Fabrication of plasmonic probes for reproducible nanospectroscopic investigation of lipid monolayers - The electrochemical etching with DC-pulsed voltage

Tip-enhanced Raman spectroscopy (TERS) is a label-free analytical technique that characterizes molecular systems, potentially even with a nanometric resolution. In principle, the metallic plasmonic probe is illuminated with a laser beam generating the localized surface plasmons, which induce a strong local electric field enhancement in close proximity to the probe. Such field enhancement improves the Raman scattering cross-section from the sample volume localized near the probe apex. TERS provides a high spatial resolution and a great sensitivity, however, it is rather rarely used due to technical limitations causing unstable enhancement and the relative lack of data reproducibility. Despite many scientific efforts for the fabrication of effective TER probes providing robust TER enhancement still requires further investigations. In this work, we explore new possibilities based on preparation of scanning tunnelling microscopy (STM) plasmonic probes, since by nature of the tunnelling effect, they potentially could offer a very high spatial resolution in STM guided TERS experiments. Here we compare two methods of STM-TERS probe preparation for effective spectra acquisition. Our results strongly indicate that an application of square pulse voltage upon the etching procedure significantly improves the quality of the TER data over those obtained with a constant voltage one. To demonstrate the efficiency of our probes we present the results of hyperspectral TER mapping of the 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) monolayer deposited on an ultra-pure and atomically flat gold substrate.

Lithocholic acid-based oligomers as drug delivery candidates targeting model of lipid raft

This study presents a new approach to designing a lithocholic acid functionalized oligomer (OLithocholicAA-X) that can be used as a drug carrier with additional, beneficial activity. Namely, this novel oligomer can incorporate an anti-cancer drug due to the application of an effective backbone as its component (lithocholic acid) alone is known to have anticancer activity. The oligomer was synthesized and characterized in detail by nuclear magnetic resonance, attenuated total reflectance Fourier-transform infrared spectroscopy, ultraviolet-visible spectroscopy, thermal analysis, and mass spectrometry analysis. We selected lipid rafts as potential drug carrier-membrane binding sites. In this respect, we investigated the effects of OLithocholicAA-X on model lipid raft of normal and altered composition, containing an increased amount of cholesterol (Chol) or sphingomyelin (SM), using Langmuir monolayers and liposomes. The surface topography of the studied monolayers was additionally investigated by atomic force microscopy (AFM). The obtained results showed that the investigated oligomer has affinity for a system that mimics a normal lipid raft (SM:Chol 2:1). On the other hand, for systems with an excess of SM or Chol, thermodynamically unfavorable fluidization of the films occurs. Moreover, AFM topographies showed that the amount of SM determines the bioavailability of the oligomer, causing fragmentation of its lattice.

The Structure of Oxysterols Determines Their Behavior at Phase Boundaries: Implications for Model Membranes and Structure-Activity Relationships

The presence of an additional polar group in the cholesterol backbone increases the hydrophilicity of resulting compounds (oxysterols), determines their arrangement at the phase boundary, and interactions with other lipids and proteins. As a result, physicochemical properties of biomembranes (i.e., elasticity, permeability, and ability to bind proteins) are modified, which in turn may affect their functioning. The observed effect depends on the type of oxysterol and its concentration and can be both positive (e.g., antiviral activity) or negative (disturbance of cholesterol homeostasis, signal transduction, and protein segregation). The membrane activity of oxysterols has been successfully studied using membrane models (vesicles, monolayers, and solid supported films). Membrane models, in contrast to the natural systems, provide the possibility to selectively examine the specific aspect of biomolecule-membrane interactions. Moreover, the gradual increase in the complexity of the used model allows to understand the molecular phenomena occurring at the membrane level. The interest in research on artificial membranes has increased significantly in recent years, mainly due to the development of modern and sophisticated physicochemical methods (static and dynamic) in both the micro- and nanoscale, which are applied with the assistance of powerful theoretical calculations. This review provides an overview of the most important findings on this topic in the current literature.

