Self-Assembled Peptide Gels for 3D Cell Culture

Under specific conditions short peptides modified with an N-terminal fluorenyl-9-methoxycarbonyl (Fmoc) group can self-assemble into hydrogel scaffolds similar in properties to the natural extracellular matrix. Fmoc-diphenylalanine (Fmoc-FF) for instance, has been shown to form hydrogels at physiological pH that have the ability to support 2D and 3D cell culture. The aim of this investigation is to provide further understanding of the self-assembly mechanism of such systems in order to progress towards the establishment of design rules for the preparation of scaffolds with tuneable properties.First, Fmoc-dipeptides composed of a combination of hydrophobic aromatic residues phenylalanine (F) and glycine (G) were studied with a particular emphasis on the effect of pH variations. The systems were investigated in order to assess what influence the position of such residues in the peptide sequence had on the physical properties of the molecules, and what impact the chemical structure had on the self-assembly behaviour and the gelation properties of the materials. Subsequently, phenylalanine was replaced by leucine (L), a non-aromatic amino acid that had the same relative hydrophobicity in order to determine whether the self-assembly of such molecules is driven by aromatic interactions or hydrophobic effects.Using potentiometry, the behaviour of the systems in solution has been investigated, revealing that they were all characterised by pKa shifts of up to six units above the theoretical values. Fmoc-FF exhibited two transitions whereas the other Fmoc-dipeptides only displayed one. These transitions were found to coincide with the formation of distinct self-assembled structures with differing molecular conformations and properties that were characterised using transmission electron microscopy, infrared and fluorescence spectroscopy, X-ray scattering and shear rheometry.?-stacking of the aromatic moieties was thought to be the driving force of the self-assembly mechanism, generating dimers that corresponded to the building blocks of the supramolecular structures formed. On the other hand, the peptide components were stabilised via hydrogen bonding and could form antiparallel ?-sheets depending on the amino acid sequence and the associated influence on the rigidity of the molecules. Below their (first) apparent pKa transition, Fmoc-FF, Fmoc-LL, Fmoc-FG, Fmoc-LG and Fmoc-GG formed hydrogels, with the mechanical properties and stability varying depending on the amino acid sequence. Fmoc-FF and Fmoc-LL exhibited the lowest storage modulus values (G? ~ 0.5?5 Pa) of the studied systems while Fmoc-LG displayed the highest (G? ~ 1000?2100 Pa). Fmoc-FG and Fmoc-LG had the peculiarity of being obtained upon heating and where found to be particularly stable, as opposed to Fmoc-GG gels which showed a tendency to crystallise. On the microscopic scale, these gels were all associated with the presence of entangled fibrillar networks of different size and morphology, which in some cases could self-assemble further through a lamellar organisation. Again, Fmoc-FG and Fmoc-LG distinguished from the other systems as they were the only Fmoc-dipeptides to show a supramolecular chirality in the form of twisted ribbons under specific pH conditions. In contrast, Fmoc-GF and Fmoc-GL did not form hydrogels below their apparent pKa due to the formation of sheet-like and spherical structures respectively.