Enhanced Microscopic Dynamics of a Liver Lipid Membrane in the Presence of an Ionic Liquid

Structural Reorganizations and Nanodomain Emergence in Lipid Membranes Driven by Ionic Liquids

Ionic liquids (ILs) have promising applications in pharmaceuticals and green chemistry, but their use is limited by toxicity concerns, mainly due to their interactions with cell membranes. This study examines the effects of imidazolium-based ILs on the microscopic structure and phase behavior of a model cell membrane composed of zwitterionic dipalmitoylphosphatidylcholine (DPPC) lipids. Small-angle neutron scattering and dynamic light scattering reveal that the shorter-chain IL, 1-hexyl-3-methylimidazolium bromide (HMIM[Br]), induces the aggregation of DPPC unilamellar vesicles. In contrast, this aggregation is absent with the longer alkyl chain IL, 1-decyl-3-methylimidazolium bromide (DMIM[Br]). Instead, DMIM[Br] incorporation leads to the formation of distinct IL-poor and IL-rich nanodomains within the DPPC membrane, as evidenced by X-ray reflectivity, differential scanning calorimetry, and molecular dynamics simulations. The less evident nanodomain formation with HMIM[Br] underscores the role of hydrophobic interactions between lipid alkyl tails and ILs. Our findings demonstrate that longer alkyl chains in ILs significantly enhance their propensity to form membrane nanodomains, which is closely linked to enhanced membrane permeability, as shown by dye leakage measurements. This heightened permeability likely underlies the greater cytotoxicity of longer-chain ILs. This crucial link between nanodomains and toxicity provides valuable insights for designing safer, more environmentally friendly ILs, and promoting their use in biomedical applications and sustainable industrial processes.

Lipid lateral diffusion: mechanisms and modulators

The lateral diffusion of lipids within a membrane is of paramount importance, serving as a central mechanism in numerous physiological processes including cell signaling, membrane trafficking, protein activity regulation, and energy transduction pathways. This review offers a comprehensive overview of lateral lipid diffusion in model biomembrane systems explored through the lens of neutron scattering techniques. We examine diverse models of lateral diffusion and explore the various factors influencing this fundamental process in membrane dynamics. Additionally, we offer a thorough summary of how different membrane-active compounds, including drugs, antioxidants, stimulants, and membrane proteins, affect lipid lateral diffusion. Our analysis unveils the intricate interplay between these additives and membranes, shedding light on their dynamic interactions. We elucidate that this interaction is governed by a complex combination of multiple factors including the physical state and charge of the membrane, the concentration of additives, the molecular architecture of the compounds, and their spatial distribution within the membrane. In conclusion, we briefly discuss the future directions and areas requiring further investigation in the realm of lateral lipid diffusion, highlighting the need to study more realistic membrane systems.

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.

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.

Neutron scattering studies on dynamics of lipid membranes

Neutron scattering methods are powerful tools for the study of the structure and dynamics of lipid bilayers in length scales from sub Å to tens to hundreds nm and the time scales from sub ps to μs. These techniques also are nondestructive and, perhaps most importantly, require no additives to label samples. Because the neutron scattering intensities are very different for hydrogen- and deuterium-containing molecules, one can replace the hydrogen atoms in a molecule with deuterium to prepare on demand neutron scattering contrast without significantly altering the physical properties of the samples. Moreover, recent advances in neutron scattering techniques, membrane dynamics theories, analysis tools, and sample preparation technologies allow researchers to study various aspects of lipid bilayer dynamics. In this review, we focus on the dynamics of individual lipids and collective membrane dynamics as well as the dynamics of hydration water.

Multiscale lipid membrane dynamics as revealed by neutron spectroscopy

The plasma membrane is one of the principal structural components of the cell and, therefore, one of the key components of the cellular life. Because the membrane's dynamics links the membrane's structure and function, the complexity and the broad range of the membrane's motions are essential for the enormously diverse functionality of the cell membrane. Even for the main membrane component, the lipid bilayer, considered alone, the range and complexity of the lipid motions are remarkable. Spanning the time scale from sub-picosecond to minutes and hours, the lipid motion in a bilayer is challenging to study even when a broad array of dynamic measurement techniques is employed. Neutron scattering plays a special role among such dynamic measurement techniques, particularly, because it involves the energy transfers commensurate with the typical intra- and inter- molecular dynamics and the momentum transfers commensurate with intra- and inter-molecular distances. Thus, using neutron scattering-based techniques, the spatial and temporal information on the lipid motion can be obtained and analysed simultaneously. Protium vs. deuterium sensitivity and non-destructive character of the neutron probe add to the remarkable prowess of neutron scattering for elucidating the lipid dynamics. Herein we present an overview of the neutron scattering-based studies of lipid dynamics in model membranes, with a discussion of the direct relevance and implications to the real-life cell membranes. The latter are much more complex systems than simple model membranes, consisting of heterogeneous non-stationary domains composed of lipids, proteins, and other small molecules, such as carbohydrates. Yet many fundamental aspects of the membrane behavior and membrane interactions with other molecules can be understood from neutron scattering measurements of the model membranes. For example, such studies can provide a great deal of information on the interactions of antimicrobial compounds with the lipid matrix of a pathogen membrane, or the interactions of drug molecules with the plasma membrane. Finally, we briefly discuss the recently emerging field of neutron scattering membrane studies with a reach far beyond the model membrane systems.

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.

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.

Stiffening Effect of the [Bmim][Cl] Ionic Liquid on the Bending Dynamics of DMPC Lipid Vesicles

The elastic properties of the cellular lipid membrane play a crucial role for life. Their alteration can lead to cell malfunction, and in turn, being able to control them holds the promise of effective therapeutic and diagnostic approaches. In this context, due to their proven strong interaction with lipid bilayers, ionic liquids (ILs)-a vast class of organic electrolytes-may play an important role. This work focuses on the effect of the model imidazolium-IL [bmim][Cl] on the bending modulus of DMPC lipid vesicles, a basic model of cellular lipid membranes. Here, by combining small-angle neutron scattering and neutron spin-echo spectroscopy, we show that the IL, dispersed at low concentrations at the bilayer-water interface, (i) diffuses into the lipid region, accounting for five IL-cations for every 11 lipids, and (ii) causes an increase of the lipid bilayer bending modulus, up to 60% compared to the neat lipid bilayer at 40 °C.

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.

Order-disorder in room-temperature ionic liquids probed via methyl quantum tunneling

Room-temperature ionic liquids are promising candidates for applications ranging from electrolytes for energy storage devices to lubricants for food and cellulose processing to compounds for pharmaceutics, biotransformation, and biopreservation. Due to the ion complexity, many room-temperature ionic liquids readily form amorphous phases upon cooling, even at modest rates. Here, we investigate two commonly studied imidazolium-based room-temperature ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, as well as their mixtures, to demonstrate how the complex interplay between the crystalline and amorphous phases is affected by the processing conditions, such as thermal history, liquid mixing, and applied pressure. We show that quantum tunneling in the cation methyl groups, measured by high-resolution inelastic neutron scattering, can be used to probe the order-disorder in room-temperature ionic liquids (crystalline vs amorphous state) that develops as a result of variable processing conditions.