# DBT JRF 2022 — Question 10

Exam: DBT JRF 2022 · Section: B · Topic: Biochemistry › Protein Structure - Peptide Bond · Difficulty: Medium

## Question

The CO-NH moiety between two C-alpha atoms constitutes the Peptide bond. The measured bond lengths (Angstroms) are: C(alpha)-CO (1.51); C=O (1.24); C-N(1.33); N-C(alpha)1.46. In which of these, there is a decrease from the expected bond length due to partial double bond character of the peptide bond?

## Options

- **A.** C(alpha)-CO
- **B.** C=O
- **C.** C-N
- **D.** N-C(alpha)

## Answer

Option C

## Explanation

Partial double-bond character in the peptide bond: In the peptide bond (C-N), resonance delocalises the lone pair of the nitrogen into the adjacent carbonyl C=O: $\text{C=O–N–H} \leftrightarrow \text{C–O}^- \text{=N}^+\text{H}$ Consequences of this partial double-bond character: • The C–N bond is shorter than a typical single bond: observed 1.33 Å vs. expected ~1.47 Å (a C-N single bond in amines). • The C=O bond is longer than a typical double bond: observed 1.24 Å vs. expected ~1.20 Å. • The 6 atoms of the peptide group (Cα–C(=O)–N–H–Cα) are coplanar (partial double-bond restricts rotation). So the bond where the decrease from expected length is observed due to partial double-bond character is the C–N bond (expected ~1.47 Å, observed 1.33 Å). Answer: C–N (Option 3).

Source: https://mypyqbuddy.com/questions/the-co-nh-moiety-between-two-c-alpha-atoms-constitutes-6a104cd5920734408bc0c719