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

Using a Nd
⁺³
phosphate-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-O
2
, myoglobin-CO and -O
2
were 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 O
2
compounds 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.