Photochemically active pigment-protein complexes from the green photosynthetic bacterium Prosthecochloris aestuarii.

Photochemically active pigment-protein complexes were prepared from a bacteriochlorophyll a containing membrane preparation of the green photosynthetic bacterium Prosthecochloris aestuarii. The preparations contained about 75 and 35 bacteriochlorophyll a molecules per reaction center and had molecular weights of 6 . 10(5) and 3.5 . 10(5), respectively. Some of the other properties of these preparations are described.

The antenna reaction center complex of heliobacteria: composition, energy conversion and electron transfer.

A survey is given of various aspects of the photosynthetic processes in heliobacteria. The review mainly refers to results obtained since 1995, which had not been covered earlier. It first discusses the antenna organization and pigmentation. The pigments of heliobacteria include some unusual species: bacteriochlorophyll (BChl) g, the main pigment, 8(1) hydroxy chlorophyll a, which acts as primary electron acceptor, and 4,4'-diaponeurosporene, a carotenoid with 30 carbon atoms. Energy conversion within the antenna is very fast: at room temperature thermal equilibrium among the approx. 35 BChls g of the antenna is largely completed within a few ps. This is then followed by primary charge separation, involving a dimer of BChl g (P798) as donor, but recent evidence indicates that excitation of the acceptor pigment 8(1) hydroxy chlorophyll a gives rise to an alternative primary reaction not involving excited P798. The final section of the review concerns secondary electron transfer, an area that is relatively poorly known in heliobacteria.

Fluorescence and energy transfer in phycobiliprotein-containing algae at low temperature.

Fluorescence emission spectra of Anacystis nidulans, Porphyridium cruentum and Cyanidium caldarium, three phycobiliprotein-containing algae, were measured at temperatures between 4 and 120 K in the absence and in the presence of quinones as quenchers of chlorophyll fluorescence. In all species three major emission bands were observed in the chlorophyll a region, near 685 nm (F-685), 695 nm (F-695) and between 710 and 730 nm. Additional bands were observed at shorter wavelengths; these were preferentially excited by light absorbed by the phycobiliproteins and are presumably due to phycocyanins and allophycocyanins. The amplitudes of F-685, F-695 and the long-wave emission showed a distinct increase upon cooling. For F-685 and F-695 the temperature dependence was similar to that earlier observed with spinach chloroplasts, for the long-wave emission it appeared to depend on the location of the emission bands, which was different for different species. All three bands were strongly quenched by quinones. These and other data suggest that the origin of these bands is the same as in higher plants, and that the fluorescence increase upon cooling can be explained by a lowering of the efficiency of energy transfer between chlorophyll molecules. It is concluded that at most a small percentage of the emission at 685 nm can be ascribed to allophycocyanin B, and that the efficiency of energy transfer between allophycocyanin B and chlorophyll a probably exceeds 99% both at 77 and 4 K. Experiments with isolated phycobilisomes suggest that energy transfer from allophycocyanin to allophycocyanin B occurs with an efficiency of about 90% at low temperature. The effect of quenchers can be understood by the assumption that the quenching is caused by the formation of non-fluorescent traps in the bulk chlorophyll. Of three quinones tested, the strongest quenching was observed with dibromothymoquinone, which quenched F-685, F-695 and the long-wave emission approximately equally. Menadione and 1,4-naphthoquinone, however, preferentially quenched the long-wave bands, indicating a stronger interaction with Photosystem I than with Photosystem II chlorophylls.

Charge accumulation at the reducing side of system 2 of photosynthesis.

A study was made of the reactions between the primary and secondary electron acceptors of Photosystem 2 by measurements of the increase of chlorophyll fluorescence induced in darkness by dithionite or by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). The experiments were done either with chloroplasts to which hydroxylamine or carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP) was added, or with chloroplasts treated with tris(hydroxymethyl)aminomethane (Tris) to which phenylenediamine and ascorbate were added as donor system. Under these conditions the fluorescence increase induced by dithionite or DCMU added after illumination with short light flashes was dependent on the flash number with a periodicity of two; it was large after an uneven number of flashes, and small after a long darktime or after an even number of flashes. The results are interpreted in terms of a model which involves a hypothetical electron carrier situated between Q and plastoquinone; this electron carrier is thought to equilibrate with plastoquinone in a two-electron transfer reaction; the results obtained with DCMU are explained by assuming that its midpoint potential is lowered by this inhibitor.

