TIME-RESOLVED MACROMOLECULAR CRYSTALLOGRAPHY. K. Moffat Department of Biochemistry and Molecular Biology, and the Consortium for Advanced Radiation Sources, The University of Chicago, 920 E. 58th St., Chicago, IL 60637, USA.
Time-resolved crystallography is aimed at a better understanding of reaction mechanisms, by initiation of a structural reaction in the crystal and the sequential generation of intermediates along the reaction path from reactants to products1. Progress along this reaction coordinate is monitored through the time dependent change in X-ray diffraction intensities, and often in another parameter such as optical absorbance. Experiments may be classed by the means of reaction initiation (e.g. photoactivation of light-sensitive systems, or reactant diffusion in a flow cell); by whether intermediates are trapped or not2 and if so, by the means of trapping (chemical or physical); by the time scale of intermediate lifetimes (at present from nanoseconds3 to kiloseconds or longer) and hence by whether the pulsed or quasi-continuous nature of the synchrotron X-ray source is exploited; by the use of Laue or monochromatic techniques; and by the mode of analysis and presentation of the results ( a time-dependent structural average, or progression through the sequence of time-independent structures). Although time-resolved experiments are challenging, successful general strategies have been identified and applied to several macromolecular systems. Surprisingly, in many respects macromolecular systems are easier to study than small organic or inorganic systems4.
1 Cruickshank, D. W. J., Helliwell, J. R. & Johnson, L. N. (Eds.) Time-resolved
macromolecular crystallography. Oxford Science Publications (1992).
2 Moffat, K. & Henderson, R., Curr. Opin. Struc. Biol. 5, 656-663 (1995).
3 Bourgeois, D. et al., J. Synch. Rad., in press (1996); Srajer, V. et al., manuscript in preparation.
4 Moffat, K., SPIE 2521, 182-187 (1995).