Shift In Equilibrium Research Articles

Opportunities and challenges for direct electrification of chemical processes with protonic ceramic membrane reactors

Abstract The necessity of developing sustainable energy storage and process electrification technologies has built an unprecedented momentum for protonic ceramic membrane reactors (PCMRs). PCMRs are practically electrolytic cells (or even fuel cells in case of cogeneration) that extend beyond the classical approach of electrolysis towards producing a variety of value-added chemicals or fuels. The use of a ceramic electrolyte membrane to electrochemically supply or remove hydrogen offers unique advantages, such as process intensification, cogeneration of chemicals and electricity, as well as the shift of the chemical equilibrium to the desired products. During the last few years, rapid progress has not only been made in the cell components, but also for upscaling, which reveals their high potential in terms of efficiency and flexibility. Herein, we discuss recent innovations and breakthroughs in the PCMR concepts and components for different processes, while we attempt to identify challenges that may hinder their wide deployment. Closer to commercialization is the production of pressurized hydrogen from sustainable sources, i.e. biogas and ammonia, while significant advancements have been made in reversible H2O electrolysis systems. CO2/H2O co-electrolysis, hydrocarbon conversion and ammonia synthesis have been also successfully demonstrated, albeit with different obstacles related to the product selectivity and stability of the cell reactors. We conclude that future projects should target beyond the experimental discovery of materials, such as, multiscale modeling that would aid optimization of the involved surfaces, interfaces, and the operating parameters towards enhancing the viability of electrosynthesis in PCMRs.

Predictive calculation of the stability constants of silver(I) ion with pyridine and 2,2′-bipyridyl in a solvent environment

In the present study, the stability constants of silver(I) coordination compounds with 2,2′-bipyridyl (bpy, L) in mixed methanol–acetonitrile solvents (MeOH-AN, χAN = 0.0 ÷ 1.0 mol fr.) were determined by the potentiometric method at 25.0 °C. The influence of reagents solvation on the change in stability of [Ag(bpy)]+ and [Ag(bpy)2]+ complexes upon changing the composition of the solvent MeOH→(MeOH-AN) was analyzed. It has been established that the determining factor in the shift of the complex formation equilibrium along the first and second steps of coordination with a change in the composition of the solvent is the contribution from the central ion resolvation. The obtained results and data on the thermodynamics of complex formation reactions of Ag+ with bpy and pyridine (py, L) in a few binary non-aqueous solvents were compared. The possibility of carrying out an estimated calculation of the stability constants of [Ag(L)]+ in non-aqueous solvents was confirmed.

Magnesium 4, 5, and 6 coordinate complexes with ligands bound via sp or sp2 hybridized atoms

As a metallic element of high natural abundance, magnesium finds a wide range of uses in both stoichiometric and more recently catalytic applications. This often takes advantage of the basic or nucleophilic properties of its compounds or their ability to co-complex with other organometallic compounds. However, the homoleptic chemistry of Mg(II) is heavily skewed towards alkyl and aryl ligands bound via sp2 and sp3 hybridized atoms. Here, we report our combined NMR spectroscopic and X-ray crystallographic study into much rarer alternative THF solvates of homoleptic magnesium complexes using ligands which bind via sp (alkynyl) and sp2 (imido) atoms. Specifically, we exploit the high acidity of terminal alkynes and diphenylacetonitrile to prepare tetra-solvated distorted octahedral complexes Mg(CCC6H4R-p)2(THF)4 (R = Me, CF3) and Mg(NCCPh2)2(THF)4 starting from the commercial alkyl reagent MgBu2. Adopting a similar deprotonative strategy using benzophenoneimine Ph2CNH affords the heteroleptic trinuclear complex Mg3(NCPh2)4nBu2(THF)2 with distorted tetrahedral Mg centres. Related imidomagnesium halide complexes can be accessed either by deprotonation of the imine with a Grignard reagent, or nucleophilic addition of a Grignard reagent to a nitrile, but these unusual five-coordinate complexes are unresponsive to a 1,4-dioxane induced Schlenk equilibrium shift. Employing a higher reflux temperature switching from a THF to a toluene medium permits access to the THF solvate of a homoleptic imido complex which also possesses a trinuclear constitution in Mg3(NCPh2)6(THF)2.

