What are peptides?

Peptides are short chains of amino acids. That’s the whole definition. The interesting parts are what happens inside that chain and at its edges — because those details determine whether a peptide is a signaling molecule, a structural fragment, a hormone, an antibiotic, or none of the above.

Amino acids, chains, and bonds

Every peptide starts with amino acids — small organic molecules with a specific shape that lets them link together. When two amino acids join, the reaction produces a peptide bond and releases a water molecule. Chain two amino acids, you have a dipeptide. Three, a tripeptide. Longer than that and biology usually calls it a peptide until around 50 residues, at which point it starts being called a protein. That boundary is fuzzy and not biologically meaningful — just a convention.

What’s meaningful is the sequence. Twenty canonical amino acids mean the number of possible 20-residue sequences is 20²⁰ — more than a million trillion. The sequence determines how the peptide folds, what it binds to, how quickly it’s broken down, and whether it triggers any biological response at all.

Where peptides come from

Peptides come from three general sources:

1. Natural enzymatic cleavage. Your body routinely cuts larger proteins into smaller functional fragments. Insulin starts life as a longer precursor. Many hormones (GLP-1, ghrelin, ACTH) are fragments of larger parent proteins.
2. De novo synthesis. Ribosomes can produce peptides directly from small genes. Humanin, MOTS-c, and other mitochondrial-derived peptides are examples.
3. Chemical synthesis in a lab. Commercial peptide synthesis builds chains one amino acid at a time using automated solid-phase methods. This is how most research and pharmaceutical peptides are manufactured.

What peptides do

Peptides act as signaling molecules. They bind to receptors on cell surfaces — typically G-protein-coupled receptors — or occasionally intracellular targets. Because receptors are picky about shape, small changes in a peptide’s sequence can change what it binds to, how tightly, and for how long.

Examples that show the range:

• Insulin (51 residues): regulates blood glucose.
• Oxytocin (9 residues): affects smooth muscle contraction and social behavior.
• GLP-1 (~30 residues depending on form): stimulates insulin release and slows gastric emptying.
• Magainin (23 residues): an antimicrobial peptide found in frog skin.

Why they’re tricky as drugs

Peptides have two features that make them both attractive and difficult as medicines. They are often specific and potent — they can target a receptor that a small molecule can’t reach. But they are also:

• Poorly absorbed by mouth. Stomach acid and digestive enzymes break most peptides apart before they reach the bloodstream. This is why most peptide drugs are injected.
• Short-lived. Many peptides have half-lives measured in minutes. Engineering longer half-lives — by modifying sequences, attaching fatty acids, or bolting on polymers — is a major area of pharmaceutical chemistry.
• Expensive to make. Synthesis costs scale with length and with required purity.

Why a research-first site matters

The word “peptide” covers FDA-approved medicines like semaglutide, unapproved research compounds like BPC-157, and marketing acronyms for vendor blends that are none of the above. Reading peptide in a product label tells you almost nothing about whether the molecule is safe, tested, or legal.

That’s the work this site is trying to do: separating what the literature actually says from what the marketing says.