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A Iu Borisov

Publications and source records attributed to A Iu Borisov.

At least 19 recordsLinked to original sources
PubMed1982

[Photooxidation of P700 in photosystem 1 preparations with various amounts of antenna chlorophyll a].

A number of membrane fragments and pigment-protein complexes of photosystem 1 was obtained from pea chloroplasts, using ionic and non-ionic detergents (SDS, digitonin, Triton X-100, lauryldimethylamine-N-oxide). The ratio of chlorophyll (Chl) a to P700 varied from 220 to 30. For non-dialyzed preparations the quantum yield of P700 photooxidation (phi e) measured by the initial rate of photobleaching at 696-698 nm with excitation at the Soret band of Chla was equal to 40-60%. When the P700 photooxidation was measured at 432 nm, the phi e value showed a further decrease to 20-40% during red light excitation over the range of 660-680 nm but rose to 70-90% at the exciting light wavelengths of greater than or equal to 695 nm. On the basis of the observed dependences the red absorption band was approximated by a sum of two spectra: the spectrum of Chla photoactive in P700 photooxidation and that of photoinactive Chla. Both spectra had maxima near the absorption peak of the object. The photoinactive fraction was additionally enriched by the long-wavelength absorption forms of Chla with an absorption maximum over the range of 684-690 nm. The amount of the bulk Chla in the photoinactive fraction was no less than 40%. The phi e value for freshly dialyzed preparation at a Chla/P700 ratio of 30 was equal to 50-60% independent of the exciting light wavelength. An addition of 0.05% Triton X-100 to this preparation caused: i) a blue shift of the absorption and fluorescence maxima; ii) a decrease of the long-wavelength absorption forms content of Chla and, iii) a considerable increase in fluorescence lifetime and quantum yield due to deaggregation of Chla and its solubilization by detergent micelles. The same phenomenon seems to be responsible for the formation of photoinactive fraction of a pigment, since after addition of a detergent the above-mentioned spectral dependence of phi e appeared, i.e. phi e showed a 3-fold decrease (down to 18%) within the region of 660-680 nm and a 1,6-fold increase (up to 90%) at 705-730 nm. These results suggest that the detergents destroy the intact construction of a light-harvesting antenna rather than that of the photosystem 1 reaction center.

Chlorophyll↗
PubMed1979

[Effect of cations on the quantum yield and the lifetime of the chloroplast fluorescence].

The interrelationship between the cation-induced fluorescence changes and the state of the photosystem 2 (PS-2) reaction centers for pea chloroplasts and their osmotic fragments was studied. The effects of K+ and Mg2+ on the fluorescence quantum yield (phi f1) under varying light intensities as well as on the fluorescence lifetime (tau f1) in the saturating light were demonstrated. K+ induces the decrease in tau f1; Mg2+ exerts an opposite effect. The effects were more pronounced when the reaction centers of PS-2 were converted into an inactive state by illuminating the sample with a saturating light or by adding DCMU. Under these conditions the cations' effect on tau f1 was accompanied by proportional changes in tau f1. It was concluded that in Mg-deficient chloroplasts an efficient channel of the excitation quenching appears in antenna chlorophyll of PS-2 with the rate constant of 7 . 10(8) s-1. The simultaneous measurements of tau f1 by phase and modulation type techniques allowed to reveal the emission heterogeneity within the nanosecond time interval and the DCMU-sensitive delayed fluorescence with the lifetime exceeding 10(-7) s and the overall quantum yield approximately equal to 2 . 10(-3).

Chloroplasts↗
PubMed1979

[Separation of plant photosystems by sievorptive chromatography].