Ga-doped spinel-typed Co-Al catalysts for CO2-assisted ethane dehydrogenation

A series of Ga-doped spinel-typed Co-Al catalysts were prepared via one-step evaporation-induced self-assembly (EISA) method. These catalysts present promising catalytic performance for CO2-assisted ethane dehydrogenation, which can be ascribed to the conjunction effect of cobalt and gallium species. The incorporation of Ga into the structure of Co-Al spinel has been recognized for enhancing CO2 conversion via reverse water-gas shift reaction, leading to a favorable equilibrium shift towards ethylene, along with an improved catalyst stability as a result of coexisting Boudouard reaction. Through X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), NH3 and CO2 temperature-programmed desorption (TPD) characterizations, coordinatively unsaturated Co2+ and Ga3+ cations were identified as collaborative catalytic active sites, with gallium species taking the prominent role. These sites were closely associated with the formation of acid-base pairs on the catalyst surface, which were crucial for the adsorption and activation of the reactants.

On Assessment of the Temperature Shift of the Chemical Equilibrium

On Assessment of the Temperature Shift of the Chemical Equilibrium

Assessment of Thermodynamic variables affecting phase nucleation

Complete understanding of any natural phenomenon can only be achieved by explicitly expressing it in terms of primitive variables. The laws of thermodynamics have been extensively studied for centuries, accelerating societal progress through developments across several branches of science and technology. Phase nucleation can be observed in the melting and freezing of rocks and ice caps, abundant shapes of snow flocks, rain regimes, snowfall, casting, and metal forming, among other applications. The most challenging aspect of nucleation is the equilibrium shift controlled by the gradients in temperatures, species, work, and other variables, in accordance with the first law of thermodynamics, which defines the thermal field. In this study, the thermal field and thermal field tensor were defined in terms of equilibrium dislocations to assess their primitive variables. Theoretical simulations for the Gibbs–Thomson coefficients were applied to predict the experimental microstructure evolution.

Impact of Temperature on the Solubility of Ionic Compounds in Water in Cameroon

Purpose: The aim of the study was to assess impact of temperature on the solubility of ionic compounds in water in Cameroon. Methodology: This study adopted a desk methodology. A desk study research design is commonly known as secondary data collection. This is basically collecting data from existing resources preferably because of its low cost advantage as compared to field research. Our current study looked into already published studies and reports as the data was easily accessed through online journals and libraries. Findings: The study found that the solubility of most ionic compounds increases with an increase in temperature. This is because higher temperatures provide more kinetic energy to the ions, helping them to break free from the crystal lattice and dissolve in water. For example, the solubility of salts like potassium nitrate (KNO3) significantly increases as the temperature rises, allowing more of the compound to dissolve. However, this trend is not universal for all ionic compounds. Some, such as calcium sulfate (CaSO4), exhibit a decrease in solubility with rising temperature. This anomaly can be attributed to the exothermic nature of the dissolution process for these compounds, where heat is released when they dissolve. As temperature increases, the equilibrium shifts to favor the solid form, reducing solubility. Therefore, while temperature generally enhances the solubility of ionic compounds in water, specific behaviors can vary depending on the individual compound's dissolution characteristics and thermodynamic properties. Implications to Theory, Practice and Policy: Le Chatelier’s principle, Arrhenius theory of dissociation and solubility product constant (KSP) theory may be used to anchor future studies on assessing impact of temperature on the solubility of ionic compounds in water in Cameroon. Practical applications of temperature-dependent solubility data should be integrated into industrial processes, particularly in fields such as pharmaceuticals, fertilizers, and desalination. Policymakers should consider incorporating temperature-dependent solubility data into environmental regulations, particularly concerning water pollution and waste management.

Necessary conditions for static and dynamic bow snap-through in electrostatically actuated initially curved and latched micro-beams

A curved bistable micro-beam, subjected to electrostatic loading from an electrode facing its concave side, may produce a snap-through response with voltages as low as 54%, when compared to actuation from a convex facing electrode. Such actuation has been dubbed “bow actuation” due to the similarity of preloading an arrow onto a bow, and the resulting equilibrium shift, as “bow snap-through”. Under a certain elevation-to-thickness ratio, a bistable beam will also become latchable, allowing the beam to maintain itself in its second stable state under zero load/voltage. In the current work, necessary conditions are found for static and dynamic bow snap-through, which can be used as a tool to design and produce bow snap-through responses, promoting efficient non-volatile and low-power consumption bistable based devices. The conditions are found using an undamped dynamic single degree-of-freedom (DoF) reduced-order (RO) model, attained via Galerkin’s decomposition. Subsequent numerical calculations, conducted in the presence of ambient damping, show that the condition is necessary to attain bow snap-through responses, while also disclosing the snapping behaviour of the model.

