What are peptides, and how do they work?

A peptide is a short chain of amino acids joined end to end. A protein is built the same way, only much longer. That is the core of it, and most of what makes peptides useful in research follows from their size.

Amino acid, peptide, protein

The boundary is a convention, not a law of chemistry. Many textbooks put it near 50 amino acids, while US FDA regulations define a peptide as 40 or fewer. Insulin, at 51, gets called both.

How the chain forms

Each amino acid has an amino group at one end and a carboxyl group at the other. When two link, those groups join and release a molecule of water. The new connection is called a peptide bond, and it is where the name comes from.

Living things use 20 standard amino acids, and the order is everything. Even a chain of five has 3.2 million possible sequences (20 × 20 × 20 × 20 × 20), and swapping one position can change what a peptide binds to. Chemists can also alter the ends of the chain. TB-500, for example, is the seven-unit sequence LKKTETQ with an acetyl group added to its starting end.

How peptides act on cells

Most peptides are messengers. They are generally too large and too polar to slip through a cell membrane, so they tend to work from the outside, binding a receptor on the cell surface. The receptor then relays the message inward. The usual comparison is a key and a lock: the fit is specific, so a small amount of the right peptide can produce a large, targeted signal.

Nature uses this in many ways:

• Hormones: insulin and GLP-1 carry signals through the blood.
• Nervous system messengers: oxytocin acts on receptors in the brain and elsewhere.
• Immune chemistry: LL-37 belongs to a family of peptides that damage microbes.
• Antioxidant chemistry: glutathione, only three units long, is a major antioxidant inside cells.

Why researchers work with them

Peptides sit between small molecules and large proteins, and they borrow strengths from both. They can be highly selective for one target, and they are small enough to make by chemical synthesis and to modify one position at a time.

They also have clear limits. Enzymes called proteases cut them apart, many clear quickly from living systems, and most cross membranes poorly. Much of modern peptide chemistry is about working around those problems. More than 80 peptide drugs have reached the market worldwide, but approved drugs are a separate category from research compounds: products sold by Omnix Bioscience are for laboratory research only.

How research peptides are made

1. Synthesis. Most are built by solid-phase peptide synthesis, a method that earned Bruce Merrifield the 1984 Nobel Prize in Chemistry. The chain grows one amino acid at a time on tiny resin beads.
2. Purification. The crude product is cleaned up by HPLC, which separates the target from shortened or damaged chains.
3. Freeze-drying. The purified peptide is lyophilized into the powder you see in a vial, because dry peptide is far more stable than peptide in solution.
4. Testing. Purity and identity are checked and recorded on a certificate of analysis.

To put that powder to work, see the reconstitution math and how to store it.

Quick answers

• Are peptides natural or synthetic? Both. Living things make thousands of them, and the same sequences, or modified versions, can be made in a laboratory.
• Are collagen peptides the same thing? Collagen is a large protein. Collagen peptides are fragments of it, broken down by enzymes.
• Are peptides steroids? No. They are a different class of molecule, as explained in peptides vs steroids vs GLP-1s.

Sources

1. Muttenthaler M, et al. Trends in peptide drug discovery. Nat Rev Drug Discov, 2021. Source
2. Wang L, et al. Therapeutic peptides: current applications and future directions. Signal Transduct Target Ther, 2022. Source
3. Lau JL, Dunn MK. Therapeutic peptides: historical perspectives, current development trends, and future directions. Bioorg Med Chem, 2018. Source
4. US Code of Federal Regulations, 21 CFR 314.3 (definition of peptide). Source