Thermoresponsive self-assembly of short elastin-like polypentapeptides and their poly(ethylene glycol) derivatives

Molecular bases for temperature sensitivity in supramolecular assemblies and their applications as thermoresponsive soft materials

Thermoresponsive supramolecular assemblies have been extensively explored in diverse formats, from injectable hydrogels to nanoscale carriers, for a variety of applications including drug delivery, tissue engineering and thermo-controlled catalysis. Understanding the molecular bases behind thermal sensitivity of materials is fundamentally important for the rational design of assemblies with optimal combination of properties and predictable tunability for specific applications. In this review, we summarize the recent advances in this area with a specific focus on the parameters and factors that influence thermoresponsive properties of soft materials. We summarize and analyze the effects of structures and architectures of molecules, hydrophilic and lipophilic balance, concentration, components and external additives upon the thermoresponsiveness of the corresponding molecular assemblies.

Placement of Tyrosine Residues as a Design Element for Tuning the Phase Transition of Elastin-peptide-containing Conjugates: Experiments and Simulations

Elastin-like polypeptides (ELP) have been widely used in the biomaterials community due to their controllable, thermoresponsive properties and biocompatibility. Motivated by our previous work on the effect of tryptophan (W) substitutions on the LCST-like transitions of short ELPs, we studied a series of short ELPs containing tyrosine (Y) and/or phenylalanine (F) guest residues with only 5 or 6 pentapeptide repeat units. A combination of experiments and molecular dynamics (MD) simulations illustrated that the substitution of F with Y guest residues impacted the transition temperature (Tt) of short ELPs when conjugated to collagen-like-peptides (CLP), with a reduction in the transition temperature observed only after substitution of at least two residues. Placement of the Y residues near the N-terminal end of the ELP, away from the tethering point to the CLP, resulted in a lower Tt than that observed for peptides with the Y residues near the tethering point. Atomistic and coarse-grained MD simulations indicated an increase in intra- and inter- peptide hydrogen bonds in systems containing Y guest residues that are suggested to enhance the ability of the peptides to coacervate, with a concomitantly lower Tt. Simulations also revealed that the placement of Y-containing pentads near the N-terminus (i.e., away from CLP tethering point) versus C-terminus of the ELP led to more π-π stacking interactions at low temperatures, in agreement with our experimental observations of a lower Tt. Overall, this study provides mechanistic insights into the driving forces for the LCST-like transitions of ELPs and offers additional means for tuning the Tt of short ELPs for biomedical applications such as on-demand drug delivery and tissue engineering.

Effect of Peptide Sequence on the LCST-Like Transition of Elastin-Like Peptides and Elastin-Like Peptide-Collagen-Like Peptide Conjugates: Simulations and Experiments

Elastin-like polypeptides (ELPs) are thermoresponsive biopolymers that undergo an LCST-like phase transition in aqueous solutions. The temperature of this LCST-like transition, Tt , can be tuned by varying the number of repeat units in the ELP, sequence and composition of the repeat units, the solution conditions, and via conjugation to other biomacromolecules. In this study, we show how and why the choice of guest (X) residue in the VPGXG pentad repeat tunes the Tt of short ELPs, (VPGXG)4, in the free state and when conjugated to collagen-like peptides (CLPs). In experiments, the (VPGWG)4 chain (in short, WWWW) has a Tt < 278 K, while (VPGFG)4 or FFFF has a Tt > 353 K in both free ELP and ELP-CLP systems. The Tt for the FWWF ELP sequence decreases from being >353 K for free ELP to <278 K for the corresponding ELP-CLP system. The decrease in Tt upon conjugation to CLP has been shown to be due to the crowding of ELP chains that decreases the entropic loss upon ELP aggregation. Even though the net hydrophobicity of ELP has been reasoned to drive the Tt , the origins of lower Tt of WWWW compared to FFFF are unclear, as there is disagreement in hydrophobicity scales in how phenylalanine (F) compares to tryptophan (W). Motivated by these experimental observations, we use a combination of atomistic and coarse-grained (CG) molecular dynamics simulations. Atomistic simulations of free and tethered ELPs show that WWWW are more prone to acquire β-turn structures than FFFF at lower temperatures. Also, the atomistically informed CG simulations show that the increased local stiffness in W than F due to the bulkier side chain in W compared to F, alone does not cause the shift in the transition of WWWW versus FFFF. The experimentally observed lower Tt of WWWW than FFFF is achieved in CG simulations only when the CG model incorporates both the atomistically informed local stiffness and stronger effective attractions localized at the W position versus the F position. The effective interactions localized at the guest residue in the CG model is guided by our atomistically observed increased propensity for β-turn structure in WWWW versus FFFF and by past experimental work of Urry et al. quantifying hydrophobic differences through enthalpy of association for W versus F.

