DPPC Bilayers Research Articles

A Limited 4 Å Radial Displacement of the S4-S5 Linker Is Sufficient for Internal Gate Closing in Kv Channels

Voltage-gated ion channels are responsible for the generation of action potentials in our nervous system. Conformational rearrangements in their voltage sensor domains in response to changes of the membrane potential control pore opening and thus ion conduction. Crystal structures of the open channel in combination with a wealth of biophysical data and molecular dynamics simulations led to a consensus on the voltage sensor movement. However, the coupling between voltage sensor movement and pore opening, the electromechanical coupling, occurs at the cytosolic face of the channel, from where no structural information is available yet. In particular, the question how far the cytosolic pore gate has to close to prevent ion conduction remains controversial. In cells, spectroscopic methods are hindered because labeling of internal sites remains difficult, whereas liposomes or detergent solutions containing purified ion channels lack voltage control. Here, to overcome these problems, we controlled the state of the channel by varying the lipid environment. This way, we directly measured the position of the S4-S5 linker in both the open and the closed state of a prokaryotic Kv channel (KvAP) in a lipid environment using Lanthanide-based resonance energy transfer. We were able to reconstruct the movement of the covalent link between the voltage sensor and the pore domain and used this information as restraints for molecular dynamics simulations of the closed state structure. We found that a small decrease of the pore radius of about 3-4 Å is sufficient to prevent ion permeation through the pore.

Free cholesterol induces higher β-sheet content in Aβ peptide oligomers by aromatic interaction with Phe19.

Accumulating experimental evidence support an enhancing effect of free cholesterol on amyloid-beta (Aβ) aggregation. To probe the mechanisms of cholesterol-mediated Aβ aggregation, we applied all-atom molecular dynamic simulations on Aβ42 peptides in presence of free cholesterol. Several control systems were also designed to examine the specificity of cholesterol-residue interactions, including mutation on aromatic residue, substitution of cholesterol with sphingomyelin (SM) and DPPC bilayer, and a mixing SM and cholesterol. Each system was performed 4 independent simulations, with a total time of 560 ns. It was found that cholesterol increased β-sheet formation by 4 folds, but the Phe19→Ser mutation on Aβ42 peptide totally eliminated cholesterol’s effect. A stable contact was recognized between the steroid group of cholesterol and the Benzyl group of Phe19. Interestingly, our simulation revealed a regular 1 ns time interval between the establishment of cholesterol-phenylalanine contact and consequent β-sheet formation, suggesting an important role of steroid-benzyl interaction in cholesterol-mediated aggregation. The presence of SM slightly increased β-sheet formation, but the mixture of cholesterol and SM had a strong induction effect. Also, the measurement of Phe19-lipid distance indicates that aromatic side chains of peptides prone to bind to cholesterol on the surface of the mixed micelle. In the DPPC system, polar chains were attracted to the surface of membrane, yielding moderate increase of β-sheet formation. These results shed light on the mechanism of cholesterol-mediated fibrillogenesis, and help to differentiate the effects of cholesterol and other lipids on β-sheet formation process.

Influence of Cholesterol on the Phase Transition of Lipid Bilayers: A Temperature-Controlled Force Spectroscopy Study

Cholesterol (Chol) plays the essential function of regulating the physical properties of the cell membrane by controlling the lipid organization and phase behavior and, thus, managing the membrane fluidity and its mechanical strength. Here, we explore the model system DPPC:Chol by means of temperature-controlled atomic force microscopy (AFM) imaging and AFM-based force spectroscopy (AFM-FS) to assess the influence of Chol on the membrane ordering and stability. We analyze the system in a representative range of compositions up to 50 mol % Chol studying the phase evolution upon temperature increase (from room temperature to temperatures high above the T(m) of the DPPC bilayer) and the corresponding (nano)mechanical stability. By this means, we correlate the mechanical behavior and composition with the lateral order of each phase present in the bilayers. We prove that low Chol contents lead to a phase-segregated system, whereas high contents of Chol can give a homogeneous bilayer. In both cases, Chol enhances the mechanical stability of the membrane, and an extraordinarily stable system is observed for equimolar fractions (50 mol % Chol). In addition, even when no thermal transition is detected by the traditional bulk analysis techniques for liposomes with high Chol content (40 and 50 mol %), we demonstrate that temperature-controlled AFM-FS is capable of identifying a thermal transition for the supported lipid bilayers. Finally, our results validate the AFM-FS technique as an ideal platform to differentiate phase coexistence and transitions in lipid bilayers and bridge the gap between the results obtained by traditional methods for bulk analysis, the theoretical predictions, and the behavior of these systems at the nanoscale.