Anodische H2O2‐Erzeugung in Elektrolyten auf Karbonatbasis – Mechanistische Erkenntnisse Mittels Elektrochemischer Rastermikroskopie

AbstractFür die anodische H2O2‐Bildung kann die Elektrolytzusammensetzung den Reaktionsweg in Richtung einer erhöhten Produktbildung lenken. Frühere Untersuchungen zur Optimierung der Elektrolytzusammensetzung haben gezeigt, dass die Auswirkungen des molaren Anteils der Karbonatspezies für verschiedene Anodenmaterialien unterschiedlich sind, und daher gibt es weiterhin Kontroversen über die Reaktionswege und die an der H2O2‐Bildung beteiligten Spezies. In Anbetracht der Tatsache, dass die Wasseroxidation zur Freisetzung von Protonen in der Anodenmikroumgebung führt, würde die entsprechende Ansäuerung eine Gleichgewichtsverschiebung zwischen den Karbonatspezies bewirken, was wiederum den Reaktionsweg modulieren könnte. Wir haben die Veränderungen im Anteil der Karbonatspezies in der Nähe einer arbeitenden Anode bestimmt, indem wir lokale pH‐Messungen mit einer Au‐Nanoelektrode, die mittels elektrochemischer Scherkraft‐Rastermikroskopie (SECM) in unmittelbarer Nähe einer sich in Betrieb befindlichen Anode positioniert war, durchgeführt haben. Es konnte bestätigt werden, dass die wichtigste anionische Spezies an der Grenzfläche HCO3− ist, und zwar bei Potentialen, bei denen H2O2 bevorzugt gebildet wird, unabhängig vom pH‐Wert im Volumen der Lösung. Die gleichzeitige Verwendung einer Au−Pt‐Doppelmikroelektrode in Generator‐Kollektor‐SECM‐Messungen zeigt, dass die lokale HCO3−Konzentration insgesamt durch den Oxidationsstrom, die Pufferkapazität und den pH‐Wert des Elektrolyten bestimmt wird.

Anodic H2O2 Generation in Carbonate-Based Electrolytes-Mechanistic Insight from Scanning Electrochemical Microscopy.

For the anodic H2O2 generation, it has been shown that the electrolyte composition can steer the reaction pathway toward increased H2O2 generation. Previous efforts made on composition optimization found that the impact of the molar fraction of carbonate species varies for different anodes, and therefore, controversies remain concerning the reaction pathways as well as the species involved in H2O2 formation. Considering that water oxidation results in the liberation of protons within the anode microenvironment, the corresponding acidification would cause an equilibrium shift between carbonate species, which in turn may modulate the reaction pathway. We determined the changes in the fraction of carbonate species in the vicinity of an anode by performing local pH measurements using a Au nanoelectrode positioned in close proximity to an operating anode by shear-force scanning electrochemical microscopy (SECM). It could be confirmed that the main anionic species at the interface is HCO3 -, at potentials where H2O2 is preferentially formed, regardless of the pH value in the bulk. The simultaneous use of a Au-Pt double barrel microelectrode in generator-collector SECM measurements demonstrates that the local HCO3 - concentration is collectively determined by the oxidation current, buffer capacity, and bulk pH of the electrolyte.

Phase behaviour and heat capacities of DBN and DBU based protic ionic liquids – Insights into the ionic character and nanostructuration

Herein, we report the thermal behavior and high-precision heat capacity values, at T = 298.15 K, and in the range from 283 to 363 K, for several protic ionic liquids (PILs), derived from the 1:1 liquid mixtures of the organic superbases 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with some carboxylic acids (propionic, butyric, hexanoic and octanoic). Glass transition, Tg, crystallization, Tc, cold-crystallization, Tcc, and melting, Tm, temperatures were determined for the PILs studied, showing marked differences with the base – Tg is lower for all DBN PILs, and no Tm, Tc, and Tcc could be obtained for the DBU PILs. The standard molar heat capacities, Cp,m0, were obtained with an uncertainty of less than ±0.3 % and their dependence on the base and on the acid’s chain length was studied in detail. The Cp,m0 of the DBU PILs were found to be significantly higher than those of the corresponding DBN PILs, which corroborates the greater ionic character of DBU PILs. The heat capacity data suggested the existence of a trend-shift with the chain length of the carboxylic acid. Similar to aprotic ionic liquids, the shift occurs around n = 5 (pentanoic acid) and suggests the existence of some degree of nanostructuration into polar and non-polar domains in PILs with larger acids. Moreover, PILs can display abnormally high excess heat capacities, resulting from the formation of an ionic mixture from neutral species. Analysis of the calculated excess heat capacities indicates that PILs tend to become less ionic as the temperature increases, which goes in accordance with the acid-base equilibrium shift.

