As the pH of the solution increases, Aspartic acid undergoes three deprotonation reactions, starting from the fully protonated form H₃A⁺. The -COOH group, with the lowest pKa, initially releases its proton, forming the zwitterionic species H₂A. As the pH rises, the -COOH side chain deprotonates, forming the HA⁻ species. Finally, the -NH₃⁺ group loses its proton, producing the A²⁻ form. These deprotonation steps are shown in Figure .