Fibrillar peptide gels in biotechnology and biomedicine

Tuning gelation kinetics and mechanical rigidity of β-hairpin peptide hydrogels via hydrophobic amino acid substitutions

Self-assembling peptide hydrogels with faster gelation kinetics and higher mechanical rigidity are favorable for their practical applications. A design strategy to control the folding, self-assembly, and hydrogelation of β-hairpin peptides via hydrophobic amino acid substitutions has been explored in this study. Isoleucine has higher hydrophobicity and stronger propensity for β-sheet hydrogen bonding than valine. After the valine residues of MAX1 (VKVKVKVKV(D)PPTKVKVKVKV-NH2) were replaced with isoleucines, oscillatory rheometry and circular dichroism (CD) spectroscopy characterizations indicated that the variants had clearly faster self-assembly and hydrogelation rates and that the resulting gels displayed higher mechanical stiffness. Transmission electron microscopy (TEM) indicated the parent MAX1 and its variants all formed networks of long and entangled fibrils with the similar diameters of ∼3 nm, suggesting little effect of hydrophobic substitutions on the self-assembled morphology. The MAX1I8 (IKIKIKIKV(D)PPTKIKIKIKI-NH2) hydrogel showed the fastest gelation rate (within 5 min) and the highest gel rigidity with the series, supporting the homogeneous cell distribution within its 3D scaffold. In addition, the MAX1I8 hydrogel showed quick shear-thinning and rapid recovery upon cessation of shear strain, and the MTT and immunological assays indicated its low cytotoxicity and good biocompatibility. These features are highly attractive for its widespread use in 3D cell culturing and regenerative medical treatments.

Sequence effects of self-assembling multidomain peptide hydrogels on encapsulated SHED cells

Here we report three new nanofibrous, self-assembling multidomain peptide (MDP) sequences and examine the effect of sequence on the morphology and expansion of encapsulated Stem cells from Human Exfoliated Deciduous teeth (SHED). We modified our previously reported set of serine-based MDPs, changing the serine residues in the amphiphilic region to threonine. The three new threonine-based sequences self-assemble into antiparallel β-sheet nanofibers, confirmed by CD and IR. AFM and negative-stained TEM show that the nanofibers formed by the new sequences are more curved than their serine-containing predecessors. Despite this change in nanofiber morphology, SEM illustrates that all three new sequences still form porous hydrogels. K(TL)2SLRG(TL)3KGRGDS, with a designed cleavage site, is able to be degraded by Matrix Metalloprotease 2. We then examine SHED cell response to these new sequences as well as their serine-based predecessors. We observe faster cell attachment and spreading in hydrogels formed by K2(SL)6K2GRGDS and K(SL)3RG(SL)3KGRGDS. By day 3, the SHEDs in all of the serine-based sequences exhibit a fibroblast-like morphology. Additionally, the SHED cells expand more rapidly in the serine-based gels while the cell number remains relatively constant in the threonine-based peptides. In hydrogels formed by K2(TL)6K2GRGDS and K(TL)2SLRG(TL)3KGRGDS, this low expansion rate is accompanied by changes in morphology where SHEDs exhibit a stellate morphology after 3 days in culture; however, by day 7 they appear more fibroblast-shaped. Throughout the duration of the experiment, the SHED cells encapsulated in the K2(TL)6K2 hydrogels remain rounded. These results suggest that the basic MDP structure easily accommodates modifications in sequence and, for SHED cells, the threonine-containing gels require the integrin-binding RGDS sequence for cell attachment to occur, while the serine-based gels are less selective and support an increase in cell number, regardless of the presence or absence of RGDS.

Peptoids for biomaterials science

Reports of peptoid structures and interfaces highlighting their potential as synthetically convenient, multifunctional, modular and precisely tunable biomaterials are reviewed.

Reversible photocontrol of self-assembled peptide hydrogel viscoelasticity

Peptide hydrogels are promising biomaterials for applications ranging from drug delivery to tissue engineering.

One-pot aqueous phase synthesis of peptide–CdTe quantum dots

We report the design and synthesis of short peptide (hexapeptide)-capped CdTe quantum dots (peptide–QDs) by a one-pot method with excellent stability in acidic and high salt solutions.

