Biological crystals are crystals built from biological macromolecules — proteins, nucleic acids or virus particles. Because biological macromolecules are large and contain 103—104 atoms, biological crystals have very large periods of the crystal lattice (50 — 200 Å) compared with ordinary crystals, and in viruses these reach 1000Å and more. The most important feature of biological crystals is that they consist not only of the macromolecules that build them but also contain, inside, between the molecules, the mother liquor (35—80%) from which they were crystallised, usually water with one or another kind of ion (Fig. 1). Biological crystals exist only in equilibrium with such a solution; when biological crystals are dried, the molecules and the crystal as a whole undergo denaturation (destruction of the structure). The regularity with which the molecules are packed in biological crystals is determined by the electrostatic interaction of the charged atomic groups on the surface of the molecules. The solvent molecules adjacent to the surface are ordered, while in the space between the molecules they are arranged at random.
Fig. 1. The packing of the molecules in a protein crystal.
Biological crystals sometimes form in living organisms — in vivo; the main method of obtaining them, however, is the crystallisation of proteins and other biological macromolecules isolated from living organisms and carefully purified (Figs. 2 and 3).
Fig. 2. Crystals of leghaemoglobin.
Fig. 3. Electron micrographs of the packing of the molecules in protein crystals (from top to bottom): catalase (X5•105): tobacco necrosis virus; the separation of protein crystals from the microorganism Bacillus thwingiensis.
The enormous size of biological macromolecules makes it possible to observe their packing in the crystal lattice directly by the methods of electron microscopy (Fig. 3). The main method of studying the structure of biological crystals is X-ray structural analysis, which makes it possible to determine the extremely complex spatial configuration of the molecules that build them. The X-ray diffraction patterns of biological crystals contain an enormous number of reflections (>100000); interpreting them is an exceptionally difficult process. The study of biological crystals has established the structure of more than 100 proteins. The molecule of a globular protein is a polypeptide chain folded in a complicated way and made up of amino-acid residues (the dotted line, Fig. 4) characterised by twenty kinds of side radical R. The number of such residues in the chain is roughly 100 to 500 in different proteins. A chain packed compactly into a globule may have, over particular stretches, the so-called α-helical structure or the β-structure, in which the stretches of the chain lie parallel to one another (Fig. 5). Interpreting the structure of protein crystals has given molecular biology information about the mechanism of the biological activity of enzymes and other proteins. Crystals of transfer ribonucleic acid (t-RNA) have also been obtained and studied.
Fig. 5. The structure of protein molecules: on the left — myoglobin, which consists mainly of α-helical stretches (the amino-acid residues are shown as dots); on the right — cytochrome c, in which stretches with α- and β-structures alternate (the amino-acid residues are shown as links of the chain).
Besides true three-dimensionally periodic biological crystals, there are biological crystals with a different kind of ordering. Thus deoxyribonucleic acid (DNA) forms textured gels — liquid crystals whose X-ray analysis made it possible to build a spatial model of DNA and to establish the nature of the transmission of genetic information. Liquid crystals are also formed by the fibrillar proteins of muscle — myosin and actin. Some globular proteins (catalase and others) crystallise to form tubes with monomolecular walls, in which the molecules are packed according to a helical symmetry. The study of crystals of spherical viruses by the methods of electron microscopy and X-ray analysis made it possible to establish the mutual packing and the structure of the protein molecules that form the shell of the virus (Fig. 6), which are packed in the virus particle according to an icosahedral symmetry with fivefold axes (see Symmetry of crystals).
Fig. 6. Electron micrograph of the packing of the molecules in the shell of a spherical herpes virus (X 640 000)