AQA A-Level Chemistry Paper 3, June 2022: Question 32

✅ Correct Answer

B — hydrogen bonds

The α-helix is held rigidly in shape by hydrogen bonds formed between the partially positive hydrogen of the N–H group of one peptide link and the lone pair on the partially negative oxygen of the C=O group of another peptide link four residues earlier along the chain.

💡 Key Knowledge: Protein Hierarchy

• Primary (1°): Sequence of amino acids joined by covalent peptide bonds ( –CONH– ).
• Secondary (2°): Local folding into an α-helix or β-pleated sheet, exclusively maintained by hydrogen bonds between backbone C=O and N–H groups.
• Tertiary (3°): 3D folding caused by interactions between R-group side chains:
  • Disulfide bridges (covalent)
  • Ionic bonds / salt bridges
  • Hydrogen bonds
  • van der Waals (dispersion) forces

🧠 Exam Technique

• Spot the structural level: The prompt specifies "secondary structure". Secondary structure is always stabilised by hydrogen bonding between the main polypeptide backbone.
• Eliminate tertiary interactions: Disulfide bridges (covalent), ionic interactions, and van der Waals forces between non-polar side chains are responsible for the tertiary structure, not the secondary structure.
• Speed strategy: Recall questions like this in Section B should take less than 30 seconds, leaving valuable time for multi-step organic synthesis or titration calculations.

❌ Common Errors & Trap Options

• Confusing Secondary and Tertiary: Selecting C (ionic interactions) or D (van der Waals forces) because they stabilise proteins — but these stabilise 3° structure, not 2°.
• Confusing Peptide Backbone Bonds with Secondary Stabilisation: Selecting A (covalent bonds). Covalent peptide bonds link the amino acids together into a chain (primary structure), but they do not fold it into an α-helix.
• Misattributing Hydrogen Bonding to Side Chains: In secondary structures, hydrogen bonds involve the backbone amide links, not side-chain R groups.