The distinct effects of two imidazolium-based ionic liquids, [C4mim][OAc] and [C6mim][OAc], on the phase behaviours of DPPC

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.

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.

A Novel Strategy for the Characterization of Self-Assembled Structures Using the Static Solid-State Phosphorus Nuclear Magnetic Resonance Technique

Structural characterization of assemblies in solutions is essential for understanding the relationship between the structure and material properties. In this study, we introduce a novel approach to investigate amphiphilic self-assemblies in solutions using the phospholipid molecule 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (Lyso PC) as a 31P NMR probe. The high natural abundance and gyromagnetic ratio of 31P make it one of the most sensitive nuclei in the low-frequency region, enabling efficient detection even in dilute solutions. Lyso PC can readily co-assemble with amphiphilic molecules and ions in aqueous solutions, forming various structures, such as hexagonal, lamellar, and micellar assemblies. The characteristic line shapes of these assemblies reflect the chemical environment around the probe and provide insights into the different phase states of the assemblies. This strategy offers a simple, cost-effective, and static method for obtaining structural information about various assemblies. Our work not only introduces a sensitive probe for characterizing assemblies in a solvent environment but also inspires new ideas for the development of similar spectroscopic probes.

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.

Ionic liquids revolutionizing biomedicine: recent advances and emerging opportunities

Ionic liquids (ILs), due to their inherent structural tunability, outstanding miscibility behavior, and excellent electrochemical properties, have attracted significant research attention in the biomedical field. As the application of ILs in biomedicine is a rapidly emerging field, there is still a need for systematic analyses and summaries to further advance their development. This review presents a comprehensive survey on the utilization of ILs in the biomedical field. It specifically emphasizes the diverse structures and properties of ILs with their relevance in various biomedical applications. Subsequently, we summarize the mechanisms of ILs as potential drug candidates, exploring their effects on various organisms ranging from cell membranes to organelles, proteins, and nucleic acids. Furthermore, the application of ILs as extractants and catalysts in pharmaceutical engineering is introduced. In addition, we thoroughly review and analyze the applications of ILs in disease diagnosis and delivery systems. By offering an extensive analysis of recent research, our objective is to inspire new ideas and pathways for the design of innovative biomedical technologies based on ILs.

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.

The Nanostructure of Alkyl-Sulfonate Ionic Liquids: Two 1-Alkyl-3-methylimidazolium Alkyl-Sulfonate Homologous Series

The functionalization of polymers with sulfonate groups has many important uses, ranging from biomedical applications to detergency properties used in oil-recovery processes. In this work, several ionic liquids (ILs) combining 1-alkyl-3-methylimidazolium cations [C_nC₁im]⁺ (4 ≤ n ≤ 8) with alkyl-sulfonate anions [C_mSO₃]^- (4 ≤ m ≤ 8) have been studied using molecular dynamics simulations, totalizing nine ionic liquids belonging to two homologous series. The radial distribution functions, structure factors, aggregation analyses, and spatial distribution functions reveal that the increase in aliphatic chain length induces no significant change in the structure of the polar network of the ILs. However, for imidazolium cations and sulfonate anions with shorter alkyl chains, the nonpolar organization is conditioned by the forces acting on the polar domains, namely, electrostatic interactions and hydrogen bonding.

How 1,n-Bis(3-alkylimidazolium-1-yl) Alkane Interacts with the Phospholipid Membrane and Impacts the Toxicity of Dicationic Ionic Liquids

Ionic liquids based on doubly charged cations, often termed dicationic ionic liquids (DILs), offer robust physicochemical properties and low toxicity than conventional monocationic ionic liquids. In this design-based study, we used solid-state NMR spectroscopy to provide the interaction mechanism of two DILs, 1,n-bis(3-alkylimidazolium-1-yl) alkane dibromide ([C_2n(C_7-nIM)₂]²⁺·2Br^-, n = 1, 6), with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) phospholipid membranes, to explain the low toxicity of DILs toward HeLa, Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae cell lines. Dications with a short linker and long terminal chains cause substantial perturbation to the bilayer structure, making them more membrane permeabilizing, as shown by fluorescence-based dye leakage assays. The structural perturbation is even higher than [C₁₂(MIM)]⁺ monocations, which carry a single 12-carbon long chain and exhibit a much higher membrane affinity, permeability, and cytotoxicity. These structural details are a crucial contribution to the design strategies aimed at harnessing the biological activity of ionic liquids.

Self-Assembly of a C16M[Mn] Magnetic Surfactant in Water

A magnetic surfactant, which combines the properties of a surfactant with magnetic responsiveness, shows great potential in biotechnology, separation, adsorption, and catalysis, especially in non-invasive manipulation through a magnetic field. However, a molecularly magnetic surfactant is usually paramagnetic for the amorphous and less ordered structures. In this work, magnetic surfactant 1-methyl-3-hexadecane-imidazolium [MnCl₂Br] (C₁₆M[Mn]) is reported to self-assemble in water. The C₁₆M[Mn] magnetic surfactant self-assembles in water to form a lamellar hydrogel from 10 to 50 wt % at and below room temperature. The hydrogel changes from a gel to a sol at 30 °C, and the hexadecane chains in the hydrogel change from noncrystalline to crystalline at 0 °C. In the hydrogel state, the lamellar domain spacing is varied from 36 to 45 nm depending on the concentration and self-assembly temperature. After self-assembly, the magnetic susceptibility of the freeze-dried magnetic surfactant is increased. Most important is the fact that the freeze-dried sample at a high concentration (40-50 wt %) shows the highest magnetic susceptibility, which is related to the closer molecular packing and the more ordered structures. The self-assembly-induced increase in magnetic susceptibility provides a method for improving the magnetic properties of a magnetic surfactant.