Advantages of the classical thermodynamic analysis of single-and multi-component Langmuir monolayers from molecules of biomedical importance-theory and applications

The Langmuir monolayer technique has been successfully used for decades to model biological membranes and processes occurring at their interfaces. Classically, this method involves surface pressure measurements to study interactions within membrane components as well as between external bioactive molecules (e.g. drugs) and the membrane. In recent years, surface-sensitive techniques were developed to investigate monolayers in situ; however, the obtained results are in many cases insufficient for a full characterization of biomolecule-membrane interactions. As result, description of systems using parameters such as mixing or excess thermodynamic functions is still relevant, valuable and irreplaceable in biophysical research. This review article summarizes the theory of thermodynamics of single- and multi-component Langmuir monolayers. In addition, recent applications of this approach to characterize surface behaviour and interactions (e.g. orientation of bipolar molecules, drug-membrane affinity, lateral membrane heterogeneity) are presented.

Site of the Hydroxyl Group Determines the Surface Behavior of Bipolar Chain-Oxidized Cholesterol Derivatives─Langmuir Monolayer Studies Supplemented with Theoretical Calculations

Cholesterol oxidation products (called oxysterols) are involved in many biological processes, showing both negative (e.g., neurodegenerative) and positive (e.g., antiviral and antimicrobial) effects. The physiological activity of oxysterols is undoubtedly closely related to their structure (i.e., the type and location of the additional polar group in the cholesterol skeleton). In this paper, we focus on determining how a seemingly minor structural change (introduction of a hydroxyl moiety at C(24), C(25), or C(27) in the isooctyl chain of cholesterol) affects the organization of the resulting molecules at the phase boundary. In our research, we supplemented the classic Langmuir monolayer technique, based on the surface pressure and electric surface potential isotherms, with microscopic (BAM) and spectroscopic (PM-IRRAS) techniques, as well as theoretical calculations (DFT and MD). This allowed us to show that 24-OH behaves more like cholesterol and forms stable, rigid monolayers. On the other hand, 27-OH, similar to 25-OH, undergoes the phase transition from monolayer to bilayer structures. Theoretical calculations enabled us to conclude that the formation of bilayers from 27-OH or 25-OH is possible due to the hydrogen bonding between adjacent oxysterol molecules. This observation may help to understand the factors responsible for the unique biological activity (including antiviral and antimicrobial) of 27-OH and 25-OH compared to other oxysterols.

Comprehensive Approach to the Interpretation of the Electrical Properties of Film-Forming Molecules

This paper presents a general protocol for the interpretation of the electric surface potential of Langmuir monolayers based on a three-layer capacitor model. The measured values were correlated with the results from DFT molecular dynamics simulations, and, as a result, the local dielectric permittivities and dipole-moment components of molecules organized in the monolayer were obtained. The main advantage of the developed approach is applicability to amphiphiles of any type; irrespective of the structure of the polar head as well as the molecular organization and inclination in the surface film. The developed methodology was successively applied to an atypical surface-active compound, perfluorodecyldecane, and its derivatives containing the hydroxyl, thiol, and carboxyl moiety. The following contributions to the apparent dipole moments connected with the reorientation of water molecules and local dielectric permittivities in the vicinity of polar and apolar molecule parts, respectively, were determined: μ^w/ε_w = −0.85 D, ε_p = 5.00, and ε_a = 1.80. Moreover, the investigated perfluorodecyldecane derivatives were comprehensively characterized in terms of their surface activity, film rheology, and effective surface dissociation equilibria. The proposed methodology may be crucial for the process of the design and the preliminary characterization of molecules for sensor and material science applications.

