Amino Acid Structures: Name Them from the Picture

Glycine is the only standard amino acid whose side chain is a single hydrogen atom, so its central carbon carries just H atoms besides the amino and carboxyl groups. That also makes it the only standard amino acid that is not chiral.

A CH2 group links the backbone to a plain benzene ring with no other atoms attached. That bare ring makes phenylalanine one of the most hydrophobic amino acids.

The side chain is one CH2 group followed by a carboxyl group (COOH), so the molecule has two acid groups. At body pH that extra group loses its proton, which makes aspartate negatively charged.

The side chain branches right at the first carbon into two methyl groups, a shape called isopropyl. Valine is one of the three branched-chain amino acids.

A straight chain of four CH2 groups ends with an amino group (NH2). That terminal amino group carries a positive charge in the body.

The side chain is just CH2 with a hydroxyl group (OH) on the end. It is the simplest amino acid with an alcohol group in its side chain.

The side chain ends in indole, a benzene ring fused to a five-membered ring containing nitrogen. It is the largest side chain of all the standard amino acids.

Two CH2 groups lead to an amide group, a C=O with an NH2 attached. Because the amide is not acidic, glutamine's side chain carries no charge.

Two CH2 groups are followed by a sulfur atom and a methyl group (–CH2–CH2–S–CH3). The sulfur sits inside the chain, so unlike an SH group it cannot form sulfur bridges.

The side chain loops back and bonds to the backbone nitrogen, closing a five-membered ring. No other standard amino acid has its amino group locked in a ring.

The side chain branches at the first carbon into a hydroxyl group (OH) and a methyl group. This makes threonine one of only two standard amino acids with two chiral carbons.

Three CH2 groups lead to a guanidino group, a carbon bonded to three nitrogens. It is the most basic side chain and stays positively charged in the body.

The side chain is a single methyl group (CH3). Alanine is the smallest chiral amino acid.

A CH2 group leads to a benzene ring with a hydroxyl group (OH) on the far side. That OH group is where enzymes can attach a phosphate.

The side chain is CH2 with a thiol group (SH) on the end. Two such groups can join into a disulfide bridge that holds proteins in shape.

One CH2 group is followed by a branch into two methyl groups, so the fork sits one carbon further from the backbone. That position is what tells leucine apart from its isomer, isoleucine.

Two CH2 groups lead to a carboxyl group (COOH), so the molecule has two acid groups. At body pH it loses a proton and becomes the negatively charged glutamate.

A CH2 group leads to imidazole, a five-membered ring with two nitrogen atoms. Its charge switches easily near body pH, which makes it common in enzyme active sites.

A single CH2 group leads to an amide group, a C=O with an NH2 attached. The amide makes the side chain polar but uncharged.

The side chain branches right at the first carbon into a methyl group and an ethyl group. Like threonine, it has a second chiral carbon in its side chain.