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The increasingly severe environmental pollution and energy shortage issues have demanded the production of renewable and sustainable biofuels to replace conventional fossil fuels. Lignocellulosic (LC) biomass as an abundant feedstock for second-generation biofuel production can help overcome the shortcomings of first-generation biofuels related to the “food versus fuel” debate and feedstock availability. Embracing the “circular bioeconomy” concept, an integrated biorefinery platform of LC biomass can be performed by employing different conversion technologies to obtain multiple valuable products. This review provides an overview of the principles and applications of thermochemical processes (pyrolysis, torrefaction, hydrothermal liquefaction, and gasification) and biochemical processes (pretreatment technologies, enzyme hydrolysis, biochemical conversion processes) involved in LC biomass biorefinery for potential biofuel applications. The engineering perspective of LC biofuel production on separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), simultaneous saccharification and co-fermentation (SSCF), and consolidated bioprocessing (CBP) were also discussed.

The effects of the novel [CuL2]Br-2 complex (L = bis{4H-1,3,5-triazino [2,1-b]benzothiazole-2-amine,4-(2-imidazole)}copper(II) bromide complex) on the photosystem II (PSII) activity of PSII membranes isolated from spinach were studied. The absence of photosynthetic oxygen evolution by PSII membranes without artificial electron acceptors, but in the presence of [CuL2]Br-2, has shown that it is not able to act as a PSII electron acceptor. In the presence of artificial electron acceptors, [CuL2]Br-2 inhibits photosynthetic oxygen evolution. [CuL2]Br-2 also suppresses the photoinduced changes of the PSII chlorophyll fluorescence yield (F-V) related to the photoreduction of the primary quinone electron acceptor, Q(A). The inhibition of both characteristic PSII reactions depends on [CuL2]Br-2 concentration. At all studied concentrations of [CuL2]Br-2, the decrease in the F-M level occurs exclusively due to a decrease in Fv. [CuL2]Br-2 causes neither changes in the F-0 level nor the retardation of the photoinduced rise in F-M, which characterizes the efficiency of the electron supply from the donor-side components to Q(A) through the PSII reaction center (RC). Artificial electron donors (sodium ascorbate, DPC, Mn2+) do not cancel the inhibitory effect of [CuL2]Br-2. The dependences of the inhibitory efficiency of the studied reactions of PSII on [CuL2]Br-2 complex concentration practically coincide. The inhibition constant Ki is about 16 mu M, and logKi is 4.8. As [CuL2]Br-2 does not change the aromatic amino acids' intrinsic fluorescence of the PSII protein components, it can be proposed that [CuL2]Br-2 has no significant effect on the native state of PSII proteins. The results obtained in the present study are compared to the literature data concerning the inhibitory effects of PSII Cu(II) aqua ions and Cu(II)-organic complexes.

Although more and more precise and efficient methods of scientific research regularly arise, inhibitory analysis still remains far from being the least among the extensively used experimental approaches. Many current investigations aplly chemical inhibitors of photosystem II as instruments of scientific cognition. Novel chemical inhibitors are being screened, educed, and/or designed. New effects of the already known inhibitors are being discovered and/or significantly corrected. The review presents the generalized information on both the known inhibitors, such as diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea), dinoseb (6‑sec-butyl-2,4-dinitrophenol), and ADRY agents (agents causing Acceleration of the Deactivation Reactions of the water-splitting enzyme system Y) and recently revealed chemical inhibitors of PSII (derivatives of perfluoroisopropyldinitrobenzene and metal-organic complexes). The action of carbonic anhydrase inhibitors on a photochemical activity of PSII is also discussed.

Soil salinity is a major threat to agricultural sustainability and a global food security. Until now, most research has concentrated around stomatal limitation to photosynthesis, while non-stomatal...

