Showing papers on "Membrane published in 1995"

Calixarenes, Macrocycles with (Almost) Unlimited Possibilities

Abstract: The condensation reaction between p-tert-butylphenol and formaldehyde leads in a single step to good yields of cyclic oligomers in which, depending on the reaction conditions, either four, six, or eight phenol units are joined by methylene bridges. The beakerlike shape of the most stable conformation of the tetramer has led to their being given the name “calixarenes” (calix = chalice). Resorcinol can undergo condensation in a similar manner with a variety of aldehydes to afford cyclic tetramers with the same basic structure (the resorcarenes). In both cases the reaction does not require the use of dilution techniques, so that large quantities of product can be readily obtained. In addition, the parent compounds can be modified in various ways, in particular at the phenolic hydroxy groups or the phenyl residues; these approaches can be used separately or in combination. Calixarenes are thus ideal starting materials for the synthesis of various types of host molecules and can also act as building blocks for the construction of larger molecular systems with defined structures and functions. Their potential applications range from use as highly specific ligands for analytical chemistry, sensor techniques and medical diagnostics to their use in the decontamination of waste water and the construction of artificial enzymes and the synthesis of new materials for non-linear optics or for ultrathin layers and sieve membranes with molecular pores.

Thermal adaptation in biological membranes: is homeoviscous adaptation the explanation?

Abstract: The phase behavior and physical properties of lipids in biological membranes are exquisitely sensitive to changes in temperature (50). Because membranes (a) act as physical barriers to solute diffusion, (b) mediate the transmembrane movement of specific solutes, (c) regulate the utilization of energy stored in transmembrane ion gradients, Cd) provide an organizing matrix for the assem­ bly of multicomponent metabolic and signal transduction pathways, and (e) supply precursors for the generation of lipid-derived second messengers, tem­ perature-induced perturbations in membrane organization pose a serious chal­ lenge to the maintenance of physiological function in poikilotherms. However, poikilotherms exploit the diversity of lipid structure to fashion membranes with physical properties appropriate to their thermal circumstance and, in this way, restore membrane function following thermal challenge. Based on the finding that membrane lipids of Escherichia coli grown at 43 and 15°C dis­ played similar physical properties when compared at their respective growth temperatures, Sinensky concluded that membrane fluidity was defended as growth temperature changes and referred to this cellular homeostatic response as homeoviscous adaptation CHV A) (94). Since the original exposition of this hypothesis, HV A has emerged as the most commonly employed paradigm to assess the efficacy of thermal adaptation in biological membranes and to explain patterns of temperature-induced change in membrane lipid composi-

Metal Nanotubule Membranes with Electrochemically Switchable Ion-Transport Selectivity

Abstract: Membranes containing cylindrical metal nanotubules that span the complete thickness of the membrane are described. The inside radius of the nanotubules can be varied at will; nanotubule radii as small as 0.8 nanometer are reported. These membranes show selective ion transport analogous to that observed in ion-exchange polymers. Ion permselectivity occurs because excess charge density can be present on the inner walls of the metal tubules. The membranes reject ions with the same sign as the excess charge and transport ions of the opposite sign. Because the sign of the excess charge on the tubule can be changed potentiostatically, a metal nanotubule membrane can be either cation selective or anion selective, depending on the potential applied to the membrane.

Structural basis for sugar translocation through maltoporin channels at 3.1 A resolution.

Abstract: Trimeric maltoporin (LamB protein) facilitates the diffusion of maltodextrins across the outer membrane of Gram-negative bacteria. The crystal structure of maltoporin from Escherichia coli, determined to a resolution of 3.1 angstroms, reveals an 18-stranded, antiparallel beta-barrel that forms the framework of the channel. Three inwardly folded loops contribute to a constriction about halfway through the channel. Six contingent aromatic residues line the channel and form a path from the vestibule to the periplasmic outlet. Soaking of a crystal with maltotriose revealed binding of the sugar to this hydrophobic track across the constriction, which suggests that maltose and linear oligosaccharides may be translocated across the membrane by guided diffusion along this path.

