Strength Of Electrostatic Interactions Research Articles

Modeling the formation of ordered nano-assemblies comprised by dendrimers and linear polyelectrolytes: The role of Coulombic interactions

Models of mixtures of peripherally charged dendrimers with oppositely charged linear polyelectrolytes in the presence of explicit solvent are studied by means of molecular dynamics simulations. Under the influence of varying strength of electrostatic interactions, these systems appear to form dynamically arrested film-like interconnected structures in the polymer-rich phase. Acting like a pseudo-thermodynamic inverse temperature, the increase of the strength of the Coulombic interactions drive the polymeric constituents of the mixture to a gradual dynamic freezing-in. The timescale of the average density fluctuations of the formed complexes initially increases in the weak electrostatic regime reaching a finite limit as the strength of electrostatic interactions grow. Although the models are overall electrically neutral, during this process the dendrimer/linear complexes develop a polar character with an excess charge mainly close to the periphery of the dendrimers. The morphological characteristics of the resulted pattern are found to depend on the size of the polymer chains on account of the distinct conformational features assumed by the complexed linear polyelectrolytes of different length. In addition, the length of the polymer chain appears to affect the dynamics of the counterions, thus affecting the ionic transport properties of the system. It appears, therefore, that the strength of electrostatic interactions together with the length of the linear polyelectrolytes are parameters to which these systems are particularly responsive, offering thus the possibility for a better control of the resulted structure and the electric properties of these soft-colloidal systems.

Is water the universal solvent for life?

There are strong reasons to believe that the laws, principles and constraints of physics and chemistry are universal. It is much less clear how this universality translates into our understanding of the origins of life. Conventionally, discussions of this topic focus on chemistry that must be sufficiently rich to seed life. Although this is clearly a prerequisite for the emergence of living systems, I propose to focus instead on self-organization of matter into functional structures capable of reproduction, evolution and responding to environmental changes. In biology, most essential functions are largely mediated by noncovalent interactions (interactions that do not involve making or breaking chemical bonds). Forming chemical bonds is only a small part of what living systems do. There are specific implications of this point of view for universality. I will concentrate on one of these implications. Strength of non-covalent interactions must be properly tuned. If they were too weak, the system would exhibit undesired, uncontrolled response to natural fluctuations of physical and chemical parameters. If they were too strong kinetics of biological processes would be slow and energetics costly. This balance, however, is not a natural property of complex chemical systems. Instead, it has to be achieved with the aid of an appropriate solvent for life. In particular, potential solvents for life must be characterized by a high dielectric constant to ensure solubility of polar species and sufficient flexibility of biological structures stabilized by electrostatic interactions. Among these solvents, water exhibits a remarkable trait that it also promotes solvophobic (hydrophobic) interactions between non-polar species, typically manifested by a tendency of these species to aggregate and minimize their contacts with the aqueous solvent. Hydrophobic interactions are responsible, at least in part, for many self-organization phenomena in biological systems, such as the formation of cellular boundary structures or protein folding. Strengths of electrostatic and hydrophobic interactions are similar and can be balanced over a wide range of temperatures, which considerably increases the repertoire of interactions that can be used to modulate biological functions. Some properties of water, e.g. its chemical activity against polymerization reactions, are considered as unfavorable to life. In actuality, this might be a favorable trait because life requires a balance between constructive and destructive processes. For example, molecules synthesized in response to specific conditions must be degraded once these conditions change. Otherwise regulation of biological processes would be virtually impossible. Water might not be the only liquid with favorable properties for supporting life. It has been proposed that formamide, which might be present elsewhere in the universe in sufficient quantities to warrant interest, could be a potential alternative to water for the origin of life. However, this will remain highly hypothetical until it is demonstrated in further studies on its physical, chemical and biological properties it is capable of mediating self-organization of matter and providing proper balance between different types of non-covalent interactions.

Charge renormalization of bilayer elastic properties

By combining molecular dynamics simulations and analytical arguments, we investigate the elastic properties of charged lipid bilayers. We show that electrostatic interactions between the head groups can lead to solidification of the lipid bilayer that would otherwise be in a liquid state if the charges were absent. All elastic parameters of the bilayer such as the bending rigidity κ and the two-dimensional bulk modulus λ and Young's modulus Y are found to depend on the values of the charges assigned to the lipid head groups. To extract κ and λ, we fit the molecular dynamics data to a standard elastic model for lipid bilayers. Moreover, we analytically obtain the dependence of the Young modulus Y on the relative strengths of electrostatic and van der Waals interactions in the zero temperature limit.

