The 20 standard amino acids
| Amino acid | Three-letter | One-letter | Residue mass (monoisotopic, Da) |
|---|---|---|---|
| Alanine | Ala | A | 71.03711 |
| Arginine | Arg | R | 156.10111 |
| Asparagine | Asn | N | 114.04293 |
| Aspartic acid | Asp | D | 115.02694 |
| Cysteine | Cys | C | 103.00919 |
| Glutamic acid | Glu | E | 129.04259 |
| Glutamine | Gln | Q | 128.05858 |
| Glycine | Gly | G | 57.02146 |
| Histidine | His | H | 137.05891 |
| Isoleucine | Ile | I | 113.08406 |
| Leucine | Leu | L | 113.08406 |
| Lysine | Lys | K | 128.09496 |
| Methionine | Met | M | 131.04049 |
| Phenylalanine | Phe | F | 147.06841 |
| Proline | Pro | P | 97.05276 |
| Serine | Ser | S | 87.03203 |
| Threonine | Thr | T | 101.04768 |
| Tryptophan | Trp | W | 186.07931 |
| Tyrosine | Tyr | Y | 163.06333 |
| Valine | Val | V | 99.06841 |
The residue mass is the mass of the amino acid as it sits inside a chain, after the water lost in forming each peptide bond. To get a peptide's mass, add the residue masses and then add one water (18.01056 Da) for the two free ends. Our peptide calculator does this for you.
Why some letters are not the first letter
Several amino acids start with the same letter, so some codes had to be found elsewhere. A few are easy to remember by sound: F for phenylalanine, R for arginine, W for tryptophan ("double ring"), Y for tyrosine.
Leucine and isoleucine have the same mass. Mass spectrometry alone cannot tell them apart.
Other symbols you will meet
| Symbol | Meaning |
|---|---|
| U, Sec | Selenocysteine, the 21st amino acid |
| X, Xaa | Any or unknown amino acid |
| Aib | α-Aminoisobutyric acid, not one of the standard twenty |
| Nle | Norleucine |
| pGlu | Pyroglutamic acid, a ring-closed glutamic acid at the start of a chain |
| D- before a code | The mirror-image (D) form of that amino acid |
| Ac- at the start | An acetylated N-terminus |
| -NH₂ at the end | An amidated C-terminus |
Are there 20, 21 or 22 amino acids?
Twenty are the standard set, found in the proteins of every living thing. Selenocysteine is counted as the 21st and pyrrolysine as the 22nd; both are built into proteins by some organisms through a special reading of the genetic code. Many more amino acids exist that are not used to build proteins, and synthetic peptides often contain them.
Reading a sequence
GEPPPGKPADDAGLV in one-letter code is Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val in three-letter code: fifteen amino acids, which is the sequence of BPC-157. Each of our product pages gives the sequence both ways.
Related: what are peptides?