Our recent studies showed that A. thaliana L. phyAphyB double mutants (DM) grown under red light (RL) and white light (WL) had higher photosystem II (PSII) vulnerability to UV-B than wild type (WT). The present work was aimed at revealing the mechanistic basis for this difference by analyzing the content of UV-absorbing pigments (UAPs) and chloroplast ultrastructure in leaves of DM and WT grown under different light conditions, of different photoperiods (12, 16 and 24 h) and light quality (WL vs RL). The content of UAPs in leaves of WT plants grown under RL showed a strong dependence of photoperiod and decreased in a sequence 24 h > 16 h > 12 h. In all treatments, contents of UAPs were higher in WT compared to mutant plants. While 1 h of UV treatment had only a small impact on chloroplast ultrastructure, it substantially inhibited PSII activity (maximum and effective PSII quantum yields). In all treatments, the UV-induced decreases of PSII activities were compared with each other. At any photoperiod, decreases in PSII activity were smaller in WT compared to that in mutant plants. Higher UAPs contents led to lesser PSII inhibition, while low UAPs contents resulted in strong decline in PSII activity. The results demonstrate that content of UAPs significantly contributes to PSII resistance to short-time UV-B exposures, and decreased content of UAPs in phytochrome double mutant can explain the reduced PSII resistance of these plants to UV-B radiation.

Biofuels are the promising alternative to exhaustible, environmentally unsafe fossil fuels. Algal biomass is attractive raw for biofuel production. Its cultivation does not compete for cropland with agricultural growing of food crop for biofuel and does not require complex treatment methods in comparison with lignocellulose-enriched biomass. Many microalgae are mixotrophs, so they can be used as energy source and as sewage purifier simultaneously. One of the main steps for algal biofuel fabrication is the cultivation of biomass. Photobioreactors and open-air systems are used for this purpose. The formers allow the careful cultivation control, but the latter ones are cheaper and simpler. Biomass conversion processes may be divided to the thermochemical, chemical, biochemical methods and direct combustion. For biodiesel production, triglyceride-enriched biomass undergoes transetherification. For bioalcohol production, biomass is subjected to fermentation. There are three methods of biohydrogen production in the microalgal cells: direct biophotolysis, indirect biophotolysis, fermentation.

The structure and functional organization of the photosystem I (PSI) reaction center (RC) donor side has a significant similarity to the reaction centers of purple bacteria (bRCs), despite the fact that they belong to different types of RCs. Moreover, the redox potential values of their primary electron donors are identical (~0.5 V). In our earlier reports [Khorobrykh et al. (2008) Phylos. Trans. R. Soc. B., 363, 1245-1251; Terentyev et al. (2011) Biochemistry (Moscow., 76, 1360-1366; Khorobrykh et al. (2018) ChemBioChem, 14, 1725-1731], we have demonstrated redox interaction of low-potential Mn2+-bicarbonate complexes with bRCs, which might have been one of the first steps in the evolutionary origin of Mn-cluster of the photosystem II water-oxidizing complex that occurred in the Archean (over 3 billion years ago). In this study, we investigated redox interactions between Mn2+-bicarbonate complexes and PSI. Such interactions were almost absent in the original PSI preparations and emerged only in preoxidized PSI preparations containing ~50% oxidized RCs. The interaction between Mn2+-bicarbonate complexes and PSI required increased Mn2+ concentrations, while its dependence on the HCO3– concentration indicated involvement of electroneutral low-potential [Mn(HCO3)2] complex in the process. Analysis of the PSI crystal structure revealed steric hindrances on the RC donor side, which could block the redox interaction between Mn2+-bicarbonate complexes and oxidized primary electron donor. Comparison of structures of RCs from the PSI and ancient RCs from heliobacteria belonging to the same type of RCs suggested that such hindrances should be absent in the primitive PSI in the Archean and allowed to explain their evolutionary origin as a consequence of PSI RCs into the united electron transport chain (ETC) of the photosynthetic membrane that was accompanied by the evolutionary loss of PSI capacity for the redox interaction with Mn2+-bicarbonate complexes.

Complexes of some metals are known to be inhibitors of various physiological processes in photosynthesizing organisms. In order to search for new efficient inhibitors, six novel antimony(III) complexes are synthesized and studied for inhibition of the photochemical activity of photosystem II (PSII) and key chloroplast enzymes, such as carbonic anhydrase and glutathione reductase. It is found that several complexes efficiently inhibit the carbonic anhydrase activity of PSII but inhibit the photochemical activity of this photosystem less efficiently. All the complexes inhibit the activity of chloroplast glutathione reductase.

