15. In a polypeptide chain, a cis-to-trans rearrangement of two consecutive proline
residues can be achieved by rotating the:
a. $ angle to 10°
b. % angle to 180°
c. & angle to 0°
d. ‘1 angle to 60°

The cis-to-trans rearrangement of a peptide bond between two consecutive proline residues is achieved by rotation of the peptide bond dihedral, the ω (omega) angle, from about 0° (cis) to about 180° (trans). Therefore, the correct option is: b. % angle to 180° (where “%” corresponds to the ω angle in this question’s notation).​

Introduction

In polypeptide chains, the conformation around each peptide bond is described mainly by three backbone torsion angles: phi (ϕ), psi (ψ) and omega (ω). For X–Pro peptide bonds (where the next residue is proline), unique cis–trans isomerization occurs, and switching between cis and trans forms involves rotation about the peptide bond itself, captured by the ω angle.​

Backbone Torsion Angles and Cis–Trans Isomerization

In proteins, three key backbone dihedral angles define conformation:

• Φ (phi): rotation around the N–Cα bond

• Ψ (psi): rotation around the Cα–C (carbonyl) bond

• Ω (omega): rotation around the C–N peptide bond (amide bond)​

Most peptide bonds are in the trans conformation, where ω is close to 180°, while the cis conformation has ω near 0°. Cis–trans isomerization of X–Pro peptide bonds is therefore described as a change in the ω dihedral from about 0° (cis) to about 180° (trans).​

Correct Option: b. % Angle to 180°

If “% angle” in the question denotes the ω (omega) angle, then:

• In the cis form of a peptide bond, the ω angle is approximately 0°, placing the Cα atoms of adjacent residues on the same side of the peptide bond.​

• In the trans form, the ω angle is approximately 180°, putting the Cα atoms on opposite sides, which is energetically favored in most peptide bonds.​

Therefore, a cis-to-trans rearrangement of the X–Pro peptide bond (between two consecutive prolines or an amino acid followed by proline) is achieved by rotating the ω angle from ~0° to ~180°.​
Hence, option b is correct.

Why the Other Options Are Incorrect

The other listed angles correspond (by standard protein nomenclature) to Φ or Ψ–like rotations, which alter backbone conformation but do not convert a cis peptide bond into trans; only ω controls cis–trans at the peptide bond.​

a. $ angle to 10°

If the “$ angle” is intended to represent ϕ (phi):

• ϕ is the torsion around the N–Cα bond and can span a wide range of values (e.g., around −60°, −120°, etc., in secondary structures).​

• Changing ϕ to 10° alters backbone orientation but does not flip the peptide bond itself from cis to trans because cis–trans is defined by ω, not ϕ.​

Thus, rotating a phi-like angle to 10° cannot accomplish cis-to-trans rearrangement, so option a is incorrect.

c. & angle to 0°

If the “& angle” is interpreted as ψ (psi):

• ψ is the torsion around the Cα–C (carbonyl) bond and, like ϕ, determines the local backbone conformation and secondary structure type.​

• Setting a ψ-like angle to 0° may dramatically change local geometry, but it does not change whether the peptide bond is cis or trans, which is governed solely by the ω dihedral.​

Hence, option c is also incorrect.

d. ‘1 angle to 60°

If “‘1 angle” refers to a side-chain χ1 dihedral (e.g., N–Cα–Cβ–Cγ of proline):

• χ1 (and other χ angles) describe side-chain orientation, such as endo/exo puckering in proline, but not the cis–trans state of the peptide bond.​

• Rotating χ1 to 60° can alter ring puckering or side-chain positioning, yet cis vs. trans of the X–Pro peptide bond remains defined by ω, not by side-chain dihedrals.​

Therefore, option d is incorrect, since χ1 rotation alone cannot convert cis to trans at the peptide bond.

Summary Table: Angles and Their Roles

Only rotation of ω from ~0° to ~180° converts a cis X–Pro peptide bond to trans, matching option b.​