A peptide is a short chain of amino acids held together by peptide bonds. Research peptides are such chains, made synthetically and intended for laboratory research only. This article covers the basics and points you to the articles on the individual compounds.
Building blocks: amino acids
An amino acid has an amino group, an acid group (a carboxyl group), and a side chain that differs from one amino acid to the next. The proteins in living cells are built from a fixed set of twenty amino acids, as the textbook Molecular Biology of the Cell by Alberts and colleagues describes. Each of the twenty has a three-letter code and a one-letter code, such as Gly and G for glycine. How to read those codes is explained in How to read a peptide sequence.
The peptide bond
Two amino acids join when the acid group of one reacts with the amino group of the other. A molecule of water is released in the process. The bond that forms is an amide bond; between amino acids it is called a peptide bond. That is why the IUPAC Gold Book defines peptides as amides derived from two or more amino carboxylic acids, with formal loss of water.
A chain has two different ends. At the start is a free amino group, the N-terminus. At the end is a free acid group, the C-terminus. A sequence is always written from N to C. A chain of two amino acids is a dipeptide, of three a tripeptide, and of four a tetrapeptide.
Peptide or protein?
There is no sharp boundary. The MeSH thesaurus of the US National Library of Medicine calls chains of about 2 to 12 amino acids oligopeptides and chains of about 13 or more polypeptides. According to MeSH, proteins are larger versions of these that can form complex structures, such as enzymes and receptors. The IUPAC Gold Book puts the lower limit for proteins at a molecular weight of about 10,000 and adds that this limit is not precise.
One example shows how vague the line is. Thymosin beta-4 has 43 amino acids and a calculated molecular weight of 4,963 (PubChem). That is below the Gold Book limit, yet the protein database UniProt lists it as a protein. More on this in What is TB-500?
Peptides in nature
Peptides occur throughout living organisms. Two examples with a known structure:
- Glutathione is a tripeptide of glutamic acid, cysteine, and glycine (PubChem CID 124886). It is an unusual example: according to PubChem, the glutamic acid is attached to the rest of the chain through its side chain, not through its usual acid group.
- Oxytocin is a hormone of the posterior pituitary and consists of nine amino acids (PubChem CID 439302). The cysteines at positions 1 and 6 form a disulfide bridge, so part of the chain forms a ring. The C-terminus is an amide.
Oxytocin also has a place in the history of peptide chemistry. Vincent du Vigneaud received the 1955 Nobel Prize in Chemistry, in part for the first synthesis of a polypeptide hormone: oxytocin.
Modified and synthetic analogues
Research peptides are built in a laboratory, one amino acid at a time, and then purified. How that works is covered in Peptide synthesis and purification. Synthesis makes it possible to use building blocks outside the twenty standard amino acids and to change the ends or the side chains. Examples from the catalog, with the structure as PubChem gives it:
- D-amino acids and Aib. Ipamorelin (H-Aib-His-D-2Nal-D-Phe-Lys-NH2) starts with Aib (α-aminoisobutyric acid) and contains two D-amino acids, the mirror-image form of the usual L-amino acids.
- A modified tyrosine. SS-31 (H-D-Arg-Dmt-Lys-Phe-NH2) starts with D-arginine and contains Dmt, 2,6-dimethyltyrosine.
- An amide at the end. In oxytocin and ipamorelin, the C-terminus is an amide (-NH2) instead of an acid.
- An acetyl group at the start. TB-500 is listed in PubChem as Ac-LKKTETQ: the N-terminus carries an acetyl group.
- A fatty acid chain. Retatrutide and cagrilintide each carry a long fatty chain, a diacid with twenty carbon atoms. Kruse and colleagues (2021) describe cagrilintide as a long-acting analogue of the peptide hormone amylin.
Modifications like these change the mass and the charge of a peptide. That is why they belong in the sequence on the product sheet and on the certificate of analysis.