Combining self-assembling peptide gels with three-dimensional elastomer scaffolds

Some of the problems raised by the combination of porous scaffolds and self-assembling peptide (SAP) gels as constructs for tissue engineering applications are addressed for the first time. Scaffolds of poly(ethyl acrylate) and the SAP gel RAD16-I were employed. The in situ gelation of the SAP gel inside the pores of the scaffolds was studied. The scaffold-cum-gel constructs were characterized morphologically, physicochemically and mechanically. The possibility of incorporating an active molecule (bovine serum albumin, taken here as a model molecule for others) in the gel within the scaffold's pores was assessed, and the kinetics of its release in phosphate-buffered saline was followed. Cell seeding and colonization of these constructs were preliminarily studied with L929 fibroblasts and subsequently checked with sheep adipose-tissue-derived stem cells intended for further preclinical studies. Static (conventional) and dynamically assisted seedings were compared for bare scaffolds and the scaffold-cum-gel constructs. The SAP gel inside the pores of the scaffold significantly improved the uniformity and density of cell colonization of the three-dimensional (3-D) structure. These constructs could be of use in different advanced tissue engineering applications, where, apart from a cell-friendly extracellular matrix -like aqueous environment, a larger-scale 3-D structure able to keep the cells in a specific place, give mechanical support and/or conduct spatially the tissue growth could be required.

Formation of ordered biomolecular structures by the self-assembly of short peptides

In nature, complex functional structures are formed by the self-assembly of biomolecules under mild conditions. Understanding the forces that control self-assembly and mimicking this process in vitro will bring about major advances in the areas of materials science and nanotechnology. Among the available biological building blocks, peptides have several advantages as they present substantial diversity, their synthesis in large scale is straightforward, and they can easily be modified with biological and chemical entities(1,2). Several classes of designed peptides such as cyclic peptides, amphiphile peptides and peptide-conjugates self-assemble into ordered structures in solution. Homoaromatic dipeptides, are a class of short self-assembled peptides that contain all the molecular information needed to form ordered structures such as nanotubes, spheres and fibrils(3-8). A large variety of these peptides is commercially available. This paper presents a procedure that leads to the formation of ordered structures by the self-assembly of homoaromatic peptides. The protocol requires only commercial reagents and basic laboratory equipment. In addition, the paper describes some of the methods available for the characterization of peptide-based assemblies. These methods include electron and atomic force microscopy and Fourier-Transform Infrared Spectroscopy (FT-IR). Moreover, the manuscript demonstrates the blending of peptides (coassembly) and the formation of a "beads on a string"-like structure by this process.(9) The protocols presented here can be adapted to other classes of peptides or biological building blocks and can potentially lead to the discovery of new peptide-based structures and to better control of their assembly.

Self-assembly and gelation properties of glycine/leucine Fmoc-dipeptides

Self-assembly of aromatic peptide amphiphiles is known to be driven by a combination of π-π stacking of the aromatic moieties and hydrogen bonding between the peptide backbones, with possible stabilisation from the amino acid side chains. Phenylalanine-based Fmoc-dipeptides have previously been reported for their characteristic apparent pKa transitions, which were shown to coincide with significant structural and morphological changes that were peptide sequence dependent. Here, phenylalanine was replaced by leucine and the effect on the self-assembling behaviour of Fmoc-dipeptides was measured using potentiometry, fluorescence and infrared spectroscopy, transmission electron microscopy, X-ray scattering and shear rheometry. This study provides additional cues towards the elucidation of the sequence-structure relationship in self-assembling aromatic peptide amphiphiles.

ECM-incorporated hydrogels cross-linked via native chemical ligation to engineer stem cell microenvironments

Limiting the precise study of the biochemical impact of whole molecule extracellular matrix (ECM) proteins on stem cell differentiation is the lack of 3D in vitro models that can accommodate many different types of ECM. Here we sought to generate such a system while maintaining consistent mechanical properties and supporting stem cell survival. To this end, we used native chemical ligation to cross-link poly(ethylene glycol) macromonomers under mild conditions while entrapping ECM proteins (termed ECM composites) and stem cells. Sufficiently low concentrations of ECM were used to maintain constant storage moduli and pore size. Viability of stem cells in composites was maintained over multiple weeks. ECM of composites encompassed stem cells and directed the formation of distinct structures dependent on ECM type. Thus, we introduce a powerful approach to study the biochemical impact of multiple ECM proteins (either alone or in combination) on stem cell behavior.

