Changes In Solution Ionic Strength Research Articles

Interplay between epigallocatechin-3-gallate and ionic strength during amyloid aggregation.

The formation and accumulation of protein amyloid aggregates is linked with multiple amyloidoses, including neurodegenerative Alzheimer’s or Parkinson’s disease. The mechanism of such fibril formation is impacted by various environmental conditions, which greatly complicates the search for potential anti-amyloid compounds. One of these factors is solution ionic strength, which varies between different aggregation protocols during in vitro drug screenings. In this work, we examine the interplay between ionic strength and a well-known protein aggregation inhibitor—epigallocatechin-3-gallate. We show that changes in solution ionic strength have a major impact on the compound’s inhibitory effect, reflected in both aggregation times and final fibril structure. We also observe that this effect is unique to different amyloid-forming proteins, such as insulin, alpha-synuclein and amyloid-beta.

Interfacial Adsorption of Silk Fibroin Peptides and Their Interaction with Surfactants at the Solid-Water Interface.

Regenerated silk fibroin (RSF) is a Food and Drug Administration-approved material and has been widely used in many biomedical and cosmetic applications. Because of the amphiphilic nature of the primary repeat amino acid sequence (e.g., AGAGAS), RSF peptides can significantly reduce the water surface tension and therefore have the potential to be used as a surface active component for many applications, particularly in the biomedical, cosmetic, pharmaceutical, and food industries. In this paper, the adsorption of RSF peptides separated into molecular fractions of 5-30, 30-300, and >300 kDa has been studied at the solid-water interface by neutron reflection and spectroscopic ellipsometry to assess its surface active behavior. A stable layer of RSF was found to be irreversibly adsorbed at the hydrophilic SiO2-water interface. Changes in solution concentration, pH, and ionic strength all had an impact on the final adsorbed amount found at the interface. There were no significant differences between the final adsorbed amounts or layer structure among the three RSF molecular fractions studied; however, >300 kDa RSF was more stable to changes in solution ionic strength. Adsorption of conventional anionic and cationic surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (C12TAB), to the preadsorbed 5-30 kDa RSF revealed penetration of the surfactant into the RSF layer, at concentrations below the critical micellar concentration (CMC). SDS was found in the preadsorbed RSF layer and gradually removed RSF from the surface with an increase in SDS concentration. At concentrations above the CMC, there is near complete removal of RSF by SDS at the interface. C12TAB adsorbed into the preadsorbed RSF layer with considerably less removal of RSF from the interface compared to SDS. At concentrations above the CMC, both C12Tab and RSF were found to coexist at the interface, forming a less thick layer but with a considerable amount of RSF still present.

Electrostatic Effects in Filamentous Protein Aggregation

Electrostatic forces play a key role in mediating interactions between proteins. However, gaining quantitative insights into the complex effects of electrostatics on protein behavior has proved challenging, due to the wide palette of scenarios through which both cations and anions can interact with polypeptide molecules in a specific manner or can result in screening in solution. In this article, we have used a variety of biophysical methods to probe the steady-state kinetics of fibrillar protein self-assembly in a highly quantitative manner to detect how it is modulated by changes in solution ionic strength. Due to the exponential modulation of the reaction rate by electrostatic forces, this reaction represents an exquisitely sensitive probe of these effects in protein-protein interactions. Our approach, which involves a combination of experimental kinetic measurements and theoretical analysis, reveals a hierarchy of electrostatic effects that control protein aggregation. Furthermore, our results provide a highly sensitive method for the estimation of the magnitude of binding of a variety of ions to protein molecules.

