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In the last decade, the task of developing environmentally friendly and cost-effective methods for obtaining stable superhydrophobic coatings has become topical. In this study, we examined the effect of the concentrations of filler and polymer binder on the hydrophobic properties and surface roughness of composite coatings made from organic-aqueous compositions based on hexyl methacrylate (HMA) and glycidyl methacrylate (GMA) copolymers. Silicon dioxide nanoparticles were used as a filler. A single-stage "all-in-one" aerosol application method was used to form the coatings without additional intermediate steps for attaching the adhesive layer or texturing the substrate surface, as well as pre-modification of the surface of filler nanoparticles. As the ratio of the mass fraction of polymer binder (Wn) to filler (Wp) increases, the coatings show the lowest roll-off angles among the whole range of samples studied. Coatings with an optimal mass fraction ratio (Wn/Wp = 1.2 ÷ 1.6) of the filler to polymer binder maintained superhydrophobic properties for 24 h in contact with a drop of water in a chamber saturated with water vapor and exhibited roll-off angles of 6.1° ± 1°.

The natural oscillations of the electromagnetic field in a particle made from left-handed metamaterial, where both permittivity and permeability are negative, are considered. Based on the exact solution of the sourceless Maxwell equations, it is shown that due to the opposite directions of the phase and group velocities in the metamaterial, natural oscillations in such particles decay exponentially at infinity, that is, these natural oscillations can be considered as trapped modes with a finite energy. The manifestation of such modes in experiments with Bessel beams is also discussed.

This paper discusses the influence of the structure of copolymers based on glycidyl methacrylate and alkyl methacrylates with C 6 -C 18 hydrocarbon side groups on the wettability and sorption properties of surface-modified chitosan aerogels. The grafting of copolymers onto the surface of aerogels was confirmed by elemental analysis, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy. As a result of the modification, with an increase in the amount of the hydrocarbon substituent alkyl methacrylate, the surface of the resulting materials became hydrophobic with contact angles in the range of 146-157°. At the same time, the water absorption of the aerogels decreased by a factor of 30 compared to that for unmodified aerogels, while the sorption capacity for light oil, diesel fuel, and synthetic motor oil remained at the level of more than 30 g/g. Chitosan aerogels with grafted copolymers based on glycidyl methacrylate and alkyl methacrylates retain biodegradation capacity; however, compared to unmodified chitosan, this process has an induction period.

This study proposes to use reactive copolymers based on glycidyl methacrylate and fluoroalkyl methacrylates with a low fluorine content in the monomer unit as agents to reduce the surface free energy (SFE). This work reveals the effect of the structure and composition of copolymers on the SFE and water-repellent properties of these coatings. On a smooth surface, coatings based on copolymers of glycidyl methacrylate and fluoroalkyl methacrylates with fluorine atoms in the monomer unit ranging from three to seven are characterized by SFE values in the range from 25 to 13 mN/m, which is comparable to the values for polyhedral oligomeric silsesquioxanes and perfluoroalkyl acrylates. On textured aluminum surfaces, the obtained coatings provide time-stable superhydrophobic properties with contact angles up to 170° and sliding angles up to 2°. The possibility of using copolymers based on glycidyl methacrylate and fluoroalkyl methacrylates for the creation of self-cleaning polymer coatings is shown.

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 HCO 3 - /CO 2 as well as the suppression of carbonic anhydrase activity of PSII by a known inhibitor of α‑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 α‑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 HCO 3 - /CO 2 . Addition of exogenous HCO 3 - 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., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Increasing inefficiency of production of important agricultural plants raises one of the biggest problems in the modern world. Herbicide application is still the best method of weed management. Traditional herbicides blocking only one of the plant metabolic pathways is ineffective due to the rapid growth of herbicide-resistant weeds. The synthesis of novel compounds effectively suppressing several metabolic processes, and therefore achieving the synergism effect would serve as the alternative approach to weed problem. For this reason, recently, we synthesized a series of nine novel Cu(II) complexes and four ligands, characterized them with different analyses techniques, and carried out their primary evaluation as inhibitors of photosynthetic electron transfer in spinach thylakoids (design, synthesis, and evaluation of a series of Cu(II) based metal-organic complexes as possible inhibitors of photosynthesis, J Photochem Photobiol B, submitted). Here, we evaluated in vitro inhibitory potency of these agents against: photochemistry and carbonic anhydrase activity of photosystem II (PSII); α-carbonic anhydrase from bovine erythrocytes; as well as glutathione reductase from chloroplast and baker's yeast. Our results show that all Cu(II) complexes excellently inhibit glutathione reductase and PSII carbonic anhydrase activity. Some of them also decently inhibit PSII photosynthetic activity.

Earlier the catalase-insensitive formation of organic hydroperoxides (via the interaction of organic radicals produced due to redox activity of P 680 +· (or TyrZ · ) with molecular oxygen) has been found in Mn-depleted PS2 preparations (apo-WOC-PS2) by Khorobrykh et al. (Biochemistry 50:10658-10665, 2011). The present work describes a second pathway of the photoproduction of organic peroxides on the donor side of PS2. It was shown that illumination of CaCl 2 -treated PS2 membranes (deprived of the PS2 extrinsic proteins without removal of the Mn-containing water-oxidizing complex) (CaCl 2 -PS2) led to the photoproduction of highly lipophilic organic hydroperoxides (LP-OOH) (in amount corresponding to 1.5 LP-OOH per one reaction center of PS2) which significantly increased upon the addition of exogenous electron acceptor potassium ferricyanide (to 4.2 LP-OOH per one reaction center). Addition of catalase (200 U/ml) before illumination inhibited ferricyanide-induced photoproduction of hydroperoxides while no effect was obtained by adding catalase after illumination or by adding inactivated catalase before illumination. The hydroperoxide photoproduction was inhibited by the addition of exogenous electron donor for PS2, diphenylcarbazide or diuron (inhibitor of the electron transfer in PS2). The addition of exogenous hydrogen peroxide to the CaCl 2 -PS2 led to the production of highly lipophilic organic hydroperoxides in the dark (3.2 LP-OOH per one reaction center). We suggest that the photoproduction of highly lipophilic organic hydroperoxides in CaCl 2 -PS2 preparations occurs via redox activity of H 2 O 2 produced on the donor side of PS2.

