Conformation Of Polyelectrolyte Research Articles

Study of the conformation of polyelectrolyte aggregates using coarse-grained molecular dynamics simulations.

The conformation of polyelectrolyte aggregates as a function of the backbone rigidity is investigated by coarse-grained molecular dynamics simulation. The polyelectrolyte is represented by a bead-spring chain with charged side chains. The simulations start from the uniform distributions of the polyelectrolytes, and the resultant polyelectrolyte conformation after a few microseconds exhibits spherical self-aggregates, clusters, or bending bundle-like aggregates, depending on the backbone rigidity. The interaggregate structures on a large scale are featured by the static structure factor (SSF). The simulated SSFs of the bending bundle-like aggregates are consistent with those of the small angle X-ray scattering (SAXS) measurement so we successfully assign the microscopic structures of polyelectrolytes to the SAXS measurement. The power-law of the SSFs for the bundle conditions is steeper than that of the conventional cylinder model. The present study finds that such discrepancy in the power-law results from the bending of the bundle-like aggregates. In addition, the relaxation behavior includes slow dynamics. The present study proposes that such slow dynamics results from diffusion-limited aggregation and from gliding processes to reduce local metastable folding within the aggregates.

Insight into the electrical properties and chain conformation of spherical polyelectrolyte brushes by dielectric spectroscopy

We report here a dielectric study on three kinds of anionic spherical polyelectrolyte brush (SPBs, consisting of a polystyrene (PS) core and three different poly (acrylic acid) chains grafted onto the core) suspensions over a frequency ranging from 40 Hz to 110 MHz. The relaxation behavior of the SPB suspensions shows significant changes in the brush-layer properties when the mass fraction of SPBs and the pH of the suspensions change. Two definite relaxations related to the interfacial polarization are observed around 100 kHz and 10 MHz. A single-layer spherical-shell model is applied to describe the SPB suspensions wherein the suspended SPB is modeled as a spherical-shell composite particle in which an insulated PS sphere is surrounded by a conducting ion-permeable shell (the polyelectrolyte chain layer). We developed the curve-fitting procedure to analyze the dielectric spectrum in order to obtain the dielectric properties of the components of the SPBs, especially the properties of the polyelectrolyte brush. Based on this method and model, the permittivity and conductivity of the brush layer, ζ potential, etc are calculated. The ordered orientation of the water molecules in the layer leads to an additional electrical dipole moment; increasing pH causes the brush layer to swell. In addition, the repulsive force between the SPB particles are evaluated using the brush-layer thickness, which is obtained by fitting dielectric spectra, combined with relative theoretical formulas. Increasing PH values or SPB concentration would improve the stability of the SPBs dispersion.

Impact of Monovalent Counter-ions on the Conformation of Flexible Polyelectrolytes Having Different Molecular Architectures

ABSTRACTWe explore the impact of monovalent counter-ions on the molecular conformation of highly charged flexible polyelectrolytes for a range of molecular topologies (linear chains, stars, and unknotted and trefoil rings) by molecular dynamics simulations that include an explicit solvent having short range interaction with the polyelectrolyte. In particular, we investigate how the counter-ions near the polyelectrolytes with variable mass influence the average molecular shape. We also characterize the interfacially “bound” counter-ions by calculating the time-averaged number of interfacial counter-ions, as well as the degree to which the polyelectrolytes wrap around the counter-ions by calculating the number of contacts between the counter-ions and the polyelectrolyte.

Quantifying the Ion Atmosphere of Unfolded, Single-Stranded Nucleic Acids

Secondary structure formation in nucleic acids (NAs) is strongly dependent on the local ion atmosphere, which screens the negative charge of the NA and reduces self-repulsion. Consideration of such interactions must include both the folded and unfolded states, as both contribute to the free energy of folding. The interaction of ions with folded, double-helical NAs has been studied with a variety of experimental techniques and, at the level of monovalent ions, is well captured by solutions to the Poisson-Boltzmann equation. However, comparatively little is known, experimentally or theoretically, about ion interactions with the unfolded, random-coil state.Here, we report on the ion atmosphere of unfolded, single-stranded DNA and RNA by measuring the ion excess: the number of cations surrounding the negatively charged macromolecule above the number due to the bulk salt concentration. We used two independent approaches to make these measurements. In one, we took advantage of the existing results for folded NAs and used a single-molecule differential ion counting approach to connect those results to the unfolded state. In the other, we directly measured the ion excess of the unfolded state through equilibrium dialysis read out by atomic emission spectroscopy. The two methods are complementary and give consistent results, which we interpret in light of recent findings on the conformation of flexible polyelectrolytes.

