The solubility of a particular protein depends on the physico-chemical environment, for example pH, ionic species and ionic strength of the solution. A slow continuous increase of the ionic strength (of a protein mixture) will selectively drive proteins out of solution. This phenomenon is known as “salting-out”. A wide variety of agents, with different “salting out” potencies are available. Chaotropic series with increasing “salting out” effects of negatively (I) and positively (II) charged molecules are given below (von Hippel et al., 1964):
Ammonium sulfate is highly soluble in cold aqueous solutions and is frequently used in “salting-out” purification.
Another method to precipitate proteins is to use water-miscible organic solvents (change in the dielectric constant). Examples of precipitating agents are polyethylene glycol and trichloracetic acid. Under certain conditions, chitosan and non-ionic polyox-yethylene detergents also induce precipitation (Cartwright, 1987; Homma et al., 1993; Terstappen et al., 1993). Precipitation is a scalable, simple and relatively economical procedure for the recovery of a product from a dilute feedstock. It has been widely used for the isolation of proteins from culture super-natants. Unfortunately, with most bulk precipitation methods, the gain in purity is generally limited. Moreover, extraneous components are introduced which must be eliminated later. Finally, large quantities of precipitates may be difficult to handle. Despite these limitations, recovery by precipitation has been used with considerable success for some products.