Because the variable set is so extensive over which successful crystallization conditions must be sought, and because the amount of protein available is often severely limiting, it is essential to conduct as efficient and economical a series of initial trials as possible. One approach to this problem is to set out a selection of parallel trials using a minimum quantity of protein to identify as quickly as possible the type of precipitant, pH, and temperature likely to yield useful results, and to eliminate those regions of parameter space that are clearly of no value. Described here is a simple scheme that has been used with some success over the past two years and that can be carried out with roughly 1 mg of protein. A second approach to the expeditious search for profitable conditions is to expose a single, larger sample, to a range of several different parameters simultaneously. While this approach is now rather hypothetical, we have been developing such techniques and have obtained crystals in several cases. This approach is based on the isoelectric focusing of protein samples on concentration gradients of common precipitating agents. © 1992.
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