Department of Physics and Astronomy and LaserLaB, Faculty of Science, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.
Department of Chemistry, Kennedy College of Science, University of Massachusetts-Lowell, One University Avenue, Lowell, Massachusetts 01854, United States.
J Am Chem Soc. 2020 Oct 14;142(41):17346-17355. doi: 10.1021/jacs.0c04619. Epub 2020 Sep 16.
Photosynthesis in plants starts with the capture of photons by light-harvesting complexes (LHCs). Structural biology and spectroscopy approaches have led to a map of the architecture and energy transfer pathways between LHC pigments. Still, controversies remain regarding the role of specific carotenoids in light-harvesting and photoprotection, obligating the need for high-resolution techniques capable of identifying excited-state signatures and molecular identities of the various pigments in photosynthetic systems. Here we demonstrate the successful application of femtosecond stimulated Raman spectroscopy (FSRS) to a multichromophoric biological complex, trimers of LHCII. We demonstrate the application of global and target analysis (GTA) to FSRS data and utilize it to quantify excitation migration in LHCII trimers. This powerful combination of techniques allows us to obtain valuable insights into structural, electronic, and dynamic information from the carotenoids of LHCII trimers. We report spectral and dynamical information on ground- and excited-state vibrational modes of the different pigments, resolving the vibrational relaxation of the carotenoids and the pathways of energy transfer to chlorophylls. The lifetimes and spectral characteristics obtained for the S1 state confirm that lutein 2 has a distorted conformation in LHCII and that the lutein 2 S1 state does not transfer to chlorophylls, while lutein 1 is the only carotenoid whose S1 state plays a significant energy-harvesting role. No appreciable energy transfer takes place from lutein 1 to lutein 2, contradicting recent proposals regarding the functions of the various carotenoids (Son et al. , (3), 575-584). Also, our results demonstrate that FSRS can be used in combination with GTA to simultaneously study the electronic and vibrational landscapes in LHCs and pave the way for in-depth studies of photoprotective conformations in photosynthetic systems.
植物的光合作用始于光捕获复合物（LHCs）对光子的捕获。结构生物学和光谱学方法已经绘制出 LHC 色素之间的结构和能量转移途径图谱。然而，关于特定类胡萝卜素在光捕获和光保护中的作用仍存在争议，这需要高分辨率技术来识别激发态特征和光合作用系统中各种色素的分子身份。在这里，我们成功地将飞秒受激拉曼光谱（FSRS）应用于多色生物复合物 LHCII 三聚体。我们展示了全局和目标分析（GTA）在 FSRS 数据中的应用，并利用它来量化 LHCII 三聚体中的激发迁移。这种强大的技术组合使我们能够从 LHCII 三聚体的类胡萝卜素中获得有关结构、电子和动态信息的有价值的见解。我们报告了不同色素的基态和激发态振动模式的光谱和动力学信息，解析了类胡萝卜素的振动弛豫和能量转移到叶绿素的途径。获得的 S1 态寿命和光谱特征证实，叶黄素 2 在 LHCII 中具有扭曲构象，叶黄素 2 的 S1 态不会向叶绿素转移，而叶黄素 1 是唯一其 S1 态发挥重要能量收集作用的类胡萝卜素。叶黄素 1 到叶黄素 2 没有可观的能量转移，这与最近关于各种类胡萝卜素功能的提议相矛盾（Son 等人，(3)，575-584）。此外，我们的结果表明，FSRS 可以与 GTA 结合使用，同时研究 LHC 中的电子和振动景观，并为深入研究光合作用系统中的光保护构象铺平道路。
