Section 4.1.2.6. Crystallization in gelled media

Because convection depends on viscosity, crystallization in gels represents an essentially convection-free environment (Henisch, 1988). Thus, the quality of crystals may be improved in gels. Whatever the mechanism of crystallization in gels, the procedure will produce changes in the nucleation and crystal-growth processes, as has been verified with several proteins (Robert & Lefaucheux, 1988; Miller et al., 1992; Cudney et al., 1994; Robert et al., 1994; Thiessen, 1994; Vidal et al., 1998a,b). Two types of gels have been used, namely, agarose and silica gels. The latter seem to be the most adaptable, versatile and useful for proteins (Cudney et al., 1994). With silica gels, it is possible to use a variety of different crystallizing agents, including salts, organic solvents and polymers such as PEG. The method also allows the investigator to control pH and temperature. The most successful efforts have involved direct diffusion arrangements, where the precipitant is diffused into a protein-containing gel, or vice versa. As one might expect, nucleation and growth of crystals occur at slower rates, and their number seems to be reduced and their size increased. This finding is supported by small-angle neutron-scattering data showing that silica gels act as nucleation inhibitors for lysozyme (Vidal et al., 1998a). Unexpectedly, in agarose gels, the effect is reversed. Here, the gel acts as a nucleation promoter and crystallization has been correlated with cluster formation of the lysozyme molecules (Vidal et al., 1998b).

Crystals grown in gels require special methods for mounting in X-ray capillaries, but this can, nonetheless, be done quite easily since the gels are soft (Robert et al., 1999). Gel growth, because it suppresses convection, has also proven to be a useful technique for analysing concentration gradients around growing crystals by interferometric techniques (Robert & Lefaucheux, 1988; Robert et al., 1994). In one respect, gel growth mimics crystallization under microgravity conditions (Miller et al., 1992). Finally, it is a useful approach to preserving crystals better once they are grown.