Bioinspired functional block copolymers

Thermoresponsive Self-Assembly of Twofold Fluorescently Labeled Block Copolymers in Aqueous Solution and Microemulsions

A nonionic double hydrophilic block copolymer with a long permanently hydrophilic and a small thermoresponsive block is synthesized by reversible addition-fragmentation chain-transfer polymerization (RAFT). By employing a specifically designed chain-transfer agent, the polymer is functionalized with complementary end groups which are suited for Förster resonance energy transfer (FRET). The end group attached to the permanently hydrophilic block of poly(N,N-dimethylacrylamide) pDMAm is designed as a permanently hydrophobic segment ("sticker") comprising a long alkyl chain and the 4-aminonaphthalimide fluorophore. The other end attached to the thermoresponsive block of poly(N-isopropylacrylamide) pNiPAm incorporates a coumarin fluorophore. The temperature-dependent self-assembly of the twofold fluorescently labeled copolymer is studied in pure aqueous solution as well as in an o/w microemulsion by several techniques including turbidimetry, dynamic light scattering (DLS), and fluorescence spectroscopy. It is compared to the behaviors of the analogous twofold-labeled pDMAm and pNiPAm homopolymer references. The findings indicate that the block copolymer behaves as a polymeric surfactant at low temperatures, with one relatively small hydrophobic end block and an extended hydrophilic chain forming "hairy micelles". At elevated temperatures above the LCST phase transition of the pNiPAm block, however, the copolymer behaves as an associative telechelic polymer with two nonsymmetrical hydrophobic end blocks, which do not mix. Thus, instead of a network of bridged "flower micelles", large dynamic aggregates are formed. These are connected alternatingly by the original micellar cores as well as by clusters of the collapsed pNiPAm blocks. This type of structure is even more favored in the o/w microemulsion than in pure aqueous solution, as the microemulsion droplets constitute an attractive anchoring point for the hydrophobic dodecyl sticker but not for the collapsed pNiPAm chains.

Hierarchical Nanomaterials Assembled from Peptoids and Other Sequence-Defined Synthetic Polymers

In nature, the self-assembly of sequence-specific biopolymers into hierarchical structures plays an essential role in the construction of functional biomaterials. To develop synthetic materials that can mimic and surpass the function of these natural counterparts, various sequence-defined bio- and biomimetic polymers have been developed and exploited as building blocks for hierarchical self-assembly. This review summarizes the recent advances in the molecular self-assembly of hierarchical nanomaterials based on peptoids (or poly-N-substituted glycines) and other sequence-defined synthetic polymers. Modern techniques to monitor the assembly mechanisms and characterize the physicochemical properties of these self-assembly systems are highlighted. In addition, discussions about their potential applications in biomedical sciences and renewable energy are also included. This review aims to highlight essential features of sequence-defined synthetic polymers (e.g., high stability and protein-like high-information content) and how these unique features enable the construction of robust biomimetic functional materials with high programmability and predictability, with an emphasis on peptoids and their self-assembled nanomaterials.

Supramolecular polymer bottlebrushes

The field of supramolecular chemistry has long been known to generate complex materials of different sizes and shapes via the self-assembly of single or multiple low molar mass building blocks. Matching the complexity found in natural assemblies, however, remains a long-term challenge considering its precision in organizing large macromolecules into well-defined nanostructures. Nevertheless, the increasing understanding of supramolecular chemistry has paved the way to several attempts in arranging synthetic macromolecules into larger ordered structures based on non-covalent forces. This review is a first attempt to summarize the developments in this field, which focus mainly on the formation of one-dimensional, linear, cylindrical aggregates in solution with pendant polymer chains - therefore coined supramolecular polymer bottlebrushes in accordance with their covalent equivalents. Distinguishing by the different supramolecular driving forces, we first describe systems based on π-π interactions, which comprise, among others, the well-known perylene motif, but also the early attempts using cyclophanes. However, the majority of reported supramolecular polymer bottlebrushes are formed by hydrogen bonds as they can for example be found in linear and cyclic peptides, as well as so called sticker molecules containing multiple urea groups. Besides this overview on the reported motifs and their impact on the resulting morphology of the polymer nanostructures, we finally highlight the potential benefits of such non-covalent interactions and refer to promising future directions of this still mostly unrecognized field of supramolecular research.

