Photogeneration and quenching of singlet molecular oxygen by bacterial C40 carotenoids with long chain of conjugated double bonds

Measurement of photosensitized luminescence of singlet oxygen has been applied to studies of singlet oxygen generation and quenching by C40 carotenoids (neurosporene, lycopene, rhodopin, and spirilloxanthin) with long chain of conjugated double bonds (CDB) using hexafluorobenzene as a solvent. It has been found that neurosporene, lycopene, and rhodopin are capable of the low efficient singlet oxygen generation in aerated solutions upon photoexcitation in the spectral region of their main absorption maxima. The quantum yield of this process was estimated to be (1.5-3.0) × 10-2. This value is near the singlet oxygen yields in solutions of ζ-carotene (7 CDB) and phytoene (3 CDB) and many-fold smaller than in solutions of phytofluene (5 CDB) (Ashikhmin et al. Biochemistry (Mosc) 85:773-780, https://doi.org/10.1134/S0006297920070056 , 2020, Biochemistry (Mosc) 87:1169-1178, 2022, https://doi.org/10.1134/S00062979221001082022 ). Photogeneration of singlet oxygen was not observed in spirilloxanthin solutions. A correlation was found between the singlet oxygen yields and the quantum yields and lifetimes of the fluorescence of the carotenoid molecules. All carotenoids were shown to be strong physical quenchers of singlet oxygen. The rate constants of 1O2 quenching by the carotenoids with long chain of CDB (9-13) were close to the rate constant of the diffusion-limited reactions ≈1010 M-1 s-1, being many-fold greater than the rate constants of 1O2 quenching by the carotenoids with the short chain of CDB (3-7) phytoene, phytofluene, and ζ-carotene studied in prior papers of our group (Ashikhmin et al. 2020, 2022). To our knowledge, the quenching rate constants of rhodopin and spirilloxanthin have been obtained in this paper for the first time. The mechanisms of 1O2 photogeneration by carotenoids in solution and in the LH2 complexes of photosynthetic cells, as well as the efficiencies of their protective action are discussed.

Phytofluene as a Highly Efficient UVA Photosensitizer of Singlet Oxygen Generation

Phytoene and phytofluene - uncolored C₄₀ carotenoids with short chain of conjugated double bonds (3 and 5, respectively) - are known to be universal precursors in biosynthesis of colored carotenoids in photosynthesizing organisms. It is commonly recognized that C₄₀ carotenoids are photoprotectors of cells and tissues. We have shown that phytofluene is an exception to this rule. By measuring photosensitized phosphorescence of singlet oxygen (¹O₂) we found out that phytofluene was very effective photosensitizer of ¹O₂ formation in aerated solutions under UVA irradiation (quantum yield of 85 ± 5%), whereas phytoene was almost inactive in this process. It was demonstrated that both carotenoids quench singlet oxygen in the dark. The obtained quenching rate constants [(4 ± 1) × 10⁶ M^-1·s^-1 for phytoene and (2 ± 0.5) × 10⁷ M^-1·s^-1 for phytofluene] were smaller than the rate constant of the diffusion-controlled reactions by 3-4 orders of magnitude. Thus, both carotenoids displayed rather weak protector properties. Moreover, phytofluene due to its high photosensitizing activity might be considered as a promoter of cell photodamage and a promising UVA photosensitizer for medical purposes.

The LH2 Complexes (B800-850 and B800-830) Are Assembled in the Cells of the Sulfur Bacterium Thiorhodospira sibirica Strain Kir-3 without Carotenoids

The results of assembling the light-harvesting complexes in the cells of the purple sulfur bacterium Thiorhodospira (T.) sibirica strain Kir-3 while suppressing the biosynthesis of carotenoids with diphenylamine (DPA) were studied. LH2 complexes (B800-850 and B800-830) with different carotenoid composition were isolated from the cells obtained. Maximum inhibition of carotenoid biosynthesis (~90% of the control) was reached at an inhibitor concentration of 53.25 μM (9 mg/L). It was established that changes in the qualitative and quantitative composition of carotenoids do not affect the assembly of B800-830 and B800-850 complexes. It is assumed that, in the population of DPA-LH2 complexes from T. sibirica strain Kir-3, both the carotenoidless complexes and the complexes containing one or two carotenoid molecules can be assembled. These results support the hypothesis that carotenoids are not required for assembling B800-850 and B800-830 complexes.

