peptide bond

Meaning: A peptide bond is an amide linkage between the α-carboxyl group of one amino acid and the α-amino group of the next. It forms by condensation (dehydration) and is broken by hydrolysis. Resonance shares the nitrogen lone pair with the carbonyl, giving the C–N bond partial double-bond character. The six atoms around it (two α-carbons, C, O, N, H) lie in one plane, and the trans arrangement is strongly favored; bonds preceding proline adopt cis more often than others, though trans still predominates. Rotation occurs about the N–Cα bond (φ) and Cα–C bond (ψ), as in a Ramachandran plot.

Equation: R–COOH + R′–NH₂ → R–CO–NH–R′ + H₂O, where R and R′ are the rest of each amino acid.

On the MCAT: Expect questions that count bonds (a linear chain of n amino acids has n − 1 peptide bonds and releases n − 1 H₂O), name the N-terminus and C-terminus, or predict hydrolysis products with a protease or strong acid. The planar, rigid bond also limits backbone folding.

Common confusion: Denaturation by heat, urea or pH change disrupts noncovalent interactions, not peptide bonds, so primary structure stays intact. The amide nitrogen is not basic, so peptide charge comes from the termini and side chains.