Physical Properties Of Model Membranes Research Articles

Plant Alkaloids Inhibit Membrane Fusion Mediated by Calcium and Fragments of MERS-CoV and SARS-CoV/SARS-CoV-2 Fusion Peptides.

To rationalize the antiviral actions of plant alkaloids, the ability of 20 compounds to inhibit calcium-mediated fusion of lipid vesicles composed of phosphatidylglycerol and cholesterol was investigated using the calcein release assay and dynamic light scattering. Piperine, tabersonine, hordenine, lupinine, quinine, and 3-isobutyl-1-methylxanthine demonstrated the most potent effects (inhibition index greater than 50%). The introduction of phosphatidylcholine into the phosphatidylglycerol/cholesterol mixture led to significant changes in quinine, hordenine, and 3-isobutyl-1-methylxanthine efficiency. Comparison of the fusion inhibitory ability of the tested alkaloids, and the results of the measurements of alkaloid-induced alterations in the physical properties of model membranes indicated a potent relationship between a decrease in the cooperativity of the phase transition of lipids and the ability of alkaloids to prevent calcium-mediated vesicle fusion. In order to use this knowledge to combat the novel coronavirus pandemic, the ability of the most effective compounds to suppress membrane fusion induced by fragments of MERS-CoV and SARS-CoV/SARS-CoV-2 fusion peptides was studied using the calcein release assay and confocal fluorescence microscopy. Piperine was shown to inhibit vesicle fusion mediated by both coronavirus peptides. Moreover, piperine was shown to significantly reduce the titer of SARS-CoV2 progeny in vitro in Vero cells when used in non-toxic concentrations.

The insecticide fipronil affects the physical properties of model membranes: A combined experimental and molecular dynamics simulations study in Langmuir monolayers

Fipronil is a widely used commercial insecticide whose action mechanism consists in blocking the influx of chloride ions through the γ-aminobutyric acid type A receptor (GABAA-R), an integral membrane protein. The present study investigates the interaction of fipronil with phospholipid Langmuir monolayers, in order to characterize the effects that its partition could exert on the physical properties of these model membranes. A combined experimental and molecular dynamics (MD) simulations approach was performed. MD simulations were conducted in such a way that they resemble an experimental compression isotherm of DPPC in the presence of fipronil in the aqueous subphase. Both the experimental and the simulated compression isotherm showed that the partition of fipronil between DPPC molecules induces an expansion of the monolayer. Experimental results also showed that fipronil can penetrate lipid monolayers even in condensed packing states. MD simulations showed that fipronil induces an ordering effect in the acyl chains of DPPC in the liquid-condensed phase. In addition, the simulations indicate that fipronil orients parallel to the plane of the monolayer and that it establishes hydrogen bonds with the glycerol region of DPPC. Free energy profiles of the partition of fipronil into the monolayers, obtained by means of umbrella sampling, indicated that its penetration is thermodynamically favorable, being the interphase between the glycerol region and the acyl chains of DPPC its most favorable location. Our results suggest that fipronil could modulate the supramolecular organization of biological membranes surrounding GABAA-R, contributing, at least in part, to its action mechanism.

