Natural and Artificial Chlorosomal Light-Harvesting Antenna: Relationship between the Supramolecular Organisation and the Properties of the Electronic Excitations.

光合作用的成功激发了许多研究人员对有机质进行太阳能转换的研究。然而，一个简单的数量级估计表明，在最佳条件下，一个典型的有机分子每秒只能吸收几个光子。因此，利用有机物生产任何类型的太阳能都需要一个高效的光收集设备--天线--来收集尽可能多的光子。自然界中发现的最有效的天线系统之一是绿硫细菌的天线系统，它们在极低的光照条件下进行光合作用。在那里，光被细菌-叶绿素分子的超分子排列所吸收，这种超分子排列被称为叶绿体。不幸的是，叶绿体具有很大程度的结构可变性，这阻碍了迄今为止用原子分辨率来解析这些组件的结构。为了减少样品的异质性，研究人员要么开发了色素含量控制更好的突变体，要么合成了结构类似于天然同类的化学定义明确的模型系统。然而，绿小体的结构仍然是一个持续争论的问题。由于单体之间的分子间相互作用，这种分子组装的最低电子激发态被描述为Frenkel激子，对应于许多分子相干共享的离域激发。由于这种激子状态的光物理性质主要取决于颜料的相互排列，因此有关超分子组织的信息也可以通过光学光谱来获取。然而，样品的高度异质性导致光谱的不均匀展宽，可能具有特定结构特征的微妙特征由于集合平均而被掩盖。本项目的目的是利用单分子光谱技术对天然(野生型和突变体)以及人工叶绿体进行系统研究。这包括吸收光谱、荧光激发光谱和发射光谱，以及对单个物体的圆二色光谱的发展。这种方法将最小化系综的不均匀性，并提供有关激子跃迁的光谱位置、激子跃迁的相对强度比、跃迁-偶极矩的相互取向以及色素排列的手性的信息。同时，实验结果将与计算机模拟的预测进行比较，计算机模拟将作为单体几何排列的函数进行。我们的目标是区分文献中讨论的各种结构模型，并找出是否存在作为色素组成的函数的叶绿体形态的系统变化。

The success of photosynthesis has inspired many researchers to study organic matter for solar energy conversion. However, a simple order-of-magnitude estimate reveals that under optimum conditions a typical organic molecule would absorb only a few photons per second. Hence, employing organic matter for any kind of solar driven energy production requires an efficient light-harvesting apparatus - an antenna - for collecting as many photons as possible. One of the most efficient antenna systems found in nature is that of the green-sulphur bacteria, which thrive photosynthetically under extremely low illumination conditions. There the light is absorbed in supramolecular arrangements of bacteriochlorophyll molecules that are referred to as chlorosomes. Unfortunately, chlorosomes feature a large degree of structural variability, which has hampered to resolve the structure of these assemblies with atomic resolution to date. In order to reduce the sample heterogeneity researchers either developed mutants with better controlled pigment content, or synthesized chemically well-defined model systems that structurally resemble their natural counterparts. Though, the structure of the chlorosomes is still a matter of an ongoing debate.Owing to the intermolecular interactions between the monomers, the lowest electronically excited states of such molecular assemblies are described as Frenkel excitons, which correspond to delocalised excitations that are coherently shared by many molecules. Since the photophysical properties of such exciton states depend crucially on the mutual arrangement of the pigments, information about the supramolecular organisation can be accessed also by optical spectroscopy. However, the great heterogeneity of the samples leads to inhomogeneous broadening of the spectra and subtle features, that might be charateristic for specific structural properties, are masked due to ensemble averaging.Aim of this project is a systematic study of natural (wild type and mutants) as well as artificial chlorosomes by single-molecule spectroscopic techniques. This includes absorption, fluorescence-excitation, and emission spectroscopy as well as the development of circular dichroism spectroscopy on an individual object. This approach will minimize the ensemble heterogeneity and provide information about the spectral positions of the exciton transitions, the relative intensity ratios of the exciton transitions, the mutual orientation of their transition-dipole moments, and the chirality of the pigment arrangement. In parallel, the experimental results will be compared with the predictions from computer simulations that will be conducted as a function of the geometrical arrangement of the monomers. Our goal is to discriminate between the various structural models discussed in the literature and to find out whether there is a systematic variation of the morphology of the chlorosomes as a function of the pigment composition.