Session M19: Self-Assembly and Phase Separation of Polymers and Charged Soft Matter I

Phase separation in alpha-synuclein solution through micro-structural organization

alpha-synuclein has the characteristics of an intrinsically disordered protein due to highly concentrated charges at the termini and a low overall hydrophobicity. It can misfold and aggregate into highly structured and beta-sheet containing amyloid fibrils, the hallmarks of Parkinson's disease (PD). Involvement in the pathogenesis of PD has led to significant experimental and theoretical efforts to understand its aggregation mechanism. Although a few studies suggest droplet formation and a subsequent phase transition to amyloid fibrils, the exact phase behavior is not well understood. To shed light onto the structural organization of alpha-synuclein aggregates, we investigate the phase behavior using multi-scale numerical simulations. We observe that micro-structural organization emerges in the solution beyond a threshold protein concentration. This organization of the protein chains is mainly driven by the electrostatic interactions between the residues in N-terminal and C-terminal of the proteins. Moreover, the presence of salt ions loosens up the compactness of the micro-structure, which can be a promising pathway to hinder aggregation into droplets and ultimately amyloid formation.   

The importance of the σ-hole in the self-assembly of halogenated peptoid nanosheets

Halogens are gaining increasing importance in designing functional polymeric materials. The most prominent feature of halogens is their high electronegativity, which facilitates the exploration of halogen-containing self-assembled structures. However, when halogens are involved in a covalent bond, the halogen atom tends to polarize and forms a region of positive electrostatic potential on its surface. This region of positive potential is referred to as a σ-hole. In this study, a series of amphiphilic diblock copolypeptoids were synthesized with a standard phenyl or a halogenated phenyl in the side chain. These diblock copolypeptoids self-assemble into crystalline nanosheets in water, and high resolution cryo-TEM imaging of the nanosheets reveals a surprising change in the crystal motifs of the halogenated nanosheets. This change in the crystal motifs was only observed by doing direct, atomic-scale imaging on the radiation sensitive materials. Molecular dynamics simulations are used to understand the intermolecular forces that gave rise to the varying crystal motifs. This combination of atomic-scale imaging and atomic-scale simulations can be broadly applied to study the effect that halogens have on the self-assembly of polymeric materials.     

Interplay between physical interactions controls chiral discrimination during the self-assembly of chiral metal-organic cages

Cationic Pd₁₂L₂₄ metal-organic cages (MOCs), based on hydrophobic amino acids including D- and L-alanine, D- and L-valine, and D- and L-leucine, can self-assemble into supramolecular structures through counterion-mediated attractions by the addition of extra nitrates. Previously, we showed that presence of N-(tert-Butoxycarbonyl) (Boc)-protected D- and L-alanine as chiral counterions can induce totally different self-assembly behavior. For example, L-alanine can suppress the self-assembly of D-MOCs based on alanine; while, L-alanine does not change the self-assembly behavior i.e., chiral discrimination exists. Here, it is revealed that chiral discrimination can be induced only when chiral counterion is less bulky or as bulky as the amino acid used in the MOCs’ structure. For example, only Boc-alanine can induce chiral discrimination to alanine-based MOCs; while, Boc-alanine, -valine, and -leucine induce chiral discrimination to leucine-based MOCs. Besides, utilizing alanine-based anionic surfactants indicated that increasing hydrophobicity of chiral counterion vanishes chiral discrimination. This study unveils the interplay of physical intermolecular interactions and their effect on the self-assembly of homochiral supramolecular structures.

    

Self-assembled Polypeptoid Nanostructures Revealed by 3-D Cryogenic Electron Microscopy

Amphiphilic diblock polypeptoids were synthesized with the same block ratio and chain length but different N-terminal capping groups. By tuning these end groups, distinct self-assembled structures, including micelles, nanofibers and nanosheets, can be obtained in polar solvents. The molecular structure of the peptoid nanofibers remains unknown. Low-dose cryogenic electron microscopy micrographs were obtained from the frozen specimens. Single particle analysis was applied to reconstruct the 3-D morphology of the self-assembled nanofibers. The spatial arrangement of the individual molecules was directly observed in the nanofibers with the spatial resolution of ~3.6 Å. The nanofibers, which result from peptoids with an uncapped N-terminus, are shown to exhibit different packing geometries (bilayer vs. interdigitated monolayer) and degrees of order depending on the solvent conditions.  When the N-terminus is acetylated, nanosheets with long-range order are formed. Our imaging approach is robust and enables the direct 3-D visualization of the arrangement of polypeptoid backbones and side chains in the self-assembled nanostructures. It reveals the effect of end group chemistry on the hierarchical structures of polypeptoids at the atomic level.   

