Strength Of Electrostatic Interactions Research Articles

Molecular Dynamics Simulations to Explore the Structure and Rheological Properties of Normal and Hyperconcentrated Airway Mucus.

We develop the first molecular dynamics model of airway mucus based on the detailed physical properties and chemical structure of the predominant gel-forming mucin MUC5B. Our airway mucus model leverages the LAMMPS open-source code [https://lammps.sandia.gov], based on the statistical physics of polymers, from single molecules to networks. On top of the LAMMPS platform, the chemical structure of MUC5B is used to superimpose proximity-based, non-covalent, transient interactions within and between the specific domains of MUC5B polymers. We explore feasible ranges of hydrophobic and electrostatic interaction strengths between MUC5B domains with 9 nanometer spatial and 1 nanosecond temporal resolution. Our goal here is to propose and test a mechanistic hypothesis for a striking clinical observation with respect to airway mucus: a 10-fold increase in non-swellable, dense structures called flakes during progression of cystic fibrosis disease. Among the myriad possible effects that might promote self-organization of MUC5B networks into flake structures, we hypothesize and confirm that the clinically confirmed increase in mucin concentration, from 1.5 to 5 mg/mL, alone is sufficient to drive the structure changes observed with scanning electron microscopy images from experimental samples. We post-process the LAMMPS simulated datasets at 1.5 and 5 mg/mL, both to image the structure transition and compare with scanning electron micrographs and to show that the 3.33-fold increase in concentration induces closer proximity of interacting electrostatic and hydrophobic domains, thereby amplifying the proximity-based strength of the interactions.

Spatial segregation of mixed-sized counterions in dendritic polyelectrolytes

Langevin dynamics simulations are utilized to study the structure of a dendritic polyelectrolyte embedded in two component mixtures comprised of conventional (small) and bulky counterions. We vary two parameters that trigger conformational properties of the dendrimer: the reduced Bjerrum length, lambda _B^*, which controls the strength of electrostatic interactions and the number fraction of the bulky counterions, f_b, which impacts on their steric repulsion. We find that the interplay between the electrostatic and the counterion excluded volume interactions affects the swelling behavior of the molecule. As compared to its neutral counterpart, for weak electrostatic couplings the charged dendrimer exists in swollen conformations whose size remains unaffected by f_b. For intermediate couplings, the absorption of counterions into the pervaded volume of the dendrimer starts to influence its conformation. Here, the swelling factor exhibits a maximum which can be shifted by increasing f_b. For strong electrostatic couplings the dendrimer deswells correspondingly to f_b. In this regime a spatial separation of the counterions into core–shell microstructures is observed. The core of the dendrimer cage is preferentially occupied by the conventional ions, whereas its periphery contains the bulky counterions.

Temporal Changes in Interparticle Interactions Drive the Formation of Transiently Stable Nanoparticle Precipitates.

We demonstrate the power of fine-tuned interparticle interactions, emanating from appropriately functionalized surfaces, in creating self-assembled structures that show a transient switching between completely precipitated and redispersed stages of nanoparticles (NPs). The pH-dependent temporal changes in the strength of electrostatic interactions are explored to unveil a transient self-assembly response in plasmonic NPs. The assembly process was triggered by the electrostatic attraction between positively charged gold NPs (AuNPs) and an aggregating agent, ethylenediaminetetraacetic acid (EDTA). The autonomous changes in the pH and ionic strength of the solution, under the influence of atmospheric CO2, weaken the aggregating ability of EDTA and initiate the complete disassembly of [+] AuNP-EDTA precipitates. The use of a nondestructive mode of autonomous disassembly helped in achieving some of the desirable feats in the field of transient self-assembly such as easy removal of waste, formation of a transiently stable precipitate state, and negligible dampness in redispersion. The chemical strategy adopted in the present work, to introduce transientness, can act as a generic tool in creating the next generation of complex matter.

