[Show abstract][Hide abstract] ABSTRACT: The self-assembly of peptide-nanotubes formed by an L-glutamic acid-based bolaamphiphile is shown to proceed via a remarkable mechanism where the peptide conformation changes from [small beta]-sheet to unordered. The kinetics of this process are elucidated via X-ray scattering and UV circular dichroism methods. The reverse transition from unordered to [small beta]-sheet structures is triggered by UV radiation.
Chemical Communications 06/2015; DOI:10.1039/C5CC03640B · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Insight is provided into the thermodynamics and kinetics of peptide fibrillization, a process of particular interest in biomedicine due to its direct relationship to amyloid diseases. The studied isomeric tetrapeptides are terminally end-capped and present two hydrophobic phenylalanine residues and two ionizable hydrophilic aspartic acid residues. These compounds afford gels in water which are composed of self-assembled fibrillar networks. Potentiometric titrations have afforded acid-base and solubility constants associated to the fibrillization process. A remarkable pKa shift of the peptide side chains is observed linked to aggregation, affording fibers at pH values around neutrality. The magnitude of the pKa shift is directly related to the solubility of the tetrapeptides namely, to the free energy change associated with fibrillization. Therefore potentiometric titration emerges as a simple tool to evaluate the thermodynamic parameters of the process. Additionally kinetic measurements with NMR, fluorescence spectroscopy and SANS reveal that initial peptide dimerization is most likely to be the fibrillization rate determining step. The aggregation process in all cases presents a relatively long lag time of ca. 1-3 hours and takes more than 8 hours to complete. No correlation is observed between kinetic and thermodynamic parameters. Finally, kinetically controlled self-sorting of a mixture of two isomeric tetrapeptides is described
Chemistry of Materials 04/2015; 27(9):150413162716003. DOI:10.1021/acs.chemmater.5b00580 · 8.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The self-assembly and bioactivity of a peptide amphiphile (PA) incorporating a 13-residue sequence derived from the last 13 amino acids of the C-terminus of lumican, C16- YEALRVANEVTLN, attached to a hexadecyl (C16) lipid chain have been examined. Lumican is a proteoglycan found in many types of tissue, and is involved in collagen fibril organization. A critical aggregation concentration (cac) for the PA was determined through pyrene fluorescence measurements. The structure of the aggregates was imaged using electron microscopy and twisted and curved nanotapes were observed. In situ small-angle X-ray scattering, and fibre X-ray diffraction, reveal that these tapes contain interdigitated bilayers of the PA molecules. FTIR and circular dichroism spectroscopies and fibre X-ray diffraction indicate that the lumican sequence in the PA adopts a β-sheet secondary structure. Cell assays using human dermal fibroblasts show that, below the cac, the PA displays good biocompatibility and also stimulated collagen production over a period of three weeks, exceeding a two-fold enhancement for several concentrations. Thus this PA has promise in future biological applications, in particular in tissue engineering.
[Show abstract][Hide abstract] ABSTRACT: A model octapeptide peptide consisting of an alternating sequence of arginine (Arg) and phenylalanine (Phe) residues, namely [Arg-Phe]4 was prepared, and its self-assembly in solution solution. The simple alternating [Arg-Phe]4 peptide sequence allows for unique insights into the aggregation process and the structure of the self-assembled motifs. Fluorescence and UV-Vis assays were used to determine critical aggregation concentrations, corresponding to the formation of oligomeric species and -sheet rich structures organized into both spheroidal aggregates and highly-ordered fibrils. Electron and atomic force microscopy images show globular aggregates and long unbranched fibres with diameters ranging from ~ 4 nm up to ~ 40 nm. Infrared and circular dichroism spectroscopy show the formation of β-sheet structures. X-ray diffraction on oriented stalks show that the peptide fibres have an internal lamellar structure, with an orthorhombic unit cell with parameters a ~ 27.6 Å, b ~ 9.7 Å and c ~ 9.6 Å. In situ small-angle X-ray scattering (SAXS) shows the presence of low molecular weight oligomers in equilibrium with mature fibres which are likely made up from 5 or 6 intertwined proto-filaments. Finally, weak gel solutions are probed under gentle shear suggesting the ability of these arginine-rich fibres to form networks.
