Sonication-Induced Coiled Fibrous Architectures of Boc-L-Phe-L-Lys(Z)-OMe

An ultra-short peptide Boc-L-Phe-L-Lys(Z)-OMe (Z=carbobenzyloxy) was shown to act as a highly efficient and versatile low molecular weight gelator (LMWG) for a variety of aliphatic and aromatic solvents under sonication. Remarkably, this simple dipeptide is not only able to form coiled fibres but also demonstrates self-healing and thermal chiroptical switching behaviour. The formation of coiled assemblies was found to be influenced by the nature of the solvent and the presence of an additive. By exploiting these properties it was possible to modulate the macroscopic and microscopic properties of the organogels of this ultra-short peptide, allowing the formation of highly ordered single-domain networks of helical fibres with dimeric or alternatively fibre-bundle morphology. The organogels were characterized by using FTIR, SEM, NMR and circular dichroism (CD) spectroscopy. Interestingly, CD experiments showed that the organogels of Boc-L-Phe-L-Lys(Z)-OMe in aromatic solvents exhibit thermal chiroptical switching. This behaviour was hypothesized to stem from changes in the morphology of the gel accompanied by conformational transformation of the gelling agent. The fact that such a small peptide can demonstrate hierarchical assemblies and the possibility of controlling the self-association is rather intriguing. The self-healing ability, chiroptical switching and more importantly the formation of helical assemblies by Boc-L-Phe-L-Lys(Z)-OMe under sonication, make this dipeptide an interesting example of the self-assembly ability of ultra-short peptides.