Block copolymer vesicle permeability measured by osmotic swelling and shrinking

Vesicle response to osmotic shock provides insight into membrane permeability, a highly relevant value for applications ranging from nanoreactor experimentation to drug delivery. The osmotic shock approach has been employed extensively to elucidate the properties of phospholipid vesicles (liposomes) and of varieties of polymer vesicles (polymersomes). This study seeks to compare the membrane response for two varieties of polymersomes, a comb-type siloxane surfactant, poly(dimethylsiloxane)-g-poly(ethylene oxide) (PDMS-g-PEO), and a diblock copolymer, polybutadiene-b-poly(ethylene oxide) (PBut-b-PEO). Despite similar molecular weights and the same hydrophilic block (PEO), the two copolymers possess different hydrophobic blocks (PBut and PDMS) and corresponding glass transition temperatures (-31 and -123 °C, respectively). Dramatic variations in membrane response are observed during exposure to osmotic pressure differences, and values for polymer membrane permeability to water are extracted. We propose an explanation for the observed phenomena based on the respective properties of the PBut-b-PEO and PDMS-g-PEO membranes in terms of cohesion, thickness, and fluidity.     

Selective complexation and transport of europium ions at the interface of vesicles

The aim of the present work was to design functionalized lipidic membranes that can selectively interact with lanthanide ions at the interface and to exploit the interaction between membranes induced by this molecular-recognition process with a view to building up self-assembled vesicles or controlling the permeability of the membrane to lanthanide ions. Amphiphilic molecules bearing a beta-diketone unit as head group were synthesized and incorporated into phospholipidic vesicles. Binding of Eu(III) ions to the amphiphilic ligand can lead to formation of a complex involving ligands of the same vesicle membrane (intravesicular complex) or of two different vesicles (intervesicular complex). The effect of Eu(III) ions on vesicle behavior was studied by complementary techniques such as fluorimetry, light scattering, and electron microscopy. The formation of an intravesicular luminescent Eu/beta-diketone ligand (1/2) complex was demonstrated. The linear increase in the binding constant with increasing concentration of ligands in the membrane revealed a cooperative effect of the ligands distributed in the vesicle membrane. The luminescence of this complex can be exploited to monitor the kinetics of complexation at the interface of the vesicles, as well as ion transport across the membrane. By encapsulation of 2,6-dipicolinic acid (DPA) as a competing ligand which forms a luminescent Eu/DPA complex, the kinetics of ion transport across the membrane could be followed. These functional vesicles were shown to be an efficient system for the selective transport of Eu(III) ions across a membrane with assistance by beta-diketone ligands.     

Electrochemical characterization of pore formation by bacterial protein toxins on hybrid supported membranes

The interaction of pore-forming streptolysin O (SLO) with biomimetic lipid membranes has been studied by electrochemical methods. Phosphatidylcholine lipid vesicles were deposited onto gold electrodes modified with supporting layers of hexyl thioctate (HT) or thioctic acid tri(ethylene glycol) ester (TA-TEGE), and integrity and permeability of the resulting membranes were characterized by cyclic voltammetry and impedance spectroscopy. Both positively and negatively charged electrochemical probes, potassium ferrocyanide, hexaammineruthenium(III) chloride, and ferrocene carboxylic acid (FCA), were employed to evaluate their suitability to probe the membrane permeability properties, with FCA exhibiting ideal behavior and thus employed throughout the work. Fusion of vesicles incubated with SLO on the electrodes yielded membranes that showed a distinctive response pattern for FCA as a function of SLO concentration. A direct dependence of both the currents and peak separation of FCA in the cyclic voltammograms was observed over a concentration range of 0-10 hemolytic units (HU)/microL of the toxin. The interaction of SLO with preformed supported lipid membranes was also investigated, and much lower response was observed, suggesting a different extent of membrane-toxin interactions on such an interface. Nonionic surfactant Triton was found to disrupt the vesicle structure but could not completely remove a preformed membrane to fully restore the electrode response. The information reported here offers some unique insight into toxin-surface interactions on a hybrid membrane, facilitating the development of electrochemically based sensing platforms for detecting trace amounts of bacterial toxins via the perforation process.     

Molecular exchange through vesicle membranes: a pulsed field gradient nuclear magnetic resonance study

The permeability of block copolymer vesicles is studied using pulsed field gradient nuclear magnetic resonance spectroscopy together with a numerical data analysis procedure. Polyethylene oxide molecules of various molecular masses are used to sample the permeability of the vesicle membrane by observing the trans-membrane exchange process under equilibrium conditions. For shorter polyethylene oxide chains, the analysis yields a nearly linear dependence of the logarithmic trans-membrane exchange rate on the hydrodynamic radius of the sample molecules.     

