Double-layer structure of the Pt(111)-aqueous electrolyte interface.

We present detailed measurements of the double-layer capacitance of the Pt(111)-electrolyte interface close to the potential of zero charge (PZC) in the presence of several different electrolytes consisting of anions and cations that are considered to be nonspecifically adsorbed. For low electrolyte concentrations, we show strong deviations from traditional Gouy-Chapman-Stern (GCS) behavior that appear to be independent of the nature of the electrolyte ions. Focusing on the capacitance further away from PZC and the trends for increasing ion concentration, we observe ion-specific capacitance effects that appear to be related to the size or hydration strength of the ions. We formulate a model for the structure of the electric double layer of the Pt(111)-electrolyte interface that goes significantly beyond the GCS theory. By combining two existing models, namely, one capturing the water reorganization on Pt close to the PZC and one accounting for an attractive ion-surface interaction not included in the GCS model, we can reproduce and interpret the main features the experimental capacitance of the Pt(111)-electrolyte interface. The model suggests a picture of the double layer with an increased ion concentration close to the interface as a consequence of a weak attractive ion-surface interaction, and a changing polarizability of the Pt(111)-water interface due to the potential-dependent water adsorption and orientation.

The surface conductance of red blood cells and platelets is modulated by the cell membrane potential.

Dielectrophoretic analysis of cell electrical properties via the Clausius-Mossotti model has been widely used to estimate values of the membrane conductance, membrane capacitance and cytoplasm conductivity of cells. However, although the latter two values produced by this method compare well to those acquired by other electrophysiological methods, the membrane conductance is often substantially larger than that acquired by methods such as patch clamp. In this paper, the electrical properties of red blood cells (RBC) are analysed at two conductivities and following membrane-altering treatments, to develop a mathematical model of membrane conductance. Results suggest that the RBC "membrane conductance" term is primarily dominated by surface conduction, comprising an element related to medium conductivity augmented by conduction in the electrical double layer, which is in turn altered by the cell membrane potential. Validation of the relationship between membrane potential and membrane conductance was performed using platelets, where a similar relationship was observed. This sheds new light on the origin and significance of the membrane conductance term and explains for the first time phenomena of alterations in the parameter counter to changes in membrane potential or cytoplasm conductivity.

Modeling the Interaction and Uptake of Cd-As(V) Mixture to Wheat Roots Affected by Humic Acids, in Terms of root cell Membrane Surface Potential (ψ0).

The joint toxicological effects of Cd2+ and As(V) mixture on wheat root as affected by environmental factors, such as pH, coexisting cations, and humic acids etc., were investigated using hydroponic experiments. The interaction and toxicological mechanisms of co-existing Cd2+ and As(V) at the interface of solution and roots in presence of humic acid were further explored by incorporating root cell membrane surface potential ψ0 into a mechanistic model of combined biotic ligand model (BLM)-based Gouy-Chapman-Stern (GCS) model and NICA-DONNAN model. Besides, molecular dynamics (MD) simulations of lipid bilayer equilibrated with solution containing Cd2+ and H2AsO4- further revealed the molecular distribution of heavy metal(loid) ions under different membrane surface potentials. H2AsO4- and Cd2+ can be adsorbed on the surface of the membrane alone or as complexes, which consolidate the limitation of the macroscopic physical models.

Double Layer at the Pt(111)-Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy-Chapman Screening.

We report, for the first time, the observation of a Gouy-Chapman capacitance minimum at the potential of zero charge of the Pt(111)-aqueous perchlorate electrolyte interface. The potential of zero charge of 0.3 V vs. NHE agrees very well with earlier values obtained by different methods. The observation of the potential of zero charge of this interface requires a specific pH (pH 4) and anomalously low electrolyte concentrations (<10-3 m). By comparison to gold and mercury double-layer data, we conclude that the diffuse double layer structure at the Pt(111)-electrolyte interface deviates significantly from the Gouy-Chapman theory in the sense that the electrostatic screening is much better than predicted by purely electrostatic mean-field Poisson-Boltzmann theory.

