Single-Molecule Studies on Purple Bacterial Antenna Complexes
Final Report Abstract
The total annual global energy consumption is set at least to double from its current level by 2050 and it appears that mankind urgently needs to find new ways to produce clean, renewable sources of energy that do not produce green house gases. Given the fact that about 0.05% of the energy provided by the sun would be sufficient to cover the global needs, it becomes clear that solar energy conversion provides a sustainable, environmental clean, longterm solution for that problem. With the invention of photosynthesis Nature demonstrates that organic materials indeed can be exploited successfully for that purpose - on large scales and under various environmental conditions. In this regard the natural systems provide beautiful model systems to study energy- and charge transfer processes in well-defined structures that might serve as blueprints for the design of such an antenna structure. In this project we investigated pigment-protein complexes from the light-harvesting apparatus of photosynthetic purple bacteria. In these species the light reactions take place in the peripheral LH2 complex and the core RC-LH1 complex. Unfortunately, it appears very difficult to obtain highly-resolved structural information about the LH complexes, and it was the aim of this project to develop a deeper understanding of the structural basis of the range of natural variation of the spectroscopic forms of the antenna complexes. In order to do so we combined biochemical preparations, low-temperature single-molecule spectroscopy, and computer simulations to investigate individual LH2 and RC-LH1 complexes from various species where no or only little structural information is available as yet. Interestingly, when grown under low-light conditions some species of purple bacteria thrive for a better coverage of the solar spectrum by expressing additional so-called low-light (LL) LH2 complexes. In this project we were able to elucidate that Nature follows different strategies with the LL LH2 complexes. For some species we find a mixture of LH2 complexes that absorb in different spectral ranges, whereas for other species each LH2 complex features a mixed composition allowing for absorption over a broad spectral range. Another result obtained within this project was that we could show that the RC-LH1 complexes from two different species of purple bacteria could be described by the same geometrical model. Finally, we addressed a question that was not part of the original proposal, and which concerns details of the electronic excitations within the LH structures, so-called exciton self trapping. From our results we found direct evidence for correlations between the electronic excitations and the nuclear motions, which is indicative that exciton self trapping might indeed be effective for some of the LH2 complexes.
Publications
- Single-Molecule spectroscopy reveals that individual low-light LH2 complexes from Rhodopseudomonas palustris 2.1.6. have a heterogeneous polypeptide composition. Biophys. J. 97 (2009) 1491-1500
T. Brotosudarmo, R. Kunz, P. Böhm, A.T. Gardiner, V. Moulisova, R.J. Cogdell, J. Köhler
- Sunlight, Purple Bacteria and Quantum Mechanics: How Purple Bacteria Harness Quantum Mechanics for efficient Light Harvesting in: Quantum Efficiency in Complex Systems, Part I: Biomolecular systems (Semiconductor and Semimetals) eds: Weber, Würfel, Thorwart, Vol. 83 (2010) 77-94
R.J. Cogdell, J. Köhler
- The Influence of Symmetry on the Electronic Structure of the Photosynthetic Pigment- Protein Complexes from Purple Bacteria Nobel Conference: Single Molecule Spectroscopy in Chemistry, Physics and Biology; Eds. A. Gräslund, R. Rigler, J. Widengren, Springer Series in Chemical Physics, 96 (2010), 513 - 533
M.F. Richter, J. Baier, R.J. Cogdell, S. Oellerich, J. Köhler
- Exciton Self Trapping in Photosynthetic Pigment-Protein Complexes studied by Single-Molecule Spectroscopy. J. Phys. Chem. B 116 (2012) 11017-11023
R. Kunz, K. Timpmann, J. Southall, R. Codgell, A. Freiberg, J. Köhler
(See online at https://doi.org/10.1021/jp3040456) - Single-Molecule Spectroscopy on RC-LH1 Complexes of Rhodopseudomonas acidophila strain 10050. J. Phys. Chem. B. 117 (2013) 3120-3126
P. Böhm, J. Southall, R.J. Cogdell, J. Köhler
(See online at https://doi.org/10.1021/jp4005218)
