Interaction of POPG membranes with ionic liquids containing 1-Dodecyl-3-methylbenzimidazolium and 1-Dodecyl-1-methylmorpholinium Cations: Structural details from 31P and 2H-based solid-state NMR spectroscopy

Interactions of ionic liquids (ILs) with phospholipid membranes play a crucial role in their biological activity be it their cytotoxicity or their application as drug delivery agents. To develop the design principles for task-specific applications of ionic liquids, structural details of this interaction with atomic resolution must be obtained. In this context, 31P and 2H-based solid-state NMR spectroscopy of phospholipid membranes is a very useful technique. Here, we have used it to study the impact of benzimidazolium (BNZ) and morpholinium (MPH) cation-based ionic liquids on the head-group orientation and acyl chain disorder of negatively charged POPG membranes. Owing to the large-size and aromatic nature of its head group, membrane partitioning of BNZ cations increases the disorder of the entire POPG acyl chain. Comparisons are drawn with the behavior of widely studied imidazolium (IMI) cation which, like MPH, has a contrasting impact in the upper and lower halves of POPG chains. The observations are used to rationalize the higher potential of BNZ to permeabilize and fuse POPG vesicles, which was determined using fluorescence-based dye leakage and lipid mixing assays.