Biochemistry—Peptide Bond Cis/Trans Preference Which statement about peptide bond conformation is most accurate?
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APeptide bonds are usually cis unless the next residue is proline.
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BPeptide bonds are usually trans; X–Pro bonds show a higher fraction of cis than others.
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CPeptide bonds are planar only because of steric hindrance, not resonance.
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DPeptide bonds occur only between proline residues.
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EPeptide bonds are typically nonplanar and freely rotating.
Answer
Correct Answer: Peptide bonds are usually trans; X–Pro bonds show a higher fraction of cis than others.
Explanation
Introduction / Context:Most peptide bonds adopt the trans configuration to minimize steric clashes across the peptide bond. This question probes knowledge of the special case involving proline.
Given Data / Assumptions:
- Resonance restricts rotation, making peptide bonds planar.
- Trans configuration is overwhelmingly favored for most residues.
- X–Pro bonds have a notable minority population of cis compared with other residues.
Concept / Approach:Recognize the general rule (trans) and the exception (X–Pro shows more cis than usual). Select the option that captures both ideas without overstating cis prevalence.
Step-by-Step Solution:
Step 1: Establish the baseline: peptide bonds are usually trans.Step 2: Consider the exception: proline's ring reduces the trans–cis energy gap, increasing the cis fraction.Step 3: Choose the statement that includes both truths.Verification / Alternative check:Protein structure statistics show cis is rare overall, but enriched at X–Pro positions compared with others.
Why Other Options Are Wrong:
- “Usually cis” is false.
- Planarity is due largely to resonance, not merely steric hindrance.
- Only between prolines: false.
- Nonplanar and freely rotating: contradicts resonance constraints.
Common Pitfalls:Interpreting the special case as the rule; confusing φ/ψ flexibility with ω rigidity.
Final Answer:Peptide bonds are usually trans; X–Pro bonds show a higher fraction of cis than others.