Photochemical activities of reaction centers from Rhodopseudomonas sphaeroides at low temperature and in the presence of chaotropic agents.

Light-induced absorbance changes were measured at low temperatures in reaction center preparations from Rhodopseudomonas sphaeroides. Absorbance difference spectra measured at 100 degrees K show that ubiquinone is photoreduced at this temperature, both by continuous light and by a short actinic flash. The reduction occurred with relatively high efficiency. These results give support to the idea that ubiquinone is involved in the primary photochemical reaction in Rhodopseudomonas sphaeroides. Reduction of ubiquinone was accompanied by a shift of the infrared absorption band of bacteriopheophytin. The rate of decay of the primary photoproducts (P+870 and ubisemiquinone) appeared to be approximately independent of temperature below 180 degrees K and above 270 degrees K; in the region between 180 and 270 degrees K it increased with decreasing temperature. The rate of decay was not affected by 0-phenanthroline. Secondary reactions were inhibited by lowering the temperature. The light-induced absorbance changes were inhibited by chaotropic agents, like thiocyanate and perchlorate. It was concluded that these agents lower the efficiency of the primary photoconversion. The kinetics indicated that the degree of inhibition was not the same for all reaction centers. The absorption spectrum of the photoconverted reaction centers appeared to be somewhat modified by thiocyanate.

Photosynthetic electron transport and electrochromic effects at sub-zero temperatures.

Spinach chloroplasts, suspended in a liquid medium containing ethyleneglycol, showed reversible absorbance changes near 700 and 518 nm due to P-700 and "P-518" in the region from -35 to -50 degrees C upon illumination. The kinetics were the same at both wavelengths, provided absorbance changes due to Photosystem II were suppressed. At both wavelengths, the decay was slowed down considerably, not only by the System I electron acceptor methyl viologen, but also by silicomolybdate. The effect of the latter compound is probably not due to the oxidation of the reduced acceptor of Photosystem I by silicomolybdate, but to the enhanced accessibility of the acceptor to some other oxidant. In the presence of both an electron donor and acceptor for System I, a strong stimulation of the extent of the light-induced absorbance increase at 518 nm was observed. The most effective donor tested was reduced N-methylphenazonium methosulphate (PMS). The light-induced difference spectrum was similar to spectra obtained earlier at room temperature, and indicated electrochromic band shifts of chlorophylls a and b and carotenoid, due to a large potential over the thylakoid membrane, caused by sustained electron transport. It was estimated that steady-state potentials of up to nearly 500 mV were obtained in this way; the potentials reversed only slowly in the dark, indicating a low conductance of the membrane. This decay was accelerated by gramicidin D. The absorbance changes were linearly proportional to the membrane potential.

Temperature and preillumination dependence of delayed fluorescence of spinach chloroplasts.

Delayed fluorescence (luminescence) from spinach chloroplasts, induced by short saturating flashes, was studied in the temperature region between 0 and minus 40 degrees C. At these temperatures, in contrast to what is observed at room temperature, luminescence at 40 ms after a flash was strongly dependent, with period four, on the number of preilluminating flashes (given at room temperature, before cooling). At minus 35 degrees C luminescence of chloroplasts preilluminated with two flashes (the optimal preillumination) was about 15 times larger than that of dark-adapted chloroplasts. The intensity of luminescence obtained with preilluminated chloroplasts increased steeply below minus 10 degrees C, presumably partly due to accumulation of reduced acceptor (Q minus), and reached a maximum at minus 35 degrees C. In the presence of 50 mM NH4Cl; at temperatures below minus 20 degrees C luminescence at 40 ms was decreased by NH4C1. At room temperature a strongly enhanced 40-ms luminescence was observed after the third and following flashes. The results indicate that both the S2 to S3 and the S3 to S4 conversion are affected by NlH4Cl. Inhibitors of Q minus reoxidation, like 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea, did only slightly affect the preillumination dependence of luminescence at sub-zero temperatures if they were added after the preillumination. This indicates that these substances by themselves do not accelerate the deactivation of S2 and S3.

