Supramolecular Hydrogel Formation in a Series of Self-Assembling Lipopeptides with Varying Lipid Chain Length

The self-assembly in aqueous solution of three lipopeptides comprising a bioactive motif conjugated at the N terminus to dodecyl, tetradecyl or hexadecyl lipid chains has been examined. The bioactive motif is the peptide block YEALRVANEVTLN; a C-terminal fragment of the lumican proteoglycan. This study was motivated by our previous studies on the hexadecyl homologue C₁₆-YEALRVANEVTLN, which showed aggregation into β-sheet structures above a critical aggregation concentration (cac), but most remarkably, we found that these aggregates were stable to dilution below the cac. Here we find that the C₁₂- and C₁₄-homologues also self-assemble above a cac into β-sheet nanotapes based on bilayer packing. The cac decreases with increasing lipopeptide hydrophobicity. Unexpectedly, the β-sheet secondary structure is present upon dilution and the aggregates are thermally stable. These results indicate that the dilution trapping of β-sheet secondary structure is not associated with lipid chain melting behavior. Instead, we associate it with pH-dependent favorable intermolecular electrostatic interactions. Investigation of the pH-dependence of aggregation led to the discovery of conditions for formation of lipopeptide hydrogels (initial sample preparation at pH 10 in NaOH solution, followed by reduction to pH ∼ 1 by addition of HCl). The lipopeptide hydrogels comprise networks of bilayer-based peptide nanotape bundles and to our knowledge this type of hydrogel is unprecedented. These hydrogels may have future applications based on processes such as encapsulation and release that involve fast switches between solution and hydrogel nanostructures.