Bacteriochlorophyll Interaction with Singlet Oxygen in Membranes of Purple Photosynthetic Bacteria: Does the Protective Function of Carotenoids Exist?

The direct action of singlet oxygen on the bacteriochlorophyll (BChl) of light-harvesting complexes in the membranes of four species of purple non-sulfur and sulfur photosynthesizing bacteria with and without carotenoids was studied. It was found that BChl in carotenoidless samples is generally more resistant to the action of singlet oxygen compared to the control. It is assumed that carotenoids are not required to protect BChl of bacterial light-harvesting complexes from singlet oxygen, and in the classic work by Griffith et al. [1] the apoptosis process in carotenoidless mutant cells, which involves the destruction of complexes, the appearance of monomeric BChl, and the generation of singlet oxygen caused by BChl, followed by BChl oxidation, was mistakenly attributed to the protective function of carotenoids.

The Influence of the Number of Conjugated Double Bonds in Carotenoid Molecules on the Energy Transfer Efficiency to Bacteriochlorophyll in Light-Harvesting Complexes LH2 from Allochromatium vinosum Strain MSU

Seven different carotenoids with the number of conjugated double bonds (N) from 5 to 11 were incorporated in vitro into carotenoidless complexes LH2 of the sulfur bacterium Allochromatium vinosum strain MSU. The efficiency of their incorporation varied from 4 to 99%. The influence of N in the carotenoid molecules on the energy transfer efficiency from these pigments to bacteriochlorophyll (BChl) in the modified LH2 complexes was studied for the first time. The highest level of energy transfer was recorded for rhodopin (N = 11) and neurosporene (N = 7) (37 and 51%, respectively). In the other carotenoids, this parameter ranged from 11 to 33%. In the LH2 complexes studied, we found no direct correlation between the decrease in N in carotenoids and increase in the energy transfer efficiency from these pigments to BChl.

Excitation energy transfer from the bacteriochlorophyll Soret band to carotenoids in the LH2 light-harvesting complex from Ectothiorhodospira haloalkaliphila is negligible

Pathways of intramolecular conversion and intermolecular electronic excitation energy transfer (EET) in the photosynthetic apparatus of purple bacteria remain subject to debate. Here we experimentally tested the possibility of EET from the bacteriochlorophyll (BChl) Soret band to the singlet S₂ level of carotenoids using femtosecond pump-probe measurements and steady-state fluorescence excitation and absorption measurements in the near-ultraviolet and visible spectral ranges. The efficiency of EET from the Soret band of BChl to S₂ of the carotenoids in light-harvesting complex LH2 from the purple bacterium Ectothiorhodospira haloalkaliphila appeared not to exceed a few percent.

Distribution of rhodopin and spirilloxanthin between LH1 and LH2 complexes when incorporating carotenoid mixture into the membrane of purple sulfur bacterium Allochromatium minutissimum in vitro

Carotenoid mixture enriched by rhodopin and spirilloxanthin was incorporated in LH2 and LH1 complexes from Allochromatium (Alc.) minutissimum in vitro. The maximum incorporating level was ~95%. Rhodopin (56.4%) and spirilloxanthin (13.8%) were incorporated into the LH1 complex, in contrast to the control complex, which contained primarily spirilloxanthin (66.8%). After incorporating, the LH2 complex contained rhodopin (66.7%) and didehydrorhodopin (14.6%), which was close to their content in the control (67.4 and 20.5%, respectively). Thus, it was shown that carotenoids from the total pool are not selectively incorporated into LH2 and LH1 complexes in vitro in the proportion corresponding to the carotenoid content in the complexes in vivo.

