Quenching of Energy Transfer in Chlorosomes from Chloroflexus by the Addition of Synthetic Quinones (original) (raw)
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Photochemistry and Photobiology, 2007
The quenching of bacteriochlorophyll (BChl) c fluorescence in chlorosomes isolated from Chloroflexus aurantiacus was examined by the addition of various benzoquinones, naphthoquinones (NQ), and anthraquinones (AQ). Many quinones showed strong quenching in the micromolar or submicromolar range. The number of quinone molecules bound to the chlorosomes was estimated to be as small as one quinone molecule per 50 BChl c molecules. Quinones which exhibit a high quenching effect have sufficient hydrophobicity and one or more hydroxyl groups in the alpha positions of NQ and AQ. Chlorobiumquinone has been suggested to be essential for the endogenous quenching of chlorosome fluorescence in Chlorobium tepidum under oxic conditions. We suggest that the quenching effect of chlorobiumquinone in chlorosomes from Chl. tepidum is related to the 1-oxo group neighboring the dicarbonyl group.
Biochemistry, 1987
The photosynthetic antenna of Chloroflexus aurantiacus includes bacteriochlorophyll (BChl) c740 and BChl a792, both of which occur in chlorosomes, and B808-866 (containing BChl aSo8 and BChl ~2866)~ which is membrane-located (subscripts refer to near-infrared absorption maxima in vivo). BChl a792 is thought to mediate excitation transfer from BChl c740 to BChl aSo8. Lifetimes of fluorescence from BChl c740 and BChl a792 were measured in isolated and membrane-bound chlorosomes in order to study energy transfer from these pigments. In both preparations, the lifetime of BChl c740 fluorescence was at or below the instrumental limit of temporal resolution (about 30-50 ps), implying extremely fast excitation transfer from this pigment. Attempts to disrupt excitation transfer from BChl c740, either by conversion of part of this pigment to a monomeric form absorbing at 671 nm or by partial destruction of BChl a792 by oxidation with K3Fe(CN),, had no discernible effects on the lifetime of BChl c740 fluorescence. Most (usually >90%) of the fluorescence from BChl a792 decayed with a lifetime of 93 f 21 ps in membrane-attached chlorosomes and 155 f 22 ps in isolated chlorosomes at room temperature. Assuming that the only difference between these preparations is the occurrence of excitation transfer from BChl a792 to B808-866, a 41% efficiency was calculated for this process. This value is lower than the 60% efficiency of excitation transfer from BChl c740 to B808-866 determined by comparison of fluorescence excitation and absorption spectra of membranes with attached chlorosomes and compares even less favorably with the 100% efficiency of excitation transfer found in whole cells by the same method. Furthermore, measurements at 77 K (on different samples) did not show an increased lifetime of BChl a792 fluorescence when isolated chlorosomes were compared with membrane-bound chlorosomes. These results imply either that BChl a792 is not an obligatory intermediate in energy transfer from BChl c740 to B808-866 or (more probably) that chlorosome isolation introduces new processes for quenching fluorescence from BChl a792.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2000
Electronic energy transfer processes in chlorosomes isolated from the green sulphur bacterium Chlorobium tepidum and from the green filamentous bacterium Chloroflexus aurantiacus have been investigated. Steady-state fluorescence excitation spectra and time-resolved triplet-minus-singlet (TmS) spectra, recorded at ambient temperature and under non-reducing or reducing conditions, are reported. The carotenoid (Car) pigments in both species transfer their singlet excitation to bacteriochlorophyll c (BChlc) with an efficiency which is high (between 0.5 and 0.8) but smaller than unity; BChlc and bacteriochlorophyll a (BChla) transfer their triplet excitation to the Car's with nearly 100% efficiency. The lifetime of the Car triplet states is approximately 3 ms, appreciably shorter than that of the Car triplets in the light-harvesting complex II (LHCII) in green plants and in other antenna systems. In both types of chlorosomes the yield of BChlc triplets (as judged from the yield of the Car triplets) remains insensitive to the redox conditions. In notable contrast the yield of BChlc singlet emission falls, upon a change from reducing to non-reducing conditions, by factors of 4 and 35 in Cfx. aurantiacus and Cb. tepidum, respectively. It is possible to account for these observations if one postulates that the bulk of the BChlc triplets originate either from a large BChlc pool which is essentially non-fluorescent and non-responsive to changes in the redox conditions, or as a result of a process which quenches BChlc singlet excitation and becomes more efficient under non-reducing conditions. In chlorosomes from Cfx. aurantiacus whose Car content is lowered, by hexane extraction, to 10% of the original value, nearly one-third of the photogenerated BChlc triplets still end up on the residual Car pigments, which is taken as evidence of BChlc-to-BChlc migration of triplet excitation; the BChlc triplets which escape rapid static quenching contribute a depletion signal at the long-wavelength edge of the Q y absorption band, indicating the existence of at least two pools of BChlc.