Phospholipid Translocation as Driver of Cholesterol (Re)Distribution

The asymmetric distribution of the major phospholipid classes between the two leaflets of the mammalian plasma membrane (PM) has been known for almost 50 years. In contrast, the interleaflet distribution of cholesterol, the single most abundant component of the PM, has been actively debated but remains unclear. Using advanced enzyme-assisted lipidomics, we performed a detailed quantification of the phospholipid distribution in the two PM leaflets of red blood cells. These measurements revealed a significant imbalance in the total abundance of phospholipids, with the cytoplasmic leaflet containing ∼50% more phospholipids than the exoplasmic leaflet. Because the total area of the two leaflets in a bilayer must be equalized, another lipid component is likely asymmetrically distributed to buffer the area difference. Cholesterol, a lipid capable of rapid flip-flop and comprising a large fraction of the bilayer, may perform this balancing act by enriching in the exoplasmic leaflet. Furthermore, due to its ability to rapidly flip between leaflets, cholesterol may dynamically buffer PM properties in response to changes in the phospholipid leaflet composition. Thus, we hypothesize that cholesterol redistribution relieves the tension arising from phospholipid flip-flop and test this hypothesis both computationally and experimentally. Coarse-grained and all-atom molecular dynamics simulations of biomimetic bilayers reveal that cholesterol indeed changes its distribution in response to phospholipid flip-flop and by doing so, reduces the accumulated leaflet tension. To understand and fully characterize the resulting changes in bilayer properties, we further analyze the distinct effects of phospholipid and cholesterol flip-flop on the bilayer elastic moduli, spontaneous curvature and acyl chain order parameters. These findings are complemented with experimental measurements of dynamics in the morphology and physical properties of model membranes with and without cholesterol. Thus, our studies provide mechanistic insights into the mechanisms of cholesterol's distribution in a biological membrane.

Nitrated Fatty Acids Modulate the Physical Properties of Model Membranes and the Structure of Transmembrane Proteins.

Nitrated fatty acids (NO2 -FAs) act as anti-inflammatory signal mediators, albeit the molecular mechanisms behind NO2 -FAs' influence on diverse metabolic and signaling pathways in inflamed tissues are essentially elusive. Here, we combine fluorescence measurements with surface-specific sum frequency generation vibrational spectroscopy and coarse-grained computer simulations to demonstrate that NO2 -FAs alter lipid organization by accumulation at the membrane-water interface. As the function of membrane proteins strongly depends on both, protein structure as well as membrane properties, we consecutively follow the structural dynamics of an integral membrane protein in presence of NO2 -FAs. Based on our results, we suggest a molecular mechanism of the NO2 -FA in vivo activity: Driven by the NO2 -FA-induced lipid layer reorganization, the structure and function of membrane-associated (signaling) proteins is indirectly affected.

Physical Properties of Model Membranes Containing Pope and Phytosterol

We have studied the effect of phytosterol on the physical properties of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) multilamellar vesicles using deuterium nuclear magnetic resonance (2H NMR). The sn-1 chain of POPE is deuterium labeled. The NMR spectra were taken as a function of temperature and phytosterol concentration. The order of POPE-d31 membranes, measured through the spectral first moment, is almost not affected by the addition of phytosterol in the gel phase, while it increases with phytosterol concentration in the liquid-crystalline phase. A significant difference in the ability of phytosterol to disorder the gel-phase and to order the liquid-phase POPE membranes is observed. This finding differs from those observed in POPE/chol and other lipid/sterol systems. Furthermore, the temperature-composition phase diagram will be discussed.

Membrane effects of lysozyme amyloid fibrils

The influence of mature lysozyme fibrils on the structural and physical properties of model membranes composed of phosphatidylcholine (PC) and its mixtures with cardiolipin (CL) (10mol%) and cholesterol (Chol) (30mol%) was studied using fluorescent probes DPH, pyrene, Laurdan and MBA. Analysis of pyrene fluorescence spectra along with the measurements of DPH fluorescence anisotropy revealed that the structure of hydrocarbon chains region of lipid bilayer is not affected by the fibrillar aggregates of lysozyme. In contrast, probing the membrane effects by Laurdan and MBA showed the rise of both the generalized polarization of Laurdan and the MBA fluorescence anisotropy, suggesting that amyloid protein induces reduction of bilayer hydration and increase of lipid packing in the interfacial region of model membranes.

