Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Proteins absorb light maximum at 260 nm
Reason (R) : Maximum light absorption for both tryptophan and tyrosine occurs near a wavelength of 280 nm
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:

- Remember 280 nm for Proteins (Tryptophan/Tyrosine absorption).
- Remember 260 nm for Nucleic acids (Purine/Pyrimidine absorption).
This distinction is the basis of the \(A_{260}/A_{280}\) purity ratio check.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
UV-visible spectrophotometry is a standard technique used to quantify proteins and nucleic acids based on their characteristic light absorption profiles.

Step 2: Detailed Explanation:
- Assertion (A) evaluation: Proteins absorb UV light maximally at 280 nm, primarily due to the presence of aromatic amino acids. Nucleic acids (DNA and RNA) absorb UV light maximally at 260 nm due to their purine and pyrimidine bases. Thus, Assertion (A) is false.
- Reason (R) evaluation: The aromatic amino acids tryptophan and tyrosine contain conjugated double bonds (benzene and indole rings) that undergo \(\pi \rightarrow \pi^*\) transitions, resulting in a strong absorption peak near 280 nm. Thus, Reason (R) is true.

Step 3: Final Answer:
Assertion (A) is false but Reason (R) is true, corresponding to option (D).