Why Gratzel’s cell works so well

In Gratzel’s cell, the electrons injected by the photo-excitation of dye molecules, anchored to a mesoporous TiO₂ film, efficiently diffuse to the back contact achieving solar energy conversion at efficiencies exceeding 10%. The mesoporous TiO₂ surface constituted of randomly arranged nanocrystallites with a roughness factor of the order 1000 is heavily populated with traps, defects and adsorbed species which act as recombination centers. Nevertheless, the cell functions, mitigating recombination expected to occur via the interaction electrons at the surface. Evidence based mainly on 1/f noise measurements is presented to show that dye bonded to the TiO₂ surface passivates recombination centers. Furthermore the suppression of trapping-detrapping events at the surface increases the diffusion coefficient of the electrons through the nanocrystalline matrix facilitating electron transport to the back contact. The Gratzel cell is also unique, none of the high bandgap oxide materials other than TiO₂ yield energy conversion and quantum efficiencies as high as that of the cells based on TiO₂. 1/f noise measurements also reveal a distinct difference between TiO₂ and ZnO mesoporous films suggesting that the films made from the latter material are more intensely populated with surface states that mediate recombination.     

Tunable Nanoporous Network Polymer Nanocomposites having Size‐Selective Ion Transfer for Dye‐Sensitized Solar Cells

Nanoporous network polymer nanocomposites with tunable pore size for size‐dependent selective ion transport are successfully prepared via the surface‐induced cross‐linking polymerization of methyl methacrylate (MMA) and 1,6‐hexanediol diacrylate (HDDA) on the surfaces of nanocrystalline TiO₂ particles. The morphologies of the porous network polymer layer and nanopores were investigated by transmission electron microscopy (TEM), field emission scanning electron microscopy (FE‐SEM), and Brunauer–Emmett–Teller (BET) experiments. The porous layer size‐selectively screened the ions that contacted the nanocrystalline TiO₂ particles, as demonstrated by ion conductivity measurements, electrochemical impedance spectroscopy (EIS), and transient absorption spectroscopy (TAS).     

Band Edge Modulated Polymer Layer to Decrease Back Electron Transfer and Increase Efficiency in Sensitized Solar Cells

Recombination of charges residing in the TiO₂ and redox electrolyte is one of the factors affecting the efficiency of dye sensitized solar cells (DSSCs). To circumvent this recombination, inorganic oxide barrier layers and organic silanes have been coated on TiO₂/dyes. Due to the insulating nature of these layers, the efficiency increase is not very impressive. Conducting polymers with different band edges are used to suppress the charge recombination. Amongst the four polymers that are used as barrier layers, a polymer with a highest occupied molecular orbital energy at ?5.8 eV and lowest unoccupied molecular orbital at ?3.1 eV is found to increase the electron life time at TiO₂ and decrease the charge recombination. The electron life time is found to be 88 ms. In addition to the long electron life time, the recombination resistance of this polymer is also high (91 Ω). This resistance is 18% higher than that measured for DSSCs without polymer barrier layer. These factors impact the efficiency of DSSCs. DSSCs fabricated with this polymer as barrier layer exhibit an efficiency of 9.2%, which is 22% higher than that of DSSCs without polymer barrier layer.     

Photoinduced absorption spectroscopy as a tool in the study of dye-sensitized solar cells

Photoinduced absorption (PIA) spectroscopy, where the excitation is provided by a square-wave modulated (on/off) monochromatic light source, is a versatile tool in the study of dye-sensitized solar cells. Spectra of transient species, such as the oxidized dye, can easily be obtained and their kinetics can be explored using frequency or time-resolved techniques. Experimental PIA conditions can be kept close to typical solar cell operating conditions, allowing extraction of relevant time constants. PIA is also a suitable method to study the quality of pore filling in case of solid hole conductors. Dye molecules that are not in direct contact with the hole conductor will have long lifetimes in their oxidized state and appear clearly in the PIA spectrum. The basic principles of PIA are explained using the example of electron injection and recombination in dye-sensitized TiO₂ in the absence of redox electrolyte.     

