What are peptides?
Short chains of amino acids, large enough for specific interactions and small enough for chemical synthesis. An introduction to structure, nomenclature and research.
Peptides are chains of amino acids linked by peptide bonds. The bond forms when the carboxyl group of one amino acid reacts with the amino group of the next, releasing water. This creates a backbone with a beginning (N-terminus) and an end (C-terminus).
Peptide or protein?
The boundary is fluid. In practice, chains of up to about 50 amino acids are called peptides, and above that, proteins. More important than the number is the structure: proteins usually fold into stable three-dimensional shapes, while short peptides are often flexible in solution.
| Designation | Number of amino acids | Example |
|---|---|---|
| Dipeptide | 2 | Carnosine |
| Oligopeptide | up to about 20 | BPC-157 (15) |
| Polypeptide | about 20 to 50 | Glucagon (29) |
| Protein | over 50 | Insulin is considered a borderline case (51) |
How to read a sequence
Sequences are always written from the N- to the C-terminus, usually in single-letter code. The pentadecapeptide BPC-157, for example, reads GEPPPGKPADDAGLV: glycine, glutamic acid, three prolines and so on. The molecular formula and molar mass can be calculated exactly from the sequence. It is precisely this calculated mass that mass spectrometry later checks.
Why peptides are interesting for research
Peptides combine two properties: they are large enough to interact very specifically with receptors or enzymes, and small enough to be fully synthesised chemically. Research uses them as tools to study signalling pathways, binding sites and structure-activity relationships. Reviews such as that by Muttenthaler et al. (2021) show how broad the field has become.
How research peptides are made
Nearly all research peptides are produced by solid-phase peptide synthesis (SPPS): the first amino acid is bound to a resin, then building block after building block is attached. After cleavage from the resin, purification by preparative HPLC follows, along with freeze-drying into the lyophilisate, the white powder in the vial.
Each of these steps can generate by-products. That is why the analysis of every single batch is not an add-on but the core of quality. More on this in Understanding HPLC and Typical impurities.