Biophysical properties of ergosterol-enriched lipid rafts in yeast and tools for their study: characterization of ergosterol/phosphatidylcholine membranes with three fluorescent membrane probes

In this work, binary mixtures of phospholipid/ergosterol (erg) were studied using three fluorescent membrane probes. The phospholipid was either saturated (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) or monounsaturated (1-palmitoyl-2-dioleoyl-sn-glycero-3-phosphocholine, POPC) phosphatidylcholine, to evaluate the fluorescence properties of the probes in gel, liquid ordered (lo) and liquid disordered (ld) phases. The probes have been used previously to study cholesterol-enriched domains, but their photophysical properties in erg-enriched membranes have not been characterized.N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (NBD-DPPE) presents modest blue-shifts upon erg addition, and the changes in the fluorescence lifetime are mainly due to differences in the efficiency of its fluorescence dynamic self-quenching. However, the steady-state fluorescence anisotropy of NBD-DPPE presents well-defined values in each lipid phase.N-(lissamine rhodamine B sulfonyl)-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (Rhod-DOPE) presents a close to random distribution in erg-rich membranes. There are no appreciable spectral shifts and the steady-state fluorescence anisotropy presents complex behavior, as a result of different photophysical processes. The probe is mostly useful to label ld domains in yeast membranes.4-(2-(6-(Dibutylamino)-2-naphthalenyl)ethenyl)-1-(3-sulfopropyl)-pyridinium (di-4-ANEPPS) is an electrochromic dye with excitation spectra largely insensitive to the presence of erg, but presenting a strong blue-shift of its emission with increasing concentrations of this sterol. Its partition coefficient is favorable to lo domains in POPC/erg mixtures. Although the fluorescence properties of di-4-ANEPPS are less sensitive to erg than to chol, in both cases the fluorescence lifetime responds monotonically to sterol mole fraction, becoming significantly longer in the presence of sterol as compared to pure POPC or DPPC bilayers. The probe displays a unique sensitivity to sterol–lipid interaction due to the influence of hydration and H-bonding patterns at the membrane/water interface on its fluorescence properties. This makes di-4-ANEPPS (and possibly similar probes) potentially useful in the study of erg-enriched domains in more complex lipid mixtures and in the membranes of living yeast cells.

An application of improved force field to fully hydrated DPPC and POPC bilayers in a tensionless NPT ensemble: a test of CHARMM 27-based new force field by Högberg et al.

Recently developed all-atom potential model by Högberg et al. has been applied to 1,2-di-palmitoyl-phosphatidylcholine (DPPC) and 1-palmitoryl-2-oleoyl-phosphatidylcholine (POPC) fully hydrated lipid bilayers. Tensionless NPT ensemble calculations have been performed over 40 ns to test the performance of the force field at 1 atm and at 323.15 and 303.15 K, respectively, where we focused our attention on the membrane area and the order parameter. The calculated membrane area per lipid molecule was 0.669 and 0.653 nm2 for DPPC and POPC, respectively, which shows a good agreement with the experimental values 0.642 and 0.683 nm2, respectively. This is an extensive improvement over the original CHARMM 27 or CHARMM 27r, which gives gel-like very small value of the membrane area 0.49 and 0.574 nm2, respectively. Furthermore, the calculated order parameter of C–H vector in acyl tails and electron density profile also show good correspondence with the experiment.

Structural Changes of DPPC Bilayers Induced by Gemini Surfactant

The phospholipid gemini surfactant systems are promising agents for construction of lipoplexes for DNA delivery systems in gene therapy. In this work the in uence of a gemini surfactant 1,1′-(1,6-hexan)bis3octyloxymethylimidazolium di-chloride) (IMI_Cl_C6_C8) on the structure and phase behaviour of aqueous suspensions of the fully hydrated phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were characterised using small angle scattering of synchrotron radiation (SAXS), di erential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The surfactant studied caused a destabilisation of the lamellar phase typical of DPPC. Addition of the surfactant also shifted to lower temperatures and extended the range of the main phase transition

Interaction of Tau Protein with Model Lipid Membranes Induces Tau Structural Compaction and Membrane Disruption

