TCB Publications - Abstract

Ana Damjanović, Thorsten Ritz, and Klaus Schulten. Energy transfer between carotenoids and bacteriochlorophylls in a light harvesting protein. Physical Review E, 59:3293-3311, 1999.

DAMJ99 In photosynthetic light harvesting systems carotenoids and chlorophylls jointly absorb light and transform its energy within about a picosecond into electronic singlet excitations of the chlorophylls only. This paper investigates this process for the light harvesting complex II of the purple bacterium Rs. molischianum, for which a structure and, hence, the exact arrangement of the participating chlorophylls and carotenoids have recently become known. Based on this structure and on CI expansions of the electronic states of individual chromophores (chlorophylls and carotenoids) as well as on an exciton description of a circular aggregate of chlorophylls, the excitation transfer between carotenoids and chlorophylls is described by means of Fermi's golden rule. The electronic coupling between the various electronic excitations is determined for all orders of multipoles (generalized Förster mechanism) and includes the electron exchange (dexter mechanism) term. The rates and efficiencies for different pathways of excitation transfer, e.g., $1^{1}B^{+}_{u}$ (carotenoid) $\rightarrow$ chlorophyll aggregate and $2^{1}A^{-}_{g}$ (carotenoid) $\rightarrow$ chlorophyll aggregate, are compared. The results show that in LH-II the generalized Förster mechanism is dominant for the transfer of singlet excitations and that exciton splitting of chlorophyll $Q_{y}$ excitations plays a crucial role in accelerating the lycopene $\rightarrow$ chlorophyll excitation transfer. Photoprotection of chlorophylls through triplet quenching is investigated, too. The results suggest that eight of the 24 chlorophylls in LH-II of Rs. molischianum are protected well by eight carotenoids observed in the X-ray structure of the protein, but that the remaining chlorophylls are protected by eight further carotenoids which are known to be bound to the protein, but have not yet been resolved in an X-ray structure.

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