Pattern formation in electro-responsive polymer ionic liquid blends

Self-assembly has been one of the widely used approaches for designing multifunctional soft materials. Here, we explore polymer-ionic liquid (PIL) systems and demonstrate the formation of intricate ordered patterns in these systems using an external electric field. First, we establish the equilibrium phase diagram of binary electro-responsive PIL blends and subsequently, use the Poisson-Nernst-Planck equation to model the kinetics of pattern formation. Our simulations reveal the formation of ordered patterns that are oriented in the direction of electric field. We also demonstrate that by tuning the direction of the electric field, ordering of the PIL domains can be dynamically controlled. The underlying mechanism is the change in the interactions between various PIL moieties namely, cation-anion, polymer-anion, and polymer-cation. We believe that our methodology provides a simplistic mechanism for creating ordered patterns in soft materials through reaction-diffusion phenomena that can be harnessed for other similar stimuli-responsive systems.   

Micelle dimerization in melts of asymmetric diblock copolymer chains

Experiments on asymmetric diblock copolymer melts have reported the formation of metastable Frank-Kasper and Laves phases, which are presumed to emerge as a consequence of arrested dynamics upon rapidly cooling a disordered micellar system. Although these reports have generated a renewed interest in the disordered micellar regime, a definitive understanding is still lacking. In this work, we perform coarse-grained molecular dynamics simulations on asymmetric diblock copolymer chains having a minority block volume fraction of 0.125. Analysis of the aggregation numbers and the shape anisotropies of clusters reveals frequent formation and breaking of bridges between two or more micelles in both the ordered and disordered states. Due to the rapid formation and breaking of bridges, we expect this phenomenon to complicate the identification of micelle fusion and fission in molecular simulations. These provocative results may be difficult to verify experimentally, yet will play an important role in the dynamics associated with the formation and transformation of disordered and ordered morphologies.   

Effect of end-block asymmetry on the packing of ABA triblock copolymers

We examine the packing of polystyrene-b-poly(ethylene-alt-propylene)-b-polystyrene (SEPS) copolymers at constant composition (25% polystyrene) but varying the ratio of the length of one styrene block to the other, from 0 (i.e., a diblock) to 1 (a symmetric triblock). Samples were prepared in the EP-selective solvent squalane, with polymer concentrations ranging from 10% up to the melt. Morphologies were characterized by SAXS, supplemented by viscoelastic measurements. For the diblock and symmetric triblock, the expected phase behavior was found: body-centered cubic from 10% to 30%, hexagonal from 50% to 80%, and lamellae at 90% and 100%. Remarkably, the asymmetric triblocks show no long-range order up to at least 30% concentration, even after considerable annealing. Clear form-factor features and structure factor peaks suggest a liquid-like packing. Several possible explanations for this suppression of lattice formation will be evaluated.   

Self-Assembly of Coil-Bottlebrush Block Polymers

Diblock copolymers self-assemble into a variety of periodic nanostructures, such as spherical micelles, cylinders, lamellae, and 3D networks. Network structures consist of reoccurring interpenetrating nanodomains. The bicontinuous nature of the network domains provides the opportunity to design materials with several combined properties. However, access to applications has been limited because self-assembly of linear diblocks into network phases tends to occur only over narrow compositions. Moreover, the slow self-assembly kinetics at high molecular weight places upper limits the accessible pore size of the network structures. Polymers with a bottlebrush architecture are expected to provide access to larger domains due to an extended backbone chain conformation and faster ordering kinetics. We have synthesized diblock copolymers with a coil-brush architecture through ring-opening metathesis polymerization. By combining the coil and the brush architectures in one diblock, we change the overall shape of the molecule, and affect the polymer packing into network phases. By a combination of molecular and structural characterization methods, we show that the shape of the polymer and the identity of the coil block give significant control over the compositions, compositional window size, and molecular weights available for network formation.   

Understanding molecular driving force of self-assembly in highly charged soft materials systems via predictive molecular simulations

The self-assembly of highly charged polymeric systems into ordered nanostructures is of fundamental interest in polymer physics and has practical importance to optimize the ion transport properties for next generation electrolytes. In this work, I'll present our ongoing efforts to understand these highly charged systems through predictive molecular simulations, with a focus on an acid-tethered block copolymer system incorporating ionic liquids, for which experiments showed diverse morphology including low symmetry A15 phase with varying conditions. Our molecular dynamic (MD) simulations enables monitoring the detailed molecular structure and dynamics on the micellar interface between hydrophobic core adn charged ionic domains, which revealed the formation of fascinating, thin ionic shell layers composed of ionic complexes. Importantly, the stability of A15 phase was strongly affected by the interfacial properties and concentration fluctionation of the ions and charged block, resulting in radical changes in the conductivity by an order of magnitude. Our collaborative work provides novel routes to develop advanced polymer electrolyte having tailor-made interfaces.   

