Biochemistry—α-Helix Geometry Which statement correctly describes hydrogen bonds in α-helices?

Introduction / Context:
The α-helix is stabilized by a regular pattern of backbone hydrogen bonds. Understanding their orientation and nature is fundamental for predicting helix stability and behavior.

Given Data / Assumptions:

• Backbone N-H donors and C=O acceptors form i → i+4 hydrogen bonds.
• Side-chain identities generally do not interrupt the backbone H-bond pattern, though they can modulate stability.
• Helical hydrogen bonds align approximately parallel to the helix axis.

Concept / Approach:
Identify which option aligns with the canonical i → i+4 backbone hydrogen bonding geometry and orientation relative to the helix axis.

Step-by-Step Solution:

Verification / Alternative check:
Helical wheel and cylinder diagrams show H-bond vectors with a significant axial component; crystallographic structures confirm repeating i → i+4 contacts.

Why Other Options Are Wrong:

• Absent at Phe residues: false; backbone H-bonds are sequence-agnostic.
• Stair-step analogy emphasizes shape, not H-bond alignment; the axial description is more accurate.
• “Negligible” contradicts their central stabilizing role.
• Side-chain H-bonds are not the defining pattern of α-helices.

Common Pitfalls:
Confusing side-chain interactions with backbone hydrogen bonding; over-interpreting side-chain effects as obligatory breaks.

Final Answer:
They are roughly parallel to the helix axis and stabilize the helical cylinder.