A peptide is a short chain of amino acids joined together by chemical bonds called peptide bonds. Amino acids are the small building blocks that also make up proteins. When just a few to around fifty amino acids are linked in a chain, scientists usually call the result a peptide.
Peptides are found throughout nature, and thousands have been described in scientific literature. In the lab, researchers study them to learn how molecules interact, signal and bind to receptors.
Peptides vs. proteins
The difference between a peptide and a protein is mostly about size and structure. Peptides are shorter chains, while proteins are long chains, often hundreds of amino acids, that fold into complex three-dimensional shapes.
There is no single strict cutoff, but chains under about 50 amino acids are commonly called peptides. Very short chains have their own names: a dipeptide has two amino acids, a tripeptide has three (for example KPV or GHK), and a pentadecapeptide has fifteen (for example BPC-157).
How peptides are described
Each peptide has a sequence, which is the exact order of its amino acids. Scientists write sequences using one-letter or three-letter codes, so a three-amino-acid peptide might be written as K-P-V or Lys-Pro-Val.
Researchers also describe peptides by molecular weight, measured in daltons (Da) or grams per mole. Molecular weight is what a mass spectrometry test checks to confirm a peptide's identity.
How research peptides are made
Most research peptides are produced by solid-phase peptide synthesis (SPPS). In this process, amino acids are added one at a time to a growing chain attached to a solid resin bead. When the chain is complete, it is cut from the resin and purified.
Purification is usually done with HPLC, which separates the target peptide from incomplete chains and by-products. The purified material is then freeze-dried (lyophilized) into a stable powder and sealed in a vial.
Modified peptides and analogs
Many peptides studied in research are analogs, meaning versions of a natural sequence that have been changed slightly. Changes might swap one amino acid, add a chemical group, or attach a fatty acid chain to alter how the molecule behaves in experiments.
Blends combine more than one peptide in a single vial. A blend's label should list each component and its amount, and its COA should report results for the blend.
Why quality testing matters
Because synthesis can leave behind incomplete sequences or other impurities, independent testing is the only reliable way to know what a vial contains. A good Certificate of Analysis confirms both identity (by mass spectrometry) and purity (by HPLC) for a specific lot.
You can read more in our guides on what a COA is and how to read HPLC purity results.