What are peptides? A beginner's guide

Peptides come up constantly in biochemistry, but the basics are rarely explained in one place. This guide covers what a peptide is, how peptides differ from proteins, how research peptides are made, and why they arrive as a dry powder. A short glossary at the end defines the terms you'll see on product pages and certificates of analysis.

What a peptide is

A peptide is a short chain of amino acids joined end to end. Amino acids are small organic molecules, each with an amino group, a carboxyl group and a side chain that gives it its character. There are twenty standard amino acids, and the order in which they're linked is the peptide's sequence.

The link between two amino acids is called a peptide bond. It forms when the carboxyl group of one amino acid reacts with the amino group of the next, releasing a molecule of water. Chain enough of these together and you have a peptide.

Every peptide has two ends. The N-terminus is the end with a free amino group, and the C-terminus is the end with a free carboxyl group. Sequences are written from N to C by convention.

Peptides and proteins

Peptides and proteins are made of the same building blocks. The difference is mainly size, and the dividing line is a convention rather than a rule:

• Peptides are generally chains of up to about 50 amino acids. GHK-Cu is a copper complex of a three-amino-acid peptide; BPC-157 is fifteen amino acids long.
• Proteins are longer chains, often hundreds of amino acids, that fold into complex three-dimensional shapes.

Some molecules sit right on the boundary. Insulin, at 51 amino acids across two chains, is described as both a peptide hormone and a small protein.

Size affects how a molecule is made and handled. Short peptides can be built entirely by chemical synthesis, while most proteins are produced in living cells using recombinant methods.

How research peptides are made

Most research peptides are produced by solid-phase peptide synthesis (SPPS), a method developed in the 1960s that is still the standard today. The idea is simple: build the chain on a tiny plastic bead so it can be washed between steps without being lost.

1. The first amino acid, the one at the C-terminus, is attached to a resin bead.
2. A protecting group on its amino end is removed.
3. The next amino acid, with its own amino end protected, is coupled on.
4. Excess reagents are washed away, and the cycle repeats until the full sequence is built.
5. The finished chain is cleaved from the resin and its remaining protecting groups are removed.

Each cycle is highly efficient but not perfect. A small fraction of chains miss a step, which is where truncated and deletion sequences come from. That's why the crude product is then purified, usually by HPLC, and tested. Our article on peptide purity goes into what those impurities are and how they're measured.

Why peptides are supplied as a powder

In water, peptides slowly break down through reactions such as hydrolysis, oxidation and deamidation. Removing the water slows those reactions dramatically. So after purification, peptides are freeze-dried, or lyophilized, into a light white powder or "cake" and sealed in a vial.

A lyophilized peptide in a sealed vial, kept cold and dark, is far more stable than the same peptide in solution. That's the form in which research peptides are shipped and stored. Our articles on lyophilization and storing sealed vials cover both in more detail.

Where peptides appear in research

Peptides are studied across many fields of biochemistry and cell biology. Some of the areas where the compounds on this site appear in published research:

• Incretin and metabolic signaling research, which includes GLP-1, GIP and glucagon receptor studies.
• Growth hormone secretagogue research, which looks at peptides that interact with GHRH and ghrelin receptors.
• Tissue research, including studies of cell migration and extracellular matrix.
• Mitochondrial and cellular research, including mitochondrial-derived peptides such as MOTS-c.
• Immunology and neuroendocrine research, including thymic peptides and kisspeptin.

These are fields of study, not uses. Peptides sold for research are reagents for laboratory work.

Glossary

• Amino acid: the building block of peptides and proteins.
• Residue: an amino acid once it's part of a chain.
• Sequence: the order of amino acids in a peptide, written N-terminus to C-terminus.
• Peptide bond: the amide bond linking one amino acid to the next.
• SPPS: solid-phase peptide synthesis, the standard way to make peptides chemically.
• Salt form: the counter-ion that comes with the peptide, such as acetate or TFA.
• Lyophilization: freeze-drying; removing water from a frozen sample under vacuum.
• HPLC: high-performance liquid chromatography, used to measure purity. Often coupled to mass spectrometry as LC-MS or LC-MS/MS.
• Mass spectrometry (MS): a method that measures molecular weight to confirm identity.
• COA: certificate of analysis, the lab report for a specific lot.
• Lot number: the identifier that ties a vial to its production run and its COA.

Where to go next

If you're new to the subject, the most useful next step is learning to read a COA, since that's how you check what's actually in a vial. Our COA walkthrough covers it section by section. Every lot we sell is tested by an independent laboratory before sale, and its report is shown on the product page under a "View COA" button.

All products are sold for laboratory research use only and are not for human or veterinary use.