The removal selectivity of heavy metal cations in micellar-enhanced ultrafiltration: A study based on critical micelle concentration

Micellar enhanced ultrafiltration (MEUF) is a process to remove heavy metal by complexing it with the counterionic surfactant micelle, and the complex formed was subsequently removed using ultrafiltration. The surfactant is usually added in excess, which renders high rejection (low selectivity) for all metal cations in the mixture. This work emphasises MEUF conducted at surfactant dosage around the critical micelle concentration (CMC). Few studies revealed that the presence of counterions could lower the CMC of a charged surfactant. The magnitude of reduction depends on the strength of electrostatic interaction between the counterions and surfactant. Such observation suggests that the MEUF process's selectivity could be altered based on the dosage of surfactant and the types of cations presented in the solution. This work started with determining CMC of sodium dodecyl sulfate (SDS) in the presence of single or binary metal cations at varying pH (pH 3, pH 10 and without pH adjustment). The lowest CMC (4.00 mM) was obtained in the solution containing 100 ppm copper (II) at pH 3, about 42% lower compared to CMC of SDS obtained in pure water without pH adjustment (7.00 mM). Then, the filtration was performed using 7 mM of SDS, under varying pH (pH 3, pH 10 and no pH adjustment), initial metal concentration of 100 mg/L, pressure of 2.5 bar using polyethersulfone (PES) membrane of 10 k Da molecular weight cut-off (MWCO). Overall, the highest metal rejection was obtained at pH 10, with 66% of copper and 26% of chromium (III) was rejected. The highest selectivity of copper (II) over chromium (III) was observed at neutral pH. Such findings could be due to the higher diffusivity of copper cations over chromium cations in aqueous since copper cations can reach the micelle surface and form metal-micelle complex faster than chromium cations.

Dynamic allostery controls the peptide sensitivity of the Ly49C natural killer receptor

Using a variety of activating and inhibitory receptors, natural killer (NK) cells protect against disease by eliminating cells that have downregulated class I major histocompatibility complex (MHC) proteins, such as in response to cell transformation or viral infection. The inhibitory murine NK receptor Ly49C specifically recognizes the class I MHC protein H-2Kb. Unusual among NK receptors, Ly49C exhibits a peptide-dependent sensitivity to H-2Kb recognition, which has not been explained despite detailed structural studies. To gain further insight into Ly49C peptide sensitivity, we examined Ly49C recognition biochemically and through the lens of dynamic allostery. We found that the peptide sensitivity of Ly49C arises through small differences in H-2Kb-binding affinity. Although molecular dynamics simulations supported a role for peptide-dependent protein dynamics in producing these differences in binding affinity, calorimetric measurements indicated an enthalpically as opposed to entropically driven process. A quantitative linkage analysis showed that this emerges from peptide-dependent dynamic tuning of electrostatic interactions across the Ly49C–H-2Kb interface. We propose a model whereby different peptides alter the flexibility of H-2Kb, which in turn changes the strength of electrostatic interactions across the protein–protein interface. Our results provide a quantitative assessment of how peptides alter Ly49C-binding affinity, suggest the underlying mechanism, and demonstrate peptide-driven allostery at work in class I MHC proteins. Lastly, our model provides a solution for how dynamic allostery could impact binding of some, but not all, class I MHC partners depending on the structural and chemical composition of the interfaces.

Modulating Microphase Separation of Lamellae-Forming Diblock Copolymers via Ionic Junctions.

We present a molecular dynamics simulation study investigating the phase behavior of lamellae-forming diblock copolymers with a single ionic junction on the backbone. Our results show qualitative agreement with experimental findings regarding enhanced microphase separation with the introduction of an ionic junction at the conjunction point, while further revealing nonmonotonic changes in domain spacing and order-disorder transition as a function of the electrostatic interaction strength. This highlights the dominant roles of entropic and binding effects of counterions under weak and strong ionic correlations, respectively. The location of the ionic junction is found to effectively modulate the charge distribution and chain conformation in the ordered domains; its presence in the middle of a block promotes folding of the block, leading to a smaller domain size. These findings demonstrate the interplay of ionic coupling with steric hindrance and chain end effects, which enhances our understanding of the delicate control over the microphase domain features.

