Electrometric and Electron Paramagnetic Resonance Measurements of a Difference in the Transmembrane Electrochemical Potential: Photosynthetic Subcellular Structures and Isolated Pigment-Protein Complexes

A transmembrane difference in the electrochemical potentials of protons (ΔμH+) serves as a free energy intermediate in energy-transducing organelles of the living cell. The contributions of two components of the ΔμH+ (electrical, Δψ, and concentrational, ΔpH) to the overall ΔμH+ value depend on the nature and lipid composition of the energy-coupling membrane. In this review, we briefly consider several of the most common instrumental (electrometric and EPR) methods for numerical estimations of Δψ and ΔpH. In particular, the kinetics of the flash-induced electrometrical measurements of Δψ in bacterial chromatophores, isolated bacterial reaction centers, and Photosystems I and II of the oxygenic photosynthesis, as well as the use of pH-sensitive molecular indicators and kinetic data regarding pH-dependent electron transport in chloroplasts, have been reviewed. Further perspectives on the application of these methods to solve some fundamental and practical problems of membrane bioenergetics are discussed.

Thioflavin-T does not report on electrochemical potential and memory of dormant or germinating bacterial spores

IMPORTANCE

Bacillus and Clostridium spores cause food spoilage and disease because of spores' dormancy and resistance to microbicides. However, when spores "come back to life" in germination, their resistance properties are lost. Thus, understanding the mechanisms of spore germination could facilitate the development of "germinate to eradicate" strategies. One germination feature is the memory of a pulsed germinant stimulus leading to greater germination following a second pulse. Recent observations of increases in spore binding of the potentiometric dye thioflavin-T early in their germination of spores led to the suggestion that increasing electrochemical potential is how spores "remember" germinant pulses. However, new work finds no increased thioflavin-T binding in the physiological germination of Coatless spores or of intact spores germinating with dodecylamine, even though spore memory is seen in both cases. Thus, using thioflavin-T uptake by germinating spores to assess the involvement of electrochemical potential in memory of germinant exposure, as suggested recently, is questionable.

Potentials in Li-Ion Batteries Probed by Operando Ambient Pressure Photoelectron Spectroscopy

The important electrochemical processes in a battery happen at the solid/liquid interfaces. Operando ambient pressure photoelectron spectroscopy (APPES) is one tool to study these processes with chemical specificity. However, accessing this crucial interface and identifying the interface signal are not trivial. Therefore, we present a measurement setup, together with a suggested model, exemplifying how APPES can be used to probe potential differences over the electrode/electrolyte interface, even without direct access to the interface. Both the change in electron electrochemical potential over the solid/liquid interface, and the change in Li chemical potential of the working electrode (WE) surface at Li-ion equilibrium can be probed. Using a Li₄Ti₅O₁₂ composite as a WE, our results show that the shifts in kinetic energy of the electrolyte measured by APPES can be correlated to the electrochemical reactions occurring at the WE/electrolyte interface. Different shifts in kinetic energy are seen depending on if a phase transition reaction occurs or if a single phase is lithiated. The developed methodology can be used to evaluate charge transfer over the WE/electrolyte interface as well as the lithiation/delithiation mechanism of the WE.

The respiratory supercomplex from C. glutamicum

Corynebacterium glutamicum is a preferentially aerobic gram-positive bacterium belonging to the phylum Actinobacteria, which also includes the pathogen Mycobacterium tuberculosis. In these bacteria, respiratory complexes III and IV form a CIII₂CIV₂ supercomplex that catalyzes oxidation of menaquinol and reduction of dioxygen to water. We isolated the C. glutamicum supercomplex and used cryo-EM to determine its structure at 2.9 Å resolution. The structure shows a central CIII₂ dimer flanked by a CIV on two sides. A menaquinone is bound in each of the Q_N and Q_P sites in each CIII and an additional menaquinone is positioned ∼14 Å from heme b_L. A di-heme cyt. cc subunit electronically connects each CIII with an adjacent CIV, with the Rieske iron-sulfur protein positioned with the iron near heme b_L. Multiple subunits interact to form a convoluted sub-structure at the cytoplasmic side of the supercomplex, which defines a path for proton transfer into CIV.

