Which one of the following corresponds to the angle of rotation per residue in a 3₁₀​ helix and 3.6₁₃​ helix, respectively?

Correct option is D

Correct Answer:  (d)
Explanation: The angle of rotation per residue in a helical structure is given by 360° divided by the number of residues per turn. A 3₁₀​ helix has 3 residues per turn, resulting in a rotation of 120° per residue. A 3.6₁₃​ helix (α-helix) has approximately 3.6 residues per turn, giving a rotation of about 100° per residue. Hence, the correct pair is 120° and 100°.
Information Booster
· The 3₁₀ helix is tighter and more compact than the α-helix.
· The α-helix is also referred to as a 3.6₁₃​ helix.
· Helical geometry is defined by residues per turn and hydrogen bonding pattern.
· The α-helix has hydrogen bonds between residues i and i+4.
· Rotation angle per residue determines side-chain orientation along the helix.
Additional Knowledge
Different helical conformations arise due to variations in backbone dihedral angles and hydrogen bonding patterns. The 3₁₀​ helix has hydrogen bonds between residues i and i+3, making it less stable and less common than the α-helix. Understanding these geometric parameters is important for interpreting protein secondary structure and stability.