At a physiological pH of 7.4, amino acids exist as zwitterions. At a very low pH (in an acid solution), both the amine group and the carboxylic acid group on an amino acid are protonated. Therefore, the following must be true: the pK_a of the ________ group is larger than the pK_a of the ________ group.

Question

Quizplus · Accepted Answer

At a very low pH, both the amine group (NH3+) and the carboxylic acid group (COOH) are protonated. The pK_a is the pH at which 50% of the group is protonated and 50% is deprotonated. Since the amine group is already protonated in an acid solution, its pK_a must be lower than the carboxylic acid group, which is why the pK_a of the carboxylic acid group is larger than that of the amine group. Therefore, choice A is correct.

At a Physiological PH of 7