Amino Acid Chart
The twenty standard amino acids with codes, side-chain class, residue mass, pKa and codons.
Showing 20 of 20.
| 1 | 3 | Name | Side chain | Residue mass |
|---|---|---|---|---|
| A | Ala | Alanine | Nonpolar | 71.0788 Da |
| R | Arg | Arginine | Basic (positive) | 156.1875 Da |
| N | Asn | Asparagine | Polar, uncharged | 114.1038 Da |
| D | Asp | Aspartic acid | Acidic (negative) | 115.0886 Da |
| C | Cys | Cysteine | Polar, uncharged | 103.1388 Da |
| E | Glu | Glutamic acid | Acidic (negative) | 129.1155 Da |
| Q | Gln | Glutamine | Polar, uncharged | 128.1307 Da |
| G | Gly | Glycine | Nonpolar | 57.0519 Da |
| H | His | Histidine | Basic (positive) | 137.1411 Da |
| I | Ile | Isoleucine | Nonpolar | 113.1594 Da |
| L | Leu | Leucine | Nonpolar | 113.1594 Da |
| K | Lys | Lysine | Basic (positive) | 128.1741 Da |
| M | Met | Methionine | Nonpolar | 131.1926 Da |
| F | Phe | Phenylalanine | Aromatic | 147.1766 Da |
| P | Pro | Proline | Nonpolar | 97.1167 Da |
| S | Ser | Serine | Polar, uncharged | 87.0782 Da |
| T | Thr | Threonine | Polar, uncharged | 101.1051 Da |
| W | Trp | Tryptophan | Aromatic | 186.2132 Da |
| Y | Tyr | Tyrosine | Aromatic | 163.1760 Da |
| V | Val | Valine | Nonpolar | 99.1326 Da |
Residue mass is the mass the amino acid contributes inside a chain — the free amino acid minus the water lost when the peptide bond forms. Add one water, 18.015 Da, to the sum of a whole sequence to get the protein’s molecular weight. Codons are written as mRNA, so U appears where DNA would have T.
Twenty amino acids build every protein. They share a backbone and differ only in the side chain, which is what sorts them into nonpolar, polar, acidic, basic and aromatic groups. That side chain decides where a residue sits in a folded protein — hydrophobic inside, charged outside.
One backbone, twenty side chains
Every amino acid has the same core: a central carbon carrying an amino group, a carboxyl group, a hydrogen and a variable R group. Only the R group differs, and every property that matters downstream — solubility, charge, size, reactivity — follows from it. Learning the twenty is really learning twenty side chains.
Grouping them by side chain is what makes the list tractable. Nonpolar residues are greasy and end up buried in the protein core away from water. Polar ones sit comfortably at the surface. Acidic and basic ones carry charge at physiological pH and drive salt bridges, catalysis and binding. Aromatic ones are bulky and flat, and two of them — tryptophan and tyrosine — are the reason proteins absorb at 280 nm.
The pKa values that matter
Seven side chains ionise, and their pKa values say at which pH. Aspartate and glutamate are deprotonated and negative above pH 4; lysine and arginine are protonated and positive well past pH 10. Histidine is the interesting one: its pKa of about 6 sits near physiological pH, so it can switch between charged and neutral under ordinary conditions. That is exactly why histidine turns up so often in enzyme active sites, shuttling protons during catalysis.
The five side-chain classes
The grouping that explains where each residue ends up in a folded structure.
| Class | Members | Behaviour |
|---|---|---|
| Nonpolar | G, A, V, L, I, M, P | Hydrophobic — buried in the core |
| Polar, uncharged | S, T, C, N, Q | Hydrogen-bond at the surface |
| Acidic | D, E | Negative at pH 7 |
| Basic | K, R, H | Positive at pH 7, though histidine is borderline |
| Aromatic | F, Y, W | Bulky rings; Y and W absorb at 280 nm |
Special cases worth knowing
Residues whose behaviour does not follow from their class alone.
| Residue | Why it is different |
|---|---|
| Glycine (G) | Side chain is a single hydrogen, so the backbone can bend where others cannot |
| Proline (P) | Its side chain loops back to the backbone, kinking the chain and breaking helices |
| Cysteine (C) | Forms disulfide bridges with another cysteine, covalently locking a fold |
| Histidine (H) | pKa near 6, so it gains or loses a proton at physiological pH |
| Methionine (M) | Coded by AUG, which is also the start codon — so it begins nearly every chain |
| Tryptophan (W) | The largest residue and the strongest absorber at 280 nm |
Reading the mass and codon columns
The mass listed is the residue mass — the free amino acid minus the water lost when the peptide bond forms. Summing those along a sequence and adding one water back gives the protein’s molecular weight. The average across the twenty is around 110 Da, which is where the familiar shortcut of multiplying residue count by 110 comes from.
The codons are written as mRNA, so U appears wherever DNA would have T. The code is degenerate: leucine, serine and arginine each have six codons while methionine and tryptophan have one apiece. Most of that redundancy sits in the third position, which is why a mutation there so often changes nothing — the silent mutations that make the third base the most tolerant position in a gene.