Biological Small Angle X-ray Scattering (SAXS) became one of the standard techniques in structural biology. As a solution scattering method SAXS does not rely on high quality protein crystals and is not limited in protein size and folding state. On the other side, SAXS cannot provide high resolution protein structures, however modern scattering data evaluation permits model building of macromolecular assemblies in the range down to 15 Angström. High quality SAXS data can be recorded at standard lab based SAXS devices, but the full advantage of solution scattering can be obtained at dedicated SAXS synchrotron beamlines. Using SAXS, environmental parameters can be altered and parameters such as temperature, pH-value and salinity adjusted to the protein activity. The most fascinating and challenging approach is the investigation of kinetics and dynamics of protein reaction. Starting from simply fast mixing experiments using protein and ligand solutions for investigation of protein kinetics, modern synchrotron beamlines allow as well pumped-probed experiments for exploring the dynamic of protein-protein interactions and protein domain motions. Following this idea of investigation on biological systems “at work” new possibilities are provided by Free Electron Lasers. These upcoming high brilliance X-ray sources are able to record X-ray diffraction data in the fs time range, which is suitable to record protein dynamics in detail.
In this lecture the basics of biological SAXS will be explained and high end application of the SAXS method introduced. Some technical details of Free Electron Lasers will be given and future prospective applications of this method discussed.