Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : In the secondary structure of proteins, the parallel \(\beta\)-pleated sheets are less stable than antiparallel \(\beta\)-pleated sheets.
Reason (R) : The hydrogen bonds of parallel \(\beta\)-pleated sheets are distorted in comparison to those of antiparallel \(\beta\)-pleated sheets.
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:

Linear hydrogen bonds are stronger and more stable than bent ones.
Since antiparallel \(\beta\)-sheets feature linear bonds, they are always more stable than parallel \(\beta\)-sheets.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Beta-pleated sheets are a common form of secondary structure in proteins. They can be arranged in parallel or antiparallel directions depending on the alignment of the polypeptide strands.

Step 2: Detailed Explanation:
In antiparallel \(\beta\)-sheets, adjacent polypeptide strands run in opposite directions. This allows the hydrogen bonds between the carbonyl groups and the amine groups of adjacent strands to be linear and perpendicular to the strand axis, which optimizes their strength.
In parallel \(\beta\)-sheets, adjacent strands run in the same direction, which forces the hydrogen bonds to form at an angle (non-linear or distorted geometry).
These bent hydrogen bonds are weaker and less stable than the linear hydrogen bonds found in antiparallel sheets.
Therefore, parallel \(\beta\)-sheets are less stable than antiparallel sheets.
This means Assertion (A) is true, Reason (R) is true, and (R) correctly explains why parallel sheets are less stable.

Step 3: Final Answer:
Both statements are true and (R) is the correct explanation of (A), corresponding to option (A).