There are 20 standard amino acids in proteins, and this amino acid chart lists each with its three-letter and one-letter code, side-chain class, formula and residue mass. In plain terms, a residue mass is the amino acid's weight minus one water. That is what each one adds to a peptide chain.
- There are 20 standard amino acids, each with a three-letter and one-letter IUPAC-IUB code.
- A residue mass is the free amino acid's molecular weight minus one water, 18.015 g/mol.
- Leucine and isoleucine have the same formula and mass.
- IMGT groups the side chains into seven chemical classes: aliphatic, aromatic, sulfur, hydroxyl, basic, acidic and amide.
- B, Z and X are IUPAC symbols for ambiguous or unknown residues; U is selenocysteine.
Twenty building blocks
Every protein in your body, and every peptide on a lab shelf, is a string built from the same short list of parts. There are 20 of them in the standard set, and each has a name, a three-letter code and a one-letter code.
All 20 share one backbone: an amino group, a carboxyl group and a hydrogen on a central carbon. What makes each one different is the side chain, also called the R group, hanging off that carbon. Our guide to the R group in amino acids covers that part in detail.
This page is a reference table. It puts the codes, the side-chain class and the masses in one place, with each value taken from a named source, so you can check a sequence or a molecular weight without flipping between books.
- Write the sequence in one-letter code
- Look up each residue mass
- Add the residue masses
- Add one water for the two ends
- Adjust for terminal modifications
The full chart
| Amino acid | 3-letter | 1-letter | Side-chain class | Hydropathy class | Formula (free) | MW, free (g/mol) | Residue mass, average (Da) | Residue mass, monoisotopic (Da) |
|---|---|---|---|---|---|---|---|---|
| Glycine | Gly | G | Aliphatic | Neutral | C2H5NO2 | 75.07 | 57.05 | 57.0215 |
| Alanine | Ala | A | Aliphatic | Hydrophobic | C3H7NO2 | 89.09 | 71.08 | 71.0371 |
| Valine | Val | V | Aliphatic | Hydrophobic | C5H11NO2 | 117.15 | 99.14 | 99.0684 |
| Leucine | Leu | L | Aliphatic | Hydrophobic | C6H13NO2 | 131.17 | 113.15 | 113.0841 |
| Isoleucine | Ile | I | Aliphatic | Hydrophobic | C6H13NO2 | 131.17 | 113.15 | 113.0841 |
| Proline | Pro | P | Aliphatic | Neutral | C5H9NO2 | 115.13 | 97.11 | 97.0528 |
| Phenylalanine | Phe | F | Aromatic | Hydrophobic | C9H11NO2 | 165.19 | 147.18 | 147.0684 |
| Tryptophan | Trp | W | Aromatic | Hydrophobic | C11H12N2O2 | 204.22 | 186.20 | 186.0793 |
| Tyrosine | Tyr | Y | Aromatic | Neutral | C9H11NO3 | 181.19 | 163.18 | 163.0633 |
| Cysteine | Cys | C | Sulfur | Hydrophobic | C3H7NO2S | 121.16 | 103.14 | 103.0092 |
| Methionine | Met | M | Sulfur | Hydrophobic | C5H11NO2S | 149.21 | 131.19 | 131.0405 |
| Serine | Ser | S | Hydroxyl | Neutral | C3H7NO3 | 105.09 | 87.08 | 87.0320 |
| Threonine | Thr | T | Hydroxyl | Neutral | C4H9NO3 | 119.12 | 101.11 | 101.0477 |
| Asparagine | Asn | N | Amide | Hydrophilic | C4H8N2O3 | 132.12 | 114.11 | 114.0429 |
| Glutamine | Gln | Q | Amide | Hydrophilic | C5H10N2O3 | 146.14 | 128.12 | 128.0586 |
| Aspartic acid | Asp | D | Acidic | Hydrophilic | C4H7NO4 | 133.10 | 115.08 | 115.0269 |
| Glutamic acid | Glu | E | Acidic | Hydrophilic | C5H9NO4 | 147.13 | 129.12 | 129.0426 |
| Lysine | Lys | K | Basic | Hydrophilic | C6H14N2O2 | 146.19 | 128.18 | 128.0950 |
| Arginine | Arg | R | Basic | Hydrophilic | C6H14N4O2 | 174.20 | 156.19 | 156.1011 |
| Histidine | His | H | Basic | Neutral | C6H9N3O2 | 155.15 | 137.13 | 137.0589 |
Codes are from the IUPAC-IUB recommendations. Formulas, free-acid molecular weights and monoisotopic masses are from PubChem. Side-chain and hydropathy classes follow the IMGT classification. Residue masses are the PubChem values minus one water (18.015 g/mol average, 18.0106 Da monoisotopic, also from PubChem), rounded as shown.
Why do the one-letter codes not match the names?
Only some one-letter codes can use the first letter, because several names start with the same letter. The three-letter codes are mostly the first three letters of the name: Gly, Ala, Val, Leu. A few break the pattern to avoid clashes. Asparagine is Asn, glutamine is Gln, isoleucine is Ile and tryptophan is Trp.
The one-letter codes are where people get caught. Only some match the first letter. When two amino acids share a first letter, one of them gets something else: phenylalanine is F, arginine is R, tyrosine is Y, tryptophan is W, lysine is K, aspartic acid is D, glutamic acid is E, asparagine is N and glutamine is Q.
IUPAC also defines a few extra symbols. B means aspartic acid or asparagine when the two have not been told apart, Z does the same for glutamic acid and glutamine, and X stands for an unknown or other amino acid. Selenocysteine was later given U. None of these appear in the table because they are not among the 20 standard residues.
