Showing papers on "Membrane published in 2003"
TL;DR: The structure of KvAP, a voltage-dependent K+ channel from Aeropyrum pernix, is presented and a crystal structure of the full-length channel at a resolution of 3.2 Å is determined, which suggests that the voltage-sensor paddles move in response to membrane voltage changes, carrying their positive charge across the membrane.
Abstract: Voltage-dependent K+ channels are members of the family of voltage-dependent cation (K+, Na+ and Ca2+) channels that open and allow ion conduction in response to changes in cell membrane voltage. This form of gating underlies the generation of nerve and muscle action potentials, among other processes. Here we present the structure of KvAP, a voltage-dependent K+ channel from Aeropyrum pernix. We have determined a crystal structure of the full-length channel at a resolution of 3.2 A, and of the isolated voltage-sensor domain at 1.9 A, both in complex with monoclonal Fab fragments. The channel contains a central ion-conduction pore surrounded by voltage sensors, which form what we call 'voltage-sensor paddles'-hydrophobic, cationic, helix-turn-helix structures on the channel's outer perimeter. Flexible hinges suggest that the voltage-sensor paddles move in response to membrane voltage changes, carrying their positive charge across the membrane.
1,758 citations
TL;DR: In this article, a review of the area encompassing modified PFSA membranes, alternative sulfonated polymer and their composite membranes, and acid−base complex membranes is presented. But the authors do not discuss the performance of the composite membranes.
Abstract: The state-of-the-art of polymer electrolyte membrane fuel cell (PEMFC) technology is based on perfluorosulfonic acid (PFSA) polymer membranes operating at a typical temperature of 80 °C. Some of the key issues and shortcomings of the PFSA-based PEMFC technology are briefly discussed. These include water management, CO poisoning, hydrogen, reformate and methanol as fuels, cooling, and heat recovery. As a means to solve these shortcomings, high-temperature polymer electrolyte membranes for operation above 100 °C are under active development. This treatise is devoted to a review of the area encompassing modified PFSA membranes, alternative sulfonated polymer and their composite membranes, and acid−base complex membranes. PFSA membranes have been modified by swelling with nonvolatile solvents and preparing composites with hydrophilic oxides and solid proton conductors. DMFC and H2/O2(air) cells based on modified PFSA membranes have been successfully operated at temperatures up to 120 °C under ambient pressure...
1,595 citations
TL;DR: It is shown that high-resolution fluorescence imaging using two dyes preferentially labelling different fluid phases directly provides a correlation between domain composition and local membrane curvature, which is able to provide experimental estimates of boundary tension between fluid bilayer domains.
Abstract: Lipid bilayer membranes--ubiquitous in biological systems and closely associated with cell function--exhibit rich shape-transition behaviour, including bud formation and vesicle fission. Membranes formed from multiple lipid components can laterally separate into coexisting liquid phases, or domains, with distinct compositions. This process, which may resemble raft formation in cell membranes, has been directly observed in giant unilamellar vesicles. Detailed theoretical frameworks link the elasticity of domains and their boundary properties to the shape adopted by membranes and the formation of particular domain patterns, but it has been difficult to experimentally probe and validate these theories. Here we show that high-resolution fluorescence imaging using two dyes preferentially labelling different fluid phases directly provides a correlation between domain composition and local membrane curvature. Using freely suspended membranes of giant unilamellar vesicles, we are able to optically resolve curvature and line tension interactions of circular, stripe and ring domains. We observe long-range domain ordering in the form of locally parallel stripes and hexagonal arrays of circular domains, curvature-dependent domain sorting, and membrane fission into separate vesicles at domain boundaries. By analysing our observations using available membrane theory, we are able to provide experimental estimates of boundary tension between fluid bilayer domains.
1,535 citations
TL;DR: Comparison with previously reported membranes shows that these microstructurally optimized films have superior performance for the separation of organic mixtures with components that have small differences in size and shape, such as xylene isomers.
Abstract: A seeded growth method for the fabrication of high-permeance, high-separation-factor zeolite (siliceous ZSM-5, [Si 96 O 192 ]-MFI) membranes is reported. The method consists of growing the crystals of an oriented seed layer to a well-intergrown film by avoiding events that lead to a loss of preferred orientation, such as twin overgrowths and random nucleation. Organic polycations are used as zeolite crystal shape modifiers to enhance relative growth rates along the desirable out-of-plane direction. The polycrystalline films are thin (∼1 micrometer) with single grains extending along the film thickness and with large in-plane grain size (∼1 micrometer). The preferred orientation is such that straight channels with an open diameter of ∼5.5 angstroms run down the membrane thickness. Comparison with previously reported membranes shows that these microstructurally optimized films have superior performance for the separation of organic mixtures with components that have small differences in size and shape, such as xylene isomers.