Molecular exchange through the vesicle membrane of siloxane surfactant in water/glycerol mixed solvents

The effect of glycerol on the permeability of vesicle membranes of a siloxane surfactant, the block copolymer polyethyleneoxide-b-polydimethylsiloxane-polyethyleneoxide, (EO)15-(DMS)15-(EO)15, was studied with freeze-fracture transmission electron microscopy (FF-TEM) and pulsed-field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy. The FF-TEM results show that, in pure water, the surfactant can form small vesicles with diameters of less than 25 nm, as well as a few multilamellar vesicles with diameters larger than 250 nm. Gradual substitution of water with glycerol to a glycerol content of 40% leads to significant structural transformations: small vesicles are gradually swollen, and large multilamellar vesicles disappear. A glycerol content of 60% results in the complete disintegration of the vesicles into membrane fragments. PFG-NMR measurements indicate that the vesicle membrane does not represent an effective barrier for water molecules on the NMR time scale; hence, the average residence time of water in the encapsulated state is below tau b = 2 ms. In contrast, the average residence time of glycerol molecules in the encapsulated state can be as large as tau b = 910 ms. The permeability of the vesicle membrane increases with increasing glycerol concentration in the solvent: At a concentration of 40%, the residence time tau b is lowered to approximately 290 ms. After vesicle destruction at higher glycerol concentrations, a small glycerol fraction is still bound by membrane fragments that are formed after the disintegration of the vesicles.     

Heterogeneous domains and membrane permeability in phosphatidylcholine-phosphatidic acid rigid vesicles as a function of pH and lipid chain mismatch

Heterogeneous lipid membranes tuned by pH were evaluated at 37 degrees C in the form of PEGylated vesicles composed of lipid pairs with dipalmitoyl ( n = 16) and distearoyl ( n = 18) chain lengths. One lipid type was chosen to have the titratable moiety phosphatidic acid on its headgroup, and the other lipid type was chosen to have a phosphatidylcholine headgroup. The effect of pH on the formation of lipid heterogeneities and on membrane permeability was studied on vesicles composed of lipid pairs with matching and nonmatching chain lengths. The formation of lipid heterogeneities increases with decreasing pH in membranes composed of lipid pairs with either matching or nonmatching chain lengths. Increased permeability with decreasing pH was exhibited only by membranes composed of lipid pairs with nonmatching chain lengths. Permeability rates correlate strongly with the predicted extent of interfacial boundaries of heterogeneities, suggesting defective packing among nonmatching acyl chains of lipids. In heterogeneous mixtures with one lipid type in the fluid state ( n = 12), the dependence of membrane permeability on pH is weaker. In the presence of serum proteins, PEGylated gel-phase vesicles containing lipid pairs with nonmatching chain lengths exhibit faster release rates with decreasing pH compared to measured release rates in phosphate buffer, suggesting a second mechanism of formation of separated phases. PEGylated vesicles composed of lipid pairs with nonmatching chain lengths labeled with internalizing anti-HER2/neu antibodies that target overexpressed antigens on the surface of SKOV3-NMP2 ovarian cancer cells exhibit specific cancer cell targeting, followed by extensive internalization (more than 84% of bound vesicles) and fast release of contents intracellularly. These PEGylated vesicles composed of rigid membranes for long blood circulation times that exhibit pH-dependent release of contents intracellularly could become potent drug delivery carriers for the targeted therapy of solid tumors.     

Effect of electron beam irradiation on morphology and sieving characteristics of nylon-66 membranes

Nylon-66 is a typical semicrystalline polymer that can be crosslinked through electron beam (EB) irradiation. Crosslinking can dramatically change polymer properties. The objective of this research was to observe how EB irradiation affects morphology and sieving characteristics of nylon-66 membranes. EB irradiation was carried out in air at 60, 70 and 80 kGy doses. Scanning electron microscopy (SEM), swelling and gel content studies evidenced the morphological changes and crosslinking of nylon-66 membranes. Sieving characteristics were also measured using pure water permeation, and rejection of raffinose, vitamin B12, and mono- and divalent salts. These results show that nylon-66 membrane surface and permeation characteristics changed with different irradiation doses. The nylon-66 surface became denser and the gel content increased with increasing irradiation dose. Furthermore, pure water permeation decreased and small molecules were increasingly rejected with greater irradiation doses. The amount of rejection was between 33% and 88.4% for vitamin B12 and between 16% and 83% for raffinose. The highest vitamin B12 and raffinose rejection was seen with a N-80 membrane, and no rejection was measured with N-0 or N-60 membranes. Salt rejection, however, was very low, especially for NaCl, with only 10.51-46% rejected. Based on flux, permeability and uncharged solute (vitamin B12 and raffinose) rejection data, nylon-66 type N-70 and N-80 membranes were estimated to be in the nanofiltration (NF) range.     

