Salting out

Salting out (also known as antisolvent crystallization, precipitation crystallization, or drowning out)^[1] is an effect based on the electrolyte-nonelectrolyte interaction, in which the non-electrolyte could be less soluble at high salt concentrations. It is used as method of separating proteins. The salt concentration needed for the protein to precipitate out of the solution differs from protein to protein. This process is also used to concentrate dilute solutions of proteins. Dialysis can be used to remove the salt if needed.

Principle

There are hydrophobic amino acids and hydrophilic amino acids in protein molecules. After protein folding in aqueous solution, hydrophobic amino acids usually form protected hydrophobic areas while hydrophilic amino acids interact with the molecules of solvation and allow proteins to form hydrogen bonds with the surrounding water molecules. If enough of the protein surface is hydrophilic, the protein can be dissolved in water.

When the salt concentration is increased, some of the water molecules are attracted by the salt ions, which decreases the number of water molecules available to interact with the charged part of the protein. As a result of the increased demand for solvent molecules, the protein-protein interactions are stronger than the solvent-solute interactions; the protein molecules coagulate by forming hydrophobic interactions with each other. This process is known as salting out.

Soaps are easily precipitated by concentrated salt solution, the metal ion in the salt reacts with the fatty acids forming back the soap and glycerol.

Application

As different proteins have different compositions of amino acids, different protein molecules precipitate at different concentrations of salt solution.

Unwanted proteins can be removed from a protein solution mixture by salting out as long as the solubility of the protein in various concentrations of salt solution is known. After removing the precipitate by filtration or centrifugation, the desired protein can be precipitated by altering the salt concentration to the level at which the desired protein becomes insoluble.

Caution

Certain ions can increase the solubility of a protein when the concentration of the ions increases, instead of decreasing the solubility of the protein. Also some ions can denature certain proteins so if assays on the function of proteins are intended then either a different ion or an alternative purification method should be used. In attempting to remove a product from water, NaCl is often used to increase the ionic strength of water, thereby increasing its polarity, and then the product is moved into the organic layer where it can be extracted.

See also

• Ionic strength
• Protein precipitation
• Salting in
• Ammonium sulfate precipitation
• Hofmeister series

References

• Physical biochemistry, David Sheehan, Wiley Blackwell(2009) p 285
• Miller, S. A.; Dykes, D. D.; Polesky, H. F. (1988). "A simple salting out procedure for extracting DNA from human nucleated cells". Nucleic Acids Research 16 (3): 1215. doi:10.1093/nar/16.3.1215. PMC 334765. PMID 3344216. 
• C. Beldie Activity coefficients in ternary systems : non-electrolyte - water - electrolyte, Revue Roum. Chim. 15, 221-228, 1970

External links

• Make The Most of Antisolvent Crystallization
• Salting out on UC Davis ChemWiki