Nonphotosynthetic reduction of the intersystem electron transport chain of chloroplasts following heat stress. Steady-state rate

The consequence of elevated temperatures in the range of 39-51 degrees C on the steady-state rate of light-induced electron transport through photosystem I (PSI) supported by stromal reductants was studied in intact barley leaves using photoacoustic and chlorophyll fluorescence techniques. Measurable electron flow through PSI in diuron-treated leaves occurred only after exposure to temperatures above 37 degrees C. The steady-state rate of the above diuron-insensitive electron flow with methyl viologen as electron acceptor was estimated to be 3.7 mu eq m-2 s-1 or 0.018 mu eq mumol chlorophyll-1 s-1 in leaves exposed for 5 min to 45 degrees C.     

Stabilization of oxygen evolution and primary electron transport reactions in photosystem II against heat stress with glycinebetaine and sucrose

The protective action of co-solutes, such as sucrose and glycinebetaine, against the thermal inactivation of photosystem II function was studied in untreated and Mn-depleted photosystem II preparations. It was shown that, in addition to the reactions that depend on the oxygen evolving activity of the photosystem, those that implicate more intimately the reaction center itself are protected by high concentrations of osmolytes. However, the temperature required to inhibit oxygen evolution totally in the presence of osmolytes is lower than that required to eliminate reactions, such as P680 (primary electron donor in photosystem II) photo-oxidation and pheophytin photo reduetion, which only involve charge separation and primary electron transport processes. The energy storage measured from the thermal dissipation yield during photoacoustic experiments and the yield of variable fluorescence are also protected to a significant degree (up to 30%) at temperatures at which oxygen evolution is totally inhibited. It is suggested that a cyclic electron transport reaction around photosystem II may be preserved under these conditions and may be responsible for the energy storage measured at relatively high temperatures. This interpretation is also supported by thermoluminescence data involving the recombination between reduced electron acceptors and oxidized electron donors at - 30 and - 55 °C. The data also imply that a high concentration of osmolyte allows the stabilization of the photosystem core complex together with the oxygen-evolving complex. The stabilization effect is understood in terms of the minimization of protein-water interactions as proposed by the theory of Arakawa and Timasheff (Biophys. J., 47 (1985) 411--414).     

A radical-anion chain mechanism following dissociative electron transfer reduction of the model prostaglandin endoperoxide, 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]heptane

The model prostaglandin endoperoxide, 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]heptane (3), was investigated in N,N-dimethylformamide at a glassy carbon electrode using various electrochemical techniques. Reduction of 3 occurs by a concerted dissociative electron transfer (ET) mechanism. Electrolysis at -1.6 V yields 1,3-diphenyl-cyclopentane-cis-1,3-diol in 97% by a two-electron mechanism; however, in competition with the second ET from the electrode, the resulting distonic radical-anion intermediate undergoes a beta-scission fragmentation. The rate constant for the heterogeneous ET to the distonic radical-anion is estimated to occur on the order of 2 x 10(7) s(-1). In contrast, electrolyses conducted at potentials more negative than -2.1 V yield a mixture of primary and secondary electrolysis products including 1,3-diphenyl-cyclopentane-cis-1,3-diol, 1,3-diphenyl-1,3-propanedione, trans-chalcone and 1,3-diphenyl-1,3-hydroxypropane by a mechanism involving less than one electron equivalent. These observations are rationalized by a catalytic radical-anion chain mechanism, which is dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for beta-scission fragmentation from oxygen-centred biradicals and analogous distonic radical-anions is presented and the results of the calculations provide insight into the reactivity of prostaglandin endoperoxides.     

