Session A16: Sequence Controlled Polymers and Structures

Leveraging Repetitive Protein Polymers to Engineer Calcium-Responsive Soft Materials

Ion-responsive polymers are useful for applications ranging from tissue engineering to personal care products. Calcium-responsive Repeats-in-Toxin (RTX) proteins comprise repetitive sequences characterized as GGXGXDXUX, which indicates positions where glycine (G) and aspartic acid (D) are highly conserved. For positions that are less conserved, U represents an aliphatic amino acid and X represents any amino acid. Calcium ions bind near the conserved aspartic acid, triggering a reversible conformation change from a random coil to a β-roll structure. We explored the impact of monomer-scale manipulations of this sequence by creating a panel of RTX proteins with the repetitive sequence GGAGXDTYL, substituting X with various amino acids to study the impact of electrostatic interactions and hydrophobicity on calcium-responsive behavior. We genetically fused modular RTX domains between associative domains to promote hydrogel formation. RTX proteins demonstrate distinct structural and mechanical properties in the presence and absence of calcium, as characterized by turbidometry, circular dichroism, and shear rheology of hydrogels.   

Supramolecular self-assembly of periodic dynamic polymers

Polymer networks formed through dynamic noncovalent or covalent bonds exhibit robust and tunable mechanical properties (e.g., tough, self-healable, and stimuli-responsive). In nature, these networks are often hierarchically-ordered to perform precise functions and assemble via cooperative interactions of many weak bonds as opposed to independent association of a few strong bonds. Here, we use these principles to design linear, flexible polymer chains with periodically-placed and directional dynamic bonds that collectively assemble into supramolecular nanofibers. We show that when the overall molecular weight (M_n) is below the polymer’s critical entanglement molecular weight (M_c), robust self-assembly of supramolecular nanofibers occurs. The formation of nanofibers increases the bulk film modulus by over an order of magnitude and delays the onset of terminal flow by more than 100°C. We also apply these molecular design principles to design a novel shape memory polymer with record-high recovery stress and energy density based on the formation of strain-induced supramolecular nanostructures. While initially polymer chains adopt an amorphous structure (M_n > M_c), during strain the polymer chains align and form strong directional dynamic bonds, which trap the stretched polymer chains in a highly elongated state. These examples highlight the potential to use ordered supramolecular networks formed from polymers with defined backbone periodicity to enhance and control material properties.   

Structure and dynamics of complex random heteropolymers

Random heteropolymers (RHPs) have been a theoretical model system to understand properties of proteins and other sequence-defined polymers. In real life, RHPs have been used to tailor the properties of plastics and solutions for different applications. Recently, however, a new class of complex synthetic RHPs has displayed protein-like properties, which in turn has reinvigorated our imagination of what is possible with RHPs. In this talk, we will present our work on the atomistic structure and dynamics of a class of complex random heteropolymers composed of 4 or more chemically distinct monomers. Our work shows that these polymers display protein-like properties in terms of having a heterogenous surface, frozen backbones, and complex unfolding behavior. These results are also dependent on the backbone chemistry and medium. In the melt state, RHPs display a subtle glass transition, which can be tuned by the composition as well as the chemistry of the counterions. Their adsorption propensity to interfaces is also of much interest. Here we find that their adsorption potential is strongly dependent on temperature due to energy barriers in the reorganization pathway, yet one can accelerate the process by orders of magnitude with minimal amounts of “molecular plasticizers” or by working at liquid-liquid interfaces. To end this talk we will discuss our findings in light of different important applications where such RHPs may be of use.   