Self-assembly of asymmetric poly(styrene)-block-poly(methacrylic acid) polyelectrolyte-neutral diblock copolymer in aqueous solution by explicit atomistic MD simulations

The micelle mechanism, structure, hydration, and thermodynamics of asymmetric poly(styrene)-block-poly(methacrylic acid) polyelectrolyte-neutral block copolymer chains in salt-free aqueous solution were investigated as a function of copolymer composition (X_PS) for unionized (f = 0) and fully ionized (f = 1) states of PMA block by molecular dynamics simulations. The mechanism of micelle formation is a combined approach of unimer insertion and cluster fusion/fission mechanism, which is examined by the population of unimers and clusters formation across the simulation trajectory. Micelle formation takes a long time for copolymer at charged state in comparison to uncharged with relatively short hydrophobic PS block. The shape of micelle transforms from spherical to ellipsoid with increase in X_PS, the radius of core and entire micelle increased with increase in X_PS. Solvation behavior of micelle increases with the quantity of charge on PMA units and insignificant change by PS fraction of copolymer chains. The interaction of PMA with water becomes more favorable at fully-charged state in comparison to an uncharged state due to the presence of COO¯ groups. The solvent accessible surface area (SASA) of micelle increases with X_PS in both states, commensurate with its shape transformation. The SASA per corona chain shows linear behavior with X_PS. The poly(methacrylic) chains are located on the surface of PS core at uncharged state (f = 0) and attain an extended structure as classical corona at fully charged state (f = 1). The results of atom density profiles, solvation enthalpy, and RDFs of copolymer-Na⁺ ion pairs confirm the existence of the polyelectrolyte micelle in the osmotic regime, at all copolymer compositions, in agreement with results of mean-field theory^.1

Keywords: Self-assembly, Polyelectrolyte​​​​​​.

References:

1. Shusharina, N. P.; Linse, P.; Khokhlov, A. R. Lattice Mean-Field Modeling of Charged Polymeric Micelles. Macromolecules 2000, 33, 8488–8496.   

Electrostatic Stabilization of Low Symmetry Morphology in Block Copolymer Electrolytes

In present study, we investigated a role of electrostatic interactions in phase behavior of block copolymer electrolytes. We prepared two block copolymers having identical molecular weights and block composition, but tethered with different types of acid functional moieties. With added various nonstoichiometric ionic liquids, we observed rich phase transitions among lamellae, hexagonal cylinder, and A15 structures. Intriguingly, radical phase shifts between lamellar and A15 structures were obtained with small changes in the volume fraction of ionic domains. Key to this stems from the amount of ionic liquid cations in the block copolymer/ionic liquid mixtures upon inducing dissimilar strength of electrostatic interactions. Notably, the type of acid functional group in polymer matrix played a crucial role in determining the stability window of the A15 structures, which becomes widened when the acid function groups are strongly interacting with ionic liquid cations. Computational simulations revealed that the thin ionic shell layers were formed at the micellar interfaces, which is the key to stabilizing anisotropic micelles to develop the low symmetry morphology. The resultant A15 phases of block copolymer electrolytes enhanced ion transport efficiency with 3-dimensionally connected ion-conducting pathways.   

Surprises in the liquid-liquid phase separation of multivalent DNA complexes

The liquid-liquid phase separation of multivalent DNA complexes--often called DNA nanostars--follows the upper critical solution temperature (UCST) behavior. At temperatures below the binodal curve, DNA nanostars phase separate into a dense liquid-like phase and a dilute gas-like phase. Above the binodal curve, the interactions between nanostars are negligibly weak and the system enters a one-phase region. In this talk, I will describe a straightforward microfluidics-based approach for constructing the phase diagram of DNA nanostars. Using our approach, we quantify the equilibrium densities of both the liquid and gas phases as a function of temperature for different star architectures, thereby constructing the phase diagrams in the density-temperature plane. Surprisingly, we find that the phase behavior of DNA nanostars is much more complex than originally anticipated, which we rationalize using an associating fluid-based model. Our results could help shed light on the interplay between the self-assembly of biomolecular complexes and liquid-liquid phase separation in other material platforms, including proteins and other nucleic acids.   

Bulk light-scattering measurements of viral capsid self-assembly around RNA.

Self-assembly is a vital part of the RNA virus life cycle. The assembly of viral coat proteins around viral RNA occurs both in vivo and in vitro, suggesting that viral capsid assembly may be driven by minimization of free energy. To better understand this process, we modify the interactions between coat proteins and between the coat proteins and RNA of MS2 bacteriophage in vitro by varying the capsid protein concentration, RNA concentration, ionic strength and pH, and we study the assembly using dynamic and static light scattering. From dynamic light scattering we determine the assembly yield and the size distribution of assembled products. From static light scattering, we measure the kinetics of assembly in bulk. By comparing the results from these two different techniques to each other and to results from gel electrophoresis, we infer features of the assembly pathway.