- Pigment-Pigment Interactions and Protein Dynamics in Light-Harvesting Complexes: a Single-Molecule Study (2004)
- Light harvesting complexes that are involved in the first steps of photosynthesis in purple bacteria were studied with low-temperature optical single molecule spectroscopy. In one series of experiments the spectral properties of the bacteriochlorophyll a molecules within the complexes were studied in the view of model systems of molecular aggregates. It was found that the excitations in the B800 band of the light harvesting 2 complex are mainly localised on individual chromophores although evidence was found for an electronic coupling in the weak to intermediate range between individual bacteriochlorophyll a molecules. In contrast, for the B850 band of the light harvesting complex 2 as well as for the B870 band of the light harvesting complex 1 it was found that the assembly of bacteriochlorophyll molecules represents a strongly coupled system and that the excitation is coherently delocalised over a substantial part of the chromophores. By performing Monte-Carlo simulation an estimate on the amount of random and correlated energetic disorder in the site energies of the chromophores as well as on structural properties of the complexes could be given. In experiments on individual LH2-LH1-RC complexes, the energy transfer within a single photosynthetic unit was observed. In further experiments the chromophores were used as local probes to monitor conformational fluctuations of the protein residues in their binding pocket. Looking at the spectral diffusion of individual chromophores allowed to elucidate the organisation of the protein-energy landscape in tiers. In addition a clear correlation for the transition rates between those states and the energy separation of the levels involved could be uncovered. To simplify and automatise the analysis of a large number of consecutively recorded spectra a pattern recognition approach using multivariate statistical analysis proved to be a very useful tool. Apart from elucidating spectral diffusion processes detailed information about the line shape of individual chromophore absorptions could be gained from which it was inferred that the electron-phonon coupling in the B800 pigment pool is very weak. In summary, this thesis demonstrates that low-temperature single-molecule spectroscopy provides a unique method to reveal details of pigment-pigment interactions in the weak to intermediate as well as strong coupling limit that are inaccessible by other experimental methods.