Atomic‐Layer‐Deposited Aluminum and Zirconium Oxides for Surface Passivation of TiO2 in High‐Efficiency Organic Photovoltaics

The reduction in electronic recombination losses by the passivation of surfaces is a key factor enabling high‐efficiency solar cells. Here a strategy to passivate surface trap states of TiO₂ films used as cathode interlayers in organic photovoltaics (OPVs) through applying alumina (Al₂O₃) or zirconia (ZrO₂) insulating nanolayers by thermal atomic layer deposition (ALD) is investigated. The results suggest that the surface traps in TiO₂ are oxygen vacancies, which cause undesirable recombination and high electron extraction barrier, reducing the open‐circuit voltage and the short‐circuit current of the complete OPV device. It is found that the ALD metal oxides enable excellent passivation of the TiO₂ surface followed by a downward shift of the conduction band minimum. OPV devices based on different photoactive layers and using the passivated TiO₂ electron extraction layers exhibit a significant enhancement of more than 30% in their power conversion efficiencies compared to their reference devices without the insulating metal oxide nanolayers. This is a result of significant suppression of charge recombination and enhanced electron extraction rates at the TiO₂/ALD metal oxide/organic interface.     

A High Voltage Aqueous Zinc–Organic Hybrid Flow Battery

Water‐soluble redox‐active organic molecules have attracted extensive attention as electrical energy storage alternatives to redox‐active metals that are low in abundance and high in cost. Here an aqueous zinc–organic hybrid redox flow battery (RFB) is reported with a positive electrolyte comprising a functionalized 1,4‐hydroquinone bearing four (dimethylamino)methyl groups dissolved in sulfuric acid. By utilizing a three‐electrolyte, two‐membrane configuration this acidic positive electrolyte is effectively paired with an alkaline negative electrolyte comprising a Zn/[Zn(OH)₄]^2? redox couple and a hybrid RFB is operated at a high operating voltage of 2.0 V. It is shown that the electrochemical reversibility and kinetics of the organic redox species can be enhanced by an electrocatalyst, leading to a cyclic voltammetry peak separation as low as 35 mV and enabling an enhanced rate capability.     

Chilling-enhanced photooxidation: The production,action and study of reactive oxygen species produced during chilling in the light

Chilling-enhanced photooxidation is the light- and oxygen-dependent bleaching of photosynthetic pigments that occurs upon the exposure of chilling-sensitive plants to temperatures below approximately 10 °C. The oxidants responsible for the bleaching are the reactive oxygen species (ROS) singlet oxygen (¹O₂), superoxide anion radical (O ₂ ,hydrogen peroxide (H₂O₂), the hydroxyl radical (OH·), and the monodehydroascorbate radical (MDA) which are generated by a leakage of absorbed light energy from the photosynthetic electron transport chain. Cold temperatures slow the energy-consuming Calvin-Benson Cycle enzymes more than the energy-transducing light reactions, thus causing leakage of energy to oxygen. ROS and MDA are removed, in part, by the action of antioxidant enzymes of the Halliwell/Foyer/Asada Cycle. Chloroplasts also contain high levels of both lipid- and water-soluble antioxidants that act alone or in concert with the HFA Cycle enzymes to scavenge ROS. The ability of chilling-resistant plants to maintain active HFA Cycle enzymes and adequate levels of antioxidants in the cold and light contributes to their ability to resist chilling-enhanced photooxidation. The absence of this ability in chilling-sensitive species makes them susceptible to chilling-enhanced photooxidation. Chloroplasts may reduce the generation of ROS by dissipating the absorbed energy through a number of quenching mechanisms involving zeaxanthin formation, state changes and the increased usage of reducing equivalents by other anabolic pathways found in the stroma. During chilling in the light, ROS produced in chilling-sensitive plants lower the redox potential of the chloroplast stroma to such a degree that reductively-activated regulatory enzymes of the Calvin Cycle, sedohepulose 1,7 bisphosphatase (EC 3.1.3.37) and fructose 1,6 bisphosphatase (EC 3.1.3.11), are oxidatively inhibited. This inhibition is reversible in vitro with a DTT treatment indicating that the enzymes themselves are not permanently damaged. The inhibition of SBPase and FBPase may fully explain the inhibition in whole leaf gas exchange seen upon the rewarming of chilling-sensitive plants chilled in the light. Methods for the study of ROS in chilling-enhanced photooxidation and challenges for the future are discussed.Abbreviations ASP ascorbate-specific peroxidase - -TH reduced -tocopherol - DTT dithiothreitol - FBP fructose 1,6 bisphosphate - FBPase fructose 1,6 bisphosphatase (EC 3.1.3.11) - HFA Cycle the Halliwell/Foyer/Asada Cycle responsible for the enzymatic removal of ROS in the chloroplast stroma - MDA monodehydroascorbate radical - MDAR monodehydroascorbate reductase - ROS reactive oxygen species - SBP sedohepulose 1,7 bisphosphate - SBPase sedohepulose 1,7 bisphosphatase (EC 3.1.3.37) - SOD superoxide dismutase     

