The Short Answer
A peptide bond is a covalent bond linking two amino acids. It forms when the carboxyl group of one amino acid reacts with the amino group of another, releasing water. This condensation reaction creates an amide bond (–CO–NH–) that holds together every protein in every living organism.
How It Forms
The nitrogen of the amino group acts as a nucleophile, attacking the electrophilic carbon of the carboxyl group. The ribosome catalyzes this reaction using 23S rRNA (a ribozyme). Energy comes from aminoacyl-tRNA.
For n amino acids, there are (n−1) peptide bonds and (n−1) water molecules released. A 100-AA protein has 99 peptide bonds.
Why It's Special
The peptide bond has ~40% double-bond character from resonance: the nitrogen lone pair delocalizes into the C=O system. Three consequences: (1) the bond is planar (6 atoms coplanar), (2) it's shorter than a single bond (1.33 vs 1.47 Å), and (3) nitrogen is sp² hybridized.
'The peptide bond is planar because of hydrogen bonding' — WRONG. H-bonds stabilize secondary structure. Planarity comes from resonance.
Trans vs Cis
Trans is favored ~1000:1 because R groups on opposite sides minimize steric clashes. Exception: X-Pro bonds (~30:1) because proline's ring reduces the steric difference.
Breaking the Bond
Hydrolysis breaks peptide bonds. Proteases catalyze this: trypsin (after Arg/Lys), chymotrypsin (after Phe/Trp/Tyr), pepsin (hydrophobic residues, pH 2). Without enzymes, half-life is 350–600 years (metastable: thermodynamically unstable but kinetically stable).
Why It Matters
Every protein drug — from insulin to semaglutide — is designed around peptide bond chemistry. Understanding formation, rigidity, and cleavage is fundamental to all of biochemistry and drug design.