The transverse stability of strongly nonlinear ion-acoustic structures, described by the general Sagdeev potential, is investigated. It is shown, that the multi-scale stability analysis by [Sov. Phys. Dokl. 15 (1970) 539] for solitary waves, described by

An investigation of the ion density produced inside the Chi Kung reactor at 250 W and 0.30 mtorr was performed through Langmuir probe measurements. Three-dimensional interpolation combined these data to produce a 3-D density volume. The resulting image wa

The effects of cobalt ion on the isolated heart of the guinea-pig were studied. The addition of cobalt to the incubation medium ceases the contractions of the isolated heart. The effect of cobalt ion could be prevented by the addition to the incubation medium of calcium ion in equimolar concentrations. Cobalt ion prevented the effects of 1-epinephrine and 1-norepinephrine and these effects of the cation, were antagonized by the presence of calcium ion in amounts not sufficient to antagonize completely the effects of cobalt ion. The mechanism of action of cobalt ions on the isolated heart was discussed. It was proposed that cobalt ion prevents the utilization of calcium ion in the physiological process supporting the mechanical activity of the isolated heart.; O artigo apresenta resumo em inglês.

Continuum theories of electrolytes are widely used to describe physical processes in various biological systems. Although these are well-established theories in macroscopic situations, it is not clear from the outset that they should work in small systems whose dimensions are comparable to or smaller than the Debye length. Here, we test the validity of the mean-field approximation in Poisson-Boltzmann theory by comparing its predictions with those of Brownian dynamics simulations. For this purpose we use spherical and cylindrical boundaries and a catenary shape similar to that of the acetylcholine receptor channel. The interior region filled with electrolyte is assumed to have a high dielectric constant, and the exterior region representing protein a low one. Comparisons of the force on a test ion obtained with the two methods show that the shielding effect due to counterions is overestimated in Poisson-Boltzmann theory when the ion is within a Debye length of the boundary. As the ion gets closer to the boundary, the discrepancy in force grows rapidly. The implication for membrane channels, whose radii are typically smaller than the Debye length, is that Poisson-Boltzmann theory cannot be used to obtain reliable estimates of the electrostatic potential energy and force on an ion in the channel environment.

We have used deep level transient spectroscopy and capacitance-voltage measurements to study the influence of low-energy hydrogen ion implantation on the creation of defects in n-Si. In particular, we have studied the ion fluence dependence of the free carrier compensation at room temperature, and we have measured the generation of VO-H complex and VP-pair in ion implanted samples. The 7.5 keV H ions created defects in the top 0.3 μm of samples, which resulted in carrier compensation to depths exceeding 1 μm. This effect is not due to defects created by ion channeling but is rather due to the migration of defects as demonstrated using binary collision code MARLOWE.

Ion channels are gated, i.e. they can switch conformation between a closed and an open state. Molecular dynamics simulations may be used to study the conformational dynamics of ion channels and of simple channel models. Simulations on model nanopores reveal that a narrow (<4 Å) hydrophobic region can form a functionally closed gate in the channel and can be opened by either a small (∼1 Å) increase in pore radius or an increase in polarity. Modelling and simulation studies confirm the importance of hydrophobic gating in K channels, and support a model in which hinge-bending of the pore-lining M2 (or S6 in Kv channels) helices underlies channel gating. Simulations of a simple outer membrane protein, OmpA, indicate that a gate may also be formed by interactions of charged side chains within a pore, as is also the case in ClC channels.

The ion mass effect on the dominating vacancy related deep levels in Cz-Si implanted with 4-MeV C ions and 6-MeV Si ions has been investigated by deep level transient spectroscopy (DLTS). It is found that the intensity of the DLTS signal for the doubly negative charge state of the divacancy [V2(=/-)] deviates from a one-to-one correlation with that of the singly negative charge state of the divacancy [V2(-/0)] and decreases, compared to V2(-/0), with increasing ion mass. Capture kinetics studies reveal that the electron-capture rate for V2(=/-) decreases with increasing ion mass, while that for V2(-/0) has a weaker dependence on ion mass. In this work, we suggest a model to explain most of the known experimental observations of the ion mass effect for V2(=/-). The model assumes a local compensation of the carrier concentration in highly disordered regions located within the collision cascades. In addition, it has been observed that the DLTS signal of the vacancy-oxygen pair exhibits a trend similar to that for V2(=/-) regarding the ion mass effect.

