Structural dynamic of a self-assembling peptide d-EAK16 made of only D-amino acids

Dipeptide nanostructures: Synthesis, interactions, advantages and biomedical applications

Short peptides are important in the design of self-assembled materials due to their versatility and flexibility. Self-assembled dipeptides, a group of peptide nanostructures, have highly attractive uses in the field of biomedicine. Recently these materials have proved to be important nanostructures because of their biocompatibility, low-cost and simplicity of synthesis, functionality/easy tunability and nano dimensions. Although there are different studies on peptide and protein-based nanostructures, more information about self-assembled nanostructures for dipeptides is still required to discover the advantages, challenges, importance, synthesis, interactions, and applications. This review describes and discusses the self-assembled dipeptide nanostructures especially for biomedical applications.

Single-conformation spectroscopy of cold, protonated DPG-containing peptides: switching β-turn types and formation of a sequential type II/II' double β-turn

D-Proline (^DPro, ^DP) is widely utilized to form β-hairpin loops in engineered peptides that would otherwise be unstructured, most often as part of a ^DPG sub-unit that forms a β-turn. To observe whether ^DPG facilitated this effect in short protonated peptides, conformation specific IR-UV double resonance photofragment spectra of the cold (∼10 K) protonated ^DP and ^LP diastereomers of the pentapeptide YAPGA was carried out in the hydride stretch (2800-3700 cm^-1) and amide I/II (1400-1800 cm^-1) regions. A model localized Hamiltonian was developed to better describe the 1600-1800 cm^-1 region commonly associated with the amide I vibrations. The CO stretch fundamentals experience extensive mixing with the N-H bending fundamentals of the NH₃⁺ group in these protonated peptides. The model Hamiltonian accounts for experiment in quantitative detail. In the ^DP diastereomer, all the population is funneled into a single conformer which presented as a type II β-turn with A and ^DP in the i + 1 and i + 2 positions, respectively. This structure was not the anticipated type II' β-turn across ^DPG that we had hypothesized based on solution-phase propensities. Analysis of the conformational energy landscape shows that both steric and charge-induced effects play a role in the preferred formation of the type II β-turn. In contrast, the ^LP isomer forms three conformations with very different structures, none of which were type II/II' β-turns, confirming that ^LPG is not a β-turn former. Finally, single-conformation spectroscopy was also carried out on the extended peptide [YAA^DPGAAA + H]⁺ to determine whether moving the protonated N-terminus further from ^DPG would lead to β-hairpin formation. Despite funneling its entire population into a single peptide backbone structure, the assigned structure is not a β-hairpin, but a concatenated type II/type II' double β-turn that displaces the peptide backbone laterally by about 7.5 Å, but leaves the backbone oriented in its original direction.

Enzyme-Responsive Amphiphilic Peptide Nanoparticles for Biocompatible and Efficient Drug Delivery

Self-assembled peptide nanostructures recently have gained much attention as drug delivery systems. As biomolecules, peptides have enhanced biocompatibility and biodegradability compared to polymer-based carriers. We introduce a peptide nanoparticle system containing arginine, histidine, and an enzyme-responsive core of repeating GLFG oligopeptides. GLFG oligopeptides exhibit specific sensitivity towards the enzyme cathepsin B that helps effective controlled release of cargo molecules in the cytoplasm. Arginine can induce cell penetration, and histidine facilitates lysosomal escape by its buffering capacity. Herein, we propose an enzyme-responsive amphiphilic peptide delivery system (Arg-His-(Gly-Phe-Lue-Gly)₃, RH-(GFLG)₃). The self-assembled RH-(GFLG)₃ globular nanoparticle structure exhibited a positive charge and formulation stability for 35 days. Nile Red-tagged RH-(GFLG)₃ nanoparticles showed good cellular uptake compared to the non-enzyme-responsive control groups with d-form peptides (LD (^LRH-^D(GFLG)₃), DL (^DRH-L(GFLG)₃), and DD (^DRH-^D(GFLG)₃). The RH-(GFLG)₃ nanoparticles showed negligible cytotoxicity in HeLa cells and human RBCs. To determine the drug delivery efficacy, we introduced the anticancer drug doxorubicin (Dox) in the RH-(GFLG)₃ nanoparticle system. LL-Dox exhibited formulation stability, maintaining the physical properties of the nanostructure, as well as a robust anticancer effect in HeLa cells compared to DD-Dox. These results indicate that the enzyme-sensitive RH-(GFLG)₃ peptide nanoparticles are promising candidates as drug delivery carriers for biomedical applications.

Peptide-based nanomaterials: Self-assembly, properties and applications

Peptide-based materials that have diverse structures and functionalities are an important type of biomaterials. In former times, peptide-based nanomaterials with excellent stability were constructed through self-assembly. Compared with individual peptides, peptide-based self-assembly nanomaterials that form well-ordered superstructures possess many advantages such as good thermo- and mechanical stability, semiconductivity, piezoelectricity and optical properties. Moreover, due to their excellent biocompatibility and biological activity, peptide-based self-assembly nanomaterials have been vastly used in different fields. In this review, we provide the advances of peptide-based self-assembly nanostructures, focusing on the driving forces that dominate peptide self-assembly and assembly mechanisms of peptides. After that, we outline the synthesis and properties of peptide-based nanomaterials, followed by the applications of functional peptide nanomaterials. Finally, we provide perspectives on the challenges and future of peptide-based nanomaterials.

