Ramachandran plot: what does it illustrate about permissible backbone dihedral angles in proteins? In protein structure analysis, the Ramachandran plot maps the sterically allowed regions for the backbone dihedral angles phi (φ) and psi (ψ) of peptide units. Which statement best captures the central fact shown by this plot?

Introduction:
Proteins adopt specific three-dimensional shapes constrained by their backbone geometry. The Ramachandran plot is a foundational tool that visualizes the sterically allowed combinations of the backbone dihedral angles phi (φ) and psi (ψ).

Given Data / Assumptions:

• Backbone dihedrals considered: phi (N–Cα) and psi (Cα–C’).
• Peptide bond itself is planar due to resonance.
• Steric clashes between atoms restrict many φ, ψ combinations.

Concept / Approach:

The plot reveals allowed basins corresponding to secondary structures such as right-handed α-helix, β-sheet, and left-handed helix. Disallowed regions arise from steric overlap among backbone and side-chain atoms.

Step-by-Step Solution:

Verification / Alternative check:

Inspection of known structures shows residues in α-helices around φ ≈ -60°, ψ ≈ -45°, and in β-sheets around φ ≈ -120°, ψ ≈ 120° (approximate ranges), confirming clustering.

Why Other Options Are Wrong:

“Peptide bond planar” is true but not what the plot primarily illustrates. “Any value” ignores sterics. “Only a single value” contradicts observed diversity. “Depends only on charge” ignores steric packing.

Common Pitfalls:

Confusing peptide bond planarity (ω) with φ, ψ flexibility; ignoring glycine/proline outliers.

Final Answer:

The phi (φ) and psi (ψ) angles cluster into a few allowed regions (e.g., α-helix, β-sheet, left-handed helix).