Dependence Of Peak Currents Research Articles

Activation and desensitization of the recombinant P2X1 receptor at nanomolar ATP concentrations.

Activation and desensitization kinetics of the rat P2X1 receptor at nanomolar ATP concentrations were studied in Xenopus oocytes using two-electrode voltage-clamp recording. The solution exchange system used allowed complete and reproducible solution exchange in <0.5 s. Sustained exposure to 1–100 nM ATP led to a profound desensitization of P2X1 receptors. At steady-state, desensitization could be described by the Hill equation with a K1/2 value of 3.2 ± 0.1 nM. Also, the ATP dependence of peak currents could be described by a Hill equation with an EC50 value of 0.7 μM. Accordingly, ATP dose-effect relationships of activation and desensitization practically do not overlap. Recovery from desensitization could be described by a monoexponential function with the time-constant τ = 11.6 ±1.0 min. Current transients at 10–100 nM ATP, which elicited 0.1–8.5% of the maximum response, were compatible with a linear three-state model, C-O-D (closed-open-desensitized), with an ATP concentration-dependent activation rate and an ATP concentration-independent (constant) desensitization rate. In the range of 18–300 nM ATP, the total areas under the elicited current transients were equal, suggesting that P2X1 receptor desensitization occurs exclusively via the open conformation. Hence, our results are compatible with a model, according to which P2X1 receptor activation and desensitization follow the same reaction pathway, i.e., without significant C to D transition. We assume that the K1/2 of 3.2 nM for receptor desensitization reflects the nanomolar ATP affinity of the receptor found by others in agonist binding experiments. The high EC50 value of 0.7 μM for receptor activation is a consequence of fast desensitization combined with nonsteady-state conditions during recording of peak currents, which are the basis of the dose-response curve. Our results imply that nanomolar extracellular ATP concentrations can obscure P2X1 receptor responses by driving a significant fraction of the receptor pool into a long-lasting refractory closed state.

Study on the Electrochemical Behavior of Melatonin with an Activated Electrode

AbstractIt was found that melatonin could be incorporated and accumulated on the surface of the glassy carbon electrode which was activated electrochemically by pretreatment in sodium hydroxide solution by means of cycling the potential well into the positive limit of the solvent. In Britton‐Robinson buffer solution (pH 6.7), melatonin gave a sensitive oxidation wave at a potential of +0.65 V (vs.Ag/AgCl) by using Osteryoung square‐wave stripping voltammetry (OSWSV). The oxidation process has been shown to be irreversible and adsorption‐controlled at this electrode by means of cyclic voltammetry and linear sweep voltammetry. A chronocoulometric characterization of the adsorption characteristics of melatonin at this electrode is also presented. The factors affecting the peak current were optimized, and the dependence of peak currents on the concentration of melatonin was found to be linear in the range 8.0×10−7−1.0×10−5 mol L−1. A detection limit of 5.0×10−8 mol L−1 was obtained (signal‐to‐noise ratio of 3). This method was applied to the assay of melatonin in tablets and capsules with good recoveries (98–100%).

Anomalous Dependence of Peak Currents on Scanning Rate in Voltammetry of Co2+, Fe2+, and Ni2+Ions in Presence of Thiols

In the voltammetry of complexes of substituted thiols with Co, Fe and Ni, in which the metals are deposited on the electrode, a new faradaic current appears which increases with decreasing rate of potential scan; it is apparently due to a catalytic evolution of hydrogen.

The electrochemical response of manganese hydroxide–oxide films in slightly alkaline solutions. II. Voltammetric and ellipsometric studies

The potentiodynamic behaviour of colloidal manganese hydroxide–oxide films obtained by chemical precipitation on a conducting substrate in alkaline solutions is described. The voltammetric response of cycled electrodes shows a main redox couple which is associated with the Mn(III)/Mn(IV) reaction occurring in solid state. The observed dependence of peak currents and peak potentials on sweep rate, charge and solution pH is similar to that reported for manganese and iridium oxides obtained by cycling the base metal. The optical response of cycled hydrous manganese oxide films is studied by in-situ ellipsometry. Data can be explained using an anisotropic single layer model. The optical anisotropy is related to the morphology of the hydrous oxide layer. The electroreduction process produces a decrease of the refractive indices and a pronounced diminution of the extinction coefficients yielding a practically transparent film.

