Inhibition Of Electron Transfer Research Articles

Adaptive changes in chlorophyll content and photosynthetic features to low light in Physocarpus amurensis Maxim and Physocarpus opulifolius "Diabolo".

The present study aims to investigate the differences in leaf pigment content and the photosynthetic characteristics under natural and low light intensities between the Chinese native Physocarpus amurensis Maxim and the imported Physocarpus opulifolius “Diabolo” from North America. We aim to discuss the responses and the adaptive mechanism of these two cultivars of Physocarpus to a low light environment. The results show that the specific leaf area (SLA) and the chlorophyll content were significantly increased in the leaves of both Physocarpus cultivars in response to a low light intensity, and the SLA and chlorophyll content were higher in the leaves of low light-treated P. opulifolius “Diabolo” compared with the leaves of low light-treated P. amurensis Maxim. Moreover, the content of anthocyanin was markedly reduced in the leaves of P. opulifolius “Diabolo” under low light intensity, which allowed for a greater capacity of photon capture under the low light condition. Under natural light, the photosynthetic carbon assimilation capacity was greater in the leaves of P. amurensis Maxim compared with the leaves of P. opulifolius “Diabolo” that were rich with anthocyanin. However, in response to low light, AQY, Pmax, LCP and LSP decreased to a lesser extent in the leaves of P. opulifolius “Diabolo” compared with the leaves of P. amurensis Maxim. These results suggest that P. opulifolius “Diabolo” exhibits a greater ability in adaption to low light, and it is probably related to the relatively higher chlorophyll content and the smaller SLA in the leaves of P. opulifolius “Diabolo.” In addition, the low light intensity resulted in a reduced photochemical activity of photosystem (PS) II in the leaves of both Physocarpus, as evidenced by increased values of the relative variable fluorescence at point J and point I on the OJIP curve. This result suggests that the electron acceptor in PS II was the major responsive site to the low light stress in the leaves of both Physocarpus cultivars, and that the low light intensity significantly inhibited electron transfer on the acceptor side of PS II and reduced the activity of the oxygen-evolving complex (OEC) in the leaves of both Physocarpus cultivars. The PS II function in P. opulifolius “Diabolo” was higher than that in P. amurensis Maxim in response to low light. Under low light, the composition of photosynthetic pigments was altered in the leaves of P. opulifolius “Diabolo” in order to maintain a relatively high activity of primary photochemical reactions, and this is the basis of the greater photosynthetic carbon assimilation capacity and one of the main reasons for the better shade-tolerance in P. opulifolius “Diabolo.”

An electrochemical aptasensor based on TiO2/MWCNT and a novel synthesized Schiff base nanocomposite for the ultrasensitive detection of thrombin

A sensitive aptasensor based on a robust nanocomposite of titanium dioxide nanoparticles, multiwalled carbon nanotubes (MWCNT), chitosan and a novel synthesized Schiff base (SB) (TiO2/MWCNT/CHIT/SB) on the surface of a glassy carbon electrode (GCE) was developed for thrombin detection. The resultant nanocomposite can provide a large surface area, excellent electrocatalytic activity, and high stability, which would improve immobilization sites for biological molecules, allow remarkable amplification of the electrochemical signal and contribute to improved sensitivity. Thrombin aptamers were simply immobilized onto the TiO2-MWCNT/CHIT-SB nanocomposite matrix through simple π - π stacking and electrostatic interactions between CHIT/SB and aptamer strands. The electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to analyze the surface characterization of unmodified GCE and TiO2-MWCNT/CHIT-SB modified GCE, and also the interaction between aptamer and thrombin. In the presence of thrombin, the aptamer on the adsorbent layer captures the target on the electrode interface, which makes a barrier for electrons and inhibits electron transfer, thereby resulting in decreased DPV and increased impedance signals of the TiO2-MWCNT/CHIT-SB modified GCE. Furthermore, the proposed aptasensor has a very low LOD of 1.0fmolL−1 thrombin within the detection range of 0.00005–10nmolL−1. The aptasensor also presents high specificity and reproducibility for thrombin, which is unaffected by the coexistence of other proteins. Clinical application was performed with analysis of the thrombin levels in blood and CSF samples obtained from patients with MS, Parkinson, Epilepsy and Polyneuropathy using both the aptasensor and commercial ELISA kit. The results revealed the proposed system to be a promising candidate for clinical analysis of thrombin.