The misfolding and aggregation of the intrinsically disordered, microtubule-associated tau protein into neurofibrillary tangles is implicated in the pathogenesis of Alzheimer's disease. However, the mechanisms of tau aggregation and toxicity remain unknown. Recent work has shown that anionic lipid membranes can induce tau aggregation and that membrane permeabilization may serve as a pathway by which protein aggregates exert toxicity, suggesting that the plasma membrane may play dual roles in tau pathology. This prompted our investigation to assess tau's propensity to interact with membranes and to elucidate the mutually disruptive structural perturbations the interactions induce in both tau and the membrane. We show that although highly charged and soluble, the full-length tau (hTau40) is also highly surface active, selectively inserts into anionic DMPG lipid monolayers and induces membrane morphological changes. To resolve molecular-scale structural details of hTau40 associated with lipid membranes, X-ray and neutron scattering techniques are utilized. X-ray reflectivity indicates hTau40s presence underneath a DMPG monolayer and penetration into the lipid headgroups and tailgroups, whereas grazing incidence X-ray diffraction shows that hTau40 insertion disrupts lipid packing. Moreover, both air/water and DMPG lipid membrane interfaces induce the disordered hTau40 to partially adopt a more compact conformation with density similar to that of a folded protein. Neutron reflectivity shows that tau completely disrupts supported DMPG bilayers while leaving the neutral DPPC bilayer intact. Our results show that hTau40s strong interaction with anionic lipids induces tau structural compaction and membrane disruption, suggesting possible membrane-based mechanisms of tau aggregation and toxicity in neurodegenerative diseases.

In search of a novel antifungal agent: probing molecular interactions of fluconazole and its analogues with model membranes by NMR and DSC techniques

In search of a novel antifungal agent with high susceptibility and increased antifungal potency it is necessary to increase the overall lipophilicity of these agents. In view of that, we have synthesized different carboxylic acid ester analogues of fluconazole, such as fluconazole-benzoate, fluconazole-p-nitrobenzoate, fluconazole-p-methoxybenzoate and fluconazole-toluate, with varying degrees of lipophilicity. In order to probe molecular level interactions of these molecules with biomembrane, lipid bilayers prepared from l-α-dipalmitoyl phosphatidyl choline (DPPC) as the model membrane were used. Multinuclear and multidimensional nuclear magnetic resonance, differential scanning calorimetry and transmission electron microscopy was used to investigate the changes in the thermotropic properties, organization of the membrane and intermolecular interactions. Fluconazole and its analogues show varying degrees of changes in the DPPC bilayer's architecture and physico-chemical characteristics. This might influence important biological features of fungal biomembranes that could be responsible for their respective antifungal effects. The study indicates that fluconazole-p-methoxybenzoate is the most active among all analogues and therefore could be the most promising antifungal candidate.

Multiscale Simulations of the Antimicrobial Peptide Maculatin 1.1: Water Permeation through Disordered Aggregates

The antimicrobial peptide maculatin 1.1 (M1.1) is an amphipathic α-helix that permeabilizes lipid bilayers. In coarse-grained molecular dynamics (CG MD) simulations, M1.1 has previously been shown to form membrane-spanning aggregates in DPPC bilayers. In this study, a simple multiscale methodology has been applied to allow sampling of important regions of the free energy surface at higher resolution. Thus, by back-converting the CG configurations to atomistic representations, it is shown that water is able to permeate through the M1.1 aggregates. Investigation of aggregate stoichiometry shows that at least six peptides are required for water permeation. The aggregates are dynamically disordered structures, and water flux occurs through irregular, fluctuating channels. The results are discussed in relation to experimental data and other simulations of antimicrobial peptides.

Orientation and depth of surfactant protein B C-terminal helix in lung surfactant bilayers

SP-BCTERM is a cationic amphipathic helical peptide and functional fragment composed of residues 63 to 78 of surfactant protein B (SP-B). Static oriented and magic angle spinning solid state NMR, along with molecular dynamics simulation was used to investigate its structure, orientation, and depth in lipid bilayers of several compositions, namely POPC, DPPC, DPPC/POPC/POPG, and bovine lung surfactant extract (BLES). In all lipid environments the peptide was oriented parallel to the membrane surface. While maintaining this approximately planar orientation, SP-BCTERM exhibited a flexible topology controlled by subtle variations in lipid composition. SP-BCTERM-induced lipid realignment and/or conformational changes at the level of the head group were observed using 31P solid-state NMR spectroscopy. Measurements of the depth of SP-BCTERM indicated the peptide center positions ~8Å more deeply than the phosphate headgroups, a topology that may allow the peptide to promote functional lipid structures without causing micellization upon compression.

