Extraction of Fast Changes in the Structure of a Disordered Ensemble of Photoexcited Biomolecules

Using pump-probe experiments of varying time intervals between pump and probe, the method of time-resolved crystallography has given many insights into the fast time variations of crystallized molecules as a result of photoexcitation. We show here that quantities extractable from multiple diffraction patterns of dissolved molecules in random orientations, as measured using powerful ultrashort pulses of X-rays, also contain information about structural changes of a molecule on photoexcitation. 1. Introduction The X-ray free-electron laser (XFEL) is a new instrument which promises to revolutionize our study of the atomic architecture of matter [1, 2]. The brightness of the X-rays produced by this instrument is some 10 billion times greater than any existing X-ray source (including present-day synchrotrons). This allows the possibility of measuring signals from scattered X-rays of even large single molecules, like proteins. The traditional limitation of X-ray flux for fragile biomolecules can be circumvented completely due to the fact that this very bright radiation is delivered in ultra-short pulses [3–5]. Although the molecules under study will undoubtedly suffer catastrophic radiation damage, the shortness of the pulse enables a signal to be measured from the particle before its disintegration. This enables structure determination of reproducible biomolecules by essentially an unlimited X-ray flux and is in one sense a complete solution of the radiation damage problem. It should also be possible to combine the ultra brightness [6] of the radiation with the ultra shortness of its duration [7] to enable the gathering of information never been before possible, for example, the changes in the structure of an uncrystallized biomolecule as a result of some stimulus, such as photoexcitation, as a function of time since the photoexcitation. Since this time can be very short, the possibility then exists of experimentally following the course of rapid chemical reactions of such uncrystallized biomolecules, as with crystallized ones [7] by the technique of time-resolved diffraction [8–10]. This may allow for the first time the study of biochemical reactions of molecules in aqueous solution in which they occur in nature. An idea proposed for sample delivery of hydrated molecules to an XFEL beam is to inject a continuous stream of solutions containing the molecules into the sample chamber [11–13]. The incident X-rays then scatter off the protein solution. The design specification of the LCLS is to produce an X-ray beam of perhaps 0.1 microns in diameter at the