Titanium Dioxide Nanoparticles Trigger Loss of Function and Perturbation of Mitochondrial Dynamics in Primary Hepatocytes

Titanium dioxide (TiO₂) nanoparticles are one of the most highly manufactured and employed nanomaterials in the world with applications in copious industrial and consumer products. The liver is a major accumulation site for many nanoparticles, including TiO₂, directly through intentional exposure or indirectly through unintentional ingestion via water, food or animals and increased environmental contamination. Growing concerns over the current usage of TiO₂ coupled with the lack of mechanistic understanding of its potential health risk is the motivation for this study. Here we determined the toxic effect of three different TiO₂ nanoparticles (commercially available rutile, anatase and P25) on primary rat hepatocytes. Specifically, we evaluated events related to hepatocyte functions and mitochondrial dynamics: (1) urea and albumin synthesis using colorimetric and ELISA assays, respectively; (2) redox signaling mechanisms by measuring reactive oxygen species (ROS) production, manganese superoxide dismutase (MnSOD) activity and mitochondrial membrane potential (MMP); (3) OPA1 and Mfn-1 expression that mediates the mitochondrial dynamics by PCR; and (4) mitochondrial morphology by MitoTracker Green FM staining. All three TiO₂ nanoparticles induced a significant loss (p < 0.05) in hepatocyte functions even at concentrations as low as 50 ppm with commercially used P25 causing maximum damage. TiO₂ nanoparticles induced a strong oxidative stress in primary hepatocytes. TiO₂ nanoparticles exposure also resulted in morphological changes in mitochondria and substantial loss in the fusion process, thus impairing the mitochondrial dynamics. Although this study demonstrated that TiO₂ nanoparticles exposure resulted in substantial damage to primary hepatocytes, more in vitro and in vivo studies are required to determine the complete toxicological mechanism in primary hepatocytes and subsequently liver function.     

Microsomal metabolism of hydroxyl radical scavenging agents: Relationship to the microsomal oxidation of alcohols