[Show abstract][Hide abstract] ABSTRACT: We describe a bioactive lipopeptide that combines the capacity to promote the adhesion and subsequent self-detachment of live cells, using template-cell-environment feedback interactions. This self-assembling peptide amphiphile comprises a diene-containing hexadecyl lipid chain (C16e) linked to a matrix metalloprotease-cleavable sequence, Thr-Pro-Gly-Pro-Gln-Gly-Ile-Ala-Gly-Gln, and contiguous with a cell-attachment and signalling motif, Arg-Gly-Asp-Ser. Biophysical characterisation revealed that the PA self-assembles into 3 nm diameter spherical micelles above a critical aggregation concentration (cac). In addition, when used in solution at 5-150 nM (well below the cac), the PA is capable of forming film coatings that provide a stable surface for human corneal fibroblasts to attach and grow. Furthermore, these coatings were demonstrated to be sensitive to metalloproteases expressed endogenously by the attached cells, and consequently to elicit the controlled detachment of cells without compromising their viability. As such, this material constitutes a novel class of multi-functional coating for both fundamental and clinical applications.
[Show abstract][Hide abstract] ABSTRACT: The need to source live human tissues for research and clinical applications has been a major driving force for the development of new biomaterials. Ideally, these should elicit the formation of scaffold-free tissues with native-like structure and composition. Here we describe a biologically interactive coating that combines the fabrication and subsequent self-release of live purposeful tissues using template-cell-environment feedback. This smart coating was formed from a self-assembling peptide amphiphile comprising a protease-cleavable sequence contiguous with a cell-attachment and signaling motif. This multi-functional material was subsequently used not only to instruct human corneal or skin fibroblasts to adhere and deposit discreet multiple layers of native extracellular matrix, but also govern their own self-directed release from the template solely via the action of endogenous metalloproteases. Tissues recovered through this physiologically relevant process were carrier-free, and structurally and phenotypically equivalent to their natural counterparts. This technology contributes to a new paradigm in regenerative medicine, whereby materials are able to actively direct and respond to cell behavior. The novel application of such materials as a coating, capable of directing the formation and detachment of complex tissues solely under physiological conditions can have broad use for fundamental research and in future cell and tissue therapies.
Tissue Engineering Part A 02/2015; 21(11-12). DOI:10.1089/ten.TEA.2014.0671 · 4.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The self-assembled structure of Toll-like receptor agonist lipopeptides containing the CSK4 peptide sequence is examined in aqueous solution. A remarkable dependence of morphology on the number of attached hexadecyl lipid chains is demonstrated, with spherical micelle structures for mono- and di-lipidated structures, but flexible wormlike micelles for the homologue containing three lipid chains. The distinct modes of assembly may have an important influence on the bioactivity of this class of lipopeptide.
Chemical Communications 10/2014; 50(100). DOI:10.1039/C4CC07511K · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The interaction of a designed bioactive lipopeptide C16-GGGRGDS, comprising a hexadecyl lipid chain attached to a functional heptapeptide, with the lipid-free apoliprotein, Apo-AI, is examined. This apolipoprotein is a major component of high density lipoprotein and it is involved in lipid metabolism and may serve as a biomarker for cardiovascular disease and Alzheimers’ disease. We find via isothermal titration calorimetry that binding between the lipopeptide and Apo-AI occurs up to a saturation condition, just above equimolar for a 10.7 μM concentration of Apo-AI. A similar value is obtained from circular dichroism spectroscopy, which probes the reduction in α-helical secondary structure of Apo-AI upon addition of C16-GGGRGDS. Electron microscopy images show a persistence of fibrillar structures due to self-assembly of C16-GGGRGDS in mixtures with Apo-AI above the saturation binding condition. A small fraction of spheroidal or possibly “nanodisc” structures was observed. Small-angle X-ray scattering (SAXS) data for Apo-AI can be fitted using a published crystal structure of the Apo-AI dimer. The SAXS data for the lipopeptide/Apo-AI mixtures above the saturation binding conditions can be fitted to the contribution from fibrillar structures coexisting with flat discs corresponding to Apo-AI/lipopeptide aggregates.
