This amino acid chart lists the one- and three-letter codes used to write peptide sequences, with the formula and mass of each amino acid residue, the modified residues found in several research peptides, and the mass changes for common terminal groups and bridges. A residue is an amino acid as it sits in a chain: the free amino acid less one molecule of water. The codes follow the IUPAC-IUB recommendations (IUPAC-IUB JCBN, 1984). Every mass here is calculated from the formulas, and the worked examples reproduce the published formulas of compounds on this site.
Amino acid chart: the 20 standard amino acids
| Amino acid | Three-letter | One-letter | Side chain | Residue formula | Average residue mass (Da) | Monoisotopic residue mass (Da) |
|---|---|---|---|---|---|---|
| Alanine | Ala | A | Methyl | C₃H₅NO | 71.08 | 71.0371 |
| Arginine | Arg | R | Guanidino (basic) | C₆H₁₂N₄O | 156.19 | 156.1011 |
| Asparagine | Asn | N | Carboxamide | C₄H₆N₂O₂ | 114.10 | 114.0429 |
| Aspartic acid | Asp | D | Carboxylic acid (acidic) | C₄H₅NO₃ | 115.09 | 115.0269 |
| Cysteine | Cys | C | Thiol | C₃H₅NOS | 103.14 | 103.0092 |
| Glutamine | Gln | Q | Carboxamide | C₅H₈N₂O₂ | 128.13 | 128.0586 |
| Glutamic acid | Glu | E | Carboxylic acid (acidic) | C₅H₇NO₃ | 129.12 | 129.0426 |
| Glycine | Gly | G | Hydrogen (no side chain) | C₂H₃NO | 57.05 | 57.0215 |
| Histidine | His | H | Imidazole (basic) | C₆H₇N₃O | 137.14 | 137.0589 |
| Isoleucine | Ile | I | sec-Butyl | C₆H₁₁NO | 113.16 | 113.0841 |
| Leucine | Leu | L | Isobutyl | C₆H₁₁NO | 113.16 | 113.0841 |
| Lysine | Lys | K | 4-Aminobutyl (basic) | C₆H₁₂N₂O | 128.18 | 128.0950 |
| Methionine | Met | M | Thioether | C₅H₉NOS | 131.19 | 131.0405 |
| Phenylalanine | Phe | F | Benzyl (aromatic) | C₉H₉NO | 147.18 | 147.0684 |
| Proline | Pro | P | Ring closed onto the backbone nitrogen | C₅H₇NO | 97.12 | 97.0528 |
| Serine | Ser | S | Hydroxymethyl | C₃H₅NO₂ | 87.08 | 87.0320 |
| Threonine | Thr | T | Hydroxyethyl | C₄H₇NO₂ | 101.11 | 101.0477 |
| Tryptophan | Trp | W | Indole (aromatic) | C₁₁H₁₀N₂O | 186.21 | 186.0793 |
| Tyrosine | Tyr | Y | Phenol (aromatic) | C₉H₉NO₂ | 163.18 | 163.0633 |
| Valine | Val | V | Isopropyl | C₅H₉NO | 99.13 | 99.0684 |
Average masses use the standard atomic weights C 12.011, H 1.008, N 14.007, O 15.999 and S 32.06; monoisotopic masses use the most abundant isotope of each element. Other published tables can differ in the second decimal place because of rounding. Leucine and isoleucine have the same formula and mass, so a mass alone cannot tell them apart.
Codes for ambiguous or unknown positions
The same recommendations define letters for positions that are not fully resolved (IUPAC-IUB JCBN, 1984). B stands for aspartic acid or asparagine (Asx), Z for glutamic acid or glutamine (Glx), and X for an unknown or unspecified residue (Xaa). Papers and certificates usually mark the D form of a residue with a "D-" prefix, as in D-Phe, and some write it as a lowercase one-letter code.
Modified and non-standard residues in research peptides
Several research peptides use residues outside the standard 20. They change shape, charge or stability while keeping a peptide backbone (peptide modifications explained).
| Residue | Abbreviation | Residue formula | Average (Da) | Monoisotopic (Da) | Where it appears |
|---|---|---|---|---|---|
| α-Aminoisobutyric acid | Aib | C₄H₇NO | 85.11 | 85.0528 | Ipamorelin, position 1 |
| Norleucine | Nle | C₆H₁₁NO | 113.16 | 113.0841 | Melanotan 2 and PT-141, position 1 |
| 3-(2-Naphthyl)alanine | 2-Nal | C₁₃H₁₁NO | 197.24 | 197.0841 | Ipamorelin, as D-2-Nal |
| 2′,6′-Dimethyltyrosine | Dmt | C₁₁H₁₃NO₂ | 191.23 | 191.0946 | SS-31, position 2 |
D-amino acids, such as the D-Phe in ipamorelin, melanotan 2 and PT-141, the D-Ala at position 2 of CJC-1295 and the D-Arg at position 1 of SS-31, have the same formula and mass as their L forms. They differ only in the arrangement of atoms around the alpha carbon.
