Experimental and theoretical studies on the excess capacity of Photosystem II

It has been recently suggested that compensatory changes in Photosystem II (PS II) electron turnover rates can protect photosynthesis from photoinhibition [Behrenfeld et al. (1998) Photosynth Res 58: 259–268]. We have further explored this feature of PS II using a rate electrode for simultaneous measurements of the steady-state rate of oxygen evolution and the oxygen flash yield depending on the background irradiance in both control and photoinhibited algal cells of Chlorella Böhm. Theoretical simulations based on the two-electron gate model agree qualitatively with experimental data if we assume an increase of the electron turnover rate in the remaining functional PS II centers of the photoinhibited sample. Our results confirm the hypothesis that the compensatory effect enables cells to maintain the maximal rates of photosynthesis even in the presence of moderate photoinhibition (decrease of up to 50% in the number of functional centers) and that the effect originates from the inner capacity of electron transport through PS II. The origin of the compensatory effect is briefly discussed.     

Building Electron/Proton Nanohighways for Full Utilization of Water Splitting Catalysts

Low electron/proton conductivities of electrochemical catalysts, especially earth‐abundant nonprecious metal catalysts, severely limit their ability to satisfy the triple‐phase boundary (TPB) theory, resulting in extremely low catalyst utilization and insufficient efficiency in energy devices. Here, an innovative electrode design strategy is proposed to build electron/proton transport nanohighways to ensure that the whole electrode meets the TPB, therefore significantly promoting enhance oxygen evolution reactions and catalyst utilizations. It is discovered that easily accessible/tunable mesoporous Au nanolayers (AuNLs) not only increase the electrode conductivity by more than 4000 times but also enable the proton transport through straight mesopores within the Debye length. The catalyst layer design with AuNLs and ultralow catalyst loading (≈0.1 mg cm^?2) augments reaction sites from 1D to 2D, resulting in an 18‐fold improvement in mass activities. Furthermore, using microscale visualization and unique coplanar‐electrode electrolyzers, the relationship between the conductivity and the reaction site is revealed, allowing for the discovery of the conductivity‐determining and Debye‐length‐determining regions for water splitting. These findings and strategies provide a novel electrode design (catalyst layer + functional sublayer + ion exchange membrane) with a sufficient electron/proton transport path for high‐efficiency electrochemical energy conversion devices.     

Nano-Anatase Relieves the Inhibition of Electron Transport Caused by Linolenic Acid in Chloroplasts of Spinach

Linolenic acid is an inhibitor of electron transport in chloroplasts of higher plants. It has obvious effects on the structure and function of chloroplasts. In the present paper, we investigated the nano-anatase relieving the inhibition of photoreduction activity and oxygen evolution caused by linolenic acid in spinach chloroplasts. The results showed that linolenic acid in various concentrations could obviously reduce the whole chain electron transport and the photoreduction activity of two photosystems, especially on the oxidative reside and reduce reside of photosystem II (PS II). After adding nano-anatase to chloroplasts treated by linolenic acid, the whole chain electron transport rate, the photoreduction activity of two photosystems, and the oxygen evolution rate were increased significantly, indicating that nano-anatase could obviously decrease the inhibition of linolenic acid on the electron transport, photoreduction activity, and oxygen evolution of spinach chloroplasts.     

Atomic Modulation of FeCo–Nitrogen–Carbon Bifunctional Oxygen Electrodes for Rechargeable and Flexible All‐Solid‐State Zinc–Air Battery

Rational design and exploration of robust and low‐cost bifunctional oxygen reduction/evolution electrocatalysts are greatly desired for metal–air batteries. Herein, a novel high‐performance oxygen electrode catalyst is developed based on bimetal FeCo nanoparticles encapsulated in in situ grown nitrogen‐doped graphitic carbon nanotubes with bamboo‐like structure. The obtained catalyst exhibits a positive half‐wave potential of 0.92 V (vs the reversible hydrogen electrode, RHE) for oxygen reduction reaction, and a low operating potential of 1.73 V to achieve a 10 mA cm^?2 current density for oxygen evolution reaction. The reversible oxygen electrode index is 0.81 V, surpassing that of most highly active bifunctional catalysts reported to date. By combining experimental and simulation studies, a strong synergetic coupling between FeCo alloy and N‐doped carbon nanotubes is proposed in producing a favorable local coordination environment and electronic structure, which affords the pyridinic N‐rich catalyst surface promoting the reversible oxygen reactions. Impressively, the assembled zinc–air batteries using liquid electrolytes and the all‐solid‐state batteries with the synthesized bifunctional catalyst as the air electrode demonstrate superior charging–discharging performance, long lifetime, and high flexibility, holding great potential in practical implementation of new‐generation powerful rechargeable batteries with portable or even wearable characteristic.     

