Long-range Electrostatic Repulsion Research Articles

Two-Dimensional Colloidal Alloys

We study the structure of mixed monolayers of large (3 μm diameter) and small (1 μm diameter) very hydrophobic silica particles at an octane-water interface as a function of the number fraction of small particles ξ. We find that a rich variety of two-dimensional hexagonal super-lattices of large (A) and small (B) particles can be obtained in this system due to strong and long-range electrostatic repulsions through the nonpolar octane phase. The structures obtained for the different compositions are in good agreement with zero temperature calculations and finite temperature computer simulations.

Nanotubes and bilayers in a model peptide system

The trifluoroacetate (tfa) salt of the peptide (ala)6lys self-assembles in water into very long, hollow nanotubes with a radius R = 26 nm, above a critical aggregation concentration (volume fraction), ϕcac = 0.10. The peptides carry a net positive charge that ensures colloidal stability of the self-assembly structures through a long-range electrostatic repulsion. There is only a weak temperature dependence of ϕcac from which an enthalpy of aggregation of −kBT per molecule is estimated. SAXS data show that the thickness of the nanotube wall, δ, is less than 1 nm indicating that the peptides form a monolayer in the nanotube wall. The nanotubes have a very large aspect ratio and form an ordered nematic or hexagonal phase. Because of the low δ/R ratio, the nanotube volume fraction grows very rapidly with increasing peptide concentration, ϕ, and reaches close packing already at ϕ = 0.15. When increasing the concentration further, there is a phase transition to a novel lamellar phase where the peptide molecules form bilayers consisting of two, presumably oppositely oriented, monolayers.

Impact of the virulence-associated MAb3/1 epitope on the physicochemical surface properties of Legionella pneumophila sg1: An issue to explain infection potential?

The relationship between the presence/absence of the virulence-associated MAb3/1 epitope of sixteen Legionella pneumophila serogroup 1 strains and their respective surface physicochemical properties is evidenced from electrokinetic measurements (microelectrophoresis) performed as a function of KNO 3 electrolyte concentration (range 1–100 mM, pH ∼ 6.5). Among the bacteria selected, nine original strains constitute the Dresden reference panel and differ according to the presence/absence of the virulence-associated monoclonal antibody MAb3/1 of the O-specific chain of the lipopolysaccharides (LPS). Five isogenic Lens strains, also investigated in the current study, present the epitope MAb3/1 of their LPS and were involved to some extent in the outbreak that stroke the Nord Pas-de-Calais region (France) in 2004. All bacteria exhibit the typical electrokinetic features of soft (permeable) particles. On the basis of Ohshima's model, analysis of the electrophoretic mobility data allows evaluating the intraparticular flow penetration length 1/ λ 0 and the (negative) volume charge density ρ 0 that both reflect the structure and chemical composition of the soft bacterial component. Our results show that the virulent MAb3/1 positive strains are characterized on average by 1/ λ 0 and ǀ ρ 0ǀ values that are about 1.5 times larger and 5 times lower, respectively, than those derived for lesser virulent (MAb3/1 negative) strains. In other words, on average the soft surface layer of MAb3/1 positive strains is significantly less charged and more permeable than those of MAb3/1 negative strains. The intimate correlation between virulence-associated MAb3/1 epitope and charge density carried by the bacterial envelop was further confirmed by lower 1/ λ 0 and greater ǀ ρ 0ǀ values for lag-1 mutant CS332 strain, lacking the MAb3/1 epitope, compared to the parental strain AM511. A closer inspection of the dispersion in 1/ λ 0 and ǀ ρ 0ǀ data over the ensemble of analysed bacteria together with the reported number of Legionnaires’ disease cases they are responsible for, points out the charge density ǀ ρ 0ǀ as the parameter that is most suitable for discriminating highly virulent (MAb3/1 positive) from less virulent (MAb3/1 negative) strains. Although short-range interaction determines infection process, our results suggest that the infection potential of Legionella pneumophila serogroup 1 may be also controlled significantly by non-specific long-range electrostatic repulsion the bacteria undergo when approaching negatively charged host cells to be infected.

