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We report femtosecond transient absorption studies of energy transfer dynamics in the B800-850 lightharvesting complex (LHC) of Rhodobacter sphaeroides 2.4.1. For complexes solubilized in lauryldimethylamine-N-oxide (LDAO), the carotenoid to bacteriochlorophyll (Bchl) B800 and carotenoid to Bchl B850 energy transfer times are 0.34 and 0.20 ps, respectively. The B800 to B850 energy transfer time is 2.5 ps. For complexes treated with lithium dodecyl sulfate (LDS), a carotenoid to B850 energy transfer time of c0.2 ps is seen, and a portion of the total carotenoid population is decoupled from Bchl. In both LDAO-solubilized and LDS-treated complexes an intensity-dependent picosecond decay component of the excited B850 population is ascribed to excitation annihilation within minimal units of the LHC.The B800-850 light-harvesting complex (LHC) serves as the principal antenna complex in many purple photosynthetic bacteria (1) and has been extensively studied in Rhodobacter sphaeroides and related genera (1-12). The B800-850 LHC contains bacteriochlorophyll (Bchl) and carotenoid pigments in a ratio of 2:1 (2, 3). The Bchl is partitioned between two binding sites; the Qy absorption of B800 is near 800 nm, that of B850 is near 850 nm. In B800-850 solubilized in lauryldimethylamine-N-oxide (LDAO), referred to as B800-850/ LDAO, both absorptions are present; in B800-850 treated with lithium dodecyl sulfate (LDS), referred to as B800-850/LDS, the 800-nm absorption is absent (4, 5). Models for the structure of the B800-850 complex suppose the complex to be composed of replications of a minimal pigmentpolypeptide unit (6, 7). The minimal unit of ref. 6 contains four B850, two B800, and three carotenoid molecules. Two of the carotenoids are coupled only to B850 molecules and the third carotenoid is coupled only to B800 molecules.Previous work has provided some understanding of singlet energy transfer processes in B800-850 LHCs. In B800-850/LDAO isolated from Rb. sphaeroides 2.4.1, total carotenoid to B850 energy transfer efficiency is 95% ± 5% (4, 6). At 4 K, 15-25% of the carotenoid excitations are transferred to B800 with subsequent, nearly 100% efficient, B800 to B850 energy transfer; the remaining carotenoid excitations are transferred directly to B850 (8). Measurements of the B800 to B850 transfer have put the time at (i) 1-2 ps at 77 K and <1 ps at 295 K as determined by picosecond transient absorption spectroscopy (9) and (ii) :3.3 ps at 4 K as calculated from B800 fluorescence emission measurements (8). For B800-850/LDS the total carotenoid to B850 energy transfer efficiency is only 72% ± 3% (4, 6). The dynamics of carotenoid to Bchl energy transfer have been studied by picosecond transient absorption in two strains of Rhodopseudomonas acidophila revealing carotenoid to Bchl transfer times of :'5 ps (10) and ==3 ps (11). The mechanism of carotenoid to Bchl singlet energy transfer is thought to be exchange coupling (13) with transfer occurring from an energetically low-lying carotenoid electronic state analogous t...

Resonance Raman (RR) spectra are reported for the photosynthetic reaction center (RC) protein from Rhodobacter sphaeroides 2.4.1. The spectra were obtained with a variety of excitation wavelengths, spanning the UV, violet, and yellow-green regions of the absorption spectrum, and at a number of temperatures ranging from 30 to 270 K. The RR data indicate that the frequencies of certain vibrational modes of the bacteriochlorin pigments in the RC shift with temperature. These shifts are reversible and do not depend on external factors such as solvent or detergent. The acetyl carbonyl bands exhibit the largest shifts with temperature. These shifts are attributed to thermal effects involving the torsional vibrations of the acetyl groups of several (or all) of the bacteriochlorins rather than to specific pigment-protein interactions. The frequency of the structure-sensitive skeletal mode near 1610 cm-1 of one of the two bacteriopheophytins (BPhs) in the RC is also sensitive to temperature. In contrast, no temperature sensitivity is observed for the analogous modes of the bacteriochlorophylls or other BPhs. Over the range 160-100 K, the skeletal mode of the BPh upshifts by approximately 4 cm-1. This upshift is attributed to a flattening of the macrocycle at low temperatures. It is suggested that the BPh active in the electron-transfer process is the pigment whose structure is temperature dependent. It is further suggested that such structural changes could be responsible in part for the temperature dependence of the electron-transfer rates in photosynthetic RCs.

-Carotenoids from phototrophic bacteria cany out light-harvesting in antenna proteins via carotenoid-to-bacteriochlorophyll singlet-singlet energy transfer and photoprotection in the reaction center via bacteriochlorophyll-to-carotenoid triplet-triplet energy transfer. Spectroscopic studies have permitted elucidation of the explicit routes of these transfers in pigment-protein complexes obtained from the bacterium Rhodobacter sphaeroides. The molecular details of these mechanisms are presented and discussed in conjunction with studies revealing the structural features of the complexes.

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