Peptides in the knowledge base
Each article below covers one compound: structure, origin, research, and regulatory status, with sources. The length comes from the structure in PubChem.
| Compound | Type | Amino acids | PubChem CID |
|---|---|---|---|
| GHK-Cu | tripeptide with copper(II) | 3 | 71587328 |
| KPV | tripeptide | 3 | 125672 |
| Epithalon | tetrapeptide | 4 | 219042 |
| SS-31 | tetrapeptide with D-arginine and Dmt | 4 | 11764719 |
| Ipamorelin | pentapeptide with Aib and D-amino acids | 5 | 9831659 |
| TB-500 | acetylated peptide | 7 | 62707662 |
| Oxytocin | peptide with a disulfide bridge | 9 | 439302 |
| BPC-157 | linear peptide | 15 | 9941957 |
| MOTS-c | linear peptide | 16 | 146675088 |
| AOD-9604 | peptide with a disulfide bridge | 16 | 71300630 |
| CJC-1295 | peptide, with or without DAC | 29, with DAC 30 | 56841945, 91971820 |
| Cagrilintide | lipidated peptide | 37 | 171397054 |
| Retatrutide | lipidated peptide | 39 | 171934787 |
| NAD+ | dinucleotide, not a peptide | none | 5892 |
| 5-Amino-1MQ | small molecule, not a peptide | none | 950107 |
Not a peptide, but in the catalog
Two compounds in the table are not peptides. According to PubChem, NAD+ (nicotinamide adenine dinucleotide) is a dinucleotide of adenine and nicotinamide: two nucleotides joined through a diphosphate bridge. It contains no peptide bond. 5-Amino-1MQ is a small molecule. PubChem describes it as the cation 5-amino-1-methylquinolinium, with the molecular formula C10H11N2+. Both are in the knowledge base because they are in the catalog, not because they are peptides.
Research peptides and the rules
The peptides from PulseChem are research chemicals labeled Research Use Only (RUO): for laboratory research only, for example in vitro. They are not intended for humans or animals and are not offered as medicines, food supplements, or cosmetic products. What RUO means in practice is explained in RUO explained. The legal definition of a medicinal product, by presentation and by function, and how PulseChem deals with it, are set out on the Liability and research use page.
More in the knowledge base. The terms peptide, amino acid, and sequence are explained briefly in the glossary. The research map shows the research areas in which the compounds appear in the literature. How to check the quality of a batch is covered in What is a COA?
Sources
Consulted in November 2026.
- peptides (IUPAC Gold Book)
- proteins (IUPAC Gold Book)
- Peptides and Oligopeptides (MeSH, National Library of Medicine), D010455 and D009842
- Alberts B. et al., Molecular Biology of the Cell, 4th edition, chapter 3: The Shape and Structure of Proteins (NCBI Bookshelf)
- Nomenclature and Symbolism for Amino Acids and Peptides (IUPAC-IUB, JCBN)
- Glutathione (PubChem), CID 124886
- Oxytocin (PubChem), CID 439302
- The Nobel Prize in Chemistry 1955 (NobelPrize.org)
- Thymosin Beta 4 (PubChem), CID 45382195, and Thymosin beta-4 (UniProt), P62328
- Ipamorelin, CID 9831659; Elamipretide (SS-31), CID 11764719; TB-500, CID 62707662 (PubChem)
- Retatrutide (sodium salt), CID 171934787; Cagrilintide, CID 171397054 (PubChem)
- Kruse T. et al. (2021), Development of Cagrilintide, a Long-Acting Amylin Analogue, J Med Chem 64:11183-11194 (PubMed), PMID 34288673
- BPC-157, CID 9941957; Prezatide copper (GHK-Cu), CID 71587328; Lys-Pro-Val (KPV), CID 125672; Epitalon, CID 219042; MOTS-c, CID 146675088; AOD-9604, CID 71300630; CJC-1295 without DAC, CID 56841945; CJC-1295, CID 91971820 (PubChem)
- Nadide (NAD+), CID 5892; 5-Amino-1-methylquinolinium, CID 950107 (PubChem)
This article is background information on laboratory research. It is not medical advice or instructions for use. All products are for research use only.
