Effects of imidazolium-based ionic liquids on the stability and dynamics of gramicidin A and lipid bilayers at different salt concentrations

• We simulated membranes consisting of imidazolium-based ionic liquids and lipids.
• We aim to understand toxicity of ionic liquids for industrial & environmental applications.
• IL concentration and salt condition modulate stability and mobility of membranes.
• Simulations explain the conflicting experimental results regarding on ion permeability.

Gramicidin A (gA) dimers with bilayers, which consist of phospholipids and ionic liquids (ILs) at different molar ratios, were simulated at different salt concentrations of 0.15 and 1 M NaCl. Bilayer thickness is larger than the length of a gA dimer, and hence lipids around the gA dimer are significantly disordered to adapt to the gA dimer, yielding membrane curvature. As the IL concentration increases, the bilayer thickness decreases and becomes closer to the gA length, leading to less membrane curvature. Also, ILs significantly increase lateral diffusivities of the gA dimer and lipids at 0.15 M NaCl, but not at 1 M NaCl because strong electrostatic interactions between salt ions and lipid head groups suppress an increase in the lateral mobility of the bilayer at high salt concentration. These findings help explain the conflicting experimental results that showed the increased ion permeability in electrophysiological experiments at 1 M NaCl, but the reduced ion permeability in fluorescent experiments at 0.15 M NaCl. ILs disorder lipids and make bilayers thinner, which yields less membrane curvature around the gA dimer and thus stabilizes the gA dimer, leading to the increased ion permeability. This IL effect predominantly occurs at 1 M NaCl, where ILs only slightly increase the bilayer dynamics because of the strong electrostatic interactions between salt ions and lipids. In contrast, at 0.15 M NaCl, ILs do not only stabilize the curved bilayer but also significantly increase the lateral mobility of gA dimers and lipids, which can reduce gA-induced pore formation, leading to the decreased ion permeability.

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