Low-molecular-weight gelators consisting of hybrid cyclobutane-based peptides

Some hybrid tetrapeptides consisting of (1R,2S)-2-aminocyclobutane-1-carboxylic acid and glycine, β-alanine, or γ-aminobutyric acid (GABA) joined in alternation, compounds 1-3, respectively, have been investigated to gain information on the non-covalent interactions responsible for their self-assembly to form ordered aggregates, as well as on parameters such as their morphology and size. All three peptides formed nice gels in many organic solvents and significant difference in their behaviour was not observed. Scanning electron microscopy (SEM) and circular dichroism (CD) pointed out that peptide 1, which contains the shortest C2 linear residue, presented the most defined fibril network and afforded nanoscale helical aggregates. Tetrapeptide 3, with C4 linear residues in its structure, also showed bundles of fibres whereas a homogeneous spherulitic network was observed for tetrapeptide 2, with a C3 spacer between cyclobutane residues. Computational calculations for 1 allowed us to model the self-assembly of the molecules and suggested a head-to-head arrangement to give helical structures corresponding to hydrogen-bonded single chains. These features were corroborated by a high-resolution NMR spectroscopy study of the dynamics of the gelation process in toluene-d8 which evidenced that molecules self-assemble to afford ordered aggregates with a supramolecular chirality.

Controlled release of simvastatin from in situ forming hydrogel triggers bone formation in MC3T3-E1 cells

Simvastatin (SIM), a drug commonly administered for the treatment of hypercholesterolemia, has been recently reported to induce bone regeneration/formation. In this study, we investigated the properties of hydrogel composed of gelatin-poly(ethylene glycol)-tyramine (GPT) as an efficient SIM delivery vehicle that can trigger osteogenic differentiation. Sustained delivery of SIM was achieved through its encapsulation in an injectable, biodegradable GPT-hydrogel. Cross-linking of the gelatin-based GPT-hydrogel was induced by the reaction of horse radish peroxidase and H(2)O(2). GPT-hydrogels of three different matrix stiffness, 1,800 (GPT-hydrogel1), 5,800 (GPT-hydrogel2), and 8,400 Pa (GPT-hydrogel3) were used. The gelation/degradation time and SIM release profiles of hydrogels loaded with two different concentrations of SIM, 1 and 3 mg/ml, were also evaluated. Maximum swelling times of GPT-hydrogel1, GPT-hydrogel2, and GPT-hydrogel3 were observed to be 6, 12, and 20 days, respectively. All GPT-hydrogels showed complete degradation within 55 days. The in vitro SIM release profiles, investigated in PBS buffer (pH 7.4) at 37°C, exhibited typical biphasic release patterns with the initial burst being more rapid with GPT-hydrogel1 compared with GPT-hydrogel3. Substantial increase in matrix metalloproteinase-13, osteocalcin expression levels, and mineralization were seen in osteogenic differentiation system using MC3T3-E1 cells cultured with GPT-hydrogels loaded with SIM in a dose-dependent manner. This study demonstrated that controlled release of SIM from a biodegradable, injectable GPT-hydrogel had a promising role for long-term treatment of chronic degenerative diseases such as disc degenerative disease.

Controlled release of dexamethasone from peptide nanofiber gels to modulate inflammatory response

New biomaterials that have the ability to locally suppress an immune response could have broad therapeutic use in the treatment of diseases characterized by acute or chronic inflammation or as a strategy to facilitate improved efficacy in cell or tissue transplantation. We report here on the preparation of a modular peptide amphiphile (PA) capable of releasing an anti-inflammatory drug, dexamethasone (Dex), by conjugation via a labile hydrazone linkage. This molecule self-assembled in water into long supramolecular nanofibers when mixed with a similar PA lacking the drug conjugate, and the addition of calcium salt to screen electrostatic repulsion between nanofibers promoted gel formation. These nanofiber gels demonstrated sustained release of soluble Dex for over one month in physiologic media. The Dex released from these gels maintained its anti-inflammatory activity when evaluated in vitro using a human inflammatory reporter cell line and furthermore preserved cardiomyocyte viability upon induced oxidative stress. The ability of this gel to mitigate the inflammatory response in cell transplantation strategies was evaluated using cell-surrogate polystyrene microparticles suspended in the nanofiber gel that were then subcutaneously injected into mice. Live animal luminescence imaging using the chemiluminescent reporter molecule luminol showed a significant reduction in inflammation at the site where particles were injected with Dex-PA compared to the site of injection for particles within a control PA in the same animal. Histological evidence suggested a marked reduction in the number of infiltrating inflammatory cells when particles were delivered within Dex-PA nanofiber gels, and very little inflammation was observed at either 3 days or 21 days post-implantation. The use of Dex-PA could facilitate localized anti-inflammatory activity as a component of biomaterials designed for various applications in regenerative medicine and could specifically be a useful module for PA-based therapies. More broadly, these studies define a versatile strategy for facile synthesis of self-assembling peptide-based materials with the ability to control drug release.

