Transmembrane Electrical Potential Research Articles

Proton gradient across the chloroplast thylakoid membrane governs the redox regulatory function of ATP synthase

Chloroplast ATP synthase (CFoCF1) synthesizes ATP by using a proton electrochemical gradient across the thylakoid membrane, termed ΔμH+, as an energy source. This gradient is necessary not only for ATP synthesis but also for reductive activation of CFoCF1 by thioredoxin, using reducing equivalents produced by the photosynthetic electron transport chain. ΔμH+comprises two thermodynamic components: pH differences across the membrane (ΔpH) and the transmembrane electrical potential (ΔΨ). In chloroplasts, the ratio of these two components in ΔμH+ is crucial for efficient solar energy utilization. However, the specific contribution of each component to the reductive activation of CFoCF1 remains unclear. In this study, an invitro assay system for evaluating thioredoxin-mediated CFoCF1 reduction is established, allowing manipulation of ΔμH+components in isolated thylakoid membranes using specific chemicals. Our biochemical analyses revealed that ΔpH formation is essential for thioredoxin-mediated CFoCF1 reduction on the thylakoid membrane, whereas ΔΨ formation is nonessential.

Electrochemical analysis of ion-exchange membranes fouling during electrodialysis treatment of real shale gas flowback water

Electrodialysis (ED) is considered an effective technology for desalinating shale gas flowback fluid toward environmental risk control and resource recovery. However, membrane fouling remains a significant challenge and the particular fouling behavior of ion-exchange membranes (IEMs) in real scenario remains unclear. In this study, we employed interface characterization techniques including electrochemical impedance spectroscopy (EIS) to investigate the fouling behavior of both anion-exchange membranes (AEMs) and cation-exchange membranes (CEMs) during the desalination process. Notably, a positive correlation was observed between the transmembrane electric potential (TMEP) of the target membrane and the extent of membrane fouling during ED processing. EIS data revealed the membrane bulk exhibited the most pronounced increase in modeled electrical resistance, suggesting that internal fouling more significantly inhibited mass transfer compared to external fouling. Under varying current densities, the resistance of the AEM and CEM membrane bulk increased from an initial value of 3.93–16.53 Ω cm2 and from 3.90 to 9.87 Ω cm2, respectively, corresponding to their maximum TMEP increase by factors of 6.0 and 2.0 during the ED treatment. Our results demonstrate that AEM undergone a more severe organic fouling than CEM (scaling) during desalination of real shale gas flowback fluid. Effective strategies to mitigate the fouling issue in IEMs were finally proposed. These findings provide valuable guidance on future process optimizing and fouling mitigation in the ED treatment of real shale gas flowback fluid.

Use of bacterial photosynthetic vesicles to evaluate the effect of ionic liquids on the permeability of biological membranes

Ionic liquids (ILs) are salts composed of a combination of organic or inorganic cations and anions characterized by a low melting point, often below 100 °C. This property, together with an extremely low vapor pressure, low flammability and high thermal stability, makes them suitable for replacing canonical organic solvents, with a reduction of industrial activities impact on the environment. Although in the last decades the eco-compatibility of ILs has been extensively verified through toxicological tests performed on model organisms, a detailed understanding of the interaction of these compounds with biological membranes is far from being exhaustive. In this context, we have chosen to evaluate the effect of some ILs on native membranes by using chromatophores, photosynthetic vesicles that can be isolated from Rhodobacter capsulatus, a member of the purple non‑sulfur bacteria. Here, carotenoids associated with the light-harvesting complex II, act as endogenous spectral probes of the transmembrane electrical potential (ΔΨ). By measuring through time-resolved absorption spectroscopy the evolution of the carotenoid band shift induced by a single excitation of the photosynthetic reaction center, information on the ΔΨ dissipation due to ionic currents across the membrane can be obtained. We found that some ILs cause a rather fast dissipation of the transmembrane ΔΨ even at low concentrations, and that this behavior is dose-dependent. By using two different models to analyze the decay of the carotenoid signals, we attempted to interpret at a mechanistic level the marked increase of ionic permeability caused by specific ILs.

