Photochemical properties of photosystem 1 immobilized in a mesoporous semiconductor matrix

The pigment-protein complex of photosystem 1 (PS1) isolated from cyanobacterium Synechocystis sp. PCC 6803 has been adsorbed on a solid mesoporous film made from TiO₂ nanoparticles. was on The TiO₂ film supported on a glass substrate with a surface area of 1 cm² adsorbs up to 0.045 nmol of PS1. PS1 molecules are distributed in the pores of the mesoporous support. Immobilization has an insignificant effect on the optical and photochemical properties of PS1. A reversible photoinduced EPR signal from the oxidized primary electron donor P700 of immobilized PS1 has been detected. It has been shown by photoelectrochemical methods that the photoexcitation of PS1 results in electron injection from PS1 to the conduction band of TiO₂.     

Proton Transport in the Outer-Membrane Flavocytochrome Complex Limits the Rate of Extracellular Electron Transport

The microbial transfer of electrons to extracellularly located solid compounds, termed extracellular electron transport (EET), is critical for microbial electrode catalysis. Although the components of the EET pathway in the outer membrane (OM) have been identified, the role of electron/cation coupling in EET kinetics is poorly understood. We studied the dynamics of proton transport associated with EET in an OM flavocytochrome complex in Shewanella oneidensis MR-1. Using a whole-cell electrochemical assay, a significant kinetic isotope effect (KIE) was observed following the addition of deuterated water (D₂O). The removal of a flavin cofactor or key components of the OM flavocytochrome complex significantly increased the KIE in the presence of D₂O to values that were significantly larger than those reported for proton channels and ATP synthase, thus indicating that proton transport by OM flavocytochrome complexes limits the rate of EET.     

Proton Transport in the Outer‐Membrane Flavocytochrome Complex Limits the Rate of Extracellular Electron Transport

The microbial transfer of electrons to extracellularly located solid compounds, termed extracellular electron transport (EET), is critical for microbial electrode catalysis. Although the components of the EET pathway in the outer membrane (OM) have been identified, the role of electron/cation coupling in EET kinetics is poorly understood. We studied the dynamics of proton transport associated with EET in an OM flavocytochrome complex in Shewanella oneidensis MR‐1. Using a whole‐cell electrochemical assay, a significant kinetic isotope effect (KIE) was observed following the addition of deuterated water (D₂O). The removal of a flavin cofactor or key components of the OM flavocytochrome complex significantly increased the KIE in the presence of D₂O to values that were significantly larger than those reported for proton channels and ATP synthase, thus indicating that proton transport by OM flavocytochrome complexes limits the rate of EET.     

Effect of different Fe(III) compounds on photosynthetic electron transport in spinach chloroplasts and on iron accumulation in maize plants

Synthesis and spectral characteristics of [Fe(nia)₃Cl₃] and [Fe(nia)₃(H₂O)₂](ClO₄)₃ are described. The effect of these compounds as well as of FeCl₃·6H₂O on photosynthetic electron transport in spinach chloroplasts was investigated using EPR spectroscopy. It was found that due to the interaction of these compounds with tyrosine radicals situated at the 161^st position in D₁ (Tyr_Z) and D₂ (Tyr_D) proteins located at the donor side of photosystem (PS) II, electron transport between the photosynthetic centres PS II and PS I was interrupted. In addition, the treatment with [Fe(nia)₃(H₂O)₂](ClO₄)₃ resulted in a release of Mn(II) from the oxygen evolving complex situated on the donor side of PS II. Moreover, the effect of the Fe(III) compounds studied on some production characteristics of hydroponically cultivated maize plants and on Fe accumulation in plant organs was investigated. In general, the production characteristic most inhibited by the presence of Fe(III) compounds was the leaf dry mass and [Fe(nia)₃(H₂O)₂](ClO₄)₃ was found to be the most effective compound. The highest Fe amount was accumulated in the roots, and the leaves treated with Fe(III) compounds contained more Fe than the stems. The treatment with FeCl₃·6H₂O caused the most effective translocation of Fe into the shoots. Comparing the effect of nicotinamide complexes, [Fe(nia)₃(H₂O)₂](ClO₄)₃ was found to facilitate the translocation of Fe into the shoots more effectively than [Fe(nia)₃Cl₃]. This could be connected with the different structure of these complexes. [Fe(nia)₃(H₂O)₂](ClO₄)₃ has ionic structure and, in addition, coordinated H₂O molecules can be easily substituted by other ligands. Dedicated to Professor Milan Melník on the occasion of his 70th birthday.     