Physical Properties of Lipid Membranes Containing Sterol Studied by Deuterium NMR and Fluorescence Microscopy

We study the physical properties of model membranes composed of phosphatidylcholines and sterol. The morphology and phase behavior of membranes were investigated by deuterium NMR and fluorescence microscopy. In binary mixtures, coexistence of solid-ordered and liquid-disordered phases was observed in a wide temperature and composition range. The results for ternary mixtures containing sterol show that addition of sterol promotes the formation of the liquid-ordered phase. Sterol has strong influence on the morphology and the phase behavior of membranes. A partial phase diagram of will be presented.

Comparison of the Effects of Cholesterol or 3β-Hydroxy-5-Oxo-5,6-Secocholestan-6-Al on the Thermotropic and Structural Properties of Mixtures of Phosphatidylethanolamine and Phosphatidylcholine

The oxidation of cholesterol with ozone produces 3β-hydroxy-5-oxo-5,6-secocholestan-6-al. This oxysterol has been implicated in a number of pathological conditions in vivo including atherosclerotic plaque formation and amyloidogenesis. We have shown previously that this oxysterol strongly modifies the physical properties of model membranes composed of different phosphatidylethanolamines or phosphatidylserine. In the present work we have extended our studies to ternary mixtures composed of phosphatidylethanolamine and phosphatidylcholine with sterols, either 3β-hydroxy-5-oxo-5,6- secocholestan-6-al or cholesterol. We use differential scanning calorimetry and small angle X-ray diffraction to characterize the phase behavior of mixtures of dipalmitoleoylphosphatidylethanolamine (diPoPE) and dipalmitoleoylphosphatidylcholine (diPoPC) or 1-palmitoyl-2-oleoyl-phosphatidylethanoloamine (POPE) and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC). We compare the effect of the two sterols on the temperature of the transition of the ternary system from the liquid crystalline to the hexagonal phase (TH) and on the curvature of the resulting cylindrical micelles. Addition of low concentrations of diPoPC increases TH while adding cholesterol to this mixture significantly lowers TH. The effect of 3β-hydroxy-5-oxo-5,6-secocholestan-6-al is much weaker than that of cholesterol. With regard to the curvature of the cylindrical micelles, the addition of diPoPC and 3β-hydroxy-5-oxo-5,6-secocholestan-6-al have opposing effects, while cholesterol does not effect the curvature at all. Low concentrations of POPC in POPE cause an increase in TH and decrease the curvature of the cylindrical micelles, while cholesterol has the opposite effect.

Effects of (+)-totarol, a diterpenoid antibacterial agent, on phospholipid model membranes

(+)-Totarol, a highly hydrophobic diterpenoid isolated from Podocarpus spp., is inhibitory towards the growth of diverse bacterial species. (+)-Totarol decreased the onset temperature of the gel to liquid-crystalline phase transition of DMPC and DMPG membranes and was immiscible with these lipids in the fluid phase at concentrations greater than 5 mol%. Different (+)-totarol/phospholipid mixtures having different stoichiometries appear to coexist with the pure phospholipid in the fluid phase. At concentrations greater than 15 mol% (+)-totarol completely suppressed the gel to liquid-crystalline phase transition in both DMPC and DMPG vesicles. Incorporation of increasing amounts of (+)-totarol into DEPE vesicles induced the appearance of the H II hexagonal phase at low temperatures in accordance with NMR data. At (+)-totarol concentrations between 5 and 35 mol% complex thermograms were observed, with new immiscible phases appearing at temperatures below the main transition of DEPE. Steady-state fluorescence anisotropy measurements showed that (+)-totarol decreased and increased the structural order of the phospholipid bilayer below and above the main gel to liquid-crystalline phase transition of DMPC respectively. The changes that (+)-totarol promotes in the physical properties of model membranes, compromising the functional integrity of the cell membrane, could explain its antibacterial effects.

