Charging Spot Research Articles

Solution structure of toxin 2 from Centruroides noxius Hoffmann, a β-scorpion neurotoxin acting on sodium channels

We have determined the solution structure of Cn2, a β-toxin extracted from the venom of the New World scorpion Centruroides noxius Hoffmann. Cn2 belongs to the family of scorpion toxins that affect the sodium channel activity, and is very toxic to mammals (LD50 = 0.4 μg/20 g mouse mass). The three-dimensional structure was determined using 1H-1H two-dimensional NMR spectroscopy, torsion angle dynamics, and restrained energy minimization. The final set of 15 structures was calculated from 876 experimental distance constraints and 58 angle constraints. The structures have a global r.m.s.d. of 1.38 Å for backbone atoms and 2.21 Å for all heavy atoms. The overall fold is similar to that found in the other scorpion toxins acting on sodium channels. It is made of a triple-stranded antiparallel β-sheet and an α-helix, and is stabilized by four disulfide bridges. A cis-proline residue at position 59 induces a kink of the polypeptide chain in the C-terminal region. The hydrophobic core of the protein is made up of residues L5, V6, L51, A55, and by the eight cysteine residues. A hydrophobic patch is defined by the aromatic residues Y4, Y40, Y42, W47 and by V57 on the side of the β-sheet facing the solvent. A positively charged patch is formed by K8 and K63 on one edge of the molecule in the C-terminal region. Another positively charged spot is represented by the highly exposed K35. The structure of Cn2 is compared with those of other scorpion toxins acting on sodium channels, in particular Aah II and CsE-v3. This is the first structural report of an anti-mammal β-scorpion toxin and it provides the necessary information for the design of recombinant mutants that can be used to probe structure-function relationships in scorpion toxins affecting sodium channel activity.

Determination of the surface potential of a dielectric layer on a target bombarded by an ion beam

The electric field at the surface of a charge spot created by an ion beam on a dielectric coating of a target is calculated. An expression is obtained which relates the surface potential of the insulator to the potential of the collector corresponding to saturation of the collector secondary-electron current. It permits determination of the potential drop across the oxide layer of a cold cathode without introducing complications in the construction of the experimental apparatus.

Dielectric charging by an electron beam

Experimental discovery of a charge spot field effect (CSFE) has altered considerably our understanding of dielectric charging by an electron beam, under conditions typical for the operation of a large class of cathode ray tubes (CRT). Dielectric charging by an electron beam was studied using a specific pulse method for the measurement of the potential. The accuracy of this method is discussed. Measurements were made of the potential relief of a positively charged spot on muscovite mica (quartz, aluminum oxide). The potential at the spot center, under conditions described in the paper and at a relatively long charging time, was shown to be considerably lower than that of the collector. Potential dependence on charging time, determined under the same conditions, is shown for the charged spot center and a number of adjacent points. During creation of the charged spot charging current was measured. The results of the measurements are discussed. A new mechanism of dielectric charging by electron beam is proposed. A CSFE is formulated, and its significance for the operation of CRT is stated. Criticism is given of in-plane grid effect. The paper presents calculations of fields for grid target models determining the mechanism of dielectric charging by electron beam.

Importance of anionic reactive intermediates for lubricant component reactions with friction surfaces

AbstractTo understand the chemical behaviour of lubricant components during boundary lubrication, a general concept of negative ionradical reactive intermediates formation for these components has been proposed. The concept is based upon the ionisation mechanism of these compounds caused by the action of electrons of law energy (1–4 eV). Electrons of such energy (exoelectrons) are spontaneously emitted from most fresh surfaces formed during friction. The principal thesis of the model is that lubricant components form anions which are then chemisorbed on the positively charged areas of rubbing surfaces.The formation of negative ions and their decomposition process are simulated by electron attachment mass spectrography. This type of mass spectrography uses electrons of an energy range similar to those of exoelectrons. The proposed model encompasses the following major stages: (a) low‐energy electron emission process and creation of positively charged spots, (b) action of emitted electrons with lubricant components (anion and radical formation), (c) reactions of anions with metal surfaces forming a film protecting the surface from wear, (d) cracking of chemical bonds producing other radicals. The model explains many lubrication phenomena of hydrocarbons, oxygen‐containing compounds, and many types of chemicals used as antiwear and extreme‐pressure additives.