Coordination between manganese and nitrogen within the ligands in the manganese complexes facilitates the reconstitution of the water-oxidizing complex in manganese-depleted photosystem II preparations

The water-oxidizing complex (WOC) within photosystem II (PSII) can be reconstituted with synthetic manganese complexes by a process called photoactivation; however, the key factors affecting the efficiency of synthetic manganese complexes in reconstitution of electron transport and oxygen evolution activity in manganese-depleted PSII remain unclear. In the present study, four complexes with different manganese coordination environments were used to reconstitute the WOC, and an interesting relationship was found between the coordination environment of the manganese atom in the complexes and their efficiency in restoring electron transport and oxygen evolution. If Mn(II) is coordinated to nitrogen atoms within the ligand, it can restore significant rates of electron transport and oxygen evolution; however, if the manganese atom is coordinated only to oxygen atoms instead of nitrogen atoms, it has no capability to restore electron transport and oxygen evolution. So, our results demonstrate that the capability of manganese complexes to reconstitute the WOC is mainly determined by the coordination between nitrogen atoms from ligands and the manganese atom. It is suggested from our results that the ligation between the nitrogen atom and the manganese atom within the manganese complex facilitates the photoligation of the manganese atom to histidyl residues on the apo-protein in manganese-depleted PSII during photoactivation.     

The Appearance and Development of Photosynthetic Activity in Etiolated Barley Leaves and Isolated Etio-chloroplasts

The appearance and development of the oxygen exchanging capacity of greening barley leaves were measured using a manometric technique and an oxygen race electrode. An oxygen evolution could first be detected after one hour of greening. During the first hour of greening a light-dependent oxygen uptake was observed. The oxygen evolving capacity, calculated on a chlorophyll weight basis, showed a fast rise in activity during the first hours of greening. A maximal activity was reached after 5 to 10 hours of greening; the oxygen evolution then declined. Using oxygen electrodes different aspects of the electron transport in etio-chloroplasts prepared from the greening barley leaves were also investigated. The activity in photosystem I and II, as well as the cooperation between the two photosystems, were studied by measuring the oxygen exchange from the etio-chloroplasts in the absence and presence of added oxidants and reductants. An activity in photosystem I could be detected already after 5 minutes of greening. The activity of photosystem I, when calculated on a chlorophyll basis, had the same appearance as the oxygen evolution from the intact plant material. An activity in photosystem II and a cooperation between the two photosystems were first detected after 3 hours of greening. After about 15 hours of greening a cooperation corresponding to that from chloroplasts prepared from normal green leaves was observed.     

Hg ++ - a DCMU independent electron acceptor of photosystem II

Mercuric chloride functions as a direct electron acceptor from the quencher of fluorescence in Photosystem II. The photoreduction of ferricyanide, dichlorophenol-indophenol or methyl viologen is inhibited by mercuric ion while oxygen evolution is uneffected. Mercuric chloride supported oxygen evolution (mercury Hill reaction) is not prevented by DCMU or other similar electron transport inhibitors.     

Photosynthetic Electron Transport in Single Guard Cells as Measured by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) is a powerful new tool for studying chemical and biological processes. It records changes in faradaic current as a microelectrode ([less than equal]7 [mu]m in diameter) is moved across the surface of a sample. The current varies as a function of both distance from the surface and the surface's chemical and electrical properties. We used SECM to examine in vivo topography and photosynthetic electron transport of individual guard cells in Tradescantia fluminensis, to our knowledge the first such analysis for an intact plant. We measured surface topography at the micrometer level and concentration profiles of O2 evolved in photosynthetic electron transport. Comparison of topography and oxygen profiles above single stomatal complexes clearly showed photosynthetic electron transport in guard cells, as indicated by induction of O2 evolution by photosynthetically active radiation. SECM is unique in its ability to measure topography and chemical fluxes, combining some of the attributes of patch clamping with scanning tunneling microscopy. In this paper we suggest several questions in plant physiology that it might address.     

