What Are Peptides? A Plain-English Guide

Peptides are short chains of amino acids, the same building blocks that make up proteins. Your body makes thousands of them, and they carry some of its most important messages: insulin, oxytocin and the hormones that control hunger, growth and stress are all peptides.

This guide explains what peptides are, how they differ from proteins and amino acids, what they do in the body, how peptide drugs came about, and what “research peptides” actually means.

The short answer

• A peptide is a chain of amino acids linked by peptide bonds, usually somewhere between 2 and about 50 amino acids long.
• Proteins are longer chains, typically folded into stable three-dimensional shapes. The cut-off between the two is a convention, not a sharp line.
• The body uses peptides as hormones, neurotransmitters, immune defences and growth signals. Insulin, oxytocin, GLP-1 and endorphins are all peptides.
• Since insulin was introduced about a century ago, more than 80 peptide drugs have reached the market, according to a 2021 review in Nature Reviews Drug Discovery.
• Research peptides are synthetic peptides made for laboratory study. They are reagents, not medicines.

Amino acids: the alphabet

Living things build their proteins and peptides from a standard set of 20 amino acids. Each has the same backbone, an amino group at one end and a carboxyl group at the other, plus a side chain that gives it its character: some are oily and avoid water, some carry a positive or negative charge, and a few have special chemistry, such as the sulfur in cysteine that can form bridges between chains.

Scientists write peptides with a one-letter code for each amino acid. Our amino acid code chart lists all 20 with their one-letter and three-letter codes. The copper peptide GHK is glycine, histidine and lysine. Epitalon is AEDG: alanine, glutamic acid, aspartic acid and glycine. Change one letter and you have a different molecule, often with completely different behaviour.

The peptide bond

Amino acids join when the carboxyl group of one reacts with the amino group of the next, releasing a molecule of water. The link that forms is an amide bond, which chemists call a peptide bond. The word itself comes from Emil Fischer, who made the first synthetic dipeptide, glycylglycine, in 1901 and coined “peptide” in 1902, from the Greek for “digested”.

Because every amino acid has a distinct start and end, a peptide chain has a direction: an N-terminus with a free amino group and a C-terminus with a free carboxyl group. The order of amino acids along that chain, the sequence, is what defines a peptide.

Peptides vs proteins

You will also see the word polypeptide. It simply means a longer, unbranched chain of amino acids, usually used for chains of more than about 20 residues, and every protein is made of one or more polypeptide chains. Shorter chains are sometimes called oligopeptides. In practice, “peptide”, “polypeptide” and “protein” describe points along a continuum of length rather than separate categories.

Many natural peptides are cut out of larger precursor proteins. LL-37 is clipped from a protein called hCAP18, and the tripeptide KPV is the tail end of the hormone alpha-MSH, itself cut from a large precursor called POMC. Our guides to LL-37 and KPV follow both stories.

What peptides do in the body

Peptides are the body’s short-range and long-range messages. A few of the main families:

• Hormones. Insulin and glucagon regulate blood sugar. GLP-1 signals fullness after a meal. GHRH and ghrelin control growth hormone release, which our growth hormone peptides map explains.
• Neuropeptides. Endorphins dampen pain, oxytocin shapes social bonding and labour, and kisspeptin starts puberty.
• Immune defences. Antimicrobial peptides such as LL-37 attack microbes directly and also steer immune cells.
• Tissue signals. Small peptides such as the copper complex GHK-Cu have been studied for their effects on collagen and gene expression; see our GHK-Cu guide.

Most peptide signals work by binding a receptor on a cell’s surface, often a G protein-coupled receptor, which then relays the message inside the cell. That lock-and-key specificity is why peptides interest drug developers: they can be extremely selective.

A short history of peptides

1. 1901–1902Emil Fischer synthesizes the first dipeptide and coins the word “peptide”.
2. 1922Insulin, extracted at the University of Toronto, is first used to treat diabetes, the first peptide medicine. Our history of Canada’s century of peptide discovery tells that story.
3. 1953Vincent du Vigneaud’s group synthesizes oxytocin, the first peptide hormone made in a lab. He receives the 1955 Nobel Prize in Chemistry.
4. 1963Bruce Merrifield introduces solid-phase peptide synthesis, building chains on resin beads one amino acid at a time. It earns the 1984 Nobel Prize and still underpins how research peptides are made; see how research peptides are made.
5. 2021A review in Nature Reviews Drug Discovery counts more than 80 approved peptide drugs, over 150 in clinical development and 400 to 600 in preclinical research.

Why peptides make tricky drugs

Peptides are potent and selective, but natural ones are fragile. Enzymes in blood and tissue break many of them down within minutes, and the digestive system treats them like food, which is why most peptide drugs are given by injection rather than as pills. Much of modern peptide chemistry is about solving those problems: swapping in unusual amino acids, capping the ends, attaching fatty-acid chains so the peptide hitches a ride on albumin, or adding polymer coats. Our guide to how peptides are made to last covers those tricks, from CJC-1295’s albumin-binding group to semaglutide’s fatty acid.

What research peptides are

“Research peptides” are synthetic peptides sold as laboratory reagents: for cell studies, receptor assays, analytical work and animal research under appropriate oversight. Some are copies of natural peptides, some are fragments, and some are engineered analogues that were drug candidates and never reached approval. They are not medicines, they are not approved for human or veterinary use, and their quality depends entirely on the supplier’s synthesis and testing. Our guide to research peptides in Canada covers what research-grade should mean and how to verify a vial against its certificate of analysis.

That last point matters. A research peptide is only as useful as the evidence that it is the right molecule at the stated purity. A credible supplier provides a batch-specific certificate of analysis showing identity by mass spectrometry and purity by HPLC. Our guides to the peptide COA, HPLC testing and spotting fake or underdosed peptides explain what to look for. The same checks apply when you buy research peptides in Canada, from us or from anyone else.

Four common misconceptions

Research peptides in our catalogue

Our catalogue runs from very short chains, such as the three-residue GHK-Cu and the four-residue Epithalon, to longer ones such as the 15-residue BPC-157. Each ships lyophilized for laboratory research only, and its Janoshik test report is listed on our Lab Testing page.

Frequently asked questions