Summary
The peptide bond exhibits resonance between a neutral form (C=O, C-N) and a zwitterionic form (C-O⁻, C=N⁺). This delocalization imparts partial double-bond character to the C-N linkage, shortening its bond length to 1.32 Å and restricting rotation.
Key Points
- 1Partial double-bond character shortens C-N bond to 1.32 Å
- 2Peptide unit is planar and rigid
- 3Trans configuration favored 1000:1 over cis
- 4Backbone freedom limited to φ and ψ angles
The peptide bond is far more than a simple covalent linkage between amino acids—it is a sophisticated chemical structure whose properties fundamentally shape protein architecture.
Resonance Structures
The peptide bond exists as a resonance hybrid between two canonical forms:
1. Neutral Form: Features a C=O double bond and a C-N single bond
2. Zwitterionic Form: Features a C-O⁻ single bond and a C=N⁺ double bond
This resonance delocalization has profound structural consequences. The partial double-bond character of the C-N linkage shortens its bond length from the typical 1.47 Å of a single bond to approximately 1.32 Å.
Planarity and Rigidity
The most significant consequence of peptide bond resonance is the planarity of the peptide unit. The six atoms involved (Cα, C, O, N, H, and the next Cα) lie in a single plane. This rigidity:
- Limits backbone conformational freedom to the φ (phi) and ψ (psi) angles
- Creates a strong dipole moment across the peptide bond
Trans vs Cis Configuration
The planar peptide bond can adopt either trans or cis configurations:
- Trans configuration (ω ≈ 180°): Strongly favored (~1000:1) due to minimized steric clash between adjacent Cα atoms
- Cis configuration (ω ≈ 0°): Rare, except in proline residues where the energy difference is smaller
This preference for the trans configuration is crucial for the regular patterns observed in protein secondary structures.