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

• Applicant: Professor Dr. Jürgen Köhler
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: from 2014 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 256462505

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.

• DFG Programme: Research Grants
• International Connection: USA
• Participating Persons: Professor Dr. Donald Bryan; Professor Dr. Frank Würthner