The influence of directed hydrogen bonds on the self-assembly of amphiphilic polymers in water

HYPOTHESIS

Molecules forming directed intermolecular hydrogen bonds, such as the well-known benzene-1,3,5-tricarboxamides (BTA) motif, are known to self-assemble into long fibrous structures. However, only a few of these systems have so far demonstrated the ability to form such anisotropic nanostructures, if they are combined with hydrophilic polymers to create an amphiphilic material. Here, we designed BTA-polymer conjugates to investigate whether the directionality of the hydrogen bonds or the ratio of hydrophobic to hydrophilic parts of the molecule, and thus the packing parameter, is decisive for obtaining anisotropic supramolecular structures in water.

EXPERIMENTS

Poly(ethylene glycol) was conjugated to BTA moieties with varying lengths of hydrophobic alkyl spacers ranging from two to twelve methylene units. The resulting amphiphilic materials were characterized in aqueous solution by light and small-angle neutron scattering, analytical ultracentrifugation, and cryo-transmission electron microscopy.

FINDINGS

While spherical micelles were observed for C₆ and C₁₀ alkyl spacers, anisotropic structures were only present in case of the C₁₂ spacer. The comparison to an analogous material, which lacks the directed hydrogen bonds, revealed that the BTA motif cannot provide a sufficient driving force to induce anisotropic structures, but increases the packing density in the hydrophobic part. Therefore, the packing parameter governs the appearance of anisotropic aggregates.

Supramolecular assembly of functional peptide–polymer conjugates

The following review gives an overview about synthetic peptide–polymer conjugates as macromolecular building blocks and their self-assembly into a variety of supramolecular architectures, from supramolecular polymer chains, to anisotropic 1D arrays, 2D layers, and more complex 3D networks.

RAFT polymerization of a RGD peptide-based methacrylamide monomer for cell adhesion

The present study provides a robust method for the preparation of RGD peptide-based polymers that has important implications in the synthesized biomaterials that support cell adhesion.

Folding induced supramolecular assembly into pH-responsive nanorods with a protein repellent shell

ABA′ triblock peptide–polysarcosine–peptide conjugates fold into antiparallel β-sheets, which promotes the self-assembly into polysarcosine-shielded core–shell nanorods with protein repellent properties.

Self-Assembly of Temperature-Responsive Protein-Polymer Bioconjugates

We report a simple temperature-responsive bioconjugate system comprising superfolder green fluorescent protein (sfGFP) decorated with poly[(oligo ethylene glycol) methyl ether methacrylate] (PEGMA) polymers. We used amber suppression to site-specifically incorporate the non-canonical azide-functional amino acid p-azidophenylalanine (pAzF) into sfGFP at different positions. The azide moiety on modified sfGFP was then coupled using copper-catalyzed "click" chemistry with the alkyne terminus of a PEGMA synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The protein in the resulting bioconjugate was found to remain functionally active (i.e., fluorescent) after conjugation. Turbidity measurements revealed that the point of attachment of the polymer onto the protein scaffold has an impact on the thermoresponsive behavior of the resultant bioconjugate. Furthermore, small-angle X-ray scattering analysis showed the wrapping of the polymer around the protein in a temperature-dependent fashion. Our work demonstrates that standard genetic manipulation combined with an expanded genetic code provides an easy way to construct functional hybrid biomaterials where the location of the conjugation site on the protein plays an important role in determining material properties. We anticipate that our approach could be generalized for the synthesis of complex functional materials with precisely defined domain orientation, connectivity, and composition.

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.

Molecular dynamics studies of PEGylated α-helical coiled coils and their self-assembled micelles

