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Structural aspects of muscle cross-bridge dynamics

Direct observation of protein movements is difficult as few techniques have the required spatial and temporal resolution. Nevertheless the understanding of the molecular basis of muscle contraction requires such measurements. With X-ray diffraction the ordered packing of muscle proteins in skeletal muscle fibres results in spatial sampling of the diffracted X-rays, thus providing spatial resolution of a few nanometres. Using a bright X-ray source, time resolution on the time scale of the molecular events underlying muscle contraction can be achieved. The research combines time-resolved X-ray diffraction with biophysical techniques to disrupt the equilibrium state of isolated muscle fibres, so to observe the time-course of induced structural and physiological changes, with the purpose of determining the structural basis of the cross-bridge power stroke, namely of the molecular events which underlie the conversion of chemical energy into work.

The low-angle X-ray diffraction of frog muscle fibres was obtained after permeabilization of the cells. It was found that the diffraction pattern closely matched that of intact muscle fibres, under a variety of physiological states. The surprising and pleasing results were that if appropriate measures are taken, the a highly ordered structure can be maintained during force generation, and also in rigor. Slight cross-linking of the muscle fibre with EDC maintains the structure during long periods of activation, and also preserves the fast mechanical characteristics. The procedures thus will make it possible to study changes in the muscle structure, on the millisecond time-scale, which are responsible for the development of force.

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