Theory of Light-Harvesting in Photosynthesis

A principal aim of the present project was the elucidation of building principles of photosynthetic antenna and reaction center complexes that can explain their high quantum e?ciency and robustness. For this purpose dynamical theories of optical spectra and excitation energy transfer and microscopic methods for the structure-based calculations of the parameters of these theories were developed further. A breakthrough occurred in the structure-based calculation of the spectral density of the pigment-protein coupling, which describes how protein dynamics modulates the pigment-pigment and pigment-protein interactions. For the ?rst time it became possible to calculate the spectral density in semi-quantitative agreement with experimental data. On the basis of these calculations we obtained a microscopic understanding of the dissipation of the pigments electronic excess energy by protein vibrations. The modulation of pigment-pigment couplings turned out to be one order of magnitude weaker than that of the pigment-protein coupling. A detailed analysis of the pigment-protein coupling revealed that it is the uncorrelated modulation of pigment transition energies that allows the protein to dissipate the excess energy of the pigments. The pigment-pigment coupling delocalizes the excited states formed after light absorption in these complexes and these delocalized states are in?uenced by the pigment-protein coupling. One distinguishes diagonal coupling that describes the ?uctuation of energies of these delocalized states and o?-diagonal coupling that couples di?erent delocalized states. The calculation of the spectral density revealed that the o?-diagonal couplings are one order of magnitude smaller than the diagonal couplings. This result directly went into the development of dynamical theories of optical spectra and energy transfer, in which the diagonal couplings were treated exactly and the o?-diagonal couplings were described in perturbation theory. Our structure-based parameterization methods and dynamical theories were successfully applied to various photosynthetic pigment-protein complexes. Energy sinks in light-harvesting antennae CP29, CP43 and CP47 of photosystem II could be uncovered, as well as the molecular identity of di?erent functional states of the BLUF photoreceptor, which is responsible for gene regulation in photosynthesis. In addition, theories were developed for the description of new types of optical spectra that can be used to verify the structure-based parameterization of light-harvesting systems. Besides a theory for the description of circularly polarized ?uorescence, we have developed a theory of hole-burning spectra of multi-pigment protein complexes, which for the ?rst time allows one to analyze the whole spectrum and to extract important information about the system like the lifetime of excited states that previously was available only from time-resolved spectroscopy.