This peptide terms glossary defines the 50 words that turn up most often on vial labels, on certificates of analysis, and in peptide literature, organized into five blocks: chemistry, pharmacology, handling, quality, and regulation.
The rest of the site runs on the definitions collected in this peptide terms glossary. When an article reaches for one of these terms, it means exactly this.
| Block | How many terms | Where they appear |
|---|---|---|
| Chemistry and structure | 13 | Technical sheet, compound sequence |
| Pharmacology and mechanism | 15 | Mechanism articles, literature |
| Handling and reconstitution | 11 | Vial label, handling guides |
| Quality and analysis | 11 | Certificate of analysis |
| Regulation and status | 6 | Label, legal framework |
Chemistry and structure
Amino acid. A molecule carrying an amino group and a carboxyl group. Amino acids are the links in peptides and proteins; twenty of them belong to the standard genetic code.
Peptide bond. The covalent link between the carboxyl group of one amino acid and the amino group of the next, releasing a molecule of water.
Peptide. A short chain of amino acids, conventionally up to about 50. Below 40, the FDA regulates it as a drug and not as a biologic.
Protein. A long amino acid chain folded into a stable three-dimensional structure. The border with the peptide is a convention, not a change in nature. The practical distinction is covered in the article on what a research peptide is.
Sequence. The order of the amino acids. It fixes the identity of the molecule: two peptides built from the same amino acids in a different order are different compounds.
Residue. Each amino acid inside the chain, once the bond has formed.
N-terminus and C-terminus. The two ends of the chain: the one with a free amino group and the one with a free carboxyl group. Sequences are written from N to C.
Analog. A molecule modified from a natural one to change some property: enzyme resistance, duration, affinity.
Fragment. A portion of a larger molecule. It gets studied when that portion keeps part of the activity of the whole.
Acetate / salt. The form in which the peptide is stabilized. It explains part of the gap between the gross weight of the contents and the mass of pure peptide.
SPPS (solid-phase peptide synthesis). A manufacturing method that anchors the chain to a resin and adds amino acids one at a time. Published by Merrifield in 1963.
Recombinant. Produced by a genetically modified organism instead of synthesized chemically. Used for long chains.
Molecular weight. The mass of the molecule, calculable from its formula. It is what mass spectrometry confirms.
Pharmacology and mechanism
Receptor. A protein that recognizes one specific molecule and triggers a cellular response when that molecule binds.
Agonist. A substance that binds to a receptor and activates it, mimicking the effect of the natural molecule.
Antagonist. Binds to the receptor without activating it, and by occupying it keeps another molecule from doing so.
Dual / triple agonist. A single molecule that activates two or three different receptors. This is the design behind the modern GLP-1 class.
Affinity. The strength with which a molecule binds to its receptor.
Secretagogue. A substance that stimulates the secretion of another. A growth hormone secretagogue supplies no GH: it makes the body release its own.
Half-life. The time the blood concentration takes to fall by half. It sets the dosing frequency.
Bioavailability. The fraction of what is administered that reaches systemic circulation. By mouth, for most peptides, it is very low.
Pharmacokinetics. What the body does to the substance: absorption, distribution, metabolism, and elimination.
Pharmacodynamics. What the substance does to the body.
DPP-4. An enzyme that degrades natural incretins quickly. Synthetic analogs are modified to resist it.
Incretin. An intestinal hormone that primes the insulin response before glucose arrives. GLP-1 and GIP are the two main ones.
Subcutaneous. Deposited in the fatty tissue under the skin.
Intramuscular. Deposited in the muscle. Absorption differs from the subcutaneous route.
First-pass effect. The liver metabolism that anything absorbed in the intestine goes through before reaching general circulation.
Handling and reconstitution
Lyophilized. Dried by freezing and vacuum sublimation. It is the state the vial arrives in.
Cake. The porous disc the lyophilized material forms at the bottom of the vial. Its appearance is an indicator of correct handling.
Reconstitution. Dissolving the lyophilized material in a sterile diluent. The full procedure has its own guide.
Diluent. The liquid that gets added. Usually bacteriostatic water, sterile water, or saline.
Bacteriostatic water. Sterile water with 0.9% benzyl alcohol as a preservative, which allows several entries into the same vial.