Oxysterols can act antiviral through modification of lipid membrane properties - The Langmuir monolayer study

In this paper we tested how oxysterols influence on fusion process between viral lipid envelope and host cells membranes. For this purpose, the Zika virus was selected, while dendritic cell (DC) and neural cell (NC) membranes were chosen as target membranes. The investigated systems were modeled as multicomponent Langmuir monolayers and characterized using surface manometry and imaging in micro- (Brewster angle microscopy, BAM) and nanoscale (Atomic Force Microscopy, AFM) to monitor local heterogeneity. The fusion process was conducted by mixing viral and host cell membranes devoid and in the presence of oxysterols: 25-hydroxycholesterol (25-OH) and 7β-hydroxycholesterol (7β-OH) as representatives of chain- and ring-oxidized oxysterols, respectively. Our results show that oxysterols hinder the fusion with host cell membranes by modifying their biophysical properties. Moreover, oxysterols applied to an already infected membrane reverse the changes caused by the infection. It could therefore be concluded that oxysterols may display antiviral activity in two ways: they prevent the healthy membrane from viral infection by blocking the fusion process; and protect already infected membrane from pathological changes induced by the virus.

Revealing local molecular distribution, orientation, phase separation, and formation of domains in artificial lipid layers: Towards comprehensive characterization of biological membranes

Lipids, together with molecules such as DNA and proteins, are one of the most relevant systems responsible for the existence of life. Selected lipids are able to assembly into various organized structures, such as lipid membranes. The unique properties of lipid membranes determine their complex functions, not only to separate biological environments, but also to participate in regulatory functions, absorption of nutrients, cell-cell communication, endocytosis, cell signaling, and many others. Despite numerous scientific efforts, still little is known about the reason underlying the variability within lipid membranes, and its biochemical significance. In this review, we discuss the structural complexity of lipid membranes, as well as the importance to simplify studied systems in order to understand phenomena occurring in natural, complex membranes. Such systems require a model interface to be analyzed. Therefore, here we focused on analytical studies of artificial systems at various interfaces. The molecular structure of lipid membranes, specifically the nanometric thickens of molecular bilayer, limits in a major extent the choice of highly sensitive methods suitable to study such structures. Therefore, we focused on methods that combine high sensitivity, and/or chemical selectivity, and/or nanometric spatial resolution, such as atomic force microscopy, nanospectroscopy (tip-enhanced Raman spectroscopy, infrared nanospectroscopy), phase modulation infrared reflection-absorption spectroscopy, sum-frequency generation spectroscopy. We summarized experimental and theoretical approaches providing information about molecular structure and composition, lipid spatial distribution (phase separation), organization (domain shape, molecular orientation) of lipid membranes, and real-time visualization of the influence of various molecules (proteins, drugs) on their integrity. An integral part of this review discusses the latest achievements in the field of lipid layer-based biosensors.

Different effects of oxysterols on a model lipid raft - Langmuir monolayer study complemented with theoretical calculations

Three oxysterols (7β-hydroxycholesterol; 7β-OH, 7-ketocholesterol; 7-K and 25-hydroxycholesterol, 25-OH) differing in the site of oxidation (ring system versus chain) and kind of polar group (hydroxyl versus carbonyl) were studied in lipid raft environment using the Langmuir monolayer technique complemented with theoretical calculations. Experiments were performed for the unmodified raft system, composed of sphingomyelin (SM) and cholesterol (Chol), and in the next step the raft was modified by the incorporation of oxysterol in different proportions. In the examined three-component system (Chol:SM:oxysterol), apart from interactions between the lipid raft components, the affinity of Chol to its oxidized derivatives also plays an important role. 25-OH was found to enhance interactions between SM and Chol and thus stabilize the raft, contrary to 7β-OH and 7-K, which exterted the fluidizing effect as well as the destabilization of the raft. Different action of oxysterols on model raft was observed. 7β-OH and 7-K, which are highly potent inducers of cell dath caused raft destabilization, while 25-OH, which is the least toxic of the investigated oxysterols, was found to stabilize the raft.

