Question

The side chains of glutamic acid (pK 4.3) and arginine (pK 12.5) can form an ionic bond under certain conditions. Draw the relevant portions of the side chains and indicate whether an ionic bond could form at the following: (a) pH 4; (b) pH 7; (c) pH 12; (d) pH 13.

Answer 1

For ionic interaction, there should be an electrostatic attraction between oppositely charged ions/ species. In amino acid, the basic amine group can exist as a positively charged quarternary ammonium species in a pH range. Similarly carboxylic acid exist as carboxylate anion beyond a certain pH. This is actually an equilibrium. The pH at which the concentration of respective neutral amine/carboxylic acid is equal to the concentration of corresponding charged species is called pKa of that group. Above the pka of acid, it exist as carboxylate anion. Below the pka of amine, it exist as quarternary ammonium species.