Ultrafast X-ray scattering: structural dynamics from diatomic to protein molecules

Recent years have witnessed the birth of picosecond pump-probe X-ray diffraction and scattering techniques, thanks to the technological developments in the third generation synchrotron beamlines and advances in theory and data analysis by combining quantum calculations, molecular dynamics simulations and global fitting analysis. Our laboratories have employed this technique to study structural dynamics and spatiotemporal kinetics of many molecular systems in solution including diatomic molecules, haloalkanes, organometallic complexes and protein molecules over timescales from picoseconds (ps) to milliseconds. The visualising power and unbiased sensitivity of X-ray scattering proved to be instrumental in identifying global reaction pathways and in some cases capturing detailed three-dimensional structures of reaction intermediates. Many results have accumulated from which we have selected some interesting examples to be reviewed here. The structural dynamics of Br2 and I2 are compared and the reaction pathways for HgBr2 and HgI2 are compared. Solvents may affect the reaction pathways as illustrated in the photolysis of CH2I2 in two different solvents. How does the excitation wavelength affect the reaction pathways is another important aspect in photochemistry as shown for Ru3(CO)12. Applications to the folding of cytochrome-c and the structural dynamics of myoglobin and bacteriorhodopsin are also reviewed. The time resolution is currently limited to about 100 ps, the X-ray pulse width available from synchrotron sources. In the near future, X-ray free electron lasers (XFELs) will deliver 100 fs or shorter X-ray pulses. In femtosecond (fs) X-ray scattering experiments with this higher resolution, real-time observation of ultrafast chemical events, such as bond-breaking and bond-making will be possible. So far, gas-phase reactions, which are the main targets for ultrafast electron diffraction due to the high scattering power of electrons, have not yet been studied with time-resolved X-ray scattering, but in principle this discipline will become feasible with the coming XFEL sources. We thus discuss potential fs X-ray scattering experiments for gas phase as well as solution phase reactions. In addition, the high photon flux and the coherence of XFEL-generated X-ray pulses might open up new research areas, such as single-molecule diffraction.