Impact of Ionic Liquids on the Physicochemical Behavior of Vesicles

The effects of two ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and 1-butyl-1-methyl pyrrolidinium tetrafluoroborate ([bmp]BF4), on a mixture of phospholipids (PLs) 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), and 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) (6:3:1, M/M/M, 70% PL) in combination with 30 mol % cholesterol (CHOL) were investigated in the form of a solvent-spread monolayer and bilayer (vesicle). Surface pressure (π)-area (A) isotherm studies, using a Langmuir surface balance, revealed the formation of an expanded monolayer, while the cationic moiety of the IL molecules could electrostatically and hydrophobically bind to the PLs on the palisade layer. Turbidity, dynamic light scattering (size, ζ-potential, and polydispersity index), electron microscopy, small-angle X-ray/neutron scattering, fluorescence spectroscopy, and differential scanning calorimetric studies were carried out to evaluate the effects of IL on the structural organization of bilayer in the vesicles. The ILs could induce vesicle aggregation by acting as a "glue" at lower concentrations (<1.5 mM), while at higher concentrations, the ILs disrupt the bilayer structure. Besides, ILs could result in the thinning of the bilayer, evidenced from the scattering studies. Steady-state fluorescence anisotropy and lifetime studies suggest asymmetric insertion of ILs into the lipid bilayer. MTT assay using human blood lymphocytes indicates the safe application of vesicles in the presence of ILs, with a minimal toxicity of up to 2.5 mM IL in the dispersion. These results are proposed to have applications in the field of drug delivery systems with benign environmental impact.

The Physics of Antimicrobial Activity of Ionic Liquids

The bactericidal potency of ionic liquids (ILs) is well-established, yet their precise mechanism of action remains elusive. Here, we show evidence that the bactericidal action of ILs primarily involves the permeabilization of the bacterial cell membrane. Our findings reveal that ILs exert their effects by directly interacting with the lipid bilayer and enhancing the membrane dynamics. Lateral lipid diffusion is accelerated, which in turn augments membrane permeability, ultimately leading to bacterial death. Furthermore, our results establish a significant connection: an increase in the alkyl chain length of ILs correlates with a notable enhancement in both lipid lateral diffusion and antimicrobial potency. This underscores a compelling correlation between membrane dynamics and antimicrobial effectiveness, providing valuable insights for the rational design and optimization of IL-based antimicrobial agents in healthcare applications.

Role of Cholesterol in Interaction of Ionic Liquids with Model Lipid Membranes and Associated Permeability

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 ([C12MIM]+Br-) ionic liquids using various biophysical techniques. On interacting with the membrane, [C12MIM]+Br- leads to enhanced membrane permeability and induces membrane fusion, leading to an increase in vesicle size. The 2H-based solid-state NMR investigations of cholesterol-containing lipid membranes reveal that [C12MIM]+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 [C12MIM]+Br- decreases. The effect of [C12MIM]+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 [C12MIM]+Br- varies based on the lipid composition. The findings suggest that the presence of cholesterol can modulate the effect of [C12MIM]+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 [C12MIM]+Br-.

Mechanisms of Biological Effects of Ionic Liquids: From Single Cells to Multicellular Organisms

The review presents a detailed discussion of the evolving field studying interactions between ionic liquids (ILs) and biological systems. Originating from molten salt electrolytes to present multiapplication substances, ILs have found usage across various fields due to their exceptional physicochemical properties, including excellent tunability. However, their interactions with biological systems and potential influence on living organisms remain largely unexplored. This review examines the cytotoxic effects of ILs on cell cultures, biomolecules, and vertebrate and invertebrate organisms. Our understanding of IL toxicity, while growing in recent years, is yet nascent. The established findings include correlations between harmful effects of ILs and their ability to disturb cellular membranes, their potential to trigger oxidative stress in cells, and their ability to cause cell death via apoptosis. Future research directions proposed in the review include studying the distribution of various ILs within cellular compartments and organelles, investigating metabolic transformations of ILs in cells and organisms, detailed analysis of IL effects on proteins involved in oxidative stress and apoptosis, correlation studies between IL doses, exposure times and resulting adverse effects, and examination of effects of subtoxic concentrations of ILs on various biological objects. This review aims to serve as a critical analysis of the current body of knowledge on IL-related toxicity mechanisms. Furthermore, it can guide researchers toward the design of less toxic ILs and the informed use of ILs in drug development and medicine.

