Polyelectrolyte Domains Research Articles

3D Printing Highly Efficient Ion‐Exchange Materials via a Polyelectrolyte Microphase Separation Strategy

Ion‐exchange materials are commonly composed of polyelectrolyte networks in which crosslinking preserves macroscopic geometry and prevents dissolution in aqueous conditions. However, crosslinking inherently inhibits efficient swelling and mass transfer during ion‐exchange processes. Herein, a one‐step polymerization‐induced microphase separation (PIMS) approach directly using water and linear polystyrene sulfonate macromolecular chain transfer agents (macroCTAs) is developed to engineer bicontinuous nanostructured materials with rapid ion‐exchange capabilities. These materials feature water‐swollen liquid‐like polyelectrolyte domain embedded in a rigid polymer network, where the domain spacing, as determined by small angle X‐ray scattering experiments, is precisely modulated between 15 and 89 nm based on the molecular weight of the macroCTA used. As the bicontinuous nanostructure enables rapid mass transfer throughout the material bulk, the 3D printed PIMS materials are able to rapidly remove model charged dyes from solution, exhibiting a mass transfer coefficient ≈35 times higher than commercially available counterparts. This work is the first example demonstrating direct self‐assembly of water into continuous nanochannels in a well‐controlled manner as supported by time‐resolved small‐angle neutron scattering experiments during polymerization. Moreover, these nanostructured materials are readily produced using commercially available 3D printers, enabling unparalleled high‐resolution fabrication of targeted complex structures, including accurately controllable macroporous geometries and surface areas.

Cartilage polymers: From viscoelastic solutions to weak gels*

AbstractThe dynamic properties of the polymeric components of cartilage are important for understanding the many biological functions of this biological tissue. Knowledge of the rheology of these materials is also crucial for the development of therapeutic treatments for diseases, such as arthritis, in which cartilage loses its functional physiological properties. In the present work complementary noninvasive techniques, rheology and dynamic light scattering (DLS) are used to probe the structural and dynamic properties of solutions of the principal proteoglycan components of cartilage extracellular matrix at different levels of structural hierarchy (chondroitin sulfate, aggrecan, aggrecan‐hyaluronic acid complex). It is shown that the bottlebrush shaped aggrecan molecules form large, loosely organized polyelectrolyte domains comprised of molecules organized into diffuse branched polymer network structures permeated by a diffuse cloud of counter‐ions. The rheological behaviors of these polymer solutions vary from viscoelastic solutions, consistent with the formation of dynamic polymer clusters in solution, to “weak gel” materials exhibiting approximate power law shear stress relaxation or creep over a wide range of timescales.

Bicontinuous Ion-Exchange Materials through Polymerization-Induced Microphase Separation.

Polymerization-induced microphase separation has been used to prepare solid cross-linked monoliths containing bicontinuous and nanostructured polymer domains. We use this process to fabricate a monolith containing either a negatively or positively charged polyelectrolyte domain inside of the neutral styrene/divinylbenzene-derived matrix. First, the materials are made with a neutral pre-ionic polymer containing masked charged groups. The monoliths are then functionalized to a charged state by treatment with trimethylamine; small-angle X-ray scattering shows no significant morphological change in the microphase-separated structure upon postpolymerization modification. By exchanging dyes with the counterions in the material, we corroborated the continuity of the charged domains. Using ion-exchange capacity measurements, we estimate the number of accessible charges within the material based on macro-chain transfer agent molar mass and loading.

Tuning the Interactions in Multiresponsive Complex Coacervate-Based Underwater Adhesives.

In this work, we report the systematic investigation of a multiresponsive complex coacervate-based underwater adhesive, obtained by combining polyelectrolyte domains and thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) units. This material exhibits a transition from liquid to solid but, differently from most reactive glues, is completely held together by non-covalent interactions, i.e., electrostatic and hydrophobic. Because the solidification results in a kinetically trapped morphology, the final mechanical properties strongly depend on the preparation conditions and on the surrounding environment. A systematic study is performed to assess the effect of ionic strength and of PNIPAM content on the thermal, rheological and adhesive properties. This study enables the optimization of polymer composition and environmental conditions for this underwater adhesive system. The best performance with a work of adhesion of 6.5 J/m2 was found for the complex coacervates prepared at high ionic strength (0.75 M NaCl) and at an optimal PNIPAM content around 30% mol/mol. The high ionic strength enables injectability, while the hydrated PNIPAM domains provide additional dissipation, without softening the material so much that it becomes too weak to resist detaching stress.