Cholesterol Protects the Liquid-Ordered Phase of Raft Model Membranes from the Destructive Effect of Ionic Liquids

Ionic liquids (ILs), although being a class of promising green solvents, have received many reports on the toxicity to living organisms. In this work, aiming at elucidating the disruptive effect of ILs to cell membrane lipid rafts, we investigated the effect of three 1-octylimidazolium-based ILs on the properties of the liquid ordered phase (L_o, a commonly used lipid raft model) of egg sphingomyelin (SM)-cholesterol model membrane. We found that, in the absence of cholesterol, a very low IL:SM molar ratio of 0.01:1 could disrupt the integrity of the bilayer structure. In sharp contrast, the presence of cholesterol in lipid bilayers helps the L_o phase resist the damaging effect of the ILs. For the role of the IL headgroup, we found that the mono- and trisubstituted species show a stronger destructive effect on the structures of the model rafts than the commonly used disubstituted counterpart.

Thermodynamics and In-Plane Viscoelasticity of Anionic Phospholipid Membranes Modulated by an Ionic Liquid

This article presents the effects of an imidazolium-based ionic liquid (IL) on the thermodynamics and in-plane viscoelastic properties of model membranes of anionic phospholipids. The negative Zeta potential of multilamellar vesicles of 14 carbon lipid 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG) is observed to reduce due to the presence of few mole % of an IL 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]). The effect was found to be stronger on enhancing the chain length of the lipid. The surface pressure-area isotherms of lipid monolayer formed at air-water interface are modified by the IL reducing the effective area per molecule. Further, the equilibrium elasticity of the film is altered depending upon the thermodynamic phase of the lipids. While the presence of the IL in the DMPG lipid makes it ordered in the gel phase by reducing the entropy, the effect is opposite in the fluid phase. The in-plane viscoelastic parameters of the lipid film is quantified by dilation rheology using the oscillatory barriers of a Langmuir trough. Even though the low chain lipid DMPG does not show any effect of IL on its storage and loss moduli, the longer chain lipids exhibit a prominent effect in the liquid extended (LE) phase. Further, the dynamic response of the lipid film is found to be distinctly different in the liquid condensed (LC) phase from that of the LE phase.

Absorption of the [bmim][Cl] Ionic Liquid in DMPC Lipid Bilayers across Their Gel, Ripple, and Fluid Phases

Lipid bilayers are a key component of cell membranes and play a crucial role in life and in bio-nanotechnology. As a result, controlling their physicochemical properties holds the promise of effective therapeutic strategies. Ionic liquids (ILs)─a vast class of complex organic electrolytes─have shown a high degree of affinity with lipid bilayers and can be exploited in this context. However, the chemical physics of IL absorption and partitioning into lipid bilayers is yet to be fully understood. This work focuses on the absorption of the model IL [bmim][Cl] into 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers across their gel, ripple, and fluid phases. Here, by small-angle neutron scattering, we show that (i) the IL cations are absorbed in the lipid bilayer in all its thermodynamic phases and (ii) the amount of IL inserted into the lipid phase increased with increasing temperature, changing from three to four IL cations per 10 lipids with increasing temperature from 10 °C in the gel phase to 40 °C in the liquid phase, respectively. An explicative hypothesis, based on the entropy gain coming from the IL hydration water, is presented to explain the observed temperature trend. The ability to control IL absorption with temperature can be used as a handle to tune the effect of ILs on biomembranes and can be exploited in bio-nanotechnological applications.

1,3 Dialkylated Imidazolium Ionic Liquid Causes Interdigitated Domains in a Phospholipid Membrane

Amphiphilic imidazolium-based ionic liquids (ILs) have proven their efficacy in altering the membrane integrity and dynamics. The present article investigates the phase-separated domains in a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membrane induced by 1,3 dialkylated imidazolium IL. Isotherm measurements on DPPC monolayers formed at the air-water interface have shown a decrease in the mean molecular area with the addition of this IL. The positive value of the excess Gibbs free energy of mixing indicates an unfavorable mixing of the IL into the lipid. This leads to IL-induced phase-separated domains in the multilayer of the lipid confirmed by the occurrence of two sets of equidistance peaks in the X-ray reflectivity data. The electron density profile along the surface normal obtained by the swelling method shows the bilayer thickness of the newly formed IL-rich phase to be substantially lower (∼34 Å) than the DPPC phase (∼45.8 Å). This IL-rich phase has been confirmed to be interdigitated, showing an enhanced electron density in the tail region due to the overlapping hydrocarbon chains. Differential scanning calorimetry measurements showed that the incorporation of IL enhances the fluidity of the lipid bilayer. Therefore, the study indicates the formation of an interdigitated phase with a lower order compared to the gel phase in the DPPC membrane supplemented with the IL.