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物质进行太阳能转换的研究。然而，一个简单的数量级估计显示，在最佳条件下，一个典型的有机分子每秒只吸收几个光子。因此，在任何一种太阳能驱动的能源生产中使用有机物质都需要一种高效的光收集装置——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，它们在极低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吸收，这些分子被称为叶绿体。不幸的是，叶绿体具有很大程度的结构变异性，这阻碍了迄今为止用原子分辨率解决这些组装的结构。为了减少样品的异质性，研究人员要么开发具有更好控制色素含量的突变体，要么合成化学上定义良好的模型系统，在结构上与天然对应物相似。尽管如此，叶绿体的结构仍然是一个持续争论的问题。由于单体之间的相互作用，这种分子组合的最低电子激发态被描述为Frenkel激子，它对应于许多分子相干共享的离域激发。由于这种激子态的光物理性质在很大程度上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谱学获得。然而，样品的巨大非均质性导致光谱的非均匀展宽，并且由于系综平均而掩盖了可能具有特定结构特性的细微特征。本项目旨在利用单分子光谱技术对天然（野生型和突变型）和人工叶绿体进行系统的研究。这包括吸收光谱、荧光激发光谱和发射光谱，以及在单个物体上发展圆二色光谱。这种方法将最大限度地减少系综不均匀性，并提供有关激子跃迁的光谱位置、激子跃迁的相对强度比、它们的过渡偶极矩的相互取向以及颜料排列的手性的信息。同时，实验结果将与计算机模拟的预测结果进行比较，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功能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.