An electrometrical technique was used to investigate electron transfer between synthetic binuclear manganese (Mn) complexes, designated M − 2 and M − 3, and the redox-active neutral tyrosine radical (Y Z • ) in proteoliposomes containing Mn-depleted photosystem II (PS II) core particles in response to single laser flashes. In the absence of Mn-containing compounds, the observed flash-induced membrane potential (ΔΨ) decay was mainly due to charge recombination between the reduced primary quinone acceptor Q A − and the oxidized Y Z • . More significant slowing down of the ΔΨ decay in the presence of lower concentrations of M − 2 and M − 3 associated with electron donation from Mn in the Mn-binding site to Y Z • indicates that these synthetic compounds are more effective electron donors than MnCl 2 . The exponential fitting of the kinetics of additional electrogenic components of ΔΨ rise in the presence of Mn-containing compounds revealed the following relative amplitudes ( A ) and lifetimes (τ): for MnCl 2 - A ∼ 3.5, τ∼150 μs, for M − 2 - A ∼5%, τ∼1.4 ms, and for M − 3 - A ∼5.5%, τ∼150 μs. This suggests that the efficiency of the manganese complexes in electron donation depends on the chemical nature of ligands. The experiments with EDTA-treated samples indicated that the ligands for M − 2 and M − 3 are required for their tight binding with the PS II reaction center. The obtained results demonstrate the importance of understanding the molecular mechanism(s) of flash-induced electrogenic reduction of the tyrosine radical Y Z • by synthetic Mn complexes capable of splitting water into oxygen and reducing equivalents.

Over the last two decades many reports have been published on diverse types of biohybrid electrodes utilizing components of the photosynthetic apparatus. Currently, the development of such devices does not extend beyond laboratory research. In the future, these electrodes could be used in biosensors, solar cells, and as a new technique to investigate photosynthetic pigment-protein complexes. Efficiency of light-to-current conversion is particularly important for solar cell applications. Selection of a suitable substrate for special pigment-protein complexes is a significant challenge for building an inexpensive and efficient device. Various combinations of pigment-protein complexes and substrates, as well as different measurement conditions make it difficult to directly compare performance of various solar cells. However, it has been shown, that one of the possible substrate materials, namely nanostructured TiO2, is the most preferred material for the immobilization of pigment-protein complexes in terms of both cost and efficiency. The photocurrent values reaching several mA, were reported for TiO2-based biohybrid electrodes. However, the efficiency of TiO2-based biohybrid is still far from its potential maximum value due to fundamental challenges related to designing an optimum interface between TiO2 nanostructure and pigment-protein complexes containing electron transferring cofactors. To date, counterproductive back reactions, also referred to as charge recombination, still dominate and lower internal quantum efficiency of these systems.

The effects of UV-B radiation (1 W m–2, 1 and 2 h) on PSII activity, chloroplast structure, and H2O2 contents in leaves of 26-d-old Arabidopsis thaliana phyA phyB double mutant (DMut) compared with the wild type (WT) were investigated. UV-B decreased PSII activity and affected the H2O2 content in WT and DMut plants grown under white light (WL). The chloroplast structure changes in DMut plants exposed to UV were more significant than that in WT. Reductions in maximal and real quantum photochemical yields and increase in the value of thermal dissipation of absorbed light energy per PSII RC and the amount of QB-nonreducing centers of PSII were bigger in mutant compared to WT plants grown both under WL and red light. Such difference in action of UV-B on WT and DMut can be explained by higher content of UV-absorbing pigments and carotenoids in WT leaves compared with DMut.

