2. In the diagram for peptide shown below, angle ϕ represents bonds between

    

   

   (1) N – C’

   (2) C’ – C
   (3) C – O

   (4) N – H

Understanding Phi (ϕ) and Psi (ψ) Angles in Peptides

Proteins are made up of amino acids linked by peptide bonds, and their function is directly related to their three-dimensional structure. The torsion angles in peptide chains determine how the backbone of a protein folds into specific conformations. One such crucial angle is the phi (ϕ) angle.

What is the Phi (ϕ) Angle?

In a peptide structure, the phi (ϕ) angle represents the rotation around the N – Cα bond. This angle plays a crucial role in determining the flexibility and allowed conformations of the polypeptide chain.

Answer to the Question:

The correct answer is:

(4) N – Cα

Other Torsion Angles in Peptides

Apart from the phi (ϕ) angle, two other significant angles define the protein backbone:

• Psi (ψ) Angle: Rotation around the Cα – C’ bond
• Omega (ω) Angle: Rotation around the C’-N bond (usually fixed at ~180° due to the partial double-bond character of the peptide bond)

Importance of Phi (ϕ) and Psi (ψ) Angles in Protein Structure

1. Ramachandran Plot

The combination of ϕ and ψ angles determines the sterically allowed conformations of a protein. The Ramachandran plot is used to visualize these allowable regions, which help predict:

• α-helices (specific ranges of ϕ and ψ values)
• β-sheets (different ranges of ϕ and ψ values)
• Random coils and turns

2. Influence on Protein Folding

Since ϕ and ψ angles control the spatial arrangement of amino acids, they dictate the overall secondary and tertiary structures of proteins. Improper folding due to incorrect angles can lead to misfolded proteins, which are implicated in diseases like Alzheimer’s and Parkinson’s.

3. Protein Flexibility and Stability

• Some ϕ and ψ angle combinations lead to stable structures like helices and sheets.
• Steric hindrance restricts certain angle combinations, preventing overlap of atoms.

Conclusion

The phi (ϕ) angle represents the bond between N – Cα, playing a critical role in determining the structure of proteins. Together with the psi (ψ) and omega (ω) angles, it defines the conformation of peptide chains and influences protein folding, function, and stability. Understanding these angles is fundamental in structural biology, molecular modeling, and drug design.