Self-Assembly of Short Elastin-like Amphiphilic Peptides: Effects of Temperature, Molecular Hydrophobicity and Charge Distribution

A novel type of self-assembling peptides has been developed by introducing the basic elastomeric β-turn units of elastin protein into the amphiphilic peptide molecules. The self-assembly behaviors of such peptides are affected by the overall molecular hydrophobicity, charge distribution and temperature. The molecules with higher hydrophobicity exhibit better self-assembling capability to form long fibrillar nanostructures. For some peptides, the temperature increase can not only promote the self-assembly process but also change the self-assembly routes. The self-assembly of the peptides with two charges centralized on one terminal show higher dependence on temperature than the peptides with two charges distributed separately on the two terminals. The study probes into the self-assembly behaviors of short elastin-like peptides and is of great help for developing novel self-assembling peptides with thermo sensitivity.

Engineering Elastin-Like Polypeptide-Poly(ethylene glycol) Multiblock Physical Networks

Hybrids of protein biopolymers and synthetic polymers are a promising new class of soft materials, as the advantages of each component can be complementary. A recombinant elastin-like polypeptide (ELP) was conjugated to poly(ethylene glycol) (PEG) by macromolecular coupling in solution to form multiblock ELP-PEG copolymers. The hydrated copolymer preserved the thermoresponsive properties from the ELP block and formed hydrogels with different transition temperatures depending on salt concentration. Small angle scattering indicates that the copolymer hydrogels form sphere-like aggregates with a "fuzzy" interface, while the films form a fractal network of nanoscale aggregates. The use of solutions with different salt concentrations to prepare the hydrogels was found to influence the transition temperature, the mechanical properties, and the size of the nanoscale structure of the hydrogel without changing the secondary structure of the ELP. The salt variation and the addition of a plasticizer also affected the nanoscale structure and the mechanical characteristics of the films.

Collagen-Like Peptide Bioconjugates

Collagen-like peptides (CLPs), also known as collagen-mimetic peptides (CMPs), are short synthetic peptides that mimic the triple helical conformation of native collagens. Traditionally, CLPs have been widely used in deciphering the chemical basis for collagen triple helix stabilization, mimicking collagen fibril formation and fabricating other higher-order supramolecular self-assemblies. While CLPs have been used extensively for elucidation of the assembly of native collagens, less work has been reported on the use of CLP-polymer and CLP-peptide conjugates in the production of responsive assemblies. CLP triple helices have been used as physical cross-links in CLP-polymer hydrogels with predesigned thermoresponsiveness. The more recently reported ability of CLP to target native collagens via triple helix hybridization has further inspired the production of CLP-polymer and CLP-peptide bioconjugates and the employment of these conjugates in generating well-defined nanostructures for targeting collagen substrates. This review summarizes the current progress and potential of using CLPs in biomedical arenas and is intended to serve as a general guide for designing CLP-containing biomaterials.

Noncovalent Modulation of the Inverse Temperature Transition and Self-Assembly of Elastin-b-Collagen-like Peptide Bioconjugates

Stimuli-responsive nanostructures produced with peptide domains from the extracellular matrix offer great opportunities for imaging and drug delivery. Although the individual utility of elastin-like (poly)peptides and collagen-like peptides in such applications has been demonstrated, the synergistic advantages of combining these motifs in short peptide conjugates have surprisingly not been reported. Here, we introduce the conjugation of a thermoresponsive elastin-like peptide (ELP) with a triple-helix-forming collagen-like peptide (CLP) to yield ELP-CLP conjugates that show a remarkable reduction in the inverse transition temperature of the ELP domain upon formation of the CLP triple helix. The lower transition temperature of the conjugate enables the facile formation of well-defined vesicles at physiological temperature and the unexpected resolubilization of the vesicles at elevated temperatures upon unfolding of the CLP domain. Given the demonstrated ability of CLPs to modify collagens, our results not only provide a simple and versatile avenue for controlling the inverse transition behavior of ELPs, but also suggest future opportunities for these thermoresponsive nanostructures in biologically relevant environments.