Flexible polyelectrolyte conformation in the presence of cationic and anionic surfactants

In this work we have studied the conformation of flexible polyelectrolyte chains in the presence of cationic and anionic surfactant molecules. We developed a simple theoretical model for the formation of the polyelectrolyte–cationic surfactant complexes and mixed micelles formed by cationic and anionic surfactant molecules, in the framework of the Debye–Hückel–Bjerrum–Manning and Flory theories, with the hydrophobic interaction included explicitly as an effective short-ranged attraction between the surfactant hydrocarbon tails. This simple model allows us to calculate the extension of the polyelectrolyte–cationic surfactant complexes as a function of the anionic surfactant concentration, for different types of cationic and anionic surfactant molecules. A discrete conformational transition from a collapsed state to an elongated coil was found, for all surfactant chain lengths we have considered, in agreement with the experimental observations for the unfolding of ​DNA–cationic surfactant complexes.

Compositional redistribution and dynamic heterogeneity in mixed lipid membrane induced by polyelectrolyte adsorption: effects of chain rigidity.

Monte Carlo simulation is employed to investigate the interaction between a polyelectrolyte and a fluid mixed membrane containing neutral (phosphatidyl-choline, PC), monovalent anionic (phosphatidylserine, PS), and multivalent anionic (phosphatidylinositol, PIP2) lipids. The effects of the intrinsic polyelectrolyte rigidity and solution ionic strength on the lateral rearrangement and dynamics of different anionic lipid species are systematically studied. Our results show that, the increase of polyelectrolyte chain rigidity reduces the loss of polyelectrolyte conformational entropy and the energy gains in electrostatic interaction, but raises the demixing entropy loss of the segregated anionic lipids. Therefore, the polyelectrolyte/membrane adsorption strength exhibits a non-monotonic dependence on the polyelectrolyte rigid parameter k ang, and there exists a certain optimal k ang for which the adsorption strength is maximal. Because the less loss of chain conformational entropy dominates the increase of the demixing entropy loss of the segregated anionic lipids and the decreases of the electrostatic energy gains, the semiflexible polyelectrolyte adsorbs onto the membrane more firmly than the flexible one. Whereas, for the adsorption of rigid polyelectrolyte, larger anionic lipid demixing entropy loss and less energy gain in the electrostatic interaction dominate over the decrease of the polyelectrolyte conformation entropy loss, leading to the desorption of the chain from the membrane. By decreasing the ionic concentration of the salt solution, the certain optimal k ang shifts to larger values. The cooperative effects of the adsorbing polyelectrolyte beads determine the concentration gradients and hierarchical mobility of the bound anionic lipids, as well as the polyelectrolyte dynamics.

Physicochemical study by multi-angle light scattering of water-soluble methacryloylaminophenylarsonate copolymers

The properties of aqueous solutions of polyelectrolytes of sodium o- and p-methacryloylaminophenylarsonate (o- and p- MAPHA-Na) with acrylamide (AAD) or sodium methacrylate (MA-Na): poly(sodium o-methacryloylaminophenylarsonate) (poly(o-MAPHA-Na)), poly(sodium o-methacryloylaminophenylarsonate-co-acrylamide) (poly(o-MAPHA-Na-co-AAD)) (26:74), poly(sodium o-methacryloylaminophenylarsonate-co-sodium methacrylate) (poly(o- MAPHA-Na-co-AM-Na) (35:65) and (14:86), and poly(sodium p-methacryloylaminophenylarsonate-co-sodium methacrylate) (poly(p- MAPHA-Na-co-AM-Na)) (34:66) were studied by off-line, multi-angle light scattering, and in-line using size exclusion chromatography coupled with multi-angle light scattering (SEC-MALS) and refractometric (RI) detection. The Mw value corresponding to the homopolymer poly(o-MAPHA-Na) was 1,300,000 g/mol and for the copolymers the Mw and gyration radius values were decreased when the AAD or AM-Na comonomer composition increased. The macromolecular parameters obtained using SEC-MALS were in agreement with the values measured in the off-line mode. Also, SEC-MALS experiments allowed examination of the radius of gyration-Mw relationship in order to determine the polyelectrolyte conformation (ρ) in solution; these values fell between 0.7-0.53, indicating that the polyelectrolytes are random coils at θ-conditions. In addition, the effect of the pH on the macromolecular parameters was evaluated. At pH 4.5, the Mw and gyration radius values corresponding to poly(o-MAPHA-Na) and poly(o-MAPHA-Na-co-AAD) (26/74) remained very close to those measured at pH 7. However, <r g 2 > values for poly(o-MAPHA-Na-co-AM-Na) copolymers at pH 4.5 were less than pH 7, indicating that the presence of AM-Na reduced the size of the copolymer through intermolecular interactions.

