Fig. 6: Scheme of the spin cross-over (SCO) and reverse SCO characterizing the photocycle of MbNO.

The low-spin ground state MbNO (S = 1/2) haem (lower left panel) undergoes prompt dissociation of NO upon π−π* excitation. The porphyrin Q-state decays to a triplet state (S = 1, upper left panel) of the pentacoordinated deoxyMb haem in <100 fs⁴⁷. Further relaxation to the quintet state of the pentacoordinated haem occurs in ~500 ± 250 fs (right “High-Spin” panel). The entire process (NO dissociation and relaxation from the Q-state to the high-spin quintet state) occurs in ~800 fs. Upon recombination of NO to deoxyMb leading to a hexacoordinated HS-domed haem (right “ligated high-spin” panel), relaxation back to the LS planar ground state occurs in ~30 ps. The initial low spin structure is PDB entry 2FRJ (Fe = orange, N = blue, O = red and C = grey), the sizes of the Fe atom, the NO molecule and the doming are exaggerated in order to highlight the key changes in the photocycle. The actual structural differences between MbNO and deoxyMb are shown in Supplementary Fig. 1.