Fibre diffraction

Abstract

Fibre diffraction, diffraction from a specimen in which the diffracting units are randomly oriented about an axis (the fibre axis), is a powerful technique for determining the structural details of helical polymers, often at atomic resolution. Fibre specimens may be noncrystalline or polycrystalline. Expressions are given for diffraction from both types of fibre. Diffraction patterns are recorded photographically or using imaging plates or charge-coupled devices (CCDs). Data processing includes correction for experimental effects, background subtraction and integration of reflections. Initial models, constructed so as to be consistent with the helical parameters or determined directly, are refined against diffraction data by various methods. Difference Fourier–Bessel syntheses are used to locate missing fragments of the structure, including ions and water molecules. Developments in theory and practice and the availability of fast computers have led to descriptions of the molecular architecture of a wide variety of biopolymers, ranging from simple polypeptides, polynucleotides and polysaccharides to complex filamentous viruses, cytoskeletal filaments and other large macromolecular assemblies. This chapter is also available as HTML from the International Tables Online site hosted by the IUCr.