Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus

We have studied the pigment arrangement in purified cytoplasmic membranes of the thermophilic green bacterium Chloroflexus aurantiacus. The membranes contain 30–35 antenna bacteriochlorophyll a molecules per reaction center; these are organized in the B808–866 light-harvesting complex, together with carotenoids in a 2:1 molar ratio. Measurements of linear dichroism in a pressed polyacrylamide gel permitted the accurate determination of the orientation of the optical transition dipole moments with respect to the membrane plane. Combination of linear dichroism and low temperature fluorescence polarization data shows that the Q_y transitions of the BChl 866 molecules all lie almost perfectly parallel to the membrane plane, but have no preferred orientation within the plane. The BChl 808 Q_y transitions make an average angle of about 44° with this plane. This demonstrates that there are clear structural differences between the B808–866 complex of C. aurantiacus and the B800–850 complex of purple bacteria. Excitation energy transfer from carotenoid to BChl a proceeds with about 40% efficiency, while the efficiency of energy transfer from BChl 808 to BChl 866 approaches 100%. From the minimal energy transfer rate between the two spectral forms of BChl a, obtained by analysis of low temperature fluorescence emission spectra, a maximal distance between BChl 808 and BChl 866 of 23 was derived.Abbreviations BChl bacteriochlorophyll - BPheo bacteriopheophytin - CD circular dichroism - LD linear dichroism - Tris Tris(hydroxymethyl)aminomethane     

Giant circular dichroism of chlorosomes fromChloroflexus aurantiacus treated with 1-hexanol and proteolytic enzymes

The circular dichroism (CD) spectrum of isolated chlorosomes fromChloroflexus aurantiacus showed a conservative, S-shaped signal with a negative maximum at 723 nm, a positive maximum at 750 nm and a zero-crossing at 740 nm. Proteolytic treatment of chlorosomes with trypsin at 37°C did not change the CD signal or the absorption spectrum in contrast to treatment with proteinase K, where a twofold increase in rotational strength and a slight decrease of the absorption band at 740 nm were observed. Treatment with saturating 1-hexanol concentrations resulted in a blue shift of the absorption band at 740 nm as well as in changes of the CD spectrum. These changes reversed when the sample was diluted to half the saturating 1-hexanol concentration. In contrast to that, we observed an irreversible formation of a giant CD signal using the combination of 1-hexanol and proteinase K treatment. Electron micrographs of chlorosomes treated with both 1-hexanol and proteinase K showed large aggregates of multiple chlorosome size. By comparison of proteinase K induced effects with trypsin effects it appeared that the 5.7 kDa polypeptide has a structural role in the organisation of BChlc in the chlorosome.     

Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus

The transfer of excitation energy and the pigment arrangement in isolated chlorosomes of the thermophilic green bacterium Chloroflexus aurantiacus were studied by means of absorption, fluorescence and linear dichroism spectroscopy, both at room temperature and at 4 K. The low temperature absorption spectrum shows bands of the main antenna pigments BChl c and carotenoid, in addition to which bands of BChl a are present at 798 and 613 nm. Fluorescence measurements showed that excitation energy from BChl c and carotenoid is transferred to BChl a, which presumably functions as an intermediate in energy transfer from the chlorosome to the cytoplasmic membrane. Measurements of fluorescence polarization and the use of two different orientation techniques for linear dichroism experiments enabled us to determine the orientation of several transition dipole moments with respect to each other and to the three principal axes of the chlorosome. The Q_y transition of BChl a is oriented almost perfectly perpendicular to the long axis of the chlorosome. The Q_y transition of BChl c and the -carotene transition dipole are almost parallel to each other. They make an angle of about 40° with the long axis and of about 70° with the short axis of the chlorosome; the angle between these transitions and the BChl a Q_y transition is close to the magic angle (55°).Abbreviations BChl bacteriochlorophyll - CD circular dichroism - LD linear dichroism Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement.     

