Q27 A molecule that forms a donor–acceptor energy transfer pair with the dansyl
group is ______
(A) Aspartate
(B) Histidine
(C) Lysine
(D) Tryptophan
Tryptophan is the correct answer (D).
The dansyl group, a fluorescent label commonly attached to proteins, forms an efficient donor-acceptor energy transfer pair (via FRET) with tryptophan due to optimal spectral overlap between tryptophan’s emission (~350 nm) and dansyl’s absorption (~340-380 nm).
FRET Basics
Förster Resonance Energy Transfer (FRET) requires spectral overlap, proper donor-acceptor distance (1-10 nm), and favorable orientation. Dansyl typically acts as the acceptor, quenched by nearby donors or quenching donors like tryptophan.
Option Analysis
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(A) Aspartate: Lacks a chromophore for energy transfer; its carboxylate group does not overlap spectrally with dansyl.
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(B) Histidine: Imidazole ring absorbs UV but poor overlap with dansyl emission/absorption; quenches via electron transfer, not FRET.
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(C) Lysine: No aromatic chromophore; ε-amino group used for dansyl labeling, but free lysine does not participate in FRET pairs.
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(D) Tryptophan: Indole ring fluoresces strongly (~350 nm), overlapping perfectly with dansyl absorption, enabling efficient FRET in protein studies.
Introduction to Dansyl-Tryptophan FRET Pair
The dansyl group donor-acceptor energy transfer pair with tryptophan exemplifies a classic FRET system in biochemistry. This MCQ from CSIR NET Life Sciences tests understanding of fluorescence spectroscopy for protein conformation studies. Tryptophan’s emission spectrum overlaps dansyl’s absorption, enabling non-radiative energy transfer.
Why Tryptophan Excels as FRET Partner
Tryptophan residues serve as natural donors due to high quantum yield and ~348 nm emission peak matching dansyl’s ~340 nm excitation. Multiple studies confirm this pair’s use in measuring protein distances (R₀ ≈ 20-25 Å). In contrast, other amino acids lack suitable chromophores.
| Amino Acid | Chromophore Present? | Spectral Overlap with Dansyl? | FRET Efficiency |
|---|---|---|---|
| Aspartate | No | None | Negligible |
| Histidine | Imidazole (weak) | Poor | Low |
| Lysine | None | None | None |
| Tryptophan | Indole (strong) | Excellent (~350 nm) | High |
Applications in Protein Research
This dansyl group donor-acceptor energy transfer pair monitors unfolding, prion conformations, and enzyme dynamics. Exciting tryptophan quenches its fluorescence while enhancing dansyl emission if proximal.
CSIR NET Exam Insights
Such questions assess FRET principles from biophysical chemistry. Focus on spectral properties: donors emit where acceptors absorb. Tryptophan-dansyl remains a gold standard over synthetic dyes.