A mathematical model of electron and proton transport in oxygenic photosynthetic systems

Results of simulation of electron and proton transport in higher plant chloroplasts, taking into account the lateral heterogeneity of their lamellar system, were summarized. The existence of heterogeneous lateral profiles of pH inside thylakoids and in gaps between the thylakoids of grana was predicted. The basic kinetic relationships were simulated for photoinduced redox transformations of P₇₀₀, the primary electron donor for PS1. It was shown that, along with changes in pH inside thylakoids, an essential role in controlling the electron transport in chloroplasts can belong to alkalinization of the gap between thylakoids of grana, caused by deceleration of diffusion of hydrogen ions from the stroma to the PS2 complexes in thylakoids of grana.     

Kinetics and mechanism of adenosine 5′-triphosphate hydrolysis catalyzed by the Cu2+ ion: The role of conformation and the catalytic effect of the OH− ion

The kinetics of 5′-ATP hydrolysis catalyzed by the Cu²⁺ ion has been investigated by HPLC in the pH range 5.6–7.8 at 25°C. Two series of experiments differing in the initial [Cu · ATP]₀ (1: 1) concentration have been carried out. The reaction was being conducted up to ≈40% ATP conversion. The (CuATP^2?)₂OH^??ub;DOH^??ub; complex, which consists of two monomeric Cy(CuATP^2?) molecules (in which the N7 atom and the γ-phosphate group are coordinated to Cu²⁺), is responsible for the formation of CuADP^? + P_i (P_i is an inorganic phosphate). The highest possible DOH^? concentration at a given pH is reached at the initial stage of hydrolysis. The pH value at which the highest initial rate of ADP formation is reached (pH_max (w _{0, ADP})) decreases as the D concentration increases. At pH > pH_max, the decrease in the ADP formation rate in the course of the processes is pH-independent and, once an ATP conversion of 20–26% is reached, hydrolysis proceeds in a steady-state regime such that ADP and AMP form from ATP by parallel reactions. The participation of the OH^? ion in the catalysis of the formation of hydrolysis intermediates is considered.     

5-Bromo- and 3,5-dibromo-2-hydroxy-N-phenylbenzamides — inhibitors of photosynthesis

5-Bromo-(Br-PBA) and 3,5-dibromo-2-hydroxy-N-phenylbenzamides (Br₂-PBA) inhibited photosynthetic electron transport (PET) and their inhibitory efficiency depended on the compound lipophilicity as well as on the electronic properties of the R substituent in the N-phenyl moiety. Br-PBA showed higher PET inhibiting activity than Br₂-PBA with the same R substituent. The most effective inhibitors in the tested series were the derivatives with R = 3-F (Br-PBA; IC₅₀ = 4.3 μmol dm^?3) and R = 3-Cl (Br₂-PBA; IC₅₀ = 8.6 μmol dm^?3). Bilinear dependence of the PET inhibiting activity on the lipophilicity of the compounds as well as on the Hammett constant, σ, of the R substituent was observed for both investigated series. Using EPR spectroscopy it was found that the site of action of the tested compounds in the photosynthetic apparatus is situated on the donor side of PS 2, in D^· or in the Z^·/D^· intermediates. Interaction of the studied compounds with chlorophyll a and aromatic amino acids present in the pigment-protein complexes mainly in photosystem 2 was documented by fluorescence spectroscopy.     