Circular dichroism of green bacterial chlorosomes

Positive and negative bands in previously measured circular dichroism (CD) spectra of Chlorobium limicola chlorosomes appeared to be sign-reversed relative to those of Chloroflexus aurantiacus chlorosomes in the 740–750 nm spectral region where bacteriochlorophyll (BChl) c absorbs maximally. It was not clear, however, whether this difference was intrinsic to the chlorosomes or was due to differences in the procedures used to prepare them. We therefore repeated the CD measurements using chlorosomes isolated from both Cb. limicola f. thiosulfatophilum and Cf. aurantiacus using the method of Gerola and Olson (1986, Biochim. Biophys. Acta 848: 69–76). Contrary to the earlier results, both types of chlorosomes had very similar CD spectra, suggesting that both have similar arrangements of BChl c molecules. The previously reported difference between the CD spectra of Chlorobium and Chloroflexus chlorosomes is due to the instability of Chlorobium chlorosomes, which can undergo a hypsochromic shift in their near infrared absorption maximum accompanied by an apparent inversion in their near infrared CD spectrum during isolation. Treating isolated chlorosomes with the strong ionic detergent sodium dodecylsulfate, which removes BChl a, does not alter the arrangement of BChl c molecules in either Chloroflexus or Chlorobium chlorosomes, as indicated by the lack of an effect on their CD spectra.Abbreviations BChl bacteriochlorophyll - Cb. Chlorobium - CD circular dichroism - Cf. Chloroflexus - NIR near infrared     

Bacteriochlorophyll organization and energy transfer kinetics in chlorosomes from Chloroflexus aurantiacus depend on the light regime during growth

We have used measurements of fluorescence and circular dichroism (CD) to compare chlorosome-membrane preparations derived from the green filamentous bacterium Chloroflexus aurantiacus grown in continuous culture at two different light-intensities. The cells grown under low light (6 mol m^–2 s^–1) had a higher ratio of bacteriochlorophyll (BChl) c to BChl a than cells grown at a tenfold higher light intensity; the high-light-grown cells had much more carotenoid per bacteriochlorophyll.The anisotropy of the Q_Y band of BChl c was calculated from steady-state fluorescence excitation and emission spectra with polarized light. The results showed that the BChl c in the chlorosomes derived from cells grown under high light has a higher structural order than BChl c in chlorosomes from low-light-grown cells. In the central part of the BChl c fluorescence emission band, the average angles between the transition dipole moments for BChl c molecules and the symmetry axis of the chlorosome rod element were estimated as 25° and 17° in chlorosomes obtained from the low- and high-light-grown cells, respectively.This difference in BChl organization was confirmed by the decay associated spectra of the two samples obtained using picosecond single-photon-counting experiments and global analysis of the fluorescence decays. The shortest decay component obtained, which probably represents energy-transfer from the chlorosome bacteriochlorophylls to the BChl a in the baseplate, was 15 ps in the chlorosomes from high-light-grown cell but only 7 ps in the preparation from low-light grown cells. The CD spectra of the two preparations were very different: chlorosomes from low-light-grown cells had a type II spectrum, while those from high-light-grown cells was of type I (Griebenow et al. (1991) Biochim Biophys Acta 1058: 194–202). The different shapes of the CD spectra confirm the existence of a qualitatively different organization of the BChl c in the two types of chlorosome.Abbreviations BChl bacteriochlorophyll - CD circular dichroism - DAS decay associated spectrum - PMSF phenylmethylsulfonyl fluoride     

Antenna organization and evidence for the function of a new antenna pigment species in the green photosynthetic bacterium Chloroflexus aurantiacus