Electrochemical reduction of substituted cobalt phthalocyanines adsorbed on graphite

Unsubstituted phthalocyaninatocobalt(II) [PcCo] and the cobalt complexes of the substituted ligands octabutoxyphthalocyanine [Pc(OBu) 8Co], octacyanophthalocyanine [Pc(CN) 8Co], phthalocyaninetetracarboxylic acid [Pc(COOH) 4Co] and tetrapyridotetraazaporphyrin [TPyTAPCo] were absorbed on the basal plane of pyrolytic graphite (BPG) from solutions in organic solvents to form ultrathin films. The films were studied by cyclic voltammetry in the presence of an inert aqueous electrolyte to yield the redox characteristics of the films. The films undergo reduction and reoxidation at the ligand inner π system as well as at the metal centre with a linear dependence of peak currents on scan rate that is typical for surface-confined species. The kinetics of the two reactions are further discussed on the basis of the analysis of peak shapes and charge found in the different waves in the cyclic voltammograms. The redox potentials of both reactions are shifted according to the electron donating or withdrawing character of the ligand's periphery as seen by comparison to molecular orbital (MO) calculations and ultraviolet photoelectron spectroscopy (UPS) results. During the reduction step, protons from the electrolyte are interacting with the films and are desorbed during reoxidation, leading to a strong pH dependence of the redox potential. The acid—base properties of the ligands determine a threshold pH above which the potential of the metal-centred first reduction is no longer dependent on pH. The ligand-centred second reduction, however, led to interaction with protons over the whole accessible pH range. Both electrode reactions are discussed in view of chemical interactions of the protons with the various ligand systems.

Cyclic voltammetry at microstructured electrodes

We present the theoretical treatment of cyclic voltammograms at microstructured electrodes. Calculations of voltammograms permit the determination of electrochemical parameters of redox systems in a single cell in parallel with the determinations of the spectroscopic parameters. The structural parameters of the electrode can be determined using the theoretical treatment presented if the electrochemical parameters of the redox system are known. Furthermore, lithographic-galvanic (LIGA) structures can be used as a model for microporous electrodes. Regression analysis was used to compare experimental and calculated cyclic voltammograms as well as to determine the electrochemical and spectroscopic parameters. A modified Randles-Sevčik equation has been derived to described the peak current dependence of cyclic voltammograms at micro-structured electrodes for both reversible and quasi-reversible charge transfer.

Effect of scattering on the resonant-tunneling current in double-barrier structures

The scattering effect on resonant tunneling current for double-barrier resonant tunneling diode is investigated. It is clearly shown that while the elastic scattering effect is negligible, the inelastic scattering will suppress the resonant tunneling current by breaking the phase coherence. An expression for the current suppression is obtained. The inelastic scattering effect is also dominant for the temperature dependence of peak current. The numerical result is in good agreement with the experiment result.

Zinc dissolution in lisbon tap water

Zinc oxidation and passivation in Lisbon tap water was studied using cyclic voltammetry and electrochemical impedance spectroscopy. An anodic peak corresponding to the formation of ZnO or Zn(OH) 2 was characterized with peak current dependence on sweep rate suggesting diffusion controlled dissolution and film formation. According to the obtained evidence by X-ray diffraction, i.r. spectroscopy and d.c. electrochemistry a mechanism that includes the formation of Zn 5(CO 3) 2(OH) 6 and CaCO 3 preceded by the formation of zinc oxides and hydroxides is discussed. Comparison is established with results obtained in bicarbonate solutions with good agreement between d.c. and a.c. data. An apparent activation energy of 18.4 kJ mol −1 was also estimated.

