A peptide bond is the covalent amide (–C(O)–NH–) linkage that joins consecutive amino-acid residues in a peptide or protein. It is formed by a condensation (dehydration) reaction between the α-carboxyl of one amino acid and the α-amino of the next, releasing one molecule of water.
The peptide bond has partial double-bond character due to delocalisation of the carbonyl π-electrons onto the nitrogen. As a result the bond is planar and predominantly adopts the trans-configuration, restricting rotation and constraining backbone conformation to the φ / ψ angles described by the Ramachandran plot.
Peptide-bond geometry determines backbone folding and, together with side-chain chemistry, sets the conformational landscape a peptide can occupy — a prerequisite for receptor binding.