Peptides are short chains of amino acids joined end to end by amide bonds, called peptide bonds. Most working definitions put a peptide somewhere between 2 and about 40 or 50 amino acids; longer chains that fold into a stable three-dimensional shape are usually called proteins. Peptides occur naturally as hormones, neurotransmitters, antimicrobial molecules and fragments of larger proteins, and they can also be made chemically, one amino acid at a time, which is how nearly all research peptides are produced.
The rest of this article covers what a peptide is built from, how the chain is put together and written down, where the line between a peptide and a protein is drawn, the main ways peptides are classified, and what that means for a laboratory working with them.
The building blocks: amino acids
Every peptide is built from amino acids. An amino acid has a central carbon atom (the alpha carbon) carrying four groups: an amino group (–NH2), a carboxylic acid group (–COOH), a hydrogen atom, and a side chain. The side chain, often written R, is what makes one amino acid different from another.
Twenty amino acids are encoded by the standard genetic code. Their side chains range from a single hydrogen atom (glycine) to large aromatic rings (tryptophan), and they differ in charge, polarity and size. That variety is why a chain of only a few amino acids can have very specific chemistry: a peptide rich in lysine and arginine carries positive charge at neutral pH, while one rich in aspartic and glutamic acid carries negative charge.
With the exception of glycine, every standard amino acid is chiral. The form used in natural proteins is the L form. Synthetic peptides can include D amino acids, non-standard amino acids, or chemical groups that do not occur in nature, which is one reason the word "peptide" covers such a broad range of molecules.
| Side chain property | Examples | What it contributes to a peptide |
|---|---|---|
| Nonpolar, aliphatic | Glycine, alanine, valine, leucine, isoleucine | Hydrophobic surfaces; lower water solubility in long runs |
| Aromatic | Phenylalanine, tyrosine, tryptophan | UV absorbance at 280 nm (Trp, Tyr), ring-ring contacts |
| Positively charged | Lysine, arginine, histidine | Net positive charge, salt formation with counter-ions |
| Negatively charged | Aspartic acid, glutamic acid | Net negative charge, metal binding |
| Polar, uncharged | Serine, threonine, asparagine, glutamine | Hydrogen bonding with water |
| Sulfur-containing | Cysteine, methionine | Disulfide bridges (cysteine), oxidation sensitivity |
| Cyclic backbone | Proline | Kinks in the chain, restricted flexibility |
How amino acids link into a chain
Two amino acids join when the carboxyl group of one reacts with the amino group of the next. The reaction releases a molecule of water and leaves an amide bond, the peptide bond, between them. Repeating that step builds a chain with a regular repeating backbone (nitrogen, alpha carbon, carbonyl carbon) and a side chain hanging off every alpha carbon.
Because each link consumes one amino group and one carboxyl group, the finished chain has a free amino group at one end and a free carboxyl group at the other. These are the N-terminus and the C-terminus, and by convention a sequence is always written from the N-terminus to the C-terminus. Once an amino acid is part of a chain it is called a residue, so a peptide made from fifteen amino acids is a 15-residue peptide.
Sequences are written with three-letter codes separated by hyphens (Gly-His-Lys) or with one-letter codes (GHK). The ends are often shown explicitly: H- for the free amine at the N-terminus and -OH for the free acid at the C-terminus, or -NH2 when the C-terminus is an amide.
The peptide bond has partial double-bond character, which holds the atoms around it in a flat plane and limits how the backbone can twist. That constraint, together with the side chains, decides which shapes a chain can take. It is covered in more depth in a separate article on the peptide bond in this series.
How long is a peptide?
There is no single agreed cut-off between a peptide and a protein. A 2021 review of short peptides in Molecules compared the definitions in use and found them inconsistent: IUPAC describes oligopeptides as having fewer than about 10 to 20 residues and polypeptides as longer, other dictionaries stretch oligopeptides to about 40 residues, and the authors themselves proposed that short peptides should not exceed 45 amino acids.
For regulatory purposes in the United States the line is drawn precisely. Under 21 CFR 600.3, the FDA defines a protein as "any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size." Chains that are associated with each other as they are in nature are counted together, so insulin, with a 21-residue A chain and a 30-residue B chain joined by disulfide bonds, counts as 51 amino acids and falls on the protein side of that definition.
| Term | Typical length | Example |
|---|---|---|
| Dipeptide | 2 residues | Carnosine (beta-alanyl-histidine) |
| Tripeptide | 3 residues | Glutathione, GHK |
| Oligopeptide | A few to roughly 20 residues | Oxytocin (9), BPC-157 (15) |
| Polypeptide | Longer chains, roughly 20 residues and up | Glucagon (29), amylin (37) |
| Protein | Usually defined as above 40 or 50 residues, often folded | Insulin (51 across two chains), growth hormone (191) |
In practice, the words describe a range rather than hard categories, and a paper may call the same 30-residue molecule a peptide in one sentence and a polypeptide in the next.