Mechanism of influence of phosphorylation on serine 124 on a decrease of catalytic activity of human thymidylate synthase

Regulation by phosphorylation is a well-established mechanism for controlling biological activity of proteins. Recently, phosphorylation of serine 124 in human thymidylate synthase (hTS) has been shown to lower the catalytic activity of the enzyme. To clarify a possible mechanism of the observed influence, molecular dynamics (MD), essential dynamics (ED) and MM-GBSA studies were undertaken. Structures derived from the MD trajectories reveal incorrect binding alignment between the pyrimidine ring of the substrate, dUMP, and the pterine ring of the cofactor analogue, THF, in the active site of the phosphorylated enzyme. The ED analysis indicates changes in the behavior of collective motions in the phosphorylated enzyme, suggesting that the formation of the closed ternary complex is hindered. Computed free energies, in agreement with structural analysis, predict that the binding of dUMP and THF to hTS is favored in the native compared to phosphorylated state of the enzyme. The paper describes at the structural level how phosphorylation at the distant site influences the ligand binding. We propose that the ‘phosphorylation effect’ is transmitted from the outside loop of Ser 124 into the active site via a subtle mechanism initiated by the long-range electrostatic repulsion between the phosphate groups of dUMP and Ser124. The mechanism can be described in terms of the interplay between the two groups of amino acids: the link (residues 125–134) and the patch (residues 189–192), resulting in the change of orientation of the pyrimidine ring of dUMP, which, in turn, prevents the correct alignment between the latter ring and the pterin ring of THF.

On the Stability of the Soluble Amyloid Aggregates

Many amyloid proteins form metastable soluble aggregates (or protofibrils, or protein nanoparticles, with characteristic sizes from ∼10 to a few hundred nm). These can coexist with protein monomers and amyloid precipitates. These soluble aggregates are key determinants of the toxicity of these proteins. It is therefore imperative to understand the physical basis underlying their stability. Simple nucleation theory, typically applied to explain the kinetics of amyloid precipitation, fails to predict such intermediate stable states. We examine stable nanoparticles formed by the Alzheimer's amyloid- β peptide (40 and 42 residues), and by the protein barstar. These molecules have different hydrophobicities, and therefore have different short-range attractive interactions between the molecules. We also vary the pH and the ionic strength of the solution to tune the long-range electrostatic repulsion between them. In all the cases, we find that increased long-range repulsion results in smaller stable nanoparticles, whereas increased hydrophobicity produces the opposite result. Our results agree with a charged-colloid type of model for these particles, which asserts that growth-arrested colloid particles can result from a competition between short-range attraction and long-range repulsion. The nanoparticle size varies superlinearly with the ionic strength, possibly indicating a transition from an isotropic to a linear mode of growth. Our results provide a framework for understanding the stability and growth of toxic amyloid nanoparticles, and provide cues for designing effective destabilizing agents.

Structure, Stability, and Formation Pathways of Colloidal Gels in Systems with Short-Range Attraction and Long-Range Repulsion

We study colloidal gels formed upon centrifugation of dilute suspensions of spherical colloids (radius 446 nm) that interact through a long-range electrostatic repulsion (Debye length approximately 850 nm) and a short-range depletion attraction (approximately 12.5 nm), by means of confocal scanning laser microscopy (CSLM). In these systems, at low colloid densities, colloidal clusters are stable. Upon increasing the density by centrifugation, at different stages of cluster formation, we show that colloidal gels are formed that significantly differ in structure. While significant single-particle displacements do not occur on the hour time scale, the different gels slowly evolve within several weeks to a similar structure that is at least stable for over a year. Furthermore, while reference systems without long-range repulsion collapse into dense glassy states, the repulsive colloidal gels are able to support external stress in the form of a centrifugal field of at least 9g.

Assembly of Magnetically Tunable Photonic Crystals in Nonpolar Solvents

We have successfully assembled superparamagnetic colloids into ordered structures with magnetically tunable photonic properties in nonpolar solvents by establishing long-range electrostatic repulsive forces using charge control agents. Reverse micelles resulted from the introduction of charge control agents such as AOT molecules can enhance the charge separation on the surfaces of n-octadecyltrimethoxysilane modified Fe(3)O(4)@SiO(2) particles. The significantly improved long-range electrostatic repulsion can counterbalance the magnetically induced attraction and therefore allow ordering of superparamagnetic colloids in nonpolar solvents. This system possesses a fast and fully reversible optical response to the external magnetic fields, long-term stability in performance, and good diffraction intensity.

