Carotenoid and bacteriochlorophyll energy transfer in the B808-866 complex from chloroflexus aurantiacus

The B 808-866 light-harvesting complex of the filamentous anoxygenic phototrophic green bacterium Chloroflexus aurantiacus has characteristics of both the LH1 and LH2 antenna complexes found in purple photosynthetic bacteria. Energy transfer kinetics in this complex were studied using ultrafast transient absorption spectroscopy and time-resolved fluorescence spectroscopy, including the excited singlet states of the γ-carotene present in the B808-866 complex and energy transfer to the B866 Bchl a. Energy transfer from the carotenoid Si state to the B866 Bchl a was observed and found to be ∼ 12-15% efficient. A separate pathway, populating the previously described S* state, was also observed as a precursor to carotenoid triplet state formation. While the energy transfer efficiency is similar to what has been reported for LH1 complexes of Rhodospirillum rubrum, the kinetic scheme for energy relaxation and transfer is somewhat different than that seen in either LH1 from Rhodospirillum rubrum or LH2 of Rhodobacter sphaeroides.