Dielectric Breakdown of Cell Membranes

With human and bovine red blood cells and Escherichia coli B, dielectric breakdown of cell membranes could be demonstrated using a Coulter Counter (AEG-Telefunken, Ulm, West Germany) with a hydrodynamic focusing orifice. In making measurements of the size distributions of red blood cells and bacteria versus increasing electric field strength and plotting the pulse heights versus the electric field strength, a sharp bend in the otherwise linear curve is observed due to the dielectric breakdown of the membranes. Solution of Laplace's equation for the electric field generated yields a value of about 1.6 V for the membrane potential at which dielectric breakdown occurs with modal volumes of red blood cells and bacteria. The same value is also calculated for red blood cells by applying the capacitor spring model of Crowley (1973. Biophys. J. 13:711). The corresponding electric field strength generated in the membrane at breakdown is of the order of 4 · 10⁶ V/cm and, therefore, comparable with the breakdown voltages for bilayers of most oils. The critical detector voltage for breakdown depends on the volume of the cells. The volume-dependence predicted by Laplace theory with the assumption that the potential generated across the membrane is independent of volume, could be verified experimentally. Due to dielectric breakdown the red blood cells lose hemoglobin completely. This phenomenon was used to study dielectric breakdown of red blood cells in a homogeneous electric field between two flat platinum electrodes. The electric field was applied by discharging a high voltage storage capacitor via a spark gap. The calculated value of the membrane potential generated to produce dielectric breakdown in the homogeneous field is of the same order as found by means of the Coulter Counter. This indicates that mechanical rupture of the red blood cells by the hydrodynamic forces in the orifice of the Coulter Counter could also be excluded as a hemolysing mechanism. The detector voltage (or the electric field strength in the orifice) depends on the membrane composition (or the intrinsic membrane potential) as revealed by measuring the critical voltage in E. coli B harvested from the logarithmic and stationary growth phases. The critical detector voltage increased by about 30% for a given volume on reaching the stationary growth phase.     

Molecular dynamics simulation of liquid water under the influence of an external electric field

Molecular dynamics simulations of liquid water were performed at 258K and a density of 1.0?g/cm³ under various applied external electric field, ranging 0～10¹⁰?V/m. The influence of external field on structural and dynamical properties of water was investigated. The simple point charge (SPC) model is used for water molecules. An enhancement of the water hydrogen bond structure with increasing strength of the electric field has been deduced from the radial distribution functions and the analysis of hydrogen bonds structure. With increasing field strength, water system has a more perfect structure, which is similar to ice structure. However, the electrofreezing phenomenon of liquid water has not been detected since the self-diffusion coefficient was very large. The self-diffusion coefficient decreases remarkably with increasing strength of electric field and the self-diffusion coefficient is anisotropic.     

Cooperative equilibrium transitions coupled with a slow annealing step explain the sharpness and hysteresis of collagen folding.

Heat and guanidinium-induced denaturation curves of collagen III and its fragments were fitted by theoretical models to explain the extreme sharpness and the hysteresis between unfolding and refolding. It was shown that a recently proposed kinetic model for collagen denaturation does not account for the observed steepness, with physically reasonable values of activation energy and frequency factors in the Arrhenius equation. The extreme slope, which amounts to 0.38 per centigrade for collagen III at the midpoint of its transition, can only be explained by a highly cooperative equilibrium model. The refolding curve is shifted to lower temperatures by 6 degrees C for collagen III and reversible unfolding matching the initial profile of the native protein is observed only after long-time annealing. A simple formalism is proposed by which experimental denaturation and refolding curves are quantitatively described. The transition proceeds via many cooperative steps with slightly different equilibrium constants for unfolding and refolding. Hysteresis and annealing are caused by very slow steps, which are probably connected with a rearrangement of misfolded regions. These slow steps disappear with decreasing size of collagen fragments and hysteresis is not found for collagen model peptides.     