Aggregation of poly(acrylic acid)-containing elastin-mimetic copolymers

Polymer-peptide conjugates were produced via the copper-catalyzed azide-alkyne cycloaddition of poly(tert-butyl acrylate) (PtBA) and elastin-like peptides. An azide-functionalized polymer was produced via atom transfer radical polymerization (ATRP) followed by conversion of bromine end groups to azide groups. Subsequent reaction of the polymer with a bis-alkyne-functionalized, elastin-like peptide proceeded with high efficiency, yielding di- and tri-block conjugates, which after deprotection, yielded poly(acrylic acid) (PAA)-based diblock and triblock copolymers. These conjugates were solubilized in dimethyl formamide, and addition of phosphate buffered saline (PBS) induced aggregation. The presence of polydisperse spherical aggregates was confirmed by dynamic light scattering and transmission electron microscopy. Additionally, a coarse-grained molecular model was designed to reasonably capture inter- and intramolecular interactions for the conjugates and its precursors. This model was used to assess the effect of the different interacting molecular forces on the conformational thermodynamic stability of the copolymers. Our results indicated that the PAA's ability to hydrogen-bond with both itself and the peptide is the main interaction for stabilizing the diblocks and triblocks and driving their self-assembly, while interactions between peptides are suggested to play only a minor role on the conformational and thermodynamic stability of the conjugates.

Changes in cell wall properties coincide with overexpression of extensin fusion proteins in suspension cultured tobacco cells

Extensins are one subfamily of the cell wall hydroxyproline-rich glycoproteins, containing characteristic SerHyp4 glycosylation motifs and intermolecular cross-linking motifs such as the TyrXaaTyr sequence. Extensins are believed to form a cross-linked network in the plant cell wall through the tyrosine-derivatives isodityrosine, pulcherosine, and di-isodityrosine. Overexpression of three synthetic genes encoding different elastin-arabinogalactan protein-extensin hybrids in tobacco suspension cultured cells yielded novel cross-linking glycoproteins that shared features of the extensins, arabinogalactan proteins and elastin. The cell wall properties of the three transgenic cell lines were all changed, but in different ways. One transgenic cell line showed decreased cellulose crystallinity and increased wall xyloglucan content; the second transgenic cell line contained dramatically increased hydration capacity and notably increased cell wall biomass, increased di-isodityrosine, and increased protein content; the third transgenic cell line displayed wall phenotypes similar to wild type cells, except changed xyloglucan epitope extractability. These data indicate that overexpression of modified extensins may be a route to engineer plants for bioenergy and biomaterial production.

Designing protein-based biomaterials for medical applications

Biomaterials produced by nature have been honed through billions of years, evolving exquisitely precise structure-function relationships that scientists strive to emulate. Advances in genetic engineering have facilitated extensive investigations to determine how changes in even a single peptide within a protein sequence can produce biomaterials with unique thermal, mechanical and biological properties. Elastin, a naturally occurring protein polymer, serves as a model protein to determine the relationship between specific structural elements and desirable material characteristics. The modular, repetitive nature of the protein facilitates the formation of well-defined secondary structures with the ability to self-assemble into complex three-dimensional architectures on a variety of length scales. Furthermore, many opportunities exist to incorporate other protein-based motifs and inorganic materials into recombinant protein-based materials, extending the range and usefulness of these materials in potential biomedical applications. Elastin-like polypeptides (ELPs) can be assembled into 3-D architectures with precise control over payload encapsulation, mechanical and thermal properties, as well as unique functionalization opportunities through both genetic and enzymatic means. An overview of current protein-based materials, their properties and uses in biomedicine will be provided, with a focus on the advantages of ELPs. Applications of these biomaterials as imaging and therapeutic delivery agents will be discussed. Finally, broader implications and future directions of these materials as diagnostic and therapeutic systems will be explored.

