Charged Solid Surfaces Research Articles

Multiprotein interactions during surface adsorption: a molecular dynamics study of lysozyme aggregation at a charged solid surface.

Multiprotein adsorption of hen egg white lysozyme at a model charged ionic surface is studied using fully atomistic molecular dynamics simulations. Simulations with two, three, and five proteins, in various orientations with respect the surface, are performed over a 100 ns time scale. Mutated proteins with point mutations at the major (Arg128 and Arg125) and minor (Arg68) surface adsorption sites are also studied. The 100 ns time scale used is sufficient to observe protein translations, rotations, adsorption, and aggregation. Two competing processes of particular interest are observed, namely surface adsorption and protein–protein aggregation. At low protein concentration, the proteins first adsorb in isolation and can then reorientate on the surface to aggregate. At high concentration, the proteins aggregate in the solution and then adsorb in nonspecific ways. This work demonstrates the role of protein concentration in adsorption, indicates the residues involved in both types of interaction (protein–protein and protein–surface), and gives an insight into processes to be considered in the development of new functionalized material systems.

The Effect of Salt on the Water Structure at a Charged Solid Surface: Differentiating Second- and Third-order Nonlinear Contributions

We have used visible-infrared sum-frequency generation spectroscopy to reveal fundamental characteristics of water structure at the fused silica surface. By studying a wide range of ionic strengths, from 0.05 mM to 4 M, we are able to comment on the contributions of second- and third-order nonlinearities to the spectroscopic response. Spectra obtained from extremely dilute salt concentrations provide evidence of the previously sought increasing surface charge with ionic strength. This is followed by a screening regime where the extent to which the surface field penetrates into the bulk is limited by the electrolyte. Data from intermediate salt concentrations reveal a few strongly ordered layers of water immediately adjacent to the surface. At high concentrations, we observe a significant disruption of solvent ordering. Together, the observation of these four distinct regimes provides a unified understanding of interfacial water structure in the presence of salt that consolidates previous reports in the literature.

Film Forming Characteristics of Common Ionic Liquid Lubricants

Ionic liquids now receive increasing attention in the mechanical lubrication research area.However,the film forming properties of ionic liquids are still rarely discussed.The film forming properties of several common phosphate ionic liquids with different side chain lengths of the cation are investigated under different pressures.Also,the lubrication failure and compression resistance of these ionic liquids are discussed by taking silicon oils with similar viscosities as comparative objects.The results show that the lubrication behaviors of these ionic liquids accord with the elastohydrodynamic lubrication(EHL) theory at high entraining speed and they accord with the thin film lubrication(TFL) theory at low entraining speed.With the increase of the side chain length of the cation,the difference between the measured central film thickness in the contact region and the calculated value according to the EHL theory tends to reduce in the TFL regime.It is primarily due to the decrease in the number of ions per unit volume and the effect of steric hindrance.When the entraining speed decreases to about 0.1 mm?s?1,no obvious lubrication failure point is observed for the tested ionic liquids.The ionic liquids possess better compression resistance than silicon oils with similar viscosities as a result of the strong electrostatic interaction between ions and charged solid surfaces.

