28 - Crystal Structure by X-Ray Diffraction

Abstract

Crystal structure determination from a whole crystal composed of small molecules is an established practice. The various methods, or combinations of procedures, that can be employed are documented and described in detail in many texts on the subject; see for example refs. 1–9. The availability of electronic computers and experimental techniques such as insertion of heavy atoms into the periodic structure to ease the phase problem has allowed the determination of the three-dimensional positions of the atoms in more complicated molecules. If a number of similar crystals with different heavy atoms inserted can be engineered (a technique known as isomorphous replacement), the individual atomic positions of exceedingly complex macromolecules, such as the globular proteins and enzymes, can be determined7 exclusively by X-ray crystallography via Fourier synthesis. The crucial ingredient for a classical crystallographic structure determination of this type is the availability of a single (macroscopic) crystal, in which the molecule, or a chemically repeating segment of the molecule, forms a periodic lattice.