Submicron structure in L-alpha-dipalmitoylphosphatidylcholine monolayers and bilayers probed with confocal, atomic force, and near-field microscopy

X-ray reflectivity study of the heat shock protein Hsp70 interaction with an artificial cell membrane model

Membrane-bound heat shock protein 70 (Hsp70) apart from its intracellular localization was shown to be specifically expressed on the plasma membrane surface of tumor but not normal cells. Although the association of Hsp70 with lipid membranes is well documented the exact mechanisms for chaperone membrane anchoring have not been fully elucidated. Herein, we addressed the question of how Hsp70 interacts with negatively charged phospholipids in artificial lipid compositions employing the X-ray reflectivity (XRR) studies. In a first step, the interactions between dioleoylphosphatidylcholine (DOPC) in the presence or absence of dioleoylphosphatidylserine (DOPS) and Hsp70 had been assessed using Quartz crystal microbalance measurements, suggesting that Hsp70 adsorbs to the surface of DOPC/DOPS bilayer. Atomic force microscopy (AFM) imaging demonstrated that the presence of DOPS is required for stabilization of the lipid bilayer. The interaction of Hsp70 with DOPC/DOPS lipid compositions was further quantitatively determined by high energy X-ray reflectivity. A systematic characterization of the chaperone-lipid membrane interactions by various techniques revealed that artificial membranes can be stabilized by the electrostatic interaction of anionic DOPS lipids with Hsp70.

Nanoarchitectonic approaches for measuring the catalytic behavior of a membrane anchored enzyme. From Langmuir-Blodgett to a novel Langmuir-Schaefer based nanofilm building device

Self-organized lipid monolayers at the air-water interface (Langmuir films, LF) are commonly used for measuring the catalytic properties of membrane-bound enzymes. This methodology allows to provide a consistent flat topography molecular density, packing defects and thickness. The aim of the present work was to show the methodological advantages of using the horizontal transfer method (Langmuir-Schaefer) with respect to the vertical transfer method (Langmuir-Blodgett) when mounting a device to measure catalytic activity of membrane enzymes. Based on the results obtained we can conclude that it is possible to prepare stable Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) films from Bovine Erythrocyte Membranes (BEM) preserving the catalytic activity of its native Acetylcholinesterase (BEA). In comparison, the LS films showed Vmax values more similar to the enzyme present in the vesicles of natural membranes. In addition, it was much easier to produce large amounts of transferred areas with the horizontal transfer methodology. It was possible to decrease the time required to mount an assay with numerous activity points, such as building activity curves as a function of substrate concentration. The present results show that LSBEM provides a proof of concept for the development of biosensors based on transferred purified membrane for the screening of new products acting on an enzyme embedded on its natural milieu. In the case of BEA, the application of these enzymatic sensors could have medical interest, providing drug screening tools for the treatment of Alzheimer's disease.

Molecular interactions of hesperidin with DMPC/cholesterol bilayers

Since cell membranes are complex systems, the use of model lipid bilayers is quite important for the study of their interactions with bioactive molecules. Mammalian cell membranes require cholesterol (CHOL) for their structure and function. For this reason, the mixtures of phospholipid and cholesterol are necessary to use in model membrane studies to better simulate the real systems. In the present study, we investigated the effect of the incorporation of hesperidin in model membranes consisting of dimyristoylphosphatidylcholine (DMPC) and CHOL by using differential scanning calorimetry (DSC), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and atomic force microscopy (AFM). ATR-FTIR results demonstrated that hesperidin increases the fluidity of the DMPC/CHOL binary system. DSC findings indicated that the presence of 5 mol% hesperidin induces a broadening of the main phase transition consisting of three overlapping components. AFM experiments showed that hesperidin increases the thickness of DMPC/CHOL lipid bilayer model membranes. In addition to experimental results, molecular docking studies were conducted with hesperidin and human lanosterol synthase (LS), which is an enzyme found in the final step of cholesterol synthesis, to characterize hesperidin's interactions with its surrounding via its hydroxyl and oxygen groups. Then, hesperidin's ADME/Tox (absorption, distribution, metabolism, excretion and toxicity) profile was computed to see the potential impact on living system. In conclusion, considering the data obtained from experimental studies, this work ensures molecular insights in the interaction between a flavonoid, as an antioxidant drug model, and lipids mimicking those found in mammalian membranes. Moreover, computational studies demonstrated that hesperidin may be a great potential for use as a therapeutic agent for hypercholesterolemia due to its antioxidant property.

