Formation Of Membrane Structures Research Articles

Does more solvent in bore liquid create more open inner surface in hollow fiber membranes?

AbstractThe solvent content in the bore liquid controls the inner structure of a hollow fiber membrane. Based on the theory of liquid–liquid phase separation, at a higher solvent concentration, the inner surface is more open and the transport resistance is lower. However, the theory does not describe clearly the membrane structure formation at a very high solvent concentration. A logic deduction is that more solvent in the bore liquid results in more open inner structure. In this report, a polymer solution, polysulfone (PSf)/N‐methylpyrrolidone (NMP)/diethylene glycol (DegOH) was used to investigate the relationship between the membrane resistance and solvent content in the bore liquid. A triple orifice spinneret was used to prepare an open outer surface, minimizing the transport resistance in the membrane outer layer. Bore liquid solutions with NMP concentration close to the critical solvent concentration (CSC) were used. The membranes were characterized by scanning electron microscopy (SEM), pore size and pore size distribution, nitrogen and pure water permeability. The results indicated that at a concentration far below the CSC, a higher NMP content corresponds to a more open inner surface, but the aggressive addition of solvent to a concentration close to CSC tends to form closed cells; thus water and gas permeability significantly decreases. This phenomenon was explained by nuclei formation and growth, followed by the formation of a dense skin and closed cell structure in a mild coagulant. Copyright © 2007 John Wiley & Sons, Ltd.

Poly(vinylidene fluoride) Crystallization Behavior and Membrane Structure Formation Via Thermally Induced Phase Separation with Benzophenone Diluent

Microporous poly(vinylidene fluoride) (PVDF) membranes were prepared by thermally induced phase separation (TIPS) at different quenching temperatures with benzophenone as the diluent. The crystallization behavior and crystal structure of PVDF in PVDF/benzophenone systems were investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). The different PVDF concentrations had a remarkable effect on PVDF crystallization behavior and resulted in different membrane structures. Spherulitic structures were vague when the PVDF/benzophenone solution was quenched to −8°C; however, discernable spherulitic structures were obtained when quenched to 34 and 49°C. Additionally, two phase separation mechanisms (solid–solid (S–S) and solid–liquid (S–L) phase separation) were observed during membrane preparation. It was revealed by scanning electron microscopy (SEM) that microporous membranes had more discernable spherulitic structures formed by S–L phase separation than by S–S phase separation, which induced macrovoids and irregular pores on the fracture surfaces of membranes.

Multivesicular Bodies as a Platform for Formation of the Marburg Virus Envelope

The Marburg virus (MARV) envelope consists of a lipid membrane and two major proteins, the matrix protein VP40 and the glycoprotein GP. Both proteins use different intracellular transport pathways: GP utilizes the exocytotic pathway, while VP40 is transported through the retrograde late endosomal pathway. It is currently unknown where the proteins combine to form the viral envelope. In the present study, we identified the intracellular site where the two major envelope proteins of MARV come together as peripheral multivesicular bodies (MVBs). Upon coexpression with VP40, GP is redistributed from the trans-Golgi network into the VP40-containing MVBs. Ultrastructural analysis of MVBs suggested that they provide the platform for the formation of membrane structures that bud as virus-like particles from the cell surface. The virus-like particles contain both VP40 and GP. Single expression of GP also resulted in the release of particles, which are round or pleomorphic. Single expression of VP40 led to the release of filamentous structures that closely resemble viral particles and contain traces of endosomal marker proteins. This finding indicated a central role of VP40 in the formation of the filamentous structure of MARV particles, which is similar to the role of the related Ebola virusVP40. In MARV-infected cells, VP40 and GP are colocalized in peripheral MVBs as well. Moreover, intracellular budding of progeny virions into MVBs was frequently detected. Taken together, these results demonstrate an intracellular intersection between GP and VP40 pathways and suggest a crucial role of the late endosomal compartment for the formation of the viral envelope.

A Salmonella protein causes macrophage cell death by inducing autophagy.