High-Efficiency Iron Photosensitizer Explained with Quantum Wavepacket Dynamics.

Fe(II) complexes have long been assumed unsuitable as photosensitizers because of their low-lying nonemissive metal centered (MC) states, which inhibit electron transfer. Herein, we describe the excited-state relaxation of a novel Fe(II) complex that incorporates N-heterocyclic carbene ligands designed to destabilize the MC states. Using first-principles quantum nuclear wavepacket simulations we achieve a detailed understanding of the photoexcited decay mechanism, demonstrating that it is dominated by an ultrafast intersystem crossing from (1)MLCT-(3)MLCT proceeded by slower kinetics associated with the conversion into the (3)MC states. The slowest component of the (3)MLCT decay, important in the context of photosensitizers, is much longer than related Fe(II) complexes because the population transfer to the (3)MC states occurs in a region of the potential where the energy gap between the (3)MLCT and (3)MC states is large, making the population transfer inefficient.

The ubiquitin signal and autophagy: an orchestrated dance leading to mitochondrial degradation

The quality of mitochondria, essential organelles that produce ATP and regulate numerous metabolic pathways, must be strictly monitored to maintain cell homeostasis. The loss of mitochondrial quality control systems is acknowledged as a determinant for many types of neurodegenerative diseases including Parkinson's disease (PD). The two gene products mutated in the autosomal recessive forms of familial early-onset PD, Parkin and PINK1, have been identified as essential proteins in the clearance of damaged mitochondria via an autophagic pathway termed mitophagy. Recently, significant progress has been made in understanding how the mitochondrial serine/threonine kinase PINK1 and the E3 ligase Parkin work together through a novel stepwise cascade to identify and eliminate damaged mitochondria, a process that relies on the orchestrated crosstalk between ubiquitin/phosphorylation signaling and autophagy. In this review, we highlight our current understanding of the detailed molecular mechanisms governing Parkin-/PINK1-mediated mitophagy and the evidences connecting Parkin/PINK1 function and mitochondrial clearance in neurons.

High-performance, stable and low-cost mesoscopic perovskite (CH3NH3PbI3) solar cells based on poly(3-hexylthiophene)-modified carbon nanotube cathodes

This work explores the use of poly(3-hexylthiophene) (P3HT) modified carbon nanotubes (CNTs@P3HT) for the cathodes of hole transporter free, mesoscopic perovskite (CH3NH3PbI3) solar cells (PSCs), simultaneously achieving high-performance, high stability and low-cost PSCs. Here the thin P3HT modifier acts as an electron blocker to inhibit electron transfer into CNTs and a hydrophobic polymer binder to tightly cross-link the CNTs together to compact the carbon electrode film and greatly stabilize the solar cell. On the other hand, the presence of CNTs greatly improve the conductivity of P3HT. By optimizing the concentration of the P3HT modifier (2 mg/mL), we have improved the power conversion efficiencies (PCEs) of CNTs@P3HT based PSCs up to 13.43% with an average efficiency of 12.54%, which is much higher than the pure CNTs based PSCs (best PCE 10.59%) and the sandwich-type P3HT/CNTs based PSCs (best PCE 9.50%). In addition, the hysteresis of the CNTs@P3HT based PSCs is remarkably reduced due to the intimate interface between the perovskite and CNTs@P3HT electrodes. Degradation of the CNTs@ P3HT based PSCs is also strongly retarded as compared to cells employing the pure CNTs electrode when exposed to the ambient condition of 20%–40% humidity.