Hydrodynamic radius coincides with the slip plane position in the electrokinetic behavior of lysozyme.

The zeta potential (ζ) is the effective charge energy of a solvated protein, describing the magnitude of electrostatic interactions in solution. It is commonly used in the assessment of adsorption processes and dispersion stability. Predicting ζ from molecular structure would be useful to the structure-based molecular design of drugs, proteins, and other molecules that hold charge-dependent function while remaining suspended in solution. One challenge in predicting ζ is identifying the location of the slip plane (XSP ), a distance from the protein surface where ζ is theoretically defined. This study tests the hypothesis that the XSP can be estimated by the Stokes-Einstein hydrodynamic radius (Rh ), using globular hen egg white lysozyme as a model system. Although the XSP and Rh differ in their theoretical definitions, with the XSP being the position of the ζ during electrokinetic phenomena (e.g., electrophoresis) and the Rh being a radius pertaining to the edge of solvation during diffusion, they both represent the point where water and ions no longer adhere to a molecule. This work identifies the limited range of ionic strengths in which the XSP can be determined using diffusivity measurements and the Stokes-Einstein equation. In addition, a computational protocol is developed for determining the ζ from a protein crystal structure. At low ionic strengths, a hyperdiffusivity regime exists, requiring direct measurement of electrophoretic mobility to determine ζ. This work, therefore, supports a basic tenant of EDL theory that the electric double layer during diffusion and electrophoresis are equivalent in the Stokes-Einstein regime.

Label-free probe of HIV-1 TAT peptide binding to mimetic membranes.

The transacting activator of transduction (TAT) protein plays a key role in the progression of AIDS. Studies have shown that a +8 charged sequence of amino acids in the protein, called the TAT peptide, enables the TAT protein to penetrate cell membranes. To probe mechanisms of binding and translocation of the TAT peptide into the cell, investigators have used phospholipid liposomes as cell membrane mimics. We have used the method of surface potential sensitive second harmonic generation (SHG), which is a label-free and interface-selective method, to study the binding of TAT to anionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-1'-rac-glycerol (POPG) and neutral 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes. It is the SHG sensitivity to the electrostatic field generated by a charged interface that enabled us to obtain the interfacial electrostatic potential. SHG together with the Poisson-Boltzmann equation yielded the dependence of the surface potential on the density of adsorbed TAT. We obtained the dissociation constants Kd for TAT binding to POPC and POPG liposomes and the maximum number of TATs that can bind to a given liposome surface. For POPC Kd was found to be 7.5 ± 2 µM, and for POPG Kd was 29.0 ± 4.0 µM. As TAT was added to the liposome solution the POPC surface potential changed from 0 mV to +37 mV, and for POPG it changed from -57 mV to -37 mV. A numerical calculation of Kd, which included all terms obtained from application of the Poisson-Boltzmann equation to the TAT liposome SHG data, was shown to be in good agreement with an approximated solution.

Aluminium alleviates manganese toxicity to rice by decreasing root symplastic Mn uptake and reducing availability to shoots of Mn stored in roots.