Alteration of the Myometrial Plasma Membrane Cholesterol Content with .beta.-Cyclodextrin Modulates the Binding Affinity of the Oxytocin Receptor

Abstract: To investigate the effect of cholesterol on the oxytocin receptor function in myometrial membranes, we developed a new method to alter the membrane cholesterol content. Using a methyl-substituted beta-cyclodextrin, we were able to selectively deplete the myometrial plasma membrane of cholesterol. Vice versa, incubating cholesterol-depleted membranes with a preformed soluble cholesterol-methyl-beta-cyclodextrin complex restored the cholesterol content of the plasma membrane. Binding experiments showed that, with the removal of cholesterol from the membrane, the dissociation constant for [3H]oxytocin is enhanced 87-fold (from Kd = 1.5 nM to Kd = 131 nM), therefore shifting the oxytocin receptor from high to low affinity. Increasing the cholesterol content of the cholesterol-depleted membrane again restored the high-affinity binding (Kd = 1.2 nM). The presence of 0.1 mM GTP gamma S did not significantly change the number of high-affinity binding sites for [3H]oxytocin in native plasma membranes, in membranes depleted of cholesterol, and in plasma membranes with restored cholesterol content. The number of high-affinity binding sites for the oxytocin antagonist [3H]PrOTA was dependent in the same way on the cholesterol content as for [3H]oxytocin. Substitution of the membrane cholesterol with other steroids showed a strong dependence of the oxytocin receptor function on the structure of the cholesterol molecule. The detergent-solubilized oxytocin receptor was not saturable with [3H]oxytocin even at concentrations up to 10(-6) M of radioligand. Addition of the cholesterol-methyl-beta-cyclodextrin complex to the detergent-solubilized oxytocin receptor induced a saturation of the solubilized binding sites (Bmax = 0.98 pmol/mg) for oxytocin (Kd = 16 nM).(ABSTRACT TRUNCATED AT 250 WORDS)

Neovascularization of synthetic membranes directed by membrane microarchitecture

Abstract: Transplantation of tissues enclosed within a membrane device designed to protect the cells from immune rejection (immunoisolation) provides an opportunity to treat a variety of disease conditions. Successful implementation of immunoisolation has been hampered by the foreign-body reaction to biomaterials. We screened a variety of commercially available membranes for foreign-body reactions following implantation under the skin of rats. Histologic analysis revealed that neovascularization at the membrane-tissue interface occurred in several membranes that had pore sizes large enough to allow complete penetration by host cells (0.8-8 microns pore size). When the vascularization of the membrane-tissue interface of 5-microns-pore-size polytetrafluoroethylene (PTFE) membranes was compared to 0.02-microns-pore-size PTFE membranes, it was found that the larger pore membranes had 80-100-fold more vascular structures. The increased vascularization was observed even though the larger pore membrane was laminated to a smaller pore inner membrane to prevent cell entry into the prototype immunoisolation device. This significantly higher level of vascularization was maintained for 1 year in the subcutaneous site in rats.

Membrane separations technology : principles and applications

Abstract: 1 Microfiltration and ultrafiltration (W Eykamp) 2 Polarization phenomena and membrane fouling (MHV Mulder) 3 Vapor permeation (Y Cen, RN Lichtenthaler) 4 Reverse osmosis (CJD Fell) 5 Pervaporation (J Neel) 6 Electrodialysis and related processes (H Strathmann) 7 Liquid membranes (liquid pertraction) (L Boyadzhiev, Z Lazarova) 8 Membrane bioseparations (SL Matson) 9 Food and beverage industry applications (M Cheryan, JR Alvarez) 10 Membrane contactors (BW Reed, MJ Semmens, EL Cussler) 11 Analysis and design of membrane permeators for gas separation (A Sengupta, KK Sirkar) 12 Gas separation using inorganic membranes (K Keizer, RJR Uhlhorn, AJ Burggraaf) 13 Economics of gas separation membrane processes (R Spillman) 14 Catalytic membrane reactors (JL Falconer, RD Noble, D Sperry) Subject index