Water transport and mechanical response of block copolymer ion-exchange membranes for water purification

A membrane inflation experiment was used to measure the mechanical response of ABCBA pentablock copolymer membranes designed to transport water through the coalescence of water-swollen polyelectrolyte domains within the polymer. Creep resistance of the block copolymers originates from glassy A blocks of either poly(p-methyl styrene) or poly(t-butyl styrene), and water permeability is attributed to the polystyrene C block that is partially sulfonated. The creep resistance of the membranes in the wet state was enhanced by replacing the poly(t-butyl styrene) A blocks with poly(p-methylstyrene), or by decreasing the molecular weight of the polyelectrolyte C block. Both of these substitutions increased the creep resistance of the membranes in a wet state while retaining a high degree of water permeability.

Properties of electrically responsive hydrogels as a potential dynamic tool for biomedical applications

ABSTRACTHydrogels with electric responsive properties are gaining research focus due to increasing demand for miniaturized devices that can be precisely controlled using an external stimulus. Such systems are well suited due to their ability to expand and contract when in contact with different types of fluid. This study reports on the synthesis of a “smart” electroresponsive network, using a neutral, “non‐smart,” biocompatible hydrogel forming building block, Pluronic F127 (PF127), as a starting molecule. The PEO–PPO–PEO copolymer was modified with telechelic methacrylic end functionalities to form a triblock linear prepolymer with crosslinkable end groups (crosslinker). This bifunctional prepolymer, PF127 bismethacrylate (PF127BMA), was copolymerized covalently with anionic methacrylic acid sodium salt groups into a nonsoluble 3D hydrogel network in the presence of redox initiators. The polyelectrolyte domains in the pluronic hydrogel afforded controllable swelling capabilities with volumetric expansion exceeding 8500% in deionized water or 1400% in Krebs solution. The hydrogels were further assessed for their mechanical and electroactive response as a function of increasing acid salt content. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41195.

Polyelectrolyte Domains and Intrinsic Disorder within the Prismatic Asprich Protein Family

A number of fully functional proteins have been identified to exist in a partially or fully disordered state. These intrinsically disordered proteins (IDP) are recognized as an important sequence class that fulfill many roles. A number of biomineral-associated proteins, particularly those which possess polyelectrolyte domains, represent potential members of the IDP class. This report describes a bioinformatics study of a ten member polyanionic sequence biomineralization protein family, Asprich, and the experimental characterization of the conserved N- and C-terminal regions found within seven members of this family. Using protein disorder prediction algorithms (DPROT, PONDR, GLOBPLOT), we confirm that all ten Asprich protein sequences are disordered, and that two polyelectrolyte domains within each protein contribute to the disorder scoring. Using synthetic peptides which model the conserved N- (F1,48 AA) and C-terminal (F2, 42 AA) domains, we determine that both domains are globally disordered and remain so in the presence of Ca(II). However, F1 and F2 possess differing proportions of extended beta strand relative to random coil structure and sequence spacing of Asp, Glu residues. As a result, the F2 sequence possesses a higher anionic surface charge density, solvent accessibility, and greater degree of local conformational response to Ca(II). These differences may explain why F1 and F2 differ with regard to step growth kinetics, mineral modulation, and metal ion complexation, and possibly distinguish the molecular role(s) that each domain conveys to the Asprich protein family. Structural and surface charge density features may also control the function of Asp, Glu polyelectrolyte domains within other IDP proteins.

Controlling cell adhesion to surfaces via associating bioactive triblock proteins

A surface functionalization strategy that produces substrates with well-controlled ligand density is critical to investigating the role of cell–substrate interactions in regulating cell adhesion, viability, migration, proliferation and differentiation. Towards this end, we have designed and synthesized a triblock protein, CRC, comprising a polyelectrolyte domain flanked by two amphiphilic leucine zipper domains. The amphiphilic end domains of CRC adsorb onto surfaces and preferentially associate into trimeric aggregates, forming a hydrogel coating layer. Under serum-free conditions, the CRC coating was found to render both 2D substrates and 3D scaffolds non-adhesive to cells. A RGDS sequence was inserted in the middle domain of CRC (generating the protein CRC-RGDS) and found to introduce cell-binding activity. Incorporation of the RGDS sequence did not significantly impact the surface activity of CRC, allowing us to titrate the RGDS surface density simply by adjusting the relative ratios of the two proteins. Ligand density dependent cell–substrate interactions were demonstrated in human foreskin fibroblasts, human umbilical vein endothelial cells, and rat neural stem cells. The versatility to functionalize a range of different substrate surfaces, combined with the ease of controlling surface ligand density, makes these triblock proteins an attractive tool for developing cell-specific surface coatings with tailored biofunctional attributes.

