Geminate recombination of carbon monoxide to myoglobin

Abstract

Transient absorption spectra of myoglobin, following photolysis of the carbon monoxide complex at room temperature, were measured using a newly developed, sensitive nanosecond absorption spectrometer. The Soret spectrum of the immediate photoproduct is almost identical to that of deoxymyoglobin at equilibrium, suggesting that the heme group has changed from a planar to a domed structure in less than about 3 ns. About 4% of the photodissociated carbon monoxide molecules rebind to the hemes to which they were initially bound, with a relaxation time of 180 ns. Duddell et al. (1980) observed a geminate yield of 27% and a relaxation time of approximately 55 ns for the photolysis of oxymyoglobin. Comparison of the two results using the simplest kinetic model suggests that the 30-fold more rapid overall association rate for the reaction of oxygen with myoglobin compared to carbon monoxide results mainly from faster binding at the heme, with a small contribution from more rapid entry of oxygen into the protein from the solvent. The data on carbon monoxide are also compared with predictions from low-temperature studies of Frauenfelder and co-workers. This comparison points to the need for further experiments to demonstrate the correspondence between the ligand rebinding processes observed at high and low temperatures.