Stimuli-Responsive Peptide-Based ABA-Triblock Copolymers: Unique Morphology Transitions With pH

Genetically Fused Resilin-like Polypeptide-Coiled Coil Bundlemer Conjugates Exhibit Tunable Multistimuli-Responsiveness and Undergo Nanofibrillar Assembly

Peptide-based materials are diverse candidates for self-assembly into modularly designed and stimuli-responsive nanostructures with precisely tunable compositions. Here, we genetically fused computationally designed coiled coil-forming peptides to the N- and C-termini of compositionally distinct multistimuli-responsive resilin-like polypeptides (RLPs) of various lengths. The successful expression of these hybrid polypeptides in bacterial hosts was confirmed through techniques such as gel electrophoresis, mass spectrometry, and amino acid analysis. Circular dichroism spectroscopy and ultraviolet-visible turbidimetry demonstrated that despite the fusion of disparate structural and responsive units, the coiled coils remained stable in the hybrid polypeptides, and the sequence-encoded differences in thermoresponsive phase separation of the RLPs were preserved. Cryogenic transmission electron microscopy and coarse-grained modeling showed that after thermal annealing in solution, the hybrid polypeptides adopted a closed loop conformation and assembled into nanofibrils capable of further hierarchically organizing into cluster structures and ribbon-like structures mediated by the self-association tendency of the RLPs.

Nanoscale polymer discs, toroids and platelets: a survey of their syntheses and potential applications

Polymer self-assembly has become a reliable and versatile workhorse to produce polymeric nanomaterials. With appropriate polymer design and monomer selection, polymers can assemble into shapes and morphologies beyond well-studied spherical and cylindrical micellar structures. Steadfast access to anisotropic polymer nanoparticles has meant that the fabrication and application of 2D soft matter has received increasing attention in recent years. In this review, we focus on nanoscale polymer discs, toroids, and platelets: three morphologies that are often interrelated and made from similar starting materials or common intermediates. For each morphology, we illustrate design rules, and group and discuss commonly used self-assembly strategies. We further highlight polymer compositions, fundamental principles and self-assembly conditions that enable precision in bottom-up fabrication strategies. Finally, we summarise potential applications of such nanomaterials, especially in the context of biomedical research and template chemistry and elaborate on future endeavours in this space.

Chirality in peptide-based materials: From chirality effects to potential applications

Chirality is ubiquitous in nature with primary cellular functions that include construction of right-/left-handed helix and selective communications among diverse biomolecules. Of particularly intriguing are the chiral peptide-based materials that can be deliberately designed to change physicochemistry properties via tuning peptide sequences. Critically, understanding their chiral effects are fundamental for the development of novel materials in chemistry and biomedicine fields. Here, we review recent researches on chirality in peptide-based materials, summarizing relevant typical chiral effects towards recognition, amplification, and induction. Driven forces for the chiral discrimination in affinity interaction as well as the handedness preferences in supramolecular structure formation at both the macroscale and microscale are illustrated. The implementation of such chirality effects of artificial copolymers, assembled aggregates and their composites in the fields of bioseparation and bioenrichment, cell incubation, protein aggregation inhibitors, chiral smart gels, and bionic electro devices are also presented. At last, the challenges in these areas and possible directions are pointed out. The diversity of chiral roles in the origin of life and chirality design in different organic or composite systems as well as their applications in drug development and chirality detection in environmental protection are discussed.

Synthesis and Characterization of a Leucine-Based Block Co-Polypeptide: The Effect of the Leucine Zipper on Self-Assembly

The self-assembly behavior of an ABC triblock copolypeptide consisting of poly(ethylene oxide-b-(leucine-s-valine)-b-lysine) (PEO-PLV-PK) was examined via dynamic light scattering in dilute aqueous solution. Leucine is a hydrophobic, α-helix forming polypeptide that exhibits a "zipper effect" in coiled-coil dimers. We hypothesize that the specific interaction afforded by the leucine zipper dominates the thermodynamics of self-assembly through the side-by-side ordering of α-helices, which drives vesicle formation in a polymer with only 6 wt % hydrophobic content. Additionally, a multitude of assembly sizes and morphologies were attainable from a single polymer, depending on the solution processing method. Thermodynamic effects of the leucine zipper can be interpreted, in part, from solubility parameters determined from molecular modeling. The combination of synthesis, solvent processing, and computational studies helps to elucidate the thermodynamic effects of this unique assembly motif on classical self-assembly processes.