We have obtained analytical expressions for the radiative decay rate of the spontaneous emission of a chiral molecule located near a dielectric spherical particle with a chiral nonconcentric spherical shell made of a bi-isotropic material. Our numerical and graphical analyses show that material composition, thickness and degree of non-concentricity of the shell can influence significantly the spontaneous radiation of the chiral molecule. In particular, the radiative decay rates can differ in orders of magnitude for a chiral molecule located near the thin and thick parts of a nonconcentric shell as well as near a concentric shell made of chiral metamaterial. We also find that the radiative decay rates of the "right" and "left" chiral molecule enantiomers located near a nanoparticle with a chiral metamaterial shell can differ pronouncedly from each other. Our findings therefore suggest a way to tune the spontaneous emission of chiral molecules by varying the material composition, thickness and degree of non-concentricity of the shell in the nearby composite nanoparticle and also to enhance the chirality selection of chiral molecules in racemic mixtures.

Nineteen antimony(III) complexes were obtained and examined as possible herbicides. Six of these were synthesized for the first time, and their structures were identified using elemental analyses, 1 H-NMR, 13 C-NMR, FTIR, LCMS, magnetic susceptibility, and conductivity measurement techniques. For the nineteen examined antimony(III) complexes their most-stable forms were determined by DFT/B3LYP/LanL2DZ calculation method. These compounds were examined for effects on photosynthetic electron transfer and carbonic anhydrase activity of photosystem II, and glutathione reductase from chloroplast as well were investigated. Our results indicated that all antimony(III) complexes inhibited glutathione reductase activity of chloroplast. A number of these also exhibited good inhibitory efficiency of the photosynthetic and carbonic anhydrase activity of Photosystem II.

Recently, it has been shown that the addition of 1M trehalose leads to the increase of the rate of oxygen photoconsumption associated with activation of electron transport in the reaction center of photosystem 2 (PS2) in Mn-depleted PS2 membranes (apo-WOC-PS2) [37]. In the present work the effect of trehalose on photoinhibition of apo-WOC-PS2 preparations (which are characterized by a high sensitivity to the donor side photoinhibition of PS2) was investigated. The degree of photoinhibition was estimated by the loss of the capability of exogenous electron donor (sodium ascorbate) to reactivate the electron transport (measured by light-induced changes of chlorophyll fluorescence yield (∆F)) in apo-WOC-PS2. It was found that 1M trehalose enhanced the Mn 2+ -dependent suppression of photoinhibition of apo-WOC-PS2: in the presence of trehalose the addition of 0.2μM Mn 2+ (corresponding to 2 Mn 2+ per one reaction center) was sufficient for an almost complete suppression of the donor side photoinhibition of the complex. In the absence of trehalose it was necessary to add 100μM Mn 2+ to achieve a similar result. The effect of trehalose was observed during photoinhibition of apo-WOC-PS2 at low (15μmolphotons -1 m -2 ) and high (200μmolphotons -1 m -2 ) light intensity. When Mn 2+ was replaced by other PS2 electron donors (ferrocyanide, DPC) as well as by Ca 2+ the protective effect of trehalose was not observed. It was also found that 1M trehalose decreased photoinhibition of apo-WOC-PS2 if the samples contained endogenous manganese (1-2 Mn ions per one RC was enough for the maximum protection effect). It is concluded that structural changes in PS2 caused by the addition of trehalose enhance the capability of photochemical reaction centers of apo-WOC-PS2 to accept electrons from manganese (both exogenous and endogenous), which in turn leads to a considerable suppression of the donor side photoinhibition of PS2., (Copyright © 2016 Elsevier B.V. All rights reserved.)

It has been shown earlier (Khorobrykh and Klimov, 2015) that molecular oxygen is directly involved in the general mechanism of the donor side photoinhibition of photosystem II (PSII) membranes. In the present work the effect of oxygen on photoassembly ("photoactivation") of the functionally active inorganic core of the water-oxidizing complex (WOC) in Mn-depleted PSII preparations (apo-WOC-PSII) in the presence of exogenous Mn(2+), Ca(2+) as well as ferricyanide was investigated. It was revealed that the efficiency of the photoassembly of the WOC was considerably increased upon removal of oxygen from the medium during photoactivation procedure using the enzymatic oxygen trap or argon flow. The lowering of O2 concentration from 250μM to 75μM, 10μM and near 0μM results in 29%, 71% and 92%, respectively, stimulation of the rate of O2 evolution measured after the photoactivation. The increase in the intensity of light used during the photoactivation was accompanied by a decrease of both the efficiency of photoassembly of the WOC and the stimulation effect of removal of O2 (that may be due to the enhancement of the processes leading to the photodamage to PSII). It is concluded that the enhancement in photoactivation of oxygen-evolving activity of apo-WOC-PSII induced by oxygen removal from the medium is due to the suppression of the donor side photoinhibition of PSII in which molecular oxygen can be involved., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Two-dimensional lattices of chiral nanoholes in a plasmonic film with lattice constants being slightly larger than light wavelength are proposed for effective control of polarization and spatial properties of light beams. Effective polarization conversion and strong circular dichroism in non-zero diffraction orders in these chiral metafilms are demonstrated by electromagnetic simulations. These interesting effects are found to result from interplay between radiation pattern of single chiral nanohole and diffraction pattern of the planar lattice, and can be manipulated by varying wavelength and polarization of incoming light as well as period of metastructure and refractive indexes of substrate and overlayer. Therefore, this work offers a novel paradigm for developing planar chiral metafilm-based optical devices with controllable polarization state, spatial orientation and intensity of outgoing light.

