Question

The amino acid Tryptophan exhibits maximum UV absorption at what approximate wavelength

• A. 220 nm
• B. 260 nm
• C. 280 nm
• D. 340 nm

Hint:

Remember the key wavelengths for biomolecules: Proteins at 280 nm (due to Trp/Tyr) and Nucleic Acids at 260 nm. The A280/A260 ratio is often used to assess the purity of a protein sample.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The aromatic amino acids—phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp)—contain conjugated ring systems that absorb ultraviolet (UV) light. This property is commonly used to detect and quantify proteins. Each has a characteristic absorption maximum.
Step 2: Detailed Explanation:
The approximate UV absorption maxima for the aromatic amino acids are:
Phenylalanine (Phe): \(\sim\)257 nm (with a very low molar absorptivity)
Tyrosine (Tyr): \(\sim\)274 nm
Tryptophan (Trp): \(\sim\)280 nm (with the highest molar absorptivity)
Because Tryptophan's absorption is the strongest, the overall absorption of most proteins is dominated by their tryptophan content, and the standard wavelength used to measure protein concentration via UV spectroscopy is 280 nm.
Other wavelengths in the options correspond to:
220 nm: Absorption by the peptide bond.
260 nm: Maximum absorption for nucleic acids (DNA and RNA).
340 nm: Absorption maximum for the reduced coenzyme NADH.
Step 3: Final Answer:
Tryptophan has its maximum UV absorption at approximately 280 nm.