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What Are Peptides? A Science Overview

By BioLabs Research · Last reviewed October 2026

A plain-language science guide to what peptides are, what they are made of, how they are classified, made and analyzed, and where they appear in biology and research.

A peptide is a chain of amino acids joined by peptide bonds. Every living cell makes peptides, and chemists can also build them in the laboratory, adding one amino acid at a time to reach an exact sequence. This guide explains what peptides are made of, how they are classified, how they are made and checked, and where they appear in biology and research.

What is a peptide?

The National Library of Medicine defines peptides as compounds made of amino acids joined by peptide bonds into linear, branched or cyclic structures^[1]. Chains of roughly 2–12 amino acids are called oligopeptides, chains of roughly 13 or more are called polypeptides, and proteins are larger versions that can fold into complex structures such as enzymes and receptors^[1]. A peptide made of two amino acids is a dipeptide^[2].

The line between a peptide and a protein is a naming convention, not a change in chemistry. US regulation draws one precise line: under FDA's definition, a protein is an alpha amino acid polymer with a specific, defined sequence of more than 40 amino acids, and naturally associated chains are counted together^[3]. Insulin has two chains of 21 and 30 amino acids^[4], so for this purpose it counts as a protein, and a 2009 law moved insulin from regulation as a drug to regulation as a biologic, with FDA accepting biosimilar insulin applications from March 23, 2020^[5].

What is a peptide made of?

• Amino acids. Only 20 standard amino acids are commonly found in proteins. Each has an amino group (−NH₂), a carboxyl group (−COOH) and a variable side chain, the R group, that gives it its own chemical character^[6].
• Peptide bonds. These link one amino acid to the next to form a linear chain^[6].
• Backbone and side chains. The repeating backbone carries amide N–H and C=O groups; hydrogen bonds between them create local folding patterns, while the side chains are not part of that backbone pattern^[6].

Sequence is the key idea. The same types and numbers of amino acids, arranged in a different order, make a different molecule^[6]. That is why a catalog name alone says little: the sequence and the measured mass on a lot's analytical report are what identify the material in a vial.

Four levels of structure

Biochemists describe the structure of amino acid chains on four levels^[7]:

• Primary: the amino acid sequence.
• Secondary: local motifs such as alpha helices and beta sheets.
• Tertiary: the overall three-dimensional fold of a single chain.
• Quaternary: the assembly of several chains into one unit.

How peptides are classified

• By length: dipeptides, oligopeptides (about 2–12 residues) and polypeptides (about 13 or more)^[1].
• By shape: linear, branched or cyclic^[1]. Oxytocin and vasopressin are both nine-residue peptides closed into a ring by a disulfide bond between the cysteines at positions 1 and 6^[8][9].
• By end-group chemistry: oxytocin ends in a glycine amide rather than a free acid^[8], and the fragment sold as TB-500 carries an acetyl group on its first residue (Ac-LKKTETQ)^[10].
• By origin: gene-encoded peptides made by cells, and synthetic peptides made by chemical synthesis^[11].
• By biological role: hormones, neuropeptides, antimicrobial peptides and structural or binding fragments.

What do peptides do in biology?

Cells use peptides as signals and regulators. A few well-characterized examples:

• Insulin. A hormone of 51 amino acids across two chains^[4]. Clinical studies of purified pancreatic extract began in Toronto in January 1922^[5][12], and on October 28, 1982 FDA approved the first human insulin made with recombinant DNA technology, the first approved medical product of any kind derived from that technology^[5].
• Glucagon. It is cut from a larger precursor protein, proglucagon, along with several related peptides^[13].
• Oxytocin and vasopressin. Nine-residue hormones, each made as part of a larger precursor that also contains its carrier protein, a neurophysin^[8][9].
• Enkephalins. Two related five-residue peptides identified in brain tissue in 1975^[14].
• Antimicrobial peptides. Host-defense peptides made by many multicellular organisms, including plants and insects^[15].

How are peptides made?

In cells, many peptides are cut from larger precursor proteins, as the oxytocin, vasopressin and proglucagon examples show^[8][13]. In the laboratory, most peptides today are made by solid-phase peptide synthesis (SPPS)^[11]. R. B. Merrifield published the method in 1963^[16] and received the 1984 Nobel Prize in Chemistry for developing chemical synthesis on a solid matrix^[17].

Synthesis is not perfectly clean. A 2014 review groups the impurities found in synthetic peptides: deletion or insertion of amino acids, racemized residues, leftover protecting groups, oxidized side chains, dimers, counter ions such as trifluoroacetate, and degradation products such as pyroglutamate or succinimide forms^[11]. The same review notes that these impurities can distort early laboratory studies and lead to erroneous conclusions^[11], which is why lot-level analysis matters.

How peptides are analyzed

Analytical laboratories pair separation with mass measurement. High-performance liquid chromatography coupled to mass spectrometry, for example, was used to characterize the acetylated thymosin beta-4 fragment found in TB-500^[10]. On a certificate of analysis (COA), purity is usually reported as HPLC area percent at a stated wavelength, and identity is confirmed by mass spectrometry (MS).

Lot COAs are available on request and are being added to our COA library. For a field-by-field walkthrough, see how to read a peptide COA.

What are peptides used for?

In medicine, some peptides and proteins are approved drugs or biologics; insulin is the classic example^[5]. In research, peptides serve as laboratory tools: to map which residues of a molecule carry a property, as mutational work did for the actin-binding motif of thymosin beta-4^[18], to develop and validate analytical methods^[10], and to study impurities and stability^[11]. BioLabs Research sells its compounds for laboratory research only.

