Terpene-Based Ionic Liquids from Natural Renewable Sources As Selective Agents in Antifungal Therapy

In this study, we present a new approach toward the design of ionic liquids with biological activity. Structural analysis of bioactive compounds was performed to design—in a technological and economic manner—salts with potential antifungal properties. The length of the alkyl chain as well as the task-specific component in the cation, the type of amine core, and the type of anion were considered as having an essential impact on achieving desired biological activity. Herein, we present the synthesis and characterization of ionic liquids based on monoterpene derivatives—namely, (1R,2S,5R)-(−)-menthol or bicyclic (1R)-endo-(+)-fenchol—from renewable sources. These new salts were synthesized with high yields (>96%) in mild conditions via a two-step procedure. Physicochemical properties (i.e., melting point, thermal stability, crystal shape, specific rotation, surfactant content, solubility, and surface activity) were analyzed in detail. The obtained results suggested the influence of the steric hindrance of the discussed salts on the reactivity, solubility, thermal stability, and surface properties of the studied compounds. Their potential selectivity in antifungal therapy was studied using Langmuir monolayer mimicking fungal (ergosterol) and mammalian (cholesterol) membranes. The model study confirmed the selective destabilizing activity of terpene-based ionic liquids on the fungus membrane.