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This review summarizes the advances of peptide-based nanomaterials, focusing on the mechanisms, properties, and applications.

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Outlining the synthesis and properties of peptide nanomaterials is helpful for the relevant research fields.

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The peptide-based nanomaterials show potential applications in many fields.

Interactions of two enantiomers of a designer antimicrobial peptide with structural components of the bacterial cell envelope

Antimicrobial peptides (AMPs) have great potential in treating multi-drug resistant bacterial infections. The antimicrobial activity of d-enantiomers is significantly higher than l-enantiomers and sometimes selectively enhanced against Gram-positive bacteria. Unlike phospholipids in the bacterial plasma membrane, the role of other bacterial cell envelop components is often overlooked in the mode of action of AMPs. In this work, we explored the structural interactions between the main different structural components in Gram-negative/Gram-positive bacteria and the two enantiomers of a designer AMP, GL13K. We observed that both l-GL13K and d-GL13K formed self-assembled amyloid-like nanofibrils when the peptides interacted with lipopolysaccharide and lipoteichoic acid, components of the outer membrane of Gram-negative bacteria and cell wall of Gram-positive bacteria, respectively. Another cell wall component, peptidoglycan, showed strong interactions exclusively with d-GL13K and formed distinct laminar structures. This specific interaction between peptidoglycans and d-GL13K might contribute to the enhanced activity of d-GL13K against Gram-positive bacteria as they have a much thicker peptidoglycan layer than Gram-negative bacteria. A better understanding of the specific role of bacterial cell envelop components in the AMPs mechanism of action can guide the design of more effective Gram-selective AMPs.

Self-Assembling Peptides: From Design to Biomedical Applications

Self-assembling peptides could be considered a novel class of agents able to harvest an array of micro/nanostructures that are highly attractive in the biomedical field. By modifying their amino acid composition, it is possible to mime several biological functions; when assembled in micro/nanostructures, they can be used for a variety of purposes such as tissue regeneration and engineering or drug delivery to improve drug release and/or stability and to reduce side effects. Other significant advantages of self-assembled peptides involve their biocompatibility and their ability to efficiently target molecular recognition sites. Due to their intrinsic characteristics, self-assembled peptide micro/nanostructures are capable to load both hydrophobic and hydrophilic drugs, and they are suitable to achieve a triggered drug delivery at disease sites by inserting in their structure's stimuli-responsive moieties. The focus of this review was to summarize the most recent and significant studies on self-assembled peptides with an emphasis on their application in the biomedical field.

Peptide-based supramolecular vaccine systems

Currently approved replication-competent and inactivated vaccines are limited by excessive reactogenicity and poor safety profiles, while subunit vaccines are often insufficiently immunogenic without co-administering exogenous adjuvants. Self-assembling peptide-, peptidomimetic-, and protein-based biomaterials offer a means to overcome these challenges through their inherent modularity, multivalency, and biocompatibility. As these scaffolds are biologically derived and present antigenic arrays reminiscent of natural viruses, they are prone to immune recognition and are uniquely capable of functioning as self-adjuvanting vaccine delivery vehicles that improve humoral and cellular responses. Beyond this intrinsic immunological advantage, the wide range of available amino acids allows for facile de novo design or straightforward modifications to existing sequences. This has permitted the development of vaccines and immunotherapies tailored to specific disease models, as well as generalizable platforms that have been successfully applied to prevent or treat numerous infectious and non-infectious diseases. In this review, we briefly introduce the immune system, discuss the structural determinants of coiled coils, β-sheets, peptide amphiphiles, and protein subunit nanoparticles, and highlight the utility of these materials using notable examples of their innate and adaptive immunomodulatory capacity.

Lysozyme-dalargin self-organization at the aqueous-air and liquid-liquid interfaces

An experimental study of protein-peptide binding was performed by means of radiochemical and spectroscopic methods. Lysozyme and dalargin were chosen due to their biological and physiological importance. By means of tensiometry and radiochemical assays, it was found that dalargin possesses rather high surface activity at the aqueous-air and aqueous-p-xylene interfaces to be substituted by protein. Dalargin forms a hydrophobic complex with lysozyme in which the secondary structure of lysozyme is preserved. When lysozyme forms a mixed adsorption layer with dalargin at the aqueous-air surface, the peptide prevents protein from concentrating in the subsurface monolayer. In the presence of p-xylene protein in the interface, reorganization occurs quickly, so there is no lag in the interfacial tension time dependence. The interfacial tension in this case is controlled by protein and/or protein-peptide complexes. An increase in the enzymatic activity of lysozyme in the presence of dalargin was confirmed by a docking model that suggests the formation of hydrogen bonds between dalargin and amino acid residues in the active site.

Self-assembling peptide scaffolds in the clinic

Well-defined scaffold hydrogels made of self-assembling peptides have found their way into clinical products. By examining the properties and applications of two self-assembling peptides-EAK16 and RADA16-we highlight the potential for translating designer biological scaffolds into commercial products.

