Ammonium sulfate is useful in protein purification because
A) it contains nitrogen and sulfur, both of which occur in proteins
B) it is sparingly soluble in water, causing proteins to co-precipitate with it
C) very pure proteins are obtained when it is used
D) it forms ion-dipole interactions with water, making proteins less soluble and more likely to precipitate

Question

Quizplus · Accepted Answer

Ammonium sulfate is used in protein purification primarily because it reduces the solubility of proteins in solution. This is achieved through the mechanism of "salting out," where the high concentration of ammonium sulfate increases the ionic strength of the solution. This, in turn, disrupts the water-protein interactions (via ion-dipole interactions) and makes the proteins less soluble, causing them to precipitate out of the solution. This method does not rely on the sparing solubility of ammonium sulfate itself (as suggested in option B) nor directly on its nitrogen and sulfur content (as suggested in option A). The purity of proteins obtained (option C) is a result of the precipitation process but is not the primary reason why ammonium sulfate is used.

Ammonium Sulfate Is Useful in Protein Purification Because