Negative Surface Charge Density Research Articles

Effect of the global electroneutrality condition on electromigration Taylor-Aris dispersion in a microcapillary with finite Debye layer thickness.

In this work, the electromigration dispersion (EMD) due to the charged electrolytes in microchannels is considered without using the thin electrical double layer assumption. The electrokinetic flow and transport of ions are actuated within a rectangular micro-/nanochannel having a negative surface charge density under the influence of an external electric field. Due to the local variation of the conductivity and the nonvalidity of the electroneutrality condition, the local electric field varies as a function of the solute concentration, wall surface charge density, valency, and Debye layer length. The resulting electrokinetic flow due to the external electric field drives the fluid along with the charged species, where the Taylor-Aris dispersion separates the solutes into their different constituents. The local concentration dependence of the electric field leads to the formation of concentration profiles that are slightly asymmetric with respect to the standard Gaussian distribution. Including a finite Debye layer thickness has an effect on the advection of the species as well as the diffusion of the species. It is found that in cases where Debye layers are thicker, the species advects faster within the microchannel. This might give valuable insights into the nature of the EMD. Our model aims to predict the evolution of ionic concentration at all positions within the channel. A study of the higher-order statistics in skewness and kurtosis has also been conducted to obtain a better understanding of the idealized model consisting of a buffer solution.

Constructing blocked-nanolayer by surface charge inversion over anion exchange membrane for improved antifouling performance

Anion exchange membranes (AEMs) with excellent antifouling property are essential for treating highly saline organic wastewater through electrodialysis (ED). In this study, we proposed a strategy of surface charge inversion to construct a blocked- nanolayer based on in-suit dopamine (DA) deposition followed by grafting negative-charged 2-acrylamide-2-methylpropanesulfonic acid (AMPS) for promoting the antifouling performance of AEM. It's indicated that the surface hydrophilicity and negative charge density was gradually enhanced with the increase of AMPS grafting density without sacrifices of ion exchange capability, swelling ratio and desalination performance. In the simulated fouling experiments, the optimal AEM-AMPS-105 membrane exhibited excellent antifouling property due to the improved surface hydrophilicity and high electrostatic repulsion against organic matter. It's highlighted that the transmembrane voltage of modified membrane after operating 200 min was reduced from 7.12 V to 4.85 V with a significant decrease of energy consumption by 55.1 % while maintaining a high current efficiency level of 85.5 %. Moreover, the modified membrane performed significantly enhanced salt/contaminant selectivity and high stability in dye wastewater treatment. We anticipated that the construction of charge-blocked nanolayer by surface charge inversion offered a practical method for improving the organic fouling-resistance of AEMs.

Nanometer-Resolved Operando Photo-Response of Faceted BiVO4 Semiconductor Nanoparticles.

Photo(electro)catalysis with semiconducting nanoparticles (NPs) is an attractive approach to convert abundant but intermittent renewable electricity into stable chemical fuels. However, our understanding of the microscopic processes governing the performance of the materials has been hampered by the lack of operando characterization techniques with sufficient lateral resolution. Here, we demonstrate that the local surface potentials of NPs of bismuth vanadate (BiVO4) and their response to illumination differ between adjacent facets and depend strongly on the pH of the ambient electrolyte. The isoelectric points of the dominant {010} basal plane and the adjacent {110} side facets differ by 1.5 pH units. Upon illumination, both facets accumulate positive charges and display a maximum surface photoresponse of +55 mV, much stronger than reported in the literature for the surface photo voltage of BiVO4 NPs in air. High resolution images reveal the presence of numerous surface defects ranging from vacancies of a few atoms, to single unit cell steps, to microfacets of variable orientation and degree of disorder. These defects typically carry a highly localized negative surface charge density and display an opposite photoresponse compared to the adjacent facets. Strategies to model and optimize the performance of photocatalyst NPs, therefore, require an understanding of the distribution of surface defects, including the interaction with ambient electrolyte.