Light-induced absorbance changes due to photosystems 1 and 2 in spinach chloroplasts at minus 50 degrees C.

Absorbance changes in the region 500-565 nm and at 702 nm, brought about by excitation of Photosystems 1 and 2, respectively, were measured in spinach chloroplasts at minus 50 degrees C. Either dark-adapted chloroplasts were used or chloroplasts preilluminated with a number of short saturating flashes just before cooling. Both photosystems were found to cause a light-induced increase of absorbance at 518 nm (due to "P518"). The System 1-induced change was not affected by pre-illumination. It decayed within 1 s in the dark and showed similar kinetics as P700. Experiments in the presence of external electron acceptors (methylviologen or Fe(CN)6-3-) suggested that P518 was not affected by the redox state of the primary electron acceptor of System 1. The absorbance increase at 518 nm due to System 2 decayed in the dark with a half-time of several min. The kinetics were similar to those of C-550, the presumed indicator of the primary electron acceptor of System 2. After two flashes preillumination the changes due to P518 and C-550 were reduced by about 40%, and a relatively slow, System 2-induced oxidation of cytochrome b559 occurred which proceeded at a similar rate as the increase in yield of chlorophyll a fluorescence. The results indicate that at minus 50 degrees C two different photoreactions of System 2 occur. One consists of a photoreduction of the primary electron acceptor associated with C-550, accompanied by the oxidation of an unknown electron donor; the other is less efficient and results in the photooxidation of cytochrome b559.

Electrochromic absorbance changes of photosynthetic pigments in Rhodopseudomonas sphaeroides. I. Stimulation by secondary electron transport at low temperature.

Light-induced absorbance changes were measured at temperatures between --30 and --55 degrees C in chromatophores of Rhodopseudomonas sphaeroides. Absorbance changes due to photooxidation of reaction center bacteriochlorophyll (P-870) were accompanied by a red shift of the absorption bands of a carotenoid. The red shift was inhibited by gramicidin D. The kinetics of P-870 indicated electron transport from the "primary" to a secondary electron acceptor. This electron transport was slowed down by lowering the temperature or increasing the pH of the suspension. Electron transport from soluble cytochrome c to P-870+ occurred in less purified chromatophore preparations. This electron transport was accompanied by a relatively large increase of the carotenoid absorbance change. This agrees with the hypothesis that P-870 is located inside the membrane, so that an additional membrane potential is generated upon transfer of an electron from cytochrome to P-870+. A strong stimulation of the carotenoid changes (more than 10-fold in some experiments) and pronounced band shifts of bacteriochlorophyll B-850 were observed upon illumination in the presence of artifical donor-acceptor systems. Reduced N-methylphenazonium methosulphate (PMS) and N,N,N',N'-tetramethyl-p-phenylene-diamine (TMPD) were fairly efficient donors, whereas endogenous ubiquinone and oxidized PMS acted as secondary acceptor. These results indicate the generation of large membrane potentials at low temperature, caused by sustained electron transport across the chromatophore membrane. The artificial probe, merocyanine MC-V did not show electrochromic band shifts at low temperature.

Energy transfer and bacteriochlorophyll fluorescence in purple bacteria at low temperature.

