Peptide bond
The amide linkage between the α-carboxyl of one amino acid and the α-amino group of the next; the structural backbone of all peptides.
For laboratory and research use only — not for human consumption.
A peptide bond is the amide linkage formed by condensation between the α-carboxyl group of one amino acid and the α-amino group of the next, releasing one molecule of water. The resulting C–N bond has partial double-bond character (~40%) due to resonance with the adjacent carbonyl, which constrains rotation around the bond and produces a planar, largely trans peptide-bond geometry that defines protein and peptide backbone conformation. Cis peptide bonds occur predominantly at proline residues (where the cyclic side chain reduces the cis/trans energy difference). The distinction between 'peptide' and 'protein' is historically conventional rather than biochemical — peptides are typically defined as chains of fewer than ~50 amino acid residues, while proteins are longer — but no sharp cut-off exists. The number of amino acids defines naming conventions: dipeptide (2 residues), tripeptide (3, e.g. KPV, GHK), tetrapeptide (4, e.g. epitalon, AC-SDKP), oligopeptide (≤10), polypeptide (>10). The peptide-bond plane is also a primary site of enzymatic proteolysis: pepsin, trypsin, chymotrypsin, and the various endo- and exopeptidases all cleave specific peptide bonds, which is why oral peptide delivery is generally limited unless the molecule is engineered for protease resistance or delivered via a protected route.