Regulation of transport in artificial membranes by environmental hydrogen-ion concentration

Transport phenomena in biological membranes and in biocatalysis (immobilized enzymes) are influenced by the structure of the matrix. This structure is not invariable but may depend on parameters of adjacent solutions. Weakly acidic ion-exchange membranes are useful tools for modelling ionic influences on such charged matrices. The pH dependence of tracer permeabilities of [³H] water and [¹⁴C] non-electrolytes was determined on polymethacrylic acid/polyhydroxyethylmethacrylate membranes as a function of their cross-linking and hydrophobicity. At lower buffer concentrations, the non-electrolyte permeabilities increase between 20 and 500% if the pH is raised from 6.5 to 7.5. The relative increase depends on membrane properties and the molecular weights of the permeants. Water permeabilities were measured between pH 5 and 8, the whole interval of pH controllability of the ion-exchange membranes used. The results are similar to those for the [¹⁴C] non-electrolytes. This control of permeability experimentally established by pH-variation is consistent with assumptions about the influence of the kind of counter ions (H⁺, Me⁺, Me²⁺) on the degree of dissociation of carboxyl groups within the membrane. Thus the stage of dissociation is responsible for the structure and transport properties of weak ion exchangers.     

Chemical surface, diffusional, electrical and elastic characterizations of two different dense regenerated cellulose membranes

A comparison of NaCl transport across two dense cellulosic membranes from different suppliers is presented. Hydraulic and diffusional permeabilities were determined from volume flow-applied pressure and concentration-time relationships, while cation transport number and membrane conductivity were determined from electromotrice force and impedance spectroscopy measurements, respectively. Chemical surface differences between both membranes are correlated to transport parameters and morphology, but differences in elastic properties of both membranes might also be considered in order to get a more complete picture of membrane behaviors and to obtain structural-transport parameters correlations.     

Gas permeation through water-swollen hydrogel membranes

This work deals with water-swollen hydrogel membranes for potential CO₂ separation applications, with an emphasis on elucidating the role of water in the membrane for gas permeation. A series of hydrogel membranes with a wide range of water contents (0.9–10 g water/g polymer) were prepared from poly(vinyl alcohol), chitosan, carboxyl methyl cellulose, alginic acid and poly(vinylamine), and the permeation of CO₂, H₂, He and N₂ through the membranes at different pressures (200–800 kPa) was studied. The gas permeabilities through the dry dense membranes were measured as well to evaluate the resistance of the polymer matrix in the hydrogel membranes. It was shown that the gas permeability in water-swollen membrane is lower than the gas permeability in water, and the selectivity of the water-swollen membranes to a pair of gases is close to the ratios of their permeabilities in water. The permeability of the water-swollen membranes increases with an increase in the swelling degree of the membrane, and the membrane permeability tends to level off when the water content is sufficiently high. A resistance model was proposed to describe gas permeation through the hydrogel membranes, where the immobilized water retained in the polymer matrix was considered to form transport passageways for gas permeation through the membrane. It was shown that the permeability of hydrogel membranes was primarily determined by the water content in the membrane. The model predictions were consistent with the experimental data for various hydrogel membranes with a wide range of water contents (0.4–10 g water/g polymer).     

A Comparative Study of the Electric Transport of Ions and Water in Sulfonated Cation-Exchange Polymeric Membranes of the New Generation

This work presents the results of the studies concerning the electric transport of ions and water through sulfonated cation-exchange membranes synthesized on the basis of polysulfone (PS) and poly(ether–ether–ketone) (PEEK). The concentration dependences of the water absorption capacity, specific conductance, and diffusion and electroosmotic permeabilities measured in sodium chloride solutions are compared to the analogous characteristics of some commercial membranes (MK-40, MF-4SK, CL-25T) under the same experimental conditions. The model concepts concerning the permeability of ion-conducting membranes as disperse systems are found to be applicable for interpreting the set of the electric transport properties of the membrane samples studied. A cluster–channel type of the membrane structure is identified. The polymeric films based on PS and PEEK are shown to possess characteristics comparable to those of commercial ion-exchange membrane samples and can be recommended for producing polymer compositions with an optimum set of electric transport properties.     