Salmonella enterica, the causative agent of food poisoning and typhoid fever, induces programmed cell death in macrophages, a process found to be dependent on a type III protein secretion system, and SipB, a protein with membrane fusion activity that is delivered into host cells by this system. When expressed in cultured cells, SipB caused the formation of and localized to unusual multimembrane structures. These structures resembled autophagosomes and contained both mitochondrial and endoplasmic reticulum markers. A mutant form of SipB devoid of membrane fusion activity localized to mitochondria, but did not induce the formation of membrane structures. Upon Salmonella infection of macrophages, SipB was found in mitochondria, which appeared swollen and devoid of christae. Salmonella-infected macrophages exhibited marked accumulation of autophagic vesicles. We propose that Salmonella, through the action of SipB, kills macrophages by disrupting mitochondria, thereby inducing autophagy and cell death.

Modification of a Ubiquitin-like Protein Paz2 Conducted Micropexophagy through Formation of a Novel Membrane Structure

Microautophagy is a versatile process in which vacuolar or lysosomal membranes directly sequester cytosolic targets for degradation. Recent genetic evidence suggested that microautophagy uses molecular machineries essential for macroautophagy, but the details of this process are still unknown. In this study, a ubiquitin-like protein Paz2 essential for micropexophagy in the yeast Pichia pastoris has been shown to receive modification through the function of Paz8 and Gsa7, yielding a modified form Paz2-I, similar to the ubiquitin-like lipidation of Aut7 that is essential for macroautophagy in Saccharomyces cerevisiae. We identified a novel membrane structure formed after the onset of micropexophagy, which we suggest is necessary for the sequestration of peroxisomes by the vacuole. Assembly of this newly formed membrane structure, which is followed by localization of Paz2 to it, was found to require a properly functioning Paz2-modification system. We herein show that Paz2 and its modification system conduct micropexophagy through formation of the membrane structure, which explains the convergence between micropexophagy and macroautophagy with regard to de novo membrane formation.

Nanotubules Formed by Highly Hydrophobic Amphiphilic α-Helical Peptides and Natural Phospholipids

We previously reported that the 18-mer amphiphilic α-helical peptide, Hel 13-5, consisting of 13 hydrophobic residues and five hydrophilic amino acid residues, can induce neutral liposomes (egg yolk phosphatidylcholine) to adopt long nanotubular structures and that the interaction of specific peptides with specific phospholipid mixtures induces the formation of membrane structures resembling cellular organelles such as the Golgi apparatus. In the present study we focused our attention on the effects of peptide sequence and chain length on the nanotubule formation occurring in mixture systems of Hel 13-5 and various neutral and acidic lipid species by means of turbidity measurements, dynamic light scattering measurements, and electron microscopy. We designed and synthesized two sets of Hel 13-5 related peptides: 1) Five peptides to examine the role of hydrophobic or hydrophilic residues in amphiphilic α-helical structures, and 2) Six peptides to examine the role of peptide length, having even number residues from 12 to 24. Conformational, solution, and morphological studies showed that the amphiphilic α-helical structure and the peptide chain length (especially 18 amino acid residues) are critical determinants of very long tubular structures. A mixture of α-helix and β-structures determines the tubular shapes and assemblies. However, we found that the charged Lys residues comprising the hydrophilic regions of amphiphilic structures can be replaced by Arg or Glu residues without a loss of tubular structures. This suggests that the mechanism of microtubule formation does not involve the charge interaction. The immersion of the hydrophobic part of the amphiphilic peptides into liposomes initially forms elliptic-like structures due to the fusion of small liposomes, which is followed by a transformation into tubular structures of various sizes and shapes.

The relationship between the antioxidant and the antibacterial properties of galloylated catechins and the structure of phospholipid model membranes.