Mechanism of Hen Egg White Lysozyme Adsorption on a Charged Solid Surface

The mechanism of hen egg white lysozyme (HEWL) adsorption on a negatively charged, hydrophilic surface has been studied using atomistic molecular dynamics (MD) simulation. Sixteen 90 ns trajectories provide adequate data to allow a detailed description of the adsorption process to be formulated. Two distinct adsorption sites have been identified. The main one is located on the N,C-terminal protein face and comprises Arg128 (the crucial one), supplemented by Arg125, Arg5, and Lys1; the minor one is used accidentally and contains only Arg68. Adsorption of this protein is driven by electrostatics, where the orientation of the protein dipole moment defines the direction of protein movement. The diffusion range on the surface depends on protein side-chain penetration through the surface water layers. This is facilitated by the long-range electric field of the charged surface, which can align polar side chains to be perpendicular to the surface. A simulation of adsorption onto a neutral ionic surface shows no such surface water layer penetration. Therefore, protein flexibility is seen to be an important factor, and to adsorb the HEWL has to adjust its structure. Nevertheless, at a flat surface only a slight loss of α-helical content is required. The adsorbed HEWL molecule is oriented between side-on and end-on ways, where the angle between the protein long axis (which mostly approximates the dipole moment) and the surface varies between 45° and 90°. Simulations with targeted mutations confirm the picture that emerges from these studies. The active site is located on the opposite face to the main adsorption site; hence, the activity of the immobilized HEWL should not be affected by the surface interactions. Our results provide a detailed insight into the adsorption mechanism and protein mobility at the surface. This knowledge will aid the proper interpretation of experimental results and the design of new experiments and functional systems.

Studies of temperature influence on adsorption behaviour of nonionic polymers at the zirconia–solution interface

The temperature influence (15–35 °C) on the adsorption mechanism and conformation of nonionic polymers (polyethylene glycol (PEG), polyethylene oxide (PEO) and polyvinyl alcohol (PVA)) on the zirconium dioxide surface was examined. The applied techniques (spectrophotometry, viscosimetry, potentiometric titration and microelectrophoresis) allowed characterization of the changes in structure and thickness of polymer adsorption layers with the increasing temperature. The rise of temperature favours more stretched conformation of polymer chains on the ZrO2 surface, which results in higher adsorption and thicker adsorption layer. Moreover, these conformational changes of adsorbed macromolecules affect the electric (solid surface charge density) and electrokinetic (zeta potential) properties of the zirconia–polymer interface. The obtained data indicate that the polyvinyl alcohol adsorption has a greater influence on zirconia properties in comparison to that of PEG and PEO. It is due to the presence of acetate groups in the PVA macromolecules (degree of hydrolysis 97.5%), which undergo dissociation.

What Governs Protein Adsorption and Immobilization at a Charged Solid Surface?

The adsorption of hen egg white lysozyme at a model charged surface is studied using fully atomistic molecular dynamics simulations. The simulations are performed over a 90 ns time scale which is sufficient to observe rotational and translational steps in the adsorption process. Electrostatics is found to play a key role in guiding the protein to the favorable binding orientation with the N,C-terminal face against the substrate. However, full immobilization appears to only occur through the strong interaction of Arg128 with the surface, facilitated by the protein’s flexibility at the terminal face. Simulated mutation at this residue confirms its crucial role. This work demonstrates that electrostatics alone might not be sufficient to guide the development of material systems that exploit protein adsorption and immobilization.

Effect of Calcium and Magnesium Addition on Arsenic Leaching from Paddy Field Soil of Bangladesh

Arsenic (As) that has been accumulated from irrigation groundwater to paddy field soil by sorption has the potential for food contamination by plant uptake and recontamination of the groundwater. This study evaluated the effect of calcium (Ca) and magnesium (Mg) addition on As leaching from paddy field soil collected from the southwest region of Bangladesh. Batch experiments were employed to systematically investigate the role of Ca and Mg addition on the leaching behavior of As under different concentrations of Ca and Mg, pH conditions and anaerobic incubation. Results indicated that As leaching was highly decreased with the increase of Ca and Mg addition, at pH greater than 9.0 and during anaerobic incubation. In contrast, Iron (Fe) leaching was decreased by Ca and Mg addition. Adsorption of Ca and Mg was observed and significant correlation with adsorbed As was obtained in all batches. The probable mechanism was precipitation of As due to the increase in the positive surface charge of the Fe hydroxide solids by Ca and Mg adsorption. This study also indicated that Ca and Mg addition could decrease As leaching even under the presence of phosphorus.