Detection of DNA by an ion channel switch biosensor has been demonstrated in a model system, using single-stranded oligonucleotide sequences of 52-84 bases in length. Two different biotinylated probes are bound, via streptavidin, either to the outer region of a gramicidin ion channel dimer or to an immobilized membrane component. The ion channels are switched off upon detection of DNA containing complementary epitopes to these probes, separated by a nonbinding region, at nanomolar levels. The DNA cross- links the ion channel to the immobilized species, preventing ions passing through the channel. Addition of DNase I after the target DNA has been added switches the ion channels on. The DNA response is dependent on the rate of hybridization of the individual probes to their complementary epitopes, as shown by using a single probe against DNA containing a repeat of the complementary epitope. These results were correlated with hybridization rates determined using surface plasmon resonance (BIAcore 2000), and with free energies of dimer formation for the probes. (C) 2000 Academic Press.

This brief review focuses on defect and microstructural issues of importance for applications of ion irradiation of semiconductors. Ion irradiation of semiconductors can lead to a variety of disordering behaviour that depends not only on the semiconductor material but also on implantation parameters such as ion fluence, flux, energy and mass as well as the implantation temperature. In some cases, such as silicon implanted at or below room temperature, ion disorder leads readily to amorphization, whereas some other semiconductors, such as aluminium arsenide and gallium nitride, undergo very efficient dynamics annealing even at liquid nitrogen implantation temperatures and are difficult or even impossible to amorphize. In cases of efficient dynamic annealing a rich array of defects can result during implantation. This presentation gives examples of the defect structures that can arise during implantation and subsequent annealing of semiconductors and illustrates how disorder can be exploited to advantage in both electronic and optoelectronic device applications. In particular, open volume defects such as voids and cavities can be generated in silicon and these can be used to selectively remove metal impurities. In compound semiconductors such as zinc oxide...

We demonstrated previously that the two continuum theories widely used in modeling biological ion channels give unreliable results when the radius of the conduit Is less than two Debye lengths. The reason for this failure is the neglect of surface charges on the protein wall induced by permeating ions. Here we attempt to improve the accuracy of the Poisson-Boltzmann and Polsson-Nernst-Planck theories, when applied to channel-like environments, by including a specific dielectric self-energy term to overcome spurious shielding effects inherent in these theories. By comparing results with Brownian dynamics simulations, we show that the inclusion of an additional term in the equations yields significant qualitative improvements. The modified theories perform well in very wide and very narrow channels, but are less successful at intermediate sizes. The situation is worse In multi-ion channels because of the inability of the continuum theories to handle the ion-to-ion interactions correctly. Thus, further work is required if these continuum theories are to be reliably salvaged for quantitative studies of biological ion channels in all situations.

The influence of ion-neutral collisions on the ion and neutral distribution functions in low field radio frequency (rf) heated argon discharges was investigated. In order to obtain some insight into the ion heating mechanism and to resolve apparent inconsistencies between probe and spectroscopic measurements of the ion temperature, the measurements were initially undertaken. It was found that the measurements of the ion distribution function reveal a substantially elevated fraction of low energy particles, associated with charge exchange and ionization. The ion and neutral distribution functions were modeled using the Boltzmann equation including, collision operators for ion-ion and ion-neutral collisions, ionization, heating and particle loss.

We consider the origin of the ion specificity found for the physical properties of micelles. Ions in solution have a polarizability different from that of the surrounding water. The excess polarizability is different for different ions and gives rise to ion specific dispersion forces toward, or away from, interfaces. We show that ionic dispersion forces have important influences on self-consistent potentials, ion distributions, and surface adsorption excess per headgroup on micelles.

We test the validity of the mean-field approximation in Poisson-Nernst- Planck theory by contrasting its predictions with those of Brownian dynamics simulations in schematic cylindrical channels and in a realistic potassium channel. Equivalence of the two theories in bulk situations is demonstrated in a control study. In simple cylindrical channels, considerable differences are found between the two theories with regard to the concentration profiles in the channel and its conductance properties. These differences are at a maximum in narrow channels with a radius smaller than the Debye length and diminish with increasing radius. Convergence occurs when the channel radius is over 2 Debye lengths. These tests unequivocally demonstrate that the mean- field approximation in the Poisson-Nernst-Planck theory breaks down in narrow ion channels that have radii smaller than the Debye length.

Surface tension of salt solutions and double layer forces between charged interfaces show marked specific ion effects not accommodated by the classical double layer theory for charged interfaces. We will demonstrate how this can be partly remedied when the ionic dispersion potential that acts between ion and interface is included on the same level as the electrostatic potential. We will focus on the important influence of these dispersion forces on ionic distributions near air-water and mica-water interfaces. Dispersion potentials that accommodate the experimental surface tension of KBr also gives the physical reason why Pashley et al. had to postulate a 90% binding of bromide ions to the charged mica surfaces to explain their measured force.

