Electrostatic Double Layer Theory Research Articles

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.

Surface and foam property of perfluoroalkyl polyoxyethylene ether phosphate salt in aqueous-ethanol system

Foam is used in many fields with excellent performance. Moreover, disinfectants with ethanol as the main ingredient have increasingly been used in people's lives. However, the foam research of an ethanol-water mixed system is still in the initial stage. In this work, a perfluoroalkyl polyoxyethylene ether phosphate salt surfactant (FEP-Na) was synthesized through a new molecular design. The surfactant has good interfacial adsorption behavior in 50% ethanol-water mixing system, and the approximately spherical aggregates are observed in the volume phase of the solution. Moreover, FEP-Na showed good foaming property in 50% ethanol-water mixed solution. Interestingly, high concentration solutions with high surface activity have lower foam stability. And under the microscope, we see that the bubble burst is dominated by the phenomenon of coalescence. In this regard, we explain the mechanism of foam stability by electrostatic double layer theory and interfacial dilation rheology. It provides a new choice for the synthesis of foaming agent in the ethanol-water mixed system at a concentration of 50%.

Binding of monovalent alkali metal ions with negatively charged phospholipid membranes

We have systematically investigated the effect of various alkali metal ions with negatively charged phospholipid membranes. Size distributions of large unilamellar vesicles have been confirmed using dynamic light scattering. Zeta potential and effective charges per vesicle in the presence of various alkali metal ions have been estimated from the measured electrophoretic mobility. We have determined the intrinsic binding constant from the zeta potential using electrostatic double layer theory. The reasonable and consistent value of the intrinsic binding constant of Na+, found at moderate NaCl concentration (10–100mM), indicates that the Gouy–Chapman theory cannot be applied for very high (>100mM) and very low (<10mM) electrolyte concentrations. The isothermal titration calorimetry study has revealed that the net binding heat of interaction of the negatively charged vesicles with monovalent alkali metal ions is small and comparable to those obtained from neutral phosphatidylcholine vesicles. The overall endothermic response of binding heat suggests that interaction is primarily entropy driven. The entropy gain might arise due to the release of water molecules from the hydration layer vicinity of the membranes. Therefore, the partition model which does not include the electrostatic contribution suffices to describe the interaction. The binding constant of Na+ (2.4±0.1M−1), obtained from the ITC, is in agreement with that estimated from the zeta potential (~2.0M−1) at moderate salt concentrations. Our results suggest that hydration dynamics may play a vital role in the membrane solution interface which strongly affects the ion–membrane interaction.

Nonelectrostatic Ionic Forces between Dissimilar Surfaces: A Mechanism for Colloid Separation

The interaction between two dissimilar surfaces across an electrolyte is re-examined. The focus is on effects of ion-specific dispersion forces missing from classical electrostatic double-layer theory. The pressure between two flat surfaces is derived by two alternate methods (midpoint and whole domain approaches). Significant differences emerge from expectations of classical theory. These are illustrated by model interactions across electrolytes of mica and oil-like surfaces. A novel consequence that emerges from inclusion of ionic dispersion forces is the possible separation of mixed colloidal suspensions at moderate (0.1 M) concentrations of divalent salt. Repulsion between the model oil and mica surfaces is found to be due to entropic repulsion driven by high adsorption of both counterions and co-ions at the mica surface. Co-ion adsorption is a consequence of electric field reversal (charge reversal), caused by attractive ionic dispersion interactions of the counterion to the mica surface. Charge reversal is also found with monovalent electrolyte but only at impracticably high concentrations.

Variation of Surface Charge along the Surface of Wool Fibers Assessed by High-Resolution Force Spectroscopy

In this study, we have mapped the surface charge of wool fibers using chemically specific high-resolution force spectroscopy in order to better understand the dispersion of amino acids in relation to fiber morphology. The inter-surface forces between standard atomic force microscopy (AFM) probe tips (tip radius ~ 50 nm) functionalized with COOH and NH(3) terminated alkanethiol self assembling monolayers and the wool surface were used to estimate the surface charge per unit area using linear Poisson-Boltzmann-based electrostatic double layer theory. The positional measurement of nano-scale surface charge showed a correlation between the surface charge and fiber morphology, indicated that basic amino acids are located near the scale edges.

Nanoscale variation in surface charge of synthetic hydroxyapatite detected by chemically and spatially specific high-resolution force spectroscopy

The normal intersurface forces between nanosized probe tips functionalized with COO −- and NH 3 +-terminated alkanethiol self-assembling monolayers and dense polycrystalline phase pure synthetic hydroxyapatite (HA) were measured via a powerful nanomechanical technique called chemically specific high-resolution force spectroscopy. The data taken on approach of the probe tip to the HA surface was compared to the nonlinear Poisson–Boltzmann-based electrostatic double layer theory to predict the surface charge per unit area of the HA, σ HA (C/m 2), as a function of ionic strength, position within a variety of grains, and across grain boundaries. The average σ HA was found to be ∼−0.02 C/m 2 and to vary from −0.0037 to −0.072 C/m 2 with nanoscale position in relation to grain boundaries and crystal planes up to −0.19 C/m 2/μm. Positional measurement of nanoscale surface properties holds great promise in elucidating the molecular origins of physicochemical processes occurring at the biomaterial interface.