Cyanobacterial Light-Harvesting Phycobilisomes Uncouple From Photosystem I During Dark-To-Light Transitions.

Photosynthetic organisms cope with changes in light quality by balancing the excitation energy flow between photosystems I (PSI) and II (PSII) through a process called state transitions. Energy redistribution has been suggested to be achieved by movement of the light-harvesting phycobilisome between PSI and PSII, or by nanometre scale rearrangements of the recently discovered PBS-PSII-PSI megacomplexes. The alternative ‘spillover’ model, on the other hand, states that energy redistribution is achieved by mutual association/dissociation of PSI and PSII. State transitions have always been studied by changing the redox state of the electron carriers using electron transfer inhibitors, or by applying illumination conditions with different colours. However, the molecular events during natural dark-to-light transitions in cyanobacteria have largely been overlooked and still remain elusive. Here we investigated changes in excitation energy transfer from phycobilisomes to the photosystems upon dark-light transitions, using picosecond fluorescence spectroscopy. It appears that megacomplexes are not involved in these changes, and neither does spillover play a role. Instead, the phycobilisomes partly energetically uncouple from PSI in the light but hardly couple to PSII.

Mitochondrial Disease-related Mutation G167P in Cytochrome b of Rhodobacter capsulatus Cytochrome bc1 (S151P in Human) Affects the Equilibrium Distribution of [2Fe-2S] Cluster and Generation of Superoxide

Cytochrome bc1 is one of the key enzymes of many bioenergetic systems. Its operation involves a large scale movement of a head domain of iron-sulfur protein (ISP-HD), which functionally connects the catalytic quinol oxidation Qo site in cytochrome b with cytochrome c1. The Qo site under certain conditions can generate reactive oxygen species in the reaction scheme depending on the actual position of ISP-HD in respect to the Qo site. Here, using a bacterial system, we show that mutation G167P in cytochrome b shifts the equilibrium distribution of ISP-HD toward positions remote from the Qo site. This renders cytochrome bc1 non-functional in vivo. This effect is remediated by addition of alanine insertions (1Ala and 2Ala) in the neck region of the ISP subunit. These insertions, which on their own shift the equilibrium distribution of ISP-HD in the opposite direction (i.e. toward the Qo site), also act in this manner in the presence of G167P. Changes in the equilibrium distribution of ISP-HD in G167P lead to an increased propensity of cytochrome bc1 to generate superoxide, which becomes evident when the concentration of quinone increases. This result corroborates the recently proposed model in which "semireverse" electron transfer back to the Qo site, occurring when ISP-HD is remote from the site, favors reactive oxygen species production. G167P suggests possible molecular effects of S151P (corresponding in sequence to G167P) identified as a mitochondrial disease-related mutation in human cytochrome b. These effects may be valid for other human mutations that change the equilibrium distribution of ISP-HD in a manner similar to G167P.

Two-Electron Oxidation of Dopamine Controlled by Surface Modification of Few-Layer Graphene

Oxidation of dopamine (DA) at classical electrodes is suggested to go through three steps and then form melanin-like compounds, which would inhibit electron transfer and result in the fouling of electrode. In addition, the selective oxidation of DA in the presence of ascorbate (VC) should also be considered for the design of electrode. Here, graphenes modified with different surface groups are used to investigate the oxidation behaviour of DA. Compared with classical electrodes, negatively charged few-layer graphene (AA-GN) shows a good performance in selective oxidation of DA and can also control the oxidation of DA in the first step, which would further prevent the fouling of electrode. The rate constant and number of electron transfer are calculated to be about 7.38s−1 and 2, respectively, which further indicates that the oxidation of DA was controlled in the first step. Furthermore, AA-GN-modified electrode is used to selectively detect DA with a detection limit of 10nM in the prescence of excess VC. At last, AA-GN-modified macro-electrode is used to monitor DA release from living cells for the first time.

