The 20 Standard Amino Acids

Indole side chain (fused benzene-pyrrole rings). Largest of the standard amino acids.

Branched aliphatic side chain with two methyl groups. One of the branched-chain amino acids (BCAAs).

Thiol group (-SH) as the side chain. The only standard amino acid with a reactive sulfur.

Primary amide group on an extended side chain (one more CH2 than asparagine).

Single methyl group (CH3) as the side chain. The simplest of the hydrophobic amino acids.

Guanidinium group on an extended side chain. Strongly positively charged at physiological pH.

Look for just an H as the side chain: no carbon at all. The only achiral amino acid.

Carboxyl group on an extended side chain (one more CH2 than aspartate).

Imidazole ring (five-membered aromatic ring with two nitrogens) as the side chain.

Sulfur atom (as thioether, not thiol) in an extended aliphatic side chain. The start amino acid in translation.

Primary amide group (-CONH2) as the side chain. Asparagine is the amide of aspartic acid.

Simple hydroxyl group (-OH) as the side chain. One of the smallest polar amino acids.

Hydroxyl group on a carbon that also bears a methyl group (secondary alcohol).

Benzene ring with a hydroxyl group attached (phenolic hydroxyl).

Four-carbon aliphatic side chain with branching. Larger than valine. One of the BCAAs.

Unique cyclic structure: the side chain loops back and connects to the backbone nitrogen. The only imino acid.

Benzene ring attached via a methylene group. One of the aromatic amino acids.

Branched aliphatic side chain with a chiral center on the side chain itself. One of the BCAAs.

Carboxyl group (-COOH) as the side chain. Negatively charged at physiological pH.

Primary amine group on an extended aliphatic side chain. Positively charged at physiological pH.