Fig. 8: Schematic representation of the two-channel photodissociation of CO from ferrous HbCO.

The excited-state species HbCO^∗_I and HbCO^∗_II are assumed to be formed sequentially from the ground-state species HbCO(¹A₁) after the photoexcitation to the ¹ππ^∗(Q) state, HbCO(¹Q), of the CO-bound protein. τ_tr is the time constant for the transition from HbCO^∗_I to HbCO^∗_II. τ_I and τ_II are the time constants for the formation of the deoxy-like species Hb from HbCO^∗_I and HbCO^∗_II, respectively. g(τ) is the lifetime distribution describing both the geminate and bimolecular ligand rebinding to the heme protein. To simplify the diagram and focus only on the CO photodissociation, protein conformational transitions are not shown. The diagram is labeled for the case of Hb, but it is generally applicable for each Hb subunit in a chosen conformational substate. For the α and β chains in the A₁ conformational substate, the lifetime distributions g(τ) are shown in Fig. 7 (black and blue line, respectively). The A₁ conformational substate is characterized by the infrared absorption band of the bound CO, ν_CO(A₁) ≈ 1951 cm⁻¹; the E₀ state, ν_CO(E₀) ≈ 1954 cm⁻¹. The ground state of HbCO and Hb is a singlet (¹A₁) and quintet (⁵B₂), respectively. The corresponding ground state electronic configurations for the ferrous iron ion are shown at the bottom of the figure (low- and high-spin state, respectively).