BACKGROUND AND AIMS: Manganese (Mn) and aluminium (Al) phytotoxicities occur mainly in acid soils. In some plant species, Al alleviates Mn toxicity, but the mechanisms underlying this effect are obscure. METHODS: Rice (Oryza sativa) seedlings (11 d old) were grown in nutrient solution containing different concentrations of Mn(2+) and Al(3+) in short-term (24 h) and long-term (3 weeks) treatments. Measurements were taken of root symplastic sap, root Mn plaques, cell membrane electrical surface potential and Mn activity, root morphology and plant growth. KEY RESULTS: In the 3-week treatment, addition of Al resulted in increased root and shoot dry weight for plants under toxic levels of Mn. This was associated with decreased Mn concentration in the shoots and increased Mn concentration in the roots. In the 24-h treatment, addition of Al resulted in decreased Mn accumulation in the root symplasts and in the shoots. This was attributed to higher cell membrane surface electrical potential and lower Mn(2+) activity at the cell membrane surface. The increased Mn accumulation in roots from the 3-week treatment was attributed to the formation of Mn plaques, which were probably related to the Al-induced increase in root aerenchyma. CONCLUSIONS: The results show that Al alleviated Mn toxicity in rice, and this could be attributed to decreased shoot Mn accumulation resulting from an Al-induced decrease in root symplastic Mn uptake. The decrease in root symplastic Mn uptake resulted from an Al-induced change in cell membrane potential. In addition, Al increased Mn plaques in the roots and changed the binding properties of the cell wall, resulting in accumulation of non-available Mn in roots.

Involvement of the alpha-subunit N-terminus in the mechanism of the Na+,K+-ATPase.

Previous studies have shown that cytoplasmic K+ release and the associated E2 â†’ E1 conformational change of the Na+,K+-ATPase is a major rate-determining step of the enzyme's ion pumping cycle and hence a prime site of acute regulatory intervention. From the ionic strength dependence of the enzyme's distribution between the E2 and E1 states, it has also been found that E2 is stabilized by an electrostatic attraction. Any disruption of this electrostatic attraction would, thus, have profound effects on the rate of ion pumping. The aim of this paper is to identify the location of this interaction. Using enhanced-sampling molecular dynamics simulations with a predicted N-terminal structure added to the X-ray crystal structure of the Na+,K+-ATPase, a previously postulated salt bridge between Lys32 and Glu233 (rat sequence numbering) of the enzyme's α-subunit can be excluded. The residues never approach closely enough to form a salt bridge. In contrast, strong interactions with anionic lipid head groups were seen. To investigate the possibility of a protein-lipid interaction experimentally, the surface charge density of Na+,K+-ATPase-containing membrane fragments was estimated from zeta potential measurements to be 0.019 (± 0.001) C m-2. This is in good agreement with the charge density previously determined to be responsible for stabilization of the E2 state of 0.023 (± 0.009) C m-2 and the membrane charge density estimated here from published electron-microscopic images of 0.018C m-2. The results are, therefore, consistent with an interaction of the Na+,K+-ATPase α-subunit N-terminus with negatively-charged lipid head groups of the neighbouring cytoplasmic membrane surface as the origin of the electrostatic interaction stabilising the E2 state.

Predicting the size of salt-containing aqueous Na-AOT reverse micellar water-in-oil microemulsions with consideration for specific ion effects.

HYPOTHESIS: Reverse micellar solutions are thermodynamically stable systems in which surfactant molecules surround water droplets within a continuous organic phase. Among their many applications, they can be used for the synthesis of nanoparticles of controlled agglomeration. Here, we consider the role specific ion effects play in reverse micelle size reduction. EXPERIMENTS: Dynamic light scattering measurements and the Gouy-Chapman electrical double layer model were combined to study water/AOT/isooctane reverse micellar systems (wo = 10). Linear relationships between the solvodynamic diameter (D) of reverse micelles containing various concentrations of FeSO4, Mg(NO3)2, CuCl2, Al(NO3)3, Fe(NO3)3, Y(NO3)3, NaBH4, ZrOCl2, and NH4OH, and their calculated Debye screening lengths, κ-1, were observed with decreasing D and increasing salt concentration (c). FINDINGS: By comparing the linear fits for reverse micelle size as a function of c-1/2, we determined the size can be described as a function of the Debye screening length, cation valency (z), and specific anion hydrated radius (ran), where D = 3.1z κ-1 + bi, and bi is linearly related to ran. Our model accurately predicts reverse micelle sizes with the addition of monovalent, divalent, and trivalent salts for which the primary hydrolyzed cation species has a charge that is equal to the cation valency.