Vyacheslav Vasilevich (V.V.) Klimov (or Slava, as most of us called him) was born on January 12, 1945 and passed away on May 9, 2017. He began his scientific career at the Bach Institute of Biochemistry of the USSR Academy of Sciences (Akademy Nauk (AN) SSSR), Moscow, Russia, and then, he was associated with the Institute of Photosynthesis, Pushchino, Moscow Region, for about 50 years. He worked in the field of biochemistry and biophysics of photosynthesis. He is known for his studies on the molecular organization of photosystem II (PSII). He was an eminent scientist in the field of photobiology, a well-respected professor, and, above all, an outstanding researcher. Further, he was one of the founding members of the Institute of Photosynthesis in Pushchino, Russia. To most, Slava Klimov was a great human being. He was one of the pioneers of research on the understanding of the mechanism of light energy conversion and of water oxidation in photosynthesis. Slava had many collaborations all over the world, and he is (and will be) very much missed by the scientific community and friends in Russia as well as around the World. We present here a brief biography and some comments on his research in photosynthesis. We remember him as a friendly and enthusiastic person who had an unflagging curiosity and energy to conduct outstanding research in many aspects of photosynthesis, especially that related to PSII.

Inhibitory effects of polycyclic aromatic hydrocarbons (PAHs) on plants were studied in pea leaves in order to elucidate the mechanisms of action of PAHs such as naphthalene (Naph) and phenanthrene (Phen) on activity of photosystem II (PSII). The changes in different Chl fluorescence parameters were calculated on the basis of Chl fluorescence induction curves. H2 O2 content was measured in leaf homogenates with the luminol-dependent chemiluminescence method. We demonstrated that following PAH treatment, total energy dissipation (DI0 /ABS) and amount of QB -non-reducing complexes of PSII significantly increased. Non-photochemical quenching (NPQ) also increased, when weak oxidative stress after PAH application developed. In leaves, a two-step increase in H2 O2 was found with time of incubation in the presence of PAHs, which may be associated with damage to the lipid bilayer of the plasma membrane and then violation of lipid bilayer membranes of cell organelles. A hypothesis for the mode of action of PAHs is provided that involves the role of ROS, membrane permeability and associated functional changes in PSII.

Biofuels are the promising alternative to exhaustible, environmentally unsafe fossil fuels. Algal biomass is attractive raw for biofuel production. Its cultivation does not compete for cropland with agricultural growing of food crop for biofuel and does not require complex treatment methods in comparison with lignocellulose-enriched biomass. Many microalgae are mixotrophs, so they can be used as energy source and as sewage purifier simultaneously. One of the main steps for algal biofuel fabrication is the cultivation of biomass. Photobioreactors and open-air systems are used for this purpose. The formers allow the careful cultivation control, but the latter ones are cheaper and simpler. Biomass conversion processes may be divided to the thermochemical, chemical, biochemical methods and direct combustion. For biodiesel production, triglyceride-enriched biomass undergoes transetherification. For bioalcohol production, biomass is subjected to fermentation. There are three methods of biohydrogen production in the microalgal cells: direct biophotolysis, indirect biophotolysis, fermentation.

The finding of unique Chl d- and Chl f-containing cyanobacteria in the last decade was a discovery in the area of biology of oxygenic photosynthetic organisms. Chl b, Chl c, and Chl f are considered to be accessory pigments found in antennae systems of photosynthetic organisms. They absorb energy and transfer it to the photosynthetic reaction center (RC), but do not participate in electron transport by the photosynthetic electron transport chain. However, Chl d as well as Chl a can operate not only in the light-harvesting complex, but also in the photosynthetic RC. The long-wavelength (Qy) Chl d and Chl f absorption band is shifted to longer wavelength (to 750 nm) compared to Chl a, which suggests the possibility for oxygenic photosynthesis in this spectral range. Such expansion of the photosynthetically active light range is important for the survival of cyanobacteria when the intensity of light not exceeding 700 nm is attenuated due to absorption by Chl a and other pigments. At the same time, energy storage efficiency in photosystem 2 for cyanobacteria containing Chl d and Chl f is not lower than that of cyanobacteria containing Chl a. Despite great interest in these unique chlorophylls, many questions related to functioning of such pigments in primary photosynthetic processes are still not elucidated. This review describes the latest advances in the field of Chl d and Chl f research and their role in primary photosynthetic processes of cyanobacteria.

