Self-assembled networks of ribbons in molecular hydrogels of cationic deoxycholic acid analogues

Abstract

Aqueous gels derived from three cationic 24-nor 3,12-dihydroxy cholane (DC) derivatives with N-methyl-2-pyrrolidinone (NMP), N-methylmorpholine (NMM), 1,4-diazabicyclo[2.2.2]octane (DABCO) at the side chain positions have been exhaustively characterized by small-angle neutron-scattering experiments. Although the molecular structures differ slightly by the heterocycle grafted to the steroid core; the derived gels exhibit a range of structural behaviors at the nanoscale that depart from those observed with simple deoxycholate systems. The NMM-DC aggregates are ribbons with a bimolecular thickness of t = 37 Å an anisotropy of the section b/a = 0.1 DABCO-DC exhibits a remarkable transition from ribbons (t = 29.5 Å b/a = 0.8) to thicker cylindrical fibers (R = 59 Å), involving four original ribbons; upon a concentration increase. The NMP-DC system forms thick cylindrical fibers (R=68 Å) with steroid molecules organized in a specific morphology. Bilayered or interdigited structures are formed; favored by the presence of multiple polar interaction centers in the DC molecules. Secondary aggregation mechanisms are invoked in the formation of bundles having a lower cross-sectional anisotropic symmetry; exhibiting Bragg peaks corresponding to molecular length periodicities. The relations between the structural information; the rheological properties are discussed.