Threshold ionic site concentrations required for Nernstian potentiometric responses of neutral ionophore-incorporated ion-selective liquid membranes.
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An equation that can describe the concentrations of ionic sites required for a Nernstian potentiometric response slope of neutral ionophore-incorporated ion-selective liquid membranes is presented. This equation is derived from a model based on electrical diffuse layers on both the membrane and the aqueous sides of the interface, in which the phase boundary potential is correlated to the surface charge density as well as the salt concentrations in the bulk membrane and aqueous solution. To experimentally and accurately confirm the validity of this equation, response characteristics of field effect transistors covered by neutral ionophore-based liquid membranes with varying concentrations of a derivative of tetraphenylborate as an anionic site but free of ionic impurities were examined. The observed membrane potentials and the response slopes for membranes with various concentrations of anionic sites were in good agreement with the values calculated from the theory presented in this paper with the measured complexation stability constants for the relevant systems. This result indicates that the theoretical prediction based on the proposed equation for the anionic site concentration is accessible for the preparation of neutral ionophore-incorporated ion-selective liquid membranes, which show Nernstian response slopes for the primary ions. (+info)
Brownian dynamics study of an open-state KcsA potassium channel.
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Three-dimensional Brownian dynamics simulations are used to study conductance of the KcsA potassium channel using the known crystallographic structure. Employing an open-state channel created by molecular dynamics simulations, current-voltage and current-concentration curves broadly consistent with experimental measurements are obtained. In the absence of an applied potential, the channel houses three potassium ions at positions that are in close agreement with X-ray diffraction maps. (+info)
Influence of electrochemical properties in determining the sensitivity of [4Fe-4S] clusters in proteins to oxidative damage.
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Interconversion between [4Fe-4S] cubane and [3Fe-4S] cuboidal states represents one of the simplest structural changes an iron-sulphur cluster can undertake. This reaction is implicated in oxidative damage and in modulation of the activity and regulation of certain enzymes, and it is therefore important to understand the factors governing cluster stability and the processes that activate cluster conversion. In the present study, protein film voltammetry has been used to induce and monitor the oxidative conversion of [4Fe-4S] into [3Fe-4S] clusters in different variants of Azotobacter vinelandii ferredoxin I (AvFdI; the 8Fe form of the native protein), and DeltaThr(14)/DeltaAsp(15), Thr(14)-->Cys (T14C) and C42D mutants. The electrochemical results have been correlated with the differing oxygen sensitivities of [4Fe-4S] clusters, and comparisons have been drawn with other ferredoxins (Desulfovibrio africanus FdIII, Clostridium pasteurianum Fd, Thauera aromatica Fd and Pyrococcus furiosus Fd). In contrast with high-potential iron-sulphur proteins (HiPIPs) for which the oxidized species [4Fe-4S](3+) is inert to degradation and can be isolated, the hypervalent state in these ferredoxins (most obviously the 3+ level) is very labile, and the reduction potential at which this is formed is a key factor in determining the cluster's resistance to oxidative damage. (+info)
Comparisons of pKa and log P values of some carboxylic and phosphonic acids: synthesis and measurement.
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The changes in the physiochemical properties accompanying the substitution of a phosphonic acid group for a carboxylic acid group on various heterocyclic platforms was determined. A series of low molecular weight heterocyclic carboxylic and phosphonic acids was prepared, and the acid dissociation content (pKa) and log P values were measured potentiometrically. These values were compared to those of substituted benzene phosphonic acids, carboxylic acids, sulfonamides, and tetrazoles. The carboxylic acids included 3 pyrazoles, an imidazole, a triazole, 2 pyrimidines, and 6 aryl compounds. The phosphonic acids included a triazole, 2 pyrazoles, 4 pyrimidines, a thiophene, and 6 aryl compounds. Most of the compounds synthesized had adequate water solubility, although a simple methyl substituent in 2 series had a great effect, completely changing the properties. Log P values for the synthesized carboxylic and phosphonic acid compounds were below 2, and pK1 values for the heterocyclic phosphonic acids were generally 2 to 3 log units lower than for the heterocyclic carboxylic acids. (+info)
Axon voltage-clamp simulations. I. Methods and tests.
