Showing papers on "Membrane published in 1994"

Self-organized honeycomb morphology of star-polymer polystyrene films

Abstract: AN important challenge in the preparation of porous polymer membranes for technological applications is to control both the size distribution and the relative positions of the pores. We have found a way to generate polymer films with an essentially monodisperse pore size, in which the pores are organized spontaneously into periodic hexagonal arrays. The films, which are 10–30 um thick, are produced by evaporating solutions of star-shaped polystyrene or polystyrene-polyparaphenylene block copolymers in carbon di-sulphide under a flow of moist gas. Empty spherical cells, about 0.2–10 µm in diameter, appear spontaneously in a hexagonal array, and the cells are open at the film surface. The use of star polymers, or of polymeric micelles, seems to be essential for obtaining this morphology. These membranes might find application in controlled release of drugs or other bioactive species, or as materials with useful optical properties, moulds or scaffolding for forming ordered microstructures, and model substrates for surface science.

Simulation of Water Transport through a Lipid Membrane

Abstract: To obtain insight in the process of water permeation through a lipid membrane, we performed molecular dynamics simulations on a phospholipid (DPPC)/water system with atomic detail. Since the actual process of permeation is too slow to be studied directly, we deduced the permeation rate indirectly via computation of the free energy and diffusion rate profiles of a water molecule across the bilayer. We conclude that the permeation of water through a lipid membrane cannot be described adequately by a simple homogeneous solubility-diffusion model. Both the excess free energy and the diffusion rate strongly depend on the position in the membrane, as a result from the inhomogeneous nature of the membrane. The calculated excess free energy profile has a shallow slope and a maximum height of 26 kJ/mol. The diffusion rate is highest in the middle of the membrane where the lipid density is low. In the interfacial region almost all water molecules are bound by the lipid headgroups, and the diffusion turns out to be 1 order of magnitude smaller. The total transport process is essentially determined by the free energy barrier. The rate-limiting step is the permeation through the dense part of the lipid tails, where the resistance is highest. We found a permeation rate of 7(±3) × 10-2 cm/s at 350 K, comparable to experimental values for DPPC membranes, if corrected for the temperature of the simulation. Taking the inhomogeneity of the membrane into account, we define a new “four-region” model which seems to be more realistic than the “two-phase” solubility-diffusion model.

Transport of macromolecules across microvascular walls: the two-pore theory

Abstract: In this review we summarized the evidence favoring the concept that the major plasma proteins are passively transported across vascular walls through water-filled pathways by means of convection and diffusion. With regard to solute transport, a majority of microvascular walls seems to show a bimodal size selectivity. This implies the presence of a high frequency of functional small pores, restricting proteins, and an extremely low number of non-size-selective pathways, permitting the passage of macromolecules from blood to tissue, here denoted large pores. We discussed the general behavior of such a heteroporous system. A major consequence of two-pore heteroporosity is that large-solute transport must mainly occur due to convection through large pores at low filtration rates, that is, at normal or even zero lymph flows. Indeed, convection must be the predominating transport mode for most solutes across large pores when the net filtration rate is zero. Under these (transient) conditions, the convective leak of macromolecules across large pores will be counterbalanced by absorption of essentially protein-free fluid through protein-restrictive pores. In a heteroporous membrane, proteins can thus be transported by solvent drag across vascular walls in the absence of a net convection. Normally the steady-state transcapillary fluid flow (lymph flow) is about equally partitioned among small and large pores, which makes lymph essentially a "half and half" mixture of protein-free ultrafiltrate and plasma. With increasing fluid flows, however, the plasma filtrate will be progressively diluted, eventually reaching a protein concentration largely in proportion to the fractional hydraulic conductance accounted for by the large pores (alpha L). Under these high lymph flow conditions, not only the large-pore transport but also the small-pore transport (of smaller macromolecules) will become convective. At low lymph flows, however, the small-pore transport of smaller macromolecules is usually mostly diffusive. An important implication of capillary heteroporosity is that single-pore formalism is inadequate for correctly evaluating the capillary sieving characteristics. With the use of homoporous transport formalism, the "lumped" macromolecular PS and sigma will therefore vary as a function of transcapillary fluid flow (Jv). However, it is approximately correct to use single-pore formalism for conditions when Jv is very high during steady state. Thus, if minimal sieving coefficients can be measured for macromolecules, then these values will accurately reflect (1 - sigma).(ABSTRACT TRUNCATED AT 400 WORDS)