Nano-mechanical properties of clay-armoured emulsion droplets

There has been a growing interest in understanding the stabilization mechanism of particle-armoured emulsion droplets over the last few decades due to their importance in many everyday products. Here, the mechanical properties of clay-armoured emulsion droplets were investigated using laser scanning confocal microscopy with an in situatomic force microscopy measurement. This combination allows the visualization of droplet shape as a function of applied force. The emulsion droplets were found to be mechanically robust, stable against coalescence during drop collisions and able to recover from large deformations without disintegration. A Hookean constitutive law was used to extract the surface Young's modulus of the clay-armoured droplets as a function of a range of solution conditions. The clay-armoured droplets were relatively insensitive to changes in solution ionic strength and pH. However, in the presence of cationic surfactants, the surface Young's modulus decreases and shows significant reduction well above the critical micelle concentration. These changes are most likely due to desorption of clay platelets from the oil–water interface after charge neutralisation and the eventual solubilisation of the oil droplet. The elasticity measurements in this study should help illuminate the impact of the clay-armoured droplets on macroscopic properties of emulsions including rheological properties and emulsion stability.

Self-assembled nanostructures in mixed anionic–neutral double hydrophilic block copolymer/cationic vesicle-forming surfactant solutions

The self-assembled nanostructures formed in mixed solutions of a double hydrophilic anionic–neutral block copolymer, poly[(2-sulfamate-3-carboxylate)isoprene-b-ethylene oxide] (SCIEO), and the vesicle-forming surfactant, didodecyldimethylammonium bromide (DDAB), are investigated. In these solutions electrostatic interactions exist between the anionic poly[(2-sulfamate-3-carboxylate)isoprene] block and the cationic surfactant. Combined static and dynamic light scattering measurements indicate that at low copolymer concentration vesicles of DDAB with adsorbed block copolymer chains are present in the solutions. As block copolymer concentration increases the structure of the nanoassemblies transforms to a core–shell, micellar-like structure, with a poly(ethylene oxide) corona and a core formed by the complex of poly[(2-sulfamate-3-carboxylate)isoprene] chains and DDAB molecules. This transformation of global aggregate structure should be attributed to a collapse of the DDAB vesicle bilayer as the number of complexed surfactant head groups to anionic polymeric segments increases. This happens in order to satisfy the conformational requirements of the complexing polymeric block and the steric requirements in the PEO corona, which apparently overcome the existing interactions between surfactant molecules in the ordered bilayer of initial DDAB vesicles. Block copolymer decorated vesicles show lower stability to an increase in solution temperature compared to micellar-like aggregates. Both nanostructures are found to be stable to changes in solution ionic strength, due to the combination of electrostatic and hydrophobic interactions acting within the mixed aggregates.

Extraction and Analysis of Extracellular Polymeric Substances: Comparison of Methods and Extracellular Polymeric Substance Levels inSalmonella pullorumSA 1685

Extracellular polymeric substances (EPS) production and composition for Salmonella pullorum SA 1685 exposed to artificial groundwater (AGW) has been examined utilizing three EPS extraction methods: lyophilization, ethanol, and sonication. Experiments were carried out to evaluate the robustness of three EPS extraction methods and the sensitivity of each to subtle changes in solution ionic strength (IS) and duration of exposure. EPS extraction and analysis was conducted via sugar and protein analyses using the phenol sulfuric acid and Lowry methods, respectively, after 0-, 6-, 12-, 18-, and 24-h incubation times in AGW with 10−2.5, 10−2, and 10−1.5 M IS. Lyophilization and ethanol methods resulted in a greater amount of EPS extracted than the sonication method (mass of EPS/cell), yet these methods fluctuated to a greater extent in the total amount—or level—of EPS extracted under the various test conditions. Systematic comparisons and extensive statistical analyses were conducted between the various experimental conditions. To our knowledge, this is the first study systematically comparing EPS extraction techniques utilizing Salmonella. As we investigated the relative EPS content in Salmonella SA1685 under conditions simulating groundwater, our results provide insight into the suitability of each method for detection of environmentally induced changes in bacteria suspended in simulated or real subsurface aquatic systems.