Whole cells and isolated chlorosomes (antenna complex) of the green photosynthetic bacterium Chloroflexus aurantiacus have been studied by absorption spectroscopy (77 K and room temperature), fluorescence spectroscopy, circular dichroism, linear dichroism and electron spin resonance spectroscopy. The chlorosome absorption spectrum has maxima at 450 (contributed by carotenoids and bacteriochlorophyll (BChl) a Soret), 742 (BChl c) and 792 nm (BChl a) with intensity ratios of 20:25. The fluorescence emission spectrum has peaks at 748 and 802 nm when excitation is into either the 742 or 450 nm absorption bands, respectively. Whole cells have fluorescence peaks identical to those in chlorosomes with the addition of a major peak observed at 867 nm. The CD spectrum of isolated chlorosomes has an asymmetric-derivative-shaped CD centered at 739 nm suggestive of exciton interaction at least on the level of dimers. Linear dichroism of oriented chlorosomes shows preferential absorption at 742 nm of light polarized parallel to the long axis of the chlorosome. This implies that the transition dipoles are also oriented more or less parallel to the long axis of the chlorosome. Treatment with ferricyanide results in the appearance of a 2.3 G wide ESR spectrum at g 2.002. Whole cells grown under different light conditions exhibit different fluorescence behavior when absorption is normalized at 742 nm. Cells grown under low light conditions have higher fluorescence intensity at 748 nm and lower intensity at 802 nm than cells grown under high light conditions. These results indicate that the BChl c in chlorosomes is highly organized, and transfers energy from BChl c (742 nm) to a connector of baseplate BChl B792 (BChl a) presumably located in the chlorosome baseplate adjacent to the cytoplasmic membrane.     

Excitation energy transfer in chlorosomes of Chlorobium phaeobacteroides strain CL1401: the role of carotenoids

The role of carotenoids in chlorosomes of the green sulfur bacterium Chlorobium phaeobacteroides, containing bacteriochlorophyll (BChl) e and the carotenoid (Car) isorenieratene as main pigments, was studied by steady-state fluorescence excitation, picosecond single-photon timing and femtosecond transient absorption (TA) spectroscopy. In order to obtain information about energy transfer from Cars in this photosynthetic light-harvesting antenna with high spectral overlap between Cars and BChls, Car-depleted chlorosomes, obtained by inhibition of Car biosynthesis by 2-hydroxybiphenyl, were employed in a comparative study with control chlorosomes. Excitation spectra measured at room temperature give an efficiency of 60–70% for the excitation energy transfer from Cars to BChls in control chlorosomes. Femtosecond TA measurements enabled an identification of the excited state absorption band of Cars and the lifetime of their S₁ state was determined to be 10 ps. Based on this lifetime, we concluded that the involvement of this state in energy transfer is unlikely. Furthermore, evidence was obtained for the presence of an ultrafast (>100 fs) energy transfer process from the S₂ state of Cars to BChls in control chlorosomes. Using two time-resolved techniques, we further found that the absence of Cars leads to overall slower decay kinetics probed within the Q_y band of BChl e aggregates, and that two time constants are generally required to describe energy transfer from aggregated BChl e to baseplate BChl a.This revised version was published online in October 2005 with corrections to the Cover Date.     

Molecular organization of bacteriochlorophyll in chlorosomes of the green photosynthetic bacteriumChloroflexus aurantiacus: Studies of fluorescence depolarization accompanied by energy transfer processes

Examination was made of changes in fluorescence polarization plane by energy transfer in the chlorosomes of the green photosynthetic bacterium,Chloroflexus aurantiacus. Fluorescence anisotropy in the picosecond (ps) time region was analyzed using chlorosomes suspended in solution as well as those oriented in a polyacrylamide gel. When the main component of BChlc was preferentially excited, the decay of fluorescence anisotropy was found to depend on wavelength. In the chlorosome suspension, the anisotropy ratio of BChlc changed from 0.31 to 0.24 within 100 ps following excitation. In the baseplate BChla region, this ratio decreased to a negative value (–0.09) from the initial 0.14. In oriented samples, the degree of polarization remained at 0.68 for BChlc, and changed from 0.25 to –0.40 for the baseplate BChla by excitation light whose electric vector was parallel to the longest axis of chlorosomes. In the latter case, there was a shift from 0.30 to –0.55 by excitation perpendicular to the longest axis. Time-resolved fluorescence polarization spectra clearly indicated extensive changes in polarization plane accompanied by energy transfer. The directions of polarization plane of emission from oriented samples were mostly dependent on chlorosome orientation in the gel but not on that of the polarization plane of excitation light. Orientations of the dipole moment of fluorescence components was consistent with that of absorption components as determined by the linear dichroism (Matsuura et al. (1993) Photochem. Photobiol. 57: 92–97). A model for molecular organization of BChlc anda in chlorosomes is proposed based on anisotropic optical properties.     

