Question

what is the principle behind salt fractionation of proteins

Answer 1

Answer:

The solubility of the given protein varies according to the ionic strength of the solution. Therefore, the salt concentration of the solution determines the protein's solubility in the solution.

Based on this property two distinct effects can be observed: at low salt concentrations, the solubility of the protein increases with increasing salt concentration (that is increasing ionic strength). This effect is termed salting in. As the salt concentration (ionic strength of the solution) is increased further, the solubility of the protein begins to decrease. Therefore, protein will be completely precipitated at sufficiently high ionic strength from the solution. This effect is termed salting out.

Explanation:

Proteins differ in the charged, polar and hydrophobic amino acids that they display on their surface. Charged polar groups on the surface of the protein are solvated by water molecules, whereas hydrophobic residues are masked by water molecules. Since the solubility is a consequence of solvation of charged and polar groups on the surface of the proteins. This follows that under a particular set of conditions, proteins precipitate differentially from solution on the addition of species such as neutral salts or organic solvents.

During salt fractionation of proteins (Ammonium sulphate commonly used) increasing salt is added to a protein solution, so the salt ions are solvated by water molecules of the solution. As a result, increased concentration of salt leads to scarcity of freely available water molecules that can solvate the ions. At this stage, those water molecules that have been forced into contact with hydrophobic groups on the proteins are the next freely available water molecules than that one is bound to the polar and charged groups. Therefore, this exposes the hydrophobic patches of protein and further increased ionic strength leads to protein aggregate and precipitation.