Colloid transport and remobilization in porous media during infiltration and drainage

Colloids are potential vectors of many contaminants in porous media. Understanding colloid transport is critical for assessing the migration of contaminants (e.g., pathogens) in the vadose zone. In this study, a series of column experiments were conducted to investigate the coupled effects of flow velocity, water content, and solution ionic strength on transport and remobilization of a model colloid (montmorillonite clay) in a model porous medium (Accusand) during transient unsaturated flow and steady-state saturated flow. The unsaturated transport experiments included a series of infiltration and drainage pulses (e.g., infiltration with colloids, followed by drainage of colloid suspensions, followed by infiltration with a colloid-free solution and drainage of the solution). Saturated flow experiments included only the infiltration of the colloid and colloid-free solutions. Tests were repeated for a variety of solution ionic strengths. Results showed that colloid transport was more sensitive to changes in solution ionic strength at low infiltration rates, and the effect of infiltration rate was more significant at high ionic strength. As a result, increased flow velocities and water content, resulting from high infiltration rates, enhanced colloid transport and remobilization under ionic strength conditions (e.g., 100 mM) that would otherwise lead to strong colloid retention. This observation conceptually suggests that chemical threshold values for preventing colloid movement in porous media might be larger for transient flow conditions than for uniform flow conditions. In addition, drainage was found to induce remobilization of the retained colloids, suggesting transport of colloids even after termination of injection. Overall, the study experimentally highlights the complicated interdependence of the effects of water content, flow velocity, and solution chemistry on colloid transport and remobilization.

Low cetyltrimethylammonium bromide concentrations induce reversible amorphous aggregation of tobacco mosaic virus and its coat protein at room temperature

Ordered and amorphous protein aggregation causes numerous diseases. Tobacco mosaic virus coat protein for many decades serves as the classical model of ordered protein aggregation (“polymerization”). It was also found to be highly prone to heat-induced amorphous aggregation and the rate of this aggregation could be easily manipulated by changes in solution ionic strength and temperature. Here, we report that rapid amorphous aggregation of this protein can be induced at 25 °C in phosphate buffer by low micromolar (start at about 15 μM) concentrations of cationic surfactant cetyltrimethylammonium bromide. At equilibrium four surfactant molecules bound to the protein subunit. As judged by circular dichroism and fluorescence spectroscopy data, the coat protein molecules retained their native structure upon the cetyltrimethylammonium bromide induced aggregation. No aggregation was observed at the higher surfactant concentrations (above 300 μM). Micromolar concentrations of anionic surfactant sodium dodecylsulfate rapidly reversed the cetyltrimethylammonium bromide induced aggregation of the coat protein due to formation of mixed surfactant–surfactant micelles. Cetyltrimethylammonium bromide (100–300 μM) also induced the reversible intact tobacco mosaic virus virion aggregation. The possible liability to the cetyltrimethylammonium bromide induced amorphous aggregation of other ordered aggregate-producing proteins has been discussed.

Interpreting Second-Harmonic Generation Images of Collagen I Fibrils

Fibrillar collagen, being highly noncentrosymmetric, possesses a tremendous nonlinear susceptibility. As a result, second-harmonic generation (SHG) microscopy of collagen produces extremely bright and robust signals, providing an invaluable tool for imaging tissue structure with submicron resolution. Here we discuss fundamental principles governing SHG phase matching with the tightly focusing optics used in microscopy. Their application to collagen imaging yields several biophysical features characteristic of native collagen structure: SHG radiates from the shell of a collagen fibril, rather than from its bulk. This SHG shell may correspond to the supporting element of the fibril. Physiologically relevant changes in solution ionic strength alter the ratio of forward-to-backward propagating SHG, implying a resulting change in the SHG shell thickness. Fibrillogenesis can be resolved in immature tissue by directly imaging backward-propagating SHG. Such findings are crucial to the design and development of forthcoming diagnostic and research tools.