The oxidation/reduction kinetics of the plastoquinone pool controls the appearance of the I-peak in the O-J-I-P chlorophyll fluorescence rise: effects of various electron acceptors

Quantitative analysis of the fluorescence induction (FI) rise was used in this study to elucidate the complex effects of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) on thylakoids. Reduced TMPD molecules, responsible for the ADRY agent effect, caused an increase in the amplitude of the O-J rise. Also, only oxidized TMPD molecules were shown to have the ability to bind the Q(B) pocket of photosystem II (PSII). On the other hand, the I-P rise was slowed in proportion with the oxidized TMPD concentration, inducing the clear appearance of the I-peak. While this property was previously thought to be unique to TMPD, this study shows that some artificial electron acceptors of PSII, silicomolybdate, 2,5-dichloro-p-benzoquinone, and phenyl-p-benzoquinone, have a similar effect. These results demonstrated a major role of the oxido-reduction kinetics of the PQ-pool in the resolution of J-I and I-P phases in the FI of isolated thylakoids.     

The optimal size of the exchange hole and reduction to one-particle Hamiltonians

We use the concept of the exchange hole introduced by Slater to bound the energy of atoms, molecules, and other systems interacting by Coulomb forces from below by one-particle Hamiltonians with an effective screening potential and an exchange hole around each electron. Interestingly enough the optimal size of the exchange hole is smaller than Slater proposed: the best lower bound is obtained when the exchange hole carries charge 1/2 instead of 1. To highlight the quality of our estimate we show that the Dirac exchange energy with a slightly different constant bounds the exchange–correlation energy from below, an estimate previously derived by Lieb and later improved by Lieb and Oxford. Acknowledgements The authors thank Walter Farkas and Christian Hainzl for helpful discussions. Financial support by the European Union under its IHP network program, grant HPRN-CT-2002-00277, is gratefully acknowledged.© By the authorsThe author A.M. Klaus Müller is deceased     

Mechanism study on the origin of delayed fluorescence by an analytic modeling of the electronic reflux for photosynthetic electron transport chain

A mathematical-physical analysis model, which describes individually the electronic reflux of several significant components in the photosynthesis electron transport chain, was firstly developed. The process of electrons flowing back to the oxidized reaction center P(680)(+) was simulated by a series of photochemical reaction equations, resulting in getting the linked differential equations of delayed fluorescence (DF) intensity. MATLAB provided a computationally efficient method to solve these linked equations. Simulations based on this model showed that the decay kinetics of DF accord with double exponential. DF components decaying in the millisecond range (fast phase) are related to the charge recombination of P(680)(+) and Q(A)(-). The components decaying in the seconds range are associated with the recombination of P(680)(+) with Q(B)(2-). The developed model was tested in maize leaves treated with different electron blockers to induce changes in photosynthesis electron transport chain. The experimental results demonstrated that the developed model can accurately determine the regulatory effects of electron blockers on photosynthesis electron transport chain. Therefore, the model presented here could be potentially useful for studying the electron transfer in plant. It also provides an experimental workbench for testing hypotheses as to the underlying mechanism controlling the change for different phases of DF.     

Chromatographic characterization of substance P endopeptidase in the rat brain reveals affected enzyme activity following heat stress

This paper describes a study of substance P endopeptidase (SPE)-like activity in various regions of the brain from male rats subjected to heat stress (HS). The enzyme activity was found to be affected in several brain areas including cerebellum, cerebral cortex, hippocampus, hypothalamus[sol ]thalamus and the spinal cord following HS. Significant increases in SPE activity were observed in, for example, hippocampus and the spinal cord. SPE-containing extracts from hippocampus were pooled and subsequently purified by size exclusion chromatography (using a Superdex 75 HR column) and by anion-exchange chromatography (using Resource Q column). The gel permeation chromatography separated the SPE-like activity into two fractions, one of which was suggested to be identical to neutral endopeptidase owing to its molecular size and inhibitory profile. The other active enzyme fraction behaved in conformity with SPE, previously identified in human cerebrospinal fluid. The activity of the purified fraction of these two enzymes was found to be increased (27%) in HS-treated animals.     