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     

Determining the orientation and localization of membrane-bound peptides

Many naturally occurring bioactive peptides bind to biological membranes. Studying and elucidating the mode of interaction is often an essential step to understand their molecular and biological functions. To obtain the complete orientation and immersion depth of such compounds in the membrane or a membrane-mimetic system, a number of methods are available, which are separated in this review into four main classes: solution NMR, solid-state NMR, EPR and other methods. Solution NMR methods include the Nuclear Overhauser Effect (NOE) between peptide and membrane signals, residual dipolar couplings and the use of paramagnetic probes, either within the membrane-mimetic or in the solvent. The vast array of solid state NMR methods to study membrane-bound peptide orientation and localization includes the anisotropic chemical shift, PISA wheels, dipolar waves, the GALA, MAOS and REDOR methods and again the use of paramagnetic additives on relaxation rates. Paramagnetic additives, with their effect on spectral linewidths, have also been used in EPR spectroscopy. Additionally, the orientation of a peptide within a membrane can be obtained by the anisotropic hyperfine tensor of a rigidly attached nitroxide label. Besides these magnetic resonance techniques a series of other methods to probe the orientation of peptides in membranes has been developed, consisting of fluorescence-, infrared- and oriented circular dichroism spectroscopy, colorimetry, interface-sensitive X-ray and neutron scattering and Quartz crystal microbalance.     

Effects of cross-linkers with different molecular weights in cross-linked Matrimid 5218 and test temperature on gas transport properties

The poly(ethylene oxide) (PEO) was introduced by the cross-linking method in the commercial Matrimid 5218. The two kinds of membranes were prepared from the Matrimid 5218 and the cross-linkers poly(propylene glycol) block poly(ethylene glycol) block poly(propylene glycol) diamine (PPG/PEG/PPGDA) with different molecular weights. The cross-linking reaction process was monitored by FTIR. The cross-linked Matrimid 5218 membranes display excellent CO₂ permeability and CO₂/light gas selectivity. The effects of cross-linkers with different molecular weights on gel content, thermal properties and H₂, CO₂, N₂ and CH₄ gas transport properties were reported. The effect of temperature on gas transport properties was also reported, and the permeabilities of these materials as a function of temperature were compared with other gas membrane materials.     

Sodium chloride concentration by electrodialysis with hybrid organic-inorganic ion-exchange membranes: An investigation of the process

This study examines how conditions for modifying homogeneous MF-4SK and heterogeneous MK-40 membranes with tetraethoxysilane affect membrane properties. The microstructure of the bulk membrane and its surface, both before and after exposure to the modifying agent, is examined by scanning electron microscopy, spark spectrophotometry, and standard contact porosimetry. The process of sodium chloride concentration by electrodialysis with hybrid organic-inorganic membranes in cells with noncirculating concentration compartments is investigated, and a mathematical model of the concentration process by electrodialysis is used to determine transport properties: current efficiency, diffusion and osmotic permeabilities, and the salt hydration number. For highly hydrophilic membranes, it is shown that water transport occurs both in ion hydration shells and also as free water. It is established that after modified membranes undergo additional heat treatment, the transport of free water ceases, and the water transport number decreases. This is in accord with an increase in the salt content of the concentrate during concentration by electrodialysis.     

Electrokinetic parameters of ion transport across isolated pepper cuticular membranes

The transport of NaCl and CaCl₂ solutions across isolated pepper cuticular membranes was studied by means of conductivity, membrane potential and diffusion experiments. Some characteristic membrane parameters such as the electrical resistance, ionic and salt permeabilities were obtained as a function of the electrolyte concentrations. Cuticle morphological asymmetry accounts for differences in membrane potential values under external reverse gradients. The influence of temperature on the membrane structure was also considered, but only small changes in the electrokinetic parameters were obtained. From the NaCl diffusion experiments two activation energies were determined (54.8 kJ/mol for temperature ranging between 15 and 35°C, and 20.6 kJ/mol for the interval of temperature between 40 and 60°C), which could be associated to thermal transitions in the molecular structure of the cuticle for the interval 30–40°C.     