Gas separation using polymer membranes: an overview

Abstract: This overview article discusses fundamental principles of gas sorption and transport in rubbery and glassy polymers and material selection guidelines for gas separation membranes. Comparisons between the performance of membrane-based gas separation systems and more conventional technologies in key commercial applications are provided. Companion articles in this special edition focus on state-of-the-art reviews and descriptions of theoretical and experimental developments important in the technology of gas separations using polymeric membranes.

Water Uptake of Perfluorosulfonic Acid Membranes from Liquid Water and Water Vapor

Abstract: The water uptake of several perfluorosulfonic acid membranes from liquid water over the temperature range 25 to 130°C and from water vapor at 80°C was determined. For water uptake from liquid water, the water uptake depended on the immersion temperature, the ion exchange capacity of the membrane, and the pretreatment of the membrane. The effects of pretreatment were not significant at immersion temperatures higher than 100 to 110°C. For water uptake from water vapor at 80°C, the sorption isotherms were similar in shape to those reported by previous investigators for 18.5 to 30°C, although the water uptake at 80°C was less than that reported for the lower temperatures. The water uptake from water vapor of some membranes that have been found to give relatively good performance when used in polymer electrolyte fuel cells was higher then that observed with, e.g., Nafion®117.

Fluid delivery apparatus

Abstract: A fluid container assembly (12) which can be aseptically filled in the field with selected fluids and one which is specially designed for sterile coupling and use with fluid dispensing and delivery devices of the character that embody stored energy sources (58) such as compressible cellular masses and distendible elastomeric membranes that form, in conjunction with a cooperating base (56), fluid chambers for containing the fluid to be dispensed.

Dimethyl sulphoxide : a review of its applications in cell biology

Abstract: Dimethyl sulphoxide is a water miscible solvent that has wide applications in cell biology. It acts as a cryoprotective agent in a variety of cells and tissues allowing prolonged storage at subzero temperatures. The action of dimethyl sulphoxide on the stability of the liquid matrix of cell membranes appears to be responsible for its effects and this appears also to be true for related effects on membrane permeability and fusion. Dimethyl sulphoxide is also known to act as an inducer of cellular differentiation and as a free radical scavenger and radioprotectant. A review of the underlying molecular basis of all these effects of dimethyl sulphoxide is presented.

Methane steam reforming in asymmetric Pd- and Pd-Ag/porous SS membrane reactors

Abstract: This work is devoted to applying electrolessly deposited Pd- and Pd-Ag/porous stainless steel composite membranes in methane steam reforming The methane conversion is significantly enhanced by the partial removal of hydrogen from the reaction location as a result of diffusion through the Pd-based membranes For example, at a total pressure of 136 kPa, a temperature of 500°C, a molar steam-to-methane ratio of 3, and in the presence of a commercial Ni/Al2O3 catalyst together with continuous pumping on the permeation side, a methane conversion twice as high as that in a non-membrane reactor was reached by using a Pd/SS membrane These effects were examined under a variety of experimental conditions A computer model of the membrane reactor was also developed to predict the effects of membrane separation on methane conversion