Adsorption of Pb (II) ions on variable charge oxidic calcined substrates with chemically modified surface

Background: The adsorption process is an alternative method for treating natural and waste waters, with heavy metals. Oxidic lithological materials, rich in iron and aluminum amphoteric oxides, with pH-dependent surface charges, are a reliable medium for ionic adsorption. Being thermally resistant, these materials can be used to prepare a calcined substrate which is chemically treated in an acid or alkaline solution to enlarge surface positive or negative charge density, making it possible anion as well as cation adsorption reactions from aqueous solutions. Oxidic lithological materials use is a low-cost alternative for filtering system because of its availability and ease of preparation and application. Methods: Present paper shows results of the adsorption reaction of Pb+2 ions on calcined substrates prepared with oxidic lithologic material. The study was performed on the substrate with chemically modified surface in alkaline medium as well as on non-treated surface. Results: Results show L-type isotherms for the adsorption on the activated substrate, indicative affinity between adsorbate and adsorbent. Average value of adsorption capacity (k) for activated substrate is around 3.7 times greater (1791.73±13.06) compared to the respective average k value for the non-activated substrate (491.54±31.97), during the adsorption reaction, 0.35 and 0.26 mmolH+ of proton are produced on the activated and non-activated substrate respectively using a 1 mM Pb+2 solution and 72.2 and 15.6 mmolH+ using a 10 mM Pb+2 solution. This acidification agrees with the theoretic model of transitional metals chemisorption on amphoteric oxides of Fe, Al, Ti and Mn present in lithological material used for the preparation of adsorbent substrates confirming the information given by the L-type isotherms. Conclusions: Results suggest that these oxidic lithologic materials show great potential as an alternative technique for water treatment and heavy metal retention from contaminated waters using a low-cost and reliable adsorption system.

A serendipity of nanofiltration membrane modification using a simple approach: Limited sulfonamidation, remarkable improvements

Surface modification via secondary interfacial polymerization (IP) is a promising technology for the enhancement of nanofiltration (NF) membrane performance such as rejection selectivity. In this study, sulfuryl chloride (SC), containing sulfur element, was employed to introduce sulfonic groups into the active layer of membranes, with a main purpose of investigating the secondary IP mechanisms as well as the variation of membrane structural and physicochemical properties. Moreover, the SC-modified membranes would be endowed with favorable performance. Results showed that SC in a hexane environment behaved similarly with small and reactive diacyl dichloride in enhancing membrane surface negative charge density and contracting membrane pores. Membrane surface characterization showed that the sulfur element had a low concentration with an S/N ratio less than 0.04. This indicated that, on the one hand, the residual amine groups in the nascent active layer for secondary IP were quite limited, and on the other hand, the improvement of membrane performance was remarkably affected by the introduced sulfonyl groups. Consequently, SC in the hexane environment could further enhance the crosslinking degree of the active layer. In addition, SC modification could increase the membrane surface hydrophilicity and narrow the pore size distribution. The SC-modified membranes would be suitable for advanced treatment of drinking water. An optimized membrane with SC modification had a molecular weight cutoff of ∼350 Da and a surface zeta potential of ∼−55.8 mV, and could effectively reject five negatively charged per- and polyfluoroalkyl substances (with molecular weight ranging from 214 to 414 Da) by over 80% while with a low rejection of MgCl2 below 30%..