It is known, that the multi-subunit complex of photosystem II (PSII) and some of its single proteins exhibit carbonic anhydrase activity. Previously, we have shown that PSII depletion of HCO3-/CO2 as well as the suppression of carbonic anhydrase activity of PSII by a known inhibitor of alpha-carbonic anhydrases, acetazolamide (AZM), was accompanied by a decrease of electron transport rate on the PSII donor side. It was concluded that carbonic anhydrase activity was required for maximum photosynthetic activity of PSII but it was not excluded that AZM may have two independent mechanisms of action on PSII: specific and nonspecific. To investigate directly the specific influence of carbonic anhydrase inhibition on the photosynthetic activity in PSII we used another known inhibitor of alpha-carbonic anhydrase, trifluoromethanesulfonamide (TFMSA), which molecular structure and physicochemical properties are quite different from those of AZM. In this work, we show for the first time that TFMSA inhibits PSII carbonic anhydrase activity and decreases rates of both the photo-induced changes of chlorophyll fluorescence yield and the photosynthetic oxygen evolution. The inhibitory effect of TFMSA on PSII photosynthetic activity was revealed only in the medium depleted of HCO3-/CO2. Addition of exogenous HCO3- or PSII electron donors led to disappearance of the TFMSA inhibitory effect on the electron transport in PSII, indicating that TFMSA inhibition site was located on the PSII donor side. These results show the specificity of TFMSA action on carbonic anhydrase and photosynthetic activities of PSII. In this work, we discuss the necessity of carbonic anhydrase activity for the maximum effectiveness of electron transport on the donor side of PSII.

Photosynthesis is a process which converts light energy into energy contained in the chemical bonds of organic compounds by photosynthetic pigments such as chlorophyll (Chl a, b, c, d, f) or bacteriochlorophyll. It occurs in phototrophic organisms, which include higher plants and many types of photosynthetic bacteria, including cyanobacteria. In the case of the oxygenic photosynthesis, water is a donor of both electrons and protons, and solar radiation serves as inexhaustible source of energy. Efficiency of energy conversion in the primary processes of photosynthesis is close to 100%. Therefore, for many years photosynthesis has attracted the attention of researchers and designers looking for alternative energy systems as one of the most efficient and eco-friendly pathways of energy conversion. The latest advances in the design of optimal solar cells include the creation of converters based on thylakoid membranes, photosystems, and whole cells of cyanobacteria immobilized on nanostructured electrode (gold nanoparticles, carbon nanotubes, nanoparticles of ZnO and TiO2). The mode of solar energy conversion in photosynthesis has a great potential as a source of renewable energy while it is sustainable and environmentally safety as well. Application of pigments such as Chl f and Chl d (unlike Chl a and Chl b), by absorbing the far red and near infrared region of the spectrum (in the range 700-750 nm), will allow to increase the efficiency of such light transforming systems. This review article presents the last achievements in the field of energy photoconverters based on photosynthetic systems.

This review describes the phytochrome system in higher plants and cyanobacteria and its role in regulation of photosynthetic processes and stress protection of the photosynthetic apparatus. A relationship between the content of the different phytochromes, the changes in the ratios of the physiologically active forms of phytochromes to their total pool and the resulting influence on photosynthetic processes is reviewed. The role of the phytochromes in the regulation of the expression of genes encoding key photosynthetic proteins, antioxidant enzymes and other components involved in stress signaling is elucidated.

The photosynthetic responses of 25-day-old Arabidopsis phyA phyB double mutant (DM) compared with the wild type (WT) to UV-B radiation (1Wm-2, 30min) were investigated. UV-B irradiation led to reduction of photosystem 2 (PS-2) activity and the photosynthetic rate. In plants grown under both white and red light (λm - 660nm) the reduction was greater in DM plants compared to the WT. Without UV-B irradiation a decrease in PS-2 activity was observed in DM grown under RL only. It is assumed that the lower content of UV-absorbing pigments and carotenoids observed in DM may be one of the reasons of reduced PS-2 resistance to UV-B. Higher decrease in activities under UV in DM plants grown under RL compared to DM plants grown under white light is likely due to the lack of activity of cryptochromes in plants grown under red light. Rates of post-stress recovery of photosynthetic activity of DM compared with WT plants under white and red light of low intensity were studied. Almost complete recovery of the activity was found which was not observed under dark conditions and in the presence of a protein synthesis inhibitor, chloramphenicol. It is assumed that phytochrome system participates in stress-protective mechanisms of the photosynthetic apparatus to UV-radiation.