We performed coarse-grained (CG) molecular dynamics simulations of trimeric α-helical coiled coils grafted with poly(ethylene glycol) (PEG) of different sizes and conjugate positions and the self-assembled micelle of amphiphilic trimers. The CG model for the trimeric coiled coil is verified by comparing the α-helical structure and interhelical distance with those calculated from all-atom simulations. In CG simulations of PEGylated trimers, the end-to-end distances and radii of gyration of PEGs grafted to the sides of peptides become shorter than those of free PEGs in water, which agrees with experiments. This shorter size of the grafted PEGs is also confirmed by calculating the thickness of the PEG layer, which is less than the size of the mushroom. These indicate the adsorption of PEG chains onto coiled coils since hydrophobic residues in the exterior sites of coiled coils tend to be less exposed to water and thus interact with PEGs, leading to the compact conformation of adsorbed PEGs. Simulations of the self-assembly of amphiphilic trimers show that the randomly distributed trimers self-assemble to micelles. The outer radius and hydrodynamic radius of the micelle, which were calculated respectively from radial densities and diffusion coefficients, are ∼7 nm, in agreement with the experimental value of ∼7.5 nm, while the aggregation number of amphiphilic molecules per micelle is lower than the experimentally proposed value. These simulations predict the experimentally measured size of PEGs grafted to the trimeric coiled coils and their self-assembled amphiphilic micelles and suggest that the aggregation number of the micelle may be lower, which needs to be confirmed by experiments.

Emerging bioinspired polymers: glycopolypeptides

This article highlights the very recent advances in glycopolypeptide synthesis via NCA polymerization and first studies on stimuli-responsive solution behavior and self-assembling structures. Yet glycopolypeptides are almost exclusively considered as smart biofunctional materials for use in biomedical applications, for instance in targeted drug delivery, but also have high potential for usage as structural materials to fabricate bioinspired hierarchical structures.

Hierarchically structured vanadium pentoxide-polymer hybrid materials

Biomimetic composite materials consisting of vanadium pentoxide (V2O5) and a liquid crystal (LC) "gluing" polymer were manufactured exhibiting six structural levels of hierarchy, formed through LC phases. The organic matrix was a polyoxazoline with pendant cholesteryl and carboxyl units, forming a lyotropic phase with the same structural orientation extending up to hundreds of micrometers upon shearing, and binding to V2O5 via hydrogen bridges. Composites consisting of V2O5-LC polymer hybrid fibers with a pronounced layered structuring were obtained. The V2O5-LC polymer hybrid fibers consist of aligned V2O5 ribbons, composed of self-assembled V2O5 sheets, encasing a chiral nematic polymer matrix. The structures of the V2O5-LC polymer composites strongly depend on the preparation method, i.e., the phase-transfer method from aqueous to organic medium, in which the polymer forms LC phases. Notably, highly defined micro- and nanostructures were obtained when initiating the synthesis using V2O5 tactoids with preoriented nanoparticle building units, even when using isotropic V2O5 dispersions. Shear-induced hierarchical structuring of the composites was observed, as characterized from the millimeter and micrometer down to the nanometer length scales using complementary optical and electron microscopy, SAXS, μCT, and mechanical nanoindentation.

Fluorescent amphiphilic PEG-peptide-PEG triblock conjugate micelles for cell imaging

A fluorescent amphiphilic poly(ethylene glycol)-peptide-fluorophore-peptide-poly(ethylene glycol) (PEG-Pep-F-Pep-PEG) triblock conjugate with a hydrophobic fluorophore moiety at the centre of the chain is synthesized by "grafting to" technique based on Schiff-base coupling chemistry. The conjugate is characterized by nuclear magnetic resonance (NMR), circular dichroism (CD), and fluorescence spectroscopy techniques. The aqueous solution of the triblock conjugate emits blue light and exhibits a fluorescence emission band at 430 nm. The amphiphilic conjugate molecules undergo self-assembly into micelles (D ≈ 15-20 nm) in aqueous solution as confirmed from transmission electron microscopy (TEM) and dynamic light scattering (DLS). The critical aggregation concentration is determined by pyrene fluorescence assay and is found to be 0.051 mg mL(-1) . The highly stable and low toxic fluorescent PEG-Pep-F-Pep-PEG conjugate micelles are used for imaging of HeLa cells.

