Mechanosensitive peptide gelation: mode of agitation controls mechanical properties and nano-scale morphology

In this study we demonstrate the sensitivity of fibrillar self-assembly and gelation of aromatic peptide amphiphiles to different modes of mechanical agitation. In particular, we show how different homogenization techniques (vortex vs. manual or orbital agitation) during the gelation process can dramatically influence the mechanical properties and the nano-scale organization of self-assembled materials. The pH-induced self-assembly of mixtures of fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF) and Fmoc-diglycine (Fmoc-GG) to gel phase materials was performed under different agitation conditions (vortex vs. manual or orbital agitation). We demonstrate that mechanically induced differences of up to one order of magnitude in shear modulus could be obtained for chemically identical samples as measured using oscillatory rheology and in creep and recovery experiments. AFM imaging and nanoindentation were then employed to highlight morphological, mechanical and physico-chemical differences in the fibrillar elements (ribbons). Furthermore, we have demonstrated that although Fmoc-diglycine alone does not undergo fibrillar aggregation, nor is capable of gelation at neutral pH, its presence influences the properties of gels at both fibrillar and macroscopic level; in particular, a Fmoc-diglycine/Fmoc-diphenylalanine 1 : 1 molar ratio appeared to provide the highest moduli and the most regular fibers, suggesting a specific mode of intercalation of Fmoc-diglycine in Fmoc-diphenylalanine fibrils.