Emission spectra of bacteriochlorophyll a fluorescence and absorption spectra of various purple bacteria were measured at temperatures between 295 and 4 K. For Rhodospirillum rubrum the relative yield of photochemistry was measured in the same temperature region. In agreement with earlier results, sharpening and shifts of absorption bands were observed upon cooling to 77 K. Below 77 K further sharpening occurred. In all species an absorption band was observed at 751-757 nm. The position of this band and its amplitude relative to the concentration of reaction centers indicate that this band is due to reaction center bacteriopheophytin. The main infrared absorption band of Rhodopseudomonas sphaeroides strain R26 is resolved in two bands at low temperature, which may suggest that there are two pigment-protein complexes in this species. Emission bands, like the absorption bands, shifted and sharpened upon cooling. The fluorescence yield remained constant or even decreased in some species between room temperature and 120 K, but showed an increased below 120 K. This increase was most pronounced in species, such as R. rubrum, which showed single banded emission spectra. In Chromatium vinosum three (835, 893 and 934 nm) and in Rps. sphaeroides two (888 and 909 nm) emission bands were observed at low temperature. The temperature dependence of the amplitudes of the short wavelength bands indicated the absence of a thermal equilibrium for the excitation energy distribution in C. vinosum and Rps. sphaeroides. In all species the increased in the yield was larger when all reaction centers were photochemically active than when the reaction centers were closed. In R. rubrum the increase in the fluorescence yield was accompanied by a decrease of the quantum yield of charge separation upon excitation of the antenna but not of the reaction center chlorophyll. Calculation of the Förster resonance integral at various temperatures indicated that the increase in fluorescence yield and the decrease in the yield of photochemistry may be due to a decrease in the rate of energy transfer between antenna bacteriochlorophyll molecules. The energy transfer from carotenoids to bacteriochlorophyll was independent of the temperature in all species examined. The results are discussed in terms of existing models for energy transfer in the antenna pigment system.

Fluorescence emission spectra of chloroplasts and subchloroplast preparations at low temperature.

A study was made of the chlorophyll fluorescence spectra between 100 and 4.2 K of chloroplasts of various species of higher plants (wild strains and chlorophyll b mutants) and of subchloroplast particles enriched in Photosystem I or II. The chloroplast spectra showed the well known emission bands at about 685, 695 and 715--740 nm; the System I and II particles showed bands at about 675, 695 and 720 nm and near 685 nm, respectively. The effect of temperature lowering was similar for chloroplasts and subchloroplast particles; for the long wave bands an increase in intensity occurred mainly between 100 and 50 K, whereas the bands near 685 nm showed a considerable increase in the region of 50--4.2 K. In addition to this we observed an emission band near 680 nm in chloroplasts, the amplitude of which was less dependent on temperature. The band was missing in barley mutant no. 2, which lacks the light-harvesting chlorophyll a/b-protein complex. At 4.7 K the spectra of the variable fluorescence (Fv) consisted mainly of the emission bands near 685 and 695 nm, and showed only little far-red emission and no contribution of the band at 680 nm. From these and other data it is concluded that the emission at 680 nm is due to the light-harvesting complex, and that the bands at 685 and 695 nm are emitted by the System II pigment-protein complex. At 4.2 K, energy transfer from System II to the light-harvesting complex is blocked, but not from the light-harvesting to the System I and System II complexes. The fluorescence yield of the chlorophyll species emitting at 685 nm appears to be directly modulated by the trapping state of the reaction center.

Fluorescence emission spectra of cells and subcellular preparations of a green photosynthetic bacterium. Effects of dithionite on the intensity of the emission bands.

Fluorescence emission spectra were measured of intact cells and subcellular preparations of the green photosynthetic bacterium Prosthecochloris aestuarii in the presence and in the absence of dithionite. A 3--5-fold increase in bacteriochlorophyll a fluorescence at 816 nm occurred upon addition of dithionite in a membrane vesicle preparation (Complex I), in a photochemically active pigment-protein complex and in a bacteriochlorophyll a protein complex free from reaction centers. The pigment-protein complex showed a relatively strong long-wave emission band (835 nm) of bacteriochlorophyll a, which was preferentially excited by light absorbed at 670 nm and was not stimulated by dithionite. With Complex I, which contains some bacteriochlorophyll c in addition to bacteriochlorophyll a, a 3--4-fold stimulation of bacteriochlorophyll c emission was also observed. Emission bands at shorter wavelengths, probably due to artefacts, were quenched by dithionite. With intact cells, the effect of dithionite was smaller, and consisted mainly of an increase of bacteriochlorophyll a emission. The results indicate that the strong increase in the yield of bacteriochlorophyll emission that occurred upon generating reducing conditions is, at least mainly, due to a direct effect on the light-harvesting systems, and does not involve the reaction center as had been earlier postulated.

The primary charge separation, cytochrome oxidation and triplet formation in preparations from the green photosynthetic bacterium Prosthecochloris aestuarii.

