Tables for
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 5.2, p. 117   | 1 | 2 |

Section 5.2.2. Solvent in macromolecular crystals

E. M. Westbrooka*

aMolecular Biology Consortium, Argonne, Illinois 60439, USA
Correspondence e-mail:

5.2.2. Solvent in macromolecular crystals

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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]; Perutz, 1946[link]; Crick, 1957[link]). 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[link]). 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[link]; Perutz, 1946[link]; Edsall, 1953[link]; Coleman & Matthews, 1971[link]; Kuntz & Kaufmann, 1974[link]; Scanlon & Eisenberg, 1975[link]) 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[link]) 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.


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