[Show abstract][Hide abstract] ABSTRACT: Amyloid fibrils are formed by a model surfactant-like peptide (Ala)10-(His)6 containing a hexa-histidine tag. This peptide undergoes a remarkable two-step self-assembly process with two distinct critical aggregation concentrations (cacs), probed by fluorescence techniques. A micromolar range cac is ascribed to the formation of pre-fibrillar structures, whereas a millimolar range cac is associated with the formation of well defined but more compact fibrils. We examine the labelling of these model tagged amyloid fibrils using Ni-NTA functionalized gold nanoparticles (Nanogold). Successful labelling is demonstrated via electron microscopy imaging. The specificity of tagging does not disrupt the β-sheet structure of the peptide fibrils. Binding of fibrils and Nanogold is found to influence the circular dichroism associated with the gold nanoparticle Plasmon absorption band. These results highlight a new approach to the fabrication of functionalized amyloid fibrils and the creation of peptide/ nanoparticle hybrid materials.
[Show abstract][Hide abstract] ABSTRACT: We studied the self-assembly of peptide A(6)RGD (A: alanine, R: arginine, G: glycine, D: aspartic acid) in water, and the use of A(6)RGD substrates as coatings to promote the attachment of human cornea stromal fibroblasts (hCSFs). The self-assembled motif of A(6)RGD was shown to depend on the peptide concentration in water, where both vesicle and fibril formation were observed. Oligomers were detected for 0.7 wt% A(6)RGD, which evolved into short peptide fibres at 1.0 wt% A(6)RGD, while a co-existence of vesicles and long peptide fibres was revealed for 2-15 wt% A(6)RGD. A(6)RGD vesicle walls were shown to have a multilayer structure built out of highly interdigitated A(6) units, while A(6)RGD fibres were based on beta-sheet assemblies. Changes in the self-assembly motif with concentration were reflected in the cell culture assay results. Films dried from 0.1-1.0 wt% A(6)RGD solutions allowed hCSFs to attach and significantly enhanced cell proliferation relative to the control. In contrast, films dried from 2.5 wt% A(6)RGD solutions were toxic to hCSFs.
[Show abstract][Hide abstract] ABSTRACT: The self-assembly of the alanine-rich amphiphilic peptides Lys(Ala)6Lys (KA6K) and Lys(Ala)6Glu (KA6E) with homotelechelic or heterotelechelic charged termini respectively has been investigated in aqueous solution. These peptides contain hexa-alanine sequences designed to serve as substrates for the enzyme elastase. Electrostatic repulsion of the lysine termini in KA6K prevents self-assembly, whereas in contrast KA6E is observed, through electron microscopy, to form tape-like fibrils, which based on X-ray scattering contain layers of thickness equal to the molecular length. The alanine residues enable efficient packing of the side-chains in a beta-sheet structure, as revealed by circular dichroism, FTIR and X-ray diffraction experiments. In buffer, KA6E is able to form hydrogels at sufficiently high concentration. These were used as substrates for elastase, and enzyme-induced de-gelation was observed due to the disruption of the beta-sheet fibrillar network. We propose that hydrogels of the simple designed amphiphilic peptide KA6E may serve as model substrates for elastase and this could ultimately lead to applications in biomedicine and regenerative medicine.
[Show abstract][Hide abstract] ABSTRACT: We examine the self-assembly of a peptide A6H comprising a hexa-alanine sequence A6 with a histidine (H) "head group", which chelates Zn2+ cations. We study the self assembly of A6H and binding of Zn2+ ions in ZnCl2 solutions, under acidic and neutral conditions. A6H self assembles in nanotapes held together by a β-sheet structure in acidic aqueous solutions. By dissolving A6H in acidic ZnCl2 solutions, the carbonyl oxygen atoms in A6H chelate the Zn2+ ions, and allow for β-sheet formation at lower concentrations, consequently reducing the onset concentration for nanotape formation. A6H mixed with water or ZnCl2 solutions under neutral conditions produce short sheets or pseudocrystalline tapes respectively. The imidazole ring of A6H chelates Zn2+ ions in neutral solutions. The internal structure of nanosheets and pseudocrystalline sheets in neutral solutions is similar to the internal structure of A6H nanotapes in acidic solutions. Our results show that is possible to induce dramatic changes in the self assembly and chelation sites of A6H, by changing the pH of the solution. However, it is likely that the amphiphilic nature of A6H determines the internal structure of the self assembled aggregates independent from changes in chelation.