Synchrotron small-angle X-ray scattering on biologicalmacromolecules in solution

Abstract

Small-angle X-ray scattering (SAXS) is a powerful method for the structural analysis of macromolecular solutions, allowing one to study the structure of native particles and complexes and to rapidly analyze structural changes in response to variations in external conditions. On synchrotrons, SAXS benefits enormously from the high brilliance of the radiation, providing a considerable advantage for the timescale of measurements (allowing also for time-resolved experiments) and the small amounts of material required. Emerging automation of the scattering experiment, data processing, and interpretationmake synchrotron solution SAXS a streamline tool for large-scale structural studies in molecular biology. In the present chapter, a brief account will be given of the basic principles of SAXS by macromolecular solutions and of the synchrotron SAXS instrumentation. The main concepts of SAXS data analysis from monodisperse solutions will be considered and the methods for computation of the overall structural parameters and ab initio low-resolution shape reconstructions will be presented. Further, approaches combining SAXS with other structural, biophysical, and biochemical techniques including validation of predicted or experimentally obtained high-resolution models in solution and identification of biologically active oligomers will be considered. Modeling methods of the quaternary structure of macromolecular complexes in terms of rigid body movements/rotations of individual subunits or domains will be reviewed. The approaches will also be considered to study oligomeric mixtures and to quantitatively characterize flexible macromolecular systems, including intrinsically unfolded proteins. The new methodological developments in SAXS will be illustrated by examples of practical applications.