Molecular dynamics simulations are carried out to obtain estimates of diffusion coefficients of biologically important Na+, K+, Ca2+ and Cl- ions in hydrophobic cylindrical channels with varying radii and large reservoirs. Calculations for the cylindrical channels are compared to those for the KcsA potassium channel, for which the protein structure has recently been determined from X-ray diffraction experiments. Our results show that ion diffusion is maintained at reasonably high levels even within narrow channels, and does not support the very small diffusion coefficients used in some continuum models in order to fit experimental data. The present estimates of ion diffusion coefficients are useful in the calculation of channel conductance using the Poisson-Nernst-Planck theory, or Brownian dynamics.

Brownian dynamics (BD) simulations provide a practical method for the calculation of ion channel conductance from a given structure. There has been much debate about the implementation of reservoir boundaries in BD simulations in recent years, with claims that the use of improper boundaries could have large effects on the calculated conductance values. Here we compare the simple stochastic boundary that we have been using in our BD simulations with the recently proposed grand canonical Monte Carlo method. We also compare different methods of creating transmembrane potentials. Our results confirm that the treatment of the reservoir boundaries is mostly irrelevant to the conductance properties of an ion channel as long as the reservoirs are large enough.

A 63 residue peptide, p7, encoded by hepatitis C virus was synthesised and tested for ion channel activity in lipid bilayer membranes. Ion channels formed by p7 had a variable conductance: some channels had conductances as low as 14 pS. The reversal potential of currents flowing through the channels formed by p7 showed that they were permeable to potassium and sodium ions and less permeable to calcium ions. Addition of Ca2+ to solutions made channels formed by p7 less potassium- or sodium-selective. Hexamethylene amiloride, a drug previously shown to block ion channels formed by Vpu encoded by HIV-1, blocked channels formed by p7. In view of the increasing number of peptides encoded by viruses that have been shown to form ion channels, it is suggested that ion channels may play an important role in the life cycle of many viruses and that drugs that block these channels may prove to be useful antiviral agents.

The protein calmodulin (apoCaM) undergoes a conformational change when it binds calcium. This structure of the protein (Ca4CaM) is a dumbbell-shaped molecule that undergoes a further profound conformational change on binding of the antipsychotic drug trifluoperazine (TFP). Experimental conditions were developed to prepare samples of apoCaM, Ca4CaM and Ca4CaM/TFP that were substantially free of sodium. The effects of the conformational changes of calmodulin on the charge-state distributions observed in positive ion and negative ion electrospray ionization (ESI) mass spectra were examined. Conversion of apoCaM into Ca4CaM was concomitant with a change in the negative ion ESI mass spectrum whereby the 16- ion was the most abundant ion observed for the apo form and the 8- ion was the most abundant for the complex. In contrast, in the positive ion ESI mass spectra of apoCaM and Ca4CaM, the most abundant species in each case was the 8+ ion. When a complex of Ca4CaM with TFP was prepared, the most abundant species was the 5+ ion. This is consistent with a conformational change of Ca4CaM that rendered some basic sites inaccessible to ionization in the ESI process. Using the same Ca4CaM/TFP mixture, no complex with TFP was observed in negative ion ESI mass spectra. These observations are discussed in the context of the structural changes that are known to occur in calmodulin...

Photoluminescence from Si implanted silica is studied as a function of ion dose and annealing ambient. The photoluminescence emission intensity from samples as-implanted and after annealing at 1000°C in Ar is shown to exhibit a distinct maximum as a func

Ion channels are formed by specific proteins embedded in the cell membrane and provide pathways for fast and controlled flow of selected ions down their electrochemical gradient. This activity generates action potentials in nerves, muscles and other excitable cells, and forms the basis of all movement, sensation and thought processes in living beings. While the functional properties of ion channels are well known from physiological studies, lack of structural knowledge has hindered development of realistic theoretical models necessary for understanding and interpretation of these properties. Recent determination of the molecular structures of potassium and mechanosensitive channels from x-ray crystallography has finally broken this impasse, heralding a new age in ion channel studies where study of structure-function relationships takes a central stage. In this paper, we present a critical review of various approaches to modelling of ion transport in membrane channels, including continuum theories, Brownian dynamics, and classical and ab initio molecular dynamics. Strengths and weaknesses of each approach are discussed and illustrated with applications to some specific ion channels.