The effects of suppression of the carbonic anhydrase (CA) activity by a CA-inhibitor, acetazolamide (AA), on the photosynthetic activities of photosystem II (PS II) particles from higher plants were investigated. AA along with CA-activity inhibits the PS II photosynthetic electron transfer and the AA-induced suppression is totally reversed by the addition of bicarbonate (3-5 mM). Similar effect of recovery in the PS II photosynthetic activity was also revealed upon the addition of known artificial electron donors (potassium ferrocyanide and TMPD). Significance and possible functions of CA for the PS II donor side are discussed.

Among various technologies for hydrogen production, the use of oxygenic natural photosynthesis has a great potential as can use clean and cheap sources—water and solar energy. In oxygenic photosynthetic microorganisms electrons and protons produced from water and redirected by the photosynthetic electron-transport chain via ferredoxin to the hydrogen-producing enzymes hydrogenase or nitrogenase. By these enzymes, e− and H+ recombine and form molecular hydrogen. Hydrogenase activity can be very high but is extremely sensitive to the photosynthetically evolved O2 that leads to reduced and unstable H2 production. However, presently, several approaches are developed to improve the energetic efficiency to generate H2. This review examines the main available pathways to improve the photosynthetic H2 production.

It is shown that pretreatment of wheat seeds with chlorocholine chloride reduces inhibition of photosystem 2 activity in the first leaves of 10�d ayold wheat variety Moskovskaya 35 seedlings caused by heating at 39-41°C for 5-20 min and improves its poststress recovery, probably due to activation of antioxi� dant enzymes and increase in the level of low molecular weight antioxidants.

An electrometric technique was used to investigate the generation of a photovoltage (Deltapsi) by Mn-depleted spinach photosystem II (PS II) core particles incorporated into liposomes. In the presence of MnCl2, the fast kinetically unresolvable phase of Deltapsi generation, related to electron transfer between the redox-active tyrosine YZ and the primary plastoquinone acceptor QA was followed by an additional electrogenic phase (tau approximately 20 micros, approximately 5% of the phase attributed to YZoxQA-). The latter phase was ascribed to the transfer of an electron from the Mn, bound to the Mn-binding site of the PS II reaction center to the YZox. An additional electrogenicity observed upon addition of synthetic trinuclear Mn complex-1 has a tau approximately 50 micros (approximately 4% of the YZoxQA) and tau approximately 160 ms (approximately 25%). The fast electrogenic component could be ascribed to reduction of YZox by Mn, delivered to the Mn-binding site in Mn-depleted samples after the release of the tripod ligands from the complex-1 while the slow electrogenic phase to the electron transfer from the Mn-containing complex-1 attached to the protein-water boundary to the oxidized Mn at the protein-embedded Mn-binding site.

Four synthetic manganese complexes in which Mn atoms have different coordination environments and valence states were used to reconstitute water-oxidizing complex (WOC) in Mn-depleted photosystem 2 preparations. Three Mn-complexes restored a significant rate of electron transfer and oxygen evolution except one complex in which Mn atom ligated to the O-atoms within the ligands by covalent linkage. The effect of coordination environment of the Mn-atom within the Mn-complexes on their efficiencies in reconstituting the electron transport and oxygen evolution was analysed.

