Question

Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R).
Assertion (A) : A polypeptide chain with a long block of Arginine at pH 7.0 can not form an $\alpha$-helix in the portion containing long block of arginine.
Reason (R) : Positively charged side chains of arginine at pH 7.0 repel each other strongly.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Any long block of similarly charged residues (e.g., poly-Lys, poly-Arg at pH 7.0, or poly-Glu, poly-Asp at pH 7.0) will fail to form an $\alpha$-helix due to electrostatic repulsion between their charged side chains.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The $\alpha$-helix is stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid residue and the amide hydrogen of the amino acid four residues further along the chain.
The stability of this conformation is influenced by the properties of the amino acid side chains (R-groups).
Detailed Explanation:
- Evaluating Reason (R): Arginine is a basic amino acid with a guanidino side chain that has a very high $\text{p}K_a$ value (approximately $12.5$).
At physiological pH (pH 7.0), this guanidino group is completely protonated and carries a net positive charge.
When several arginine residues are grouped together, these adjacent positive charges repel each other strongly. Thus, Reason (R) is true.
- Evaluating Assertion (A): The electrostatic repulsion between the positively charged side chains of adjacent arginine residues destabilizes the polypeptide backbone.
This prevents it from adopting the tight, structured helical twist of an $\alpha$-helix.
Instead, the steric and electrostatic repulsion forces the chain into an open, random coil conformation. Thus, Assertion (A) is true.
- Connecting A and R: The strong electrostatic repulsion of the positively charged side chains (R) is the direct physical cause for the inability of the poly-arginine segment to form an $\alpha$-helix (A).
Therefore, (R) is the correct explanation of (A).

Step 2: Final Answer:
Both (A) and (R) are true and (R) is the correct explanation of (A). This corresponds to option (A).