The effects of four catechins, (+)-catechin (C), (-)-epicatechin (EC), (-)-epicatechin gallate (ECG), and (-)-epigallocatechin gallate (EGCG), on the physical properties of phospholipid model membranes and the correlation to their antioxidant and antibacterial capacities have been studied by using differential scanning calorimetry (DSC), fluorescence spectroscopy, infrared spectroscopy (IR), AAPH-induced oxidation, and leakage experiments. DSC data revealed that galloylated catechins, especially ECG, affected the physical properties of both the phosphatidylcholine (PC) and phosphatidylethanolamine (PE) bilayers dramatically. Galloylated catechins showed higher phospholipid/water partition coefficients than their homologues and were immersed in the phospholipid palisade intercalating within the hydrocarbon chains, ECG being at the deepest position. In contrast, nongalloylated catechins presented a shallow location close to the phospholipid/water interface. ECG also exhibited the highest antioxidant capacity against lipid peroxidation, which correlated with its strong effect on DPH fluorescence anisotropy (as observed by the increase of the lipid order of fluid PC bilayers) and with the presence of highly cooperative transitions as seen by DSC. We propose that the high antioxidant capacity of some galloylated catechins such as ECG could be partially due to the formation of membrane structures showing resistance to detergent solubilization and in which the phospholipids have tightly packed acyl chains and highly hydrated phosphate groups. Significantly, PE was found to be essential to the promotion of carboxyfluorescein leakage from bacterial model membranes by galloylated catechins, indicating that their bactericidal activity, at least at the membrane level, could be due to the specific effect of these catechins on PE.

The increased bactericidal activity of a fatty acid-modified synthetic antimicrobial peptide of human cathepsin G correlates with its enhanced capacity to interact with model membranes

The bactericidal potency of a synthetic peptide (CG 117–136) of human lysosomal cathepsin G (cat G) can be substantially increased by covalent attachment to its N- or C-termini, of saturated, linear fatty acids (FAs), namely those with C-8, C-10 and C-12 hydrocarbon chains. In order to understand better the mechanism by which FA moieties increase the bactericidal activity of CG 117–136, the interaction of N-terminally FA-modified peptides with artificial membranes was studied. First, the content of secondary structure motifs in the modified and unmodified peptides was determined by circular dichroism (CD). A marked increase in the propensity of FA-modified CG 117–136 to form an α-helix structure was observed for the C-8, C-10 and C-12 derivatives compared with unmodified/short-chain and long-chain (C-14, C-16, C-18) derivatives. These effects were observed both in the presence of large unilamellar liposomes or in trifuluoroethanol, a membrane-stimulating agent. Second, the capacity of peptides to insert into large unilamellar liposomes as a function of FA length was determined by their ability to release a trapped fluorescent dye. FA derivatives with the highest α-helical content were found to be the most effective in releasing a fluorescent dye, compared with an unmodified peptide and/or derivatives having a low α-helical content. The ability of the peptides to attain α-helical structure in the membrane-like environment and the ability to disrupt the liposomal membrane, therefore correlate remarkably well with their increased ability to kill bacteria. A plausible explanation for improved bactericidal action of the modified peptide is that the FA moiety facilitates formation of the peptide with an α-helical structure formation in membranes, which is essential for disrupting the integrity of the bacterial cytoplasmic membrane.

Modeling the dynamics of membrane structure formation in quenched polymer solutions

Calculations are presented to illustrate the use of droplet growth rate data, obtained from light scattering measurements and/or spinodal decomposition calculations, to model the evolution of pore size gradients in solutions undergoing temperature-induced (TIPS) phase inversion. A simplified one-dimensional heat transport equation is used to obtain transient temperature profiles in quenched thin films, and these are combined with pore growth rates as a function of temperature and composition, to quantify droplet (i.e. pore) sizes in the final membrane. Results demonstrate the effects of quench temperature on the transient and final pore size distribution in the quenched films.

Folding of β-sheet membrane proteins: a hydrophobic hexapeptide model

Beta-sheets, in the form of the β-barrel folding motif, are found in several constitutive membrane proteins (porins) and in several microbial toxins that assemble on membranes to form oligomeric transmembrane channels. We report here a first step towards understanding the principles of β-sheet formation in membranes. In particular, we describe the properties of a simple hydrophobic hexapeptide, acetyl-Trp-Leu 5 (AcWL 5), that assembles cooperatively into β-sheet aggregates upon partitioning into lipid bilayer membranes from the aqueous phase where the peptide is strictly monomeric and random coil. The aggregates, containing 10 to 20 monomers, undergo a relatively sharp and reversible thermal unfolding at ∼60°C. No pores are formed by the aggregates, but they do induce graded leakage of vesicle contents at very high peptide to lipid ratios. Because β-sheet structure is not observed when the peptide is dissolved in n-octanol, trifluoroethanol or sodium dodecyl sulfate micelles, aggregation into β-sheets appears to be an exclusive property of the peptide in the bilayer membrane interface. This is an expected consequence of the hypothesis that a reduction in the free energy of partitioning of peptide bonds caused by hydrogen bonding drives secondary structure formation in membrane interfaces. But, other features of interfacial partitioning, such as side-chain interactions and reduction of dimensionality, must also contribute. We estimate from our partitioning data that the free energy reduction per residue for aggregation is about 0.5 kcal mol −1. Although modest, its aggregate effect on the free energy of assembling β-sheet proteins can be huge. This surprising finding, that a simple hydrophobic hexapeptide readily assembles into oligomeric β-sheets in membranes, reveals the potent ability of membranes to promote secondary structure in peptides, and shows that the formation of β-sheets in membranes is more facile than expected. Furthermore, it provides a basis for understanding the observation that membranes promote self-association of β-amyloid peptides. AcWL 5 and related peptides thus provide a good starting point for designing peptide models for exploring the principles of β-sheet formation in membranes.

