Cholesterol Modulates Ionic Liquid–Lipid Membrane Interactions

Understanding the role of cholesterol in ionic liquid (IL)-membrane interactions is essential for advancing biomedical applications of ILs, including the development of innovative antimicrobial agents. In this study, we explore the intricate and multifaceted role of cholesterol in modulating IL-membrane interactions employing a comprehensive suite of biophysical techniques. We systematically examine how IL alkyl chain length and membrane physical state influence the impact of cholesterol on the IL-lipid membrane interaction. The incorporation of ILs is shown to increase the area per lipid in both pristine dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol membranes. Cholesterol modulates the impact of ILs on the lipid conformation, membrane viscoelasticity, and phase behavior. Small-angle neutron scattering and dynamic light scattering measurements reveal that cholesterol mitigates IL-induced structural perturbations in the vesicles. Interestingly, while cholesterol significantly weakens IL binding, membranes containing cholesterol exhibit a greater permeabilization. This counterintuitive behavior arises from cholesterol's ordering effect on the lipid bilayer, which, while stabilizing the structure, increases its vulnerability to stress-induced defects. Our results underscore the complex and nonuniversal interplay between lipid composition, IL alkyl chain length, and membrane phase state. These insights provide a deeper understanding of cholesterol's role in IL-membrane interactions, paving the way for the design of advanced applications of ILs in antimicrobial therapy and drug delivery.