Among the following bonds where rotation is not allowed in a peptide chains?
(1) ψ
(2) ω
(3) x
(4) φ

In a peptide chain, rotation is restricted due to partial double-bond character of certain bonds. Let’s analyze each option:

1. ψ (Psi) angle – Rotation Allowed ✅

• This is the Cα–C bond (between the α-carbon and carbonyl carbon).
• It can freely rotate, influencing secondary structure formation.

2. ω (Omega) angle – Rotation NOT Allowed ❌

• This is the peptide bond (C–N bond), which has partial double-bond character due to resonance.
• It is usually either 180° (trans) or 0° (cis) and does not rotate freely.

3. χ (Chi) angle – Rotation Allowed ✅

• This represents side-chain rotations, which vary for different amino acids.

4. φ (Phi) angle – Rotation Allowed ✅

• This is the N–Cα bond, which can rotate and contributes to secondary structure formation.

Correct Answer:

(2) ω (Omega) – because the peptide bond has restricted rotation due to partial double-bond character.