Explain resonance stabilization and how it affects peptide bond planarity and protein secondary structure.

Explain resonance stabilization and how it affects peptide bond planarity and protein secondary structure.

• A. Single bond with free rotation around the C-N.
• B. Partial double-bond character from resonance makes the C-N bond planar.
• C. Triple bond character restricts rotation.
• D. No resonance effect on rotation.

Answer: (B)

Resonance stabilization gives the amide C–N bond partial double-bond character, so it behaves more like a double bond than a single bond. This delocalization of electrons across the carbonyl and amide nitrogen restricts rotation around the peptide bond, making it effectively planar. That planarity fixes the backbone geometry so that the atoms involved lie in a single plane, which in turn sets the allowed phi and psi dihedral angles of the peptide chain. With this rigid, planar backbone, the chain can align in regular, repeating patterns that support backbone hydrogen bonding between neighboring units, giving rise to protein secondary structures like alpha helices and beta sheets. While the bond geometry favors trans configurations for most peptide bonds, the key idea is that the resonance-induced partial double-bond character is what immobilizes the peptide bond and underpins the characteristic geometry of protein structure.

Resonance stabilization gives the amide C–N bond partial double-bond character, so it behaves more like a double bond than a single bond. This delocalization of electrons across the carbonyl and amide nitrogen restricts rotation around the peptide bond, making it effectively planar. That planarity fixes the backbone geometry so that the atoms involved lie in a single plane, which in turn sets the allowed phi and psi dihedral angles of the peptide chain. With this rigid, planar backbone, the chain can align in regular, repeating patterns that support backbone hydrogen bonding between neighboring units, giving rise to protein secondary structures like alpha helices and beta sheets. While the bond geometry favors trans configurations for most peptide bonds, the key idea is that the resonance-induced partial double-bond character is what immobilizes the peptide bond and underpins the characteristic geometry of protein structure.