Amino acid codes
and notation.
The reference table of one-letter and three-letter codes for the twenty standard L-amino acids, plus notes on N-terminal and C-terminal modifications, salt forms, and post-translational modifications. The table working researchers reach for when reading a peptide sequence.
The twenty standard L-amino acids.
Each entry shows the full name, the standard three-letter code, the standard one-letter code, and key chemical properties. These are the codes used to write peptide sequences across all institutional and research literature. BPC-157’s sequence in one-letter code is GEPPPGKPADDAGLV; in three-letter code, the same sequence is Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val. Both encode the same fifteen-amino-acid peptide.
| Amino acid | 3-letter | 1-letter | Properties |
|---|---|---|---|
| Alanine | Ala | A | Hydrophobic, small, neutral |
| Arginine | Arg | R | Positively charged, basic |
| Asparagine | Asn | N | Polar, neutral |
| Aspartic acid | Asp | D | Negatively charged, acidic |
| Cysteine | Cys | C | Polar, thiol-containing, disulfide bridge capable |
| Glutamic acid | Glu | E | Negatively charged, acidic |
| Glutamine | Gln | Q | Polar, neutral |
| Glycine | Gly | G | Smallest, no side chain |
| Histidine | His | H | Positively charged at low pH, aromatic, basic |
| Isoleucine | Ile | I | Hydrophobic, branched chain |
| Leucine | Leu | L | Hydrophobic, branched chain |
| Lysine | Lys | K | Positively charged, basic |
| Methionine | Met | M | Hydrophobic, sulfur-containing, oxidation-sensitive |
| Phenylalanine | Phe | F | Hydrophobic, aromatic |
| Proline | Pro | P | Imino acid, ring structure, conformationally rigid |
| Serine | Ser | S | Polar, hydroxyl-containing, phosphorylation site |
| Threonine | Thr | T | Polar, hydroxyl-containing, phosphorylation site |
| Tryptophan | Trp | W | Hydrophobic, aromatic, largest amino acid, fluorescent |
| Tyrosine | Tyr | Y | Polar, aromatic, hydroxyl-containing, phosphorylation site |
| Valine | Val | V | Hydrophobic, branched chain |
Sequence direction and termini.
Peptide sequences are written N-terminus to C-terminus by convention, left to right. The N-terminus is the free amine end (free -NH₂ group); the C-terminus is the free carboxyl end (free -COOH group). When a peptide is described as “N-terminally acetylated,” the free amine is replaced by -NH-CO-CH₃ (an acetyl group). When described as “C-terminally amidated,” the free carboxyl is replaced by -CO-NH₂ (a primary amide).
Modifications and prefixes.
Common modifications appear as prefixes or suffixes in peptide names. Ac- at the start indicates N-terminal acetylation. -NH₂ at the end indicates C-terminal amidation. D- before a residue indicates the D-stereoisomer (rare in biology, used in peptide engineering for protease resistance). p before a serine, threonine, or tyrosine indicates phosphorylation (e.g., pSer, pTyr).
Disulfide bridges and cyclization.
Disulfide bridges between two cysteine residues are denoted by brackets: (Cys^1-Cys^7) indicates a disulfide between cysteine residues at positions 1 and 7 of the sequence. Cyclized peptides are denoted by cyclo(...). Head-to-tail cyclization joins the N-terminus to the C-terminus.
Salt forms and counter-ions.
Peptide names sometimes include a salt-form designation: peptide.acetate, peptide.TFA, peptide.HCl, peptide.NH₄. Cresten Labs ships peptides as acetate salts by default. Alternate salt forms are available on quote.
Pyroglutamyl (pGlu) peptides.
When N-terminal glutamine (Gln, Q) or glutamic acid (Glu, E) is present, the residue can spontaneously cyclize to form pyroglutamate (pGlu). Pyroglutamyl peptides are denoted by pGlu- at the N-terminus or by (pyroglutamyl) notation. The pGlu form is reported as part of HPLC purity, not as an impurity, and is a stable, biologically relevant variant.
How a peptide name actually decodes.
Take a name like Ac-Gly-Pro-Glu-Lys-NH₂. Reading left to right: Ac- is N-terminal acetylation; Gly is glycine at position 1; Pro is proline at position 2; Glu is glutamic acid at position 3; Lys is lysine at position 4; -NH₂ is C-terminal amidation. Total: a four-residue peptide with both terminal modifications.