Hierarchical Fractal Assemblies from Poly(ethylene oxide- b-lysine- b-leucine)

Poly(ethylene oxide₄₃- b-lysine₆₂- b-leucine₇₂) (wherein subscripts denote the degree of polymerization) was synthesized via ring-opening polymerization of N-carboxyanhydrides using an amine-terminated poly(ethylene oxide) macroinitiator, with polypeptide blocks produced by sequential monomer addition. Infrared and circular dichroism spectroscopy indicated that the peptide blocks in this polymer formed α-helices in the solid and solution states, respectively. In the aqueous solution, this polymer self-assembled into spherical micelles with a hydrodynamic radius of approximately 90 nm at concentrations between 0.05 and 0.20% w/w and pH values between 2 and 6.5. Upon preparation of transmission electron microscopy (TEM) grids, the micelles at pH 2 underwent hierarchical assembly to produce fractal assemblies, whereas small clusters were observed for micellar solutions at pH 6.5. Cryogenic-TEM of solutions showed spherical micelles, and dynamic light scattering showed no large (∼1 μm) aggregates in the solution, which suggests that fractal formation was a result of the drying process, and that fractals were not present in the solution. This system provides a facile route to nanostructured surfaces, which can be used for applications such as modulating cell adhesion or promoting the growth of neurons.

Formation of disk-like micelles of triblock copolymers in frustrating solvents

Coil–coil block copolymers rarely self-assemble into flat low-curvature micelles due to the lack of proper interchain association. Here, we report a facile route to prepare disk-like micelles through the self-assembly of amphiphilic polystyrene-b-polybutadiene-b-poly(2-vinylpyridine) triblock copolymers in a mixture of acetone and cyclohexane, which shows distinct selectivity towards the PS, PB and P2VP blocks. Subtle solvation/aggregation of these blocks in this frustrating solvent system provides access to low-curvature micellar structures, and thus favors the formation of uniform disk-like micelles. Further variation of the volume ratio of the mixed solvents also leads to the emergence of other interesting morphologies, including disk arrays, disk clusters and perforated disk-like micelles. This work provides a complementary insight into the solution self-assembly of block copolymers in a view of selective solvents and demonstrates a distinctive pathway to unconventional micellar nanostructures through the use of complex solvent systems.

Self-assembly of amphiphilic triblock copolymers in a frustrating solvent system leads to the formation of various low-curvature micellar structures.

UV-induced vesicle to micelle transition: a mechanistic study

The morphology of self-assembled block copolymer aggregates is highly dependent on the relative volume fraction of the hydrophobic block.

Probing Membrane Hydration at the Interface of Self-Assembled Peptide Amphiphiles Using Electron Paramagnetic Resonance

The relative hydrophilicity at the interface of a nanoparticle was measured utilizing electron paramagnetic resonance (EPR) spectroscopy. The supramolecular structure was assembled from spin-labeled peptide amphiphiles (PA) derived from N-carboxy anhydrides (NCA). Cyanuric chloride, or 2,4,6-trichloro-1,3,5-triazine (TCT), was used as a modular platform to synthesize the spin-labeled, lipid-mimetic macroinitiator used for the ring-opening polymerization of γ-benzyl-l-glutamic acid NCA to produce polyglutamate-b-dodecanethiol₂. Through static and dynamic light scattering, as well as transmission electron microscopy, PAs with DP of 50 and 17 were shown to assemble into stable nanoparticles with an average hydrodynamic radius of 117 and 84 nm, respectively. Continuous wave EPR spectroscopy revealed that the mobility parameter (h_-1/h₀) and 2A_iso of the nitroxide radical increased with increasing pH, in concert with the deprotonation of the PE side chains and associated helix-coil transition. These results are consistent with an increase in the relative hydration and polarity at the nanoparticle interface, which would be dependent on the secondary structure of the polypeptide. This research suggests that a pH stimulus could be used to facilitate water diffusion through the membrane.