Predicting the packing parameter for lipids in monolayers with the use of molecular dynamics

Lipid molecules form the backbone of biological membranes. Due to their amphiphilic structure, they can self-organize in a plethora of different structures when in contact with water. The type of self-assembled structure and its curvature depend on so-called shape factor or critical packing parameter, CPP, that can be derived knowing the molecular volume of a lipid (V), optimal surface area (a₀) and critical chain length (l_c) (see Intermolecular and Surface Forces by Jacob N. Israelachvili, Third Edition, 2011). The value of CPP allows not only to predict the type of self-assembled structure but also is a key factor for molecular interactions, which play a great role both in physiological and pathological conditions. The greatest difficulties arise when calculating the a₀ parameter, and although for some typical membrane lipids these values have been determined, there are a number of derivatives for which this parameter, and thus CPP, are unknown. The value of CPP allows not only to predict the type of self-assembled structure but also is a key factor for molecular interactions, which play a great role both in physiological and pathological conditions. So far, the determination of the packing parameter required the use of theoretical models with assumptions deviating from the physical conditions. Here we report a method based on molecular dynamics, which was applied to simulate lipid membranes consisting of cholesterol, oxysterols, sphingolipids, phosphatidylcholines, and phosphatidylethanolamines. For lipid molecules for which CPPs have already been determined, high compliance has been demonstrated. This proves that the method presented herein can be successfully used to determine packing parameters for other membrane lipids and amphiphilic molecules.

Can oxysterols work in anti-glioblastoma therapy? Model studies complemented with biological experiments

Despite the progress made in recent years in the field of oncology, the results of glioblastoma treatment remain unsatisfactory. In this paper, cholesterol derivatives - oxysterols - have been investigated in the context of their anti-cancer activity. First, the influence of three oxysterols (7-K, 7β-OH and 25-OH), differing in their chemical structure, on the properties of a model membrane imitating glioblastoma multiforme (GBM) cells was investigated. For this purpose, the Langmuir monolayer technique was applied. The obtained results clearly show that oxysterols modify the structure of the membrane by its stiffening, with the 7-K effect being the most pronounced. Next, the influence of 7-K on the nanomechanical properties of glioblastoma cells (U-251 line) was verified with AFM. It has been shown that 7-K has a dose-dependent cytotoxic effect on glioblastoma cells leading to the induction of apoptosis as confirmed by viability tests. Interestingly, significant changes in membrane structure, characteristic for phospholipidosis, has also been observed. Based on our results we believe that oxysterol-induced apoptosis and phospholipidosis are related and may share common signaling pathways. Dysregulation of lipids in phospholipidosis inhibit cell proliferation and may play key roles in the induction of apoptosis by oxysterols. Moreover, anticancer activity of these compounds may be related to the immobilization of cancer cells as a result of stiffening effect caused by oxysterols. Therefore, we believe that oxysterols are good candidates as new therapeutic molecules as an alternative to the aggressive treatment of GBM currently in use.

25-hydroxycholesterol interacts differently with lipids of the inner and outer membrane leaflet - The Langmuir monolayer study complemented with theoretical calculations

25-hydroxycholesterol (25-OH), a molecule with unusual behavior at the air/water interface, being anchored to the water surface alternatively with a hydroxyl group at C(3) or C(25), has been investigated in mixtures with main membrane phospholipids (phosphatidylcholines - PCs, and phosphatidylethanolamines - PEs), characteristic of the outer and inner membrane leaflet, respectively. To achieve this goal, the classical Langmuir monolayer approach based on thermodynamic analysis of interactions was conducted in addition to microscopic imaging of films (in situ with BAM and after transfer onto mica with AFM), surface-sensitive spectroscopy (PM-IRRAS), as well as theoretical calculations. Our results show that the strength of interactions is primarily determined by the kind of polar group (strong, attractive interactions leading to surface complexes formation were found to occur with PCs while weak or repulsive ones with PEs). Subsequently, the saturation of phosphatidylcholines apolar chain(s) was found to be crucial for the structure of the formed complexes. Namely, saturated PC (DPPC) does not have preferences regarding the orientation of 25-OH molecule in surface complexes (which results in the two possible 25-OH arrangements), while unsaturated PC (DOPC) enforces one specific orientation of oxysterol (with C(3)-OH group). Our findings suggest that the transport of 25-OH between inner and outer membrane leaflet can proceed without orientation changes, which is thermodynamically advantageous. This explains results found in real systems showing significant differences in the rate of transmembrane transport of 25-OH and the other chain-oxidized oxysterols compared to their ring-oxidized analogues or cholesterol.