Determination of cell membrane permeability coefficients: Comparison of models in the case of oocytes

Values of cell membranes permeability coefficients for water and molecules of cryoprotective agents (CPAs) are the necessary characteristics for developing physical-mathematical models describing mass transfer processes through cell membranes in order to predict optimal cell cooling rates. We carried out a comparative analysis of the permeability coefficients of mouse oocyte membranes for molecules of water, ethylene glycol (EG), propane-1,2-diol (1,2-PD) and dimethyl sulfoxide (Me2SO), determined by applying the classical Kedem-Katchalsky model, which considers only the penetration of non-electrolyte molecules (water and CPA) through the membrane, and the model developed by us, which takes into account the transmembrane transfer of ions and the associated changes in the transmembrane electric potential. We shown that calculations based on the developed modified model provide lower values of the permeability coefficients of the oocyte membrane for water and CPA molecules. What is important that the obtained by our modified model permeability coefficients for water molecules do not depend on the type of cryoprotectant, while the application of the classical model both in our studies and works of other authors always gave different values of these coefficients in solutions with different cryoprotectants. Our modified model also makes it possible to determine the dynamics of the transmembrane electric potential of the cell under the conditions of transmembrane mass transfer and the duration of the membrane being influenced by the changes in electric potential, that is a parameter that can directly affect the viability of cells.

A real-time analysis of protein transport via the twin arginine translocation pathway in response to different components of the protonmotive force

The twin arginine translocation (Tat) pathway transports folded protein across the cytoplasmic membrane in bacteria, archaea, and across the thylakoid membrane in plants as well as the inner membrane in some mitochondria. In plant chloroplasts, the Tat pathway utilizes the protonmotive force (PMF) to drive protein translocation. However, in bacteria, it has been shown that Tat transport depends only on the transmembrane electrical potential (Δψ) component of PMF invitro. To investigate the comprehensive PMF requirement in Escherichia coli, we have developed the first real-time assay to monitor Tat transport utilizing the NanoLuc Binary Technology in E.coli spheroplasts. This luminescence assay allows for continuous monitoring of Tat transport with high-resolution, making it possible to observe subtle changes in transport in response to different treatments. By applying the NanoLuc assay, we report that, under acidic conditions (pH= 6.3), ΔpH, in addition to Δψ, contributes energetically to Tat transport invivo in E.coli spheroplasts. These results provide novel insight into the mechanism of energy utilization by the Tat pathway.

Time-Dependent Features of Mass Transfer and Transmembrane Potential in Erythrocytes During Equilibration in Cryoprotective Solutions

On the basis of the developed physical and mathematical model of mass transfer, which takes into account the transmembrane transfer of non-electrolytes, basic ions and the associated changes in the transmembrane potential, the redistribution of osmotically active substances during equilibration of erythrocytes in cryoprotective solutions was investigated. Time parameters of changes in concentrations of osmotically active substances inside and outside cells, as well as transmembrane electric potential, were calculated. It is shown that during the exposure of human erythrocytes to 1M solutions of glycerol, 1,2-propanediol (1,2-PD) and dimethylsulfoxide (DMSO), the sign of their transmembrane electric potential changes three times, and in solutions of ethylene glycol (EG) and of acetamide (AA) – once. The analysis of the obtained results showed that the most acceptable for further cryopreservation from the point of view of erythrocytes reaching a state close to equilibrium in a 1M solution of glycerol was their equilibration for 5.5 min, and in solutions of DMSO, AA, EG and 1,2-PD with the same concentration – 1 min. At the same time, the cells remain somewhat dehydrated (by 5.5–7.5%), and the concentrations of cryoprotectants inside erythrocytes change insignificantly during longer exposure. The indicated degree of dehydration does not affect cell viability, but reduces the likelihood of intracellular ice formation during subsequent freezing.

CCCP Facilitates Aminoglycoside to Kill Late Stationary-Phase Escherichia coli by Elevating Hydroxyl Radical.

