Screening Surface Charges Research Articles

Colloidal Behavior of Cellulose Nanocrystals in Presence of Sodium Chloride

AbstractAggregation and gelation of cellulose nanocrystals (CNCs) induced by sodium chloride (NaCl) were investigated as a function of NaCl and CNC concentrations. Incorporation of NaCl improved CNCs ability to form clusters via screening surface charges of CNCs. Transmission electron microscopy (TEM) images revealed the formation of porous CNC clusters following NaCl addition. The confocal laser scanning microscopy (CLSM) micrographs indicated the presence of regions with colloid‐rich and colloid‐poor patterns in CNC clusters. Fluorescent brightener 28 was found to have a strong hydrogen bonding to the cellulose surface and used as the staining agent in CLSM. The CLSM images also indicated a dynamic structure for gels, continually rearranging over the course of time. Zeta potential data coupled with CLSM images confirmed the impact of NaCl on the gel formation of CNCs.

Label-free quantification of the effects of lithium niobate polarization on cell adhesion via holographic microscopy.

The surface of a c- cut ferroelectric crystal at room temperature is characterized by the so-called screening surface charges, able to compensate the charge due to the spontaneous polarization. Recently, these charges inspired the investigation of the interaction affinity of live cells with lithium niobate and lithium tantalate crystals. However, different knowledge gaps still remain that prevent a reasonable application of these materials for biological applications. Here, a label-free holographic total internal reflection microscopy is shown; the technique is able to evaluate quantitatively the contact area of live fibroblast cells adhering onto the surface of a ferroelectric lithium niobate crystal. The results show values of contact area significantly different between cells adhering onto the positive or negative face of the crystal. This reinforces the reasons for using the polarization charge of these materials to study and/or control cellular processes and, thus, to develop an innovative platform based on polar dielectric functional substrates.

Extracellular Linkers Completely Transplant the Voltage Dependence from Kv1.2 Ion Channels to Kv2.1

The transmembrane voltage needed to open different voltage-gated K (Kv) channels differs by up to 50 mV from each other. In this study we test the hypothesis that the channels’ voltage dependences to a large extent are set by charged amino-acid residues of the extracellular linkers of the Kv channels, which electrostatically affect the charged amino-acid residues of the voltage sensor S4. Extracellular cations shift the conductance-versus-voltage curve, G(V), by interfering with these extracellular charges. We have explored these issues by analyzing the effects of the divalent strontium ion (Sr2+) on the voltage dependence of the G(V) curves of wild-type and chimeric Kv channels expressed in Xenopus oocytes, using the voltage-clamp technique. Out of seven Kv channels, Kv1.2 was found to be most sensitive to Sr2+ (50 mM shifted G(V) by +21.7 mV), and Kv2.1 to be the least sensitive (+7.8 mV). Experiments on 25 chimeras, constructed from Kv1.2 and Kv2.1, showed that the large Sr2+-induced G(V) shift of Kv1.2 can be transferred to Kv2.1 by exchanging the extracellular linker between S3 and S4 (L3/4) in combination with either the extracellular linker between S5 and the pore (L5/P) or that between the pore and S6 (LP/6). The effects of the linker substitutions were nonadditive, suggesting specific structural interactions. The free energy of these interactions was ∼20 kJ/mol, suggesting involvement of hydrophobic interactions and/or hydrogen bonds. Using principles from double-layer theory we derived an approximate linear equation (relating the voltage shifts to altered ionic strength), which proved to well match experimental data, suggesting that Sr2+ acts on these channels mainly by screening surface charges. Taken together, these results highlight the extracellular surface potential at the voltage sensor as an important determinant of the channels’ voltage dependence, making the extracellular linkers essential targets for evolutionary selection.

Dispersing Zwitterions into Comb Polymers for Nonviral Transfection: Experiments and Molecular Simulation.

Polymer-based gene delivery vehicles benefit from the presence of hydrophilic groups that mitigate the inherent toxicity of polycations and that provide tunable polymer-DNA binding strength and stable complexes (polyplexes). However, hydrophilic groups screen charge, and as such can reduce cell uptake and transfection efficiency. We report the effect of embedding zwitterionic sulfobetaine (SB) groups in cationic comb polymers, using a combination of experiments and molecular simulations. Ring-opening metathesis polymerization (ROMP) produced comb polymers with tetralysine (K4) and SB pendent groups. Dynamic light scattering, zeta potential measurements, and fluorescence-based experiments, together with coarse-grained molecular dynamics simulations, described the effect of SB groups on the size, shape, surface charge, composition, and DNA binding strength of polyplexes formed using these comb polymers. Experiments and simulations showed that increasing SB composition in the comb polymers decreased polymer-DNA binding strength, while simulations indicated that the SB groups distributed throughout the polyplex. This allows polyplexes to maintain a positive surface charge and provide high levels of gene expression in live cells. Notably, comb polymers with nearly 50 mol % SB form polyplexes that exhibit positive surface charge similarly as polyplexes formed from purely cationic comb polymers, indicating the ability to introduce an appreciable amount of SB functionality without screening surface charge. This integrated simulation-experimental study demonstrates the effectiveness of incorporating zwitterions in polyplexes, while guiding the design of new and effective gene delivery vectors.

