Dendritic Elastin-like Peptides: The Effect of Branching on Thermoresponsiveness

Advances in MRI: Peptide and peptidomimetic-based contrast agents

Magnetic resonance imaging (MRI) is a common medical imaging technique that provides three-dimensional body images. MRI contrast agents improve image contrast by raising the rate of water proton relaxation in specific tissues. Peptides and peptidomimetics act as scaffolds for MRI imaging agents because of their increased size and offer the possibility to engine a higher hydration value within the design. The design of a new Gd-based contrast agent must take into account high stability constants to avoid free Gd(III), with the subsequent nephrotoxicity, and high relaxivity values. This review analyzes various synthetic approaches, reports studies of relaxometric parameters, and focuses on the description and application of Gd(III)-chelates based on peptide and peptidomimetic scaffolds. In addition, the X-ray molecular structures of three DOTA complexes will be reported to emphasize the necessity of using the X-ray diffraction analysis to identify the coordination sphere of the metals and the mechanism of action of the compounds.

Encoding Structure in Intrinsically Disordered Protein Biomaterials

In nature, proteins range from those with highly ordered secondary and tertiary structures to those that completely lack a well-defined three-dimensional structure, termed intrinsically disordered proteins (IDPs). IDPs are generally characterized by one or more segments that have a compositional bias toward small hydrophilic amino acids and proline residues that promote structural disorder and are called intrinsically disordered regions (IDRs). The combination of IDRs with ordered regions and the interactions between the two determine the phase behavior, structure, and function of IDPs. Nature also diversifies the structure of proteins and thereby their functions by hybridization of the proteins with other moieties such as glycans and lipids; for instance, post-translationally glycosylated and lipidated proteins are important cell membrane components. Additionally, diversity in protein structure and function is achieved in nature through cross-linking proteins within themselves or with other domains to create various topologies. For example, an essential characteristic of the extracellular matrix (ECM) is the cross-linking of its network components, including proteins such as collagen and elastin, as well as polysaccharides such as hyaluronic acid (HA). Inspired by nature, synthetic IDP (SynIDP)-based biomaterials can be designed by employing similar strategies with the goal of introducing structural diversity and hence unique physiochemical properties. This Account describes such materials produced over the past decade and following one or more of the following approaches: (1) incorporating highly ordered domains into SynIDPs, (2) conjugating SynIDPs to other moieties through either genetically encoded post-translational modification or chemical conjugation, and (3) engineering the topology of SynIDPs via chemical modification. These approaches introduce modifications to the primary structure of SynIDPs, which are then translated to unique three-dimensional secondary and tertiary structures. Beginning with completely disordered SynIDPs as the point of origin, structure may be introduced into SynIDPs by each of these three unique approaches individually along orthogonal axes or by combinations of the three, enabling bioinspired designs to theoretically span the entire range of three-dimensional structural possibilities. Furthermore, the resultant structures span a wide range of length scales, from nano- to meso- to micro- and even macrostructures. In this Account, emphasis is placed on the physiochemical properties and structural features of the described materials. Conjugates of SynIDPs to synthetic polymers and materials achieved by simple mixing of components are outside the scope of this Account. Related biomedical applications are described briefly. Finally, we note future directions for the design of functional SynIDP-based biomaterials.

Activating hidden signals by mimicking cryptic sites in a synthetic extracellular matrix

Cryptic sites are short signaling peptides buried within the native extracellular matrix (ECM). Enzymatic cleavage of an ECM protein reveals these hidden peptide sequences, which interact with surface receptors to control cell behavior. Materials that mimic this dynamic interplay between cells and their surroundings via cryptic sites could enable application of this endogenous signaling phenomenon in synthetic ECM hydrogels. We demonstrate that depsipeptides ("switch peptides") can undergo enzyme-triggered changes in their primary sequence, with proof-of-principle studies showing how trypsin-triggered primary sequence rearrangement forms the bioadhesive pentapeptide YIGSR. We then engineered cryptic site-mimetic synthetic ECM hydrogels that experienced a cell-initiated gain of bioactivity. Responding to the endothelial cell surface enzyme aminopeptidase N, the inert matrix transformed into an adhesive synthetic ECM capable of supporting endothelial cell growth. This modular system enables dynamic reciprocity in synthetic ECMs, reproducing the natural symbiosis between cells and their matrix through inclusion of tunable hidden signals.