Supramolecular structure of helical ribbons self-assembled from a β-sheet peptide

We have investigated the supramolecular structure of helical ribbons formed during self-assembly of a β-sheet peptide using computer simulation. We tested a wide range of molecular packing geometries consistent with the experimental dimensions to identify the most stable structure, and then systematically changed the helical geometry to investigate its energy landscape. The effect of pH was incorporated by scaling the amount of charge on the side chains based on the electrostatic double layer theory. Our results suggest that these left-handed helical ribbons are comprised of a double β-sheet and that the experimentally measured dimensions correspond to a local energy minimum. Side chain interactions are found to be critical in determining the stability and curvature of the helix. Our approach has general applicability to the study of self-assembled nanostructures from β-sheet peptides where high resolution data are not yet available.

Adhesion between cerebroside bilayers.

The structure, hydration properties, and adhesion energy of the membrane glycolipid galactosylceramide (GalCer) were studied by osmotic stress/X-ray diffraction analysis.(1) Fully hydrated GalCer gave a repeat period of 67 A, which decreased less than 2 A with application of applied osmotic pressures as large as 1.6 x 10(9) dyn/cm(2). These results, along with the invariance of GalCer structure obtained by a Fourier analysis of the X-ray data, indicated that there was an extremely narrow fluid space (less than the diameter of a single water molecule) between fully hydrated cerebroside bilayers. Electron density profiles showed that the hydrocarbon chains from apposing GalCer monolayers partially interdigitated in the center of the bilayer. To obtain information on the adhesive properties of GalCer bilayers, we incorporated into the bilayer various mole ratios of the negatively charged lipid dipalmitoylphosphatidylglycerol (DPPG) to provide known electrostatic repulsion between the bilayers. Although 17 and 20 mol % DPPG swelled (disjoined) the GalCer bilayers by an amount predictable from electrostatic double-layer theory, 5, 10, 13, and 15 mol % DPPG did not disjoin the bilayers. By calculating the magnitude of the electrostatic pressure necessary to disjoin the bilayers, we estimated the adhesion energy for GalCer bilayers to be about -1.5 erg/cm(2), a much larger value than that previously measured for phosphatidylcholine bilayers. The observed discontinuous disjoining with increased electrostatic pressure and this relatively large value for adhesion energy indicated the presence of an attractive interaction, in addition to van der Waals attraction, between cerebroside bilayers. Possible attractive interactions are hydrogen bond formation and hydrophobic interactions between the galactose headgroups of apposing GalCer bilayers.

Lipopolysaccharide bilayer structure: effect of chemotype, core mutations, divalent cations, and temperature.

Lipopolysaccharide (LPS), the primary lipid on the surface of Gram-negative bacteria, is thought to act as a protective and permeability barrier. X-ray diffraction analysis of osmotically stressed LPS multilayers was used to determine the structure and interactive properties of LPSs from strains containing the minimum number of sugars necessary for bacterial survival (Re chemotype) to the maximum number of sugars found in rough bacteria (Ra chemotype). At 20 degrees C in the absence of divalent cations, LPS suspensions gave a sharp wide-angle reflection at 4.23 A and a broad low-angle band centered at 50-68 A depending on the chemotype, indicating the presence of gel phase bilayers separated by large fluid spaces. As osmotic pressure was applied, the apposing bilayers were squeezed together and lamellar diffraction at 6 A resolution was obtained. At low applied pressures (<10(6) dyn/cm2), the total repulsive pressure between bilayers could be explained by electrostatic double layer theory. At higher applied pressures, there was a sharp upward break in each pressure-distance relation, indicating the presence of a hydrophilic steric barrier whose range depended strongly on the LPS chemotype. The positions of these upward breaks, along with electron density profiles, showed that the sugar core width systematically increased from 10 A for the Re chemotype to 27 A for the Ra chemotype. In excess buffer, the addition of divalent cations brought the bilayers into steric contact. Electron density profiles were used to determine the locations of cation binding sites and polar substituents on the LPS oligosaccharide core. The area per hydrocarbon chain was approximately 26 A2 in liquid-crystalline LPS bilayers, an indication of an acyl chain packing that is much tighter than that found in bilayers composed of typical membrane lipids. This unusually tight packing could be a critical factor in the permeability barrier provided by LPS.