Critical Instability Leads To Labyrinthine Transition In Binary Lipid/Polymer Monolayers

Near critical points, the microscopic structures of fluid mixtures become unimportant and systems are characterized by strong susceptibility to small perturbations, large fluctuations and mesoscopic ordering. In biological membranes, these fluctuations in physical properties (composition, density, and curvature) may provide a spatially and temporally defined structural means to organize important functions such as formation of lipid rafts, caveolar invaginations or molecular colocalization. We report an observation of characteristic critical fluctuations in lipid monolayers composed of binary mixtures of 1,2 dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) and polyoxyethylene stearate (PegxS of varying PEG chain length) confined to an air/water interface. Using fluorescence and Brewster angle microscopy, compositional changes that occur during the expansion of an initially phase-separated binary monolayer reveal a passage through a critical composition at vanishing lateral surface pressure. The corresponding morphological transformation from an optically homogeneous liquid expanded state to a gas bubble phase via a striped, labyrinthine phase reflects the competition between long-range electrostatic repulsion and line tension forces caused by interactions between DPPC and PegxS.

Ordered Suspensions of Charged Liposomes

Accurate determination of liposome and colloidal-vesicle charge remains an unsolved problem. Electrophoretic measurements are subject to slip-plane and surface-structure uncertainties. We propose a charge measurement that avoids hydrodynamics in the vicinity of the shear layer so is independent of the slip plane, eliminates diffuse-layer electroosmosis by avoiding applied electric fields, and is insensitive to surface structure. We form ordered electrostatic gels of monodisperse unilamellar PG/PC liposomes by high-pressure extrusion through polycarbonate membranes in deionized water. The gel consists of a strongly-coupled suspension of liposomes, stabilized by mutual long-range electrostatic repulsions in a confined volume. The gel structure is that of an ordered liquid, as seen by angle-dependent static light scattering, freeze-fracture electron microscopy, and optical Bragg scattering. The liposome charge Z is screened by dissociated H+ counterions. Z thus determines both the strength and range of the interactions, hence determines the elastic properties of the gel. Measurements of shear modulus by a mechanical resonance technique and of osmotic compressibility by dynamic light scattering yield an effective particle charge Z∗. The relation of Z∗ to Z may be calculated by numerical iteration of the Poisson-Boltzmann equation for any Z∗ and particle concentration. The technique is most accurate in the Debye-Hückel regime where Z ∼ Z∗. Addition of H+ and salt permits measurement of protonation and counterion binding to the surface. This method of determining liposome charge by measuring the bulk elastic properties of a strongly-coupled suspension differs markedly from that of measuring the mobilities of individual liposomes in an uncoupled suspension.

Colloidal systems with competing interactions: from an arrested repulsive cluster phase to a gel

We report an extensive numerical study of a charged colloidal system with competing short-range depletion attraction and long-range electrostatic repulsion. By analyzing the cluster properties, we identify two distinct regions in the phase diagram: a state composed of stable finite-size clusters, whose relative interactions are dominated by long-range repulsion, and a percolating network. Both states are found to dynamically arrest at low temperatures, providing evidence of the existence of two distinct non-ergodic states in these systems: a Wigner glass of clusters and a gel.

Mediation of the Nanotribological Properties of Cellulose by Chitosan Adsorption

Cellulosic model surfaces functionalized with chitosan, a naturally occurring cationic biomacromolecule, by in situ adsorption have been studied with an atomic force microscope (AFM) in colloidal probe configuration. The interaction forces on approach and separation, as well as the nanotribological properties, were shown to be highly pH-dependent, and a significant difference in the behavior was seen before and after chitosan adsorption. In general, all forces on approach showed a highly repulsive interaction at shorter distances due to deformation of the probe. At high pH, before chitosan adsorption, a long-range electrostatic repulsion was observed, consistent with DLVO theory. However, at low pH no electrostatic contribution was found before adsorption, probably due to charge neutralization of carboxyl groups. After chitosan adsorption, repulsive forces acting over a much longer distance than predicted by DLVO theory were present at low pH. This effect was ascribed to chain extension of the chitosan species of which the magnitude and the range of the force increased dramatically with higher charge at low pH. In all cases, a typical saw-tooth patterned adhesion was present, with pull-off events occurring at different separations. The frequency of these events after chitosan adsorption was greatly increased at longer distances. Additionally, the adsorbed chitosan markedly reduced the friction, where the largest effect was a 7-fold decrease of the friction coefficient observed at low pH.

