Dimyristoylphosphatidylcholine Membranes Research Articles

Cholesterol and Melatonin Exert Opposite Effects on the Interaction of Model DMPC Membranes with the SARS‐CoV‐2 S‐Protein: A SANS Study

AbstractAn effect of receptor‐binding domain (RBD) of SARS‐CoV‐2 S‐protein on structural parameters of model lipid membranes presented by dimyristoylphosphatidylcholine (DMPC) systems with cholesterol and melatonin impurities is studied by small angle neutron scattering (SANS). It is shown that an increase in melatonin concentration in the lipid membrane leads to a decrease in the thickness of the lipid bilayer, while an increase in the concentration of cholesterol leads to an increase in its thickness. It is suggested that increasing the concentration of melatonin in a membrane prevents the interaction of coronaviral S‐protein with a lipid membrane of a cell. In the presence of cholesterol in the system, the interaction of a lipid membrane with an active part of S‐protein occurs depending on a phase state of the lipid: in the case of a gel phase, there is no changes in structural parameters, but at higher temperatures in the case of a liquid crystal phase, an addition of RBD SARS‐CoV‐2 to the system causes a reduce in the membrane thickness.

How Ethanolic Disinfectants Disintegrate Coronavirus Model Membranes: A Dissipative Particle Dynamics Simulation Study.

We have developed dissipative particle dynamics models for pure dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), and dimyristoylphosphatidylcholine (DMPC) as well as their binary and ternary mixed membranes, as coronavirus model membranes. The stabilities of pure and mixed membranes, surrounded by aqueous solutions containing up to 70 mol % ethanol (alcoholic disinfectants), have been investigated at room temperature. We found that aqueous solutions containing 5–10 mol % ethanol already have a significant weakening effect on the pure and mixed membranes. The magnitude of the effect depends on the membrane composition and the ethanol concentration. Ethanol permeabilizes the membrane, causing its lateral swelling and thickness shrinking and reducing the orientational order of the hydrocarbon tail of the bilayer. The free energy barrier for the permeation of ethanol in the bilayers is considerably reduced by the ethanol uptake. The rupture-critical ethanol concentrations causing the membrane failure are 20.7, 27.5, and 31.7 mol % in the aqueous phase surrounding pure DMPC, DOPC, and DPPC membranes, respectively. Characterizing the failure of lipid membranes by a machine-learning neural network framework, we found that all mixed binary and/or ternary membranes disrupt when immersed in an aqueous solution containing a rupture-critical ethanol concentration, ranging from 20.7 to 31.7 mol %, depending on the composition of the membrane; the DPPC-rich membranes are more intact, while the DMPC-rich membranes are least intact. Due to the tight packing of long, saturated hydrocarbon tails in DPPC, increasing the DPPC content of the mixed membrane increases its stability against the disinfectant. At high DPPC concentrations, where the DOPC and DMPC molecules are confined between the DPPC lipids, the ordered hydrocarbon tails of DPPC also induce order in the DOPC and DMPC molecules and, hence, stabilize the membrane more. Our simulations on pure and mixed membranes of a diversity of compositions reveal that a maximum ethanol concentration of 32 mol % (55 wt %) in the alcohol-based disinfectants is enough to disintegrate any membrane composed of these three lipids.

Insights into molecular mechanism of action of citrus flavonoids hesperidin and naringin on lipid bilayers using spectroscopic, calorimetric, microscopic and theoretical studies