Electron flow at the polarized mercury-water interface in the presence of membrane fragments rich in Na+−K+-Activated ATPase

Summary Measurements of interfacial electron flow indicate that membrane fragments rich in Na⁺–K⁺-ATPase are capable of absorbing and releasing electrons in the form of random currents at an electrode surface. The electron transporting system, which functions in the presence or absence of substrate and activating ions, may be part of or in contact with the enzyme system, but it is not related to the ATPase activity. The observed electron transport at an electrode surface resembles physiological electron transport processes in being reversible, in extending over the same range of potential, and in being affected by some of the chemicals that interfere with electron transport and oxidative phosphorylation in mitochondria. Our experiments do not provide sufficient evidence to identify the substances that are responsible for the random currents, but the results suggest that the electro-active substances are similar to those which are involved in the reactions at the second phosphorylation site in mitochondria. Experiments with this technique provide a new approach to the study of the mechanism of biological electron transport processes and their possible relation to ATP synthesis and hydrolysis.Supported by U.S. Public Health Service Research Career Development Award K3-GM-8158.     

Chronocoulometric techniques in the study of computerized enzyme electrode systems

The electrochemical transient of a two-substrate enzyme electrode was studied theoretically and experimentally. Operation of such electrodes in the chronocoulometric mode leads to increased electrode sensitivity and makes possible the retrieval of useful information on transport and kinetics parameters. Digital simulation was used to solve the kinetics and transport equations and to produce the theoretical chronocoulometric response. A glucose electrode based on glucose oxidase crosslinked to different matrices was tested with air oxygen and p-benzoquinone as the cosubstrate. A computerized electrochemical system was employed for electrode potential control and data acquistion and analysis.     

2,6-Dichloro-phenol indophenol prevents switch-over of electrons between the cyanide-sensitive and -insensitive pathway of the mitochondrial electron transport chain in the presence of inhibitors

To evolve a simple oxygen electrode-based method to estimate alternative respiration, one needs to develop a procedure to prevent switch-over of electrons to either pathway upon inhibition by cyanide or salicylhydroxamic acid. It was hypothesized that the inclusion of appropriate electron acceptor, possessing redox potential close to one of the electron transport carriers in between ubiquinone (branch point) and cytochrome a-a3, should be able to stop switch-over of electrons to either pathway by working as an electron sink. To test the hypothesis, 2,6-dichloro-phenol indophenol (DCPIP; redox potential +0.217 V), an artificial electron acceptor having a redox potential quite similar to the site near cytochrome c1 (redox potential +0.22 V) on the cyanide-sensitive pathway, was used with isolated mitochondria and leaf discs in the absence and presence of inhibitors (potassium cyanide, antimycin A, and salicylhydroxamic acid). Polarographic data confirmed electron acceptance by DCPIP only from the inhibited (by cyanide or salicylhydroxamic acid) mitochondrial electron transport chain, hence preventing switch-over of electrons between the cyanide-sensitive and cyanide-insensitive pathway of respiration. Results with antimycin A and reduction status of DCPIP further confirmed electron acceptance by DCPIP from the mitochondrial electron transport chain. Possible implications of the results have been discussed.     

Effects of Some Chemicals on the Oxygen Evolution and Oxygen Uptake in Isolated Wheat Chloroplasts Irradiated without the Addition of an Oxidant

The oxygen exchange, obtained when isolated chloroplasts of Triticum aestivum, wheat, are irradiated without the addition of a Hill oxidant has been investigated using an oxygen electrode. Ascorbate, catalase, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone(DBMIB), diethyldithio-carbamate (DEDT), dichlorophenylmethylurea (DCMU), and potassium cyanide were added to the Chloroplasts in order to investigate the oxygen exchange. At least two oxygen uptake reactions, one sensitive to catalase and one catalase-insensitive, appeared upon irradiation. Hydrogen peroxide was the product of the oxygen uptake in the former process, and water was the reductant. The formation of hydrogen peroxide was probably associated with photosystem I. The other oxygen consuming reaction was found to be insensitive to both catalase and potassium cyanide. After the chloroplasts had been treated with DCMU, it was possible to show that the catalase-insensitive oxygen uptake was localized in photosystem I, and that a cyclic electron transport system or some endogenous reductant (-s) acted in the oxygen uptake. Addition of ascorbate or DEDT to the chloroplasts led to an enhanced oxygen uptake in 710 nm light. This was probably due to the effect of these compounds on the superoxide radical ion formed in photosystem I. The stimulated oxygen uptake was only weakly affected by catalase, indicating that hydrogen peroxide was not a product of this oxygen uptake. Addition of DEDT and potassium cyanide inhibited (strongly respectively weakly) the oxygen uptake when photosystem II was functioning. The effect of these compounds was probably due to an inhibition of the electron transport at the plastocyanin. DBMIB inhibited the oxygen uptake reactions and the cooperation between the two photosystems. The cooperation between the photosystems was also studied in DCMU-treated chloroplasts. The reactions in photosystem II, measured as oxygen evolution, were more inhibited than the coupling between the photosystems. The oxygen “gush” appearing upon irradiation in light of 650 nm was not affected by a DBMIB-treatment, showing that the oxygen evolution was due to the reduction of plastoquinone. The reoxidation in the dark of the plastoquinone pool was stimulated by DBMIB and potassium cyanide indicating that an oxygen uptake could be associated with plastoquinone. The sites of interaction of oxygen with the electron transport pathways in chloroplasts, and the different reductants for the oxygen consuming reactions are discussed.     