Assessing the Applicability of Singlet Oxygen Photosensitizers in Leaf Studies

Singlet oxygen (¹O₂) is of special interest in plant stress physiology. Studies focused on internal, chlorophyll‐mediated production are often complemented with the use of artificial ¹O₂ photosensitizers. Here, we report a comparative study on the effects of Rose Bengal (RB), Methylene Violet (MVI), Neutral Red (NR) and Indigo Carmine (IC). These were infiltrated into tobacco leaves at concentrations generating the same fluxes of ¹O₂ in solution. Following green light‐induced ¹O₂ production from these dyes, leaf photosynthesis was characterized by Photosystem (PS) II and PSI electron transport and oxidative damage was monitored as degradation of D1, a PSII core protein. Cellular localizations were identified on the basis of the dyes’ fluorescence using confocal laser scanning microscopy. We found that RB and NR were both localized in chloroplasts but the latter had very little effect, probably due to its pH‐dependent photosensitizing. Both RB and intracellular, nonplastid MVI decreased PSII electron transport, but the effect of RB was stronger than that of MVI and only RB was capable of damaging the D1 protein. Intercellularly localized IC had no significant effect. Our results also suggest caution when using RB as photosensitizer because it affects PSII electron transport.     

Natural products from Pluchea sagittalis act as inhibitors of photosynthesis in vitro

Four compounds were isolated from roots and aerial parts of Pluchea sagittalis (Asteraceae), 3, 5-dihydroxy-6, 7, 3′, 4′-tetramethoxiflavunol (1), 5-hydroxymethylfurfural (2), 3, 4-dimethoxybenzaldehyde (3) and 2, 3, 4-trihydroxybenzaldeyde (4). Their herbicidal potential was detected by polarographic techniques. All of them inhibited the non-cyclic electron transport on basal, phosphorylating and uncoupled conditions from H₂O to methylviologen (MV); thus, they act as Hill reaction inhibitors. Studies on fluorescence of chlorophyll a (ChL a) indicated they have different modes of interaction and inhibition sites on the photosystem II electron transport chain; 1–3 have interacted with the acceptor side while 4 has interacted at the donor side.     

BASF 13.338 INDUCED CHANGES IN STRUCTURE and FUNCTION OF THE PHOTOSYNTHETIC APPARATUS OF WHEAT SEEDLINGS

Abstract— Growing wheat seedlings in the presence of BASF 13.338 [4-chloro-5-dimethylamino-2-phenyl-3(2H)pyridazinone], a PS II inhibitor of the pyridazinone group, brought about notable changes in the structure and functioning of photosynthetic apparatus. In BASF 13.338 treated plants, there was a decrease in the ratio of Chi a/Chl b, an increase in xanthophyll/carotene ratio and an increase in the content of Cyt b 559 (HP + LP). Chl/p700 ratio increased when measured with the isolated chloroplasts but not with the isolated PS I particles of the treated plants. The SDS-PAGE pattern of chloroplast preparations showed an increase in the CPII/CP I ratio. The F685/F740 ratio in the emission spectrum of chloroplasts at -196°C increased. The difference absorption spectrum of chloroplasts between the control and the treated plants showed a relative increase of a chlorophyll component with a peak absorption at 676 nm and a relative decrease of a chlorophyll component with a peak absorption at 692 nm for the treated plants. The excitation spectra of these chloroplast preparations were similar. Chloroplasts from the treated plants exhibited a greater degree of grana stacking as measured by the chlorophyll content in the 10 K pellet. The rate of electron transfer through photosystem II at saturating light intensity in chloroplast thylakoids isolated from the treated plants increased (by 50%) optimally at treatment of 125 μM BASF 13.338 as compared to the control. This increase was accompanied by an increase in (a) I₅₀ value of DCMU inhibition of photosystem II electron transfer; (b) the relative quantum yield of photosystem II electron transfer; (c) the magnitude of C550 absorbance change; and (d) the rate of carotenoid photobleaching. These observations were interpreted in terms of preferential synthesis of photosystem II in the treated plants. The rate of electron transfer through photosystems I and through the whole chain (H₂O → methyl viologen) also increased, due to an additional effect of BASF 13.338, namely, an increase in the rate of electron transfer through the rate limiting step (between plastoquinol and cytochrome f). This was linked to an enhanced level of functional cytochrome f. The increase in the overall rate of electron transfer occurred in spite of a decrease in the content of photosystem I relative to photosystem II. Treatment with higher concentrations (> 125 μM) of BASF 13.338 caused a further increase in the level of cytochrome f, but the rate of electron transfer was no greater than in the control. This was due to an inhibition of electron transfer at several sites in the chain.     