Inclusion and fractionated release of nucleic acids using microcapsules made from plant cells.

The encapsulation and fractionated release of nucleic acids on vesicular packing (VP) materials have been investigated. The earlier described dependence of the permeation of nucleic acid molecules through the vesicle membranes on the salt concentration is a necessary precondition for both encapsulation and fractionation. Encapsulation is achieved by applying a suitable sample onto a VP column that has been equilibrated with a high-salt buffer. In that buffer the sample molecules are permeable. Immediately after sample application, elution is started with a low-salt buffer, from which the sample molecules are excluded. At the front between the two buffers the permeability changes, and some of the sample molecules distributed inside the vesicles cannot pass through the membranes. These encapsulated molecules can be released by increasing the salt concentration in the eluent. If the encapsulated nucleic acid sample is polydisperse, a stepwise or linear increase in the salt concentration leads to a fractionated release. The fractions obtained differ in their molecular size composition.     

PSSA pore-filled PVDF membranes by simultaneous electron beam irradiation: Preparation and transport characteristics of protons and methanol

Polystyrene sulfonic acid (PSSA) pore-filled poly(vinylidene fluoride) (PVDF) membranes have been prepared using simultaneous electron irradiation method. Porous PVDF films were grafted by pre-swelling in styrene solution and subsequent irradiation with an electron beam (EB) under nitrogen atmosphere and at ambient temperature. The grafted films i.e. polystyrene (PS) pore-filled PVDF were subsequently sulfonated with a diluted mixture of chlorosufonic acid. The effects of the reaction parameters on the content of PS grafted in the pores of PVDF films were investigated. The chemical and morphological properties of the membranes in comparison with their un-grafted and grafted counterparts were studied by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The transport properties of these membranes such as ionic conductivity and methanol permeability were evaluated and correlated with the content of PS grafted in the pores of the PVDF films. The PSSA pore-filled PVDF membranes with PS content in the pores of 40% and above showed superior performance characteristics compared to Nafion 117 membrane and therefore can be potential alternatives to improve the performance of direct methanol fuel cell (DMFC).     

Mimicking the cell membrane with block copolymer membranes

Cell membranes are essential barriers in Nature. To understand their properties and functions and to develop desirable applications, a simple and elegant approach is to study membranes that mimic the cell membrane. Lipid bilayers represent simple models that are physiologically representative when in the form of mixtures of various lipids, but they are not adequately stable even when covered with amphipathic proteins or when combined with polymers, thus preventing technological applications. This makes necessary the design of completely synthetic membranes. In this respect, amphiphilic copolymers that self‐assemble under dilute aqueous conditions and generate supramolecular polymer vesicles or films are ideal candidates for synthetic membranes. Their versatility in terms of chemistry and properties (permeability, mechanical stability, thickness), if appropriately designed, enable the insertion of biological molecules, such as membrane proteins and biopores, or the attachment of biomolecules at their surfaces. Here, we present the domain of synthetic membranes based on amphiphilic copolymers beginning with their generation and up to their applications in medicine, the food industry, and technology. Even though significant progress has been made in combining them with membrane proteins, open questions remain with respect to desired properties that could accommodate biological molecules and support further development of the field, from both the point of view of fundamental understanding and of applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Electrostatic Localization of RNA to Protocell Membranes by Cationic Hydrophobic Peptides

Cooperative interactions between RNA and vesicle membranes on the prebiotic earth may have led to the emergence of primitive cells. The membrane surface offers a potential platform for the catalysis of reactions involving RNA, but this scenario relies upon the existence of a simple mechanism by which RNA could become associated with protocell membranes. Here, we show that electrostatic interactions provided by short, basic, amphipathic peptides can be harnessed to drive RNA binding to both zwitterionic phospholipid and anionic fatty acid membranes. We show that the association of cationic molecules with phospholipid vesicles can enhance the local positive charge on a membrane and attract RNA polynucleotides. This phenomenon can be reproduced with amphipathic peptides as short as three amino acids. Finally, we show that peptides can cross bilayer membranes to localize encapsulated RNA. This mechanism of polynucleotide confinement could have been important for primitive cellular evolution.     

Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane

Two types of poly(phenylene oxide) (PPO) membranes were prepared: one by chemical modification through sulfonation using chlorosulfonic acid and another by physical incorporation with a heteropolyacid (HPA), viz., phosphotungstic acid. These membranes were tested for the separation of CO₂/CH₄ mixtures. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction techniques were used to confirm the modified structure of PPO as well as to understand its interactions with gaseous molecules. Scanning electron microscopy (SEM) was used to investigate the membrane morphology. Thermal stability of the modified polymers was assessed by differential scanning calorimetry (DSC), while the tensile strength was measured to evaluate their mechanical stability. Both chemical and physical modifications did not adversely affect the thermally and mechanical stabilities. Experiments with pure CO₂ and CH₄ gases showed that CO₂ selectivity (27.2) for SPPO increased by a factor of 2.2, while the PPO–HPA membrane exhibited 1.7 times increase in selectivity with a reasonable permeability of 28.2 Barrer. An increase in flux was observed for the binary CO₂/CH₄ mixture permeation with an increasing feed concentration (5–40 mol%) of CO₂. An enhancement in feed pressure from 5 to 40 kg/cm² resulted in reduced CO₂ permeability and selectivity due to the competitive sorption of methane. Both the modified PPO membranes were found to be promising for enrichment of methane despite exhibiting lower permeability values than the pristine PPO membrane.     

Light-regulated permeability of rhodopsin-phospholipid membrane vesicles.

Abstract—Light absorption by rhodopsin in receptor cell membranes initiates the excitation of the receptor cell. Rhodopsin-phospholipid membrane vesicles were studied to localize initial transduction events. Rhodopsin-phospholipid recombinant membranes are thermally stable and light sensitive and may be chemically regenerated after bleaching in the same manner as receptor cell membranes. Rhodopsin-containing vesicles prepared from unsaturated phosphatidylcholine (PCho) or PCho and phosphatidylethanolaminc display kinetics for the metarhodopsin I to II transition which are comparable to those of receptor cell membranes. NMR spectroscopy was used to examine the permeability of the membrane vesicles to added shift (Eu³⁺) or relaxation reagents (Mn²⁺, Co²⁺). Unexposed rhodopsin-phospholipid vesicles are sealed to ion movement and become permeable after light exposure. Selected ions (Ca²⁺, Mn²⁺, Co²⁺) may be photoreleased from the interior of loaded membrane vesicles. The quantity released is proportional to the initial ionic concentration. The number of ions released/rhodopsin bleached is dependent on the light intensity, and high yields (40–160) of Ca²⁺/rhodopsin bleached are observed at low levels of light bleaching. The present results indicate that rhodopsin spans the phospholipid bilayer membrane, and are consistent with an increase in the permeability of the membrane initiated by light excitation of rhodopsin.     

Planar block copolymer membranes by vesicle spreading

An easy route to planar solid-supported polymer membranes by vesicle spreading is described. Pre-organized poly(butadiene)-block-poly(ethylene oxide)(PB-PEO) assemblies were spread on two different supports, i.e. strongly hydrophilic glass surfaces and ultrasmooth gold substrates. Polymer membranes were produced on a hydrophilic support by spreading hydroxyl-functionalized polymer vesicles, while covalently immobilized polymer membranes were obtained by spreading LA-functionalized polymer vesicles on gold substrates. Covalently bound membranes were further incubated with the peptide polymyxin B. Interactions with the polymer membrane were detected by EIS. These systems are of great interest to fundamental membrane science and have potential in technological applications, such as drug screening and (bio)sensing.     

Poly(vinyl alcohol) (PVA)/sulfonated polyhedral oligosilsesquioxane (sPOSS) hybrid membranes for direct methanol fuel cell applications

Organic/inorganic hybrid membranes based on poly(vinyl alcohol) (PVA) and sulfonated polyhedral oligosilsesquioxane (sPOSS), crosslinked by ethylenediaminetetraacetic dianhydride (EDTAD), were prepared as candidate materials for proton exchange membranes in direct methanel fuel cell (DMFC) applications. Fourier transform infrared (FT‐IR) spectroscopy and ion exchange capacity measurements for the prepared networks clearly revealed sPOSS incorporation. We found that proton conductivity increased and methanol permeability decreased with increasing sPOSS content in the hybrid membrane. In particular, our hybrid membranes demonstrated proton conductivities as high as 0.042 S/cm, which is comparable to that of Nafion?, while exhibiting two orders of magnitude lower methanol permeability as compared to Nafion?. We postulate that the polar sulfonic acid groups of the incorporated sPOSS cages assemble to provide ion conduction paths while the hydrophobic portions of the same sPOSS cages combine to form a barrier to methanol permeation with improved thermal stability of the hybrid membrane. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of Sulfonation Degree on Performance of Proton Exchange Membranes for Direct Methanol Fuel Cells

