Mechanism of Energy Transfer from Carotenoids to Bacteriochlorophyll:  Light-Harvesting by Carotenoids Having Different Extents of π-Electron Conjugation Incorporated into the B850 Antenna Complex from the Carotenoidless Bacterium Rhodobacter sphaeroides R-26.1

Abstract

Spheroidene and a series of spheroidene analogues with extents of π-electron conjugation ranging from 7 to 13 carbon−carbon double bonds were incorporated into the B850 light-harvesting complex of Rhodobacter sphaeroides R-26.1. The structures and spectroscopic properties of the carotenoids and the dynamics of energy transfer from the carotenoid to bacteriochlorophyll (BChl) in the B850 complex were studied by using steady-state absorption, fluorescence, fluorescence excitation, resonance Raman, and time-resolved absorption spectroscopy. The spheroidene analogues used in this study were 5‘,6‘-dihydro-7‘,8‘-didehydrospheroidene, 7‘,8‘-didehydrospheroidene, and 1‘,2‘-dihydro-3‘,4‘,7‘,8‘-tetradehydrospheroidene. These data, taken together with results from 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene, and spheroidene already published (Frank, H. A.; Farhoosh, R.; Aldema, M. L.; DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lugtenburg, J. Photochem. Photobiol. 1993, 57, 49. Farhoosh, R.; Chynwat, V.; Gebhard, R.; Lugtenburg, J.; Frank, H. A. Photosynth. Res. 1994, 42, 157), provide a systematic series of molecules for understanding the molecular features that determine the mechanism of energy transfer from carotenoids to BChl in photosynthetic bacterial light-harvesting complexes. The data support the hypothesis that only carotenoids having 10 or less carbon−carbon double bonds transfer energy via their 2Ag (S1) states to BChl to any significant degree. Energy transfer via the 11Bu (S2) state of the carotenoid becomes more important than the S1 route as the number of conjugated carbon−carbon double bonds increases. The results also suggest that the S2 state associated with the Q x transition of the B850 BChl is the most likely acceptor state for energy transfer originating from both the 21Ag (S1) and 11Bu (S2) states of all carotenoids.

Carotenoid-binding Sites of the Major Light-harvesting Complex II of Higher Plants

“…These values indicate that the environment of the different xanthophylls in the three xanthophyll-binding sites is similar. Comparable red shift values were observed for spheroidene in LH2 proteins (33).…”

Electronic and vibrational properties of carotenoids: from in vitro to in vivo

“…These carotenoids can have different functions depending on their configuration and environment, either as auxiliary LH molecules [100,101] or as photoprotective species quenching Chl exited states [33,90,91]. In native LH pigment -protein complexes, spheroidene and neurosporene bind to LH2 from Rhodobacter sphaeroides, strains 2.4.1 (grown anaerobically) and G1C, respectively, whereas spirilloxanthin is present in LH1 from Rhodospirillum rubrum strain S1 [90,91,102].…”

Quantum Chemical Description of Absorption Properties and Excited‐State Processes in Photosynthetic Systems

“…[308] In the LH2 complex (see Section 6.1), the total efficiency of these channels drops if the carotenoid contains more than ten double bonds. [309] This finding was related to the energy lowering of electronic states in carotenoids leading to a closure of energy-transfer channels to BChl pigments. [309,310] This closed channel might either be the 1B u + (S 2 )-to-Q x [310] or the 2A g À (S 1 )-to-Q y pathway.…”