Potential-dependent Processes Research Articles

Potential Distribution across Model Membranes.

Membrane models assembled on electrodes are widely used tools to study potential-dependent molecular processes at or in membranes. However, the relationship between the electrode potential and the potential across the membrane is not known. Here we studied lipid bilayers immobilized on mixed self-assembled monolayers (SAM) on Au electrodes. The mixed SAM was composed of thiol derivatives of different chain lengths such that between the islands of the short one, mercaptobenzonitrile (MBN), and the tethered lipid bilayer an aqueous compartment was formed. The nitrile function of MBN, which served as a reporter group for the vibrational Stark effect (VSE), was probed by surface-enhanced infrared absorption spectroscopy to determine the local electric field as a function of the electrode potential for pure MBN, mixed SAM, and the bilayer system. In parallel, we calculated electric fields at the VSE probe by molecular dynamics (MD) simulations for different charge densities on the metal, thereby mimicking electrode potential changes. The agreement with the experiments was very good for the calculations of the pure MBN SAM and only slightly worse for the mixed SAM. The comparison with the experiments also guided the design of the bilayer system in the MD setups, which were selected to calculate the electrode potential dependence of the transmembrane potential, a quantity that is not directly accessible by the experiments. The results agree very well with estimates in previous studies and thus demonstrate that the present combined experimental-theoretical approach is a promising tool for describing potential-dependent processes at biomimetic interfaces.

ANT1 Activation and Inhibition Patterns Support the Fatty Acid Cycling Mechanism for Proton Transport.

Adenine nucleotide translocase (ANT) is a well-known mitochondrial exchanger of ATP against ADP. In contrast, few studies have shown that ANT also mediates proton transport across the inner mitochondrial membrane. The results of these studies are controversial and lead to different hypotheses about molecular transport mechanisms. We hypothesized that the H+-transport mediated by ANT and uncoupling proteins (UCP) has a similar regulation pattern and can be explained by the fatty acid cycling concept. The reconstitution of purified recombinant ANT1 in the planar lipid bilayers allowed us to measure the membrane current after the direct application of transmembrane potential ΔΨ, which would correspond to the mitochondrial states III and IV. Experimental results reveal that ANT1 does not contribute to a basal proton leak. Instead, it mediates H+ transport only in the presence of long-chain fatty acids (FA), as already known for UCPs. It depends on FA chain length and saturation, implying that FA’s transport is confined to the lipid-protein interface. Purine nucleotides with the preference for ATP and ADP inhibited H+ transport. Specific inhibitors of ATP/ADP transport, carboxyatractyloside or bongkrekic acid, also decreased proton transport. The H+ turnover number was calculated based on ANT1 concentration determined by fluorescence correlation spectroscopy and is equal to 14.6 ± 2.5 s−1. Molecular dynamic simulations revealed a large positively charged area at the protein/lipid interface that might facilitate FA anion’s transport across the membrane. ANT’s dual function—ADP/ATP and H+ transport in the presence of FA—may be important for the regulation of mitochondrial membrane potential and thus for potential-dependent processes in mitochondria. Moreover, the expansion of proton-transport modulating drug targets to ANT1 may improve the therapy of obesity, cancer, steatosis, cardiovascular and neurodegenerative diseases.

Building Ab Initio Interface Pourbaix diagrams to Investigate Electrolyte Stability in the Electrochemical Double Layer: Application to Magnesium Batteries.

Insights into the electrochemical processes occurring at the electrode-electrolyte interface are a crucial step in most electrochemistry domains and in particular in the optimization of the battery technology. However, studying potential-dependent processes at the interface is one of the biggest challenges, both for theoreticians and experimentalists. The challenge is pushed further when stable species also depend on the concentration of specific ligands in the electrolyte, such as chlorides. Herein, we present a general theoretical ab initio methodology to compute a Pourbaix-like diagram of complex electrolytes as a function of electrode potential and anion's chemical potential, that is, concentration. This approach is developed not only for the bulk properties of the electrolytes but also for electrode-electrolyte interfaces. In the case of chlorinated magnesium complexes in dimethoxyethane, we show that the stability domains of the different species are strongly shifted at the interface compared to the bulk of the electrolyte because of the strong local electric fields and charges occurring in the double layer. Thus, as the interfacial stability domains are strongly modified, this approach is necessary to investigate all interface properties that often govern the reaction kinetics, such as solvent degradation at the electrode. Interface Pourbaix diagram is used to give some insights into the improved stability at the Mg anode induced by the addition of chloride. Because of its far-reaching insights, transferability, and wide applicability, the methodology presented herein should serve as a valuable tool not only for the battery community but also for the wider electrochemical one.

