Photolysis of the Histidine-Heme-CO Complex

Photodissociation of the histidine-heme-CO complex has been investigated with picosecond to millisecond absorption difference spectra. The results suggest that both ligands of the heme group, histidine and carbon monoxide, are released after excitation. No independent phase for the dissociation of the histidine was observed, indicating that both axial ligands are rapidly dissociated. The recombination rates and kinetic difference spectra are consistent with the following scenario: After photodissociation the heme group recombines first with a water molecule and then with carbon monoxide; the final event is the replacement of the water molecule by histidine. The tetracoordinated heme has a Soret absorption peak at 432 nm and recombines with water with a rate of 2 × 10⁴ M⁻¹ s⁻¹.