Precipitation
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.
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