A family of low-molecular-weight hydrogelators based on L-lysine derivatives with a positively charged terminal group

A family Of L-lysine-based low-molecular-weight compounds with various positively charged terminals (pyridinium and imidazolium derivatives) was synthesized and its gelation behavior in water was investigated. Most of the compounds can be very easily synthesized in high yields (total yields > 90%), and they function as excellent hydrogelators that form hydrogels below 1 wt %; particularly, N-epsilon-lauroyl-N-alpha-[11-(4-tert-butylpyridinium)undecanoyl]-L-lysine ethyl ester (2c) and N-epsilon-lauroyl-N-alpha-[11-(4-phenylpyridinium)un-decanoyl]-L-lysine ethyl ester (2d) which are able to gel water at concentration of only 0.2 wt %. This corresponds to a gelator molecule that entraps more than 20 000 water molecules. All hydrogels are very stable and maintain the gel state for at least 9 months. TEM observations demonstrated that these hydrogelators self-assemble into a nanoscaled fibrous structure; a three-dimensional network is then formed by the entanglement of the nanofibers. An FTIR study in [D-6]DMSO/D2O and in CHCl3 revealed the existence of intermolecular hydrogen bonding between the amide groups. This was further supported by a H-1 NMR study in [D-6]DMSO/H2O. A luminescence study, in which ANS (1-anilino-8-naphthar-enesulfonic acid) was used as a probe, indicated that the hydrogelators self-assemble into nanostructures possessing hydrophobic pockets at a very low concentration. Consequently, it was found that the driving forces for self-assembly into a nanofiber are hydrogel bonding and hydrophobic interactions.