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In a polypeptide chain, a cis-to-trans rearrangement of two consecutive proline residues can be achieved by rotating the:

(1) O angle to 10°

(2) ω angle to 180°

(3) Ψ angle to 0°

(4) x angle to 60°

In a polypeptide chain, cis-to-trans isomerization primarily involves rotation around the peptide bond (ω angle).

• ω (omega) angle represents the dihedral angle around the peptide bond.
• In a cis conformation, ω ≈ 0°.
• In a trans conformation, ω ≈ 180°.

Analyzing the options:

1. O angle to 10° → Incorrect, as O is not a standard torsion angle in peptide bonds.
2. ω angle to 180° → Correct, as flipping from cis (ω ≈ 0°) to trans (ω ≈ 180°) directly achieves the rearrangement.
3. Ψ angle to 0° → Incorrect, because Ψ (psi) refers to the Cα–C bond rotation and does not determine cis-trans isomerization.
4. χ (chi) angle to 60° → Incorrect, as χ angles refer to side-chain rotations, not peptide bond isomerization.

Correct Answer:

(2) ω angle to 180°