In biochemistry, what is the best description of the peptide bond's nature between two amino acids in a polypeptide chain?

Introduction:
Peptide bonds connect amino acids to form polypeptides and proteins. Understanding the chemical character of this bond is fundamental to explaining protein rigidity, planarity, and secondary structure preferences such as alpha helices and beta sheets. The key concept tested here is the resonance that imparts partial double-bond character to the peptide linkage, thereby restricting rotation and enforcing planarity.

Given Data / Assumptions:

• Peptide bond forms between the carboxyl carbon of one amino acid and the amino nitrogen of the next.
• Water is eliminated during formation (condensation reaction).
• Backbone atoms relevant: C=O and N–H with the intervening C–N bond.

Concept / Approach:
Resonance delocalizes electron density between the carbonyl oxygen, carbonyl carbon, and amide nitrogen. This delocalization gives the C–N bond partial double-bond character, making it shorter than a typical C–N single bond and restricting rotation. The atoms lie approximately in one plane, creating a trans preference and enabling predictable phi and psi torsion angle behavior in proteins.

Step-by-Step Solution:

Verification / Alternative check:
Measured bond lengths show C–N in peptides is shorter than a single C–N bond but longer than a true C=N, matching partial double-bond character. X-ray and cryo-EM structures support planarity of the peptide group.

Why Other Options Are Wrong:

• Truly double bond: would be too rigid and not match observed bond length.
• Hydrogen bond: occurs between backbone groups but is not the peptide bond itself.
• van der Waals interaction: weak noncovalent contact, not a covalent peptide linkage.
• Pure single bond: would allow free rotation, contradicting observed planarity.

Common Pitfalls:
Confusing hydrogen bonding patterns of the backbone with the covalent nature of the peptide bond, and assuming free rotation across all backbone bonds.

Final Answer:
partial double bond