Photosynthesis-inhibiting effects of 2-benzylsulphanylbenzimidazoles in spinach chloroplasts

Inhibition of photosynthetic electron transport (PET) in spinach chloroplasts by nineteen 2-benzylsulphanylbenzimidazoles (BZA) was studied. BZA were found to inhibit photosynthetic electron transport (PET) and for their inhibitory efficiency, electronic properties of the R substituent on the benzyl moiety are decisive. The PET inhibiting activity of the studied BZA expressed as IC50 varied in the range from 28.5 ??M (R = 3,5-(CF₃)₂) to 394.5 ??M (R = 2,4-(NO₂)₂). For compounds with R = H, 4-CH₃, 3-CH₃, 3-OCH₃, 4-F, 3-F, 4-Cl, 3-Cl, 2-Cl, 4-Br, 3-Br, 3,4-F₂, 3,4-Cl₂, 3-CF₃, 3,5-(CF₃)₂ linear increase of the inhibitory activity with the increasing value of the substituent??s ?? constant up to 0.86 was observed. Further increase of the ?? constant resulted in a sharp activity decrease of the corresponding compounds (R = 2-F-6-Cl, 2-NO₂, 3,5-(NO₂)₂, 2,4-(NO₂)₂). Using EPR spectroscopy and an artificial electron donor diphenyl carbazide it was found that the site of BZA action in the photosynthetic apparatus is situated on the donor side of PS 2, prior to the Z^·/D^· intermediate.     

Time-programmable pH: decarboxylation of nitroacetic acid allows the time-controlled rising of pH to a definite value

In this report it is shown that nitroacetic acid 1 (O₂NCH₂CO₂H) can be conveniently used to control the pH of a water solution over time. Time-programmable sequences of the kind pH_1(high)–pH_2(low)–pH_3(high) can be achieved, where both the extent of the initial pH jump (pH_1(high)–pH_2(low)) and the time required for the subsequent pH rising (pH_2(low)–pH_3(high)) can be predictably controlled by a judicious choice of the absolute and relative concentrations of the reagents (acid 1 and NaOH). Successive pH_1(high)–pH_2(low)–pH_3(high) sequences can be obtained by subsequent additions of acid 1. As a proof of concept, the method is applied to control over time the pH-dependent host–guest interaction between alpha-cyclodextrin and p-aminobenzoic acid.

Predictable and time-programmable sequences of the kind pH_1(high)–pH_2(low)–pH_3(high) in water solution are obtained by a judicious choice of the concentration of nitroacetic acid undergoing decarboxylation.     

The electrode responses of a tungsten bronze electrode differ in potentiometry and voltammetry and give access to the individual contributions of electron and proton transfer

A tungsten wire covered with Na_0.75WO₃ acts in potentiometry as a reversible pH electrode having a pH dependent open-circuit potential E_ocp with nernstian slope. The mid-peak potential E_mp of cyclic voltammograms also depends on pH. At low pH (e.g., pH 2) and slow scan rates (e.g., 2 mV s^–1) the voltammetric response is almost completely reversible. At higher pH and faster scan rates, the voltammetric systems exhibit features of increasing irreversibility. Under the conditions of reversibility, the E_ocp and E_mp differ significantly. E_ocp is determined by the proton transfer at the electrode surface; whereas E_mp is determined by the electron transfer equilibrium tungsten(VI)/tungsten(V) and the proton transfer at the electrode surface. The difference between E_ocp and E_mp provides the individual thermodynamic contributions of electron and proton transfer to the overall pH dependent redox electrode. This is the first time that both contributions can be separated for an insertion electrochemical system (thin surface layer). It is also shown for the first time that the mechanism of an ion-sensitive electrode can differ in potentiometry and voltammetry.     