The determination of diffusion coefficients and rate constants from the dependence of the peak separation and peak current on the scan rate of cyclic voltammograms at micro-cylindrical electrodes

The theoretical dependence of the peak separation and the peak current on the radius of micro-cylindrical electrodes has been determined in dimensionless form for the evaluation of the rate constant of heterogeneous charge transfer and the diffusion coefficient. The data were determined from calculated cyclic voltammograms. The calculation is based on the numerical solution of the integral equation for a simple charge transfer and diffusion to a micro-cylindrical electrode. The dependences found are described by hyperbolic functions. Replacing the dimensionless quantities in the approximation function leads to a dependence of the peak separation and the peak current on the scan rate, Δ E p( v) and I p( v), respectively. Both functions include the following parameters: diffusion coefficient, rate constant, electrode radius, number of transferred electrons and temperature. This makes it possible to determine the diffusion coefficient and the rate constant from the experimental peak separation and peak current dependence on the scan rate by non-linear regression. The diffusion coefficient and the rate constant were determined for the reduction of potassium hexacyanoferrate(III) at carbon fibre electrodes in the way described. The parameters determined are compared with published data.

Sodium current block caused by group IIb cations in calf Purkinje fibres and in guinea-pig ventricular myocytes

The action of group IIb cations [Cadmium (Cd2+), Zinc (Zn2+), Mercury (Hg2+)] on the cardiac fast sodium current (INa) was investigated in calf Purkinje fibres and in ventricular cells isolated from guinea-pig hearts. In calf Purkinje fibres, INa was depressed by submillimolar concentrations of Zn2+ and Hg2+. With both cations, the current reduction occurred at all voltages in the range of current activation and the voltage dependence of peak current was unchanged. The degree of peak current inhibition depended on the cation concentration but not on voltage. The position of the inactivation curve on the voltage axis was unaltered at cation concentrations giving substantial current inhibition, and moved to the right only with concentration exceeding 1-1.5 mM. These effects can be interpreted as due to INa channel blockade. The action of Zn2+ and Hg2+ was similar to that described earlier of Cd2+ on Purkinje fibres (DiFrancesco et al. 1985b). INa was also inhibited by group IIb cations in isolated guinea-pig ventricular cells. Depression of INa by Cd2+, Zn2+ and Hg2+ was essentially voltage-independent, in agreement with its being caused by channel block. The dependence of INa block by Cd2+ upon external Na concentration [Na+0] was investigated in ventricular myocytes. The fraction of INa block by 0.1 mM CdCl2 was 0.50 at 140 mM, 0.81 at 70 mM and 0.83 at 35 mM [Na+]0. A similar increase of block efficiency at low [Na+0] was observed with 0.05 mM CdCl2. In both the Purkinje fibre and the ventricular cell, the order of potency of INa block by group IIb cations was Hg2+ greater than Zn2+ greater than Cd2+. Manganese (Mn2+, 2-5 mM), an ion of group VIIa, also depressed the INa in Purkinje fibres and ventricular myocytes. This effect was however due mainly to a positive shift on the voltage dependence of current kinetics rather than to a reduction of the conductance of the channel (GNa), and can be accounted for by an ion-screening action of Mn2+ on the external membrane surface. The block by group IIb cations is a typical property of cardiac Na+ channels and characterizes the cardiac as opposed to other types of Na+ channel.

An experimental study of phase‐sensitive ac voltammetry in continuous‐flow streams—reversible systems

AbstractFundamental harmonic ac voltammetry at flow‐by wall electrodes was found experimentally to be independent of the flow rate for rates from 0 to 2.1 L/min. Measurements were made with phase‐sensitive instrumentation. The ac peak current dependence on the concentration, the frequency and amplitude of the applied ac voltage, and the dc scan rate was the same in flowing or stationary solutions. The shape of the ac voltammogram was also not influenced by the flow rate. These results are general for experimental conditions that produce unrestricted diffusion and that maintain simple, reversible electron‐transfer mechanisms.

Theory of square-wave stripping voltammetry with adsorptive accumulation

A model of the anion-induced adsorptive accumulation combined with a square-wave voltammetric stripping is developed. The dependence of peak currents and peak potentials on the reversibility of redox reactions and on the reaction mechanism is demonstrated. The sensitivity of the method is estimated.

