Repulsive Hydration Forces Research Articles

Long Range Interactions between Apoferritin Molecules

Recent light scattering experiments regarding the apoferritin molecules (the hollow shells of the iron-storage protein ferritin) indicated a surprising dependence of the repulsion between proteins on the electrolyte concentration (NaCH3COO). The second virial coefficient decreased to a value close to that corresponding to hard spheres for 0.15 M but increased to a very large value at 0.25 M. The results are difficult to be interpreted in the classical framework through the addition of double layer and hydration repulsive forces. While the double layer theory can predict the behavior of the virial coefficient at low electrolyte concentrations, only an abnormally large charge can explain the values of the virial coefficient at high ionic strengths. Alternatively, the traditional hydration force should increase with orders of magnitude between 0.15 and 0.25 M, to be consistent with experiment, and this is unlikely to happen. In this paper, it is shown that a unitary treatment of the repulsion (the double layer and the polarization-based hydration repulsions) might explain the unexpected values of the second virial coefficient and the corresponding long-ranged repulsion.

Fluidity of bound hydration layers.

We have measured the shear forces between solid surfaces sliding past each other across aqueous salt solutions, at pressures and concentrations typical of naturally occurring systems. In such systems the surface-attached hydration layers keep the compressed surfaces apart as a result of strongly repulsive hydration forces. We find, however, that the bound water molecules retain a shear fluidity characteristic of the bulk liquid, even when compressed down to films 1.0 +/- 0.3 nanometer thick. We attribute this to the ready exchange (as opposed to loss) of water molecules within the hydration layers as they rub past each other under strong compression.

The role of hydration force on the stability of the suspension of Saccharomyces cerevisiae–application of the extended DLVO theory

Including with the term of repulsive hydration force, the extended DLVO theory adopted in the present paper can successfully describe the stability of cell suspension, Saccharomyces cerevisiae THB001, over a wide range of pH and NaCl concentration. By using the experimental results of zeta potential and turbidity measurements, the parameter values of the hydration energy equation are estimated. It is found that, since the turbidity of cell suspension decreases with the increase of NaCl concentration at isoelectric point, hence the hydration energy existed between cells is repulsive and is an inherent surface property of S. cerevisiae THB001.

Role of the Hydration Force in the Stability of Colloids at High Ionic Strengths

It is shown that the repulsion between colloidal particles or emulsion droplets depends both on the surface charge density and on the surface dipole density, the latter being a result of the presence of ion pairs on the surface. For illustration purposes, one considers the case in which a surfactant, such as sodium dodecyl sulfate, is adsorbed on the surface of droplets or particles. As the concentration of electrolyte (NaCl) increases, the charge on the surface decreases, and the number of ion pairs increases, because of the association−dissociation equilibrium. At relatively low salt concentrations, the repulsion due to the double layer is dominant and decreases as the electrolyte concentration increases. At relatively high electrolyte concentrations, the hydration repulsive force due to the ion pairs present on the surface becomes dominant. Consequently, as the salt concentration increases, the total repulsion decreases and passes through a minimum, after which it increases. If the hydration repulsion is large, the emulsion or colloidal system will remain stable at any electrolyte concentration. If the hydration repulsion is small, the system will be stable only for sufficiently low electrolyte concentrations. At intermediate strength of the hydration repulsion, the stability depends on the size of the particles or droplets and the Hamaker constant. The rate of coagulation of particles of small radii and small Hamaker constants reaches a maximum and then decreases with increasing ionic strength. For particles of large radii, the increase of the maximum of the interaction energy with increasing electrolyte concentration can be so large (∼30 kT) that it can forbid the coagulation at the primary minimum; however, the particles can aggregate at the secondary minimum, which is deep.

Hydration Forces between Phospholipid Bilayers at Subzero Temperatures

A hydration repulsive force plays an important role in various phenomena. In order to study its temperature dependence, the hydration force of dipalmitoylphosphatidylcholine bilayers was investigated at temperatures below freezing. Using known chemical potential of ice, the strength of the interbilayer repulsive force of dipalmitoylphosphatidylcholine bilayers at sub-zero temperatures was estimated and X-ray diffraction was used to estimate the interbilayer spacing. An exponentially decaying repulsive force with a decay constant of 0.15 nm was observed. The present subzero temperature data are slightly greater than those measured at temperatures about 20°C.