Dynamic Properties of Artificial Protein Hydrogels Assembled through Aggregation of Leucine Zipper Peptide Domains

Network relaxation dynamics of hydrogels formed from a genetically engineered multidomain protein (AC_(10)A, where A is an associative leucine zipper domain and C_(10) is a random-coil polyelectrolyte domain) were investigated by shear rheometry. Physical gels form by tetrameric association of the leucine zipper end-blocks (A). The longest stress relaxation time (τ_r) of these gels varies strongly with pH, increasing from τ_r ≈ 80 s at pH 8.0 to τ_r ≈ 1000 s at pH 7.0. The rate of strand exchange of the end-blocks was studied by using fluorescence quenching of the labeled form of the A domain. Fluorescence is quenched in solutions of fluorescein-labeled A; dequenching occurs when labeled A is mixed with a 60-fold excess of the unlabeled peptide. The dequenching transient after mixing reveals the characteristic strand exchange time (τ_e) of the A domain. As pH decreases from 8.0 to 7.0, τ_e increases from ca. 200 s to ca. 4500 s. Thus, τ_r of AC_(10)A hydrogels and τ_e of the A domain vary in parallel with pH. The strong correlation between macroscopic and molecular properties indicates that network relaxation is regulated by the lifetime of associations in the transient network. Because the rate of leucine zipper strand exchange is sensitive to interstrand electrostatic interactions, the relaxation behavior of artificial protein hydrogels can be engineered systematically by genetic programming of the amino acid sequence.

Preparation and characterization of novel pH‐sensitive binary grafted polymeric blends of gelatin and poly(vinyl alcohol): Water sorption and blood compatibility study

AbstractIn this study, we attempted the synthesis and characterization of novel biocompatible hydrogels of binary polymeric blends of crosslinked poly(acrylic acid) grafted onto poly(vinyl alcohol) and gelatin by a redox polymerization technique. The end polymer was characterized by IR spectral analysis, differential scanning calorimetry measurements, and scanning electron microscopy. The prepared smart, environment‐responsive hydrogels, containing polyelectrolyte domains, were assessed for their water sorption potential under various experimental conditions and were further used to evaluate important network parameters such as the crosslink density, number of elastically effective chains, and molecular mass between crosslinks. The diffusion mechanism of the solvent–polymer interaction was also analyzed to predict the behavior of continuously relaxing chains containing several carboxylate ions. The blood compatibility of premeditated hydrogels was also judged by in vitro methods such as protein adsorption, blood clot formation, and hemolysis percentage assay measurement. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 599–617, 2006

Equilibrium particle aggregates in attractive colloidal suspensions

The formation of equilibrium clusters in colloidal particle suspensions, due tothe balance of a short-range attraction and a long-range repulsion interaction,has been recently widely discussed, considering its important role in many areasof material science and soft-matter physics. In this work, we describe a simplephenomenological model in which particles interact with a screened electrostatic repulsiveinteraction modelled by a Yukawa potential and with a short-ranged attractivepotential (‘charge patch’ attraction) due to the strong lateral correlations among theadsorbed polyions at the surface of the approaching particles. This ‘correlatedadsorption’ generates a more or less ordered ‘patchwork-like’ surface structurewith excess negative charge domains (adsorbed polyelectrolyte domains) andexcess positive charge domains (polyelectrolyte free domains), which give rise to ashort-range attractive potential. In low-density colloidal liposome suspensions, particlesform stable equilibrium clusters. We investigated the cluster formation and thegrowth in the cluster size in cationic liposomal particle suspensions (built up byDOTAP and DOTAP/DOPE mixtures), by means of a dynamic light scatteringtechnique, in several experimental conditions (adsorbing polyions of differentmolecular weights, liposomes of different surface charge densities, presence of addedsalt), and a generally good agreement with the model predictions has been found.

Polyelectrolyte Nanoring Structures: Critical Parameters Governing Formation and Structural Analysis

Recently we have presented new polyelectrolyte nanoring structures formed by self-assembly of poly(ethylenimine) and poly(sodium 4-styrenesulfonate) during sequential adsorption observed by AFM microscope in liquid cell technique. In this work, filter pore size and carbonate ion concentration are identified as critical parameters for their formation. We show how these two parameters modulate the nanorings formation as a consequence of the hydrophobic polyelectrolyte domains formed into the polyelectrolyte solutions and the screening effect produced by the divalent carbonate ions present also in solutions. Also, we prove how the nanorings size is controlled through competition between electrostatic and hydrophobic interactions where the charge density of the substrate plays an important role.

