Biochemistry—Peptide Bond Cis/Trans Preference Which statement about peptide bond conformation is most accurate?

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:

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.