Antenna organization and energy transfer in membranes of Heliobacterium chlorum

Absorption, fluorescence emission and fluorescence excitation spectra of membranes of the recently discovered photosynthetic bacterium Heliobacterium chlorum (Gest, H. and Favinger, J.L. (1983) Arch. Microbiol. 136, 11–16) showed that at 4 K at least three spectroscopically different forms of bacteriochlorophyll g (BChl g 778, BChl g 793 and BChl g 808) can be discerned in the antenna system. Efficient energy transfer occurs from the short-wave-absorbing bacteriochlorophylls to BChl g 808. Energy transfer to bacteriochlorophyll, albeit with lower efficiency (70%), also occurred from the main carotenoid, neurosporene, and from a pigment absorbing at 670 nm. The complex structure of the antenna system is also reflected by fluorescence polarization and linear and circular dichroism spectra. Significant circular dichroism was only observed for BChl g 793, and different orientations were observed for the various Q_y transition dipoles, the one of BChl g 808 making a smaller angle with the plane of the membrane than those of the other bacteriochlorophylls.     

Regulatory properties of the ADPglucose pyrophosphorylase from Rhodopseudomonas sphaeroides and from Rhodopseudomonas gelatinosa

Highly purified fractions of chlorosomes and cytoplasmic membranes were isolated from Chloroflexus aurantiacus Ok-70-fl and Chlorobium limicola 6230. These fractions were comparatively analyzed for their pigmentation, phospholipid, glycolipid, and cytochrome c content as well as for their specific activities of succinate dehydrogenase and NADH-oxidase. The data showed that there are some differences in pigmentation and phospholipid content between the isolated fractions of Chloroflexus and Chlorobium. Chlorosomes of Chloroflexus contained a specific BChl a-complex with a characteristic absorption maximum at about 790 nm. This BChl a-complex could not be detected in spectra of chlorosomes from Chlorobium. The near infrared region of the spectra of the isolated cytoplasmic membranes of both organisms revealed considerable differences: The BChl a-complexes of Chloroflexus membranes exhibited peaks at 806 and 868 nm whereas the membranes of Chlorobium had a single BChl a-peak at 710 nm. In contrast to the findings with Chlorobium the chlorosomes of Chloroflexus contained at least twice as much phospholipids as did the cytoplasmic membranes. In Chlorobium the phospholipid content of cytoplasmic membranes is three times that of their chlorosomes. The distribution of all other components (carotenoid composition, enzyme activities, cytochrome c content, and glycolipids) was about the same in both strains. From the data it was concluded that differences in the organization of the photosynthetic apparatus are mainly based on differences of the organization of the photosynthetic units in the cytoplasmic membrane and probably the kind of linkage of the light harvesting system in the chlorosomes with the reaction center in the cytoplasmic membranes.Abbreviations BChl c bacteriochlorophyll c - BChl a bacteriochlorophyll a - DSM Deutsche Sammlung von Mikrorganismen     

Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus. III. Energy transfer in whole cells