Formation of a Subunit Form of the Core Light-Harvesting Complex from Sulfur Purple Bacteria Ectothiorhodospira haloalkaliphila with Different Carotenoid Composition

B820 subunits from a purple sulfur bacterium Ectothiorhodospira. haloalkaliphila strain ATCC 51935T were obtained by treatment of Carotenoid free LH I-RC complexes of this bacterium with P--octylglu- copyranoside (β-OG). The same complexes with 100% carotenoid content were unable to dissociate to B820 subunits, but disintegrated to monomeric bacteriochlorophyll (BChl) regardless of their carotenoid compo- sition. The degree of dissociation of the LH 1-RC complexes with an intermediate content of carotenoids (the' B820 formation) was directly dependent on the quantity of carotenoids in the samples. The resulting B820 subunits did not contain carotenoids. B820 subunits easily aggregated to form a complex with an absorption . peak at 880 nm at decreased β-OG concentration. Analysis of the spectra of the LH I-RC complexes isolated from the cells with different'levels of carotenogenesis inhibition led to the conclusion of the heterogeneity of the samples with a predominance in them of (a) the fraction with 100% of carotenoids and (b) the fraction of carotenoid free complexes.

Embedding carotenoids of spheroidene-branch biosynthesis into antenna complexes of sulfur photosynthetic bacteria

The possibility of embedding the carotenoids of spheroidene-branch biosynthesis (spheroidene and spheroidenone) from non-sulfur bacteria into the diphenylamine antenna complexes (DPA-complexes) from the sulfur bacteria Allochromatium minutissimum and Ectothiorhodospira haloalkaliphila with carotenoid synthesis inhibited by diphenylamine (DPA) was studied for the first time. It was found that spheroidene was embedded into the DPA-complexes from these bacteria at a level of 75-87%, with spheroidene embedding efficiency being 41-68% for the LH1-RC DPA-complexes and 71-89% for the LH2 DPA-complexes. The energy transfer efficiency from carotenoids to bacteriochlorophyll was shown to depend not only on the type of carotenoid but also on the very structure on the antenna complex.

Effect of Illumination Intensity and Inhibition of Carotenoid Biosynthesis on Assembly of Peripheral Light-Gathering Complexes in Purple Sulfur Bacteria C Allochromatium vinosum ATCC 17899

Effect of illumination intensity and inhibition of carotenoid biosynthesis on assemblage of different spectral types of LH2 complexes in a purple sulfur bacterium Allochromatium (Alc.) vinosum ATCC 17899 was studied. Under illumination of 1200 and 500 lx, the complexes B800-850 and B800-840 and B800-820 were assembled. While rhodopine was the major carotenoid in all spectral types of the LH2 complex, a certain- increase in the content of carotenoids with higher numbers of conjugated double bonds (anhydrorhodovibrin and didehydrorhodovibrin) was observed in the B800-820 complex. At 1200 lx, the cells grew slowly at diphe- nylamine (DPA) concentrations not exceeding 53 .iM, while at illumination intensity decreased to 500 Ix they could grow at 71 jiM DPA (DPA cells). Independent on illumination level, the inhibitor is supposed to impair the functioning of phytoine synthetase (resulting in a decrease in the total carotenoid content) and of phyto- ine desturase, which results in formation of neurosporene hydroxy derivatives and ;-carotene. In the cells grown at 500 lx, small amounts of spheroidene and.OH-spheroidene were detected. These carotenoids were originally found under conditions of carotenoid synthesis inhibition in bacteria with spirilloxanthin as the major carotenoid. Carotenoid content in the LH2 complexes isolated from the DPA cells was -15% of the control (without inhibition) for the B800-850 and -20%of the control for the B800-820 and B800-840 DPA complexes. Compared to the DPA pigment-containing membranes, the DPA complexes were enriched with -carotenoids due to- disintegration of some carotenoid-free complexes in the course of isolation. These results support the supposition that some of the B800-820, B800-840, and B800-850 complexes may be Assembled in the cells of Alc. vinosum ATCC 17899 without carotenoids. Comparison of the characteristics obtained for Alc. vinosum ATCC 17899 and the literature data on strain D of the same bacteria shows that they belong to two different strains, rather than to one as was previously supposed.