Effect of PEG additive on membrane formation by phase inversion

This study investigates the effect of PEG additive as a pore-former on the structure formation of membranes and their permeation properties connected with the changes of thermodynamic and kinetic properties in phase inversion process. The membranes were prepared by using polysulfone (PSf)/ N-methyl-2-pyrrolidone (NMP)/poly(ethylene glycol) (PEG) casting solution and water coagulant. The resulting membranes prepared by changing the molecular weight of PEG additive and the ratio of PEG to NMP were characterized by scanning electron microscope observations, measurements of water flux and PEG rejection. The thermodynamic and kinetic properties of membrane-forming system were studied through coagulation value, light transmittance and viscosity. The correlations between the final membrane structure/permeation properties and thermodynamic/kinetic properties of membrane forming system are discussed extensively.

Biofilm formation of Staphylococcus aureus strains isolated from impetigo and furuncle: role of fibrinogen and fibrin

The formation of membranous structure (thickness from the plastic tissue-culture coverslip (hematoxylin-eosin) >1 mm; periodic acid-Schiff-positive) was more prominent with Staphylococcus aureus ( S. aureus) strains isolated from impetigo (coagulase types I·V origin) than with S. aureus strains isolated from furuncle (coagulase type IV origin) ( P<0.05) in the plastic tissue-culture coverslip in human plasma after 72 h. Attachment of S. aureus cells to a plastic tissue-culture coverslip was more marked in 0.3% fibrinogen/tryptic soy broth (TSB) than in plasma ( P<0.05). The formation of the membranous structure was observed on the plastic tissue-culture coverslip with 0.3% fibrinogen/human serum but not with 0.3% fibrinogen+5% glucose/TSB. Electron microscopy revealed abundant fibrin around S. aureus cells at 4 h and Ruthenium red-positive materials increased at 24 and 72 h in plasma. Staphylococcus aureus cell attachment to the plastic tissue-culture coverslip in plasma decreased by addition of levofloxacin (LVFX) at 1/2 minimum inhibitory concentration (MIC) and clarithromycin (CAM) at 1/4 MIC. Polysaccharide production of S. aureus cells on the plastic tissue-culture coverslip in plasma decreased with the addition of CAM at 1/4 MIC. Fibrinogen is closely related to initiation of infection but biofilm formation requires the conversion of fibrinogen to fibrin. Thus, attachment of S. aureus cells to the plastic tissue-culture coverslip, conversion of fibrinogen to fibrin by coagulase-prothrombin complex, and production of abundant glycocalyx by S. aureus cells are at least required for the production of biofilm in staphylococcal skin infection.

Chromoplast structures inThunbergia flowers

The differentiation of tubulous chromoplasts in developing flowers ofThunbergia alata was studied by ultrastructural, pigment and protein analyses. The way of chromoplast formation in the mesophyll differed from that in the adaxial epidermis. While, in mesophyll cells, the chloroplasts were directly transformed into chromoplasts of the tubulous type, characteristic membranes and the tubular reticulum appeared in the adaxial epidermal cells, before the formation of tubules. The disappearance of the photosynthetic apparatus, the formation of membranous structures and the appearance of tubules were studied. The tubulous chromoplasts contained a 32 kDa protein, an unidentified carotene, and small quantities of lutein and β-carotene.