Hesperidin and naringin are predominant flavonoids in citrus fruits and exert various biological activities. However, mechanism of these activities has not been completely investigated at cellular and molecular levels. For this reason, we aim to systematically elucidate molecular interactions between citrus flavonoids, hesperidin and naringin, and model lipid membranes composed of dimyristoylphosphatidylcholine (DMPC) using combinatorial experimental biophysical techniques, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, differential scanning calorimetry (DSC), atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) as a function of citrus flavonoid concentrations, and computational study. ATR-FTIR showed that both flavonoids significantly increase the fluidity of DMPC membranes with the increase in the bandwidth values of the CH2 antisymmetric stretching vibration. While hesperidin exhibits weakly disordering effect in hydrophobic region, naringin has an ordering effect in this region. Both flavonoids induce alterations in the arrangement of polar heads of lipids. DSC revealed that naringin significantly influences the cooperativity and thermotropic phase behavior of DMPC compared to hesperidin. Furthermore, microscopic observations are in cooperation with the spectroscopic and calorimetric results indicating that naringin is more effective than hesperidin in terms of inducing changes in the membrane organization. Density functional theory (DFT) study demonstrated that both DMPC − Hesperidin and DMPC − Naringin systems form strong hydrogen bonding via PO2− moieties. Methodologies showed that small structural differences can influence the membrane bilayers distinctly. Consequently, this study implies that variations in structural and dynamical properties of lipid membranes can be critical for processes involving the inhibition of lipid peroxidation and antioxidant mechanism.

Hydrolysable tannins change physicochemical parameters of lipid nano-vesicles and reduce DPPH radical - Experimental studies and quantum chemical analysis

Tannins belong to plant secondary metabolites exhibiting a wide range of biological activity. One of the important aspects of the realization of the biological effects of tannins is the interaction with lipids of cell membranes. In this work we studied the interaction of two hydrolysable tannins: 1,2,3,4,6-penta-O-galloyl-β-d-glucose (PGG) and 1,2-di-O-galloyl-4,6-valoneoyl-β-d-glucose (T1) which had the same number of both aromatic rings (5) and hydroxyl groups (15) but differing in flexibility due to the presence of valoneoyl group in the T1 molecule with DMPC (dimyristoylphosphatidylcholine) lipid nano-vesicles (liposomes). Tannins-liposomes interactions were investigated using fluorescence spectroscopy, differential scanning calorimetry, laser Doppler velocimetry, dynamic light scattering and Fourier Transform Infra-Red spectroscopy. It was shown that more flexible PGG molecules stronger decreased the microviscosity of the liposomal membranes and increased the values of negative zeta potential in comparison with the more rigid T1. Both compounds diminished the phase transition temperature of DMPC membranes, interacted with liposomes via PO groups of head of phospholipids and their hydrophobic regions. These tannins neutralized DPPH free radicals with the stoichiometry of the reaction equal 1:1.The effects of the studied compounds on liposomes were discussed in relation to tannin quantum chemical parameters calculated by molecular modeling.

Thermodynamics and Intermolecular Interactions during the Insertion of Anionic Naproxen into Model Cell Membranes.

The insertion process of Naproxen into model dimyristoylphosphatidylcholine (DMPC) membranes is studied by resorting to state-of-the-art classical and quantum mechanical atomistic computational approaches. Molecular dynamics simulations indicate that anionic Naproxen finds an equilibrium position right at the polar/nonpolar interphase when the process takes place in aqueous environments. With respect to the reference aqueous phase, the insertion process faces a small energy barrier of ≈5 kJ mol-1 and yields a net stabilization of also ≈5 kJ mol-1. Entropy changes along the insertion path, mainly due to a growing number of realizable microstates because of structural reorganization, are the main factors driving the insertion. An attractive fluxional wall of noncovalent interactions is characterized by all-quantum descriptors of chemical bonding (natural bond orbitals, quantum theory of atoms in molecules, noncovalent interaction, density differences, and natural charges). This attractive wall originates in the accumulation of tiny transfers of electron densities to the interstitial region between the fragments from a multitude of individual intermolecular contacts stabilizing the tertiary drug/water/membrane system.