The pigment-protein complexes enriched with photosystem I (PPC-1) and photosystem II (PPC-2) were obtained using sievorptive chromatography on DEAE-Sephadex. Both types of complexes contain chlorophyll a, beta-carotene and minor quantities of chlorophyll b. Red absorption maxima for PPC-1 and PPC-2 are located at 676 and 673 nm, respectively. Using differential spectrophotometry, the degree of the reaction centre enrichment was established: PPC-1 has one P700 per 35 bulk chlorophyll a molecules, PPC-2 contains one P680 per 18 bulk chlorophyll a molecules. The yield of PPC-2 is 7-10 times lower than that of PPC-1 and equals to 0,3% of the chlorophyll content of the initial chloroplasts. The amount of P680 in PPC-1 preparations does not exceed 7% after a single chromatographic procedure; the amount of P700 in PPC-2 makes up to 2%. The method proposed is more advantageous with respect to higher reaction centre enrichment of PPC-1 and PPC-2 as compared to ion-exchange chromatography on DEAE-cellulose.

Chlorophyll↗
PubMedDate not supplied

[Picosecond energy transfer between the spectral forms of pigments from the reaction center of Rhodospirillum rubrum].

Absorption changes of reaction centers from Rhodospirillum rubrum at 748, 796 and 870 nm induced by 532 and 870 nm picosecond light pulses were investigated with a picosecond spectrometer. Kinetics of absorption changes at 748 and 796 had an additional bleaching when induced by the 532 nm pulse, in comparison with those at 870 nm. The additional bleaching was interpreted as a result of the excitation energy transfer via spectral forms of pigments of reaction centers. The experimental results fit the mathematical simulation of the additional bleaching for the following set of rate constant values: intrinsic conversion to the lowest excited singlet state in bacteriopheophytine molecule--10(13) s-1, energy transfer from bacteriopheophytine to P800 3.10(12) s-1, from P800 to P870--2.10(12) s-1.

Chlorophyll↗
PubMedDate not supplied

[Modeling of energy migration and trapping in purple bacteria. Analysis of extreme formulae].

Homogeneous pigment ensembles similar to those of purple bacteria Rhodospirillum rubrum were studied. Two formulae were advanced for the limiting values of excitation lifetime and quantum yield of excitation trapping in these ensembles, provided all reaction centers are in an active state. It was demonstrated by mathematical modeling that these limiting values strictly depend on three parameters of molecular ensembles: the numbers of core-bacteriochlorophyll molecules per reaction center, the values of rate constants for excitation trapping in reaction centers, and excitation wasteful deactivation in all molecules. The excitation lifetime and quantum yield were proved to approach their limiting values as the rate constants of excitation intermolecular migration increase. The closeness of experimental values for two above mentioned functions to their calculated limiting values proves the migration-limited type of the photosynthetic unit investigated and a high efficiency of excitation trapping in its reaction centers.

Algorithms↗
PubMedDate not supplied

[Comparative genetics and evolutionary morphology of symbiosis formed by plants with nitrogen-fixing microbes and endomycorrhizal fungi].

Results of comparative morphological and genetic analyses are described for two major plant-microbe endosymbioses: N2-fixing nodules (with rhizobia or actinomycetes Frankia) and arbuscular mycorrhiza (with Glomales fungi). Development from the primordia formed de novo in root tissues is common for all known types of N2-fixing nodules. However, their structure varies greatly with respect to: (i) tissue topology (location of vascular bundles is peripheral in legumes but central in non-legumes); (ii) position of nodule primordium (inner or outer cortex in legumes, whereas pericycle in non-legumes); (iii) stability of apical meristem (persistent in the indeterminate nodules, transient in the determinate ones). In addition, legumes vary in ability to form compartments harboring endosymbiotic rhizobia that can be located intercellularly (infection threads) and intracellularly (symbiosomes). Using pea (Pisum sativum) symbiotic mutants, the nodule developmental program is dissected into a range of spatially and temporarily differentiated steps composing four sub-programs (development of endosymbiotic compartments; nodule histogenesis; autoregulation of nodulation; bacteroid differentiation). The developmental mutations are suggested in some cases to reverse the endosymbiotic system into the morphologically simpler forms some of which may correspond to the ancestral stages of nodule evolution. Origination of legume-rhizobial and actinorhizal symbioses is suggested to be based on a set of preadaptations many of which had been evolved in angiosperms during coevolution with arbuscular mycorrhizal fungi (e.g. inter- and intracellular maintenance of symbionts, their control via defence-like reactions and recognition of chitin-like molecules). Analysis of parallel morphological variation in symbiotic mutants and wild-growing legume species enables us to reconstruct the major stages of evolution for N2-fixing symbioses. This evolution proceeded to a sufficient degree independently from the basic physiological function of nodules (symbiotic N2-fixation) and possibly a recruiting of plant genes that initially fulfilled various "non-symbiotic" functions into the genetic networks monitoring plant-microbe interactions.

