Biochemistry—Peptide Bond Geometry Which statement best describes the peptide bond in proteins?

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:

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.