A new class of thiolipids for the attachment of lipid bilayers on gold surfaces

Abstract: A new class of lipid mols. is synthesized, based on two dipalmitoylphosphatidic mols., each extended at the lipid phosphate by a hydrophilic spacer chain of ethoxy groups of variable length, which are then coupled as a bilipid via a terminal disulfide group at the hydrophilic spacer. These anchor-bearing qthiolipidsq can attach to gold substrates by forming stable gold-sulfur bonds. In this way, the authors can couple lipid bilayers to gold surfaces, with the possibility of preserving a water layer between the support and the first monolayer. The thiolipid mols. are characterized on a Langmuir film balance using fluorescence microscopy. The mol. areas of the thiolipids on the water surface are 80-90 .ANG.2 at a fully compressed state. The thiolipid monolayers show a typical first-order phase transition on the water surface with regular, starlike domains. The formation of thiolipid-attached mono- and bilayers on gold surfaces was studied by surface plasmon resonance (SPR), impedance measurements, and cyclic voltammetry. Four different supported membrane systems are studied in detail: (1) pure thiolipid layers; (2) mixed lipid bilayers contg. a first pure thiolipid monolayer and a second one of conventional phospholipids; (3) bilayers, where the first gold-attached monolayer is composed of a mixt. of thio- and conventional phospholipids with another second phospholipid layer on top; (4) monolayers of pure 1-hexadecanethiol and layers with a second phospholipid film on top of the 1-hexadecanethiol. The electrochem. expts. reveal elec. blocking layers for all lipid systems investigated with specific resistances of 104-105 W cm2. The capacitance values for pure thiolipid bilayers are in the range of 0.5-0.7 mF/cm2 for the pure thiolipid bilayers and 0.7-0.8 mF/cm2 for the mixed thiolipid/phospholipid bilayers, which is comparable to the values found for unsupported, so-called black lipid membranes. SPR measurements confirm qual. the results of the electrochem. expts. [on SciFinder (R)]

C60 in Model Biological Systems. A Visible-UV Absorption Study of Solvent-Dependent Parameters and Solute Aggregation

Abstract: A study was made of the solubilization of C[sub 60] in various solvents and systems of biological interest, i.e., octanols, micelles, and liposomes, using visible-UV absorption spectroscopy as a diagnostic tool. The state of incorporation of C[sub 60] molecules in micellar and colloidal liposome solutions was monitored using a number of spectroscopic criteria of solute-solvent and solute-solute interactions based on comparison with spectra obtained in alkane and octanol solvents and from thin films of C[sub 60]. Spectral red shifts and intensity modifications of C[sub 60] absorption and C[sub 60] aggregation are discussed in terms of environment-dependent physical parameters. The results indicate that C[sub 60] can be dispersed in micellar solutions of Triton X-100 and Triton X-100 R-S, the fullerene molecules being localized in the inner hydrophobic part of the micelles. C[sub 60] was shown to be incorporated, mainly as aggregates, into phosphatidylcholine liposome colloidal solutions. It is concluded that micellar and liposome solutions can be prepared which could be used to transfer individual C[sub 60] molecules, or groups of molecules, to biological cells. 38 refs., 9 figs., 2 tabs.

Proton migration along the membrane surface and retarded surface to bulk transfer

Abstract: SINCE the proposal of the chemiosmotic theory1 there has been a continuing debate about how protons that have been pumped across membranes reach another membrane protein that utilizes the established pH gradient. Evidence has been gathered in favour of a 'delocalized' theory, in which the pumped protons equilibrate with the aqueous bulk phase before being consumed, and a 'localized' one, in which protons move exclusively along the membrane surface2,3. We report here that after proton release by an integral membrane protein, long-range proton transfer along the membrane surface is faster than proton exchange with the bulk water phase. The rate of lateral proton diffusion can be calculated by considering the buffer capacity of the membrane surface. Our results suggest that protons can efficiently diffuse along the membrane surface between a source and a sink (for example H+-ATP synthase) without dissipation losses into the aqueous bulk.

Hydrophobic barriers of lipid bilayer membranes formed by reduction of water penetration by alkyl chain unsaturation and cholesterol.

