Section 19.3.1. Introduction

Mechanistic biology is frequently confronted with an experimental paradox. High-resolution structures are required to develop a chemical description of macromolecular interactions, but the processes themselves are dynamic and not amenable to high-resolution methods. Chemists and molecular biologists have been successful in generating homogeneous components and entire complex systems that are amenable to crystallization and high-resolution analysis. If the dynamics of the systems are small, as in the case of some enzymes and electron-transfer reactions, time-resolved studies can be performed in the context of the crystal lattice using Laue diffraction (Moffat, 1997; Genick et al., 1997; Srajer et al., 1996). If the dynamic features of the biological reactions are large, the motility must be studied in solution. During the last decade, small-angle X-ray scattering (SAXS) has emerged as an important method for studying large-scale dynamic processes, ranging from protein folding to virus particle polymorphism. The renaissance of this method has resulted from a variety of advances in molecular biology and X-ray instrumentation, and these have dramatically increased the information content of the derived results. Modern synchrotron X-ray sources and advanced detector systems have lead to higher-resolution data in both the spatial and time domains. Computational analyses of the data have improved dramatically, resulting, in some favourable cases, in de novo three-dimensional density functions from SAXS data that are comparable to a 20 Å-resolution electron-microscopy reconstruction. Model-based approaches to interpreting SAXS data have expanded their usefulness to the validation of mechanistic hypotheses involving movement or association of components independently determined at high resolution. Finally, SAXS studies have benefited from the same molecular-biology advances that provide large quantities of homogeneous material for crystallographic and NMR studies. The purpose of this chapter is to address practical aspects of SAXS as they relate to and complement macromolecular crystallography.