How the replacement of cholesterol by 25-hydroxycholesterol affects the interactions with sphingolipids: The Langmuir Monolayer Study complemented with theoretical calculations

In this paper, a representative of chain-oxidized sterols, 25-hydroxycholesterol (25-OH), has been studied in Langmuir monolayers mixed with the sphingolipids sphingomyelin (SM) and ganglioside (GM₁) to build lipid rafts. A classical Langmuir monolayer approach based on thermodynamic analysis of interactions was complemented with microscopic visualization of films (Brewster angle microscopy), surface-sensitive spectroscopy (polarization modulation-infrared reflection-absorption spectroscopy) and theoretical calculations (density functional theory modelling and molecular dynamics simulations). Strong interactions between 25-OH and both investigated sphingolipids enabled the formation of surface complexes. As known from previous studies, 25-OH in pure monolayers can be anchored to the water surface with a hydroxyl group at either C(3) or C(25). In this study, we investigated how the presence of additional strong interactions with sphingolipids modifies the surface arrangement of 25-OH. Results have shown that, in the 25-OH/GM₁ system, there are no preferences regarding the orientation of the 25-OH molecule in surface complexes and two types of complexes are formed. On the other hand, SM enforces one specific orientation of 25-OH: being anchored with the C(3)-OH group to the water. The strength of interactions between the studied sphingolipids and 25-OH versus cholesterol is similar, which indicates that cholesterol may well be replaced by oxysterol in the lipid raft system. In this way, the composition of lipid rafts can be modified, changing their rheological properties and, as a consequence, influencing their proper functioning.

Electrical Properties of Membrane Phospholipids in Langmuir Monolayers

Experimental surface pressure (π) and electric surface potential (ΔV) isotherms were measured for membrane lipids, including the following phosphatidylcholines (PCs)-1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC); 1,2-diarachidoyl-sn-glycero-3-phosphocholine (DAPC); and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). In addition, other phospholipids, such as phosphatidylethanolamines (represented by 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE)) and sphingolipids (represented by N-(hexadecanoyl)-sphing-4-enine-1-phosphocholine (SM)) were also studied. The experimental apparent dipole moments (μAexp) of the abovementioned lipids were determined using the Helmholtz equation. The particular contributions to the apparent dipole moments of the investigated molecules connected with their polar (μ⟂p) and apolar parts (μ⟂a) were theoretically calculated for geometrically optimized systems. Using a three-layer capacitor model, introducing the group's apparent dipole moments (calculated herein) and adopting values from other papers to account for the reorientation of water molecules (μ⟂w/εw), as well as the for the local dielectric permittivity in the vicinity of the polar (εp) and apolar (εa) groups, the apparent dipole moments of the investigated molecules were calculated (μAcalc). Since the comparison of the two values (experimental and calculated) resulted in large discrepancies, we developed a new methodology that correlates the results from density functional theory (DFT) molecular modeling with experimentally determined values using multiple linear regression. From the fitted model, the following contributions to the apparent dipole moments were determined: μ⟂w/εw=-1.8±1.4 D; εp=10.2±7.0 and εa=0.95±0.52). Local dielectric permittivity in the vicinity of apolar groups (εa) is much lower compared to that in the vicinity of polar moieties (εp), which is in line with the tendency observed by other authors studying simple molecules with small polar groups. A much higher value for the contributions from the reorientation of water molecules (μ⟂w/εw) has been interpreted as resulting from bulky and strongly hydrated polar groups of phospholipids.