Polypeptide self-assemblies: nanostructures and bioapplications

Polypeptide copolymers can self-assemble into diverse aggregates. The morphology and structure of aggregates can be varied by changing molecular architectures, self-assembling conditions, and introducing secondary components such as polymers and nanoparticles. Polypeptide self-assemblies have gained significant attention because of their potential applications as delivery vehicles for therapeutic payloads and as additives in the biomimetic mineralization of inorganics. This review article provides an overview of recent advances in nanostructures and bioapplications related to polypeptide self-assemblies. We highlight recent contributions to developing strategies for the construction of polypeptide assemblies with increasing complexity and novel functionality that are suitable for bioapplications. The relationship between the structure and properties of the polypeptide aggregates is emphasized. Finally, we briefly outline our perspectives and discuss the challenges in the field.

Secondary structures of synthetic polypeptide polymers

Synthetic peptide-based polymers can fold into different secondary structures in the same way as do proteins. This review article presents how tuning the polypeptide secondary structure could be a key step to modulate various properties in advanced polymeric materials (size, rigidity, self-assembly,etc.).

Synthesis and Self-Assembled Behavior of pH-Responsive Chiral Liquid Crystal Amphiphilic Copolymers Based on Diosgenyl-Functionalized Aliphatic Polycarbonate

The morphological control of polymer micellar aggregates is an important issue in applications such as nanomedicine and material science. Stimuli responsive soft materials have attracted significant attention for their well-controlled morphologies. However, despite extensive studies, it is still a challenge to prepare nanoscale assemblies with responsive behaviors. Herein, a new chiral liquid crystal (LC) aliphatic polycarbonate with side chain bearing diosgenyl mesogen, named mPEG₄₃-PMCC₂₅-P(MCC-DHO)₁₅, was synthesized through the ring-opening polymerization and coupling reaction. The self-assembled behavior of the LC copolymer was explored. In aqueous solution, the functionalized copolymer could self-organize into different nanostructures with changing pH value, such as nanospheres and nanofibers. This would offer new possibilities in the design of nanostructured organic materials.

Gas-Responsive Polymers

Gas-responsive polymers have inspired much interest over the past ten years. Gas triggers can interact with functionalities on polymer chains and, thus, modulate their chain structures, architectures, and aggregation states. This review summarizes the latest research progresses in the theme of developing different gas triggers for fine control over some critical properties of polymers, as well as their potential applications in various areas. We focus on the interactions/reactions between gases and gas-responsive functionalities of polymers and highlight some state-of-art developments, which provided good insight and understanding of each particular gas-responsive polymer. We also offer a perspective point of view on future research directions on gas-responsive polymers, both in fundamental studies and in potential application developments.

Biosynthetic Polymers as Functional Materials

The synthesis of functional polymers encoded with biomolecules has been an extensive area of research for decades. As such, a diverse toolbox of polymerization techniques and bioconjugation methods has been developed. The greatest impact of this work has been in biomedicine and biotechnology, where fully synthetic and naturally derived biomolecules are used cooperatively. Despite significant improvements in biocompatible and functionally diverse polymers, our success in the field is constrained by recognized limitations in polymer architecture control, structural dynamics, and biostabilization. This Perspective discusses the current status of functional biosynthetic polymers and highlights innovative strategies reported within the past five years that have made great strides in overcoming the aforementioned barriers.

Poly(PEGA)-b-poly(L-lysine)-b-poly(L-histidine) Hybrid Vesicles for Tumoral pH-Triggered Intracellular Delivery of Doxorubicin Hydrochloride

A series of poly(ethylene glycol) methyl ether acrylate-block-poly(L-lysine)-block-poly(L-histidine) [p(PEGA)30-b-p(Lys)25-b-p(His)n] (n = 25, 50, 75, 100) triblock copolypeptides were designed and synthesized for tumoral pH-responsive intracellular release of anticancer drug doxorubicin hydrochloride (Dox). The tumoral acidic pH-responsive hybrid vesicles fabricated were stable at physiological pH 7.4 and could gradually destabilize in acidic pH as a result of pH-induced swelling of the p(His) block. The blank vesicles were nontoxic over a wide concentration range (0.01-100 μg/mL) in normal cell lines. The tumor acidic pH responsiveness of these vesicles was exploited for intracellular delivery of Dox. Vesicles efficiently encapsulated Dox, and pH-induced destabilization resulted in the controlled and sustained release of Dox in CT26 murine cancer cells, and dose-dependent cytotoxicity. The tumor-specific controlled release Dox from vesicles demonstrates this system represents a promising theranostic agent for tumor-targeted delivery.

