Peptide Sequence Database

What a Peptide Sequence Is

A peptide is a chain of amino acids linked by peptide bonds. The sequence is the ordered list of those amino acids — the most fundamental description of the molecule. Everything else about a peptide, its molecular weight, its structure, its function, follows from the sequence.

Sequences are written in a fixed direction: from the N-terminus (the amino end) on the left to the C-terminus (the carboxyl end) on the right. This is the same direction in which peptides are chemically synthesized and the universal convention in which they are read and recorded.

This page does two things. First, it provides the reference tools for working with sequences — the amino acid code table and the notation conventions. Second, it provides verified sequences for major research peptides. It is the sequence-focused companion to the Peptide Database, the Molecular Weight Database, and the Half-Life Reference.

The Amino Acid Code Table

The 20 standard amino acids each have a full name, a three-letter code, and a single-letter code. This is the foundational reference for reading any peptide sequence.

Common non-standard residues in research peptides

Synthetic research peptides frequently contain amino acids that are not among the standard 20. These have no single-letter code and are always written out:

How to Read a Peptide Sequence

Take the BPC-157 sequence as a worked example. In three-letter code:

The same sequence in single-letter code:

Reading it: the chain starts at glycine (Gly / G) at the N-terminus on the left, proceeds through fifteen residues, and ends at valine (Val / V) at the C-terminus on the right. Both notations describe the identical molecule — the choice between them is about readability. Three-letter code is clearer when a sequence includes modified residues; single-letter code is more compact for long sequences and is standard in databases.

Terminal & Structural Modifications

Most research peptides are not just bare chains of standard amino acids. They carry modifications — and the notation records them. The four you will encounter constantly:

Ac- — N-terminal acetylation

An Ac- prefix means an acetyl group has been added to the N-terminus. This is a stabilizing modification: it blocks one of the routes by which enzymes degrade peptides. The true TB-500 fragment is written Ac-LKKTETQ — the acetylation is part of what the molecule is.

-NH2 — C-terminal amidation

An -NH2 suffix means the C-terminus is amidated — the terminal carboxyl group is converted to an amide. Like acetylation, it improves resistance to enzymatic degradation. Ipamorelin and the GHRPs are all C-terminally amidated.

D- — D-amino acids

Naturally occurring amino acids are almost all the L-form. A D- prefix on a residue (D-Ala, D-Phe) marks the mirror-image D-form. Synthetic peptides use D-amino acids deliberately: degradation enzymes are shape-specific and do not process D-residues well, so a D-amino acid at a key position slows clearance.

Non-standard residues — Aib and others

Residues like Aib (alpha-aminoisobutyric acid) are not among the standard 20 and are written out in full. Aib is one of the most important in modern peptide design — it resists cleavage by the enzyme DPP-4, and its presence in Semaglutide and Tirzepatide is part of why those peptides have multi-day half-lives.

Verified Sequence Reference

Amino acid sequences for major research peptides, each verified against multiple independent published sources. Sequences are given in three-letter code with modifications noted; the residue count is the count of amino acids in the chain.

The same sequences in single-letter code

Single-letter code is shown only for peptides composed entirely of standard amino acids. Peptides containing non-standard residues — Ipamorelin (Aib, D-2-Nal), GHRP-6 (D-Trp, D-Phe), CJC-1295 and Sermorelin (D-Ala, terminal modifications) — are not cleanly representable in single-letter code and are best read in three-letter form above.

Sequence and Function

The sequences above are not arbitrary — and a few patterns in them are worth noticing, because they connect sequence to behavior.

Fragments of larger molecules. Several of these peptides are sequences lifted from larger natural proteins. Sermorelin is the first 29 amino acids of growth hormone-releasing hormone, a 44-residue hormone — the "1-29" in its name is literal. The TB-500 fragment Ac-LKKTETQ is a seven-residue stretch of the 43-residue Thymosin β4. BPC-157 is a fifteen-residue partial sequence of a larger protein found in gastric juice.

Engineered variants of fragments. CJC-1295 (no DAC) is Sermorelin with a small number of amino acid substitutions — compare the two sequences in the table above and the differences are visible at specific positions. Those substitutions are why one clears in minutes and the other resists degradation longer, despite being nearly the same chain.

Proline-rich short peptides. Selank (TKPRPGP) and Semax (MEHFPGP) both end in -Pro-Gly-Pro. Proline's rigid ring makes a chain harder for enzymes to process; that shared PGP tail is a stabilizing motif, not a coincidence.

Why This Database Verifies Sequence by Sequence

This page publishes sequences only where they have been verified against multiple independent published sources. That is a deliberate constraint, and it is worth explaining.

An amino acid sequence is an exact claim. A single wrong residue describes a different molecule. Unlike an approximate molecular weight or a half-life range — where "approximately" is honest and useful — a sequence is either exactly right or it is wrong. There is no useful approximate sequence.

For that reason, this database does not attempt to list a sequence for every one of the 45+ compounds in the broader Peptide Database. It publishes the sequences that are firmly verified, and it expands the verified set over time. Additional sequences are confirmed and published on individual compound pages, each alongside its source citations and PubChem identifier, as those pages are built.