From the first studies showing the feasibility of macromolecular crystal growth in gels (1), an increasing attention has been paid to applications of gel techniques to the domain of biological macromolecules. Confidence in these techniques is such that kits of crystallization in gels are now commercially available (Hampton Research, Laguna Hills, CA, USA). Basically, the protein crystallization process consists of two consecutive steps: • first, the transport of growth units towards the surface of the crystals • second, the incorporation of the growth units into a crystal surface position of high bond strength. The whole growth process is dominated by the slowest of these two steps and is either transport controlled or surface controlled. Avoiding convection in the growth environment will increase the possibility of growing the crystal under slow diffusive mass transport providing that the surface interaction kinetics are faster than the characteristic diffusive flow of macromolecules (in the range of 10-6 cm2/sec for proteins). The ratio between transport to surface kinetics, which can be tuned by either enhancing or reducing transport processes in the solution, has been shown (2) to control the amplitude of growth rate fluctuations (which is thought to reduce crystal quality). These are the main reasons why gels (as well as capillaries and microgravity conducted experiments), if correctly designed, are expected to enhance the quality of crystals. This quality enhancement (3), as well as the possibility of getting crystals when conventional solution techniques failed (4), have been experimentally demonstrated. However, up to now, gel methods have been used on a rather empirical basis, as a simple transposition of solution techniques, and recent fundamental studies of nucleation and growth in gels show that the situation is not as simple as first expected (5, 6). After summarizing the main characteristics of crystal growth in gels, we will examine what are the best conditions using a gel method. Recipes for the preparation of different gel growth experiments will be given. Considering gel growth as a possible simulation of experiments under reduced gravity, recent results of space experiments will be reviewed. Mention will also be made to growth under hypergravity conditions.
