The central mystery of muscle contraction is how the enzymatic mysoin heads (crossbridges) convert the energy stored into mechanical work. X-ray diffraction is one of the few biophysical techniques which allows collection of structural information on the physiologically relevant ms time scale and these kinds of experiments were in fact one of the first uses of synchrotron radiation. One of the goals of these experiments has been to detect structural changes due to the action of muscle crossbridges synchronized by rapid mechanical transients. Such experiments so far have been limited to very strong reflections such as the myosin-based 14.3 nm reflection and the equator. We recently obtained 2 ms time resolved data that showed, for the first time, intensity changes in actin -based reflections (mainly on the 5,9 and 5.1 nm layer lines) due to synchronized action of muscle crossbridges.. By measuring changes in the position of the 2.72 nm meridional reflection as a function of the residual force after a given quick release, we were also able to measure the instantaneous elasticity of muscle filaments, quantities of intense interest to muscle physiologists who wish to model muscle. We were able to obtain excellent quality data on the BioCAT beamline and are in the process of attempting to model this data using the known crystal structures of actin, myosin and tropomysosin.
