31.
Column I                                         Column II
W                                                           i. 𝜓
X                                                            ii. 𝜒
Y                                                            iii. 𝜙
Z                                                            iv. 𝜔
Which of the following identifies the correctly matched pairs?
(A) W-iii ; X-i ; Y-iv ; Z-ii
(B) W-i ; X-iii ; Y-iv ; Z-ii
(C) W-i ; X-iii ; Y-ii ; Z-iv
(D) W-iii ; X-i ; Y-ii ; Z-iv

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Protein Backbone Torsion Angles: Concept Overview

Proteins have a repeating polypeptide backbone composed of the sequence:

–N–Cα–C(=O)–

Rotation around specific bonds in this backbone gives rise to characteristic torsion (dihedral) angles, which define protein conformation.

The four important torsion angles are:

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Understanding the Given Diagram

In the diagram:

• R₁, R₂, R₃ represent amino acid side chains.

• W, X, Y, Z label different rotating bonds.

• Column II lists possible torsion angles.

Correct Identification of Each Label

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Correct Matching

• W → χ (ii)

• X → φ (iii)

• Y → ψ (i)

• Z → ω (iv)

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✅ Correct Answer

Option (C)

W–ii ; X–iii ; Y–i ; Z–iv

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Explanation of All Options

Option (A): W–iii ; X–i ; Y–iv ; Z–ii ❌

• Assigns φ to side chain rotation (incorrect).

• ψ and ω are also mismatched.

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Option (B): W–i ; X–iii ; Y–iv ; Z–ii ❌

• W incorrectly labeled as ψ instead of χ.

• Y incorrectly labeled as ω.

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Option (C): W–ii ; X–iii ; Y–i ; Z–iv ✅

• Correctly identifies:

  • Side chain → χ

  • N–Cα → φ

  • Cα–C → ψ

  • Peptide bond → ω
    ✔ Scientifically accurate and structurally correct

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Option (D): W–iii ; X–i ; Y–ii ; Z–iv ❌

• Side chain and backbone angles are confused.

• ψ and χ wrongly assigned.

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Key Exam Takeaways

• φ and ψ define protein secondary structure (α-helix, β-sheet)

• ω is usually fixed due to peptide bond planarity

• χ controls side-chain orientation and protein functionality