PB/PANI-modified electrode used as a novel oxygen reduction cathode in microbial fuel cell

This study focuses on the preparation of a new type of Prussian Blue/polyaniline (PB/PANI)-modified electrode as oxygen reduction cathode, and its availability in microbial fuel cell (MFC) for biological power generation. The PB/PANI-modified electrode was prepared by electrochemical and chemical methods, both of which exhibited good electrocatalytical reactivity for oxygen reduction in acidic electrolyte. The MFC with PB/PANI-modified cathode aerated by either oxygen or air was shown to yield a maximum power density being the same with that of the MFC with liquid-state ferricyanide cathode, and have an excellent duration as indicated by stable cathode potential for more than eight operating circles. This study suggests a promising potential to utilize this novel electrode as an effective alternative to platinum for oxygen reduction in MFC system without losing sustainability.     

Estimation of oxygen evolution by marine phytoplankton from measurement of the efficiency of Photosystem II electron flow

The relation between photosynthetic oxygen evolution and Photosystem II electron transport was investigated for the marine algae t Phaeodactylum tricornutum, Dunaliella tertiolecta, Tetraselmis sp., t Isochrysis sp. and t Rhodomonas sp.. The rate of Photosystem II electron transport was estimated from the incident photon flux density and the quantum efficiency of Photosystem II electron transport as determined by chlorophyll fluorescence. The relation between the estimated rate of Photosystem II electron transport and the rate of oxygen evolution was investigated by varying the ambient light intensity. At limiting light intensities a linear relation was found in all species. At intensities approaching light saturation, the relation was found to deviate from linearity. The slope of the line in the light-limited range is species dependent and related to differences in absorption cross-section of Photosystem II. The observed non-linearity at high irradiances is not caused by photorespiration but probably by a Mehler-type of oxygen reduction. The relationship could be modelled by including a redox-state dependent oxygen uptake. In the diatom t Phaeodactylum tricornutum, the photochemical efficiency of dark adapted open Photosystem II centers was found to be temperature-dependent with an optimum near 10°C.     

Integration of FeOOH and Zeolitic Imidazolate Framework‐Derived Nanoporous Carbon as an Efficient Electrocatalyst for Water Oxidation

As a cost‐effective catalyst for the oxygen evolution reaction (OER), the potential use of FeOOH is hindered by its intrinsic poor electron conductivity. Here, the significant enhancement of OER activity and long‐term stability of electrodeposited FeOOH on zeolitic imidazolate framework‐derived N‐doped porous carbons (NPCs) are reported. In alkaline media, FeOOH/NPC supported on nickel foam as a 3D electrode delivers a current density of 100 mA cm^?2 at a small overpotential of 230 mV and exhibits a low Tafel slope of 33.8 mV dec^?1 as well as excellent durability, making it one of the most active OER catalysts. Such high performance is attributed to a combined effect of the excellent electron conductivity of NPC and the synergy between FeOOH and NiO derived from Ni substrate.     

Comparison of methods for measuring oxygen consumption in tumor cells in vitro

The oxygen consumption rate of tumor cells affects tumor oxygenation and response to therapies. Highly sensitive methods for determining cellular oxygen consumption are, therefore, needed to identify treatments that can modulate this parameter. We compared the performances of three different methods for measuring cellular oxygen consumption: electron paramagnetic resonance (EPR) oximetry, the Clark electrode, and the MitoXpress fluorescent assay. To compare the assays, we used K562 cells in the presence of rotenone and hydrocortisone, compounds that are known to inhibit the mitochondrial electron transport chain to different extents. The EPR method was the only one that could identify both rotenone and hydrocortisone as inhibitors of tumor cell oxygen consumption. The Clark electrode and the fluorescence assay demonstrated a significant decrease in cellular oxygen consumption after administration of the most potent inhibitor (rotenone) but failed to show any significant effect of hydrocortisone. EPR oximetry is, therefore, the most sensitive method for identifying inhibitors of oxygen consumption on cell assays, whereas the Clark electrode offers the unique opportunity to add external compounds during experiments and still shows great sensitivity in studying enzyme and chemical reactions that consume oxygen (non-cell assays). Finally, the MitoXpress fluorescent assay has the advantage of a high-sample throughput and low bulk requirements but at the cost of a lower sensitivity.     

