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. Which acid group is the stronger acid?

Question

Quizplus · Accepted Answer

The carboxylic acid group (COOH) is the stronger acid, as it will donate a proton more readily than the amine group (NH2) in the low pH conditions described. This is due to the fact that the carboxylic acid group has a higher pKa value (around 2) than the amine group (around 9-10), meaning it requires less energy to donate a proton. In contrast, the amine group would require a significantly lower pH (i.e. more acidic conditions) to be fully protonated.

At a Physiological PH of 7