Ammonium sulphate fractionation of proteins
The solubility of proteins is markedly affected by the ionic strength of the medium. As the ionic strength is increased, protein solubility at first increases. This is referred to as 'salting in'. However, beyond a certain point the solubility begins to decrease and this is known as 'salting out'.
At low ionic strengths the activity coefficients of the ionizable groups of the proteins are decreased so that their effective concentration is decreased. This is because the ionizable groups become surrounded by counter ions which prevent interaction between the ionizable groups. Thus protein-protein interactions are decreased and the solubility is increased.
At high ionic strengths much water becomes bound by the added ions that not enough remains to properly hydrate the proteins. As a result, protein-protein interactions exceed protein-water interactions and the solubility decreases.
Because of differences in structure and amino acid sequence, proteins differ in their salting in and salting out behavior. This forms the basis for the fractional precipitation of proteins by means of salt.
Ammonium sulphate is a particularly useful salt for the fractional precipitation of proteins. It is available in highly purified form, has great solubility allowing for significant changes in the ionic strength and is inexpensive. Changes in the ammonium sulphate concentration of a solution can be brought about either by adding solid substance or by adding a solution of known saturation, generally, a fully saturated (100%) solution.