Biochemistry, 1987
The photosynthetic antenna of Chloroflexus aurantiacus includes bacteriochlorophyll (BChl) c740 and BChl a792, both of which occur in chlorosomes, and B808-866 (containing BChl aSo8 and BChl ~2866)~ which is membrane-located (subscripts refer to near-infrared absorption maxima in vivo). BChl a792 is thought to mediate excitation transfer from BChl c740 to BChl aSo8. Lifetimes of fluorescence from BChl c740 and BChl a792 were measured in isolated and membrane-bound chlorosomes in order to study energy transfer from these pigments. In both preparations, the lifetime of BChl c740 fluorescence was at or below the instrumental limit of temporal resolution (about 30-50 ps), implying extremely fast excitation transfer from this pigment. Attempts to disrupt excitation transfer from BChl c740, either by conversion of part of this pigment to a monomeric form absorbing at 671 nm or by partial destruction of BChl a792 by oxidation with K3Fe(CN),, had no discernible effects on the lifetime of BChl c740 fluorescence. Most (usually >90%) of the fluorescence from BChl a792 decayed with a lifetime of 93 f 21 ps in membrane-attached chlorosomes and 155 f 22 ps in isolated chlorosomes at room temperature. Assuming that the only difference between these preparations is the occurrence of excitation transfer from BChl a792 to B808-866, a 41% efficiency was calculated for this process. This value is lower than the 60% efficiency of excitation transfer from BChl c740 to B808-866 determined by comparison of fluorescence excitation and absorption spectra of membranes with attached chlorosomes and compares even less favorably with the 100% efficiency of excitation transfer found in whole cells by the same method. Furthermore, measurements at 77 K (on different samples) did not show an increased lifetime of BChl a792 fluorescence when isolated chlorosomes were compared with membrane-bound chlorosomes. These results imply either that BChl a792 is not an obligatory intermediate in energy transfer from BChl c740 to B808-866 or (more probably) that chlorosome isolation introduces new processes for quenching fluorescence from BChl a792.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1991
Chloroflexus aurantiacus strain Ok-70-fl obtained by two different isolation procedures. The gel-electrophoretic filtration procedure yields cldorosomes that are essentially free of BChl a790 and proteins, while isolation by sucrose density gradient centrifugation yields the conventional chlorosome preparations. The LD spectra of the two kinds of preparation were very similar. In both cases the Qy LD signals correspond to an average angle between the BChl-c-Qy transition and the long axis of the chlorosome of approx. 15 + I0 °. In contrast to the LD spectra, the CD spectra of different preparations (membranes, BChl-a-free chlorosomes, BChl-a-containing chlorosomes) show pronounced differences both in the ellipticity as well as in the shape of the spectra and the number of maxima. However, these differences are not caused by the isolation procedure or the detergents used. We show that even freshly prepared membranes (of different, parallel grown batch cultures) give rise to very different CD spectra. The set of different CD spectra we obtained could be simulated well by linear combinations of two basic spectra. This strongly suggests that the variations in the CD spectra are caused by a variation in the relative amounts of two different species, two different types of chlorosome, or possibly by two different types of pigment aggregate within the chlorosomes.
Photochemistry and Photobiology, 1999
Chlorosomes isolated from two types of green sulfur bacteria, Chlorobium tepidum which contains bacteriochlorophyll c (BChl c ) and the BChl e-containing Chlorobium phaeobacteroides, were subjected to alkaline treatment (pH 12.7 at 40°C for 20 min). This caused selective degradation of BChl a, whereas BChl c or e were not affected. Chlorobiumquinone in the chlorosomes was partially degraded by the alkaline treatment but menaquinone was unchanged. Fluorescence decay kinetics showed that alkaline treatment disrupted energy transfer from BChl c or e to BChl a under reducing conditions. However, this did not give rise to any substantial increase in the excited state lifetime of BChl e in C. phaeobacteroides chlorosomes, while for C. tepidum a decrease in the BChl c lifetime was found. The steady-state fluorescence of chlorosomes is highly dependent on the redox potential such that emission is quenched in oxidizing environments. Alkaline treatment diminished this quenching effect and caused a doubling in the BChl c or e emission intensity under aerobic conditions. Single-photon timing experiments confirmed that alkaline treatment inhibits the energy trapping process operative under aerobic conditions. These effects of alkaline treatment on the fluorescence intensity and decay kinetics are likely to be related to the depletion in BChl a or in chlorobiumquinone or a combination of these.