Redox state control of a metalloprotein to generate multi-level signals for applications to bioelectronic devices

In this research, we investigate the redox state control of metalloprotein immobilized on a metal surface for the generation of different output signals. Ferredoxin, as a type of metalloprotein, is immobilized on an Au substrate by a chemical linking method using organic materials for the fabrication of a stable protein thin film. To control ionic state of the immobilized ferredoxin, the external electrochemical potentials for oxidation and reduction are applied selectively to the ferredoxin layer. The initial output signal is enhanced or suppressed by the reverse potential, which can inhibit electron transfer by restricting the number of redoxreactive molecules. Consequently, multi-level signals are achieved using a single protein film, and this proposed concept can be used widely to fabricate biosensors for multi-target detection and bioelectronic devices with new functionalities.

Directionality of electron transfer in cyanobacterial photosystem I at 298 and 77 K

Electron transfer processes in cyanobacterial photosystem I particles from Synechocystis sp. PCC 6803 with a high potential naphthoquinone (2,3-dichloro-1,4-naphthoquinone) incorporated into the A1 binding site have been studied at 298 and 77K using time-resolved visible and infrared difference spectroscopy. The high potential naphthoquinone inhibits electron transfer past A1, and biphasic P700+A1− radical pair recombination is observed. The two phases are assigned to P700+A1B− and P700+A1A− recombination. Analyses of the transient absorption changes indicate that the ratio of A- and B-branch electron transfer is 95:5 at 77 K and 77:23 at 298K.

Carbon nanotube–cyclodextrin adducts for electrochemical recognition of tartaric acid

Since β-Cyclodextrin (CD) is the chiral selector, and carbon nanotube (CNT) is a good adsorbent and a good conductor for electron transfer, CD was covalently attached on CNT during the reduction of oxidized CNT with hydrazine hydrate for electrochemical recognition of the chiral enantiomers. The obtained CNT–CD could disperse well in water because of the hydrophilic groups from CD. A modified electrode — composed of CNT–CD, methylene blue (MB), and a glassy carbon electrode (GCE), was designed for electrochemical recognition of chiral tartaric acid (TA). Since β-CD selectively holds L-TA, this stable complex inhibited electron transfer and linearly decreased the redox current of MB. The concentrations of L-TA or D-TA in aqueous solution could be estimated from the linear equation between the difference of the redox peak current (ΔI) and the concentration, and the enantiomeric ratio of L-TA in TA samples could be calculated by determining ΔI because of the linear relationship, ΔI (μA)=0.068×(L-TA ratio×100%)+8.2.

An electrochemical aptasensor based on a TiO2/three-dimensional reduced graphene oxide/PPy nanocomposite for the sensitive detection of lysozyme.

A sensitive aptasensor based on a nanocomposite of hollow titanium dioxide nanoball, three-dimensional reduced graphene oxide, and polypyrrole (TiO2/3D-rGO/PPy) was developed for lysozyme detection. A lysozyme aptamer was easily immobilized onto the TiO2/3D-rGO/PPy nanocomposite matrix by assembling the aptamer onto graphene through simple π-stacking interactions and electrostatic interactions between PPy molecular chains and aptamer strands. In the presence of lysozyme, the aptamer on the adsorbent layer catches the target on the electrode interface, which generates a barrier for electrons and inhibits electron transfer, subsequently resulting in decreased electrochemically differential pulse voltammetric signals of a gold electrode modified with TiO2/3D-rGO/PPy. Using this strategy, a low limit of detection of 0.085 ng mL(-1) (5.5 pM) for detecting lysozyme was observed within the detection range of 0.1-50 ng mL(-1) (0.007-3.5 nM). The aptasensor also presents high specificity for lysozyme, which is unaffected by the coexistence of other proteins. Such an aptasensor opens a rapid, selective, and sensitive route to lysozyme detection. This finding indicates that the TiO2/3D-rGO/PPy nanocomposite could be used as an electrochemical biosensor for detecting proteins in the biomedical field.