Effects of Embedded Carbon Nanotube on Properties of Biomembrane

We investigated the interaction between embedded nanotube and biomembrane using molecular dynamics (MD) simulations. The effects of embedded nanotube on biomembrane were characterized by investigating the influence on the conformational fluctuation of individual lipid molecules, the organization of membrane molecules, the diffusion behavior of lipid molecules, and the diffusion behavior of penetrants inside biomembrane. The steric interaction with the nanotube leads to an entropy reduction of interfacial membrane molecules, while the long-range electrostatic interaction with the N-DWCNT enhances the conformational fluctuation of lipid molecules. The curvature of embedded nanotube could also influence the flexibility of lipid molecules. When the interaction between nanotube and the membrane molecules is weak, the packing density of the membrane is almost unaffected. On the contrary, when the attraction between nanotube and the membrane molecules significantly increases, the attraction among the membrane molecules decreases effectively, which leads to a relaxation of the organization of membrane. With the increase of the strength of electrostatic interaction between nanotube and small polar molecules, interaction-modified friction increases, which leads to the decrease of the diffusion constant of penetrants inside the biomembrane.

DNA conformation in nanochannels: Monte Carlo simulation studies using a primitive DNA model

We have performed canonical ensemble Monte Carlo simulations of a primitive DNA model to study the conformation of 2.56 ~ 21.8 μm long DNA molecules confined in nanochannels at various ionic concentrations with the comparison of our previous experimental findings. In the model, the DNA molecule is represented as a chain of charged hard spheres connected by fixed bond length and the nanochannels as planar hard walls. System potentials consist of explicit electrostatic potential along with short-ranged hard-sphere and angle potentials. Our primitive model system provides valuable insight into the DNA conformation, which cannot be easily obtained from experiments or theories. First, the visualization and statistical analysis of DNA molecules in various channel dimensions and ionic strengths verified the formation of locally coiled structures such as backfolding or hairpin and their significance even in highly stretched states. Although the folding events mostly occur within the region of ~0.5 μm from both chain ends, significant portion of the events still take place in the middle region. Second, our study also showed that two controlling factors such as channel dimension and ionic strength widely used in stretching DNA molecules have different influence on the local DNA structure. Ionic strength changes local correlation between neighboring monomers by controlling the strength of electrostatic interaction (and thus the persistence length of DNA), which leads to more coiled local conformation. On the other hand, channel dimension controls the overall stretch by applying the geometric constraint to the non-local DNA conformation instead of directly affecting local correlation. Third, the molecular weight dependence of DNA stretch was observed especially in low stretch regime, which is mainly due to the fact that low stretch modes observed in short DNA molecules are not readily accessible to much longer DNA molecules, resulting in the increase in the stretch of longer DNA molecules.

Colorimetric recognition and sensing of nitrite with unmodified gold nanoparticles based on a specific diazo reaction with phenylenediamine

A colorimetric sensor for nitrite ion with high selectivity and sensitivity by unmodified citrate-capped gold nanoparticles (Au NPs) is presented. Recognition of nitrite is developed on the basis of a highly specific diazo reaction between nitrite and phenylenediamine (PDA). PDA caused the Au NPs to aggregate owing to the strong covalent NH-Au bond, with a clear color change of solution from red to blue being visualized. In the presence of phosphoric acid and nitrite, the amines of PDA would readily be converted to diazo bonds, and a red solution was observed after the subsequent addition of Au suspension due to the much less strength of electrostatic interaction between the positive diazo groups and the negative citrate-capped Au NPs. With this colorimetric "light-up" method, <1 ppm of nitrite can be easily detected within 5 min at room temperature without instrumentation. Since the diazo reaction and the colorimetric response are separate, this approach features the use of pristine Au NPs in an assay where acidic environment is a necessity, making it a more convenient and cost-effective method for the sensing of nitrite when compared with those utilizing chemically modified Au NPs.

Monte Carlo Simulation to Investigate the Cascade Transition of a Permuted Polyelectrolyte Chain

Using Monte Carlo simulations, the cascade transition of a single polyelectrolyte chain having permuted charge distribution is investigated. The polyelectrolyte chain is simulated at various solvent regimes. The effect of the electrostatic interaction strength is studied. The structures are analyzed with respect to the scaling laws. The effect of permuted charge distribution is investigated in comparison to the other charge distribution in the literature.

