Intermolecular Electrostatic Forces Research Articles

Polyelectrolyte-protein synergism: pH-responsive polyelectrolyte/insulin complexes as versatile carriers for targeted protein and drug delivery

The co-assembly of polyelectrolytes (PE) with proteins offers a promising approach for designing complex structures with customizable morphologies, charge distribution, and stability for targeted cargo delivery. However, the complexity of protein structure limits our ability to predict the properties of the formed nanoparticles, and our goal is to identify the key triggers of the morphological transition in protein/PE complexes and evaluate their ability to encapsulate multivalent ionic drugs. A positively charged PE can assemble with a protein at pH above isoelectric point due to the electrostatic attraction and disassemble at pH below isoelectric point due to the repulsion. The additional hydrophilic block of the polymer should stabilize the particles in solution and enable them to encapsulate a negatively charged drug in the presence of PE excess. We demonstrated that diblock copolymers, poly(ethylene oxide)-block-poly(N,N-dimethylaminoethyl methacrylate) and poly(ethylene oxide)-block-poly(N,N,N-trimethylammonioethyl methacrylate), consisting of a polycation block and a neutral hydrophilic block, reversibly co-assemble with insulin in pH range between 5 and 8. Using small-angle neutron and X-ray scattering (SANS, SAXS), we showed that insulin arrangement within formed particles is controlled by intermolecular electrostatic forces between protein molecules, and can be tuned by varying ionic strength. For the first time, we observed by fluorescence that formed protein/PE complexes with excess of positive charges exhibited potential for encapsulating and controlled release of negatively charged bivalent drugs, protoporphyrin-IX and zinc(II) protoporphyrin-IX, enabling the development of nanocarriers for combination therapies with adjustable charge, stability, internal structure, and size.

Quantitative proteomics provides a new perspective on the mechanism of network structure depolymerization during egg white thinning

As a typical colloidal solution system, egg white (EW) will naturally thin during storage. This paper discussed mechanism of EW thinning and protein depolymerization from the perspective of “protein composition and molecular structure”. The results of rheology showed that viscoelasticity of EW declined substantially. Analysis of EW protein in gel system demonstrated that arrangement of EW thermal gel gradually tightened with dissociation of skeleton protein during storage. Molecular characteristics of EW protein in solution showed that particle size and free sulfhydryl content decreased. The increase of disulfide bonds enhanced intermolecular electrostatic force and hindered molecular aggregation, which improved solubility of molecules and reduced surface hydrophobicity. Quantitative proteomic analysis indicated the reduced abundance of β-ovomucin (OVO) might be the direct cause of EW thinning. Notably, some proteins extensively involved in the aggregation of proteins during later storage. The results can provide scientific basis for depolymerization and aggregation of EW during storage.

Geometry influenced adsorption of fluoxetine over the surface of RuFeO3 and CeFeO3 nanoparticles: Kinetics and thermodynamic studies

Novel mesoporous ferrite nanoparticles of ruthenium (RuFeO3) and cerium (CeFeO3) with large surface area were successfully fabricated by hydrothermal method and applied for the removal of fluoxetine from aqueous solution. The morphology and physicochemical properties of meso-porous metal ferrites were characterized by different analytical techniques. The obtained results showed that the particle size of 20–30 nm for meso-CeFeO3, and 80–100 nm for meso-RuFeO3 respectively. Study of the adsorption of fluoxetine over metal ferrite nanoparticles was explored using UV–vis spectroscopy at different experimental conditions such as temperature, pH, initial fluoxetine concentration and time. The adsorption data were fitted and analyzed with kinetic and different adsorption isotherm models, and the results showed that both the metal ferrites effectively adsorbed fluoxetine (>99%). The maximum adsorption capacity of 729.6 mg g−1 for CeFeO3, and 683.5 mg g−1 for RuFeO3 at neutral pH and at 25 °C were observed. The amount adsorbed over the surface of metal ferrite nanoparticles increased with increasing initial fluoxetine concentration, reaction time, and temperature, and the adsorption performance improved under neutral conditions (pH 7 and 8). The pseudo-second order kinetic and Langmuir isotherm models satisfactorily fitted the adsorption data, showing that the adsorption of fluoxetine involved physical adsorption through intermolecular electrostatic force between the metal ferrites and fluoxetine which is further confirmed by intra-particle diffusion model. The observed wide band/gap, 3.815eV for CeFeO3, and 3.360eV for RuFeO3 respectively showed their stability towards visible light suggesting their real sample applicability, recovery and reuse capability involving physisorption with fluoxetine molecules. In addition, the feasible adsorption process of fluoxetine is analyzed with its geometry that is determined by theoretical energy calculations using DFT.