A series of proton exchange membranes based on sulfonated polyarylene ether ketones(SPAEKs) was used to study the effect of sulfonation degree on proton conductivity, methanol permeation and performance of direct methanol fuel cells(DMFCs). Dependences of physical characteristics of the membranes, i. e., proton conductivity, water uptake, swelling ratio, methanol permeability and ion exchange capacity(IEC) were systematically studied. Both methanol permeability and proton conductivity of the SPAEK membrane grow rapidly as the increase in sulfonation degree since methanol molecules and protons share the same transfer channel. However,the methanol permeability plays more important role comparing to proton conductivity. As a result, the SPAEK membrane with a medium sulfonation degree(60%) was found to yield the best performance in a DMFC due to the acquirement of balanced conductivity and methanol permeability.     

A molecular-dynamics study of lipid bilayers: effects of the hydrocarbon chain length on permeability

In this paper, we investigate the effects of the hydrocarbon chain length of lipid molecules on the permeation process of small molecules through lipid bilayers. We perform molecular-dynamics simulations using three kinds of lipid molecules with different chain length: dilauroylphosphatidylcholine, dimyristoylphosphatidylcholine, and dipalmiltoylphosphatidylcholine. Free-energy profiles of O2, CO, NO, and water molecules are calculated by means of the cavity insertion Widom method and the probability ratio method. We show that the lipid membrane with longer chains has a larger and wider energy barrier. The local diffusion coefficients of water across the bilayers are also calculated by the force autocorrelation function method and the velocity autocorrelation function method. The local diffusion coefficients in the bilayers are not altered significantly by the chain length. We estimate the permeability coefficients of water across the three membranes according to the solubility-diffusion model; we find that the water permeability decreases modestly with increasing chain length of the lipid molecules.     

PEO-PPO block copolymer vectors do not interact directly with DNA but with lipid membranes

Small angle neutron (SANS) and light scattering was used to study the interaction between fragments of double stranded deoxyribonucleic acid (DNA) and a synthetic triblock [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)] amphiphilic polymer, known as L64, a potential vector for gene therapy. The mechanism of action of this vector is yet unknown. The contrast variation method was used to separate the partial structure factors of the different components in mixtures of triblock and DNA. It has been found that the copolymer and DNA molecules exhibit repulsive interactions. Further, the interaction between the copolymer and a model lipid membrane was investigated in order to explain the action of the vector. Electrical measurements on black lipid membranes indicated that the main effect of L64 as a vector is to permeabilize the cell's membrane.     

磺化聚醚醚酮/磷钨酸复合膜的导电和甲醇渗透性能

通过磺化反应制备了磺化聚醚醚酮,1H-NMR测试表明其磺化度分别为0.65和0.73.用共混的方法制备了磺化聚醚醚酮/磷钨酸复合质子交换膜.研究了磺化聚醚醚酮的磺化度和磷钨酸的含量对复合膜的吸水性能?电导率,甲醇渗透性能的影响.随着磺化度和磷钨酸含量的增加,电导率逐渐增大,最高达到1.36×10-2S/cm(20℃),高于相同测试条件下NafionR○117膜的电导率(1.0×10-2S/cm).对复合膜的横向和纵向电导率进行了测试和比较,两者相差接近一个数量级.磷钨酸的掺杂虽然没有降低复合膜的甲醇渗透系数,但是仍然都低于相同条件下测得的NafionR○117膜的甲醇渗透系数.     

Shear-induced permeation and fusion of lipid vesicles

This paper introduces a novel approach to controlling membrane permeability in free unilamellar vesicles using shearing in the presence of a detergent with a large head-group to tune pore formation. Such shear-induced permeation could offer a simple means of postencapsulating bioactive molecules to prepare vesicle vectors for drug delivery. Using UV absorption, fluorescence emission, dynamic light scattering, and electron microscopy, we investigated the membrane permeability and the morphology of unilamellar lipid vesicles (diameter in the range 50-400 nm) subjected to a shear stress in the presence of a small amount of nonionic surfactant (Brij 76). Shear-induced leakage and fusion events were observed. We analyzed the significance of the vesicle size, the shear rate, and the surfactant-to-lipid ratio for the observed phenomena. The present approach is evaluated for postloading of preformed vesicles.