Side-chain classes
The IMGT classification sorts the 20 into seven chemical classes by what the side chain is made of. Aliphatic side chains are plain carbon and hydrogen: glycine, alanine, valine, leucine, isoleucine and proline. The aromatic class, with its ring structures, holds phenylalanine, tryptophan and tyrosine. Cysteine and methionine carry sulfur. Serine and threonine carry a hydroxyl group.
The remaining three classes are the ones that change with pH or hydrogen bonding. Arginine, histidine and lysine are basic. Aspartic acid and glutamic acid are acidic. Asparagine and glutamine carry an amide. Our guide to acidic and basic amino acids explains why those five hold a charge, and the polar amino acids guide covers the polar and nonpolar split.
Hydropathy is a second, separate grouping. IMGT puts eight residues in the hydrophobic class (A, C, I, L, M, F, W, V), six in a neutral class (G, H, P, S, T, Y) and six in the hydrophilic class (R, N, D, Q, E, K). Cysteine sits with the hydrophobic group even though its chemical class is sulfur.
IMGT also gives a charge grouping. Arginine, histidine and lysine are listed as positively charged, aspartic acid and glutamic acid as negatively charged, and the other fifteen as uncharged. That split matches the basic and acidic chemical classes exactly, which makes it easy to count the likely charge of a short sequence straight from the chart.
What is the difference between free mass and residue mass?
Free mass is the whole amino acid, while residue mass is that weight minus one water. The molecular weight PubChem lists for an amino acid is the free molecule, with its full amino and carboxyl groups. When two amino acids join, a peptide bond forms and one water leaves. So each amino acid inside a chain weighs its free mass minus 18.015 g/mol. That smaller number is the residue mass.
Glycine is the easy example. Free glycine is 75.07 g/mol in PubChem. Subtract one water and the glycine residue is 57.05. Tryptophan is the heaviest: 204.22 free, 186.20 as a residue.
The two mass columns serve different jobs. Average mass uses the natural mix of isotopes and is what a balance or a label weight reflects. Monoisotopic mass uses only the most common isotope of each element, and it is what a high-resolution mass spectrometer reports for a small peptide's main peak.
Using the chart to weigh a peptide
To estimate a peptide's mass, add the residue masses of every amino acid in the sequence, then add one water for the two free ends. The water goes back on because the first residue keeps its full amino group and the last keeps its full carboxyl group.
Take the tripeptide glycyl-histidyl-lysine, written GHK. From the average column: 57.05 + 137.13 + 128.18 = 322.36. Add 18.015 and the free tripeptide comes to about 340.4 g/mol. The monoisotopic version is 57.0215 + 137.0589 + 128.0950 + 18.0106 = 340.1860 Da.
Two cautions apply. Terminal changes such as an acetyl group or a C-terminal amide shift the total, and a salt form adds the counterion's mass to the powder but not to the peptide. A certificate's theoretical mass should state which form it describes.
Isoleucine, leucine and other pairs
Leucine and isoleucine have the same formula, C6H13NO2, and identical masses in the table. They differ only in where a methyl group sits on the side chain. A mass reading cannot tell them apart, so a peptide that swaps one for the other has the same molecular weight. Sequencing or comparison with a reference standard is needed to settle it.
Lysine and glutamine are a closer call. Their residue masses are both near 128 Da on average but differ at the second decimal place in the monoisotopic column, 128.0950 against 128.0586. Low-resolution instruments can blur them; high-resolution ones separate them.
These pairs are why identity testing does not stop at one mass number. The HPLC purity versus identity guide covers what else a laboratory checks.
Reading a sequence on a label or certificate
Peptide sequences are written from the amino end to the carboxyl end, left to right. So GHK means glycine first, lysine last, and KHG would be a different compound with the same mass. The same rule holds whether the sequence is printed in one-letter code or as three-letter codes joined by hyphens, such as Gly-His-Lys.
Longer sequences are almost always given in one-letter code because they fit on a line. A 20-residue peptide in three-letter code runs to about 80 characters with hyphens; in one-letter code it is 20. When you check a certificate, convert the one-letter string back to names with the chart, count the residues, and compare the count with the length the document claims.
Watch for prefixes and suffixes. A prefix such as Ac- or a suffix such as -NH2 marks a terminal change. Those details change the theoretical mass, so they belong in any calculation made from the table.
How many amino acids exist?
Twenty amino acids make up the standard set used to build proteins, but chemistry knows far more. Twenty is the standard set used to build proteins, and it is what nearly every sequence you read uses. It is not the total number of amino acids in chemistry, which runs far higher once modified and non-protein amino acids are counted.
Selenocysteine has its own IUPAC symbol, U, which is why some sources call it a 21st amino acid. Research peptides also use residues outside the 20, such as D-amino acids or modified side chains, and those are written with extended notation on a certificate rather than a single standard letter.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION.
Common questions
How many amino acids are in the standard chart?
Why do some one-letter codes not match the first letter?
What is the difference between molecular weight and residue mass?
Which amino acids have the same mass?
More science guides
Sources
- IUPAC-IUB Joint Commission on Biochemical Nomenclature, Nomenclature and Symbolism for Amino Acids and Peptides, one-letter and three-letter symbols. Three-letter and one-letter codes for the 20 standard amino acids, plus B, Z, X and U.
- PubChem PUG REST: formula, molecular weight and monoisotopic mass for the 20 standard amino acids. Free-acid formula, average molecular weight and monoisotopic mass for each amino acid.
- PubChem PUG REST: water. Average (18.015) and monoisotopic (18.0106) mass of water, subtracted to give residue masses.
- IMGT Education, IMGT classes of the 20 common amino acids. Chemical, hydropathy and charge classes used in the chart.
