Phenylalanine-Based Amphiphilic Self-Assembled Materials: Gels or Crystals?

We prepared three simple molecules, that we chose as representative examples of amphiphilic and bolamphiphilic amino acid derivatives: N-lauroyl-L-phenylalanine (Lau-Phe-OH), N-palmitoyl-L-phenylalanine (Pal-Phe-OH), N,N-azeloyl-L-diphenylalanine Az-(Phe-OH)2, to study the influence of the aliphatic side chain on the formation of supramolecular materials. We found that Pal-Phe-OH is a very efficient gelator in contrast with Az-(Phe-OH)2 that efficiently forms crystals, while Lau-Phe-OH forms metastable hydrogels that slowly become crystals. We demonstrated by X-ray diffraction that Lau-Phe-OH and Pal-Phe-OH easily form hetero-intermolecular hydrogen bonds between the carboxylic and amidic groups, while Az-(Phe-OH)2 forms homo-intermolecular hydrogen bonds, i. e., the typical carboxylic ring dimer and chains between the amidic functions, which leads to an extended and robust 2D hydrogen bonding network. Moreover, Lau-Phe-OH is more ordered than Pal-Phe-OH and the comparison of these results clearly indicates that the reduced order of Pal-L-Phe-OH is the main reason for the efficiency of this molecule as supergelator.