CONTROLLING NUCLEATION IN PROTEIN SOLUTIONS

The first step in the growth of any type of crystal from aqueous solution involves providing a suitable solid surface upon which the crystal can grow to the required size and structural perfection. Two procedures can be employed for this purpose; one involves placing a small seed crystal into a solution whose composition and temperature is carefully controlled, while the other involves inducing a sufficient amount of supersaturation in the solution so that crystals of the type desired nucleate directly in the liquid phase. The former method requires the availability of high quality seed crystals with clean surfaces and the availability of such seeds for protein crystal growth is a major problem. The latter suffers from the fact that the supersaturation needed for nucleation is often significantly greater than that needed for the subsequent growth process and, therefore, both the number of nuclei formed and the resulting growth rates of the nucleated crystals are difficult to control. A method for controlling the nucleation and crystal growth process in small, unseeded aqueous solutions is discussed. In this method, a homogeneous aqueous solution held just near saturation conditions can be selectively supersaturated in a particular locale within the solution so that the number of nuclei formed and their location can be more easily controlled. This can be accomplished by controlling either the temperature or solution concentration within this small region of the solution. The results of preliminary experiments using large localized temperature gradients in water, Rochelle salt, and lysozyme solutions are also discussed.