Adhesion between Phosphatidylethanolamine Bilayers

The goal of this study is to provide additional information on the short-range interactions that determine the adhesion energy between bilayer surfaces. The specific problem concerns the hydration properties of bilayers of the membrane lipid phosphatidylethanolamine (PE), which imbibe much less water than bilayers composed of the other common ziwtterionic membrane lipid, phosphatidylcholine (PC). The osmotic stress/X-ray diffraction method was used to measure pressure−distance relations for PE and PC bilayers containing known mole fractions of the charged lipid phosphatidic acid (PA). The addition of 20 mol % PA to either PC or PE bilayers swelled the bilayers by an amount predictable from electrostatic double-layer theory. However, whereas the addition of 5 mol % PA disjoined PC bilayers, it did not change the fluid space between PE bilayers. By calculating the magnitude of the electrostatic pressure necessary to disjoin the bilayers, we estimate the adhesion energy for gel phase PE bilayers to be about −0.7 erg/cm2, a value considerably larger than previously measured values for gel phase PC bilayers. The magnitude of the adhesion energy indicates that, in addition to the attractive van der Waals pressure, there is another attractive pressure between adjacent PE layers that prevents them from swelling to the extent of PC bilayers. We argue that a small fraction of direct electrostatic interbilayer interactions or indirect hydrogen-bonded water interactions between the N+H3 group in one bilayer and the PO4- group in the apposing bilayers could account for the additional attractive interactions in PE bilayers.

The application of pH-sensitive spin labels to studies of surface potential and polarity of phospholipid membranes and proteins

The effects of pH titration on the EPR spectra of imidazolidine nitroxides located at the surface of mixed bilayers composed of dimyristoylphosphatidylglycerol (DMPG) and dimyristoylphosphatidylcholine (DMPC), and at the surface of the protein, human serum albumin (HSA), have been investigated. It is found that the shift in p K a of the amino group of the imidazolidine radical from its value of 4.6 in water depends both on the interfacial polarity ( Δp K pol a) and on the electrostatic surface potential ( Δp K el a) when it is positioned at the bilayer/water interface by an anchoring hydrocarbon tail. The polarity shift is determined to be: Δp K pol a = −1.3 units at the surface of DMPC bilayers at 17°C, corresponding to an effective interfacial dielectric constant of ε ≈ 37, and depends on the temperature with a coefficient of d Δp K pol a/d T ≈ −0.01 per degree. The electrostatic shift at the surface of DMPG bilayers is Δp K el a = +1.6 units in 0.1 M KCl, which corresponds to an electrostatic surface potential of −95 mV. This electrostatic shift depends strongly both on ionic strength and on the fraction of charged lipid in the DMPC/DMPG mixtures, in a manner that agrees with the predictions of electrostatic double-layer theory. It is found that the shift in p K a of an imidazolidine radical covalently bound at the surface of HSA is determined mainly by the surface electrostatics ( Δp K pol a ≈ 0) and corresponds to an electrostatic potential of +32 mV in 0.01 M KCl at a pH below the isoelectric point of the protein.

On the measurement of weak repulsive and frictional colloidal forces by reflection interference contrast microscopy

terference contrast (RIC) microscopy in combination with real-time image processing. The temporal fluctuations of the absolute sphere-to-substrate distance are determined from changes of interference fringe pattern (Newtonian rings). Both the shape about its minimum and the absolute minimum equilibrium distance of the interaction potential can be obtained by analyzing the distribution of distances in terms of a Boltzmann distribution. The timeautocorrelation function of distances yields the hydrodynamic friction. The method has been applied to the interaction of latex spheres with glass substrates in salt solutions of different ionic strength. The results correspond to classical electrostatic double layer theory that leads to a characteristic dependence of the mean separation distance and the mean square displacement on the radius of the spheres. The hydrodynamic friction close to a wall exhibits the predicted inverse proportionality to the sphere-wall distance. It is demonstrated that the method can be applied to study the interaction between biologically relevant objects such as giant vesicles with bilayer covered substrates.

Measurement of the repulsive force between polyelectrolyte molecules in ionic solution: hydration forces between parallel DNA double helices.

We have measured the repulsive force between B-form double helices in parallel packed arrays of polymer-condensed DNA in the presence of 0.005-1.0 M ionic solutions. Molecular repulsion is consistently exponential with a 2.5-3.5 A decay distance, when the separation between DNA surfaces is 5-15 A. Only weakly dependent on ionic strength and independent of molecular size, this intermolecular repulsion does not obey the predictions of electrostatic double-layer theory. Rather, it strongly resembles the "hydration force" first recognized and quantified between phospholipid bilayers. Only beyond 15 A separation between molecules is there evidence of electrostatic double-layer forces. The quantitative failure of electrostatic double-layer theory seen here must gravely affect accepted analyses of other polyelectrolyte systems. Because the packing of condensed DNA resembles the spacings of DNA in many bacteriophages, our results permit estimation of the "DNA pressure" in phage heads.