Effect of cholesterol on the ion-membrane interaction: Zeta potential and dynamic light scattering study

Cholesterol in a bio-membrane plays a significant role in many cellular event and is known to regulate the functional activity of protein and ion channel. In this study we report a significant effect of cholesterol on the ion-membrane interaction. We prepare large unilamellar vesicles, composed of zwitterionic lipid DOPC and anionic lipid DOPG with different cholesterol concentration. Electrostatics of anionic membranes containing cholesterol in the presence of NaCl has systematically been explored using dynamic light scattering and zeta potential. Negative zeta potential of the membrane decreases its negative value with increasing ion concentration for all cholesterol concentrations. However, zeta potential itself decreases with increasing cholesterol content even in the absence of monovalent ions. Electrostatic behaviour of the membrane is determined from well-known Gouy Chapmann model. Negative surface charge density of the membrane decreases with increasing cholesterol content. Binding constant, estimated from the electrostatic double layer theory, is found to increase significantly in the presence of cholesterol. Comparison of electrostatic parameters of the membrane in the presence and absence of cholesterol suggests that cholesterol significantly alter the electrostatic behaviour of the membrane.

Electro-osmotic flow in different phosphorus nanochannels

The electrokinetic transport in a neutral system consists of an aqueous NaCl solution confined in a nanochannel with two similar parallel phosphorene walls and is investigated for different black, blue, red, and green phosphorene allotropes in the presence of an external electric field in the directions x (parallel to the walls roughness axis) and y (perpendicular to the walls roughness axis). The results show that irrespective of the electric field direction, the thickness of the Stern layer increases with the increase in the magnitude of the negative electric surface charge density (ESCD) on the nanochannel walls, and it also increases with the increase in the roughness ratio for different allotropes. Moreover, three different regimes of Debye–Hückel (DH), intermediate, and flow reversal appear as the absolute value of the negative ESCD on the walls grows. With the increase in the absolute value of the negative ESCD, in the DH regime, the flow velocity grows, then in the intermediate regime, it decreases, and finally, at sufficiently high ESCD, the flow reversal occurs. When the external electric field is applied in the y direction, the dynamics of the system are slower than that of the x direction; therefore, the flow reversal occurs at the smaller absolute values of the negative ESCD.

Fabrication of Loose Nanofiltration Membranes with High Rejection Selectivity between Natural Organic Matter and Salts for Drinking Water Treatment.

Loose nanofiltration (LNF) membranes with a molecular weight cut-off (MWCO) of about 1000 Da and high surface negative charge density have great application potential for drinking water treatment pursuing high rejection selectivity between natural organic matter (NOM) and mineral salts. This study was conducted to exploit the novel method coupling non-solvent induced phase separation (NIPS) and interfacial polymerization (IP) for the preparation of high-performance LNF membranes. A number of LNF membranes were synthesized by varying the polyethersulfone (PES) and piperazine (PIP) concentrations in the cast solution for the PES support layer preparation. Results showed that these two conditions could greatly affect the membrane water permeance, MWCO and surface charge. One LNF membrane, with a water permeance as high as 23.0 ± 1.8 L/m2/h/bar, when used for the filtration of conventional process-treated natural water, demonstrated a rejection of NOM higher than 70% and a low rejection of mineral salts at about 20%. Both the mineral salts/NOM selectivity and permselectivity were superior to the currently available LNF membranes as far as the authors know. This study demonstrated the great advantage of the NIPS–IP method for the fabrication of LNF membranes, particularly for the advanced treatment of drinking water.

Loose nanofiltration membranes for selective rejection of natural organic matter and mineral salts in drinking water treatment

Loose nanofiltration (LNF) is a promising technology for advanced treatment of drinking water, primarily owing to the potentially high rejection of natural organic matter (NOM) and high passage of mineral salts, particularly divalent cations, along with the low operating pressure. This study was devoted to relating the performance with the properties of LNF membranes, and obtaining the membrane properties most suitable for drinking water treatment. Cluster analysis indicated that the commercially available LNF membranes would behave quite differently, mainly depending on the molecular weight cut-off (MWCO). By using a treated natural water as the feed, a best-performing membrane had a mineral salts rejection lower than 30% and an NOM rejection higher than 70%, with the membrane water permeance at ∼17 L/m2/h/bar. The LNF membranes with high mineral salts/NOM selectivity should have an MWCO of around 1000 Da, sufficiently uniform pore sizes, and high surface negative charge density. The rejection characteristics for mineral ions by LNF membranes were similar with the relatively dense conventional NF membranes, but with a lower rejection difference among the ions, manifesting less strong co-ion and counter-ion competition effects. The LNF membranes with an MWCO of ∼1000 Da could satisfactorily remove (>80%) disinfection byproduct precursors, with small-sized protein-like and other substances remaining in the permeate water. This study helps to understand the rejection characteristics and mechanisms of LNF membranes, which will benefit the synthesis of high-performance LNF membranes for drinking water treatment and other applications.

