Peptide

A peptide is a chain of amino acids linked by peptide bonds. The dividing line between a peptide and a protein is convention rather than chemistry: chains under about fifty residues are called peptides, and anything longer, or anything that folds into a stable three-dimensional structure, is called a protein. The compounds on this site range from three residues (KPV, GHK-Cu) to forty-plus (the tesamorelin GHRH analogue), with a few non-peptides carried for completeness and labelled as such.

Peptides are attractive research compounds because they are the body's own signalling language. Hormones such as insulin and GLP-1 are peptides, and synthetic analogues can be designed to bind the same receptors with a longer half-life or altered selectivity. That is the whole story of the incretin class: semaglutide, tirzepatide and retatrutide are engineered peptides built on a natural hormone's backbone.

The same properties that make peptides useful make them fragile. They are digested in the gut, degraded by enzymes in blood, and sensitive to heat and repeated freezing, which is why research-grade peptides ship lyophilised, are reconstituted before use, and are kept on a cold chain.