Read Elizabeth L, Engel Gregory S, Calhoun Tessa R, Mancal Tomás, Ahn Tae Kyu, Blankenship Robert E, Fleming Graham R
Department of Chemistry, University of California, Berkeley, CA 94720, USA.
Proc Natl Acad Sci U S A. 2007 Sep 4;104(36):14203-8. doi: 10.1073/pnas.0701201104. Epub 2007 Jun 4.
Intermolecular electronic coupling dictates the optical properties of molecular aggregate systems. Of particular interest are photosynthetic pigment-protein complexes that absorb sunlight then efficiently direct energy toward the photosynthetic reaction center. Two-dimensional (2D) ultrafast spectroscopy has been used widely in the infrared (IR) and increasingly in the visible to probe excitonic couplings and observe dynamics, but the off-diagonal spectral signatures of coupling are often obscured by broad diagonal peaks, especially in the visible regime. Rotating the polarizations of the laser pulses exciting the sample can highlight certain spectral features, and the use of polarized pulse sequences to elucidate cross-peaks in 2D spectra has been demonstrated in the IR for vibrational transitions. Here we develop 2D electronic spectroscopy using cross-peak-specific pulse polarization conditions in an investigation of the Fenna-Matthews-Olson light harvesting complex from green photosynthetic bacteria. Our measurements successfully highlight off-diagonal features of the 2D spectra and, in combination with an analysis based on the signs of features arising from particular energy level pathways and theoretical simulation, we characterize the dominant response pathways responsible for the spectral features. Cross-peak-specific 2D electronic spectroscopy provides insight into the interchromophore couplings, as well as into the energetic pathways giving rise to the signal. With femtosecond resolution, we also observe dynamical processes that depend on these couplings and interactions with the protein environment.
分子间电子耦合决定了分子聚集体系统的光学性质。特别令人感兴趣的是光合色素 - 蛋白质复合物，它们吸收太阳光，然后有效地将能量导向光合反应中心。二维（2D）超快光谱已在红外（IR）领域广泛应用，在可见光领域的应用也日益增多，用于探测激子耦合并观察动力学过程，但耦合的非对角光谱特征常常被宽的对角峰所掩盖，尤其是在可见光区域。旋转激发样品的激光脉冲的偏振方向可以突出某些光谱特征，并且在红外区域用于振动跃迁的二维光谱中，使用偏振脉冲序列来阐明交叉峰的方法已得到证实。在此，我们利用交叉峰特定的脉冲偏振条件开发了二维电子光谱，用于研究绿色光合细菌中的费纳 - 马修斯 - 奥尔森光捕获复合物。我们的测量成功地突出了二维光谱的非对角特征，并结合基于特定能级路径产生的特征符号的分析和理论模拟，我们表征了负责光谱特征的主要响应路径。交叉峰特定的二维电子光谱为发色团间的耦合以及产生信号的能量路径提供了深入了解。借助飞秒分辨率，我们还观察到了依赖于这些耦合以及与蛋白质环境相互作用的动力学过程。