Direct Measurement of Sub-Debye-Length Attraction between Oppositely Charged Surfaces

Using a surface force balance with fast video analysis, we have measured directly the attractive forces between oppositely charged solid surfaces (charge densities sigma(+), sigma(-)) across water over the entire range of interaction, in particular, at surface separations D below the Debye screening length lambda(S). At very low salt concentration we find a long-ranged attraction between the surfaces (onset ca. 100 nm), whose variation at D<lambda(S) agrees well with predictions based on solving the Poisson-Boltzmann theory, when due account is taken of the independently-determined surface charge asymmetry (sigma(+) not equal to |sigma(-)|).

Coincidence measurements of secondary ions and scattered ions in collision between slow Ar6+ and a GaN(0001) surface

An experimental apparatus has been developed to investigate interactions between highly charged ions (HCIs) and solid surfaces under off-normal incidence. The apparatus consists of a secondary ion mass spectrometer (SIMS) and a low-energy ion scattering spectrometer (LEIS) equipped with a Wien filter (E × B velocity filter). It allows us to carry out simultaneous SIMS and LEIS analysis. It successfully measured the time-of-flight SIMS spectrum in coincidence with scattered Ar0 and Ar+ in collisions between Ar6+ (15.0 keV) and a GaN(0001) surface. Protons and hydrocarbons, N+, Ga+ and GaN+ ions were observed in the SIMS spectra taken in coincidence with the LEIS spectrum of Ar0 and Ar+. The kinetic energy of Ar+ measured in coincidence with protons emitted from the surface was distributed over an energy range from 13.6 to 15.0 keV. This fact directly shows that kinetic processes play a significant role in proton emissions.

Protein−Solid Interactions: Important Role of Solvent, Ions, Temperature, and Buffer in Protein Binding to α-Zr(IV) Phosphate

The interaction of proteins with a solid surface involves a complex set of interactions, and elucidating the details of these interactions is essential in the rational design of solid surfaces for applications in biosensors, biocatalysis, and biomedical applications. We examined the enthalpy changes accompanying the binding of met-hemoglobin, met-myoglobin, and lysozyme to layered alpha-Zr(IV)phosphate (20 mM NaPipes, 1 mM TBA, pH 7.2, 298 K) by titration calorimetry, under specific conditions. The corresponding binding enthalpies for the three proteins are -24.2 +/- 2.2, -10.6 +/- 2, and 6.2 +/- 0.2 kcal/mol, respectively. The binding enthalpy depended on the charge of the protein where the binding of positively charged proteins to the negatively charged solid surface was endothermic while the binding of negatively charged proteins to the negatively charged solid was exothermic. These observations are contrary to a simple electrostatic model where binding to the oppositely charged surface is expected to be exothermic. The binding enthalpy depended on the net charge on the protein, ionic strength of the medium, the type of buffer ions present, and temperature. The temperature dependence studies of binding enthalpies resulted in the estimation of heat capacity changes accompanying the binding. The heat capacity changes observed with Hb, Mb, and lysozyme are 1.4 +/- 0.3, 0.89 +/- 0.2, and 0.74 +/- 0.1 kcal/(mol.K), respectively, and these values depended on the net charge of the protein. The enthalpy changes also depended linearly on the enthalpy of ionization of the buffer, and the numbers of protons released per protein estimated from this data are 12.6 +/- 2, 6.0 +/- 1.2, and 1.2 +/- 0.5 for Hb, Mb, and lysozyme, respectively. Binding enthalpies, independent of buffer ionization, are also estimated from these data. Entropy changes are related to the loss in the degrees of freedom when the protein binds to the solid and the displacement of solvent molecules/protons/ions from the protein-solid interface. Proton coupled protein binding is one of the major processes in these systems, which is novel, and the binding enthalpies can be predicted from the net charge of the protein, enthalpy of buffer ionization, ionic strength, and temperature.

