Q. 30 Which one of the following amino acids is responsible for the intrinsic fluorescence of proteins?
(A) Pro
(B) Met
(C) His
(D) Trp
Intrinsic fluorescence in proteins arises from specific amino acids that emit light upon UV excitation, with tryptophan being the primary contributor due to its strong quantum yield. The correct answer to this question is (D) Trp, as it dominates protein fluorescence spectra around 350 nm.
Correct Answer
Tryptophan (Trp) is responsible for the intrinsic fluorescence of proteins. Its indole ring absorbs UV light at 280 nm and emits strongly, making it ideal for studying protein folding and dynamics in techniques like fluorescence spectroscopy.
Option Analysis
Proteins exhibit intrinsic fluorescence mainly from aromatic amino acids, but among the given choices, only one fits.
| Option | Amino Acid | Fluorescence Role | Explanation |
|---|---|---|---|
| (A) Pro | Proline | None | Proline lacks an aromatic ring; its cyclic structure disrupts secondary structures but does not fluoresce. |
| (B) Met | Methionine | None | Methionine’s thioether side chain absorbs poorly in UV and shows no significant emission. |
| (C) His | Histidine | Negligible | Histidine’s imidazole ring fluoresces weakly if at all, far below tryptophan’s intensity. |
| (D) Trp | Tryptophan | Primary | Trp has the highest quantum yield (~0.13-0.20), with emission sensitive to microenvironment, dominating ~90% of protein fluorescence. |