Inconsistency in the triple layer model description of ionic strength dependent boron adsorption

Understanding anion adsorption mechanisms is necessary to allow prediction of anion adsorption behavior. This study was conducted to evaluate the ability of the triple layer model, a chemical surface complexation model, to describe the effect of changes in solution ionic strength (0.01–1.0 M NaCl) and solution pH (3–11) on B adsorption by the iron oxide, goethite, the aluminum oxide, gibbsite, the clay minerals, kaolinite and montmorillonite, and two arid zone soils. Ionic strength dependence of adsorption suggests an inner-sphere adsorption mechanism for goethite, kaolinite, montmorillonite, and the two soils and an outer-sphere adsorption mechanism for gibbsite. The triple layer model, containing an inner-sphere adsorption mechanism, was able to describe B adsorption on goethite, kaolinite, montmorillonite, and the two soils. The model was able to describe B adsorption on gibbsite using an outer-sphere adsorption mechanism. A problematic inconsistency exists in the triple layer model description of ionic strength dependent B adsorption between the type of B surface complex defined in the model and the ionic strength dependence of the model result. That is, postulating an inner-sphere adsorption mechanism in the triple layer model resulted in an ionic strength dependence appropriate for the formation of outer-sphere surface complexes and vice versa. Additional tests of the ability of the triple layer model to describe ionic strength dependent adsorption of additional ions are needed to establish whether the inconsistencies are limited to the B system or are of concern in other triple layer model applications.

Salinity and pH effect on the colloidal properties of suspended particles in super intensive aquaculture systems

A large amount of feed and other organic material is found as suspended matter in super intensive fish production tanks. Both treatment and recycling of these organic materials depend on the size and stability of the particles in suspension. The stability of suspended material and the states of dispersion or flocculation were studied extensively by colloid chemists. The effects of pH and electrolyte concentration on colloidal stability of suspended particles collected in super intensive hybrid tilapia ( Oreochromis niloticus× Oreochromis aureus) tanks, operated as an active suspension pond (ASP), were studied here. Higher salinity led to better flocculation of the suspended matter. The point of zero salt effect (PZSE) of the particles was found to be at a pH range of 2.8–4.2, similar to the isoelectric point (IEP) range commonly found in bacteria. Using multiple regression analysis, 92% of the colloidal dispersion variation was attributed to changes in solution ionic strength; addition of pH to model increased the model adjusted- R 2 from 0.92 to 0.98. Our results suggest that in intensive fish production tank, the lower the water salinity and the higher the pH, the more stable would be the colloidal suspension.

Effect of Interparticle Electrostatic Double Layer Interactions on Permeate Flux Decline in Crossflow Membrane Filtration of Colloidal Suspensions: An Experimental Investigation

A systematic study on the effect of electrostatic double layer interaction on permeate flux decline and deposit cake formation in crossflow membrane filtration of colloidal suspensions is reported. Three monodisperse silica suspensions with diameters of 47, 110, and 310 nm were used as model colloids, and a tubular zirconia membrane with an average pore diameter of 20 nm was used as a model membrane. The magnitude and range of the electrostatic double layer interactions were controlled via changes in solution ionic strength and pH. The coupling between colloidal interactions and hydrodynamic forces was investigated by changing the transmembrane pressure and particle size. The results indicate that the rate of flux decline is strongly dependent on solution ionic strength and, to a much lesser degree, on solution pH (for the investigated pH range 6.1–10.0). Variations in flux decline rate with solution ionic strength are especially significant as the particle size decreases. Particle cake thickness, permeability, and porosity generally increased with a decrease in solution ionic strength for a given particle size. For given physical and chemical conditions, the cake layer porosity increased with decreasing particle size, while cake permeability decreased with decreasing particle size. These trends are consistent with the increased importance of double layer repulsive forces in controlling the cake layer structure as the solution ionic strength and particle size decrease. Pressure relaxation experiments indicated that the particle cake layer is reversible, implying no irreversible deposition (attachment) of silica colloids onto the zirconia membrane surface.

Effect of solution ionic strength on lipid peroxidation initiation by the perhydroxyl (xanthine oxidase-derived) and peroxyl radicals.