Molecular Dynamics Simulations of Hen Egg White Lysozyme Adsorption at a Charged Solid Surface

Hen egg white lysozyme (HEWL) adsorption on negatively charged, hydrophilic surfaces has been investigated using atomistic molecular dynamics. Analysis of six 20 ns trajectories performed at pH 7 and ionic strength 0.02 M (NaCl) reveals that conformational alterations are required for HEWL adsorption, and that upon adsorption the protein loses some alpha-helical content. Simulations for a few different initial orientations show that the HEWL protein adsorbs on a flat surface with an angle between the protein long axis and the surface of about 45 degrees . The main adsorption site is located on the N,C-terminal part of the HEWL surface; the major role is played by Lys1, Arg5, Arg14, and Arg128. Adsorption is not found with contrary orientations. Two additional 20 ns trajectories calculated with 0.5 M ionic strength suggest that the main force governing adsorption is electrostatic attraction between parts of the protein and the surface. A trajectory obtained for the protein situated inside a cubic box built from the charged surfaces shows that the adsorption pattern is different for flat and nonflat surfaces, and in particular, adsorption on the nonflat surface requires tertiary structure alterations and partial unfolding. The observed trends are consistent with both experimental and previous computational studies.

Negative charges at the air/water interface and their consequences for aqueous wetting films containing surfactants

Negative charges at the air/water interface have been proven previously by the authors via disjoining pressure measurements of wetting films. In the present study the surface charges of wetting film are modified in order to control the film stability. Studies at different pH values support the assumption that the negative charges are caused by an excess of hydroxide ions. Addition of different types of salt anions seems to lead to an increase in surface charges with decreasing hydration shell. Both water-soluble and water-insoluble amphiphiles replaces the charges. The air/water surface remains negatively charged in case of non-ionic surfactant (dodecyl-beta maltoside, C12G2) and anionic ampihiphiles (stearic acid and SDS). The charge will be reversed in case of cationic amphiphiles. Under all of those conditions, films against likely charged solid surfaces are stable and unstable against oppositely charged solid substrates. For this purpose, the charge of silicon wafers was precoated with polyelectrolytes before. At concentrations well below the CMC cationic surfactants are not able to reverse the charges at the air/water interface. Respective wetting films are less stable and show a partial dewetting even on likely charged surfaces. These results lead to the conclusion that the films are stabilised by electrostatic repulsion. Potential adsorption of water-soluble surfactants on oppositely charged surfaces is also taken into account and is studied by contact-angle measurements.

On the phase behavior and structural properties of polyelectrolyte solutions

Correlations between structural properties and phase behavior of polyelectrolyte solutions were discussed along the line of the work reported by Châtellier and Joanny [Joanny JF, Châtellier X. J Phys France II 1996;6:1669]. A multicomponent system made of polyions, salt ions and counterions was considered under poor solvent conditions. Unlike this reference, partial structure factors were derived from the celebrated Zimm’s formula written in the matrix form including the effects of finite chain length. These effects were found to generate significant shifts in phase diagrams and qualitative changes in structural properties. The presence of a charged solid surface was briefly discussed. Here also, the phase diagram was found to shift with an increasing amount as the polyion chain length decreased.

Adsorption of Molecular Brushes with Polyelectrolyte Backbones onto Oppositely Charged Surfaces: A Self-Consistent Field Theory

The two-gradient version of the Scheutjens-Fleer self-consistent field (SF-SCF) theory is employed to model the interaction between a molecular bottle brush with a polyelectrolyte backbone and neutral hydrophilic side chains and an oppositely charged surface. Our system mimics graft-copolymers with a cationic main chain (among which poly( L-lysine)- graft-poly(ethylene glycol) (PLL- g-PEG) or poly( L-lysine)- graft-polyoxazoline are well-known examples) interacting with negatively charged (metal oxide) solid surfaces. We aim to analyze the copolymer-surface interaction patterns as a function of the molecular architecture parameters. Two regimes are investigated: First, we compute the effective interaction potential versus the distance from the surface for individual bottle brush macromolecules. Here, depending on the grafting ratio and the degree of polymerization of the side chains, the interplay of electrostatic attractions of the main chain to the surface and the steric repulsion of the grafts results in different patterns in the interaction potential and, therefore, in qualitatively different adsorption behavior. In particular, we demonstrate that, at high side chain grafting density and short grafts, the molecular brushes are strongly adsorbed electrostatically onto negatively charged substrates, whereas, in the opposite case of low grafting ratio and high molecular weight of grafts, the steric repulsion of the side chains from the surface dominates the polymer-surface interaction. At intermediate grafting ratios, the adsorption/depletion scenario depends essentially on the ratio between the electrostatic screening length and the thickness of the molecular bottle brush. We further have analyzed the equilibrium adsorbed amount as a function of the macromolecular architecture. Our results are based on a detailed account of attractive and repulsive (including intermolecular in-plane) interactions, and suggest a nonmonotonic dependence of the adsorbed amount on the grafting ratio, in good agreement with the experimental studies for PLL- g-PEG adsorption onto Nb2O5 surfaces. The results of the theoretical modeling are discussed in the context of the optimization of the PLL-g-PEG molecular design parameters in order to create protein-resistant surfaces.