A novel biocomposite scaffold with antibacterial potential and the ability to promote bone repair

Clinical treatment of bone defects caused by trauma, tumor resection and other bone diseases, especially bone defects that can lead to infection, remains a major challenge. Currently, autologous bone implantation is the gold standard for treatment of bone defects, but it is limited by secondary trauma and insufficient autologous material. Moreover, postoperative infection is an important factor affecting bone healing.AcN-RADARADARADARADA-CONH2 (RADA) is a new type of self-assembling peptide(SAP) composed of Arg,Ala,Asp and other amino acids was designed and prepared. The "RADA" self-assembling peptide hydrogels has excellent biological activity and it's completely biodegradable and non-toxic.It is also have been confirmed to promote cell proliferation, wound healing, tissue repair, and drug delivery. To promote bone regeneration and simultaneously prevent bacterial infection, we designed biocomposite scaffolds comprising RADA and calcium phosphate cement (CPC), termed RADA-CPC. The morphological features of the scaffold were characterized by scanning electron microscopy (SEM). In vitro studies demonstrated that RADA-CPC enhances osteoblast proliferation, differentiation and mineralization. In addition, the scaffold was used as a drug delivery system to treat postoperative infections by sustained release of ciprofloxacin (CIP). The RADA-CPC scaffold may have potential application prospects in orthopedics field because of its role in promoting bone repair and as a sustained-release drug carrier to prevent infections.

Chemically-Induced Cross-Linking of Peptidic Fibrils for Scaffolding Polymeric Particles and Macrophages

EAK16-II (EAK) is a self-assembling peptide (SAP) that forms β-sheets and β-fibrils through ionic-complementary interactions at physiological ionic strengths. The soft materials can be injected in vivo, creating depots of drugs and cells for rendering pharmacological and biological actions. The scope of the applications of EAK is sought to extend to tissues through which the flow of extracellular fluid tends to be limited. In such anatomical locales the rate and extent of the fibrilization are limited insofar as drug delivery and cellular scaffolding would be impeded. A method is generated utilizing a carbodiimide cross-linker by which EAK fibrils are pre-assembled yet remain injectable soft materials. It is hypothesized that the resulting de novo covalent linkages enhance the stacking of the β-sheet bilayers, thereby increasing the lengths of the fibrils and the extent of their cross-linking, as evidenced in Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy, scanning electron microscopy, and atomic force microscopy analyses. The cross-linked EAK (clEAK) retains polymeric microspheres with an average diameter of 1 µm. Macrophages admixed with clEAK remain viable and do not produce the inflammatory mediator interleukin-1β. These results indicate that clEAK should be investigated further as a platform for delivering particles and cells in vivo.

Design of RGDS Peptide-Immobilized Self-Assembling β-Strand Peptide from Barnacle Protein

We designed three types of RGD-containing barnacle adhesive proteins using self-assembling peptides. In the present study, three types of RGD-containing peptides were synthesized by solid-phase peptide synthesis, and the secondary structures of these peptides were analyzed by CD and FT-IR spectroscopy. The mechanical properties of peptide hydrogels were characterized by a rheometer. We discuss the correlation between the peptide conformation, and cell attachment and cell spreading activity from the viewpoint of developing effective tissue engineering scaffolds. We created a peptide-coated cell culture substrate by coating peptides on a polystyrene plate. They significantly facilitated cell adhesion and spreading compared to a non-coated substrate. When the RGDS sequence was modified at N- or C-terminal of R-Y, it was found that the self-assembling ability was dependent on the strongly affects hydrogel formation and cell adhesion caused by its secondary structure.

Self-assembling peptides: From a discovery in a yeast protein to diverse uses and beyond

Well-defined nanofiber scaffold hydrogels made of self-assembling peptides have found their way into various 3D tissue culture and clinical products. I reflect initial puzzlement of the unexpected discovery, gradual understanding of how these peptides undergo self-assembly, to eventually translating designer biological scaffolds into commercial products. Peptides are ubiquitous in nature and useful in many fields. They are found as hormones, pheromones, antibacterial, and antifungal agents in innate immunity systems, toxins, as well anti-inset pesticides. However, the concept of peptides as materials was not recognized until 1990 when a self-assembling peptide as a repeating segment in a yeast protein was serendipitously discovered. The peptide materials have bona fide materials properties and are made from simple amino acids with well-ordered nanostructures under physiological conditions. Some current applications include: (a) Real 3D tissue cell cultures of diverse tissue cells and various stem cells; (b) reparative and regenerative medicine as well as tissue engineering; (c) 3D tissue printing; (d) sustained releases of small molecules, growth factors and monoclonal antibodies; and (e) accelerated wound healing of skin and diabetic ulcers as well as instant hemostasis in surgery. Self-assembling peptide nanobiotechnology will likely continue to expand in many directions in the coming years. I will also briefly introduce my current research using a simple QTY code for membrane protein design. I am greatly honored and humbled to be invited to contribute an Award Winner Recollection of the 2020 Emil Thomas Kaiser Award from the Protein Society.