Metal Ion Induced Adsorption and Ordering of Charged Macromolecules at the Aqueous/Hydrophobic Liquid Interface

Whether a charged macromolecule remains solvated or adsorbs to the interface between two immiscible fluids depends on a variety of complex factors. Ions, in particular, have the capacity to affect the degree of polyelectrolyte hydrophobicity and conformation through electrostatic screening and site-specific interactions. Here, surface-specific vibrational spectroscopy is utilized to probe the roles of ions in the oil–water interfacial behavior of a model carboxylic acid-containing macromolecule. Specifically, the adsorption time dependence, conformation, and extent of carboxylate–cation interactions of poly(methacrylic acid) at the carbon tetrachloride–water interface are deduced as a function of CaCl2 and KCl ionic strength. Additionally, the role of the polymer backbone configuration in interfacial behavior is explored through isomer-specific studies. Results show that ion binding and charge screening induce the adsorption of the isotactic isomer of poly(methacrylic acid) to the oil–water interface under solution conditions unfavorable to surface adsorptivity and that the degree of cation–carboxylate interactions strongly affects the adsorption dynamics and interfacial structure of the adsorbed polymer. The backbone structure of syndiotactic poly(methacrylic acid) does not allow for strong interactions with cations, and therefore, it remains water-solvated in the presence of salt. This work has significance for understanding the role of metal ions in the assembly of natural charged macromolecules at biological and environmental interfaces.

Modeling of Ionization and Conformations of Starlike Weak Polyelectrolytes

The target of this work is to study conformational properties of starlike polyelectrolytes with pH-sensitive (annealed) dissociation in salt-free solutions. We confront hybrid Monte Carlo (HMC) simulations with computationally less expensive approximate numerical self-consistent field (SCF) calculations and with analytical theories. We demonstrate when the mean-field results are reliable and their advantage over MC in terms of efficiency can be exploited and when not. In the interior of the star, where inter-arm interactions dominate over intra-arm ones, the mean-field approximation works well and SCF agrees with the MC results. Intra-arm interactions dominate at star periphery, and their role is underestimated by the mean field. Here, conformations and dissociation resemble those of linear polyelectrolytes. Consequently, the dissociation profile along the chain contour is qualitatively different between MC and SCF. Comparison of the two methods and a distinction between intra-arm and inter-arm contributi...

Conformation of polyelectrolytes in poor solvents: variational approach and quantitative comparison with scaling predictions.

We present the results of variational calculations of a polyelectrolyte solution with low salt in poor solvent conditions for a polymer backbone. By employing the variation method, we quantitatively determined the diagram of the state of the polyelectrolyte in poor solvents as a function of the charge density and the molecular weight. The exact structure and diagram of the polyelectrolyte were compared to the scaling predictions of the necklace model developed by Dobrynin and Rubinstein [Prog. Polym. Sci. 30, 1049-1118 (2005); Dobrynin and Rubinstein, Macromolecules 32, 915-922 (1999); Dobrynin and Rubinstein, Macromolecules 34, 1964-1972 (2001)]. We find that the scaling necklace model may be used as a rather good estimation and analytical approximation of the exact variational model. It is also pointed out that the molecular connection of polymer is crucial for ellipsoid and necklace conformation.

A theoretical investigation on the pH-induced switching of mixed polyelectrolyte brushes

A theoretical investigation on the pH-induced switching of mixed polyelectrolyte brushes was performed by using a molecular theory. The results indicate that the switching properties of mixed polyelectrolyte brushes are dependent on the pH values. At low pH, negatively charged chains adopt a compact conformation on the bottom of the brush while positively charged chains are highly stretched away from the surface. At high pH values, the inverse transformation takes place. The role of pH determining the polymer chains conformation and charge behavior of mixed polyelectrolyte brushes was analyzed. It is found that there exists a mechanism for reducing strong electrostatic repulsions: stretching of the chains. The H+ and OH− units play a more important role as counterions of the charged polymers do. The collapse of the polyelectrolyte chains for different pH values could be attributed to the screening of the electrostatic interactions and the counterion-mediated attractive interaction along the chains.

