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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. 5.2, p. 117
Section 5.2.2. Solvent in macromolecular crystals
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Molecular Biology Consortium, Argonne, Illinois 60439, USA |
Crystals of biological macromolecules differ from crystals of smaller molecules in that a significant fraction of their volume is occupied by solvent (Adair & Adair, 1936![[link]](/graphics/greenarr.gif)
; Perutz, 1946; Crick, 1957). This solvent is not homogeneous: a part binds tightly to the macromolecule as a hydration shell, and the remainder remains free, indistinguishable from the solvent surrounding the crystal.
Hydration is essential for macromolecular stability: bound solvent is part of the complete macromolecule's structure (Tanford, 1961). Diffraction-based studies of macromolecular crystals verify the presence of well defined bound solvent. Typically, 8–10% of the atomic coordinates in each Protein Data Bank file are those of bound water molecules. The consensus observation of protein hydration (Adair & Adair, 1936; Perutz, 1946; Edsall, 1953; Coleman & Matthews, 1971; Kuntz & Kaufmann, 1974; Scanlon & Eisenberg, 1975) is that every gram of dry protein is hydrated by 0.2–0.3 g of water: this is consistent both with the presence of a shell of hydration, the thickness of which is about one water molecule (2.5–3 Å), and with the rule-of-thumb that approximately one water molecule is found for every amino-acid residue in the protein's crystal structure. Matthews (1974) suggests setting this hydration ratio, w, to 0.25 g water per gram of protein as a reasonable estimate for typical protein crystals.
Crystallographic structures also exhibit empty regions of `free' solvent. Such voids are to be expected: closely packed spheres occlude just 74% of the space they occupy, so to the extent that proteins are spherical, tight packing in their crystals would leave 26% of the crystal volume for free solvent. Although the distinction between free and bound solvent is not sharp (solvent-binding-site occupancies vary, as do their refined B factors), it is a useful convention and is consistent with many observed physical properties of these crystals.
References
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