Three‐Way Cooperativity in d8 Metal Complexes with Ligands Displaying Chemical and Redox Non‐Innocence

Reversible proton‐ and electron‐transfer steps are crucial for various chemical transformations. The electron‐reservoir behavior of redox non‐innocent ligands and the proton‐reservoir behavior of chemically non‐innocent ligands can be cooperatively utilized for substrate bond activation. Although site‐decoupled proton‐ and electron‐transfer steps are often found in enzymatic systems, generating model metal complexes with these properties remains challenging. To tackle this issue, we present herein complexes [(cod_?H)M(μ‐L^2?) M (cod_?H)] (M=Pt^II, [ 1 ] or Pd^II, [ 2 ], cod=1,5‐cyclooctadiene, H₂L=2,5‐di‐[2,6‐(diisopropyl)anilino]‐1,4‐benzoquinone), in which cod acts as a proton reservoir, and L^2? as an electron reservoir. Protonation of [ 2 ] leads to an unusual tetranuclear complex. However, [ 1 ] can be stepwise reversibly protonated with up to two protons on the cod_?H ligands, and the protonated forms can be stepwise reversibly reduced with up to two electrons on the L^2? ligand. The doubly protonated form of [ 1 ] is also shown to react with OMe^? leading to an activation of the cod ligands. The site‐decoupled proton and electron reservoir sources work in tandem in a three‐way cooperative process that results in the transfer of two electrons and two protons to a substrate leading to its double reduction and protonation. These results will possibly provide new insights into developing catalysts for multiple proton‐ and electron‐transfer reactions by using metal complexes of non‐innocent ligands.     

Hydrophobic “pocket” on photosynthesizing membrane near photosystem II

Studies concerning the effect of 2-alkyl pyridine N-oxides with different substituents on electron transfer, phosphorylation, and light-dependent proton transport in photosynthesizing membranes of chloroplasts were conducted. It is determined that 2-alkyl pyridine N-oxides with short alkyl chains stimulate the Hill reaction and light-dependent proton transport inside chloroplasts. Compounds having alkyl residues with 6-10 carbon atoms in the chain inhibit electron transport, ADP phosphorylation, and reduction of ferrocyanide in thylakoids. A conclusion is drawn on the presence of the hydrophobic “pocket,” which is of importance for organization of the electron-transport chain of chloroplasts, near photosystem II.     

Nitrosation of Phenolic Compounds: Effects of Alkyl Substituents and Solvent

Summary.  Nitrosation reactions of phenol, o-cresol, 2,6-dimethylphenol, o-tert-butylphenol, 2-hydroxyacetophenone, and 2-allylphenol in water and water/acetonitrile were studied. Kinetic monitoring of the reactions was accomplished by spectrophotometric analysis of the nitrosated products at 345 nm. The dominant reaction was C-nitrosation via a mechanism consisting of an attack on the nitrosatable substrate by NO⁺/NO₂H₂ ⁺ followed by a slow proton transfer. The values of the rate constants of phenolic C-nitrosation were increased by electron donating substituents, and a good Hammett correlation was observed with ρ = −6.1. The results also revealed the strong effect of pH and the permitivity of the reaction medium on the rate constant, whose maximum values were observed for pH ≈ 3, decreasing strongly for higher pH values. The study in water/acetonitrile with up to 25% acetonitrile showed that it is possible to inhibit the reaction strongly by increasing the percentage of the organic component. The conclusions drawn show that (i) it is possible to predict the rate of nitrosation of phenolics as a function of the meta-substituents on the phenol ring and (ii) the nitrosation of phenolics can be strongly inhibited by increasing the pH of the reaction medium as well as by lowering its dielectric constant. Received July 13, 2001. Accepted (revised) September 18, 2001     

Fabrication and characterization of Grass pea (Lathyrus sativus) protein isolate-Alyssum homolocarpum seed gum complex coacervate

The present work was carried out to fabricate Grass pea (Lathyrus sativus) protein isolate (GPPI) and Alyssum homolocarpum seed gum (AHSG) complex coacervate and investigate its morphological and structural properties. Formation of complex coacervates were studied by zeta potentiometry and turbidimetric analysis as function of different pHs (7.0–2.0) and GPPI to AHSG ratios (3:1 to 1:3). The critical pH values associated with the formation of soluble (pH_c) and insoluble (pH_φ1) complexes, and complete dissociation (pH_φ2) at GPPI to AHSG ratio (1:1) were found to be 4.8, 4.0, and 2.5, respectively. Formation of insoluble complex coacervate was maximum at pH 3.2 and GPPI to AHSG ratio of 1:1, where the highest yield (69%) was observed. Scanning electron microscopic (SEM) demonstrated that GPPI-AHSG complex had a rough branched-like structure. Results also showed that the complexation between GPPI and AHSG were formed through the electrostatic interaction and hydrogen bonding. Circular dichroism spectroscopy (CD) indicated that the β-sheet and random coil content in GPPI increased when AHSG molecules were added to protein solution. The fading of pure peaks of GPPI and AHSG in X-Ray diffraction (XRD) patterns of GPPI-AHSG complexes confirmed the alterations in the physical state of mixture from crystalline to amorphous. GPPI-AHSG complex coacervates had lower weight loss compared to individual GPPI and AHSG.     