Novel and effective hemostats based on graphene oxide-polymer aerogels: In vitro and in vivo evaluation.

In this study, graphene oxide (GO)-based aerogels cross-linked with chitosan (CS), gelatin (GEL), and polyvinyl alcohol (PVA) were characterized and their hemostatic efficiencies both in vitro and in vivo were investigated and compared to commercial materials (ChitoGauze®XR and Spongostan™). All aerogels exhibited highly porous structures and a negative surface charge density favorable to their interaction with blood cells. The in vitro studies showed that all aerogels coagulated >60% of the blood contained in their structures after 240s of the whole-blood clotting assay, the GO-CS aerogel being the one with the highest blood clotting. All aerogels showed high hemocompatibility, with hemolytic rates <5%, indicating their use as biomaterials. Among them, the GO-GEL aerogel exhibited the lowest hemolytic activity, due possibly to its high GEL content compared to the GO amount. According to their blood clotting activity, aerogels did not promote coagulation through extrinsic and intrinsic pathways. However, their surfaces are suitable for accelerating hemostasis by promoting alternative routes. All aerogels adhered platelets and gathered RBCs on their surfaces, and in addition the GO-CS aerogel surface also promoted the formation of filamentous fibrin networks adhered on its structure. Furthermore, in vivo evaluations revealed that all aerogels significantly shortened the hemostatic times and reduced the blood loss amounts compared both to the Spongostan™ and ChitoGauze®XR commercial materials and to the gauze sponge (control group). The hemostatic performance in vitro and in vivo of these aerogels suggests that they could be used as hemostats for controlling profuse bleedings.

Insight into the melamine-derived freeze-dried nanostructured g-C3N4 for expeditious photocatalytic degradation of dye pollutants

Efficient g-C3N4 photocatalysts have been successfully fabricated with different synthesis approaches. The XRD patterns confirm formation of g-C3N4 nanostructures. The freeze-dried samples exhibit the porous sheet like morphology, and FTIR analysis confirms the CN heterocycles. The zeta potential studies reveal that g-C3N4 nanostructures have more negative surface charge density. The catalysts degrade the organic pollutant Methylene Blue (MB) and Rhodamine B (RhB) in the presence of visible light irradiation. Freeze-dried g-C3N4 catalyst demonstrations expeditious photocatalytic degradation the MB (98.8 %) and RhB (98.6 %) dye in 40 min. The maximum photocatalytic degradation performance because of its low e-h recombination rate, wide range of absorption light, more photo-generated electrons and larger specific surface area. Trapping test confirms that photogenerated electrons will be the main reason for the efficient degradation of organic dyes (MB). The repeatability experiment results evidently confirm the prepared catalyst have high photo-stability. The obtained degradation efficiency results were competing with the many previously reported photocatalyst. These attempts will be useful to realize the photocatalytic mechanism of g-C3N4 with respective different synthesis routes.