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This is the first in a series of four papers in which we present the numerical simulation of the application of the voltage clamp technique to excitable cells. In this paper we describe the application of the Crank-Nicolson (1947) method for the solution of the parabolic partial differential equations that describe a cylindrical cell in which the ionic conductances are functions of voltage and time (Hodgkin and Huxley, 1952). This method is compared with other methods in terms of accuracy and speed of solution for a propagated action potential. In addition, differential equations representing a simple voltage-clamp electronic circuit are presented. Using the voltage clamp circuit equations, we simulate the voltage clamp of a single isopotential membrane patch and show how the parameters of the circuit affect the transient response of the patch to a step change in the control potential. The stimulation methods presented in this series of papers allow the evaluation of voltage clamp control of an excitable cell or a syncytium of excitable cells. To the extent that membrane parameters and geometrical factors can be determined, the methods presented here provide solutions for the voltage profile as a function of time. (+info)
Axon voltage-clamp simulations. II. Double sucrose-gap method.
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This is the second in a series of four papers on the simulation of the voltage clamp of cylindrical excitable cells. In this paper we evaluate the double sucrose-gap voltage-clamp technique for the squid and lobster giant axons. Using the Crank-Nicolson method of solution of the cable equations and differential equations representing the voltage clamp circuit we studied the effect of length of the sucrose gap "node" on the voltage profile along an excitable cell during a simulated voltage clamp. The voltage gradients along the region of the cell within the node produce "notches" in the current recording as well as changes in the magnitude of the sodium and potassium current for a given voltage step. Our results show that good voltage clamp control requires node lengths less than one-half the axon diameter. (+info)
Axon voltage-clamp simulations. A multicellular preparation.
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In this paper we extend the simulation of the voltage clamp of a single nerve fiber to a bundle of axons. These simulations included not only the description of the voltage clamp circuit and a single unidimensional cable to represent the preparation in the "node" region of a double sucrose gap used previously but also a series resistance and a shunt pathway. The output of the voltage control amplifier is applied across the membrane plus the series resistance, producing a voltage drop across the series resistance due to the current generated by the membrane in response to a depolarizing voltage step. Since the membrane current has an inward and an outward phase, voltage drops of opposite sign are produced across the series resistance. During the transient current and at all points along an axon, the potential deviation produced by the series resistance is opposite to the deviation produced by the longitudinal gradient. Only at a command potential equal to the sodium equilibrium potential, the membrane potential transiently matches the command potential. For the attempted voltage clamp of an axon, values of series resistance larger than 50 omega-cm2 allowed propagated action potentials in the membrane. In spite of the presence of propagated action potentials at the calbe membrane, the recorded current does not show "notches" and it has a phase of inward current and a phase of outward current. It is concluded that, in a multicellular preparation with series resistance, the recording of a square voltage pulse does not indicate voltage control of the transmembrane potential. The presence of a shunt pathway produces inaccurate values of current density. Neither series or shunt resistance produce "notches" in the current records. (+info)
The study on a PVC membrane electrode for gemfibrozil.
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In this paper, a poly(vinyl chloride) (PVC) membrane electrode is prepared for gemfibrozil, 2, 2-dimethyl-5-(2,5-xylyloxy) valeric acid, based on its ion pair complexes with hexadecyltrioctyl ammonium iodide (HTOA). The membrane composition of the electrode was optimized by using the sequential level elimination method for orthogonal experimental design. The electrode has a Nernstian response range from 2.5 x 10(-5) to 0.1 mol/l with an average slope of 55.3 mV/decade. The limit of detection is 7.1 x 10(-6) mol/l. The electrode responses were not affected by pH in the range 10.0-12.3. A Na2B4O7-Na2CO3 buffer of pH = 11.0 was selected as the background electrolyte solution for potentiometric measurements. The electrode was used for determining gemfibrozil in pharmaceutical preparations with satisfactory results. (+info)