Improving the efficacy of existing antibiotics is significant for combatting antibiotic resistance that poses a major threat to human health. Carbonyl cyanide m-chlorophenylhydrazine (CCCP), a well-known protonophore for dissipating proton motive force (PMF), has been widely used to block the PMF-dependent uptake of aminoglycoside antibiotics and thus suppress aminoglycoside lethality. Here, we report that CCCP and its functional analog FCCP, but not other types of protonophores, unprecedently potentiate aminoglycosides (e.g., tobramycin and gentamicin) by 3-4 orders of magnitude killing of Escherichia coli, Staphylococcus aureus, Shigella flexneri, and Vibrio alginolyticus cells in stationary phase but not these cells in exponential phase nor other 12 bacterial species we examined. Overall, the effect of CCCP on aminoglycoside lethality undergoes a gradual transition from suppression against E. coli exponential-phase cells to potentiation against late stationary-phase cells, with the cell growth status and culture medium being crucial. Consistently, disturbance of the PMF by changing transmembrane proton gradient (ΔpH) or electric potential (ΔΨ) also potentiates tobramycin. Nevertheless, CCCP neither increases the intracellular concentration of tobramycin nor decreases the MIC of the antibiotic, thus excluding that CCCP acts as an efflux pump inhibitor to potentiate aminoglycosides. Rather, we show that the combined treatment dramatically enhances the cellular level of hydroxyl radical under both aerobic and anaerobic culturing conditions, under which the antioxidant N-acetyl cysteine fully suppresses both hydroxyl radical accumulation and cell death. Together, these findings open a new avenue to develop certain protonophores as aminoglycoside adjuvants against pathogens in stationary phase and also illustrate an essential role of hydroxyl radical in aminoglycoside lethality regardless of aerobic respiration.

Interactions of Surfactants with Biomimetic Membranes-2. Generation of Electric Potential with Non-Ionic Surfactants.

It is known that noncharged surfactants lead to electric effects that interact with biomimetic membranes made of nitrocellulose filters, which are impregnated with fatty acid esters. At a surfactant concentration as low as 64 micrometers in one of the solutions, they lead to the transient formation of transmembrane electric potential. Maximum changes of this potential are proportional to the log of noncharged surfactant concentrations when it changes by three orders of magnitude. We explain this new and nontrivial effect in terms of an earlier suggested physicochemical mechanics approach and noncharged surfactants transient changes induced by membrane permeability for inorganic ions. It could be used to imitate the interactions of non-ionic drugs with biological membranes. The effect may also be used in determining the concentration of these surfactants and other non-ionic chemicals of concern, such as pharmaceuticals and personal care products.

Fouling behavior of anion-exchange membrane during electrodialysis of biogas slurry: Correlation to applied current

Electrodialysis draws recent attention toward valorization of biogas slurry, especially in terms of inorganic nutrient selectivity. Nevertheless, wide application of this process is impeded by organic fouling of anion-exchange membrane (AEM) dealing with such a complex stream, which lacks of a systematic investigation so far. Herein, AEM fouling by real biogas slurry under varying current density was delicately evaluated in terms of transmembrane electrical potential and membrane bulk resistance, beyond the conventional monitoring on dilute conductivity and solution pH for this desalination process. Our results showed that TMEP of the target AEM was very sensitive to the applied current, but a negligible variation of desalination rate and solution pH indicated a limited occurrence of water splitting. Then, the existence of apolar and large sized foulant in biogas slurry (probably the detected humic-like substances) and few salt crystals from the membranes together suggested that organic fouling was mainly responsible for the fouling issue. Finally, EIS data revealed Rm as the main component of membrane resistance for the fouled membrane, indicating a non-negligible internal fouling, which was also positively related to the applied current. Our result implied operating electrodialysis at a lower current density for biogas slurry is recommended from the perspective of membrane fouling mitigation and operation cost optimization in pursuit of long-term efficacy for nutrient recovery and valorization.

Insights into the antibacterial mechanism of ozone water combined with tea polyphenols against Shewanella putrefaciens: membrane disruption and oxidative stress

SummaryShewanella putrefaciens (S. putrefaciens) is the main specific spoilage organism (SSO) of marine fish during cold logistic process. To explore the antimicrobial mechanism of ozone water combined with tea polyphenol (TP) against S. putrefaciens, the growth condition, endogenous oxidative stress, changes of membrane integrity and energy metabolism enzyme were evaluated, respectively. As evidenced by the growth curve and confocal laser scanning microscopy (CLSM) analysis, bacterial growth was inhibited, and death threat occurred. The increase of reactive oxygen species (ROS) suggested intracellular oxidative stress was activated. Additionally, membrane integrity and metabolism functions were destroyed according to the results of alkaline phosphatase (AKPase), β‐galactosidase, transmembrane electrical potential, scanning electron microscope (SEM), adenosine triphosphatase (ATPase) and lactate dehydrogenase (LDHase). In summary, the inactivation of S. putrefaciens could be induced by the combination of ozone water and TP, which provided a theoretical basis for the sterilisation in aquatic products.