Lack of Dependence of the Sizes of the Mesoscopic Protein Clusters on Electrostatics

Protein-rich clusters of steady submicron size and narrow size distribution exist in protein solutions in apparent violation of the classical laws of phase equilibrium. Even though they contain a minor fraction of the total protein, evidence suggests that they may serve as essential precursors for the nucleation of ordered solids such as crystals, sickle-cell hemoglobin polymers, and amyloid fibrils. The cluster formation mechanism remains elusive. We use the highly basic protein lysozyme at nearly neutral and lower pH as a model and explore the response of the cluster population to the electrostatic forces, which govern numerous biophysical phenomena, including crystallization and fibrillization. We tune the strength of intermolecular electrostatic forces by varying the solution ionic strength I and pH and find that despite the weaker repulsion at higher I and pH, the cluster size remains constant. Cluster responses to the presence of urea and ethanol demonstrate that cluster formation is controlled by hydrophobic interactions between the peptide backbones, exposed to the solvent after partial protein unfolding that may lead to transient protein oligomers. These findings reveal that the mechanism of the mesoscopic clusters is fundamentally different from those underlying the two main classes of ordered protein solid phases, crystals and amyloid fibrils, and partial unfolding of the protein chain may play a significant role.

Viscosity of Collagen Solutions: Influence of Concentration, Temperature, Adsorption, and Role of Intermolecular Interactions

An investigation of the influences of the adsorption, electrolyte, shear rate, and temperature on the viscometric behavior of collagen solutions was performed using a photoelectric viscometer. The experimental results showed an abnormal behavior of the reduced viscosity (ηsp/C) of dilute collagen solutions measured by a viscometer with or without a hydrophobic surface treatment; the reduced viscosity increased with decreasing concentration. This phenomenon can be completely eliminated by increasing the concentration of an added salt. It indicated that the abnormal viscosity resulted from a long-ranged, inter-molecular electrostatic force. The reduced viscosity decreased as the shear rate increased. The shear-thinning cavitation phenomenon resulted from the high aspect ratio of the collagen molecules. The reduced viscosity of the collagen solutions increased with an increase in temperature, which was due to the association of collagen molecules at high temperature.

Ab initio study of HXeF dimer and trimer

In this work, the HXeF molecule and its neutral clusters were studied at MP2/Aug-cc-pVTZ. Due to their strong ionizing character, HXeF molecules can aggregate to form stable clusters, the stability of which originates from both intermolecular electrostatic forces and charge transfer. A periodic boundary conditions (PBCs) calculation was also carried out to prove the existence of larger clusters. In addition, a new linear compound containing two Xe atoms was also found and analyzed by the NBO method. The new xenon compound is more stable than the HXeF molecule, with a stabilization energy of −10.50kcal/mol.