Freeze-fracture replica electron microscopy combined with SDS digestion for cytochemical labeling of integral membrane proteins. Application to the immunogold labeling of intercellular junctional complexes

Abstract: We propose a new electron microscopic method, the sodium dodecylsulphate (SDS)-digested freeze-fracture replica labeling technique, to study the two-dimensional distribution of integral membrane proteins in cellular membranes. Unfixed tissue slices were frozen with liquid helium, freeze-fractured, and replicated in a platinum/carbon evaporator. They were digested with 2.5% SDS to solubilize unfractured membranes and cytoplasm. While the detergent dissolved unfractured membranes and cytoplasm, it did not extract fractured membrane halves. After SDS-digestion, the platinum/carbon replicas, along with attached cytoplasmic and exoplasmic membrane halves, were processed for cytochemical labeling, followed by electron microscopic observation. As an initial screening, we applied this technique to the immunogold labeling of intercellular junction proteins: connexins (gap junction proteins), occludin (tight junction protein), desmoglein (desmosome protein), and E-cadherin (adherens junction protein). The immunogold labeling was seen superimposed on the image of a fracture face visualized by platinum/carbon shadowing. The immunoreaction was specific, and only the structures where the proteins were expected were labeled. For instance, anti-occludin immunogold complexes were observed immediately adjacent to the tight junction strands on the protoplasmic and exoplasmic fracture faces. No significant levels of gold label were associated with non-tight-junctional regions of plasma membranes. The procedures of the SDS-digested freeze-fracture replica labeling and its potential significance are discussed.

Lipids in biological membrane fusion.

Abstract: The results reviewed suggest that membrane fusion in diverse biological fusion reactions involves formation of some specific intermediates: stalks and pores. Energy of these intermediates and, consequently, the rate and extent of fusion depend on the propensity of the corresponding monolayers of membranes to bend in the required directions. Proteins and peptides can control the bending energy of membrane monolayers in a number of ways. Monolayer lipid composition may be altered by different phospholipases [50, 85, 90], flipases and translocases [4, 50]. Proteins and peptides can change monolayer spontaneous curvature or hydrophobic void energy by direct interaction with membrane lipids [20, 32, 111]. Proteins may also provide some barriers for lipid diffusion in the plane of the monolayer [83, 141]. If diffusion of lipids at some specific membrane sites (e.g., in the vicinity of fusion protein) is somehow hindered, the energy of the bent fusion intermediates would reflect the elastic properties of these particular sites rather than the spontaneous curvature of the whole monolayers. Proteins may deform membranes while bringing them locally into close contact. The alteration of the geometric (external) curvature will certainly change the elastic energy of the initial state and, thus affect the energetic barriers of the formation of the intermediates [143]. In addition, the area and the energy of the stalk can be reduced by preliminary bending of the contacting membranes [111]. The possible effects of proteins and polymers on local elastic properties and local shapes of the membranes have been recently analyzed [22, 39, 45, 63]. These studies may provide a good basis for future development of theoretical models of protein-mediated fusion. Various models for biological fusion have been presented as hypothetical sequences of intermediate conformations of proteins, with membrane lipids just covering the empty spaces between the proteins. Although the results discussed above do not allow us to draw an allexplaining cartoon of the fusion mechanism, they do indicate which properties of membrane lipid bilayers (if modified by fusion proteins) would get these bilayers to fuse. In addition, these data suggest a specific geometry to bent fusion intermediates (stalks and pores) and imply a contribution by lipids to the energy of these intermediates. We think that the synthesis of rapidly developing structural information on fusion proteins with the analysis of the physics of membrane rearrangement may soon yield a real understanding of the fascinating and fundamental phenomenon of membrane fusion.