It has been shown by Khorobrykh et al. (Biochemistry (Moscow) 67:683-688, 2002); Yanykin et al. (Biochim Biophys Acta 1797:516-523, 2010); Khorobrykh et al. (Biochemistry 50:10658-10665, 2011) that Mn-depleted photosystem II (PSII) membrane fragments are characterized by an enhanced oxygen photoconsumption on the donor side of PSII which is accompanied with hydroperoxide formation and it was suggested that the events are related to the oxidative photoinhibition of PSII. Experimental confirmation of this suggestion is presented in this work. The degree of photoinhibition was determined by the loss of the capability of exogenous electron donors (Mn(2+) or sodium ascorbate) to the reactivation of electron transport [measured by the light-induced changes of chlorophyll fluorescence yield (∆F)] in Mn-depleted PSII membranes. The transition from anaerobic conditions to aerobic ones significantly activated photoinhibition of Mn-depleted PSII membranes both in the absence and in the presence of exogenous electron acceptor, ferricyanide. The photoinhibition of Mn-depleted PSII membranes was suppressed upon the addition of exogenous electron donors (Mn(2+), diphenylcarbazide, and ferrocyanide). The addition of superoxide dismutase did not affect the photoinhibition of Mn-depleted PSII membranes. It is concluded that the interaction of molecular oxygen (rather than superoxide anion radical formed on the acceptor side of PSII) with the oxidized components of the donor side of PSII reflects the involvement of O2 in the donor-side photoinhibition of Mn-depleted PSII membranes.

It is known that the removal of manganese from the water-oxidizing complex (WOC) of photosystem 2 (PS2) leads to activation of oxygen photoconsumption (OPC) [Khorobrykh et al., 2002; Yanykin et al., 2010] that is accompanied by the formation of organic hydroperoxides on the electron-donor side of PS2 [Khorobrykh et al., 2011]. In the present work the effect of trehalose on the OPC in Mn-depleted PS2 preparations (apo-WOC-PS2) was investigated. A more than two-fold increase of the OPC is revealed upon the addition of 1M trehalose. Drastic (30%-70%) inhibition of the OPC upon the addition of either electron acceptor or electron donor indicates that the trehalose-induced activation of the OPC occurs on both donor and acceptor sides of PS2. A two-fold increase in the rate of superoxide-anion radical photoproduction on the electron-acceptor side of PS2 was also shown. Applying the "variable" chlorophyll fluorescence (ΔF) it was shown that the addition of trehalose induces: (i) a significant increase in the ability of exogenous Mn(2+) to donate electrons to the reaction center of PS2, (ii) slowing down the photoaccumulation of the primary quinone electron acceptor of PS2 (QA(-)) under aerobic conditions, (iii) acceleration of the reoxidation of QA(-) by QB (and by QB(-)) as well as the replacement of QB(2-) by a fully oxidized plastoquinone, and (iv) restoration of the electron transfer between the quinone electron carriers in the so-called "closed reaction centers of PS2" (their content in the apo-WOC-PS2 is 41%). It is suggested that the trehalose-induced increase in efficiency of the O2 interaction with the electron-donor and electron-acceptor sides of apo-WOC-PS2 is due to structural changes leading to both a decrease in the proportion of the "closed PS2 reaction centers" and an increase in the electron transfer rate in PS2., (Copyright © 2015 Elsevier B.V. All rights reserved.)

The photosynthetic water oxidation in photosystem II (PS II) takes place in a special water-oxidizing complex (WOC) that consists of a catalytic center, Mn4CaO5 cluster, and also includes a group of extrinsic proteins needed for its stability. The most important of these is PsbO, which binds to the donor side of PS II near the Mn cluster and is directly involved in the regulation of its stability and activity. However, the molecular mechanism of PsbO involvement in photosynthetic water oxidation remains unclear. One of the main approaches to solving this problem is site-directed mutagenesis. Until recently, the effect of mutations in PsbO in vivo has been studied only in cyanobacteria (prokaryotes). In eukaryotic organisms, such studies (site-directed mutagenesis of PsbO) have not been carried out, though it is known that the role of PsbO protein in plants and cyanobacteria may be different. In this review, we consider the possibility of using for this purpose the unicellular green alga Chlamydomonas reinhardtii, a eukaryotic organism with a set of extrinsic proteins of the WOC similar to that of the higher plants. However, in contrast to higher plants, the ΔpsbO mutant of C. reinhardtii is viable. Another reason to use this alga is that the ΔpsbO strain of C. reinhardtii grown in the dark (heterotrophically) is able to build the minimal photochemically active complex of PS II, allowing investigation of the role of individual amino acid substitutions in PsbO in vivo without damaging PS II due to photoinactivation.

Thirty novel chemical compounds were designed and synthesized expecting that they would be possible inhibitors. From this number eleven were organic bases, twenty-four were their organic derivatives and fourteen were metal complexes. Screening of these chemicals by their action on photosynthetic electron transfer (PET) and carbonic anhydrase (CA) activity (CAA) of photosystem II (PSII), α-CA, as well as β-CA was done. Several groups were revealed among them. Some of them are capable to suppress either one, two, three, or even all of the measured activities. As example, one of the Cu(II)-phenyl sulfonylhydrazone complexes (compound 25) suppresses CAA of α-CA by 88%, CAA of β-CA by 100% inhibition; CAA of PSII by 100% and the PSII photosynthetic activity by 66.2%. The Schiff base compounds (12, 15) and Cu(II)-phenyl sulfonylhydrazone complexes (25, 26) inhibited the CAA and PET of PSII significantly. The obtained data indicate that the PSII donor side is a target of the inhibitory action of these agents. Some physico- or electrochemical properties such as diffusion coefficient, number of transferred electrons, peak potential and heterogeneous standard rate constants of the compounds were determined in nonaqueous media. pKa values were also determined in nonaqueous and aqueous media. Availability in the studied group of novel chemical agents possessing different inhibitory activity allow in future to isolate the "active part" in the structure of the inhibitors responsible for different inhibitory mechanisms, as well as to determine the influence of side substituters on its inhibitory efficiency., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Eigenmodes of a chiral sphere placed in a dielectric medium were investigated in details. Excitation of these eigenmodes by a plane wave and a chiral molecule radiation was studied both analytically and numerically. It was found that decay rates of "right" and "left" enantiomers are different in the presence of the chiral sphere. Strong dependence of radiation pattern of the chiral molecule placed in the vicinity of the chiral sphere on chirality strength was also demonstrated. An interesting correlation between chirality of sphere and spatial spirality (helicity, vorticity ...) of the electromagnetic fields in the presence of chiral sphere was observed for the first time.

