From fibres to networks using self-assembling peptides

We have investigated the self-assembly and gelation properties of the octapeptide FEFEFKFK (F: phenylalanine; E: glutamic acid; K lysine) as a function of media pH. Temperature vs. concentration phase diagrams were constructed using the test tube tilting method at each pH. The fibre morphology and network topology formed were investigated using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), transmission electron microscopy (TEM) small angle X-ray scattering (SAXS) and oscillatory rheology. In this work we show that changing the charge modulus carried by the peptide through modification of the media pH leads to a change in the self-assembly and gelation behaviour of the peptide. At low (<6) and high (>8) pHs different fibre morphologies were encountered resulting in the formation of hydrogels with different mechanical properties. This change in fibre morphology from rigid to twisted is thought to be due to a change in the intrinsic twist of the beta-sheet ladder formed by the peptide. At low pH the network topology formed by the fibres was shown to affect the concentration dependency of the storage modulus. In particular, the tendency to form laterally associated thicker fibres was shown to lead to hydrogels with higher moduli. The lateral assembly of the fibre is controlled by the charge modulus carried by the peptide; as the charge modulus decreases, the electrostatic repulsion between fibres decreases. As a result the tendency of the fibres to associate laterally increases due to their intrinsic hydrophobicity, which leads to the formation of stronger gels. In the pH range 6-8 the peptide becomes non-charged and large bundles of aggregated fibres are observed.