A study of the photodissociation of the CO and O2forms of hemoglobin and myoglobin by picosecond absorption spectroscopy

Using a Nd⁺³phosphate-glass laser in a picosecond, pump-probe double-beam absorption apparatus (λ pump = 531 nm, FWHM= 8 ps, T = 4°C), hemoglobin-CO was observed to pass through a series of four intermediates during the first 50 ps after photon absorption, each state possessing a distinct absorption spectrum. Hemoglobin-O2, myoglobin-CO and -O2were also examined, and were all seen to go through the same sequence of intermediates, although a range of decay rates was apparent. Two phases of geminate recombination were also observed: a "rapid" recombination during the first 20 ps, concurrent with the third intermediate; and a "slow" recombination, extending into the nanosecond regime, concurrent with the fourth intermediate. The "rapid" recombination is, in general, more prevalent with the O2compounds than with the CO compounds, and is more apparent for Hb than Mb. A kinetic model, involving convolution of the laser pulses with a photodissociation scheme, was fit to the data to obtain approximate lifetimes and decay rates.