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 measures light absorption by substances. The amount of absorption depends on the substance's chemical structure and the wavelength of light.
Proteins absorb light primarily in the ultraviolet (UV) range due to specific amino acids.
Step 2: Detailed Explanation:
The absorption of UV light by proteins is due to the aromatic side chains of three amino acids: Tryptophan (Trp), Tyrosine (Tyr), and Phenylalanine (Phe).


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

Tryptophan and Tyrosine have absorption maxima around 280 nm. Phenylalanine's maximum is at a lower wavelength (~260 nm), and its contribution is minor.

Because most proteins contain Trp and Tyr, the overall absorbance spectrum of a protein solution shows a characteristic peak at approximately 280 nm.

The other wavelengths are incorrect: 90 nm is in the far-UV, while 420 nm and 550 nm are in the visible spectrum.

Step 3: Final Answer:
The strong absorption of light by proteins rich in aromatic amino acids occurs at a wavelength of 280 nm.