Experimental and modeling studies of sorption of tetracycline onto iron oxides-coated quartz

The interaction of tetracycline (TET) with pure quartz (Q) and two iron oxides-coated quartz including quartz/ferrihydrite (Q 1) and quartz/goethite (Q 2) was investigated. The iron oxides-coated quartz were synthesized by precipitation suspension method and characterized by XRD, Fourier transform infrared spectroscopy (FTIR), surface area measurements and chemical analyses. Sorption of TET onto mixed oxides was studied under a wide range of conditions (time, sorbate concentration, pH, ionic strength, foreign divalent cations (Ca 2+, Pb 2+) and temperature). Kinetic data were treated using the pseudo-second-order model. The sorption isotherm of TET was fitted using a Langmuir model for both solids. The presence of complexing cation divalents decreased the sorption of TET to solid surface, but the variation of ionic strength (NaCl addition) was of less importance. The adsorption of TET was strongly affected by the pH at three temperatures with maximum adsorption close to the p K a values. The adsorption increased with temperature increasing at low pH values, while at high pH values, it decreased with temperature increasing. Thermodynamic parameters (Δ G°, Δ H° and Δ S°) calculated from the temperature-dependent sorption data showed that the sorption reactions are endothermic and spontaneous for both mineral oxides. The experimental data of TET sorption can be described by surface complexation model assuming one type of site and binuclear surface complexes. The results of this study suggest that pH, temperature, surface charge of solids and presence of complexing divalent cations should be considered when predicting sorption of TET onto mineral oxides and its fate in soil and environment.

Adsorption of Some Alkyloxyethylene Pyridinium Chlorides at Solid-Water Interface

The adsorption of cetylalkyl pyridinium and alkyloxyethylene pyridinium chlorides at the silica-alumina water interface at ph =7 and 25 0C was analyzed to determine the type of interaction between surfactant molecules and solid substrat between adsorbed adiacent molecules and with the surfactant molecules in the equilibrium water solution. The experimental adsorption isotherm for the entire concentration range consists of four regions with the maximum adsorption attained at the critical micelle concentration of each surfactant in aqueous solutions. Different models and equations of adsorption suitable to describe different types of forces implicated in adsorption of cationic surfactant at the opposite charged solid surfaces were applied. The free energy of adsorption, free energy of hydrophobic interaction and mean surface aggregation number were calculated. Insertion of oxyethylene groups leads to the increase of the hydrophobicity of these cationic surfactants. From the calculated free energy of adsorption through electrostatic and van der Waals interactions different surface structures were attributed to each region: neutralization of electrical charges, formation of hemimicelles, admicelles and bilayers.

