Biochemistry—Peptide Bond Geometry Which statement best describes the peptide bond in proteins?
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APlanar and usually found in a trans conformation.
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BNonpolar and fixed in a trans conformation with no resonance.
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CPlanar but freely rotates to three preferred dihedral angles.
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DUsually cis unless the next residue is proline.
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EOnly forms between proline residues.
Answer
Correct Answer: Planar and usually found in a trans conformation.
Explanation
Introduction / Context:The peptide bond links amino acids in proteins. Its electronic structure imposes geometric constraints that strongly influence secondary structure. This question targets the planarity and cis/trans preference of the peptide bond.
Given Data / Assumptions:
- Peptide bond exhibits resonance between the carbonyl C=O and the C–N bond.
- Resonance imparts partial double-bond character to C–N.
- Trans conformation minimizes steric clashes across the peptide bond.
Concept / Approach:Because of resonance, rotation about the peptide C–N bond is restricted, making the peptide unit planar. Most peptide bonds adopt the trans configuration; cis is rare (more frequent only for X-Pro bonds).
Step-by-Step Solution:
Step 1: Recognize resonance restricts rotation, enforcing planarity.Step 2: Recognize steric advantages of trans over cis for most residues.Step 3: Select the option that states “planar, usually trans,” which matches standard protein chemistry.Verification / Alternative check:Structural databases show peptide ω dihedral angle near 180 degrees (trans) in the vast majority of residues.
Why Other Options Are Wrong:
- Nonpolar and fixed with no resonance: ignores the key resonance feature.
- Freely rotating with preferred angles: rotation about C–N is restricted, not free.
- Usually cis unless next is proline: opposite of reality.
- Only between prolines: false.
Common Pitfalls:Confusing φ/ψ rotations (around Cα) with the peptide ω angle (around C–N). Assuming cis is common.
Final Answer:Planar and usually found in a trans conformation.