Effects of imidazolium ionic liquids on skin barrier lipids - Perspectives for drug delivery

Ionic liquids (ILs) have great potential to facilitate transdermal and topical drug delivery. Here, we investigated the mechanism of action of amphiphilic ILs 1-methyl-3-octylimidazolium bromide (C8MIM) and 3-dodecyl-1-methylimidazolium bromide (C12MIM) in skin barrier lipid models in comparison to their complex effects in human skin. C8MIM incorporated in a skin lipid model was a better permeation enhancer than C12MIM for water and model drugs, theophylline and diclofenac. Solid state 2H NMR and X-ray diffraction indicated that both ILs prefer the cholesterol-rich regions in skin lipids without significantly perturbing their lamellar arrangement and that C8MIM induces the formation of an isotropic lipid phase to a greater extent compared to C12MIM. C12MIM applied topically to the lipid model or human skin as a pretreatment was more potent than C8MIM. When co-applied with the drugs to human skin, aqueous C12MIM was more potent than C8MIM in enhancing theophylline permeation, but neither IL affected (even decreased) diclofenac permeation. Thus, the IL's ability to permeabilize skin lipid barrier is strongly modulated by its ability to reach the site of action and its interactions with drug and solvent. Such an interplay is far from trivial and requires detailed investigation to realize the full potential of ILs.

Ionic liquids meet lipid bilayers: a state-of-the-art review

In the past 25 years, a vast family of complex organic salts known as room-temperature ionic liquids (ILs) has received increasing attention due to their potential applications. ILs are composed by an organic cation and either an organic or inorganic anion, and possess several intriguing properties such as low vapor pressure and being liquid around room temperature. Several biological studies flagged their moderate-to-high (cyto)-toxicity. Toxicity is, however, also a synonym of affinity, and this boosted a series of biophysical and chemical-physical investigations aimed at exploiting ILs in bio-nanomedicine, drug-delivery, pharmacology, and bio-nanotechnology. Several of these investigations focused on the interaction between ILs and lipid membranes, aimed at determining the microscopic mechanisms behind their interaction. This is the focus of this review work. These studies have been carried out on a variety of different lipid bilayer systems ranging from 1-lipid to 5-lipids systems, and also on cell-extracted membranes. They have been carried out at different chemical-physical conditions and by the use of a number of different approaches, including atomic force microscopy, neutron and X-ray scattering, dynamic light scattering, differential scanning calorimetry, surface quartz microbalance, nuclear magnetic resonance, confocal fluorescence microscopy, and molecular dynamics simulations. The aim of this "2023 Michèle Auger Award" review work is to provide the reader with an up-to-date overview of this fascinating research field where "ILs meet lipid bilayers (aka biomembranes)," with the aim to boost it further and expand its cross-disciplinary edges towards novel high-impact ideas/applications in pharmacology, drug delivery, biomedicine, and bio-nanotechnology.

Effects of ionic liquids on biomembranes: A review on recent biophysical studies

Ionic liquids (ILs) have been emerged as a versatile class of compounds that can be easily tuned to achieve desirable properties for various applications. The ability of ILs to interact with biomembranes has attracted significant interest, as they have been shown to modulate membrane properties in ways that may have implications for various biological processes. This review provides an overview of recent studies that have investigated the interaction between ILs and biomembranes. We discuss the effects of ILs on the physical and chemical properties of biomembranes, including changes in membrane fluidity, permeability, and stability. We also explore the mechanisms underlying the interaction of ILs with biomembranes, such as electrostatic interactions, hydrogen bonding, and van der Waals forces. Additionally, we discuss the future prospects of this field.

Fusion Landscape of Mycobacterial Envelope-Derived Lipid Vesicles with Intact Bacteria Dictates High Intracellular Drug Retention

Membrane vesicles are critical regulators of pathogenic diseases. In tubercular infections, the use of mycobacteria derived vesicles as delivery vehicles to overcome drug resistance and complex treatment regimens has never been attempted. Here, we first address how these vesicles interact with their target cells, especially via membrane fusion. Membrane fusion between alike mycobacterial outer and inner membrane layer-derived lipid vesicles is shown to be driven by the structural, geometrical, and biophysical attributes of constituent lipids. The increased fusion of outer-membrane-derived vesicles with intact bacteria ensures enhanced intracellular drug levels and is presented as a "natural" antitubercular drug delivery vehicle.

Chronic and intergenerational toxic effects of 1-decyl-3-methylimidazolium hexafluorophosphate on the water flea, Moina macrocopa

With the increasing use and production of "green solvents" ionic liquids (ILs) and their known stability in the environment, the potential adverse effects of ILs have become a focus of research. In the present study, acute, chronic, and intergenerational toxic effects of an imidazolium-based ionic liquid, 1-decyl-3-methylimidazolium hexafluorophosphate ([Demim]PF6), on Moina macrocopa were investigated following the parental exposure. The results showed that [Demim]PF6 exhibited high toxicity to M. macrocopa, and the long-term exposure significantly inhibited the survivorship, development, and reproduction of the water flea. Furthermore, it is also observed that [Demim]PF6 induced toxic effects in the following generation of M. macrocopa, resulting in the complete cessation of reproduction in the first offspring generation, and the growth of the organisms was also significantly affected. These findings provided a novel insight into the intergenerational toxicity induced by ILs to crustaceans and suggested that these compounds pose potential risks to the aquatic ecosystem.