Tubular Structures of Calcium Carbonate: Formation, Characterization, and Implications in Natural Mineral Environments

Chemical gardens are self-assembled tubular precipitates formed by a combination of osmosis, buoyancy, and chemical reaction, and thought to be capable of catalyzing prebiotic condensation reactions. In many cases, the tube wall is a bilayer structure with the properties of a diaphragm and/or a membrane. The interest in silica gardens as microreactors for materials science has increased over the past decade because of their ability to create long-lasting electrochemical potential. In this study, we have grown single macroscopic tubes based on calcium carbonate and monitored their time-dependent behavior by in situ measurements of pH, ionic concentrations inside and outside the tubular membranes, and electrochemical potential differences. Furthermore, we have characterized the composition and structure of the tubular membranes by using ex situ X-ray diffraction, infrared and Raman spectroscopy, as well as scanning electron microscopy. Based on the collected data, we propose a physicochemical mechanism for the formation and ripening of these peculiar CaCO₃ structures and compare the results to those of other chemical garden systems. We find that the wall of the macroscopic calcium carbonate tubes is a bilayer of texturally distinct but compositionally similar calcite showing high crystallinity. The resulting high density of the material prevents macroscopic calcium carbonate gardens from developing significant electrochemical potential differences. In the light of these observations, possible implications in materials science and prebiotic (geo)chemistry are discussed.

First-Principles Study of Microscopic Electrochemistry at the LiCoO2 Cathode/LiNbO3 Coating/β-Li3PS4 Solid Electrolyte Interfaces in an All-Solid-State Battery

High interfacial resistance between electrode and solid electrolyte (SE) is one of the major challenges for the commercial application of all-solid-state batteries (ASSBs), and coating at the interface is an effective way for decreasing the resistance. However, microscopic electrochemistry especially for the electrochemical potential and the distribution of Li⁺ at the interface has not been well established yet, impeding the in-depth understanding of interfacial Li⁺ transport. Herein, we have introduced a potential energy profile for Li⁺, η_Li⁺, and demonstrated that the interfacial η_Li⁺ can be evaluated from the calculated interfacial Li vacancy formation energy or the bulk vacancy formation energy and the interface band alignment. Through computational analysis of the representative LiCoO₂ cathode/LiNbO₃ coating/β-Li₃PS₄ SE interfaces using the novel interface structure prediction scheme based on the CALYPSO method, we found that η_Li⁺ at the LiCoO₂/β-Li₃PS₄ interface is highly disordered under the influence of the interface reconstruction and is rather electronic conductive. Insertion of LiNbO₃ coating can effectively decrease the preference of ion mixing. Besides, the appropriate changes in band alignments lead to a decrease of difference in the interfacial η_Li⁺ and lower resistances at the interfaces. The results provide a reliable explanation for the effectiveness of the coating layer observed experimentally. Furthermore, our study provides a guidance for the future simulation of the microscopic electrochemistry at the electrode/SE interfaces in ASSBs.

Selective Growth of Interface Layers from Reactions of Sc(MeCp)2(Me2pz) with Oxide Substrates

The transformation of an oxide substrate by its reaction with a chemical precursor during atomic layer deposition (ALD) has not attracted much attention, as films are typically deposited on top of the oxide substrate. However, any modification to the substrate surface can impact the electrical and optical properties of the device. We demonstrate herein the ability of a precursor to react deep within an oxide substrate to form an interfacial layer that is distinct from both the substrate and deposited film. This phenomenon is studied using a scandium precursor, Sc(MeCp)₂(Me₂pz) (1, MeCp = methylcyclopentadienyl, Me₂pz = 3,5-dimethylpyrazolate), and five oxide substrates (SiO₂, ZnO, Al₂O₃, TiO₂, and HfO₂). In situ Fourier transform infrared (FTIR) spectroscopy shows that at moderate temperatures (∼150 °C) the pyrazolate group of 1 reacts with the surface hydroxyl groups of OH-terminated SiO₂ substrates. However, at slightly higher temperatures (≥225 °C) typically used for the ALD of Sc₂O₃, there is a direct reaction between 1 and the SiO₂ layer, in addition to chemisorption at the surface hydroxyl groups. This reaction is sustained by sequential exposures of 1 until an ∼2 nm thick passivating interface layer is formed, indicating that 1 reacts with oxygen derived from SiO₂. A shift of the Si 2p core level position, measured by ex situ X-ray photoelectron spectroscopy, is consistent with the formation of a ScSi _xO _y layer. Similar observations are made following the exposure of a ZnO substrate to 1 at 275 °C. In contrast, Al₂O₃, TiO₂, and HfO₂ substrates remain resistant to reaction with 1 under similar conditions, except for a surface reaction occurring in the case of TiO₂. These striking observations are attributed to the differences in the electrochemical potentials of the elements comprising the oxide substrates to that of scandium. Precursor 1 can react with SiO₂ or ZnO substrates, since the constituent elements of these oxides have less-negative electrochemical potentials than do aluminum, titanium, and hafnium. Additionally, Sc₂O₃ and surface carbonates are deposited on all substrates by gas-phase reactions between 1 and residual water vapor in the reactor. The extent of gas-phase reactions contributing to film growth is governed by the relative pressure of water vapor in the presence of 1. These results suggest caution when using very reactive, oxophilic precursors such as 1 to avoid misinterpreting unconventional film deposition as that resulting from a standard ALD process.

