Abstract
We report time-resolved X-ray absorption measurements after photolysis of carbonmonoxy myoglobin performed at the LCLS X-ray free electron laser with nearly 100 fs (FWHM) time resolution. Data at the Fe K-edge reveal that the photoinduced structural changes at the heme occur in two steps, with a faster (∼70 fs) relaxation preceding a slower (∼400 fs) one. We tentatively attribute the first relaxation to a structural rearrangement induced by photolysis involving essentially only the heme chromophore and the second relaxation to a residual Fe motion out of the heme plane that is coupled to the displacement of myoglobin F-helix.
Highlights
Investigating chemical reactions in the femtosecond timescale with direct structural sensitive techniques is one of the scientific challenges that can be approached thanks to the advent of X-ray free electron lasers (XFELs)
Structural changes at the level of the heme chromophore, such as the elongation of the bonds between the Fe ion and the pyrrole N atoms are expected to induce a shift in the Fe K-edge position that can be probed by monitoring the time evolution of the absorption at 7123 eV
The $70 fs relaxation time obtained in the present experiment is shorter than the $170 fs characteristic timescale of the signal change in iron coordination complexes (Bressler et al, 2009; Lemke et al, 2013) at the same X-ray photon energy and attributed to the structural change accompanying the iron low-spin to high-spin transition, but is longer than a quarterperiod of the Fe-His stretching vibration (Fe-His $ 220 cmÀ1, corresponding to an oscillation period of $150 fs) observed in both equilibrium deoxygenated Mb and photolyzed MbCO (Findsen et al, 1985; Franzen et al, 1995a)
Summary
Investigating chemical reactions in the femtosecond timescale with direct structural sensitive techniques is one of the scientific challenges that can be approached thanks to the advent of X-ray free electron lasers (XFELs). The propagation of the perturbation from photoexcited chromophores to the global polypeptide chain conformation has been recently observed through XFEL time-resolved X-ray scattering both in a bacterial reaction center (Arnlund et al, 2014) and in the hemeprotein myoglobin (Mb) (Levantino et al, 2015). In view of the limited spatial resolution of X-ray solution scattering (Cammarata et al, 2008), these studies were not able to provide information on the time evolution of the chromophore structural rearrangement that occurs after light absorption and precedes the polypeptide response (Martin and Vos, 1992). In the case of Mb, one of the most extensively studied systems in biology (Frauenfelder et al, 2003), information relevant to ultrafast structural changes of the heme chromophore has been obtained through time-resolved spectroscopy (Martin et al, 1983; Findsen et al, 1985; Petrich et al, 1988; Zhu et al, 1994; Franzen et al, 1995a; Mizutani and Kitagawa, 2001; and Sato et al, 2007). It is wellknown that the bond between the protein and physiologically relevant diatomic ligands (O2, CO a)Contributed paper, published as part of the special topic issue “Invited Papers of the 2nd International BioXFEL
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