Molecular insight into neurodegeneration - Langmuir monolayer study on the influence of oxysterols on model myelin sheath

Systematic studies on the influence of selected ring-oxidized (7α-hydroxycholesterol, 7α-OH; 7β-hydroxycholesterol, 7β-OH; 7-ketocholesterol, 7-K) and chain-oxidized (25-OH) sterols on lipid layer of myelin were performed. Myelin sheath was modeled as five-component Langmuir monolayer (Chol:PE:SM:PS:PC 50:20:12:9:9). Particular oxysterols have been incorporated into the model myelin sheath by replacing cholesterol totally or partially (1:1). The effect of oxysterol incorporation was characterized with surface pressure and electric surface potential - area isotherms and visualized with Brewster angle microscopy (BAM) and atomic force microscopy (AFM). It has been noticed that model myelin loses its homogeneous structure (due to the appearance of domains) at physiological bilayer conditions (30-35 mN/m). In the presence of oxysterols, the fluidity of myelin model increases and the organization of lipids is altered, which is reflected in the decrease of electric surface potential changes (ΔV). The strongest myelin/oxysterol interactions have been observed for 7-K and 25-OH, being the most cytotoxic oxysterols found in biological tests.

Terpene-Based Ionic Liquids from Natural Renewable Sources As Selective Agents in Antifungal Therapy

In this study, we present a new approach toward the design of ionic liquids with biological activity. Structural analysis of bioactive compounds was performed to design-in a technological and economic manner-salts with potential antifungal properties. The length of the alkyl chain as well as the task-specific component in the cation, the type of amine core, and the type of anion were considered as having an essential impact on achieving desired biological activity. Herein, we present the synthesis and characterization of ionic liquids based on monoterpene derivatives-namely, (1R,2S,5R)-(-)-menthol or bicyclic (1R)-endo-(+)-fenchol-from renewable sources. These new salts were synthesized with high yields (>96%) in mild conditions via a two-step procedure. Physicochemical properties (i.e., melting point, thermal stability, crystal shape, specific rotation, surfactant content, solubility, and surface activity) were analyzed in detail. The obtained results suggested the influence of the steric hindrance of the discussed salts on the reactivity, solubility, thermal stability, and surface properties of the studied compounds. Their potential selectivity in antifungal therapy was studied using Langmuir monolayer mimicking fungal (ergosterol) and mammalian (cholesterol) membranes. The model study confirmed the selective destabilizing activity of terpene-based ionic liquids on the fungus membrane.

Effect of selected B-ring-substituted oxysterols on artificial model erythrocyte membrane and isolated red blood cells

In this paper, systematic studies concerning the influence of selected oxysterols on the structure and fluidity of human erythrocyte membrane modeled as Langmuir monolayers have been performed. Three oxidized cholesterol derivatives, namely 7α-hydroxycholesterol (7α-OH) 7β-hydroxycholesterol (7β-OH) and 7-ketocholesterol (7-K) have been incorporated in two different proportions (10 and 50%) into artificial erythrocyte membrane, modeled as two-component (cholesterol:POPC) Langmuir monolayer. All the studied oxysterols were found to alter membrane fluidity and the effect was more pronounced for higher oxysterol content. 7α-OH increased membrane fluidity while opposite effect was observed for 7β-OH and 7-K. Experiments performed on model systems have been verified in biological studies on red blood cells (RBC). Consistent results have been found, i.e. under the influence of 7α-OH, the elasticity of erythrocytes increased, and in the presence of other investigated oxysterols - decreased. The strongest effect was noticed for 7-K. Change of membrane elasticity was associated with the change of erythrocytes shape, being most noticeable under the influence of 7-K.

