Every entry in this library that describes an actual peptide describes a chain. This page is about what that word means, how such a chain is written and counted, and — using this library's own entries as the sample — how long the chains in the compounds people actually encounter turn out to be.
The chain itself
An amino acid has an amino group at one end and a carboxyl group at the other. Join the carboxyl of one to the amino of the next and you get a peptide bond, with one molecule of water released in the process. Do it again, and again, and the result is a chain.
Once an amino acid is inside a chain it is called a residue — because it is the residue of the free amino acid, the part left after the water was removed. This is why sequence lengths are given in residues rather than in amino acids: the things in the chain are no longer quite the things that went in.
A chain of n residues contains n − 1 peptide bonds. Fifteen residues, fourteen bonds. That arithmetic is the basis of the mass check described in the peptide bond entry: each bond released 18.015 daltons of water, so a chain weighs that much less than its free amino acids summed.
The two ends, and why direction matters
One end of the chain still has a free amino group. The other still has a free carboxyl group. They are called the N-terminus and the C-terminus, and they are not interchangeable.
Chains are always written and numbered from the N-terminus. BPC-157's sequence GEPPPGKPADDAGLV starts at glycine, position 1, at the N end, and finishes at valine, position 15, at the C end. Fragment names inherit the convention: when a clinical record names BNP(4-27), it means residues 4 through 27 counted from the parent chain's N-terminus.
The termini are also where modifications usually sit, which is where sequence letters start to mislead. The seven-residue fragment sold as TB-500 is described as the heptapeptide LKKTETQ, and its published mass does not match those seven residues — because the molecule carries an acetyl group on its N-terminus that the letters have no way to record. A sequence is a description of the backbone, not a complete description of the molecule.
Where a peptide becomes a protein
There is no chemical event at the boundary. The bonds in a 40-residue chain and a 400-residue chain are the same bonds.
The convention most often quoted puts the line at about 50 residues; some fields use 40, others 100. Below it, "peptide"; above it, "protein". The honest version is that this is a naming habit that grew up around what could be synthesised and what had to be extracted, and it survives because it is useful rather than because it is true.
The practical consequence for anyone reading about these compounds: the same molecule can appear as a peptide in one source and a small protein in another without either being wrong.
What the real distribution looks like
Because this library states a chain length in every compound entry, the distribution can be measured rather than asserted. Read across all entries on 2026-09-19:
- 63 entries state a length.
- 44 resolve to a plain residue count.
- Median: 10 residues. Range: 2 to 315.
| Chain length | Entries |
|---|---|
| 10 residues or fewer | 24 |
| 11–20 | 5 |
| 21–50 | 11 |
| More than 50 | 4 |
The typical compound here is very short. A clear majority — 24 of 44 — is ten residues or fewer: GHK-Cu at three, KPV at three, glutathione at three, noopept at two, oxytocin at nine, selank at seven. The chains are small enough to write out in full, which is unusual among biologically active molecules and is a large part of why they can be synthesised and sold at all.
The long tail is mostly not peptides. Only four entries exceed 50 residues, and bimagrumab at around 300 is a full-size IgG antibody — as is trevogrumab — a molecule of an entirely different class that appears in this library because it is discussed in the same conversations. Follistatin at 288 or 315, IGF-1 LR3 at 83 and hCG at 92 and 145 residues across two subunits are proteins by any convention.
And some have no chain at all. Of the 19 entries whose stated length does not resolve to a residue count, seven describe substances containing no amino acid chain whatsoever — NAD, orforglipron, AICAR, tesofensine, 5-Amino-1MQ and SLU-PP-332 among them. Each is sold in the same places as the peptides and is regularly described as one. Roughly one entry in ten in this library carries the word peptide in its surroundings and no peptide bond in its structure.
Others in that group of 19 are mixtures rather than molecules — KLOW, GLOW, cerebrolysin, the Wolverine stack — where asking for the chain length is the wrong question, because there are several chains in the vial.
Why the count is worth having
The measurement above is small, reproducible and specific to this library, and it says something the general literature does not: the compounds that reach the retail peptide market are, as a population, very short chains, and the exceptions to that are largely molecules that were never peptides.
That shapes how to read a claim. A three-residue molecule and a 300-residue antibody do not behave alike, are not made alike, and are not absorbed alike — and a page that discusses both under one word is doing something misleading whether or not it means to.
Related: the peptide bond for the chemistry of a single link and the mass arithmetic; the amino acid chart for the twenty building blocks; and what peptides are for the definition this page unpacks.