959 citations
TL;DR: In this paper, a general overview of several aspects of the dissolution of amorphous polymers is provided, including experimentally observed dissolution phenomena and mechanisms reported to this date, solubility behavior of polymers and their solvents, models used to interpret and understand polymer dissolution, and techniques used to characterize the dissolution process.
885 citations
TL;DR: The observed flow rates are high, comparable to those through the transmembrane protein aquaporin-1, and are practically independent of the length of the nanotube, in contrast to predictions of macroscopic hydrodynamics.
Abstract: We use molecular dynamics simulations to study osmotically driven transport of water molecules through hexagonally packed carbon nanotube membranes. Our simulation setup comprises two such semipermeable membranes separating compartments of pure water and salt solution. The osmotic force drives water flow from the pure-water to the salt-solution compartment. Monitoring the flow at molecular resolution reveals several distinct features of nanoscale flows. In particular, thermal fluctuations become significant at the nanoscopic length scales, and as a result, the flow is stochastic in nature. Further, the flow appears frictionless and is limited primarily by the barriers at the entry and exit of the nanotube pore. The observed flow rates are high (5.8 water molecules per nanosecond and nanotube), comparable to those through the transmembrane protein aquaporin-1, and are practically independent of the length of the nanotube, in contrast to predictions of macroscopic hydrodynamics. All of these distinct characteristics of nanoscopic water flow can be modeled quantitatively by a 1D continuous-time random walk. At long times, the pure-water compartment is drained, and the net flow of water is interrupted by the formation of structured solvation layers of water sandwiched between two nanotube membranes. Structural and thermodynamic aspects of confined water monolayers are studied.
851 citations
TL;DR: In pressure-driven membrane processes, a pressure exerted on the solution at one side of the membrane serves as a driving force to separate it into a permeate and a retentate as discussed by the authors.
Abstract: In pressure-driven membrane processes (reverse osmosis, nanofiltration, ultrafiltration, and microfiltration) a pressure exerted on the solution at one side of the membrane serves as a driving force to separate it into a permeate and a retentate. The permeate is usually pure water, whereas the retentate is a concentrated solution that must be disposed of or treated by other methods. Membranes may be polymeric, organo-mineral, ceramic, or metallic, and filtration techniques differ in pore size, from dense (no pores) to porous membranes. Depending on the type of technique, salts, small organic molecules, macromolecules, or particles can be retained, and the applied pressure will differ. This paper reviews the principles behind the different techniques, the types of membranes used, rejection mechanisms, and process modeling. Applications of pressure-driven membrane processes are also considered, including reverse osmosis and nanofiltration for the treatment of wastewater from landfills and composting plants, nanofiltration in the textile industry, and ultrafiltration and microfiltration in drinking water production and wastewater treatment.
Lastly, the paper discusses recent developments, including techniques to prevent membrane fouling by modifications affecting surface roughness or hydrophilicity/hydrophobicity, or by cleaning the membranes, and methods for treating or disposing of the retentate.
822 citations
TL;DR: Recent studies have shown that Ras proteins interact dynamically with specific microdomains of the plasma membrane as well as with other internal cell membranes, highlighting the complex interplay between Ras location and function.
Abstract: Ras signalling has classically been thought to occur exclusively at the inner surface of a relatively uniform plasma membrane. Recent studies have shown that Ras proteins interact dynamically with specific microdomains of the plasma membrane as well as with other internal cell membranes. These different membrane microenvironments modulate Ras signal output and highlight the complex interplay between Ras location and function.
816 citations
TL;DR: Histological examination indicates that the rate of epithelialization is increased and the dermis becomes well organized if wounds are covered with electrospun nanofibrous membrane, which has potential applications for wound dressing based upon its unique properties.
Abstract: Produced via electrospinning, polyurethane membrane, which has a unique property, has been of interest in medical fields. Electrospinning is a process by which nanofibers can be produced by an electrostatically driven jet of polymer solution. Electrospun fibers are collected in the form of membranes. The porous structured electrospun membrane is particularly important for its favorable properties: it exudates fluid from the wound, does not build up under the covering, and does not cause wound desiccation. The electrospun nanofibrous membrane shows controlled evaporative water loss, excellent oxygen permeability, and promoted fluid drainage ability, but still it can inhibit exogenous microorganism invasion because its pores are ultra-fine. Histological examination indicates that the rate of epithelialization is increased and the dermis becomes well organized if wounds are covered with electrospun nanofibrous membrane. This electrospun membrane has potential applications for wound dressing based upon its unique properties.