Bacterial Physiological Phenomena↗
PubMedDate not supplied

[Two-level heterogenous energy migration. Modeling and application to purple bacteria].

The dynamics of migration of electronic excitations and the efficiency of their trapping in two-dimensional ensembles of molecules were analyzed. Molecules were characterized using the following parameters: the width of long-wavelength bands, the values of extinction and rate constant of deactivation of electronic excitations, critical distances of migration close to those of dye molecules, in particular, bacteriochlorophyll a and purple bacteria. A comparative analysis of two-dimensional models of energy migration made it possible to chose a model with an optimum light-harvesting on traps from the largest numbers of light-absorbing molecules. It was shown that in ensembles of molecules having different spectral characteristics (spectral shifts between the short- and long-wavelength fractions of the molecules are hear 800 cm-1) the efficiency of excitation trapping is approximately 90 and 80% for the number of light-harvesting molecules per one trap 210 and 580, respectively.

Algorithms↗
PubMedDate not supplied

[A discrepancy between the experimental and theoretical data on energy migration from B800 to B850 in LH-2 antennary complexes in purple bacteria].

A discrepancy between the times of excitation transfer from B800 to B850 bacteriochlorophyll fractions in LH-2 complexes of purple bacteria was revealed. The experimental value (0.7-0.8 ps from literature sources) are at least four times lower than that (> 3.2 ps) calculated theoretically on the basis of recently obtained atomic structure of LH2. Possible reasons for this discrepancy are discussed.

Bacterial Proteins↗
PubMedDate not supplied

[Kinetics and spectra of photo-induced changes in the absorption of pigment-protein complexes of photosystem 1 in a picosecond range].

The energy transfer from the light-harvesting antenna chlorophylls to the reaction center molecules and subsequent charge separation were investigated using a difference picosecond spectrophotometer with selective excitation. The objects were the pigment-protein complexes of photosystem 1 (Chl/P700 = 60) isolated from bean leaves. The difference absorption spectra of the excited states of light-harvesting antenna chlorophylls and the P700 photooxidation were measured. It was shown that the excited states of antenna chlorophylls were generated within 10 ps and deactivated with three-component kinetics: tau 1 = 20--45 ps, tau 2 = 100--300 ps, tau 3 greater than 500 ps. The process of the P700 photooxidation induced by the 650 nm exciting pulse was approximately monoexponential with tau equal to 15--30 ps. It is established that the P700 photooxidation is due to the efficient transfer of excitation energy from antenna chlorophylls to reaction centers.

Chlorophyll↗
PubMedDate not supplied

[Intermediate states formed during discharge separation in the reaction centers of Rhodospirillum rubrum in the presence of a low-redox potential].

The intermediate short-lived states arising in reaction centre preparations (RC) of purple bacterium Rhodospirillum rubrum are investigated under the conditions of low redox potential. Excitation by 353 and 530 nm laser pulses produced two states characterized by optical absorption changes in the range of 350--650 nm and lifetimes: 10--30 ns for the first state and 2.5 +/- 0.5 microseconds for the second one. The first state is similar to the state PF, described previously by Parson et al. for RC from Rps. sphaeroides. Carotenoid extraction with isooctane resulted in changing the spectrum with tau = 2.5 microseconds and in the appearance of new absorption changes similar to those for the R state observed before in carotenoidless bacterial strains within microsecond time range. The comparison of the microsecond spectra with difference spectra (continuous light minus dark) of RC from R. rubrum in the range of 350--650 nm made it possible to identify the states with tau = 2.5 microseconds as carotenoid triplet states. The ratio of quantum yields of PR and carotenoid triplet states production was determined as being 1 : 1. The conclusion was made that triplet-triplet energy transfer from state PR to carotenoid is responsible for the production of carotenoid triplet states.

Binding Sites↗