Electron transport system associated with direct glucose oxidation in Gluconobacter oxydans

Summary The mode of electron transport associated with the dehydrogenase enzymes located on the cytoplasmic membrane inGluconobacter oxydans (ATCC 9937) has been postulated. High turnover of dehydrogenases under oxygen enrichment conditions is explained on the basis of a simplistic electron transport chain comprising cytochrome c553 (MW 23000) as a subunit of dehydrogenase and a cytochrome b562. The electron transport chain under low dissolved oxygen tension (DOT) is shown to comprise a number of cytochrome c species with very low midpoint potential difference.     

Electrospun Carbon Nanofibers Encapsulated with NiCoP: A Multifunctional Electrode for Supercapattery and Oxygen Reduction,Oxygen Evolution,and Hydrogen Evolution Reactions

Functionalizing nanostructured carbon nanofibers (CNFs) with bimetallic phosphides enables the material to become an active electrode for multifunctional applications. A facile electrospinning technique is utilized for the first time to develop NiCoP nanoparticles encapsulated CNFs that are used as an energy storage system of supercapattery, and as an electrocatalyst for oxygen reduction, oxygen evolution, and hydrogen evolution reaction in KOH electrolyte. Evolving from the inclusion of bimetallic phosphide nanoparticles, the NiCoP/CNF electrode unveils superior‐specific capacitance (333 Fg^?1 at 2 Ag^?1) and rate capability (87%). The fabricated supercapattery device offers a voltage of 1.6 V that supplies a remarkable energy density (36 Wh kg^?1) along with an improved power density (4000 W kg^?1) and unwavering cyclic stability (25 000 cycles). Meanwhile, the NiCoP/CNF electrode has simultaneously performed well as a multifunctional electrocatalyst for oxygen reduction reaction at a half‐wave potential of 0.82 V versus reversible hydrogen electrode and can attain a current density of 10 mA cm^?2 at a very low overpotential of 268 and 130 mV for the oxygen evolution reaction and hydrogen evolution reaction, respectively. Thus, the NiCoP/CNF with all its inimitable electrode properties has profoundly proved its proficiency at handling multifunctional challenges in terms of both storage and conversion.     

Electron transport in xanthine oxidase. A model for other biological electron transport chains.

Part of the catalytic function of xanthine oxidase (XO) involves the transfer of two electrons from a substrate to a molybdenum ion on the enzyme followed by equilibration of these electrons among other electron resting sites on the enzyme. The electrons are removed from the enzyme at a flavin by oxygen to form hydrogen peroxide. This paper considers mechanisms which allow the electrons to equilibrate between the different resting sites on the enzyme. The mechanisms are chosen to be consistent with known properties of the enzyme (relative reduction potentials, electron transfer rates, and the estimated separation of these resting sites). Tunneling appears to be a good candidate to account for most of the electron transport. It is shown that the XO electron transport system is similar in many respects to sections of mitochondrial electron transport chains and can serve as a nice model for parts of these more complicated biological electron transport systems.     

NiFe‐Based Metal–Organic Framework Nanosheets Directly Supported on Nickel Foam Acting as Robust Electrodes for Electrochemical Oxygen Evolution Reaction

It is of great significance to develop highly efficient and superior stable oxygen evolution reaction (OER) electrocatalysts for upcoming electrochemical conversion technologies and clean energy systems. Here, an assembled 3D electrode is synthesized by a one‐step solvothermal process using such an original OER electrocatalyst. During the solvothermal process, Ni ions released from Ni foam in acidic solution and Fe ions added exogenously act as metal centers and coordinate with terephthalic acid (TPA) organic molecules by robust coordinate bonds, and finally, NiFe‐based metal–organic framework (MOF) nanosheets in situ grown on Ni foam, i.e., MIL‐53(FeNi)/NF, are prepared. This binder‐free 3D electrode shows superior OER activity with high current density (50 mA cm^?2) at an overpotential of 233 mV, a Tafel slope of 31.3 mV dec^?1, and excellent stability in alkaline aqueous solution (1 m KOH). It is discovered that introduction of Fe into MIL‐53 structure increases electrochemically‐active areas as well as reaction sites, accelerated electron transport capability, and modulated electronic structure to enhance catalytic performance. Besides, first principles calculations show that MIL‐53(FeNi) is more favorable for foreign atoms' adsorption and has increased 3d orbital electron density boosting intrinsic activity. This work elucidates a promising electrode for electrocatalysts and enriches direct application of MOF materials.