Question

The decreasing order of the numerical values of Molar Extinction Coefficient of following amino acids at 280 nm is:
(A). Cysteine
(B). Phenylalanine
(C). Tyrosine
(D). Tryptophan
Choose the correct answer from the options given below:

• A. (D), (C), (A), (B).
• B. (B), (C), (D), (A).
• C. (C), (D), (B), (A).
• D. (D), (B), (C), (A).

Hint:

Remember the general sequence for UV absorption of amino acids at 280 nm: \(\text{Trp} > \text{Tyr} > \text{Cys (disulfide)} > \text{Phe}\).
Phenylalanine has negligible absorption at this wavelength because its peak absorption occurs at 257 nm.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Aromatic amino acids absorb light in the ultraviolet region of the electromagnetic spectrum.
The measurement of absorbance at 280 nm is a standard method used to estimate protein concentrations, which is primarily determined by the molar extinction coefficients of these aromatic residues.

Step 2: Detailed Explanation:
Tryptophan (D) contains an indole ring and exhibits the highest molar extinction coefficient at 280 nm, which is approximately \(5600 \text{ M}^{-1}\text{cm}^{-1}\) due to its highly conjugated \(\pi\)-electron system.
Tyrosine (C) contains a phenol group and exhibits a moderate molar extinction coefficient of approximately \(1400 \text{ M}^{-1}\text{cm}^{-1}\) at 280 nm.
Cysteine (A) does not contain an aromatic ring, but in its oxidized form as cystine (containing disulfide bonds), it exhibits weak absorbance at 280 nm with an extinction coefficient of around \(125 \text{ M}^{-1}\text{cm}^{-1}\).
Phenylalanine (B) absorbs ultraviolet light primarily at 257 nm, and its absorbance at 280 nm is essentially negligible.
Arranging these amino acids in decreasing order of their molar extinction coefficients at 280 nm yields Tryptophan (D) \(>\) Tyrosine (C) \(>\) Cysteine (A) \(>\) Phenylalanine (B).

Step 3: Final Answer:
Therefore, the decreasing order of the numerical values of the molar extinction coefficients is represented by option (A).