Multiple geminate ligand recombinations in human hemoglobin

The geminate ligand recombination reactions of photolyzed carbonmonoxyhemog lobin were studied in a nanosecond double-excitation-pulse time-resolved ab sorption experiment. The second laser pulse, delayed by intervals as long a s 400 ns after the first, provided a measure of the geminate kinetics by re photolyzing ligands that have recombined during the delay time. The peak-to -trough magnitude of the Soret band photolysis difference spectrum measured as a function of the delay between excitation pulses showed that the room temperature kinetics of geminate recombination in adult human hemoglobin ar e best described by two exponential processes, with lifetimes of 36 and 162 ns. The relative amounts of bimolecular recombination to T- and R-state he moglobins and the temperature dependence of the submicrosecond kinetics bet ween 283 and 323 K are also consistent with biexponential kinetics for gemi nate recombination. These results are discussed in terms of two models: gem inate recombination kinetics modulated by concurrent protein relaxation and heterogeneous kinetics arising from alpha and beta chain differences.