Session K30: Organization and Dynamics of Functional Liquid Crystals, Polymers, and Biological Assemblies II

DYNAMIC SELF-ASSEMBLY OF VIRUS CAPSIDS

The assembly and disassembly of virus capsids, composed of many subunits, are fundamental steps in the viral life cycle. The complete set of possible capsid intermediates is immense, yet the concerted assembly process is done with high fidelity and leads to stable capsids that can efficiently encapsulate and protect genetic material, and when needed, dissociate and release their cargo. Virus capsids are therefore stable and flexible dynamic structures. To better understand and predict the outcomes of these apparently contradictory processes, we precisely analyzed the structure, kinetics, and thermodynamic stability of the experimentally tractable virus assembly reaction, in vitro. High-resolution modern synchrotron solution X-ray scattering measurements of assembly reactions provided statistically reliable and rich structural data. We rigorously analyzed the data by integrating our home-developed state-of-the-art scattering data analysis software D+ (https://scholars.huji.ac.il/uriraviv/software/d-software) with simulations and theory of macromolecular self-assembly. Our accurate and comprehensive analysis provided new insight into the mechanisms of viral self-assembly and the boundaries where thermodynamic products can be realized and function, and when kinetically trapped metastable states may form. This insight could be important for designing antiviral therapeutics as well as noncapsules or nanoreactors.
  

Counterion-dependent Dynamics of Nanoconfined Water in Lyotropic Liquid Crystalline Mesophases

Understanding how the interfacial chemical functionalities influence nanoconfined water dynamics could potentially inform the design of next-generation permselective and ion-transporting membranes for energy applications. However, lacking access to well-defined systems with tunable interfacial chemistries and nanoconfinement geometries has hampered conclusive studies of water dynamics therein. Derived from the water concentration-dependent self-assembly of small molecule surfactants, lyotropic liquid crystals (LLCs) offer a platform for studying confined water dynamics in nanopores (diameters ~0.7-2.5 nm) lined with well-defined chemical functionalities. In this contribution, we describe the synthesis and aqueous phase behavior of ionic gemini dicarboxylate surfactants, which form technologically-useful, normal double gyroid phases in which water is nanoconfined between two carboxylate-lined, convex interfaces. Quasielastic neutron scattering (QENS) measurements indicate that the gyroid water dynamics are significantly slower than in bulk water, and that the observed dynamics are very sensitive to the nature of the charge-compensating counterion (Na⁺, K⁺, Me₄N⁺) associated with the carboxylate surfactant headgroup.   

Phase transformations in lipids confined to colloidally stable nanoscale particles

Lipids – biological, amphiphilic molecules with ability to self-organize into bilayer structures – have found lasting therapeutic application in drug-laden nanoparticles. Of recent interest is the internal structure of these nanoparticles, which influences their endosomal escape in cellular delivery pathways. Lipid nanoparticles with bicontinuous cubic internal structure show improved gene knockdown efficiency over more traditional liposomal particles [1]. Inverse hexagonal-phase lipid materials show further improved fusogenic properties, but are toxic due to pore formation with the plasma membrane [2]. Here we report a lipid-based material with phase-triggering character to avoid such toxicity limitations. In this system, a change from bicontinuous cubic to inverse hexagonal within the nanoparticles can potentially be triggered at any delivery step without requiring endosomal acidification. Instead, local heating is achieved via near-infrared radiation of incorporated gold nanorods, inciting localized surface plasmon resonance. The formulated lipid systems are characterized with cryogenic TEM and small-angle X-ray scattering (SAXS).

References:
[1] Kim, H. & Leal, C. (2015) ACS Nano. [2] Bouxsein, N. F. et al. (2007) Biochemistry.   

Modeling the Properties of Liquid Crystal Electrolytes with Replica Exchange Molecular Dynamics

Ionic liquid crystals exist at the intersection of ionic liquids and liquid crystals; properties from both classes of materials can be utilized in various applications, from nonlinear optical and photonic devices to dye-sensitized solar cells. One intriguing use has been suggested where the ionic liquid crystals act as anisotropic battery electrolytes, where the liquid crystalline order provides global structure and the ionic liquid character provides a medium through which charge transport is facilitated. The ionic liquid crystals in question have bulky ionic groups and mesogenic “tails” with little or no effective charge, qualitatively similar to charged surfactants. This class of materials is relatively unprobed experimentally, as many are nontrivial to synthesize. We leverage atomistic molecular dynamics simulations of these systems as a preliminary way to study their phase behavior, utilizing replica exchange methods to pinpoint and elucidate the observed phase transitions.   