FeIIaq–FeIIIoxide electron transfer and Fe exchange: effect of organic carbon

Environmental context Concerns about carbon-driven climate change make it critical to better understand how carbon interacts with soils. We examined whether carbon compounds commonly found in soils affect the behaviour of soil iron minerals, and found that iron minerals remain dynamic and mix with the surrounding water, despite the presence of carbon in the water. Our findings suggest that both carbon and trace elements in the minerals may be more mobile and less stable than we previously thought. Abstract The close association of organic carbon and Fe oxides has been recognised for decades and recently interest in the role of Fe oxides in organic C sequestration has increased. Here we explore the effect of natural organic C on electron transfer and exchange of aqueous FeII and Fe in the bulk structure of goethite and magnetite. Using 57Fe isotope experiments coupled with 57Fe Mössbauer spectroscopy it is found that electron transfer occurs between aqueous FeII and structural FeIII in both goethite and magnetite in the presence of most organic C compounds, including natural organic matter (NOM), extracellular polysaccharides (EPS), and cell materials. Only a long-chain (39-C) phospholipid containing organic C significantly inhibited electron transfer. Despite evidence for FeII–FeIIIoxide electron transfer in the presence of NOM, exchange of Fe between the aqueous phase and bulk Fe was less than observed in the absence of NOM. Our findings suggest that electron transfer between aqueous FeII and bulk structural Fe in goethite and magnetite is a robust process likely to occur in a variety of organic-rich biogeochemical environments but that Fe exchange may be inhibited by the presence of some C compounds.

Abstract 4314: Metabolic manipulation of hypoxia and radiotherapy response by electron transport inhibitors

Abstract The presence of hypoxia in solid tumors is correlated with poor treatment outcome. Even small population of hypoxic cells within tumors can significantly decrease the efficacy of radiation therapy. One approach to treating hypoxia in tumours is to reduce overall tumor cell oxygen consumption by modulating mitochondrial respiration. This could reoxygenate hypoxic cells within the tumor and restore radiation sensitivity. To guide in vivo application we tested the effects of three respiratory chain inhibitors; the biguanides metformin and phenformin and rotenone on oxygen consumption in 3D tissue engineered discs. Experiments were carried out to evaluate the effect of respiratory inhibition on the hypoxic fraction of HCT116 and HT29 tissue discs using immunostaining with the hypoxic cell marker pimonidazole. An oxygen sensing phosphorescence lifetime probe assessed pO2 on the basal side of the disc as a function of time after inhibitors were exposed to the apical surface. Clonogenic assays were then performed to confirm the radiosensitization of hypoxic cells within the tissue discs. Pimonidazole staining and direct pO2 sensing confirmed that all three electron transfer inhibitors decrease the hypoxic fraction by reducing cellular oxygen consumption. Rotenone was found to completely eliminate the hypoxic fraction at nanomolar concentrations compared to metformin and phenformin which require milli to micromolar concentrations respectively. Furthermore, rotenone was found to have an effect on oxygen utilization within minutes of drug exposure compared to both metformin and phenformin, which took hours to be effective. In addition, clonogenics assays determined that all three electron transfer inhibitors radiosensitize tumor cells with rotenone being the most efficient compound on a molar basis. The mitochondrial inhibitors of complex I metformin, phenformin and rotenone decrease oxygen utilization by tumor cells in a concentration dependent manner resulting in reduction of the hypoxic fraction and increased radiosensitivity. Citation Format: Maria Jose Gandolfo, Alastair H. Kyle, Andrew I. Minchinton. Metabolic manipulation of hypoxia and radiotherapy response by electron transport inhibitors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4314. doi:10.1158/1538-7445.AM2014-4314

Possible involvement of a tetrahydrobiopterin in photoreception for UV-B-induced anthocyanin synthesis in carrot.

