Showing papers on "Membrane published in 2002"

A Photoactivatable GFP for Selective Photolabeling of Proteins and Cells

Abstract: We report a photoactivatable variant of the Aequorea victoria green fluorescent protein (GFP) that, after intense irradiation with 413-nanometer light, increases fluorescence 100 times when excited by 488-nanometer light and remains stable for days under aerobic conditions. These characteristics offer a new tool for exploring intracellular protein dynamics by tracking photoactivated molecules that are the only visible GFPs in the cell. Here, we use the photoactivatable GFP both as a free protein to measure protein diffusion across the nuclear envelope and as a chimera with a lysosomal membrane protein to demonstrate rapid interlysosomal membrane exchange.

X-ray structure of a ClC chloride channel at 3.0 A reveals the molecular basis of anion selectivity.

Abstract: The ClC chloride channels catalyse the selective flow of Cl- ions across cell membranes, thereby regulating electrical excitation in skeletal muscle and the flow of salt and water across epithelial barriers. Genetic defects in ClC Cl- channels underlie several familial muscle and kidney diseases. Here we present the X-ray structures of two prokaryotic ClC Cl- channels from Salmonella enterica serovar typhimurium and Escherichia coli at 3.0 and 3.5 A, respectively. Both structures reveal two identical pores, each pore being formed by a separate subunit contained within a homodimeric membrane protein. Individual subunits are composed of two roughly repeated halves that span the membrane with opposite orientations. This antiparallel architecture defines a selectivity filter in which a Cl- ion is stabilized by electrostatic interactions with alpha-helix dipoles and by chemical coordination with nitrogen atoms and hydroxyl groups. These findings provide a structural basis for further understanding the function of ClC Cl- channels, and establish the physical and chemical basis of their anion selectivity.

Ultrapermeable, Reverse-Selective Nanocomposite Membranes

Abstract: Polymer nanocomposites continue to receive tremendous attention for application in areas such as microelectronics, organic batteries, optics, and catalysis. We have discovered that physical dispersion of nonporous, nanoscale, fumed silica particles in glassy amorphous poly(4-methyl-2-pentyne) simultaneously and surprisingly enhances both membrane permeability and selectivity for large organic molecules over small permanent gases. These highly unusual property enhancements, in contrast to results obtained in conventional filled polymer systems, reflect fumed silica-induced disruption of polymer chain packing and an accompanying subtle increase in the size of free volume elements through which molecular transport occurs, as discerned by positron annihilation lifetime spectroscopy. Such nanoscale hybridization represents an innovative means to tune the separation properties of glassy polymeric media through systematic manipulation of molecular packing.

Phospholipids undergo hop diffusion in compartmentalized cell membrane

Abstract: The diffusion rate of lipids in the cell membrane is reduced by a factor of 5-100 from that in artificial bilayers. This slowing mechanism has puzzled cell biologists for the last 25 yr. Here we address this issue by studying the movement of unsaturated phospholipids in rat kidney fibroblasts at the single molecule level at the temporal resolution of 25 micros. The cell membrane was found to be compartmentalized: phospholipids are confined within 230-nm-diameter (phi) compartments for 11 ms on average before hopping to adjacent compartments. These 230-nm compartments exist within greater 750-nm-phi compartments where these phospholipids are confined for 0.33 s on average. The diffusion rate within 230-nm compartments is 5.4 microm2/s, which is nearly as fast as that in large unilamellar vesicles, indicating that the diffusion in the cell membrane is reduced not because diffusion per se is slow, but because the cell membrane is compartmentalized with regard to lateral diffusion of phospholipids. Such compartmentalization depends on the actin-based membrane skeleton, but not on the extracellular matrix, extracellular domains of membrane proteins, or cholesterol-enriched rafts. We propose that various transmembrane proteins anchored to the actin-based membrane skeleton meshwork act as rows of pickets that temporarily confine phospholipids.

