Showing papers on "Membrane published in 2000"
TL;DR: An atomic model of human red cell AQP1 is described, providing a possible molecular explanation to a longstanding puzzle in physiology—how membranes can be freely permeable to water but impermeable to protons.
Abstract: Human red cell AQP1 is the first functionally defined member of the aquaporin family of membrane water channels. Here we describe an atomic model of AQP1 at 3.8A resolution from electron crystallographic data. Multiple highly conserved amino-acid residues stabilize the novel fold of AQP1. The aqueous pathway is lined with conserved hydrophobic residues that permit rapid water transport, whereas the water selectivity is due to a constriction of the pore diameter to about 3 A over a span of one residue. The atomic model provides a possible molecular explanation to a longstanding puzzle in physiology-how membranes can be freely permeable to water but impermeable to protons.
1,662 citations
TL;DR: In this paper, the authors present an overview of the synthesis, chemical and electrochemical properties, and polymer electrolyte fuel cell applications of new proton-conducting polymers based on hydrocarbon polymers.
1,476 citations
TL;DR: In this paper, a series of composite membranes based on sulfonated polyether ether ketone with embedded heteropolycompounds were studied and their electrochemical and thermal properties were studied, showing that an increase in degree of sulfonation and introduction of these fillers resulted in increased Tg and enhanced membrane hydrophilicity, bringing about a substantial gain in proton conductivity.
838 citations
TL;DR: In this article, the authors focus on composite membranes which consist of a zeolite top layer on a mesoporous ceramic or metal support, and special attention is given to supported MFI membranes.
779 citations
TL;DR: In this article, the structural evolution of perfluorosulfonated ionomer (PFSI) membranes from dry materials to highly swollen membranes and solutions using mainly small-angle scattering techniques was investigated.
762 citations
TL;DR: Detergent insolubility is an invaluable tool for studying cellular rafts and characterizing their composition and can depend on lipid composition, detergent and temperature.
689 citations
TL;DR: This study suggests that plasma membrane PIP2 controls dynamic membrane functions and cell shape by locally increasing and decreasing the adhesion between the actin-based cortical cytoskeleton and the plasma membrane.
678 citations
TL;DR: In this article, the performance of a thin-film composite aromatic polyamide nanofiltration membrane and its relation to membrane surface charge (electrokinetic) characteristics were investigated.
Abstract: The performance (i.e., water flux and solute rejection) of a thin-film composite aromatic polyamide nanofiltration membrane and its relation to membrane surface charge (electrokinetic) characteristics were investigated. Membrane performance and streaming potential measurements were carried out as a function of pH for several solution chemistries, including an indifferent electrolyte, humic acid, and anionic and cationic surfactants. Performance results for the membrane were interpreted by relating the water flux and salt/ion rejection to the membrane charge characteristics. In the case of the indifferent electrolyte (NaCl), water flux and salt passage were maximal at the membrane pore isoelectric point (pH 5) primarily due to decreased electrostatic repulsion and increased pore volume (size) in the cross-linked polymer network. Ion rejection is directly related to the membrane pore charge and is attributed to co-ion electrostatic repulsion (exclusion). At low pH, negative rejection of protons was observed...
659 citations
TL;DR: This review focuses on the self-aggregation properties of drugs, as well as on their interaction with membranes, and it is seen that drug-membrane interactions are analogous to the interactions between membranes and classical detergents.
639 citations
TL;DR: Both the liquid‐ordered phase characteristics and the sizes of domains match properties of membrane fractions described as detergent‐resistant membranes (DRMs), strongly suggesting that the domains seen are the in vivo correlate of DRMs and thus may be identified as lipid rafts.
Abstract: The lateral motion of single fluorescence labeled lipid molecules was imaged in native cell membranes on a millisecond time scale and with positional accuracy of ∼50 nm, using ‘single dye tracing’. This first application of single molecule microscopy to living cells rendered possible the direct observation of lipid-specific membrane domains. These domains were sensed by a lipid probe with saturated acyl chains as small areas in a liquid-ordered phase: the probe showed confined but fast diffusion, with high partitioning (∼100-fold) and long residence time (∼13 s). The analogous probe with mono-unsaturated chains diffused predominantly unconfined within the membrane. With ∼15 saturated probes per domain, the locations, sizes, shapes and motions of individual domains became clearly visible. Domains had a size of 0.7 μm (0.2–2 μm), covering ∼13% of total membrane area. Both the liquid-ordered phase characteristics and the sizes of domains match properties of membrane fractions described as detergent-resistant membranes (DRMs), strongly suggesting that the domains seen are the in vivo correlate of DRMs and thus may be identified as lipid rafts.