Molecular cooking: physical transformations in Chinese ‘century’ eggs

Over two thousand years ago the Chinese developed a method to preserve eggs such that they remain edible for many months. The room temperature, physico-chemical preservation process that is used to prepare ‘century’ eggs transforms the egg white into a yellow, transparent gel with optical and mechanical properties that are very different to those of the familiar white protein aggregate that forms upon boiling a raw egg. Here we show that boiled egg white gels can be further transformed into an elastic and transparent gel using the Chinese preservation method. We demonstrate that the resulting protein gel is made of fine-stranded globular assemblies of partially denatured protein, and resembles the aggregates formed by colloidal particles interacting through long-range electrostatic repulsion combined with short-range attraction. These gels are not only highly deformable but are also very stable, maintaining their structure even when boiled. We suggest that the mechanism responsible for gelation in century eggs illustrates a non-specific aggregation pathway available to globular proteins.

Bundle Formation in Polyelectrolyte Brushes

Bundle formation of the vertically oriented polyelectrolytes within polyelectrolyte brushes is studied with x-ray reflectivity and grazing-incidence diffraction as a function of grafting density and ion concentration. At 0.8 Molar monomer concentration and without added salt, a bundle consists of two chains and is 50 A long. On the addition of up to 1M CsCl, the aggregation number increases up to 15 whereas the bundle length approaches a limiting value, 20 A. We suggest that the bundle formation is determined by a balance between long-ranged electrostatic repulsion, whose range and amplitude is decreased on salt addition, and short-ranged attraction.

Beyond Molecular Recognition: Using a Repulsive Field to Tune Interfacial Valency and Binding Specificity between Adhesive Surfaces

Surface-bound biomolecular fragments enable "smart" materials to recognize cells and other particles in applications ranging from tissue engineering and medical diagnostics to colloidal and nanoparticle assembly. Such smart surfaces are, however, limited in their design to biomolecular selectivity. This feature article demonstrates, using a completely nonbiological model system, how specificity can be achieved for particle (and cell) binding, employing surface designs where immobilized nanoscale adhesion elements are entirely nonselective. Fundamental principles are illustrated by a model experimental system where 11 nm cationic nanoparticles on a planar negative silica surface interact with flowing negative silica microspheres having 1.0 and 0.5 microm diameters. In these systems, the interfacial valency, defined as the number of cross-bonds needed to capture flowing particles, is tunable through ionic strength, which alters the range of the background repulsion and therefore the effective binding strength of the adhesive elements themselves. At high ionic strengths where long-range electrostatic repulsions are screened, single surface-bound nanoparticles capture microspheres, defining the univalent regime. At low ionic strengths, competing repulsions weaken the effective nanoparticle adhesion so that multiple nanoparticles are needed for microparticle capture. This article discusses important features of the univalent regime and then illustrates how multivalency produces interfacial-scale selectivity. The arguments are then generalized, providing a possible explanation for highly specific cell binding in nature, despite the degeneracy of adhesion molecules and cell types. The mechanism for the valency-related selectivity is further developed in the context of selective flocculation in the colloidal literature. Finally, results for multivalent binding are contrasted with the current thinking for interfacial design and the presentation of adhesion moieties on engineered surfaces.