Mechanics of the cell

Abstract: 1. Introduction to the cell Part I. Rods and Ropes: 2. Polymers 3. Two-dimensional networks 4. Three-dimensional networks Part II. Membranes: 5. Biomembranes 6. Membrane undulations Part III. The Whole Cell: 7. The simplest cells 8. Intermembrane forces 9. Dynamic filaments 10. Mechanical designs Appendix A. Animal cells and tissues Appendix B. The cell's molecular building blocks Appendix C. Elementary statistical mechanics Appendix D. Elasticity References Index.

Dynamics of cell wall structure in Saccharomyces cerevisiae.

Abstract: The cell wall of Saccharomyces cerevisiae is an elastic structure that provides osmotic and physical protection and determines the shape of the cell. The inner layer of the wall is largely responsible for the mechanical strength of the wall and also provides the attachment sites for the proteins that form the outer layer of the wall. Here we find among others the sexual agglutinins and the flocculins. The outer protein layer also limits the permeability of the cell wall, thus shielding the plasma membrane from attack by foreign enzymes and membrane-perturbing compounds. The main features of the molecular organization of the yeast cell wall are now known. Importantly, the molecular composition and organization of the cell wall may vary considerably. For example, the incorporation of many cell wall proteins is temporally and spatially controlled and depends strongly on environmental conditions. Similarly, the formation of specific cell wall protein–polysaccharide complexes is strongly affected by external conditions. This points to a tight regulation of cell wall construction. Indeed, all five mitogen-activated protein kinase pathways in bakers’ yeast affect the cell wall, and additional cell wall-related signaling routes have been identified. Finally, some potential targets for new antifungal compounds related to cell wall construction are discussed.

Electrospun Nanofibrous Membranes for Highly Sensitive Optical Sensors

Abstract: The first use of electrospun nanofibrous membranes as highly responsive fluorescence quenching-based optical sensors for metal ions (Fe3+ and Hg2+) and 2,4-dinitrotoluene (DNT) is reported. A fluorescent polymer, poly(acrylic acid)−poly(pyrene methanol) (PAA−PM), was used as a sensing material. Optical chemical sensors were fabricated by electrospinning PAA−PM and thermally cross-linkable polyurethane latex mixture solutions. These sensors showed high sensitivities due to the high surface area-to-volume ratio of the nanofibrous membrane structures.

Membrane Fouling in Membrane Bioreactors for Wastewater Treatment

Abstract: Membrane bioreactors (MBRs), in which membranes are applied to biological wastewater treatment for biomass separation, provide many advantages over conventional treatment. However, membrane fouling in MBRs restricts their widespread application because it reduces productivity and increases maintenance and operating costs. Recently much research and development has taken place to investigate, model, and control membrane fouling processes. However, unified and well-structured theories on membrane fouling are not currently available because of the complexity of the biomass matrix, which is highly heterogeneous and includes living microorganisms. Membrane fouling in MBR systems can be reversible (i.e., removable by physical washing) or irreversible (removable by chemical cleaning only), and can take place on the membrane surface or into the membrane pores. Although establishing a general model to describe membrane fouling in such a process is made extremely difficult by the inherent heterogeneity of the syste...

Organization in lipid membranes containing cholesterol.

Abstract: A fundamental attribute of raft formation in cell membranes is lateral separation of lipids into coexisting liquid phases. Using fluorescence microscopy, we observe spontaneous lateral separation in free-floating giant unilamellar vesicles. We record coexisting liquid domains over a range of composition and temperature significantly wider than previously reported. Furthermore, we establish correlations between miscibility in bilayers and in monolayers. For example, the same lipid mixtures that produce liquid domains in bilayer membranes produce two upper miscibility critical points in the phase diagrams of monolayers.

