Isomorphous replacement and anomalous scattering

Abstract

When two or more crystals have the same structure except for the replacement or addition of one or a few (usually heavy) atoms, phase determination using isomorphous replacement can be resorted to. In most practical instances, the atomic form factor is a real number. When the absorption edge of an atom is close to the wavelength of the incident radiation, the form factor becomes complex and the atom becomes an anomalous scatterer. Anomalous scattering can be used for phase determination and also for determining the absolute configuration. The effects of isomorphous replacement and anomalous scattering are often complementary. Although they have been in use from the 1930s, the most useful applications of the two approaches have been in macromolecular crystallography from the 1950s. Robust methods have been developed for their use in the determination and refinement of heavy-atom positions in protein heavy-atom derivatives and in the calculation of phase angles. With the advent of tunable synchrotron radiation, methods based only on anomalous scattering have become prominent. This chapter describes the development in the area up to the mid-1980s. The subsequent developments in isomorphous replacement and anomalous scattering have been almost exclusively concerned with macromolecular crystallography. They are discussed in International Tables for Crystallography Volume F .