797 citations
TL;DR: DHA's interaction with other membrane lipids, particularly cholesterol, may play a prominent role in modulating the local structure and function of cell membranes.
780 citations
TL;DR: In this article, the authors provide an overview of progress made in the development of proton-conducting hydrocarbon and heterocyclic-based polymers for direct methanol fuel cells and describe the various approaches made to polymer modification/synthesis and salient properties of the materials formed.
Abstract: ▪ Abstract The past 10 years have witnessed a tremendous acceleration in research devoted to non-fluorinated polymer membranes, both as competitive alternatives to commercial perfluorosulfonic acid membranes operating in the same temperature range and with the objective of extending the range of operation of polymer fuel cells toward those more generally occupied by phosphoric acid fuel cells. Important requirements are adequate membrane mechanical strength at levels of functionalization (generally sulfonation) and hydration allowing high proton conductivity, and stability in the aggressive environment of a working fuel cell, in particular thermohydrolytic and chemical stability. This review provides an overview of progress made in the development of proton-conducting hydrocarbon and heterocyclic-based polymers for proton exchange and direct methanol fuel cells and describes the various approaches made to polymer modification/synthesis and salient properties of the materials formed, including those relati...
TL;DR: In this article, the authors used infrared-laser atomic-force microscopy to reveal the native arrangement of rhodopsin, which forms paracrystalline arrays of dimers in mouse disc membranes.
Abstract: Neat rows of paired photon receptors are caught on camera in their natural state. In vertebrate retinal photoreceptors, the rod outer-segment disc membranes contain densely packed rhodopsin molecules for optimal light absorption and subsequent amplification by the visual signalling cascade1, but how these photon receptors are organized with respect to each other is not known. Here we use infrared-laser atomic-force microscopy to reveal the native arrangement of rhodopsin, which forms paracrystalline arrays of dimers in mouse disc membranes. The visualization of these closely packed rhodopsin dimers in native membranes gives experimental support to earlier inferences about their supramolecular structure2,3 and provides insight into how light signalling is controlled.
TL;DR: The phenomenology and the pathways of the well-characterized reactions of biological remodeling, such as fusion mediated by influenza hemagglutinin, are compared with those studied for protein-free bilayers and some proteins involved in fusion and fission are considered.
Abstract: Disparate biological processes involve fusion of two membranes into one and fission of one membrane into two. To formulate the possible job description for the proteins that mediate remodeling of biological membranes, we analyze the energy price of disruption and bending of membrane lipid bilayers at the different stages of bilayer fusion. The phenomenology and the pathways of the well-characterized reactions of biological remodeling, such as fusion mediated by influenza hemagglutinin, are compared with those studied for protein-free bilayers. We briefly consider some proteins involved in fusion and fission, and the dependence of remodeling on the lipid composition of the membranes. The specific hypothetical mechanisms by which the proteins can lower the energy price of the bilayer rearrangement are discussed in light of the experimental data and the requirements imposed by the elastic properties of the bilayer.
TL;DR: In this paper, a microdevice consisting of two adjacent symmetric silicon nitride membranes suspended by long silicon-nitride beams for measuring thermophysical properties of one-dimensional manostructures (nanotubes, nanowires, and mmobelts) bridging the two membranes is fabricated.
Abstract: We have batch-fabricated a microdevice consisting of two adjacent symmetric silicon nitride membranes suspended by long silicon nitride beams for measuring thermophysical properties of one-dimensional manostructures (nanotubes, nanowires, and mmobelts) bridging the two membranes. A platinum resistance heater/thermometer is fabricated on each membrane. One membrane can be Joule heated to cause heat conduction through the sample to the other membrane. Thermal conductance, electrical conductance, and Seebeck coefficient can be measured using this microdevice in the temperature range of 4-400 K of an evacuated Helium cryostat. Measurement sensitivity, errors, and uncertainty are discussed. Measurement results of a 148 nm and a 10 nm-diameter single wall carbon nanotube bundle are presented.
TL;DR: Carbon molecular sieves (CMSs) have been incorporated into two different polymer matrices to form mixed matrix membrane films for gas separations as mentioned in this paper, which have an intrinsic CO2/CH4 selectivity of 200 with a CO2 permeability of 44 −Barrers and an O2/N2 selectivity with 133 with an O 2 permeability at 35 −C.
