Aging-Induced Structural Transition of Nanoscale Oleanolic Acid Amphiphiles and Selectivity against Gram-Positive Bacteria

Triterpenoids are among the largest groups of functional plant secondary metabolites but with intrinsically low water solubility. Because of their rigid backbone, multiple chiral centers, and functional groups, they are suitable for synthesizing water-soluble and conformationally rigid triterpenoid amphiphiles with unique self-assembly behavior. In this context, we present the aqueous self-assembly, structural transition, and antimicrobial properties of nanoscale oleanolic acid-spermine conjugates (2-4). The conjugates contain either one or two spermine moieties connected through a 1,4-disubstituted 1,2,3-triazole linker. We use cryogenic transmission electron microscopy (cryo-TEM) imaging to show that conjugates 2 and 3 self-assemble in water initially into kinetically favored metastable micellar nanoparticles (d approximate to 6-10 nm). The nanoparticles further reorganize to form thermodynamically stable helical nanofibers. Notably, cryo-TEM imaging also suggests the formation of spherulite-like structures. Time-dependent infrared (IR) spectroscopy reveals the role of hydration and dehydration in the structural transition of initial micelle-like structures into thermodynamically stable nanofibers. Electronic and vibrational circular dichroism (ECD and VCD, respectively) spectroscopy in the solution state suggests the formation of chiral superstructures with a left-handed helical twist. The conjugates display antibacterial properties with high selectivity against Gram- positive bacterial strains. The results help us understand fibrillar network formation in supramolecular gels, and demonstrate that the position and number of spermine groups influence the self-assembly behavior of the conjugates in aqueous media and their biological properties.