Dependence of the quantum efficiency for photolysis of carboxyhemoglobin on the degree of ligation.

A combined stopped flow-laser photolysis apparatus was used to measure the quantum efficiency for removal of carbon monoxide bound to human hemoglobin as a function of fractional CO saturation. This flow-flash technique allows the properties of partially liganded hemoglobin molecules, which are sparsely populated under equilibrium conditions, to be conveniently studied. Experiments performed at pH 7 and 20 degrees C both in the presence and absence of phosphates gave a similar dependence of quantum efficiency on fractional saturation. The observed quantum efficiency was 0.90 +/- 0.06 at 10% saturation and decreased to 0.47 +/- 0.02 as full saturation was approached. An allosteric model in which Hb(CO)1 has a quantum efficiency of 0.99 while other liganded species have quantum efficiencies of 0.47 was used to produce a good simulation of the results.