Copper sulfate and carbon tetrachloride induces a uniform response at the level of the redox system and the nature of this response depends on age

The concept of oxidative stress, which is a development of D. Harman’s idea of random harmful tissue damage by free radicals, remains one of the most popular in the study of pathological processes, including age-dependent chronic changes. The work tested the hypothesis according to which, a shift in equilibrium towards pro-oxidants, i.e. oxidative stress, is the primary adaptive response of the body to exogenous toxic environmental factors. To test this, a number of indicators of the redox system were determined as a response to hepatotoxic compounds of different nature (copper sulfate and carbon tetrachloride) in young and old animals. The amount of lipid hydroperoxides and the activity of a number of antioxidant enzymes were determined in the blood serum of young (3 months) and old (20 months) rats before exposure (initial level) and after repeated sequential injections (3 injections) of copper sulfate at a dose of 1 mg/100 g body weight and carbon tetrachloride in a dose of 0.1 mL in 50% vegetable oil. It was found that in intact (initial level) old animals, the indicators of the redox system are shifted towards antioxidants. After three consecutive administrations of various hepatotoxic compounds, with an interval of 48 hours between administrations, the balance shifted towards pro-oxidants, regardless of the inducer, however, this effect was more pronounced in old animals compared to young ones, relative to their initial level. Such different reactivity of redox system indicators in animals of different ages led to the “evening out” of the initially different redox system indicators. We came to the conclusion that changing the balance in the pro-oxidant-antioxidant system is a universal, primary reaction of the body to endogenous or exogenous factors that perform regulatory functions and, depending on the temporal and functional characteristics of the body, are “transformed” into specific physiological manifestations.

Accelerated Degradation of DiXyl-α,ε-Lys-ARP via Interaction between Extra-Added Xylose and Monosubstituted Lys-ARPs during Maillard Reaction.

Lysine (Lys) is capable of forming a di-substituted Amadori rearrangement product (ARP) with xylose (Xyl), designated as diXyl-α,ε-Lys-ARP. DiXyl-α,ε-Lys-ARP degradation was characterized by two steps: Initially, Xyl-α- and Xyl-ε-Lys-ARP were formed through elimination or hydrolysis at specific Nα/Nε positions of the corresponding enol and imine intermediates, which were then further degraded to dicarbonyl compounds and regenerated Lys. Xyl-α- or Xyl-ε-Lys-ARP had a reactive free amino group (ε-NH2 or α-NH2), both of which were still highly reactive and able to undergo further reactions with Xyl. Therefore, the diXyl-α,ε-Lys-ARP/Xyl model system was established to explore the impact of extra-added Xyl on diXyl-α,ε-Lys-ARP degradation behavior. Extra-added Xyl remarkably affected the degradation pathway of diXyl-α,ε-Lys-ARP by capturing the Xyl-α- and Xyl-ε-Lys-ARP to regenerate diXyl-α,ε-Lys-ARP. This interaction between Xyl and mono-substituted Lys-ARPs promoted the shift of chemical equilibrium toward the degradation of diXyl-α,ε-Lys-ARP, thereby accelerating its degradation rate. This degradation was markedly facilitated by the elevated temperature and pH values. Interestingly, the yield of Xyl-α- and Xyl-ε-Lys-ARP was particularly dependent on the pH during diXyl-α,ε-Lys-ARP degradation. Xyl-ε-Lys-ARP was the dominant product at pH 5.5-7.5 while Xyl-α-Lys-ARP possessed a relatively higher content under weak alkaline conditions, which was related to the reactivities of the Nα/Nε positions under various reaction conditions.