Proton exchange membrane fuel cell cathode contamination – Acetylene

Acetylene adsorption on PEMFC electrodes and contamination in single cells are investigated with 300 ppm acetylene at a cathode held at 80 °C. The results of adsorption experiments suggest that acetylene adsorbs readily on electrodes and is reduced to ethylene and ethane under an open circuit potential of H2/N2, as the adsorbates can be electro-oxidized at high potentials. The cell voltage response shows that 300 ppm acetylene results in a cell performance loss of approximately 88%. The voltage degradation curve is divided into two stages by an inflection point, which suggests that potential-dependent processes are involved in acetylene poisoning. These potential-dependent processes may include acetylene oxidation and reduction as well as accumulation of intermediates on the electrode surface. Electrochemical impedance spectroscopy analysis suggests that acetylene affects the oxygen reduction reaction and may also affect mass transport processes. Acetylene also may be reduced in the steady poisoning state of the operating cell. After neat air operation, the cyclic voltammetry results imply that the cathode catalyst surface is almost completely restored, with no contaminant residues remaining in the MEA. Linear scanning voltammetry measurements show no change in hydrogen crossover caused by contamination, and polarization curves confirm complete recovery of cell performance.

Electrochemically-Driven Large Amplitude pH Cycling for Acid–Base Driven DNA Denaturation and Renaturation

In this paper, we present an electrochemically driven large amplitude pH alteration method based on a serial electrolytic cell involving a hydrogen permeable bifacial working electrode such as Pd thin foil. The method allows solution pH to be changed periodically up to ±4~5 units without additional alteration of concentration and/or composition of the system. Application to the acid-base driven cyclic denaturation and renaturation of 290 bp DNA fragments is successfully demonstrated with in situ real-time UV spectroscopic characterization. Electrophoretic analysis confirms that the denaturation and renaturation processes are reversible without degradation of the DNA. The serial electrolytic cell based electrochemical pH alteration method presented in this work would promote investigations of a wide variety of potential-dependent processes and techniques.

Multidimensional electrochemical imaging in materials science

In the past 20 years the characterization of electroactive surfaces and electrode reactions by scanning probe techniques has advanced significantly, benefiting from instrumental and methodological developments in the field. Electrochemical and electrical analysis instruments are attractive tools for identifying regions of different electrochemical properties and chemical reactivity and contribute to the advancement of molecular electronics. Besides their function as a surface analytical device, they have proved to be unique tools for local synthesis of polymers, metal depots, clusters, etc. This review will focus primarily on progress made by use of scanning electrochemical microscopy (SECM), conductive AFM (C-AFM), electrochemical scanning tunneling microscopy (EC-STM), and surface potential measurements, for example Kelvin probe force microscopy (KFM), for multidimensional imaging of potential-dependent processes on metals and electrified surfaces modified with polymers and self assembled monolayers.

Novel Time-Resolved Surface-Enhanced (Resonance) Raman Spectroscopic Technique for Studying the Dynamics of Interfacial Processes: Application to the Electron Transfer Reaction of Cytochrome c at a Silver Electrode

