Question

The addition of a small amount of a salt, such as (NH₄)₂SO₄ to a solution containing a charged protein increases the solubility of the protein in water. This observation is called the salting-in effect. However, the addition of large amounts of salt can decrease the solubility of the protein to such an extent that the protein precipitates from solution. This observation is called the salting-out effect and is widely used by biochemists to isolate and purify proteins. Consider the equilibrium PX_n(s) ↔ Pⁿ⁺(aq) + nX^–(aq), where Pⁿ⁺ is a polycationic protein of charge +n and X^– is its counter-ion. Use Le Châtelier's principle and principles behind the Debye-Hückel theory to provide a molecular interpretation for the salting-in and salting-out effects.

Answer 1

‘Salting In’ is a phenomenon of increasing the solubility of a solute (which depend upon the concentrations of dissolved salts, the polarity of the solvent, the ph, and the temperature), such as a protein, by increasing the ionic strength of the solution. In ‘Salting in’, as salt is added the solubility of a protein at low ion concentrations increases. The additional ions shield the protein's multiple ionic charges, thereby weakening the attractive forces between individual protein molecules (such forces can lead to aggregation and precipitation). However, as more salt is added, particularly with sulfate salts, the solubility of protein again decreases. This "salting out" effect is primarily a result of the competition between the added salt ions and the other dissolved solutes (protein molecules) for molecules of solvent (water).

Because a protein contains multiple charged groups, its solubility depends on the concentrations of dissolved salts, the polarity of the solvent, the ph, and the temperature. Some or all of these variables can be manipulated to selectively precipitate certain proteins while others remain soluble.

The solubility of a protein at low ionic strength generally increases with increasing salt concentration. The process is called as salting in. It occurs due to the binding of salt ions to the proteins ionizable groups which decrease the interaction between oppositely charged groups on the protein molecules.hence creating the demand of protien in the solution as per le chateliers principles.