COMPARATIVE STUDY OF THE DYNAMIC BEHAVIOR OF BACTERIAL PHOTOREGULATED ADENYLATE CYCLASES BPAC AND OAPAC

Photoregulated adenylate cyclases bPAC and OaPAC are compared using classical molecular dynamics simulations in both states, before and after blue light irradiation. The pathways of signal transduction from the photoreceptor to the catalytic domain are determined. The more pronounced acceleration of the catalytic reaction rate as a result of light irradiation in the case of bPAC is explained both by a shift in equilibrium towards a closed conformation and by a larger number of suboptimal pathways of allosteric signal transduction from photoreceptor to catalytic domain

Self-Neutralizing Melamine-Urea-Formaldehyde-Citric Acid Resins for Wood Panel Adhesives.

In this study, we used a self-neutralizing system to counteract too acidic a pH, unsuitable for wood adhesives, and tested it on MUF resins augmented by the addition of citric acid or other organic acids, based on the addition of small percentages of hexamine or another suitable organic base to form an acid-base buffer. In this manner, the pH of the adhesive was maintained above the minimum allowed value of 4, and the strength results of wood particleboard and plywood bonded with this adhesive system increased due to the additional cross-linking imparted by the citric acid. Thus, the wood constituents at the wood/adhesive interface were not damaged/degraded by too low a pH, thus avoiding longer-term service failure of the bonded joints. The addition of the buffering system increased the strength of the bondline in both the plywood and particleboard, both when dry and after hot water and boiling water tests. The IB strength of the particleboard was then increased by 15-17% when dry but by 82% after boiling. For the plywood, the shear strengths when dry and after 3 h in hot water at 63 °C were, respectively, 37% and 90% higher than for the control. The improvement in the bonded panel strength is ascribed to multiple reasons: (i) the slower, more regular cross-linking rate due to the action of the buffer; (ii) the shift in the polycondensation-degradation equilibrium to the left induced by the higher pH and the long-term stability of the organic buffer; (iii) the additional cross-linking by citric acid of some of the MUF resin amine groups; (iv) the already known direct linking of citric acid with the carbohydrates and lignin constituents at the interface of the wood substrate; and (v) the likely covalent linking to the interfacial wood constituents of the prelinked MUF-citric acid resin by some of the unreacted citric acid carboxyl groups.

Genetic characteristics of producing bulls of the Kholmogorsky breed, depending on their linear affiliation and the level of Holstein

Relevance. Genetic analysis using STR markers makes it possible to effectively use various techniques to control the breeding process in a population.Methods. The material for DNA research was the deep-frozen sperm of 103 bulls-producers of the Kholmogorsky breed, with different blood levels of Holstein cattle belonging to five main genealogical lines. Molecular genetic studies were conducted in the laboratory of DNA technologies at VNIIPlem.Results. The animals of the VBA (I = 1,557) and Nl (I = 1,556) lines had the maximum genetic diversity, and the Chl lines had the lowest (the indicator is lower by 0.229 (р ≤ 0.05) relative to VBA). A pronounced discrepancy between the observed and expected heterozygosity towards heterozygote deficiency was found among animals of the VBA line (F = 0.173), which is higher by 0.220–0262 relative to RS and Chl (р ≤ 0.01–0.001). The bulls of the Chl and RS lines have an equilibrium shift towards an excess of heterozygosity (F = -0.111 and F = -0.054 respectively). The most related lines are Nl and VBA (DN = 0.097), as well as Nl and Lm (DN = 0.101), and the least VBA and Chl (DN = 0.247), as well as VBA and RS (DN = 0.218). When differentiating the samples by the level of Holstein, it was found that the greatest genetic diversity was characterized by bulls with a blood density of 25–49% (1,633), and the minimum was animals with a degree of Holstein above 74% (which is lower by 0.299–0.334 relative to other samples, р ≤ 0.05–0.01). With an increase in the degree of Holstein, an increase in the level of genetic inbred was observed (in crossbreeds with a bloodline above 74%, the indicator was 0.142–0.229 more (р ≤ 0.05–0.001) compared with other samples).

Effects of Macromolecular Cosolutes on the Kinetics of Huntingtin Aggregation Monitored by NMR Spectroscopy.