Mechanisms to Twist-Bend and Splay-Bend Nematic Phases

Twist bend phases of liquid crystals have been studied extensively experimentally. However, the mechanism behind their emergence remains debated. Theoretical models have predicted the existence of twist-bend and splay-bend phases as stable states arising from a common mechanism. With the recent experimental observation of splay-bend phases, we look at bond orientational order, such as planar hexatic order, as a way to arrive at splay-bend and twist-bend phases. We find that phase chirality, and the nematic symmetry play a key role in looking at possible interactions of nematic order and bond orientational order.   

Length segregation in mixtures of spherocylinders induced by imposed topological defects

We explore length segregation in binary mixtures of spherocylinders of lengths L₁ and L₂ which are tangentially confined on a spherical surface of radius R. The orientation of spherocylinders is constrained along an externally imposed direction field which is either along the longitude or the latitude lines of the sphere. We show that the integer orientational defects at the poles induce a complex segregation picture also depending on the length ratio factor γ=L₂/L₁ and the total packing fraction η of the spherocylinders. In longitude preoriented cases shorter rods tend to accumulate at the poles whereas longer rods occupy central equatorial area of the spherical surface. In lattitude preoriented cases longer rods are predominantly both in the cap and in the equatorial areas and shorter rods are localized in between. A reference situation with a defect-free flat plane does not show any length segregation at similar γ and η. We also develop an Onsager-like density functional theory which is capable to predict length segregation in ordered mixtures. At low density, the results of this theory are in good agreement with the simulation data.   

Temperature-induced coordinated transformation of block copolymer micelles on fcc lattices to hcp structures

We discovered coordinated phase transformation of the strongly-segregated block copolymer micelles in aqueous solution on fcc lattices to hcp structures. The fcc structures formed by direct dissolution of poly(1,2-butadiene-b-ethylene oxide) diblock copolymer in water were gently shear-aligned and contained in sealed thin-wall capillaries. Heating the shear-aligned fcc crystallites to a higher temperature induced highly-coordinated hcp crystallites aligned in the same direction which we believe cannot happen by a nucleation and growth mechanism due to the high symmetry of fcc crystals. Careful analysis of the hcp structures suggests that the phase transformation occurred by sliding specific two-dimensional hexagonal micelle layers of the shear-aligned fcc crystallites likely with the smallest cross-section area.   

A mean field approach to determine the statistics of bundles of wormlike chains

Crosslinked bundles of macromolecules play an important role in a variety of biologically relevant systems, including the flagella that provide locomotion and the microtubules and actin filaments that compose the cytoskeleton. In this talk, I describe a novel method of studying the statistics of weakly-bending, weakly-shearing bundles of stiff polymers using a mean field approach. I show that the imposition of the constraints of inextensibility and inter-filament separation on average leads to an analytically tractable free energy determined by a single wormlike Hamiltonian coupled to an effective Hamiltonian of a cylindrically confined chain, the latter representing the crosslinking between filaments. This gives rise to a bundle free energy that deviates from a typical wormlike chain, depending on a deflection length that couples the filament stiffness and bundle radius. The free energy of an intrinsically twisted bundle is determined for stiff bundles as well, which produces a significant change in the dependence of the free energy on filament stiffness and cross-sectional width. This mean field approach provides new insight into the statistics of bundled macromolecules with a variety of geometric constraints, useful in a number of biological contexts.   

Configurable Self-Assembly of Block Copolymers at the Liquid-Liquid Interface

Self-assembly of charged block copolymers has a range of potential applications such as fabrication of reconfigurable patterns in liquid-liquid systems, thin-film nanopatterning, bottom-up nanofabrication, demulsifying and antifoaming in extraction methods, drug delivery, protein encapsulation, among many others. Here we investigate the configurations of amphiphilic block copolymers at the water-chloroform interface using molecular dynamics simulations. The copolymers are constituted by one hydrophilic block and one hydrophobic block. A fraction of monomers (f_q) in the hydrophilic block bear a positive elementary charge which is balanced by free counterions. Our model represents the block copolymer poly(styrene)-poly(2-vinylpyridine) (PS-b-P2VP) with a degree of quaternization (f_q). A variety of structures going from circular domains to elongated stripes is observed by varying the polymer charge fraction and the hydrophilic/hydrophobic ratio. The adsorption and the structural changes are driven by a combination of effects such as the dielectric mismatch at the liquid-liquid interface, ionic correlations, hydrophilic-hydrophobic forces, and solvation effects.
  