Anomalous self-assembly of architecturally semiflexible block copolymers

Block copolymer (BCP) self-assembly is a fundamental process in which incompatible blocks spontaneously form organized microstructures with broad practical applications. Classical understanding is that the domain spacing is limited by the contour length of the polymer backbone. Here, using a combination of molecular design, chemical synthesis, small-/wide-angle X-ray scattering, transmission electron microscopy, and electron tomography, we discover that this molecular picture does not hold for architecturally semiflexible BCPs. For strongly segregated linear–semiflexible bottlebrush–linear triblock copolymers, the size of the bottlebrush domain can be twice the bottlebrush backbone contour length. The mechanism of such anomalous self-assembly is likely that the interfacial repulsion between the incompatible blocks is large enough to pull a part of the linear end blocks into the bottlebrush domain. This effectively increases the bottlebrush domain size. Moreover, the semiflexible bottlebrush widens the regime for the cylinder morphology that is associated with the volume fraction of the end blocks f_C^SFB ∈ (0.10, >0.41). This window is much wider than that for flexible linear BCPs, f_C^F ∈ (0.14, 0.35), and that predicted by the recent self-consistent field theory for linear-bottlebrush BCPs of the same chemistry and molecular architecture. Our experimental findings reveal previously unrecognized mechanisms for the self-assembly of architecturally complex BCPs.   

Predicting Sequence-Structure Correlations of a Copolymer using Evolutionary Computing

The correlations between the sequence of monomers in a copolymer and its three-dimensional (3D) structure is a grand challenge in polymer science and biology. The properties and functions of copolymers depend on their 3D shape that has appeared to be dictated by their monomer sequence. However, the progress towards understanding the sequence-structure-property correlations and their utilization in materials engineering are slow because it is almost impossible to characterize astronomically large number of possible sequences of a copolymer using traditional experimental and simulation methods. To address this problem, here, we combine evolutionary computing and coarse-grained molecular dynamics (CGMD) simulation and study the sequence-structure correlations of a model AB type copolymer system. The CGMD based evolutionary algorithm (EA) employs evolutionary mechanisms – elitism, selection, crossover and mutations successively to generate new polymer sequences that have superior properties. The CGMD based evolutionary algorithm (EA) screens the sequence space of a single chain copolymer efficiently and identifies wide range of single molecule structures including extremal radius of gyrations (R_gs). The data is utilized to establish new sequence-structure correlations of a copolymer. We further report correlations between monomer sequence and multimolecular assemblies and bulk phases of copolymers. The work demonstrate how monomer level sequence control can be utilized to design polymer microstructure.   

Conformations of Sequence-Specific Polyampholytes with Zero and Nonzero Global Charges

This work combines scaling theory and coarse-grained simulations to reveal how monomer sequences control single-chain conformational behaviors of PAs in salt-free solutions. We consider PAs with the charge statistics given by the first-order Markov process. In these PAs, charge “blockiness” is defined by the correlation λ along the chain. Markov sequences cover a wide spectrum of sequences ranging from alternating (λ = −1) to ideally random (λ = 0) to diblock PA for the charge-balanced case or (homo)polyelectrolyte for the charge-imbalanced case (λ → 1).

In the first part of the work, we consider globally neutral PAs with zero net charges. We demonstrate that any sufficiently long PAs form globules under theta solvent conditions, but the physical properties of these globules are strongly sequence-dependent. The higher the clustering of the opposite charges, i.e., the value of λ, the higher the density of the PA globule. We distinguish three different scaling regimes, which correspond to almost alternating, substantially random. and highly blocky polyampholytes. The predicted scaling laws for the dimensions of the PA globules in these regimes are confirmed coarse-grained molecular dynamics simulations.

The second part of the present work is devoted to charge-imbalanced Markov PAs. In this case, the global charge of PAs results in their pearl-necklace, tadpole-like, or fully stretched (polyelectrolyte-like) conformations. The developed scaling approach shows that the formation, structure, and dimensions of PA necklaces are controlled by the charge statistics. The sequence-dependent structure of PA necklaces is supported by coarse-grained simulations. The present work provides an insight into sequence-encoded conformations and conformational transitions in single-chain synthetic PAs and intrinsically disordered proteins (IDPs).    

Polymers constructed from the self-assembly of computationally designed coiled-coil peptide via designed assembly pathways

A series of net positively charged 29 amino-acids peptides were computationally designed to self-assemble into tetrameric, anti-parallel coiled-coil peptide bundles, or 'bundlemers'. Specifically, peptides with a net +4 charge were synthesized with either a thiol or maleimide present at the peptide N-terminus.  In order to form polymer chains, peptides are firstly self-assembled into coiled-coil bundlemer particles. Then the thiol and maleimide bundles are subsequently chemically conjugated to form rigid peptide bundle chains. This pathway is called a physical-chemical pathway to chain formation. Another chemical-physical pathway is possible to fabricate the rod-like chains where the two peptides are first dissolved in organic solvent and conjugated together. Subsequent water addition causes the peptides to self-assemble into coiled-coil bundlemer chains. Transmission electron microscopy (TEM) was used to investigate the length distribution of peptide rods, and Small-angle X-ray scattering (SAXS) was utilized to characterize the size and structure of the nanostructure at different solvent conditions. The merits of both pathways in the control of chain formation and the control of amino acid sequence that constitutes the resultant chains will be discussed.   

