Illustrated Glossary of Organic Chemistry

Alanine is a nonacidic α-amino acid. Its side chain (a methyl group) is neither acidic (it is not more acidic than water) nor basic (it does not have a nitrogen atom lone pair that is not delocalized by resonance).


Cysteine is an acidic amino acid because its side chain (CH₂SH; pK_a 10) is more acidic than water (pK_a 15.7).


Lysine is a basic amino acid because its side chain (CH₂CH₂CH₂NH₂) has a nitrogen atom lone pair that is not delocalized by resonance.