Membrane Charge Density Research Articles

Translocating the blood-brain barrier using electrostatics.

Mammalian cell membranes regulate homeostasis, protein activity, and cell signaling. The charge at the membrane surface has been correlated with these key events. Although mammalian cells are known to be slightly anionic, quantitative information on the membrane charge and the importance of electrostatic interactions in pharmacokinetics and pharmacodynamics remain elusive. Recently, we reported for the first time that brain endothelial cells (EC) are more negatively charged than human umbilical cord cells, using zeta-potential measurements by dynamic light scattering. Here, we hypothesize that anionicity is a key feature of the blood-brain barrier (BBB) and contributes to select which compounds cross into the brain. For the sake of comparison, we also studied the membrane surface charge of blood components—red blood cells (RBC), platelets, and peripheral blood mononuclear cells (PBMC). To further quantitatively correlate the negative zeta-potential values with membrane charge density, model membranes with different percentages of anionic lipids were also evaluated. From all the cells tested, brain cell membranes are the most anionic and those having their lipids mostly exposed, which explains why lipophilic cationic compounds are more prone to cross the blood-brain barrier.

Simultaneous Determination of Pore Size and Surface Charge Density of Microfiltration Membranes by Streaming Potential Measurement

Simultaneous Determination of Pore Size and Surface Charge Density of Microfiltration Membranes by Streaming Potential Measurement

Self-assembly of onion-like vesicles induced by charge and rheological properties in anionic–nonionic surfactant solutions

Surface and bulk properties of mixed nonionic tetraethylene glycol monododecyl ether (C12EO4) and dialkyl anionic sodium bis(2-ethyl hexyl)sulfosuccinate (AOT) mixtures in aqueous solution were studied and self-assembly of onions (multilamellar vesicles) with uncountable bilayers induced by charge were obtained. An attractive net interaction exists between the two surfactant molecules and a negative interaction parameter values (βm and βs) indicate a synergy in the non-ideal mixing behavior. The onion-like structures, i.e., the densely packed multilamellar vesicles with uncountable bilayers are directly observed in a bluish sample solution by charging the swollen nonionic lamellar phases with addition of anionic surfactant, AOT. The addition of ionic surfactants into the nonionic lamellar phase, through electrostatic repulsion of the ionic headgroups, will suppress the Helfrich undulation and induce the transition from planar lamellae to vesicles. Similarly, in an AOT-rich region, upon addition of the nonionic ones, the planar lamellar gel phases of AOT–H2O mixture can also be transformed into closed vesicle gel-phases due to the reduction of the Coulombic force. Rheological measurements show that both of the storage modulus (G′) and the loss modulus (G′′) increase with the increase of membrane charge density. Interestingly, the addition of NaCl into the mixed systems causes the phase transition from micelles to vesicles. On increasing the amount of salts, the bilayers of the lamellar phase exhibit more flexibility due to the electrostatic shielding of electrolyte, along with a decrease of the solution viscosity, the G′, and the G′′. It is believed that the ionic charges play crucial roles in the flexibility of bilayer membranes. The obtained multilamellar architectures are excellent for the long-term storage of multiple components and can effectively slow their release.

Membrane Curvature Sensing by Amphipathic Helices: A SINGLE LIPOSOME STUDY USING α-SYNUCLEIN AND ANNEXIN B12

Preferential binding of proteins on curved membranes (membrane curvature sensing) is increasingly emerging as a general mechanism whereby cells may effect protein localization and trafficking. Here we use a novel single liposome fluorescence microscopy assay to examine a common sensing motif, the amphipathic helix (AH), and provide quantitative measures describing and distinguishing membrane binding and sensing behavior. By studying two AH-containing proteins, α-synuclein and annexin B12, as well as a range of AH peptide mutants, we reveal that both the hydrophobic and hydrophilic faces of the helix greatly influence binding and sensing. Although increased hydrophobic and electrostatic interactions with the membrane both lead to greater densities of bound protein, the former yields membrane curvature-sensitive binding, whereas the latter is not curvature-dependent. However, the relative contributions of both components determine the sensing of AHs. In contrast, charge density in the lipid membrane seems important primarily in attracting AHs to the membrane but does not significantly influence sensing. These observations were made possible by the ability of our assay to distinguish within our samples liposomes with and without bound protein as well as the density of bound protein. Our findings suggest that the description of membrane curvature-sensing requires consideration of several factors such as short and long range electrostatic interactions, hydrogen bonding, and the volume and structure of inserted hydrophobic residues.

