What Is a Peptide? Structure, Bond and Examples

A peptide is a molecule made of two or more amino acids joined by peptide bonds. The chain has a defined direction (N-terminus to C-terminus) and a defined sequence, which together determine its biological behaviour.

A peptide bond is an amide linkage formed when the carboxyl group (–COOH) of one amino acid reacts with the amino group (–NH₂) of the next, releasing a molecule of water. This condensation reaction is the chemistry behind every protein and peptide in biology.

Two-residue chains are dipeptides, three are tripeptides, and so on. The umbrella term 'oligopeptide' usually covers chains up to about ten residues; 'polypeptide' covers longer chains; and once a chain folds into a stable, functional 3D structure it is conventionally called a protein. The boundary is convention, not a hard chemical line.

Familiar peptides include insulin (51 amino acids, often classified as a small protein), oxytocin (9 amino acids), glutathione (3 amino acids), and modern research peptides such as semaglutide, tirzepatide, BPC-157 and GHK-Cu. Each of these has a precisely defined sequence that determines its behaviour.

Because peptide identity is defined by sequence, two peptides with the same molecular weight but different sequences are not interchangeable. This is why research-grade peptides should always be verified by mass spectrometry and HPLC, with a lot-matched COA, before any quantitative work.