[21] Analysis of diffuse scattering and relation to molecular motion

This chapter discusses the features of diffuse scattering from macromolecules. Diffuse studies are important because they provide the only way to distinguish among different models for correlated movement. This chapter uses interchangeably the terms movement and displacement, reflecting the mixing of space and time averages in an X-ray scattering experiment. Despite this ambiguity, much of the disorder in macromolecular crystals is dynamic. Even if the interconversion of conformations within crystallized molecules does not occur on fast time scales, diffuse scattering studies still reveal the conformational flexibility of the molecules, which is crucial to understanding their biological function. Any variation in the periodicity of the electron density of a crystal, not just the disorder introduced by atomic displacements, gives rise to diffuse scattering. The chapter presents a survey of what the diffuse scattering looks like from various kinds of crystal disorder. For example, both mosaicity and substitution disorder produce diffuse scattering. Although all imperfections are interesting from a condensed-matter perspective, displacement disorder is most relevant to the biological behavior of molecules. This is the focus of this chapter.