The solvation and ion condensation properties for sulfonated polyelectrolytes in different solvents—a computational study

In contrast to the broad knowledge about aqueous polyelectrolyte solutions, less is known about the properties in aprotic and apolar solvents. We therefore investigate the behavior of sulfonated polyelectrolytes in sodium form in the presence of different solvents via all-atom molecular dynamics simulations. The results clearly reveal strong variations in ion condensation constants and polyelectrolyte conformations for different solvents like water, dimethyl sulfoxide (DMSO) and chloroform. The binding free energies of the solvent contacts with the polyelectrolyte groups validate the influence of different solvent qualities. With regard to the ion condensation behavior, the numerical findings show that the explicit values for the condensation constants depend on the preferential binding coefficient as derived by the evaluation of Kirkwood–Buff integrals. Surprisingly, the smallest ion condensation constant is observed for DMSO compared to water, whereas in the presence of chloroform, virtually no free ions are present, which is in good agreement to the donor number concept. In contrast to the results for the low condensation constants, the sodium conductivity in DMSO is smaller compared to water. We are able to relate this result to the observed smaller diffusion coefficient for the sodium ions in DMSO.

Sizes and conformations of hydrophilic and hydrophobic polyelectrolytes in solutions of various ionic strengths

The hydrodynamic characteristics of macromolecules of a random copolymer of N-methyl-N-vinylacetamide and N-methyl-N-vinylamine hydrochloride containing 43.6% charged units in the molecular-mass range of 27 × 103 to 355 × 103 are studied. For solutions in 0.2 M NaCl, sedimentation and translational-diffusion coefficients are determined. For salt-free solutions and for solutions in 0.2 and 5.0 M NaCl, the intrinsic viscosities of the fractions are found. The lengths of the statistical segments of the chains are estimated in terms of the Gray-Bloomfield-Hearst theory. The behavior of the polycation, whose uncharged counterpart is a hydrophilic polymer, is compared to the behavior of poly(sodium 4-styrenesulfonate), whose uncharged counterpart is a hydrophobic polymer. The comparison is based on normalized scaling relations. It is shown that the level of compaction of macromolecules of strong polyelectrolytes at a high ionic strength is determined by the degree of hydrophobicity of their polymer chains. Polyelectrolytes based on hydrophilic polymers cannot be compacted into a preglobular state; their chains preserve a swollen coil conformation up to maximally high values of ionic strength.

Titrimetric methods for the determination of surface and total charge of functionalized nanofibrillated/microfibrillated cellulose (NFC/MFC)

Total and surface charge of three different carboxymethylated nanofibrillated/microfibrillated cellulose (NFC/MFC) samples were investigated by using titrimetric methods (conductometric and polyelectrolyte (PE) titrations). Conductometric titration was found to be suitable method for the NFC total charge measurements when the back titration with HCl was applied. Surface charge measurements of NFC/MFC were conducted by using both indirect and direct PE titrations. The direct PE titration was found to be a more suitable method for the surface charge determination of NFC/MFC whereas the indirect PE titration produced too high surface charge values. This is presumably due to kinetically locked polyelectrolyte conformations on the NFC/MFC surfaces or entrapment of residual polymer after adsorption onto the NFC/MFC gel network. Finally, NFC was propargyl-functionalized and the changes in surface and total charge were successfully monitored and compared to those of propargyl-functionalized pulp. A good correlation between the titrimetric methods and elemental analysis was observed.

Yield Stress and Scaling of Polyelectrolyte Multilayer Modified Suspensions: Effect of Polyelectrolyte Conformation during Multilayer Assembly

The yield stress of polyelectrolyte multilayer modified suspensions exhibits a surprising dependence on the polyelectrolyte conformation of multilayer films. The rheological data scale onto a universal master curve for each polyelectrolyte conformation as the particle volume fraction, φ, and the ionic strength of the background fluid, I, are varied. It is shown that rough films with highly coiled, brushy polyelectrolytes significantly enhance the yield stress. Moreover, via the ionic strength I of the background fluid, the dynamic yield stress of brushy polyelectrolyte multilayers can be finely adjusted over 2 decades.

Coarse-grained simulations of the salt dependence of the radius of gyration of polyelectrolytes as models for biomolecules in aqueous solution

The salt dependent radius of gyration of a polyelectrolyte in aqueous solution is calculated in an environment where the polyelectrolyte is surrounded by a permeable membrane that exchanges only solvent particles with the bulk. We obtain additionally the scaling exponent of the gyration radius as a function of the polymerization degree, and find that the polyelectrolyte retains a stretched conformation during the condensation and re-expansion process, indicating that these effects are of an electrostatic nature. The solvent quality is also shown to affect the polyelectrolyte conformation, especially for the poor solvent case. These results are obtained using a hybridized Monte Carlo technique with the coarse-grained, dissipative particle dynamics method with fluctuating number of solvent particles. The full range of the electrostatic interactions is included in the simulations, using the Ewald sum method, and the counterions and solvent molecules are included explicitly. In the complex systems mentioned above, the electrostatic interactions and the solvent quality play a key role in understanding phenomena that do not occur in uncharged systems. Our results are compared and validated with the behavior of some biomolecules under similar environments.