Cationic liposome–nucleic acid complexes: liquid crystal phases with applications in gene therapy

Cationic liposome (CL) carriers of nucleic acids are studied primarily because of their applications in gene delivery and gene silencing with CL-DNA and CL-siRNA (short-interfering RNA) complexes, respectively, and their implications for ongoing clinical gene therapy trials worldwide. A series of synchrotron-based small-angle X-ray scattering studies, dating back to 1997, has revealed that CL-nucleic acid complexes spontaneously assemble into distinct novel liquid crystalline phases of matter. Significantly, transfection efficiency (TE; a measure of expression of an exogenous gene that is transferred into the cell by the lipid carrier) has been found to be dependent on the liquid crystalline structure of complexes, with lamellar complexes showing strong dependence on membrane charge density (σM) and non-lamellar complexes exhibiting TE behaviour independent of σM. The review describes our current understanding of the structures of different liquid crystalline CL-nucleic acid complexes including the recently described gyroid cubic phase of CL-siRNA complexes used in gene silencing. It further makes apparent that the long-term goal of developing optimised liquid crystalline CL-nucleic acid complexes for successful medical applications requires a comprehensive understanding of the nature of the interactions of distinctly structured complexes with cell membranes and events leading to release of active nucleic acids within the cell cytoplasm.

Formation of Kinetically Trapped Nanoscopic Unilamellar Vesicles from Metastable Nanodiscs

Zwitterionic long-chain lipids (e.g., dimyristoyl phosphatidylcholine, DMPC) spontaneously form onion-like, thermodynamically stable structures in aqueous solutions (commonly known as multilamellar vesicles, or MLVs). It has also been reported that the addition of zwitterionic short-chain (i.e., dihexanoyl phosphatidylcholine, DHPC) and charged long-chain (i.e., dimyristoyl phosphatidylglycerol, DMPG) lipids to zwitterionic long-chain lipid solutions results in the formation of unilamellar vesicles (ULVs). Here, we report a kinetic study on lipid mixtures composed of DMPC, DHPC, and DMPG. Two membrane charge densities (i.e., [DMPG]/[DMPC] = 0.01 and 0.001) and two solution salinities (i.e., [NaCl] = 0 and 0.2 M) are investigated. Upon dilution of the high-concentration samples at 50 °C, thermodynamically stable MLVs are formed, in the case of both weakly charged and high salinity solution mixtures, implying that the electrostatic interactions between bilayers are insufficient to cause MLVs to unbind. Importantly, in the case of these samples small angle neutron scattering (SANS) data show that, initially, nanodiscs (also known as bicelles) or bilayered ribbons form at low temperatures (i.e., 10 °C), but transform into uniform size, nanoscopic ULVs after incubation at 10 °C for 20 h, indicating that the nanodisc is a metastable structure. The instability of nanodiscs may be attributed to low membrane rigidity due to a reduced charge density and high salinity. Moreover, the uniform-sized ULVs persist even after being heated to 50 °C, where thermodynamically stable MLVs are observed. This result clearly demonstrates that these ULVs are kinetically trapped, and that the mechanical properties (e.g., bending rigidity) of 10 °C nanodiscs favor the formation of nanoscopic ULVs over that of MLVs. From a practical point of view, this method of forming uniform-sized ULVs may lend itself to their mass production, thus making them economically feasible for medical applications that depend on monodisperse lipid-based systems for therapeutic and diagnostic purposes.

Characterisation and application of a novel positively charged nanofiltration membrane for the treatment of textile industry wastewaters

The present study demonstrates the high potential for the application of a novel self assembled positively charged nanofiltration membrane, PA6DT-C, in processes such as the recovery of valuable cationic macromolecules in the bioprocess and pharmaceutical industries or removal of multi-valent cations such as dyes and heavy metals in the paper and pulp, textiles, nuclear, and automotive industries. The nanofiltration membrane, prepared in this laboratory, is further characterised and then tested for the removal and recovery of Methylene Blue from a synthetic dye house wastewater. The characterisation process involved the construction of a rejection profile for NaCl over a wide range of pH and concentration, which illustrates that the optimal process conditions for the removal of small cations using this membrane is in the region pH <8.0 and concentration less than 15 mol m −3. The salt rejection data was used to calculate the magnitude of the effective membrane charge density and this was found to be significantly higher for the PA6DT-C membrane than two commercially available membranes (Desal-DK and Nanomax-50). The membrane flux for this new membrane is also superior to the commercial membranes with an approximate increase of 3–4 fold. The PA6DT-C membrane was successful in removal of Methylene Blue dye from synthetic dye house wastewaters achieving 98% rejection and a membrane flux of ∼17 LMH bar −1. Thus, this new membrane both adds to and complements the existing short supply of positively charged NF membranes.