Competition between Entropy and Electrostatic Interactions in a Binary Colloidal Mixture of Spheres and Platelets

We describe the phase behavior of an aqueous mixture of discotic nanoparticles of laponite and spherical magnetic nanoparticles of maghemite. To obtain stable mixtures from a chemical point of view, the maghemite nanoparticles are first coated by a thin layer of silica in order to adapt their surface chemistry to that of laponite nanoparticles: this enables one to raise volume fractions of maghemite Phi mag in the laponite suspensions up to several percent. Although the system is out of equilibrium, a "fluid-solid" state diagram was established showing that the mixtures undergo a fluid-solid transition, similar to that of pure suspensions of laponite, over a given volume fraction of laponite Phi lap and over a given Phi mag. An increase in Phi mag shifts Phi lap toward the lower values. When a solid sample is just above Phi lap, the application of an external magnetic field gradient induces a solid-to-liquid transition if the sample is located not too far from Phi lap on the state diagram. The structure of the mixtures, determined either at small scale by small-angle neutron scattering (SANS) or at intermediate scales by optical microscopy, shows that the solid samples are phase separated at a local scale: they are made of densely connected domains of laponite nanoparticles surrounding liquid pockets of maghemite nanoparticles. The size of the pockets grows with time. The magnetic liquid pockets are responsible for the rupture of the solid samples when an external magnetic field gradient is applied since their deformation induces local mechanical stress, internally damaging the network formed by the solid domains of laponite. The microscopic phase separation is the result of two opposite effects: (i) entropic effects that tend to phase separate the system macroscopically when the packing entropy overcomes the orientational entropy and (ii) long-range electrostatic repulsions that freeze the system.

Interactions between a Polystyrene Particle and Hydrophilic and Hydrophobic Surfaces in Aqueous Solutions

The interaction between a colloidal polystyrene particle mounted on an AFM cantilever and a hydrophilic and a hydrophobic surface in aqueous solution is investigated. Despite the apparent simplicity of these two types of systems a variety of different types of interactions are observed. The system containing the polystyrene particle and a hydrophilic surface shows DLVO-like interactions characteristic of forces between charged surfaces. However, when the surface is hydrophobized the interaction changes dramatically and shows evidence of a bridging air bubble being formed between the particle and the surface. For both sets of systems, plateaus of constant force in the force curves are obtained when the particle is retracted from the surface after being in contact. These events are interpreted as a number of individual polystyrene molecules that are bridging the polystyrene particle and the surface. The plateaus of constant force are expected for pulling a hydrophobic polymer in a bad (hydrophilic) solvent. The plateau heights are found to be of uniform spacing and independent of the type of surface, which suggests a model by which collapsed polymers are extended into the aqueous medium. This model is supported by a full stretching curve showing also the backbone elasticity and a stretching curve obtained in pentanol, where the plateau changes to a nonlinear force response, which is typical for a polymer in a good or neutral solvent. We suggest that these polymer bridges are important in particular for the interaction between polystyrene and the hydrophilic surface, where they to some extent counteract the long-range electrostatic repulsion.

Study of the Coalescence of Acoustic Bubbles as a Function of Frequency, Power, and Water-Soluble Additives

The effect that surface-active solutes, such as aliphatic alcohols and sodium dodecyl sulfate (SDS), have on the extent of bubble coalescence in liquids under different sonication conditions has been investigated by measuring the volume change of the solution following a period of sonication. In general, the adsorption of surface-active solutes onto the bubble surface retards bubble coalescence. Within the limitations of the measurement method and the systems studied, bubble coalescence does not appear to be dependent on the applied acoustic power. Also, varying the applied acoustic frequency has a minimal effect on the extent of bubble coalescence in systems where long-range electrostatic repulsion between bubbles, imparted by the adsorbed surface-active solutes, dominates. However, when short-range steric repulsion (or other short-range repulsive forces) is the primary factor in inhibiting bubble coalescence, the dependence on the applied acoustic frequency becomes apparent, with less coalescence inhibition at higher frequencies. It is also concluded that SDS does not reach an equilibrium adsorption level at the bubble/solution interface under the sonication conditions used. On the basis of this conclusion, a method is proposed for estimating nonequilibrium surface excess values for solutes that do not fully equilibrate with the bubble/solution interface during sonication. For the case of SDS in the presence of excess NaCl, the method was further employed to estimate the maximum lifetime of bubbles in a multibubble field. It was concluded that an acoustic bubble in a multibubble field has a finite lifetime, and that this lifetime decreases with increasing applied frequency, ranging from up to 0.35 +/- 0.05 ms for 213 kHz to 0.10 +/- 0.05 ms for 1062 kHz. These estimated lifetimes equate to a bubble in a multibubble field undergoing an upper limit of 50-200 oscillations over its lifetime for applied ultrasound frequencies between 200 kHz and 1 MHz.

