摘要:
Picosecond kinetic studies of complexes of protoheme (PH), protoheme dimethyl ester (PHDME), and chelated protoheme (MCPH) are reported. The photolysis of carbon monoxide (CO) from complexes BHmCO where B is 1-methylimidazole, 1,2-dimethylimidazole, methanol, and 2-propanol all displayed no observable CO return on picosecond time scales in common nonviscous solvents. Myoglobin-CO and chelated protoheme-CO gave similar results. At high viscosity in a glycerol solution, photolysis of 1-methylimidazole HmCO resulted-in partial carbon monoxide return to the bound state with a rate constant of 10(9) s-1. These results suggest that the bond-making step in the reaction of CO with five-coordinated hemes and heme proteins has a rate constant around 10(9) s-1 compared to > 10(10) s-1 for other common ligands. Thus carbon monoxide reactions are the only ones studied which are not diffusion-controlled in normal solvents.