Hierarchical structuring of liquid crystal polymer-Laponite hybrid materials

Biomimetic organic-inorganic composite materials were fabricated via one-step self-organization on three hierarchical levels. The organic component was a polyoxazoline with pendent cholesteryl and carboxyl (N-Boc-protected amino acid) side chains that was able to form a chiral nematic lyotropic phase and bind to positively charged inorganic faces of Laponite. The Laponite particles formed a mesocrystalline arrangement within the liquid-crystal (LC) polymer phase upon shearing a viscous dispersion of Laponite nanoparticles and LC polymer in DMF. Complementary analytical and mechanical characterization techniques (AUC, POM, TEM, SEM, SAXS, μCT, and nanoindentation) covering the millimeter, micrometer, and nanometer length scales reveal the hierarchical structures and properties of the composite materials consisting of different ratios of Laponite nanoparticles and liquid-crystalline polymer.

pH-Switchable ampholytic supramolecular copolymers

β-sheet-encoded anionic and cationic dendritic peptide amphiphiles form supramolecular copolymers when self-assembled in a 1:1 feed ratio of the monomers. These ampholytic materials have been designed for on-off polymerization in response to pH triggers. The cooperative supramolecular self-assembly process is switched on at a physiologically relevant pH value and can be switched off by increasing or decreasing the pH value.

Peptoid polymers: a highly designable bioinspired material

Bioinspired polymeric materials are attracting increasing attention due to significant advantages over their natural counterparts: the ability to precisely tune their structures over a broad range of chemical and physical properties, increased stability, and improved processability. Polypeptoids, a promising class of bioinspired polymer based on a N-substituted glycine backbone, have a number of unique properties that bridge the material gap between proteins and bulk polymers. Peptoids combine the sequence specificity of biopolymers with the simpler intra/intermolecular interactions and robustness of traditional synthetic polymers. They are highly designable because hundreds of chemically diverse side chains can be introduced from simple building blocks. Peptoid polymers can be prepared by two distinct synthetic techniques offering access to two material subclasses: (1) automated solid-phase synthesis which enables precision sequence control and near absolute monodispersity up to chain lengths of ~50 monomers, and (2) a classical polymerization approach which allows access to higher molecular weights and larger-scale yields, but with less control over length and sequence. This combination of facile synthetic approaches makes polypeptoids a highly tunable, rapid polymer prototyping platform to investigate new materials that are intermediate between proteins and bulk polymers, in both their structure and their properties. In this paper, we review the methods to synthesize peptoid polymers and their applications in biomedicine and nanoscience, as both sequence-specific materials and as bulk polymers.

Materiomics: an -omics approach to biomaterials research

The past fifty years have seen a surge in the use of materials for clinical application, but in order to understand and exploit their full potential, the scientific complexity at both sides of the interface--the material on the one hand and the living organism on the other hand--needs to be considered. Technologies such as combinatorial chemistry, recombinant DNA as well as computational multi-scale methods can generate libraries with a very large number of material properties whereas on the other side, the body will respond to them depending on the biological context. Typically, biological systems are investigated using both holistic and reductionist approaches such as whole genome expression profiling, systems biology and high throughput genetic or compound screening, as already seen, for example, in pharmacology and genetics. The field of biomaterials research is only beginning to develop and adopt these approaches, an effort which we refer to as "materiomics". In this review, we describe the current status of the field, and its past and future impact on the biomedical sciences. We outline how materiomics sets the stage for a transformative change in the approach to biomaterials research to enable the design of tailored and functional materials for a variety of properties in fields as diverse as tissue engineering, disease diagnosis and de novo materials design, by combining powerful computational modelling and screening with advanced experimental techniques.

Nanoparticle Formation from Hybrid, Multiblock Copolymers of Poly(Acrylic Acid) and VPGVG Peptide

Elastin-mimetic hybrid copolymers with an alternating molecular architecture were synthesized via the step growth polymerization of azide-functionalized, telechelic poly(tert-butyl acrylate) (PtBA) and an alkyne-terminated, valine and glycine-rich peptide with a sequence of (VPGVG)2 (VG2). The resultant hybrid copolymer, [PtBA-VG2]n, contains up to six constituent building blocks and has a polydispersity index (PDI) of ~1.9. Trifluoroacetic acid (TFA) treatment of [PtBA-VG2]n gave rise to an alternating copolymer of poly(acrylic acid) (PAA) and VG2 ([PAA-VG2]n). The modular design permits facile adjustment of the copolymer composition by varying the molecular weight of PAA (22 and 63 repeat units). Characterization by dynamic light scattering indicated that the multiblock copolymers formed discrete nanoparticles at room temperature in aqueous solution at pH 3.8, with an average diameter of 250-270 nm and a particle size distribution of 0.34 for multiblock copolymers containing PAA22 and 0.17 for those containing PAA63. Upon increasing the pH to 7.4, both types of particles were able to swell without being disintegrated, reaching an average diameter of 285-300 nm for [PAA22-VG2]n and 330-350 nm for [PAA63-VG2]n, respectively. The nanoparticles were not dissociated upon the addition of urea, further confirming their unusual stability. The nanoparticles were capable of sequestering a hydrophobic fluorescent dye (pyrene), and the critical aggregation concentration (CAC) was determined to be 1.09 × 10-2 or 1.05 × 10-2 mg/mL for [PAA22-VG2]n and [PAA63-VG2]n, respectively. We suggest that the multiblock copolymers form through collective H-bonding and hydrophobic interactions between the PAA and VG2 peptide units, and that the unusual stability of the multiblock nanoparticles is conferred by the multiblock architecture. These hybrid multiblock copolymers are potentially useful as pH-responsive drug delivery vehicles, with the possibility of drug loading through concerted H-bonds and hydrophobic interactions.