[Singlet-triplet excitation fission in light-harvesting complexes of photosynthetic bacteria and in isolated carotenoids]

Time-resolved electron paramagnetic resonance was used to study the properties of carotenoid triplet states populated in LH2 light-harvesting complexes of phototrophic bacteria Allochromatium minutissimum, Rhodopseudomonas palustris, and in carotenoid films free of bacteriochlorophyll. The study was performed on purified LH2 preparations not contaminated by reaction centers, and under selective pigment excitation. The obtained results enable a conclusion that the carotenoid triplet states, both in LH2 complexes and films, are populated in the process of homofission of singlet excitation into two triplets, which involves only carotenoid molecules. It is observed that the fission process in magnetic field leads to predominant population of the T0 spin sublevel of the triplet. One molecular spin sublevel of the triplet is demonstrated to possess an increased probability of intersystem crossing to the ground state, independent of the carotenoid configuration. Pigment composition of the LH2 protein heterodimers is discussed, and a conclusion of the possible

[Roseococcus suduntuyensis sp. nov., a new aerobic bacteriochlorophyll A-containing bacterium isolated from a low-mineralized soda lake of Eastern Siberia]

A novel strain, SHET, of aerobic bacteriochlorophyll a-containing bacteria was isolated from the surface layer of bottom sediments from the soda lake Shuluutai-Ekhe-Torom (Chita oblast, Eastern Siberia, Russia). The lake water has a total mineralization of 3.0 g/l and a pH of 9.2. The cells of strain SHET are cocci or short rods, which reproduce by uniform division. The cells are motile by means of flagella. The cell wall structure is of the gram-negative type. Sparse intracytoplasmic membrane vesicles are located close to the cell wall. The new isolate is an obligate aerobe and facultative alkaliphile which grows in a pH range of 7.5-9.5 (with an optimum at pH 8.5-9.0). The best growth of strain SHET occurred at 2.0 g/l NaCl and 23-28 degrees C. Photosynthetic pigments are represented by bacteriochlorophyll a, with the maximum absorption at 865 nm in the in vivo spectrum, and carotenoids (spirilloxanthin derivatives). Analysis of the 16S rRNA gene sequences demonstrated that strain SHET is closely related to Roseococcus thiosulfatophilus of the alpha-1 subclass of Proteobacteria (98.6 % similarity). The DNA G + C base content is 69.1 mol %. Unlike Rsc. thiosulfatophilus, strain SHET grows well on sugars and glycerol and is not capable of utilizing thiosulfate as an energy source. The new isolate is a facultative alkaliphile and reduces nitrates to nitrites. On the basis of its phenotypic and genetic characteristics, strain SHET was described as a new species of the genus Roseococcus, Rsc. suduntuyensis sp. nov.

[Assembly of LH2 light-harvesting complexes in Rhodopseudomonas palustris cells illuminated by blue and red light]

We investigated the formation of the B800-850 complex in cells of the bacterium Rhodopseudomonas palustris AB illuminated by red and blue light under anaerobic growth conditions. Under red illumination, the B800-850 complex was assembled with a reduced absorption band at 850 nm. The results of re-electrophoresis of the B800-850 complex and oxidation in the presence of potassium iridate suggest its heterogeneity. It may be a mixture of two complexes (B800 and B800-850). The B800-850 complex lacks the capacity for conformational transitions if assembled under blue illumination. Accordingly, the light-harvesting complex assembled in the blue light contains polypeptides that are not synthesized under normal conditions or at increased or decreased light intensities. The mechanism of regulation of the synthesis of the polypeptides of light-harvesting B800-850 complex and its dependence on the spectral composition of the light is discussed.

