Time-resolved structures of macromolecules at the ESRF: Single-pulse Laue diffraction, stroboscopic data collection and femtosecond flash photolysis

We review the time structure of synchrotron radiation and its use for fast time-resolved diffraction experiments in macromolecular photocycles using flash photolysis to initiate the reaction. The source parameters and optics for ID09 at ESRF are presented together with the phase-locked chopper and femtosecond laser. The chopper can set up a 900 Hz pulse train of 100 ps purses from the hybrid bunch-mode and, in conjunction with a femtosecond laser, it can be used for stroboscopic data collection with both monochromatic and polychromatic beams. Single-pulse Lane data from cutinase, a 22 kD lipolic enzyme, are presented which show that the quality of single-pulse Lane patterns are sufficient to refine the excited state(s) in a reaction pathway from a known ground state. The flash photolysis technique is discussed and an example is given for heme proteins. The radiation damage from a laser pulse in the femto and picosecond range can be reduced by triggering at a wavelength where the interaction is strong. We propose the use of microcrystals in the range 25-50 mu m for efficient photolysis with femto and picosecond pulses. The performance of circular storage rings is compared with the predicted performance of an X-ray free electron laser (XFEL). The combination of micro beams, a gain of 10(5) photons per pulse and an ultrashort pulse length of 100 fs is likely to improve pulsed diffraction data very substantially. It may be used to image coherent nuclear motion at atomic resolution in ultrafast uni-molecular reactions.