Ultrafine fibrous cellulose membranes from electrospinning of cellulose acetate

Abstract: Three solvents, that is, acetone, acetic acid, and dimethylacetamide (DMAc), with a range of solubility parameter δ, surface tension γ, viscosity η and boiling temperature were used to generate mixtures for electrospinning cellulose acetate (CA) (degree of substitution, DS = 2.45). Although none of these solvents alone enables continuous formation of fibers, mixing DMAc with either acetone or acetic acid produced suitable solvent systems. The 2:1 acetone:DMAc mixture is the most versatile mixture because it allows CA in the 12.5–20% concentration range to be continuously electrospun into fibrous membranes. These CA solutions have η between 1.2 and 10.2 poise and γ around 26 dyne/cm and produce smooth fibers with diameters from 100 nm to ∼1 μm. Fiber sizes generally decrease with decreasing CA concentrations. The nature of the collectors affects the morphology as well as packing of fibers. Fibers collected on paper have more uniform sizes, smooth surfaces, and fewer defects, whereas fibers collected on water are more varied in size. Electrically conductive solid collectors, such as Al foil and water, favor more tightly packed and less porous membranes. Porous collectors, like paper and copper mesh, produce highly porous membranes. The pores in membranes collected on the Al foil and paper are much better interconnected in the planar directions than those in membranes collected on water. There is evidence that electrospinning induces order in the fibers. Deacetylation of CA membranes is more efficient and complete in NaOH/ethanol than in aqueous NaOH, producing DS values between 0.15 and 2.33 without altering fiber surfaces, packing, or organization. The fully regenerated cellulose membranes are similarly hydrophilic as commodity cellulose fibrous matrices but absorb nearly 10 times as much water. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2119–2129, 2002

Antibody-based bio-nanotube membranes for enantiomeric drug separations.

Abstract: Synthetic bio-nanotube membranes were developed and used to separate two enantiomers of a chiral drug. These membranes are based on alumina films that have cylindrical pores with monodisperse nanoscopic diameters (for example, 20 nanometers). Silica nanotubes were chemically synthesized within the pores of these films, and an antibody that selectively binds one of the enantiomers of the drug was attached to the inner walls of the silica nanotubes. These membranes selectively transport the enantiomer that specifically binds to the antibody, relative to the enantiomer that has lower affinity for the antibody. The solvent dimethyl sulfoxide was used to tune the antibody binding affinity. The enantiomeric selectivity coefficient increases as the inside diameter of the silica nanotubes decreases.

Role of membranes in the activities of antimicrobial cationic peptides

Abstract: Cationic amphiphilic peptides that are found throughout nature have very broad-spectrum activities against microbes. The initial sites of interaction are with microbial membranes. Although dogma suggests that their lethal action involves disruption of the cytoplasmic membranes, a number of cationic peptides can traverse intact membranes to interact with internal targets.

DL_POLY: Application to molecular simulation

Abstract: DL_POLY is a general-purpose molecular dynamics simulation package, which was developed by Daresbury Laboratory in the mid-1990s for the molecular simulation community in the United Kingdom. The package now has a world-wide user base and applications in many areas of molecular simulation. In this article we briefly review the history and design of the package and highlight some recent applications in the areas of; liquids and solutions; spectroscopy; ionic solids; molecular crystals; polymers; glasses; membranes; proteins; solid and liquid interfaces; catalysis; liquid crystals; intercalation and clathrates; and novel systems. The strengths and weaknesses of the code and its future in the near term are also discussed.

Novel Sulfonated Polyimides as Polyelectrolytes for Fuel Cell Application. 1. Synthesis, Proton Conductivity, and Water Stability of Polyimides from 4,4‘-Diaminodiphenyl Ether-2,2‘-disulfonic Acid

Abstract: A sulfonated diamine monomer, 4,4‘-diaminodiphenyl ether-2,2‘-disulfonic acid (ODADS), was successfully synthesized by direct sulfonation of a commercially available diamine, 4,4‘-diaminodiphenyl ether (ODA), using fuming sulfuric acid as the sulfonating reagent. A series of sulfonated polyimides were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), ODADS, and common nonsulfonated diamines. The resulting sulfonated polyimides displayed much better stability toward water than those derived from the widely used sulfonated diamine 2,2‘-benzidinedisulfonic acid (BDSA). This is because ODADS-based polyimide membranes have a more flexible structure than the corresponding BDSA-based ones. Fenton's reagent test revealed that ODADS-based polyimide membranes also had fair good stability to oxidation. Polyimide membranes with good water stability as well as high proton conductivity were developed. NTDA−ODADS/BAPB(1/1) copolyimide membrane (BAPB refers to 4,4‘-bis(4-aminophenoxy)biphenyl)), for ex...