Microencapsulation of hemp seed oil by pea protein isolate−sugar beet pectin complex coacervation: Influence of coacervation pH and wall/core ratio

The objective of this study was to investigate the impact of coacervation formation pH (3.5 and 2.5) and wall/core ratio (1:1, 2:1 and 4:1) on physicochemical properties of spray-dried hemp seed oil (HSO) microcapsules by means of pea protein isolate (PPI)–sugar beet pectin (SBP) complex coacervates. In general, both phase behaviors and viscoelastic properties of PPI−SBP coacervates were affected by the wall/core ratio. The coacervates structures were highly dependent on the coacervation formation pH, which greatly impacted the spray drying efficiency (PY), encapsulation efficiency (EE), oil distributions in microcapsules and oxidative stability of HSO. At the pHopt (3.5), PPI−SBP complex coacervates possess denser structure owing to the greater electrostatic interaction, whereas softer structure was formed at pH 2.5 as evidenced by rheological properties and FTIR. Because of the homogeneous large pore size and smooth inner surface of liquid coacervates formed at pH 2.5, spray-dried microcapsules had a remarkable higher EE than that formed at pH 3.5. However, the oxidative stability of HSO in PPI−SBP microcapsules prepared at pH 2.5 was significantly shortened due to the weaker electrostatic interaction strength between PPI−SBP, resulting in the partially broken PPI−SBP network caused by the spray drying process. As such, some hollow particle and incomplete particle shape was observed in SEM and CLSM image, which promoted the lipid oxidation of encapsulated HSO. In terms of wall/core ratio, the lower wall/core ratio had lower EE, higher percentage of surface oil on microcapsules, and lower oxidative stability of HSO.

Flow driven transitions of polyelectrolytes

The nonequilibrium properties of uniformly charged linear polymers in the presence of explicit counterions under shear flow are studied by coarse-grained mesoscale hydrodynamics simulations. The conformational properties of the polyelectrolyte (PE) are quantified by the gyration tensor, the distribution of the end-to-end distance, and alignment with the flow, which display rather universal behavior for small and moderate electrostatic interaction strengths in the regime of condensed counterions. In the limit of strong counterion condensation, shear flow leads to a globule-coil transition and polymer stretching, associated with an increase of the effective PE charge. The polyelectrolytes exhibit a pronounced tumbling motion with cyclic stretched and collapsed conformations. The average tumbling-time period decreases with increasing shear rate by a power-law with the exponent −2/3 for PEs in the coiled state. The tumbling time exhibits a plateaulike regime over nearly a decade of shear rates for PEs in the globular state. In addition, we identify various characteristic PE structures under flow in the globule and coil limits determined by the condensed counterions.

Integral Representation of Electrostatic Interactions inside a Lipid Membrane.

Interactions between charges and dipoles inside a lipid membrane are partially screened. The screening arises both from the polarization of water and from the structure of the electric double layer formed by the salt ions outside the membrane. Assuming that the membrane can be represented as a dielectric slab of low dielectric constant sandwiched by an aqueous solution containing mobile ions, a theoretical model is developed to quantify the strength of electrostatic interactions inside a lipid membrane that is valid in the linear limit of Poisson-Boltzmann theory. We determine the electrostatic potential produced by a single point charge that resides inside the slab and from that calculate charge-charge and dipole-dipole interactions as a function of separation. Our approach yields integral representations for these interactions that can easily be evaluated numerically for any choice of parameters and be further simplified in limiting cases.

Charged Dendrimers with Finite-Size Counterions.

We report on the structure of dendritic polyelectrolytes accompanied by counterions in a good, salt-free, implicit solvent using Langevin dynamics simulations and a Flory-type approach. Our focus is on the modification of charged dendrimer conformations via the strength of electrostatic interactions and the counterion excluded volume. We study the effects caused by charges by varying the reduced Bjerrum length, λB*, between the extremes of weak and strong electrostatic interactions. The counterion excluded volume was controlled by the size of ions. We investigate counterions ranging from conventional ones, with the size comparable to the monomer size, to bulky ions. Our results indicate that, as compared to neutral dendrimers, dendritic polyelectrolytes exist in swollen conformations, and the degree of swelling changes non-monotonically with increasing λB*. For weak electrostatic couplings, counterion density within dendrimers is minor and their radius of gyration subtly exceeds the size of neutral dendrimers. For intermediate electrostatic couplings, Coulomb attraction between opposite charges promotes absorption of ions into dendrimers' pervaded volume and counterion condensation on charged monomers. As a result, counterion density inside dendrimers abruptly increases and the ionic size starts to play a crucial role. In this regime, we observe that swelling of dendrimers reaches its maximum and is more pronounced for bulky counterions. For strong electrostatic couplings, complete condensation of conventional counterions proceeds, whereas for bulky ions condensation remains partial. In this regime, dendrimers deswell. In particular, in the presence of conventional ions, dendrimers collapse into globules, while, for bulky counterions, deswelling is suppressed.

