Among these 20 genetically coded amino acids are: glycine, which has only a hydrogen atom for a side group; valine, which has a hydrocarbon side chain; phenylalanine, which has an aromatic ring; polar but uncharged serine; negatively charged aspartic acid; and basic lysine, which has a positive charge. These side chains, -R, are:









These samples of the repertory of amino acids are illustrated in the polypeptide chain shown on the right. We will see later how important the side chains can be to the chemical behaviour of a protein.