A novel approach for time-resolved (TR) surface-enhanced (resonance) Raman (SE(R)R) spectroscopy is presented for probing potential-dependent processes of molecules adsorbed on a silver electrode. TR SE(R)R spectroscopy offers the unique advantage of providing structural and kinetic data exclusively of the adsorbed species and their reactions. These processes are initiated by a rapid potential jump and monitored by SE(R)R spectroscopy after a delay time δ for the probe interval Δ t. The synchronization is achieved by a mechanical chopper that triggers the potential jump via a photodiode and gates the exciting continuous-wave laser beam. After the probe event, the potential is reset to its initial value. Thus, the original equilibrium is restored to allow a continuous repetition of the sequence of potential jumps and probe events. During the entire experiment, the detection system, a liquid nitrogen-cooled charge-coupled device (CCD) detector, is active so that the signal-to-noise ratio (SNR) can be iteratively improved. This mode of detection does not limit the time resolution, so that the present approach allows TR SE(R)R experiments down to the microsecond time scale without lowering the SNR. The possibilities and limitations of this method are discussed. As an example we present preliminary results of a TR SERR study on yeast iso-1 cytochrome c (Cyt- c) adsorbed on a Ag electrode by applying a potential jump from −0.4 V to +0.05 V (vs. saturated calomel electrode). The experiments are carried out with a rotating electrode to avoid photoinduced degradation and desorption processes. The SERR spectra, which were measured with delay times between 45 to 175 ms, were analyzed quantitively in terms of the various states of the adsorbed Cyt- c that are formed in this potential range. The results show that under these conditions the relaxation processes include the electron transfer of the adsorbed Cyt- c to the electrode and a subsequent conformational transition. The analysis of the data reveals a heterogenous oxidation rate constant of 10.3 s−1 and rate constant for the conformational transition of 4.3 s−1, supporting the view that the biological electron transfer of Cyt- c is coupled with conformational transitions.

Potential-dependent surface enhanced resonance Raman spectroscopy of cytochromec552 fromThermus thermophilus

Surface-enhanced resonance Raman (SERR) spectroscopy was employed to study the potential-dependent processes of the electron-transferring heme protein cytochrome c552 (Cyt-c552) of Thermus thermophilus adsorbed on a silver electrode. In the reduced state, the SERR spectrum of Cyt-c552 is very similar to the resonance Raman (RR) spectrum of the dissolved species, ruling out substantial conformational changes due to adsorption. The adsorbed oxidized form, however, exists in different conformational states including species in a five-coordinated high-spin state and a six-coordinated low-spin state which is different from that of the dissolved species. Based on the SERR spectra measured in the potential range between 0.0 and -0.2 V (vs. a saturated calomel electrode), an apparent redox potential of -0.097 V was obtained which is significantly more negative than the value determined in solution. This discrepancy and the non-Nernstian behavior could be attributed to the coupling of electron-transfer reactions and conformational transitions. These findings, which are closely related to those obtained previously for mitochondrial cytochrome c, are discussed on the basis of the specific structural properties of Cyt-c552. © 1998 John Wiley & Sons. Ltd.

Quantitative Solid-State NMR Spectra of CO Adsorbed from Aqueous Solution onto a Commercial Electrode

In this communication we present quantitative, solid-state NMR spectra of CO adsorbed from solution onto a commercial Pt/C fuel cell electrode; the CO spectra are presented as a function of surface coverage, as modified by electrochemical oxidation of surface-bound CO. These data clearly portend the widespread application of quantitative solid-state NMR techniques for studies of practical electrochemical processes. In conclusion, we report quantitative {sup 13}C NMR spectra for CO adsorbed onto commercial Pt/C fuel cell electrodes. Each spectrum shown required approximately 14 h of instrument time to acquire; we are further optimizing both the NMR parameters and probe configuration for improved signal-to-noise ratios. The number and character of CO adsorption sites are clearly accessible via this approach; if used in conjunction with in situ potential control, the number of possible studies of potential-dependent processes appears to be vast. 6 refs., 2 figs.

Corrosion of aluminium in acidic and neutral solutions

Polarization and impedance measurements were performed to study the corrosion behaviour of aluminium in acidic and nearly neutral solutions. The impedance diagram shows at least three time constants or semi-circles of which the size are dependent on the electrode potential. This indicates that at least three potential dependent processes are occurring. The capacitive time constant at high frequencies may be related to the oxide film itself. The second capacitive time constant at low frequencies can be attributed to the dissolution of the oxide film.

Differential effects of M 1- and M 2-muscarinic drugs on striatal dopamine release and metabolism in freely moving rats

