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DPA membranes from Allochromatium vinosum cells, in which carotenoid biosynthesis was inhibited using diphenylamine (DPA) were obtained, into which rhodopin was incorporated. The LH2 complex with rhodopin content of 85% was isolated. Using a test for the thermal stability of LH2 complexes (DPA and with incorporated rhodopin), it was established that carotenoids of the early stages of biosynthesis (≤1 molecules per complex) did not interfere with rhodopin incorporation. It was found that when the LH2 complex with incorporated rhodopin was irradiated with light at the wavelength of 502 nm, BChl850 was photobleached at a rate close to that in the control LH2 complex. This indicates that rhodopin, after being incorporated into the DPA LH2 complex, is capable of generating singlet oxygen under illumination. Previously obtained data on heterogeneity of the carotenoid composition in DPA LH2 complexes (variation in the number of individual carotenoids molecules per complex in the general population) and our earlier suggestion about the structural role of carotenoids, namely, their ability to stabilize the LH2 complexes, were confirmed. Based on analysis of our results, as well as of the literature data, the interaction of singlet oxygen and carotenoids is discussed. [ABSTRACT FROM AUTHOR]

The effect of the qualitative and quantitative content of carotenoids on the assembly of light-harvesting complexes isolated from the heat-resistant purple sulfur bacterium Thermochromatium (T.) tepidum was studied. To decrease the content of carotenoids in the photosynthetic membranes and complexes, an inhibitor of carotenonegesis, diphenylamine (DPA), was used. The minimum carotenoid content of 40% was observed in T. tepidum cells grown in the presence of 120 μM DPA. At higher DPA concentrations, the growth of the culture stopped. Photosynthetic membranes and LH2 and LH1-RC complexes were isolated from DPA-treated cells; carotenoid content of these structures was 45 and 55%, respectively. Their spectral characteristics in the near IR region did not depend on the amount of carotenoids in the complexes and were similar to those of the native wild-type complexes. Simultaneously with a general decrease in the content of carotenoids, carotenoids from earlier stages of biosynthesis, such as neurosporene, ζ-carotene, and their hydroxy derivatives, appeared in the DPA-treated complexes. Emergence in DPA-treated complexes of carotenoids with a shorter chain of conjugated double bonds than in the control resulted in an increase in the efficiency of energy transfer from carotenoids to bacteriochlorophyll. [ABSTRACT FROM AUTHOR]

Regardless of cultivation conditions, two types of LH2 complexes are always assembled in photosynthetic membranes of Thiorhodospira (T.) sibirica: short-wave B800−830 and long-wave B800−850. The present work studied the effect of qualitative and quantitative content of carotenoids on assembling of these complexes. Diphenylamine (DPA), an inhibitor of carotenoid biosynthesis, was used to decrease the content of carotenoids in photosynthetic membranes and the complexes. In the cells of T. sibirica strain A12T, inhibition of carotenoid biosynthesis ranged from 40 to 50%. Complete suppression of carotenoid biosynthesis was achieved only once, which resulted in absorption spectrum changes of the DPA-LH2 complexes and lower stability of their structure; they were often destroyed during the isolation. The use of DPA with the cells of T. sibirica strain Kir-3 was more efficient. From inhibited cells of this strain, B800−850 and B800−830 complexes were isolated, in which the carotenoid content decreased to 80, 50, and 10%. Their spectral characteristics in the near infrared region were independent of the carotenoid quantity in the complexes and were similar to those of native complexes of the wild-type. At higher DPA concentration in the cultivation medium, accompanied by a decrease of the overall carotenoid content, such carotenoids of the earlier stages of biosynthesis as neurosporene, ζ-carotene, and their hydroxy derivatives, were found in the complexes. The efficiency of energy transfer from carotenoids to bacteriochlorophyll in all LH2 complexes was not shown to depend on the composition and quantity of carotenoids in the samples and was ~40%. Since the near-infrared spectrum of the LH2-complex of the B800−850 type was similar to that of the B800−850 complexes from Rhodospirillum molishianum, with 8 pairs of α/β-heterodimers forming the basis of their structure, it is possible to assume that the structure of the studied B800−850 complex from T. sibirica also consists of 8 pairs of α/β-heterodimers. A single carotenoid molecule is known to occur per the α/β-heterodimer; therefore, on average less than one carotenoid molecule (8 × 0.1 = 0.8) occurs per an LH2 complex from T.sibirica with 10% carotenoid content. This implies that the population of LH2 complexes contains both carotenoidless LH2 complexes and the complexes containing one or more carotenoid molecules. Obviously, LH2 complexes of both types are assembled regardless of the quantity of carotenoids per a complex, and carotenoids are not the required component for their correct assembly in vivo. [ABSTRACT FROM AUTHOR]

The possibility of singlet oxygen generation in five species of photosynthetic bacteria under irradiation with light was studied using the Singlet Oxygen Sensor Green (SOSG) dye that, when singlet oxygen is bound, is transformed into a fluorescent endoperoxide form. Irradiation was carried out using light of different spectral composition: red (absorption of bacteriochlorophyll, BChl) and white (absorption of BChl and carotenoids). No singlet oxygen generation was detected under red light irradiation, which indicated that BChl was not involved in this process. Under white light, as the irradiation time increased, an increase in SOSG fluorescence was recorded in the membranes of four bacterial species: Allochromatium vinosum MSU, Rhodobacter sphaeroides G1C, Rba. blasticus K-1 and Rhodopseudomonas faecalis. In Rba. sphaeroides, no increase in SOSG fluorescence was observed. It is assumed that in the white light this process occurs due to the pigments that absorb in the blue-green region of the spectrum, i.e. carotenoids with 8‒11 conjugated double bonds (neurosporene, spheroidene, lycopene, and rhodopin). Similar to Trolox, SOSG may slow down the bleaching process of BChl850 in the membranes of Alc. vinosum strain MSU and may therefore effectively perform the role of a singlet oxygen trap. [ABSTRACT FROM AUTHOR]

The effect of singlet oxygen was studied in the system of Allochromatium (Alc.) vinosum MSU membranes, rose bengal, and light (547−600nm) with and without a quencher. In the system without a quencher, bleaching of the BChl850 band of the LH2 light-harvesting complex was observed and an absorption peak emerged at 698 nm, which belonged to the oxidized product, 3-acetyl-chlorophyll. The efficiency of five quenchers of singlet oxygen for neutralization of the effect of singlet oxygen on bacteriochlorophyll in the light-harvesting complexes of the membranes Alc. vinosum MSU was studied: sodium ascorbate, histidine, imidazole, trolox, and sodium azide. All of them, with the exception of sodium azide, neutralized successively the effect of singlet oxygen. [ABSTRACT FROM AUTHOR]