Synthetic peptide hydrogels as 3D scaffolds for tissue engineering

The regeneration of tissues and organs poses an immense challenge due to the extreme complexity in the research work involved. Despite the tissue engineering approach being considered as a promising strategy for more than two decades, a key issue impeding its progress is the lack of ideal scaffold materials. Nature-inspired synthetic peptide hydrogels are inherently biocompatible, and its high resemblance to extracellular matrix makes peptide hydrogels suitable 3D scaffold materials. This review covers the important aspects of peptide hydrogels as 3D scaffolds, including mechanical properties, biodegradability and bioactivity, and the current approaches in creating matrices with optimized features. Many of these scaffolds contain peptide sequences that are widely reported for tissue repair and regeneration and these peptide sequences will also be discussed. Furthermore, 3D biofabrication strategies of synthetic peptide hydrogels and the recent advances of peptide hydrogels in tissue engineering will also be described to reflect the current trend in the field. In the final section, we will present the future outlook in the design and development of peptide-based hydrogels for translational tissue engineering applications.

Delivery of MSCs with a Hybrid β-Sheet Peptide Hydrogel Consisting IGF-1C Domain and D-Form Peptide for Acute Kidney Injury Therapy

Purpose

By providing a stem cell microenvironment with particular bioactive constituents in vivo, synthetic biomaterials have been progressively successful in stem cell-based tissue regeneration by enhancing the engraftment and survival of transplanted cells. Designs with bioactive motifs to influence cell behavior and with D-form amino acids to modulate scaffold stability may be critical for the development and optimization of self-assembling biomimetic hydrogel scaffolds for stem cell therapy.

Materials and Methods

In this study, we linked naphthalene (Nap) covalently to a short D-form peptide (Nap-^DF^DFG) and the C domain of insulin-like growth factor-1 (IGF-1C) as a functional hydrogel-based scaffolds, and we hypothesized that this hydrogel could enhance the therapeutic efficiency of human placenta-derived mesenchymal stem cells (hP-MSCs) in a murine acute kidney injury (AKI) model.

Results

The self-assembling peptide was constrained into a classical β-sheet structure and showed hydrogel properties. Our results revealed that this hydrogel exhibited increased affinity for IGF-1 receptor. Furthermore, cotransplantation of the β-IGF-1C hydrogel and hP-MSCs contributed to endogenous regeneration post-injury and boosted angiogenesis in a murine AKI model, leading to recovery of renal function.

Conclusion

This hydrogel could provide a favorable niche for hP-MSCs and thereby rescue renal function in an AKI model by promoting cell survival and angiogenesis. In conclusion, by covalently linking the desired functional groups to D-form peptides to create functional hydrogels, self-assembling β-sheet peptide hydrogels may serve as a promising platform for tissue-engineering and stem cell therapy.

Point your SmartPhone at the code above. If you have a QR code reader the video abstract will appear. Or use:

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Controlled release of basic fibroblast growth factor from a peptide biomaterial for bone regeneration

Self-assembled peptide scaffolds based on D-RADA16 are an important matrix for controlled drug release and three-dimensional cell culture. In this work, D-RADA16 peptide hydrogels were coated on artificial bone composed of nano-hydroxyapatite/polyamide 66 (nHA/PA66) to obtain a porous drug-releasing structure for treating bone defects. The developed materials were characterized via transmission electron microscopy and scanning electron microscopy. The proliferation and adhesion of bone mesenchymal stem cells (BMSCs) were examined by confocal laser microscopy and CCK-8 experiments. The osteogenic ability of the porous materials towards bone BMSCs was examined in vitro by staining with Alizarin Red S and alkaline phosphatase, and bioactivity was evaluated in vivo. The results revealed that nHA/PA66/D-RADA16/bFGF reduces the degradation rate of D-RADA16 hydrogels and prolongs sustained release of bFGF, which would promote BMSCs proliferation, adhesion and osteogenesis in vitro and bone repair in vivo. Thus, it deserves more attention and is worthy of further research.

Hydrogel formation by short D-peptide for cell-culture scaffolds

The present study reports that a short oligopeptide D-P1, consisting of only five D-amino acids, self-assembled into entangled nanofibers to form a hydrogel that functioned as a scaffold for cell cultures. D-P1 (Ac-D-Phe-D-Phe-D-Phe-Gly-D-Lys) gelated aqueous buffer solution and water at a minimum gelation concentration of 0.5 wt%. The circular dichroism (CD) measurements demonstrated the formation of a β-sheet structure in the self-assembly of D-P1. We investigated the gelation properties and CD spectra of both the D- and L-forms of the oligopeptide, and found only a minimal difference between them. The D-P1 hydrogel was resistant to a protease, whereas the L-P1 hydrogel was rapidly degraded. Both oligopeptides exhibited nontoxic properties to human cancer cells and embryoid bodies (EBs) derived from human-induced pluripotent stem cells. Additionally, we succeeded in forming spheroids of HeLa cells on the D-P1 hydrogel, which indicates the potential of this hydrogel for 3-dimensional cell culture.

D-RADA16-RGD-Reinforced Nano-Hydroxyapatite/Polyamide 66 Ternary Biomaterial for Bone Formation

BACKGROUND

Nano-hydroxyapatite/polyamide 66 (nHA/PA66) is a composite used widely in the repair of bone defects. However, this material is insufficient bioactivity. In contrast, D-RADA16-RGD self-assembling peptide (D-RADA16-RGD sequence containing all D-amino acids is Ac-RADARADARADARADARGDS-CONH₂) shows admirable bioactivity for both cell culture and bone regeneration. Here, we describe the fabrication of a favorable biomaterial material (nHA/PA66/D-RADA16-RGD).

