Triterpenoid Building Blocks for Functional Nanoscale Assemblies: A Review

Naturally abundant, renewable, and sustainable plant triterpenoids have gained considerable attention in biological application, including antitumor activities. However, due to their lipophilic nature, natural triterpenoids display limited solubility in aqueous media. Notably, the rigid backbone and the presence of functional groups also offer the possibilities toward a strategic design of a class of conformationally rigid amphiphiles. Such derivatives may improve their bioavailability, and they display unique self-assembly properties relevant for soft functional materials. In recent years, a formation of nano-assemblies has been studied with plant triterpenoids and their semisynthetic derivatives to investigate ways of their applications in materials science. For example, naturally amphiphilic glycyrrhizic acid has been shown to form hydrogels. Similarly, other natural triterpenoid-based phosphates or heterocyclic-based quaternary conjugates have also been studied for supramolecular gelation. Triterpenoid-based supramolecular gels are important types of soft materials, displaying rapid self-healing, thixotropic behaviors, aging-induced transition, and self-assembly-induced anticancer and antimicrobial properties. In this review, we will provide a summary of (i) unique structural and functional properties of natural triterpenoids, (ii) self-assembly and gelation properties of natural triterpenoids, (iii) gelation properties of rationally designed triterpenoid derivatives, (iv) structural transition in aqueous media, and (v) their biological effects.