13. Acetyl-(Ala)₁₈-CONH₂ exists in α-helical conformation in solution. Most of the backbone dihedral angles (φ, Ψ) will be.
    (1) -60⁰, -30⁰
    (2) 60⁰,30⁰
    (3) -60⁰ ,-30⁰ (50%) and 60⁰, 30⁰ (50%)
    (4) -80⁰, 120⁰

Backbone Dihedral Angles (ϕ, Ψ) in α-Helical Peptides: Acetyl-(Ala)18-CONH2

Proteins and peptides adopt specific secondary structures, such as α-helices, based on their dihedral angles (ϕ, Ψ). The Acetyl-(Ala)18-CONH2 peptide, rich in alanine residues, naturally adopts an α-helical conformation in solution. Understanding these angles helps in analyzing protein folding and stability.

Correct Answer:

The correct option is:

(3) -60°, -30° (50%) and 60°, 30° (50%)

Explanation of the Answer Choices

1. Typical ϕ and Ψ Values in α-Helices

• In a right-handed α-helix, the dihedral angles are typically:
  • ϕ ≈ -60°
  • Ψ ≈ -30°
• In a left-handed α-helix (less common), the angles would be:
  • ϕ ≈ 60°
  • Ψ ≈ 30°
• Since 50% of the peptide may adopt each conformation, the correct answer includes both sets of angles.

2. Why Do Alanine-Rich Peptides Form α-Helices?

• Alanine has a high helix-forming propensity due to its small, non-bulky side chain.
• The peptide Acetyl-(Ala)18-CONH2 is long enough to form stable intra-helical hydrogen bonds.
• Such homopolypeptides are often used in studies of peptide folding.

3. Incorrect Answer Choices

• (1) -60°, -30° (Only right-handed helix, but not accounting for both conformations).
• (2) 60°, 30° (Only left-handed helix, incorrect for the majority of α-helical peptides).
• (4) -80°, 120° (These angles correspond to extended β-sheet structures, not α-helices).

Significance of Dihedral Angles in Protein Studies

1. Role in Protein Folding

• Incorrect φ and Ψ angles lead to misfolding and aggregation, seen in diseases like Alzheimer’s and Parkinson’s.

2. Importance in Structural Bioinformatics

• Ramachandran plots are used to validate protein models in X-ray crystallography and NMR studies.

3. Applications in Peptide Engineering

• Knowledge of dihedral angles helps in designing synthetic peptides for drug development and biomaterials.

Conclusion

Acetyl-(Ala)18-CONH2 exists predominantly in an α-helical conformation, with backbone dihedral angles of -60°, -30° (right-handed helix) and 60°, 30° (left-handed helix), each appearing in 50% of the population. Understanding these angles is crucial for protein modeling, bioinformatics, and drug discovery.