TL;DR: The results suggest that the photokilling reaction is initiated by a partial decomposition of the outer membrane, followed by disordering of the cytoplasmic membrane, resulting in cell death.
Abstract: In order to elucidate the mechanism for photokilling of Escherichia coli (E. coli) cells on titanium dioxide (TiO2) thin film, the survival of intact cells and the spheroplasts was investigated as a function of photo-illumination time. The photokilling reaction for intact cells was observed to involve two steps, an initial lower rate photokilling step followed by a higher rate one. In contrast, the reaction for spheroplasts, which do not have cell wall, exhibited only a single step kinetics with a higher rate, suggesting that the cell wall of E. coli cell acts as a barrier to the photokilling process. Changes in concentration of the cell wall components during illumination further showed that the outer membrane serves as a barrier, while the peptidoglycan layer does not have a barrier function. Moreover, atomic force microscopy measurements of cells on illuminated TiO2 film showed that the outer membrane decomposed first, and with further illumination, the cells completely decomposed. These results suggest that the photokilling reaction is initiated by a partial decomposition of the outer membrane, followed by disordering of the cytoplasmic membrane, resulting in cell death.
TL;DR: It is found that the detergents differ considerably in their ability to selectively solubilize membrane proteins and to enrich sphingolipids and cholesterol over glycerophospholipid as well as saturated over unsaturated phosphatidylcholine.
Abstract: Partial resistance of cell membranes to solubilization with mild detergents and the analysis of isolated detergent-resistant membranes (DRMs) have been used operationally to define membrane domains. Given the multitude of detergents used for this purpose, we sought to investigate whether extraction with different detergents might reflect the same underlying principle of domain formation. We therefore compared the protein and lipid content of DRMs prepared with a variety of detergents from two cell lines. We found that the detergents differ considerably in their ability to selectively solubilize membrane proteins and to enrich sphingolipids and cholesterol over glycerophospholipids as well as saturated over unsaturated phosphatidylcholine. In addition, we observed cell type-dependent variations of the molecular characteristics of DRMs and the effectiveness of particular detergents. These results make it unlikely that different detergents reflect the same aspects of membrane organization and underscore both the structural complexity of cell membranes and the need for more sophisticated analytical tools to understand their architecture.
TL;DR: A method that allows for the concurrent proteomic analysis of both membrane and soluble proteins from complex membrane-containing samples and coupling protease protection strategies to this method permits characterization of the relative sidedness of the hydrophilic domains of membrane proteins.
Abstract: We describe a method that allows for the concurrent proteomic analysis of both membrane and soluble proteins from complex membrane-containing samples. When coupled with multidimensional protein identification technology (MudPIT), this method results in (i) the identification of soluble and membrane proteins, (ii) the identification of post-translational modification sites on soluble and membrane proteins, and (iii) the characterization of membrane protein topology and relative localization of soluble proteins. Overlapping peptides produced from digestion with the robust nonspecific protease proteinase K facilitates the identification of covalent modifications (phosphorylation and methylation). High-pH treatment disrupts sealed membrane compartments without solubilizing or denaturing the lipid bilayer to allow mapping of the soluble domains of integral membrane proteins. Furthermore, coupling protease protection strategies to this method permits characterization of the relative sidedness of the hydrophilic domains of membrane proteins.
TL;DR: In this paper, the nanoscale structure of composite polyamide reverse osmosis (RO) and nanofiltration (NF) membranes was investigated by transmission electron microscopy and atomic force microscopy.
Abstract: The nanoscale structure of composite polyamide reverse osmosis (RO) and nanofiltration (NF) membranes was investigated by transmission electron microscopy and atomic force microscopy. The study demonstrated that the polymer density and charge are distributed across the active polyamide layer in a highly nonuniform fashion. The polyamide films appear to be built of a negatively charged outer layer sitting on top of an inner layer possessing a small positive charge. This picture appears to be fairly general for all types of composite membranes and seems to reconcile previously reported contradictory experimental facts concerning measurements of charge for this type of membrane. The sharp boundary between the layers roughly corresponds to the region of the highest polymer density, that is, the actual selective barrier. The location of this barrier deep inside the RO films indicates that formation of the RO polyamide is not limited solely by monomer diffusion through the film, as was suggested previously, but...
TL;DR: A model that accounts for both hindered diffusion of salt ions and altered hydrodynamics within colloidal deposit layers is presented and successfully links permeate flux and salt rejection to cake-enhanced concentration polarization and provides new insight into the mechanisms through which salt-rejecting membranes foul.