Dilute-solution structure of charged arborescent graft polymer

The solutions of charged G1 arborescent polystyrene- graft-poly(2-vinylpyridine) copolymers in methanol- d4 and D 2O were investigated over a dilute concentration range ϕ=0.005–0.05 ( ϕ: mass fraction) using small-angle neutron scattering (SANS). Upon addition of acid (HCl) arborescent graft polymers became charged and a peak appeared in SANS data. The interparticle distance ( d exp) calculated from a peak position corresponded to the expected value ( d uni) for a uniform particle distribution. This indicates the formation of liquid-like ordering due to long-range Coulombic repulsions. The smaller dielectric constant of methanol- d4 resulted in long-range electrostatic repulsions persisting to lower polymer concentration than in D 2O. The slow mode scattering was observed by dynamic light scattering measurements for the same polymer solutions, indicating the presence of structural inhomogeneity in the solutions. Both the peak and slow mode disappeared by addition of NaCl or excess HCl into the solutions due to the screening of electrostatic interactions. The G1 polymer grafted with longer P2VP chains ( M w∼30,000 versus 5000 g mol) formed a gel on addition of HCl. This result reveals that molecular expansion is more significant for arborescent polymers with longer ( M w∼30,000) linear polyelectrolyte branches, resulting in gelation for ϕ>0.01. Upon addition of NaCl or excess HCl a gel transformed back to a liquid resulted from the screening of electrostatic interactions.

Second Virial Coefficient Studies of Cosolvent-Induced Protein Self-Interaction

Protein self-interaction is important in protein crystal growth, solubilization, and aggregation, both in vitro and in vivo, as with protein misfolding diseases, such as Alzheimer’s. Although second virial coefficient studies can supply invaluable quantitative information, their emergence as a systematic approach to evaluating protein self-interaction has been slowed by the limitations of traditional measurement methods, such as static light scattering. Comparatively, self-interaction chromatography is an inexpensive, high-throughput method of evaluating the osmotic second virial coefficient ( B) of proteins in solution. In this work, we used self-interaction chromatography to measure B of lysozyme in the presence of various cosolvents, including sucrose, trehalose, mannitol, glycine, arginine, and combinations of arginine and glutamic acid and arginine and sucrose in an effort to develop a better fundamental understanding of protein self-interaction in complex cosolvent systems. All of these cosolvents, alone or in combination, increased B, indicating a reduction in intermolecular attraction. However, the magnitude of cosolvent-induced changes in B was found to be largely dependent on the ability to control long-range electrostatic repulsion. To the best of our knowledge, this work represents the most comprehensive virial coefficient study to date focusing on complex cosolvent-induced effects on the self-interaction of lysozyme.

Structure and Stability of Silica Particle Monolayers at Horizontal and Vertical Octane−Water Interfaces

Monolayers of silica particles at horizontal and vertical octane-water interfaces have been studied by microscopy. It is found that their structure and stability depend strongly on the particle hydrophobicity. Very hydrophobic silica particles, with a contact angle of 152 degrees measured through the water, give well-ordered monolayers at interparticle distances larger than 5 particle diameters which are stable toward aggregation and sedimentation. In contrast, monolayers of less-hydrophobic particles are disordered and unstable. Two-dimensional particle sedimentation has been observed in the case of vertical monolayers. The results have been analyzed with a simple two-particle model considering the sedimentation equilibrium as a balance between the long-range electrostatic repulsion through the oil, the gravity force, and the capillary attraction due to deformation of the fluid interface around particles. The value of the charge density at the particle-octane interface, 14.1 muC/m(2), found for the most hydrophobic particles is reasonable. It drastically decreases for particles with lower hydrophobicity, which is consistent with the order-disorder transition in monolayer structure reported by us before. The pair interactions between particles at a horizontal octane-water interface have been analyzed including the capillary attraction due to undulated three-phase contact line caused by nonuniform wetting (the contact angle hysteresis). The results are in agreement with the great stability of very hydrophobic silica particle monolayers detected experimentally, even at low pH at the point of zero charge of the particle-water interface, and with the aggregated structure of hydrophilic particle monolayers.