Oxylipins are signaling molecules formed enzymatically or spontaneously from unsaturated fatty acids in all aerobic organisms. Oxylipins regulate growth, development, and responses to environmental stimuli of organisms. The oxylipin biosynthesis pathway in plants includes a few parallel branches named after first enzyme of the corresponding branch as allene oxide synthase, hydroperoxide lyase, divinyl ether synthase, peroxygenase, epoxy alcohol synthase, and others in which various biologically active metabolites are produced. Oxylipins can be formed non-enzymatically as a result of oxygenation of fatty acids by free radicals and reactive oxygen species. Spontaneously formed oxylipins are called phytoprostanes. The role of oxylipins in biotic stress responses has been described in many published works. The role of oxylipins in plant adaptation to abiotic stress conditions is less studied; there is also obvious lack of available data compilation and analysis in this area of research. In this work we analyze data on oxylipins functions in plant adaptation to abiotic stress conditions, such as wounding, suboptimal light and temperature, dehydration and osmotic stress, and effects of ozone and heavy metals. Modern research articles elucidating the molecular mechanisms of oxylipins action by the methods of biochemistry, molecular biology, and genetics are reviewed here. Data on the role of oxylipins in stress signal transduction, stress-inducible gene expression regulation, and interaction of these metabolites with other signal transduction pathways in cells are described. In this review the general oxylipin-mediated mechanisms that help plants to adjust to a broad spectrum of stress factors are considered, followed by analysis of more specific responses regulated by oxylipins only under certain stress conditions. New approaches to improvement of plant resistance to abiotic stresses based on the induction of oxylipin-mediated processes are discussed.

Photosystem II (PSII) is a pigment-protein complex of thylakoid membrane of higher plants, algae, and cyanobacteria where light energy is used for oxidation of water and reduction of plastoquinone. Light-dependent reactions (generation of excited states of pigments, electron transfer, water oxidation) taking place in PSII can lead to the formation of reactive oxygen species. In this review attention is focused on the problem of interaction of molecular oxygen with the donor site of PSII, where after the removal of manganese from the water-oxidizing complex illumination induces formation of long-lived states (P680(+•) and TyrZ(•)) capable of oxidizing surrounding organic molecules to form radicals.

Influence of bicarbonate on the efficiency of the electron donation from Mn(2+) to P₆₈₀(+) in isolated D1/D2/cytochrome b559 complex was investigated. All the experiments were carried out in a medium depleted of HCO₃(-)/CO₂. Kinetics of photoinduced absorbance changes (ΔA) at different wavelengths and decrease of chlorophyll fluorescence yield (-ΔF) related to photoaccumulation of reduced pheophytin, the intermediary electron acceptor of photosystem II (PSII), in the presence of Mn(2+) under anaerobic conditions were measured. Addition of bicarbonate (1 mM) increased the amplitude of these ΔA and -ΔF at least by a factor of 3. Measurements of the photoinduced ΔA, related to photooxidation of the primary electron donor of PSII, chlorophyll P₆₈₀, were done in the presence of silicomolybdate as electron acceptor. These results show that the addition of 0.05 mM Mn(2+) alone or jointly with 1 mM bicarbonate induces a 20% and 70%-decrease of the magnitude of the ΔA at 680 nm. The effect of Mn(2+) (in the presence and absence of bicarbonate) was completely eliminated by the addition of 12 mM EDTA. All these bicarbonate effects were not observed if MgCl₂ or formate were used instead of MnCl₂ and bicarbonate, respectively. In the absence of Mn(2+), bicarbonate induced none of the mentioned above effects (increase of photoaccumulation of reduced pheophytin and decrease of photooxidation of P680). The presented data suggest that bicarbonate stimulates the electron donation from Mn(2+) to D1/D2/cyt b559 reaction center evidently due to formation of easily oxidizable Mn-bicarbonate complexes., (Copyright © 2013 Elsevier B.V. All rights reserved.)

It has been shown that removal of manganese from the water-oxidizing complex (WOC) of photosystem II (PSII) leads to flash-induced oxygen consumption (FIOC) which is activated by low concentration of Mn(2+) (Yanykin et al., Biochim Biophys Acta 1797:516-523, 2010). In the present work, we examined the effect of transition and non-transition divalent metal ions on FIOC in Mn-depleted PSII (apo-WOC-PSII) preparations. It was shown that only Mn(2+) ions are able to activate FIOC while other transition metal ions (Fe(2+), V(2+) and Cr(2+)) capable of electron donation to the apo-WOC-PSII suppressed the photoconsumption of O2. Co(2+) ions with a high redox potential (E (0) for Co(2+)/Co(3+) is 1.8 V) showed no effect. Non-transition metal ions Ca(2+) by Mg(2+) did not stimulate FIOC. However, Ca(2+) (in contrast to Mg(2+)) showed an additional activation effect in the presence of exogenic Mn(2+). The Ca(2+) effect depended on the concentration of both Mn(2+) and Ca(2+). The Ca effect was only observed when: (1) the activation of FIOC induced by Mn(2+) did not reach its maximum, (2) the concentration of Ca(2+) did not exceed 40 μM; at higher concentrations Ca(2+) inhibited the Mn(2+)-activated O2 photoconsumption. Replacement of Ca(2+) by Mg(2+) led to a suppression of Mn(2+)-activated O2 photoconsumption; while, addition of Ca(2+) resulted in elimination of the Mg(2+) inhibitory effect and activation of FIOC. Thus, only Mn(2+) and Ca(2+) (which are constituents of the WOC) have specific effects of activation of FIOC in apo-WOC-PSII preparations. Possible reactions involving Mn(2+) and Ca(2+) which could lead to the activation of FIOC in the apo-WOC-PSII are discussed.

