Surface Charge Potential Research Articles

Importance of the Liposomal Cationic Lipid Content and Type in Tumor Vascular Targeting: Physicochemical Characterization and In Vitro Studies Using Human Primary and Transformed Endothelial Cells

Using cationic liposomes to deliver cytotoxic molecules to the tumor microvasculature is currently being developed for the treatment of cancer and other angiogenesis-related diseases. To improve on their beneficial properties, the authors have examined whether the particular cationic lipid type and lipid content employed are important factors influencing cellular interactions and formulation effects. The authors prepared different PEG (polyethylene glycol)-modified cationic liposomes (PCLs) with varying percent cationic lipid content and lipid type, and evaluated liposome size, surface charge (zeta) potential, and cellular properties in vitro. The cell lines used were human umbilical vein (HUVEC), lung microvascular (HMVEC-L and HPVE-26), coronary microvascular (HMVEC-C), dermal microvascular (HMVEC-D), and immortalized dermal microvascular (HMEC-1) endothelial cells. In vitro experiments consisted of cellular uptake and cytotoxicity studies, fluorescence-activated cell sorting (FACS) analysis, fluorescence, and transmission electron microscopic analysis. Liposome size and zeta potential analysis of five different PCLs revealed significant differences in their physicochemical properties. Some cationic lipids formed relatively toxic liposomes compared to others. The efficiency of loading chemotherapeutic drugs (doxorubicin hydrochloride, etoposide), affinity of PCLs for endothelial cells, and formulation effects varied according to cationic lipid content and the lipid type. Cellular uptake was observed in lung, dermal, and coronary endothelial cells. Heparan sulfate proteoglycans were found present on HMEC-1 cells, which may have enabled PCL uptake. In conclusion, physicochemical properties of cationic liposomes and their ability to interact with endothelial cells are important factors to consider during the early stages of formulation development for the treatment of cancer and other angiogenesis-dependent diseases.

Crystal structure of T4 endonuclease VII resolving a Holliday junction

Holliday proposed a four-way DNA junction as an intermediate in homologous recombination, and such Holliday junctions have since been identified as a central component in DNA recombination and repair. Phage T4 endonuclease VII (endo VII) was the first enzyme shown to resolve Holliday junctions into duplex DNAs by introducing symmetrical nicks in equivalent strands. Several Holliday junction resolvases have since been characterized, but an atomic structure of a resolvase complex with a Holliday junction remained elusive. Here we report the crystal structure of an inactive T4 endo VII(N62D) complexed with an immobile four-way junction with alternating arm lengths of 10 and 14 base pairs. The junction is a hybrid of the conventional square-planar and stacked-X conformation. Endo VII protrudes into the junction point from the minor groove side, opening it to a 14 A x 32 A parallelogram. This interaction interrupts the coaxial stacking, yet every base pair surrounding the junction remains intact. Additional interactions involve the positively charged protein and DNA phosphate backbones. Each scissile phosphate that is two base pairs from the crossover interacts with a Mg2+ ion in the active site. The similar overall shape and surface charge potential of the Holliday junction resolvases endo VII, RuvC, Ydc2, Hjc and RecU, despite having different folds, active site composition and DNA sequence preference, suggest a conserved binding mode for Holliday junctions.

A new method to evaluate polydisperse kaolinite clay particle removal in roughing filtration using colloid filtration theory

Previous application of colloid filtration theory to roughing filtration has not considered a reliable method for determining a representative attachment factor for a polydisperse suspension (of constant particle density). Establishment of such a method would broaden the application of trajectory modelling in roughing filtration, and progress the development of a comprehensive database of attachment factors and surface charge potentials for various particle and fluid types. This study establishes a methodology for the application of colloid filtration theory to roughing filtration and incorporates recent advancements in theoretical single-collector efficiency. A polydisperse kaolinite clay suspension was passed through a series of four gravel upflow roughing filters and removal efficiencies were calculated. Both the classical and Tufenkji and Elimelech's more recent correlation equations were used to calculate theoretical single-collector efficiencies and associated attachment factors for three different filter media sizes, flow rates, and suspended solids concentrations (0.137±0.023). The use of Tufenkji and Elimelech's modified correlation equation resulted in reduced variability in the estimation of theoretical single-collector efficiencies.

