Role of cationic head-group in cytotoxicity of ionic liquids: Probing changes in bilayer architecture using solid-state NMR spectroscopy

Abstract

The effect of cationic head-group of ionic liquid on the structure and dynamics of phospholipid bilayer was studied to provide insights into the mechanism of ionic liquid-membrane interaction. The effect was observed using six ionic liquids containing benzimidazolium, imidazolium, pyrrolidinium, piperidinium, ammonium, and morpholinium based amphiphilic cations carrying a dodecyl alkyl chain. Unilamellar and multilamellar vesicles composed of zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were used. Permeability of POPC bilayer was found to have a strong dependence on ionic liquid head-group structure. To probe the structural details of interaction, 31P and 2H based solid-state NMR measurements were performed. The cations differed in terms of their effect on the orientation and disorder in the phosphocholine moiety in lipid head-group as revealed by chemical shift anisotropy of 31P. Cations carrying an unshielded charge like benzimidazolium, imidazolium, and ammonium result in strong reorientation of phosphocholine moiety in lipid head-group. Large sized cations like benzimidazolium and piperidinium result in enhanced lipid chain dynamics as revealed by order parameter calculations of deuterated lipid chains. Relatively polar head-group of morpholinium cation neither impacts the phospholipid head-group nor chain packing. Our results suggest that there exists a direct correlation between ionic liquid head-group induced structural changes in bilayer and their ability to permeabilize/disrupt the membrane and be cytotoxic.