Deciphering Reversible Homogeneous Catalysis of the Electrochemical H2 Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects**

Nickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so-called DuBois’ catalysts) capable of bidirectional/reversible electrocatalytic oxidation and production of dihydrogen. This unique behaviour is directly linked to the presence of proton relays installed close to the metal center. We report here for the arginine derivative [Ni(P₂^CyN₂^Arg)₂]⁶⁺ on a mechanistic model and its kinetic treatment that may apply to all DuBois’ catalysts and show that it allows for a good fit of experimental data measured at different pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from balanced equilibria related to hydrogen uptake/evolution on one side and (metal)-hydride installation/capture on the other side, both controlled by concentration effects resulting from the presence of proton relays and connected by two square schemes corresponding to proton-coupled electron transfer processes. We show that the catalytic bias is controlled by the kinetic of the H₂ uptake/evolution step. Reversibility does not require that the energy landscape be flat, with redox transitions occurring at potentials up to 250 mV away for the equilibrium potential, although such large deviations from a flat energy landscape can negatively impacts the rate of catalysis when coupled with slow interfacial electron transfer kinetics.     

Nitrosation of Phenolic Compounds: Effects of Alkyl Substituents and Solvent

 Nitrosation reactions of phenol, o-cresol, 2,6-dimethylphenol, o-tert-butylphenol, 2-hydroxyacetophenone, and 2-allylphenol in water and water/acetonitrile were studied. Kinetic monitoring of the reactions was accomplished by spectrophotometric analysis of the nitrosated products at 345 nm. The dominant reaction was C-nitrosation via a mechanism consisting of an attack on the nitrosatable substrate by NO⁺/NO₂H₂ ⁺ followed by a slow proton transfer. The values of the rate constants of phenolic C-nitrosation were increased by electron donating substituents, and a good Hammett correlation was observed with ρ = −6.1. The results also revealed the strong effect of pH and the permitivity of the reaction medium on the rate constant, whose maximum values were observed for pH ≈ 3, decreasing strongly for higher pH values. The study in water/acetonitrile with up to 25% acetonitrile showed that it is possible to inhibit the reaction strongly by increasing the percentage of the organic component. The conclusions drawn show that (i) it is possible to predict the rate of nitrosation of phenolics as a function of the meta-substituents on the phenol ring and (ii) the nitrosation of phenolics can be strongly inhibited by increasing the pH of the reaction medium as well as by lowering its dielectric constant.     

Effect of end group chemistry on surface molecular motion of monodisperse polystyrene films

The surface molecular motion of monodisperse polystyrene (PS) with various chain end groups was investigated on the basis of temperature‐dependent scanning viscoelasticity microscope (TDSVM). The surface glass transition temperatures, T_g^ss for the proton‐terminated PS (PS‐H) films with number‐average molecular weight, M_n of 4.9k–1,450k measured by TDSVM measurement were smaller than those for the bulk one, with corresponding M_ns, and the T_g^ss for M_n smaller than ca. 50k were lower than room temperature (293 K). In the case of M_n = ca. 50k, the T_g^ss for the α,ω‐diamino‐terminated PS (α,ω‐PS(NH₂)₂) and α,ω‐dicarboxy‐terminated PS (α,ω‐PS(COOH)₂) films were higher than that of the PS‐H film. On the other hand, the T_g^s for the α,ω‐perfluoroalkylsilyl‐terminated PS (α,ω‐PS(SiC₂C^F₆)₂) film with the same M_n was much lower than those for the PS films with all other chain ends. The change of T_g^s for the PS film with various chain end groups can be explained in terms of the depth distribution of chain end groups at the surface region.