Peptide Academy · Research use only
Speak peptide: basic terms, explained simply
Peptide Academy · Chapter 6 of 7
Peptide talk is full of jargon. Here are the words you'll meet most often, grouped so they build on each other. Each one gets a plain-English definition, then a line on why it matters.
The building blocks
Amino acid A building block of peptides and proteins. There are 20 standard amino acids commonly found in proteins. Each has an amino group, a carboxyl group and a side chain (the "R group") that gives it its own chemistry[1]. Why it matters: every peptide is just amino acids in a particular order.
Peptide bond The link that joins one amino acid to the next, forming a chain[1]. Think of it as the "snap" between two LEGO bricks. Why it matters: it's literally where the word "peptide" comes from.
Peptide A chain of amino acids joined by peptide bonds. The US National Library of Medicine describes chains of roughly 2–12 amino acids as oligopeptides and roughly 13 or more as polypeptides[2]. Why it matters: see Chapter 1 for how peptides differ from proteins.
Protein A longer amino-acid chain that usually folds into a complex 3D shape. For US regulatory purposes, the FDA defines a protein as a defined-sequence chain of more than 40 amino acids[3]. Why it matters: the peptide/protein line is a naming convention, not a change in chemistry.
Sequence The order of amino acids in the chain, written from start to end. The same building blocks in a different order make a different molecule[1]. Why it matters: a sequence identifies a peptide far better than a product name.
N-terminus and C-terminus The two ends of a chain. The N-terminus is the start, with a free amino group; the C-terminus is the end, with a free carboxyl group. Sequences are written from N to C. Why it matters: small chemical changes at the ends, such as an acetyl "cap", create a different molecule. The fragment sold as TB-500, for example, is acetylated at its N-terminus[4].
Kinds of peptides
Analogue A modified version of a natural peptide, with one or more building blocks changed or added. Semax, for example, combines a fragment of a natural hormone with an added Pro-Gly-Pro tail[5]. Why it matters: an analogue is a different molecule from the natural one, with its own identity data.
Fragment A piece cut from, or modelled on part of, a larger molecule. AOD-9604 is based on amino acids 177–191 at the end of the growth-hormone molecule, with an extra tyrosine added[6]. Why it matters: a fragment isn't the parent molecule, and it shouldn't be described as if it were.
Receptor A protein, often on a cell's surface, that a signalling molecule binds to. The classic analogy: the receptor is a lock and the signalling molecule is the key. Why it matters: a lot of peptide research is about how molecules interact with receptors.
Agonist A molecule that binds a receptor and activates it: a key that turns the lock. (A molecule that binds without activating, and blocks others, is an antagonist.) Why it matters: you'll see the word in research papers describing how compounds are studied.
In the lab
Lyophilized Freeze-dried. The material is frozen, then water is removed under vacuum, leaving a dry powder[7]. Same basic idea as astronaut ice cream. Why it matters: dry powder degrades more slowly, though it still needs cold, dry, dark storage (Chapter 3).
Molecular weight How much one molecule weighs, usually given in daltons (Da) or grams per mole. It can be calculated from the sequence. Why it matters: mass spectrometry compares the measured mass with the expected one to confirm identity (Chapter 5).
HPLC High-performance liquid chromatography: a test that separates a sample into its parts and shows each as a peak. The main peak's share of the total area is reported as purity. Why it matters: it's the standard way purity appears on a certificate.
Mass spectrometry (MS) A test that measures the mass of molecules. When combined with HPLC it's called LC-MS, an approach used, for example, to characterise the TB-500 fragment[4]. Why it matters: it answers "is this the right molecule?"
Purity The share of the peptide material that is the right peptide, compared with peptide-related impurities, as measured by HPLC under a stated method. Why it matters: it's not the same as net peptide content (next).
Net peptide content The share of the powder's total weight that is peptide at all. The rest can include water and counter-ions such as trifluoroacetate, a common leftover from synthesis and purification[8]. Why it matters: people often confuse it with purity. A high purity figure does not mean the powder is 100% peptide by weight.
Impurity Anything in the sample that isn't the target molecule: missing or extra building blocks, leftover chemical caps, oxidised side chains and more[8]. Why it matters: impurities are what testing is designed to catch.
Paperwork and identifiers
COA (certificate of analysis) The test report for one specific lot. It should list the lot number, the tests, the methods and the results (Chapter 4). Why it matters: a COA from a different lot says nothing about your vial.
Lot (batch) number The ID for one production run. It should appear on both the vial label and the COA. Why it matters: it's what ties a vial to its test results.
SDS (safety data sheet) A document covering a material's hazards, handling, storage and incompatibilities. Ours are in the SDS library. Why it matters: storage conditions come from the lot documents, not from rules of thumb.
CAS number A unique identifier assigned to a chemical substance by CAS, a division of the American Chemical Society. Think of it as a chemical's ID card number. Why it matters: it helps make sure everyone is talking about the same substance.
Research and rules
In vitro Studied in cells or test tubes, outside a living organism.
Preclinical Research done before any human studies, typically in cells or animals. Why it matters: much of the published research on popular research peptides is preclinical (Chapter 2).
RUO (Research Use Only) Sold for laboratory research only, not for use in people or animals. Important: a label alone doesn't settle the legal question. Regulators look at how a product is marketed (Chapter 7).
Frequently asked questions
What does lyophilized mean? Freeze-dried: frozen, then dried under vacuum to remove water, leaving a powder.
What is the difference between purity and net peptide content? Purity is the share of peptide material that is the right peptide. Net peptide content is the share of the powder's weight that is peptide at all; the rest can be water and counter-ions.
What does RUO mean? Research Use Only: sold for laboratory research, not for use in people or animals.
What is a CAS number? A unique identifier for a chemical substance, assigned by CAS, a division of the American Chemical Society.
Series navigation: ← Chapter 5: HPLC and mass spec explained · Hub · Next: Chapter 7: The state of the market →
For laboratory research use only. Not for human or veterinary use. Not a drug, food, cosmetic or dietary supplement.
References
- Sanvictores T, Farci F. Biochemistry, Primary Protein Structure. StatPearls; 2025. PMID 33232013
- National Library of Medicine. MeSH D010455, Peptides. meshb.nlm.nih.gov/record/ui?ui=D010455
- 21 CFR 600.3(h)(6). ecfr.gov/current/title-21/section-600.3
- Esposito S, et al. Drug Test Anal. 2012;4(9):733–738. PMID 22962027
- Synthetic peptides based on natural regulatory peptides (Semax). Neuroscience & Medicine, 2013. scirp.org/pdf/NM_2013120914223729.pdf
- Heffernan M, et al. Int J Obes. 2001. PMID 11673763
- Tang X, Pikal MJ. Design of freeze-drying processes for pharmaceuticals: practical advice. Pharm Res. 2004;21(2):191–200. PMID 15032301
- D'Hondt M, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2–30. PMID 25044089