Handbook of Industrial Membranes

Abstract: Introduction to Membrane Separations. Introduction. Contamination, particle size and separation. Membrane separation processes. Polarisation and fouling. Module designs. Membrane process equipment. Electrodialysis cell stacks and design. Laboratory equipment. Membrane Materials, Preparation and Characterisation. Introduction. Characterisation of membranes. Electrodialysis and ion exchange membranes. Gas Separations. Air and Gas Filtration and Cleaning. Separation of Liquid Mixtures/Pervaporation. Separation of Organic Vapour/Air Mixtures. Microfiltration. Analytical Application of Membranes. Water Desalination. Water Purification. Introduction. Laboratory water purification. Industrial Waste Water and Effluent Treatment. Introduction. Pulp, paper and textile industries. Absorption, Desorption and Extraction with Membranes. Waste Water Treatment and Liquid Membranes. Biotechnology and Medical Applications. Medical Applications. Recovery of Salts, Acids and Bases. Food Industry. Membranes for Electrochemical Cells. Electrokinetic Separations. Appendix. Indices.

Dielectrophoretic separation of cancer cells from blood

Abstract: Measurements have demonstrated that the dielectric properties of cells depend on their type and physiological status. For example, MDB-MB-231 human breast cancer cells were found to have a mean plasma membrane specific capacitance value of 26 nF/m/sup 2/, more than double the value observed for resting T-lymphocytes. When an inhomogeneous DC electric field is applied to a particle, a dielectrophoretic (DEP) force arises that depends on the particle dielectric properties. Therefore, cell having different dielectric characteristics will experience differential DEP forces when subjected to such a field. In this article we demonstrate the use of these differential DEP forces for the reparation of several different cancerous cell types from blood. These separations were accomplished using thin, flat chambers having microelectrode arrays on the bottom wall. DEP forces generated by the application of AC fields to the electrodes were used to influence the rate of elution of cells from the chamber by hydrodynamic forcer from a parabolic fluid flow profile. Electrorotation measurements were first made on the various cell types found within cell mixtures to be separated and theoretical modelling was used to derive the cell dielectric parameters. Optimum separation conditions were then predicted from the frequency and suspension conductivity dependencies of cell DEP responses defined by these dielectric parameters. Cell separations were then undertaken for various ratios of cancerous to normal cells at different concentrations. Fluted cells were characterized in terms of separation efficiency, cell viability, and separation speed. For example, after the separation of mixtures of MDA-MB-231 cells and normal whole human blood (ratio 1:3), the eluted normal blood cell fraction contained less than 0.01% of the starting concentration of cancerous cells, cell viability was not compromised, and separation speeds of at least 10/sup 3/ cells/sec were achieved. Theoretical and experimental criteria for the design and operation of such separators are presented.

The relationship between membrane fluidity and permeabilities to water, solutes, ammonia, and protons.

Abstract: Several barrier epithelia such as renal collecting duct, urinary bladder, and gastric mucosa maintain high osmotic pH and solute gradients between body compartments and the blood by means of apical membranes of exceptionally low permeabilities. Although the mechanisms underlying these low permeabilities have been only poorly defined, low fluidity of the apical membrane has been postulated. The solubility diffusion model predicts that lower membrane fluidity will reduce permeability by reducing the ability of permeant molecules to diffuse through the lipid bilayer. However, little data compare membrane fluidity with permeability properties, and it is unclear whether fluidity determines permeability to all, or only some substances. We therefore studied the permeabilities of a series of artificial large unilamellar vesicles (LUV) of eight different compositions, exhibiting a range of fluidities encountered in biological membranes. Cholesterol and sphingomyelin content and acyl chain saturation were varied to create a range of fluidities. LUV anisotropy was measured as steady state fluorescence polarization of the lipophilic probe DPH. LUV permeabilities were determined by monitoring concentration-dependent or pH-sensitive quenching of entrapped carboxyfluorescein on a stopped-flow fluorimeter. The relation between DPH anisotropy and permeability to water, urea, acetamide, and NH3 was well fit in each instance by single exponential functions (r > 0.96), with lower fluidity corresponding to lower permeability. By contrast, proton permeability correlated only weakly with fluidity. We conclude that membrane fluidity determines permeability to most nonionic substances and that transmembrane proton flux occurs in a manner distinct from flux of other substances.