Structural and Functional Characterization of a Secreted Hookworm Macrophage Migration Inhibitory Factor (MIF) That Interacts with the Human MIF Receptor CD74

Hookworms, parasitic nematodes that infect nearly one billion people worldwide, are a major cause of anemia and malnutrition. We hypothesize that hookworms actively manipulate the host immune response through the production of specific molecules designed to facilitate infection by larval stages and adult worm survival within the intestine. A full-length cDNA encoding a secreted orthologue of the human cytokine, Macrophage Migration Inhibitory Factor (MIF) has been cloned from the hookworm Ancylostoma ceylanicum. Elucidation of the three-dimensional crystal structure of recombinant AceMIF (rAceMIF) revealed an overall structural homology with significant differences in the tautomerase sites of the human and hookworm proteins. The relative bioactivities of human and hookworm MIF proteins were compared using in vitro assays of tautomerase activity, macrophage migration, and binding to MIF receptor CD74. The activity of rAceMIF was not inhibited by the ligand ISO-1, which was previously determined to be an inhibitor of the catalytic site of human MIF. These data define unique immunological, structural, and functional characteristics of AceMIF, thereby establishing the potential for selectively inhibiting the hookworm cytokine as a means of reducing parasite survival and disease pathogenesis.

Synthesis and Structure‐Property Relationships of Fluorinated Poly(ether imide)s as Electret Materials

AbstractThe aim of this work was to establish and better understand the structure‐property relationship of fluorinated aromatic poly(ether imide)s (PEI) with respect to their electret performance. Therefore two series of PEIs with increasing fluorine content were synthesized. All PEIs are amorphous materials with glass transition temperatures above 220 °C. Thin films with thicknesses around 100 µm were compression molded and corona charged, and isothermal surface potential decay was investigated at 90 °C as an accelerated test for the electret performance. In comparison to the corresponding non‐fluorinated PEI, an improvement of up to thirty‐fold was achieved by the incorporation of trifluoromethyl moieties. Out of the investigated fluorinated PEIs, the polymer with the best charge storage performance maintained 87% of the initial surface potential charge after annealing at 90 °C for 24 h. It was also found that, within the investigated series, the highest fluorine content does not necessarily result in the best charge‐storage properties.magnified image

Surface charges on alumina in vacuum with varying surface roughness and electric field distribution

We investigated the charging characteristics of alumina in vacuum with varying surface roughness, and the electric field distribution. The charging on the alumina surface in vacuum is, in general, strongly influenced by field electron emission (FEE) and secondary electron emission avalanche (SEEA). We varied the surface roughness and the electric field distribution on the alumina surface in order to control the FEE and the SEEA. Under these conditions, we measured the 2-dimensional distribution of surface charging potential on the alumina surface. From the measurement results, we quantitatively discussed charge polarity and charge density on the surface for various types of electric field distributions. Finally, we successfully propose a concept of a charging control technique on the alumina insulator in vacuum for the improvement of electrical insulation performance on the alumina surface.

Cleaning results of new and fouled nanofiltration membrane characterized by zeta potential and permeability

In the membrane process industries, membrane cleaning is one of the most important concerns from both economical and scientific points of view. The characterisation of membrane surface by using zeta potential technique provides a more appropriate tool to study the cleaning performance and membrane integrity. The use of zeta potential technique to characterize the cleaning effect on membrane surfaces has very little precedence in literature. In this study the cleaning performance has been studied with membrane zeta potential and hydraulic permeability measurement. Virgin nanofiltration (NF) membranes provided by GE Osmonics (DK, HL and DL), as well as fouled DK membranes were used in this study. Several cleaning agents were investigated, some of them were analytical grade, such as HCl, NaOH and others were commercial grade agents that are already in use in commercial plants. Zeta potential of virgin and fouled NF membranes before and after chemical cleaning were measured and compared. The results of the zeta potential measurements with and without chemical cleaning of virgin and fouled membrane indicated that zeta potential may be used to characterise the membrane surface cleanliness and permeability. The effects of temperature, pH and the cleaning agents on membrane zeta potential were also investigated. The temperature was not found to have any significant effect on membrane zeta potential; however, the effect of pH and cleaning agents had pronounced effects on the surface charge and zeta potential. At high pH values mixed cleaning agents of mixture of trisodium phosphate, sodium tripolyphosphate and EDTA (recommended by membrane manufacturer) had a marked effect on zeta potential of HL and DL membranes compared to other cleaning agents. A DK fouled membrane was found to be almost negatively charged with no isoelectric point compared to a virgin DK membrane. At low pH, the zeta potential values of a fouled DK membrane were in close agreement with those obtained with an untreated virgin membrane after cleaning by NaOH followed by HCl. The zeta potential of all virgin membranes cleaned with SDS changed to more negative for the entire pH range compared to untreated virgin membranes. The study revealed that the SDS cleaning agent had the highest effect on surface charge for both virgin and fouled NF membranes.