Nonlinear discharge dynamics in small bore current channels

General properties of current and voltage dynamics for discharges having narrow current channels are studied using a generic transmission-line model with active parameters. The line parameters for the inductance and capacitance per unit length are, except for positivity and some physically motivated monotonicity conditions, left as unspecified functions of local line current and voltage. The resistance per unit length is taken to vary inversely with current, thereby assuring dynamical properties common to transient electrical discharges. Emphasis is placed on determining global properties of current front dynamics, i.e., the speed and direction of motion of the free boundary interfaces between instantaneously active (current-carrying) and passive (current-free) regions of the discharge current channel, and the space-time paths across which current reversal occurs. The results are applied to surface electrical discharges initiated at a charge spot.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stability of colloidal particles with a nonhomogeneous surface potential: Application to colloidal nickel hydroxy carbonate

The effect of a nonhomogeneous surface potential on the stability of a colloidal dispersion is evaluated within the framework of the classical DLVO theory. The surface potential of the particles is assumed to consist of a periodically varying potential superimposed on a constant background. The stability factor for the dispersion is evaluated as a function of potential periodicity and amplitude. Calculations show that dispersions of particles with a nonhomogeneous surface potential are less stable than dispersions of homogeneously charged particles, which can be attributed to the presence of additional attraction forces between positively and negatively charged spots on the surface of the particles. The validity of the approach was tested by stability measurements for colloidal nickel hydroxy carbonate at various electrolyte concentrations. If a nonhomogeneous surface potential is assumed, the aggregation behavior of the system can be described much better than when a homogeneous surface potential is assumed. By comparing experimental and theoretical results, information concerning the details of the potential distribution is obtained.

A theory of weak transient electrical discharges on dielectric surfaces

The nonlinear evolution of transient electrical discharges initiated from a small charge spot on dielectric surfaces is analyzed with a transmission line model. The relation between the resistance per unit length, R̂, and the current, I, is assumed to be given by a local Arc Welder’s Ansatz: R̂‖I‖=E*, where E* is a positive constant. Comparison is made with a similar study of discharges initiated from a large charge spot. While both studies predict conditions under which charge is, or is not, transported to a dielectric edge, significant differences in the two cases are revealed. For example, if charge is not transported to an edge, current reversal is possible if the charge spot is small, but can only be unidirectional if the spot is large.

Theory of electrical discharges initiated from a large charge spot on dielectric surfaces

The nonlinear dynamics of charge transport due to an electric discharge on a dielectric surface is analyzed using a transmission line model. The relation between the resistance per unit length, R̂, and the current, I, is assumed to be given by the local arc-welder’s ansatz, R̂‖I‖=E*, where E* is a positive constant. The model predicts that a discharge initiated in the vicinity of a charge spot can propagate partway down a current channel and abruptly terminate before transporting charge to the dielectric edge. This behavior is similar to leader phenomena observed in lightning and other electrical discharges. We show that the direction of the current along the current channel is constant throughout such a discharge. The minimum voltage at the charge spot that allows charge to be transported to the dielectric edge is determined. This critical voltage Vl depends on the length l of the current channel. We show that the ‘‘average field,’’ Vl/l, decreases as l increases. When the charge spot voltage is less than the critical voltage, we obtain upper and lower bounds for both the arc duration time and amount of charge removed from the charge spot.

One Single Lysine Residue Is Responsible for the Special Interaction between Polyphosphate and the Outer Membrane Porin PhoE of Escherichia coli

Site-directed mutagenesis was performed with the phosphate starvation-inducible outer membrane porin PhoE of Escherichia coli K-12 to study the molecular basis of its anion selectivity. Lysines 18, 29, 64, and 125 were replaced by glutamic acids, and the properties of the mutant porins were investigated in in vivo and in vitro experiments. Lipid bilayer experiments showed that all these mutations had no influence on the pore structure because PhoE and the mutants had the same single channel conductance in KCl solution. Selectivity measurements revealed that the mutations changed the ionic selectivity of PhoE, but the change was dependent on the location of the lysine. Replacement of Lys18 and Lys29 by glutamic acid had a relatively small influence. The effect of the Lys64 substitution was somewhat larger, and the effect of the replacement of Lys125 resulted in the most drastic change in selectivity and in the loss of the interaction of PhoE with polyphosphate, whereas the replacement of the other lysines had no effect on the polyphosphate interaction behavior. The results are consistent with the assumption that the charge spot in PhoE consists of only 1 lysine per monomer, located in position 125 of the primary sequence and probably close to the pore interior.

Wall charge spreading on planar insulators in external electrode discharge devices

Three special cases of ohmic dissipation are considered where wall charges are deposited on a planar insulating barrier of surface resistance eta which separates the external electrodes from the discharge plasma in a typical external electrode discharge device. It is shown that the electrical potential Vs produced inside the discharge vessel by wall charges associated with small electrodes behaves as if it were produced by a point charge which recedes from the discharge at a constant speed 1/2 eta epsilon 0, where epsilon 0 is the vacuum permittivity. The other two cases relate to planar electrodes covered by an insulator of thickness h. For circular wall charge spots with gaussian radial charge density distributions of initial average radius r0, it is shown that the electrical potential developed by small spots (r0<<h) behaves in the same way as Vs: when r0>>h the area of the wall charge spots increases linearly with time at a rate proportional to h/ eta epsilon 0. It is shown that the size dependence of the behaviour of the wall charge spots is due to the interaction of the wall charges with the image charges required to keep the electrode surfaces as equipotentials.

