Section 5.1.1.4. Packing of molecules in crystals

Growth kinetics of the different faces should be correlated with the structural anisotropy of the intermolecular contacts. It has been found that a judicious mutation of a single surface residue of a protein can markedly affect its solubility and hence crystallizability. This method has been used with great success for crystallizing a retroviral integrase (Dyda et al., 1994).

The relationship between crystal morphology and internal crystal structure was examined in the mid-1950s (Hartman & Perdok, 1955a,b,c). It was shown that the morphology of a crystal is determined by `chains' of strong intermolecular interactions (hydrogen bonding, van der Waals contacts, molecular stacking) running through the entire crystal. For a crystal to grow in the direction of a strong interaction (`bond'), these bonds must form an uninterrupted chain through the structure, giving rise to the periodic bond chain theory. The stronger the interaction between molecules, the more likely the crystal is to be elongated in that direction. If a bond chain contains interactions of different kinds, its influence on the shape of the crystal is determined by the weakest interaction present in a particular chain. Prominent faces are parallel to at least two high-energy bond chains. This enables a correlation to be made between the crystal lattice and the crystal morphology, based on the fact that direct protein–protein contacts, reinforced by well ordered solvent molecules, are important in determining crystal packing (Frey et al., 1988). Studies of the morphology of tetragonal lysozyme (Nadarajah & Pusey, 1996; Nadarajah et al., 1997) showed that the crystallizing unit is a helical tetramer (centred on the 4₃ crystallographic axes).