Molecular dynamics simulation of photodissociation of carbon monoxide from hemoglobin.

Abstract

A molecular dynamics simulation of the photodissociation of CO from the .alpha. subunit of Hb is described. To initiate photodissociation, trajectories of the liganded molecule were interrupted, the Fe CO bond was broken, and the parameters of the Fe-N bonds were simultaneously, altered to produce a deoxyheme conformation. Heme potential functions were used that reproduce the energies and forces for the Fe out-of-plane motion obtained from quantum mechanical calculations. The effect of the protein on the rate and extent of the displacement of the Fe from the porphyrin plane was assessed by comparing the results with those obtained for an isolated complex of heme with imidazole and CO. The half-time for the displacement of the Fe from the porphyrin plane was found to be 50-15 fs for both the protein and the isolated complex. These results support the interpretation of optical absorption studies using 250-fs laser pulses that the Fe is displaced from the porphyrin plane within 350 fs in both Hb and a free heme complex in solution.