Nanoscale Chemical Imaging of Supported Lipid Monolayers using Tip-Enhanced Raman Spectroscopy

Visualizing the molecular organization of lipid membranes is essential to comprehend their biological functions. However, current analytical techniques fail to provide a non‐destructive and label‐free characterization of lipid films under ambient conditions at nanometer length scales. In this work, we demonstrate the capability of tip‐enhanced Raman spectroscopy (TERS) to probe the molecular organization of supported DPPC monolayers on Au (111), prepared using the Langmuir–Blodgett (LB) technique. High‐quality TERS spectra were obtained, that permitted a direct correlation of the topography of the lipid monolayer with its TERS image for the first time. Furthermore, hyperspectral TERS imaging revealed the presence of nanometer‐sized holes within a continuous DPPC monolayer structure. This shows that a homogeneously transferred LB monolayer is heterogeneous at the nanoscale. Finally, the high sensitivity and spatial resolution down to 20 nm of TERS imaging enabled reproducible, hyperspectral visualization of molecular disorder in the DPPC monolayers, demonstrating that TERS is a promising nanoanalytical tool to investigate the molecular organization of lipid membranes.

Sensing molecular organizational changes through the catalytic activity of acetylcholinesterase from erythrocyte membranes in Langmuir-Blodgett films

Langmuir films prepared from bovine erythrocyte membranes (LF_BEM) were studied and transferred to alkylated glasses (Langmuir-Blodgett films, LB_BEM) in order to assess the effects of membrane molecular packing on Bovine Erythrocyte Acetylcholinesterase (BEA) catalytic activity. Surface pressure (π) vs Area isotherms showed three 2D-transitions at ~7, ~18 and ~44 mN/m and a collapse pressure at π_c = 49 mN/m. The 0-12-0 mN/m compression-decompression cycles resulted reversible while those 0-40-0 mN/m exhibited a significant hysteresis. Taken together, EFM, BAM and AFM images and the stability of the film after 3C-D cycles, we can suggest that over the air-water interface as well as over the silanized glass substrate the surface is mostly covered by a monolayer with a few particles dispersed. Acetylthiocholine hydrolysis was assayed with BEA in bovine erythrocyte membrane suspensions (S_BEM) and in LB_BEM packed at 10 (LB_BEM,10) and 35 mN/m (LB_BEM,35), which gave the following kinetic parameters: V_max = 3.41 ± 0.15, 0.021 ± 0.002 and 0.030 ± 0.003 nmol.min^-1·μg prot^-1 and K_M = 0.11 ± 0.02, 0.047 ± 0.017 and 0.026 ± 0.017 mM, respectively. Although from S_BEM to LB_BEM we lost active enzyme, the catalytic efficiency (V_max/K_M) increased ~750 times. Eugenol and 1,8-cineol inhibited BEA catalytic activity in LB_BEM,35. Our results demonstrate the transmission of information between the membrane and the environment within the subphase immediately below the membrane, where anchored proteins are hosted. This was reflected by the membrane packing-induced modulation of BEA catalytic activity. Furthermore, LB_BEM provides a proof of concept for the development of biosensors to screen new green pesticides acting through BEA interaction.