A generalized model for the stability of polymer colloids

A generalized model has been proposed to describe the stability of polymer colloids stabilized with ionic surfactants by accounting simultaneously for the interactions among three important physicochemical processes: colloidal interactions, surfactant adsorption equilibrium, and association equilibria of surface charge groups with counterions at the particle–liquid interface. A few Fuchs stability ratio values, determined experimentally for various salt types and concentrations through measurements of the doublet formation kinetics, are used to estimate the model parameters, such as the surfactant adsorption and counterion association parameters. With the estimated model parameters, the generalized model allows one to monitor the dynamics of surfactant partitioning between the particle surface and the disperse medium, to analyze the variation of surface charge density and potential as a function of the electrolyte type and concentration, and to predict the critical coagulant concentration for fast coagulation. Three fluorinated polymer colloids, stabilized by perfluoropolyether-based carboxylate surfactant, have been used to demonstrate the feasibility of the proposed colloidal stability model.

The potential toxicity of nanomaterials—The role of surfaces

Nanotechnology has attracted considerable attention in the scientific community ever since its emergence as a powerful basic and applied science tool. While beneficial aspects of nanomaterials are well visioned, several reports have suggested the negative impact of nanomaterials on living cells. The diverse array of surface properties achieved due to reduction in particle size that catalyzes the surface chemistry of nanoparticles is responsible for their toxic potential. Physical parameters such as surface area, particle size, surface charge, and zeta potential are very important for providing mechanistic details in the uptake, persistence, and biological toxicity of nanoparticles inside living cells. This short review provides insights into the physical, chemical, and interfacial parameters on the toxic potential of nanomaterials. While nanotechnology has promised invaluable progress in science and technology, the onus rests on the scientific community to predict the unknown outcome on the biological system for its safe proliferation.

Surface Charge Density and Electrokinetic Potential of Highly Charged Minerals: Experiments and Monte Carlo Simulations on Calcium Silicate Hydrate

In this paper, we are concerned with the charging and electrokinetic behavior of colloidal particles exhibiting a high surface charge in the alkaline pH range. For such particles, a theoretical approach has been developed in the framework of the primitive model. The charging and electrokinetic behavior of the particles are determined by the use of a Monte Carlo simulation in a grand canonical ensemble and compared with those obtained through the mean field theory. One of the most common colloidal particles has been chosen to test our theoretical approach. That is calcium silicate hydrate (C-S-H) which is the main component of hydrated cement and is known for being responsible for cement cohesion partly due to its unusually high surface charge density. Various experimental techniques have been used to determine its surface charge and electrokinetic potential. The experimental and simulated results are in excellent agreement over a wide range of electrostatic coupling, from a weakly charged surface in contact with a reservoir containing monovalent ions to a highly charged one in contact with a reservoir with divalent ions. The electrophoretic measurements show a charge reversal of the C-S-H particles at high pH and/or high calcium concentration in excellent agreement with simulation predictions. Finally, both simulation and experimental results clearly demonstrate that the mean field theory fails not only quantitatively but also qualitatively to describe a C-S-H dispersion under realistic conditions.

Structure of the N-terminal fragment of topoisomerase V reveals a new family of topoisomerases

Topoisomerases are involved in controlling and maintaining the topology of DNA and are present in all kingdoms of life. Unlike all other types of topoisomerases, similar type IB enzymes have only been identified in bacteria and eukarya. The only putative type IB topoisomerase in archaea is represented by Methanopyrus kandleri topoisomerase V. Despite several common functional characteristics, topoisomerase V shows no sequence similarity to other members of the same type. The structure of the 61 kDa N-terminal fragment of topoisomerase V reveals no structural similarity to other topoisomerases. Furthermore, the structure of the active site region is different, suggesting no conservation in the cleavage and religation mechanism. Additionally, the active site is buried, indicating the need of a conformational change for activity. The presence of a topoisomerase in archaea with a unique structure suggests the evolution of a separate mechanism to alter DNA.

An amino-terminal lysine residue of rat connexin40 that is required for spermine block.