Peptide-based coacervates in therapeutic applications

Coacervates are droplets formed by liquid‒liquid phase separation. An increasing number of studies have reported that coacervates play an important role in living cells, such as in the generation of membraneless organelles, and peptides contribute to condensate droplet formation. Peptides with versatile functional groups and special secondary structures, including α-helices, β-sheets and intrinsically disordered regions, provide novel insights into coacervation, such as biomimetic protocells, neurodegenerative diseases, modulations of signal transmission, and drug delivery systems. In this review, we introduce different types of peptide-based coacervates and the principles of their interactions. Additionally, we summarize the thermodynamic and kinetic mechanisms of peptide-based coacervates and the associated factors, including salt, pH, and temperature, affecting the phase separation process. We illustrate recent studies on modulating the functions of peptide-based coacervates applied in biological diseases. Finally, we propose their promising broad applications and describe the challenges of peptide-based coacervates in the future.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

Adaptive Recombinant Nanoworms from Genetically Encodable Star Amphiphiles

Recombinant nanoworms are promising candidates for materials and biomedical applications ranging from the templated synthesis of nanomaterials to multivalent display of bioactive peptides and targeted delivery of theranostic agents. However, molecular design principles to synthesize these assemblies (which are thermodynamically favorable only in a narrow region of the phase diagram) remain unclear. To advance the identification of design principles for the programmable assembly of proteins into well-defined nanoworms and to broaden their stability regimes, we were inspired by the ability of topologically engineered synthetic macromolecules to acess rare mesophases. To test this design principle in biomacromolecular assemblies, we used post-translational modifications (PTMs) to generate lipidated proteins with precise topological and compositional asymmetry. Using an integrated experimental and computational approach, we show that the material properties (thermoresponse and nanoscale assembly) of these hybrid amphiphiles are modulated by their amphiphilic architecture. Importantly, we demonstrate that the judicious choice of amphiphilic architecture can be used to program the assembly of proteins into adaptive nanoworms, which undergo a morphological transition (sphere-to-nanoworms) in response to temperature stimuli.

Dendronized Gelatins Showing Both LCST and UCST-type Thermoresponsive Behavior

Modification of natural polymers with stimuli-responsive synthetic moieties witnesses convergences of superior properties from natural polymers and stimuli-responsiveness to generate new intelligent materials. This was usally performed through synthetic polymers...

Recent Advances in Thermoresponsive OEGylated Poly(amino acid)s

Thermoresponsive polymers have been widely studied in the past decades due to their potential applications in biomedicine, nanotechnology, and so on. As is known, poly(N-isopropylacrylamide) (PNIPAM) and poly(oligo(ethylene glycol)methacrylates) (POEGMAs) are the most popular thermoresponsive polymers, and have been studied extensively. However, more advanced thermoresponsive polymers with excellent biocompatibility, biodegradability, and bioactivity also need to be developed for biomedical applications. OEGylated poly(amino acid)s are a kind of novel polymer which are synthesized by attaching one or multiple oligo(ethylene glycol) (OEG) chains to poly(amino acid) (PAA).These polymers combine the great solubility of OEG, and the excellent biocompatibility, biodegradability and well defined secondary structures of PAA. These advantages allow them to have great application prospects in the field of biomedicine. Therefore, the study of OEGylated poly(amino acid)s has attracted more attention recently. In this review, we summarized the development of thermoresponsive OEGylated poly(amino acid)s in recent years, including the synthesis method (such as ring-opening polymerization, post-polymerization modification, and Ugi reaction), stimuli-response behavior study, and secondary structure study. We hope that this periodical summary will be more conducive to design, synthesis and application of OEGylated poly(amino acid)s in the future.

Simple Regulation of the Self-Assembling Ability by Multimerization of Elastin-Derived Peptide (FPGVG) n Using Nitrilotriacetic Acid as a Building Block

Elastin comprises hydrophobic repetitive sequences, such as Val-Pro-Gly-Val-Gly, which are thought to be important for the temperature-dependent reversible self-association (coacervation). Elastin and elastin-like peptides (ELPs), owing to their characteristics, are expected to be applied as base materials for the development of new molecular tools, such as drug-delivery system carrier and metal-scavenging agents. Recently, several studies have been reported on the dendritic or branching ELP analogues. Although the topological difference of the branched ELPs compared to their linear counterparts may lead to useful properties in biomaterials, the available information regarding the effect of branching on molecular architecture and thermoresponsive behavior of ELPs is scarce. To obtain further insight into the thermoresponsive behavior of branched ELPs, novel ELPs, such as nitrilotriacetic acid (NTA)-(FPGVG) _n conjugates, that is, (NTA)-Fn analogues possessing 1-3 (FPGVG) _n (n = 3, 5) molecule(s), were synthesized and investigated for their coacervation ability. Turbidity measurement of the synthesized peptide analogues revealed that (NTA)-Fn analogues showed strong coacervation ability with various strengths. The transition temperature of NTA-Fn analogues exponentially decreased with increasing number of residues. In the circular dichroism measurements, trimerization did not alter the secondary structure of each peptide chain of the NTA-Fn analogue. In addition, it was also revealed that the NTA-Fn analogue possesses one peptide chain that could be utilized as metal-scavenging agents. The study findings indicated that multimerization of short ELPs via NTA is a useful and powerful strategy to obtain thermoresponsive molecules.