Benzyl alcohol. The preservative. It inhibits microbial growth; it does not sterilize.
Concentration. Milligrams of peptide per milliliter of solution. Calculated by dividing the milligrams in the vial by the milliliters added.
IU (international unit). A measure of biological activity. On an insulin syringe, the marks measure volume, not activity: 100 marks = 1 mL. This is the costliest confusion in the field, and it has an article of its own.
U-100 / U-40. Insulin syringe scales. On U-100, 1 unit = 0.01 mL. On U-40, 1 unit = 0.025 mL.
Gauge (G). The thickness of the needle. The higher the number, the thinner it is.
Cold chain. Holding the storage temperature from manufacture through to use.
Denaturation. Loss of the molecule's conformation. Shaking and heat cause it.
Lipohypertrophy. Thickening of the subcutaneous tissue from repeated injections at the same spot; it alters absorption.
Quality and analysis
COA (certificate of analysis). The document that reports identity, purity, and net content for one specific lot. How to read one.
HPLC. High-performance liquid chromatography. It separates the components of a sample and makes it possible to calculate purity.
Chromatogram. The plot HPLC produces. Without it, a purity figure rests on nothing.
Retention time. The moment a component leaves the column. It is characteristic of each substance under a given set of conditions.
Mass spectrometry. Measures molecular mass and confirms identity. It complements HPLC: purity and identity are separate questions.
Purity. The area of the main peak divided by the total area, as a percentage. It does not include salts, water, or solvents, which do not absorb in UV.
Net content. The actual milligrams of peptide in the vial. Different from purity, and complementary to it.
Lot. A unit of production. Each lot has its own analysis: a COA without a lot number is not traceable.
Endotoxin. A component of the gram-negative bacterial wall that provokes a febrile response. It calls for a specific assay; it does not appear on a purity COA.
Stability. The capacity to hold identity and potency under given conditions over a given time.
Regulation and status
Research use only (RUO). The label indicating supply for laboratory research, with no evaluation and no approval for human use. What it means and what it does not.
Marketing authorization. The approval a national authority grants (INVIMA in Colombia) to sell a medicine.
Compounding. Custom preparation by a licensed pharmacy starting from bulk substances.
Health claim. Stating that a product diagnoses, treats, cures, or prevents. It is the line that turns a product into a drug for regulatory purposes.
Surrogate endpoint. A marker assumed to predict a clinical outcome. Improving it is not the same as improving the outcome, as the article on levels of evidence explains.
S0 (WADA). The Prohibited List category covering any non-approved substance. It admits no therapeutic use exemption.
Frequently asked questions
Are purity and net content the same thing?
No. Purity is a proportion: what fraction of what was detected is the correct compound. Net content is a quantity: how many milligrams are there. A vial can be 99% pure and still hold fewer milligrams than the label declares.
What is the difference between an agonist and a secretagogue?
An agonist activates the receptor of the pathway directly. A secretagogue acts one step earlier: it stimulates the body to release its own hormone, which then acts on the receptor.
Why do syringes talk about "units" if the peptide comes in milligrams?
Because those marks were calibrated for insulin. With any other substance they measure volume and nothing else, and the milligrams they hold depend on the concentration the vial was reconstituted to.
Are a peptide and a protein the same thing?
Chemically they are the same: chains of amino acids joined by peptide bonds. The distinction is one of length and it is conventional, with a cutoff that each organization places at a slightly different point.
References
- IUPAC. Compendium of Chemical Terminology (Gold Book). goldbook.iupac.org
- U.S. Food and Drug Administration. ANDAs for Certain Highly Purified Synthetic Peptide Drug Products — Guidance for Industry. fda.gov
- U.S. Pharmacopeia. General Chapter <621> Chromatography. usp.org
- World Anti-Doping Agency. The Prohibited List. wada-ama.org
Written by the Bionic Editorial Team. Last reviewed: August 2026.
How we work: our editorial policy and source hierarchy.
This content is strictly educational and does not constitute medical advice, diagnosis or a therapeutic recommendation. The compounds mentioned are research products (Research Use Only) and are not approved by INVIMA, FDA, EMA or ANSM for therapeutic use in humans. Any health-related decision should be made with a licensed medical professional.