Vapor diffusion is the standard method used for protein crystallization, because of the small volumes used, easily set-up and quickly checked. Typically, a hanging or sitting drop, containing a solution of protein plus precipitant at a concentration insufficient to precipitate the protein. The drop is equilibrated against a larger reservoir of solution containing precipitant or another dehydrating agent. After sealing, the solution equilibrates to achieve supersaturating concentrations of protein and thereby induce crystallization in the drop [Weber (1991)].
The equilibrium dialysis can be used for the crystallization of proteins at low ionic strength and high ionic strength. At very low ionic strength a phenomenon known as 'salting in' occurs. When the concentration of ions is reduced the protein molecules seek to balance their electrostatic requirements through interactions among themselves [McPherson (1990)]. The salting-in effect can be used to crystallize proteins when it is known that the solubility of the protein is lower at low ionic strength.
Depending on the amount of protein available, one of the following dialysis methods can be used.