Mass transfer simulation of ion separation by nanofiltration considering electrical and dielectrical effects

In this work, transport of mono and divalent ions through nanofiltration membranes using Donnan-steric pore model and dielectric exclusion (DSPM-DE) has been simulated. Dielectric effects were based on Born and image force effects. Donnan equilibrium was used for electrical effects. The model predicts rejection of ions, Donnan potential, and interaction energy due to image forces in the nanofiltration process. The model findings predict the rejection of symmetric monovalent ions as a function of the membrane charge density. It is indicated that a significant decrease in the interactions among the ions and their images causes the Donnan exclusion to be predominant at higher membrane charge densities. Consequently, higher membrane rejections are obtained. The findings of the developed model are valuable in predicting the nanofiltration behavior at various operating conditions as well as different membrane characteristics.

Preparation and characterization of ion-selective polyvinyl chloride based heterogeneous cation exchange membrane modified by magnetic iron–nickel oxide nanoparticles

In this research, polyvinylchloride/styrene–butadiene-rubber blend heterogeneous cation exchange membranes were prepared by solution casting technique using tetrahydrofuran as solvent and cation exchange resin powder as functional groups agent. Iron–nickel oxide (Fe 2NiO 4) nanoparticle was also employed as additive in membrane fabrication. The effect of additive concentration on properties of home-made membranes was studied. SOM images showed uniform particles distribution and relatively uniform surfaces for the membranes. The increase of Fe 2NiO 4 concentration in casting solution caused to decrease of membrane water content. Membrane ion exchange capacity initially was increased by additive increasing to 1 wt.% and then showed decreasing trend with higher additive content. Membrane potential, charge density, permselectivity and transport number were enhanced initially by increase of additive loading to 2 wt.% and then they began to decrease by more additive concentration. Membranes exhibited lower potential, selectivity and transport number for bivalent ions compared to monovalent ones. Membrane permeability, flux and conductivity were enhanced initially in monovalent ionic solution by increase in additive concentration to 0.5 wt.% and then began to decrease by more additive loading. Permeability was improved for bivalent ions with increase in additive concentration. Modified membranes showed higher permeability for bivalent ions compared to monovalent type.

Dielectric analysis of nanofiltration membrane in electrolyte solutions: Influences of permittivity of wet membrane and volume charge density on ion permeability

Dielectric measurements were carried out for the systems composed of nanofiltration membranes and electrolyte solutions over a frequency range of 40Hz to 10MHz, and double dielectric relaxations were observed at about 102–106Hz. Based on the interfacial polarization theory, the dielectric relaxation at high frequency was analyzed theoretically, and the permittivity and conductivity of the membrane and electrolyte solutions, were obtained. By using the permittivity of wet membrane, the ion solvation energy barrier in eight electrolyte solutions was calculated. The results showed that the ion solvation energy barrier of 2-2 type of electrolyte solution was the biggest among all types of electrolyte. The influence of solvation energy barrier on separation performance was also evaluated. Because the permittivity of wet membrane takes the place of that of dry membrane, the calculations can ensure more accurate results close to practical separation process. Furthermore, the variation of the ratio of membrane/solution conductivity, κm/κw, with the electrolyte concentration was explained by Donnan equilibrium theory, and the ratio together with the TMS model was used to calculate the volume charge density of membrane. On the basis of this, by using the volume charge density of membrane and the existing transport models, such as TMS, TMS-DE, DSPM model, four transport parameters were calculated. It was concluded that in the process of ion permeation through the membrane, Donnan exclusion is the most important influencing factor and steric effect is secondary in low electrolyte concentration, whereas dielectric exclusion plays a dominant role in high electrolyte concentration.

Investigation of Mg 2+/Li + Separation by Nanofiltration

The Mg 2+/Li +/Cl − solutions were filtrated with a commercially available DK nanofiltration membrane to investigate the possibility to enrich the lithium component. The investigation was significant as such an approach might be a competing substitute for the present lithium purification industry and the environmental protection purpose. The Donnan steric pore model (DSPM) was implemented for the prediction. The separation of Mg 2+/Li + was mainly affected by the working pressure (or the permeation flux) and a limiting separation factor was found around 0.31. The effective membrane charge density was evaluated and its dependence on the permeation flux as well as the ion pattern was discussed. For predicting an actual separation of electrolytes, the experimental investigation seems necessary for the reliability and efficiency.