How a single residue in individual β-thymosin/WH2 domains controls their functions in actin assembly

β-Thymosin (βT) and WH2 domains are widespread, intrinsically disordered actin-binding peptides that display significant sequence variability and different regulations of actin self-assembly in motile and morphogenetic processes. Here, we reveal the structural mechanisms by which, in their 1:1 stoichiometric complexes with actin, they either inhibit assembly by sequestering actin monomers like Thymosin-β4, or enhance motility by directing polarized filament assembly like Ciboulot βT. We combined mutational, functional or structural analysis by X-ray crystallography, SAXS (small angle X-ray scattering) and NMR on Thymosin-β4, Ciboulot, TetraThymosinβ and the long WH2 domain of WASP-interacting protein. The latter sequesters G-actin with the same molecular mechanisms as Thymosin-β4. Functionally different βT/WH2 domains differ by distinct dynamics of their C-terminal half interactions with G-actin pointed face. These C-terminal interaction dynamics are controlled by the strength of electrostatic interactions with G-actin. At physiological ionic strength, a single salt bridge with actin located next to their central LKKT/V motif induces G-actin sequestration in both isolated long βT and WH2 domains. The results open perspectives for elucidating the functions of βT/WH2 domains in other modular proteins.

Effect of ageing on the microfiltration performance of ceramic membranes

The present work considers the influence of time or ageing on the electrokinetic characteristics of ceramic microfiltration and their subsequent impact on the microfiltration performances of whey suspensions. The filtration performance is assessed both in terms of permeate flux and total organic carbon (TOC). As the ageing period increases, the zeta potential of the membrane increases and plateaus towards the end of the ageing period. Higher permeate fluxes are found over the ageing period. The effect of ageing seems to have the greatest influence on the zeta potential and fluxes at the alkaline pH compared to the acidic pH and at the pH corresponding to the isoelectric point. Highest TOC removals were found at alkaline conditions. The main consequences of these results concern the interactions between membrane and whey particles. Despite the fact that the surface charge only depends on the pH, the various surface states of alumina may induce different strengths of electrostatic interactions or hydrophobic interactions between membrane and particles.

Hydrogels in Poor Solvents: A Molecular Dynamics Study

AbstractThe equilibrium swelling behavior of a crosslinked polyelectrolyte gel under poor‐solvent conditions is examined. To this end, MD simulations of a coarse‐grained network model with a diamond‐like topology are employed where the counterions are explicitly taken into account. A large range of different parameter combinations in the fraction f of charged monomers of the network, the strength of electrostatic interactions measured via the Bjerrum length $\ell _{{\rm B}} $, and various strand lengths are studied. The results agree with data on single PE chains, in particular the theoretically predicted pearl necklace conformations can be identified as well as a sausage‐like regime in the strong‐coupling region.magnified image

Simulation of a Symmetric Binary Mixture of Charged Dendrimers Under Varying Electrostatic Interactions: Static and Dynamic Aspects

A symmetric binary mixture of terminally charged trifunctional-core/difunctional-branched dendrimers of the third and the fourth generation isexaminedinexplicitsolventsolution bymeans ofmoleculardynamics simulations.Thestaticand dynamicresponseof the components is monitored under a varying strength of electrostatic interactions. The response of the dendrimer constituents shares common features as the intensity of Coulombic interactions increases, but characteristic differences are noted in the individual structural arrangement and their dynamic response. An effective-charge modulation develops across the dendritic structure and at the immediate vicinity of each molecule. The asymmetry in size and in the overall charge between the two components affects the packing properties of the molecules resulting in different morphologies compared to the respective single component systems. In addition, individual translational and rotational motion of the dendrimers exhibits distinct behavior in different Coulombic regimes. The motional contrast between the two components differentiates their relative dynamic response as the electrostatic interactions grow stronger. The increase of Coulombic interactions drives the components of the mixture to a dynamically arrested state, but the eventual vitrification takes place at different levels of electrostatic interactions depending on dendrimer generation. The findings of the present study elucidate several aspects of the behavior of these charged soft-colloidal materials and offer new insight toward the control of their long-range order and of their dynamic properties.