Exploring membrane viscosity at the headgroup region utilizing a hemicyanine-based fluorescent probe

Membrane viscosity is an important determinant for several cellular functions. In spite of having vast literature on the interfacial and acyl chain region viscosity of the membrane, there is hardly any report on the membrane viscosity at the headgroup region. In this present work, we have exploited the photophysical properties of a hemicyanine dye covalently linked with a C16 alkyl chain (HC16) to explore the membrane viscosity at the headgroup region. The chromophoric group of HC16 locates at the headgroup region due to its overall positive charge. Methanol-glycerol mixture of known composition and viscosity were used to develop the standard curve of apparent rotational correlation time against viscosity, and utilized this standard curve to evaluate viscosity of dimyristoylphosphatidylcholine (DMPC) membrane as a function of temperature. The result exhibited that there is a 4-fold change in headgroup viscosity of the DMPC membrane between gel and fluid phase; with a sharp change during the phase transition. Moreover, we have monitored the effect of cholesterol on the viscosity of the headgroup region of a biologically relevant lipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), keeping in mind the role of cholesterol on membrane organization, dynamics, and cellular functions. Interestingly, the headgroup viscosity of POPC membrane does not show a significant change in presence of various concentrations of cholesterol, though literature showed remarkable change in interfacial and acyl chain region viscosity in presence of similar concentrations of cholesterol.

The effects of amino acid sequence and solvent polarity on the self-assembling of cyclic peptide nanotubes and molecular channel formation inside the lipid bilayer

In this article, the effects of amino acid sequence and solvent polarity on the self-assembling process of cyclic peptides (CPs) were investigated by employing molecular dynamic (MD) simulations and quantum chemistry calculations. As a result, CP dimers are not stable in water, because of hydrogen bond (H-bond) lost between the CP units, while chloroform increases the stability of the CP dimers. MM-PBSA and MM-GBSA calculations confirmed that solvent polarity has an important effect on the stability of the CP dimers. Dynamical behavior of the cyclic peptide nanotubes (CPNTs) in chloroform indicates that CPNTs composed of leucine and phenylalanine are better molecular containers than that of isoleucine. At the next step, the ability of these CPNTs in molecular channel formation inside a fully hydrated DMPC (dimyristoylphosphatidylcholine) bilayer was investigated during 50 ns MD simulations. The obtained results show that only CPNT composed of isoleucine can form a molecular channel inside the DMPC membrane because isoleucine has a greater hydrophobicity than leucine and phenylalanine. This property increases the interactions between the CPNT and lipid residues, which elevates the stability of the CPNT inside the DMPC bilayer. Quantum chemistry calculations and non-covalent interactions analysis indicate that the solvent changes the stability and dynamical behavior of the CPNTs through the change in the H-bond strength. Finally, according to the different analyses, it can be concluded that the amino acid sequence in the CP units has an important role in designing specific nanostructures.

Mechanism of Action of the Antimicrobial Peptide Caerin1.1

AbstractCaerin1.1 is a 25‐residue antimicrobial peptide secreted by the tree frog Litoria splendida. It is classified as an α‐helical cationic membrane disrupting peptide, with a good activity against Gram positive bacteria. Given its length, it is believed that caerin1.1 acts via transmembrane pore formation. However, the exact mechanism of its action is unknown. Here the interactions of caerin1.1 with biomimetic membranes were studied to establish the molecular mechanism of action. The activity of the peptide against different model membranes was determined by dye leakage experiments. Caerin1.1 showed weak activity against neat dimyristoyl phosphatidylcholine (DMPC) membrane and minimal activity against negatively charged or cholesterol containing membrane mixtures. Circular dichroism spectroscopy revealed that the peptide exhibited only 8‐16% helicity even in the DMPC membrane, yet quartz crystal microbalance fingerprinting suggested that the peptide was able to penetrate the membrane in each case. The results suggest that caerin1.1 is a membrane penetrating peptide but not a membrane disruptor, only causing incidental dye leakage while diffusing across the membrane. Consistently the main mode of action of caerin1.1 is likely exerted through an intracellular target.

