Finding optimal conditions for crystal growth

The two most important keys to success of a crystallographic project are purity and quantity of the macromolecule under study. Impure samples will not make suitable crystals, and even for proteins of the highest purity, repeated trials will be necessary before good crystals result. 

The complexity of this problem is illustrated in Fig. 3.3, which shows the effects of varying just two parameters, the concentrations of protein (in this case, the enzyme lysozyme) and precipitant (NaCI). Notice the effect of slight changes in concentration of either protein or precipitant on the rate of crystallization, as well as the size and quality of the resulting crystals. 

An example of this approach is illustrated in Fig. 3.4. The graph in the center is a two-dimensional slice of a four-dimensional surface over which [protein], ([protein] X [precipitant]), pH, and temperature were varied, in attempts to find optimal crystallization conditions for the enzyme tryptophanyl-tRNA synthetase. Note that this surface samples the rectangular region [protein] versus ([protein] X [precipitant]), mentioned earlier. The height of the surface is the score for the crystallization. Surrounding the graph are photos of typical crystals obtained in multiple trials of each set of conditions. None of the trial conditions were near the peak of the surface. The 

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In efforts to obtain crystals, or to find optimal conditions for crystal growth, crystallographers sometimes obtain a protein or other macromolecule in more than one crystalline form. Compare, for instance, Figs. 3.1a and 3.le, which... 

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