High-Resolution X-ray Diffraction of Muscle Using Undulator Radiation from the Tristan Main Ring at KEK

High-resolution X-ray diffraction studies on striated muscle fibres were performed using a hard X-ray undulator installed in the Tristan main ring at KEK, Tsukuba, Japan. The performance of the undulator, along with an example experiment which exploited the unique characteristics of undulator radiation, are reported. The vertical divergence angle of the first harmonic of the undulator was ∼10 µrad under 8 GeV multi-bunch operating conditions and the peak photon flux density was estimated to be ∼3 × 10¹⁶ photons s⁻¹ mrad⁻² (0.1% bandwidth)⁻¹ (10 mA)⁻¹. The well collimated X-ray beam from the undulator made it possible to resolve clearly, with high angular resolution (∼700 nm), the closely spaced diffraction peaks on the meridional axis in the X-ray patterns arising from the thick filaments of a striated muscle under static conditions. By fitting the meridional intensity pattern, a model for the molecular arrangement of the constituent proteins in the thick filaments is proposed. These studies of muscle demonstrate the promise of undulator radiation from third-generation sources for high-resolution diffraction studies.