Tunable exciton binding energy in 2D hybrid layered perovskites through donor-acceptor interactions within the organic layer.

The strength of electrostatic interactions within semiconductors strongly affects their performance in optoelectronic devices. An important target is the tuning of a material's exciton binding energy-the energy binding an electron-hole pair through the electrostatic Coulomb force-independent of its electronic band gap. Here, we report on the doping of a family of two-dimensional hybrid perovskites, in which inorganic lead halide sheets alternate with naphthalene-based organic layers, with tetrachloro-1,2-benzoquinone (TCBQ). For four out of seven n = 1 perovskites, the incorporation of the electron-accepting TCBQ dopant into the organic sublattice containing the electron-donating naphthalene species enabled the tuning of the materials' 1s exciton binding energy. The naphthalene-TCBQ electron donor-acceptor interactions increased the electrostatic screening of the exciton, in turn lowering its binding energy relative to the undoped perovskite-by almost 50% in one system. Structural and optical characterization showed that the inorganic lattice is not significantly perturbed even though the layer-to-layer spacing increases upon molecular dopant incorporation.

Long effective tea tree oil/mesoporous silica sustained release system decorated by polyethyleneimine with high antibacterial performance

Mesoporous silica MCM-41 was prepared and decorated by polyethyleneimine (PEI) with different molecule weight before loading tea tree oil (TTO) to construct an environment-friendly antimicrobial sustained release system. The characterization results proved that the PEI was successfully introduced to pores of the mesoporous silica without destruction of the structure integrity. The strength of electrostatic interaction between PEI and MCM-41 was measured by the value of zeta potential which indicated that PEI with low molecule weigh was easier to be adsorbed by MCM-41 pores due to its low viscosity. The interaction of MCM-41 with TTO was strengthened after PEI modification. As a result, TTO/PEI600-MCM-41 performed the lowest cumulative release rate of 46.88% in 12 h due to the strongest interaction with TTO among all the samples. Kinetics study indicated that PEI constructed a new barrier for drug diffusion and volatilization through winding and swelling. The kinetic model for TTO/MCM-41 sustained release changed from first-order to Weibull model after PEI modification. What’s more, the service life of TTO in antimicrobial was also obviously prolonged after supported on mesoporous silica modified by PEI.

Correlations between temperature-dependent rheology and electrostatic interactions in reverse wormlike micelles induced by inorganic salts.

Previous studies have shown that the plateau modulus Gp of the wormlike micelles formed in water driven by hydrophobic interactions is a constant upon heating, similar to polymer solutions, and Gp of the reverse worms formed in oils driven by hydrogen bonding decreases with increasing temperature. In this work, we investigated the reverse worms induced by three chloride salts that bind lecithin through different strengths of electrostatic interactions, in the order of LaCl3 > CaCl2 > LiCl. We correlated the interaction strengths with the temperature-dependent rheological properties and found that upon heating, Gp for all the reverse worms driven by electrostatic interactions decays slower than that driven by the weak temperature-sensitive hydrogen bonding. Furthermore, the decay rates of Gp follow an order in the inverse relation to the interaction strength, LaCl3≤ CaCl2 < LiCl, indicating that the dependence of Gp on temperature can reflect the strength of the driving forces for micellization. We utilized Fourier transform infrared spectroscopy (FTIR) to confirm the weakening of the interaction and the small angle X-ray scattering (SAXS) technique to reveal the decrease in the lengths of the reverse worms as temperature increases, both of which echo the changes in the rheological properties.

Polycation-globular protein complex: Ionic strength and chain length effects on the structure and properties

AbstractThe effect of high salt concentration and pH on the binding of globular protein to polycation at different molar masses has been investigated by dynamic light scattering (DLS), turbidimetry and electrostatic modeling for the protein. In dilute concentration regime, DLS and pH titrations showed three remarkable pH transitions: pHc, the pH where soluble complexes of bovine serum albumin (BSA) and linear synthetic Polyethylenemine (PEI) are formed, pHc’ presents the end of primary soluble complex and pHφ presents the first appearance of microcoacervate droplets. All pH transitions increase with increasing NaCl concentration. The Adaptive Poisson-Boltzmann Solver (APBS) identify with precision the functional sites at the surface of BSA and shows that the strength of electrostatic interactions depends hugely on the variation of pH and ionic strength. At a very high concentration of salt, no remarkable effect on a mixture formed of a long chain of polycation and globular protein.

