The building block and the bond
All amino acids share one layout. There is an amino group, a carboxylic acid group, and a side chain that differs from one amino acid to the next and decides its chemical character. Join the carboxyl end of one to the amino end of another and you get an amide link, with a molecule of water as the by-product. That link is the peptide bond. Repeat the reaction and a chain grows.
The bond is flat and rather rigid, because its carbon, oxygen and nitrogen atoms share electrons. That stiffness is part of the reason helices and turns show up so often in peptide structures.
Chains are directional. The start, called the N-terminus, has a free amino group; the far end, the C-terminus, has a free carboxyl group. Every sequence on a label or a certificate is written from N to C.
Peptide or protein: where the line falls
A dipeptide has two amino acids, a tripeptide three, and a few dozen still make a peptide. Near 50 residues usage switches to protein. That cut-off is custom, not chemistry. Insulin, with its 51 amino acids spread over two chains, is still generally called a peptide hormone, whereas the US Food and Drug Administration counts anything longer than 40 amino acids as a protein.
Shape is the more useful criterion. A short peptide is flexible in solution and tends to settle into a definite form only on binding its target. Proteins fold into a stable three-dimensional structure, commonly braced by disulfide bridges, and depend on that fold for their activity.
Mass rises with length. The lightest residue, glycine, adds about 57 daltons and the heaviest, tryptophan, 186; the average is roughly 110 Da. The peptides discussed on this site cover most of the range.
- GHK-Cu: its peptide part, the tripeptide GHK, is 340.4 Da.
- BPC-157: 15 residues, 1419.5 Da.
- Semaglutide: a GLP-1 backbone of 31 residues.
- Tesamorelin: 44 residues.
- IGF-1 LR3: 83 residues and about 9.1 kDa, a small protein by most definitions.
Decoding the sequence on a label
Four conventions cover most of the notation on product pages and certificates.
| Notation | Meaning | Purpose |
|---|---|---|
| Gly-His-Lys, GHK | Glycine, histidine and lysine, first in three-letter and then in one-letter code | Long and short form of one sequence |
| D-Phe, D-Trp | The mirror-image form of the residue. Natural proteins are built from the L form; glycine alone has no mirror image | Put into a designed peptide so that enzymes fail to cut there |
| Ac- in front, -NH2 behind | An acetyl group on the N-terminus; an amide in place of the free acid on the C-terminus | End caps against enzymes that nibble a chain from its ends |
| Aib, Nle, 2Nal | Aminoisobutyric acid, norleucine, 2-naphthylalanine | Building blocks unknown in natural proteins yet frequent in designed peptides |
From resin bead to freeze-dried powder
Research peptides are almost all produced by solid-phase peptide synthesis. The method is the work of Bruce Merrifield at the Rockefeller Institute; he published it in 1963 and was awarded the 1984 Nobel Prize in Chemistry for it. The trick is to fix the first amino acid to an insoluble resin bead. Further amino acids are coupled one by one, and after every coupling the leftover reagents are simply washed off. The chain stays put on its bead, so there is never an intermediate to isolate.
Synthesis proceeds from the C-terminus towards the N-terminus. A cycle has two moves: take off the temporary protecting group at the chain end, then couple the next amino acid, whose side chain wears a more permanent protecting group of its own. Fmoc chemistry, the dominant variant today, removes the temporary group with a mild base. When the chain is complete, trifluoroacetic acid (TFA) cuts it from the resin and clears the side-chain protection in one operation. Preparative reverse-phase HPLC purifies the crude product and freeze-drying finishes the job. Hence the lyophilised powder in the vial, often a TFA salt.
Small losses compound. Suppose each coupling is 99% complete. Over 40 residues only about two-thirds of the chains reach full length (0.9940 ≈ 0.67). The others are deletion and truncated sequences. Purification takes out most of them, seldom all, which is why each lot needs a purity measurement of its own. Checking a Certificate of Analysis, Line by Line shows how that measurement is reported.
Receptors: how a peptide passes on its signal
A research peptide works by docking onto a specific receptor, which may sit on the cell surface or inside the cell. Docking starts a signalling cascade within the cell, and the cascade alters gene expression, enzyme activity and cell behaviour. Since a given peptide fits few receptors, it can switch on one pathway while leaving others alone, which is what makes it a research tool.
This site sorts its profiles by the biological system addressed, not by chemistry; the peptide index shows all of them.
| Receptors and targets | Group | Profiles | How far the evidence reaches |
|---|---|---|---|
| GLP-1, GIP and glucagon receptors; mitochondrial signalling | Metabolic and incretin | Semaglutide, tirzepatide, retatrutide, MOTS-c | Ranges from authorised medicines through a phase 3 programme to preclinical work |
| GHRH receptor, ghrelin receptor (GHS-R1a), IGF-1 receptor | Growth hormone axis | Ipamorelin, GHRP-2, GHRP-6, hexarelin, CJC-1295, tesamorelin, IGF-1 LR3 | Several have human pharmacology studies. No EU authorisation for any |
| Blood vessel growth, actin, collagen, telomerase (reported) | Repair and longevity | BPC-157, TB-500, GHK-Cu, epithalon | Chiefly cell and animal work. GHK-Cu is used as a cosmetic ingredient |
| BDNF and GABA signalling in the brain; melanocortin receptors | Neuro and melanocortin | Selank, semax, melanotan II, PT-141 | Registrations in Russia and a single US approval. No EU authorisation for any |
Large randomised controlled trials stand behind the incretin peptides, whereas the repair group mostly has animal data to show and not much else. Two comparison pages go deeper into single groups: Semaglutide, Tirzepatide and Retatrutide Compared and GH Secretagogues Compared.