Separating Sequence-specific from Composition Effects on Adsorption of Amphiphilic Polypeptoids at Fluid-Fluid Interfaces

Adsorption of surface-active polymers to fluid interfaces involves the formulation of stable solutions or suspensions and the subsequent equilibration of an adjacent interface.  By measuring adsorption at long times as equilibrium is approached, interfacial properties are related to design properties like block structure and molecular weight.  However,  polydispersity and large molecular weight making it difficult to determine the impact of subtle changes along the polymer chain on interfacial properties.  We avoid these difficulties by choosing four sequences of compositionally uniform, amphiphilic polypeptoids (4 kg/mol).  Subtle variations in sequence possessed by these four isomers (tapered, inverse tapered, blocky and distributed) do not manifest in distinct values of surface tension or dilatational modulus at the air/solution interface when adsorbing from a selective solvent (25/75 ACN/water). When the polypeptoid solutions are rinsed out with water, a nonsolvent, while the adjacent polypeptoid-laden bubble is preserved more chains are driven to the interface, and surface pressure increases rapidly.  Variation in chain sequence appears in the measurement of dilatational elasticity only after processing has driven the adsorption of additional chains to the surface.   

Strategy for Synthesis of Statistically Sequence-Controlled Uniform PLGA, and Effects of Sequence Distribution on Interaction and Drug Release Properties

Extensive studies have been conducted to elucidate the effects of such parameters as molecular weight, polydispersity and composition on the controlled release properties of poly(D,L-lactic-co-glycolic acid) (PLGA). However, studies dealing with the effect of monomer sequence distribution have been sparse mainly because of the difficulty of precisely controlling monomer sequence in PLGA. Herein, we present a new semi-batch copolymerization strategy (i.e., a slow simultaneous comonomer addition method) that enables production of statistically sequence-controlled “uniform PLGA” polymers through control of the rate of comonomer addition. Using this method, a series of PEG-PLGA samples having an identical molecular weight and composition but different sequence distributions (uniform vs. gradient) were prepared. The properties of these materials (PEG crystallization/melting, hygroscopicity, aqueous sol-gel transition, drug release kinetics) were found to significantly vary, demonstrating that sequence control only at the statistical level still significantly influences the properties of PLGA. Most notably, uniform PLGA exhibited the more sustained drug release behavior, compared to gradient PLGA.   

Understanding the structure-property relationship of polymer-grafted nanosheets upon crumpling

Understanding the crumpling behavior of polymer-grafted nanosheet materials is of fundamental importance in engineering and technological applications. Here, we report the results of a systematic coarse-grained molecular dynamics (CG-MD) simulation study of the crumpling process of polymer-grafted graphene sheets at various polymer grafting density. We find that polymer grafted graphene sheets with larger grafting density exhibit more wrinkling in equilibrium than pristine graphene sheet and sheets with smaller grafting density. By evaluating the evolution of potential energy of the sheet during the crumpling process, our results show that the grafted polymer chains significantly reduce the adhesion properties of the graphene sheets. Notably, various structural properties of the crumpled polymer grafted graphene sheets, including the radius of gyration, hydrodynamic radius, and intrinsic viscosity, can be quantitatively described by a power-law scaling relationship about the radius of confining sphere during the crumpling process. Moreover, the shape descriptor analysis shows less self-folding and self-adhering upon crumpling for sheets with larger grafting density. In addition, the evaluation of stress distribution of the sheet further reveals the stress heterogeneity arising from grafted polymers and crumpling. Our findings highlight the critical role of grafted polymer in the crumpling process of graphene sheets, which has significant implications for the tailored design of crumpled polymer functionalized nanosheets.