CONCENTRATION GRADIENTS INSIDE MICRODROPLETS.

Small water microdroplets in microfluidic systems have a high surface charge density resulting from charged surfactants. As a result, an electric double layer forms inside the droplet. Depletion of ions from the center of the droplet to form the double layer can shift the concentration of ions dramatically from that of the microdroplet precursor solution. Here we show numerical solutions to the Gouy-Chapman model in spherical coordinates. Some notable effects include: 1) large percentages of the microdroplet volume experience very large DC electric fields; 2) many ions get forced into a Stern layer giving dramatically different conditions from the bulk.

Broad considerations concerning electrochemical electrodes in primarily fluid environments.

This review is variously a presentation, reflection, synthesis and report with reference to more recent developments of an article - in a journal which has ceased publication - entitled "Some Electrode Theorems with Experimental Corroboration, Inclusive of the Ag/AgCl System" Internet Journal of Chemistry, (http://www.ijc.com), Special Issues: Vol. 2 Article 24 (1999). The results from new lemmas relating charge densities and capacitance in a metallic electrode in equilibrium with an ionic solution are used to explain the data and observed effects due to Esin, Markov, Grahame, Lang and Kohn. Size effects that vary the measured e.m.f. of electrodes due to changes in the electronic chemical potential are demonstrated in experiment and theory implying the need for standardization of electrodes with respect to geometry and size. The widely used Stern modification of the Gouy-Chapman theory is shown to be mostly inapplicable for many of the problems where it is employed. Practical consequences of the current development include the possibility of determining the elusive single-ion activity coefficients of solution ions directly from the expression given by a simplified capacitance theorem, the potential of zero charge and the determination of single ion concentrations of active species in the electrode reactions from cell e.m.f. measurements.

Interaction of salt with ether- and ester-linked phospholipid bilayers.

A distinguishing feature of Archaeal plasma membranes is that their phospholipids contain ether-links, as opposed to bacterial and eukaryotic plasma membranes where phospholipids primarily contain ester-links. Experiments show that this chemical difference in headgroup-tail linkage does produce distinct differences in model bilayer properties. Here we examine the effects of salt on bilayer structure in the case of an ether-linked lipid bilayer. We use molecular dynamics simulations and compare equilibrium properties of two model lipid bilayers in NaCl salt solution - POPC and its ether-linked analog that we refer to as HOPC. We make the following key observations. The headgroup region of HOPC "adsorbs" fewer ions compared to the headgroup region of POPC. Consistent with this, we note that the Debye screening length in the HOPC system is ∼ 10% shorter than that in the POPC system. Herein, we introduce a protocol to identify the lipid-water interfacial boundary that reproduces the bulk salt distribution consistent with Gouy-Chapman theory. We also note that the HOPC bilayer has excess solvent in the headgroup region when compared to POPC, coinciding with a trough in the electrostatic potential. Waters in this region have longer autocorrelation times and smaller lateral diffusion rates compared to the corresponding region in the POPC bilayer, suggesting that the waters in HOPC are more strongly coordinated to the lipid headgroups. Furthermore, we note that it is this region of tightly coordinated waters in the HOPC system that has a lower density of Na+ ions. Based on these observations we conclude that an ether-linked lipid bilayer has a lower binding affinity for Na+ compared to an ester-linked lipid bilayer.

Method for Studying the Physical Effect of Extracellular Matrix on Voltage-Dependent Ion Channel Gating.

Divalent cations can change the actual electrical potential at the outer surface of the plasma membrane. They do so by the so-called Gouy-Chapman-Stern effect which is due to the electrical "masking" that certain ions, especially divalents, can exert onto the electrically negative charged polar heads of the membrane phospholipids.Chondroitin sulfates can chelate free calcium ions to a different extent based on the spatial arrangement of their sulfate groups and can thus alter the actual availability of screening divalent ions at the outer membrane surface.Voltage-dependent ion channels sense the actual potential difference between the two sides of the plasma membrane and are thus exquisite and extremely sensitive "devices" able to react to changes in the electrical potential across the membrane.Hence, by recording the shift in the activation curve of well-known voltage-dependent ionic channels it will be possible to study the physical effect of ECM chondroitin sulfates on membrane conductances.

