The short definition
A peptide is a short chain of amino acids joined by peptide bonds — the amide linkage formed when the carboxyl group of one amino acid condenses with the amino group of the next, releasing water. Chains are conventionally called peptides up to roughly 50 residues and proteins above that, though the boundary is a convention rather than a physical property.
Most compounds handled as research material are far shorter than that limit — commonly between three and forty residues. The order of the residues is what defines the molecule, and a single substitution produces a different compound entirely.
Peptide, protein, or small molecule?
The distinction is practical rather than academic, because it determines how a compound behaves at the bench.
| Small molecules | Typically under 900 Da. Generally chemically stable and membrane-permeable. Usually synthesised by conventional organic chemistry. |
|---|---|
| Peptides | Roughly 200–5,000 Da. Chemically synthesised residue by residue, and generally susceptible to enzymatic and hydrolytic degradation, which is why they are supplied lyophilised and stored cold. |
| Proteins | Above roughly 5,000 Da. Too large for practical chemical synthesis; produced in expression systems and prone to misfolding. |
Peptides sit in the middle: small enough to synthesise chemically and modify residue by residue, large enough to be handled and stored quite differently from a conventional small-molecule reagent.
How research peptides are made
Nearly all are produced by solid-phase peptide synthesis (SPPS), the method Bruce Merrifield introduced in 1963 and won a Nobel Prize for. The chain is assembled on an insoluble resin bead, one residue at a time, from the C-terminus backwards:
- The first amino acid is anchored to the resin.
- Its protecting group is removed, exposing a reactive amine.
- The next amino acid — protected, and chemically activated — is coupled on.
- Excess reagent is washed away. The cycle repeats.
- When the sequence is complete, the chain is cleaved from the resin and the side-chain protecting groups are removed.
The critical detail is that no cycle is 100% efficient. At 99% coupling efficiency per residue, a 15-residue peptide finishes at about 86% theoretical yield of full-length product. The remainder consists of deletion sequences — chains missing one or more residues. These are structurally similar to the target, which makes them difficult to separate and easy to miss.
Crude SPPS product is never acceptable research material. Preparative HPLC purification is what separates the target from its own truncated relatives.
After purification the material is lyophilised — freeze-dried under vacuum — into the powder cake you receive. Lyophilisation removes water, which is the main driver of hydrolytic degradation, and is why properly stored powder is stable far longer than a reconstituted solution.
Why purity determines reproducibility
Impure material is a common and under-recognised source of irreproducible results. Consider what can be present in a poorly purified vial alongside the intended compound:
- Deletion sequences left over from incomplete coupling.
- Residual solvents from synthesis and purification, trifluoroacetic acid in particular.
- Bacterial endotoxin introduced during handling.
- Water, invisible on the balance, meaning the mass in the vial is not all compound.
None of these are visible in the vial. All of them can shift a result. This is the entire reason we publish independent test results against every lot rather than a single headline number — see how we test.
What documentation to expect
Whoever you buy from, the minimum you should be able to obtain is a certificate of analysis that carries the lot number printed on your vial, issued by a laboratory independent of the seller. A generic PDF that is not tied to your specific lot tells you nothing about the material in front of you.
Check that the purity figure states its detection wavelength, that the certificate names the testing laboratory, and that the lot number on the certificate, the vial and the seller's published record all agree. If they do not, do not use the material.
Ours are published openly in the COA library, without an account, for every lot we release.