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

Abstract

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 S2 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.

Reconstitution of Okenone into Light Harvesting Complexes from Allochromatium minutissimum

“…Consequently, the carotenoid environment (i.e., complex accommodating the reconstituted carotenoid) plays a decisive role in this process. Such conclusion is consistent with experimental results [32], demonstrating the formation of carotenoid triplets; the latter depended on the type of complex used for reconstitution rather than the type of carotenoid mol ecule.…”

“…sphaeroides (strain R26.1) compared with the control samples from Rsp. rubrum [32]. In the first case, the spirilloxanthin mole cule had planar configuration, whereas in the second case it had twisted configuration.…”

“…minutissimum is con sistent with literature data. For example, Papagiannakis et al [32] reported reconstitution of only 5 10% of spiril loxanthin to the B850 complex of Rba. sphaeroides (strain R26.1).…”

Triplet Excitons of Carotenoids Formed by Singlet Fission in a Membrane

“…[30] There are no significant changes in the hydrogen out-of-plane wagging modes, which would otherwise indicate structural distortion of the polyene chain. [12,30,31] The HOOP bands of zeaxanthin at~874 and~959 cm À1 are nearly identical in the fluid and gel phases, and reveal that the carotenoids are not distorted in the bilayers. The findings support the conclusion that exciton coupling in the gel phase is the cause of the enhanced blue absorption in the UV/Vis spectrum.…”

Ultrafast time-resolved vibrational spectroscopies of carotenoids in photosynthesis

“…Another type of intermediate excited state, termed as S* has been found with carotenoids both free in solution and bound to lightharvesting complexes, and the things become even more complicated [13][14][15][16][17]. At the higher-energy side of the S 1 → S n transition, a new transient absorption band was detected by means of pump-probe timeresolved absorption spectroscopy and subsequent spectral analysis using SVD (singular value decomposition) and global fitting.…”