Q.35 In Ramachandran Plot, the angle of rotation lies between:

1. -120° to +180°
2. -180° to +180°
3. -90° to +90°
4. -180° to +120°

   Ramachandran Plot Angle Range: Full Guide and MCQ Answer

   The correct answer to the Ramachandran Plot angle range question is -180° to +180°. This reflects the full 360° rotation possible for phi (φ) and psi (ψ) dihedral angles in protein backbones.​​

   Ramachandran Plot Basics

   Ramachandran plots map allowed φ and ψ torsion angles based on steric constraints. These angles define backbone flexibility, excluding the fixed omega (ω) angle near 180° due to peptide bond planarity. Plots span a full circle from -180° to +180°, identifying core, allowed, and disallowed regions for α-helices (-60°, -45°), β-sheets (-120°, +120°), and outliers.​

   Correct Answer Explanation

   Option: -180° to +180°
   Dihedral angles rotate fully across 360°, conventionally plotted from -180° to +180° (0°/360° equivalent). This captures all conformations without steric clashes, as confirmed in G.N. Ramachandran’s hard-sphere models. Most residues fall in ~20% of this space.​

   Incorrect Options Breakdown

   • -120° to +180°: Misses left-handed helix regions (~ -60°, +45°) and full β-sheet extensions; incomplete span ignores symmetry.

   • -90° to +90°: Far too narrow; excludes dominant right-handed α-helices and β-strands, ignoring most observed structures.​

   • -180° to +120°: Covers much but omits positive ψ extremes in extended chains; asymmetric and non-standard.​

   Option	Coverage Issue	Real-World Impact
   -120° to +180°	Misses negative ψ extremes	Excludes some β-turns ​
   -90° to +90°	Only central quadrants	Ignores 80% of residues ​
   -180° to +120°	Lacks full positive ψ	Limits sheet validation ​
   -180° to +180° (Correct)	Full circle	Matches all PDB data ​