High zeolite loading mixed matrix membrane for effective removal of ammonia from surface water

While zeolite-based mixed matrix membrane (MMM) has been proven effective to remove the ammonia in the wastewater by adsorption, its adsorption capacity is limited by the low zeolite loading due to the need of a high concentration of polymer matrix to maintain the mechanical strength. To break the bottleneck, in this study we proposed a facile solvent evaporation method instead of conventional phase inversion method to prepare the zeolite-based MMMs. With this new preparation method, the loading of zeolite could reach up to ∼90wt.% while the MMM still maintained a good mechanical property. The zeolite-based MMM could treat 910 L·m−2 of feedwater before reaching the ammonia breakthrough point (0.5 mg-N·L−1) when treating the synthetic wastewater water. In addition, it showed a high rejection of turbidity and natural organic material (NOM) (∼90%), mainly due to its high negative surface charge density. When applied to treat real surface water, the membrane demonstrated a high normalized treatable capacity (∼900 L·m−2) with a high rejection to NOM (87.4%). Moreover, the MMM even showed a higher fouling resistance than the PVDF microfiltration membrane. Regeneration and cleaning with NaClO could efficiently recover the adsorption capacity and water flux of the MMM. After four cycles of operation, the MMM still maintained a high treatable capacity (850 L·m−2) with a high NOM rejection. This study provides a new strategy for the preparation of high-loading zeolite-based MMM for the effective removal of ammonia from surface water.

Surface composite modification of anion exchange membrane by electrodeposition and self-polymerization for improved antifouling performance

A novel composite modified membrane (PSS&PDA-M) was prepared by constructing polydopamine (PDA) self-polymerization modified layer covering electrodeposited poly (sodium 4-styrene sulfonate) (PSS) modified layer. The improvement on surface hydrophilicity and negative charge density of PSS&PDA-M mainly benefited from electrodeposited PSS modified layer. PDA self-polymerization modification layer not only improved the stability of the composite modified layer but also reduced the surface roughness of modified membrane. The results of desalination and antifouling experiments indicated that PSS&PDA-M had the best ability to inhibit fouling by sodium dodecyl sulfonate without affecting desalination performance. And it still had excellent antifouling performance after 120 h rinsing test. Moreover, the surface hydrophilicity and negative charge density of PSS&PDA-M with the best antifouling properties were slightly inferior to electrodeposited PSS modified AEM (ePSS-M). Therefore, enhancing the stability of modified layer was also crucial to improving the antifouling performance of AEM.

Triboelectric charging of melt-blown nonwoven filters with high filtration efficiency

As a novel technology to convert low-frequency energy into electric power, the triboelectric nanogenerator is a hot research topic recently. However, the nature of charge carriers and their transfer mechanisms still remain poorly understood, especially for the cases of liquid–solid triboelectric nanogenerator. In this paper, charges produced by a triboelectric charging process were designed to provide melt-blown nonwoven fabrics with high filtration efficiency by making full use of the electrostatic attraction filtration mechanism. Influences of water conductivity and drying temperature on the filtration efficiency of melt-blown nonwoven fabrics were investigated. And the corresponding properties such as the surface charge potential and charge stability were analyzed by using the electrostatic voltmeter, bio atomic force microscope and thermally stimulated discharge technique. In addition, metal and inorganic elements in the masterbatch and water before and after triboelectric charging were measured in order to uncover the charge transfer mechanism. Melt-blown nonwoven fabrics with filtration efficiency as high as 96.8% was obtained through the triboelectric charging treatment by using water with the conductivity as low as 1.1 μS/cm for the first time. Negative and positive surface charge density appeared randomly on both sides of melt-blown nonwoven fabrics after the triboelectric charging treatment from the bio atomic force microscope measurement while only one kind of surface charge density can be achieved in the research of TENG, that is, negative or positive. It seems there are both electron and ion transfers during the triboelectric charging process and electron transfer seems to have more important contribution for the generation of charges.