[Rhodobaca barguzinensis sp. nov., a new alkaliphilic purple nonsulfur bacterium isolated from a soda lake of the Barguzin Valley (Buryat Republic, eastern Siberia)]

A novel strain, alga-05, of alkaliphilic purple nonsulfur bacteria was isolated from sediments of a small saline (60 g/l) soda lake near Lake Algin (Barguzin Valley, Buryat Republic, Russia). These bacteria contain bacteriochlorophyll a and carotenoids of the alternative spirilloxanthin group with predominating demethylspheroidenone. They are facultative anaerobes; their photosynthetic structures are of the vesicular type and arranged along the cell periphery. Growth of this strain is possible in a salinity range of 5-80 g/l NaCl, with an optimum at 20 g/l NaCl. Best growth occurred at 20-35 degrees C. Analysis of the 16S rRNA gene sequences demonstrated that the studied isolate is closely related to the alkaliphilic purple nonsulfur bacterium Rhodobaca bogoriensis (99% similarity) isolated from soda lakes of the African Rift Zone. According to the results of DNA-DNA hybridization, strain alga-05 has a 52% similarity with the type species of the genus Rhodobaca. On the basis of the obtained genotypic data and some phenotypic properties (dwelling in a hypersaline soda lake of Siberia, moderate halophily, ability to grow at relatively low temperatures, etc.), the isolated strain of purple bacteria was described as a new species of the genus Rhodobaca, Rca. barguzinensis sp. nov.

Reconstitution of Okenone into Light Harvesting Complexes from Allochromatium minutissimum

Okenone was reconstituted into light harvesting (LH) complexes of the purple photosynthetic bacterium Allochromatium minutissimum possessing the spirilloxanthin pathway for carotenoid biosynthesis. Suppression of this pathway by diphenylamine, an inhibitor of carotenogenesis, yielded nearly carotenoidless complexes preserving their native spectral properties. Using a previously developed technique, okenone was readily reconstituted into LH1 complex (>90%) whereas its reconstitution into LH2 complex was of low efficacy (10-20%). The absorption band of the reconstituted okenone was shifted to shorter wavelength compared with its position in vivo. This is typical for other reconstituted carotenoids. The reconstitution of okenone was confirmed by Li-DS electrophoresis (in contrast to free okenone the reconstituted okenone migrated with complexes), circular dichroism spectra (reconstituted okenone exhibited optical activity), and fluorescence excitation spectrum (energy transfer from okenone to bacteriochlorophyll was at the control level).

Effects of carotenoid inhibition on the photosynthetic RC-LH1 complex in purple sulphur bacterium Thiorhodospira sibirica

Core complexes (LH1-RC) were isolated using preparative gel electrophoresis from photosynthetic membranes of the purple bacterium, Thiorhodospira sibirica, grown in the absence or presence of the carotenoid biosynthesis inhibitor, diphenylamine. The biosynthesis of carotenoids is affected by diphenylamine both quantitavely and qualitatively: after inhibition, the level of carotenoids in core complexes reaches only 10% of the normal content, as analyzed by HPLC and absorption spectroscopy. The normally grown bacterium biosynthesizes spirilloxanthin, rhodopin, anhydrorhodovibrin and lycopene, whereas after inhibition only neurosporene, zeta-carotene and their derivatives are found in the complexes. There is no concomitant accumulation of appreciable amounts of colorless carotenoid precursors. Interestingly, the main absorption band of the core light harvesting complex isolated from carotenoid-inhibited cells, shows a red shift to 889 nm, instead of a blue shift observed in many carotenoid-deficient species of purple photosynthetic bacteria. The stability of isolated core complexes against n-octyl-beta-D: -glucopyranoside clearly depends on the presence of carotenoids. Subcomplexes resulting from the detergent treatment, were characterized by non-denaturating gel electrophoresis combined with in situ absorption spectroscopy. Core complexes with the native carotenoid complement dissociate into three subcomplexes: (a) LH1 complexes partially depleted of carotenoids, with an unusual spectrum in the NIR region (lambdamax = 791, 818, 847 and 875 nm), (b) reaction centers associated with fragments of LH1, (c) small amounts of a carotenoidless B820 subcomplex. The core complex from the carotenoid-deficient bacterium is much less stable and yields only the two sub-complexes (b) and (c). We conclude that carotenoids contribute critically to stability and interactions of the core complexes with detergents.

