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There exists a variety of methods allowing to slow down reaction rates in proteins in order to match the lifetime of intermediates with the data collection time from standard X-ray diffraction techniques: slow substrates or less reactive mutants can be produced and cryocrystallography has proved extremely useful. Unfortunately, these methods run the risk of trapping an enzymatic reaction in the abnormal state. The development of synchrotron sources, allowing the simultaneous use of a broad range of X rays, led to a renewal of the old Laue technique. In this technique, diffraction occurs simultaneously from all lattices planes fulfilling the Bragg condition for some wavelength present in the incident X-ray beam, resulting in a considerable gain in acquisition speed. Although involving serious complication in data processing (spatial and harmonic overlaps, wavelength normalization, low resultion hole, high sensitivity to crystal disorder), the Laue method has been used successfully. With the third generation os synchrotron radiation sources like the ESRF, providing a spectacular increase in brilliance, the accessible timescales can be lowered down to the lifetime of transient species involved in genuine enzymatic reactions. Taking advantage of the low divergence, high flux, wide bandpath and temporal structure of X-ray beams delivered by these sources, one can attack the challenging task of solving the 3D structure of short lived intermediates building up on the milli, micro and even 100 psec timescale, and tertiary structural changes in proteins can be captured. The White Beam Station (BL3) at the ESRF uses the X-rays emitted by a single electron bunch (50 psec rms, ESRF single bunch mode, 5-15 mA) or by an electron "superbunch" (1 usec pulse, ESRF standard mode, 150 mA) to record Laue diffraction patterns from protein crystals on the usec to 100 psec timescales, and thus open the possibility of following conformational changes in these range of timescales. The potential of time resolved macromolecular crystallography not only depends on the existence of a high quality X-ray source, but also on the parallel development of new instrumental methods, detectors and data processing techniques. Furthermore, its success primarily relies on the availability of biochemical techniques allowing the simultaneous and uniform triggering of a transient process ini a molecules of a crystal. Temperature, pressure or pH jumps might be used to start a reaction, but the fastest timescales seem to be reachable only by photochemical activation. BL3 provides a YAG/DYE laser system synchronized witht eESRF bunchclock to trigger photodissociation of substrates or caged compounds on the nanosecond timescale.
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