Effect of SDS on Casein Micelles: SDS‐Induced Milk Gel Formation

ABSTRACT: The addition of sodium dodecyl sulfate (SDS) during skim‐milk reconstitution contributed to a modification of hydrophobic interactions and, consequently, to change in the micellar structure. SDS‐induced modifications in casein micelles were investigated by biochemical measurements (soluble mineral and protein analyses, granulometric and electrokinetic potential measurements, and casein micelle solvation). SDS induced micellar κ‐casein dissociation and caused a decrease in steric, hydration, and electrostatic repulsive forces between casein micelles and as a result altered micellar stability. Consequently, SDS‐modified micelle aggregation occurred. Mineral analysis indicated that Ca, PO43‐ and Mg partitioning between aqueous phase and curd is similar, suggesting a possible bridging mechanism via minerals and SDS molecules.

Short-Range Interactions of Globular Proteins at High Ionic Strengths

We compare the interactions of betalactoglobulin and lysozyme under different salt concentrations of ammonium sulfate by means of photon correlation spectroscopy measurements. The diffusion coefficient depends linearly upon protein concentration (1-25 g/L), and no evident aggregation occurs even at the highest salt concentrations. The slope of the diffusion coefficient is larger for lysozyme than for betalactoglobulin, especially at high salt concentration. The analysis of the interaction coefficients by means of the DLVO theory does not offer an unified picture of the protein interactions for the two systems. We discuss the possibility that specific (hydrogen bonding, salt bridges, etc.) or nonspecific (hydration forces) interactions, that go beyond the DLVO approximation, might be the origin of the larger van der Waals attraction shown by lysozyme. The data are consistent with a contribution of repulsive hydration forces that are smaller for lysozyme than for betalactoglobulin. This effect is in agreement with the difference in the protein size and charge.

Heteroflocculation of Amidine Polystyrene Latex and Anticarsia gemmatalis Nucleopolyhedrovirus as a Model System for Studying Sunlight Protection

Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV) is a baculovirus specific for the control of an important soybean defoliator. The baculovirus is comprised of double-stranded DNA, occluded in a proteinaceous structure called a polyhedron. Ultraviolet sunlight is the most destructive factor that affects the persistence of the virus in the field. In the present study, we use a model system wherein the pathogen is covered by another particle of opposite charge in order to test the effectiveness of a physical barrier as a protection against sunlight. Heteroflocculation experiments were carried out using two different age batches of AgMNPV and amidine polystyrene latex particles. The assessment of heteroflocculation was achieved by zeta potential and adsorption isotherm measurements, and by scanning electron microscopy. Despite the great difference in potentials between latex particles and the baculovirus, low-affinity isotherms were obtained in both pure water and 0.1 mM KCl. Adsorbed latex particles were easily washed out from the polyhedron surface. This low affinity could be attributed to the presence of a strongly repulsive hydration force of short range operating on the system. The results suggest that the failure to obtain a good physical barrier against sunlight might be attributed to the difficulty in keeping the polyhedron surface covered.

Formation of reverse hexagonal phase in water-polyoxyethylene oleyl ether system

In polyoxyethylene oleyl ether–water systems, an effective cross-sectional area per surfactant molecule at a hydrophobic interface of liquid crystal can be geometrically calculated from the interlayer spacing d of the liquid crystal. As is expected, a s increases with increasing the EO-chain length due to the increase of the hydration repulsion between the hydrophilic moieties. Consequently, the structures of surfactant self-assemblies transform from reverse hexagonal (H 2) to normal hexagonal (H 1) phases via a lamellar structure (L α). The magnitude of a s is determined by the balance between attractive (interfacial tension) and repulsive (hydration) forces acting at the hydrophobic interfaces of the aggregates. The H 2 phase is only observed in the very narrow region of EO-chain length, and the estimated a s values are almost independent with increasing surfactant concentration. According to the theory of the local free energy of the surfactant, the negative curvature of the H 2 phase would lead to the increase of the chemical potential μ because the EO chains must be closely packed. This increase of μ, however, is probably compensated by the rather decrease of attraction force between the hydrophobic parts with curvature. With increase of EO chain length, the increase of repulsion force becomes dominant the reduction of attraction force, and finally the H 2–L α phase transition takes place. The theory which the curvature effect is considered can well explain the change of a s in the water–oleyl surfactant systems.

