Peptide sequence notation, explained
One-letter and three-letter codes, direction, Ac- and -NH2, D-residues and Aib, fragment ranges, and worked readings from the catalogue.
A peptide sequence on a specification table carries a lot of information in a small space, and the conventions are not obvious to anyone who has not worked with them. This note decodes the notation: one-letter and three-letter codes, direction, modifications, D-residues, non-natural residues and the shorthand for lipidation, using the Alphex catalogue as worked examples. All Alphex products are for laboratory and in-vitro research only.
Direction
Sequences are always written N-terminus first, C-terminus last, matching the direction of biosynthesis. Confusingly, solid-phase synthesis builds in the opposite direction (C to N), but the written sequence never changes. So GEPPPGKPADDAGLV starts at the free amino group and ends at the carboxyl.
One-letter and three-letter codes
Each of the twenty standard amino acids has both. Gly = G, Glu = E, Pro = P, Lys = K, Ala = A, Asp = D, Leu = L, Val = V, His = H, Thr = T, Gln = Q, Phe = F. Three-letter codes are used when clarity matters or when non-natural residues appear (they have no one-letter code); one-letter codes are compact for long sequences. BPC-157 is written both ways: Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val, or GEPPPGKPADDAGLV.
Modifications and prefixes
- Ac- at the start: N-terminal acetylation. TB-500 is Ac-LKKTETQ, an acetylated heptapeptide.
- -NH2 at the end: C-terminal amidation. Ipamorelin ends in Lys-NH2.
- -OH at the end, or nothing: free C-terminal acid, the default.
- D- prefix on a residue: the D stereoisomer. Ipamorelin contains D-2-Nal and D-Phe.
- Aib: alpha-aminoisobutyric acid, a non-natural residue with no one-letter code.
- 2-Nal: 2-naphthylalanine. Others you may meet: Nle (norleucine), Orn (ornithine), Sar (sarcosine), Cha (cyclohexylalanine).
- Brackets and superscripts in modified natural peptides, for example [Aib8]-GLP-1(7-37), meaning the GLP-1 fragment from residue 7 to 37 with Aib substituted at position 8.
- Fragment ranges: "thymosin beta-4 (17-23)" means residues 17 through 23 of the parent protein. See thymosin beta-4 versus TB-500.
- Lipidation is usually described in words (C20 fatty diacid via a gamma-Glu/PEG linker on Lys17, for example) rather than in the sequence string, because there is no compact standard notation. See lipidated peptides.
- Disulfides: written as a bracketed pairing, for example Cys1-Cys6, or drawn. None of the Alphex catalogue contains cysteine.
Worked readings
- Ac-LKKTETQ: acetylated N-terminus; Leu-Lys-Lys-Thr-Glu-Thr-Gln; free acid C-terminus. Seven residues, 889.0 g/mol. That is TB-500.
- Aib-His-D-2-Nal-D-Phe-Lys-NH2: non-natural Aib at the N-terminus; histidine; D-naphthylalanine; D-phenylalanine; lysine with an amidated C-terminus. Five residues, 711.9 g/mol. That is Ipamorelin.
- Gly-His-Lys : Cu(II): a tripeptide written with its coordinated metal after a colon. That is GHK-Cu; see GHK versus GHK-Cu.
Why it matters when comparing suppliers
Two listings for "the same" peptide can differ in ways only the sequence notation reveals: amidated versus free acid, acetylated versus not, D versus L at a position, a fragment range that differs by one residue. Each of those is a different compound with a different mass, and the mass on the COA is what settles it. A supplier who cannot give you the full sequence notation for what they are selling is a supplier who has not asked. See questions to ask before ordering.
Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.
Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.
Published 2 March 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.