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 Crosslinker Topology on Enzymatic Degradation of Hydrogels

Despite the widespread use of hydrogels in biomedical applications, little is known regarding the effect of crosslinker topology on hydrogel degradation. Dendritic and linear elastin-like peptides (ELPs) were used as crosslinkers for hyaluronic acid (HA) hydrogels, and their enzymatic degradation was studied using trypsin. Rheological studies revealed that hydrogels crosslinked with ELP dendrimers (HA_denELPs) degraded more slowly than those crosslinked with the otherwise equivalent linear ELPs (i.e., both molecules have the same number of pentamers and peripheral lysine residues). The origin of this phenomenon was evaluated using solution studies in which various dendritic and linear ELPs were treated with trypsin. Apart from the expected steric hindrances due to the dendritic topology, we identified the dual directionality of the peptide sequences (generated by a central branching lysine residue) and the likelihood of cleaving a productive crosslinking point as two additional contributors to the lesser degradability of HA_denELPs. Overall, these results highlight how the molecular design of crosslinker topology represents a novel strategy to tune the degradation rate of hydrogels.

Hydrogel scaffolds for tissue engineering: the importance of polymer choice

We explore the design and synthesis of hydrogel scaffolds for tissue engineering from the perspective of the underlying polymer chemistry. The key polymers, properties and architectures used, and their effect on tissue growth are discussed.

Peptide-based building blocks as structural elements for supramolecular Gd-containing MRI contrast agents

Magnetic resonance imaging (MRI) is one of the most important clinic diagnostic tool used to obtain high-quality body images. The administration of low-molecular-weight Gd complex-based MRI contrast agents (CAs) permits to increase the ¹ H relaxation rate of nearby water molecules, thus modulating signal intensity and contrast enhancement. Even if highly accurate, MRI modality suffers from its low sensitivity. Moreover, low-molecular-weight CAs rapidly equilibrate between the intravascular and extravascular spaces after their administration. In order to improve their sensitivity and limit the extravasation phenomenon, several macromolecular and supramolecular multimeric gadolinium complexes (dendrimers, polymers, carbon nanostructures, micelles, and liposomes) have been designed until now. Because of their biocompatibility, low immunogenicity, low cost, and easy synthetic modification, peptides are attractive building blocks for the fabbrication of novel materials for biomedical applications. We report on the state of the art of supramolecular CAs obtained by self-assembly of three different classes of building blocks containing a peptide sequence, a gadolinium complex, and, if necessary, a third functional portion achieving the organization process.

Hydrogels composed of hyaluronic acid and dendritic ELPs: hierarchical structure and physical properties

Hydrogels that mimic the native extracellular matrix were prepared from hyaluronic acid (HA) and amine-terminated dendritic elastin-like peptides (denELPs) of generations 1, 2, and 3 (G1, 2, and 3) as crosslinking units. The physical properties of the hydrogels were investigated by rheology, scanning electron microscopy, swelling tests, small-angle X-ray scattering (SAXS), and model drug loading and release assays. Hydrogel properties depended on the generation number of the denELP, which contained structural segments based on the repeating GLPGL pentamer. Hydrogels with higher generation denELPs (G2 and 3) showed similar properties, but those prepared from G1 denELPs were rheologically weaker, had a larger mesh size, absorbed less model drug, and released the drug more quickly. Interestingly, most of the HA_denELP hydrogels studied here remained transparent upon gelation, but after lyophilization and addition of water retained opaque, "solid-like" regions for up to 4 d during rehydration. This rehydration process was carefully evaluated through time-course SAXS studies, and the phenomenon was attributed to the formation of pre-coacervates in the gel-forming step, which slowly swelled in water during rehydration. These findings provide important insights into the behavior of ELP-based hydrogels, in which physical crosslinking of the ELP domains can be controlled to tune mechanical properties, highlighting the potential of HA_denELP hydrogels as biomaterials.

