Department of Physics and Astronomy, Aarhus University, Denmark.
State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, P. R. China.
Phys Chem Chem Phys. 2020 May 20;22(19):11095-11100. doi: 10.1039/d0cp01287d.
Förster Resonance Energy Transfer (FRET) between a photoexcited and a ground-state dye is dictated by how far apart the two dyes are compared to the Förster distance. While there is a significant number of studies on the process for biomacromolecules in solution, there are only a few reports on gas-phase FRET. Here we report on a simple gas-phase model system, synthesized with the rhodamine 575 (R575+) and rhodamine 640 (R640+) FRET pair and a covalent linker with four methylenes, R575+-(CH2)4-R640+. Each dye carries a positive charge which allows for mass-spectroscopy experiments. We have recorded gas-phase dispersed fluorescence spectra of the mass-selected dications excited at different wavelengths using the homebuilt LUNA (LUminescence iNstrument in Aarhus) setup and find in all cases that emission is exclusively from the R640+ acceptor dye. The linker does not interfere electronically with the dyes and simply acts as a spacer. We can therefore establish the direct effect of the interaction between the two dyes when it comes to emission band maximum. Indeed, we find that R640+ experiences a significant shift in its maximum from 560 ± 1 nm for the monomer cation to 577 ± 2 nm in the presence of R575+, independent of initial excitation of R575+ or R640+. This redshift is ascribed to the large polarizability along the long axis of the xanthene core structure, and that this polarizability is larger in the excited state than in the ground state. Experiments were also done on a triply charged 11-mer peptide labelled with the same two dyes, R575+-(Gly-Gln)5-Lys-R640+ + H+ (Gly = glycine, Gln = glutamine, and Lys = lysine) where the extra positive charge is located on the peptide. Again a redshifted emission spectrum of the donor is observed with maximum at 582 ± 2 nm. Our work clearly demonstrates strong sensitivity of the photophysics of one dye to the nearby environment, and that caution is needed when using the energy transfer efficiency to infer dye-dye separations in gas-phase experiments.
Förster 共振能量转移（FRET）在光激发态和基态染料之间发生，这取决于两个染料之间的距离与 Förster 距离相比有多远。虽然有大量关于生物大分子在溶液中过程的研究，但只有少数关于气相 FRET 的报道。在这里，我们报告了一个简单的气相模型系统，该系统由罗丹明 575（R575+）和罗丹明 640（R640+）FRET 对以及带有四个亚甲基的共价连接体 R575+-(CH2)4-R640+合成。每个染料都带有正电荷，允许进行质谱实验。我们使用自制的 LUNA（奥胡斯的 LUminescence iNstrument）装置记录了在不同波长下激发的质量选择双离子的气相分散荧光光谱，并在所有情况下发现发射仅来自 R640+受体染料。连接体不会与染料发生电子干扰，而只是充当间隔物。因此，当涉及到发射带最大值时，我们可以确定两个染料之间相互作用的直接影响。事实上，我们发现 R640+的最大位移显著，从单体阳离子的 560±1nm 变为 R575+存在时的 577±2nm，而与 R575+或 R640+的初始激发无关。这种红移归因于 xanthene 核结构长轴的大极化率，并且该极化率在激发态比在基态时更大。实验还在带有相同两个染料的三价 11 肽上进行，R575+-(Gly-Gln)5-Lys-R640+ + H+（Gly = 甘氨酸，Gln = 谷氨酰胺，Lys = 赖氨酸），其中额外的正电荷位于肽上。再次观察到供体的红移发射光谱，其最大发射波长为 582±2nm。我们的工作清楚地表明，一个染料的光物理性质对附近环境非常敏感，并且在气相实验中使用能量转移效率推断染料-染料分离时需要谨慎。
