Initial steps of the photodissociation of the CO ligated heme group

Upon irradiation of CO ligated hemoglobin or myoglobin with a laser pulse of 570 nm (2.17 eV), the proteins are electronically excited into the Q states and undergo dissociation of the CO ligand within 50 fs. Because the Q state is well-known to correspond to a pi-pi* transition localized at the iron porphyrin (heme) group, this should not greatly affect the binding of the CO ligand. It is shown by means of time-dependent density functional theory that, in fact, the Q states decay into the 5 A" and 3 A' singlet excited states. The latter states are repulsive along the Fe-CO stretch coordinate and, consequently, lead to the dissociation of the heme-CO bond. The nature of the repulsive states is analyzed with attachment/detachment density plots. At the equilibrium geometry of heme-CO, they can be understood as an excitation from a pi-back-bonding orbital into an anti-back-bonding orbital which nicely explains their repulsive nature. However, at a separation distance of 2.5 Angstrom, the state has charge-transfer character from the iron to the nitrogens of the porphyrin ring as well as the CO and imidazole ligand. This is in accord with a previous experimental assignment of the first intermediate of the photodissociation process to be a charge-transfer state.