Molecular Dynamics Simulations of Linear and Cyclic Amphiphilic Polymers in Aqueous and Organic Environments

Topology-controlled self-assembly of amphiphilic block copolymers

Contemporary synthetic chemistry approaches can be used to yield a range of distinct polymer topologies with precise control. The topology of a polymer strongly influences its self-assembly into complex nanostructures however a clear mechanistic understanding of the relationship between polymer topology and self-assembly has not yet been developed. In this work, we use atomistic molecular dynamics simulations to provide a nanoscale picture of the self-assembly of three poly(ethylene oxide)-poly(methyl acrylate) block copolymers with different topologies into micelles. We find that the topology affects the ability of the micelle to form a compact hydrophobic core, which directly affects its stability. Also, we apply unsupervised machine learning techniques to show that the topology of a polymer affects its ability to take a conformation in response to the local environment within the micelles. This work provides foundations for the rational design of polymer nanostructures based on their underlying topology.

The pH Response of a Peptoid Oligomer

Polypeptoids are N-substituted glycine polymers, which differ from peptides in the placement of the side chain on the amide nitrogen rather than the C_α carbon. A peptoid with a chiral side chain containing both an aromatic group and carboxylic acid has a structure that responds to pH changes. All-atom molecular dynamics simulations using a force field specifically tuned for peptoids were carried out with an advanced sampling method for the peptoid (S)-N-(1-carboxy-2-phenylethyl)glycine in the high and low pH limits. The simulations show that the structure changes from mostly cis amide bonds at low pH to mostly trans bonds at high pH. The structural changes are driven by side chain-backbone hydrogen bonds at low pH and side chain repulsions and increased water contact at high pH.

Characterizing the Interplay between Polymer Solvation and Conformation

Conformational transitions of flexible molecules, especially those driven by hydrophobic effects, tend to be hindered by desolvation barriers. For such transitions, it is thus important to characterize and understand the interplay between solvation and conformation. Using specialized molecular simulations, here we perform such a characterization for a hydrophobic polymer solvated in water. We find that an external potential, which unfavorably perturbs the polymer hydration waters, can trigger a coil-to-globule or collapse transition, and that the relative stabilities of the collapsed and extended states can be quantified by the strength of the requisite potential. Our results also provide mechanistic insights into the collapse transition, highlighting that the bottleneck to polymer collapse is the formation of a sufficiently large cluster, and the collective dewetting of such a cluster. We also study the collapse of the hydrophobic polymer in octane, a nonpolar solvent, and interestingly, we find that the mechanistic details of the transition are qualitatively similar to that in water.

Polarity of Cationic Lignin Polymers: Physicochemical Behavior in Aqueous Solutions and Suspensions

The structure of cationic monomers can significantly impact the polarity of lignin after polymerization. Cationic hydrolysis lignin (CHL) polymers were produced by polymerizing hydrolysis lignin (HL) with [3-(methacryloylamino)propyl] trimethylammonium chloride (MAPTAC) or [2-(methacryloyloxy)ethyl] trimethyl ammonium chloride (METAC). The METAC monomer has an oxygen atom, with larger electronegativity, in its molecular structure, whereas the MAPTAC monomer contains a nitrogen atom, as well as an extra nonpolar CH₂ group, facilitating investigation into the effects of the polarity of CHLs on their physicochemical performance in an aqueous system. CHL polymers are analyzed and their interactions with clay particles are determined in colloidal systems. CHLs are designed to have similar charge densities (2.1-2.2 mmol g^-1) and molecular weights (55000-60000 g mol^-1 ). The hydrodynamic radius (H_y) and radius of gyration, (R_g) of HL-METAC are larger than those of HL-MAPTAC, implying a more 3-dimensional structure of HL-METAC in aqueous solution. The stability ratio of kaolin particles affirms the better performance of HL-METAC in comparison to HL-MAPTAC, which reflects the better flocculation efficiency of HL-METAC. The results also reveal that salt and urea aqueous solutions affect the H_y, R_g, and configuration of CHL polymers, which alters the flocculation efficiency of HL-METAC and HL-MAPTAC polymers in kaolin suspensions.

Modular and rapid access to amphiphilic homopolymers via successive chemoselective post-polymerization modification

A modular and simplified post-polymerization modification strategy is developed for the synthesis of amphiphilic homopolymers.

Computational and experimental approaches for investigating nanoparticle-based drug delivery systems

Most therapeutic agents suffer from poor solubility, rapid clearance from the blood stream, a lack of targeting, and often poor translocation ability across cell membranes. Drug/gene delivery systems (DDSs) are capable of overcoming some of these barriers to enhance delivery of drugs to their right place of action, e.g. inside cancer cells. In this review, we focus on nanoparticles as DDSs. Complementary experimental and computational studies have enhanced our understanding of the mechanism of action of nanocarriers and their underlying interactions with drugs, biomembranes and other biological molecules. We review key biophysical aspects of DDSs and discuss how computer modeling can assist in rational design of DDSs with improved and optimized properties. We summarize commonly used experimental techniques for the study of DDSs. Then we review computational studies for several major categories of nanocarriers, including dendrimers and dendrons, polymer-, peptide-, nucleic acid-, lipid-, and carbon-based DDSs, and gold nanoparticles. This article is part of a Special Issue entitled: Membrane Proteins edited by J.C. Gumbart and Sergei Noskov.

ATRP Synthesis of Sunflower Polymers using Cyclic Multimacroinitiators

Polymers with advanced architectures can now be readily and reproducibly synthesized using controlled living polymerization. These materials are attractive as potential drug carriers due to their tunable size, versatile methods of drug incorporation and release, and ease of functionalization with targeting ligands. In this work, we report the design and development of macrocyclic brush, or "sunflower," polymers, synthesized by controlled radical polymerization of hydrophilic "petals" from a cyclic multimacroinitiator "core." These nanostructures can be synthesized with low polydispersity and controlled sizes depending on polymerization time. We further demonstrate that folate-functionalized sunflower polymers facilitate receptor-mediated uptake into cancer cells. These materials therefore show potential as drug carriers for anti-cancer therapies.