A dialysis loop cannula was implanted into rat striatum under anesthetized condition, and the area was perfused with Ringer's solution under freely moving condition after 3 days for surgical recovery. Dopamine (DA) and 3,4-dihydroxyphenylacetic acid recovered in the dialysate were measured by high-performance liquid chromatography with electrochemical detection. The effects of M 1- and M 2-muscarinic receptor agents, which were perfused continuously into the striatum through the dialysis membrane, were investigated. Continuous perfusion of AF102B, and M 1-selective agonist, and oxotremorine, a non-selective agonist, resulted in a dose-dependent increase in the striatal DA release. Pirenzepine (10 −5 and 10 −7 M), an M 1-selective antagonist, decreased the release of DA, and the stimulatory effect of AF102B (10 −5 M) was completely inhibited by 10 −5 and 10 −7 M pirenzepine, while the stimulatory effect of oxotremorine (10 −4 M) was only partly inhibited by 10 −5 M pirenzepine. AF-DX116 (10 −5 M), an M 2-selective antagonist, increased the DA release, and showed an additive effect on the DA release evoked by AF102B (10 −5 M), whereas it produced no significant effect on oxotremorine (10 −5 M)-evoked DA release. These results suggest that in vivo DA release in the rat striatum is modulated by different subtypes of muscarinic receptors; i.e., the stimulatory effect is mainly mediated by M 1-sites and inhibitory effect is mainly mediated by M 2-sites. The changes in the DA release induced by the various drugs were prevented by pretreatment with tetrodotoxin (TTX). Since action potential-dependent DA release (exocytosis) is blocked by the pretreatment with TTX, those drugs affect DA release by means of action potential-dependent processes.

Experiments on optical second-harmonic generation as a surface probe of electrodes

Optical second-harmonic generation (SHG) has been used as an in situ probe of potential-dependent processes occurring at the metal–electrolyte interface. Differential capacitance measurements of the charging behavior of smooth silver surfaces have been made and compared with the simultaneously monitored SHG. The results show a correlation between the electrostatics of the interface and the second-harmonic response when the electrode is polarized within the double-layer charging region. The nonlinear second-harmonic response during electrochemical film formation has been compared with the potential dependence of the linear optical properties of the surface at ω and 2ω. Optoacoustic detection methods have been employed in these studies.

The effects of temperature, local anaesthetics, pH, divalent cations, and group-specific reagents on repriming and repolarization-induced contractures in frog skeletal muscle.

Contractures appear during repolarization of frog toe muscles in media containing perchlorate in place of chloride. These contractures were suppressed or delayed by certain procedures which retard the repriming of K contractures, i.e., by sufficient reduction in temperature or by alkaline pH in solutions lacking divalent cations. They also were greatly reduced without interference with repriming after treatment with a reagent which selectively modifies free amino groups. In the presence of appropriate concentrations of procaine, repriming was markedly impaired with only a small reduction in the amplitude of repolarization-induced contractures. Small contractures were produced during repolarization in chloride solutions in the presence of 10 mM procaine at pH 8.0. None of these procedures affected the changes produced by perchlorate solutions in the potential dependence and the time course of K contractures. The results support the view that activation and inactivation of contraction following depolarization are separate potential dependent processes. Tension appears to develop during repolarization when the reversal of inactivation occurs before the reversal of activation is completed, both steps being necessary to recover the reprimed resting state.

Restoration of the ability of frog skeletal muscle to develop potassium contractures in calcium-deficient media.

A number of agents were tested for their ability to restore potassium (K) contractures in calcium-free media. Effective agents included caffeine, chloroform, more polar permeant anions (e.g. nitrate and perchlorate) in place of external chloride, as well as divalent cations, e.g. Mg2+. The presence of sufficient EGTA to preclude significant increases in [Ca]0 did not affect the extent of K contracture restoration produced by these agents. The loss of K contracture capacity in calcium-free media, and its restoration by effective agents, appear to be related to the disproportionate effect of these procedures on the relation between log [K]0 and the potential-dependent processes which regulate contractile function.

Repolarization-induced reactivation of contracture tension in frog skeletal muscle.

In solutions containing perchlorate in place of chloride frog toe muscles or small bundles of semitendinosus fibers undergo maximum potassium (K) contractures of long duration at low [K]0 (5–10 mM). After relaxation at high [K]0, large protracted "reactivation" contractures (70–90% of maximum tension) again develop when repolarization is accomplished by a reduction of [K]0 to 2.5–10 mM. Somewhat smaller contractures also appear during repolarization at lower perchlorate concentrations (8–12 mM). The effects of perchlorate include disproportionate shifts in the relation between log [K]0 and K-contracture tension, and between log [K]0 and relaxation rate. Similar but smaller effects are observed in the presence of 1 mM caffeine or 1.5 mM chloroform. These observations implicate at least two potential-dependent processes in the regulation of contraction in frog twitch muscle.