Experimental signatures of a nonequilibrium phase transition near the crossover point of a Langmuir monolayer

We investigate the response of the two-dimensional (2D) continuous non-particulate film of surfactant sorbitan tristearate confined at the air-water interface under oscillatory shear deformation. The time dependence of various rheological parameters show critical-like behavior at a value of strain amplitude close to the crossover point of elastic ([Formula: see text]) and viscous ([Formula: see text]) shear moduli. Imposing oscillatory shear of different strain amplitudes ([Formula: see text]) above and below the crossover strain amplitude ([Formula: see text]) over a large number of cycles, we quantify the temporal dependence of interfacial viscous modulus, phase angle ([Formula: see text]) as well as higher harmonic components of stress. The number of shear cycles ([Formula: see text]) required for these quantities to reach the steady state value diverges near [Formula: see text]. The steady state values of the third harmonic ([Formula: see text]) show order parameter like behavior indicating the importance of higher order harmonics near the nonequilibrium transition. We further show that the energy dissipation per cycle per unit volume has a marked change near [Formula: see text], consistent with continuum level nonequilibrium shear-transformation-zone model of amorphous viscoplasticity.

Partition of common anesthetic molecules in the liquid disordered phase domain of a composite multicomponent membrane

Despite a vast clinical application of anesthetics, the molecular level of understanding of general anesthesia is far from our reach. Using atomistic molecular dynamics simulation, we study the effects of common anesthetics: ethanol, chloroform, and methanol in the fully hydrated symmetric multicomponent lipid bilayer membrane comprised of an unsaturated palmitoyl-oleoyl-phosphatidyl-choline (POPC), a saturated palmitoyl-sphingomyelin, and cholesterol, which exhibits phase coexistence of liquid-ordered (l_{o})-liquid-disordered (l_{d}) phase domains. We find that the mechanical and physical properties such as the thickness and rigidity of the membrane are reduced while the lateral expansion of the membrane is exhibited in the presence of anesthetic molecules. Our simulation shows both lateral and transverse heterogeneity of the anesthetics in the composite multicomponent lipid membrane. Both ethanol and chloroform partition in the POPC-rich l_{d} phase domain, while methanol is distributed in both l_{o}-l_{d} phase domains. Chloroform can penetrate deep into the membrane, while methanol partitions mostly at the water layer closed to the head group and ethanol at the neck of the lipids in the membrane.

Scanning resonator microscopy integrating phase sensitive detection

Scanning resonator microscopy (SRM) is a scanning probe technique that uses a small, optical resonator attached to the end of a conventional atomic force microscopy cantilever to simultaneously measure optical and topography properties of sample surfaces. In SRM, whispering gallery mode (WGM) resonances excited in the attached optical resonator shift in response to changes in surface refractive index (RI), providing a mechanism for mapping RI with high spatial resolution. In our initial report, the SRM tip was excited with a fixed excitation wavelength during sample scanning, which limits the approach. An improved method based on a wavelength modulation coupled with phase sensitive detection is reported here. This results in real-time characterization of WGM spectral shifts while eliminating complications arising from measurements based solely on signal intensity. This improved approach, combined with a modified tip design enabling integration of smaller resonators, is shown to enhance signal-to-noise and lead to sub-100 nm spatial resolution in the SRM optical image. The improved capabilities are demonstrated through measurements on thin dielectric and polymer films.

Emergence of compositionally tunable nanoscale dynamical heterogeneity in model binary lipid biomembranes

While the existence of nanoscale dynamical heterogeneity in biological membranes has been suggested to act as an active functional platform for enabling various cellular processes like signal transduction and viral or bacterial entry, it has been extremely difficult to detect the existence of such domains. Model lipid bilayer membranes have been widely used to detect such dynamical heterogeneity in order to avoid complications arising from the compositional heterogeneity of cellular membranes. However, even in model biological membranes the issue of nanoscale lipid dynamics has remained controversial and unresolved due to the difficulty of detecting the existence of such dynamical heterogeneity on the scale of 10-300 nm. Here we report direct evidence of nanoscale lipid dynamical heterogeneity in model binary lipid bilayer membranes using a combination of super-resolution stimulated emission depletion (STED) microscopy and fluorescence correlation spectroscopy (FCS). We control the phase behavior of the lipid bilayers by varying their composition and discuss how this leads to the emergence of dynamical lipid domains on the scale of 80-150 nm, which is also dependent on the lipid phase in which such dynamics are observed. Notably, our work shows that the presence of cholesterol is not required for the existence of such domains even in fluid like bilayers, as has been widely believed, and specifies the minimal conditions required for the emergence of such dynamical heterogeneity in cellular membranes. Our work will thus not only be of great significance towards understanding the nanoscale dynamic organizing principles of cellular membranes but could also be useful in understanding the dynamics of related soft matter systems and nanoparticle-cell membrane interactions.