Dynamics and Structure of Biopolyelectrolytes Characterized by Dielectric Spectroscopy

AbstractStructural properties of Na‐DNA and Na‐HA aqueous solutions can be quantified using dielectric spectroscopy in the frequency range 100 Hz–100 MHz. Two relaxation modes are typically detected that can be attributed to diffusive motion of polyion counterions. The overall study as a function of polyion length, concentration and added salt concentration demonstrates that the motion of polyion counterions detected at MHz frequencies probes collective properties, whereas the motion at kHz range probes single‐chain properties of polyelectrolytes. Fundamental length scales found to characterize the polyelectrolyte structure differ for the dilute and semidilute regime and also depend on the strength of electrostatic interactions and the flexibility. Characteristic length scales detected in the dielectric spectroscopy measurements compare well with the fundamental length scales predicted by theory and comply with those extracted from small‐angle X‐ray scattering.

Behavior of polyelectrolyte solutions in a wide range of solvent dielectric constant

We exploit the strong temperature dependence of dielectric constant of N-methylformamide to control the strength of electrostatic interactions in polyelectrolyte solutions of sodium polystyrene sulfonate, and investigate the dynamic properties of salt-free solutions over a broad temperature range, from 54 to −58 °C. The traditional fast and slow diffusion processes are observed. The ratio of diffusion coefficients, D s/ D f, increases and the ratio of their amplitudes, A s/ A f, decreases, both by a factor of about two in this temperature range confirming the expected temperature variation with the Bjerrum length.

Synthesis and Phase Structures of Mesogen-Jacketed Liquid Crystalline Polyelectrolytes and Their Ionic Complexes

Two mesogen-jacketed liquid crystalline polyelectrolytes, poly{sodium 2,5-bis[(4-sulfophenyl)aminocarbonyl]styrene} and poly{sodium 2.5-bis[(4-sulfophenyl)oxycarbonyl]styrene}, with rigid cores containing different linkages at the center and sulfonate groups on the two ends in the side chains were designed and successfully synthesized via conventional radical polymerization. X-ray scattering experiments revealed that the two polymers exhibited smectic A phases in bulk. Comb-shaped nonstoichiometric polymer–surfactant complexes were obtained by mixing the anionic sulfonated polyelectrolytes and cationic lipids of different lengths and shapes. The formation of ordered structures of the complexes depended on the length and shape of the lipids. Lamellar phases were observed when the two polyelectrolytes complexed with cetyltrimethylammonium bromide and fan-shaped 3,4,5-tris(dodecyloxy)benzenamine. Owing to the stronger bond strength of electrostatic interactions, the types of mesophases of the complexes based...

Structural selection of ionic-complementary peptides with electrostatic interactions

The structures of the peptides and their assembly are largely modulated by the environment. To discover the physical principles governing the structural modulations of peptides by the environment would be useful for many applications. As the typical examples, the structures of three kinds of ionic-complementary EAK16-family peptides under various environmental conditions are studied with simulations in this work. A model with intermediate resolution is used, in which both the backbone hydrogen bonds and electrostatic interactions are explicitly considered. The thermodynamics of these peptides (including the free energy and heat capacity) are described for various strengths of the electrostatic interactions which reflect the variation of environment. With these results, the phase diagrams of these peptides related to the temperature and the strength of electrostatic interactions are presented and compared. Based on the differences in the phase structures of the peptide, the different aggregation behaviors are explained based on the monomeric structural features of the peptides. Through the analysis on the stability of various secondary structures of these peptides, it is demonstrated that the charge pattern is the basic reason of the different responses of the EAK16-family peptides to the environmental changes. These results provide some examples and insights for the principles of structural selection by environment and may be helpful for further analysis and designs of peptide systems.

Phase Separation in Symmetric Mixtures of Oppositely Charged Rodlike Polyelectrolytes

Phase separation in salt-free symmetric mixtures of oppositely charged rodlike polyelectrolytes is studied using quasi-analytical calculations. Stability analyses for the isotropic-isotropic and the isotropic-nematic phase transitions in the mixtures are carried out and demonstrate that electrostatic interactions favor nematic ordering. Coexistence curves for the symmetric mixtures are also constructed and are used to examine the effects of linear charge density and electrostatic interaction strength on rodlike polyelectrolyte complexation. It is found that the counterions are uniformly distributed in the coexisting phases for low electrostatic interaction strengths dictated by the linear charge density of the polyelectrolytes and Bjerrum's length. However, the counterions also partition along with the rodlike polyelectrolytes with an increase in the electrostatic interaction strength. It is shown that the number density of the counterions is higher in the concentrated (or "coacervate") phase than in the dilute (or supernatant) phase. In contrast to such rodlike mixtures, flexible polyelectrolyte mixtures can undergo only isotropic-isotropic phase separation. A comparison of the coexistence curves for weakly charged rodlike mixtures with those of analogous flexible polyelectrolyte mixtures reveals that the electrostatic driving force for the isotropic-isotropic phase separation is stronger in the flexible mixtures.

