Consider the amino acid histidine, which contains a basic imidazole ring as its side chain, in addition to the -NH₃⁺ and -COOH groups (Figure ). As the pH increases, histidine undergoes three deprotonation reactions, starting from the fully protonated form, H₃A 2 ⁺ . Initially, the -COOH group with the lowest pKa releases its proton, forming the species H₂A + . As the pH rises, the imidazole side chain and -NH₃⁺ groups sequentially lose their protons, forming the zwitterionic HA and A⁻ forms, respectively. These deprotonation steps are shown in Figure .