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Q. The measured values of main chain torsion angles of a residue in a polypeptide has values (Φ=+50, Ψ=+60). What type of secondary structure is it most likely to be present in?

(1) Left-handed a-helix

(2) Right-handed a-helix

(3) Type II ẞ-turn

(4) Parallel B-sheet

The secondary structure of a polypeptide is largely determined by the torsion angles (Φ, Ψ) of the backbone. These angles are typically analyzed using a Ramachandran plot, which maps the allowed conformations for different types of secondary structures.

Given values:
Φ = +50°
Ψ = +60°

Now, let’s analyze each option:

1. Left-handed α-helix – Typically has (Φ ≈ 50°, Ψ ≈ 50°), which matches closely with the given values.
2. Right-handed α-helix – Has (Φ ≈ -60°, Ψ ≈ -40°), which does not match the given values.
3. Type II β-turn – Has (Φ ≈ -60°, Ψ ≈ 120°), which does not match exactly.
4. Parallel β-sheet – Has (Φ ≈ -120°, Ψ ≈ 105°), which does not match the given values.

Correct answer:

(1) Left-handed α-helix
This is because the given torsion angles (Φ = +50, Ψ = +60) are located in the region corresponding to a left-handed α-helix in the Ramachandran plot.