The photophysics and photochemistry of carotenoids {\it in vitro\/} and in light-harvesting pigment-protein complexes

Carotenoids supplement the light-capturing ability of (bacterio)chlorophyll in antenna pigment-protein complexes by harvesting light in the $\sim$425-500 nm visible region. Energy is then transferred from the carotenoids-to-(bacterio)chlorophyll via singlet energy transfer. To understand the molecular details of how carotenoids carry out their light-harvesting role, the spectroscopic properties of spheroidene and a series of spheroidene analogs with extents of $\pi$-electron conjugation ranging from 7 to 13 carbon-carbon double bonds were studied using steady-state and time resolved optical spectroscopy. Studies were conducted with the carotenoids in solution and incorporated into the B850 light-harvesting complex from Rhodobacter sphaeroides R-26.1. The spheroidene analogs used were $5\sp\prime,6\sp\prime$-dihydro-7$\sp \prime,8\sp\prime$-didehydrospheroidene, 7$\sp\prime,8\sp\prime$-didehydrospheroidene, and $1\sp\prime,2\sp\prime$-dihydro-3$\sp\prime,4\sp \prime,7\sp\prime,8\sp\prime$-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 (DeCoster, B., et al. (1992) Biochim. Biophys. Acta 1102, 107-114.: Frank, H. A., et al. (1993) Chem. Phys. Lett. 207, 88-92.: Frank, H. A., et al. (1993) Photochem. Photobiol. 57, 49-55.: and Farhoosh, R., et al. (1994) Photosyn. Res. 42, 157-166.) provide a systematic series of molecules for understanding the molecular features that control energy transfer to bacteriochlorophyll in photosynthetic bacterial light-harvesting complexes. Based on the results of the in vitro studies it was hypothesized that transfer from the S$\sb1$ state of the carotenoid is more significant for the shorter molecules ($\le$10 carbon-carbon double bonds) whereas for longer carotenoids ($\u3e$10 carbon-carbon double bonds), transfer from the S$\sb2$ state is more important. After the behavior of the carotenoids in solution was established the carotenoids were incorporated into the B850 light-harvesting complex. Data obtained support the hypothesis that only carotenoids having 10 or less carbon-carbon double bonds transfer energy via their S$\sb1$ $\rm(2\sp1A\sb{g})$ states to bacteriochlorophyll to any significant degree. Energy transfer via the S$\sb2$ $\rm(1\sp1B\sb{u})$ state of the carotenoid becomes more important than the S$\sb1$ route as the number of conjugated carbon-carbon double bonds increase. The results also suggest that the S$\sb2$ state associated with the Q$\rm\sb{x}$ transition of the B850 bacteriochlorophyll, is the most likely acceptor state for energy transfer originating from both the $\rm2\sp1A\sb{g}$ and $\rm 1\sp1B\sb{u}$ states of all carotenoids.