Geminate combination of oxygen with iron-cobalt hybrid hemoglobins

Photodissociation of oxygen from the ferrous subunits of hybrid hemoglobins in which the heme of either the α or the β chain has been replaced by cobalt protoporphyrin IX shows large differences between the subunits. With a 25-ns light pulse, the apparent quantum yield at the end of the flash is greater for the β-iron hybrid than for the α-iron hybrid. With the β-iron hybrid, the yield is greater when solution conditions favor the T-state. After the flash, a part of the oxygen which has been dissociated recombines with a half-time of the order of tens of nanoseconds. The proportion is greatest in the R-state at low temperature and least in the T-state. With the α-iron hybrid, oxygen is much less readily removed, and the rapid recombination is slight or absent. It is seen most clearly at low temperatures in conditions which favor the T-state. The long term (greater than 100 ns) effect is that oxygen is much more readily removed from the β-iron hybrid in the T-state than under any other condition. Analogous flash experiments performed with human hemoglobin A may be closely simulated by superposition of the results obtained with the two hybrid hemoglobins under the same conditions. Isolated human α and β - SH chains show differences similar to, but less marked than, those of the iron-cobalt hybrids.