Ultrafast X-ray Spectroscopy of Heme Proteins

With the advent of X-ray free-electron lasers (XFELs) time-resolved X-ray spectroscopic techniques have advanced to the femtosecond regime. These are element selective techniques which offer unique insight into the electronic and chemical environment and dynamics of a sample. Specifically, X-ray emission spectroscopy probes the occupied density of states and is sensitive to the spin and local structure of the element of interest, whereas X-ray absorption spectroscopy is a tool for probing the unoccupied density of electronic states which makes it sensitive to the oxidation state, ligation and local structure around a specific atom. The tunable photon energy of the X-ray from XFELs allows to selectively probe the element of interest in the sample and additionally, the high intensity (1011 to 1012 photons per pulse) makes it possible to study dilute biological samples in physiological conditions. The sample studied in this work is myoglobin with nitric oxide as ligand, which has long been used as a model system to gain deeper understanding of the class of heme proteins. These proteins all have an iron porphyrine (heme) as an active center and play a crucial role in oxygen storage and transport in all mammals for example, amongst many other functions. In heme proteins, the change of the low-spin (LS) hexacoordinated heme (ground state) to the high spin (HS) pentacoordinated domed form (excited state) is promoted by a reversible light induced ligand detachment, representing the “transition state” that ultimately drives the respiratory function. Here we investigate Myoglobin-NO (MbNO) by employing femtosecond Fe Kα and Kβ non-resonant X-ray emission spectroscopy (XES) at an XFEL upon photolysis of the Fe-NO bond. We find that the photoinduced change from the LS (S = 1/2) MbNO to the HS (S = 2) deoxy-myoglobin (deoxyMb) heme occurs in ~800 fs, and it proceeds via an intermediate (S = 1) spin state. The XES results also show that upon NO recombination to deoxyMb, the return to the planar MbNO ground state is an electronic relaxation from HS to LS taking place in ~30 ps. Thus, the en-tire ligand dissociation-recombination cycle in MbNO is a spin cross-over followed by a reverse spin cross-over process. Femtosecond X-ray absorption near edge spectroscopy (XANES) experiments also performed at an XFEL show that NO dissociates in