Cathodic corrosion of Au in aqueous methanolic alkali metal hydroxide electrolytes: Notable role of water

AbstractCathodic corrosion is an electrochemical process that induces restructuring, roughening, and etching of metal surfaces at a highly negative surface charge density, yet, details of the reaction mechanism are not fully resolved. An in‐depth fundamental understanding of the processes and parameters underlying cathodic corrosion is crucial for tailoring the surface structure of the metal electrodes and for synthesizing shape‐ and size‐controlled nanoparticles. Here, we investigate the relevance of water and hydrogen evolution in the cathodic corrosion process. To achieve this aim, Au electrodes were polarized at ‐1.6 V versus RHE in KOH and NaOH electrolytes prepared using different water + methanol mixtures. Structural changes of the Au surfaces were studied by cyclic voltammetry and monitored by scanning electron microscopy (SEM). Most importantly, cathodic corrosion does not take place in the absence of water. There is no detectable bubble formation due to the hydrogen evolution reaction on Au in purely methanolic alkali. Furthermore, the electrochemically active surface area, facet distribution, and surface morphology of Au electrodes are significantly altered upon cathodic polarization as a function of the water concentration. Cathodic corrosion features become more and more pronounced with a further increase in water content. In addition, substantial differences in the surface structure of Au are observed as a function of the nature and concentration of alkali metal cations. Overall, this study provides a more detailed understanding of the role of water and the hydrogen evolution reaction in dominating cathodic corrosion, which might advance the understanding of this phenomenon.

Understanding the anchoring interaction of coagulation factor Va light chain on zeolites: A molecular dynamics study

HypothesisFactor Va (FXa) and Xa (FVa) can assemble on the phosphatidylserine (PS) membrane (of platelet) to form prothrombinase complex and contribute to blood clotting. Very recently, we discovered that Ca-zeoliteacts as a type of reinforced activated inorganic platelet to enable assembly of prothrombinase complex and display an unusual zymogen (prothrombin) activation pattern. Inspired but not constrained by nature, it is of great interest to understand how FVa and FXa assembly on the inorganic surface (e.g., zeolites) and perform their biocatalytic function. ExperimentsGiven the important role of FVa C1-C2 domains in the assembly and activity of the prothrombinase complex, in this work, molecular dynamics simulations were performed to investigate the binding details of FVa A3-C1-C2 domains on the PS membranes and Ca2+-LTA-type (CaA) zeolite surface. FindingsWe found that different from the natural PS membrane, FVa light chain repeatedly exhibits a strong C2 domain anchoring interaction on the CaA zeolite. It mainly arises from the porous surface structure of CaA zeolite and local highly dense solvation water clusters on the CaA zeolite surface restrict the movement of some lysine residues on the C2 domain. The anchoring interaction can be suppressed by reducing the surface negative charge density, so that FVa light chain can change from single-foot (only C2 domain) to double-foot (both C1-C2 domain) adsorption states on the zeolite surface. This double-foot adsorption state is similar to natural PS membrane systems, which may make FVa have higher cofactor activity.

Monte Carlo analysis of the influence of surface charges on GaN asymmetric nanochannels: Bias and temperature dependence

In this paper, the occupancy of sidewall surface states having a clear signature in the performance of AlGaN/GaN-based self-switching diodes (SSDs) is analyzed using a semi-classical Monte Carlo (MC) simulator in a wide temperature (T) range, from 100 to 300 K. Experimental I–V curves show an unusual current decrease at low temperature attributed to surface trapping. The dependence on T of the negative surface charge density σ at the etched sidewalls of the SSDs is essential to explain the measurements. Two devices with different widths (80 and 150 nm) have been characterized and simulated in detail paying especial attention to the modeling of the surface states. At room temperature, MC simulations with a position-independent value of σ are able to qualitatively reproduce the I–V curves. However, a more complex approach is required to correctly replicate the values and shape of the DC experimental curves at low temperature, below 220 K. An algorithm where σ depends not only on T but also on the applied bias V is proposed to successfully fit the current values at every bias point. The model is able to explain the physics of the unexpected dependence of the resistance with the channel width and the sign change in the bowing coefficient, the parameters that govern the detection capabilities of the diodes.

