Peptides are short chains of amino acids held together by peptide bonds. In size they sit between small molecules and proteins; a protein is essentially a long peptide chain [1]. Peptides sold for laboratory research are synthesised chemically, usually by solid-phase synthesis, and supplied as a freeze-dried powder. This article explains the key terms, from the peptide bond and the amino acid sequence to purity, peptide content and research use only.
Amino acids and the peptide bond
Amino acids are the building blocks of peptides and proteins. Every amino acid has an amino group, a carboxyl group and a side chain that gives it its specific properties. A peptide bond forms when the carboxyl group of one amino acid links to the amino group of the next, releasing one molecule of water. Chemically, the peptide bond is an amide bond [1].
The result is a chain with a distinct beginning and end: the N-terminus (with a free amino group) and the C-terminus (with a free carboxyl group). Biochemists call each amino acid in the chain a residue [1]. The order of the residues – the sequence – determines which peptide you have, and is written using three-letter or one-letter codes. The tripeptide GHK, for example, is glycine-histidine-lysine, or Gly-His-Lys in three-letter code [5].
Two peptides containing exactly the same amino acids in a different order are therefore different substances. That is why a good product description or certificate of analysis always states the sequence or an unambiguous identifier, such as a PubChem compound ID (CID) or a CAS number.
Peptide, polypeptide or protein: where the line is drawn
There is no strict dividing line between a peptide and a protein. The widely used textbook by Berg, Tymoczko and Stryer states that most natural polypeptide chains are between 50 and 2000 amino acids long; these are usually called proteins. Chains made up of a small number of amino acids are called oligopeptides or simply peptides [1]. Regulators, however, sometimes draw a sharp line: the US FDA considers any amino acid polymer with a specific defined sequence that is greater than 40 amino acids in size to be a protein [2].
Research peptides also vary widely in length. A few examples, with data from PubChem [5]:
| Substance | Number of amino acids | Molar mass (PubChem) |
|---|---|---|
| GHK (glycyl-histidyl-lysine) | 3 | 340.4 g/mol (CID 73587) |
| TB-500 (Ac-LKKTETQ) | 7 | 889.0 g/mol (CID 62707662) |
| BPC-157 | 15 | 1419.5 g/mol (CID 9941957) |
Not everything a peptide shop sells is a peptide. NAD+, for example, is a coenzyme built from two nucleotides, not from amino acids (PubChem CID 5892) [5]. It is often sold alongside peptides in shops like ours.
How synthetic peptides are made
Most synthetic peptides today are made by solid-phase peptide synthesis (SPPS) [4]. In this method, the first amino acid is anchored to small resin beads. The chain is then extended step by step: one protected amino acid is coupled at a time, after which the protecting group is removed ready for the next step. Because the growing peptide stays attached to the resin, excess reagents can simply be washed away. Robert Bruce Merrifield received the 1984 Nobel Prize in Chemistry for this approach [3].
No coupling step is 100 per cent complete, so by-products form that closely resemble the intended peptide. A review from Ghent University lists, among other things, deletion sequences (an amino acid is missing), insertions (an extra amino acid), racemisation (an amino acid in its mirror-image form), residual protecting groups and oxidation [4]. After synthesis, the peptide is cleaved from the resin and purified. It is then usually freeze-dried (lyophilised): the solution is frozen and the ice is removed under vacuum. What remains in the vial is a dry powder or ‘cake’: the lyophilisate. In this dry form, a peptide is far more stable than in solution.
Terms on the label and the certificate of analysis
- Sequence: the order of the amino acids. Together with the molar mass and a CID or CAS number, it defines exactly which molecule you have.
- Lyophilisate: the freeze-dried powder in the vial. How best to store it is explained in Storing peptides.
- Purity (HPLC): the main peak’s share of the chromatogram, as a percentage of everything detected. It tells you about related by-products, not about the total composition of the powder.
- Peptide content: how much of the powder is actually peptide. Besides peptide, a lyophilisate also contains water and counterions. A purity of 99% therefore does not mean that the powder is 99% peptide.
- Counterion: an oppositely charged ion present alongside the peptide in the powder, such as trifluoroacetate (TFA) from synthesis or purification [4], or acetate.
- Batch number: the number of the production lot. Every Clean Peptides vial and pen carries a batch number on the label, so you can link a product to the analysis of that lot.
- Research use only (RUO): the product is intended for laboratory research, not for use in humans or animals.
How to check the purity and the mass on a certificate is explained step by step in How to read a certificate of analysis (COA).
Products
Products from our range mentioned in this article:
- GHK-Cu 50 mg and GHK-Cu 100 mg
- TB-500 5 mg and TB-500 10 mg
- BPC-157 5 mg and BPC-157 10 mg
- NAD+ 1000 mg
- The full range is available in the shop.
Further reading
- Research use only (RUO): what does it mean?
- What is BPC-157? The state of the research
- What is GHK-Cu? The copper peptide in research and cosmetics
- What is NAD+? A coenzyme in research
- Semax and Selank: what are they?
- What is PT-141 (bremelanotide)?
- Knowledge base: peptides and research materials explained
Sources
- Berg JM, Tymoczko JL, Stryer L. Biochemistry, 5th edition. New York: W.H. Freeman; 2002. Section 3.2: Primary Structure: Amino Acids Are Linked by Peptide Bonds to Form Polypeptide Chains. NCBI Bookshelf NBK22364
- U.S. Food and Drug Administration. 21 CFR 600.3(h)(6), definition of ‘protein’. ecfr.gov (accessed 30 September 2026)
- The Nobel Prize in Chemistry 1984: Robert Bruce Merrifield. nobelprize.org (accessed 30 September 2026)
- D’Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis 2014;101:2–30. doi:10.1016/j.jpba.2014.06.012
- PubChem, National Library of Medicine. Compound records CID 73587 (GHK), 62707662 (TB-500), 9941957 (BPC-157) and 5892 (NAD+). pubchem.ncbi.nlm.nih.gov (accessed 30 September 2026)
For laboratory research use only. Clean Peptides products are not intended for use in humans or animals. This article provides general information and is not medical advice.