Dissolution of gold in the DMSO—RX systems. The concept of a donor-acceptor electron transport system

Summary The liquid phase oxidation of gold in donor-acceptor organic and aqueous-organic media has been studied. The compounds [AuCl(Me₂S)], [AuBr(Me₂S)], [AuBr₃(Me₂S)], [Me₃S][AuBr₄], [Me₃S][AuBr₄(Me₂S)]·H₂O, [Me₃SO]-[AuBr₄]·H₂O, [Me₃S][Au₂Br₇(Me₂S)₂]·3H₂O, [Me₃S]₂-[Au₂Br₈]·2DMSO·H₂O, [Me₂(Bu)SO][AuBr₄]·H₂O and [Me₃S]Br were isolated by dissolution of Au⁰ in DMSO-RX mixtures (R = H or Bu; X = Cl or Br). The products were characterized by elemental analysis and i.r. spectroscopy. The nature of the Au⁰-DMSO-RX systems and the oxidant species are discussed in terms of a newly-developed concept of donor-acceptor electron transport (DAET) systems.     

The inhibitory effect of the artificial electron donor system, phenazine methosulfate-ascorbate, on bacterial transport mechanisms.

The artificial electron donor system, phenazine methosulfate (PMS)-ascorbate, inhibited active transort of solutes in Pseudomonas aeruginosa irrespective of whether the active transport systems were shock sensitive or shock resistant. N,N,N',N'-tetramethylphenylenediamine could be substituted for PMS but a higher concentration was required. PMS-ascorbate also inhibited active transport in several other bacterial species with the exception of Escherichia coli and of a nonpigmented strain of Serratia marcescens. PMS-ascorbate previously has been shown to energize active transport in isolated membrane vesicles, even those prepared from the same bacterial species in whose intact cells active transport was inhibited. The apparent Km of glucose active transport in untreated cells of P. aeruginosa was 40 micron while the Km of glucose transport in cells incubated with PMS-ascorbate was 25 mM, and PMS-ascorbate had no effect on efflux of accumulated glucose. These results strongly suggested that facilitated diffusion resulted upon exposure of the cells to PMS- ascorbate. Thus, PMS-ascorbate appeared to have an uncoupler-like effect on cells of P. aeruginosa. The experimental data also pointed out that there are fundamental differences between the response of intact cells and membrane vesicles to exogenous electron donors.     

Redox state of the photosynthetic electron transport chain in wild-type and mutant leaves of Arabidopsis thaliana: Impact on photosystem II fluorescence

In addition to the photosynthetic linear electron transport, several alternative electron transport routes exist in thylakoids of higher plants. The plastoquinone (PQ) pool acts as a common electron carrier in these pathways. In the cyclic electron flow around photosystem I (PSI), reduced ferredoxin is used by the ferredoxin-quinone reductase (FQR) to reduce the PQ pool. Chlororespiratory pathway consists in the reduction of the PQ pool by the NAD(P)H dehydrogenase (NDH). These alternative pathways and their role in photosynthesis are still not fully understood. In the present study, the accumulation kinetics of quinone acceptors was measured by fluorescence induction in leaves of Arabidopsis thaliana wild-type and mutants altered in alternative electron pathways after various light- and dark-adaptation conditions. Results show that NDH activity can be probed by fluorescence induction during light-to-dark transition of plants. Also, the activity of FQR pathway did not affect directly the FI kinetics. However, the accumulation kinetics of reduced PQ under actinic light was dependant on the redox state of PSI acceptors prior to illumination.     

Modeling hole transfer in DNA. II. Molecular basis of charge transport in the DNA chain

A theoretical multiconfigurational second-order perturbation method, CASPT2, has been employed to determine the binding energies and electronic couplings for all pairs of stacked canonical nucleobases in the standard conformation of the B-DNA. The existence and relevance of conical intersections mediating the hole transfer process has been shown in different systems in vacuo and, by using hybrid QM/MM techniques, in a more realistic biological environment, formed by a double helix of 18 oligomers of DNA surrounded by water molecules. The present results support, therefore, the cooperative micro-hopping mechanism proposed in a previous work for the migration of the hole between adjacent π-stacked 2′-deoxycytidine 5′-monophosphate (dCMP) or alternate 2′-deoxyadenosine 5′-monophosphate and dCMP oligonucleotides.     

