MECHANISM OF ENERGY-TRANSFER FROM CAROTENOIDS TO BACTERIOCHLOROPHYLL - LIGHT-HARVESTING BY CAROTENOIDS HAVING DIFFERENT EXTENTS OF PI-ELECTRON CONJUGATION INCORPORATED INTO THE B850 ANTENNA COMPLEX FROM THE CAROTENOIDLESS BACTERIUM RHODOBACTER-SPHAEROIDES R-26.1

Spheroidene and a series of spheroidene analogues with extents of pi-e lectron conjugation ranging from 7 to 13 carbon-carbon double bonds we re 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 t o bacteriochlorophyll (BChl) in the B850 complex were studied by using steady-state absorption, fluorescence, fluorescence excitation, reson ance Raman, and time-resolved absorption spectroscopy. The spheroidene analogues used in this study were 5',6'- dihydro-7',8'-didehydrospher oidene, 7',8'-didehydrospheroidene, and 1',2'-dihydro-3',4',7',8'-tetr adehydrospheroidene. These data, taken together with results from 3,4, 7,8-tetrahydsospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydros pheroidene, and spheroidene already published (Frank, H. A.; Farhoosh, R.; Aldema, M. L.; DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lug tenburg, 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 t he 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 havin g 10 or less carbon-carbon double bonds transfer energy via their 2(1) A(g) (S-1) states to BChl to any significant degree. Energy transfer v ia the 1(1)B(u) (S-2) State of the carotenoid becomes more important t han the S-1 route as the number of conjugated carbon-carbon double bon ds increases. The results also suggest that the S-2 state associated w ith the Q(x) transition of the B850 BChl is the most likely acceptor s tate for energy transfer originating from both the 2(1)A(g) (S-1) and 1(1)B(u) (S-2) states of all carotenoids.