What are peptides?

A short definition

A peptide is a short chain of amino acids joined by peptide bonds. When the chain is long and folds into a stable structure it is usually called a protein; the line between the two terms is a convention rather than a strict boundary.

That is why scientific literature uses the word “peptide” both for molecules of a few amino acids and for others with several dozen.

Amino acids and sequence

Amino acids are the units that make up peptides and proteins. The order in which they are linked is called the sequence, and it is what distinguishes one peptide from another: changing, removing or adding a single amino acid produces a different molecule.

Some peptide names describe their origin or structure; others are acronyms or laboratory codes that say nothing about how they act. It is best not to infer properties from a name.

Where they appear

The body produces many peptides that act as signals between cells, such as some hormones. Oxytocin, with 9 amino acids, and insulin, with 51, are well-known examples.

Research also studies laboratory-designed peptides, sometimes as analogues of natural molecules: versions with a modified sequence used to study how their behaviour changes.

How they are made

Most short research peptides are made by chemical synthesis, adding amino acids one at a time on a solid support. They are then purified and analysed to confirm their identity and purity.

Many are supplied lyophilised, that is, freeze-dried into a dry powder, because they keep better that way than in solution.

Basic vocabulary

Terms you will find in the product sheets and in the other guides:

• Amino acid: the unit that makes up peptides.
• Sequence: the order of amino acids in the chain.
• Analogue: a molecule designed from another one, with changes to its structure.
• Lyophilised: freeze-dried product, in powder form.
• Purity: the proportion of the intended compound relative to other substances in the sample.