The role of solution ionic strength in perhydroxyl (HOO.) and peroxyl (ROO.) radical initiated lipid peroxidation has been defined and investigated. Xanthine oxidase activity was used as the source of superoxide (O2-) and its conjugate acid (HOO.) in these experiments. While the enzyme's activity varied with changes in ionic strength, the effect could be factored out of the lipid peroxidation studies. Both HOO.- and ROO.-initiated peroxidations of linoleic acid were promoted by increases in solution ionic strength: the inclusion of 0.1 M of various alkali metal salts in the reaction resulted in up to a 4-fold increase in the overall peroxidation rate. Significant differences between alkali metal cations (Li+, Na+, K+, Cs+) and halogen anions (F-, Cl-, Br-) were not observed. Thus, the increased rates of lipid peroxidation were attributable to changes in solution ionic strength rather than specific ion-reaction interactions. Ionic stimulation of lipid peroxidation occurred only in the presence of preexisting fatty acid hydroperoxides (LOOHs), which provided additional support for the hydrogen atom transfer mechanism previously proposed [Aikens, J., and Dix, T. A. (1991) J. Biol. Chem. 266, 15091] for the HOO./LOOH initiation process. Physiologically appropriate salt concentrations were used in these studies, hence the results may have biological significance.

Adsorption of Cu, Pb and Zn by δMnO 2: applicability of the site binding-surface complexation model

A synthetic δMnO 2 was characterized with respect to intrinsic equilibrium constants and surface charge density as a function of pH. Site binding-surface complexation theory was used to model adsorption of Cu, Pb, and Zn onto the δMnO 2 surface. The model response was tested with respect to changes in solution ionic strength, total trace metal concentration, δMnO 2 in suspension, substrate surface area, and the presence of competitive trace metals. Results indicate that both freshly precipitated and aged δMnO 2 act as heterogeneous surfaces with a spectrum of binding site energies. The distribution of site energies was examined by determination of apparent equilibrium binding constants. Highest energy sites were saturated once less than 0.1% of all surface sites had been occupied. Adsorption experiments were performed for solutions which simulate conditions in the channel of a mountainous stream. The natural system is actively precipitating δMnO 2 in a watershed containing base metal sulfide deposits.

Anion adsorption on alumina

A series of inert metal complex anions with net charges of −1 to −3 have been used to examine the anion adsorption process on alumina. We have found that this process involves the simultaneous coadsorption of both hydrogen ion (from added acid) and complex anion. Adsorption is fast and depends only slightly upon changes in solution ionic strength and complex anion concentration. The adsorption process is also reversible with the addition of base. These findings are interpreted on a molecular level in the following way. A surface-bound hydroxyl group is protonated, thereby creating a positively charged surface group which can bind a complex anion by electrostatic attraction. The addition of base removes the positively charged surface group by reaction with the bound proton and releases the complex anion. This work implies that one may vary a catalyst metal loading simply by controlling the amount of acid present in a preparation where an inert metal complex anion is employed.

Electrophoretic Mobility Distributions of Normal Human T and B Lymphocytes and of Peripheral Blood Lymphoblasts in Acute Lymphocytic Leukemia: Effects of Neuraminidase and of Solvent Ionic Strength

Human T and B lymphocytes were found to be distinguishable on the basis of electrophoretic mobility, with the T cells having the higher mobility, in agreement with previous reports. The effects of the enzyme neuraminidase on the electrophoretic mobilities of T and B lymphocytes were determined. T lymphocytes showed a greater decrease in electrophoretic mobility after neuraminidase treatment; the relative mobilities of T and B cells were reversed by neuraminidase treatment, and the electrophoretic distinguishability was enhanced. The electrophoretic mobility distributions of lymphoblasts from patients with acute lymphocytic leukemia were found to differ from those of normal cells in their response to neuraminidase treatment and to changes in solution ionic strength. These results imply that the surface structure of the leukemic cells differs from that of either T or B lymphocytes from normal donors.