Chapter 5.2 Crystal-density measurements

Crystal-density measurements may be necessary to define a crystal’s molecular-packing arrangement, particularly when a crystal has an unusual packing density (very dense or very open); when there are a large number of subunits in the crystallographic asymmetric unit; when the structure consists of heterogeneous subunits, so the molecular symmetry or packing is uncertain; and for crystals of nucleic acids, nucleic acid/protein complexes and viruses. Topics covered in this chapter include: solvent in macromolecular crystals; the Matthews number; algebraic concepts related to crystal density; experimental estimation of hydration; and methods for measuring crystal denisty. A discussion of how to handle solvent density is also given.

Keywords: Archimedes’ method; Matthews number; crystal-density measurement; flotation method; gradient-tube method; estimation of hydration; immersion microbalance; pycnometry; solvent; solvent density; tomographic crystal-volume measurement; volumenometry