Recent advances in NMR spectroscopy of ionic liquids

This review presents recent developments in the application of NMR spectroscopic techniques in the study of ionic liquids. NMR has been the primary tool not only for the structural characterization of ionic liquids, but also for the study of dynamics. The presence of a host of NMR active nuclei in ionic liquids permits widespread use of multinuclear NMR experiments. Chemical shifts and multinuclear coupling constants are used routinely for the structure elucidation of ionic liquids and of products formed by their covalent interactions with other materials. Also, the availability of a multitude of NMR techniques has facilitated the study of dynamical processes in them. These include the use of NOESY to study inter-ionic interactions, pulsed-field gradient techniques for probing transport properties, and relaxation measurements to elucidate rotational dynamics. This review will focus on the application of each of these techniques to investigate ionic liquids.

Impact of Lipid Ratio on the Permeability of Mixed Phosphatidylcholine/Phosphatidylglycerol Membranes in the Presence of 1-Dodecyl-3-methylimidazolium Bromide Ionic Liquid

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 ([C₁₂MIM]⁺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. ³¹P- and ²H-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 [C₁₂MIM]⁺ 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.

Thermodynamical Origin of Nonmonotonic Inserting Behavior of Imidazole Ionic Liquids into the Lipid Bilayer

The GPU-accelerated molecular dynamics simulations are performed to explore the dynamical inserting process of ionic liquids (ILs) into the lipid bilayer. We found that the free ions and clusters coexist in the system, but only the cation can insert into the lipid bilayer. In specific, after a microsecond-scale simulation (up to 1.16 μs), the inserting rate increases first and then decreases nonmonotonic as side chain of cation (n_chain) elongates, peaking at n_chain = 10. However, the inserting free energy decreases with n_chain, indicating the inserting process is easier for the larger n_chain. Such contrary originates from the formation of cluster, where the cluster dissociating energy shows that only cluster for n_chain ≤ 10 can dissociate spontaneously. Hence, the inserting rate is determined by the balance between n_chain and cluster stability. These quantitative competition mechanisms shed light to the rational design of the biocompatible ILs toward their applications in the biochemical-related fields.

Cytotoxicity and Membrane Permeability of Double-Chained 1,3-Dialkylimidazolium Cations in Ionic Liquids

We have evaluated ionic liquids based on double-chained 1-alkyl-3-octylimidazolium cations ([C_nC₈IM]⁺, n = 2, 4, 6, 8, 10, 12) for their cytotoxicity toward various cell lines. The toxicity of ionic liquids was correlated to their ability to partition into and permeabilize phosphocholine (POPC)- or phosphoglycerol (POPG)-based large unilamellar vesicles. Membrane partitioning of ionic liquids was assessed using the ζ-potential measurements, and membrane permeability was determined using fluorescence-based dye leakage assays. Both cytotoxicity and membrane permeability of these ILs were found to increase in a sigmoidal fashion with increasing chain length on the N1 atom (n in [C_nC₈IM]⁺) cations. These results were compared with those for ionic liquids based on single-chained 1-alkyl-3-methylimidazolium cations ([C_n+8C₁IM]⁺), carrying a similar number of carbon atoms but as a single alkyl chain. Our studies show that ionic liquids containing double-chained cations are relatively less cytotoxic and membrane-permeabilizing than the cations bearing a single long alkyl chain.

Thermodynamics and structure of model bio-membrane of liver lipids in presence of imidazolium-based ionic liquids

Ionic liquids (ILs) are the attractions of researchers today due to their vast area of potential applications. For biomedical uses, it becomes essential to understand their interactions with cellular membrane. Here, the membrane is mimicked with lipid bilayer and monolayer composed of liver lipids extract. Three archetypal imidazolium based ILs, 1-decyl-3-methylimidazolium tetrafluoroborate ([DMIM][BF4] or [C10MIM][BF4]), 1-octyl-3-methylimidazolium tetrafluoroborate, ([OMIM][BF4] or [C8MIM][BF4]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4] or [C2MIM][BF4]) having different alkyl chain lengths are used in the present study. The isothermal titration calorimetry (ITC) measurements showed that [DMIM][BF4] interacts strongest with the liver lipid membrane compared to other two ILs which have relatively shorter alkyl chain length. The low values of stoichiometry ratio of ILs indicates that ILs penetrate within the core of the lipid bilayer. The interaction of ILs with the liver lipid membrane is found to be mainly driven by entropy which could be due to the change in the structure of the lipid membrane at local or global scales. Dynamic light scattering (DLS) measurements indicate that there are no changes in the size of vesicles due to addition of [DMIM][BF4] indicating stability of the vesicles. On the other hand, x-ray reflectivity (XRR) measurements showed a concentration dependent change in the monolayer structure. At low concentration of the IL, the monolayer thickness decreases, exhibiting an increase in the electron density of the layer. However, at higher concentrations, the monolayer thickness increases proving a concentration dependent effects of the IL on the arrangement of the molecules.