Question

Which of the following amino acids in a protein or peptide does not contribute significantly towards its UV absorption at 280 nm?

• A. Tryptophan
• B. Tyrosine
• C. Phenylalanine
• D. Cysteine

Hint:

Remember that only the aromatic amino acids (Tryptophan, Tyrosine, and to a lesser extent Phenylalanine) absorb light at 280 nm due to their conjugated ring systems. Non-aromatic amino acids like cysteine show no significant absorption at this wavelength.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The absorption of ultraviolet (UV) light by proteins at 280 nm is a widely used analytical property for protein concentration determination.
This UV absorption is primarily due to the presence of aromatic amino acids with conjugated double bonds that can be excited by UV radiation.

Step 2: Detailed Explanation:
Among the 20 standard amino acids, only the aromatic amino acids absorb UV light significantly in the range of 250 to 300 nm.
Tryptophan has the highest molar extinction coefficient at 280 nm due to its indole ring.
Tyrosine also absorbs significantly at 280 nm due to its phenolic group.
Phenylalanine has a lower absorption at 280 nm, with its absorption peak located around 257 nm, but it still contributes weakly.
Cysteine is a sulfur-containing aliphatic amino acid that lacks any conjugated pi-electron system or aromatic ring.
Consequently, its side chain does not absorb light at 280 nm.
Although disulfide bonds formed by oxidized cysteine residues can show a very weak, broad absorption around 250 nm, their contribution at 280 nm is negligible compared to tryptophan and tyrosine.
Therefore, cysteine does not contribute significantly to the UV absorbance of proteins at 280 nm.

Step 3: Final Answer:
Cysteine does not contribute significantly toward the UV absorption of proteins at 280 nm.