The transfer of excitation energy in intact cells of the thermophilic green photosynthetic bacterium Chloroflexus aurantiacus was studied both at low temperature and under more physiological conditions. Analysis of excitation spectra measured at 4K indicates that the minor fraction of bacteriochlorophyll a present in the chlorosome functions as an intermediate in energy transfer between the main light-harvesting pigment BChl c and the membrane-bound B808-866 antenna complex. This supports the hypothesis that BChl a is associated with the base plate which connects the chlorosome with the membrane. The overall efficiency for energy transfer from the chlorosome to the membrane is only 15% at 4K. High efficiencies of close to 100% are observed above 40°C near the temperature where the cultures are grown. Cooling to 20°C resulted in a sudden drop of the transfer efficiency which appeared to originate in the chlorosome. This decrease may be related to a lipid phase transition. Further cooling mainly affected the efficiency of transfer between the chlorosome and the membrane. This effect can only partially be explained by a decreased Förster overlap between the chlorosomal BChl a and BChl a 808 associated with the membrane-bound antenna system. The temperature dependence of the fluorescence yield of BChl a 866 also appeared to be affected by lipid phase transitions, suggesting that this fluorescence can be used as a native probe of the physical state of the membrane.     

A new bacteriochlorophyll a-protein complex associated with chlorosomes of green sulfur bacteria

Chlorosomes were prepared from Chlorobium limicola f. thiosulfatophilum by sucrose density gradient centrifugation. Cells broken in the presence of 2 M NaSCN yielded three chlorosome fractions in the gradient: low density (no sucrose), medium density (approx. 18% sucrose), and high density (approx. 26% sucrose). All fractions were stable at any chlorosome concentration. Cells broken in the absence of 2 M NaSCN also yielded three fractions, but only the high-density fraction contained stable chlorosomes. The medium-density chlorosomes were stable only when highly concentrated. Upon dilution, bacteriochlorophyll (BChl) c was degraded to bacteriopheophytin c and concomitantly a band at 794 nm (BChl a) was revealed. Two 794-nm fractions were observed with the same densities as low- and medium-density chlorosomes. The protein composition of the 794-nm fractions was similar to that of the stable chlorosome fractions. All showed a 4-5 kDa (Mr) protein as a major component, but no trace of the 40-kDa protein characteristic of the water-soluble BChl a-protein of green sulfur bacteria. BChl a was present in all types of chlorosomes, in stable chlorosomes the BChl c/BChl a ratio was approx. 90. A special BChl a-protein (794 nm) inside the chlorosome is postulated to mediate energy transfer from BChl c to the water-soluble BChl a-protein in the baseplate.     

Ultrafast excited‐state dynamics in chlorosomes isolated from the photosynthetic filamentous green bacterium Chloroflexus aurantiacus

Bacteriochlorophyll (BChl) c pigments in the aggregated state are responsible for efficient light harvesting in chlorosomes of the filamentous anoxygenic photosynthetic bacterium, Chloroflexus (Cfx.) aurantiacus. Absorption of light creates excited states in the BChl c aggregates. After subpicosecond intrachlorosomal energy transfer, redistribution and relaxation, the excitation is transferred to the BChl a complexes and further to reaction centers on the picosecond time scale. In this work, the femtosecond excited state dynamics within BChl c oligomers of isolated Cfx. aurantiacus chlorosomes was studied by double difference pump‐probe spectroscopy at room temperature. Difference (A^light ? A^dark) spectra corresponding to excitation at 725 nm (blue side of the BChl c absorption band) were compared with those corresponding to excitation at 750 nm (red side of the BChl c absorption band). A very fast (time constant 70 ± 10 fs) rise kinetic component was found in the stimulated emission (SE) upon excitation at 725 nm. This component was absent at 750‐nm excitation. These data were explained by the dynamical red shift of the SE due to excited state relaxation. The nature and mechanisms of the ultrafast excited state dynamics in chlorosomal BChl c aggregates are discussed.     

