Effect of the Alkyl Chain Length of Amphiphilic Ionic Liquids on the Structure and Dynamics of Model Lipid Membranes

• Published in: Langmuir Vol. 35; no. 37; pp. 12215 - 12223
• Main Authors: Kumar, Sandeep, Scheidt, Holger A, Kaur, Navleen, Kang, Tejwant Singh, Gahlay, Gagandeep Kaur, Huster, Daniel, Mithu, Venus S
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 17.09.2019
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/acs.langmuir.9b02128

We compare the biophysical and structural aspects of the interaction of amphiphilic ionic liquids containing 1-alkyl-3-methylimidazolium cation ([C n MIM]+, n = 8, 12, or 16) with membranes composed of zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or anionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol (POPG). Liposome affinity and permeabilization were determined using ζ-potential and fluorescence studies, correlated with the cytoxicity of [C n MIM]+Br– toward HeLa cell lines. Membrane affinity is strongest in the case of [C16MIM]+Br– followed by [C12MIM]+Br– and [C8MIM]+Br– for both membranes, and trends remained the same in the case of membrane permeability and cytotoxicity. Solid-state NMR spectroscopy was used to localize [C n MIM]+ inside the lipid bilayers and to study their impact on the head group and acyl chain structures and dynamics of the lipid molecules. The charged ring moiety of the [C n MIM]+ is localized in the lipid–water interface of the membranes irrespective of the chain length and membrane surface charge. While [C8MIM]+ binds the membrane most weakly, it induces the largest disorder in the lipid chain region. A lack of fast flip-flop motions of the amphiphiles in the case of long chain [C16MIM]+ is suggested to render the membrane unstable, which increases its permeability. Between the lipid molecules, the POPC membrane incurs larger disorder in lipid chain packing upon insertion of [C n MIM]+ molecules. The study provides structural details of the impact of increasing chain lengths in [C n MIM]+ on the structural properties of lipid bilayers.