Inverted Current–Voltage Hysteresis in Mixed Perovskite Solar Cells: Polarization,Energy Barriers,and Defect Recombination

Organic‐inorganic metal halide perovskite solar cells show hysteresis in their current–voltage curve measured at a certain voltage sweep rate. Coinciding with a slow transient current response, the hysteresis is attributed to a slow voltage‐driven (ionic) charge redistribution in the perovskite solar cell. Thus, the electric field profile and in turn the electron/hole collection efficiency become dependent on the biasing history. Commonly, a positive prebias is beneficial for a high power‐conversion efficiency. Fill factor and open‐circuit voltage increase because the prebias removes the driving force for charge to pile‐up at the electrodes, which screen the electric field. Here, it is shown that the piled‐up charge can also be beneficial. It increases the probability for electron extraction in case of extraction barriers due to an enhanced electric field allowing for tunneling or dipole formation at the perovskite/electrode interface. In that case, an inverted hysteresis is observed, resulting in higher performance metrics for a voltage sweep starting at low prebias. This inverted hysteresis is particularly pronounced in mixed‐cation mixed‐halide systems which comprise a new generation of perovskite solar cells that makes it possible to reach power‐conversion efficiencies beyond 20%.     

紫膜碎片的电二色性研究

悬浮在水中的嗜盐菌紫膜碎片,在外电场作用下产主定向排列.在20℃时,568nm的电二色性研究表明:外加电场为2kV/m时取向程度可达60%以上;大于5.5kV/m时,取向作用趋于饱和状态;饱和时简约电二色性为-0.437左右,视黄醛生色团的跃迁矩方向与电偶极矩方向形成60.9°夹角;紫膜的永久偶极短为9.2×10~(-24)C、M,剩余电极化率为3.0×10~(-27)m~2;紫膜的旋转扩散常数为0.53秒~(-1).曲线拟合分析表明,感应偶极对紫膜碎片的定向的贡献应予考虑.本文对紫膜碎片的定向机理进行了讨论.     

Enzyme reactions at the surface of living cells. I. Electric repulsion of charged ligands and recognition of signals from the external milieu

The dynamic behaviour of a polyelectrolyte-bound enzyme is studied when diffusion of substrate or diffusion of product is coupled to electric repulsion and to Michaelis-Menten enzyme reaction. The definition of the classical concepts of electric partition coefficients and Donnan potential of a polyelectrolyte membrane has been extended under global non-equilibrium conditions. This extension is permissible when a strong repulsion exists of substrate and product by the fixed negative charges of the membrane. Coupling between product diffusion, electric repulsion and enzyme reaction at constant advancement may result in a hysteresis loop of the partition coefficient as the product concentration is increased in the reservoir. This hysteresis loop vanishes as the rate of product diffusion increases. No hysteresis loop may occur when electric repulsion effects are coupled to substrate diffusion and reaction. The existence of multiple values of the partition coefficient for a fixed concentration of product implies that the membrane may store short-term memory of the former product concentration present in the external milieu. The occurrence of hysteresis generated by coupling enzyme reaction, product diffusion, electric partition effects at constant advancement of the reaction may be viewed as a sensing device of product concentration in the external milieu. Surprisingly, non-linearities required to generate this sensing device come from electrostatic effects and not from enzyme kinetics.     

β‐Hairpin folding and stability: molecular dynamics simulations of designed peptides in aqueous solution

The structural properties of a 10‐residue and a 15‐residue peptide in aqueous solution were investigated by molecular dynamics simulation. The two designed peptides, SYINSDGTWT and SESYINSDGTWTVTE, had been studied previously by NMR at 278 K and the resulting model structures were classified as 3:5 β‐hairpins with a type I + G1 β‐bulge turn. In simulations at 278 K, starting from the NMR model structure, the 3:5 β‐hairpin conformers proved to be stable over the time period evaluated (30 ns). Starting from an extended conformation, simulations of the decapeptide at 278 K, 323 K and 353 K were also performed to study folding. Over the relatively short time scales explored (30 ns at 278 K and 323 K, 56 ns at 353 K), folding to the 3:5 β‐hairpin could only be observed at 353 K. At this temperature, the collapse to β‐hairpin‐like conformations is very fast. The conformational space accessible to the peptide is entirely dominated by loop structures with different degrees of β‐hairpin character. The transitions between different types of ordered loops and β‐hairpins occur through two unstructured loop conformations stabilized by a single side‐chain interaction between Tyr2 and Trp9, which facilitates the changes of the hydrogen‐bond register. In agreement with previous experimental results, β‐hairpin formation is initially driven by the bending propensity of the turn segment. Nevertheless, the fine organization of the turn region appears to be a late event in the folding process. Copyright © 2004 European Peptide Society and John Wiley & Sons, Ltd.     