Spermine blocks connexin40 (Cx40) gap junctions, and two cytoplasmic amino-terminal domain glutamate residues are essential for this inhibitory activity. To further examine the molecular basis for block, we mutated a portion of a basic amino acid (HKH) motif on the Cx40 amino-terminal domain. Replacement of the Cx40 H15 + K16 residues with the Q15 + A16 sequence native to spermine-insensitive connexin43 (Cx43) gap junctions increased the equilibrium dissociation constant (K(d)) and reduced the maximum inhibition by spermine. The corresponding electrical distance (delta) approximation was decreased by about 50%. The transjunctional voltage (V(j))-dependent gating of homotypic Cx40 H15Q + K16A mutant gap junctions was also significantly reduced. The minimum normalized steady-state junctional conductance (G(min)) increased from 0.17 to 0.72, with an increase in the half-inactivation voltage from 48 to 60 mV. However, the unitary junctional conductance (gamma(j); 160 pS) was only slightly altered, and the relative cation/anion conductance and permeability ratios were unchanged from wild-type Cx40 gap junction channels. The relative K(+)/Cl(-) permeability (P(K)/P(Cl)) ratio increased from six to ten when [KCl] was reduced to 25% of normal. These data suggest that the HKH motif at positions 15-17 is important to the conformational structure of the putative voltage sensor and spermine receptor of Cx40, without causing significant alteration of the electrostatic surface charge potentials that contribute to the ion selectivity of this gap junction channel.

Evaluation of Respirator Filters for Asbestos Fibers

Fiber aerosols are known to have different aerodynamic behaviors than spherical particles and usually carry higher electrostatic charges. We investigated the effects of flow rate and charge status of filter cartridges on the penetration of spherical and fiber aerosols. Four types of test respirator filters were selected: two for passive respirators, one for a powered respirator, and one disposable respirator. Surface charges on respirator filters were determined using a noncontact field electrostatic field meter. Penetration tests were performed for filter cartridges before and after charge neutralization. The surface charge measurements on the respirator filters showed that some filters, including those used in disposable face masks, are charged to enhance the collection efficiency. Only high-efficiency particulate air filters performed consistently for both spherical test aerosols and the three types of asbestos fibers. The surface charge potential of filter cartridges and flow rate did not appear to affect the performance of these filters. In contrast to the high-efficiency filters, the aerosol penetration performance of low-efficiency filters and face masks deteriorated when the charge potential on the filter was removed. Our data also showed that the surface charges decreased in a high-temperature, high-humidity environment and disappeared after 1 week. Deposition of spherical particles and fibers in the charged disposable facemask filter was enhanced. For charged-neutralized, low-efficiency filter cartridges, asbestos fibers may penetrate more than spherical particles with a mean particle size of 0.3 μ m diameter.

Three-Dimensional Solution Structures of the Chromodomains of cpSRP43

Chloroplasts contain a unique signal recognition particle (cpSRP). Unlike the cytoplasmic forms, the cpSRP lacks RNA but contains a conserved 54-kDa GTPase and a novel 43-kDa subunit (cpSRP43). Recently, three functionally distinct chromodomains (CDs) have been identified in cpSRP43. In the present study, we report the three-dimensional solution structures of the three CDs (CD1, CD2, and CD3) using a variety of triple resonance NMR experiments. The structure of CD1 consists of a triple-stranded beta-sheet segment. The C-terminal helical segment typically found in the nuclear chromodomains is absent in CD1. The secondary structural elements in CD2 and CD3 include a triple-stranded antiparallel beta-sheet and a C-terminal helix. Interestingly, the orientation of the C-terminal helix is significantly different in the structures of CD2 and CD3. Critical comparison of the structures of the chromodomains of cpSRP43 with those found in nuclear chromodomain proteins revealed that the diverse protein-protein interactions mediated by the CDs appear to stem from the differences that exist in the surface charge potentials of each CD. Results of isothermal titration calorimetry experiments confirmed that only CD2 is involved in binding to cpSRP54. The negatively charged C-terminal helix in CD2 possibly plays a crucial role in the cpSRP54-cpSRP43 interaction.

Structural basis for UTP specificity of RNA editing TUTases from Trypanosoma brucei.

Trypanosomatids are pathogenic protozoa that undergo a unique form of post-transcriptional RNA editing that inserts or deletes uridine nucleotides in many mitochondrial pre-mRNAs. Editing is catalyzed by a large multiprotein complex, the editosome. A key editosome enzyme, RNA editing terminal uridylyl transferase 2 (TUTase 2; RET2) catalyzes the uridylate addition reaction. Here, we report the 1.8 A crystal structure of the Trypanosoma brucei RET2 apoenzyme and its complexes with uridine nucleotides. This structure reveals that the specificity of the TUTase for UTP is determined by a crucial water molecule that is exquisitely positioned by the conserved carboxylates D421 and E424 to sense a hydrogen atom on the N3 position of the uridine base. The three-domain structure also unveils a unique domain arrangement not seen before in the nucleotidyltansferase superfamily, with a large domain insertion between the catalytic aspartates. This insertion is present in all trypanosomatid TUTases. We also show that TbRET2 is essential for survival of the bloodstream form of the parasite and therefore is a potential target for drug therapy.