Improvement on the Repair Effect of Electrochemical Chloride Extraction Using a Modified Electrode Configuration

To improve the repair effect of electrochemical chloride extraction, a modified electrode configuration is applied in this investigation. In this configuration, two auxiliary electrodes placed in the anodic and cathodic electrolytes were used as the anode and cathode, respectively. Besides this, the steel in the mortar was grounded to protect it from corrosion. By a comparative experiment, the potential evolution, various ions concentrations (Cl^-, OH^-, Na⁺, and K⁺) in different mortar depths, the corrosion potential, and the current density of the steel were measured. The results indicate that compared to electrochemical chloride extraction with the traditional electrode configuration, this electrochemical chloride extraction method with a modified electrode configuration has a similar chloride removal ratio. Besides this, potential of steel is just about 800 mV for a saturated calomel electrode (SCE) during the treatment, which did not reach the hydrogen evolution potential. The phenomenon of the accumulation of OH^-, Na⁺, and K⁺ did not occur when the modified electrode configuration is applied. Additionally, higher corrosion potentials and lower corrosion current rates were measured after performing electrochemical chloride extraction with the modified electrode configuration. Additionally, it is a short period of time for the steel to go from activation to passivation. On this basis, the modified electrode configuration may overcome the drawbacks of electrochemical chloride extraction.

Perfect chemomechanical coupling of FoF1-ATP synthase

F_oF₁-ATP synthase (F_oF₁) couples H⁺ flow in F_o domain and ATP synthesis/hydrolysis in F₁ domain through rotation of the central rotor shaft, and the H⁺/ATP ratio is crucial to understand the coupling mechanism and energy yield in cells. Although H⁺/ATP ratio of the perfectly coupling enzyme can be predicted from the copy number of catalytic β subunits and that of H⁺ binding c subunits as c/β, the actual H⁺/ATP ratio can vary depending on coupling efficiency. Here, we report actual H⁺/ATP ratio of thermophilic Bacillus F_oF₁, whose c/β is 10/3. Proteoliposomes reconstituted with the F_oF₁ were energized with ΔpH and Δψ by the acid-base transition and by valinomycin-mediated diffusion potential of K⁺ under various [ATP]/([ADP]⋅[Pi]) conditions, and the initial rate of ATP synthesis/hydrolysis was measured. Analyses of thermodynamically equilibrated states, where net ATP synthesis/hydrolysis is zero, show linear correlation between the chemical potential of ATP synthesis/hydrolysis and the proton motive force, giving the slope of the linear function, that is, H⁺/ATP ratio, 3.3 ± 0.1. This value agrees well with the c/β ratio. Thus, chemomechanical coupling between F_o and F₁ is perfect.

Phosphonic Acid Adsorbates Tune the Surface Potential of TiO2 in Gas and Liquid Environments

Controlled attachment of molecules to the surface of a material can alter the band structure energies with respect to the surrounding environment via a combination of intrinsic and bonding-induced dipoles. Here, we demonstrate that the surface potential of an application-relevant material, anatase TiO2, can be tuned over a broad energy range of ∼1 eV using a family of dipolar phosphonic acid-based adsorbates. Using TiO2 as an example, we show with photoelectron spectroscopy that these adsorbates are stable in a liquid environment (propylene carbonate). More interestingly, the tunability is substantially retained and follows trends in the computed bound dipole. The electrochemical surface potential is shown to vary over 600 meV, the highest range in electrolytes to the best of our knowledge. Using density functional theory calculations, we rationalize the measured trends and show that the effective dipole upon molecular adsorption and not the intrinsic dipole of the isolated molecules correlates with observed changes in surface potential. Control of the effective dipole, through judicious choice of robust surface species, can allow in situ tuning of energy levels and functionality at active surfaces for energy conversion and storage, biosensing, and molecular electronics.