Light scattering of aqueous solutions of DNA, poly(acrylic acid), and tobacco mosaic virus under alternating electric field

Two new techniques, amplitude modulation (AM) and frequency modulation (FM) of an electric field, are developed for the light-scattering study of polymer solutions under ac electric fields. The AM technique makes it possible to observe accurately the frequency dependence of the intensity changes of scattered light due to the electric field. The FM one allows us to obtain directly the frequency derivative of the intensity change. The techniques are applied to DNA, poly(acrylic acid), and tobacco mosaic virus in the frequency range from 10 Hz to 100 kHz. A low-frequency relaxation is found for both DNA and poly(acrylic acid). The obsersved relaxation time of DNA agrees with that in the dielectric relaxation of DNA, which has been attributed to the rotation of the molecule with a quasipermanent dipole. In the case of poly(acrylic acid), the relaxation strength increases with increasing degree of neutralization. TMV at a concentration of 0.1% exhibits a negative relaxation at low frequencies, which indicates the rotation of TMV aggregate with a permanent dipole along its minor axis.     

Estimating the dielectric constant of the channel protein and pore

When modelling biological ion channels using Brownian dynamics (BD) or Poisson–Nernst–Planck theory, the force encountered by permeant ions is calculated by solving Poisson’s equation. Two free parameters needed to solve this equation are the dielectric constant of water in the pore and the dielectric constant of the protein forming the channel. Although these values can in theory be deduced by various methods, they do not give a reliable answer when applied to channel-like geometries that contain charged particles. To determine the appropriate values of the dielectric constants, here we solve the inverse problem. Given the structure of the MthK channel, we attempt to determine the values of the protein and pore dielectric constants that minimize the discrepancies between the experimentally-determined current–voltage curve and the curve obtained from BD simulations. Two different methods have been applied to determine these values. First, we use all possible pairs of the pore dielectric constant of water, ranging from 20 to 80 in steps of 10, and the protein dielectric constant of 2–10 in steps of 2, and compare the simulated results with the experimental values. We find that the best agreement is obtained with experiment when a protein dielectric constant of 2 and a pore water dielectric constant of 60 is used. Second, we employ a learning-based stochastic optimization algorithm to pick out the optimum combination of the two dielectric constants. From the algorithm we obtain an optimum value of 2 for the protein dielectric constant and 64 for the pore dielectric constant.     

Electric polarization of polyelectrolyte and colloid media: dielectric versus electro-optic approach

The theories of dielectric dispersion and of electric birefringence as a representative of electro-optic methods are considered and it is shown that they both depend in a similar way simply on the real part of the complex electric polarizability of the macromolecules or the particles. The latter also contains the permanent dipole moment. Experimental data on dielectric dispersion, electric birefringence and electric light scattering of strongly elongated, rod-like poly(tetrafluoroethylene) particles are compared and an attempt is made to extend the dielectric dispersion curve to lower frequencies using electric birefringence and electric light scattering data. Further, the experimental data on dielectric dispersion, electric light scattering, electro-orientation and dipolophoresis for the more complicated Escherichia coli particles are compared. Again, the possibility to extend the 10 kHz-100 MHz dielectric dispersion curve down below 1 Hz by using electric light scattering data is examined. The good matching of the dielectric dispersion and electric light scattering frequency curves found in the overlapping frequency range (10 kHz-5 MHz) essentially enhances the chance that dielectric dispersion below 1 MHz is related to alpha dispersion and not to electrode polarization. Thus it is not only possible to obtain additional information on the mechanism of polarization at lower-frequency dielectric dispersion, but also to extend our knowledge about the effective dielectric properties of biological complex fluids to frequencies essentially below 1 MHz. This could be important for the understanding of the effect of low-frequency electromagnetic fields on living matter.     

Modulation of tendon fibroplasia by exogenous electric currents

A chicken tendon explant model system has been developed to investigate the effects of extremely-low-frequency (ELF), low-amplitude, unipolar, square wave pulsed electric fields on fibroplasia in vitro. An electric field parameter set consisting of 1-Hz, 1-ms duration pulses, with a time-averaged current density of 7 mA/m2 (peak current density 7 A/m2) induced maximal (32%) increase in fibroblast proliferation in tendon explants exposed for 4 days. Exposure to the same field at an average current density of 1.8 mA/m2 had no effect on fibroblast proliferation, whereas exposure to current densities on greater than 10 mA/m2 inhibited proliferation and relative collagen synthesis, without affecting noncollagen protein synthesis. Fibroplasia was significantly increased in explants oriented parallel to applied electric fields having current densities of 3.5 or 7 mA/m2, but there was no detectable effect on explants oriented perpendicular to the same electric field. Fibroblast proliferation and relative collagen synthesis were inversely proportional to donor age for chickens in the 3- to 16-week age group used in this study. For these dependent variables (proliferation and relative collagen synthesis), there was no interaction between donor age and ELF electric field exposure.     

