresponding to the B1 state, T1∈(B1)=640±185 ps, and for a frequency of ω0=2119cm−1 corresponding to the B2 state, T1∈(B2)=590±175 ps. Although the CO relaxation rates in both the ∈- and δ-tautomers are similar in magnitude, the simulations predict that the vibrational relaxation of the CO is faster in the δ-tautomer. For CO in the myoglobin δ-tautomer, it was found that the relaxation times were identical within error for the two CO substate frequencies, T1δ(B1)=335±115 ps and T1δ(B2)=330±145 ps. These simulation results are in reasonable agreement with experimental results of Anfinrud and coworkers (unpublished results). Normal mode calculations were used to identify the dominant coupling between the protein and CO molecules. The calculations suggest that the residues of the myoglobin pocket, acting as a first solvation shell to the CO molecule, contribute the primary “doorway” modes in the vibrational relaxation of the oscillator.