577 citations
TL;DR: The results of the study suggest that the flavonoids and isoflavonoids, similar to cholesterol and alpha-tocopherol, partition into the hydrophobic core of the membrane and cause a dramatic decrease in lipid fluidity in this region of the membranes.
TL;DR: A new tethered polymer-supported planar lipid bilayer system was developed, which permitted us to reconstitute integral membrane proteins in a laterally mobile form and a model for the interaction of these proteins with the underlying polymer is discussed.
TL;DR: Results of numerical calculation for nonlinear steady state convection, developing from the respective instability, are reported along with those for a slightly wavy membrane and also the results of recent experiments with modified membranes.
Abstract: The paper is concerned with convection at an ion exchange electrodialysis membrane induced by nonequilibrium electro-osmosis in the course of concentration polarization under the passage of electric current through the membrane. Derivation of nonequilibrium electro-osmotic slip condition is recapitulated along with the linear stability analysis of quiescent electrodiffusion through a flat ion exchange membrane. Results of numerical calculation for nonlinear steady state convection, developing from the respective instability, are reported along with those for a slightly wavy membrane. Besides these results, we report those of time dependent calculations for periodic and chaotic oscillations, resulting from instability of the respective steady state flows, and also the results of recent experiments with modified membranes. These latter rule in favor of electro-osmotic versus bulk electroconvective origin of overlimiting conductance through ion exchange membranes.
TL;DR: Poliovirus-induced vesicles derive from the ER by the action of viral proteins 2BC and 3A by a mechanism that excludes resident host proteins, which is consistent with an autophagic origin for these membranes.
Abstract: All positive-strand RNA viruses of eukaryotes studied assemble RNA replication complexes on the surfaces of cytoplasmic membranes. Infection of mammalian cells with poliovirus and other picornaviruses results in the accumulation of dramatically rearranged and vesiculated membranes. Poliovirus-induced membranes did not cofractionate with endoplasmic reticulum (ER), lysosomes, mitochondria, or the majority of Golgi-derived or endosomal membranes in buoyant density gradients, although changes in ionic strength affected ER and virus-induced vesicles, but not other cellular organelles, similarly. When expressed in isolation, two viral proteins of the poliovirus RNA replication complex, 3A and 2C, cofractionated with ER membranes. However, in cells that expressed 2BC, a proteolytic precursor of the 2B and 2C proteins, membranes identical in buoyant density to those observed during poliovirus infection were formed. When coexpressed with 2BC, viral protein 3A was quantitatively incorporated into these fractions, and the membranes formed were ultrastructurally similar to those in poliovirus-infected cells. These data argue that poliovirus-induced vesicles derive from the ER by the action of viral proteins 2BC and 3A by a mechanism that excludes resident host proteins. The double-membraned morphology, cytosolic content, and apparent ER origin of poliovirus-induced membranes are all consistent with an autophagic origin for these membranes.
TL;DR: In this paper, the authors describe methanol flux measurements across Nafion, 1100 equivalent weight membranes under conditions of a direct methanoline fuel cell but in which methanols is completely electro-oxidized on the opposite side in an inert atmosphere at sufficiently high electrode potential.
Abstract: This paper describes methanol flux measurements across Nafion, 1100 equivalent weight membranes under conditions of a direct methanol fuel cell but in which methanol is completely electro-oxidized on the opposite side in an inert atmosphere at sufficiently high electrode potential. Both the diffusion coefficient and the methanol concentration in the membrane were determined from the measured transient limiting current density following a potential step. Corrections for electro-osmotic drag effects are developed and found necessary even for low MeOH concentrations. The results agree well with those obtained from nuclear magnetic resonance measurements. The partition coefficient [{rho} = [MeOH]{sub membrane}/[MeOH]{sub solution}] was approximately constant for the membranes in contact with methanol solutions of various concentration and from room temperature to 90 C. The activation energy of methanol diffusion in a fully hydrated Nafion membrane between 30 and 130 C is 4.8 kcal/mol, and that for protonic conduction under the same conditions is 2.3 kcal/mol. For a membrane dried in vacuum at above 100 C, lower values of methanol permeation rate and protonic conductance were found.
TL;DR: This review focuses on the structural features and the mechanism of major efflux pumps of Gram‐negative bacteria, which expel from the cells a remarkably broad range of antimicrobial compounds and produce the characteristic intrinsic resistance of these bacteria to antibiotics, detergents, dyes and organic solvents.
