Question

Tryptophan, tyrosine and phenylalanine, absorb ultraviolet light. Therefore, the protein rich in these amino acids strongly absorbs the light of wavelength ___________ due to aromatic side chain of these amino acids.

• A. 90 nm
• B. 420 nm
• C. 550 nm
• D. 280 nm

Hint:

Associate aromatic amino acids (Trp, Tyr, Phe) with UV light absorption. The number \textbf{280 nm} is a standard wavelength used in biochemistry labs for quick and non-destructive protein quantification (A280).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Spectrophotometry quantifies a substance's light absorption, which is influenced by its chemical composition and the light's wavelength.
Proteins primarily absorb ultraviolet (UV) light due to the presence of specific amino acids.
Step 2: Detailed Explanation:
The UV absorption of proteins stems from the aromatic side chains of Tryptophan (Trp), Tyrosine (Tyr), and Phenylalanine (Phe).


These rings contain delocalized $\pi$ electrons that absorb UV radiation.

Tryptophan and Tyrosine exhibit peak absorption around 280 nm. Phenylalanine absorbs maximally at approximately 260 nm, contributing less to the overall absorption.

Since most proteins contain Trp and Tyr, protein solutions typically show a characteristic absorbance peak near 280 nm.

Wavelengths of 90 nm are in the far-UV range, while 420 nm and 550 nm fall within the visible spectrum and are therefore incorrect.

Step 3: Final Answer:
Proteins with a high content of aromatic amino acids exhibit strong light absorption at 280 nm.