Light-induced red shift of the Qx absorption band of light-harvesting bacteriochlorophyll in Rhodopseudomonas capsulata and Rhodopseudomonas sphaeroides (original) (raw)
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Dimeric carotenoid interaction in the light-harvesting antenna of purple phototrophic bacteria
The Biochemical journal, 1991
The carotenoid content of intracytoplasmic membrane vesicles isolated from purple phototrophic bacteria was reduced to a variable extent by mild extraction with light petroleum. Using preparations obtained from Rhodobacter capsulatus strains that contained the Light Harvesting System I (LHI) complex as the only major photosynthetic holochrome, it was shown that the visible circular dichroism of the carotenoids increased with the square of the membrane carotenoid content, as expected from being caused by dimeric exciton interaction. No chirality resulting from twists of the individual planar chromophore was detected. Therefore the contribution to carotenoid optical activity of non-degenerate interactions with bacteriochlorophyll or the apoprotein does not appear to be significant. The broadening of the absorption band of the bound pigment, caused by the splitting of the monomer transition, was demonstrated in membrane vesicles of both Rb, capsulatus and Rhodospirillum rubrum as a dec...
Journal of Physical Chemistry B, 1998
Ultrafast fluorescence upconversion has been used to probe electronic excitation transfer within the B800-B820 light-harvesting antenna of Rhodopseudomonas acidophila strain 7050. Emission from the carotenoid S 2 band decays in 54 ( 8 fs, and the bacteriochlorophyll B820 Q y band rises in approximately 110 fs. The B820 Q y rise time is wavelength-dependent. Energy-transfer rates between the carotenoid and several neighboring bacteriochlorophyll are calculated. Coupling strengths are estimated through transition dipoletransition dipole, polarization, and higher-order Coulombic coupling along with a new transition density volume coupling calculation. Data are compared to calculated energy-transfer rates through the use of a four-state model representing direct carotenoid to B820 energy transfer. The carotenoid emission data bound the S 2 to Q x transfer time between 65 and 130 fs. The S 1 to Q y transfer is assumed to be mediated by polarization and Coulombic coupling rather than by exchange; the transfer time is estimated to be in the picosecond regime, consistent with fluorescence quantum yield data. † Present address:
Physical Chemistry Chemical Physics, 2014
Time-resolved fluorescence spectroscopy was used to explore the pathway and kinetics of energy transfer in photosynthetic membrane vesicles (chromatophores) isolated from Rhodobacter (Rba.) sphaeroides cells harvested 2, 4, 6 or 24 hours after a transition from growth in high to low level illumination. As previously observed, this light intensity transition initiates the remodeling of the photosynthetic apparatus and an increase in the number of light harvesting 2 (LH2) complexes relative to light harvesting 1 (LH1) and reaction center (RC) complexes. It has generally been thought that the increase in LH2 complexes served the purpose of increasing the overall energy transmission to the RC. However, fluorescence lifetime measurements and analysis in terms of energy transfer within LH2 and between LH2 and LH1 indicate that, during the remodeling time period measured, only a portion of the additional LH2 generated are well connected to LH1 and the reaction center. The majority of the additional LH2 fluorescence decays with a lifetime comparable to that of free, unconnected LH2 complexes. The presence of large LH2-only domains has been observed by atomic force microscopy in Rba. sphaeroides chromatophores (Bahatyrova et al., Nature, 2004, 430, 1058), providing structural support for the existence of pools of partially connected LH2 complexes. These LH2-only domains represent the light-responsive antenna complement formed after a switch in growth conditions from high to low illumination, while the remaining LH2 complexes occupy membrane regions containing mixtures of LH2 and LH1-RC core complexes. The current study utilized a multi-parameter approach to explore the fluorescence spectroscopic properties related to the remodeling process, shedding light on the structure-function relationship of the photosynthetic assembles. Possible reasons for the accumulation of these largely disconnected LH2-only pools are discussed.
2001
The photosystem I (PSI) core complex of oxygenic photosynthesis is an integral pigment-protein complex that incorporates both the antenna and the reaction center (RC). It binds about 100 Chl a and 20 -carotene molecules. In the PSI core complex of the cyanobacterium Synechococcus elongatus, a total of about 9 antenna Chl a molecules are red-shifted with respect to the primary electron donor, which absorbs at 700 nm. We have studied energy transfer and trapping processes in trimeric PSI complexes of this species at femtosecond resolution by means of the fluorescence-upconversion technique. By simultaneously analyzing the fluorescence upconversion results and those obtained with a streak camera with picosecond resolution and multichannel detection (Gobets, B.; et al. Biophys. J., in press), we have mapped out the energy transfer processes that follow immediately after photon absorption. Equilibration among Chl a pigments in the bulk antenna was found to occur with a time constant of 360 fs. A major energy equilibration phase between bulk Chl a and the red-shifted antenna Chls occurs in 3.6 ps. A slow phase in energy equilibration takes place in 9.8 ps, after which the excitations are trapped by the RC in 38 ps. Fluorescence anisotropy measurements indicated an initial anisotropy of 0.30, which decayed biphasically with a major fast phase of 160 fs and a minor slow phase of 1.8 ps to a final anisotropy of 0.06. The 160 fs phase is assigned to elementary energy transfer steps in the bulk Chl a antenna, and the 1.8 ps phase to further equilibration processes, possibly involving energy transfer to or among red-shifted Chls. Energy transfer from -carotene to Chl a was found to proceed both from the S 2 state and the S 1 state, with the majority of transferred excitations (60%) originating from the S 2 state, resulting in an estimated overall yield of ∼90%. A comparison is made with the PSII core antenna protein CP47, which binds the same pigments but has a substantially lower carotenoid-Chl a energy transfer yield of ∼35% (van Dorssen R. J.; et al. Biochim. Biophys. Acta 1987, 893, 267).
The carotenoid shift in Rhodopseudomonas sphaeroides. Change induced under continuous illumination
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1977
The spectrum of the carotenoid shift generated under continuous illumination in the G1C mutant of Rhodopseudomonas sphaeroides, which has a single carotenoid, has been examined under a variety of conditions expected to alter the size of the membrane potential. If the difference spectrum observed was due to a species with the spectrum of the bulk pigment, it would correspond to a change of a variable proportion of the pigment to a form absorbing at a higher wavelength. The maximal change induced by light could be described as a shift of about 10 ~o of the pigment by 7 nm to the red, assuming that the shifted species was speetrally identical to the bulk carotenoid.