Previous studies provided indirect evidence that hydroxyl radicals are involved in the oxidation of primary aliphatic alcohols by rat liver microsomes. In the current study, three ·OH scavengers were used as chemical probes to evaluate ·OH production by microsomes. The scavengers and their products were 3-thiomethylpropanal (methional) and 2-keto-4-thiomethylbutyric acid, which yield ethylene gas, and dimethylsulfoxide, which yields methane gas. We observed that microsomes actively generate the appropriate hydrocarbon gas from each scavenger when electron transport is initiated with NADPH. Hydrocarbon gas production is augmented by 0.5 mm azide, an agent which inhibits catalase and, thereby, permits H₂O₂ to accumulate. However, no metabolism of scavengers occurs when H₂O₂ is added in the absence of microsomes. These results are consistent with a presumed role for H₂O₂ as a precursor of hydroxyl radicals. In addition, no metabolism of scavengers occurs when azide and H₂O₂ are added either to boiled microsomes or to intact microsomes in the absence of electron transport (NADPH-generating system omitted). Therefore, both H₂O₂ and simultaneous electron transport are required. Ethanol inhibits the metabolism of the scavengers. Similarly, the scavengers inhibit the oxidation of ethanol to acetaldehyde; inhibition in the presence of azide is competitive. These latter results indicate a competition between the scavengers and ethanol for metabolically generated ·OH in microsomes. The specificity of this interaction is evident from the observation that the scavengers do not affect the activities of microsomal aminopyrine demethylase or aniline hydroxylase. Two model ·OH-generating systems (Fenton's reagent and iron-EDTA-ascorbate) were also studied and they produced acetaldehyde from ethanol and hydrocarbon gases from the scavengers. These results, as a whole, tend to verify a role for ·OH in the microsomal oxidation of alcohols.     

Extended π‐Bridge in Organic Dye‐Sensitized Solar Cells: the Longer,the Better?

The elongation of π‐conjugated bridges between the donor (D) and the acceptor (A) represents a feasible strategy towards enhancement of light‐harvesting in both breadth and depth of organic D‐π‐A dyes suitable for nanocrystalline TiO₂‐based dye‐sensitized solar cells (DSSCs). Here, a series of organic dyes with elongating conjugated bridges is synthesized and characterized. DSSC devices employing a cobalt (II/III) redox electrolyte are fabricated using these dyes as light‐harvesting sensitizers. Compared to a dye with the 3,4‐ethylenedioxythiophene (EDOT) linker ( G188 ), the three counterparts with further extended π‐bridges present gradually red‐shifted electronic absorption spectra and a persistent decrease in oxidation potential. The photocurrent action spectra show that the extension of π‐conjugated bridges decreases the open‐circuit photovoltage. The best performance is shown in G268 with a short‐circuit photocurrent density (J_sc) of 16.27 mA cm², an open‐circuit photovoltage (V_oc) of 0.83 V, and a fill factor (FF) of 0.67, corresponding to an overall conversion efficiency of 9.24%. Unexpectedly, G270, which has with the longest π‐bridge , showed the lowest J_sc, V_oc, and efficiency.     

Cooperation of photosystem I with the plastoquinone pool in oxygen reduction in higher plant chloroplasts

The possible functions of a light-induced electron transfer to oxygen in the photosynthetic electron transport chain of higher plant chloroplasts are considered. The thermodynamic preconditions, as well as the experimental data about the participations of ferredoxin, the components of photosystems I and II, and plastoquinone in oxygen reduction are examined. It is concluded that, even in the presence of ferredoxin and ferredoxin + NADP⁺, oxygen reduction is carried out mainly by the membrane-bound carriers of the photosynthetic electron transport chain. The hypothesis is put forward that most superoxides, which are produced by reduction of O₂ molecules by the intramembrane components of the acceptor side of photosystem I, are reduced within the membrane by the plastohydroquinone molecules to the hydrogen peroxide. It is assumed that the H₂O₂ molecules that originate as the result of this process serve for signaling about the redox state of the plastoquinone pool. Published in Russian in Biokhimiya, 2008, Vol. 73, No. 1, pp. 137–144.     