Currently available data about bicarbonate (BC) action on the Mn-containing water-oxidizing complex (WOC) of the photosystem II (PSII) were obtained almost solely in vitro, e.g. on subchloroplast membrane fragments enriched with PSII. To investigate the in vivo BC effect on the PSII donor side, we used the method of dark thermoinactivation of intact Chlamydomonas reinhardtii cells. Photosynthetic activity of PSII was measured as photoinduced changes in the PSII chlorophyll fluorescence yield and as the rate of photosynthetic oxygen evolution. To exclude a “direct” effect of the absence of BC on the PSII activity, before measurements of the photosynthetic activity, the concentration of BC in all samples was equalized by addition of NaHCO3 to each of them (except for those that contained 5 mM of NaHCO3 during thermoinactivation) to reach the final concentration of 5 mM. This allowed registering only so-called “irreversible” (i.e., not reversible by subsequent addition of BC) effect of the absence of BC during thermoinactivation. It was shown that, if 5 mM NaHCO3 was added to the medium before thermoinactivation, the rate of inactivation of the PSII donor side was lower than in BC-depleted medium 1.5-to 2-fold. The obtained results are interpreted as an indication that BC protects the donor side of PSII against thermoinactivation in vivo, in intact C. reinhardtii cells. This proves the correctness of the earlier proposition that BC is an integral constituent of the Mn-containing water-oxidizing complex of PSII.

A protective action of K15 (4-[methoxy-bis(trifluoromethyl)methyl]-2,6-dinitrophenylhydrazone methyl ketone), an inhibitor of electron transport in photosystem 2 (PS 2), against photoinactivation of the PS 2 reaction center (RC) D1/D2/cytochrome b(559) complex, isolated from pea chloroplasts, by red light (0.7 mmol photons/sec per m(2)) has been investigated under aerobic conditions. The inhibitor K15 causing cyclic electron transfer around PS 2 and thus prohibiting stabilization of separated charges has been shown to effectively protect RC both against the loss of photochemical activity (measured as reversible photoinduced absorbance changes related to photoreduction of pheophytin) and aggregation and degradation of the proteins D2 and D1 during photoinactivation. Comparison of the protective action of K15 and of another inhibitor of electron transfer in PS 2, diuron, against light-induced destruction of proteins D1 and D2 shows that diuron stabilizes protein D1 and K15 stabilizes protein D2. The preferential protection of D2 against photoinduced destruction revealed in our work is in accord with the concept of a specific binding of K15 with this protein. It is proposed that this binding site may be that of the primary quinone electron acceptor Q(A) located on the D2 protein (in contrast to diuron, which is known to replace the secondary electron acceptor Q(B) from its binding site on D1).

A binding site for novel inhibitors of K15 type (derivatives of perfluoroisopropyldinitrobenzene) with the components of reaction center (RC) of photosystem 2 (PS-2) of higher plants has been investigated. It has been shown that multiple washing the PS-2 submembrane chloroplast fragments (BBY-particles) treated with the K15 inhibitor, including multiple dilution in buffer in the presence of high concentrations of mono- and divalent ions, prolonged (up to 2-5 h) incubation, centrifugation, and subsequent resuspension in buffer deprived of the inhibitor, does not lead to restoration of functional activity of the PS-2. After addition of dithionite, inducing reduction and consequent decomposition of the inhibitor, and subsequent removal of dithionite by washing, the functional activity of PS-2 was completely restored. Incubation in the presence of sodium dodecyl sulfate (SDS), leading to solubilization of the sample to the level of protein components, induced the appearance of a fraction of free K15 retaining the initial inhibitory efficiency. To create a covalent binding of the inhibitor with protein, retained under the conditions of denaturing SDS polyacrylamide gel electrophoresis, the azido-containing analog of K15 (K15-N(3)) was used. The need for radioactive label for identification of K15 was avoided by the revealed ability of K15-type inhibitors to emit fluorescence, which retained its features under the experimental conditions. With the technique of photoaffinity binding and denaturing SDS-PAGE in the presence of 6 M urea of submembrane chloroplast fragments enriched in PS-2 the D2-polypeptide, an integral component of the reaction center of PS-2, has been shown to be a binding site for K15-type inhibitors. This conclusion is in agreement with a suggestion (put forward in our earlier publications) that K15-type inhibitors are bound to PS-2 reaction center, replacing Q(A) in its binding site. Hence, an agent specifically binding to polypeptide D2 has been found for the first time. The data are compared with information about inhibitory action and binding sites of the known inhibitors of electron transfer in PS-2.