Electrolyte tuning of electrode potentials: the one electron vs. two electron reduction of anthraquinone-2-sulfonate in aqueous media

The electrode potentials of quinone redox centres in aqueous solutions can be tuned by varying the electrolyte cation identity. The phenomenon is due to the ion pairing effect of the tetra-n-butylammonium cation with the semiquinone intermediate species.     

The electron as a protecting group. 3. Generation of acenaphthyne radical anion and the determination of the heat of formation of a strained cycloalkyne.

Acenaphthyne dicarboxylate (12) was transferred into the gas phase from solution via electrospray ionization and subsequently was sequentially fragmented in a Fourier transform mass spectrometer to afford acenaphthyne radical anion (9). Structural confirmation of 9 was achieved by converting it to acenaphthenone enolate (13) and demonstrating that this species is identical to the ion produced upon deprotonation of acenaphthenone (5). The reactivity of 9 was explored, and since an electron can serve as a protecting group, we were able to measure the heat of hydrogenation (98 +/- 4 kcal mol(-1)) and the heat of formation (160 +/- 4 kcal mol(-1)) of acenaphthyne (1) via the application of a thermodynamic cycle. Its strain energy (68 kcal mol(-1)) and acenaphthylene's (10H) first and second C-H bond dissociation energies (117 +/- 4 and 84 +/- 2 kcal mol(-1)) also were obtained. Ab initio and density functional theory calculations were carried out on the species of interest to explore their geometries and energetics. Our results were interpreted in comparison to cyclopentyne, and its predicted heat of formation (98 kcal mol(-1)) and strain energy (59 kcal mol(-1)) are reported.     

Isotope effect in gas transport. II. Effect of the glass transition on the size of the free volume element

A diffusive gas-transport isotope effect is used to estimate the size of the free volume element above and below the glass transition for poly(ethyl methacrylate) and poly(vinyl fluoride). The cavity size, as measured by the hydrogen probe molecule, is apparently larger in the glassy region for both polymers than it is above T_g. It is postulated that the number of free volume elements essentially decreases below T_g, so that the total free volume, which is the sum of all such elements, is smaller below the glass transition, in accord with density measurements on bulk polymers.     

The flash-quench technique in protein-DNA electron transfer: reduction of the guanine radical by ferrocytochrome c

Electron transfer from a protein to oxidatively damaged DNA, specifically from ferrocytochrome c to the guanine radical, was examined using the flash-quench technique. Ru(phen)2dppz2+ (dppz = dipyridophenazine) was employed as the photosensitive intercalator, and ferricytochrome c (Fe3+ cyt c), as the oxidative quencher. Using transient absorption and time-resolved luminescence spectroscopies, we examined the electron-transfer reactions following photoexcitation of the ruthenium complex in the presence of poly(dA-dT) or poly(dG-dC). The luminescence-quenching titrations of excited Ru(phen)2dppz2+ by Fe3+ cyt c are nearly identical for the two DNA polymers. However, the spectral characteristics of the long-lived transient produced by the quenching depend strongly upon the DNA. For poly(dA-dT), the transient has a spectrum consistent with formation of a [Ru(phen)2dppz3+, Fe2+ cyt c] intermediate, indicating that the system regenerates itself via electron transfer from the protein to the Ru(III) metallointercalator for this polymer. For poly(dG-dC), however, the transient has the characteristics expected for an intermediate of Fe2+ cyt c and the neutral guanine radical. The characteristics of the transient formed with the GC polymer are consistent with rapid oxidation of guanine by the Ru(III) complex, followed by slow electron transfer from Fe2+ cyt c to the guanine radical. These experiments show that electron holes on DNA can be repaired by protein and demonstrate how the flash-quench technique can be used generally in studying electron transfer from proteins to guanine radicals in duplex DNA.