Ab initio description of the quantum mechanics in light-harvesting complexes of purple bacteria

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The complex and subtle relationship between structure and function, i.e. light-harvesting, of photosynthetic systems has been studied conjointly with high-resolution crystallographic structures, ultrafast spectroscopy and quantum chemical calculations. The aim of the proposal is to start at the available crystal structure of the light-harvesting system 2 (LH2) of purple bacterium Rhodopseudomonas acidophila, perform a molecular dynamics simulation and then use the obtained thermal fluctuations of the nuclear positions as input for quantum chemical calculations. Based on the results of these quantum chemical calculations a refined quantum mechanical model shall be applied to the determination of optical spectra. This way one can construct a model with unprecedented accuracy and without fitting parameters. Besides static and transient spectra of ensembles of LH2 rings, another focus shall be on the theoretical determination and classification of single-molecule spectra. The model also allows for the incorporation of the carotenoids into the model and to treat them on the same footing as the bacteriochlorophylls which constitute the ring systems. Modeling of recent coherent control experiments on the energy transfer between carotenoids and the rings in the LH2 is a further goal of the project.