The enzyme that catalyzes water oxidation in oxygenic photosynthesis contains an inorganic cluster (Mn4 CaO5 ) that is universally conserved in all photosystem II (PSII) protein complexes. Its hypothesized precursor is an anoxygenic photobacterium containing a type 2 reaction center as photo-oxidant (bRC2, iron-quinone type). Here we provide the first experimental evidence that a native bRC2 complex can catalyze the photo-oxidation of Mn(2+) to Mn(3+) , but only in the presence of bicarbonate concentrations that allows the formation of (bRC2)Mn(2+) (bicarbonate)1-2 complexes. Parallel-mode EPR spectroscopy was used to characterize the photoproduct, (bRC2)Mn(3+) (CO3 (2-) ), based on the g tensor and (55) Mn hyperfine splitting. (Bi)carbonate coordination extends the lifetime of the Mn(3+) photoproduct by slowing charge recombination. Prior electrochemical measurements show that carbonate complexation thermodynamically stabilizes the Mn(3+) product by 0.9-1 V relative to water ligands. A model for the origin of the water oxidation catalyst is presented that proposes chemically feasible steps in the evolution of oxygenic PSIIs, and is supported by literature results on the photoassembly of contemporary PSIIs., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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 pI(50) values and the most significant set of the descriptors. The obtained models were validated by cross-validation (R(2)(cv)) 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 NO(2) 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., (Copyright © 2012 Elsevier B.V. All rights reserved.)

We report handedness-sensitive surface plasmon polariton (SPP) emission in mirror-symmetric ensembles of elliptical nanoholes made in a thin gold film. It is found by means of rigorous calculations and scanning near-field optical microscopy that SPP excitation direction depends on the direction of circularly polarized illumination E-vector rotation. An analytical model based on anisotropic polarizability of each nanohole is presented. Both the experimental and calculated results are in agreement with Curie's principle, and contribute to better understanding of symmetry in plasmonics.

It has been shown that thermoinactivation of the isolated D1/D2/cytochrome b(559) complex (RC) of photosystem 2 (PS-2) from pea under anaerobic conditions at 35°C in 20 mM Tris-HCl buffer (pH 7.2) depleted of HCO(3)(-), with 35 mM NaCl and 0.05% n-dodecyl-β-maltoside, results in a decrease in photochemical activity measured by photoreduction of the PS-2 primary electron acceptor, pheophytin (by 50% after 3 min of heating), which is accompanied by aggregation of the D1 and D2 proteins. Bicarbonate, formate, and acetate anions added to the sample under these conditions differently influence the maintenance of photochemical activity: a 50% loss of photochemical activity occurs in 11.5 min of heating in the presence of bicarbonate and in 4 and 4.6 min in the presence of formate and acetate, respectively. The addition of bicarbonate completely prevents aggregation of the D1 and D2 proteins as opposed to formate and acetate (their presence has no effect on the aggregation during thermoinactivation). Since the isolated RCs have neither inorganic Mn/Ca-containing core of the water-oxidizing complex nor nonheme Fe(2+), it is supposed that bicarbonate specifically interacts with the hydrophilic domains of the D1 and D2 proteins, which prevents their structural modification that is a signal for aggregation of these proteins and the loss of photochemical activity.

Site-directed mutations were introduced into PsbO protein of photosystem 2 to study the role of two lysine residues, 223 and 226 (LGAKPPK), in the green alga Chlamydomonas reinhardtii. Lysines 223 and 226 homologous to His228 and His231 from cyanobacteria are located on the protein side facing the lumen and can participate in formation of a channel connecting the Mn cluster with the intrathylakoid space. The K223E and K226E mutants were generated on the basis of the ΔpsbO strain of C. reinhardtii with the substitution of glutamic acid for the lysine residues. The K226E mutation leads to a decrease in stability of the protein and development of the ΔpsbO phenotype (the absence of both photosynthetic activity of photosystem 2 and photoautotrophic growth), with substantially decreased PsbO content in the cells. In the case of K223E, the mutant strain accumulated the normal level of PsbO protein and was able to grow photoautotrophically and to evolve oxygen. However, the rate of oxygen evolution and the F(v)/F(m) ratio were reduced by 15-20% compared to the control. Also, the time of the dark decay of F(v) in the presence of DCMU in the cells of the K223E mutant was increased, indicating impairment in the water-oxidizing complex. In general, our study shows the importance of amino acids K223 and K226 located at the lumenal surface of PsbO protein for the activity of the water-oxidizing complex.

The photoproduction of organic peroxides (ROOH) in photosystem II (PSII) membranes was studied using the fluorescent probe Spy-HP. Two types of peroxide, highly lipophilic ones and relatively hydrophilic ones, were distinguished by the rate of reaction with Spy-HP; the former oxidized Spy-HP to the higher fluorescent form Spy-HPOx within 5 min, while the latter did so very slowly (the reaction was still not completed after 180 min). The level of photoproduction of these peroxides was significantly larger in the alkaline-treated, Mn-depleted PSII membranes than that in the untreated membranes, and it was suppressed by an artificial electron donor (diphenylcarbazide or ferrocyanide) and by the electron transport inhibitor diuron. Postillumination addition of Fe(2+) ions, which degrade peroxides by the Fenton mechanism, abolished the accumulation of Spy-HPOx, but catalase did not change the peroxide level, indicating that the detected species were organic peroxides, excluding H(2)O(2). These results agreed with our previous observation of an electron transport-dependent O(2) consumption on the PSII donor side and indicated that ROOH accumulated via a radical chain reaction that started with the formation of organic radicals on the donor side. Illumination (λ > 600 nm; 1500 μmol of photons m(-2) s(-1)) of the Mn-depleted PSII membranes for 3 min resulted in the formation of nearly 200 molecules of hydrophilic ROOH per reaction center, but only four molecules of highly lipophilic ROOH. The limited formation of the latter was due to the limited supply of its precursor to the reaction, suggesting that it represented structurally fixed peroxides, i.e., either protein peroxides or peroxides of the lipids tightly bound to the core complex. These ROOH forms, likely including several species derived from lipid peroxides, may mediate the donor side-induced photoinhibition of PSII via protein modification.