Our previous studies of action spectra for UV-B-induced anthocyanin accumulation in cultured carrot cells indicated that a reduced form of pterin, possibly tetrahydrobiopterin, contributes to UV-B photoreception. In this report, we provide additional evidence for the involvement of pterin in UV-B light sensing. UV-B-induced phenylalanine ammonia-lyase (PAL) activity was considerably suppressed by N-acetylserotonin (an inhibitor of tetrahydrobiopterin biosynthesis), and this suppression was partially recovered by adding biopterin or tetrahydrobiobiopterin. In addition, protein(s) specifically bound to biopterin were detected by radiolabeling experiments in N-acetylserotonin-treated cells. Furthermore, diphenyleneiodonium, a potent inhibitor of electron transfer, completely suppressed UV-B-induced PAL activity. These results suggest the occurrence of an unidentified UV-B photoreceptor (other than UVR8, the tryptophan-based UV-B sensor originally identified in Arabidopsis) with reduced pterin in carrot cells. After reexamining published action spectra, we suggest that anthocyanin synthesis is coordinately regulated by these two UV-B sensors.

Electrochemical detection of intracellular and cell membrane redox systems in Saccharomyces cerevisiae.

Redox mediators can interact with eukaryote cells at a number of different cell locations. While cell membrane redox centres are easily accessible, the redox centres of catabolism are situated within the cytoplasm and mitochondria and can be difficult to access. We have systematically investigated the interaction of thirteen commonly used lipophilic and hydrophilic mediators with the yeast Saccharomyces cerevisiae. A double mediator system is used in which ferricyanide is the final electron acceptor (the reporter mediator). After incubation of cells with mediators, steady state voltammetry of the ferri/ferrocyanide redox couple allows quantitation of the amount of mediator reduced by the cells. The plateau current at 425 mV vs Ag/AgCl gives the analytical signal. The results show that five of the mediators interact with at least three different trans Plasma Membrane Electron Transport systems (tPMETs), and that four mediators cross the plasma membrane to interact with cytoplasmic and mitochondrial redox molecules. Four of the mediators inhibit electron transfer from S. cerevisiae. Catabolic inhibitors were used to locate the cellular source of electrons for three of the mediators.

A chloroplast retrograde signal regulates nuclear alternative splicing.

Light is a source of energy and also a regulator of plant physiological adaptations. We show here that light/dark conditions affect alternative splicing of a subset of Arabidopsis genes preferentially encoding proteins involved in RNA processing. The effect requires functional chloroplasts and is also observed in roots when the communication with the photosynthetic tissues is not interrupted, suggesting that a signaling molecule travels through the plant. Using photosynthetic electron transfer inhibitors with different mechanisms of action, we deduce that the reduced pool of plastoquinones initiates a chloroplast retrograde signaling that regulates nuclear alternative splicing and is necessary for proper plant responses to varying light conditions.

The Redox Potential of the Plastoquinone Pool of the Cyanobacterium Synechocystis Species Strain PCC 6803 Is under Strict Homeostatic Control

A method is presented for rapid extraction of the total plastoquinone (PQ) pool from Synechocystis sp. strain PCC 6803 cells that preserves the in vivo plastoquinol (PQH2) to -PQ ratio. Cells were rapidly transferred into ice-cold organic solvent for instantaneous extraction of the cellular PQ plus PQH2 content. After high-performance liquid chromatography fractionation of the organic phase extract, the PQH2 content was quantitatively determined via its fluorescence emission at 330 nm. The in-cell PQH2-PQ ratio then followed from comparison of the PQH2 signal in samples as collected and in an identical sample after complete reduction with sodium borohydride. Prior to PQH2 extraction, cells from steady-state chemostat cultures were exposed to a wide range of physiological conditions, including high/low availability of inorganic carbon, and various actinic illumination conditions. Well-characterized electron-transfer inhibitors were used to generate a reduced or an oxidized PQ pool for reference. The in vivo redox state of the PQ pool was correlated with the results of pulse-amplitude modulation-based chlorophyll a fluorescence emission measurements, oxygen exchange rates, and 77 K fluorescence emission spectra. Our results show that the redox state of the PQ pool of Synechocystis sp. strain PCC 6803 is subject to strict homeostatic control (i.e. regulated between narrow limits), in contrast to the more dynamic chlorophyll a fluorescence signal.