Cellular Toxicity of TiO2 Nanoparticles in Anatase and Rutile Crystal Phase

Titanium dioxide nanoparticles are massively produced and widely used in daily life, which has posed potential risk to human health. However, the molecular mechanism of TiO₂ nanoparticles (NPs) with different crystal phases is not clear. In this study, the characterization of two crystalline phases of TiO₂ NPs is evaluated by transmission electron microscopy and X-ray absorption fine structure spectrum; an interaction of these TiO₂ NPs with HaCaT cells is studied in vitro using transmission electron microscopy, chemical precipitation method, and X-ray absorption fine structure spectrometry. The coordination and surface properties indicate that only the anatase–TiO₂ NPs allow spontaneous reactive oxygen species (ROS) generation, but rutile–TiO₂ NPs do not after dispersion. The interaction between TiO₂ NPs and cellular components might also generate ROS for both anatase–TiO₂ NPs and rutile–TiO₂ NPs. The ROS generation could lead to cellular toxicity if the level of ROS production overwhelms the antioxidant defense of the cell or induces the mitochondrial apoptotic mechanisms. Furthermore, Ti had a direct combination with some protein or DNA after NPs enter the cell, which could also lead to cellular toxicity.     

Effect of NADP+ on light-induced cytochrome changes in membrane fragments from a blue-green alga

The effect of NADP⁺ on light-induced steady-state redox changes of membrane-bound cytochromes was investigated in membrane fragments prepared from the blue-green algae Nostoc muscorum (Strain 7119) that had high rates of electron transport from water to NADP⁺ and from an artificial electron donor, reduced dichlorophenolindophenol (DCIPH₂) to NADP⁺. The membrane fragments contained very little phycocyanin and had excellent optical properties for spectrophotometric assays. With DCIPH₂ as the electron donor, NADP⁺ had no effect on the light-induced redox changes of cytochromes: with or without NADP⁺, 715- or 664-nm illumination resulted mainly in the oxidation of cytochrome f and of other component(s) which may include a c-type cytochrome with an α peak at 549 nm. With 664 nm illumination and water as the electron donor, NADP⁺ had a pronounced effect on the redox state of cytochromes, causing a shift toward oxidation of a component with a peak at 549 nm (possibly a c-type cytochrome), cytochrome f, and particularly cytochrome b₅₅₉. Cytochrome b₅₅₉ appeared to be a component of the main noncyclic electron transport chain and was photooxidized at physiological temperatures by Photosystem II. This photooxidation was apparent only in the presence of a terminal acceptor (NADP⁺) for the electron flow from water.     

Modeling of the photosynthetic electron transport regulation in cyanobacteria

In this work we have performed a computer analysis of electron and proton transport in cyanobacterial cells using a mathematical model of light-dependent stages of photosynthesis taking into account the key stages of pH-dependent regulation of electron transport on both acceptor and donor sides of photosystem 1 (PS1). Comparison of theoretical and experimental data shows that the model adequately describes the multiphase kinetics of photoinduced redox transformations of P₇₀₀ (the primary electron donor in PS1). Our computer simulation describes the effect of variations of atmospheric gases (CO₂ and O₂) on the induction events in cyanobacteria (P₇₀₀ photooxidation, generation of transmembrane ΔpH), which strongly depends on the preillumination conditions (aerobic or anaerobic atmosphere). It has been shown that the variations of CO₂ concentration in the cell interior may noticeably affect the kinetics of electron transport, acidification of lumen, and ATP synthesis. The contributions of alternative pathways of electron transport (cyclic electron transport around PS1 and electron outflow to O₂) to the function of cyanobacterial photosynthetic apparatus have been analyzed. At the initial stage of induction period, cyclic electron flows around PS1 (“short” and “long” pathways) substantially contribute to photosynthetic electron transport. These flows, however, attenuate with the light-induced activation of the Calvin-Benson cycle reactions. In the meantime, the outflow of electrons from PS1 to O₂ (or to other metabolic chains) increases with oxygen accumulation in the medium. The effects of ferredoxin oxidation by hydrogenase catalyzing the H₂ formation on the kinetics of P700 photooxidation and distribution of electron flows on the acceptor side of PS1 have been modeled.     