Dynamic Structure of a Protein Hydrogel: A Solid-State NMR Study

^(13)C and ^(15)N solid-state NMR spectroscopy has been used to study the dynamic structure of a genetically engineered multidomain protein hydrogel that contains two leucine-zipper domains and a central polyelectrolyte domain. ^(13)C NMR spectra show that on the microsecond time scale the central domain is isotropically mobile while the leucine-zipper domains are rigid. This supports the hypothesis that the central domain acts as the flexible swelling agent of the gel network while the terminal domains form intermolecular aggregates. ^(13)C isotropic chemical shifts indicate that the terminal domains are helical, while the central domain has a random coil conformation. On the millisecond time scale, the leucine-zipper domains are highly dynamic, as determined from the ^(13)C-detected ^(15)N CODEX experiment. The motion is rigid-body in nature with a correlation time of about 80 ms at room temperature and has an average amplitude of about 50°. Several specific motional models are considered by comparing simulated and experimental exchange intensities as a function of the recoupling time for ^(15)N chemical shift anisotropy. The experimental data are consistent with two of the models considered: a random jump model and a uniaxial rotation model. The implications of this motion to strand exchange between helical bundles are discussed.

Influence of counterion valency on the scattering properties of highly charged polyelectrolyte solutions

Light and neutron scattering measurements on highly charged polyelectrolyte solutions have recently provided firm evidence for the existence of “domain structures” containing many chains, even at rather low-polymer concentrations. In the present paper, we systematically investigate the influence of counterion charge valency Zc on the scattering properties of sulfonated polystyrene (PSS) solutions in water with monovalent and divalent counterions. This study is part of a larger effort to identify essential factors governing polyelectrolyte domain formation and the geometric properties of these transient structures. Neutron scattering measurements indicate that the interchain correlation length ξd within the domains becomes larger by a factor of 1.5–2 for divalent relative to monovalent counterions. This observation is consistent with the Manning model estimate of the change in effective polymer charge density Γ* with Zc and with previous observations linking ξd [from the peak position in the scattering intensity [I(q)] with the bare polymer charge density, Γ. Light scattering measurements of the radius of gyration Rg,d of the domains indicate that their size becomes smaller for divalent counterions and with a reduction of Γ. We observe that the “fractal” dimension of the domains measured from the low-angle scaling of I(q) depends on the chemical structure of the polyelectrolyte. Zero average contrast (ZAC) neutron scattering measurements show that the radius of gyration Rg,c of individual polyelectrolyte chains is also reduced for divalent counterions, suggesting that chain rigidity is strongly influenced by Γ*. Charge valency effects on relaxation times are investigated by dynamic light scattering. As usual, two diffusive modes are observed in the light intensity autocorrelation function, G(τ). The “fast” mode becomes slower and the “slow” mode becomes faster for the divalent counterion (Mg2+), relative to the monovalent counterion (Na+). Counterion valence has a large influence on the structure and dynamics of highly charged polyelectrolyte solutions through its influence on Γ*.

Fluorescence study of the interaction between metal ions and methyl methacrylate-methacrylic acid copolymers in aqueous solutions: thallium(I), calcium(II), and terbium(III)

The binding of the cations thallium(I), calcium(II) and terbium(III) to methyl methacrylate– methacrylic acid copolymers with different fractions of acid groups (x) has been studied in aqueous solution at various pH values using the fluorescence of covalently bonded 9-vinyl anthracene as a probe. In all cases, the extent of binding increases as a function of the charge of the polymer with either increasing fraction of carboxylic acids or of pH. However, differences are observed in the behavior of the three cations. With Tl(I), quenching of the anthracene group fluorescence is observed, indicating that the thallium(I) approaches the probe and suggesting that the alkylanthracene is probably in a relatively polar region. Binding constants have been determined from anthracene quenching data and from studies with the fluo-rescent-probe sodium pyrenetetra-sulfonate. Good agreement is obtained between the two methods, and values for the binding constants increase from 250 to 950 M-1 as x increases from 0.39 to 1. It is suggested that the cation is held in the polyelectrolyte domain, partly by Debye–Huckel effects and partly by more specific interactions. Stronger binding is found with calcium(II) and terbium(III), and in this case increases in fluorescence intensity are observed on complexation due to the anthracene group being in a more hydrophobic region, probably as a result of conformational changes in the polymer chain. In the former case the stoichiometry of the interaction was determined from the fluorescence data to involve two carboxylate groups bound per calcium. Association constants were found using murexide as an indicator of free calcium to vary from 8400 to 37 000 M-1 as x increases from 0.39 to 1. It is suggested that in this case specific calcium(II)–carboxylate interactions contribute to the binding. With terbium(III), a greater increase in the probe fluorescence intensity was observed than with calcium, and it is suggested that the interaction with the polymer is even stronger, leading to a more pronounced conformational change in the polymer. It is proposed that the terbium(III) interacts with six carboxylic groups on the polymer chain, with three being coordinated and three attracted by electrostatic interactions.