Phase field modeling of domain structures and P–E hysteresis in thin ferroelectric layers with deadlayers

The phase field method was used for the simulation of the domain structure evolution and polarization–applied field hysteresis in thin ferroelectric layers with deadlayers. The simulation demonstrated that the hysteresis loop shape depends on the thickness of the deadlayer and the strength of electrostatic interactions. The systems with a thin deadlayer and/or weaker electrostatic interactions produce a rectangular loop with a large remnant polarization value. The systems with a thicker deadlayer and/or stronger electrostatic interactions produce dumbbell-shaped double loops with an approximately zero remnant polarization. It is shown that the transformation from a poled single-domain state to a polydomain state can produce either the 180° domain structure when elastic interactions in the layer are weak or the 90° c/a structure when elastic interactions are strong. It is demonstrated that the type of the domain structure produced by the single-domain to polydomain transformation can affect the possibility of further changes in the domain structure in a varying applied field, and that 180° domain structures produced by this transformation can have a columnar morphology instead of stripe domains.

Multiple time scales and cluster formation mechanisms in charge-heteroaggregation processes

We report simulation and experimental evidence of multiple cluster-formation mechanisms arising in coagulation in a binary mixture composed of oppositely charged colloids with large asymmetry in the relative particle concentration. These mechanisms can be clearly distinguished from one another as they cause the appearance of several maxima in the cluster-size distribution curves at well-separated time scales. We show that this behavior is caused by the fact that clusters of a given size can have different internal compositions. Moreover, the peaks collapse into a single maximum when electrostatic interactions are screened, which is a confirmation that this effect is very sensitive to the strength and range of the interparticle electrostatic interactions. The experimental confirmation of the simulation results represents the first evidence of non bell-shaped cluster-size distribution curves.

Conformations and Dynamics of Polypeptide Chains Revealed By Tryptophan-Cysteine Contact Formation Kinetics

Spectroscopic probes sensitive to intra-chain contact formation events in polypeptides are increasingly used to study the conformational and dynamical properties of different amino acid sequences. Quenching of the triplet state of tryptophan by close contact with cysteine enables the measure of contact formation rates without the need of extrinsic probes, thus being suitable for the study of natural proteins and peptides. We illustrate the use of this method to investigate the unfolded state of small proteins in conditions close to native and the kinetics of weakly structured protein fragments. The coexistence of different conformational states can be revealed from the non-exponential relaxation of the excited triplet, enabling the characterization of both the chain dynamics for each state and the transition kinetics. Moreover, the rate of contact formation measured for the least structured states is compared with those observed for model disordered peptides, allowing to estimate the strength of electrostatic and hydrophobic interactions between residues other than the probes. We test this approach with the widely studied GB1 15-residue C-terminal, which folds into a beta-hairpin structure. The kinetics of elementary conformational steps leading to the folded state is outlined, revealing the presence of misfolded states as proposed in recent computational works.

Origin of translocation barriers for polyelectrolyte chains

For single-file translocations of a charged macromolecule through a narrow pore, the crucial step of arrival of an end at the pore suffers from free energy barriers, arising from changes in intrachain electrostatic interaction, distribution of ionic clouds and solvent molecules, and conformational entropy of the chain. All contributing factors to the barrier in the initial stage of translocation are evaluated by using the self-consistent field theory for the polyelectrolyte and the coupled Poisson-Boltzmann description for ions without radial symmetry. The barrier is found to be essentially entropic due to conformational changes. For moderate and high salt concentrations, the barriers for the polyelectrolyte chain are quantitatively equivalent to that of uncharged self-avoiding walks. Electrostatic effects are shown to increase the free energy barriers, but only slightly. The degree of ionization, electrostatic interaction strength, decreasing salt concentration, and the solvent quality all result in increases in the barrier.