Observation of Translational Diffusion in a Planer Supported Lipid Bilayer Membrane by Total Internal Reflection-Transient Grating Method

Abstract Total internal reflection-transient grating (TIR-TG) method has been applied to diffusion coefficient measurements of trans-stilbene in a dimyristoylphosphatidylcholine (DMPC) lipid bilayer membrane at a silica/water interface. The observed TIR-TG signal consisted of four decay components. The rate constants of the third and fourth components quadratically depended on grating wavenumbers, which indicates that these components represent diffusion processes of trans-stilbene (diffusion signals). From the decay rate constants of the diffusion signals, two different diffusion coefficients, (5.7 ± 1.0) × 10−10 and (2.8 ± 0.5) × 10−12 m2s−1 were obtained for trans-stilbene in the DMPC membrane. This result indicates that the DMPC membrane is an inhomogeneous environment that can be described with two different effective viscosities.

Computational Model to Unravel the Function of Amyloid-β Peptides in Contact with a Phospholipid Membrane.

Divalent cations have a strong impact on the properties of phospholipid membranes, where amyloid-β peptides exert effects related to possible functional or pathological roles. In this work, we use an atomistic computational model of dimyristoyl-phosphatidylcholine (DMPC) membrane bilayers. We perturb this model with a simple model of divalent cations (Mg2+) and with a single amyloid-β (Aβ) peptide of 42 residues, both with and without a single Cu2+ ion bound to the N-terminus. In agreement with the experimental results reported in the literature, the model confirms that divalent cations locally destabilize the DMPC membrane bilayer and, for the first time, that the monomeric form of Aβ helps in avoiding the interactions between divalent cations and DMPC, preventing significant effects on the DMPC bilayer properties. These results are discussed in the frame of a protective role of the diluted Aβ peptide floating around phospholipid membranes.

Ordering of Saturated and Unsaturated Lipid Membranes near Their Phase Transitions Induced by an Amphiphilic Cyclodextrin and Cholesterol.

When inserted in membranes of dimyristoyl phosphatidylcholine (DMPC), methylated β-cyclodextrins with one (TrimβMLC) or two (TrimβDLC) lauryl acyl chains grafted onto the hydrophilic cavity exert a "cholesterol-like ordering effect", by straightening the acyl chains in the fluid phase at temperatures near the chain melting transition. This effect may be related to pretransitional events such as the "anomalous swelling" known to occur with saturated phosphatidylcholine membranes. To investigate this model, order profiles and bilayer thicknesses of DMPC and unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) membranes containing amphiphilic cyclodextrins or cholesterol were determined by deuterium NMR. The pure lipid membranes display both a qualitatively similar chain ordering upon cooling in the fluid phase, more important at the chain extremity, which gets more pronounced near their fluid-to-gel transitions. Both membranes show a bilayer thickness increase by ∼0.5 Å just above their transition, as observed previously with saturated phosphatidylcholines of various chain lengths. Membrane-insertion of 5% TrimβMLC or cholesterol induces an important ordering of the DMPC acyl chains just above the transition, which is also more pronounced at the chain extremity. There is an additional increase of the bilayer thickness, most probably due to a deep insertion of these amphiphilic molecules, facilitated by increased bilayer softness in the anomalous swelling regime. These effects are more important with TrimβMLC than with cholesterol. By contrast, no enhanced acyl chain ordering was observed when approaching the transition of TrimβMLC-containing POPC membranes, as a possible consequence of an eventual lack of anomalous swelling in unsaturated lipid membranes. Insertion of higher concentrations of TrimβMLC was found to induce a magnetic orientation of the DMPC membranes in the fluid phase with 10% of this derivative, coupled with the appearance of a broad isotropic component when the concentration is raised to 20%. No membrane orientation or isotropic component was detected with TrimβMLC-containing POPC membranes.

Polar Interactions Play an Important Role in the Energetics of the Main Phase Transition of Phosphatidylcholine Membranes.

