Solar energy capture and conversion in plants and bacteria is governed by structure and interactions in photosynthetic pigment-protein complexes. By rigorous comparison to experimental data, we show that describing protein environment with the polarizable effective fragment potential (EFP) method provides realistic description of excitonic interactions in one of the extensively studied photosynthetic systems, the Fenna-Matthews-Olson (FMO) pigment-protein complex. In this talk we will discuss new experimental data and modeling revealing dynamics of the triplet energy transfer and dissipation in FMO and its mutants. Triplet states generated in photosynthetic complexes present a hazard as they can readily generate highly-reactive singlet oxygen. The nature has developed a number of mechanisms to dissipate triplet state energy safely, including incorporation of carotenoids into photosynthetic complexes. However, FMO lacks any carotenoids and the photoprotection mechanism is still unknown. Our modeling shows that a possible photoprotection mechanism could be a lowering of the triplet state energy of BChl a pigments below the energy of singlet oxygen. Additional benefit of studying the triplet state dynamics lays in the fact that this allows us to mimic knock-out mutations without actually removing BChl a out of the system. A pigment in the triplet state is effectively excluded from excitonically interacting pools of pigments. We have measured triplet state dynamics by time-resolved absorption/CD spectroscopy and used these data in conjunction with computational modeling to reveal detailed excitonic structure of this protein and its mutants. The developed methodology provides structure-function relations for energy transfer in FMO and is generalizable to other photosynthetic complexes.

Triplet energy transfer in the Fenna-Matthews-Olson (FMO) pigment-protein complex: Experiment and modeling

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ACS Fall 2022

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