Abstract: The hydrophobicity profiles across phosphatidylcholine (PC)-cholesterol bilayer membranes were estimated in both frozen liposome suspensions and fluid-phase membranes as a function of alkyl chain length, unsaturation, and cholesterol mole fraction. A series of stearic acid spin labels, with the probe attached to various positions along the alkyl chain, cholesterol-type spin labels (cholestane and androstane spin labels), and Tempo-PC were used to examine depth-dependent changes in local hydrophobicity, which is determined by the extent of water penetration into the membrane. Local hydrophobicity was monitored primarily by observing the z component of the hyperfine interaction tensor (A,) of the nitroxide spin probe in a frozen suspension of the membrane at -150 "C and was further confirmed in the fluid phase by observing the rate of collision of Fe(CN)63- with the spin probe in the membrane using saturation recovery ESR. Saturated-PC membranes show low hydrophobicity (high polarity) across the membrane, comparable to 2-propanol and 1-octanol, even at the membrane center where hydrophobicity is highest. Longer alkyl chains only make the central hydrophobic regions wider without increasing the level of hydrophobicity. Introduction of a double bond at C9-C10 decreases the level of water penetration at all locations in the membrane, and this effect is considerably greater than the cis configuration than with the trans configuration. Incorporation of cholesterol (30 mol %) dramatically changes the profiles; it decreases hydrophobicity (increases water penetration) from the polar headgroup region to a depth of approximately C7 and C9 for saturated- and unsaturated-PC membranes, respectively, which is about where the bulky rigid steroid ring structure of cholesterol reaches in the membrane. Membrane hydrophobicity sharply increases at these positions from the level of methanol to the level of pure hexane, and hydrophobicity is constant in the inner region of the membrane. Thus, formation of effective hydrophobic barriers to permeation of small polar molecules requires alkyl chain unsaturation and/or cholesterol. The thickness of this rectangular hydrophobic barrier is less than 50% of the thickness of the hydrocarbon regions. Results obtained in dioleoyl-PC- cholesterol membranes in the fluid phase are similar to those obtained in frozen membranes. These results correlate well with permeability data for water and amino acids in the literature.

A dimerization motif for transmembrane α–helices

Abstract: Specific helix-helix interactions inside lipid bilayers guide the folding and assembly of many integral membrane proteins and their complexes. We report here a pattern of 7 amino acids (LIxxGVxxGVxxT) which when introduced into several hydrophobic transmembrane alpha-helices promotes their specific dimerization. Dimerization is driven by interactions that are specific, dominated by the helix-helix interface, and involve no potentially ionizable groups. The motif may provide a useful tool for the functional analysis of such interactions in a variety of systems. Further, since this particular motif is rare, whilst specific helix association is not, many other such motifs may exist, which could permit sorting within complex membranes as well as guiding folding and oligomerization.

Aquaporins: the molecular basis of facilitated water movement through living plant cells?

Abstract: Osmotic effects observed with living cells indicate that their plasma membranes are freely permeable to water while essentially creating a barrier to other molecules. The hydraulic conductivity of biological membranes is sometimes still ascribed to the simple diffusion of water molecules through the lipid bilayer. However, more than 30 years ago the idea was advanced that hydraulic water movement through living cells occurs by bulk flow of water through pores in the membrane (Sidel and Solomon, 1957). Certain membranes of animal cells are unusually permeable to water and there is now a substantial body of evidence for the existence of water transport channels in such membranes (reviewed by Finkelstein, 1987; Verkman, 1992). For example, membranes from red blood cells and renal proximal tubules are exceptionally permeable to water; the membranes of the convoluted dista1 tubules of the kidney are also highly water permeable, and water permeability can be modulated by hormones. The hydraulic conductivity of plant cells has been thoroughly investigated and numerous reviews on this subject have been published (for a recent review, see Steudle, 1992). Many observations on plants and animals support the recent discovery of proteins that form water channels (Preston et al., 1992; Fushimi et al., 1993; Maurel et al., 1993). We proposed to cal1 such proteins "aquaporins" (Agre et al., 1993). These water channel proteins belong to the MIP family, an ancient family of membrane proteins (see Reizer et al., 1993, for review). Aquaporins form water-selective channels, allowing water to pass freely while excluding ions and metabolites. In plants, aquaporins have been demonstrated in the tonoplast (vacuolar membrane) (Hofte et al., 1992), but they also may be present in the plasma membrane (Kammerloher and Schaffner, 1993). In animal cells, aquaporins are found in the plasma membranes of specific cell types. It is important to note that such channels permit or facilitate the movement of water through membranes and do not act as pumps. The driving forces behind water movement are hydraulic or osmotic in nature. .