What Are Research Peptides?

Peptides and proteins

A peptide is a chain of amino acids joined end to end. Two make a dipeptide, three a tripeptide, and chains of a few dozen are still peptides. Somewhere around 50 amino acids the word protein takes over, although that boundary is a convention rather than a law of chemistry. Insulin, with 51 amino acids in two chains, sits right on it and is usually called a peptide hormone. Regulators draw their own lines: the US Food and Drug Administration, for example, treats chains of more than 40 amino acids as proteins.

The practical difference is structure. Short peptides are flexible in solution and often take on a defined shape only when they bind their target. Proteins fold into stable three-dimensional structures, frequently held together by disulfide bridges, and their activity depends on keeping that fold. Mass follows length: each amino acid adds between about 57 daltons (glycine) and 186 daltons (tryptophan), roughly 110 Da on average.

The peptides on this site cover most of that range. GHK, the copper-binding tripeptide, weighs 340.4 Da. BPC-157 has 15 amino acids and weighs 1419.5 Da. Semaglutide is built on a 31-residue GLP-1 backbone, tesamorelin has 44 residues, and IGF-1 LR3, with 83 amino acids and a mass of about 9.1 kDa, is a small protein by most definitions.

How the peptide bond works

Every amino acid carries an amino group at one end, a carboxylic acid group at the other and a side chain that gives it its character. When the carboxyl group of one amino acid reacts with the amino group of the next, the two join through an amide bond, the peptide bond, and a molecule of water is released. Repeating that step builds the backbone. The peptide bond is flat and fairly rigid, because electrons are shared across its carbon, oxygen and nitrogen atoms, which limits how the chain can twist and helps explain the helices and turns that recur in peptide structures.

A chain has direction. It begins at the N-terminus, with a free amino group, and ends at the C-terminus, with a free carboxyl group, and sequences are always written in that order. Profiles and certificates use a few conventions.

• Three-letter and one-letter codes. Gly-His-Lys means glycine, histidine, lysine; GHK is the same sequence in single letters.
• The D- prefix. Amino acids in natural proteins have the L configuration, and glycine has no mirror form at all. D-Phe or D-Trp marks the mirror-image version, placed there to resist enzymes.
• End caps. Ac- at the start means the N-terminus is acetylated; -NH₂ at the end means the C-terminus is an amide rather than a free acid.
• Non-standard residues. Aib (aminoisobutyric acid), Nle (norleucine) and 2Nal (2-naphthylalanine) do not occur in natural proteins but are common in designed peptides.

How research peptides are made

Most research peptides are made by solid-phase peptide synthesis, the method Bruce Merrifield developed at the Rockefeller Institute and published in 1963. It earned him the Nobel Prize in Chemistry in 1984. The first amino acid is anchored to a tiny insoluble resin bead and the others are added one at a time; after each step the excess reagents are washed away while the growing chain stays on the bead, so no intermediate has to be isolated.

The chain is built from the C-terminus towards the N-terminus. Each cycle removes a temporary protecting group and couples the next amino acid, whose own reactive side chain is blocked by a more permanent group. In the Fmoc method that dominates today, the temporary group comes off with a mild base, and at the end trifluoroacetic acid (TFA) cuts the peptide from the resin and strips the side-chain protection in the same step. The crude product is purified by preparative reverse-phase HPLC and freeze-dried, which is why research peptides arrive as a lyophilised powder, and often as a TFA salt.

Small inefficiencies add up. If every coupling step were 99% complete, a 40-residue synthesis would leave only about two-thirds of the chains at full length, because 0.99⁴⁰ ≈ 0.67. The rest are deletion and truncated sequences that purification has to remove, and traces usually survive. That arithmetic is why purity has to be measured lot by lot, as our guide to reading a certificate of analysis explains.

Why most peptides are injected in research

The digestive tract is built to take peptides apart. Pepsin in the stomach, pancreatic proteases such as trypsin and chymotrypsin in the small intestine, and enzymes on the intestinal lining cut peptide bonds until dietary protein is reduced to single amino acids and short fragments. A peptide swallowed as a tablet meets the same enzymes, and the little that survives has to cross the gut wall, which large, water-loving molecules cross poorly. For most peptides, oral bioavailability is well below 2%.

