Light harvesting by carotenoids incorporated into the B850 light-harvesting complex from Rhodobacter sphaeroides R-26.1:: Excited-state relaxation, ultrafast triplet formation, and energy transfer to bacteriochlorophyll

Spirilloxanthin and spheroidene were reconstituted into the carotenoidless B850 light-harvesting (LH) complex from the Rhodobacter (Rb.) sphaeroides R-26.1 mutant with the aim to obtain new insights in energy transfer, triplet formation, and other relaxation phenomena in photosynthetic light harvesting. Resonance Raman measurements showed that spirilloxanthin and spheroidene are bound to the B850 complex in the same planar configuration, whereas spirilloxanthin in its native LH1 complex of Rhodospirillum (Rs.) rubrum assumes a twisted configuration. Ultrafast transient absorption measurements with excitation of the carotenoid molecules to their S-2 state enabled us to identify, in both reconstituted B850 complexes, the recently found S* carotenoid singlet excited state and the direct generation of carotenoid triplet states within picoseconds through the singlet fission mechanism [Gradinaru, C. C., et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 2364-2369]. Global analysis has allowed us to quantify the formation yields of these states. In the B850 complex reconstituted with spheroidene, the triplet yield is 5-10%, similar to that found on the spheroidene-binding LH2 complex of Rb. sphaeroides 2.4.1 [Papagiannakis, E. et al. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6017-6022]. The triplet state of spirilloxanthin in the B850 complex is formed with a similar low yield, in contrast to the native LH1 complex of Rs. rubrum where the triplet yield is as large as 25-30%. This illustrates that the formation of the triplet state depends on the type of complex that binds the carotenoid and not on the carotenoid itself. More specifically, the singlet fission process that underlies ultrafast triplet formation is more efficient when spirilloxanthin is bound in a distorted configuration (in Rs. rubrum) than with either spirilloxanthin or spheroidene bound in a planar configuration (in Rb. sphaeroides). The extent of geometrical deformation of the carotenoid imposed by binding to the LH complexes partly determines the carotenoid light-harvesting function by either deactivating the excited-state energy of S* by transformation into a triplet pair or allowing this energy to flow to bacteriochlorophyll. Comparison of the energy transfer properties in the spheroidene-reconstituted B850 complex, which lacks the B800 bacteriochlorophyll, with that of the LH2 complex of Rb. sphaeroides 2.4.1 suggests that, apart from a light-harvesting function, the B800 bacteriochlorophylls in LH2 may have an important role in funneling the photon energy absorbed by carotenoids toward the reaction center.