Connection between rheological parameters and colloidal interactions of a soy protein suspension

The viscosity of a 9.1% (w/w) aqueous suspension of soy proteins at pH 7.5, which constitutes a colloidal dispersion of negatively charged and hydrated particles, is analyzed in relation to colloidal interactions. An experimental study is carried out through the classical shear rate rheometry, in the range 20–50°C. Viscosity data at different temperatures and shear rates are superimposed onto a master plot, which shows the characteristic shear-thinning behavior of colloidal suspensions. A microstructural model for the low shear limit viscosity is derived to establish a quantitative relationship between the viscosity and the interparticle forces in the suspension. Apart from the electrical forces, the hydration repulsive force must be considered in the calculation of the protein–protein interaction energy, in order to achieve a satisfactory prediction of the high viscosity values obtained experimentally. It is also observed that the relaxation time associated to small shear deformations differs from the characteristic relaxation time of the steady shear flow. The theoretical analysis carried out in this work explains the effect of colloidal interactions on rheological parameters of the soy protein suspension.

Aggregation of Dipalmitoylphosphatidylcholine Vesicles Induced by Some Metal Ions with High Activity for Hydrolysis

AlCl3, YCl3, and Pb(NO3)2 induced a reversible aggregation of vesicles prepared from dipalmitoylphosphatidylcholine (DPPC). The dependence of the aggregation behavior on both the salt concentration and temperature was similar to that previously reported for BeSO4. A nature common to these four salt species is that the metal ions have a strong tendency to be hydrolyzed in aqueous solutions. The mechanism of the DPPC vesicle aggregation induced by these metal ions is considered as follows. When these metal ions are bound to the vesicular surface, they could cause a partial destruction of the hydration shell on the surface of DPPC vesicles because of their high activity for hydrolysis. This would result in the reduction of a repulsive hydration force which is responsible for the stability of phosphatidylcholine vesicles, and accordingly, would lead to an aggregation of vesicles. The dehydration of hydrated water around DPPC headgroups is suggested by the phase-transition behavior of hydrated DPPC bilayer in the presence of these metal ions.

Influence of ethanol on the thickness and free energy of film formation of DMPC foam films

Foam films prepared from 1,2-dimirystoil-sn-glycero-3-phosphorylcholine (DMPC) dispersions in water–ethanol mixtures were investigated. Their thickness and contact angle (foam film/meniscus) were measured. Experimental results show that an increase of EtOH concentration in the film forming dispersions leads to a decrease in the film thickness. At EtOH concentrations above 40% v/v the foam films have a bilayer structure without any noticeable core of solvent. The film thickness remains constant with the further increasing of the EtOH concentration up until 50% v/v. This behaviour is corroborated by the strong increase in the contact angles (a decrease in the free energy of film formation) with increasing EtOH concentration. Ellipsometric measurements on the thickness of adsorbed DMPC monolayers and surface pressure isotherms of DMPC spread on the water–ethanol subphases show that the effective area per lipid molecule decreases, resulting in a larger monolayer thickness, when the EtOH concentration in the subphase is increased. An increase of the EtOH concentration leads to a dehydration of the DMPC molecules and a reduction in strength and range of the repulsive hydration force between the film monolayers. The film thickness and the free energy of film formation are governed by the balance of the van der Waals attraction and the repulsive hydration force between the foam film surfaces.

Non-covalent chiral fibres in aqueous gels and their functionalization

The fully reversible synthesis of noncovalent assemblies which are held together by directed bonds is called synkinesis. Micellar fibres with a distinct stereochemistry and gels are formed by amphiphiles forming strong hydrogen bonds between chiral head groups. The 3D-crystallization of such molecular assemblies, without dominating repulsive hydration forces, is prevented by the curvature of the fibres. Curvature might perhaps be directly connected with chirality. Addition of enantiomers often leads to an irreversible destruction of gels. Recent developments relating to ‘hydrophobic water’ in Ångström-wide membrane gaps are also shortly discussed.

