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 the aromatic triad—Tryptophan, Tyrosine, and Phenylalanine—absorbs UV light. Tryptophan and Tyrosine are the primary contributors to the standard protein absorption peak at 280 nm.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The absorption of ultraviolet (UV) light by proteins at a wavelength of 280 nm is a fundamental physical property.
This property is widely exploited in biochemistry for the quantification of proteins in solution.
The principal chromophores responsible for this absorption are the aromatic amino acids, which contain conjugated double-bond ring systems.

Step 2: Detailed Explanation:
The three aromatic amino acids—tryptophan, tyrosine, and phenylalanine—possess resonance-stabilized aromatic rings that absorb light in the near-UV range (240–300 nm).
Among these, tryptophan has the highest molar extinction coefficient at 280 nm, followed by tyrosine.
Phenylalanine also absorbs UV light, but its absorption maximum is at 257 nm, showing only a very weak contribution at 280 nm.
In contrast, cysteine is a sulfur-containing aliphatic amino acid lacking any conjugated pi-electron ring system.
Consequently, cysteine does not absorb UV light at 280 nm in any significant capacity.
While disulfide bonds formed by oxidized cysteine residues (cystine) exhibit weak absorption around 250 nm, their absorbance at 280 nm remains negligible compared to the aromatic residues.
Therefore, cysteine is the amino acid that does not contribute significantly to protein UV absorption at 280 nm.

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