Light harvesting mechanism of the multi-pigment reaction center with three chlorophyll species adapted to both visible and far-red light

We studied functions of a unique type-I photosynthetic reaction center complex containing three different types of chlorophyll species: bacteriochlorophyll a (BChl a), Zn-bacteriochlorophyll aʹ (Zn-BChl aʹ), and chlorophyll a (Chl a), adapted to both visible and far-red light environments, of an Acidobacterium Chloracidobacterium thermophilum reaction center (CabRC). The RC contains altogether 16 BChl a, 10 Chl a, the two molecules of Zn-BChl aʹ that form a special pair, and two lycopenes. We analyzed the excitation energy transfer (EET) dynamics on CabRC by calculating exciton Hamiltonians of all the pigments based on the structural information by using the modified Redfield theory. EET rates were predicted to be slower between Chls a and BChls a, compared to that between Chls a or between BChls a. The slow EET between exciton bands of 10 Chls a and 16 BChls a, thus, limits the overall efficiency of EET, although it is significantly enhanced by the intramolecular vibrational modes in the exciton-phonon interaction. The usage of multiple pigment species, therefore, allows the efficient usage of energy of a wide range of visible to far-red light in CabRC, which provides the highly optimized arrangements of heterogeneous pigments.