Impact of chain microstructure on solution and thin film self-assembly of PCHD-based semi-flexible/flexible diblock copolymers

Self-assembly of semi-flexible/flexible block copolymers in a selective solvent is examined using a set of diblock copolymers where the chain microstructure of the semi-flexible block is manipulated in order to tune chain stiffness. Conceptually, the reduced conformational space of the semi-flexible block is anticipated to alter the way the chains pack, potentially changing the structure of self-assembled aggregates in comparison to flexible diblock copolymer analogs. Semi-flexible/flexible diblock copolymers comprised of poly(styrene)-block-poly(1,3-cyclohexadiene) (PS-b-PCHD) having systematic changes in chain microstructure, as captured by the ratio of 1,4/1,2-linkages between cyclohexenyl repeat units, and molecular weight of the PCHD blocks were synthesized using anionic polymerization. These diblocks were dissolved in tetrahydrofuran (THF), which is a preferential solvent for PS, and the structures formed were examined using laser light scattering and complementary imaging techniques. Results show that PS-b-PCHD copolymers with a chain microstructure of 90% 1,4/10% 1,2 linkages between cyclohexenyl repeat units (referred to simply as 90/10) are able to micellize, forming spherical structures, while diblocks of 70/30 and 50/50 1,4-to-1,2 ratios remain as single chains and ill-defined aggregates, respectively, when dissolved in THF. With inferences drawn from simple structural models, we speculate that this self-assembly behavior arises due to the change in the chain configuration with increasing content of 1,2-links in the backbone. This renders the chain with higher 1,2 content incapable of swelling in response to solvent and unable to pack into well-defined self-assembled structures.

pH-switchable self-assembled materials

Self-assembled materials, which are able to respond to external stimuli, have been extensively studied over the last decades. A particularly exciting stimulus for a wide range of biomedical applications is the pH value of aqueous solutions, since deprotonation-protonation events are crucial for structural and functional properties of biopolymers. In living cells and tissues, intra- and extracellular pH values are stringently regulated, but can deviate from pH neutral as observed for example in tumorous, inflammatory sites, in endocytic pathways, and specific cellular compartments. By using a pH-switch as a stimulus, it is thereby possible to address specific targets in order to cause a programmed response of the supramolecular material. This strategy has not only been successfully applied in fundamental research but also in clinical studies. In this feature article, current strategies that have been used in order to design materials with pH-responsive properties are illustrated. This discussion only addresses selected examples from the last four years, the self-assembly of polymer-based building blocks, assemblies emerging from small molecules including surfactants or derived from biological macromolecules, and finally the controlled self-assembly of oligopeptides.

Oxygen and Carbon Dioxide Dual Responsive Nanoaggregates of Fluoro- and Amino-Containing Copolymer

We report herein a novel approach for preparing CO₂- and O₂-responsive polymer nanoaggregates. The polymer, synthesized via atom transfer radical polymerization (ATRP), has one hydrophilic poly(ethylene glycol) (PEG) block, and the other hydrophobic block composed of 88 randomly distributed units of CO₂-responsive N,N-diethylaminoethyl methacrylate (DEA) and 43 units of O₂-responsive 2,2,2-trifluoroethyl methacrylate (FMA). The amphiphilic copolymer self-assembled into vesicular nanoaggregates in water. With O₂ bubbling, the vesicles expanded eight times in volume. With CO₂ bubbling, the vesicular morphology collapsed and transformed into a small spherical micelle. The dual gas-responsivity significantly expanded the scope in designing stimuli-responsive materials and processes.