Lipid nanotube formation from streptavidin-membrane binding

A novel transformation of giant lipid vesicles to produce nanotubular structures was observed upon the binding of streptavidin to biotinylated membranes. Unlike membrane budding and tubulation processes caused by proteins involved with endocytosis and vesicle fusion, streptavidin is known to crystallize at near the isoelectric point (pI 5 to 6) into planar sheets against biotinylated films. We have found, however, that at neutral pH membranes of low bending rigidity (<10kT), such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), spontaneously produce tubular structures with widths ranging from micrometers to below the diffraction limit (<250 nm) and lengths spanning up to hundreds of micrometers. The nanotubes were typically held taut between surface-bound vesicles suggesting high membrane tension, yet the lipid nanotubes exhibited a fluidic nature that enabled the transport of entrained vesicles. Confocal microscopy confirmed the uniform coating of streptavidin over the vesicles and nanotubes. Giant vesicles composed of lipid membranes of higher bending energy exhibited only aggregation in the presence of streptavidin. Routes toward the development of these highly curved membrane structures are discussed in terms of general protein-membrane interactions.     

Studies on the Diisocyanate-Modified Polyvinyl Alcohol Reverse Osmosis Membranes

The hydroxyl group in polyvinyl alcohol (PVA) membranes had been partially reacted with diisocyanates such as 4,4′-diphenylmethane diisocyanate (MDI) or the prepolymers containing different functional groups. The transport properties toward water and salt and the stress-strain properties of these modified membranes were investigated. The results showed an improvement in salt rejection and a considerable increase in the wet strength of the modified membrane. The water absorption and permeabilities of these modified membranes depended largely upon the functional groups introduced into them.     

Unsupported palladium alloy foil membranes fabricated by electroless plating

It is desirable to create thin (<25 μm), unsupported, defect-free palladium and palladium alloy foils in a cost-effective manner in order to study intrinsic material properties exclusive of support effects. We have developed a novel technique for producing unsupported palladium films by electroless plating upon mirror-finished stainless steel supports followed by mechanical removal. High quality pure palladium films as thin as 7.2 μm were produced. Single gas steady state permeation experiments were performed using hydrogen and nitrogen to examine permeability and selectivity. The pure palladium membranes showed hydrogen permeabilities comparable to cold-rolled unsupported foils, and high H₂/N₂ selectivity. Palladium-copper membranes were prepared by sequential electroless plating of copper onto palladium foils followed by in situ annealing. The annealing process produces films of desired composition with permeabilities comparable to those in the literature. The annealing process does not appear to produce defects in the film, and the membranes thus produced have performed 15 days without increased leak rates.     

Transport properties of nanofiltration plasticized polyvinyl chloride membranes, molecular sieves

The modification of surfaces of solid-state potentiometric surfactant sensors with nanofiltration membranes (molecular sieves) with different diameters allows the detection of homologues of anionic, cationic, and nonionic surfactants. The quantitative characteristics of the membrane transport (permeability and ion flow) and the separating ability of plasticized polyvinyl chloride molecular sieves are evaluated. The permeabilities of nanofiltration membranes and ion flows through them depend on the nature of the blowing agent and the nature and concentration of the surfactants in the contacting solutions whose variation allows the separation of homologues of sodium alkyl sulfates, alkylpyridinium chlorides, and polyethoxylated nonylphenols in multicomponent mixtures.     

Third‐generation amphiphilic conetworks. III. Permeabilities and mechanical properties

While two of our earlier papers on poly(dimethyl acryl amide)/polymethylhydrosiloxane/polydimethylsiloxane (PDMAAm/PMHS/PDMS) amphiphilic conetworks concerned synthesis and biological properties, respectively, the present contribution focuses on oxygen and insulin permeabilities, and select mechanical properties. We show that by increasing the PDMAAm content from 20 to 60% (i.e., by decreasing the hydrophobic content from 80 to 40%), oxygen permeabilities decrease from ～240 to ～130 barrers. Evidently, oxygen permeability is a function of the sum of the oxyphilic components, PDMS + PMHS, in the conetworks. In contrast, insulin permeability is a function of the hydrophilic component, and reaches a desirable 1.5 × 10^?7 cm²/s at 61% PDMAAm. We also studied the permeabilities of glucose, dextran, and albumin through a PDMAAm₆₁/PMHS₆/PDMS₃₃ membrane and found, unsurprisingly, that the permeability of these molecules follows their hydrodynamic radii, and we project that the permeability of IgG is infinitesimally low. Tensile strengths and ultimate elongations of water‐swollen membranes are also a function of conetwork composition: by increasing the PDMAAm content from 30 to 60%, strengths decrease from 1.6 to 1.2 MPa, and elongations from ～60 to ～40%. Overall, the permeabilities and the mechanical properties of these membranes are appropriate for implantation and, specifically, for immunoisolation of living tissue. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4276–4283, 2007     