Unravelling the fouling behavior of anion-exchange membrane (AEM) by organic solute of varying characteristics

Organic fouling is a major obstacle in various ion-exchange membrane based electrochemical systems. Particularly, the correlation between the main characteristics of organic solute and fouling of anion-exchange membrane (AEM) is still controversial. Herein, AEM fouling by four target charged organic solutes of distinct charge, hydrophobicity, molecular weight and solute size was explored in electrodialysis, evaluated using three fouling indicators (i.e., transition time, transmembrane migration, and membrane resistance). Sodium dodecyl sulfate (SDS) compared to the other three much larger organic solutes induced immediate rising of transmembrane electrical potential (TMEP), i.e., transition time approximating zero. Surprisingly, all organic solutes penetrated across the nano-scaled membrane, supported by the total organic carbon (TOC) content of concentrate. Then, in each fouling case, membrane impedance analysis highlighted that the resistance of membrane bulk (Rm) was dominant compared to those of the interfaces and external fouling layer, thus suggesting occurrence of severe internal fouling. Finally, correlation analysis using R language demonstrated that the electronegativity and hydrophobicity of organic solutes were highly responsible for AEM fouling (contribution >70%) under each evaluation regime, thus being more important than molecule weight and size. These results bring an insight about the relative importance of and can be useful for developing future more appropriate anti-fouling strategies for ion-exchange membranes subjected to organic fouling.

Proton Motive Force Inhibitors Are Detrimental to Methicillin-Resistant Staphylococcus aureus Strains.

ABSTRACTMethicillin-resistant Staphylococcus aureus (MRSA) strains are tolerant of conventional antibiotics, making them extremely dangerous. Previous studies have shown the effectiveness of proton motive force (PMF) inhibitors at killing bacterial cells; however, whether these agents can launch a new treatment strategy to eliminate antibiotic-tolerant cells mandates further investigation. Here, using known PMF inhibitors and two different MRSA isolates, we showed that the bactericidal potency of PMF inhibitors seemed to correlate with their ability to disrupt PMF and permeabilize cell membranes. By screening a small chemical library to verify this correlation, we identified a subset of chemicals (including nordihydroguaiaretic acid, gossypol, trifluoperazine, and amitriptyline) that strongly disrupted PMF in MRSA cells by dissipating either the transmembrane electric potential (ΔΨ) or the proton gradient (ΔpH). These drugs robustly permeabilized cell membranes and reduced MRSA cell levels below the limit of detection. Overall, our study further highlights the importance of cellular PMF as a target for designing new bactericidal therapeutics for pathogens.IMPORTANCE Methicillin-resistant Staphylococcus aureus (MRSA) emerged as a major hypervirulent pathogen that causes severe health care-acquired infections. These pathogens can be multidrug-tolerant cells, which can facilitate the recurrence of chronic infections and the emergence of diverse antibiotic-resistant mutants. In this study, we aimed to investigate whether proton motive force (PMF) inhibitors can launch a new treatment strategy to eliminate MRSA cells. Our in-depth analysis showed that PMF inhibitors that strongly dissipate either the transmembrane electric potential or the proton gradient can robustly permeabilize cell membranes and reduce MRSA cell levels below the limit of detection.

Application of direct electrometry in studies of microbial rhodopsins reconstituted in proteoliposomes.

Microbial rhodopsins are the family of retinal-containing proteins that perform primarily the light-driven transmembrane ion transport and sensory functions. They are widely distributed in nature and can be used for optogenetic control of the cellular activities by light. Functioning of microbial rhodopsins results in generation of the transmembrane electric potential in response to a flash that can be measured by direct time-resolved electrometry. This method was developed by L. Drachev and his colleagues at the Belozersky Institute and successfully applied in the functional studies of microbial rhodopsins. First measurements were performed using bacteriorhodopsin from Halobacterium salinarum-the prototype member of the microbial retinal protein family. Later, direct electrometric studies were conducted with proteorhodopsin from Exiguobacterium sibiricum (ESR), the sodium pump from Dokdonia, and other proteins. They allowed detailed characterization of the charge transfer steps during the photocycle of microbial rhodopsins and provided new insights for profound understanding of their mechanism of action.

