Question

Light of 250 nm wavelength is absorbed by which amino acid?

• A. Arginine
• B. Alanine
• C. Tyrosine
• D. Histidine

Hint:

Look for the aromatic amino acid among the options.

Answer 1

The Correct Option is C

Step 1: Amino acids do not absorb visible light and are colourless. However, the aromatic amino acids, namely tryptophan, tyrosine and phenylalanine, absorb ultraviolet light in the 250 to 290 nm range because of their aromatic rings.

Step 2: Among the four options, tyrosine is the only aromatic amino acid, so it absorbs UV light at around 250 nm. Hence option 3 is correct.

Step 3: This UV absorption, with a maximum near 280 nm, is the basis for estimating protein concentration spectrophotometrically; tryptophan has the strongest absorption among the aromatic amino acids.

Step 4: Arginine, alanine and histidine lack a strongly UV-absorbing aromatic ring of this type, so options 1, 2 and 4 are wrong.