Flash-induced absorbance changes were measured in intact cells and subcellular preparations of the green photosynthetic bacterium Prosthecochloris aestuarii. In Complex I, a membrane vesicle preparation, photooxidation of the primary electron donor, P-840, and of cytochrome c-553 was observed. Flash excitation of the photosystem pigment complex caused in addition the generation of a bacteriochlorophyll a triplet. Triplet formation was the only reaction observed after flash excitation in the reaction center pigment-protein complex. The triplet had a lifetime of 90 microseconds at 295 K and of 165 microseconds at 120 K. The amount of triplet formed in a flash increased upon cooling from 295 to 120 K from 0.2 and 0.5 per reaction center to 0.45 and nearly 1 per reaction center in the photosystem pigment and reaction center pigment-protein complex, respectively. Measurements of absorbance changes in the near infrared in the reaction center pigment-protein complex indicate that the triplet is formed in the reaction center and that the reaction center bacteriochlorophyll a triplet is that of P-840. Formation of a carotenoid triplet did not occur in our preparations. Illumination with continuous light at 295 K of the reaction center pigment-protein complex produced a stable charge separation (with oxidation of P-840 and cytochrome c-553) in each reaction center, but with a low efficiency. This low efficiency, and the high yield of triplet formation is probably due to damage of the electron transport chain at the acceptor side of the reaction center of the reaction center pigment-protein complex. The halftime for cytochrome c-553 oxidation in Complex I and the photosystem pigment complex was 90 microseconds at 295 K; below 220 K no cytochrome oxidation occurred. At 120 K P-840+ was rereduced with a halftime of 20 ms, presumably by a back reaction with a reduced acceptor.

The fluorescence yield of Rhodopseudomonas viridis in relation to the redox state of the primary electron donor.

The fluorescence yield of bacteriochlorophyll (BChl) b in membranes of Rhodopseudomonas viridis was measured immediately and at a variable time-interval after a saturating laser flash to bring about charge separation. At 4 K a decrease of the yield by 28% was observed immediately after the flash. This yield recovered mono-exponentially with a time constant of 6.3 +/- 0.4 ms to approximately the original level. The same time constant was observed for the re-reduction of the primary electron donor, indicating that the fluorescence quenching can be ascribed to the oxidation of the primary donor. The extent of quenching decreased with increasing temperature and reversed to a fluorescence increase at temperatures above 50 K. These results may be explained by the presence of long-wavelength absorbing BChls b in the antenna which at low temperature transfer their excitation energy more efficiently to the oxidized than to the reduced primary donor, in support of a similar hypothesis used to explain the quenching of fluorescence by 'oxidized' reaction centers in heliobacterium chlorum (Deinum, G., Kramer, H., Aartsma, T.J. Kleinherenbrink, F.A.M. and Amesz, J. (1991) Biochim. Biophys. Acta 1058, 339-344).

EPR signals of oxidized plastocyanin in intact algae.

(1) An electron paramagnetic resonance (EPR) signal was observed at g = 2.05 in the low temperature spectra of intact cells of green, red and blue-green algae and of spinach chloroplasts. The g-value and the shape of the signal were similar to that of purified, soluble plastocyanin. (2) The amount of the copper protein, determined from the EPR signal height, was estimated to be nearly the same in all the studied organisms on the basis of the concentration of chlorophyll. Furthermore, it was found that the amount of the copper protein, determined from the EPR signal height in spinach chloroplasts corresponds with that of plastocyanin as determined chemically by Katoh, S., Suga, I., Shiratori, I. and Takamiya, A. (1961) Arch. Biochem. Biophys. 94, 136-141. (3) Experiments with far-red and red illumination show that the site of the copper protein in vivo is in the electron transport pathway between Photosystems 1 and 2. Plastocyanin is not oxidized by illumination at 77 degrees K, indicating that no electron transfer occurs between the primary electron donor of Photosystem 1, P700, and plastocyanin at that temperature. Furthermore, the experiments suggest that in the intact cells of the studied algae, plastocyanin is not only reduced by Photosystem 2 but also by cyclic electron transport around Photosystem 1.

Energy transfer and charge separation in the purple non-sulfur bacterium Roseospirillum parvum.