Conformational changes of membrane proteins are accompanied by deformation in the surrounding lipid bilayer. To gain insight into the energetics of membrane deformation, the phase behavior of dimyristoylphosphatidylcholine (DMPC) membranes in the presence of the dipole potential, ψd, modifiers was investigated by differential scanning calorimetry. 7-Ketocholesterol, which weakens ψd and reduces membrane-perpendicular dipole–dipole repulsion, causes a discrete second peak on the high-temperature side of the main transition, whereas 6-ketocholestanol, which strengthens ψd and increases membrane-perpendicular dipole–dipole repulsion, merely produces a shoulder. Measurements on pure DMPC vesicles showed that the observed temperature profile could not be explained by a single endothermic process, that is, breaking of van der Waals forces between hydrocarbon chains alone. Removal of NaCl from the buffer caused an increase in the main transition temperature and the appearance of an obvious shoulder, implicating polar interactions. Consideration of the phosphatidylcholine (PC) head group dipole moment indicates direct interactions between PC dipoles that are unlikely to account for the additional process. It seems more likely that the breaking of an in-plane hydrogen-bonded network consisting of hydrating water dipoles together with zwitterionic lipid head groups is responsible. The evidence presented supports the idea that the breaking of van der Waals forces between lipid tails required for the main phase transition of PC membranes is coupled to partial breaking of a hydrogen-bonded network at the membrane surface.

Interaction of the mononucleotide UMP with a fluid phospholipid bilayer.

Interaction between mononucleotides and lipid membranes is believed to have played an important role in the origin of life on Earth. Studies on mononucleotide-lipid systems hitherto have focused on the influence of the lipid environment on the organization of the mononucleotide molecules, and the effect of the latter on the confining medium has not been investigated in detail. We have probed the interaction of the mononucleotide, uridine 5'-monophosphate (UMP), and its disodium salt (UMPDSS) with fluid dimyristoylphosphatidylcholine (DMPC) membranes, using small-angle X-ray scattering (SAXS), cryogenic scanning electron microscopy (cryo-SEM) and computer simulations. UMP adsorbs and charges the lipid membrane, resulting in the formation of unilamellar vesicles in dilute solutions. Adsorption of UMP reduces the bilayer thickness of DMPC. UMPDSS has a much weaker effect on interbilayer interactions. These observations are in very good agreement with the results of an all-atom molecular dynamics simulation of these systems. In the presence of counterions, such as Na+, UMP forms small aggregates in water, which bind to the bilayer without significantly perturbing it. The phosphate moiety in the lipid headgroup is found to bind to the hydrogens from the sugar ring of UMP, while the choline group tends to bind to the two oxygens from the nucleotide base. These studies provide important insights into lipid-nucleotide interactions and the effect of the nucleotide on lipid membranes.

A concentration dependent spectroscopic study of binary mixtures of plant sterol stigmasterol and zwitterionic dimyristoyl phosphatidylcholine multilamellar vesicles: An FTIR study

Plant sterols such as stigmasterol and sitosterol are mostly found in plant cell walls and membranes and have been used as cholesterol-lowering agents as well as anti-inflammatory, antibacterial, antifungal, anti-ulcerative and antitumor activities. This study was conducted to examine the thermotropic and physicochemical properties of binary mixtures of plant sterol stigmasterol and zwitterionic dimyristoyl phosphatidylcholine (DMPC) multilamellar vesicles (MLVs) as a function of temperature and different stigmasterol concentrations by utilizing Fourier transform infrared (FTIR) spectroscopy. In this study, we observed that stigmasterol incorporation induces variations in the lipid thermotropic phase behavior, lipid order (acyl chain flexibility) and dynamics (fluidity) and hydration state of the head group and/or the region near the head group of DMPC MLVs. Our results suggest that stigmasterol is fully capable of interacting with zwitterionic DMPC membranes and changes the biophysical properties of them.