Relationship between fatty acid accretion, membrane composition, and biologic functions

Dietary fat affects metabolic pathways for phospholipid biosynthesis in tissues in a coordinated fashion. This may be important to aspects of development that concern phosphatidylcholine metabolism or regulatory processes that depend on signals from a changing milieu in the microenvironment of the membrane. Dietary fat influences the phosphatidylethanolamine (PE) composition in many membranes of the brain and retina and may by altered by small changes in the content of 20:4(6) and 22:6(3). Membrane PE fatty acids that contain one, four, or six double bonds and the ratio of 22:5(6) to 22:6(3) in PE that contains four to six double bonds are also affected. An increase in the omega 6 fatty acid content of membranes is associated with increased PE methyltransferase activity and decreased phosphocholine transferase activity, thus indicating a mechanism by which change in an exogenous factor (e.g., dietary fat intake) may alter neural phospholipid biosynthesis. Small changes in the composition of dietary fat intake change the composition of brain membranes during development. It is provocative to ponder whether diet could be used to induce formation of membrane structures that are more resistant to specific insults that cause degeneration of brain structural material, to ensure optimal functional compositions, or to reverse degenerative changes that occur in neural membrane structure and function.

Preparation of polyacrylonitrile ultrafiltration membranes from polymer solutions containing glycerol

The conditions for the formation of ultrafiltration membranes based on polyacrylonitrile methylmetacrylate sodium vinylsulfonate have been studied. Glycerol is used as an additive in the casting solution with a concentration of 0.5–4 wt.%. The effect of glycerol content on the structure and pore-size distribution of the membranes have been studied. It is established that with an increase in the glycerol concentration in the casting solution, membranes with a higher pore density and with smaller pore size are formed and thus increase their selectivity with respect to lower molecular weight components. The selectivities for different calibrants such a trypsin, lyzozyme, cytochrome-c are reported. The flux with respect to the mentioned calibrant solutions is as high as 230–250 l/m 2-hr. It has been established that the glycerol plays an important role in the formation of membrane structure, and leads to a better performance.

Inhibition of sendai virus fusion with phospholipid vesicles and human erythrocyte membranes by hydrophobic peptides

Hydrophobic di- and tripeptides which are capable of inhibiting enveloped virus infection of cells are also capable of inhibiting at least three different types of membrane fusion events. Large unilamellar vesicles (LUV) of N-methyl dioleoylphosphatidylethanolamine ( N-methyl DOPE), containing encapsulated 1-aminonaphthalene-3,6,8-trisulfonic acid (ANTS) and/or p-xylene bis(pyridinium bromide) (DPX), were formed by extrusion. Vesicle fusion (contents mixing) and leakage were then monitored with the ANTS/DPX fluorescence assay. Sendai virus fusion with lipid vesicles and Sendai virus fusion with human erythrocyte membranes were measured by following the relief of fluorescence quenching of virus labeled with octadecylrhodamine B chloride (R 18), a lipid mixing assay for fusion. This study found that the effectiveness of the peptides carbobenzoxy- l-Phe- l-Phe (Z- l-Phe- l-Phe), Z- l-Phe, Z-D-Phe, and Z-GIy- l-Phe- l-Phe in inhibiting N-methyl DOPE LUV fusion or fusion of virus with N-methyl DOPE LUV also paralleled their reported ability to block viral infectivity. Furthermore, Z- d-Phe- l-PheGly and Z-GIy- l-Phe inhibited Sendai virus fusion with human erythrocyte membranes with the same relative potency with which they inhibited vesicle-vesicle and virus-vesicle fusion. The evidence suggests a mechanism by which these peptides exert their inhibition of plaque formation by enveloped viruses. This class of inhibitors apparently acts by inhibiting fusion of the viral envelope with the target cell membrane, thereby preventing viral infection. The physical pathway by which these peptides inhibit membrane fusion was investigated. 31 P nuclear magnetic resonance (NMR) of proposed intermediates in the pathway for membrane fusion in LUV revealed that the potent fusion inhibitor Z- d-Phe- l-PheGly selectively altered the structure (or dynamics) of the hypothesized fusion intermediates and that the poor inhibitor Z-GIy- l-Phe did not. One possible interpretation of these 31P NMR results was that the inhibitory peptide stabilized a membrane structure with a large radius of curvature, when the fusion pathway demanded a membrane defect with a small radius of curvature. This hypothesis was tested by determining the influence of an inhibitory and a noninhibitory peptide on the formation of membraneous structures with small radii of curvature, through ultrasonic irradiation of phospholipid dispersions. The inhibitory peptide prevented the formation of membrane structures with small radii of curvature, while the noninhibitory peptide did not prevent the formation of such structures. These data taken collectively suggest that the peptides that inhibit the fusion of enveloped viruses could act, at least in part, by interfering with the formation of intermediates in membrane fusion (possibly punctate membrane defects induced by the virus) that force lipids into structures with small radii of curvature.

