CAGE ionic liquids meet biomembranes: unraveling molecular mechanisms and partitioning kinetics

Choline-based ionic liquids (ILs) have been extensively explored for drug delivery and antimicrobial applications, yet their interactions with lipid membranes remain poorly understood. Using coarse-grained molecular dynamics simulations, free energy calculations, and recently developed kinetics models, we investigate the partitioning of choline-geranic acid/geranate (CAGE) IL formulations into simple zwitterionic, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and anionic 1,2-dioleoyl-sn-glycero-3-phosphoglycerol (DOPG) phospholipid membranes over a range of compositions and concentrations. The neutral component, geranic acid (GRA), forms micelles in aqueous IL formulations and dominates membrane partitioning due to a low free energy barrier. In contrast, the ionic components, geranate (GRI) and choline (COL), remain localized near the headgroup-water interface due to higher free energy barriers. Membrane structural responses were found to scale approximately linearly with CAGE concentration following a composition dependent trend (CAGE14 > CAGE12 > CAGE11). Equilibrium membrane partitioning isotherms reveal charge dependent uptake of GRI and COL, whereas GRA partitioning is governed primarily by CAGE composition. The partitioning kinetics identifies two uptake mechanisms: (a) membrane controlled first order kinetics for GRI and COL, and (b) micelle size limited zeroth order membrane partitioning for GRA. The uptake kinetics were found to be slower in anionic DOPG membranes. Overall, our study provides a comprehensive simulation method to analyze both the dynamics and equilibrium partitioning of ILs into phospholipid membranes.