An Introduction to the Crystallogenesis of Biological Macromolecules

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Abstract
The word ‘crystal’ is derived from the Greek root ‘krustallos’ meaning ‘clear ice’. Like ice, crystals are chemically well defined, and many among of them are of transparent and glittering appearance, like quartz, which was for a long time the archetype. Often they are beautiful geometrical solids with regular faces and sharp edges, which probably explains why crystallinity, even in the figurative meaning, is taken as a symbol of perfection and purity. From the physical point of view, crystals are regular three-dimensional arrays of atoms, ions, molecules, or molecular assemblies. Ideal crystals can be imagined as infinite and perfect arrays in which the building blocks (the asymmetric units) are arranged according to well-defined symmetries (forming the 230 space groups) into unit cells that are repeated in the three-dimensions by translations. Experimental crystals, however, have finite dimensions. An implicit consequence is that a macroscopic fragment from a crystal is still a crystal, because the orderly arrangement of molecules within such a fragment still extends at long distances. The practical consequence is that crystal fragments can be used as seeds (Chapter 7). In laboratory-grown crystals the periodicity is never perfect, due to different kinds of local disorders or long-range imperfections like dislocations. Also, these crystals are often of polycrystalline nature. The external forms of crystals are always manifestations of their internal structures and symmetries, even if in some cases these symmetries may be hidden at the macroscopic level, due to differential growth kinetics of the crystal faces. Periodicity in crystal architecture is also reflected in their macroscopic physical properties. The most straightforward example is given by the ability of crystals to diffract X-rays, neutrons, or electrons, the phenomenon underlying structural chemistry and biology (for introductory texts see refs 1 and 2), and the major aim of this book is to present the methods employed to produce three-dimensional crystals of biological macromolecules, but also two-dimensional crystals (Chapter 12), needed for diffraction studies. Other properties of invaluable practical applications should not be overlooked either, as is the case of optical and electronic properties which are at the basis of non-linear optics and modern electronics (for an introduction to physical properties of molecular crystals see ref. 3).

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