The use of AFM for direct force measurements between expandable fluorine mica

An atomic force microscope has been used to measure forces interacting between a polymethyl methacrylate (PMMA) sphere and a flat plate, onto which expandable fluorine mica platelets have been coated. The surface thus behaves as a single macroscopic basal plane of expandable fluorine mica over concentrations 10 −5–10 −2 M NaCl and a range in pH of 3.5–10. Most of the measured forces fit well to theoretically calculated forces by the DLVO theory at separations larger than 4 nm. A short-range repulsive hydration force can be observed at separations below 4 nm which varied in magnitude both with NaCl concentration and pH. The diffuse-layer potential was extracted from the best fit to the forces calculated from the DLVO theory and compared with electrokinetic potential. There is good agreement between the two values in acidic pH range and at concentrations less than 10 −3 M NaCl, although slight discrepancies between the potential were recognized in the alkaline pH range and in the high NaCl concentration region. The diffuse-layer potentials were analyzed using the Gouy–Chapman–Stern–Graham model with dissociation-binding equilibria of surface groups. By assuming an extremely low value of the inner layer capacitance (∼5 μF/cm 2), the fit of the outer Helmholtz layer potentials calculated to the diffuse-layer potential obtained experimentally were improved.

H+/Li+and H+/K+Exchange on Delaminated Muscovite Mica

Muscovite mica was delaminated by heating with concentrated lithium nitrate solution. The delamination occurred through partial replacement of the alkali counterions originally present by lithium. The H+/Li+exchange was studied in dilute nitric acid. With delaminated mica it was possible to follow the ion exchange by measuring the change in conductivity of the reaction mixture. The exchange was also followed by measuring the amount of released lithium and was found to be fast. When the adsorbed lithium ions were replaced by potassium, the mica platelets collapsed to stacks with lower accessible surface area, and the ion exchange capacity was reduced to one-fifth of the original value. This was attributed to the smaller short-range repulsive (hydration) forces of the potassium ions. The H+/K+exchange on delaminated mica was slower than the corresponding H+/Li+exchange and its equilibrium constant was much smaller than that of the H+/Li+. The affinity of oxonium ions to the muscovite surface is much higher than that of lithium but is comparable to that of potassium.

Influence of surface charge on interaction forces in soluble salt flotation systems as determined by atomic force microscopy

The recent nonequilibrium electrophoretic mobility measurements by laser-Doppler electrophoresis coupled with the stability and prevalence of collector colloids in soluble-salt flotation systems suggest that the selective flotation of alkali halides is due to the adsorption of oppositely charged collector colloids by heterocoagulation. Previously reported flotation results confirm this surface charge/collector colloid adsorption model. These results also indicate that the sign of the surface charge has a significant effect on the extent of particle dispersion/aggregation. In this regard, the nature of such particle interaction forces at high ionic strengths are examined to improve our understanding of these systems. Atomic force microscopy (AFM) is now being used to directly measure the interaction forces responsible for the stability of these alkali halide aggregates observed in optical microscopy studies. Results from this study indicate that, while repulsive hydration forces exist between similarly charged hydrophilic particles, attractive forces exist between oppositely charged hydrophilic particles in high ionic strength solutions.

Interactions between dissimilar surfaces in high ionic strength solutions as determined by atomic force microscopy

The recent non-equilibrium electrophoretic mobility measurements by laser-Doppler electrophoresis coupled with the stability and prevalence of collector colloids in soluble salt flotation systems suggest that the selective flotation of alkali halides is due to the adsorption of oppositely charged collector colloids by heterocoagulation. Previously reported flotation results confirm this surface charge/collector colloid adsorption model. However, the nature of the interparticle forces responsible for heterocoagulation of such oppositely charged particles at high ionic strengths remains to be determined. In this regard, atomic force microscopy was used for interparticle force measurements at high ionic strengths. Model systems such as polystyrene/quartz and silica/sapphire were studied to measure the particle interaction forces prevalent at high ionic strengths. Results from this study indicate that while repulsive hydration forces exist between similarly charged hydrophilic particles, attractive forces exist between oppositely charged particles in high ionic strength solutions. The repulsive forces between similarly charged hydrophilic surfaces at high ionic strengths (2–4 m) are described by a double exponential function with decay lengths of 0.17 and 1.4 nm, depending upon the surfaces involved. On the other hand, attractive forces were observed between oppositely charged hydrophilic surfaces at high ionic strengths (2–4 m) and were described by a single exponential function with decay lengths of up to 9 nm, depending upon the surfaces involved.

