Acid-Base Properties of Amino Acids

Amino acids are amphoteric: because they contain both an acidic group (-COOH) and a basic group (-NH₂), they react with both acids and bases.In acidic solution (low pH) there are many H⁺ ions. The -COO⁻ group of the zwitterion accepts H⁺ and becomes -COOH, while the amine stays protonated as -NH₃⁺. The amino acid exists as a positive ion:⁺H₃N-CHR-COO⁻ + H⁺ → ⁺H₃N-CHR-COOHIn alkaline solution (high pH), OH⁻ ions remove H⁺ from the -NH₃⁺ group, which becomes -NH₂, while the acid group stays as -COO⁻. The amino acid exists as a negative ion:⁺H₃N-CHR-COO⁻ + OH⁻ → H₂N-CHR-COO⁻ + H₂OSo, as the pH increases: positive ion → zwitterion → negative ion.If the side chain contains an extra acidic or basic group, that group is also protonated in acid and deprotonated in alkali. For example, lysine has an extra -NH₂ in its side chain, so in strongly acidic solution it carries a 2+ charge.Worked example. Give the structure of glycine at pH 1 and at pH 13. At pH 1 both groups are protonated: ⁺H₃NCH₂COOH. At pH 13 both are deprotonated: H₂NCH₂COO⁻.