Measurements of the light-induced steady state electric potential generation by photosynthetic pigment-protein complexes.

In this minireview, we consider the methods of measurements of the light-induced steady state transmembrane electric potential (Δψ) generation by photosynthetic systems, e.g. photosystem I (PS I). The microelectrode technique and the detection of electrochromic bandshifts of carotenoid pigments are most appropriate for Δψ measurements in situ and in vivo. Direct electrometrical method and Δψ measurements in the photovoltaic system based on membrane filter (MF) sandwiched between semiconductor indium tin oxide electrodes (ITO) are suitable for studies of isolated pigment-protein complexes and small natural vesicles-chromatophores. In the presence of trehalose, ITO|PS I-MF|ITO system allows to keep a steady state level of ∆ψ after 1h of illumination. According to preliminary experiments, this system is capable of providing steady state light-induced ∆ψ after several months of storage in the dark at room temperature under controlled humidity in the presence of trehalose. The long-term generation of light-induced ∆ψ in relatively simple system may serve as a source of the solar-to-electric energy conversion.

Organoplatinum Compounds as Anion‐Tuneable Uphill Hydroxide Transporters

AbstractActive transport of ions uphill, creating a concentration gradient across a cell membrane, is essential for life. It remains a significant challenge to develop synthetic systems that allow active uphill transport. Here, a transport process fuelled by organometallic compounds is reported that creates a pH gradient. The hydrolysis reaction of PtII complexes results in the formation of aqua complexes that established rapid transmembrane movement (“flip‐flop”) of neutral Pt−OH species, leading to protonation of the OH group in the inner leaflet, generating OH− ions, and so increasing the pH in the intravesicular solution. The organoplatinum complex effectively transports bound hydroxide ions across the membrane in a neutral complex. The initial net flow of the PtII complex into the vesicles generates a positive electric potential that can further drive uphill transport because the electric potential is opposed to the chemical potential of OH−. The OH− ions equilibrate with this transmembrane electric potential but cannot remove it due to the relatively low permeability of the charged species. As a result, effective hydroxide transport against its concentration gradient can be achieved, and multiple additions can continuously drive the generation of OH− against its concentration gradient up to ΔpH>2. Moreover, the external addition of different anions can control the generation of OH− depending on their anion binding affinity. When anions displayed very high binding affinities towards PtII compounds, such as halides, the external anions could dissipate the pH gradient. In contrast, a further pH increase was observed for weak binding anions, such as sulfate, due to the increase of positive electric potential.

Photomodulation of Transmembrane Transport and Potential by Stiff-Stilbene Based Bis(thio)ureas.

Membrane transport proteins fulfill important regulatory functions in biology with a common trait being their ability to respond to stimuli in the environment. Various small-molecule receptors, capable of mediating transmembrane transport, have been successfully developed. However, to confer stimuli-responsiveness on them poses a fundamental challenge. Here we demonstrate photocontrol of transmembrane transport and electric potential using bis(thio)ureas derived from stiff-stilbene. UV–vis and 1H NMR spectroscopy are used to monitor E–Z photoisomerization of these bis(thio)ureas and 1H NMR titrations reveal stronger binding of chloride to the (Z)-form than to the (E)-form. Additional insight into the binding properties is provided by single crystal X-ray crystallographic analysis and DFT geometry optimization. Importantly, the (Z)-isomers are much more active in transmembrane transport than the respective (E)-isomers as shown through various assays. As a result, both membrane transport and depolarization can be modulated upon irradiation, opening up new prospects toward light-based therapeutics as well as physiological and optopharmacological tools for studying anion transport-associated diseases and to stimulate neuronal activity, respectively.

Penumbra in Acute Ischemic Stroke.

Acute ischemic stroke is one of the leading causes of disability and death worldwide. The brain tissue adjacent to the central necrotic core was first defined as ischemic penumbra characterized by reduced cerebral blood flow (CBF) with electrical failure but maintained ionic homeostasis and transmembrane electrical potential. Since then, the evolving concepts of the ischemic penumbra have been proposed based on energy metabolism, CBF thresholds, and protein synthesis, providing insight for the diagnosis and treatment of acute ischemic stroke. This paper summarizes the recent advances in the understanding of ischemic penumbra, from its discovery to the diagnosis methods based on imaging techniques and biomarkers, finally some of the treatments developed. In addition, we discussed future perspectives on therapeutic targets beyond the ischemic penumbra to develop a treatment for acute ischemic stroke.