Towards a Molecular Understanding of Phase Separation in the Lens: a Comparison of the X-ray Structures of Two HighTcγ-Crystallins, γE and γF, with Two LowTcγ-Crystallins, γB and γD

γ-Crystallins, although closely related in sequence, show intriguing differences in their temperature-dependent interactions: those that have a high or intermediateTcfor phase separation are cryoproteins whereas lowTcγ-crystallins are not. To address the molecular basis of phase separation, X-ray crystallography has been used to define the structural differences between high and lowTcγ-crystallins. A pre-requisite for this study was to clarify the assignment of bovine gene sequences to bovine γ-crystallin proteins used for biophysical measurements. Based on nucleotide sequence analyses of γE and γF bovine crystallin genes, γF corresponds to the previously crystallised highTcprotein bovine γIVa and γE corresponds to the highTcbovine protein fraction previously known as γIIIa. The γF sequence has enabled the completion of the refinement of the bovine γF crystal structure which shows that the molecule has an additional surface tryptophan explaining why γF has different spectroscopic properties from γB. A highTcprotein from rat lens, γE crystallin, has been crystallised and the X-ray structure solved at 2.3 Å resolution. Comparison of the X-ray structures of two highTcproteins, rat γE and bovine γF, with the structures of two lowTcproteins, bovine γB and bovine γD, shows that the main conformational change between high and lowTcproteins is in thecdsurface loop of motif 3. All four structures have numerous ion pairs on their surfaces leading to a high surface charge density, yet with low overall charge. Comparison of the lattice contacts of the two highTcproteins with the two lowTcγ-crystallins indicates that these highTcproteins utilise more amino-aromatic interactions such as between histidine and arginine. Comparison of the sequences of all the γ-crystallins which have been characterised for phase separation temperature indicates that only residue Arg/Lys 163 uniquely distinguishes cryo from non-cryo γ-crystallins and it is close to the altered surface loop. Although this region probably contributes to phase separation,Tcis likely to be a function of an overall global property that is responsive to overall charge distribution. Calculated dipole moments of native γ-crystallins, lowTcγ-crystallin sequences threaded into highTcγ-crystallin structures, andvice versa, show how both sequence and 3D structure contribute to this overall property. HighTcγ-crystallins have on average higher Arg/Lys ratios and higher histidine content. It is hypothesised that this increases the proportion of surface static paired charged networks which thus reduces the repulsive hydration force and so increases the attractive interactions of the protein-rich phase in binary liquid phase separation.

A comparison of the interaction forces between model alumina surfaces and their colloidal properties

Previous work has demonstrated that alumina dispersions are only destabilized by monovalent electrolytes such as KCl at concentrations <0.1 M over a wide pH range. This unusual stability has been qualitatively attributed to a repulsive hydration force that operates at distances <5 nm. Intermolecular forces, measured between an aluminium coated colloidal silica sphere and a flat alumina substrate, carried out using an atomic force microscope demonstrates that the additional repulsion is due to short range forces not expected in DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. The origin of these forces is postulated to be due to a combination of surface gel formation, probably due to polymeric Al species, and the natural hydration of the surface. The gel layer thickness determined at pH 8 was at least 15 nm. At pH ≤7 (i.e. ≤isoelectric point) the forces obtained conformed to DLVO behaviour down to separation distances of ca 3–5 nm, at smaller separations an additional repulsive force was detected. A thin gel-layer may be formed even at lower pH values which would contribute to this short range repulsion. These results may partially explain the difficulty encountered in dewatering aluminium hydroxide rich sludges generated during water treatment.

Kinetics of Protein-Induced Flocculation of Phosphatidylcholine Liposomes

The influence of protein adsorption on membrane−membrane interactions was studied regarding the kinetic stability of phosphatidylcholine (PC) liposome dispersions. The rates of flocculation were measured by the decrease in the transmitted light intensities in time. To elucidate the experimental data, we proposed an analytical method relating light transmittance to the average size of liposome flocs. The obtained relaxation time and maximum light absorbance were connected to the kinetic constant and the flocculation activation energy. These model parameters were calculated for the flocculation processes of PC liposomes in the presence of lysozyme, cytochrome c, and bovine serum albumin as a function (i) of the protein concentration and (ii) of the ionic strength. In addition to the generally accepted concept that the stability of PC liposomes is due to the action of hydration repulsive force, here we found that in the presence of soluble proteins the steric factors and electrostatics play essential roles for the colloidal stability of egg PC liposome dispersions.