The excitation energy transfer (EET) from the bacteriochlorophyll (BChl) Soret band to the second excited state(s) (S2) of carotenoids in pigment-protein complexes of purple bacteria was investigated. The efficiency of EET was determined, based on fluorescence excitation and absorption spectra of chromatophores, peripheral light-harvesting complexes (LH2), core complexes (LH1-RC), and pigments in solution. Carotenoid-containing and carotenoid-less samples were compared: LH1-RC and LH2 from Allochromatium minutissimum, Ectothiorhodospira haloalkaliphila, and chromatophores from Rhodobacter sphaeroides and Rhodospirillum rubrum wild type and carotenoid-free strains R-26 and G9. BChl-to-carotenoid EET was absent, or its efficiency was less than the accuracy of the measurements of ∼5%. Quantum chemical calculations support the experimental results: The transition dipole moments of spatially close carotenoid/BChl pairs were found to be nearly orthogonal. The structural arrangements suggest that Soret EET may be lacking for the studied systems, however, EET from carotenoids to Qx appears to be possible.

Belozersky Research Institute of Physico Chemical Biology Lomonosov Moscow State University 119991 Moscow Russia

Department of Chemical Physics and Optics Charles University Ke Karlovu 3 121 16 Prague Czech Republic

Faculty of Biology Lomonosov Moscow State University 119991 Moscow Russia

Institute of Basic Biological Problems of Russian Academy of Sciences Federal Research Center Pushchino Scientific Center for Biological Research of Russian Academy of Sciences 142290 Pushchino Russia

Institute of Chemistry and Biochemistry Freie Universität Berlin 14195 Berlin Germany

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