METHODS

Proteinase K and circular dichroism spectroscopy were employed to test the stability and secondary structural properties of peptide D-RADA16-RGD respectively. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the surface of these materials. Confocal laser scanning (CLS), cell counting kit-8 tests (CCK-8), alizarin red S staining, cell immunofluorescence analysis and Western blotting were involved in vitro. Also biosafety and bioactivity of them have been evaluated in vivo.

RESULTS

Proteinase K and circular dichroism spectroscopy demonstrated that D-RADA16-RGD in nHA/PA66 was able to form stable-sheet secondary structure. SEM and TEM showed that the D-RADA16-RGD material was 7-33 nm in width and 130-600 nm in length, and the interwoven pore size ranged from 40 to 200 nm. CLS suggests that cells in nHA/PA66/D-RADA16-RGD group were linked to adjacent cells with more actin filaments. CCK-8 analysis showed that nHA/PA66/D-RADA16-RGD revealed good biocompatibility. The results of Alizarin-red S staining and Western blotting as well as vivo osteogenesis suggest nHA/PA66/D-RADA16-RGD exhibits better bioactivity.

CONCLUSION

This study demonstrates that our nHA/PA66/D-RADA16-RGD composite exhibits reasonable mechanical properties, biocompatibility and bioactivity with promotion of bone formation.

Self-assembly dynamics and antimicrobial activity of all l- and d-amino acid enantiomers of a designer peptide

Recent studies have shown that antimicrobial peptides (AMPs) can self-assemble into supramolecular structures, but this has been overlooked as causative of their antimicrobial activity. Also, the higher antimicrobial potency of d-enantiomers compared to l-enantiomers of AMPs cannot always be attributed to their different resistance to protease degradation. Here, we tested all l- and d-amino acid versions of GL13K, an AMP derived from a human protein, to study structural links between the AMP secondary structure, supramolecular self-assembly dynamics, and antimicrobial activity. pH dependence and the evolution of secondary structures were related to a self-assembly process with differences among these AMPs. The two GL13K enantiomers formed analogous self-assembled twisted nanoribbon structures, but d-GL13K initiated self-assembly faster and had notably higher antimicrobial potency than l-GL13K. A non-antimicrobial scrambled amino acid version of l-GL13K assembled at a much higher pH to form distinctively different self-assembled structures than l-GL13K. Our results support a functional relationship between the AMP self-assembly and their antimicrobial activity.

Biofunctionalized peptide nanofiber-based composite scaffolds for bone regeneration

Bone tissue had moderate self-healing capabilities, but biomaterial scaffolds were required for the repair of some defects such as large bone defects. Peptide nanofiber scaffolds demonstrated important potential in regenerative medicine. Functional modification and controlled release of signal molecules were two significant approaches to increase the bioactivity of biofunctionalized peptide nanofiber scaffolds, but peptide scaffolds were limited by insufficient mechanical strength. Thus, it was necessary to combine peptide scaffolds with other materials including polymers, hydroxyapatite, demineralized bone matrix (DBM) and metal materials based on the requirement of different bone defects. As the development of peptide-based composite scaffolds continued to evolve, ultimate translation to the clinical environment may allow for improved therapeutic outcomes for bone repair.

Recent advances in smart biotechnology: Hydrogels and nanocarriers for tailored bioactive molecules depot

Over the past ten years, the global biopharmaceutical market has remarkably grown, with ten over the top twenty worldwide high performance medical treatment sales being biologics. Thus, biotech R&D (research and development) sector is becoming a key leading branch, with expanding revenues. Biotechnology offers considerable advantages compared to traditional therapeutic approaches, such as reducing side effects, specific treatments, higher patient compliance and therefore more effective treatments leading to lower healthcare costs. Within this sector, smart nanotechnology and colloidal self-assembling systems represent pivotal tools able to modulate the delivery of therapeutics. A comprehensive understanding of the processes involved in the self-assembly of the colloidal structures discussed therein is essential for the development of relevant biomedical applications. In this review we report the most promising and best performing platforms for specific classes of bioactive molecules and related target, spanning from siRNAs, gene/plasmids, proteins/growth factors, small synthetic therapeutics and bioimaging probes.

Discovery and design of self-assembling peptides

Peptides are ubiquitous in nature and useful in many fields, from agriculture as pesticides, in medicine as antibacterial and antifungal drugs founded in the innate immune systems, to medicinal chemistry as hormones. However, the concept of peptides as materials was not recognized until 1990 when a self-assembling peptide as a repeating segment in a yeast protein was serendipitously discovered. Peptide materials are so called because they have bona fide materials property and are made from simple amino acids with well-ordered nanostructures under physiological conditions. These structures include well-ordered nanofibres, nanotubes and nanovesicles. These peptide materials have been used for: (i) three-dimensional tissue cell cultures of primary cells and stem cells, (ii) three-dimensional tissue printing, (iii) sustained releases of small molecules, growth factors, monoclonal antibody and siRNA, (iv) accelerated wound healing in reparative and regenerative medicine as well as tissue engineering, (v) used to stabilize membrane proteins including difficult G-protein coupled receptors and photosystem I for designing nanobiodevices, (vi) a few self-assembling peptides have been used in human clinical trials for accelerated wound healings in surgical uses and (vii) in human clinical trials for siRNA delivery for treatment of cancers. It is likely that these self-assembling peptides will open doors for more and more diverse uses. The field of self-assembling peptides is growing in a number of directions in areas of materials, synthetic biology, and clinical medicine and beyond.