Identification of a structural site on acetylcholinesterase that promotes neurite outgrowth and binds laminin-1 and collagen IV

The cell adhesion and neurite outgrowth-promoting function of acetylcholinesterase has been localised to the area of the peripheral anionic site. In order to precisely determine the site involved, we used synthetic peptides representing sequences of the peripheral anionic site and its surrounds, and investigated their binding to a panel of monoclonal antibodies that inhibit cell adhesion/neurite outgrowth and/or to recognise the peripheral anionic site. Binding to laminin-1 and collagen IV was also investigated. A relationship between recognition of the sequence 37-50, representing a surface loop adjacent to the peripheral anionic site, and the degree of inhibition of cell adhesion was observed; both laminin-1 and collagen IV also bound this loop with high affinity. Neurite outgrowth on coverslips coated with this peptide was similar to those coated with acetylcholinesterase itself. Adhesion-inhibiting antibodies also recognised the omega loop 69-96, as did laminin-1 and collagen IV. Laminin also bound the sequences 55-66 and 340-353, recognised by the antibodies to varying degrees, but collagen did not. All these peptides were able to promote neurite outgrowth to some degree. No binding to the amyloid-binding omega loop 275-304 by the ligands was observed, nor did the antibodies recognise this consistently. No relationship was observed between the degree of inhibition of acetylcholinesterase and inhibition of neurite outgrowth by the antibodies from which we conclude that the neurite outgrowth function is non-cholinergic. In conclusion, we have identified a specific conformational structure on acetylcholinesterase, comprising adjacent surface loops between residues 37-50 and 69-96, with additional involvement of the sequences 55-66 and 340-353, that mediates cell adhesion and neurite outgrowth.     

Tunable Dielectric Properties of Ferrite-Dielectric Based Metamaterial

A ferrite-dielectric metamaterial composed of dielectric and ferrite cuboids has been investigated by experiments and simulations. By interacting with the electromagnetic wave, the Mie resonance can take place in the dielectric cuboids and the ferromagnetic precession will appear in the ferrite cuboids. The magnetic field distributions show the electric Mie resonance of the dielectric cuboids can be influenced by the ferromagnetic precession of ferrite cuboids when a certain magnetic field is applied. The effective permittivity of the metamaterial can be tuned by modifying the applied magnetic field. A good agreement between experimental and simulated results is demonstrated, which confirms that these metamaterials can be used for tunable microwave devices.     

褐飞虱成虫体内磁性物质检测

地磁定向是昆虫远距离迁飞定向的重要机制之一.本研究以褐飞虱Nilaparvata lugens长翅型和短翅型成虫为研究对象,利用MPMS-7型号超导量子干涉磁强计(磁场范围为±4.8 mA/m,温度范围为1.9 ～ 400 K)检测虫体内的磁性物质,明确其体内的分布状况.结果表明:褐飞虱长翅型雄成虫整个虫体的温度退磁曲...     

Antennae: the strongest magnetic part of the migratory ant

Pachycondyla marginata (P.m.), a migratory and termitophageous ant, hunting only the termite species Neocapritermes opacus, migrates significantly oriented 13 degrees with respect to the magnetic North-South axis. Results of hysteresis curves at room temperature of four Pachycondyla marginata heads, thorax, pairs of antennae and abdomens, oriented parallel to the magnetic field, indicate that the antennae give the strongest saturation magnetization, suggesting this sensory organ as being also a magnetic sensory organ. The total saturation magnetization in a whole P.m. is composed by 42 +/- 3%, 24 +/- 3%, 19 +/- 3% and 15 +/- 3% of antennae, head, thorax and abdomen contributions, respectively. The abdomen hysteresis curve presents a wasp-waisted loop with Hcr/Hc of 4.75, characteristic of mixed magnetic systems.     

