Salt out

Salting out (also known as anti-solvent crystallization, precipitation crystallization, or drowning out) is an effect based on the electrolyte-non electrolyte interaction, in which the non-electrolyte could be less soluble at high salt concentrations. It is used as a method of purification for proteins, as well as preventing protein denaturation due to excessively diluted samples during experiments. 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 solutions of proteins. Dialysis can be used to remove the salt if needed.

Salt compounds dissociate in aqueous solutions. This property is exploited in the process of salting out. 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.

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.

As a result of the increased demand for solvent molecules, the protein–protein interactions are stronger than the solvent-solute interactions; the protein molecules associates by forming hydrophobic interactions with each other. After dissociation in a given solvent, the negatively charged atoms from a chosen salt begin to compete for interactions with positively charged molecules present in the solution. Similarly, the positively charged cations compete for interactions with the negatively charged molecules of the solvent. This process is known as salting out.

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