Fast energy transfer between BChl d and BChl c in chlorosomes of the green sulfur bacterium Chlorobium limicola

We have studied energy transfer in chlorosomes of Chlorobium limicola UdG6040 containing a mixture of about 50% bacteriochlorophyll (BChl) c and BChl d each. BChl d-depleted chlorosomes were obtained by acid treatment. The energy transfer between the different pigment pools was studied using both steady-state and time-resolved fluorescence spectroscopy at room temperature and low temperature. The steady-state emission of the intact chlorosome originated mainly from BChl c, as judged by comparison of fluorescence emission spectra of intact and BChl d-depleted chlorosomes. This indicated that efficient energy transfer from BChl d to BChl c takes place. At room temperature BChl c/d to BChl a excitation energy transfer (EET) was characterized by two components of 27 and 74 ps. At low temperature we could also observe EET from BChl d to BChl c with a time constant of approximately 4 ps. Kinetic modeling of the low temperature data indicated heterogeneous fluorescence kinetics and suggested the presence of an additional BChl c pool, E790, which is more or less decoupled from the baseplate BChl a. This E790 pool is either a low-lying exciton state of BChl c which acts as a trap at low temperature or alternatively represents the red edge of a broad inhomogeneous absorption band of BChl c. We present a refined model for the organization of the spatially separated pigment pools in chlorosomes of Cb. limicola UdG6040 in which BChl d is situated distal and BChl c proximal with respect to the baseplate.     

Utilization of blue-green light by chlorosomes from the photosynthetic bacterium Chloroflexus aurantiacus: Ultrafast excitation energy conversion and transfer

Chlorosomes of photosynthetic green bacteria are unique molecular assemblies providing efficient light harvesting followed by multi-step transfer of excitation energy to reaction centers. In each chlorosome, 10⁴–10⁵ bacteriochlorophyll (BChl) c/d/e molecules are organized by self-assembly into high-ordered aggregates. We studied the early-time dynamics of the excitation energy flow and energy conversion in chlorosomes isolated from Chloroflexus (Cfx.) aurantiacus bacteria by pump-probe spectroscopy with 30-fs temporal resolution at room temperature. Both the S₂ state of carotenoids (Cars) and the Soret states of BChl c were excited at ~490 nm, and absorption changes were probed at 400–900 nm. A global analysis of spectroscopy data revealed that the excitation energy transfer (EET) from Cars to BChl c aggregates occurred within ~100 fs, and the Soret → Q energy conversion in BChl c occurred faster within ~40 fs. This conclusion was confirmed by a detailed comparison of the early exciton dynamics in chlorosomes with different content of Cars. These processes are accompanied by excitonic and vibrational relaxation within 100–270 fs. The well-known EET from BChl c to the baseplate BChl a proceeded on a ps time-scale. We showed that the S₁ state of Cars does not participate in EET. We discussed the possible presence (or absence) of an intermediate state that might mediates the Soret → Q_y internal conversion in chlorosomal BChl c. We discussed a possible relationship between the observed exciton dynamics and the structural heterogeneity of chlorosomes.     

Purification and Characterization of the B808–866 Light-harvesting Complex from Green Filamentous Bacterium Chloroflexus aurantiacus

The integral membrane light-harvesting complex B808–866 from the thermophilic green filamentous bacterium Chloroflexus aurantiacus has been isolated and characterized. Reversed-phase HPLC analysis demonstrated that the number of bacteriochlorophyll (BChl) in the B808–866 antenna complex is 36 ± 2 per reaction center. The main carotenoid type is γ-carotene, and the molar ratio of BChl to carotenoid is 3:2. The steady-state absorption and fluorescence spectroscopy of the B808–866 complex are reminiscent of the well-studied LH2 peripheral antenna of purple bacteria, whereas the protein sequence and the circular dichroism spectrum of B808–866 is more similar to the LH1 inner core antenna. The efficiency of excitation transfer from carotenoid to BChl is about 25%. The above results combined with electron microscopy and dynamic light scattering analysis suggest that the B808–866 antenna is more like the LH1, whereas surrounds the reaction center but probably consists of 24 building blocks with a ring diameter of about 20 nm. The above results suggested that there are probably two reaction centers inside the ring of B808–866. The unique properties of this light-harvesting complex may provide insights on the protein–pigment interactions in bacterial photosynthesis.     