Super-resolution optical microscopy for studying membrane structure and dynamics

Investigation of cell membrane structure and dynamics requires high spatial and temporal resolution. The spatial resolution of conventional light microscopy is limited due to the diffraction of light. However, recent developments in microscopy enabled us to access the nano-scale regime spatially, thus to elucidate the nanoscopic structures in the cellular membranes. In this review, we will explain the resolution limit, address the working principles of the most commonly used super-resolution microscopy techniques and summarise their recent applications in the biomembrane field.

DPPTE Thiolipid Self-Assembled Monolayer: A Critical Assay

Supported lipid membranes represent an elegant way to design a fluid interface able to mimic the physicochemical properties of biological membranes, with potential biotechnological applications. In this work, a diacyl phospholipid, the 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE), functionalized with a thiol group, was immobilized on a gold surface. In this molecule, the thiol group, responsible for the Au-S bond (45 kJ/mol) is located on the phospholipid polar head, letting the hydrophobic chain protrude from the film. This system is widely used in the literature but is no less challenging, since its characterization is not complete, as several discordant data have been obtained. In this work, the film was characterized by cyclic voltammetry blocking experiments, to verify the SAM formation, and by reductive desorption measurements, to estimate the molecular density of DPPTE on the gold surface. This value has been compared to that obtained by quartz crystal microbalance measurements. Ellipsometry and impedance spectroscopy measurements have been performed to obtain information about the monolayer thickness and capacitance. The film morphology was investigated by atomic force microscopy. Finally, Monte Carlo simulations were carried out, in order to gain molecular information about the morphologies of the DPPTE SAM and compare them to the experimental results. We demonstrate that DPPTE molecules, incubated 18 h below the phase transition temperature (T = 41.1 ± 0.4 °C) in ethanol solution, are able to form a self-assembled monolayer on the gold surface, with domain structures of different order, which have never been reported before. Our results make possible rationalization of the scattered results so far obtained on this system, giving a new insight into the formation of phospholipids SAMs on a gold surface.

Number of sialic acid residues in ganglioside headgroup affects interactions with neighboring lipids

Monolayers of binary mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and asialo-(GA1), disialo-(GD1b) and trisialo-(GT1b) gangliosides were used to determine the effect of ganglioside headgroup charge and geometry on its interactions with the neighboring zwitterionic lipid. Surface pressure versus molecular area isotherm measurements along with concurrent fluorescence microscopy of the monolayers at the air-water interface were complemented with atomic force microscopy imaging of monolayers deposited on solid substrates. Results were used to further develop a proposed geometric packing model that the complementary geometry of DPPC and monosialoganglioside GM1 headgroups affects their close molecular packing, inducing condensation of the layer at small mol % of ganglioside. For GA1, GD1b, and GT1b, a similar condensing effect, followed by a fluidizing effect is seen that varies with glycosphingolipid concentration, but results do not directly follow from geometric arguments because less DPPC is needed to condense ganglioside molecules with larger cross-sectional areas. The variations in critical packing mole ratios can be explained by global effects of headgroup charge and resultant dipole moments within the monolayer. Atomic force microscopy micrographs further support the model of ganglioside-induced DPPC condensation with condensed domains composed of a striped phase of condensed DPPC and DPPC/ganglioside geometrically packed complexes at low concentrations.