A highly scalable peptide-based assay system for proteomics

We report a scalable and cost-effective technology for generating and screening high-complexity customizable peptide sets. The peptides are made as peptide-cDNA fusions by in vitro transcription/translation from pools of DNA templates generated by microarray-based synthesis. This approach enables large custom sets of peptides to be designed in silico, manufactured cost-effectively in parallel, and assayed efficiently in a multiplexed fashion. The utility of our peptide-cDNA fusion pools was demonstrated in two activity-based assays designed to discover protease and kinase substrates. In the protease assay, cleaved peptide substrates were separated from uncleaved and identified by digital sequencing of their cognate cDNAs. We screened the 3,011 amino acid HCV proteome for susceptibility to cleavage by the HCV NS3/4A protease and identified all 3 known trans cleavage sites with high specificity. In the kinase assay, peptide substrates phosphorylated by tyrosine kinases were captured and identified by sequencing of their cDNAs. We screened a pool of 3,243 peptides against Abl kinase and showed that phosphorylation events detected were specific and consistent with the known substrate preferences of Abl kinase. Our approach is scalable and adaptable to other protein-based assays.

A general method for detecting protease activity via gelation and its application to artificial clotting

A modular system for detecting protease activity via enzyme-triggered gel formation is described. Protease-specific recognition sequences are utilized to achieve enzyme specificity. Artificial blood clotting is demonstrated by activating endogenous thrombin to trigger gelation in fibrinogen-deficient blood plasma.

End-to-end self-assembly of RADA 16-I nanofibrils in aqueous solutions

RADARADARADARADA (RADA 16-I) is a synthetic amphiphilic peptide designed to self-assemble in a controlled way into fibrils and higher ordered structures depending on pH. In this work, we use various techniques to investigate the state of the peptide dispersed in water under dilute conditions at different pH and in the presence of trifluoroacetic acid or hydrochloric acid. We have identified stable RADA 16-I fibrils at pH 2.0-4.5, which have a length of ∼200-400 nm and diameter of 10 nm. The fibrils have the characteristic antiparallel β-sheet structure of amyloid fibrils, as measured by circular dichroism and Fourier transform infrared spectrometry. During incubation at pH 2.0-4.5, the fibrils elongate very slowly via an end-to-end fibril-fibril aggregation mechanism, without changing their diameter, and the kinetics of such aggregation depends on pH and anion type. At pH 2.0, we also observed a substantial amount of monomers in the system, which do not participate in the fibril elongation and degrade to fragments. The fibril-fibril elongation kinetics has been simulated using the Smoluchowski kinetic model, population balance equations, and the simulation results are in good agreement with the experimental data. It is also found that the aggregation process is not limited by diffusion but rather is an activated process with energy barrier in the order of 20 kcal/mol.

Viscoelastic properties and nanoscale structures of composite oligopeptide-polysaccharide hydrogels

Biocompatible and biodegradable peptide hydrogels are drawing increasing attention as prospective materials for human soft tissue repair and replacement. To improve the rather unfavorable mechanical properties of our pure peptide hydrogels, in this work we examined the possibility of creating a double hydrogel network. This network was created by means of the coassembly of mutually attractive, but self-repulsive oligopeptides within an already-existing fibrous network formed by the charged, biocompatible polysaccharides chitosan, alginate, and chondroitin. Using dynamic oscillatory rheology experiments, it was found that the coassembly of the peptides within the existing polysaccharide network resulted in a less stiff material as compared to the pure peptide networks (the elastic modulus G' decreased from 90 to 10 kPa). However, these composite oligopeptide-polysaccharide hydrogels were characterized by a greater resistance to deformation (the yield strain γ grew from 4 to 100%). Small-angle neutron scattering (SANS) was used to study the 2D cross-sectional shapes of the fibers, their dimensional characteristics, and the mesh sizes of the fibrous networks. Differences in material structures found with SANS experiments confirmed rheology data, showing that incorporation of the peptides dramatically changed the morphology of the polysaccharide network. The resulting fibers were structurally very similar to those forming the pure peptide networks, but formed less stiff gels because of their markedly greater mesh sizes. Together, these findings suggest an approach for the development of highly deformation-resistant biomaterials.

