Rods, helices and spherulites: diverse self-assembled architectures from L-phenylalanine derivatives

Diphenylalanine (FF) has emerged as a concise self-assembling motif yielding a variety of nanostructures through simple processing. We explored the self-assembly and hydrogelation properties of some L-Phe (F) derivatives having a rotationally flexible and aromatic 1-naphthaleneacetyl moiety as the N-protecting group, while the carboxylic acid residue was converted into an amide by reaction with different amines such as hydrazine (L-NapF-Hz), ethylenediamine (L-NapF-EDA), 3-aminopropylimidazole (L-NapF-API) and hydroxyethylamine (L-NapF-HEA). All the derivatives were dissolved in hot water and the subsequent cooling of these sols had varied outcomes. For some derivatives such as L-NapF-Hz and L-NapF-EDA, molecular self-assembly yielded optically clear supramolecular hydrogels, while for others such as L-NapF-API and L-NapF-HEA, precipitates were obtained upon cooling. Field emission scanning electron microscopy (FESEM) revealed diverse morphologies such as rigid rods, helical nanofibers and spherulites which were obtained by spontaneous self-assembly of these compounds. Circular dichroism spectroscopy revealed p-p stacking as one of the critical intermolecular interactions dictating the self-assembly. Stable hydrogels entrapping the anticancer drug doxorubicin were obtained with L-NapF-EDA only, highlighting the importance of flexibility in the molecule for designing efficient hydrogelators.