Protein denaturation by interaction with acid and bases

Acid and bases mainly disrupt salt bridges in proteins by modifying the pH. When an acid or a base are in solution, they dissociate into cations (H⁺ or another positive chemical element in a base, such as Na⁺ in NaOH) and anions (OH^- or another negative chemical element in an acid, such as Cl^- in HCl). The dissociated elements interact with the positive ammonium group and the negative acid group of the amino acids. Therefore, a type of double replacement reaction occurs where the positive and negative ions in the amino acid salt change partners with the positive and negative ions in the new acid or base added (Figure 1).

Figure 1. Disruption of salt bridge interaction due to addition of hydrochloric acid.

Typically, the structures disrupted in the protein depend on the acid/base strength. A weak acid/base will normally disrupt tertiary and quaternary structures, while a strong acid/base will also disrupt the secondary ones.

There are several processes where this reaction occurs, such as in the digestive system, when the acidic gastric juices cause the curdling (coagulating) of milk, or while cooking some mayonnaise, after adding lemon juice (citric acid) to the eggs to favor a better emulsion formation.