Dendronized albumin core-shell transporters with high drug loading capacity

We describe the synthesis of a core-shell biohybrid consisting of a human serum albumin (HSA) core that serves as a reservoir for lipophilic molecules and a cationized shell region consisting of ethynyl-G2.0-PAMAM or ethynyl-G3.0-PAMAM dendrons. The binding capacity of lipophilic guests was quantified applying electron paramagnetic resonance (EPR) spectroscopy, and five to six out of seven pockets were still available compared with HSA. The attachment of ethynyl-G2.0-PAMAM dendrons to HSA yielded a nontoxic core-shell macromolecule that was clearly uptaken by A549 human epithelial cells due to the presence of the dendritic PAMAM shell. Significantly higher loading of doxorubicin was observed for dendronized G2-DHSA compared with the native protein due to the availability of binding pockets of the HSA core, and interaction with the dendritic shell. Dendronized G2-DHSA-doxorubicin displayed significant cytotoxicity resulting from high drug loading and high stability under different conditions, thus demonstrating its great potential as a transporter for drug molecules.

Conformational exploration of two peptides and their hybrid polymer conjugates: potentialities as self-aggregating materials

In this work we elucidate the conformational preferences of two amyloid-forming peptides, Arginine-Vasopressin and Neuromedin-K, and two new biomacromolecular conjugates obtained by linking the two peptides to a polyester (poly(R-lactic acid)) chain. The conformational properties of the new hybrid conjugates have been assessed through molecular dynamics simulations and compared to those of their individual components. Our results suggest that the free unconjugated peptides tend to adopt backbone arrangements which resemble a β-hairpin shape, a conformation which has been reported to facilitate amyloid self-aggregation. The backbone conformational preferences of the unlinked peptides are maintained in the peptide-polymer hybrid. Yet significant differences in the side-chains nonbonding interactions patterns were detected between the two states. This suggests that the conformational profile of the peptides' backbones is preserved when linked to the polymer, maintaining the amyloid precursor-like structure. Additionally, several hydrodynamic parameters were computed for both the polylactic acid and for the conjugates: no significant differences were observed, which suggests that the peptide moiety of the hybrid does not significantly affect the conformational tendencies of the polymer chain. Combined, our results provide a conformational exploration of two amyloid-forming peptides and first steps toward the design of two feasible self-aggregating hybrid materials.

Investigation of the reversibility of the unimer-to-aggregate transition in block copolymers by surface tension-measurements

Two DNA-block copolymers, poly(caprolactone)-DNA and poly(methyl metacrylate)-DNA, were synthesized by conjugation of a short single strand of DNA (12 or 22mer) to a single reactive group at one end of the synthetic polymer. These polymers self-assemble in water, without the need of any cosolvent, forming micelle-like aggregates that were imaged by TEM. The solution behavior of the bioconjugated polymers was investigated by surface tension measurements. In the direction of dilution, the surface tension was measured using a down-scaled Wilhelmy plate method. To proceed in the reverse direction (concentration), we measured the surface tension of a sessile drop during its evaporation. This latter method was firstly validated using ionic and non-ionic surfactants, including polymeric surfactants. It was then applied to investigate the unimer to micelles transition of the DNA-block copolymers. In all cases, a reversible transition was observed demonstrating the existence of a critical micellar concentration, close to 0.01 mmol L(-1) for all the conjugates. The CMC was only slightly influenced by the length of the hydrophilic DNA block.