Effects of Channel Forming Peptides on Lipid Bilayer Dynamics and Leaflet Coupling

Altering the material properties of lipid bilayers has been shown to affect the biological activity of a number of channel-forming peptides and proteins. A prototypical example is gramicidin, in which the channel formation and lifetime directly depends on the lipid membrane in which it is embedded. However, less is known about how a membrane-spanning channel affects the lipid bilayer properties, particularly the elasticity. Here we use neutron spin echo spectroscopy (NSE) to measure both the collective bending and thickness fluctuation dynamics in dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) bilayers containing gramicidin at low peptide/lipid ratios. At these low concentrations, gramicidin incorporation did not have a measurable effect on the average bilayer structure but significantly impacted the collective membrane dynamics. The bending modulus of DMPC membranes increased with increasing gramicidin concentration, while the trend in DPPC was non-monotonic and first decreased at low gramicidin concentrations before increasing at higher concentrations. In contrast, the thickness fluctuation amplitude increased with increasing gramicidin concentration in both the DMPC and DPPC lipid bilayers. Notably, combining the bending and thickness fluctuation results revealed that the seemingly disparate concentration trends in the DMPC and DPPC lipid bilayers were both consistent with an increase in coupling between the bilayer leaflets with increasing gramicidin concentration. Together these results demonstrate that channel formation can have considerable effects on the surrounding lipid membrane elasticity and highlight the interplay between lipids and peptides in determining the membrane dynamics.

Correction to: Temperature-Dependent Electron Paramagnetic Resonance Studies of Docosahexaenoic Acid and Gamma Linolenic Acid Effects on Phospholipid Membranes With and Without Cholesterol

Free docosahexaenoic acid (DHAn-3) and gamma linolenic acid (GLAn-6) effects on dimyristoyl phosphatidylcholine (DMPC) membranes were studied as a function of temperature by electron paramagnetic resonance (EPR) spectroscopy. 5- and 16-doxyl stearic acid (5-, 16-DS) spin labels were utilized to obtain information from the interfacial and alkyl chain region, respectively. In the studied temperature range, the presence of DHAn-3 or GLAn-6 caused decreases in maximum hyperfi ne splitting values and correlation times of DMPC membranes. Both in the interfacial region and depths of membrane, changes were more pronounced for DHAn-3 in pure DMPC. In the presence of cholesterol (CH), DHAn-3 and GLAn-6 effects were similar and more pronounced in the depths of the membrane. The changes in the structure and dynamics of samples were obtained from simulations of spectra, which indicated some changes in the number of spectral components by incorporation of DHAn-3 and GLAn-6. In the interfacial region and below the main phase transition temperature of DMPC, there was an increase in heterogeneity. For temperatures above the phase transition, a more homogeneous environment for spin label was obtained in the presence of fatty acids.

New insights into the influence of monofluorination on dimyristoylphosphatidylcholine membrane properties: A solid-state NMR study

Solid-state 19F NMR spectroscopy is a method of choice to study the interactions between lipid membranes and other molecules such as peptides, proteins or drugs. Numerous fluorine-labeled NMR probes have been developed over the last few years, especially fluorine-labeled peptides. In order to develop a new kind of NMR reporter molecule and a complementary approach to fluorine-labeling of peptides, we synthesized six monofluorinated derivatives of the lipid dimyristoylphosphatidylcholine (F-DMPC), with the fluorine atom located along the acyl chain linked to the central glycerol position. To better understand the behavior of these fluorine-labeled lipids, we report here the investigation of F-DMPC membrane properties using solid-state 2H, 15N, 19F‐ and 31P‐NMR spectroscopy. This study was carried out on pure F-DMPC bilayers as well as F-DMPC/DMPC mixtures at various ratios. Slight perturbations were observed for pure F-DMPC multilamellar vesicles (MLVs), most noticeable for lipids with the fluorine atom located at the extremities of the acyl chain. On the other hand, no significant perturbations were observed for F-DMPC/DMPC MLVs containing up to 25% F-DMPC, nor for any fluorine-labeled bilayers that were prepared as macroscopically oriented samples. To test the interaction with some representative peptides, 15N-labeled α-helical antimicrobial peptides (AMPs) were incorporated into F-DMPC/DMPC (1/3) bilayers. 15N SS-NMR analyses confirmed that the known orientation of each AMP in pure DMPC was preserved in the presence of 25% monofluorinated DMPC, irrespective of the position of the 19F-label. In summary, F-DMPC/DMPC (1/3) model membranes can be used as NMR reporter to study membrane interactions with other molecules.