Self-assembled amino acids and dipeptides as noncovalent hydrogels for tissue engineering

This review critically assesses progress in the use of self-assembling dipeptides and amino acids as hydrogel materials for tissue engineering.

Regulation of endothelial cell activation and angiogenesis by injectable peptide nanofibers

RAD16-II peptide nanofibers are promising for vascular tissue engineering and were shown to enhance angiogenesis in vitro and in vivo, although the mechanism remains unknown. We hypothesized that the pro-angiogenic effect of RAD16-II results from low-affinity integrin-dependent interactions of microvascular endothelial cells (MVECs) with RAD motifs. Mouse MVECs were cultured on RAD16-II with or without integrin and MAPK/ERK pathway inhibitors, and angiogenic responses were quantified. The results were validated in vivo using a mouse diabetic wound healing model with impaired neovascularization. RAD16-II stimulated spontaneous capillary morphogenesis, and increased β(3) integrin phosphorylation and VEGF expression in MVECs. These responses were abrogated in the presence of β(3) and MAPK/ERK pathway inhibitors or on the control peptide without RAD motifs. Wide-spectrum integrin inhibitor echistatin completely abolished RAD16-II-mediated capillary morphogenesis in vitro and neovascularization and VEGF expression in the wound in vivo. The addition of the RGD motif to RAD16-II did not change nanofiber architecture or mechanical properties, but resulted in significant decrease in capillary morphogenesis. Overall, these results suggest that low-affinity non-specific interactions between cells and RAD motifs can trigger angiogenic responses via phosphorylation of β(3) integrin and MAPK/ERK pathway, indicating that low-affinity sequences can be used to functionalize biocompatible materials for the regulation of cell migration and angiogenesis, thus expanding the current pool of available motifs that can be used for such functionalization. Incorporation of RAD or similar motifs into protein engineered or hybrid peptide scaffolds may represent a novel strategy for vascular tissue engineering and will further enhance design opportunities for new scaffold materials.

Dissolution parameters reveal role of structure and solvent in molecular gelation

The relationship between thermodynamic dissolution parameters (enthalpy and entropy) and gelation ability was examined for two different classes of compounds in three different solvent systems. In total, 11 dipeptides and 19 pyridines were synthesized and screened for gelation in aqueous and organic solvents, respectively. The dissolution parameters were determined from the variable-temperature solubilities using the van't Hoff equation. These studies revealed that the majority of gelators had higher dissolution enthalpies and entropies compared to nongelators, consistent with the notion that gelators have stronger intermolecular interactions and more order in the solid state. The dissolution parameters were also found to be solvent-dependent, suggesting that solvent-solute interactions are also important in gelation. Overall, these results indicate that converting nongelators into gelators is attainable when structural modifications or a change in solvent lead to increases in the dissolution parameters.

Chemically well-defined self-assembled monolayers for cell culture: toward mimicking the natural ECM

The extracellular matrix (ECM) is a network of biological macromolecules that surrounds cells within tissues. In addition to serving as a physical support, the ECM actively influences cell behavior by providing sites for cell adhesion, establishing soluble factor gradients, and forming interfaces between different cell types within a tissue. Thus, elucidating the influence of ECM-derived biomolecules on cell behavior is an important aspect of cell biology. Self-assembled monolayers (SAMs) have emerged as promising tools to mimic the ECM as they provide chemically well-defined substrates that can be precisely tailored for specific cell culture applications, and their application in this regard is the focus of this review. In particular, this review will describe various approaches to prepare SAM-based culture substrates via non-specific adsorption, covalent immobilization, or non-covalent sequestering of ECM-derived biomolecules. Additionally, this review will highlight SAMs that present ECM-derived biomolecules to cells to probe the role of these molecules in cell-ECM interactions, including cell attachment, spreading and 'outside-in' signaling via focal adhesion complex formation. Finally, this review will introduce SAMs that can present or sequester soluble signaling molecules, such as growth factors, to study the influence of localized soluble factor activity on cell behavior. Together, these examples demonstrate that the chemical specificity and variability afforded by SAMs can provide robust, well-defined substrates for cell culture that can simplify experimental design and analysis by eliminating many of the confounding factors associated with traditional culture substrates.

