Question

An ideal right handed $\alpha$-helix is made up of 200 amino acid residues, the length of that helix structure is

• A. 200 angstrom
• B. 240 angstrom
• C. 260 angstrom
• D. 300 angstrom

Hint:

Always memorize these three basic dimensions of the ideal $\alpha$-helix:
1. Rise per residue = $1.5\text{ \AA}$
2. Residues per turn = $3.6$
3. Pitch (Rise per turn) = $3.6 \times 1.5 = 5.4\text{ \AA}$

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The $\alpha$-helix is a common secondary structure of proteins.
To find the length of an ideal right-handed $\alpha$-helix, we must use its standard geometric parameters: the rise per amino acid residue.
Key Formula or Approach:
In an ideal right-handed $\alpha$-helix:
- The helical rise per residue ($d$) is $1.5\text{ \AA}$ ($0.15\text{ nm}$).
- There are $3.6$ residues per turn of the helix.
- The pitch (height of one full turn) is $5.4\text{ \AA}$ ($0.54\text{ nm}$).
The total length ($L$) of the helix can be calculated as:
\[ L = \text{Number of residues } (N) \times \text{rise per residue } (d) \]

Step 2: Detailed Explanation:
Given:
- Number of residues ($N$) = $200$
- Rise per residue ($d$) = $1.5\text{ \AA}$
Calculate the length:
\[ L = 200 \times 1.5\text{ \AA} = 300\text{ \AA} \]

Step 3: Final Answer:
The length of the 200-residue $\alpha$-helix is 300 \AA, corresponding to option (D).