Oxygen Bubble Nucleation Modeling in a PEM Electrolyzer Electrode

The polymer electrolyte membrane (PEM) electrolyzer is a promising technology that reduces water into hydrogen and oxygen, from which the hydrogen is captured and stored for use in fuel cells. One of the challenging issues in PEM electrolyzers is flow inhibition of liquid water within the porous gas diffusion layer (GDL), due to the formation of oxygen bubbles (1). The bubbles block pores within the GDL, limiting the transport of liquid water to the catalyst layer, which in turn negatively affects the electrolyzer performance (2). Moreover, the presence of the oxygen bubbles can increase the solution resistance, inhibit electron transfer, and consequently increase ohmic losses, leading to efficiency reduction (1).Investigation of the oxygen bubbles behavior generated at the anode side of the PEM electrolyzer in operational pressure and temperature can give us a more accurate insight for predicting its influence on the performance. A number of promising studies on bubble nucleation and detachment have been done over the past few years. Various numerical techniques were introduced and improved to increase the accuracy of the bubble interface tracking such as volume-of-fluid (VOF) (3), moment-of-fluid (MOF) (4), and level-set (LS) methods (5).In this study, a numerical simulation has been performed to mimic the nucleation, growth and detachment of the oxygen bubbles in electrolyzers. Utilizing a state-of-the-art multiphase algorithm (6), a three-dimensional, two-phase computational algorithm was developed using the LS method to track the oxygen bubble interface expansion. To simulate the multiphase system more precisely, the thermodynamic free energy of the system has been taken into account by employing the Gibbs free energy function. The behavior of the oxygen bubble through the GDL as a function of the geometrical properties of the GDL was then studied and compared to the experimental results presented by Arbabi et al. (7). Acknowledgements The authors would like to gratefully acknowledge the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC).

The importance of the hydrophilic region of PsbL for the plastoquinone electron acceptor complex of Photosystem II

The PsbL protein is a 4.5kDa subunit at the monomer–monomer interface of Photosystem II (PS II) consisting of a single membrane-spanning domain and a hydrophilic stretch of ~15 residues facing the cytosolic (or stromal) side of the photosystem. Deletion of conserved residues in the N-terminal region has been used to investigate the importance of this hydrophilic extension. Using Synechocystis sp. PCC 6803, three deletion strains: ∆(N6–N8), ∆(P11–V12) and ∆(E13–N15), have been created. The ∆(N6–N8) and ∆(P11–V12) strains remained photoautotrophic but were more susceptible to photodamage than the wild type; however, the ∆(E13–N15) cells had the most severe phenotype. The Δ(E13–N15) mutant showed decreased photoautotrophic growth, a reduced number of PS II centers, impaired oxygen evolution in the presence of PS II-specific electron acceptors, and was highly susceptible to photodamage. The decay kinetics of chlorophyll a variable fluorescence after a single turnover saturating flash and the sensitivity to low concentrations of PS II-directed herbicides in the Δ(E13–N15) strain indicate that the binding of plastoquinone to the QB-binding site had been altered such that the affinity of QB is reduced. In addition, the PS II-specific electron acceptor 2,5-dimethyl-p-benzoquinone was found to inhibit electron transfer through the quinone-acceptor complex of the ∆(E13–N15) strain. The PsbL Y20A mutant was also investigated and it exhibited increased susceptibility to photodamage and increased herbicide sensitivity. Our data suggest that the N-terminal hydrophilic region of PsbL influences forward electron transfer from QA through indirect interactions with the D–E loop of the D1 reaction center protein. Our results further indicate that disruption of interactions between the N-terminal region of PsbL and other PS II subunits or lipids destabilizes PS II dimer formation. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.