Directed intermixing in multicomponent self-assembling biomaterials

The noncovalent coassembly of multiple different peptides can be a useful route for producing multifunctional biomaterials. However, to date, such materials have almost exclusively been investigated as homogeneous self-assemblies, having functional components uniformly distributed throughout their supramolecular structures. Here we illustrate control over the intermixing of multiple different self-assembling peptides, in turn providing a simple but powerful means for modulating these materials' mechanical and biological properties. In β-sheet fibrillizing hydrogels, significant increases in stiffening could be achieved using heterobifunctional cross-linkers by sequestering peptides bearing different reactive groups into distinct populations of fibrils, thus favoring interfibril cross-linking. Further, by specifying the intermixing of RGD-bearing peptides in 2-D and 3-D self-assemblies, the growth of HUVECs and NIH 3T3 cells could be significantly modulated. This approach may be immediately applicable toward a wide variety of self-assembling systems that form stable supramolecular structures.

Evolving the use of peptides as components of biomaterials

This manuscript is part of a debate on the statement that "the use of short synthetic adhesion peptides, like RGD, is the best approach in the design of biomaterials that guide cell behavior for regenerative medicine and tissue engineering". We take the position that although there are some acknowledged disadvantages of using short peptide ligands within biomaterials, it is not necessary to discard the notion of using peptides within biomaterials entirely, but rather to reinvent and evolve their use. Peptides possess advantageous chemical definition, access to non-native chemistries, amenability to de novo design, and applicability within parallel approaches. Biomaterials development programs that require such aspects may benefit from a peptide-based strategy.

Materials from peptide assembly: towards the treatment of cancer and transmittable disease

As the prevalence of cancer and transmittable disease persists, the development of new and more advanced therapies remains a priority in medical research. An emerging platform for the treatment of these illnesses is the use of materials formed via peptide assembly where the bulk material itself acts as the therapeutic. Higher ordered peptide structures with defined chemistry are capable of cellular targeting, recognition, and internalization. Recent design efforts are being made to exploit the nanoscale definition of the materials formed by assembling peptides to target cancer and microbial cells and to function as vaccines. This review focuses on assembled peptide materials that actively participate in the biological processes important to cancer and transmittable diseases to exert an anticipated functional outcome.

Tuning β-sheet peptide self-assembly and hydrogelation behavior by modification of sequence hydrophobicity and aromaticity

Peptide self-assembly leading to cross-β amyloid structures is a widely studied phenomenon because of its role in amyloid pathology and the exploitation of amyloid as a functional biomaterial. The self-assembly process is governed by hydrogen bonding, hydrophobic, aromatic π-π, and electrostatic Coulombic interactions. A role for aromatic π-π interactions in peptide self-assembly leading to amyloid has been proposed, but the relative contributions of π-π versus general hydrophobic interactions in these processes are poorly understood. The Ac-(XKXK)(2)-NH(2) peptide was used to study the contributions of aromatic and hydrophobic interactions to peptide self-assembly. Position X was globally replaced by valine (Val), isoleucine (Ile), phenylalanine (Phe), pentafluorophenylalanine (F(5)-Phe), and cyclohexylalanine (Cha). At low pH, these peptides remain monomeric because of repulsion of charged lysine (Lys) residues. Increasing the solvent ionic strength to shield repulsive charge-charge interactions between protonated Lys residues facilitated cross-β fibril formation. It was generally found that as peptide hydrophobicity increased, the required ionic strength to induce self-assembly decreased. At [NaCl] ranging from 0 to 1000 mM, the Val sequence failed to assemble. Assembly of the Phe sequence commenced at 700 mM NaCl and at 300 mM NaCl for the less hydrophobic Ile variant, even though it displayed a mixture of random coil and β-sheet secondary structures over all NaCl concentrations. β-Sheet formation for F(5)-Phe and Cha sequences was observed at only 20 and 60 mM NaCl, respectively. Whereas self-assembly propensity generally correlated to peptide hydrophobicity and not aromatic character the presence of aromatic amino acids imparted unique properties to fibrils derived from these peptides. Nonaromatic peptides formed fibrils of 3-15 nm in diameter, whereas aromatic peptides formed nanotape or nanoribbon architectures of 3-7 nm widths. In addition, all peptides formed fibrillar hydrogels at sufficient peptide concentrations, but nonaromatic peptides formed weak gels, whereas aromatic peptides formed rigid gels. These findings clarify the influence of aromatic amino acids on peptide self-assembly processes and illuminate design principles for the inclusion of aromatic amino acids in amyloid-derived biomaterials.