Abstract: A set of multidrug efflux systems enables Gram-negative bacteria to survive in a hostile environment. This review focuses on the structural features and the mechanism of major efflux pumps of Gram-negative bacteria, which expel from the cells a remarkably broad range of antimicrobial compounds and produce the characteristic intrinsic resistance of these bacteria to antibiotics, detergents, dyes and organic solvents. Each efflux pump consists of three components: the inner membrane transporter, the outer membrane channel and the periplasmic lipoprotein. Similar to the multidrug transporters from eukaryotic cells and Gram-positive bacteria, the inner membrane transporters from Gram-negative bacteria recognize and expel their substrates often from within the phospholipid bilayer. This efflux occurs without drug accumulation in the periplasm, implying that substrates are pumped out across the two membranes directly into the medium. Recent data suggest that the molecular mechanism of the drug extrusion across a two-membrane envelope of Gram-negative bacteria may involve the formation of the membrane adhesion sites between the inner and the outer membranes. The periplasmic components of these pumps are proposed to cause a close membrane apposition as the complexes are assembled for the transport.
TL;DR: It is shown that Trp spectra can be obtained in the presence of up to 6 mM LUV that are virtually identical to spectra obtained in buffer alone, which obviates the use of SUV.
TL;DR: In this paper, experimental and simulated data for the diffusion of water across Nafion membranes as a function of the water activity gradient are presented, and the model predictions are very sensitive to the value of the Fickian diffusion coefficient of water.
Abstract: In this paper, experimental and simulated data for the diffusion of water across Nafion membranes as a function of the water activity gradient are presented. The gradient in the activity of water across the membrane was varied by changing the flow rate and pressure of nitrogen gas on one side of the membrane. The other side of the membrane was equilibrated with liquid water. It was found that the model predictions are very sensitive to the value of the diffusion coefficient of water in Nafion. Using the Fickian diffusion coefficient extracted from self-diffusion measurements reported in the literature, the model simulations matched experimental data with less than 5% error over a wide range of operating conditions.
TL;DR: In this paper, the effects of solution chemistry on the rate and extent of adsorption of a protein and humic acid onto a regenerated cellulose ultrafiltration membrane were studied.
TL;DR: Evidence is provided that cyclic nucleotides are physiological substrates for MRP5 and may represent a novel pharmacological target for the enhancement of tissue levels of cGMP.
TL;DR: Freeze-dried membranes could serve as potent candidates for antibiotic delivery in an acidic environment and had superior drug-release properties to air-d dried hydrogels.
TL;DR: Crystallization of the membrane pores provides possibilities for diffraction studies that might provide useful information on the pore structures.
TL;DR: Marked alterations in neural membrane glycerophospholipid composition have been reported to occur in neurological disorders and these processes along with the accumulation of lipid peroxides and compromised energy metabolism may be responsible for the neurodegeneration observed in neurological Disorders.
TL;DR: In this paper, the performance of perovskites based on alkaline-earth containing lanthanum cobaltites was investigated in the system La1−xSrxCo1−yFeyO3−δ.
TL;DR: In this article, the effects of humic acid adsorption, concentration polarization, and aggregate deposition on the rate and extent of fouling were investigated using a broad range of molecular weight cutoffs.
Abstract: Recent studies have shown that natural organic matter (e.g., humic and fulvic acids) is a major foulant during ultrafiltration of surface water. The objective of this study was to develop a more complete understanding of the mechanisms governing humic acid fouling, including the effects of humic acid adsorption, concentration polarization, and aggregate deposition on the rate and extent of fouling. Data were obtained with Aldrich and Suwannee River humic acids using ultrafiltration membranes with a broad range of molecular weight cutoffs. Fouled membranes were also examined using streaming potential and contact angle measurements. The extent of flux decline was greatest for the largest molecular weight cutoff membranes due to the greater relative hydraulic resistance of the humic acid deposit formed on the surface of these membranes. This humic acid deposit reduced the apparent zeta potential and increased the membrane contact angle. Simple static adsorption and concentration polarization caused relatively little flux decline. Humic acid aggregates had a significant effect on fouling only for the larger molecular weight cutoff membranes. The rate and extent of humic acid fouling increased at low pH, high ionic strength, and in the presence of calcium due to changes in intermolecular electrostatic interactions. These results provide important insights into the mechanisms of humic acid fouling during ultrafiltration.
TL;DR: Electrical pulses that cause the transmembrane voltage of fluid lipid bilayer membranes to reach at least U/sub m//spl ap/0.2 V, are hypothesized to create primary membrane "pores" with a minimum radius of -1 nm, to enhance molecular transport through tissues and into their constitutive cells.