Perfluorohexyloctane (F6H8) as a delivery agent for cyclosporine A in dry eye syndrome therapy - Langmuir monolayer study complemented with infrared nanospectroscopy

One of the key challenges in dry eye syndrome therapy is to find a suitable carrier for immunosuppressant drug - cyclosporine A (CsA) - delivery to the eye. To investigate this issue, herein we present a methodology based on the combined analysis in macro- (Langmuir monolayers), micro- (Brewster angle microscopy) and nanoscale (atomic force microscopy and infrared nano-spectroscopy). The applied approach proves that CsA affects the phospholipid part of the tear film lipid layer by loosening molecular packing. This effect can be reversed by the addition of perfluorohexyloctane (F₆H₈). We have highlighted that F₆H₈ increases the availability of CsA and therefore is appropriate carrier for CsA topical delivery to the eye in the dry eye syndrome. In addition, the applied herein procedure provides a simple, low-cost laboratory tool for preliminary studies involving membrane active pharmaceuticals, preceding in vivo tests.

Cholesterol and Cardiolipin Importance in Local Anesthetics-Membrane Interactions: The Langmuir Monolayer Study

Local anesthetics (LAs) are known to act on membrane level; however, the molecular mechanism of their activity is still not fully understood. One hypothesis holds that these drugs can incorporate into lipid membrane of nerve cells and in this way change conformation of channel proteins responsible for transport of sodium ions. However, the action of anesthetics is not limited to nerve cells. These drugs also affect other types of cells and organelles, causing severe side effects. In this paper, we applied Langmuir monolayers—as model of cellular membranes—and investigated interactions between selected amide-type local anesthetics (lidocaine prilocaine, mepivacaine and ropivacaine, in the form of hydrochlorides) and lipid components of natural membranes: cholesterol, POPC and cardiolipin (CL) and their mixtures (POPC/cholesterol and POPC/CL/cholesterol), which can serve as simplified models of nerve cell membranes, erythrocytes, and mitochondria. The influence of the drug was monitored by registering the surface pressure (π) as a function of surface area per molecule (A) in a monolayer in the presence of the drug in the subphase. The structure of lipid monolayers on subphases containing and devoid of the studied drugs were visualized with Brewster angle microscopy (BAM). Langmuir monolayer studies complemented with surface visualization technique reveal the expansion and fluidization of lipid monolayers, with the most pronounced effect observed for cardiolipin. In mixed systems, the effect of LAs was found to depend on cholesterol proportion. The observed fluidization of membranes by local anesthetics may negatively affect cells functioning and therefore can explain side effects of these drugs both on the cardiovascular and nervous systems.

Oxysterols Versus Cholesterol in Model Neuronal Membrane. I. The Case of 7-Ketocholesterol. The Langmuir Monolayer Study

Oxysterols are products of cholesterol oxidation. They can be formed endogenously (in both enzymatic and non-enzymatic reactions) as well as exogenously (delivered with food). Recent studies clearly demonstrate cytotoxic properties of these compounds, being mainly due to their incorporation into natural lipid bilayers. This process can influence mechanical and physicochemical properties of biomembrane—mainly by modifying the interactions between its components, which may result in the disruption of proper functioning of cell membrane and could lead to its degradation. Therefore, it can be assumed that oxysterols may affect the initiation of neurodegenerative diseases, including Alzheimer’s disease. However, the mode of action of these molecules at the molecular level is not fully known. To get a better understanding of the role of oxysterols in neurodegeneration, it is of great importance to examine mutual interactions between oxysterols and neuronal membrane components. One of the most promising techniques that can be used to analyze such interactions is the Langmuir monolayer technique. In this work, we have prepared an artificial neuronal membrane modeled as multicomponent Langmuir monolayer built up with cholesterol, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and sphingomyelin (SM). To examine whether there are any changes in the membrane properties under oxidative stress, in this paper we have investigated the impact of the representative ring-oxidized oxysterol: 7-ketocholesterol (7-KC). Our results show that replacing cholesterol with 7-KC increases the interaction between molecules in the model membrane.

