NEW DIRECTIONS IN MUSCLE DIFFRACTION USING INSERTION DEVICE BEAMLINES AT SYNCHROTRON SOURCES. Tom Irving, BioCAT, Illinois Institute of Technology

The modern concept of the mechanism of muscle contraction is based on structural, mechanical and biochemical studies, all indicating that relative sliding between the two filament types in the sarcomere depends on cyclic ATPase-coupled crossbridge interactions between myosin heads on the thick filaments and actin in the thin filaments. Fiber diffraction has historically and continues to play a key role studiying this system because of its ability to study muscle fibers under hydrated, physiological conditions, in fact even in the living state. Furthermore, it has the ability (as yet only partially realised) to detect global changes in sarcomere structure at the physiologically relevent millisecond and sub-millisecond time scale. Small angle X-ray diffraction of muscle make unusally high demands on X-ray sources and optics. Over 20 year ago Ken Holmes and Gerd Rosenbaum performed the first muscle experiments using a storage ring as a source of synchrtron radiation motivating the development of the first synchrotron X-ray diffraction facility. Since then, static and limited time-resolved experiments using synchrotron sources have told us much of what we know about muscle contraction. The future promise of time resolved X-ray diffraction studies of muscle can be realized in studies that produce full two-dimensional diffraction data at high time resolution during rapid mechanical transients from cellular level preparations. The major impediment to reaching this goal has been lack of flux. I will present a personal overview of experiments that have been made possible by high intenisty insertion device beamlines at CHESS, Daresbury, and an innovative beamline on the KEK Main ring in Tsukuba and where I think the field will be going with the advent of undulator based beamlines at ESRF, Spring-8 and the Advanced Photon Source.