Rheological and Microstructural Characteristics of Canola Protein Isolate-Chitosan Complex Coacervates.

Wastage of byproducts such as canola meal is a pressing environmental concern, and canola protein isolate (CPI)-chitosan (Ch) coacervates have a good potential to utilize and convert the wastes into a high value added product. Yet so far, there is very limited rheological and microstructural information to assist in proper utilization of CPI -Ch complex coacervates. The rheological and microstructural properties of the complex coacervates formed from CPI and chitosan Ch at various CPI-to-Ch mixing ratios (1:1, 16:1, 20:1, and 30:1) and pH values (5.0, 6.0, and 7.0) were therefore investigated. These CPI-Ch complex coacervate phases were found to exhibit elastic behavior as evidenced by significantly higher elastic modulus (G') compared to viscous modulus (G″) in all the tested ratios and pH ranges. They also exhibited shear-thinning behavior during viscous flow. The complex coacervates formed at the optimum CPI-to-Ch ratio of 16:1 and pH of 6.0 demonstrated the highest G', G″, and shear viscosity, which correlated well with the high strength of electrostatic interaction and thick-walled, sponge-like, less-porous microstructure at this condition. The higher shear viscosity of the coacervate at pH 6.0 was most likely induced by stronger attractive electrostatic interactions between CPI and Ch molecules, due to the formation of a rather densely packed complex coacervate structure. Hence, it can be concluded that the microstructural observations of denser structure correlated well with the rheological findings of stronger intermolecular bonds at the optimum CPI-to-Ch ratio of 16:1 and pH of 6.0. The complex coacervate phase formed at a CPI-to-Ch ratio of 16:1 and pH of 6.0 also showed glassy consistency at low temperatures and rubbery consistency above its glass-transition temperature. This study identified the potential for the newly developed CPI-Ch complex coacervate to be used as an encapsulating material due to its favorable strength. This would drastically reduce the wastage of byproducts, provide a solution to tackle the pressing global issue of wastage of byproducts, and bring about a more environmentally friendly paradigm.

Designing Electrostatic Interactions via Polyelectrolyte Monomer Sequence.

Charged polymers are ubiquitous in biological systems because electrostatic interactions can drive complicated structure formation and respond to environmental parameters such as ionic strength and pH. In these systems, function emerges from sophisticated molecular design; for example, intrinsically disordered proteins leverage specific sequences of monomeric charges to control the formation and function of intracellular compartments known as membraneless organelles. The role of a charged monomer sequence in dictating the strength of electrostatic interactions remains poorly understood despite extensive evidence that sequence is a powerful tool biology uses to tune soft materials. In this article, we use a combination of theory, experiment, and simulation to establish the physical principles governing sequence-driven control of electrostatic interactions. We predict how arbitrary sequences of charge give rise to drastic changes in electrostatic interactions and correspondingly phase behavior. We generalize a transfer matrix formalism that describes a phase separation phenomenon known as “complex coacervation” and provide a theoretical framework to predict the phase behavior of charge sequences. This work thus provides insights into both how charge sequence is used in biology and how it could be used to engineer properties of synthetic polymer systems.

DNA Sequence and Histone Core Composition Control the Unwrapping of DNA from Nucleosome Core Particles

As controlling access to DNA is a major component of transcriptional regulation, understanding the effects of interactions between core proteins and the DNA is crucial to gaining an overall understanding of transcription. We report on the use of small angle X-ray scattering with contrast variation (CV-SAXS) to analyze DNA unwrapping from nucleosome core particles. Interactions between the histone core and the DNA are destabilized using increasing concentrations of neutral salt, lowering the strength of electrostatic interactions that bind the components together. As the complex is destabilized many unwrapping states are accessible and the DNA explores possible unwrapping pathways. The CV-SAXS data reports on full range of structures present in the sample. They are fit using sequence-dependent models of unwrapped structures with thermal motions, selected by an ensemble optimization method [1]. Different combinations of DNA sequence and histone composition also alter their interactions, and the unwrapped structures reflect biophysical differences in the nucleosome compositions. 1. Mauney A., Tokuda J.M., Gloss L.M., Gonzalez O., Pollack L. Local DNA sequence controls the cooperativity and asymmetry of DNA unwrapping from nucleosome core particles. Biophys J. 2018 Sep 4;115(5):773-781