Polyprodrug amphiphiles: hierarchical assemblies for shape-regulated cellular internalization, trafficking, and drug delivery

Solution self-assembly of block copolymers (BCPs) typically generates spheres, rods, and vesicles. The reproducible bottom-up fabrication of stable planar nanostructures remains elusive due to their tendency to bend into closed bilayers. This morphological vacancy renders the study of shape effects on BCP nanocarrier-cell interactions incomplete. Furthermore, the fabrication of single BCP assemblies with built-in drug delivery functions and geometry-optimized performance remains a major challenge. We demonstrate that PEG-b-PCPTM polyprodrug amphiphiles, where PEG is poly(ethylene glycol) and PCPTM is polymerized block of reduction-cleavable camptothecin (CPT) prodrug monomer, with >50 wt % CPT loading content can self-assemble into four types of uniform nanostructures including spheres, large compound vesicles, smooth disks, and unprecedented staggered lamellae with spiked periphery. Staggered lamellae outperform the other three nanostructure types, exhibiting extended blood circulation duration, the fastest cellular uptake, and unique internalization pathways. We also explore shape-modulated CPT release kinetics, nanostructure degradation, and in vitro cytotoxicities. The controlled hierarchical organization of polyprodrug amphiphiles and shape-tunable biological performance opens up new horizons for exploring next-generation BCP-based drug delivery systems with improved efficacy.

A novel diblock copolymer with a supramolecular polymer block and a traditional polymer block: preparation, controllable self-assembly in water, and application in controlled release

A novel diblock copolymer with a hydrophobic supramolecular polymer block and a hydrophilic traditional polymer block has been prepared. Control over the chain length ratio of the two blocks is obtained by simply changing the concentration proportion of the monomer of the supramolecular polymer block to the traditional polymer block in solution. When the chain length ratio of the two blocks is changed, the formation of various self-assembly morphologies is achieved.

Endolytic, pH-responsive HPMA-b-(L-Glu) copolymers synthesized via sequential aqueous RAFT and ring-opening polymerizations

A facile synthetic pathway for preparing block copolymers with pH-responsive L-glutamic acid segments for membrane disruption is reported. Aqueous reversible addition-fragmentation chain transfer (aRAFT) polymerization was first used to prepare biocompatible, nonimmunogenic poly[N-(2-hydroxypropyl)methacrylamide]. This macro chain transfer agent (CTA) was then converted into a macroinitiator via simultaneous aminolysis and thiol-ene Michael addition using the primary amine substituted N-(3-aminopropyl)methacrylamide. This macroinitiator was subsequently utilized in the ring-opening polymerization of the N-carboxyanhydride monomer of γ-benzyl-L-glutamate. After deprotection, the pH-dependent coil-to-helix transformations of the resulting HPMA-b-(L-Glu) copolymers were monitored via circular dichroism spectroscopy. HPMA segments confer water solubility and biocompatibility while the L-glutamic acid repeats provide reversible coil-to-helix transitions at endosomal pH values (~5-6). The endolytic properties of these novel [HPMA-b-(L-Glu)] copolymers and their potential as modular components in drug carrier constructs was demonstrated utilizing red blood cell hemolysis and fluorescein release from POPC vesicles.

Stimuli-responsive copolymer solution and surface assemblies for biomedical applications

Stimuli-responsive polymeric materials is one of the fastest growing fields of the 21st century, with the annual number of papers published more than quadrupling in the last ten years. The responsiveness of polymer solution assemblies and surfaces to biological stimuli (e.g. pH, reduction-oxidation, enzymes, glucose) and externally applied triggers (e.g. temperature, light, solvent quality) shows particular promise for various biomedical applications including drug delivery, tissue engineering, medical diagnostics, and bioseparations. Furthermore, the integration of copolymer architectures into stimuli-responsive materials design enables exquisite control over the locations of responsive sites within self-assembled nanostructures. The combination of new synthesis techniques and well-defined copolymer self-assembly has facilitated substantial developments in stimuli-responsive materials in recent years. In this tutorial review, we discuss several methods that have been employed to synthesize self-assembling and stimuli-responsive copolymers for biomedical applications, and we identify common themes in the response mechanisms among the targeted stimuli. Additionally, we highlight parallels between the chemistries used for generating solution assemblies and those employed for creating copolymer surfaces.