Amino Acids: Structure and the 20 Standard Types

Twenty small molecules build almost every protein in every living thing on Earth, from the keratin in your hair to the enzymes in bacteria. These are the standard amino acids. They share a common core but differ in one part, their side chain, and those side chains decide how a protein folds, what it binds and what it does. This guide covers the shared structure, the chemistry that makes amino acids unusual, and a tour of all twenty.

The general structure

Every standard amino acid has a central alpha carbon (Cα) bonded to four groups:

1. an amino group, –NH₂;
2. a carboxyl group, –COOH;
3. a hydrogen atom;
4. a side chain, written R, which differs from one amino acid to another.

The simplest amino acid, glycine, has just a hydrogen atom as its side chain (formula C₂H₅NO₂, molar mass 75.07 g mol⁻¹). Others have side chains ranging from a single methyl group (alanine) to large ring systems (tryptophan).

Chirality: L-amino acids

Because the alpha carbon carries four different groups, it’s a chiral centre in every standard amino acid except glycine (which has two hydrogens). So each amino acid can exist as two mirror-image forms, labelled L and D.

Proteins are built almost exclusively from L-amino acids. Why life chose L rather than D is still debated, but once chosen, the choice is fixed: enzymes and ribosomes are built to handle L forms only. D-amino acids do occur in nature, for example in bacterial cell walls and some antibiotics, but not in the proteins made by ribosomes.

Zwitterions: acids and bases in one molecule

An amino acid has an acidic group (–COOH) and a basic group (–NH₂) in the same molecule. In water near neutral pH:

• the carboxyl group loses a proton → –COO⁻;
• the amino group gains a proton → –NH₃⁺.

The result is a zwitterion: a molecule with both a positive and a negative charge but no overall charge. This explains why amino acids are crystalline solids with high melting points and why they dissolve well in water but not in non-polar solvents.

As pH changes, the charge changes:

• Low pH (acidic): the carboxylate picks up a proton → overall charge +1.
• Around neutral: zwitterion → overall charge 0 (for amino acids with neutral side chains).
• High pH (basic): the ammonium group loses a proton → overall charge −1.

The pH at which an amino acid has no net charge is its isoelectric point (pI). For glycine it’s about 6.0. Differences in pI are used to separate amino acids and proteins by electrophoresis (see electrophoresis). The acid–base chemistry is covered in depth in amino acids as acids and bases.

The 20 standard amino acids by side chain

Amino acids are usually grouped by the chemistry of their side chains, because that’s what matters for protein structure. Each has a three-letter and a one-letter code.

Non-polar (hydrophobic) side chains

These tend to be buried inside folded proteins, away from water.

Amino acid	Codes	Side chain notes
Glycine	Gly, G	Just H; smallest, very flexible, not chiral
Alanine	Ala, A	Methyl group
Valine	Val, V	Branched, three carbons
Leucine	Leu, L	Branched, four carbons
Isoleucine	Ile, I	Branched, four carbons, extra chiral centre
Proline	Pro, P	Side chain loops back to bond to the nitrogen; rigid, breaks helices
Methionine	Met, M	Contains sulfur; the “start” amino acid in protein synthesis
Phenylalanine	Phe, F	Benzene ring
Tryptophan	Trp, W	Large double ring containing nitrogen; absorbs UV light

(Glycine is sometimes grouped separately because its side chain is so small.)

Polar, uncharged side chains

These can form hydrogen bonds with water and with each other.

Amino acid	Codes	Side chain notes
Serine	Ser, S	–CH₂OH; often a site of phosphorylation
Threonine	Thr, T	–OH on a branched chain
Cysteine	Cys, C	Thiol (–SH); forms disulfide bridges
Tyrosine	Tyr, Y	Phenol ring; absorbs UV light
Asparagine	Asn, N	Amide group
Glutamine	Gln, Q	Amide group, one carbon longer

Acidic side chains (negatively charged at pH 7)

Amino acid	Codes	Side chain notes
Aspartic acid (aspartate)	Asp, D	–CH₂COOH → –CH₂COO⁻
Glutamic acid (glutamate)	Glu, E	One carbon longer; glutamate is also a key brain neurotransmitter

Basic side chains (positively charged at pH 7)

Amino acid	Codes	Side chain notes
Lysine	Lys, K	Long chain ending in –NH₃⁺
Arginine	Arg, R	Guanidinium group; strongly basic
Histidine	His, H	Imidazole ring, pKa near 6; can gain or lose a proton near body pH, which makes it important in enzyme active sites

Why side chains matter

Side chains decide:

• Folding: hydrophobic side chains cluster in the core; charged and polar ones face the water. This drives protein folding.
• Bonds within proteins: cysteines can form covalent disulfide bridges; acidic and basic side chains can form ionic bonds (salt bridges); polar side chains form hydrogen bonds.
• Chemistry at active sites: histidine, serine, cysteine, aspartate and glutamate often do the catalytic work in enzymes.
• Where proteins sit: membrane proteins have stretches of hydrophobic amino acids that sit within the lipid bilayer.

Essential and non-essential amino acids

Humans can make some amino acids from other molecules but not others. The nine essential amino acids for adults are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. They must come from food. More detail, including food sources, is in essential amino acids.

Beyond the standard twenty

• Selenocysteine (like cysteine, but with selenium instead of sulfur) is built into a small number of human proteins by a special mechanism. It’s sometimes called the 21st amino acid, and pyrrolysine, found in some microbes, the 22nd.
• Many amino acids are modified after being built into proteins: phosphorylation of serine, threonine or tyrosine switches proteins on and off; collagen contains hydroxyproline, made from proline in a step that needs vitamin C (which is why vitamin C deficiency causes scurvy).
• Some amino acids that are never built into proteins are still important, such as ornithine in the urea cycle and GABA in the nervous system.

How amino acids are analysed

To find out which amino acids a protein contains, biochemists first hydrolyse it completely, usually by heating in 6 mol dm⁻³ hydrochloric acid, then separate the free amino acids by chromatography. In school, paper or thin-layer chromatography followed by spraying with ninhydrin makes amino acids visible as purple spots, and comparing Rf values with known standards identifies them (see Rf value calculations). Research labs use ion-exchange or high-performance liquid chromatography for accurate quantities. To find the order of amino acids, the sequence, scientists today mostly use mass spectrometry of protein fragments or read the sequence from the gene.

Key takeaways

• Every amino acid has an alpha carbon bearing an amino group, carboxyl group, hydrogen and side chain (R).
• All standard amino acids except glycine are chiral; proteins use the L form.
• In water, amino acids exist as zwitterions; their charge depends on pH, and the isoelectric point is where the net charge is zero.
• The 20 standard amino acids are grouped as non-polar, polar, acidic and basic according to their side chains.
• Side chains control folding, bonding and catalysis. See how they link up in the peptide bond.