The effect of a typical polyaromatic hydrocarbon, naphthalene (Naph), on photosystem 2 (PS-2) photochemical activity in thylakoid membrane preparations and 20-day-old pea leaves was studied. Samples were incubated in water in the presence of Naph (0.078, 0.21, and 0.78 mM) for 0.5-24 h under white light illumination (15 μmol photons·m(-2)·s(-1)). The PS-2 activity was determined by studying fast and delayed chlorophyll (Chl) a fluorescence. Incubation of samples in water solutions at Naph concentrations of 0.21 and 0.78 mM led to a decrease in the maximum PS-2 quantum efficiency (Fv/Fm), noticeable changes in the polyphasic induction kinetics of fluorescence (OJIP), and a decrease in the amplitudes of the fast and slow components of delayed fluorescence of Chl a. The rate of release of electrolytes from leaves that were preliminarily incubated with Naph (0.21 mM) was also increased. Significant decrease in the fluorescence parameters in thylakoid membrane preparations was observed at Naph concentration of 0.03 mM and 12-min exposure of the samples. Chlorophyll (a and b) and carotenoid content (mg per gram wet mass) was insignificantly changed. The quantum yields of electron transfer from QA to QB (φET2o) and also to the PS-1 acceptors (φRE1o) were reduced. These results are explained by the increase in the number of QB-non-reducing centers of PS-2, which increased with increasing Naph concentration and exposure time of leaves in Naph solution. The suppression of PS-2 activity was partly abolished in the presence of the electron donor sodium ascorbate. Based on these results, it is suggested that Naph distorts cell membrane intactness and acts mainly on the PS-2 acceptor and to a lesser degree on the PS-2 donor side.

Quantitative structure-activity relationship (QSAR) analysis of the twenty-six perfluoroisopropyl-dinitrobenzene (PFIPDNB) derivatives was performed to explain their ability to suppress photochemical activity of the plants photosystem II using chloroplasts and subchloroplast thylakoid membranes enriched in photosystem II, called DT-20. Compounds were optimized by semi-empirical PM3 and DFT/B3LYP/6-31G methods. The Heuristic and the Best Multi-Linear Regression (BMLR) method in CODESSA were used to select the most appropriate molecular descriptors and to develop a linear QSAR model between experimental pl(50) values and the most significant set of the descriptors. The obtained models were validated by cross-validation (R-cv(2)) and internal validation to confirm the stability and good predictive ability. The obtained eight models with five-parameter show that: (a) coefficient (R-2) value of the chloroplast samples are slightly higher than that of the DT-20 samples both of Heuristic and BMLR models: (b) the coefficients of the BMLR models are slightly higher than that of Heuristic models both of chloroplasts and DT-20 samples; (c) The YZ shadow parameter and the indicator parameter, for presence of NO2 substituent in the ring, are the most important descriptor at PM3-based and DFT-based QSAR models, respectively. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial. (C) 2012 Elsevier B.V. All rights reserved.

Photoinduced electron transfer and oxygen evolution are completely inhibited as a result of the complete (>98%) removal of Mn and water soluble proteins with m.w. 17,23 and 33 kD from pea photosystem II (PS-II) particles (DT-20) by means of I M tris-HCl (pH 8) and 0.5 M MgCl2/1–4/. Restoration of oxygen evolution after the treatment requires a joint addition of MnCl and CaCl2 /2–4/. Ca++ is highly specific in the reactivation (only Sr++ can replace it with a lower efficiency). At the same time, the restoration requires a very high concentration of Ca++ (10–20 mM) (Fig.1). The reactivation is observed in the absence of the proteins with m.w. 17,23 and.33 kD which are completely removed during extraction of Mn /3/ thus showing that all of them (including the 33 kD protein) are not the binding site for Mn and are not strictly necessary for oxygen evolution. When the sum of these proteins (isolated from DT-20 by I M CaCl2 /5/) is added to the particles, the reactivation is observed at much lower concentrations of Ca++ though the maximal rate of O2-evolution is not increased (Fig.1).