X-Ray Reflectivity and Diffuse Scattering Study of Effect of Ca2+ on Cushioned Lipid Bilayer

The effects of Ca2+ on bio-membranes are of interest because of their role in several biological processes, including facilitating the membrane fusion process. In a recent study, we have investigated a cushioned [1] DPPC bilayer in water and in different concentration CaCl2 solutions via fluid exchange. We used a polymer (PAA) cushion deposited on a single crystal silicon substrate. Using a 22 keV X-ray beam at the beamline of Sector 8 at the Advanced Photon Source, we obtained high resolution reflectivity from the bilayer and cushion through water, and were also able to study the grazing incidence x-ray diffuse scattering. The effect of Ca2+ on the bilayer structure has been obtained from the reflectivity analysis, which shows the Ca2+ ions binding to the headgroups of outer leaflet of the DPPC bilayer. The diffuse scattering is used to obtain the physical properties such as the surface tension, bending modulus, etc. Combining both, we present a complete picture of how Ca2+ affects a DPPC bilayer in water at room temperature as a function of Ca2+ concentration.

Imaging of Supported Bilayers Modified by Butanol and Hexanol, and by the Solvent Decane

Ion channels, including voltage-gated channels, are often studied in planar bilayer model membranesthat of necessity incorporate a solvent, typically decane. We are interested in the bilayer mechanics of channel function and have examined the electrophysiology of KvAP channels in bilayers modified by short chain alkanols and by cholesterol (Finol-Urdaneta et al. 2010, Biophysical Journal 98:762). Since both alkanols and cholesterol affect KvAP conductance and kinetics, we need a better understanding of how the mechanical status of these “defined” bilayers is affected by the presence of decane. We prepare supported bilayers of DPPC, DOPC/DPPC 1:1 and DOPC/egg sphingomyelin/cholesterol 2:2:1 (“DEC221”) by vesicle fusion on mica in the presence and absence of decane, and image them with tapping-mode atomic force microscopy (AFM) in aqueous solution. Decane modifies the domain morphology of DEC221 but has minimal effect on DOPC/DPPC or DPPC bilayer patches. Butanol and hexanol in the absence of decane reduce the percent coverage of the liquid-ordered phase in DEC221 and the gel phase in DOPC/DPPC, and induce an interdigitated phase of lower height in DPPC bilayer patches, consistent with literature results for ethanol. Fluorescence assays in vesicles confirm the alkanol-induced increases in fluidity. This work aims to elucidate the interplay between bilayer lipid molecules and solvent molecules in determining the response of planar bilayers to membrane-perturbing additives.

Volumetric stability of lipid bilayers

In agreement with recent reports, a commercial densimeter has yielded a gradual decrease in lipid molecular volume of DPPC multilamellar vesicle dispersions in the gel phase upon repeated thermal cycling between 10 °C and 50 °C. The considerable size of this decrease would have significant implications for the physical chemistry of biomembranes. In contrast, neutral buoyancy measurements performed with similar thermal cycling indicate no gradual change in lipid volume in the gel phase at 20 °C. Remixing the lipid in the densimeter shows that the apparent volume decrease is an artifact. We conclude that gel phase DPPC bilayers exist in a volumetrically stable phase.

Differing modes of interaction between monomeric Aβ1–40 peptides and model lipid membranes: an AFM study

Membrane interactions with β-amyloid peptides are implicated in the pathology of Alzheimer's disease and cholesterol has been shown to be key modulator of this interaction, yet little is known about the mechanism of this interaction. Using atomic force microscopy, we investigated the interaction of monomeric Aβ1–40 peptides with planar mica-supported bilayers composed of DOPC and DPPC containing varying concentrations of cholesterol. We show that below the bilayer melting temperature, Aβ monomers adsorb to, and assemble on, the surface of DPPC bilayers to form layers that grow laterally and normal to the bilayer plane. Above the bilayer melting temperature, we observe protofibril formation. In contrast, in DOPC bilayers, Aβ monomers exhibit a detergent-like action, forming defects in the bilayer structure. The kinetics of both modes of interaction significantly increases with increasing membrane cholesterol content. We conclude that the mode and rate of the interaction of Aβ monomers with lipid bilayers are strongly dependent on lipid composition, phase state and cholesterol content.

