Which types of secondary structure are favored due to peptide bond planarity?

• A. Random coils
• B. Beta turns only
• C. Triple helices
• D. Alpha helices and beta sheets

Answer: (D)

Peptide bond planarity locks the backbone in place because the bond has partial double-bond character from resonance, which severely restricts rotation around it. That rigidity forces the backbone to adopt regular angles that can maximize hydrogen bonding between backbone atoms. The two main patterns that emerge from these constraints are the alpha helix and the beta sheet. In an alpha helix, the backbone forms a right-handed coil with hydrogen bonds forming between the carbonyl oxygen of one residue and the amide hydrogen four residues away, producing a stable, repeating structure. In beta sheets, extended strands align beside each other so hydrogen bonds connect between strands, creating a sheet-like arrangement. Random coils lack a consistent hydrogen-bonding pattern and aren’t favored by the need for regular backbone geometry, while beta turns and triple helices are not the general, planarity-driven outcomes for typical protein secondary structure.