Toward atomic force microscopy and mass spectrometry to visualize and identify lipid rafts in plasmodesmata

Plant cell-to-cell communication is mediated by nanopores called plasmodesmata (PDs) which are complex structures comprising plasma membrane (PM), highly packed endoplasmic reticulum and numerous membrane proteins. Although recent advances on proteomics have led to insights into mechanisms of transport, there is still an inadequate characterization of the lipidic composition of the PM where membrane proteins are inserted. It has been postulated that PDs could be formed by lipid rafts, however no structural evidence has shown to visualize and analyse their lipid components. In this perspective article, we discuss proposed experiments to characterize lipid rafts and proteins in the PDs. By using atomic force microscopy (AFM) and mass spectrometry (MS) of purified PD vesicles it is possible to determine the presence of lipid rafts, specific bound proteins and the lipidomic profile of the PD under physiological conditions and after changing transport permeability. In addition, MS can determine the stoichiometry of intact membrane proteins inserted in lipid rafts. This will give novel insights into the role of membrane proteins and lipid rafts on the PD structure.

Lipid reassembly in asymmetric Langmuir-Blodgett/Langmuir-Schaeffer bilayers

Molecular-reorganization-induced morphology alteration in asymmetric substrate-supported lipid bilayers (SLBs) was directly visualized by means of atomic force microscopy (AFM) and total internal reflection fluorescence (TIRF) microscopy. SLB samples were fabricated on mica-on-glass and glass substrates by Langmuir-Blodgett (LB)/Langmuir-Schaeffer (LS) using binary lipid mixtures, namely, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and ternary mixtures DOPC/DPPC/1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS), labeled with 0.2 mol % Texas Red 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine triethylammonium salt (TR-DHPE) dye. Phase segregations were characterized by TIRF imaging, and DPPC-enriched domain structures were also observed. Interestingly for ∼40% (n = 6) of the samples with binary mixtures in the LB leaflet and a single component in the LS leaflet, that is, (DOPC/DPPC)(LB)+DOPC(LS), the contrast of the DPPC domains changed from the original dark (without dye) to bright (more TR dye partitioning) on TIRF images, returning to dark again. This contrast reverse was also correlated to AFM height images, where a DPPC-DPPC gel phase was spotted after the TIRF image contrast returned to dark. The rupture force mapping results measured on these binary mixture samples also confirmed unambiguously the formation of DPPC-DPPC gel domain components during the contrast change. The samples were tracked over 48 h to investigate the lipid molecule movements in both the DPPC domains and the DOPC fluid phase. The fluorescence contrast changes from bright to dark in SLBs indicate that the movement of dye molecules was independent of the movement of lipid molecules. In addition, correlated multimodal imaging using AFM, force mapping, and fluorescence provides a novel route to uncover the reorganization of lipid molecules at the solid-liquid interface, suggesting that the dynamics of dye molecules is highly structure dependent.

Near-field scanning optical microscopy for high-resolution membrane studies

The desire to directly probe biological structures on the length scales that they exist has driven the steady development of various high-resolution microscopy techniques. Among these, optical microscopy and, in particular, fluorescence-based approaches continue to occupy dominant roles in biological studies given their favorable attributes. Fluorescence microscopy is both sensitive and specific, is generally noninvasive toward biological samples, has excellent temporal resolution for dynamic studies, and is relatively inexpensive. Light-based microscopies can also exploit a myriad of contrast mechanisms based on spectroscopic signatures, energy transfer, polarization, and lifetimes to further enhance the specificity or information content of a measurement. Historically, however, spatial resolution has been limited to approximately half the wavelength due to the diffraction of light. Near-field scanning optical microscopy (NSOM) is one of several optical approaches currently being developed that combines the favorable attributes of fluorescence microscopy with superior spatial resolution. NSOM is particularly well suited for studies of both model and biological membranes and application to these systems is discussed.