Printable Fluorescent Hydrogels Based on Self-Assembling Peptides

Fluorescent hydrogels (FH) have a variety of potential applications in the field of soft electronics. However, fabrication of mechanically stable and printable fluorescent hydrogels remains challenging. Here, we report a kind of fluorescent hydrogel based on the co-assembly of peptide motif and transition metal ions. The metal ions are captured in the hydrogel network at specific positions through covalently linked ligands on the peptide hydrogelators. This efficiently prevents the aggregation and self-quenching of organometallic chromophores. In addition, the formation of metal-ligand complexes introduces additional interactions to stabilize the hydrogel network, making the FH even more stable after the incorporation of metal ions. The FH is optically transparent but highly fluorescent. By using three different metal ions, the white light fluorescent supramolecular hydrogel has been achieved. As a proof-of-principle, we demonstrate the printability of the hydrogels to various patterns. We anticipate that with the improved fluorescent performance and stability, this kind of FH can find broad applications in extrusion-based 3D printing for the construction of soft electronics.

D-amino acid-containing supramolecular nanofibers for potential cancer therapeutics

Nanostructures formed by peptides that self-assemble in water through non-covalent interactions have attracted considerable attention because peptides possess several unique advantages, such as modular design and easiness of synthesis, convenient modification with known functional motifs, good biocompatibility, low immunogenicity and toxicity, inherent biodegradability, and fast responses to a wide range of external stimuli. After about two decades of development, peptide-based supramolecular nanostructures have already shown great potentials in the fields of biomedicine. Among a range of biomedical applications, using such nanostructures for cancer therapy has attracted increased interests since cancer remains the major threat for human health. Comparing with L-peptides, nanostructures containing peptides made of D-amino acid (i.e., D-peptides) bear a unique advantage, biostability (i.e., resistance towards most of endogenous enzymes). The exploration of nanostructures containing D-amino acids, especially their biomedical applications, is still in its infancy. Herein we review the recent progress of D-amino acid-containing supramolecular nanofibers as an emerging class of biomaterials that exhibit unique features for the development of cancer therapeutics. In addition, we give a brief perspective about the challenges and promises in this research direction.

Functionalized d-form self-assembling peptide hydrogels for bone regeneration

Bone defects are very common in orthopedics, and there is great need to develop suitable bone grafts for transplantation in vivo. However, current bone grafts still encounter some limitations, including limited availability, immune rejection, poor osteoinduction and osteoconduction, poor biocompatibility and degradation properties, etc. Self-assembling peptide nanofiber scaffolds have emerged as an important substrate for cell culture and bone regeneration. We report on the structural features (eg, Congo red staining, circular dichroism spectroscopy, transmission electron microscopy, and rheometry assays) and osteogenic ability of d-RADA16-RGD peptide hydrogels (with or without basic fibroblast growth factor) due to the better stability of peptide bonds formed by these peptides compared with those formed by l-form peptides, and use them to fill the femoral condyle defect of Sprague Dawley rat model. The bone morphology change, two-dimensional reconstructions using microcomputed tomography, quantification of the microcomputed tomography analyses as well as histological analyses have demonstrated that RGD-modified d-form peptide scaffolds are able to enhance extensive bone regeneration.

Structure–mechanical property correlations of hydrogel forming β-sheet peptides

This review discusses about β-sheet peptide structure at the molecular level and the bulk mechanical properties of the corresponding hydrogels.

The Unexpected Advantages of Using D-Amino Acids for Peptide Self- Assembly into Nanostructured Hydrogels for Medicine

Self-assembled peptide hydrogels have brought innovation to the medicinal field, not only as responsive biomaterials but also as nanostructured therapeutic agents or as smart drug delivery systems. D-amino acids are typically introduced to increase the peptide enzymatic stability. However, there are several reports of unexpected effects on peptide conformation, self-assembly behavior, cytotoxicity and even therapeutic activity. This mini-review discusses all the surprising twists of heterochiral self-assembled peptide hydrogels, and delineates emerging key findings to exploit all the benefits of D-amino acids in this novel medicinal area.

Characterization of irreversible physio-mechanical processes in stretched fetal membranes

We perform bulge tests on live fetal membrane (FM) tissues that simulate the mechanical conditions prior to contractions. Experimental results reveal an irreversible mechanical behavior that appears during loading and is significantly different than the mechanical behavior that appears during unloading or in subsequent loading cycles. The irreversible behavior results in a residual strain that does not recover upon unloading and remains the same for at least 1h after the FM is unloaded. Surprisingly, the irreversible behavior demonstrates a linear stress-strain relation. We introduce a new model for the mechanical response of collagen tissues, which accounts for the irreversible deformation and provides predictions in agreement with our experimental results. The basic assumption of the model is that the constitutive stress-strain relationship of individual elements that compose the collagen fibers has a plateau segment during which an irreversible transformation/deformation occurs. Fittings of calculated and measured stress-strain curves reveal a well-defined single-value property of collagenous tissues, which is related to the threshold strain εth for irreversible transformation. Further discussion of several physio-mechanical processes that can induce irreversible behavior indicate that the most probable process, which is in agreement with our results for εth, is a phase transformation of collagen molecules from an α-helix to a β-sheet structure. A phase transformation is a manifestation of a significant change in the molecular structure of the collagen tissues that can alter connections with surrounding molecules and may lead to critical biological changes, e.g., an initiation of labor.