The change in the dark reduction rate of photooxidized reaction centers (RC) of type II from three anoxygenic bacteria (Rhodobacter sphaeroides R-26, Chromatium minutissimum, and Chloroflexus aurantiacus) having different redox potentials of the P(+)/P pair and availability of RC for exogenous electron donors was investigated upon the addition of Mn(2+) and HCO(3)(-). It was found that the dark reduction of P(870)(+) from Rb. sphaeroides R-26 is considerably accelerated upon the combined addition of 0.5 mM MnCl(2) and 30-75 mM NaHCO(3) (as a result of formation of "low-potential" complexes [Mn(HCO(3))(2)]), while MnCl(2) and NaHCO(3) added separately had no such effect. The effect is not observed either in RC from Cf. aurantiacus (probably due to the low oxidation potential of the primary electron donor, P(865), which results in thermodynamic difficulties of the redox interaction between P(865)(+) and Mn(2+)) or in RC from Ch. minutissimum (apparently due to the presence of the RC-bound cytochrome preventing the direct interaction between P(870)(+) and Mn(2+)). The absence of acceleration of the dark reduction of P(870)(+) in the RC of Rb. sphaeroides R-26 when Mn(2+) and HCO(3)(-) were replaced by Mg(2+) or Ca(2+) and by formate, oxalate, or acetate, respectively, reveals the specificity of the Mn2+-bicarbonate complexes for the redox interaction with P(+). The results of this work might be considered as experimental evidence for the hypothesis of the participation of Mn(2+) complexes in the evolutionary origin of the inorganic core of the water oxidizing complex of photosystem II., (© Pleiades Publishing, Ltd., 2011.)

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., (Copyright © 2011 Elsevier B.V. All rights reserved.)

In a previous study, we measured the redox potential of the primary electron acceptor pheophytin (Phe) a of photosystem (PS) II in the chlorophyll d-dominated cyanobacterium Acaryochloris marina and a chlorophyll a-containing cyanobacterium, Synechocystis. We obtained the midpoint redox potential (E(m)) values of -478 mV for A. marina and -536 mV for Synechocystis. In this study, we measured the redox potentials of the primary electron acceptor quinone molecule (Q(A)), i.e., E(m)(Q(A)/Q(A)(-)), of PS II and the energy difference between [P680·Phe a(-)·Q(A)] and [P680·Phe a·Q(A)(-)], i.e., ΔG(PhQ). The E(m)(Q(A)/Q(A)(-)) of A. marina was determined to be +64 mV without the Mn cluster and was estimated to be -66 to -86 mV with a Mn-depletion shift (130-150 mV), as observed with other organisms. The E(m)(Phe a/Phe a(-)) in Synechocystis was measured to be -525 mV with the Mn cluster, which is consistent with our previous report. The Mn-depleted downshift of the potential was measured to be approximately -77 mV in Synechocystis, and this value was applied to A. marina (-478 mV); the E(m)(Phe a/Phe a(-)) was estimated to be approximately -401 mV. These values gave rise to a ΔG(PhQ) of -325 mV for A. marina and -383 mV for Synechocystis. In the two cyanobacteria, the energetics in PS II were conserved, even though the potentials of Q(A)(-) and Phe a(-) were relatively shifted depending on the special pair, indicating a common strategy for electron transfer in oxygenic photosynthetic organisms.

Dependence of the electrochemical potential of oxidation of Mn(II) to Mn(III) on logarithm of bicarbonate concentration at different pH is considered. Slope values of the dependence are equal to -60, -120 and -180mV/log C(HCO)₃ at pH6.20, 6.50 and 8.35 respectively, that corresponds to binding of one, two and three HCO(3)(-) ligands to Mn(III). Extrapolation of these dependences to log C(HCO)₃=0 shows that at pH6.20 complex [Mn(III)HCO(3)](2+) with E(0)=0.99V and K(st)=6.5×10(8)M(-1), at pH6.50 complex [Mn(III)(HCO(3))(2)](+) with E(0)=0.77V and K(st)=3.5×10(12)M(-2), and at pH8.35 complex [Mn(III)(HCO(3))(3)](0) with E(0)=0.67V and K(st)=1.73×10(14)M(-3) are formed. From the dependence of oxidation potential of Mn(II) to Mn(III) on pH at initial concentration of NaHCO(3) equal to 28mM the limits of pH stability of the complexes were determined: pH5.0-6.35 for [Mn(III)HCO(3)](2+), pH6.35-7.0 for [Mn(III)(HCO(3))(2)](+) and pH7.0-8.35 for [Mn(III)(HCO(3))(3)](0). The obtained data may be important for understanding the mechanism of stimulating action of HCO(3)(-) ions on photoinduced electron transfer from Mn(II) to reaction centres of photosystem2 or to reaction centres of anoxygenic photosynthetic bacteria., (Copyright © 2010 Elsevier B.V. All rights reserved.)

The effect of reversible removal of HCO(3)(-) on structural re-arrangements in the Mn-stabilizing protein (MSP) of photosystem II, isolated from pea leaves, was studied using measurements of characteristic alterations in fluorescence of hydrophobic probe 8-anilino-1-naphthalene-sulfonic acid (ANS). It was shown that the treatments capable of removal of HCO(3)(-) (or CO(2)) from possible binding sites in MSP (pH lowering from 6.5 to 3.5, addition of a structurally similar anion HCO(3)(-) in concentration 1-20mM or air evacuation at pH 3.5) result in a significant (up to 370%) increase of ANS fluorescence (indicative of structural changes in MSP), whereas HCO(3)(-) lowers the ANS fluorescence to the initial level observed in untreated protein at pH 6.5. Since the effects are revealed at (sub)micromolar concentrations of HCO(3)(-), the specific high-affinity binding of HCO(3)(-) (or CO(2)) to MSP (required for its native structure preservation) is proposed. Possible bicarbonate binding sites and its physiological role within the water-oxidizing complex of photosystem II are discussed., (Copyright 2010 Elsevier B.V. All rights reserved.)

