WORKSHOP 3: Applications of Small Angle X-Ray Scattering

University, LLNL, and University of Wisconsin-Milwaukee. The LCLS isoperated by Stanford University on behalf of the U.S. Department of Energy,Office of Basic Energy Sciences.192-WkshpPicosecond X-Ray Scattering from Biological PhotoreceptorsKeith Moffat.University of Chicago, Chicago, IL, USA.Synchrotrons emit X-ray pulses whose duration is controlled by the lineardimensions of the electron bunches, which in turn are established by the char-acteristics of the RF system. A typical zero current electron bunch length is50 picoseconds and a more typical value for a useful current is ~100 picosec-onds. When an ultrafast pump - probe, X-ray scattering experiment is con-ducted with X-rays emitted by such bunches in pump - probe mode, thetime resolution is set at ~100 picoseconds by the X-ray probe pulse length;the pump laser pulse duration and the jitter in their relative timing aremuch shorter. This contrasts with X-ray free electron laser X-ray sourcessuch as the LCLS where the X-ray pulse duration can be much shorter, inthe femtosecond time range. However, the pump - probe approach to ultrafastkinetics is applicable to both synchrotron and free electron laser sources. AtBioCARS sector 14 of the APS, both X-ray crystallographic and SAXS/WAXS experiments have been successfully conducted on biological sampleswith ~100 picosecond time resolution, on naturally light-sensitive samplessuch as heme proteins and signaling photoreceptors. These studies will bebriefly reviewed.Since not all systems are naturally light-sensitive and appropriate for pump -probe approaches, we apply biologically-inspired protein design to conferlight sensitivity on otherwise light-inert systems. We carefully fuse a lightsensor, input domain to the effector, output domain that displays the desiredactivity. This activity is thereby made light-sensitive, an approach known asoptogenetics (Moeglich and Moffat, Photochem.Photobiol.Sci. 9, 1286-1300(2010)). Challenges in adapting this approach to femtosecond pump - probeexperiments at the LCLS will be presented.