Peptide Bond

The peptide bond is the covalent linkage that connects two consecutive amino acids within a peptide or protein. Chemically, it is an amide bond formed by reaction between the carboxyl group (-COOH) at the C-terminus of one amino acid and the primary amine (-NH2) at the N-terminus of the next. This condensation releases a water molecule (H2O) and creates a rigid C-N bond: it is the polycondensation reaction that generates the entire diversity of peptides and proteins.

The peptide bond displays three major structural features. First, its planarity: the six involved atoms (Calpha-C(=O)-N-Calpha of both residues) lie in a single plane, due to resonance that confers partial double-bond character to the C-N link. This planarity imposes strong steric constraints on peptide conformation, restricting rotation to the phi (Calpha-N) and psi (Calpha-C) bonds. The Ramachandran plot maps these allowed angles and explains why peptides preferentially adopt alpha-helix, beta-sheet, or turn secondary structures.

Second, the peptide bond is almost exclusively in trans configuration (omega angle approximately 180 ), except for proline residues where cis configurations reach 5-10% frequency. This trans preference minimizes steric clash between side chains. Third, the bond is chemically highly stable: its cleavage requires either strong acid hydrolysis (6M HCl, 110 C, 24h) or specialized enzymes called proteases (trypsin, chymotrypsin, pepsin, cellular peptidases).

In solid-phase peptide synthesis (SPPS, Merrifield method), the peptide bond is formed by chemical activation of the carboxyl via coupling reagents such as HBTU, HATU, or DIC/HOBt, followed by nucleophilic attack from the free amine. Each coupling must exceed 99.5% yield to produce >95% HPLC-purity peptide without exponential accumulation of impurities. Protecting groups (Fmoc on the amine, tert-butyl or trityl on side chains) prevent side reactions during chain elongation.

Enzymatic cleavage of peptide bonds is the key to proteolysis: trypsin specifically cleaves after lysine or arginine, chymotrypsin after aromatic residues (Phe, Trp, Tyr), while aminopeptidases and carboxypeptidases trim the extremities. This knowledge enables predicting peptide in vivo half-life and designing modifications (cyclization, D-amino acids, Aib, PEGylation) that stiffen the backbone and resist proteolysis.