Supramolecular Hydrogel Induced by Electrostatic Interactions between Polycation and Phosphorylated-Fmoc-Tripeptide

Supramolecular hydrogels formed through noncovalent interactions of low-molecular-weight hydrogelators (LMWHs) show great potential applications in different fields, such as delivery of therapeutics, injectable biomaterials, catalysis, or materials chemistry. Generally, the selfassembly of LMWHs is triggered by a sol-gel process through an external stimulus able to switch their solubility, such as temperature, pH, or solvent change and chemical or enzymatic reactions. In this work, we introduced a new strategy to trigger and control the self-assembly of Fmoc-FFpY peptides by direct electrostatic interactions with a polycation without dephosphorylation of the peptides. The resulting hydrogels show enhanced mechanical properties in comparison to gels of Fmoc-FFpY induced by enzymatic dephosphorylation. Peptide self-assembly yields beta-sheets, revealed by circular dichroism and infrared spectroscopy. Characteristic distances predicted by geometry optimization in the gas phase are in agreement with X-ray scattering data and transmission electron microscopy observations. It is proposed that core-shell cylinders are formed in which polycation chains decorate the micellar structures of Fmoc-FFpY peptides through electrostatic interactions between the charged amine groups of the polycations and the phosphate groups of the peptides. Because the gels form quickly and have superior mechanical properties, applications as injectable biomaterials are foreseen. This work opens a route toward a new class of self-assembled hydrogels, where Fmoc tripeptides can be self-assembled with specific polycations to obtain, for example, antimicrobial hydrogels.