Ultrafast spin cross-over dynamics in heme proteins

Heme proteins are essential for respiration, neural transmission and biological signaling. The binding
to and dissociation of small diatomic ligands (O 2 , CO, NO, CN) from the Fe atom of the heme
porphyrin represents the primary events of these processes. These can be mimicked by
photodissociating them with a pulse of light and monitoring the recombination with a probe pulse. With
the advent of XFELs, fs photon-in/photon-out experiments such as X-ray emission (XES), X-ray
Raman scattering (XRS) and resonant-inelastic X-ray scattering (RIXS) have become possible that
could never be achieved before due to their requirement for high fluxes and high temporal resolution.
We have investigated the case of Nitrosyl-Myoglobin (MbNO) in order to address crucial questions
about the change from the planar low spin (LS) hexacoordinated heme configuration to the domed
high spin (HS) pentacoordinated one. We excite the system into the Q-bands and probed its evolution
by fs-XES using X-ray pulses from the SACLA, SwissFEL and European-XFEL free electron lasers.
We found that the entire photocycle from planar to domed and back, after ligand recombination, is a
series of spin cross-over (SCO) and back SCO events. We also investigated the most important
electron transfer protein in our body, ferric cyctochrome c, for which no ligand dissociation was ever
reported, and therefore neither doming nor spin states. Here too we establish the photocycle as a
SCO event, and propose that doming is crucial for the electron transfer properties of the protein,
rather than heme ruffling as previously proposed.