Interactions of Surfactants with Biomimetic Membranes. 1. Ionic Surfactants

AbstractWhen impregnated with esters of fatty acids, nitrocellulose filters can be used as a biomimetic membrane. They are easy to make, cheap, and may be used not only as an imitation of biological membranes but also as a sensor to characterize surfactant interactions with dirty fabric. These impregnated filters when fixed in a thermostatically controlled chamber separated into two compartments that contain stirred aqueous solutions by the filter, may be considered as a simple model of a laundry machine. The results revealed that the addition of ionic surfactants into one of the solutions led to the formation of transmembrane electrical potential that is proportional to the logarithm of surfactant concentration. Thus, our biomimetic membranes can be used as electrochemical sensors to detect and measure surfactant concentration. When concentration is near the critical micelle concentration (CMC), a new phenomenon was observed, i.e., spontaneous oscillations of both transmembrane potential and transmembrane current, which mimics the opening and closing of aqueous channels in biological membranes. A further increase of surfactant concentration les to a sharp 100‐fold decrease of transmembrane electrical resistance and the disappearance of any transmembrane voltage. This effect, explained by the washing out of fatty acids from the pores, may be used for screening and development of new detergents.

Interaction fingerprint of transmembrane segments in voltage sensor domains

Voltage sensor domain (VSD) in channel and non-channel membrane proteins shares a common function in the detection of changes in the transmembrane electric potential. The VSD is made of four helical transmembrane segments (S1–S4) that form a structurally conserved scaffold through inter-transmembrane residue-residue interactions. Details about these interactions are yet to be fully understood in the context of the unique structural and physical characteristics of the voltage sensor unit. In this study, molecular dynamics simulations were carried out to investigate transmembrane helix-helix interactions via residue-based nonbonding energies using the activated and resting state conformations of VSD from Hv1, CiVSP, KvAP and NavAb. Inter-transmembrane interaction energies within the VSD were determined. Analysis of electrostatic and van der Waals components revealed the strengths and weaknesses of the interactions between each pair of transmembrane segments. In all cases the S4 helix had the highest electrostatic contribution to favor the key role as the voltage sensitive segment. Electrostatic interactions for the S1–S2 pair as well as the S1–S3 pair were relatively weak. Van der Waal interaction energies between adjacent segments were on average greater than that between diagonally opposite segments. Salt bridge interactions between S4-arginines and the negatively charged residues in other segments appear to contribute more to stabilizing the energy than the van der Waals interactions between nonpolar residues. The overall behavior of residue-residue contacts is similar among the transmembrane domains, reflecting the common inter- transmembrane interaction pattern in the VSD. In addition, analysis of the residue positions suggested that subtle differences in the orientation of the salt-bridges can be attributed to the difference in the inter-transmembrane interaction strengths inside the VSDs.

Fouling dynamics of anion polyacrylamide on anion exchange membrane in electrodialysis

Anion polyacrylamide (APAM) fouling towards anion exchange membrane (AEM) during electrodialysis (ED) of industrial effluent draws recent attention. The fouling dynamics was herein investigated via different approaches in monitoring the variation of the solution and membrane properties. The results showed that conductivity and pH variation is less sensitive than electrochemical signals via transmembrane electric potential (TMEP) and electrochemical impedance spectroscopy (EIS) focusing on micro-scale variation occurred on/near the membrane interface. Initially, the deposition of APAM caused few current transfer limitations as supported by the plateau region for the TMEP profile. Further building-up of APAM fouling gradually hindered the current transfer, resulting into an increased TMEP due to the formation of gel layer on AEM and/or blockage of free volume. Later, the water splitting indicated by an obvious pH variation in dilute and concentrate occurred, which may further accelerate the fouling via modifying the particle size of APAM. EIS successfully distinguished the electrochemical property of membrane bulk and boundary layer, implying the internal fouling is largely responsible for the increased membrane resistance. Future work is suggested to further clarify the external and internal fouling on ion exchange membrane (IEM) and investigate effective fouling mitigation methods in ED.