Block and gradient copoly(2-oxazoline) micelles: striking different on the inside

Herein, we provide a direct proof for differences in the micellar structure of amphiphilic diblock and gradient copolymers, thereby unambiguously demonstrating the influence of monomer distribution along the polymer chains on the micellization behavior[1]. The internal structure of amphiphilic block and gradient co poly(2-oxazolines) based on the hydrophilic poly(2-methyl-2-oxazoline) (PMeOx) and the hydrophobic poly(2-phenyl-2-oxazoline) (PPhOx) was studied in water and water-ethanol mixtures by Small-Angle X-ray Scattering (SAXS), Small-Angle Neutron Scattering (SANS), Static and Dynamic Light Scattering (SLS/DLS), and ¹H NMR spectroscopy. Contrast matching small angle neutron scattering (SANS) experiments revealed that block copolymers form micelles with a uniform density profile of the core. In contrast to popular assumption, the outer part of the core of the gradient copolymer micelles has a distinctly higher density than the middle of the core. We attribute the latter finding to back-folding of chains resulting from hydrophilic-hydrophobic interactions, leading to a new type of micelles that we refer to as micelles with a “bitterball-core” structure.
References
[1] S.K. Filippov,et.al. J. Phys. Chem. Let. 2017, 8, 3800-3804   

Ionic Phase-segregated Liquid Crystal/Polymer Electrolyte for Lithium-ion Transport

Solid polymer electrolytes have been widely studied for applications in lithium-ion batteries due to the potential for improved thermal and electrochemical stability.[1] Poly(ethylene oxide) (PEO) based electrolytes are the most widely studied solid polymer electrolyte. The conductivity of liquid-free PEO-based electrolytes is limited by ethylene oxide chain segmental motion.[2] In the present work, we seek to investigate lithium-ion transport via a different mechanism, lithium-ion transport through ionic domains. Ionic liquid crystal (LC) model small molecules and related single-ion conducting side-chain polymer electrolytes were synthesized. Small-angle X-ray scattering shows that both the LCs and polymers present segregated ionic phases. Impedance spectroscopy measurements indicate that both materials present Vogel-Tammann-Fulcher temperature-dependent conductivity. Current efforts are focused on macroscopic alignment of ionic phase for mitigation of grain boundary effects and studying the effect of anion structure on lithium-ion transport.

[1] L. Long, S. Wang, M. Xiao, and Y. Meng, Journal of Materials Chemistry A 4, 10038 (2016).
[2] W. H. Meyer, Advanced Materials 10, 439 (1992).   

Rotational Symmetry Breaking of Complex Polymeric Macromolecules

A generalized version of Flory mean-field theory, supported by numerical simulations, indicates that polymeric macromolecules in good solvent with branches and cycles can show a sequence of spontaneous rotational symmetry breaking transitions as a function of increasing strength of the self-repulsion (J. Kelly, A. Grosberg and R. Bruinsma, submitted to PRL). We present here the analysis of the effect of including thermal fuctuations in the mean-field theory on the nature of these transitions for specific polymeric structures.   

Scalable Production of Internally-Structured and Surface-Active Polymer Colloids via Flash NanoPrecipitation

Production of polymer nanocolloids with complex internal structures or anisotropic surface functionality on large scales has been a challenge which has hindered their implementation in applications from drug delivery and biosensing to oil and gas recovery. We have developed Flash NanoPrecipitation (FNP) as a scalable process which can be used to generate kilograms of such polymer colloids per day. This solvent-exchange process relies on the rapid mixing of a polymer solution with an antisolvent stream to produce supersaturated conditions which result in monodisperse polymer colloids. Additional complexity can be achieved using the same low-cost equipment by taking advantage of the phase separation of chemically distinct polymers. Janus, core-shell, patchy, and lamellar morphologies have been produced by incorporating a blend of homopolymers or block copolymers into the system, and hydrophilic coronas can be generated via the use of amphiphilic block copolymers. Amphiphilic Janus colloids have recently been created by combining the phase separation behavior of hydrophobic polymers with the adsorption of amphiphilic block copolymers on one domain. These colloids are surface-active in oil-water mixtures and show promise as Pickering emulsion stabilizers.