Use of electrochemical impedance spectroscopy to determine double-layer capacitance in doped nonpolar liquids.

Electrochemical impedance spectroscopy in a thin cell (10 µm) was used to infer conductivity, permittivity and the differential double-layer capacitance of solutions of dodecane doped with OLOA 11000 [poly(isobutylene) succinimide] for concentrations of dopant between 0.1% and 10% by weight. All spectra (frequencies between 1 Hz and 100 kHz) were well fit by an equivalent circuit having four elements including a constant-phase element representing the double-layer capacitance. Using Gouy-Chapman theory for small zeta potentials and assuming univalent charge carriers, the double-layer capacitances were converted into charge carrier concentration which was found to be directly proportional to the weight percent of dopant with a 1 wt% solution having 87 carriers/µm(3) (the concentration of either positive or negative charges). This is only 17 ppm of the total monomer concentration calculated from the average molecule weight of the dopant. Dividing the measured conductivities by the charge carrier concentration, we inferred the mobility and hydrodynamic diameters for the charged micelles. The hydrodynamic diameters of carriers were significantly larger than the average diameter of all micelles measured independently by dynamic light scattering. This suggests that only large micelles become charged.

The interaction of electric field and hydrostatic pressure in an electrical double layer: A simple "first principle" model that accounts for the finite sizes of counterions.

A simple model describing the influence of ion size in the electrical double layer (EDL) near a highly charged plane is proposed here. This model is based on the Poisson-Boltzmann equation with a single additional term representing the mechanical response of bulky ions to hydrostatic pressure. This pressure is produced by Coulomb forces, and increases to several kilobars in the vicinity of a highly charged plane. Numerical simulations demonstrate close packing as a limit for counterion concentrations. Differential capacity reaches maximum at 0.1-0.3V and remains reasonably small in wide range of potentials.

Ionizable Nitroxides for Studying Local Electrostatic Properties of Lipid Bilayers and Protein Systems by EPR.

Electrostatic interactions are known to play a major role in the myriad of biochemical and biophysical processes. Here, we describe biophysical methods to probe local electrostatic potentials of proteins and lipid bilayer systems that are based on an observation of reversible protonation of nitroxides by electron paramagnetic resonance (EPR). Two types of probes are described: (1) methanethiosulfonate derivatives of protonatable nitroxides for highly specific covalent modification of the cysteine's sulfhydryl groups and (2) spin-labeled phospholipids with a protonatable nitroxide tethered to the polar head group. The probes of both types report on their ionization state through changes in magnetic parameters and degree of rotational averaging, thus, allowing the electrostatic contribution to the interfacial pKa of the nitroxide, and, therefore, the local electrostatic potential to be determined. Due to their small molecular volume, these probes cause a minimal perturbation to the protein or lipid system. Covalent attachment secures the position of the reporter nitroxides. Experimental procedures to characterize and calibrate these probes by EPR, and also the methods to analyze the EPR spectra by simulations are outlined. The ionizable nitroxide labels and the nitroxide-labeled phospholipids described so far cover an exceptionally wide range of ca. 2.5-7.0 pH units, making them suitable to study a broad range of biophysical phenomena, especially at the negatively charged lipid bilayer surfaces. The rationale for selecting proper electrostatically neutral interface for probe calibration, and examples of lipid bilayer surface potential studies, are also described.

A model for membrane potential and intracellular ion distribution.

