11. ϕ and Ψ values for right handed a helix of L-amino acid are expected to be
    (1) ϕ Negative, Ψ negative
    (2) ϕ Negative, Ψ positive
    (3) ϕ Positive, Ψ negative
    (4) ϕ Positive, Ψ positive

Understanding ϕ and Ψ Angles in Right-Handed α-Helices of L-Amino Acids

Protein structure is determined by the backbone dihedral angles ϕ (phi) and Ψ (psi). These angles define the secondary structure of proteins, including α-helices and β-sheets. The Ramachandran plot helps visualize allowed conformations.

Correct Answer:

The correct option is:

(1) ϕ Negative, Ψ Negative

Explanation of ϕ and Ψ Values in Right-Handed α-Helices

1. Typical ϕ and Ψ Values for Right-Handed α-Helices

• In L-amino acids, a right-handed α-helix has:
  • ϕ ≈ -60°
  • Ψ ≈ -45°
• These values place α-helices in the bottom-left quadrant of the Ramachandran plot.

2. Why Are Both ϕ and Ψ Negative?

• ϕ (phi) angle: Determines the rotation around the N-Cα bond.
• Ψ (psi) angle: Determines the rotation around the Cα-C’ bond.
• Due to steric constraints, both angles must be negative in a right-handed α-helix.

Significance of Right-Handed α-Helices in Proteins

1. Structural Stability

• The right-handed α-helix is the most common helical conformation because it is energetically favorable.
• Intramolecular hydrogen bonding stabilizes the structure.

2. Found in Globular Proteins

• Myoglobin and hemoglobin contain right-handed α-helices.
• Many enzymes also utilize this structure.

3. Role in Protein Folding and Misfolding

• Incorrect φ and Ψ angles can lead to protein misfolding disorders like Alzheimer’s and prion diseases.

Conclusion

The right-handed α-helix of L-amino acids has both negative ϕ and Ψ values due to steric constraints and hydrogen bonding requirements. Understanding these angles is crucial for protein modeling, drug design, and bioengineering applications.