Combination of anti-hypertensive drugs: a molecular dynamics simulation study

The anti-hypertensive drugs amlodipine, atenolol and lisinopril, in ordinary and PEGylated forms, with different combined-ratios, were studied by molecular dynamics simulations using GROMACS software. Twenty simulation systems were designed to evaluate the interactions of drug mixtures with a dimyristoylphosphatidylcholine (DMPC) lipid bilayer membrane, in the presence of water molecules. In the course of simulations, various properties of the systems were investigated, including drug location, diffusion and mass distribution in the membrane; drug orientation; the lipid chain disorder as a result of drug penetration into the DMPC membrane; the number of hydrogen bonds; and drug surface area. According to the results obtained, combined drugs penetrate deeper into the DMPC lipid bilayer membrane, and the lipid chains remain ordered. Also, the combined PEGylated drugs, at a combination ratio of 1:1:1, enhance drug penetration into the DMPC membrane, reduce drug agglomeration, orient the drug in a proper angle for easy penetration into the membrane, and decrease undesirable lipotoxicity due to distorted membrane self-assembly and thickness. Graphical abstract ᅟ.

TMAO mediates effective attraction between lipid membranes by partitioning unevenly between bulk and lipid domains.

Under environmental duress, many organisms accumulate large amounts of osmolytes - molecularly small organic solutes. Osmolytes are known to counteract stress, driving proteins to their compact native states by their exclusion from protein surfaces. In contrast, the effect of osmolytes on lipid membranes is poorly understood and widely debated. Many fully membrane-permeable osmolytes exert an apparent attractive force between lipid membranes, yet all proposed models fail to fully account for the origin of this force. We follow the quintessential osmolyte trimethylamine N-oxide (TMAO) and its interaction with dimyristoyl phosphatidylcholine (DMPC) membranes in aqueous solution. We find that by partitioning away from the inter-bilayer space, TMAO pushes adjacent membranes closer together. Experiments and simulations further show that the partitioning of TMAO away from the volume between bilayers stems from its exclusion from the lipid-water interface, similar to the mechanism of protein stabilization by osmolytes. We extend our analysis to show that the preferential interaction of other physiologically relevant solutes (including sugars and DMSO) also correlates with their effect on membrane bilayer interactions. Our study resolves a long-standing puzzle, explaining how osmolytes can increase membrane-membrane attraction or repulsion depending on their preferential interactions with lipids.

Temperature-Dependent Electron Paramagnetic Resonance Studies of Docosahexaenoic Acid and Gamma Linolenic Acid Effects on Phospholipid Membranes With and Without Cholesterol

Free docosahexaenoic acid (DHAn-3) and gamma linolenic acid (GLAn-6) effects on dimyristoyl phosphatidylcholine (DMPC) membranes were studied as a function of temperature by electron paramagnetic resonance (EPR) spectroscopy. 5- and 16-doxyl stearic acid (5-, 16-DS) spin labels were utilized to obtain information from the interfacial and alkyl chain region, respectively. In the studied temperature range, the presence of DHAn-3 or GLAn-6 caused decreases in maximum hyperfi ne splitting values and correlation times of DMPC membranes. Both in the interfacial region and depths of membrane, changes were more pronounced for DHAn-3 in pure DMPC. In the presence of cholesterol (CH), DHAn-3 and GLAn-6 effects were similar and more pronounced in the depths of the membrane. The changes in the structure and dynamics of samples were obtained from simulations of spectra, which indicated some changes in the number of spectral components by incorporation of DHAn-3 and GLAn-6. In the interfacial region and below the main phase transition temperature of DMPC, there was an increase in heterogeneity. For temperatures above the phase transition, a more homogeneous environment for spin label was obtained in the presence of fatty acids.