Q.29 Which one of the following amino-acids has highest fluorescence quantum yield (Φ) in aqueous solution?
(D) Histidine
Tryptophan exhibits the highest fluorescence quantum yield among the listed amino acids in aqueous solution due to its rigid indole ring structure. The correct answer is (B) Tryptophan.
Correct Answer
(B) Tryptophan.
Tryptophan’s indole chromophore has Φ ≈ 0.13-0.20 in water, far exceeding others due to efficient radiative decay and minimal non-radiative quenching. Excitation ~280 nm, emission ~350 nm makes it dominant in protein fluorescence studies.
Fluorescence Basics
Intrinsic fluorescence arises from aromatic side chains: Trp > Tyr >> Phe > His. Quantum yield (Φ = photons emitted/photons absorbed) measures efficiency; water quenches via H-bonding/vibrational relaxation, but Trp’s planarity resists this best.
Option Breakdown
(A) Tyrosine
Incorrect. Tyr’s phenol ring gives Φ ≈ 0.14 (free) but drops to ~0.05 in proteins due to quenching by nearby residues. Weaker than Trp.
(B) Tryptophan
Correct. Highest Φ (~0.2), longest lifetime (~5 ns). Indole rigidity minimizes quenching; standard for protein folding/conformation probes.
(C) Phenylalanine
Incorrect. Phe’s benzene ring has lowest Φ ≈ 0.02-0.04; high vibrational freedom promotes non-radiative decay.
(D) Histidine
Incorrect. His imidazole fluoresces weakly (Φ < 0.02); protonation/deprotonation and solvent interactions quench heavily.
Quantum Yield Comparison Table
| Amino Acid | Φ (aqueous, approx.) | Emission Max (nm) | Role |
|---|---|---|---|
| Trp (B) | 0.13-0.20 | 350 | Highest |
| Tyr (A) | 0.05-0.14 | 305 | Moderate |
| Phe (C) | 0.02-0.04 | 282 | Low |
| His (D) | <0.02 | ~370 | Negligible |