Counterion association to cationic polyelectrolytes in methanol/water mixtures

AbstractThe effect of the nature of the medium on the chloride and bromide condensation to a cationic polyelectrolyte was studied. Methanol/water mixtures were used as solvent in order to vary continuously the properties of the medium. The counterion–polyion interaction was studied by the determination of the Eisenberg interaction parameter (f*) by means of electrical conductivity measurements. Results indicate that f* parameters increase while the limiting electrical conductivity is rapidly decreasing. This apparently anomalous phenomenon is interpreted as a preferential solvation of the polyelectrolyte which brings about a conformational change, as revealed by the intrinsic viscosity behavior of the solutions. This change could induce an increase in the average distance between charges without further dissociation, which could explain the observed behavior of the f* parameter. It is concluded that the actual dielectric constant which governs the polyion–counterion interaction in the polyelectrolyte domain is not the bulk dielectric constant.

Phosphophoryn, an "acidic" biomineralization regulatory protein: conformational folding in the presence of Cd(II).

The divalent cation-induced protein folding properties of the template macromolecule, bovine dentine phosphophoryn (BDPP), have been examined by 1H/31P/13C/113Cd-nmr spectroscopy. Cd(II) was employed, exploiting the sensitivity of 113Cd-nmr to ligand-binding interactions and kinetics. Cation binding was studied over the stoichiometric range of 0-50: 1 Cd(II): protein (mole ratio), well below the range of Cd(II) concentration required to induce protein precipitation. The stepwise titration of divalent cation-depleted phosphophoryn at pH 7.2 in H2O/D2O with 113CdCl2 revealed that (PSer)n, (PSerAsp)n, and (Asp)n polyelectrolyte cation-binding domains undergo two major transitions in their secondary and tertiary structures: the first transition, occurring between 1:10 and 1:1 Cd(II): protein stoichiometry, and the second, between 10:1 and 50:1. By monitoring the amide NH intensities, 31P-nmr chemical shift, and 13C Asp-C, resonances, it was concluded that Cd(II) ions exhibit a binding-site preference for polyelectrolyte cation-binding domains, in the order (PSer)n > (PSerAsp)n > (Asp)n This preference correlates with the degree of negative charge density for each sequence motif. Accompanying the backbone conformational transitions at the polyelectrolyte regions were conformational transitions in the flanking hinge domains, indicating that the hinge domains participate in the folding of the phosphophoryn molecule as divalent cation binding occurs at the polyelectrolyte domains. We were unsuccessful in detecting phosphophoryn-bound Cd(II) species by 113Cd-nmr because of chemical exchange modulation. However, using a smaller 21-residue peptide mimetic of phosphophoryn, we have observed three stoichiometric-dependent 113Cd resonances that differ in terms of the oxoanion coordination number. Our observation of multiple Cd(II) species in the presence of the peptide supports our contention that Cd(II) has many chemically distinct coordination sites on phosphophoryn, each in multiple equilibria with H2O, Cl-, and side-chain oxoatoms.

Variable Charge-Densities in Hydrophilic Weak-Base Ion-Exchange Membranes. V. Equilibrium Properties with Pure 1-1 Electrolytes. Swelling and Ion-Exclusion

Abstract The equilibrium distribution of chloride or nitrate counter-ions of K or NH4 co-ions and of water is determined experimentally at different degrees of ionization X[xbar] of hydrophylic weak-base aminated polyvinyl-alcohol membranes. These data are analyzed and explained in terms of thermodynamics interrelating the molality of fixed-charge densities and of counter-ions to that of the amount of water and of diffused salt in the membrane. Three domains have to be considered: the polyelectrolyte domain with small external concentration compared to the internal net fixed charge concentration (notion of “net-charge” densities), the domain of homogeneous interstitial liquid of the concentrated electrolyte type with sufficiently concentrated salt solution, and the intermediate domain.