Formation of Bacteriochlorophyll Form B820 in Light Harvesting 2 Complexes from Purple Sulfur Bacteria Treated with Dioxane

Treatment of some sulfur bacteria (Allochromatium minutissimum, Thiorhodospira sibirica, and Ectothiorhodospira halovacuolata WN22) with dioxane results in formation of the bacteriochlorophyll form B820 in the light harvesting complex LH2. This form characterized by absorption maximum at 820 nm has the same absorption spectrum as B820 subcomplex from LH1 complex. Appearance of the B820 form was accompanied by a sharp decrease in absorption in the carotenoid region. This phenomenon observed in all LH2 complexes investigated may be attributed to formation of colorless carotenoid aggregates. This is very similar to the previously reported dissociation of the LH1 complex with carotenoids into B820 subcomplexes. Although the B820 form corresponded the bacteriochlorophyll dimer, its circular dichroism spectrum showed that pigment molecules in this dimer exhibit different interaction than those in the B820 subcomplex. The dioxane treatment of LH2 complexes isolated from Rhodopseudomonas palustris bacteria grown under normal or low intensity illumination did not result in formation of such dimers. It is suggested that bacteriochlorophyll B820 formation is related to unique structure of LH2 complexes from the sulfur bacteria.

[Ectothiorhodosinus mongolicum gen. nov., sp. nov.,--a new purple sulfur bacterium from soda lake in Mongolia]

A new purple sulfur bacterium (strain M9) was isolated from the steppe soda Lake Dzun Uldziin Nur (pH 9.4; mineralization, 3.3%) situated in southeastern Mongolia. Individual cells appear as vibrios 0.3-0.5 x 0.7-1 micron in size. The dividing cells often do not separate from each other, forming an almost closed ring. The internal photosynthetic membranes are represented by concentric lamellae lining the cell wall. Photosynthetic pigments are bacteriochlorophyll a and carotenoids of the spirilloxanthin series. The main carotenoid (> 96%) is spirilloxanthin. Two typical light-harvesting complexes (LH1 and LH2) are present in the membranes in a 1:1 ratio. The bacterium is an anaerobe and facultative photoorganoheterotroph. Photolithoautotrophic growth on sulfide is scarce. Thiosulfate is utilized as an electron donor only in the presence of organic matter. Globules of elemental sulfur are formed as an intermediary product of sulfide and thiosulfate oxidation and are deposited outside the cells. The end product of oxidation is sulfate. In the presence of sulfide and carbonates, acetate, lactate, malate, pyruvate, propionate, succinate, and fumarate are used as the additional sources of carbon in anoxygenic photosynthesis. Vitamin are not required. The bacterium is an alkaliphile the pH optimum is at 8.3-9.1, the pH range is 7.6-10.1. The optimum NaCl concentration in the medium is 1 to 7%; the range is 0.5 to 0.9%. The optimum carbonate content in the medium is 2%; the range is 1 to 10%. The best growth occurs at 30-35 degrees C. The DNA G + C content is 57.5 mol%. According to the results of analysis of the 16S rRNA gene sequences, the new isolate M9 belongs to the phylogenetic cluster containing representatives of the family Ectothiorhodospiraceae within the class "Gammaproteobacteria." In this class, the new isolate forms a new branch, which occupies an intermediate position between the representatives of the genera Ectothiorhodospira and Thiorhodospira. Based on the phenotypic and genetic characteristics, the new purple sulfurbacterium was assigned to a new species of a new genus of the family Ectothiorhodospiraceae, Ectothiorhodosinus mongolicum gen. nov., sp. nov.