Dense ceramic membranes for partial oxidation of methane to syngas

Abstract: Several perovskite-type oxides (ABO3) containing transition metals on the B-site show mixed (electronic/ionic) conductivity. These mixed-conductivity oxides are promising materials for oxygen-permeating membranes that can operate without electrodes or external electrical circuitry. Oxides in the system LaSrFeCoO permeate large amounts of oxygen, and extruded tubes of these materials have been evaluated in a reactor operating at ca. 850°C for direct conversion of methane into syngas (CO + H2) in the presence of a reforming catalyst. Methane conversion efficiencies of > 99% were observed, and some of the reactor tubes have been operated for over 1000 h. Membrane tubes were fabricated from calcined powders by a plastic extrusion technique. Ceramic powders in the LaSrFeCoO system were made by solid-state reaction of the constituent carbonates, oxides, and/or nitrates. The chemical-phase behavior of the ceramic powders with varying stoichiometries were studied by high-temperature in-situ X-ray diffraction (XRD) as a function of oxygen partial pressure. The sintered extruded tubes were also characterized by XRD and scanning electron microscopy.

Solid multi-component membranes, electrochemical reactor components, electrochemical reactors and use of membranes, reactor components, and reactor for oxidation reactions

Abstract: Solid membranes comprising an intimate, gas-impervious, multi-phase mixture of an electronically-conductive material and an oxygen ion-conductive material and/or a mixed metal oxide of a perovskite structure are described. Electrochemical reactor components, such as reactor cells, and electrochemical reactors are also described for transporting oxygen from any oxygen-containing gas to any gas or mixture of gases that consume oxygen. The reactor cells generally comprise first and second zones separated by an element having a first surface capable of reducing oxygen to oxygen ions, a second surface capable of reacting oxygen ions with an oxygen-consuming gas, an electron-conductive path between the first and second surfaces and an oxygen ion-conductive path between the first and second surfaces. The element may further comprise (1) a porous substrate, (2) an electron-conductive metal, metal oxide or mixture thereof and/or (3) a catalyst. The reactor cell may further comprise a catalyst in the zone which comprises a passageway from an entrance end to an exit end of the element. Processes described which may be conducted with the disclosed reactor cells and reactors include, for example, the partial oxidation of methane to produce unsaturated compounds or synthesis gas, the partial oxidation of ethane, substitution of aromatic compounds, extraction of oxygen from oxygen-containing gases, including oxidized gases, ammoxidation of methane, etc. The extraction of oxygen from oxidized gases may be used for flue or exhaust gas cleanup.

The Plasma Membrane H+-ATPase (A Highly Regulated Enzyme with Multiple Physiological Functions)

Abstract: The proton-pump ATPase (H+-ATPase) of the plant plasma membrane acts as a primary transporter by pumping protons out of the cell, thereby creating pH and electrical potential differences across the plasmalemma (Fig. 1). Transport of many solutes (ions, metabolites, etc.) into and out of the cell involves secondary transporters whose ability to function is directly dependent on the proton-motive force created by the H+-ATPase. Depending on the electrical charge of the solute to be transported, the direction of its transport, and its concentration on either side of the membrane, it is possible to predict from Figure 1 the type of transport protein required. For instance, the uptake of a cation is energetically favorable because of the positive external electrical potential, and therefore requires only a diffusion facilitator, such as a channel protein or a uniport. Conversely, to be energetically favorable, the uptake of an anion must be accompanied by the uptake of one or more protons in a symport system. In addition to activating secondary transport, the H+-ATPase promotes more specialized physiological functions.