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In this work, we explored how the amount of cholesterol in the lipid membrane composed of phosphatidylcholine (POPC) or phosphatidylglycerol (POPG) affects the interaction with 1-dodecyl-3-methylimidazolium bromide ([C 12 MIM] + Br - ) ionic liquids using various biophysical techniques. On interacting with the membrane, [C 12 MIM] + Br - leads to enhanced membrane permeability and induces membrane fusion, leading to an increase in vesicle size. The 2 H-based solid-state NMR investigations of cholesterol-containing lipid membranes reveal that [C 12 MIM] + Br - decreases the lipid chain order parameters and counteracts the lipid condensation effect of cholesterol to some extent. Therefore, as the amount of cholesterol in the membrane increases, the membrane effect of [C 12 MIM] + Br - decreases. The effect of [C 12 MIM] + Br - on the membrane properties is more pronounced for POPC compared to that of POPG membranes. This suggests a dependence of these effects on the electrostatic interactions, indicating that the influence of [C 12 MIM] + Br - varies based on the lipid composition. The findings suggest that the presence of cholesterol can modulate the effect of [C 12 MIM] + Br - on membrane properties, with variations observed between POPC and POPG membranes, highlighting the importance of lipid composition. In short, this study provides insights into the intricate interplay between cholesterol, the lipid membrane, and the ionic liquid [C 12 MIM] + Br - .

Amyloid β (Aβ) is a hallmark protein of Alzheimer's disease. One physiologically important Aβ variant is formed by initial N-terminal truncation at a glutamic acid position (either E 3 or E 11 ), which is subsequently cyclized to a pyroglutamate (either pE 3 or pE 11 ). Both forms have been found in high concentrations in the core of amyloid plaques and are likely of high importance in the pathology of Alzheimer's disease. However, the molecular structure of the fibrils of these variants is not entirely clear. Solid-state NMR spectroscopy studies have reported a molecular contact between Gly 25 and Ile 31 , which would disagree with the conventional hairpin model of wildtype (WT-)Aβ 1-40 fibrils, most often described in the literature. We investigated the conformation of the monomeric unit of pE 3 -Aβ 3-40 and pE 11 -Aβ 11-40 (and for comparison also wildtype (WT)-Aβ 1-40 ) fibrils to find out whether the hairpin or a newly suggested extended structure dominates the structure of the Aβ monomers in these fibrils. To this end, solid-state NMR spectroscopy was applied probing the inter-residual contacts between Phe 19 /Leu 34 , Ala 21 /Leu 34 , and especially Gly 25 /Ile 31 using suitable isotopic labeling schemes. In the second part, the flexible turn of the Aβ 40 peptides was replaced by a (3-(3-aminomethyl)phenylazo)phenylacetic acid (AMPP)-based photoswitch, which can predefine the peptide conformation to either an extended ( trans ) or hairpin ( cis ) conformation. This enables simultaneous spectroscopic assessment of the conformation of the AMPP-photoswitch, allowing in situ structural investigations during fibrillation in contrast to structural techniques such as NMR spectroscopy or cryo-EM, which can only be applied to stable conformers. Both methods confirm an extended structure for the peptidic monomers in fibrils of all investigated Aβ variants. Especially the Gly 25 /Ile 31 contact is a decisive indicator for the extended structure along with the characteristic absorption spectra of trans -AMPP-Aβ.

We have studied the impact of the lipid ratio on the membrane permeability of mixed phosphatidylcholine (POPC)/phosphatidylglycerol (POPG) membranes induced by 1-dodecyl-3-methylimidazolium bromide ([C 12 MIM] + Br - ) ionic liquid by evaluating the role of affinity and architecture of the phospholipid bilayer. Nine different model membranes composed of negatively charged POPG and zwitterionic POPC lipids mixed in molar ratios of 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, and 1:9 have been studied. The membrane permeability of each composition has been evaluated using fluorescence-based dye leakage assays. Despite having the highest membrane affinity, POPG-rich membranes doped with 10 and 20 mol % POPC are found to be the least permeable. 31 P- and 2 H-based solid-state NMR investigations reveal that the minor POPC component is homogeneously dispersed in the PG/PC (8:2) membrane. In contrast, the lipids seem to be segregated into POPG- and POPC-rich domains in the complementary PG/PC (2:8) composition. Although [C 12 MIM] + cations have a stronger interaction with the POPG component in the mixed membranes, their insertion has a limited impact on the overall structure and dynamics of the PG/PC (8:2) composition.

The interaction of amphiphilic ionic liquids containing an 1-alkyl-3-methylimidazolium cation ([C 12 MIM] + ), which shows acute cytotoxicity toward marine and bacterial life, with zwitterionic 1-palmitoyl-2-oleoyl- sn-glycero-3-phospho-choline (POPC) and anionic 1-palmitoyl-2-oleoyl- sn-glycero-3-phospho- rac-glycerol (POPG) membranes was investigated. Biophysical parameters of this interaction were quantified by fluorescence spectroscopy, isothermal titration calorimeter, and solution-state NMR measurements. [C 12 MIM] + inserts into the membrane and induces vesicle leakage at relatively low concentration (<1 mM). Zwitterionic POPC membranes are more leakage-prone, but the binding of [C 12 MIM] + cations is stronger to anionic POPG membranes. A higher rate of exchange of membrane-bound [C 12 MIM] + is suspected to play a key role in membrane leakage. Furthermore, solid-state NMR spectroscopy was employed to determine the location of [C 12 MIM] + in lipid membranes and its impact on the structure and dynamics of the bilayer. The study provides a molecular understanding of the membrane permeabilizing effect of the [C 12 MIM] + mediated by its detergent-like structure.