Reconstitution of carotenoids into the light-harvesting complex B800-850 of Chromatium minutissimum

Chromatophores and peripheral light-harvesting complexes B800-850 with a trace of carotenoids were isolated from Chromatium minutissimum cells in which carotenoid biosynthesis was inhibited by diphenylamine. Three methods previously used for the reconstitution of carotenoids into either the light-harvesting (LH1) type complexes or reaction centers (RC) of carotenoidless mutants were examined for the possibility of carotenoid reconstitution into the carotenoid depleted chromatophores. All these methods were found to be unsuitable because carotenoid depleted complex B800-850 from Chr. minutissimum is characterized by high lability. We have developed a novel method maintaining the native structure of the complexes and allowing reconstitution of up to 80% of the carotenoids as compared to the control. The reconstituted complex has a similar CD spectrum in the carotenoid region as the control, and its structure restores its stability. These data give direct proof for the structural role of carotenoids in bacterial photosynthesis.

[Two-photon excitation fluorescence spectrum of the light-harvesting complex LH2 from Chromatium minutissimum within 650-745 nm range is determined by two-photon absorption of bacteriochlorophyll rather than of carotenoids]

Two-photon fluorescence excitation spectra of the peripheral light-harvesting complex LH2 from the purple photosynthetic bacterium Chromatium minutissimum were examined within the expected spectral range of the optically forbidden S1 singlet state of carotenoids. LH2 preparations isolated from wild-type and carotenoid-depleted cells were used. 100-fs laser pulses in the range of 1300-1490 nm with an energy of 7-9 mW (corresponding to one-photon absorption between 650 and 745 nm) were used for two-photon fluorescence excitation. It was shown that two-photon fluorescence excitation spectra of LH2 complex from wild and carotenoid-depleted cells are very similar to each other and to the two-photon fluorescence excitation spectrum of bacteriochlorophyll a in acetone. It was concluded that direct two-photon excitation of bacteriochlorophyll a determines the fluorescence of both samples within the 650-745 nm spectral range.

Model of detergent-induced spectral changes of the B800-850 complex from Chromatium minutissimum

The absorption and circular dichroism spectra of the B800-850 complex from Chromatium minutissimum before and after the Triton X-100 treatment were simulated by means of standard exciton theory, taking into account inhomogeneous broadening. To explain the spectral changes of the B800-850 complex treated with Triton X-100, we have assumed that all bacteriochlorophyll pigments absorbing at 850 nm exhibit the same additional rotation of approximately 20 degrees around the axis perpendicular to the membrane plane. This has been sufficient to fit the transformation in absorption and circular dichroism spectra induced by detergent treatment of the B800-850 complex.

Picosecond dynamics of excitations in light-harvesting complex B800-850 from Chromatium minutissimum studied using fluorescence spectrochronography

The picosecond dynamics of excitations in the isolated B800-850 light-harvesting complex of the purple sulfur bacterium Chromatium minutissimum has been studied using picosecond fluorescence spectrochronography. A short-lived component of about 20 ps lifetime has been found at 77K at the short wavelength part of the B850 fluorescence spectrum similar to that previously described for the core antenna bacteriochlorophyll band B880 of Rhodospirillum rubrum. Evidence has been presented indicating that this component is likely to reflect excitation energy relaxation step(s) involving both photoexcited bacteriochlorophyll and the protein environment. A new kinetic scheme of excitation transfer from the peripheral antenna to the photoreceptor units in purple bacteria is suggested which takes into account these findings.