Mass changes for terminal groups and bridges
| Change | Formula change | Average (Da) | Monoisotopic (Da) |
|---|---|---|---|
| Free N-terminus and C-terminus (add once per linear chain) | +H₂O | +18.02 | +18.0106 |
| C-terminal amide (-OH becomes -NH₂) | +N +H -O | -0.98 | -0.9840 |
| N-terminal acetyl group | +C₂H₂O | +42.04 | +42.0106 |
| Disulfide bridge between two cysteines | -2H | -2.02 | -2.0157 |
| Lactam bridge between a side-chain amine and carboxyl | -H₂O | -18.02 | -18.0106 |
How to calculate a peptide's molecular weight
Add the residue masses in the sequence, add one water for the two free ends of a linear chain, then apply the changes above for each modification. The result is the molecular weight of the free peptide; a salt form adds its counterions (see TFA vs acetate peptide salts).
- GHK. Gly 57.05 + His 137.14 + Lys 128.18 + water 18.02 = 340.38 Da, the formula C₁₄H₂₄N₆O₄ of the free GHK peptide given in the GHK-Cu guide.
- Melanotan 2. Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂: the seven residues (983.14 Da) plus water (18.02), plus an acetyl group (+42.04), an amide (-0.98) and a lactam bridge (-18.02) give C₅₀H₆₉N₁₅O₉ and 1024.20 Da, matching the formula in the Melanotan 2 guide; the listed weight, 1024.18, differs only by rounding.
The peptide reference table lists the published formula and weight of every compound with a guide on this site, so a calculation can be checked against it.
Average or monoisotopic mass?
Average mass reflects the natural mix of isotopes and is the figure used for weighing and for molar calculations. Monoisotopic mass counts only the most abundant isotope of each element and is what mass spectrometry reports as the lightest peak of a molecule's isotope pattern. The gap grows with size: for BPC-157 the two are about 1418.70 and 1419.55 Da.
Frequently asked questions
What are amino acids?
Small organic molecules that carry an amino group and a carboxylic acid group. The 20 in the chart above are the alpha-amino acids that cells link through peptide bonds into peptides and proteins; they differ only in their side chains, listed in the chart.
What is the one-letter code for an amino acid?
A single capital letter assigned to each standard amino acid, such as G for glycine, K for lysine, W for tryptophan and Y for tyrosine. The full set is in the table above, following the IUPAC-IUB recommendations.
How many amino acids are there?
The standard genetic code encodes the 20 amino acids in the chart above. Selenocysteine (Sec), whose codeword is UGA, is called the 21st amino acid in the genetic code (Turanov et al., 2011), and pyrrolysine, specified by UAG and known mainly from some archaea and a few anaerobic bacteria, the 22nd (Brugère et al., 2018). Research peptides also use non-standard residues such as Aib and Nle, listed in the second table.
What do B, Z and X mean in a peptide sequence?
B is aspartic acid or asparagine, Z is glutamic acid or glutamine, and X is an unknown or unspecified residue (IUPAC-IUB JCBN, 1984).
What is an amino acid residue mass?
The mass of an amino acid once it is part of a chain: the free amino acid less one water molecule (18.02 Da), because each peptide bond releases one water.
How do you calculate the molecular weight of a peptide?
Sum the residue masses, add one water for the free ends, then apply terminal and bridge changes: an amide subtracts 0.98 Da, an acetyl group adds 42.04 Da and a disulfide bridge subtracts 2.02 Da.
Do D-amino acids have a different molecular weight?
No. A D-amino acid has the same formula and mass as its L form; only the spatial arrangement differs.
What is the difference between average and monoisotopic mass?
Average mass uses standard atomic weights and suits weighing and solution math. Monoisotopic mass uses the most abundant isotopes and matches the lightest peak in a mass spectrum.
Related guides
- Peptide molecular weights
- Peptide molecular weight calculator
- What is a peptide?
- Peptide modifications explained
- TFA vs acetate peptide salts
- How research peptides are made
- What is a COA?
- Research hub
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