Examples of well-characterized peptides
The table below lists peptides whose identities are well documented in public databases. Lengths for the hormones come from the UniProt records of their precursor proteins; formulas and molecular weights come from PubChem.
| Peptide | Length | Sequence or structure note | Molecular formula | Molecular weight (g/mol) |
|---|---|---|---|---|
| Glutathione | 3 | gamma-Glu-Cys-Gly (side-chain linkage) | C10H17N3O6S | 307.33 |
| GHK | 3 | Gly-His-Lys | C14H24N6O4 | 340.38 |
| Oxytocin | 9 | Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, 1-6 disulfide | C43H66N12O12S2 | 1007.2 |
| Arg-vasopressin | 9 | Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2, 1-6 disulfide | C46H65N15O12S2 | 1084.2 |
| BPC-157 | 15 | Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val | C62H98N16O22 | 1419.5 |
| Glucagon | 29 | Cleaved from proglucagon | — | — |
| GLP-1 (7-37) | 31 | Cleaved from proglucagon | — | — |
| Amylin (IAPP) | 37 | Cleaved from its 89-residue precursor | — | — |
A few of these show how varied peptides are. Glutathione is a tripeptide, but its first bond runs through the side-chain carboxyl of glutamic acid rather than the usual alpha carboxyl, so it is not made on the ribosome at all. Oxytocin and vasopressin are nine residues long and are ring-shaped: a disulfide bond between the cysteines at positions 1 and 6 closes a loop, and the C-terminus is an amide rather than a free acid. GHK was first isolated from human plasma in 1973 and binds copper(II), and the copper complex, GHK-Cu, is a common research reagent in its own right.
BPC-157 is a synthetic 15-residue peptide first described by Sikiric and colleagues in 1993 as derived from a protein in human gastric juice. It is not approved as a drug by the FDA. Its sequence contains four prolines, whose ring-shaped side chains restrict how the backbone can rotate. Anhydrolabs lists it as BPC-157 for in-vitro work.
Where peptides come from in nature
Most natural peptides are not made as short chains. The ribosome builds a larger precursor protein, and enzymes then cut it into smaller active pieces. Proglucagon, a 180-residue precursor, is a clear example: its UniProt record lists glucagon (29 residues), GLP-1 in its 7-37 form (31 residues), GLP-2 (33 residues), oxyntomodulin and glicentin, all cut from the same parent chain. Different tissues process the same precursor into different peptides.
Pro-opiomelanocortin (POMC) works the same way. Its 267 residues yield corticotropin (39 residues), alpha-melanocyte-stimulating hormone (13), beta-endorphin (31) and Met-enkephalin (5), among others.
Other peptides are assembled without the ribosome, by dedicated enzymes. Glutathione is one; many antibiotics made by bacteria and fungi are others. These often contain D amino acids, unusual linkages or ring structures that the ribosome cannot produce.
A third source is digestion. Proteases break dietary and tissue proteins into shorter fragments, and some of those fragments have measurable activity in laboratory assays.
The main classes of peptides
Peptides are grouped in several overlapping ways. None of these schemes is exclusive, so a single molecule often belongs to more than one class.
| Classification | Groups | Examples |
|---|---|---|
| By length | Dipeptides, tripeptides, oligopeptides, polypeptides | Carnosine, glutathione, oxytocin, glucagon |
| By topology | Linear, cyclic (head-to-tail or disulfide-bridged), branched | BPC-157 (linear), oxytocin (disulfide ring) |
| By origin | Ribosomal from a precursor, non-ribosomal, synthetic | Glucagon, glutathione, research analogs |
| By role in the source organism | Hormones, neuropeptides, antimicrobial peptides, growth factors | Insulin, beta-endorphin, defensins |
| By modification | C-terminal amides, N-acetylation, lipidation, metal complexes | Oxytocin (amide), GHK-Cu (copper complex) |
Modifications matter for handling as much as for biology. A C-terminal amide removes a negative charge. A fatty-acid side chain makes a peptide bind albumin and changes its solubility. A metal complex such as GHK-Cu has a characteristic blue color in solution and is sensitive to chelating agents in buffers.
Peptides versus proteins
Length is the headline difference, but it is not the only one. Most proteins fold into a stable tertiary structure held together by many weak interactions along the chain, and that fold is what gives them enzymatic or structural function. Short peptides usually have no single stable fold in water; they sample many shapes and may only take a defined structure when bound to a receptor or a membrane.
The difference also shows in how they are made. Short and medium peptides are made chemically by solid-phase synthesis. Proteins of a few hundred residues are almost always expressed in cells, because chemical synthesis becomes impractical at that length. A later article in this series goes further into where that line falls.
How research peptides are made and supplied
Research peptides are made by solid-phase peptide synthesis. The first amino acid is anchored to an insoluble resin bead, and each following residue is added in turn, with protecting groups keeping the side chains from reacting out of order. When the chain is complete, it is cleaved from the resin, the protecting groups are removed, and the crude material is purified by reversed-phase HPLC. Identity is confirmed by mass spectrometry, and purity is reported as the area of the principal peak on the HPLC trace.
The purified peptide is then freeze-dried into a solid. A lyophilized peptide is far more stable than the same peptide in solution, because the water that drives hydrolysis and microbial growth has been removed. Anhydrolabs supplies its catalog as lyophilized powder in vacuum-sealed vials, and as 10-vial kits for laboratories that run the same compound across many experiments.
Two practical consequences follow for the laboratory. First, a peptide should be kept dry, cold and away from light until it is needed; the article on how to store peptides covers temperatures and moisture. Second, preparing a stock solution depends on the sequence: a highly charged peptide may dissolve in water, while a hydrophobic one may need a small amount of organic solvent first. The guide to how to reconstitute peptides walks through choosing a solvent for in-vitro work.
What "research use only" means for peptides
Every peptide in the Anhydrolabs catalog is sold for laboratory research: in-vitro assays, analytical method development and similar work. Anhydrolabs is not a pharmacy, and its peptides are not for human or veterinary diagnosis, treatment, or consumption. The research-use page sets out the terms, and the article on research peptides explains how the label is applied and why it matters for a supplier.
More articles on peptide chemistry, from bonds to synthesis, are collected under peptide science.