AbstractsBiology & Animal Science

The topic of this thesis was the investigation of the specific arrangement of the bacteriochlorophyll (BChl) a molecules in the light-harvesting 1 (LH1) complex from Rhodopseudomonas (Rps.) acidophila. In purple bacteria the LH1 complex forms an interconnected unit with a reaction-center (RC) complex, where the LH1 complex directly surrounds the RC to form the so called RC-LH1 core complex. As part of a cyclic electron transport pathway in bacterial photosynthesis, reduced ubiquinone molecules, Q_B H_2, have to leave the RC and pass through the LH1 complex into the membrane space. By now, it is well established that for some purple bacterial species the LH1 complex has a gap, presumably to facilitate the shuttling of Q_B H_2, whereas for other species, with a completely closed LH1 ring, channels in the LH1 aggregate might enable the transfer of Q_B H_2. Before this work, it was not known to which of these two types of RC-LH1 complexes (interrupted vs. closed LH1 ring), the RC-LH1 complex from Rps. acidophila belonged, let alone an idea of the exact arrangement of the BChl a molecules in the LH1 aggregate. In the following a short summary of the results achieved in this thesis will be presented. In a first series of experiments the RC-LH1 complex from Rps. acidophila was revisited for single-molecule spectroscopy. Thereby, the pigment-protein complexes were stabilized in the relatively mild, non-denaturating detergent dodecyl-β-D-maltoside (DDM), instead of the more deactivating detergent lauryldimethylamine N-oxide (LDAO) which was used in a previous study. In total, low-temperature (1.2 K) fluorescence-excitation spectra have been recorded from 61 individual RC-LH1 complexes stabilized in DDM, in comparison to only 24 complexes measured in the precursor study. According to their spectral characteristics, the complexes were either considered as intact or broken. The fraction of broken (dissociated) RC-LH1 complexes could be reduced significantly by using DDM instead of LDAO as detergent, namely from 37.5% in the precursor work to 11.5% in this thesis. Due to the enhanced quality of the stock solution, it was possible to perform polarization-resolved fluorescence-excitation spectroscopy, and to determine the distributions of the spectral properties for the intact RC-LH1 complexes with an improved statistics. Both these distributions and characteristic spectral features, such as a narrow line occurring on the red end of the spectra for most of the complexes, show a remarkable resemblance with the data obtained on RC-LH1 complexes from Rps. palustris in an earlier study. For the Rps. palustris RC-LH1 complex a low-resolution crystal structure is available. Hence, taking also the aforementioned similarity of the spectral characteristics into account, it is concluded that the LH1 complexes from Rps. acidophila and Rps. palustris can be described by the same structural model, namely an elliptical arrangement of BChl a molecules interrupted by a gap. Thus, by successfully applying low-temperature single-molecule…