Research peptides in our catalog

Each BioLabs Research product page lists sequence-level identity data: BPC-157, TB-500, GHK-Cu, MOTS-c, Semax, AOD-9604 and Tesamorelin. Browse the full research compound catalog, compare two identities in BPC-157 vs TB-500, or read identity notes on the blog.

For regulatory background, see are peptides safe?, are peptides steroids? and are peptides legal?.

Important note on our products: all compounds sold by BioLabs Research are supplied strictly for laboratory research use only (RUO). They are not drugs, are not for human or veterinary use, and we provide no guidance on use in people or animals.

Frequently asked questions

What is the difference between a peptide and a protein?

Both are chains of amino acids joined by peptide bonds. Proteins are larger and can fold into complex structures such as enzymes and receptors. For US regulatory purposes, FDA defines a protein as a defined-sequence chain of more than 40 amino acids.

What is a peptide made of?

Amino acids joined by peptide bonds. Each amino acid has an amino group, a carboxyl group and a side chain, and the order of the amino acids, the sequence, defines the peptide.

Are peptides natural?

Many are. Insulin, glucagon, oxytocin and the enkephalins are made by the body. Research peptides may match a natural sequence, be a fragment of one, or carry chemical modifications such as N-terminal acetylation.

How are peptides analyzed in the lab?

Most often by liquid chromatography to assess purity and mass spectrometry to confirm identity. The results for one manufacturing lot are recorded on its certificate of analysis.

Where can I find purity data for a specific compound?

On the certificate of analysis for that lot. Lot COAs are available on request and are being added to our COA library.

For laboratory research use only. Not for human or veterinary use. Not a drug, food, cosmetic or dietary supplement, and not intended to diagnose, treat, cure or prevent any condition. Nothing on this page is medical or legal advice.

References

1. National Library of Medicine. MeSH descriptor D010455, Peptides (scope note). meshb.nlm.nih.gov/record/ui?ui=D010455
2. National Library of Medicine. MeSH descriptor D004151, Dipeptides (scope note). meshb.nlm.nih.gov/record/ui?ui=D004151
3. 21 CFR 600.3(h)(6), definition of "protein" (eCFR). www.ecfr.gov/current/title-21/section-600.3
4. UniProt Consortium. UniProtKB P01308, Insulin (human). www.uniprot.org/uniprotkb/P01308/entry
5. US Food and Drug Administration, FDA History Office. 100 Years of Insulin. www.fda.gov/about-fda/fda-history-exhibits/100-years-insulin
6. Sanvictores T, Farci F. Biochemistry, Primary Protein Structure. In: StatPearls. StatPearls Publishing; updated 2025. PMID 33232013. pubmed.ncbi.nlm.nih.gov/33232013
7. Ragupathi A, Mastrogiannis AJ, Rahimi N. Biochemistry, Tertiary Protein Structure. In: StatPearls. StatPearls Publishing; updated 2025. PMID 29262204. pubmed.ncbi.nlm.nih.gov/29262204
8. UniProt Consortium. UniProtKB P01178, Oxytocin-neurophysin 1 (human). www.uniprot.org/uniprotkb/P01178/entry
9. UniProt Consortium. UniProtKB P01185, Vasopressin-neurophysin 2-copeptin (human). www.uniprot.org/uniprotkb/P01185/entry
10. Esposito S, Deventer K, Goeman J, Van der Eycken J, Van Eenoo P. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500 […]. Drug Test Anal. 2012;4(9):733–738. doi:10.1002/dta.1402. PMID 22962027. pubmed.ncbi.nlm.nih.gov/22962027
11. D'Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2–30. doi:10.1016/j.jpba.2014.06.012. PMID 25044089. pubmed.ncbi.nlm.nih.gov/25044089
12. Banting FG, Best CH, Collip JB, Campbell WR, Fletcher AA. Pancreatic extracts […]. Can Med Assoc J. 1922;12(3):141–146. PMID 20314060. pubmed.ncbi.nlm.nih.gov/20314060
13. UniProt Consortium. UniProtKB P01275, Pro-glucagon (human). www.uniprot.org/uniprotkb/P01275/entry
14. Hughes J, Smith TW, Kosterlitz HW, et al. Identification of two related pentapeptides from the brain […]. Nature. 1975;258(5536):577–580. doi:10.1038/258577a0. PMID 1207728. pubmed.ncbi.nlm.nih.gov/1207728
15. Zasloff M. Antimicrobial peptides of multicellular organisms. Nature. 2002;415(6870):389–395. doi:10.1038/415389a. PMID 11807545. pubmed.ncbi.nlm.nih.gov/11807545
16. Merrifield RB. Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. J Am Chem Soc. 1963;85(14):2149–2154. doi:10.1021/ja00897a025. doi.org/10.1021/ja00897a025
17. The Nobel Prize in Chemistry 1984: Bruce Merrifield. NobelPrize.org. www.nobelprize.org/prizes/chemistry/1984/summary
18. Van Troys M, Dewitte D, Goethals M, Carlier MF, Vandekerckhove J, Ampe C. The actin binding site of thymosin beta 4 mapped by mutational analysis. EMBO J. 1996;15(2):201–210. PMID 8617195. pubmed.ncbi.nlm.nih.gov/8617195