Pathways of energy transformation in antenna reaction center complexes of Heliobacillus mobilis

The conversion of excitation energy in the antenna reaction center complex of Heliobacillus mobilis was investigated at 10 K as well as at 275 K by means of time-resolved absorbance difference spectroscopy of isolated membranes in the (sub)picosecond time range. Selective excitation of the primary electron acceptor, chlorophyll (Chl) a 670, and of the different spectral pools of bacteriochlorophyll (BChl) g (BChl g 778, BChl g 793, and BChl g 808) was applied. At 10 K, excitation at 770 or 793 nm resulted on the one hand in rapid energy transfer to BChl g 808 and on the other hand in fast charge separation from excited BChl g 793 ( approximately 1 ps). Once the excitations were on BChl g 808, the bleaching band shifted gradually to the red, from 806 to 813 nm, and charge separation from excited BChl g 808 occurred by a very slow process ( approximately 500 ps). The main purpose of our experiments was to answer the question whether an "alternative" pathway for charge separation exists upon excitation of Chl a 670. Our measurements showed that the amount of oxidized primary donor (P798(+)) relative to that of excited BChl g produced by excitation of Chl a 670 was considerably larger than upon direct excitation of BChl g. This indicates the existence of an alternative pathway for charge separation that does not involve excited antenna BChl g. This effect occurred at 10 K as well as at 275 K. The mechanism for this process is discussed in relation to different trapping models; it is concluded that charge separation occurs directly from excited Chl a 670.     

Hole burning study of excited state structure and energy transfer dynamics of bacteriochlorophyll c in chlorosomes of green sulphur photosynthetic bacteria

Results of low temperature fluorescence and spectral hole burning experiments with whole cells and isolated chlorosomes of the green sulfur bacterium Chlorobium limicola containing BChl c are reported. At least two spectral forms of BChl c (short-wavelength and long-wavelength absorbing BChl c) were identified in the second derivative fluorescence spectra. The widths of persistent holes burned in the fluorescence spectrum of BChl c are determined by excited state lifetimes due to fast energy transfer. Different excited state lifetimes for both BChl c forms were observed. A site distribution function of the lowest excited state of chlorosomal BChl c was revealed. The excited state lifetimes are strongly influenced by redox conditions of the solution. At anaerobic conditions the lifetime of 5.3 ps corresponds to the rate of energy transfer between BChl c clusters. This time shortens to 2.6 ps at aerobic conditions. The shortening may be caused by introducing a quencher. Spectral bands observed in the fluorescence of isolated chlorosomes were attributed to monomeric and lower state aggregates of BChl c. These forms are not functionally connected with the chlorosome.Abbreviations BChl bacteriochlorophyll - EET electronic energy transfer - FWHM full width at half maximum - SDF site distribution function - RC reaction centre     

A comparative study of the optical characteristics of intact cells of photosynthetic green sulfur bacteria containing bacteriochlorophyll c,d or e

Energy transfer and pigment arrangement in intact cells of the green sulfur bacteria Prosthecochloris aestuarii, Chlorobium vibrioforme and chlorobium phaeovibrioides, containing bacteriochlorophyll (BChl) c, d or e as main light harvesting pigment, respectively, were studied by means of absorption, fluorescence, circular dichroism and linear dichroism spectroscopy at low temperature. The results indicate a very similar composition of the antenna in the three species and a very similar structure of main light harvesting components, the chlorosome and the membrane-bound BChl a protein. In all three species the Q_y transition dipoles of BChl c, d or e are oriented approximately parallel to the long axis of the chlorosome. Absorption and fluorescence excitation spectra demonstrate the presence of at least two BChl c-e pools in the chlorosomes of all three species, long-wavelength absorbing BChls being closest to the membrane. In C. phaeovibrioides, energy from BChl e is transferred with an efficiency of 25% to the chlorosomal BChl a at 6 K, whereas the efficiency of transfer from BChl e to the BChl a protein is 10%. These numbers are compatible with the hypothesis that the chlorosomal BChl a is an intermediary in the energy transfer from the chlorosome to the membrane.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - CD circular dichroism - LD linear dichroism