College of Science , George Mason University , Fairfax , Virginia 22030 , United States.
J Phys Chem B. 2018 May 17;122(19):5020-5029. doi: 10.1021/acs.jpcb.8b02134. Epub 2018 May 8.
We examine the effect of electronic coupling on the optical properties of Cy3 dimers attached to DNA duplexes as a function of base pair (bp) separation using steady-state and time-resolved spectroscopy. For close Cy3-Cy3 separations, 0 and 1 bp between dyes, intermediate to strong electronic coupling is revealed by modulation of the absorption and fluorescence properties including spectral band shape, peak wavelength, and excited-state lifetime. Using a vibronic exciton model, we estimate coupling strengths of 150 and 266 cm for the 1 and 0 bp separations, respectively, which are comparable to those found in natural light-harvesting complexes. For the strongest electronic coupling (0 bp separation), we observe that the absorption band shape is strongly affected by the base pairs that surround the dyes, where more strongly hydrogen-bonded G-C pairs produce a red-shifted absorption spectrum consistent with a J-type dimer. This effect is studied theoretically using molecular dynamics simulation, which predicts an in-line dye configuration that is consistent with the experimental J-type spectrum. When the Cy3 dimers are in a standard aqueous buffer, the presence of relatively strong electronic coupling is accompanied by decreased fluorescence lifetime, suggesting that it promotes nonradiative relaxation in cyanine dyes. However, we show that the use of a viscous solvent can suppress this nonradiative recombination and thereby restore the dimer fluorescent emission. Ultrafast transient absorption measurements of Cy3 dimers in both standard aqueous buffer and viscous glycerol buffer suggest that sufficiently strong electronic coupling increases the probability of excited-state relaxation through a dark state that is related to Cy3 torsional motion.
我们研究了电子耦合对连接到 DNA 双链体的 Cy3 二聚体光学性质的影响，作为碱基对 (bp) 分离的函数，使用稳态和时间分辨光谱法。对于接近的 Cy3-Cy3 分离，染料之间的 0 和 1 bp 之间存在中等至强的电子耦合，这表现在吸收和荧光性质的调制上，包括光谱带形状、峰值波长和激发态寿命。使用振子激子模型，我们估计 1 和 0 bp 分离的耦合强度分别为 150 和 266 cm，这与在天然光捕获复合物中发现的耦合强度相当。对于最强的电子耦合 (0 bp 分离)，我们观察到吸收带形状受到染料周围碱基对的强烈影响，其中氢键更强的 G-C 对产生了与 J 型二聚体一致的红移吸收光谱。这一效应通过分子动力学模拟进行了理论研究，预测了与实验 J 型光谱一致的直线型染料构型。当 Cy3 二聚体存在于标准水性缓冲液中时，相对较强的电子耦合伴随着荧光寿命的降低，表明它促进了菁染料的非辐射弛豫。然而，我们表明，使用粘性溶剂可以抑制这种非辐射复合，从而恢复二聚体的荧光发射。在标准水性缓冲液和粘性甘油缓冲液中的 Cy3 二聚体的超快瞬态吸收测量表明，足够强的电子耦合通过与 Cy3 扭转运动相关的暗态增加了激发态弛豫的概率。