That is why animal and human studies almost always use injection: subcutaneous or intravenous in people and larger animals, often intraperitoneal in rodents. The exceptions are instructive. Oral semaglutide (Rybelsus) is formulated with an absorption enhancer, SNAC (salcaprozate sodium), which shields the peptide from digestion locally in the stomach and helps it across the stomach lining. Even then only around 1% of each dose is absorbed, so the tablets contain far more peptide than an injection of comparable effect. Semax and Selank are registered in Russia as nasal drops, and GHK-Cu is applied to the skin as a cosmetic ingredient.

How chemists extend half-life

Natural peptide hormones are built to act briefly. Native GLP-1 survives about two minutes in the circulation, because the enzyme DPP-4 removes its first two amino acids, and small peptides are filtered out rapidly by the kidneys. Medicinal chemists use a handful of tools against both routes of loss.

The albumin strategies produce the largest gains. Albumin, the most abundant protein in blood plasma, has a half-life of about three weeks and is too large for the kidneys to filter, so an attached peptide inherits much of that protection. Semaglutide reaches a half-life of about one week and tirzepatide about five days, which makes once-weekly injection possible for the authorised products. CJC-1295 shows the effect inside a single molecule: with the DAC linker its half-life has been measured at roughly 6–8 days, while the same peptide without it, often called Mod GRF 1-29, lasts minutes. The result is a different pattern of exposure, not simply a better one: a long-acting analogue stimulates its receptor continuously, a short-acting one delivers a brief pulse.

The four classes on this site

Profiles here are grouped by the system a peptide acts on rather than by its chemistry. The full peptide list shows every profile in each group.

The evidence column matters as much as the target column: the incretin peptides carry large randomised controlled trials, most repair peptides have animal data and little else. Every profile labels the evidence behind each claim.

Authorised medicine or research compound

The same molecule can be two very different products. A semaglutide pen from a pharmacy and a vial sold as semaglutide for research may hold the same peptide, but only the first is a medicine. In the EU a medicine reaches patients through a marketing authorisation, granted by the European Commission after assessment by the European Medicines Agency, or by a national agency. The company has to demonstrate quality, safety and efficacy, and four things stand behind that authorisation.

• Clinical trials. Phase 1 studies test safety and pharmacology in small groups, phase 2 explores doses, and phase 3 trials, often with hundreds or thousands of participants, compare the medicine with placebo or an existing treatment.
• Marketing authorisation. Regulators assess the whole dossier and approve specific uses, doses and warnings, published in the summary of product characteristics.
• GMP manufacture. Production follows Good Manufacturing Practice, with validated processes, sterility testing for injectable products and certification of every batch by a qualified person.
• Pharmacovigilance. Once the medicine is in use, suspected side effects are collected and analysed continuously, and the authorisation can be amended, suspended or withdrawn.

A research compound has none of that. It is characterised by a certificate of analysis confirming identity and purity for one lot, and that is the extent of it. It has not been tested for sterility or endotoxins unless the certificate says so, it has no approved dose, and what is known comes from the published literature alone. That is not a criticism of research chemicals, simply what the label means, and it is why every page here treats them as laboratory tools and nothing else.

Frequently asked questions

Where does a peptide end and a protein begin? There is no fixed point. Around 50 amino acids is the usual rule of thumb, and proteins generally fold into stable three-dimensional structures that short peptides lack.

Why are research peptides supplied as a powder? Freeze-drying after purification removes the water that drives most degradation, so a lyophilised peptide keeps far longer than a solution.

Why do so few peptides work by mouth? Digestive enzymes break them down and the gut wall absorbs little of what survives. Oral semaglutide needs a dedicated absorption enhancer, and even then around 1% of the dose is absorbed.

Is a research peptide the same as the medicine with the same name? No. A medicine carries a marketing authorisation backed by clinical trials, GMP manufacture and safety monitoring. A research compound has only its certificate of analysis and is not sold for human use.