Differential scanning calorimetric study of the effect of vitamin D 3 on the thermotropic phase behavior of lipids model systems

Differential scanning calorimetry has been used to investigate the effects of vitamin D 3 on the physical properties of model membranes including pure phosphatidylcholines (PC's) of chain lengths from 14 to 18, pure dipalmitoylphosphatidylethanolamine (DPPE), and various mixtures of these lipids. The results demonstrate that the interactions of vitamin D 3 with PC's are dependent on acyl chain lengths. It was found that vitamin D 3 reduces the transition temperatures of PC's and PE's, broadening the transition and reducing the enthalpy, eventually abolishing the transition. The interaction of vitamin D 3 with PC varies with chain length; the transition of DMPC is abolished by only 15 mol% vitamin D 3 whereas 45 mol% vitamin D 3 is required to abolish the transition of DSPC. These variations in vitamin D 3 lipid interactions are further explored in various mixtures. In the mixture studies it is shown that vitamin D 3 affects the mixing properties of the lipids in the mixtures. The results suggest that the presence of vitamin D 3 in lipids can affect the lateral phase distributions of lipids, and thereby have important effects on membrane function.

Effects of farnesol on the thermotropic behavior of dimyristoylphosphatidylcholine

Differential scanning calorimetry (DSC) and DPH fluorescence anisotropy have been used to investigate the effects of trans-trans farnesol on the physical properties of model membranes and extracted cell lipids. Farnesol was shown to have a significant effect on the gel to liquid-crystal phase transition temperature, the enthalpy of the transition and the transition co-operativity for extruded vesicles of dimyristoylphosphatidylcholine (DMPC). The phase transition of DMPC vesicles was eliminated at 25 mol% farnesol. Farnesol decreased the fluorescence anisotropy of the lipids extracted from human leukemia line CEM-C1 cells.

Effects of antineoplastic ether lipids on model and biological membranes

Differential scanning calorimetry and electron spin resonance were utilized to measure the effects of di-ether glycerophospholipid analogs (EL) on the physical properties of model membranes and on the membrane fluidity of HL60 leukemic cells. 1-Octadecyl-2-methyl- rac-glycero-3-phosphocholine (ET-18-OMe) and 1-thiohexadecyl-2-ethyl- rac-glycero-3-phosphocholine (ET 16S-OEt) lower the transition temperature of dimyristoylphosphatidylcholine vesicles in a range of concentrations between 0.5 and 15 mol %. Studies conducted on the interaction of EL with a wide spectrum of different phospholipids, namely dipalmitoylphosphatidylcholine, 1-hexadecyl-2-palmitoylphosphatidylcholine, dipalmitoylphosphatidylethanolamine, and dielaidoylphosphatidylethanolamine confirmed the ability of EL to effect the physical properties of model membranes. Changes in calorimetric enthalpy were observed only with phosphatidylethanolamine-containing phospholipids. ET-18-OMe and ET-16S-OEt increased the membrane fluidity of HL60 leukemic cells labeled with the fatty acid spin label probe 5-nitroxystearate. These data demonstrate the ability of EL to partition into phospholipidic domains and to change their physical properties. Furthermore, they affect the membrane fluidity of whole cells. These effects indicate an interaction between EL and the plasma membrane which may be of importance in determining the cytotoxic activity against tumor cells exerted by EL.

A model mosaic membrane: Association of phospholipids and cytochrome oxidase

The structure and physical properties of model membranes formed from lipids and cytochromec oxidase have been examined. The lipid-depleted protein is in the form of 90 A rods or globules. When phospholipid is added the rods swell and then. form sheets and concentric membrane vesicles. The protein is saturated with lipid at 65 g/atoms of phosphorus per mole of hemea. Electron microscope examination by negative staining, sectioning, and freeze etching indicates a 50 A thick unit membrane with 50–60 A protein globules in the lipid bilayer. Infrared, circular dichroism and fluorescence binding studies are consistent with globular protein units surrounded with lipid. Diolein will substitute for phospholipid but the membrane formed remains as sheets rather than vesicles. Saturated phospholipids will not interact with the oxidase to form membrane. The capacity to form membrane is specific to protein associated with the hemea, and other insoluble protein in the original oxidase preparation cannot form membrane.