What a peptide is
A peptide is a short chain of amino acids joined end to end by peptide bonds. Each link in the chain is called a residue, and the order of residues — the sequence — is written from the N-terminus (the end with a free amino group) to the C-terminus (the end with a free carboxyl group), usually with one-letter codes such as G for glycine or K for lysine.
Your body makes thousands of peptides. Many are hormones and signalling molecules: insulin and glucagon regulate blood sugar, oxytocin is involved in childbirth and lactation, and GLP-1 is released by the gut after a meal. Others are antimicrobial defences, neurotransmitters or fragments cut from larger proteins.
Peptides vs proteins vs small molecules
There is no sharp line between a peptide and a protein. A common convention calls chains of up to about 50 amino acids peptides and longer ones proteins, and proteins usually fold into stable three-dimensional shapes. Small-molecule drugs, like aspirin or metformin, are far smaller still and are built from ordinary organic chemistry rather than amino acids.
| Small molecule | Peptide | Protein | |
|---|---|---|---|
| Typical size | Under ~900 Da | ~2 to 50 amino acids | 50 to thousands of amino acids |
| How it’s made | Organic synthesis | Chemical synthesis or recombinant expression | Usually recombinant expression in cells |
| Swallowed as a pill? | Often works | Usually digested; rare exceptions with special formulations | Almost always digested |
| Target precision | Variable | Often very selective for one receptor | Often very selective |
How peptides work in the body
Most signalling peptides act by binding a receptor on the surface of a cell. A molecule that switches the receptor on is an agonist; one that occupies it without switching it on is an antagonist. Because peptides fit their receptors closely, they can be highly selective — one reason drug developers like them.
The catch is durability. Natural peptides are broken down quickly by enzymes in the blood and tissues, so their half-life is often just minutes. They are also digested in the gut, which gives most of them very low oral bioavailability. That is why most peptide medicines are injected.
How peptide drugs are designed
Much of modern peptide chemistry is about making a natural signal last longer without losing its precision. Common tricks include:
- Swapping residues for unnatural amino acids that protective enzymes cannot cut, such as Aib (α-aminoisobutyric acid).
- Attaching a fatty-acid chain so the peptide binds albumin in the blood and is released slowly — the approach used in liraglutide and semaglutide.
- Adding polymers or linkers such as PEG, or chemistry that binds the peptide to albumin, as in CJC-1295 with DAC.
- Cyclising the chain or shortening it to the minimal active fragment.
25 of the 47 single entries in this library have a defined sequence on file; their fingerprints show each residue’s chemistry, with modified residues highlighted.
How peptides get studied
A new compound usually travels a long road, and most fall off it. Research tends to move through these stages:
- In vitro — cells or purified proteins in the lab show what the molecule can bind and do.
- Animal studies (in vivo) — test effects and toxicity in a living organism, most often rodents.
- Phase 1 — small human studies of safety and pharmacokinetics.
- Phase 2 — first controlled tests of whether it helps patients with a condition.
- Phase 3 — large randomised controlled trials that regulators use to decide on approval.
Many peptides discussed online have stalled at step 1 or 2 for years. Positive animal data are a reason to run human trials, not a substitute for them: results in rodents frequently fail to translate to people.
Where this library’s entries sit
Every single entry carries an evidence tier that reflects how far along that road the published research has travelled:
- Tier S: 8 entries
- Tier A: 17 entries
- Tier B: 7 entries
- Tier C: 15 entries
- Tier D: 0 entries
A low tier means the evidence is thin — not that a compound is useless or dangerous. A high tier means strong evidence exists for a specific use — not that it is right for anyone in particular.