Efficiency enhancement of the electrocatalytic reduction of CO2: fac-[Re(v-bpy)(CO)3Cl] electropolymerized onto mesoporous TiO2 electrodes

As the greenhouse effect increases, the development of systems able to convert with high efficiency CO₂ to energetically rich molecules owns a crucial weight in the technological and environmental domain. As catalyst, rhenium complexes, of the type fac-[Re(L)(CO)₃Cl] (i.e. L = 2,2′-bipyridyl or 4,4′-bipyridyl), have attracted a large interest demonstrating promising catalytic properties. fac-[Re(v-bpy)(CO)₃Cl]-based polymer deposited onto a solid support has been already investigated as heterogeneous catalyst in the reduction of CO₂. Here, we deposited by electrochemical polymerization fac-[Re(v-bpy)(CO)₃Cl] onto a nanocrystalline TiO₂ film on glass and we investigated by cyclic voltammetry the properties of such heterogeneous catalyst in the electrochemical reduction of CO₂. We demonstrated that the nanoporous nature of the substrate allows to increase the two-dimensional number of redox sites per surface area and hence to get a significant enhancement of the catalytic yield.     

Overcoming the “Light‐Soaking” Issue in Inverted Organic Solar Cells by the Use of Al:ZnO Electron Extraction Layers

A common phenomenon of organic solar cells (OSCs) incorporating metal‐oxide electron extraction layers is the requirement to expose the devices to UV light in order to improve device characteristics – known as the so‐called “light‐soaking” issue. This behaviour appears to be of general validity for various metal‐oxide layers, various organic donor/acceptor systems, and regardless if single junction devices or multi stacked cells are considered. The requirement of UV exposure of OSCs may impose severe problems if substrates with limited UV transmission, UV blocking filters or UV to VIS down‐conversion concepts are applied. In this paper, we will demonstrate that this issue can be overcome by the use of Al doped ZnO (AZO) as electron extraction interlayer. In contrast to devices based on TiO_x and ZnO, the AZO devices show well‐behaved solar cell characteristics with a high fill factor (FF) and power conversion efficiency (PCE) even without the UV spectral components of the AM1.5 solar spectrum. As opposed to previous claims, our results indicate that the origin of s‐shaped characteristics of the OSCs is the metal‐oxide/organic interface. The electronic structures of the TiO_x/fullerene and AZO/fullerene interfaces are studied by photoelectron spectroscopy, revealing an electron extraction barrier for the TiO_x/fullerene case and facilitated electron extraction for AZO/fullerene. These results are of general relevance for organic solar cells based on various donor acceptor active systems.     

Molecular Design of Interfacial Modifiers for Polymer‐Inorganic Hybrid Solar Cells

The heterojunction of poly(3‐hexylthiophene) (P3HT) and TiO₂ in hybrid solar cells is systematically engineered with four cyanoacrylic acid‐containing conjugated molecules with various lowest unoccupied molecular orbital (LUMO) levels, WL‐1 to WL‐4, which are prepared by the formylation of thiophene derivatives in a Vilsmeier–Haack reaction, followed by treatment with cyanoacetic acid. The optical characteristics, redox properties, and intrinsic dipole moments of these interfacial modifiers (IMs) are examined using UV‐vis spectrophotometry, cyclic voltammetry, and density functional theory calculations. Using cyanoacrylic acid as a terminal anchoring group in IMs increases the electron affinity in regions close to the titania surface and forms a molecular dipole that is orientated away from the TiO₂ surface, enabling both open‐circuit voltage (V_OC) and short‐circuit current density to be increased simultaneously. Photovoltaic measurements demonstrate that V_OC increases with the dipole moment of IMs along the molecular backbone. Moreover, the external quantum efficiency (EQE) spectra display a bimodal distribution, revealing that both IMs and P3HT contribute to the photocurrent. The EQE at 570 nm is identified as characteristic of P3HT. More importantly, the LUMO of the IMs decisively determines the dissociation efficiency of P3HT excitons. The device based on P3HT/WL‐4/TiO₂ exhibits the highest power conversion efficiency of 2.87%.     

