What is a peptide? A plain explanation

A peptide is a short chain of small building blocks called amino acids, joined end to end. When the chain gets long and folds into a working shape, chemists usually call it a protein instead. That is the whole idea; the rest of this page fills in the detail.

Amino acids: the building blocks

An amino acid is a small molecule with two "ends" that can link up and a side part that gives it its own character. One end is called the amino group and the other is called the carboxyl group. The side part, often called the side chain, is what makes each amino acid different. Some side chains are small, some are bulky, some carry an electric charge and some avoid water.

Living things mostly build their peptides and proteins from a set of 20 common amino acids. Chemists give each one a three-letter and a one-letter code. Glycine, for example, is "Gly" or "G", and alanine is "Ala" or "A". These codes let you write a whole chain as a short string of letters.

The peptide bond: how the links join

Two amino acids join when the carboxyl end of one reacts with the amino end of the next. A small molecule of water is released, and the link that forms is called a peptide bond. This is the same link whether the chain is two units long or two thousand.

Because each amino acid still has a free end after it joins, the chain can keep growing, one unit at a time. A chain always has a direction. One end, the N-terminus, has a free amino group. The other end, the C-terminus, has a free carboxyl group. By habit, chemists write sequences starting from the N-terminus.

Chain length: from dipeptide to protein

Short chains have simple names. Two amino acids make a dipeptide, three make a tripeptide, and a chain of up to about 20 is often called an oligopeptide ("oligo" means "a few"). Longer chains are called polypeptides.

So where does a peptide stop and a protein start? Roughly, peptides are short chains and proteins are long ones that fold into a stable three-dimensional shape. Many chemists draw the line loosely at around 50 amino acids. That number is a convention, not a law of nature. A chain of 45 units and a chain of 55 units are built in exactly the same way, and different textbooks and fields put the line in slightly different places. Shape matters too: a long chain that folds into a fixed structure behaves like a protein, while a short chain is often floppy and changes shape freely.

Sequence: why the order matters

The order of the amino acids along the chain is called the sequence. Two peptides can contain exactly the same amino acids in the same amounts and still be different molecules, because the order is different. Think of letters in a word: "listen" and "silent" use the same letters, but they are not the same word.

The sequence decides how the chain folds, which parts face outward, how it dissolves in water and how stable it is. Changing even one amino acid, or swapping two neighbours, makes a new molecule with its own properties. This is why a peptide is described by its exact sequence, and why checking that a sample has the right sequence matters so much in a lab.

How synthetic peptides are made

Most peptides made in labs today are built by a method called solid-phase peptide synthesis. The first amino acid is fixed to a tiny plastic bead that sits in a reaction vessel. The next amino acid is added with its own reactive end temporarily covered by a "protecting group", so it can only join in one place. After it joins, the protecting group is removed and the cycle repeats: add, join, wash, uncover. Because the growing chain stays stuck to the bead, leftover chemicals can simply be washed away after each step. When the chain is complete, a strong acid cuts it free from the bead and removes the remaining protecting groups. The result is a crude mixture that still needs to be cleaned up, because no step works perfectly every time and some chains come out short or flawed.

Why purity and identity are measured

Because synthesis is never perfect, a finished sample is usually a mix of the intended peptide and closely related by-products, such as chains missing one amino acid. Two questions matter: how much of the sample is the intended peptide (purity), and is it really the intended peptide at all (identity). Two standard lab methods answer them.

HPLC (high-performance liquid chromatography). The sample is dissolved and pushed under pressure through a narrow column packed with fine material. Different molecules stick to that material by different amounts, so they come out of the column at different times. A detector draws a graph with a peak for each substance. The size of the main peak compared with all the peaks gives an estimate of purity, usually written as a percentage. HPLC is good at showing how clean a sample is, but on its own it does not prove what the main peak actually is.

Mass spectrometry. This method measures the mass of molecules very precisely. Each peptide sequence has a mass that can be worked out in advance by adding up its amino acids. If the measured mass matches the expected mass, that is good evidence the sample contains the intended amino acids, though a chain with the same amino acids in a different order has the same mass. If it does not match, something is wrong, such as a missing or extra amino acid. Labs often use HPLC and mass spectrometry together: one to judge purity, the other to check identity.

Where the rules come in

The chemistry above is the same everywhere, but the law looks at peptides as more than chemistry. Health Canada's April 2026 public advisory puts it plainly:

In Canada, peptides are generally regulated as prescription drugs.

Health Canada public advisory, 9 Apr 2026, retrieved 4 October 2026.

What that means in practice, and what it does and does not cover, is set out on our page Are peptides legal in Canada?.

What this means for you

A peptide is a chain of amino acids. Its identity comes from its exact sequence, and a chain of roughly 50 or more units is usually called a protein, by convention rather than by a hard rule. Lab-made peptides are built one unit at a time on a solid support, and their purity and identity are checked with HPLC and mass spectrometry. Knowing these terms makes it easier to read a lab report or a scientific paper with a clear eye. For how Canadian law handles peptides, read Are peptides legal in Canada?.

The chemistry on this page (amino acids, peptide bonds, solid-phase synthesis, HPLC and mass spectrometry) is standard textbook material in introductory biochemistry and analytical chemistry. For the synthesis steps in a regulator's own words, see how synthetic peptides are made.

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