Determination of the glycosaminoglycan and collagen contents in tissue samples by high-resolution 1H NMR spectroscopy after DCl-induced hydrolysis

The determination of the collagen and glycosaminoglycan (GAG) contents of native and particularly bioengineered tissues is of considerable interest because the collagen-to-GAG ratio determines the water content of the tissue, which is crucial regarding its mechanical properties. (1)H NMR spectroscopy subsequent to the hydrolysis of the sample by aqueous 6 M DCl at 353 K is used to determine the GAG and collagen contents simultaneously. Under these strongly acidic conditions the biopolymers of the extracellular matrix, collagen, and GAG are fragmented into their individual monomers, that is, free amino acids from collagen and monosaccharides from the polymer repeat units of GAGs. The amino acid amount can be easily determined in the presence of an internal standard by (1)H NMR spectroscopy because amino acids proved to be stable under acidic conditions. The carbohydrates are subject to charring in the presence of concentrated DCl, but glucosamine and galactosamine were found to be sufficiently stable for quantification under the chosen conditions.     

Kinetic Hysteresis in Collagen Folding

The triple helix of collagen shows a steep unfolding transition upon heating, whereas less steep and more gradual refolding is observed upon cooling. The shape of the hysteresis loop depends on the rate of temperature change as well as the peptide concentration. Experimental heating and cooling rates are usually much faster than rates of unfolding and refolding. In this work, collagen model peptides were used to study hysteresis quantitatively. Their unfolding and refolding profiles were recorded at different heating and cooling rates, and at different peptide concentrations. Data were fitted assuming kinetic mechanisms in which three chains combine to a helix with or without an intermediate that acts as a nucleus. A quantitative fit was achieved with the same kinetic model for the forward and backward reactions. Transitions of exogenously trimerized collagen models were also analyzed with a simplified kinetic mechanism. It follows that true equilibrium transitions can only be measured at high concentrations of polypeptide chains with slow scanning rates, for example, 0.1°C/h at 0.25 mM peptide concentration of (Gly-Pro-Pro)₁₀. (Gly-Pro-4(R)Hyp)₁₀ folds ∼2000 times faster than (Gly-Pro-Pro)₁₀. This was explained by a more stable nucleus, whereas the rate of propagation was almost equal. The analysis presented here can be used to derive kinetic and thermodynamic data for collagenous and other systems with kinetically controlled hysteresis.     

AC electro-osmotic mixing induced by non-contact external electrodes

We demonstrate efficient mixing in a micro-fluidic reservoir smaller than 10 microL using ac electro-osmosis driven by field-induced polarization. Our mixing device, of that electrodes are outside of the mixing unit, consists of three circular reservoirs (3mm in diameter) connected by a 1 mm x 1 mm channel. Unlike dc electro-osmosis, whose polarization is from charged substrate functional groups, this new mechanism uses the external field to capacitively charge the surface and the surface capacitance becomes the key factor in the electrokinetic mobility. The charging and mixing are enhanced at tailor-designed channel corners by exploiting the high normal fields at geometric singularities. The induced surface dielectric polarization and the resulting electric counter-ion double layer produce an effective Zeta potential in excess of 1 V, over one order of magnitude larger than the channel Zeta potential. The resulting ac electro-osmotic slip velocity scales quadratically with respect to the applied field, in contrast to the linear scaling of dc electro-osmosis and at 1cm/s and larger, exceeds the classical dc values by two orders of magnitude. The polarization is non-uniform at the corners due to field leakage to the dielectric substrate and the inhomogeneous slip velocity produces intense mixing vortices that effectively homogenize solutes in 30s in a 3mm reservoir, in contrast to hour-long mixing by pure diffusion.     

Electrical properties of hydrated collagen. I. Dielectric properties

The effect of water on the low-frequency (10²-10⁵ H_z) complex permittivitv of native, sold-state collagen has been investigated experimentally. Measurements at ambient temperature show that dry collagen exhibits essentially no frequency or temperature dependence. As water is absorbed, both dielectric constant and loss factor increase simultaneously and rise sharply upward at a hydration level which may be associated with the completion of the primary absorption layer as determined from independent water absorption studies. The behaviour is qualitatively identical to that observed for other proteins and related materials. Temperature-dependent measurements made under vacuum conditions in the range ?196°C to +100°C are characteristic of the dielectric properties of the water in the sample. Dehydration produced by successive temperature recycling to the maximum temperature effectively eliminates any temperature or frequency dependence. A maximum in the temperature-dependent curves is found at about +40°C and is explained as the superposition of two processes: (1) the transition of water molecules from bound to free states, and (2) the difffusion of water molecules out of the system. The dielectric constant of dry collagen, after desorption at ambient temperature, is about 4.5. Desorption at elevated temperatures reduced the room temperature value to about 2.3 and the liquid nitrogen temperature value to a number indistinguishable from the optical value of n² = 2.16.