Mass transfer modeling of asymmetric membrane formation by phase inversion

AbstractA derivation is presented of a ternary diffusion model to describe the mass transfer processes associated with the quench bath period of the phase inversion process for membrane formation. The complete governing equations, initial conditions, and boundary conditions in the casting film and coagulation bath are presented. Equations for ternary chemical potentials and diffusion coefficients are consistently based on constant specific volume formulations. The model is applied to the analysis of mass transfer paths and their effects on membrane structure formation. Precipitation times are determined for given sets of conditions by superposing calculated mass transfer paths on the ternary phase diagram and observing when the miscibility gap is crossed. Comparisons are made with an earlier reported study on the membrane‐forming system: water‐acetone‐cellulose acetate (CA). Agreement between predicted and measured precipitation times is found to be excellent. The polymer film composition profile at the moment of precipitation is shown to be a useful indicator of both skin and sublayer structures, allowing distinctions to be made between conditions leading to spongelike and fingerlike morphologies. The influence of model parameters on the mass transfer paths and associated polymer profiles is also discussed.

Nasu-Hakola disease (membranous lipodystrophy): Clinical, histopathological and biochemical studies of three cases

We report 3 cases of Nasu-Hakola disease found in 2 families. These cases had identical clinical features with progressive spastic paraplegia and severe dementia after adolescence. They had no history of any skeletal symptoms, but roentgenographs of their bones presented characteristic evidence of polycystic osteodysplasia. All cases revealed not only manifestations of this condition in the central nervous system, but also peripheral neuropathy with axonal degeneration. The membranous structures in the adipose tissues appeared histochemically to be composed of a kind of compound glycolipid or glycoprotein. Histopathologically, the biopsied rectum showed the infiltration of many histiocytes in the mucosa and ultrastructurally, the granules in these histiocytes showed many membrane-bound vacuoles of different sizes. Interestingly, the histochemical reactivity of the material in the granules was very similar to that of membranous structures in adipose tissues. In the biochemical analysis of lipids in affected adipose tissues, no marked abnormalities were found in the patients. Nasu-Hakola disease is not a typical form of lysosomal storage disease, because lysosomal enzyme activities remain normal and there is no accumulation of urinary oligosaccharides and lipids, no vacuolation of lymphocytes, and no hepatosplenomegaly. However, histochemical findings suggest that the lysosomes may be secondarily involved in this disease, and that the formation of membranous structures might be related to the disturbance of glycolipid or glycoprotein metabolisms.

Analysis of nonsolvent–solvent–polymer phase diagrams and their relevance to membrane formation modeling

AbstractCalculations have been carried out, based on Flory–Huggins solution theory, to analyze the behavior of the ternary nonsolvent–solvent–polymer phase diagram for typical membrane‐forming systems. Consideration is given to the behavior of the spinodal as well as binodal curves, tie‐line slopes, and critical points as a function of various parameters, most especially those related to the concentration dependency of the interaction parameters. Implications regarding membrane structure formation are discussed, and the suitability of different functional forms for the interaction parameter concentration dependence is also analyzed.The net result of these calculations is to demonstrate the importance of the various parameters in controlling the phase‐diagram behavior and particularly to show the critical role of the concentration dependence of the solvent–polymer interaction parameter in affecting the nature of the miscibility gap.

Polymer membrane formation through the thermal-inversion process. 2. Mathematical modeling of membrane structure formation

La membrane structuree est formee quand un film polymere-solvant est coagule thermiquement a partir d'une face tandis que l'autre face est isolee thermiquement