Oxygen consumption in Mn-depleted photosystem II (PSII) preparations under continuous and pulsed illumination is investigated. It is shown that removal of manganese from the water-oxidizing complex (WOC) by high pH treatment leads to a 6-fold increase in the rate of O(2) photoconsumption. The use of exogenous electron acceptors and donors to PSII shows that in Mn-depleted PSII preparations along with the well-known effect of O(2) photoreduction on the acceptor side of PSII, there is light-induced O(2) consumption on the donor side of PSII (nearly 30% and 70%, respectively). It is suggested that the light-induced O(2) uptake on the donor side of PSII is related to interaction of O(2) with radicals produced by photooxidation of organic molecules. The study of flash-induced O(2) uptake finds that removal of Mn from the WOC leads to O(2) photoconsumption with maximum in the first flash, and its yield is comparable with the yield of O(2) evolution on the third flash measured in the PSII samples before Mn removal. The flash-induced O(2) uptake is drastically (by a factor of 1.8) activated by catalytic concentration (5-10microM, corresponding to 2-4 Mn per RC) of Mn(2+), while at higher concentrations (>100microM) Mn(2+) inhibits the O(2) photoconsumption (like other electron donors: ferrocyanide and diphenylcarbazide). Inhibitory pre-illumination of the Mn-depleted PSII preparations (resulting in the loss of electron donation from Mn(2+)) leads to both suppression of flash-induced O(2) uptake and disappearance of the Mn-induced activation of the O(2) photoconsumption. We assume that the light-induced O(2) uptake in Mn-depleted PSII preparations may reflect not only the negative processes leading to photoinhibition but also possible participation of O(2) or its reactive forms in the formation of the inorganic core of the WOC.

Water oxidation by photosystem (PS) II in oxygenic photosynthetic organisms is a major source of energy on the earth, leading to the production of a stable reductant. Mechanisms generating a high oxidation potential for water oxidation have been a major focus of photosynthesis research. This potential has not been estimated directly but has been measured by the redox potential of the primary electron acceptor, pheophytin (Phe) a. However, the reported values for Phe a are still controversial. Here, we measured the redox potential of Phe a under physiological conditions (pH 7.0; 25 degrees C) in two cyanobacteria with different special pair chlorophylls (Chls): Synechocystis sp. PCC 6803, whose special pair for PS II consists of Chl a, and Acaryochloris marina MBIC 11017, whose special pair for PS II consists of Chl d. We obtained redox potentials of -536 +/- 8 mV for Synechocystis sp. PCC 6803 and -478 +/- 24 mV for A. marina on PS II complexes in the presence of 1.0 M betaine. The difference in the redox potential of Phe a between the two species closely corresponded with the difference in the light energy absorbed by Chl a versus Chl d. We estimated the potentials of the special pair of PS II to be 1.20 V and 1.18 V for Synechocystis sp. PCC 6803 (P680) and A. marina (P713), respectively. This clearly indicates conservation in the properties of water-oxidation systems in oxygenic photosynthetic organisms, irrespective of the special-pair chlorophylls.

Four sources of carbonic anhydrase (CA) activity in submembrane preparations of photosystem II (PS II) isolated from pea leaves were examined. Three of them belong to the hydrophilic proteins of the oxygen-evolving complex of PS II with molecular mass 33 kDa (protein PsbO), 24 kDa (protein PsbP), and 18 kDa (protein PsbQ). The fourth source of CA activity is associated with a pigment-protein complex of PS II after removing three hydrophilic proteins by salt treatment. Except for protein PsbQ, the CA activity of all these proteins depends on the presence of Mn2+: the purified protein PsbO did not show CA activity before adding Mn2+ into the medium (concentration of Mn2+ required for 50% effect, EC(50), was 670 microM); CA activity of protein mixture composed of PsbP and PsbQ increased more than 5-fold upon adding Mn2+ (EC(50) was 45 microM). CA activity of purified protein PsbP increased 2-fold in the presence of 200 microM Mn2+. As indicated for the mixture of two proteins (PsbP and PsbQ), Mg2+, Ca2+, and Zn2+, in contrast to Mn2+, suppressed CA activity (both initial and Mn2+-induced activity). Since the found sources of CA activity demonstrated properties different from ones of typical CA (need for Mn2+, insensitivity or low sensitivity to acetazolamide or ethoxyzolamide) and such CA activity was found only among PS II proteins, we cannot exclude that they belong to the type of Mn-dependent CA associated with PS II.

Hydrogen can be important clean fuel for future. Among different technologies for hydrogen production, oxygenic natural and artificial photosyntheses using direct photochemistry in synthetic complexes have a great potential to produce hydrogen, since both use clean and cheap sources: water and solar energy. Artificial photosynthesis is one way to produce hydrogen from water using sunlight by employing biomimetic complexes. However, splitting of water into protons and oxygen is energetically demanding and chemically difficult. In oxygenic photosynthetic microorganisms such as algae and cyanobacteria, water is split into electrons and protons, which during primary photosynthetic process are redirected by photosynthetic electron transport chain, and ferredoxin, to the hydrogen-producing enzymes hydrogenase or nitrogenase. By these enzymes, e- and H+ recombine and form gaseous hydrogen. Biohydrogen activity of hydrogenase can be very high but it is extremely sensitive to photosynthetic O2. In contrast, nitrogenase is insensitive to O2, but has lower activity. At the moment, the efficiency of biohydrogen production is low. However, theoretical expectations suggest that the rates of photon conversion efficiency for H2 bioproduction can be high enough (>10%). Our review examines the main pathways of H2 photoproduction by using of photosynthetic organisms and biomimetic photosynthetic systems.

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.

Reconstitution of Mn-depleted photosystem II (PSII) particles was examined with synthetic trinuclear Mn complexes of newly developed tripod ligands. Rates of the electron transfer and oxygen evolution were up to 74-86 and 52-56% of those measured in native PSII. These values are higher than those for the PSII reconstituted by MnCl(2). The role of the tripod ligands during the reconstitution process was examined by (19)F NMR. Due to the high NMR sensitivity of the (19)F nucleus and the low abundance of fluorine atoms in natural PSII, it was possible to selectively observe the fluorine atoms on the tripod ligand. It was shown that the tripod ligands were released from the Mn complex after the reconstitution. We propose that the primary step in the reconstitution process is the prebinding of the Mn complex to the hydrophobic part of the PSII particle.

