Q.29 The dipeptide with least rotational barrier in the peptide bond is
Introduction
In peptides and proteins, the peptide bond (–CO–NH–) shows restricted rotation due to partial double-bond character arising from resonance between the carbonyl group and the nitrogen lone pair.
However, not all peptide bonds have the same rotational barrier. The extent of resonance — and hence the rotational barrier — depends strongly on the substitution at the nitrogen atom.
This question asks which dipeptide has the least rotational barrier in its peptide bond.
Key Concept: Rotational Barrier in Peptide Bond
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Peptide bond behaves like a partial double bond
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Resonance between C=O and N lone pair restricts rotation
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More resonance = higher rotational barrier
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Less resonance = lower rotational barrier
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Tertiary amides show least resonance because the nitrogen lone pair is less available
Option-wise Explanation
Option A
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Contains proline at the N-terminal
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Peptide bond is still a secondary amide
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Nitrogen lone pair participates in resonance
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High rotational restriction remains
❌ Not correct
Option B (✅ Correct Answer)
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Peptide bond involves proline nitrogen
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Nitrogen is part of a five-membered ring
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This makes the peptide bond a tertiary amide
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Lone pair on nitrogen is less available for resonance
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Least partial double bond character
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Lowest rotational barrier
✅ Correct Answer
Option C
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Normal dipeptide with secondary amide
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Strong resonance stabilization
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Significant partial double bond character
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High rotational barrier
❌ Not correct
Option D
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Simple glycine–glycine dipeptide
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Classic peptide bond with strong resonance
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Restricted rotation remains significant
❌ Not correct
Final Answer
✅ Correct Option: B
Reason:
The peptide bond in option B is a tertiary amide involving proline, which reduces resonance stabilization and thus has the least rotational barrier.
Exam Tip
Peptide bonds involving proline have lower rotational barriers due to reduced resonance from nitrogen lone-pair involvement.