Co-Ion and Ion Competition Effects: Ion Distributions Close to a Hydrophobic Solid Surface in Mixed Electrolyte Solutions

We consider within a modified Poisson-Boltzmann theory an electrolyte, with different mixtures of NaCl and NaI, near uncharged and charged solid hydrophobic surfaces. The parametrized potentials of mean force acting on Na+, Cl-, and I- near an uncharged self-assembled monolayer were deduced from molecular simulations with polarizable force fields. We study what happens when the surface presents negative charges. At moderately charged surfaces, we observe strong co-ion adsorption and clear specific ion effects at biological concentrations. At high surface charge densities, the co-ions are pushed away from the interface. We predict that Cl- ions can also be excluded from the surface by increasing the concentration of NaI. This ion competition effect (I- versus Cl-) may be relevant for ion-specific partitioning in multiphase systems where polarizable ions accumulate in phases with large surface areas.

Restricted primitive model for electrolyte solutions in contact with solid surface modified by grafted chains: A density functional approach

We have studied the microscopic structure and the thermodynamic and electric properties of the restricted primitive model for electrolyte solutions in contact with a chemically modified uncharged and charged solid surface. The modification of the surface is performed by the tethering of chain particles via a specific single segment at the stage preceeding the adsorption of the restricted primitive model. Some fraction of segments belonging to a chain particle can be charged such that the system models adsorption on an electrode covered by an electrolyte brush. We apply the density functional method which combines previously developed approach for inhomogeneous chain fluids, and the weighted density mean spherical approximation energy route for the description of inhomogeneous ionic fluids. The theory is formulated in the semigrand canonical ensemble. Our principal findings are concerned with the density profiles of the species, the adsorption isotherms of ions in the presence of grafted chain particles, the charge density profile and the potential of zero charge (all as the functions of the grafted density), and other parameters related to the solid surface and its modification.

Effect of low-molecular-weight organic acids on the adsorption of norfloxacin in typical variable charge soils of China

Batch equilibrium experiments were used to study the adsorption of norfloxacin (NOR) onto three kinds of variable charge soils in China, namely Rho-Udic Ferralisols collected from Yunnan, Ali-Perudic Ferrisols collected from Jiangxi and Typ-Hap-Udic Ferralisols collected from Guangdong. Results show that NOR is strongly adsorbed by the soils, with lg K d-values (linear model) of 4.41 ± 0.01, 4.50 ± 0.02, 4.44 ± 0.01 and lg K f-values (Freundlich model) of 4.32, 4.45, 3.08 for the three tested soils, respectively. Both curves of the Freundlich equation and Langmuir equation for the three soils were in excellent linear correlation. Low-molecular-weight (LMW) organic acids, including citric acid, malic acid and salicylic acid were added in the presence of acetate buffer (pH 4.5) to test their effects on NOR adsorption. It was observed that the addition of LMW organic acids inhibited the NOR adsorption process. It was assumed that several effects, including soil pH, solid surface charge and competitive adsorption of co-existing cations, dominated the NOR sorption processes onto variable charge soils. Further studies should be performed to reveal the mechanism of the adsorption.

Surface reactivity of rapidly quenched nano-quasicrystalline ribbons with respect to biomolecules

In this paper, we report on preliminary investigations about surface reactivity of quasicrystalline-based alloys with respect to small biological molecules such as amino-acids. The main purpose is to study potential applications for these materials in biomedical fields since biomolecules are well known to be sensitive to solid surface wettability, surface energy and charge. In that sense, quasicrystals surfaces could be attractive considering their unusual symmetry, their low surface energy and their peculiar electronic properties. Our investigations were focussed on superficial reactivity of as-quenched Ti 45Zr 38Ni 17 (numbers indicate at.%) ribbons, whose microstructure was analysed by transmission electron microscopy and X-ray diffraction. Surfaces were then carefully polished and immersed in a solution ( C = 10 −2 mol l −1, pH 6) of l-glutamic acid or l-lysine for duration varying from 30 min to 6 days. Molecular vibrational modes and adsorption kinetics were investigated using infrared spectroscopy. The first results reveal that l-glutamic acid seems to display faster adsorption kinetic on quasicrystalline ribbons compared to that on pure titanium (cpTi) or on β-metastable alloys (Ti–12 Mo–6 Zr–4.5 Sn), which is quite interesting since surface energy on quasicrystals is known to be low.