Electric field and humidity effects on adsorbed water behavior on BaTiO3 ferroelectric domains studied by scanning probe microscopy

Distribution of the adsorbed water on BaTiO3 ferroelectric single crystal (001) surface was investigated by means of scanning probe microscopy. Under high relative humidity, above 95%, the presence of water droplets was observed on domain surfaces. The droplets were up to 20 nm high and their morphology changed when electrical field was applied between the single crystal substrates and droplets via scanning probe microscopy. With an electric field applied parallel to the (001) top surface, the droplets on c domains spread out, followed by complete recovery upon switching the electric field off. However, few droplets on a domains tend to shrink with the electrical field application. It is shown that the screening surface charges and induced charges on droplets surface play a dominant role in droplets behavior.

Electric field and surface charge effects on ferroelectric domain dynamics in BaTiO3single crystal

The potential distribution of BaTiO${}_{3}$ single crystal ferroelectric domains was investigated by scanning Kelvin probe microscopy at room temperature with and without an electric field applied parallel to the (001) top surface. Immediate c domain surface potential inversion was observed after reaching the 6 V/mm critical electric field intensity followed by complete recovery upon switching the electric field off. Piezoresponse force microscopy was used to characterize domain structure evolution during the electric field application, which caused c domain motion. Newly formed domain patterns were stable for a month after switching the electric field off. Screening surface charges and their mobility play a dominant role in this experiment.

Electric surface potential and frozen-in field direct measurements in thermally poled silica

We report measurements of the electric surface potential (SP) and its temporal evolution in thermally poled silica samples, thus, providing a direct quantitative evidence of the frozen-in voltage. The SP value is found to be directly related to the voltage across the depletion layer and there is a clear link between frozen-in electric field and nonlinear optical coefficient, inferred from SP and second harmonic generation measurements respectively. Our studies also show the presence of screening surface charge layers. We have calculated the space charge distribution making evident that the thinner the sample, the larger the screening surface charge density.

Modulation of Neuronal Voltage-Activated Calcium and Sodium Channels by Polyamines and pH

The endogenous polyamines spermine, spermidine and putrescine are present at high concentrations inside neurons and can be released into the extracellular space where they have been shown to modulate ion channels. Here, we have examined polyamine modulation of voltage-activated Ca2+ channels (VACCs) and voltage-activated Na+ channels (VANCs) in rat superior cervical ganglion neurons using whole-cell voltage-clamp at physiological divalent concentrations. Polyamines inhibited VACCs in a concentration-dependent manner with IC50s for spermine, spermidine, and putrescine of 4.7 ± 0.7, 11.2 ± 1.4, and 90 ± 36 mM, respectively. Polyamines caused inhibition by shifting the VACC half-activation voltage (V0.5) to depolarized potentials and by reducing total VACC permeability. The shift was described by Gouy-Chapman-Stern theory with a surface charge density of 0.120 ± 0.005 e- nm-2 and a surface potential of -19 mV. Attenuation of spermidine and spermine inhibition of VACC at decreased pH was explained by H+ titration of surface charge. Polyamine-mediated effects also decreased at elevated pH due to the inhibitors having lower valence and being less effective at screening surface charge. Polyamines affected VANC currents indirectly by reducing TTX inhibition of VANCs at high pH. This may reflect surface charge induced decreases in the local TTX concentration or polyamine-TTX interactions. In conclusion, polyamines inhibit neuronal VACCs via complex interactions with extracellular H+ and Ca. Many of the observed effects can be explained by a model incorporating polyamine binding, H+ binding and surface charge screening.

The kinetics of gap junction currents are sensitive to the ionic composition of the pipette solution.

Myocytes were isolated from neonatal rat hearts using an enzymatic procedure. Cell pairs were used to control the junctional voltage, V(j), and to measure the transjunctional current, I(j), using the dual voltage-clamp method. V(j) gradients provoked I(j) signals with voltage-dependent inactivation. During voltage pulses, I(j) remained virtually constant at ¿V(j)¿ <40 mV. At ¿V(j)¿>40 mV, it inactivated with time to a residual level. The inactivation followed a single exponential. The time constant of I(j) inactivation, taui, and the size of I(j) at steady state, I(j,ss), were both sensitive to the ions in the pipette solution. I(j,ss) was smaller in the presence of tetraethylammonium aspartate (TEA+ aspartate-) than KC1, while taui was smaller in the presence of KC1 than TEA+ aspartate-. The modification of I(j,ss) is readily explained by a change in the residual conductance of the gap junction channels, gammaj,residual x The alterations in taui are correlated with a change in beta, the rate constant that describes the transition of the channel from the main state to the residual state. Pipette solutions may affect the kinetics of gap junction currents by altering the conductive and/or kinetic parameters. Computer simulations revealed a substantial influence of the latter, but only a marginal effect of the former. Conceivably, ions of the pipette solution may affect the kinetics of gap junction channels by screening surface charges of the channel wall.

