Biochemistry—α-Helix Geometry Which statement correctly describes hydrogen bonds in α-helices?
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AThey are roughly parallel to the helix axis and stabilize the helical cylinder.
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BThey are absent at phenylalanine residues because Phe disrupts the backbone pattern.
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CThey are best described as steps of a spiral staircase rather than axial bonds.
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DThey are fewer in number than van der Waals contacts and thus negligible.
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EThey primarily form between side chains on adjacent turns.
Answer
Correct Answer: They are roughly parallel to the helix axis and stabilize the helical cylinder.
Explanation
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:
Step 1: Recognize α-helix H-bonds form between backbone groups, not side chains.Step 2: Visualize the geometry: the hydrogen bonds run roughly parallel to the helix axis, stitching the turns together.Step 3: Select the option that states this orientation explicitly.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.