Polarographic reduction of aldehydes and ketones: Part XXX. Effects of acid-base, hydration-dehydration and keto-enol equilibria on reduction of α-ketoglutaric and oxalacetic acid and their esters

Oxalacetic acid is reduced in four waves, corresponding to the reduction of the following species: i 1a, keto form of the acid; i 1b, enol form of the acid; i 2, monoanion; i 3, dianion. Reduction of the acid and monoanion occurs after protonation of the CO group, that of the dianion on the unprotonated CO group. Ethyl oxalacetate is reduced in corresponding waves i 1a, i 1b and i 2, the free ester after protonation of the CO group, the monoanion without. Diethyl oxalate is also reduced in the keto ( i 1a) and enol ( i 1b) form after pre-protonation, but also in the unprotonated form in wave i 2. α-Ketoglutaric acid is reduced in three waves, i 1 (acid), i 2 (monoanion) and i 3 (dianion), the first two with pre-protonation of the CO group, the third without. Diethyl α-ketoglutarate is reduced in the accessible pH-range in a single two-electron wave, where the CO group is reduced after pre-protonation. Carbanion-enolates are not reducible. Using the dependence of peak currents obtained by LSV on ν 1 2 values of equilibrium constants for the hydration-dehydration reaction ( K d) involving the CO group were determined. α-Ketoglutaric acid exists in aqueous solution 65%, its diethyl ester 75%, oxalacetic acid and its ethyl ester 87% and diethyl oxalacetate 93% in the hydrated form. Anions and dianions (with the exception of ethyl oxalacetate) are pratically unhydrated. Hydration and acid-base catalyzed dehydration results in pH-dependence of polarographic reduction currents. The limiting current is governed by the rate of dehydration and from the intensity of the current (in the region where it is pH-dependent) and K d-values it is possible to calculate rate constants ( k d) of the dehydration (Table 8). When compared with dehydration, keto-enol equilibria are slowly established. From the ratios of the currents, i 1b: i 1a, the value of K E = [enol]/[keto] has been obtained for oxalacetic acid (0.12), its monoethyl (0.47) and diethyl ester (0.42). Structural effects on reduction potentials, keto-enol equilibria and hydration are discussed.

Zinc Oxidation and Redeposition Processes in Aqueous Alkali and Carbonate Solutions: II . Distinction Between Dissolution and Oxide Film Formation Processes

The early stages of the process of anodic zinc oxidation in , , and Cl− solutions around pH 11.5, were studied by cyclic voltammetry, as in part I. Unusual peak current dependence on sweep rate is observed at the first anodic peak. This behavior is explained in terms of a mixed process involving coupled diffusion and film formation. By correlating the shapes of peaks and peak potentials for different Zn surface preparations, and the relation of the peak current to sweep rate and to electrode rotation rate, , the roles of film formation and diffusion of solution species are distinguished. A new method of plotting out cyclic voltammetry peak currents, , as a function of sweep rate, , is proposed that treats the anodic currents in terms of parallel components for dissolution into solution, with diffusion (proportional to ), and for film formation or reduction (proportional to ). This allows a separation of diffusion‐controlled and film formation currents to be made. The RDE experiments yield and sweep rate experiments showed relations apply to the for the peak. Two processes therefore take place: diffusion‐controlled dissolution and direct film formation. The importance of studies at single‐crystal surfaces is stressed where resolution of the various processes involved is much clearer. Thus, it is shown that only the direct oxide film formation process is predominant at the single‐crystal Zn(0001) face. This compact film is responsible for passivation. The results enable a model for dissolution and coupled‐film formation, associated with passivation, at the Zn electrode, to be suggested. The participation of these two types of process depends on pH and the types of ions present.

Linear Sweep Voltammetry of Ni(II), Co(II), Cd(II), and Pb(II) at Glassy Carbon Electrodes in Molten Lithium Chloride‐Potassium Chloride Eutectic

The reduction of Ni(II), Co(II), Cd(II), and Pb(II) was studied at glassy carbon electrodes in molten lithium chloride‐potassium chloride eutectic at 450°C using the technique of linear sweep voltammetry. Peak‐shaped voltammograms were obtained at scan rates of . Linear dependence of peak currents on metal ion concentration and the square root of voltage scan rate was obtained for all the cations studied. The diffusion coefficients of Ni(II), Co(II), Cd(II), and Pb(II) were calculated from the experimental data and found to be 1.26, 1.02, 0.98, and , respectively.