Question

Given below are two statements, one is labeled as Assertion (A), and the other one is labeled as Reason (R).
Assertion (A): 2 Proline is an aromatic amino acid as it has a ring structure.
Reason (R): Aromatic amino acids have a benzene ring of its derivative side chain structures.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct 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:

Proline is an imino acid with a saturated pyrrolidine ring, not an aromatic amino acid.
The only standard aromatic amino acids are Phenylalanine, Tyrosine, and Tryptophan, which contain conjugated aromatic rings.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Amino acids are the building blocks of proteins, classified into different groups based on the chemical structure of their side chains (R-groups).
These categories include aliphatic, aromatic, acidic, basic, polar uncharged, and sulfur-containing amino acids.
Aromatic compounds are defined by the presence of a planar, conjugated ring system that obeys Hückel's rule ($4n+2$ $\pi$-electrons), typically containing a benzene ring or a similar aromatic heterocycle.
Understanding the specific structural characteristics of amino acids is crucial for biochemistry and plant physiology.

Step 2: Detailed Explanation:
Let us evaluate both statements to determine their validity:
Assertion (A) is false:
The amino acid Proline (referred to as “2 Proline” in the question) is not an aromatic amino acid.
Although proline contains a cyclic ring structure, this ring is a five-membered pyrrolidine ring, which is completely saturated and aliphatic.
Because the nitrogen atom of the amino group is incorporated directly into this ring system, proline is technically a cyclic secondary amino acid, often called an imino acid.
Since it lacks a conjugated, planar system of alternating double bonds and does not obey Hückel's rule, it is classified as an aliphatic, non-polar amino acid rather than an aromatic one.
Reason (R) is true:
Aromatic amino acids are characterized by having an aromatic ring (such as a benzene ring or its heterocyclic derivatives) in their side chain structures.
The classic aromatic amino acids found in proteins are:
1. Phenylalanine: Contains a benzene ring attached to a methyl group (benzyl side chain).
2. Tyrosine: Contains a phenol group (a benzene ring with a hydroxyl group).
3. Tryptophan: Contains an indole ring system (a benzene ring fused to a pyrrole ring).
These structures exhibit unique spectroscopic properties, such as ultraviolet light absorption at $280\text{ nm}$.
Since Proline lacks this benzene-derived aromatic ring system, Assertion (A) is false, while Reason (R) is a chemically correct statement.

Step 3: Final Answer:
Therefore, Assertion (A) is false, but Reason (R) is true.