STATEMENT OF SIGNIFICANCE

This study is driven by the hypothesis that pre-contraction mechanical stretch of the fetal membrane (FM) can lead to a change in the microstructure of the FM, which in turn induces a critical biological (hormonal) change that leads to the initiation of labor. We present mechanical characterizations of live FM tissues that reveal a significant irreversible process and a new model for the mechanical response of collagen tissues, which accounts for this process. Fittings of calculated and measured results reveal a well-defined single-value property of collagenous tissues, which is related to the threshold strain for irreversible transformation. Further discussion indicates that the irreversible deformation is induced by a phase transformation of collagen molecules that can lead to critical biological changes.

Designer functionalised self-assembling peptide nanofibre scaffolds for cartilage tissue engineering

Owing to the limited regenerative capacity of cartilage tissue, cartilage repair remains a challenge in clinical treatment. Tissue engineering has emerged as a promising and important approach to repair cartilage defects. It is well known that material scaffolds are regarded as a fundamental element of tissue engineering. Novel biomaterial scaffolds formed by self-assembling peptides consist of nanofibre networks highly resembling natural extracellular matrices, and their fabrication is based on the principle of molecular self-assembly. Indeed, peptide nanofibre scaffolds have obtained much progress in repairing various damaged tissues (e.g. cartilage, bone, nerve, heart and blood vessel). This review outlines the rational design of peptide nanofibre scaffolds and their potential in cartilage tissue engineering.

pH-dependent self-assembly of EAK16 peptides in the presence of a hydrophobic surface: coarse-grained molecular dynamics simulation

Self-assembly behavior of the three types of ionic peptide, EAK16, is studied in the presence of a hydrophobic surface using coarse-grained molecular dynamics simulations at three pH ranges of the solution. It is found that the peptide chains of all the three types assemble on the hydrophobic surface. EAK16-I and EAK16-II peptides assemble into ribbon-like structures, regardless of the value of pH. EAK16-IV peptide chains, however, assemble into ribbon-like structures at low and high pH ranges and form disc-shaped assemblies on the hydrophobic surface at the isoelectric point, pH = 7. Strong intra-chain electrostatic interactions in the case of EAK16-IV peptide play the main role in dependence of its self-assembly behavior on pH and the different morphology of its assembly relative to those of the two other types. Kinetics of growth of the assemblies on the hydrophobic surface is also studied.

Cellular membrane enrichment of self-assembling D-peptides for cell surface engineering

We occasionally found that several self-assembling peptides containing D-amino acids would be preferentially enriched in cellular membranes at self-assembled stages while distributed evenly in the cytoplasma of cells at unassembled stages. Self-assembling peptides containing only Lamino acids distributed evenly in cytoplasma of cells at both self-assembled and unassembled stages. The self-assembling peptides containing D-amino acids could therefore be applied for engineering cell surface with peptides. More importantly, by integrating a protein binding peptide (a PDZ domain binding hexapeptide of WRESAI) with the self-assembling peptide containing D-amino acids, protein could also be introduced to the cell surface. This study not only provided a novel approach to engineer cell surface, but also highlighted the unusual properties and potential applications of self-assembling peptides containing D-amino acids in regenerative medicine, drug delivery, and tissue engineering.

Designing unconventional Fmoc-peptide-based biomaterials: structure and related properties

We have recently employed L-amino acids in the lipase-catalyzed biofabrication of a class of self-assembling Fmoc-peptides that form 3-dimensional nanofiber scaffolds. Here we report that using d-amino acids, the homochiral self-assembling peptide Fmoc-D-Phe3 (Fmoc-F*F*F*) also forms a 3-dimensional nanofiber scaffold that is substantially distinguishable from its L-peptide and heterochiral peptide (F*FF and FF*F*) counterparts on the basis of their physico-chemical properties. Such chiral peptides self-assemble into ordered nanofibers with well defined fibrillar motifs. Circular dichroism and atomic force microscopy have been employed to study in depth such fibrillar peptide structures. Dexamethasone release kinetics from PLGA and CS-PLGA nanoparticles entrapped within the peptidic hydrogel matrix encourage its use for applications in drug controlled release.

Morphology of nanostructures and their long-acting properties in vivo for a novel synthetic peptide of gonadotropin-releasing hormone antagonist

OBJECTIVES

To demonstrate the correlation between the nanostructure formation and the long duration of action in vivo of peptides, the morphology of nanostructures of LXT-101, a novel synthetic amphiphilic peptide of gonadotropin-releasing hormone antagonist were observed when dissolved in different solvents, and their long-acting properties in vivo were investigated in this study.

