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International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 5.1, pp. 111-112
Section 5.1.1.1.3. Polymorphism
aThe Institute for Cancer Research, The Fox Chase Cancer Center, Philadelphia, PA 19111, USA |
Intermolecular contacts between protein molecules in the crystalline state determine the mechanical stability of the crystal. If the conditions used for crystallization vary, the number and identity of these contacts may be changed, and polymorphs will result. Polymorphism is the existence of two or more crystalline forms of a given material. Polymorphs have different unit-cell dimensions and hence different molecular arrangements within them. This property is common for biological macromolecules and can be used to select the best crystalline form for X-ray diffraction studies. Different polymorphs of a particular material are often prepared by varying the crystallization conditions. They may also develop in the same crystallizing drop because supersaturation conditions may change while a crystal is growing. Examples of polymorphs are provided by hen egg-white lysozyme, which can form tetragonal, triclinic, monoclinic or orthorhombic crystals, depending on the pH, temperature and nature of added salts in the crystallization setup (Steinrauf, 1959![[link]](/graphics/greenarr.gif)
; Ducruix & Giegé, 1992: Oki et al., 1999).
A regular surface that offers a charge distribution pattern that is complementary to a possible protein layer in a crystal can sometimes be useful in producing a starting point for the nucleation of new protein crystals. Epitaxy is the oriented growth of one material on a crystal of an entirely different material. Regularities on the surface of the first crystal can act as a nucleus for the oriented growth of the second material. Generally, there should be similar, but not necessarily exactly matching, repeat distances in the two crystals. Epitaxy has been used with considerable success for the growth of protein crystals on selected mineral surfaces (McPherson & Shlichta, 1988). For example, lysozyme crystals grow well on the surface of the mineral apophyllite. Crystals of related macromolecules can also be used as nucleation sources for protein crystallization. Epitaxy can, however, sometimes be a nuisance rather than a benefit if the crystallization setup contains surfaces with unwanted regularity.
A change in the environment around a protein crystal may also cause a change in unit-cell dimensions, and possibly even in space group. For example, the transference of a RuBisCO crystal from a high-salt, low-pH mother liquor to a low-salt, high-pH synthetic mother liquor produced a more densely packed polymorph. The overall unit-cell dimensions were smaller in the latter (Vm changed from 3.16 to 2.74 Å3 Da−1 ) (Zhang & Eisenberg, 1994).
References
Ducruix, A. & Giegé, R. (1992). Editors. Crystallization of nucleic acids and proteins. A practical approach. Oxford, New York, Tokyo: IRL Press.Google Scholar
McPherson, A. & Shlichta, P. (1988). Heterogeneous and epitaxial nucleation of protein crystals on mineral surfaces. Science, 239, 385–387.Google Scholar
Oki, H., Matsuura, Y., Komatsu, H. & Chernov, A. A. (1999). Refined structure of orthorhombic lysozyme crystallized at high temperature: correlation between morphology and intermolecular contacts. Acta Cryst. D55, 114–121.Google Scholar
Steinrauf, L. K. (1959). Preliminary X-ray data for some new crystalline forms of β-lactoglobulin and hen egg-white lysozyme. Acta Cryst. 12, 77–79.Google Scholar
Zhang, K. Y. J. & Eisenberg, D. (1994). Solid-state phase transition in the crystal structure of ribulose 1,5-bisphosphate carboxylase/oxygenase. Acta Cryst. D50, 258–262.Google Scholar