Interfacial Modification Using Hydrogenated TiO2 Electron‐Selective Layers for High‐Efficiency and Light‐Soaking‐Free Organic Solar Cells

Optimizing the interfacial contacts between the photoactive layer and the electrodes is an important factor in determining the performance of organic solar cells (OSCs). A charge‐selective layer with tailored electrical properties enhances the charge collection efficiency and interfacial stability. Here, the potential of hydrogenated TiO₂ nanoparticles (H‐TiO₂ NPs) as an efficient electron‐selective layer (ESL) material in OSCs is reported for the first time. The H‐TiO₂ is synthesized by discharge plasma in liquid at atmospheric pressure, which has the benefits of a simple one‐pot synthesis process, rapid and mild reaction conditions, and the capacity for mass production. The H‐TiO₂ exhibits high conductivity and favorable energy level formation for efficient electron extraction, providing a basis for an efficient bilayer ESL system composed of conjugated polyelectrolyte/H‐TiO₂. Thus, the enhanced charge transport and extraction efficiency with reduced recombination losses at the cathode interfacial contacts is achieved. Moreover, the OSCs composed of H‐TiO₂ are almost free of light soaking, which has been reported to severely limit the performance and stability of OSCs based on conventional TiO₂ ESLs. Therefore, H‐TiO₂ as a new efficient, stable, and cost‐effective ESL material has the potential to open new opportunities for optoelectronic devices.     

Human erythrocytes exposure to juglone leads to an increase of superoxide anion production associated with cytochrome b5 reductase uncoupling

Cytochrome b₅ reductase is an enzyme with the ability to generate superoxide anion at the expenses of NADH consumption. Although this activity can be stimulated by cytochrome c and could participate in the bioenergetic failure accounting in apoptosis, very little is known about other molecules that may uncouple the function of the cytochrome b₅ reductase. Naphthoquinones are redox active molecules with the ability to interact with electron transfer chains. In this work, we made an inhibitor screening against recombinant human cytochrome b₅ reductase based on naphthoquinone properties. We found that 5-hydroxy-1,4-naphthoquinone (known as juglone), a natural naphthoquinone extracted from walnut trees and used historically in traditional medicine with ambiguous health and toxic outcomes, had the ability to uncouple the electron transfer from the reductase to cytochrome b₅ and ferricyanide. Upon complex formation with cytochrome b₅ reductase, juglone is able to act as an electron acceptor leading to a NADH consumption stimulation and an increase of superoxide anion production by the reductase. Our results suggest that cytochrome b₅ reductase could contribute to the measured energetic failure in the erythrocyte apoptosis induced by juglone, that is concomitant with the reactive oxygen species produced by cytochrome b₅ reductase.     

Regulation of physiological aspects in plants by hydrogen sulfide and nitric oxide under challenging environment

Plants are exposed to a plethora of abiotic stresses such as drought, salinity, heavy metal and temperature stresses at different stages of their life cycle, from germination to seedling till the reproductive phase. As protective mechanisms, plants release signaling molecules that initiate a cascade of stress-signaling events, leading either to programmed cell death or plant acclimation. Hydrogen sulfide (H₂S) and nitric oxide (NO) are considered as new ‘gasotransmitter’ molecules that play key roles in regulating gene expression, posttranslational modification (PTM), as well as cross-talk with other hormones. Although the exact role of NO in plants remains unclear and is species dependent, various studies have suggested a positive correlation between NO accumulation and environmental stress in plants. These molecules are also involved in a large array of stress responses and act synergistically or antagonistically as signaling components, depending on their respective concentration. This study provides a comprehensive update on the signaling interplay between H₂S and NO in the regulation of various physiological processes under multiple abiotic stresses, modes of action and effects of exogenous application of these two molecules under drought, salt, heat and heavy metal stresses. However, the complete picture of the signaling cascades mediated by H₂S and NO is still elusive. Recent researches indicate that during certain plant processes, such as stomatal closure, H₂S could act upstream of NO signaling or downstream of NO in response to abiotic stresses by improving antioxidant activity in most plant species. In addition, PTMs of antioxidative pathways by these two molecules are also discussed.