Isolation and development of chlorosomes in the green bacterium Chloroflexus aurantiacus
Journal of …, 1981
Freeze-fracture electron microscopy was used to study further the changes in chlorosome structure during the development of the photosynthetic apparatus in Chloroflexus aurantiacus J-10-fl. During development, in response to decreased light intensity or lower oxygen tension, the number of chlorosomes per cell increased. The same conditions also led to a general thickening of chlorosomes but did not affect their length or width. The thickening of the chlorosomes paralleled increases in the bacteriochlorophyll c/bacteriochlorophyll a ratio. Semiaerobic induction of the photosynthetic apparatus did not produce a synchronous assembly of chlorosomes in all cells of a given culture. Even adjacent cells of a single filament showed great variations in the rate and extent of response. Parallel appearance of (i)-5-nm particles (in a lattice configuration) in the membrane attachment site, (ii) the crystalline baseplate material (with a periodicity of-6 nm) adjacent to the membrane attachment site, and (iii) the chlorosome envelope layer preceded addition of longitudinally oriented, rodlike elements (diameter, =6 rim) to the chlorosome core. It is estimated that each chlorosome can funnel energy into approximately 100 reaction centers. Chlorosomes could be isolated by a simple density gradient procedure only from cells grown at low light intensity. A bacteriochlorophyll a species absorbing at 790 nm was associated with isolated chlorosomes. Lithium dodecyl sulfate-polyacrylamide gel electrophoresis of chlorosomes showed only a few low-molecular-weight polypeptides (<15,000). Chlorosomes are the light-harvesting structures of the Chlorobiineae (4, 8, 9, 16). They are located in close association with the inner surface of the cytoplasmic membrane and contain all of the antenna bacteriochlorophyll (Bchl) c (d or e) within the cell (6, 17). The reaction center Bchl a is located in the cytoplasmic membrane (7; C.
FEBS Letters, 1994
Structurally different chlorosomes were isolated from the green photosynthetic bacterium Chloroflexus aurantiacus grown under different conditions. They were analysed with respect to variable pigment-protein stoichiometries in view of the presumed BChl c-binding function of the 5.7 kDa chlorosome polypeptide. Under high-light conditions on substrate-limited growth medium the pigment-protein ratio of isolated chlorosomes was several times lower than under low-light conditions on complex medium. Proteolytic degradation of the 5.7 kDa polypeptide in high-light chlorosomes led to a 60% decrease of the absorbance at 740 nm. The CD spectrum of high-light chlorosomes exhibited a sixfold lower relative intensity at 740 nm (AA/&,,) than low-light chlorosomes, but it showed a fivefold increase in intensity upon degradation of the 5.7 kDa polypeptide compared to a twofold increase in low-light chlorosomes. It seems probable that BChl c in the chlorosomes is present as oligomers bound to the 5.7 kDa polypeptide. Our data suggest further that compared to low-light chlorosomes smaller oligomers or single BChl c molecules are bound to the 5.7 kDa polypeptide in high-light chlorosomes resulting in lower rotational strength.
Journal of Physical Chemistry B, 2000
A comparative study on the isolated chlorosomes from Chloroflexus aurantiacus, a green filamentous photosynthetic bacterium and Chlorobium tepidum, a green sulfur photosynthetic bacterium, was done by ODMR (optically detected magnetic resonance). Correlation between the results obtained by fluorescence and absorption detection is shown to be a source of information about the functional interactions among pigments. Analogies and differences are pointed out between the light-harvesting systems of the two species. Triplet states are easily detected in both bacteria at 1.8 K under steady-state illumination and are assigned to BChl c, BChl a, and carotenoid molecules. Carotenoids are found to be able to quench BChl a triplet states, but no evidence of BChl c triplet states quenching by this triplet-triplet transfer mechanism is found in both systems. Then from the data it appears that some carotenoids are in close contact with BChl a molecules. The relevance of this finding to the localization of carotenoids in the chlorosomes is discussed. In Cb. tepidum three different pools of BChl c oligomers connected to BChl a were found by detection of their triplet state, while only one pool of BChl c was evidenced in Cf. aurantiacus. The latter appears to be unconnected, at least at 1.8 K, to BChl a. On the other hand, heterogeneity in the BChl a triplet population was detected in Cf. aurantiacus. Even though the two bacteria show common features in the way the light excitation induces triplet formation at low temperature, the detected triplet states show spectroscopic properties that strongly depend on the system. The results clearly indicate that differences in pigment organization exist both in the core and in the baseplate of the chlorosomes from the two different bacteria.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1990
When isolated chlorosomes from Chlorobium limicola or Chloroflexus aurantiacus are suspended in a solution saturated with l-hexanol, the far-red absorption band of bacteriochlorophyil c at 750 or 740 nm is converted completely to a band peaking at 670 nm. The cooperation of 9 to 15 hexanol molecules is required to effect this change. This conversion corresponds to a change of the pigment molecules from the aggregated form to the monomeric form in vitro and suggests that hexanol destroys the strong interaction between the chlorosome pigments by the ligation of the hydroxyl oxygen of hexanol to the magnesium atom of the chlorophyll. However, fluorescence from the monomer in the treated chlorosomes is very small in comparison to that from monomer in organic solvent or detergent treated chlorosomes and efficient energy transfer from bacteriochlorophyll c to bacteriochlorophyll a in the hexanol-treated chlorosomes is still observed. When the treated chiorosomes are diluted slowly with buffer by a factor of two or more, the hexanol effect is reversed completely. These results suggest that the red-shifted far-red bands of bacteriochlorophyll c at 740 or 750 nm are largely due to strong pigment-pigment interactions rather than pigment-protein interactions and that the far-red bands are not necessary for energy transfer to the bacteriochlorophyll a in chlorosomes.