Persistence length of polyelectrolytes with precisely located charges

The conformation of polyelectrolytes in aqueous salt solutions is closely related to their self-assembly properties. In particular, the persistence length has a large impact on how the chain can arrange itself. In this work, biomimetic poly N-substituted glycines (polypeptoids) have been designed to position charged side chains at precise distances from each other to elucidate the relationship between the spacing of the charges along the backbone, the ionic strength, and the persistence length. Using small angle neutron scattering (SANS), it is shown that at low ionic strength, polypeptoids with charged groups located closer to each other along the polymer backbone are stiffer than those with the charged groups spaced further apart. At high ionic strength, the total persistence length decreases for both macromolecules because the electrostatic repulsions between ionized groups are screened. The measured persistence lengths were compared to those calculated using a discrete chain model with bending rigidity, and it is shown that the electrostatic persistence length scales quadratically with the Debye screening length. It is also shown that the bare persistence length of a molecule with alternating ionizable and hydrophilic groups is larger than that of a molecule containing 100% ionizable groups. This difference can be attributed to the longer hydrophilic side chains that may induce local chain stiffening.

A dynamic light scattering study of counter-ions condensation on DNA

The interaction between DNA and counter-ions of different valence, including sodium chloride (Na+), magnesium chloride (Mg2+), hexammine cobalt III ([Co(NH3)6]3+), and spermine ([C10N4H30]4+), is investigated by dynamic light scattering. It is found that the ratio of electrophoretic motilities of DNA in a buffer containing Na+ and Mg2+ is about 2:1, when the concentration of counter-ions c≥ 5 mM. But the ratio of DNA motilities in a buffer containing Na+ and [Co(NH3)6]3+ is about 4.5:1. When c&lt;5 mM, the ratio grows with increasing concentration of counter-ions. DNA charge reversal can be observed in the case of quadrivalent counter-ion. The experimental results are in good agreement with the Manning counter-ions condensation theory for cases of monovalent or bivalent counter-ions. However, when the valency of counter-ions is equal to three, the experimental data deviates from the expectation of the theory significantly. For the quadrivalent counter-ions, the counter-ions condensation theory, which is based on the average field, fails. Furthermore, through the atomic force microscopy, it is found that DNA molecules will condense into compact structures when the valency of counter-ions is equal to or greater than three. Thus, the conformation of polyelectrolyte in free solution and the ion correlation play an important role in the migration process of polyelectrolyte.

Self-assembling microsized materials to fabricate multifunctional hierarchical nanostructures on macroscale substrates

It is very challenging to assemble microscale objects on a macroscale substrate due to the weak interaction and size/geometric mismatch. Herein a novel polyelectrolyte-mediated self-assembly approach with microsized ZnO nanoflowers as building blocks was successfully used to grow a hierarchical nanostructure on a substrate, which is mainly due to the loop and tail conformation of the weak polyelectrolyte used. Furthermore, a heating step was able to enhance the self-assembly process. The obtained ZnO flower hierarchical nanostructure possesses simultaneous non-light induced superhydrophilic, antifogging and antibacterial properties, thus providing great potential in applications such as biomedical devices, hospital building paints, and daily life uses. This demonstrated method could be extended to fabricate hierarchical nanostructures with other microscale nanostructured materials on various substrates for broad applications.

Conformation of polyelectrolytes using cellular automata techniques

A cellular automata model of polyelectrolytes is studied using NetLogo, a multi-agent modelling environment based on the Java virtual machine. NetLogo allows easy construction of cellular automata models, is free to use and has a detailed documentation and examples library. In order to illustrate the working principles of NetLogo, we study polyelectrolyte conformations in a liquid salt solution using the so called cluster entropy S_C as an order parameter. Our results predict the correct evolution of polyelectrolyte conformations, from collapsed clustered structures, at low charge/high salt concentration conditions, to elongated chains, at high charge/low salt concentration, as had been reported in the literature. In both of these situations the cluster entropy is approximately zero S_C ≈ 0. In intermediate charge and salt concentration conditions the well-known pearl necklace structure is observed, in this case the values of S_{C} reach a maximum. The use of NetLogo modelling environment along the cluster entropy concept, permits quick and objective descriptions of the spatial properties of polyelectrolytes.