Picosecond photodissociation and subsequent recombination processes in carbon monoxide hemoglobin.

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Abstract
Excitation of [human] HbCO by a single 6-psec 530-nm pulse results in photodissociation with a 1st-order constant of 0.89 .times. 1011 s-1. The kinetics of photodissociation, monitored by following absorbance changes in the Soret band at 440 nm, are interpreted as corresponding to predissociation followed by a crossing into a dissociative state. Subsequent recombination of CO with the porphyrin system and protein structural transformations were monitored by use of a continuous He-Cd laser beam spatially coincident with the photolysis and Soret interrogation beams at the sample. The latter events take place in 3 distinct time regions, depending on excitation pulse energy and repetition rate. Excitation of HbCO with a single pulse (0.8-5 mJ) results in a relaxation to the ground state with an associative 1st-order constant of 5 .times. 103 sec-1. With a 100-pulse train .apprx. 7.5 mJ), a new decay grows with a rate constant of 63 sec-1. For a pulse-train energy of 12 mJ or higher, a delay occurs at the onset of the 2nd (slower) recombination.