The effects of two macromolecular cosolutes, specifically the polysaccharide dextran-20 and the protein lysozyme, on the aggregation kinetics of a pathogenic huntingtin exon-1 protein (hhtex1) with a 35 polyglutamine repeat, httex1Q35, are described. A unified kinetic model that establishes a direct connection between reversible tetramerization occurring on the microsecond time scale and irreversible fibril formation on a time scale of hours/days forms the basis for quantitative analysis of httex1Q35 aggregation, monitored by measuring cross-peak intensities in a series of 2D 1H-15N NMR correlation spectra acquired during the course of aggregation. The primary effects of the two cosolutes are associated with shifts in the prenucleation tetramerization equilibrium resulting in substantial changes in concentration of "preformed" httex1Q35 tetramers. Similar effects of the two cosolutes on the tetramerization equilibrium observed for a shorter, nonaggregating huntingtin variant with a 7-glutamine repeat, httex1Q7, lend confidence to the conclusions drawn from the fits to the httex1Q35 aggregation kinetics.

Unveiling Tst3, a Multi-Target Gating Modifier Scorpion α Toxin from Tityus stigmurus Venom of Northeast Brazil: Evaluation and Comparison with Well-Studied Ts3 Toxin of Tityus serrulatus.

Studies on the interaction sites of peptide toxins and ion channels typically involve site-directed mutations in toxins. However, natural mutant toxins exist among them, offering insights into how the evolutionary process has conserved crucial sequences for activities and molecular target selection. In this study, we present a comparative investigation using electrophysiological approaches and computational analysis between two alpha toxins from evolutionarily close scorpion species of the genus Tityus, namely, Tst3 and Ts3 from T. stigmurus and T. serrulatus, respectively. These toxins exhibit three natural substitutions near the C-terminal region, which is directly involved in the interaction between alpha toxins and Nav channels. Additionally, we characterized the activity of the Tst3 toxin on Nav1.1-Nav1.7 channels. The three natural changes between the toxins did not alter sensitivity to Nav1.4, maintaining similar intensities regarding their ability to alter opening probabilities, delay fast inactivation, and induce persistent currents. Computational analysis demonstrated a preference for the down conformation of VSD4 and a shift in the conformational equilibrium towards this state. This illustrates that the sequence of these toxins retained the necessary information, even with alterations in the interaction site region. Through electrophysiological and computational analyses, screening of the Tst3 toxin on sodium isoform revealed its classification as a classic α-NaTx with a broad spectrum of activity. It effectively delays fast inactivation across all tested isoforms. Structural analysis of molecular energetics at the interface of the VSD4-Tst3 complex further confirmed this effect.

Cross-border political competition.

Individuals are increasingly exposed to news and opinion from beyond national borders. This news and opinion are often concentrated in clusters of ideological homophily, such as political parties, factions, or interest groups. But how does exposure to cross-border information affect the diffusion of ideas across national and ideological borders? Here, we develop a non-linear mathematical model for the cross-border spread of two ideologies. First, we describe the standard deterministic model where the populations of each country are assumed to be constant and homogeneously mixed. We solve the system of differential equations numerically by the Runge-Kutta method and show how small changes in the influence of a minority ideology can trigger shifts in the global political equilibrium. Second, we simulate recruitment as a stochastic differential process for each political affiliation and fit model solutions to population growth rates and voting populations in US presidential elections from 1932 to 2020. We also project the dynamics of several possible scenarios from 2020 to the end of the century. We show that cross-border influence plays a fundamental role in determining election outcomes. An increase in foreign support for a national party's ideas could change the election outcome, independent of domestic recruitment capacity. One key finding of our study suggests that voter turnout in the US will grow at a faster rate than non-voters in the coming decades. This trend is attributed to the enhanced recruitment capabilities of both major parties among non-partisans over time, making political disaffection less prominent. This phenomenon holds true across all simulated scenarios.

Dynamics underlie the drug recognition mechanism by the efflux transporter EmrE

The multidrug efflux transporter EmrE from Escherichia coli requires anionic residues in the substrate binding pocket for coupling drug transport with the proton motive force. Here, we show how protonation of a single membrane embedded glutamate residue (Glu14) within the homodimer of EmrE modulates the structure and dynamics in an allosteric manner using NMR spectroscopy. The structure of EmrE in the Glu14 protonated state displays a partially occluded conformation that is inaccessible for drug binding by the presence of aromatic residues in the binding pocket. Deprotonation of a single Glu14 residue in one monomer induces an equilibrium shift toward the open state by altering its side chain position and that of a nearby tryptophan residue. This structural change promotes an open conformation that facilitates drug binding through a conformational selection mechanism and increases the binding affinity by approximately 2000-fold. The prevalence of proton-coupled exchange in efflux systems suggests a mechanism that may be shared in other antiporters where acid/base chemistry modulates access of drugs to the substrate binding pocket.