The influence of an antitumor lipid - erucylphosphocholine - on artificial lipid raft system modeled as Langmuir monolayer

Outer layer of cellular membrane contains ordered domains enriched in cholesterol and sphingolipids, called 'lipid rafts', which play various biological roles, i.e., are involved in the induction of cell death by apoptosis. Recent studies have shown that these domains may constitute binding sites for selected drugs. For example alkylphosphocholines (APCs), which are new-generation antitumor agents characterized by high selectivity and broad spectrum of activity, are known to have their molecular targets located at cellular membrane and their selective accumulation in tumor cells has been hypothesized to be linked with the alternation of biophysical properties of lipid rafts. To get a deeper insight into this issue, interactions between representative APC: erucylphosphocholine, and artificial lipid raft system, modeled as Langmuir monolayer (composed of cholesterol and sphingomyelin mixed in 1:2 proportion) were investigated. The Langmuir monolayer experiments, based on recording surface pressure-area isotherms, were complemented with Brewster angle microscopy results, which enabled direct visualization of the monolayers structure. In addition, the investigated monolayers were transferred onto solid supports and studied with AFM. The interactions between model raft system and erucylphosphocholine were analyzed qualitatively (with mean molecular area values) as well as quantitatively (with ΔG(exc) function). The obtained results indicate that erucylphosphocholine introduced to raft-mimicking model membrane causes fluidizing effect and weakens the interactions between cholesterol and sphingomyelin, which results in phase separation at high surface pressures. This leads to the redistribution of cholesterol molecules in model raft, which confirms the results observed in biological studies.

Affinity of alkylphosphocholines to biological membrane of prostate cancer: studies in natural and model systems

The effectiveness of two alkylphosphocholines (APCs), hexadecylphosphocholine (miltefosine) and erucylphosphocholine to combat prostate cancer has been studied in vitro with artificial cancerous membrane, modelled with the Langmuir monolayer technique, and on cell line (Du-145). Studies performed with the Langmuir method indicate that both the investigated drugs have the affinity to the monolayer mimicking prostate cancer membrane (composed of cholesterol:POPC = 0.428) and the drug-membrane interactions are stronger for erucylphosphocholine as compared to hexadecylphosphocholine. Moreover, both studied drugs were found to fluidize the model membrane, which may lead to apoptosis. Indeed, biological studies confirmed that in Du-145 cell line both investigated alkylphosphocholines cause cell death primarily by apoptosis while necrotic cells constitute only a small percentage of APC-treated cells.

Interactions of alkylphosphocholines with model membranes-the Langmuir monolayer study

Alkylphosphocholines (APCs) belong to a class of synthetic antitumor lipids, which are new-generation anticancer agents. In contrast to traditional antitumor drugs, they do not attack the cell nucleus but, rather, the cellular membrane; however, their mechanism of action is not fully understood. This work compared the interactions of selected APCs [namely, hexadecylphosphocholine (miltefosine), octadecylphosphocholine and erucylphosphocholine] with the most important membrane lipids [cholesterol, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)] and examined their influence on a model membrane of tumor and normal cells. As a simple model of membranes, Langmuir monolayers prepared by mixing cholesterol either with a saturated phosphatidylcholine (DPPC), for a normal cell membrane, or with an unsaturated one (POPC), for a tumor cell membrane, have been applied. The APC–lipid interactions, based on experimental surface pressure (π) versus mean molecular area (A) isotherms, were analyzed qualitatively (with mean molecular area values) as well as quantitatively (with the ΔG^exc function). Strong attractive interactions were observed for mixtures of APCs with cholesterol, contrary to the investigated phosphatidylcholines, for which the interactions were found to be weak with a tendency to separation of film components. In ternary monolayers it has been found that the investigated model systems (cholesterol/DPPC/APC vs cholesterol/POPC/APC) differ significantly as regards the interactions between film-forming molecules. The results demonstrate stronger interactions between the components of cholesterol/POPC/APC monolayers compared to cholesterol/POPC film, mimicking tumor cell membranes. In contrast, the interactions in cholesterol/DPPC/APC films were found to be weaker than those in the cholesterol/DPPC system, serving as a model of healthy cell membranes, thus proving that the incorporation of APCs is, from a thermodynamic point of view, unfavorable for binary cholesterol/DPPC monolayers. It can be concluded that the composition of healthy cell membranes is a natural barrier preventing the incorporation of APCs into normal cells.