A transferable classical potential for the water molecule

We developed a new model for the water molecule which contains only three Gaussian charges. Using the gas-phase geometry the dipole moment of the molecule matches, the quadrupole moment closely approximates the experimental values. The negative charge is connected by a harmonic spring to its gas-phase position. The polarized state is identified by the equality of the intermolecular electrostatic force and the spring force acting on the negative charge. In each timestep the instantaneous position of the massless negative charge is determined by iteration. Using the technique of Ewald summation, we derived expressions for the potential energy, the forces, and the pressure for Gaussian charges. The only properties to be fitted are the half-width values of the Gaussian charge distributions and the parameters of the nonelectrostatic repulsion-attraction potential. We determined the properties of gas-phase clusters up to six molecules, the internal energy and density of ambient water and hexagonal ice. We calculated the equilibrium density of ice VII as a function of pressure. As an additional test, we calculated the pair-correlation function, the isotherm compressibility, the heat capacity, and the self-diffusion coefficients for ambient water. As far as we know, this is the first classical model of water which is able to estimate both ends of the phase diagram, the high pressure ice VII, and the gas clusters of water with excellent accuracy.

The Use of Anisotropic Potentials in Modeling Water and Free Energies of Hydration.

We propose a novel, anisotropic rigid-body intermolecular potential model to predict the properties of water and the hydration free energies of neutral organic solutes. The electrostatic interactions of water and the solutes are modeled using atomic multipole moments up to hexadecapole; these are obtained from distributed multipole analysis of the quantum mechanically computed charge densities and include average polarization effects in solution. The repulsion-dispersion water-water interactions are modeled with a three-site, exp-6 model fitted to the experimental liquid water density and oxygen-oxygen radial distribution function at ambient conditions. The proposed water model reproduces well several water properties not used in its parametrization, including vapor-liquid coexistence densities, the maximum in liquid water density at atmospheric pressure, the structure of ordered ice polymorphs, and the liquid water heat capacity. The model is used to compute the hydration free energy of 10 neutral organic solutes using explicit-solvent free energy perturbation. The solute-solute repulsion-dispersion intermolecular potential is obtained from previous parametrizations on organic crystal structures. In order to calculate the free energies of hydration, water-solute repulsion-dispersion interactions are modeled using Lorenz-Berthelot combining rules. The root-mean-square error of the predicted hydration free energies is 1.5 kcal mol(-1), which is comparable to the error found using a continuum mean-field quantum mechanical approach parametrized using experimental free energy of hydration data. The results are also contrasted with explicit-solvent hydration free energies obtained with an atomic charge representation of the solute's charge density computed at the same level of theory used to compute the distributed multipoles. Replacing the multipole description of the solute's charge density with an atomic charge model changes the free energy of hydration by as much as 3 kcal mol(-1) and provides an estimate for the effect of the modeling quality of the intermolecular electrostatic forces in free energy of solvation calculations.

Modeling the Optical Behavior of Complex Organic Media: From Molecules to Materials

For the past three decades, a full understanding of the electro-optic (EO) effect in amorphous organic media has remained elusive. Calculating a bulk material property from fundamental molecular properties, intermolecular electrostatic forces, and field-induced net acentric dipolar order has proven to be very challenging. Moreover, there has been a gap between ab initio quantum-mechanical (QM) predictions of molecular properties and their experimental verification at the level of bulk materials and devices. This report unifies QM-based estimates of molecular properties with the statistical mechanical interpretation of the order in solid phases of electric-field-poled, amorphous, organic dipolar chromophore-containing materials. By combining interdependent statistical and quantum mechanical methods, bulk material EO properties are predicted. Dipolar order in bulk, amorphous phases of EO materials can be understood in terms of simple coarse-grained force field models when the dielectric properties of the media are taken into account. Parameters used in the statistical mechanical modeling are not adjusted from the QM-based values, yet the agreement with the experimentally determined electro-optic coefficient is excellent.

Inhibition of J-aggregation of meso-tetra(4-sulfonatophenyl)porphyrin by an ionic liquid with π-conjugated character

UV–visible spectral observations indicate that the J-aggregation of protonated meso-tetra(4-sulfonatophenyl)porphyrin ([H2TSPP]2+) under acidic conditions is completely inhibited by the π–π counteraction between 1-butyl-pyridinium tetrafluoroborate ([bpy]BF4) and [H2TSPP]2+. The studies also suggest that the intermolecular π–π force is of relative importance for the J-aggregates of [H2TSPP]2+ and the intermolecular electrostatic force for the H-aggregates.