Naturally and synthetic smart composite biomaterials for tissue regeneration

The development of smart biomaterials for tissue regeneration has become the focus of intense research interest. More opportunities are available by the composite approach of combining the biomaterials in the form of biopolymers and/or bioceramics either synthetic or natural. Strategies to provide smart capabilities to the composite biomaterials primarily seek to achieve matrices that are instructive/inductive to cells, or that stimulate/trigger target cell responses that are crucial in the tissue regeneration processes. Here, we review in-depth, recent developments concerning smart composite biomaterials available for delivery systems of biofactors and cells and scaffolding matrices in tissue engineering. Smart composite designs are possible by modulating the bulk and surface properties that mimic the native tissues, either in chemical (extracellular matrix molecules) or in physical properties (e.g. stiffness), or by introducing external therapeutic molecules (drugs, proteins and genes) within the structure in a way that allows sustainable and controllable delivery, even time-dependent and sequential delivery of multiple biofactors. Responsiveness to internal or external stimuli, including pH, temperature, ionic strength, and magnetism, is another promising means to improve the multifunctionality in smart scaffolds with on-demand delivery potential. These approaches will provide the next-generation platforms for designing three-dimensional matrices and delivery systems for tissue regenerative applications.

New directions in thermoresponsive polymers

Interest in thermoresponsive polymers has steadily grown over many decades, and a great deal of work has been dedicated to developing temperature sensitive macromolecules that can be crafted into new smart materials. However, the overwhelming majority of previously reported temperature-responsive polymers are based on poly(N-isopropylacrylamide) (PNIPAM), despite the fact that a wide range of other thermoresponsive polymers have demonstrated similar promise for the preparation of adaptive materials. Herein, we aim to highlight recent results that involve thermoresponsive systems that have not yet been as fully considered. Many of these (co)polymers represent clear opportunities for advancements in emerging biomedical and materials fields due to their increased biocompatibility and tuneable response. By highlighting recent examples of newly developed thermoresponsive polymer systems, we hope to promote the development of new generations of smart materials.

Diethylene glycol functionalized self-assembling peptide nanofibers and their hydrophobic drug delivery potential

Self-assembling peptide nanofibers have emerged as important nanobiomaterials, with such applications as delivery of therapeutic agents and vaccines, nanofabrication and biomineralization, tissue engineering and regenerative medicine. Recently a new class of self-assembling peptides has been introduced, which takes into consideration amino acid pairing (AAP) strategies in the peptide sequence design. Even though these peptides have shown promising potential in the design of novel functional biomaterials, they have a propensity to initiate uncontrollable aggregation and be degraded by proteolytic enzymes. These present the most significant challenge in advancing self-assembling peptides for in vitro and in vivo applications. Functionalizing biomaterials with polyethylene glycol (PEG) has been shown to surmount such problems. Here the results of conjugating diethylene glycol (DEG), a short segment of PEG, to one of the AAP peptides, AAP8, with eight amino acids in sequence, are reported. The results indicate that incorporation of DEG into the peptide sequence modulates fiber self-assembly through creating more aligned and uniform nanostructures. This is associated with increasing solubility, stability, and secondary structure β-sheet content of the peptide. The DEG conjugate of AAP8 also shows reduced cellular cytotoxicity. Functionalization of AAP8 improves the capability of the peptide to stabilize and deliver a hydrophobic anticancer compound, ellipticine, in aqueous solution, consequently inducing greater cytotoxicity to lung carcinoma cells over a relatively long time, compared with non-functionalized AAP8. The presented functionalized peptide and its drug delivery application indicate a potentially useful design strategy for novel self-assembling peptide biomaterials for biotechnology and nanomedicine.

Smart self-assembled hybrid hydrogel biomaterials

Hybrid biomaterials are systems created from components of at least two distinct classes of molecules, for example, synthetic macromolecules and proteins or peptide domains. The synergistic combination of two types of structures may produce new materials that possess unprecedented levels of structural organization and novel properties. This Review focuses on biorecognition-driven self-assembly of hybrid macromolecules into functional hydrogel biomaterials. First, basic rules that govern the secondary structure of peptides are discussed, and then approaches to the specific design of hybrid systems with tailor-made properties are evaluated, followed by a discussion on the similarity of design principles of biomaterials and macromolecular therapeutics. Finally, the future of the field is briefly outlined.

Protein-based block copolymers

Advances in genetic engineering have led to the synthesis of protein-based block copolymers with control of chemistry and molecular weight, resulting in unique physical and biological properties. The benefits from incorporating peptide blocks into copolymer designs arise from the fundamental properties of proteins to adopt ordered conformations and to undergo self-assembly, providing control over structure formation at various length scales when compared to conventional block copolymers. This review covers the synthesis, structure, assembly, properties, and applications of protein-based block copolymers.