Effect of membrane charge density on the protein corona of cationic liposomes: Interplay between cationic charge and surface area

Here we investigate the effect of membrane charge density on the protein corona that forms upon exposure of cationic liposomes to human plasma. We show that the protein corona around cationic liposomes is not uniquely defined by membrane charge density, but it is deeply affected by both the overall cationic charge and the available surface area. Remarkably, changes in protein composition are largely removed when a correcting quantity accounting for the ratio between the cationic charge and the surface area of different cationic liposome formulations is considered.

An electrochemical and theoretical comparison of ionic transport through a polystyrene-based cobalt arsenate membrane

An organic–inorganic composite material, i.e. polystyrene cobalt arsenate was chemically synthesized by a sol–gel process. The physico-chemical of the membrane was determined using SEM, TGA and XRD studies. The membrane was found to be crystalline in nature with uniform arrangement of particles and no sign of visible cracks. Ionic potentials were measured across polystyrene cobalt arsenate membranes with 1:1 electrolytes at different concentrations. Observed membrane potentials were positive. The membrane potential offered by the electrolytes was in the order of KCl > NaCl > LiCl. The successful application of TMS and Kobatake theory were used for the estimation of the charge density of membranes. The order of charge density for 1:1 electrolytes solution was found to be KCl > NaCl > LiCl. Kobatake's equation was used under two limiting conditions, namely in the high concentration range and in the dilute concentration range. The two limiting forms of Kobatake's equation gave identical values of charge density for the membrane taken in this investigation. The TMS method was used to investigate the transport number of ions, mobility ratio, distribution coefficient, charge effectiveness, perm selectivity and concentration of fixed charge of the membranes.

Structure–Function Relationships of New Lipids Designed for DNA Transfection

Cationic liposome/DNA complexes can be used as nonviral vectors for direct delivery of DNA-based biopharmaceuticals to damaged cells and tissues. To obtain more effective and safer liposome-based gene transfection systems, two cationic lipids with identical head groups but different chain structures are investigated with respect to their in vitro gene-transfer activity, their cell-damaging characteristics, and their physicochemical properties. The gene-transfer activities of the two lipids are very different. Differential scanning calorimetry and synchrotron small- and wide-angle X-ray scattering give valuable structural insight. A subgel-like structure with high packing density and high phase-transition temperature from gel to liquid-crystalline state are found for lipid 7 (N'-2-[(2,6-diamino-1-oxohexyl)amino]ethyl-2,N-bis(hexadecyl)propanediamide) containing two saturated chains. Additionally, an ordered head-group lattice based on formation of a hydrogen-bond network is present. In contrast, lipid 8 (N'-2-[(2,6-diamino-1-oxohexyl)amino]ethyl-2-hexadecyl-N-[(9Z)-octadec-9-enyl]propanediamide) with one unsaturated and one saturated chain shows a lower phase-transition temperature and a reduced packing density. These properties enhance incorporation of the helper lipid cholesterol needed for gene transfection. Both lipids, either pure or in mixtures with cholesterol, form lamellar phases, which are preserved after addition of DNA. However, the system separates into phases containing DNA and phases without DNA. On increasing the temperature, DNA is released and only a lipid phase without intercalated DNA strands is observed. The conversion temperatures are very different in the two systems studied. The important parameter seems to be the charge density of the lipid membranes, which is a result of different solubility of cholesterol in the two lipid membranes. Therefore, different binding affinities of the DNA to the lipid mixtures are achieved.

Phase separation of phospholipid multilayers incorporated with cell penetrating peptides

We used X-ray reflectivity to investigate the structures of phospholipid multilayers with transcription-activating-factor-derived peptide (TDP) as a function of the membrane charge density. Mixed phospholipid multilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl -sn-glycero-3-phosphoserine (DPPS) with different mixing ratios (C:S) were used to elucidate the various charge densities in a plasma membrane. We fixed the peptide/lipid molar ratio (P/L) and varied the DPPC/DPPS molar ratio in the mixed multilayer. In the pure DPPC multilayer, the incorporation of TDP had nearly no effect on the bilayer thickness of the mixed lipid multilayer. However, in the mixed DPPC/DPPS multilayer, the incorporation of TDP decreased the bilayer thickness, suggesting that the TDP peptide had a stronger interaction with DPPS than with DPPC and caused disorder in the lamellar structure. Combining this with the refined X-ray reflectivity (XR) data, we concluded that the TDP existed more in the headgroup region of the TDP-induced segregated DPPS in the mixed multilayer and caused significant membrane thinning.

