A chain of a certain length
Amino acids link to one another through a peptide bond, and the resulting chain is called a peptide. Keep adding and at some point people start calling it a protein instead. There is no law that fixes the boundary, though somewhere around fifty residues is the convention most sources use.
That vagueness matters more than it sounds. Insulin has fifty-one residues and is usually called a protein; several compounds sold as peptides are considerably shorter than that; and one or two things sold as peptides in this market are not chains of amino acids at all. The word does real work as a category and very little as a specification.
What actually determines behaviour is the sequence. The order of the amino acids decides the shape the chain folds into, and the shape decides what it can bind to. Two peptides of identical length and similar composition can do entirely unrelated things.
The plain answer to what are peptides is that they are defined sequences: short enough to be assembled synthetically one residue at a time, long enough to fold into a shape a receptor can recognise. The insulin entries on UniProt are sequences written out residue by residue, and a synthetic research peptide is judged the same way, against its sequence and the mass that sequence predicts. Residue-by-residue assembly is also why purity is never assumed: a coupling step can fall short and leave chains missing one residue, and much of the solid-phase synthesis literature on PubMed is about suppressing those by-products and separating them out afterwards.
Four things worth knowing first
If you read nothing else on this page, read these.
What gets called a peptide but is not
Several products in this market carry the label without meeting the definition. NAD+ is a coenzyme built from nucleotides. 5-Amino-1MQ and dihexa are small molecules. Collagen peptides are a mixture of hydrolysed fragments rather than a single defined sequence.
None of that makes them worthless. It does mean that reading across from peptide research to those compounds, or the reverse, is a mistake, and a surprising number of product descriptions make exactly that mistake.
We flag it on the individual pages where it applies. It costs nothing to be accurate and it saves you from assuming a body of evidence covers something it does not.
Collagen and NAD+ cause the most confusion. The collagen peptides page explains why a hydrolysed mixture is sold as food and cannot carry a certificate in the sense used here, and the NAD+ page sets out what a coenzyme is doing in a peptide catalogue at all.
- “Peptide” used as a quality claimIt is a chemical category, not a grade. It says nothing about purity, source or evidence.
- Length quoted without sequenceFifteen amino acids describes thousands of different molecules. The sequence is the identifier.
- Research transferred across the boundaryFindings about a full protein do not automatically apply to a fragment of it, or the reverse.
- Non-peptides in a peptide catalogueCoenzymes and small molecules are sold alongside peptides everywhere. Ours are labelled.
Where to go next
Three pages that build directly on this one.
About the basics
Four questions that come up constantly on this topic.