The primary targets of thermal damage in plants are the oxygen evolving complex along with the associated cofactors in photosystem II (PSII), carbon fixation by Rubisco and the ATP generating system. Recent investigations on the combined action of moderate light intensity and heat stress suggest that moderately high temperatures do not cause serious PSII damage but inhibit the repair of PSII. The latter largely involves de novo synthesis of proteins, particularly the D1 protein of the photosynthetic machinery that is damaged due to generation of reactive oxygen species (ROS), resulting in the reduction of carbon fixation and oxygen evolution, as well as disruption of the linear electron flow. The attack of ROS during moderate heat stress principally affects the repair system of PSII, but not directly the PSII reaction center (RC). Heat stress additionally induces cleavage and aggregation of RC proteins; the mechanisms of such processes are as yet unclear. On the other hand, membrane linked sensors seem to trigger the accumulation of compatible solutes like glycinebetaine in the neighborhood of PSII membranes. They also induce the expression of stress proteins that alleviate the ROS-mediated inhibition of repair of the stress damaged photosynthetic machinery and are required for the acclimation process. In this review we summarize the recent progress in the studies of molecular mechanisms involved during moderate heat stress on the photosynthetic machinery, especially in PSII.

The effects of oxalate on PS II and PS I photochemistry were studied. The results suggested that in chloride-deficient thylakoid membranes, oxalate inhibited activity of PS II as well as PS I. To our knowledge, this is the only anion so far known which inhibits both the photosystems. Measurements of fluorescence induction kinetics, YZ* decay, and S2 state multiline EPR signal suggested that oxalate inhibited PS II at the donor side most likely on the oxygen evolving complex. Measurements of re-reduction of P700+ signal in isolated PS I particles in oxalate-treated samples suggested a binding site of oxalate on the donor, as well as the acceptor side of PS I.

It is found that dark reduction of photooxidized primary electron donor P870+ in reaction centres from purple anoxygenic bacteria (two non-sulphur Fe-oxidizing Rhodovulum iodosum and Rhodovulum robiginosum, Rhodobacter sphaeroides R-26 and sulphur alkaliphilic Thiorhodospira sibirica) is accelerated upon the addition of Mn2+ jointly with bicarbonate (30-75 mM). The effect is not observed if Mn2+ and HCO3(-) have been replaced by Mg2+ and HCO2(-), respectively. The dependence of the effect on bicarbonate concentration suggests that formation of Mn2+-bicarbonate complexes, Mn(HCO3)+ and/or Mn(HCO3)2, is required for re-reduction of P870+ with Mn2+. The results are considered as experimental evidence for a hypothesis on possible participation of Mn-bicarbonate complexes in the evolutionary origin of oxygenic photosynthesis in the Archean era.

Water oxidation in photosystem II (PSII) is still insufficiently understood and is assumed to involve HCO(3)(-). A Chlamydomonas mutant lacking a carbonic anhydrase associated with the PSII donor side shows impaired O(2) evolution in the absence of HCO(3)(-). The O(2) evolution for saturating, continuous illumination (R(O2)) was slower than in the wild type, but was elevated by HCO(3)(-) and increased further by Cah3. The R(O2) limitation in the absence of Cah3/HCO(3)(-) was amplified by H(2)O/D(2)O exchange, but relieved by an amphiphilic proton carrier, suggesting a role of Cah3/HCO(3)(-) in proton translocation. Chlorophyll fluorescence indicates a Cah3/HCO(3)(-) effect at the donor side of PSII. Time-resolved delayed fluorescence and O(2)-release measurements suggest specific effects on proton-release steps but not on electron transfer. We propose that Cah3 promotes proton removal from the Mn complex by locally providing HCO(3)(-), which may function as proton carrier. Without Cah3, proton removal could become rate limiting during O(2) formation and thus, limit water oxidation under high light. Our results underlie the general importance of proton release at the donor side of PSII during water oxidation.

Reconstitution of Mn-depleted PSII particles with synthetic binuclear Mn complexes (one Mn(II)(2) complex and one Mn(IV)(2) complex) was examined. In both cases the electron-transfer rates in the reconstituted systems were found to be up to 75-82% of that measured in native PSII but the oxygen evolution activity remained lower (<5-40%). However, hydrogen peroxide was also produced by the reconstituted samples. These samples therefore represent a new type of reconstituted PSII that generates hydrogen peroxide as the final product in reconstituted PSII centers.

The hypothesis presented here for proton transfer away from the water oxidation complex of Photosystem II (PSII) is supported by biochemical experiments on the isolated PsbO protein in solution, theoretical analyses of better understood proton transfer systems like bacteriorhodopsin and cytochrome oxidase, and the recently published 3D structure of PS II (Pdb entry 1S5L). We propose that a cluster of conserved glutamic and aspartic acid residues in the PsbO protein acts as a buffering network providing efficient acceptors of protons derived from substrate water molecules. The charge delocalization of the cluster ensures readiness to promptly accept the protons liberated from substrate water. Therefore protons generated at the catalytic centre of PSII need not be released into the thylakoid lumen as generally thought. The cluster is the beginning of a localized, fast proton transfer conduit on the lumenal side of the thylakoid membrane. Proton-dependent conformational changes of PsbO may play a role in the regulation of both supply of substrate water to the water oxidizing complex and the resultant proton transfer.

A protective effect of bicarbonate (BC) against extraction of the extrinsic proteins, predominantly the Mn-stabilizing protein (PsbO protein), during treatment of Photosystem II (PS II) membrane fragment from pea with 2 M urea, and at low pH (using incubation in 0.2 M glycine-HCl buffer, pH 3.5 or 0.5 M citrate buffer, pH 4.0-4.5) was detected. It was shown that the extraction of the proteins with Mw 24 kDa (PsbP protein) and 18 kDa (PsbQ protein) by the use of highly concentrated solutions of NaCl does not depend on the presence of BC in the medium. An optimal concentration of BC at which it produces the maximum protecting effect was shown to be between 1 mM and 10 mM. The addition of formate did not influence the protein extraction but it reduced the stabilizing effect of BC. Independence of the stabilizing effect on the presence of the functionally active Mn within the water-oxidizing complex indicates that the protecting effect of BC is not related to its interaction with Mn ions. The fact that there is a preferable sensitivity of the PsbO protein to the absence of BC in the medium during all the treatments makes it possible to suggest that either BC interacts directly with the PsbO protein or it binds to some other sites within PS II and this binding facilitates the preservation of the native structure of this protein.