Nanogyroids Incorporating Multivalent Lipids: Enhanced Membrane Charge Density and Pore Forming Ability for Gene Silencing

The self-assembly of a custom-synthesized pentavalent cationic lipid (MVL5) and glycerol monooleate (GMO) with small interfering RNA (siRNA) results in the formation of a double-gyroid bicontinuous inverted cubic phase with colocalized lipid/siRNA domains as shown by synchrotron X-ray scattering and fluorescence microscopy. The high charge density (due to MVL5) and positive Gaussian modulus of the GMO-containing membranes confer optimal electrostatic and elastic properties for endosomal escape, enabling efficient siRNA delivery and effective, specific gene silencing.

Assessment of dielectric contribution in the modeling of multi-ionic transport through nanofiltration membranes

In the present work, a knowledge model is used to investigate the possibility of predicting the separation of ionic mixtures. In the model, the transfer through the membrane is based upon the classical vision coupling the Donnan, steric and dielectric exclusions at the interfaces with the extended Nernst–Planck transport equation within the pores. The prediction of selectivity between ions requires the knowledge of the interfacial partitioning which results of three contributions, i.e. steric, electric and dielectric. Each contribution can be assessed by means of a parameter describing the exclusion phenomena. The mean pore radius ( r p ) and the membrane charge density ( X d ) which govern respectively steric and electric exclusions are usually experimentally identified by microscopy or neutral solute rejection and zetametry. The dielectric contribution through the dielectric constant of the solution within the pores ( ɛ p ) is usually fitted from rejection curves. In this paper, an experimental method based upon the membrane potential measurements is also implemented to assess ɛ p -value. The identified values are compared with those obtained by fitting experimental ternary mixtures. The agreement found between these two sets of values indicates that the theory of ionic transfer seems to be coherent and let foresee the possibility of a fully predictive model.

A transport model considering charge adsorption inside pores to describe salts rejection by nanofiltration membranes

In the present work, rejection of mineral salts by nanofiltration membranes is modeled by a new knowledge model considering a charge distribution along the membrane pores. The model is classically based upon the coupling between the extended Nernst–Planck equation of transport within the pores with the Donnan, steric and dielectric interfacial exclusion. In literature, this kind of model is currently being used to describe salt rejections and it requires assessment of the membrane charge density and the dielectric constant of the solution inside the pores or the dielectric constant of the membrane material. Experimental estimation of these parameters, especially the membrane charge density, is difficult. For this reason, these parameters are usually adjusted so as to fit experimental rejections. The novelty of the proposed approach lies with the membrane charge density, which is not considered constant along the pores. Indeed, in this work, the membrane charge density was introduced in the model by means of adsorption isotherms, which were determined beforehand from the streaming potential measurements. In this case, only the dielectric constant inside pores had to be adjusted. The good agreement found between experimental and simulated rejection curves with coherent dielectric constants proves that the model theory is consistent. It has also proved that the estimated membrane charge density was, clearly, not overestimated as it was previously reported and made it possible to foresee a fully predictive model.

DNA Condensation at Freestanding Cationic Lipid Bilayers

We describe a previously unreported coil-globule transition of DNA electrostatically bound to a freestanding fluid cationic lipid membrane [1]. The collapse of a DNA coil into a compact globule takes place after the DNA molecule attaches in an extended conformation to the membrane. DNA condensation is favored at a higher cationic lipid content, while at lower membrane charge densities coexistence of DNA random coils, partially collapsed conformations, and globules is observed.

Transport Studies with Composite Membrane by Sol-Gel Method

The preparation of polystyrene-based composite membrane at different pressures with varying amounts of material has been described. In order to understand the mechanism of transport of ions, membrane potential measurements were carried out using different concentrations of 1:1 electrolyte (KCl, NaCl, and LiCl) solutions and also to evaluate various membrane parameters such as mobility, distribution coefficient, and charge effectiveness controlling the transport phenomena. The membrane potential offered by the electrolytes is in the order of LiCl > NaCl > KCl, and the membrane is found to be cation selective. The large deviation in the value of K ± at the lower concentration of electrolytes was attributed to the high mobility of comparatively free charges of the strong electrolyte. Teorell, Meyer, and Sievers (TMS) method was used for the estimation of the thermodynamically fixed charge density of membranes. The data were then utilized to calculate membrane potential using the TMS theory. It was interesting to note that the theoretical predictions were borne out quite satisfactorily with experimental results.