Photoinduced breaking of the Fe–O$_{2}$ bond in hemoglobin: dissociation quantum yield, excited electronic states, and nonradiative relaxation processes

Bimolecular recombination of molecular oxygen with human hemoglobin upon excitation in different spectral ranges is studied by nanosecond laser kinetic spectroscopy. The results obtained are fundamentally different; i.e., rather efficient photodissociation takes place in the case of excitation into the $Q$-band of the $\pi\pi$* nature and is practically absent upon excitation into the near-IR band corresponding to the electron transfer from the porphyrin $a_{2u}$ orbital to a mixed orbital formed of the iron $d$ orbital and the free $\pi$ orbital of molecular oxygen. Analysis of these data, together with data obtained previously using the techniques of nano-, pico-, and femtosecond spectroscopy, allowed us to describe the mechanism and dynamics of the photodissociation reaction and intraheme relaxation processes.