Solid-State NMR characterization of autofluorescent fibrils formed by the elastin-derived peptide GVGVAGVG

The characterization of the molecular structure and physical properties of self-assembling peptides is an important aspect of optimizing their utility as scaffolds for biomaterials and other applications. Here we report the formation of autofluorescent fibrils by an octapeptide (GVGVAGVG) derived via a single amino acid substitution in one of the hydrophobic repeat elements of human elastin. This is the shortest and most well-defined peptide so far reported to exhibit intrinsic fluorescence in the absence of a discrete fluorophore. Structural characterization by FTIR and solid-state NMR reveals a predominantly β-sheet conformation for the peptide in the fibrils, which are likely assembled in an amyloid-like cross-β structure. Investigation of dynamics and the effects of hydration on the peptide are consistent with a rigid, water excluded structure, which has implications for the likely mechanism of intrinsic fibril fluorescence.

Self-assembly of chiral trans-cyclobutane-containing β-dipeptides into ordered aggregates

Two chiral synthetic β-dipeptides have been constructed, one with two trans-cyclobutane residues and the other with one trans and one cis fragment, 1 and 2, respectively, and investigated to get insight into the non-covalent interactions responsible for their self-assembly to form ordered aggregates, as well into parameters such as their morphology and size. Experimental evidence of the formation of these assemblies was provided by spectroscopy, microscopy and X-ray diffraction experiments that suggest the formation of nanoscale helical aggregates. This process involves a conformational change in the molecules of each dipeptide with respect to the preferred conformation of the isolated molecules in solution. A high-resolution NMR spectroscopy study allowed the determination of the dynamics of the gelation process in [D(8)]toluene and the sol-gel transition temperature, which was around 270 K in this solvent at a concentration of 15 mM. NMR spectroscopy experiments also provided some information about conformational changes involved in the sol-gel transition and also suggested a different gel packing for each dipeptide. These observations have been nicely explained by computational studies. The self-assembly of the molecules has been modelled and suggested a head-to-head molecular arrangement for 1 and a head-to-tail arrangement for 2 to give helical structures corresponding to hydrogen-bonded single chains. These chains interact with one another in an antiparallel way to afford bundles, the significant geometry parameters of which fit well to the main peaks observed in wide-angle X-ray diffraction spectra of the aggregates in the solid state.

Enzymatic cross-linking of a nanofibrous peptide hydrogel

The rheological properties of the environment in which a cell lives play a key role in how the cells will respond to that environment and may modify cell proliferation, morphology and differentiation. Effective means of modifying these properties are needed, particularly for peptide hydrogels which are generally relatively weak and soft. In this report we describe the enzymatic cross-linking of a nanofibrous multidomain peptide hydrogel. When this method was used, the storage modulus, G', could be increased to over 4000 Pa without changes in hydrogel concentration and without dramatic changes in nanostructural architecture. Enzymatic cross-linking represents a mild and simple method for increasing the mechanical strength of peptide hydrogels in applications for which the robustness of the gel is essential. This method should be suitable for a broad array of peptide hydrogels containing lysine such as those currently under study by many different groups.

A reductive trigger for peptide self-assembly and hydrogelation

Stimulus-responsive peptide self-assembly provides a powerful method for controlling self-assembly as a function of environment. The development of a reductive trigger for peptide self-assembly and subsequent hydrogelation is described herein. A self-assembling peptide sequence, Ac-C(FKFE)(2)CG-NH(2), was cyclized via disulfide bonding of the flanking cysteine residues. The macrocyclic form of this peptide enforces a conformational restraint that prevents adoption of the beta-sheet conformation that is required for self-assembly. Upon reduction of this disulfide bond, the peptide relaxes into the preferred beta-sheet conformation, and immediate self-assembly into fibrillar superstructures occurs. At sufficient peptide concentration, self-assembly is accompanied by the formation of rigid, viscoelastic hydrogels.