Enhancement of Self-Aggregation Properties of Linear Elastin-Derived Short Peptides by Simple Cyclization: Strong Self-Aggregation Properties of Cyclo[FPGVG] n, Consisting Only of Natural Amino Acids

Elastin-like peptides (ELPs) consist of distinctive repetitive sequences, such as (VPGVG) _n, exhibit temperature-dependent reversible self-assembly (coacervation), and have been considered to be useful for the development of thermoresponsive materials. Further fundamental studies evaluating coacervative properties of novel nonlinear ELPs could present design concepts for new thermoresponsive materials. In this study, we prepared novel ELPs, cyclic (FPGVG) _n (cyclo[FPGVG] _n, n = 1-5), and analyzed their self-assembly properties and structural characteristics. Cyclo[FPGVG] _n ( n = 3-5) demonstrated stronger coacervation capacity than the corresponding linear peptides. The coacervate of cyclo[FPGVG]₅ was able to retain water-soluble dye molecules at 40 °C, which implied that cyclo[FPGVG]₅ could be employed as a base material of DDS (drug delivery system) matrices and other biomaterials. The results of molecular dynamics simulations and circular dichroism measurements suggested that a certain chain length was required for cyclo[FPGVG] _n to demonstrate alterations in molecular structure that were critical to the exhibition of coacervation.

Self-Assembly of Thermoresponsive Recombinant Silk-Elastinlike Nanogels

Recombinant silk-elastinlike protein polymers (SELPs) combine the biocompatibility and thermoresponsiveness of human tropoelastin with the strength of silk. Direct control over structure of these monodisperse polymers allows for precise correlation of structure with function. This work describes the fabrication of the first SELP nanogels and evaluation of their physicochemical properties and thermoresponsiveness. Self-assembly of dilute concentrations of SELPs results in nanogels with enhanced stability over micelles due to physically crosslinked beta-sheet silk segments. The nanogels respond to thermal stimuli via size changes and aggregation. Modifying the ratio and sequence of silk to elastin in the polymer backbone results in alterations in critical gel formation concentration, stability, aggregation, size contraction temperature, and thermal reversibility. The nanogels sequester hydrophobic compounds and show promise in delivery of bioactive agents.

Multi-scale characterization of thermoresponsive dendritic elastin-like peptides

Elastin like peptides (ELPs)-polypeptides based on the protein elastin-are used widely as thermoresponsive components in biomaterials due to the presence of a sharp soluble-to-insoluble phase change at a characteristic transition temperature (T_t). While linear ELPs have been thoroughly studied, few investigations into branched ELPs have been carried out. Using lysine amino acids as branching and terminal units with 1-3 pentameric repeats between each branch, ELP dendrimers were prepared by solid-phase peptide synthesis with molecular weights as high as 14kDa. A conformation change from random coil to β-turn upon heating through the T_t, typical of ELPs, was observed by circular dichroism spectroscopy for all peptides. The high molecular weights of these peptides enabled the use of characterization techniques typically reserved for polymers. Variable-temperature small-angle X-ray scattering measurements in dilute solution revealed an increase in size and fractal dimension upon heating, even well below the T_t. These results were corroborated by cryogenic transmission electron microscopy, which confirmed the presence of aggregates below the T_t, and micro differential scanning calorimetry, which showed a broad endothermic peak below the T_t. These results collectively indicate the presence of a pre-coacervation step in the phase transition of ELP dendrimers.

Polymeric Biomaterials: Diverse Functions Enabled by Advances in Macromolecular Chemistry

Biomaterials have been extensively used to leverage beneficial outcomes in various therapeutic applications, such as providing spatial and temporal control over the release of therapeutic agents in drug delivery as well as engineering functional tissues and promoting the healing process in tissue engineering and regenerative medicine. This perspective presents important milestones in the development of polymeric biomaterials with defined structures and properties. Contemporary studies of biomaterial design have been reviewed with focus on constructing materials with controlled structure, dynamic functionality, and biological complexity. Examples of these polymeric biomaterials enabled by advanced synthetic methodologies, dynamic chemistry/assembly strategies, and modulated cell-material interactions have been highlighted. As the field of polymeric biomaterials continues to evolve with increased sophistication, current challenges and future directions for the design and translation of these materials are also summarized.

RAFT synthesis of triply responsive poly[N-[2-(dialkylamino)ethyl]acrylamide]s and their N-substitute determined response

The thermo- and pH/CO₂-responsive poly[N-[2-(dialkylamino)ethyl]acrylamide]s containing a polyacrylamide backbone but different N-substitutes of dialkylamine were synthesized and their solution properties were comparatively checked.