The antenna reaction centre system of the recently described purple non-sulfur bacterium Roseospirillum parvum strain 930I was studied with various spectroscopic techniques. The bacterium contains bacteriochlorophyll (BChl) a, 20% of which was esterified with tetrahydrogeranylgeraniol. In the near-infrared, the antenna showed absorption bands at 805 and 909 nm (929 nm at 6 K). Fluorescence bands were located at 925 and 954 nm, at 300 and 6 K, respectively. Fluorescence excitation spectra and time resolved picosecond absorbance difference spectroscopy showed a nearly 100% efficient energy transfer from BChl 805 to BChl 909, with a time constant of only 2.6 ps. This and other evidence indicate that both types of BChl belong to a single LH1 complex. Flash induced difference spectra show that the primary electron donor absorbs at 886 nm, i.e. at 285 cm(-1) higher energy than the long wavelength antenna band. Nevertheless, the time constant for trapping in the reaction centre was the same as for almost all other purple bacteria: 55+/-5 ps. The shape as well as the amplitude of the absorbance difference spectrum of the excited antenna indicated exciton interaction and delocalisation of the excited state over the BChl 909 ring, whereas BChl 805 appeared to have a monomeric nature.

Orientation of pigments and pigment-protein complexes in the green photosynthetic bacterium Prosthecochloris aestuarii.

The orientation of pigments and pigment-protein complexes of the green photosynthetic bacterium Prosthecochloris aestuarii was studied by measurement of linear dichroism spectra at 295 and 100 K. Orientation of intact cells and membrane vesicles (Complex I) was obtained by drying on a glass plate. The photochemically active pigment-protein complexes (photosystem-protein complex and reaction center pigment-protein complex) and the antenna bacteriochlorophyll a protein were oriented by pressing a polyacrylamide gel. The data indicate that the near-infrared transitions (Qy) of bacteriochlorophyll c and most bacteriochlorophyll a molecules have a relatively parallel orientation to the membrane, whereas the Qy transitions of the bacteriochlorophyll a in the antenna protein are oriented predominantly perpendicularly to the membrane. Carotenoids and the Qx transitions (590-620 nm) of bacteriochlorophyll a, not belonging to the bacteriochlorophyll a protein, have a relatively perpendicular orientation to the membrane. The absorption and linear dichroism spectra indicate the existence of different pools of bacteriochlorophyll c in the chlorosomes and of carotenoid and bacteriopheophytin c in the cell membrane. The results suggest that the photosystem-protein and reaction center pigment-protein complexes are oriented with their short axes approximately perpendicular to the plane of the membrane. The symmetry axis of the bacteriochlorophyll a protein has an approximately perpendicular orientation.

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.

Enrichment of bacteriochlorophyll g' in membranes of Heliobacterium chlorum by ether extraction. Unequivocal evidence for its existence in vivo.

Treatment of H. chlorum membrane preparations with diethyl ether of high degrees of water saturation raised the bacteriochlorophyll (BChl) g' mole fraction, as determined by HPLC analysis of their acetone extracts, toward a level of 40% of total BChl g or higher. Starting from pure BChl g, the BChl g' mole fraction should never exceed 24.6% which is the equilibrium value in diethyl ether. The existence (and possible functioning) of BChl g' in vivo is thus unequivocally demonstrated.

Excitation energy transfer in Rhodopseudomonas sphaeroides chromatophore membranes fused with liposomes.

The role of phospholipid in the structural organization of the light-harvesting complexes of Rhodopseudomonas sphaeroides was examined in photosynthetic (chromatophore) membrane vesicles fused with liposomes. Photochemically active preparations with progressive phospholipid enrichment up to greater than 15-fold were obtained by both polyethylene glycol- and acidic-pH-induced fusion. Their fluorescence emission at approximately 300 and 77 K was increased by 2-3.5-fold from the peripheral B800-850 antenna relative to that from the core B875 antenna. Up to 30-40% reduction in the efficiency of excitation energy transfer between B850 and B875 was also observed at 77 K suggesting a selective, phospholipid-induced dissociation of a portion of the B800-850 from the rest of the light-harvesting system.