Similarity transformations for decaying wall charge fields in a spherical shell

It is shown that ohmic dissipation of wall charges causes the electric field inside a spherical insulating shell to decay with a characteristic time constant, t0 =2aηε0 , where η is the surface resistance of the shell, a is its radius, and ε0 is the permittivity of a vacuum. It is demonstrated that the electric potential inside the shell at time t can be constructed by means of similarity transformations which are generated by multiplying the initial charge densities by a factor f and lengths by f 2 where f=exp(−t/2t0 ). Using this method the potential near the centers of oppositely charged disks of wall charge of radius b (b≪a) is calculated in terms of analytic expressions for the case where the externally applied field is absent. It is pointed out that the latter solution can be generalized to yield the potential distribution generated by small wall charge spots which have rotationally symmetric charge distributions. It is anticipated that the results will be useful for further studies of external electrode discharges in gases in insulating containers, Examples include low-frequency electric field sensors and gas-filled cavities in solid insulators.

About a negative-ion concept of the antiwear and antiseizure action of hydrocarbons during friction

A new negative-ion concept of the mechanism of lubrication of hydrocarbons has been derived. The thesis of the concept is based on the mechanism of ionization of saturated and aromatic hydrocarbons caused by the action of exoelectrons. The following stages have been considered in the model: (a) the low energy electron (2–3 eV) emission process and creation of positively charged spots; (b) the reaction of the emitted electrons with hydrocarbon molecules adsorbed on the rubbing surface, leading to the formation of negative ions and radicals; (c) the reaction of the negative ions with the metal surface. The proposed model explains the untypical behaviour of these hydrocarbons under different atmospheres in the friction process.

Conversion of surface charge distributions by the electrostatic induction method

The electrostatic induction method of measuring charge microdistributions is very efficient in the study of electrization of dielectric and high-resistance semiconductor materials, charge relaxation in these materials, and flaw detection. The results of experimental application of the principal relations and specific features of the conversion of surface charge distributions into an electrical signal are described in this report. Experiments on reading the charge contour of dielectric layers have been conducted with the aid of a precision lathe. In the electrostatic induction method of charge conversion the reproduction of small (as compared with the measuring electrode diameter) charge spots follows different rules than the reproduction of large charge-contour elements. The theory proves that the signal obtained from a point charge repeats the instrumental function of the probe system. As the surface charge density increases and the distance between the surface an probe decreases, the probability of discharge phenomena taking place in the gap grows.

Glycosylation as a criterion for defining subpopulations of fast-transported proteins.

The role carbohydrate residues may play in the sorting of newly synthesized fast-transported proteins during the initiation of fast axonal transport has been examined by identifying individual fast-transported glycoproteins that contain either or both fucose and galactose. [3H]Fucose or [3H]galactose was incorporated together with [35S]methionine in vitro in bullfrog dorsal root ganglia. Fast-transported proteins that accumulated proximal to a ligature on the spinal nerve were separated via two-dimensional gel electrophoresis, and 92 gel spots were analyzed quantitatively for the presence of 35S and 3H. Of these spots, 56 (61%) contained either or both fucose and galactose. Glycomoieties were generally associated with families of charged spots whose isoelectric points could be altered with neuraminidase treatment. Single spots tended to be unglycosylated and were unaffected by neuraminidase. The prevalence of glycoproteins was considerably greater in the higher-molecular weight range. Of the 55 spots analyzed with molecular weight greater than approximately 35,000 daltons, 89% were glycosylated, whereas only 19% of the 37 spots with lower molecular weight contained sugar moieties. When considered in light of previous studies in which similar subpopulations have been described, the current findings suggest that the presence or absence of glycomoieties may represent another criterion by which proteins are sorted during the initiation of fast axonal transport.

Noise in electrostatic recording

Playback noise in electrostatic recording is due to random electrical charge patterns on and in the dielectric tape record. Surface charges on the tape may be neutralized by passing the tape through an ion bath, which greatly reduces the playback noise level, particularly at low frequencies. The noise of such neutralized tapes may be interpreted in terms of a random array of charged regions in the tape having an average diameter of 0.04 cm and a charge density of 5 esu/cm2. Charge injection during recording changes this charge pattern but results in a similar array of smaller charged spots, 3 × 10-5cm in diameter. This recorded noise is due to the spark-like character of the recording discharge and may be reduced by generating a continuous discharge.

Theory of the Electroviscous Effect in Polymer Solutions

AbstractThe statistical form of a long chain molecule is modified if the macromolecule contains charged spots, e.g. by dissociation of carboxylic groups. The coil formed by the large molecule is less dense as a consequence of the repulsive force between different charged spots. This repulsion and therefore the inflation of the coil will be less pronounced if electrolyte is present which, by the formation of ionic atmospheres around the charged spots, diminishes the interaction.A quantitative theory of the average dimensions of the coil is developed based on the statistics of coil molecules and on the free energy of the electric charges on the coil.Application of the theory to the viscosity of solutions of charged polymers results in a natural explanation of the so‐called electroviscous effect.The limits of applicability of the theory due to certain approximations in the statistical theory and to the use of the Debye‐Hückel approximation in evaluating the free energy of the charges is discussed.