Abstract: Results from well-controlled colloidal fouling experiments with reverse osmosis (RO) and nanofiltration (NF) membranes suggest the existence of a new source of flux decline for salt-rejecting membr...
TL;DR: In this paper, a self-assembly of TiO2 nanoparticles with photocatalytic destructive capability on microorganisms was devised as a novel means to reduce membrane biofouling.
TL;DR: Calcium was shown to enhance the tendency of SLB formation for negatively charged and zwitterionic vesicles, and the role of vesicle-support, interbilayer, and intrabilayer interactions in the formation of SLBs is discussed.
TL;DR: This work reports significant advances in non-gel shotgun methods using liquid chromatography coupled to mass spectrometry (LC/MS) that facilitate the identification of large numbers of membrane proteins and modifications, and have the potential to provide insights into protein topology and orientation in membranes.
Abstract: Membrane proteins perform some of the most important functions in the cell, including the regulation of cell signaling through surface receptors, cell-cell interactions, and the intracellular compartmentalization of organelles. Recent developments in proteomic strategies have focused on the inclusion of membrane proteins in high-throughput analyses. While slow and steady progress continues to be made in gel-based technologies, significant advances have been reported in non-gel shotgun methods using liquid chromatography coupled to mass spectrometry (LC/MS). These latter strategies facilitate the identification of large numbers of membrane proteins and modifications, and have the potential to provide insights into protein topology and orientation in membranes.
TL;DR: The present picture of cell membranes as lipid bilayers is the legacy of a century's study that concentrated on the lipids and proteins of cell-surface membranes.
Abstract: Our present picture of cell membranes as lipid bilayers is the legacy of a century's study that concentrated on the lipids and proteins of cell-surface membranes. Recent work is changing the picture and is turning the snapshot into a video.
TL;DR: In this paper, the main obstacles to greater commercialization of polymer electrolyte fuel cells are mostly related to the low-proton conductivity at low-relative humidity of the known ionomeric membranes, to their high methanol permeability and poor mechanical properties above ∼130°C.
Abstract: ▪ Abstract The main obstacles to greater commercialization of polymer electrolyte fuel cells are mostly related to the low-proton conductivity at low-relative humidity of the known ionomeric membranes, to their high methanol permeability and poor mechanical properties above ∼130°C. A possible solution for these problems has been found in the development of composite membranes, where particles of suitable fillers are dispersed in the ionomer matrix. The preparation methods for obtaining composite membranes are described, and recent work dealing with composite ionomeric membranes containing silica, heteropolyacids, layered metal phosphates, and phosphonates is reviewed. Finally, new strategies for the preparation of nano-composite membranes and for the filling of porous polymeric membranes with highly conductive zirconium phosphonates are described. The expected influence of size and orientation of these particles on membrane properties, such as conductivity and permeability to methanol, is also discussed.
TL;DR: The use of gas bubbles and slugs in microfiltration and ultra-filtration with flow inside tubes and fibres, across flat sheets and outside fibres is discussed in this article.
TL;DR: The main effects observed are a significant ordering of the DPPC chains (as monitored by NMR type order parameters), a reduced fraction of gauche bonds, a reduced surface area per lipid, less undulations, and a reduced lateral diffusion of DPPC-lipids as well as cholesterols.
TL;DR: A cellular context for understanding Arf6 function is provided and the differences and similarities with the Arf1 structures provide some insight into mechanisms of activation and interaction with effectors.
TL;DR: In this paper, a survey of the material properties that govern the performance of mixed-conducting membranes in real operating conditions and highlights significant developments in the field is presented, where the authors mainly focus on material properties.
TL;DR: In this article, a model for predicting the Derjaguin−Landau−Verwey−Overbeek (DLVO) interaction energy between colloidal particles and rough membrane surfaces is developed.
Abstract: Recent experimental investigations suggest that interaction of colloidal particles with polymeric membrane surfaces is influenced by membrane surface morphology (roughness). To better understand the consequences of surface roughness on colloid deposition and fouling, it is imperative that models for predicting the Derjaguin−Landau−Verwey−Overbeek (DLVO) interaction energy between colloidal particles and rough membrane surfaces be developed. We present a technique of reconstructing the mathematical topology of polymeric membrane surfaces using statistical parameters derived from atomic force microscopy roughness analyses. The surface element integration technique is used to calculate the DLVO interactions between spherical colloidal particles and the simulated (reconstructed) membrane surfaces. Predictions show that the repulsive interaction energy barrier between a colloidal particle and a rough membrane is lower than the corresponding barrier for a smooth membrane. The reduction in the energy barrier is ...