Systematic Modulation of the Supramolecular Gelation Properties of Bile Acid Alkyl Amides

Abstract

The self‐assembly properties of nine low‐molecular‐weight gelators (LMWGs) based on bile acid alkyl amides were studied in detail. Based on the results, the number of hydroxyl groups attached to the steroidal backbone plays a major role in the gelation, although the nature of the aliphatic side chain also modulates the gelation abilities. Of the 50 gel systems studied, 35 are based on lithocholic acid and 15 on cholic acid derivatives. The deoxycholic acid derivatives did not form any gels. The gelation occurred primarily in aromatic solvents and the gels manifested typical fibrous or spherical morphologies. The 13C cross‐polarized magic angle spinning (CPMAS) NMR spectra measured on the crystalline materials and the corresponding wet organogels were analogous, suggesting that the chemical environments, that is, the intermolecular interactions found in the two materials were similar. The single‐crystal X‐ray structures of all nine bile‐acid amide derivatives studied revealed very similar molecular conformations in the solid state and gave insights into the possible intermolecular interactions in the gel state.

The Effect of the Side Chain on Gelation Properties of Bile Acid Alkyl Amides

“…According to our previous studies, [33,34] lithocholic acid derivatives have been more efficient gelator molecules, whereas in this study, mostly partial gels were obtained. For example, in the case of compound 1 which [34] gold (N-propyl), [33] green (N-butyl), [33] blue (N-isopentyl), [33] grey (N-cyclohexyl), and purple (N-hexyl). Co-crystallized solvent molecules as well as atom sites with minor occupancies have been omitted for clarity.…”

“…In our previous studies, [33,34] the gel systems collapsed immediately after being mechanically disturbed. Previously studied bile acid alkyl amides bearing shorter or branched side chains (butylamides and iso-pentylamides, respectively) were observed to form a larger number of gel systems [33] than the currently studied 1-6.…”

“…Bile acid alkyl amides with even shorter side chains (ethylamides and propylamides) [33,34] were less effective gelators and the gels also decomposed easily. The effectiveness of gelation should thus be evaluated by taking into account not only the number of gels formed but also the qualitative stability of the gel.…”

“…[29] During the past fifteen years, a large number of bile acid derivatives have been studied regarding their gelation properties in our research group. [17,18,[30][31][32][33][34][35][36][37][38] Recently, we have been interested in bile acid derivatives which do not have any functional group at their end of the side chain. In this respect, we wondered how the length and the shape (linear or branched) of the functional-group-free side chain affected the gel formation ability of a particular bile acid derivative.…”

“…Clearly, the number of hydroxyl groups on the steroidal skeleton of the compound has an effect on gelation, verifying previous observations. [ 17 , 18 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ]…”