Illustrated Glossary of Organic Chemistry

Amino acid side chain: The portion of alpha-amino acid structure other than the amino group (or the ammonium group in an ionic form), the alpha carbon, and the carboxylic acid group (or the carboxyl group in an ionic form). Amino acids are categorized as acidic, basic, or nonacidic (based on the acid/base properties of this side chain) and hydrophobic or hydrophilic (based upon the polarity and hydrogen bonding ability of the side chain).

Generic amino acid structure.		The side chain of alanine is a methyl group (shown in red).		The side chain of zwitterionic cysteine is a CH2SH group (shown in red).