Peptides explained: what they are, how they are made, and what their literature can establish

The word "peptide" carries almost no information on its own. It describes a three-residue fragment and a thirty-nine-residue engineered analogue equally well, and it tells you nothing about what either has been studied for, how far that study got, or whether a regulator has ever looked at it. This page is the chemistry and the vocabulary underneath everything else in this section, so that the compound-specific articles can get on with the literature.

What a peptide is, chemically

A peptide is a chain of amino acids joined end to end by amide bonds — called peptide bonds when they link residues. Each amino acid contributes an amino group and a carboxyl group, and the bond forms between the carboxyl of one and the amino group of the next, releasing water. The chain therefore has a direction, and sequences are written from the amino end (the N-terminus) to the carboxyl end (the C-terminus). Reverse a sequence and you have a different molecule.

An individual amino acid in a chain is a residue, and a peptide's identity is its sequence of residues — KPV is lysine-proline-valine, and those three letters are the whole molecule.

The line between a peptide and a protein is conventional rather than chemical. Chains up to roughly fifty residues are usually called peptides, longer ones proteins, and the cut-off shifts between textbooks.

Against that, the range in a single catalogue is instructive. KPV is three residues, GHK-Cu three plus a bound copper ion, Semax seven, BPC-157 fifteen, MOTS-c sixteen; tesamorelin and tirzepatide are both long enough to sit at the protein end of the convention. Nothing about how any of them behaves follows from the shared label. Nor is everything sold alongside peptides one: 5-Amino-1MQ is a small-molecule quinolinium salt with no peptide bond in it, and NAD+ is a dinucleotide, while glutathione, rarely called a peptide, is a tripeptide.

Where the sequences come from

Three origins account for nearly everything in this field.

Endogenous sequences and their fragments. Many of these compounds are pieces of something the body already makes. KPV is the C-terminal tripeptide of alpha-melanocyte-stimulating hormone; BPC-157 is a fifteen-residue sequence derived from a protein found in human gastric juice; GHK was identified in human plasma. Fragments are attractive because a short chain is cheap to synthesise and may retain one activity of the parent while dropping others — but "derived from" is a statement about origin, not equivalence.

Engineered analogues. Here a natural sequence is deliberately modified: a residue substituted to block the site a protease cuts, a group added to the terminus, a chain attached to slow clearance. Tesamorelin is growth hormone-releasing hormone with a trans-3-hexenoyl group on its N-terminal tyrosine, a modification whose purpose is stability. Peptides carrying a fatty-acid chain to extend their circulating life are the same idea applied to a different problem. This is where most of the approved peptide medicines live, because engineering is what makes a fragile molecule into a usable one.

Designed sequences. Selank and Semax both attach a short proline-containing tail to a fragment of a natural peptide, a strategy from Russian institutes in the 1980s.

How they are made, and why that shows up on a certificate

Almost all research peptides are made by solid-phase peptide synthesis, the method Bruce Merrifield introduced in 1963 and was awarded the Nobel Prize for in 1984. The growing chain stays anchored to an insoluble resin bead while reagents are washed over it: deprotect the terminus, couple the next activated amino acid, wash, repeat. Anchoring the chain is what makes the washing possible, and the washing is what makes the cycle repeatable by machine.

The consequence worth understanding is arithmetic. No coupling step is perfectly efficient. If each cycle completes at 99%, a fifteen-residue chain finishes at about 86% and a forty-residue chain at about 67% — and the shortfall is not empty space, it is deletion and truncation sequences that are chemically similar to the target and must be separated from it, usually by preparative reverse-phase HPLC. Longer chains are harder, which is why the longest peptide medicines are often made by recombinant expression in cells instead.

What survives purification is lyophilised — freeze-dried under vacuum — because dry peptide is far more stable than dissolved peptide. Lyophilised material also carries counter-ions from purification, commonly trifluoroacetate or acetate, plus residual water. Hence chromatographic purity and net peptide content are two different measurements: a vial can be chromatographically clean and still hold less peptide by weight than its label implies. Identity is a third question, answered by mass spectrometry rather than by any purity figure.