Molecular-dynamic simulation of DPPC bilayer in different phase state: Hydration and electric field distribution in the presence of Be2+ cations

New molecular-dynamic topology of phosphatidylcholine bilayer (DPPC) in total atomic OPLS force field was developed and used to study the structural characteristics of liquid-crystalline and gel state of lipid bilayer in the absence and in the presence of Na+ and Be2+ cations adsorbed at the interface and different in their affinity. The parameters of bilayer geometry, the amount of surface water, and the electrostatic potential distribution were estimated quantitatively from the simulation in both phase states. The azimuthal angle of hydrocarbon chains was found nearly the same in the region of each monolayer in gel state. The amount of surface water decreases upon bilayer “freezing” mainly by loss of water molecules not participating in H-bonds between lipid headgroups. The cation adsorption was shown to have a small effect on these H-bonded water molecules, whereas Be2+ appeared to retain surface water in the bilayer upon its freezing. The electric potential distribution in the normal direction to the membrane-water interface had a similar shape in any bilayer phase state regardless of the presence of the adsorbed cations. Analysis of the microscopic nature of the electric potential revealed a mutual compensation of the contributions of the main structural components of the system (lipids, water, and ions). The boundary potential increased by 116 mV for pure DPPC, by 212 mV in the presence of Na+, and by 133 mV in the presence of Be2+ upon the phase transition of bilayer to the gel state. The boundary potential difference in the presence of Na+ and Be2+ and its change at the bilayer phase transition are in a good agreement with the experimental data published earlier [Ermakov Yu.A., 1993].

Correlation between the ripple phase and stripe domains in membranes

We investigate the relationship between stripe domains and the ripple phase in membranes. These have previously been observed separately without being linked explicitly. Past results have demonstrated that solid and ripple phases exhibit rich textural patterns related to the orientational order of tilted lipids and the orientation of ripple corrugations. Here we reveal a highly complex network pattern of ripple and solid domains in DLPC, DPPC bilayers with structures covering length scales from 10nm to 100μm. Using spincoated double supported membranes we investigate domains by correlated AFM and fluorescence microscopy. Cooling experiments demonstrate the mode of nucleation and growth of stripe domains enriched in the fluorescent probe. Concurrent AFM imaging reveals that these stripe domains have a one-to-one correspondence with a rippled morphology running parallel to the stripe direction. Both thin and thick stripe domains are observed having ripple periods of 13.5±0.2nm and 27.4±0.6nm respectively. These are equivalent to previously observed asymmetric/equilibrium and symmetric/metastable ripple phases, respectively. Thin stripes grow from small solid domains and grow predominantly in length with a speed of ~3 times that of the thick stripes. Thick stripes grow by templating on the sides of thinner stripes or can emerge directly from the fluid phase. Bending and branching angles of stripes are in accordance with an underlying six fold lattice. We discuss mechanisms for the nucleation and growth of ripples and discuss a generic phase diagram that may partly rationalize the coexistence of metastable and stable phases.

Polarity-Sensitive Fluorescent Probes in Lipid Bilayers: Bridging Spectroscopic Behavior and Microenvironment Properties

We have studied the emission features of the fluorescent polarity-sensitive probes known as Prodan and Laurdan in a liquid-crystalline DPPC bilayer. To this purpose, we have combined high-level quantum mechanical electronic structure calculations with a molecular field theory for the positional-orientational-conformational distribution of the probes, in their ground and excited states, inside of the lipid bilayer, taking into account at both levels the nonuniformity and anisotropy of the environment. Thus, we can interpret the features of the fluorescence spectra of Prodan and Laurdan in relation to the position and orientation of their chromophore in the bilayer. We have found that the environment polarity is not sufficient to explain the large red shifts experimentally observed and that specific effects due to hydrogen bonding must be considered. We show that the orientation of the probe is important in determining the accessibility to water of the H-bond-acceptor group; in the case of Laurdan interesting conformational effects are highlighted.