Most cells carry a negative electric charge. It produces a potential difference across the membrane, which regulates voltage-sensitive ion transport and ATP synthesis in mitochondria. The negative charge comes partly from an excess of negative ions in the cell interior (Donnan potential) and partly from ionized groups on the membrane (surface potential). In this work we propose some important modifications to the existing theory of membrane potential. First, we calculate the concentration profile of intracellular positive ions and derive a simple equation to assess the submembrane depletion of positive ions that gives rise to the Donnan potential. The extent of depletion varies with potential, which may provide a regulatory mechanism for ion pumps and channels. Next we consider the surface component of the potential and note that the standard Gouy-Chapman theory has been developed for planar membranes, whereas real cell membranes have a closed geometry. In this case, charges on the membrane surface are not expected to generate fields extending into the cell interior. This fact calls for reinterpretation of some theoretical points as well as experimental data. In particular, the experimentally demonstrated electrostatic attraction between cationic proteins and the negative membrane must now be explained without invoking intracellular fields, and we suggest a new mechanism that can account for this interaction.

Modification of the surface of activated carbon electrodes for capacitive mixing energy extraction from salinity differences.

The "capacitive mixing" (CAPMIX) is one of the techniques aimed at the extraction of energy from the salinity difference between sea and rivers. It is based on the rise of the voltage between two electrodes, taking place when the salt concentration of the solution in which they are dipped is changed. We study the rise of the potential of activated carbon electrodes in NaCl solutions, as a function of their charging state. We evaluate the effect of the modification of the materials obtained by adsorption of charged molecules. We observe a displacement of the potential at which the potential rise vanishes, as predicted by the electric double layer theories. Moreover, we observe a saturation of the potential rise at high charging states, to a value that is nearly independent of the analyzed material. This saturation represents the most relevant element that determines the performances of the CAPMIX cell under study; we attribute it to a kinetic effect.

Theoretical voltammetric response of electrodes coated by solid polymer electrolyte membranes.

A model for the differential capacitance of metal electrodes coated by solid polymer electrolyte membranes, with acid/base groups attached to the membrane backbone, and in contact with an electrolyte solution is developed. With proper model parameters, the model is able to predict a limit response, given by Mott-Schottky or Gouy-Chapman-Stern theories depending on the dissociation degree and the density of ionizable acid/base groups. The model is also valid for other ionic membranes with proton donor/acceptor molecules as membrane counterions. Results are discussed in light of the electron transfer rate at membrane-coated electrodes for electrochemical reactions that strongly depend on the double layer structure. In this sense, the model provides a tool towards the understanding of the electro-catalytic activity on modified electrodes. It is shown that local maxima and minima in the differential capacitance as a function of the electrode potential may occur as consequence of the dissociation of acid/base molecular species, in absence of specific adsorption of immobile polymer anions on the electrode surface. Although the model extends the conceptual framework for the interpretation of cyclic voltammograms for these systems and the general theory about electrified interfaces, structural features of real systems are more complex and so, presented results only are qualitatively compared with experiments.

Capacitive mixing for the extraction of energy from salinity differences: survey of experimental results and electrochemical models.

The "capacitive mixing" (CAPMIX) technique is an emerging technology aimed at the extraction of energy from salinity differences, e.g. between sea and river waters. CAPMIX benefits from the voltage rise that takes place between two electrodes dipped in a saline solution when its salt concentration is changed. Several kinds of electrodes have been proposed so far: activated carbon materials (Brogioli, 2009), membrane-based ion-selective electrodes (Sales et al., 2010), and battery electrodes (Biesheuvel and van der Wal, 2010). The power production mainly depends on two properties of each single electrode: the amplitude of the potential rise upon salinity change, and the potential in the high-salinity solution. The various electrode materials that have been used returned different values of the two parameters, and hence to different power productions. In this paper, we apply electrokinetic and electrochemical models to qualitatively explain the experimentally observed behaviors of various materials under different experimental conditions. The analysis allows to devise techniques for tailoring new materials, particularly suited for the CAPMIX technique.