Counterdiffusion methods applied to protein crystallization.

Accumulated experience during the last years on counterdiffusion crystallization methods shows that they are a convenient and generally applicable way of optimizing solution crystal growth experiments. Irrespective of whether the objective of the experiment is to improve crystal quality or size, many experiments reporting a positive or neutral effect of counterdiffusion exists, but adverse effects are consistently absent. Thus counterdiffusion is viewed as a rational crystallization approach to minimize supersaturation and impurity levels at the crystal growth front and to ensure steadiness of both values. This control of the phase transition state is automatically achieved and sustained by a dynamic equilibrium between mass transport and aggregation kinetics. The course of this function can be implemented in any media permitting diffusive mass transport (gels, capillaries, microfluidic devices or microgravity). The counterdiffusion technique has been exploited in many recent applications revealing interesting effects on nucleation and polymorphic precipitation, hence opening further possibilities for innovative screening of crystallization conditions.