Q.28 The length of an α-helix composed of 36 amino acid residues is
(A) 10 Å
(B) 54 Å
(C) 27 Å
(D) 360 Å
The length of an α-helix with 36 amino acid residues is 54 Å, making option (B) correct. This calculation relies on the standard rise per residue in an α-helix.
α-Helix Structure Basics
An α-helix features 3.6 residues per helical turn and a pitch of 5.4 Å, meaning the helix advances 1.5 Å along its axis per amino acid residue.
For 36 residues, multiply the rise per residue by the number of residues: 36×1.5 A˚=54 A˚.
This geometry arises from hydrogen bonds between the C=O of residue i and N-H of residue i+4, stabilizing the right-handed coil.
Correct Answer Explanation
Option (B) 54 Å is accurate because the axial rise is precisely 1.5 Å/residue, confirmed across protein structure references.
A 36-residue helix spans 10 full turns (36/3.6=10), covering 10×5.4 A˚=54 A˚ or directly 36×1.5 A˚.
This matches experimental data from X-ray crystallography of proteins.
Why Other Options Are Incorrect
| Option | Length (Å) | Explanation |
|---|---|---|
| (A) 10 Å | Too short | Equals ~6-7 residues (10/1.5≈6.7); insufficient for 36 residues. |
| (C) 27 Å | Underestimates | Matches ~18 residues (27/1.5=18); ignores half the residues. |
| (D) 360 Å | Excessive | Implies 10 Å/residue rise, confusing total chain length or pitch misapplication. |
These errors often stem from mixing pitch (5.4 Å/turn) with per-residue rise or assuming extended chain lengths (~3.5 Å/residue in β-sheets).
Exam Relevance
This concept tests secondary structure parameters, vital for protein folding in biochemistry exams like NEET or CSIR NET.
Understanding the 1.5 Å rise helps predict helix dimensions in molecular modeling and bioinformatics.