Why the gut is a poor route
Digestion is designed to take peptides apart. Pepsin works in the stomach, trypsin and chymotrypsin from the pancreas continue in the small intestine, and enzymes on the gut lining complete the work. Food protein ends up as single amino acids plus some short fragments, and a swallowed peptide gets identical treatment. What is left must still pass the gut wall, and large, water-loving molecules pass it poorly. Oral bioavailability of most peptides is well below 2%.
For that reason published animal and human studies rely on injection in nearly every case: subcutaneous or intravenous for people and the larger animals, frequently intraperitoneal for rodents. Rybelsus, the oral form of semaglutide, is formulated with SNAC (salcaprozate sodium). This absorption enhancer protects the peptide locally in the stomach and eases it across the stomach lining. Even so, roughly 1% of a dose is absorbed, so a tablet holds far more peptide than an injection giving a similar effect. Russia has registered semax and selank in the form of nasal drops.
Engineering a longer half-life
Natural peptide hormones are short-lived by design. Native GLP-1 is gone from the bloodstream in about two minutes. DPP-4, an enzyme, clips the first two amino acids off the chain, and the kidneys clear small peptides fast.
| Threat | Modification | How it helps | Example |
|---|---|---|---|
| Proteases along the chain | D-amino acids | Proteases are built for L-amino acids and pass over the mirror form | D-Trp and D-Phe in GHRP-6 |
| Proteases along the chain | Cyclisation | A lactam bridge ties the chain into a ring that offers proteases little grip | Melanotan II and PT-141 |
| DPP-4 | Aib at the cleavage site | The non-natural residue stops the enzyme cutting | Semaglutide: Aib at position 8 |
| Enzymes that trim chain ends | End caps | No free end is left for them to attack | Thymosin beta-4 (acetylated); the GHRPs (amidated) |
| Kidney filtration | Fatty-acid side chain | Binds albumin reversibly, and albumin-bound peptide escapes the filter | Semaglutide (C18 diacid) and tirzepatide (C20 diacid) |
| Kidney filtration | DAC linker | Its reactive group attaches covalently to albumin after injection | CJC-1295 with DAC |
Hitching a ride on albumin gives the biggest gains. Albumin is the most abundant plasma protein. Its own half-life is about three weeks and its size keeps it out of the kidney filter, and a bound peptide shares much of that protection. The half-life of semaglutide is about one week and that of tirzepatide about five days, which allows the authorised products a once-weekly schedule. CJC-1295 shows the effect in a single molecule: roughly 6–8 days measured with DAC, minutes for the DAC-free peptide often called Mod GRF 1-29.
Longer does not mean better. A long-acting analogue stimulates its receptor continuously and a short-acting one delivers a brief pulse: two different exposures, not a good and a bad one. Pairings of peptides, and the evidence still missing for them, are covered in Peptide Combinations.
Research material versus licensed medicine
One molecule can be two products. Semaglutide in a pharmacy pen and semaglutide in a research vial may be the same peptide, yet only the pen is a medicine. Within the EU, patients receive a medicine only once it has a marketing authorisation. The European Commission grants it following assessment by the European Medicines Agency, or else a national agency does. A central authorisation applies in the Netherlands, Belgium and Luxembourg alike. The company behind it has to demonstrate quality, safety and efficacy.
| Aspect | Authorised medicine | Research material |
|---|---|---|
| Human evidence | Phase 1 (safety and pharmacology, small groups), phase 2 (dose finding) and phase 3, often hundreds or thousands of participants compared with placebo or an existing treatment | Only what the published literature holds |
| Regulatory review | Full dossier assessed by regulators; the approved uses, doses and warnings go into the summary of product characteristics | None. No approved dose exists |
| Manufacture | Good Manufacturing Practice: validated processes, sterility tests for injectables, each batch certified by a qualified person | A certificate of analysis stating identity and purity of one lot. Sterility and endotoxins untested unless the certificate states otherwise |
| After release | Pharmacovigilance: suspected side effects are collected and analysed all the time; the authorisation may be changed, suspended or withdrawn | Nothing |
The right-hand column is no criticism of research chemicals; it spells out what a research-use-only label means. The consequences for a buyer are set out in the Benelux buying guide, and handling at the bench in the storage and reconstitution protocol.
Quick answers
How long can a chain be and still count as a peptide? About 50 amino acids by convention, though the boundary is soft: insulin, at 51, is still called a peptide hormone.
Why does research material arrive dry and not as a liquid? Freeze-drying is the last manufacturing step. With the water gone, most degradation has no medium to run in, so powder keeps far longer than a solution.
Why is almost nothing in this field studied as a tablet? Digestive enzymes cut peptides apart and the gut wall lets little of the remainder through. Even oral semaglutide, which has an absorption enhancer built into the tablet, gets only around 1% of a dose absorbed.
If a research vial and a pharmacy pen hold the same molecule, are they equivalent? No. Behind the pen stand clinical trials, GMP manufacture, a marketing authorisation and continuous safety monitoring. The vial carries a certificate of analysis for one lot and is not supplied for human use.
Research use only. Everything on PeptidenBenelux.com describes peptides for laboratory research. Nothing here is medical advice. Always comply with the laws that apply in your jurisdiction.