16. An α-helix in a peptide or protein is characterized by hydrogen bonds and characteristic dihedral angles. Choose the right combination.
    (1) Hydrogen bonding between the amide CO of residue i and amide NH of residue i + 4. Dihedral angles in the region φ = -50⁰, Ψ = -60⁰
    (2) Hydrogen bonding between the amide NH of residue i and amide CO of residue i + 4. Dihedral angles in the region of φ = -50⁰, Ψ = -60.
    (3) Hydrogen bonding between the amide CO of residue i and amide NH of residue i + 4. Dihedral angles in the region of φ = -50⁰, Ψ = +60.
    (4) Hydrogen bonding between the amide CO of residue i and amide NH of residue 1 + 3. Dihedral angles in the region of φ = -50⁰, Ψ = -60

Dihedral Angles and Hydrogen Bonding in α-Helices

The α-helix is a fundamental secondary structure in proteins, stabilized by hydrogen bonds and specific dihedral angles. These angles dictate the helical twist and structural stability.

Correct Answer:

The correct option is:

(2) Hydrogen bonding between the amide NH of residue i and amide CO of residue i + 4. Dihedral angles in the region of φ = -50°, Ψ = -60°.

Explanation of the Answer Choices

1. Hydrogen Bonding in α-Helices

• In an α-helix, the amide CO of residue i forms a hydrogen bond with the amide NH of residue i + 4.
• This stabilizes the helical conformation and contributes to protein folding and function.

2. Characteristic Dihedral Angles (ϕ, Ψ) of an α-Helix

• The Ramachandran plot shows that α-helical conformations fall within a distinct region:
  • ϕ (phi) ≈ -50°
  • Ψ (psi) ≈ -60°
• These angles define the right-handed twist of α-helices, optimizing steric and electrostatic interactions.

3. Why Are Other Options Incorrect?

• (1) Incorrect: The dihedral angles Ψ = -600 is likely a typo (should be -60°).
• (3) Incorrect: Ψ = +60° does not correspond to the right-handed α-helix.
• (4) Incorrect: Hydrogen bonding between i and i+3 is characteristic of 3₁₀ helices, not α-helices.

Significance of α-Helical Structure

1. Role in Protein Stability

• Hydrogen bonds prevent the helix from unwinding.
• Proper ϕ and Ψ angles optimize helical packing.

2. Importance in Protein Function

• Found in enzymes, membrane proteins, and DNA-binding proteins.
• Essential for protein-protein interactions.

3. Applications in Bioengineering

• Designing stable synthetic peptides for biomedical research.
• Engineering artificial helical proteins for drug delivery.

Conclusion

The α-helix in peptides and proteins is stabilized by i to i+4 hydrogen bonding and characterized by dihedral angles ϕ ≈ -50° and Ψ ≈ -60°. Understanding these structural features is essential for biochemistry, molecular biology, and protein engineering.