The Peptide Bond

The peptide bond is the amide linkage that connects one amino acid's α-carboxyl group to the next amino acid's α-amine, releasing water in the process. It is the most fundamental link in biology — every protein in every living organism is held together by peptide bonds.

Formation

A peptide bond forms through condensation: the carboxyl carbon attacks the amine nitrogen, displacing water. In the cell, this reaction is catalyzed by the ribosome. In chemical synthesis (solid-phase peptide synthesis), it is driven by coupling reagents like HBTU or HATU.

Geometry

Because of resonance between the carbonyl oxygen and the amide nitrogen, the peptide bond has partial double-bond character. Six atoms lie in one plane: the two α-carbons on either side, plus the carbonyl carbon, oxygen, amide nitrogen, and amide hydrogen. Rotation around the bond itself is restricted to two main configurations — trans (overwhelmingly preferred) and cis (mostly observed before proline residues).

Consequence for structure

Planarity constrains the polypeptide chain. Between any two peptide bonds, the chain can rotate around two bonds: the N-Cα bond (the phi angle) and the Cα-C bond (the psi angle). The permissible combinations of phi/psi are plotted on a Ramachandran diagram, and they predict secondary structures — alpha helices cluster in one region, beta sheets in another.

Pharmacological implications

Because the peptide bond is cleaved by ubiquitous proteases in the GI tract, blood, and cells, most native peptides have short biological half-lives. Modern peptide drug design spends considerable effort on protecting peptide bonds: D-amino-acid substitution, N-methylation, cyclization, and fatty-acid conjugation for albumin binding all extend half-life by shielding peptide bonds from proteolytic attack.