Abstract: Electrical pulses that cause the transmembrane voltage of fluid lipid bilayer membranes to reach at least U/sub m//spl ap/0.2 V, usually 0.5-1 V, are hypothesized to create primary membrane "pores" with a minimum radius of -1 nm. Transport of small ions such as Na/sup +/ and Cl/sup -/ through a dynamic pore population discharges the membrane even while an external pulse tends to increase U/sub m/, leading to dramatic electrical behavior. Molecular transport through primary pores and pores enlarged by secondary processes provides the basis for transporting molecules into and out of biological cells. Cell electroporation in vitro is used mainly for transfection by DNA introduction, but many other interventions are possible, including microbial killing. Ex vivo electroporation provides manipulation of cells that are reintroduced into the body to provide therapy. In vivo electroporation of tissues enhances molecular transport through tissues and into their constitutive cells. Tissue electroporation, by longer, large pulses, is involved in electrocution injury. Tissue electroporation by shorter, smaller pulses is under investigation for biomedical engineering applications of medical therapy aimed at cancer treatment, gene therapy, and transdermal drug delivery. The latter involves a complex barrier containing both high electrical resistance, multilamellar lipid bilayer membranes and a tough, electrically invisible protein matrix.
TL;DR: In this paper, the authors applied powerful surface-sensitive techniques to study protein-protein recognition processes at membranes and the control of cell adhesion by the interplay of specific 'lock-and-key' forces and universal interfacial forces.
TL;DR: In this paper, the authors compared direct methanol fuel cells (DMFCs) employing two types of Nafion{reg{underscore}sign} (EW) membranes of different equivalent weight (EW).
Abstract: This paper compares direct methanol fuel cells (DMFCs) employing two types of Nafion{reg{underscore}sign} (E.I.DuPont de Nemours and Company) membranes of different equivalent weight (EW). Methanol and water uptakes in 1,100 and 1,200 EW Nafion membranes were determined by weighing P{sub 2}O{sub 5}-dried and methanol solution-equilibrated membranes. Both methanol and water uptakes in the 1,200 EW membrane were about 70--74% of those in the 1,100 EW membrane. The methanol crossover rate corresponding to that in a DMFC at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell operating conditions. After accounting for the thickness difference between the membrane samples, the methanol crossover rate through a 1,200 EW membrane was 52% of that through an 1,100 EW membrane. To resolve the cathode and anode performances in an operating DMFC, a dynamic hydrogen electrode was used as a reference electrode. Results show that in an operating DMFC the cathode can be easily flooded, as shown in a DMFC using 1,100 EW membrane. An increase in methanol crossover rate decreases the DMFC cathode potential at open circuit. At a high cell current density, the DMFC cathode potential can approach that of a H{sub 2}/air cell.
TL;DR: This strategy of membrane-based patterning, given the acronym MEMPAT for brevity, offers a more convenient way for patterning cells on surfaces and for studying cell spreading than existing methods.
Abstract: We describe the patterning of proteins and cells onto the surfaces of bacteriological Petri dishes, glass, and poly(dimethylsiloxane) (PDMS) with the use of elastomeric lift-off membranesfree-standing polymer films that have circular or square holes with diameters, sides, and height ≥50 μm. Cells are patterned within the physical constraints provided by the holes of the membranes; these constraints can be released to allow the cells to spread onto the rest of the surface or to remain in the pattern by controlling the properties of the surfaces. Careful control of the properties of the surfaces of the substrates are required to cause the cells to adhere to the substrate and not to the membrane, and to avoid damage to the cells on removing the membrane. This strategy of membrane-based patterninggiven the acronym MEMPAT for brevityoffers a more convenient way for patterning cells on surfaces and for studying cell spreading than existing methods.
TL;DR: In this article, an alternating sequence of cationic and anionic polyelectrolytes were prepared by means of electrostatic layer-by-layer adsorption and investigated on their permeability for NaCl, Na2SO4, and MgCl2 in aqueous solution.
Abstract: Ultrathin membranes consisting of an alternating sequence of cationic and anionic polyelectrolytes were prepared by means of electrostatic layer-by-layer adsorption and investigated on their permeability for NaCl, Na2SO4, and MgCl2 in aqueous solution. It is demonstrated that the multi-bipolar structure of the polyelectrolyte membranes favors the separation of mono- and divalent ions by Donnan exclusion of the divalent ions. Various effects on the rate of ion permeation and the selectivity were investigated. Addition of salt to the polyelectrolyte solutions used for membrane preparation led to improved ion separation, while an increase of the pH had the opposite effect. Use of polyelectrolytes with high charge density also improved the ion separation. Especially good results were obtained if membranes containing polyallylamine (PAH) as the cationic polyelectrolyte were used. For 60 layer pairs of PAH/polystyrenesulfonate, for example, a separation factor α for Na+/Mg2+ up to 112.5 and for Cl-/SO42- up to ...