An efficient ring polymer contraction scheme for imaginary time path integral simulations

A quantum simulation of an imaginary time path integral typically requires around n times more computational effort than the corresponding classical simulation, where n is the number of ring polymer beads (or imaginary time slices) used in the calculation. However, this estimate neglects the fact that the potential energies of many systems can be decomposed into a sum of rapidly varying short-range and slowly varying long-range contributions. For such systems, the computational effort of the path integral simulation can be reduced considerably by evaluating the long-range forces on a contracted ring polymer with fewer beads than are needed to evaluate the short-range forces. This idea is developed and then illustrated with an application to a flexible model of liquid water in which the intramolecular forces are evaluated with 32 beads, the oxygen-oxygen Lennard-Jones forces with seven, and the intermolecular electrostatic forces with just five. The resulting static and dynamic properties are within a few percent of those of a full 32-bead calculation, and yet they are obtained with a computational effort less than six times (rather than 32 times) that of a classical simulation. We hope that this development will encourage future studies of quantum mechanical fluctuations in liquid water and aqueous solutions and in many other systems with similar interaction potentials.

Self-assembly of 1-nitronaphthalene on Au(111)

We report low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure investigations of the organic molecule 1-nitronaphthalene adsorbed on the reconstructed Au(111) surface. Depending on the coverage, molecular structures of distinct dimensionality are observed. After saturation of the step edges with molecules, clusters form on the substrate terraces at coverages up to 0.15 monolayers (ML). The smallest clusters consist of three molecules and are stable at specific surface reconstruction sites only. In contrast, pinwheel-shaped clusters comprised of ten molecules are observed on various inequivalent sites of the reconstructed Au surface. For coverages larger than 0.2ML one-dimensional molecular double chains prevail. The structure of the decamers and the chains is determined by intermolecular electrostatic forces, whereas their position and orientation are affected by the surface reconstruction. At saturation coverage a periodic two-dimensional close-packed arrangement of molecules that is independent of the surface reconstruction occurs simultaneously with one-dimensional structures along the uniaxial reconstruction domains. The structural features are discussed in terms of competing effects of intermolecular forces and the interaction with the substrate.

Driving Forces for DNA Adsorption to Silica in Perchlorate Solutions

The adsorption of both plasmid and chromosomal duplex DNA to silica is investigated with the aim of determining the dominant forces involved in the binding reaction. Changes in the initial slopes and plateau values of adsorption isotherms for DNA on microcrystalline silica particles are used to establish the sensitivity of the binding reaction to ionic strength, temperature and pH, and DNA size and conformation. Binding is driven by an increase in entropy, with little or no enthalpic contribution. Adsorption isotherm results indicate that three effects, namely: (i) shielded intermolecular electrostatic forces, (ii) dehydration of the DNA and silica surfaces, and (iii) intermolecular hydrogen bond formation in the DNA–silica contact layer, make the dominant contributions to the overall driving force for adsorption.

The effect of undulations on the electrostatic potential in a polyelectrolyte system

I consider the effect of macromolecular undulation on the electrostatic potential around a rod-like molecule. This effort is set to demonstrate the use of a particular perturbation technique through application to a geometrical system of general colloidal interest. The Poisson—Boltzmann equation together with a constant charge boundary condition on the well defined surface of an undulating cylinder is reformulated in integral equation form by use of Green’s theorem. A perturbation solution appropriate to the deformed boundary can be extracted when the Green function is approximated by that relevant to a reference, undeformed cylinder. Numerical results demonstrate that undulation causes significant deviations (increases) in electrochemical properties from expected behaviour, assuming rigid cylindrical symmetry. By considering the total free energy of the system it is found that electrostatics tend to diminish the extent of the undulations. The predicted deviations are briefly discussed in light of measured intermolecular electrostatic forces acting in a condensed phase of close-packed DNA. The perturbation technique has potential applications to mathematically similar problems occurring in hydrodynamics.