Fast thermoresponsive BAB-type HEMA/NIPAAm triblock copolymer solutions for embolization of abnormal blood vessels

Thermoresponsive BAB-type HEMA/NIPAAm triblock copolymers (A = NIPAAm, B = HEMA) were prepared by atomic transfer radical polymerization (ATRP). BAB1-6 with shorter PNIPAAm blocks failed to form stable gel; while a relatively stable gel could be achieved by BAB1-8 with longer PNIPAAm blocks when copolymer aqueous solution was heated up. Introducing radiopaque agent (RA) was shown to slightly increase the transition temperature and gelation time, but the gelling ability was strengthened due to slightly weakening dehydration of copolymer in the mixture of water and RA. BAB1-8 aqueous solution about 5 wt% in the presence of RA was demonstrated to successfully occlude the cerebral rete mirabiles (RMs) and renal arteries of pigs. Within 3-month surgery, no recanalization was observed and the embolized kidney shrank considerably. Histological assay of embolized kidney demonstrated interstitial fibrosis and calcification as well as the thickening of renal small artery. This temperature sensitive copolymer with well-defined architecture holds a great potential as an embolic agent for treating arteriovenous malformations (AVMs) and renal disease due to the design flexibility of ATRP.

Peptide-directed self-assembly of hydrogels

This review focuses on the self-assembly of macromolecules mediated by the biorecognition of peptide/protein domains. Structures forming alpha-helices and beta-sheets have been used to mediate self-assembly into hydrogels of peptides, reactive copolymers and peptide motifs, block copolymers, and graft copolymers. Structural factors governing the self-assembly of these molecules into precisely defined three-dimensional structures (hydrogels) are reviewed. The incorporation of peptide motifs into hybrid systems, composed of synthetic and natural macromolecules, enhances design opportunities for new biomaterials when compared to individual components.

Self-assembling diblock copolymers of poly[N-(2-hydroxypropyl)methacrylamide] and a beta-sheet peptide

The self-assembly of hybrid diblock copolymers composed of poly(HPMA) and beta-sheet peptide P11 (CH(3)CO-QQRFQWQFEQQ-NH(2)) blocks was investigated. Copolymers were synthesized via thiol-maleimide coupling reaction, by conjugation of semitelechelic poly(HPMA)-SH with maleimide-modified beta-sheet peptide. As expected, CD and CR binding studies showed that the peptide block imposed its beta-sheet structural arrangement on the structure of diblock copolymers. TEM and AFM proved that peptide and these copolymers had the ability to self-assemble into fibrils.

Peptide/protein-polymer conjugates: synthetic strategies and design concepts

This feature article provides a compilation of tools available for preparing well-defined peptide/protein-polymer conjugates, which are defined as hybrid constructs combining (i) a defined number of peptide/protein segments with uniform chain lengths and defined monomer sequences (primary structure) with (ii) a defined number of synthetic polymer chains. The first section describes methods for post-translational, or direct, introduction of chemoselective handles onto natural or synthetic peptides/proteins. Addressed topics include the residue- and/or site-specific modification of peptides/proteins at Arg, Asp, Cys, Gln, Glu, Gly, His, Lys, Met, Phe, Ser, Thr, Trp, Tyr and Val residues and methods for producing peptides/proteins containing non-canonical amino acids by peptide synthesis and protein engineering. In the second section, methods for introducing chemoselective groups onto the side-chain or chain-end of synthetic polymers produced by radical, anionic, cationic, metathesis and ring-opening polymerization are described. The final section discusses convergent and divergent strategies for covalently assembling polymers and peptides/proteins. An overview of the use of chemoselective reactions such as Heck, Sonogashira and Suzuki coupling, Diels-Alder cycloaddition, Click chemistry, Staudinger ligation, Michael's addition, reductive alkylation and oxime/hydrazone chemistry for the convergent synthesis of peptide/protein-polymer conjugates is given. Divergent approaches for preparing peptide/protein-polymer conjugates which are discussed include peptide synthesis from synthetic polymer supports, polymerization from peptide/protein macroinitiators or chain transfer agents and the polymerization of peptide side-chain monomers.