Lipids, curvature stress, and the action of lipid prodrugs: Free fatty acids and lysolipid enhancement of drug transport across liposomal membranes

Molecular shape and its impact on bilayer curvature stress are powerful concepts for describing the effects of lipids and fatty acids on fundamental membrane properties, such as passive permeability and derived properties like drug transport across liposomal membranes. We illustrate these relationships by studying the effects of fatty acids and lysolipids on the permeation of a potent anti-cancer drug, doxorubicin, across the bilayer of a liposome in which the drug is encapsulated. Using a simple fluorescence assay, we have systematically studied the passive permeation of doxorubicin across liposomal membranes in different lipid phases: the solid-ordered phase (DPPC bilayers), the liquid-disordered phase (POPC lipid bilayers), and the liquid-ordered phase induced by high levels of cholesterol (DOPC + cholesterol lipid bilayers). The effect of different free fatty acids (FA) and lysolipids (LL), separately and in combination, on permeability was assessed to elucidate the possible mechanism of phospholipase A2-triggered release in cancer tissue of liposomal doxorubicin formulations. In all cases, FAs applied separately lead to significant enhancement of permeability, most pronounced in liquid-disordered bilayers and less pronounced in solid and solid-ordered bilayers. LLs applied separately had only a marginal effect on permeability. FA and LL applied in combination lead to a synergistic enhancement of permeability in solid bilayers, whereas in liquid-disordered bilayers, the combined effect suppressed the otherwise strong permeability enhancement due to the FAs.

Membrane orientation of MSI-78 measured by sum frequency generation vibrational spectroscopy.

Antimicrobial peptides (AMPs) selectively disrupt bacterial cell membranes to kill bacteria whereas they either do not or weakly interact with mammalian cells. The orientations of AMPs in lipid bilayers mimicking bacterial and mammalian cell membranes are related to their antimicrobial activity and selectivity. To understand the role of AMP-lipid interactions in the functional properties of AMPs better, we determined the membrane orientation of an AMP (MSI-78 or pexiganan) in various model membranes using sum frequency generation (SFG) vibrational spectroscopy. A solid-supported single 1,2-dipalmitoyl-an-glycero-3-[phospho-rac-(1-glycerol)] (DPPG) bilayer or 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) bilayer was used as a model bacterial cell membrane. A supported 1,2-dipalmitoyl-an-glycero-3-phosphocholine (DPPC) bilayer or a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer was used as a model mammalian cell membrane. Our SFG results indicate that the helical MSI-78 molecules are associated with the bilayer surface with ∼70° deviation from the bilayer normal in the negatively charged gel-phase DPPG bilayer at 400 nM peptide concentration. However, when the concentration was increased to 600 nM, MSI-78 molecules changed their orientation to make a 25° tilt from the lipid bilayer normal whereas multiple orientations were observed for an even higher peptide concentration in agreement with toroidal-type pore formation as reported in a previous solid-state NMR study. In contrary, no interaction between MSI-78 and a zwitterionic DPPC bilayer was observed even at a much higher peptide concentration (∼12,000 nM). These results demonstrate that SFG can provide insights into the antibacterial activity and selectivity of MSI-78. Interestingly, the peptide exhibits a concentration-dependent membrane orientation in the lamellar-phase POPG bilayer and was also found to induce toroidal-type pore formation. The deduced lipid flip-flop from SFG signals observed from lipids also supports MSI-78-induced toroidal-type pore formation.

The energetic and topography changes of mixed lipid bilayers deposited on glass

Characterization of the changes in wetting, energetic and topography of mixed phospholipid/sterol bilayer (BL) deposited on solid support was undertaken in these studies. Synthetic phospholipid (1,2-dipalmitoyl- sn-glycero-3-phosphocholine – DPPC) and sterol (cholesterol – Chol) were used. The lipid and mixed phospholipid/sterol bilayers were deposited on glass surface from chloroform solutions by spontaneous spreading or using Langmuir–Schaefer technique. The studied bilayers prepared by solution spreading were additionally incubated in water and then analyzed. Wettability of the films was investigated via contact angle measurements performed for three probe liquids (water, formamide and diiodomethane). The contact angles allowed evaluation of the total surface free energy and its components for particular systems applying two theoretical approaches, the one proposed by van Oss et al. (LWAB) and the other one of Chibowski (CAH). In order to visualize the changes of structure and properties of studied films their topography images were taken employing atomic force microscopy (AFM). The performed studies showed, that incorporation of cholesterol into the DPPC bilayers influenced their energetic and wetting properties, what reflected in the changes of contact angle and consequently the surface free energy of these films. The increasing sterol concentration fosters creation of rafts and organization of DPPC hydrocarbon chains on the layer surface. This causes changes in the surface free energy of mixed DPPC/Chol layers depending on the cholesterol concentration. The changes in topography of the studied layers observed with a help of AFM correlate well with the changes of their energetic properties.