The apparent quantum yield of T-state human hemoglobin. Contribution of protein and heme to rates of oxygen reactions.

The apparent quantum yield for dissociation of oxygen from T-state human hemoglobin has been determined using pulses of light 350 ns long at 540 nm. Two quantum yields were found. One was the same as for the R-state, and, like it, strongly temperature- and viscosity-dependent. The other, only slightly influenced by temperature and viscosity, was 10 times larger at 20 degrees C. Previous work (Sawicki, C. A., and Gibson, Q. H. (1977) J. Biol. Chem. 252, 7538-7547) has shown two distinct phases in binding of oxygen by T-state human hemoglobin at pH 7, 20 degrees C. When the apparent quantum yield was followed with time, the species with high quantum yield correlated with the rapidly reacting T-state species. The hemoglobin chains have different quantum yields in the T-state. Quantum yield data may serve as a measure of population of the liganded T-state in human hemoglobin, supplementing absorbance and circular dichroism data, and permit calculation of the rates of reaction at the heme in both R- and T-states.