Those three measurements, and what a certificate of analysis can and cannot cover, are worked through in the certificate-of-analysis guide. The terms themselves are defined in the glossary.

What happens to a peptide in a biological system

Peptides are made of the same bonds the body spends energy breaking. Proteases in plasma, in the gut and at the kidney cleave them, and circulating half-lives measured in minutes are ordinary rather than exceptional. They are also poorly absorbed from the gut, for the same reason, which is why peptide studies overwhelmingly use parenteral routes — a fact about how the literature was generated, not guidance of any kind.

The flip side is specificity. Peptides act on receptors evolved to recognise them, often with high affinity and little activity at anything else, which is the property that makes them interesting as tools and as medicines. How that interaction is measured, and what a binding or signalling result does and does not establish, is the subject of how peptide receptor signalling is studied.

Why nearly all of this literature is preclinical

A handful of peptides are approved medicines with registration trials behind them. The great majority of compounds sold for research are not, and their published record consists of cell culture and rodent work, frequently from a small number of laboratories. That is not a reason to dismiss the literature — it is a reason to read it for what it is. How to read a peptide study sets out the controls, endpoints and sample-size questions that decide how much a given paper is worth.

Where Canadian law sits

Health Canada regulates drugs under the Food and Drugs Act, and a product sold for human use needs market authorisation. Material supplied for laboratory research is not authorised for human use and is not a medicine, including when a compound with the same chemical name is approved somewhere as one. The detail, and the parts of it that are genuinely unsettled, are in peptides and Canadian law.

Compounds discussed on this site are supplied for laboratory research use only, and several of them — BPC-157, TB-500, KPV among them — are not approved as medicines by Health Canada, the FDA or the EMA for any indication. Janoshik-verified by HPLC with mass-spec identity confirmation; 100% synthetic. The measured purity for each lot is stated on its product page. Nothing here is medical, dosing, or therapeutic guidance.

Common questions

Is a peptide the same thing as a protein? Chemically they are the same class of molecule. The distinction is a convention about length, usually drawn near fifty residues, and it moves depending on the field and the author.

Why are peptide medicines injected rather than swallowed? Because the gut digests them. The same proteases that break dietary protein break a therapeutic peptide, and the fraction surviving to the bloodstream intact is generally too small and too variable to work with. Chemical engineering has solved this for a few specific molecules and not as a general problem.

Does "derived from a human protein" mean a compound is safe or natural? No. It describes where a sequence was found. A fragment can behave quite differently from the parent protein, synthetic material is not biologically sourced, and neither fact says anything about safety, which is established by toxicology and trials rather than by provenance.

Why does so much of this literature come from so few laboratories? Because for compounds with no commercial sponsor, the group that identified a sequence tends to be the group that keeps working on it. That concentration is historical rather than suspicious, but it means the findings share methods and assumptions, and independent replication is the thing to look for.

Selected Research References

1. Muttenthaler M, King GF, Adams DJ, Alewood PF. Trends in Peptide Drug Discovery. Nature Reviews Drug Discovery. 2021;20(4):309-325. pubmed.ncbi.nlm.nih.gov/33536635
2. Fosgerau K, Hoffmann T. Peptide Therapeutics: Current Status and Future Directions. Drug Discovery Today. 2015;20(1):122-128. pubmed.ncbi.nlm.nih.gov/25450771
3. Lau JL, Dunn MK. Therapeutic Peptides: Historical Perspectives, Current Development Trends, and Future Directions. Bioorganic & Medicinal Chemistry. 2018;26(10):2700-2707. pubmed.ncbi.nlm.nih.gov/28720325
4. Vlieghe P, Lisowski V, Martinez J, Khrestchatisky M. Synthetic Therapeutic Peptides: Science and Market. Drug Discovery Today. 2010;15(1-2):40-56. pubmed.ncbi.nlm.nih.gov/19879957
5. Merrifield RB. Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. Journal of the American Chemical Society. 1963;85(14):2149-2154.

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