Anomalous scattering in structural chemistry and biology

The uses of X-ray anomalous scattering in crystal structure analysis have undergone a major expansion due to the refinement and ease of availability of the necessary X-ray instrumentation and methods. The structural chemistry and biology fields span a similar suite of technical needs but with widely differing molecular systems. The innate synergies between the two research fields brought two of the authors (JRH and VK) together at an Erice Summer School on Synchrotron Radiation in Crystallography in 1985 and took them into a collaboration spanning already 20 years. The authors wide perspectives are therefore, if not unique, perhaps rather rare. Thus the breadth of coverage of this review is unusual. However, there are two excellent books on anomalous scattering and its uses that have been published covering the periods up to 1975 and 1994 [S. Ramaseshan, S.C. Abrahams (Eds). Anomalous Scattering , Munksgaard, Copenhagen (1975); G. Materlik, C.J. Sparks, K. Fischer (Eds). Resonant Anomalous X-ray Scattering: Theory and Applications , North Holland, Amsterdam (1994)]. As the number of examples of applications in structural biology are now so many it has only been possible to select some illustrative examples but with surveys of trends. In addition though, the development of the methodologies is described in more detail. The structural chemistry applications in, for example, microporous materials, superconductors and magnetic materials is expanding fast but still at a stage where we could attempt to provide a detailed coverage of results, which we have done. Anomalous scattering results on locating metal atoms can also be compared with other technique results and so sections on X-ray Absorption Spectroscopy (XAS), Diffraction Anomalous Fine Structure (DAFS), neutron diffraction and Nuclear Magnetic Resonance (NMR) applications are described where they relate to metal atom location and local structure. Finally anomalous scattering has also been very useful to help develop the modern synchrotron Laue method for quantitative crystal structure analysis, which is also briefly described.