Extracellular vesicles mediated exocytosis of antisense peptide nucleic acids

Peptide nucleic acids (PNAs), a synthetic DNA mimic, have been extensively utilized for antisense- and antigene-based biomedical applications. Significant efforts have been made to increase the cellular uptake of PNAs, but here we examined relatively unexplored aspects of intracellular trafficking and endocytic recycling of PNAs. For proof-of-concept, we used anti-microRNA (miR) PNA targeting miR-155. The sub-cellular localization of PNA was studied via confocal and flow-cytometry-based assays in HeLa cells. A comprehensive characterization of PNA-containing extracellular vesicles revealed spherical morphology, negative surface charge density, and the presence of tetraspanin markers. Most importantly, we investigated rab11a and rab27b GTPases’ role in regulating the exocytosis of PNAs. Organelle staining, followed by confocal imaging, showed higher localization of PNA in lysosomes. Gene-expression analysis established the enhanced functional activity of PNA after inhibition of endocytic recycling. Multiple studies report the exocytosis of single-stranded oligonucleotides, short interfering RNAs (siRNAs), and nanocarriers. To our knowledge, this is the first mechanistic study to establish that PNA undergoes endocytic recycling and exocytosis out of tumor cells. The results presented here can serve as a platform to develop and optimize strategies for improving the therapeutic efficacy of PNAs by avoiding the recycling pathways.

Impact of sodium hypochlorite on rejection of non-steroidal anti-inflammatory drugs by biomimetic forward osmosis membranes

Biomimetic forward osmosis membranes with a polyamide-based active layer containing polymersomes incorporating aquaporins were exposed to various free chlorine doses (free chlorine concentration x exposure time) and pH. Membrane chlorination was found to decrease the apolar/polar balance of the membrane surface energy and to increase its negative surface charge density. A substantial modification of the hydrogen-bond network in the membrane active layer was also highlighted. The separation performance of biomimetic membranes was also impacted by chlorine-induced chemical ageing. For the lowest free chlorine doses (100 ppm h), the reverse salt flux was found to increase with respect to the pristine membrane while the water flux dropped by 10–20%. For higher free chlorine doses (1000 and 10 000 ppm h), both the reverse salt flux and the water flux were found higher than for the pristine membrane. These behaviors were explained by the combination of several mechanisms with possible antagonist effects, namely a reduced internal concentration polarization due to ageing of the membrane support material, and modifications of the hydrogen-bond network and surface charge density of the membrane active layer. Although the degradation caused by sodium hypochlorite increased the draw-solute loss per unit of water passed, the biomimetic membranes maintained their rejection ability towards naproxen and diclofenac as rejections higher than 95.9% were measured irrespective of the ageing conditions.

Soluble TatA forms oligomers that interact with membranes: Structure and insertion studies of a versatile protein transporter

The twin-arginine translocase (Tat) mediates the transport of already-folded proteins across membranes in bacteria, plants and archaea. TatA is a small, dynamic subunit of the Tat-system that is believed to be the active component during target protein translocation. TatA is foremost characterized as a bitopic membrane protein, but has also been found to partition into a soluble, oligomeric structure of yet unknown function. To elucidate the interplay between the membrane-bound and soluble forms we have investigated the oligomers formed by Arabidopsis thaliana TatA. We used several biophysical techniques to study the oligomeric structure in solution, the conversion that takes place upon interaction with membrane models of different compositions, and the effect on bilayer integrity upon insertion. Our results demonstrate that in solution TatA oligomerizes into large objects with a high degree of ordered structure. Upon interaction with lipids, conformational changes take place and TatA disintegrates into lower order oligomers. The insertion of TatA into lipid bilayers causes a temporary leakage of small molecules across the bilayer. The disruptive effect on the membrane is dependent on the liposome's negative surface charge density, with more leakage observed for purely zwitterionic bilayers. Overall, our findings indicate that A. thaliana TatA forms oligomers in solution that insert into bilayers, a process that involves reorganization of the protein oligomer.