METHODS

The morphology of nanostructures of LXT-101 was observed by transmission electron microscopy when dissolved in different solvents, and the plasma concentrations of LXT-101 and testosterone levels were also assayed for different solutions after intramuscular injection administration in beagle dogs.

KEY FINDINGS

TEM data suggest that LXT-101 in pure water can form fibres, while in mannitol, dextrose or sodium chloride solution, they tend to form vesicles. The pharmacokinetic and pharmacodynamic results showed that the plasma concentrations of LXT-101 within 48 h were much higher but descended dramatically with mannitol, dextrose and NaCl solutions structurally composed of vesicles compared with that of pure water structurally composed of fibres. An effectively suppression of testosterone can be achieved only 2 or 3 days with the frontal three solutions, while LXT-101 in pure water maintained over a period of 7 days.

CONCLUSIONS

It may indicate that LXT-101 peptide in pure water forms fibre depot that release monomeric active peptide slowly. The correlation between the nanostructure and duration of action in vivo suggests that the addition of excipients influence self-assembly process of LXT-101 that leads to the formation of different nanostructures and exhibit various behaviours in vivo.

Molecular self-assembly guides the fabrication of peptide nanofiber scaffolds for nerve repair

The particular features render ionic self-complementary peptide-formed and peptide amphiphile-formed nanofiber scaffolds to be compelling biomaterial substrates for nerve repair.

Bone induction by biomimetic PLGA copolymer loaded with a novel synthetic RADA16-P24 peptide in vivo

Bone morphogenetic protein-2 (BMP-2) is a key bone morphogenetic protein, and poly(lactic-co-glycolic acid) (PLGA) has been widely used as scaffold for clinical use to carry treatment protein. In the previous studies, we have synthesized BMP-2-related peptide (P24) and found its capacity of inducing bone regeneration. In this research, we have synthesized a new amphiphilic peptide Ac-RADA RADA RADA RADA S[PO4]KIPKASSVPTELSAISTLYLDDD-CONH2 (RADA16-P24) with an assembly peptide RADA16-Ion the P24 item of BMP2 to form divalent ion-induced gelatin. Two methods of physisorption and chemical cross-linking were used to bind RADA16-P24 onto the surface of the copolymer PLGA to synthesize RADA16-P24-PLGA, and its capacity of attaching bone marrow stromal cells (BMSCs) was evaluated in vitro and inducing ectopic bone formation was examined in vivo. In vitro our results demonstrated that RADA16-P24-PLGA copolymer prepared by physisorbing or prepared by chemical cross-linking had a peptide binding rate of (2.0180±0.5296)% or (10.0820±0.8405)% respectively (P<0.05). In addition the BMSCs proliferated vigorously in the RADA16-P24-PLGA biomaterials. Significantly the percentage of BMSCs attached to RADA16-P24-PLGA composite prepared by chemical cross-linking and physisorbing were (71.4±7.5) % or (46.7±5.8) % (P<0.05). The in vivo study showed that RADA16-P24-PLGA chemical cross-linking could better induce ectopic bone formation compared with RADA16-P24-PLGA physisorbing and PLGA. It is concluded that the PLGA copolymer is a good RADA16-P24 carrier. This novel RADA16-P24-PLGA composite has strong osteogenic capability.

Molecular mechanisms of RADA16-1 peptide on fast stop bleeding in rat models

Ionic self-assembly of the peptide RADARADARADARADA (RADA16-1) may form a well-defined nanofiber and eventually a hydrogel scaffold, with a water content of over 99.5%. This leads to the establishment of a nanofiber barrier that can be used to achieve complete hemostasis in less than 20 s in multiple tissues and in a variety of different wounds. In the present study, the nanofiber scaffolds of RADA16-1 peptide were sonicated into smaller fragments to identify possible molecular mechanisms underlying the rapid cessation of bleeding associated with these materials. Atomic force microscopy (AFM), circular dichroism (CD), and rheometry were also used to evaluate the re-assembly kinetics of this peptide. A bleeding control experiment was performed in animal models to uncover the molecular mechanisms underlying this fast hemostasis. In this way, these sonicated fragments not only quickly reassembled into nanofibers indistinguishable from the original material, but the degree of reassembly was also correlated with an increase in the rigidity of the scaffold and increased as the time required for hemostasis increased.

Self-assembly of amphipathic β-sheet peptides: insights and applications

Amphipathic peptides composed of alternating polar and nonpolar residues have a strong tendency to self-assemble into one-dimensional, amyloid-like fibril structures. Fibrils derived from peptides of general (XZXZ)(n) sequence in which X is hydrophobic and Z is hydrophilic adopt a putative β-sheet bilayer. The bilayer configuration allows burial of the hydrophobic X side chain groups in the core of the fibril and leaves the polar Z side chains exposed to solvent. This architectural arrangement provides fibrils that maintain high solubility in water and has facilitated the recent exploitation of self-assembled amphipathic peptide fibrils as functional biomaterials. This article is a critical review of the development and application of self-assembling amphipathic peptides with a focus on the fundamental insight these types of peptides provide into peptide self-assembly phenomena.

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.

Molecular hydrogelators of peptoid-peptide conjugates with superior stability against enzyme digestion

We report on molecular hydrogelators based on peptoid-peptide conjugates with good biocompatibility to different cells and superior stability against proteinase K digestion.