Educational resource
Amino acid abbreviations
The twenty proteinogenic (standard) amino acids are the building blocks of ribosomal peptides and the alphabet behind most sequences in this database. Each residue has a full name, a three-letter code, and a one-letter code.
Residue mass versus free amino-acid mass
The residue mass in the table below is the mass contributed when that amino acid is incorporated into a peptide chain: free amino-acid mass minus one water (H2O). It is not the mass of the free zwitterion in solution. Summing residue masses and adding back the terminal water (plus corrections for amides, disulfides, and other modifications) is the usual route to peptide molecular weight — see the molecular-weight guide.
Standard residue table
Monoisotopic residue masses below use common IUPAC elemental values and are typical inputs for high-resolution peptide calculations. Compound records on this site more often store average (chemical) molecular weight when the cited monograph does.
| Name | 3-letter | 1-letter | Side chain | Residue mass (Da) |
|---|---|---|---|---|
| Alanine | Ala | A | Methyl | 71.03711 |
| Arginine | Arg | R | Guanidinium | 156.10111 |
| Asparagine | Asn | N | Amide | 114.04293 |
| Aspartic acid | Asp | D | Carboxylate | 115.02694 |
| Cysteine | Cys | C | Thiol | 103.00919 |
| Glutamic acid | Glu | E | Carboxylate | 129.04259 |
| Glutamine | Gln | Q | Amide | 128.05858 |
| Glycine | Gly | G | Hydrogen | 57.02146 |
| Histidine | His | H | Imidazole | 137.05891 |
| Isoleucine | Ile | I | sec-Butyl | 113.08406 |
| Leucine | Leu | L | Isobutyl | 113.08406 |
| Lysine | Lys | K | Butylammonium | 128.09496 |
| Methionine | Met | M | Methyl thioether | 131.04049 |
| Phenylalanine | Phe | F | Benzyl | 147.06841 |
| Proline | Pro | P | Pyrrolidine | 97.05276 |
| Serine | Ser | S | Hydroxymethyl | 87.03203 |
| Threonine | Thr | T | Hydroxyethyl | 101.04768 |
| Tryptophan | Trp | W | Indole | 186.07931 |
| Tyrosine | Tyr | Y | Phenol | 163.06333 |
| Valine | Val | V | Isopropyl | 99.06841 |
Side-chain chemistry in brief
- Nonpolar — Gly, Ala, Val, Leu, Ile, Met, Phe, Trp, Pro; often buried in folded structures.
- Polar uncharged — Ser, Thr, Asn, Gln, Tyr, Cys (thiol); hydrogen-bonding and, for Cys, disulfide formation.
- Acidic — Asp, Glu; contribute negative charge at physiological pH.
- Basic — Lys, Arg, His; contribute positive charge (His near neutrality).
Ile and Leu share the same residue mass but differ in side-chain branching; they are distinct letters (I vs L) and must not be swapped in sequences.
Ambiguous and placeholder letters
X— unknown or non-canonical residue (explained in display notation on this site)B— Asx (Asn or Asp); common in older sequencing; not used hereZ— Glx (Gln or Glu); not used hereJ— Xle (Leu or Ile); not used hereU— selenocysteine; rare; would be noted explicitly if presentO— pyrrolysine; rare; would be noted explicitly if present
Beyond the twenty
Therapeutic and microbial peptides often include Aib (α-aminoisobutyric acid), ornithine, D-amino acids, N-methyl residues, pyroglutamate, and lipidated lysines. Those species are not interchangeable with a single standard letter. This database records them in display notation and in the modifications taxonomy rather than forcing a false 1-letter identity.