Aromatic−Aromatic Interactions Induce the Self-Assembly of Pentapeptidic Derivatives in Water To Form Nanofibers and Supramolecular Hydrogels

In this paper we report the conjugation of an aromatic moiety (pyrene (P), fluorene (F), or naphthalene (N)) to pentapeptides GAGAS (1), GVPVP (2), VPGVG (3), VTEEI (4), VYGGG (5), and YGFGG (6) to afford peptidic derivatives for exploring pentapeptide-based hydrogels as potential biomaterials. Most of these compounds (1F, 1P, 2F, 2P, 4F, 4P, 4N, 5F, 5N, 6F, 6P, and 6N) behave as molecular hydrogelators and can form hydrogels at minimum concentrations of gelation from 0.5 to 2.8 wt %. The fluorescence spectra of the hydrogels exhibit a significant red shift, indicating the interactions between the aromatic moieties in those hydrogels. The circular dichroism spectra indicate that the self-assembly of the hydrogelators affords helical or beta-sheet-like structures. Transmission and scanning electron microscopic examination reveals the nanofiber networks of these hydrogelators. In addition, rheological measurements show fair to excellent viscoelastic properties of these hydrogels. The balance of intermolecular aromatic-aromatic interactions and hydrogen bonds of these hydrogelators leads to their self-assembly in water and the formation of nanofibers as the matrixes of hydrogels. A total of 6 of these 18 pentapeptide derivatives--1N, 2N, 3F, 3P, 3N, and 5P--fail to form hydrogels under the conditions tested, likely due to unbalanced intermolecular interactions. This work suggests that aromatic-aromatic interactions are useful and favorable forces for creating molecular nanofibers and supramolecular hydrogels.