Recall the structural feature of an $\alpha$-helix

The $\alpha$-helix is a common secondary structure of proteins. Its stability comes mainly from intramolecular hydrogen bonding along the peptide backbone.

Identify the specific interaction involved

In an $\alpha$-helix, the carbonyl oxygen $\left(\mathrm{C=O}\right)$ of one peptide bond forms a hydrogen bond with the amide hydrogen $\left(\mathrm{N-H}\right)$ of another peptide bond further along the chain.

This regular pattern of hydrogen bonding stabilizes the helical structure.

Evaluate the options

• A: dipolar interaction
  Dipolar interactions may exist, but they are not the main stabilizing force for the $\alpha$-helix.

• B: H-bonding interaction
  This is the correct stabilizing interaction for the $\alpha$-helix.

• C: van der Waals forces
  These can contribute generally to protein stability, but they are not the primary reason for $\alpha$-helix stability.

• D: $\pi$-stacking interaction
  $\pi$-stacking is relevant for aromatic rings, not for the backbone stabilization of an $\alpha$-helix.

Conclusion

Therefore, the stability of the $\alpha$-helix structure of proteins depends upon hydrogen bonding interactions.