Voltage coupling of primary H+ V-ATPases to secondary Na+- or K+-dependent transporters

This review provides alternatives to two well established theories regarding membrane energization by H(+) V-ATPases. Firstly, we offer an alternative to the notion that the H(+) V-ATPase establishes a protonmotive force (pmf) across the membrane into which it is inserted. The term pmf, which was introduced by Peter Mitchell in 1961 in his chemiosmotic hypothesis for the synthesis of ATP by H(+) F-ATP synthases, has two parts, the electrical potential difference across the phosphorylating membrane, Deltapsi, and the pH difference between the bulk solutions on either side of the membrane, DeltapH. The DeltapH term implies three phases - a bulk fluid phase on the H(+) input side, the membrane phase and a bulk fluid phase on the H(+) output side. The Mitchell theory was applied to H(+) V-ATPases largely by analogy with H(+) F-ATP synthases operating in reverse as H(+) F-ATPases. We suggest an alternative, voltage coupling model. Our model for V-ATPases is based on Douglas B. Kell's 1979 'electrodic view' of ATP synthases in which two phases are added to the Mitchell model - an unstirred layer on the input side and another one on the output side of the membrane. In addition, we replace the notion that H(+) V-ATPases normally acidify the output bulk solution with the hypothesis, which we introduced in 1992, that the primary action of a H(+) V-ATPase is to charge the membrane capacitance and impose a Deltapsi across the membrane; the translocated hydrogen ions (H(+)s) are retained at the outer fluid-membrane interface by electrostatic attraction to the anions that were left behind. All subsequent events, including establishing pH differences in the outside bulk solution, are secondary. Using the surface of an electrode as a model, Kell's 'electrodic view' has five phases - the outer bulk fluid phase, an outer fluid-membrane interface, the membrane phase, an inner fluid-membrane interface and the inner bulk fluid phase. Light flash, H(+) releasing and binding experiments and other evidence provide convincing support for Kell's electrodic view yet Mitchell's chemiosmotic theory is the one that is accepted by most bioenergetics experts today. First we discuss the interaction between H(+) V-ATPase and the K(+)/2H(+) antiporter that forms the caterpillar K(+) pump, and use the Kell electrodic view to explain how the H(+)s at the outer fluid-membrane interface can drive two H(+) from lumen to cell and one K(+) from cell to lumen via the antiporter even though the pH in the bulk fluid of the lumen is highly alkaline. Exchange of outer bulk fluid K(+) (or Na(+)) with outer interface H(+) in conjunction with (K(+) or Na(+))/2H(+) antiport, transforms the hydrogen ion electrochemical potential difference, mu(H), to a K(+) electrochemical potential difference, mu(K) or a Na(+) electrochemical potential difference, mu(Na). The mu(K) or mu(Na) drives K(+)- or Na(+)-coupled nutrient amino acid transporters (NATs), such as KAAT1 (K(+) amino acid transporter 1), which moves Na(+) and an amino acid into the cell with no H(+)s involved. Examples in which the voltage coupling model is used to interpret ion and amino acid transport in caterpillar and larval mosquito midgut are discussed.

Role of Na+ in alpha-aminoisobutyric acid uptake by membrane vesicles from mouse fibroblasts transformed by simian virus 40

The uptake of alpha-amino[(3)H]isobutyric acid (AIB) was studied in membrane vesicles from mouse fibroblasts transformed by simian virus 40 to examine the features of the Na(+)-stimulated and Na(+)-dependent AIB transport process. The simultaneous addition of NaCl and AIB to these vesicles produced a transient accumulation, or "overshoot," of amino acid 3-4 times the equilibrium value. Both the initial rate of uptake and the rate of fall of intravesicular AIB after maximal accumulation were sensitive to the temperature of incubation. The overshoot of AIB uptake was enhanced with Na(+) salts of highly permeant lipophilic anions, such as SCN(-) and NO(3) (-), and was decreased by the addition of SO(4) (2-), a relatively impermeant ion. Gramicidin D, which enhances the membrane conductance of Na(+) electrogenically, decreased the overshoot, while a potassium diffusion potential, induced by valinomycin (in K(+)-preloaded membrane vesicles), produced a Na(+)-dependent overshoot of AIB uptake. When vesicles were preincubated with both Na(+) and AIB, followed by the generation of an interior negative membrane potential (by the addition of SCN(-)), an overshoot of AIB uptake resulted. However, this did not occur in the absence of Na(+). It is concluded that, apart from its role in the generation of a transmembrane electrochemical potential, Na(+) is essential for the overshoot of AIB uptake.