Investigation of pH and concentration influence on layer-by-layer self-assembly for nickel(II)phthalocyanine-tetrasulfonic acid tetrasodium salt coatings

Porphyrins and phthalocyanines are widely studied molecules for various functional applications. Researchers have investigated these photoactive compounds for electrochemical, sensor, semiconductor and photodynamic therapy purposes. Layer-by-layer (LbL) self-assembly is preferred for its simple, environmentally-friendly and water-based features compared to other coating techniques in the literature. Coating thickness can be controlled on the order of nanometers by LbL mechanism. Multilayer thin film formation of diverse phthalocyanine-based molecules is examined in terms of molecular orientation and temperature dependency by the LbL method. However, as well as concentration and temperature, the pH of the coating medium is another challenging parameter in the LbL approach. Film thickness and layer distribution are influenced by pH value, changing ionic density and hence the strength of electrostatic interactions during LbL assembly. In this study, layer-by-layer deposition of branched poly(ethyleneimine)/nickel(II)phthalocyanine-tetrasulfonic acid tetrasodium salt (NiPcTS) coating pair is studied. Impact of pH and concentration of NiPcTS on thin film properties are tested for four different pH conditions. Corresponding analysis is made by UV-vis spectroscopy, surface profiler and quartz-crystal microbalance. LbL deposition of NiPcTS is homogeneously controlled and 98 nm thick films are obtained in the presence of acidic media.

Effects of reaction pH on self-crosslinked chitosan-carrageenan polyelectrolyte complex gels and sponges

Macromolecular biomaterials often require covalent crosslinking to achieve adequate stability for their given application. However, the use of auxiliary chemicals may be associated with long-term toxicity in the body. Oppositely-charged polyelectrolytes (PEs) have the advantage that they can self-crosslink electrostatically and those derived from marine organisms such as chitosan (CS) and carrageenan (CRG) are inexpensive non-toxic alternatives to glycosaminoglycans present in the extracellular matrix of human tissues. The aim of this study was to explore the properties of crosslinker-free PEC gels and freeze-dried PEC sponges based on CS and CRG precursors. We offer new insights into the optimisation of conditions and mechanisms involved in the process and offer a systematic study of property changes across a full range of pH values. Zeta-potential measurements indicated that the PECs produced at pH 2–6 had a high strength of electrostatic interaction with the highest being at pH 4–5. This resulted in strong intra-crosslinking in the PEC gels which led to the formation of higher yield, viscosity, fibre content and lower moisture content. The weaker interaction between CS and CRG at pH 7–12 resulted in higher levels of CS incorporated into the complex and the formation of more inter-crosslinking through entanglements and secondary interactions between PEC units. This resulted in the production of stable PEC sponge materials compared with the PEC materials produced at pH 6 and below. From the range of samples tested, the PECs produced at pH 7.4 appeared to show the optimum combination of yield, stability and homogeneity.

Morphological Response of a Spherical Polyelectrolyte Brush to Solvent Quality and Electrostatic Interaction Strength

We study surface morphologies of a spherical polyelectrolyte brush in the presence of trivalent counterions using molecular dynamics simulations. Solvent quality and electrostatic interaction strength are varied to generate a series of structures. Through a careful analysis on snapshots of morphologies, shape factor of tethered chains, and monomer–monomer pair correlation function we find a nonmonotonic dependence of surface morphology on electrostatic strength, which represents clearly the electrostatic correlation effect mediated by the multivalent counterions. Due to the very importance of counterions, we further study the correlation effect by classifying counterions into four states, calculating the monomer–counterion pair correlation function and diffusion coefficient of counterions. Our simulation results clearly demonstrate that ordered patterns can be induced by the electrostatic correlation effect in the presence of trivalent counterions, which is absent in the system with monovalent ions. Also,...