Biochemistry—Peptide Bond Resonance Resonance structures drawn for the peptide bond imply which of the following?
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AThe peptide C–N bond is stronger than an ordinary single bond.
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BThe peptide bond has partial double-bond character that restricts rotation.
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CBoth A and B.
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DThe electronic structure of the peptide bond is still not understood.
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EThe peptide bond is fully saturated and freely rotating.
Answer
Correct Answer: Both A and B.
Explanation
Introduction / Context:Peptide bond resonance is central to protein structure. Delocalization between the carbonyl and the amide nitrogen creates partial double-bond character and defines peptide planarity.
Given Data / Assumptions:
- Resonance implies electron delocalization between C=O and C–N.
- Partial double-bond character shortens and strengthens C–N relative to a single bond.
- Restricted rotation yields a planar peptide unit.
Concept / Approach:If resonance increases bond order above 1, the bond becomes stronger and less rotatable. Therefore both strengthening and partial double-bond character are correct.
Step-by-Step Solution:
Step 1: Consider resonance forms that place a double bond between C and N.Step 2: Infer consequences: shorter, stronger C–N bond with restricted rotation.Step 3: Choose the combined statement that includes both effects.Verification / Alternative check:Empirical peptide C–N bond lengths fall between typical single and double bonds, consistent with partial double-bond character.
Why Other Options Are Wrong:
- D is incorrect; peptide bond electronic structure is well characterized.
- E is incorrect; free rotation contradicts resonance-imposed planarity.
Common Pitfalls:Assuming resonance only affects stability but not geometry; confusing φ/ψ mobility with peptide ω rigidity.
Final Answer:Both A and B.