Synthesis of Novel N-Ferrocenesulfonyl Paracyclophane

There exists a close relationship between the field of synthetic host-guest chemistry and molecular recognition in biological systems. Artificial host-guest systems have been designed to mimic the binding of substrates by enzymes,antibodies, and receptors and the transport of ions across biological membranes mediated by ionospheres as natural carriers. [ 1]     

Inter-Vesicle Signal Transduction Using a Photo-Responsive Zinc Ionophore

Transmission of chemical information between cells and across lipid bilayer membranes is of profound significance in many biological processes. The design of synthetic signalling systems is a critical step towards preparing artificial cells with collective behaviour. Here, we report the first example of a synthetic inter-vesicle signalling system, in which diffusible chemical signals trigger transmembrane ion transport in a manner reminiscent of signalling pathways in biology. The system is derived from novel ortho-nitrobenzyl and BODIPY photo-caged Zn^II transporters, in which cation transport is triggered by photo-decaging with UV or red light, respectively. This decaging reaction can be used to trigger the release of the cationophores from a small population of sender vesicles. This in turn triggers the transport of ions across the membrane of a larger population of receiver vesicles, but not across the sender vesicle membrane, leading to overall inter-vesicle signal transduction and amplification.     

Physicochemical properties of low molecular weight alkylated chitosans: a new class of potential nonviral vectors for gene delivery

Low molecular weight chitosans grafted with N-/2(3)-(dodec-2-enyl)succinoyl groups (HM-LMW-chitosans) with a mean molecular mass of 5 kDa, a degree of acetylation of 3% and a degree of tetradecenoyl substitution (TDC) of 3–18 mol% have been synthesized. These molecules are monodisperse and soluble in water at neutral pH. Using tensiometry and Nile Red fluorescence, the HM-LMW-chitosans were found to form micelles through hydrophobic interactions involving their tetradecenoyl chains and nonprotonated glucosamine monomers. Their critical micelle concentration decreases with increasing TDC values but varies little with pH and salt. Interaction with large unilamellar vesicles taken as model membranes indicated that HM-LMW-chitosans interact mainly with vesicles mimicking the inner leaflet of biomembranes both through electrostatic and hydrophobic interactions. This preferential interaction may destabilize endosomal membranes and favor the DNA release into the cytoplasm in gene delivery applications. Moreover, since this interaction significantly decreased the membrane fluidity of these vesicles, the HM-LMC-chitosans are thought to exhibit limited lateral mobility and flip-flop ability, and thus, limited cytotoxicity. These properties suggest that the HM-LMW-chitosans may constitute a promising new class of nonviral vectors for gene therapy.     

Ion transport across biomembranes and model membranes

The milestones formerly achieved in the comprehension of ion transport across biological membranes on the basis of electrochemical concepts and/or instrumentation are briefly summarized. The various types of model membranes presently employed for the investigation of ion transport across biomembranes are reviewed and their requirements for the incorporation and functional investigation of membrane proteins are examined. The potential of model membranes for the elucidation of many problems in molecular membrane biology and for the realization of microarray sensors individually addressable to membrane proteins by electrochemical means is assessed.     

Factors influencing the transport of short-chain alcohols through mesoporous gamma-alumina membranes

The pressure-driven transport of water, ethanol, and 1-propanol through supported gamma-alumina membranes with different pore diameters is reported. Water and alcohols had similar permeabilities when they were transported through gamma-alumina membranes with average pore diameters of 4.4 and 6.0 nm, and the permeability coefficient was found to be proportional to the square of pore size, in accordance with a viscous flow mechanism. For transport through membranes with an average diameter of 3.2 nm, the behavior of water was in accordance with the viscous flow mechanism, but the permeability of the membrane for ethanol and 1-propanol was much smaller than expected and could not be explained in terms of viscous flow. Although the low permeability of the membrane with 3.2 nm pores for ethanol and 1-propanol was partly due to the presence of small amounts of water in the alcohols, the permeability coefficients were still substantially smaller when water was absent. This intrinsic difference between water and alcohol may be due to differences in molecular size, chemisorption of alcohols on the oxide pore wall, which would lead to a reduction of the effective pore size, and/or a certain degree of translational ordering of the alcohol molecules inside the membrane pores, which leads to an effectively higher viscosity and, therefore, to a higher transport resistance.