Absence of Specific Cation or Anion Effects at Low Salt Concentrations on the Charge at the Oil/Water Interface

Surfactant-free 2 vol % hexadecane-in-water emulsions have been prepared at pH 9 in the presence of various alkali-metal salts. The surface charge and zeta potential of these emulsions are independent of the identities of the monovalent cations and anions up to 0.01 M electrolyte concentrations. The surface charge density of -5 microC cm(-2) is independent of the identity of the alkali-metal cation among Li, Na, and Cs. The zeta potentials decrease with the log of the salt concentration between 0.1 and 11 mM, independent of the identity of the anion of the sodium salts of iodide, bromide, chloride, fluoride, perchlorate, or iodate or of the identity of the cation of the chloride salts of Li, Na, or Cs. These results imply that neither hydration enthalpies nor ion dispersion potentials are significant in affecting the charge created by the hydroxide ion at the pristine oil/water interface at up to 0.01 M salt concentrations.

The Impact of Geometric and Surface Electronic Properties of Pt-Catalysts on the Particle Size Effect in Electrocatalysis

The particle size effect on the formation of OH adlayer, the CO bulk oxidation, and the oxygen reduction reaction (ORR) have been studied on Pt nanoparticles in perchloric acid electrolyte. From measurements of the CO displacement charge at controlled potential, the corresponding surface charge density versus potential curves yielded the potentials of total zero charge (pztc), which shifts approximately 35 mV negative by decreasing the particle size from 30 nm down to 1 nm. As a consequence, the energy of adsorption of OH is more enhanced, that is, at the same potential the surface coverage with OH increases by decreasing the particle size, which in turn affects the catalytic reactions thereon. The impact of the electronically induced potential shift in the OH adsorption is demonstrated at the CO bulk oxidation, in which adsorbed OH is an educt species and promotes the reaction, and the ORR, where it can act as a surface site blocking species and inhibits the reaction.

The Surface Charge on Coated Superconductors Carrying Transport Current

A careful solution of Maxwell's equations for the case of a circuit carrying a forced transport current in zero applied magnetic field shows that the current in the circuit is not divergenceless. A small component of the current diverges into the surface leading to a surface charge. This current along with the surface charge it produces is computed for two parallel strips of coated conductor with one strip acting as the return path for the other. Although the component of current with divergence is too small to be of much importance for the vector potential, it is completely responsible for the surface charge and scalar potential.

Surface Charge Heterogeneities Measured by Atomic Force Microscopy

Unfavorable aggregation and deposition of colloidal particles in natural and engineered systems is still a subject of debate. Complicating factors such as surface roughness, secondary minimum aggregation, and the nature of discrete surface charge and surface potential make it difficult to attribute a specific cause to these phenomena. The presence of surface charge heterogeneity and its influence on interaction forces, which are responsible for aggregation and deposition, are studied in this work through the application of atomic force microscopy (AFM). Force-volume-mode AFM was used to map interaction forces on a surface and relate them to surface charge heterogeneities. The experimental system consisted of a silica plate and a standard silicon nitride AFM tip. Copper ions were used for sorption on the silica surface in order to modify the surface charge and cause charge reversal. Different concentrations of copper ions were selected to identify conditions of partial coverage of the silica surface. The pH and ionic strength of the solutions were varied, and the extension of the surface charge modification and its influence on the resulting interaction forces were monitored via AFM force measurements. Depending on the pH and ionic strength, the interaction force was found to change at certain regions on the surface from attraction to either weak or strong repulsion. Force imaging allowed the visual localization of zones of strong repulsive interaction that diminished in size with increasing ionic strength. X-ray photoelectron spectroscopy analysis was used to confirm the presence of copper on the surface. Local charge differences on a surface result in local differences in surface forces, not only in magnitude but also in direction. This behavior may explain the aggregation, deposition, and transport of colloidal particles under unfavorable chemical conditions.

Scanning probe investigation of surface charge and surface potential of GaN-based heterostructures

Scanning Kelvin probe microscopy (SKPM) and electrostatic force microscopy (EFM) have been employed to measure the surface potentials and the surface charge densities of the Ga- and the N-face of a GaN lateral polarity heterostructure (LPH). The surface was subjected to an HCl surface treatment to address the role of adsorbed charge on polarization screening. It has been found that while the Ga-face surface appears to be unaffected by the surface treatment, the N-face surface exhibited an increase in adsorbed screening charge density (1.6±0.5×1010cm−2), and a reduction of 0.3±0.1V in the surface potential difference between the N- and Ga-face surfaces.