A seed provides a template for the assembly of molecules to form a crystal with the same characteristics as the crystal from which it originated. Seeding has often been used as a method of last resort, rather than a standard practice. Recently, these techniques have gained popularity, in particular, macroseeding, used to enlarge the size of crystals. Seeding has many more applications, and the use of seeding in crystallization can simplify the task of the crystallographer even when crystals can be obtained without it. We will explore the various seeding techniques, and their applications, in the growth of large single crystals and the methods by which we may attempt to obtain crystals that diffract to higher resolution. Crystallogenesis can be divided into two separate phases. The first being the screening of crystallization conditions to obtain the first crystals, the second consisting of the optimization of these conditions to improve crystal size and quality. Seeding can be used advantageously in both these situations. The first stage in crystallogenesis consists of the discovery of initial crystals, crystalline aggregates, or microcrystalline precipitate. This may result from a standardized screening method (1, 2), a systematic method (3), an incomplete factorial search (see Chapter 1 and refs 4 and 5), or by extensive screening of many conditions. This may be bypassed by starting with seeds from crystals of a related molecule that has been previously crystallized. Molecules that have been obtained by genetic or molecular engineering of a previously crystallized macromolecule fall in this category. This method is termed cross-seeding. It has been used to obtain crystals of pig aspartate aminotransferase starting with crystal from the chicken enzyme (6) and between native and complexed Fab molecules (7). Whatever the method used to obtain the initial crystals, seeding may provide a fast and effective way to facilitate the optimization of growth conditions without the uncertainty which is intrinsic in the process of spontaneous nucleation. The streak seeding technique can be used to carry out a search quickly and efficiently over a wide range of growth conditions.
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