Structural features of a multisubstate cardiac mitoplast anion channel: inferences from single channel recording.

Ion channels from sheep cardiac mitoplast (inverted inner mitochondrial membrane vesicle) preparations were incorporated into voltage-clamped planar lipid bilayers. A low-conductance anion channel (approximately 40 or approximately 85 pS in symmetric 300 or 550 mM choline Cl, respectively), characterized by the presence of two well-defined substates, at approximately 25 and approximately 50% of the fully open level, was studied in detail. The substate behavior was consistent with a multibarelled channel containing four functionally coupled pores. At negative (cis-trans) membrane potentials, the putative protomers appeared to gate with substantial positive cooperativity, accounting for the apparent absence of a approximately 75% sublevel. At positive holding potentials, allosteric promoter interactions were more complicated, and the channel complex could be modeled as a dimer of dimers. The protochannels in one dimer ("dimer A") appeared to open independently of each other, and with a relatively high probability, while the monomers comprising the second dimer ("dimer B") were functionally coupled, could only open if both protomers in dimer A were open, and closed as soon as one of the monomers in dimer A shut. The channels also displayed Ca(2+)-(and Mg(2+)-) sensitive rectification related to bilayer lipid surface charge. By assuming that Ca2+ acted solely by screening surface charge, the membrane surface potential profile was used as a "microscopic ruler" to place one month of the channel within 10-11 A of the bilayer surface.

Possible Electrical Double‐Layer Contribution to the Equilibrium Thickness of Intergranular Glass Films in Polycrystalline Ceramics

The plausibility of the entropic repulsion of electrical double layers acting to stabilize an equilibrium thickness of intergranular glass films in polycrystalline ceramics is explored. Estimates of the screening length, surface potential, and surface charge required to provide a repulsive force sufficiently large to balance the attractive van der Waals and capillary forces for observable thicknesses of intergranular film are calculated and do not appear to be beyond possibility. However, it has yet to be established whether crystalline particles in a liquid‐phase sintering medium possess an electrical double layer at high temperatures. If they do, such a surface charge layer may well have important consequences not only for liquid‐phase sintering but also for high‐frequency electrical properties and microwave sintering of ceramics containing a liquid phase.

Modulation of voltage-dependent sodium and potassium currents by charged amphiphiles in cardiac ventricular myocytes. Effects via modification of surface potential.

Modulation of voltage-dependent sodium and potassium currents by charged amphiphiles was investigated in cardiac ventricular myocytes using the patch-clamp technique. Negatively charged sodium dodecylsulfate (SDS) increased amplitude of INa, whereas positively charged dodecyltrimethylammonium (DDTMA) decreased INa. Furthermore, SDS shifted the steady-state activation and inactivation of INa in the negative direction, whereas DDTMA shifted the curves in the opposite direction. These shifts provided an explanation for the changes in current amplitude. Activation and inactivation kinetics of INa were accelerated by SDS but slowed by DDTMA. These changes in both steady-state gating and kinetics of INa are consistent with a decrease of the intramembrane field by SDS and an increase of the field by DDTMA due to an alteration of surface potential after their insertion into the outer monolayer of the sarcolemma. The effect of SDS on the steady-state inactivation of INa was concentration dependent and partially reversed by screening surface charges with increased extracellular [Ca2+]. These amphiphiles also altered the activation of the delayed rectifier K+ current (IK,del), producing a shift in the negative direction by SDS but in the positive direction by DDTMA. These results suggest that the insertion of charged amphiphiles into the cell membrane alters the behavior of voltage-dependent INa and IK,del by changing the surface charge density, and consequently the surface potential and implies, although indirectly, that the lipid surface charges are important to the voltage-dependent gating of these channels.

Protonation of histidine groups inhibits gating of the of the quisqualate/kainate channel protein in isolated catfish cone horizontal cells

Increases in the extracellular hydrogen ion concentration ([H +] o) but not the intracellular concentration ([H +] i) antagonized the inward going membrane currents recorded from isolated cone horizontal cells during application of quisqualate, α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid, and kainate. The pK determined from a titration curve was 6.5 with a slope greater than 1, indicating protonation of several histidines. The reduction in membrane current was voltage-independent. The affinity of the agonist for the receptor, the single-channel conductance, and the open time were unaffected by [H +] o· [H +] o antagonism was not the result of charge neutralization such as screening surface charge. Diethylpyrocarbonate, a histidine-modifying reagent, reduced the agonist-induced current, but disulfide- and sulfhydryl-modifying reagents were ineffective. These results suggest that histidine groups on the external face of the channel protein provide a functional site regulating channel gating.

Possible involvement of surface charges in modifying the adrenomedullary secretion induced by high K depolarization

The perfused adrenal gland was challenged with excess K and the role of Ca or Na ions in the medium on this secretory response was investigated. The results suggest that univalent and divalent cations increase the electrical gradient within the membrane by screening surface charges, thereby modifying secretion in response to depolarization.