Ionic Conductivity of an Extruded Nafion 1100 EW Series of Membranes

Abstract: The proton conductivity of a series of extruded Nafion membranes @of equivalent weight ~EW! of 1100 and nominal dry thickness of 51, 89, 127, and 178 mm# has been studied. Measurements were made in 1 M H2SO4 at 298 K using a four-electrode, dc technique. The membrane area resistance increases with thickness, as expected, from 0.07 to 0.16 V cm2 for Nafion 112 and Nafion 117, respectively. However, in contrast to the published literature, after correcting for the membrane thickness, the conductivity of the membranes decreases with decreasing membrane thickness. For example, values of 0.083 and 0.16 S cm21 were obtained for Nafion 112 and 117 membranes, respectively. In situ current-interrupt measurements in a proton exchange membrane fuel cell confirmed the relatively poor conductivity of the membrane electrode assemblies ~MEAs! based on the thinner membranes. While a high contact resistance to the electrodes may have contributed to the in situ MEA resistance, water balance measurements over the MEA showed that the high resistance was not due to a low water content or to an uneven water distribution in the MEAs. The implications of the findings for the understanding of the membrane properties are discussed.

Silicon Oxide Nafion Composite Membranes for Proton-Exchange Membrane Fuel Cell Operation at 80-140°C

Abstract: Silicon oxide/Nafion composite membranes were studied for operation in hydrogen/oxygen proton-exchange membrane fuel cells (PEMFCs) from 80 to 140°C. The composite membranes were prepared either by an impregnation of Nafion 115 via sol-gel processing of tetraethoxysilane or by preparing a recast film, using solubilized Nafion 115 and a silicon oxide polymer/gel Tetraethoxysilane, when reacted with water in an acidic medium, undergoes polymerization to form a mixture of SiO 2 and siloxane polymer with product hydroxide and ethoxide groups. This material is referred to as SiO s /-OH/-OEt. When Nafion is used as the acidic medium, the SiO 2 /siloxane polymer forms within the membrane. All composite membranes had a silicon oxide content of less than or equal to 10 wt %. The silicon oxide improved the water retention of the composite membranes, increasing proton conductivity at elevated temperatures Attenuated total reflectance-Fourier transform infrared spectroscopy and scanning electron microscopy experiments indicated an evenly distributed siloxane polymer of SiO 2 /-OH/-OEt in the composite membranes. At a potential of 0.4 V, silicon oxide/Nafion 115 composite membranes delivered four times the current density obtained with unmodified Nafion 115 in a H 2 /O 2 PEMFC at 130°C and a pressure of 3 atm. Furthermore, silicon oxide-modified membranes were more robust than the control membranes (unmodified Nafion 115 and recast Nafion), which degraded after high operation temperature and thermal cycling.

Polyunsaturated fatty acids in lipid bilayers: intrinsic and environmental contributions to their unique physical properties.

Abstract: Polyunsaturated lipids are an essential component of biological membranes, influencing order and dynamics of lipids, protein-lipid interaction, and membrane transport properties. To gain an atomic level picture of the impact of polyunsaturation on membrane properties, quantum mechanical (QM) and empirical force field based calculations have been undertaken. The QM calculations of the torsional energy surface for rotation about vinyl-methylene bonds reveal low barriers to rotation, indicating an intrinsic propensity toward flexibility. Based on QM and experimental data, empirical force field parameters were developed for polyunsaturated lipids and applied in a 16 ns molecular dynamics (MD) simulation of a 1-stearoyl-2-docosahexaenoyl-sn-glyerco-3-phosphocholine (SDPC) lipid bilayer. The simulation results are in good agreement with experimental data, suggesting an unusually high degree of conformational flexibility of polyunsaturated hydrocarbon chains in membranes. The detailed analysis of chain conformation and dynamics by simulations is aiding the interpretation of experimental data and is useful for understanding the unique role of polyunsaturated lipids in biological membranes. The complete force field is included as Supporting Information and is available from http://www.pharmacy.umaryland.edu/faculty/amackere/research.html.