Correlative optical and scanning probe microscopies for mapping interactions at membranes

Innovative approaches for real-time imaging on molecular-length scales are providing researchers with powerful strategies for characterizing molecular and cellular structures and dynamics. Combinatorial techniques that integrate two or more distinct imaging modalities are particularly compelling as they provide a means for overcoming the limitations of the individual modalities and, when applied simultaneously, enable the collection of rich multi-modal datasets. Almost since its inception, scanning probe microscopy has closely associated with optical microscopy. This is particularly evident in the fields of cellular and molecular biophysics where researchers are taking full advantage of these real-time, in situ, tools to acquire three-dimensional molecular-scale topographical images with nanometer resolution, while simultaneously characterizing their structure and interactions though conventional optical microscopy. The ability to apply mechanical or optical stimuli provides an additional experimental dimension that has shown tremendous promise for examining dynamic events on sub-cellular length scales. In this chapter, we describe recent efforts in developing these integrated platforms, the methodology for, and inherent challenges in, performing coupled imaging experiments, and the potential and future opportunities of these research tools for the fields of molecular and cellular biophysics with a specific emphasis on the application of these coupled approaches for the characterization of interactions occurring at membrane interfaces.

Atomic force microscopy: a versatile tool to probe the physical and chemical properties of supported membranes at the nanoscale

Atomic force microscopy (AFM) was developed in the 1980s following the invention of its precursor, scanning tunneling microscopy (STM), earlier in the decade. Several modes of operation have evolved, demonstrating the extreme versatility of this method for measuring the physicochemical properties of samples at the nanoscopic scale. AFM has proved an invaluable technique for visualizing the topographic characteristics of phospholipid monolayers and bilayers, such as roughness, height or laterally segregated domains. Implemented modes such as phase imaging have also provided criteria for discriminating the viscoelastic properties of different supported lipid bilayer (SLB) regions. In this review, we focus on the AFM force spectroscopy (FS) mode, which enables determination of the nanomechanical properties of membrane models. The interpretation of force curves is presented, together with newly emerging techniques that provide complementary information on physicochemical properties that may contribute to our understanding of the structure and function of biomembranes. Since AFM is an imaging technique, some basic indications on how real-time AFM imaging is evolving are also presented at the end of this paper.

Difference in lipid packing sensitivity of exchangeable apolipoproteins apoA-I and apoA-II: an important determinant for their distinctive role in lipid metabolism

Exchangeable apolipoproteins A-I and A-II play distinct roles in reverse cholesterol transport. ApoA-I interacts with phospholipids and cholesterol of the cell membrane to make high density lipoprotein particles whereas apolipoprotein A-II interacts with high density lipoprotein particles to release apolipoprotein A-I. The two proteins show a high activity at the aqueous solution/lipid interface and are characterized by a high content of amphipathic α-helices built upon repetition of the same structural motif. We set out to investigate to what extent the number of α-helix repeats of this structural motif modulates the affinity of the protein for lipids and the sensitivity to lipid packing. To this aim we have compared the insertion of apolipoproteins A-I and A-II in phospholipid monolayers formed on a Langmuir trough in conditions where lipid packing, surface pressure and charge were controlled. We also used atomic force microscopy to obtain high resolution topographic images of the surface at a resolution of several nanometers and performed statistical image analysis to calculate the spatial distribution and geometrical shape of apolipoproteins A-I and A-II clusters. Our data indicate that apolipoprotein A-I is sensitive to packing of zwitterionic lipids but insensitive to the packing of negatively charged lipids. Interestingly, apolipoprotein A-II proved to be insensitive to the packing of zwitterionic lipids. The different sensitivity to lipid packing provides clues as to why apolipoprotein A-II barely forms nascent high density lipoprotein particles while apolipoprotein A-I promotes their formation. We conclude that the different interfacial behaviors of apolipoprotein A-I and apolipoprotein A-II in lipidic monolayers are important determinants of their distinctive roles in lipid metabolism.

Biophysical interaction between corticosteroids and natural surfactant preparation: implications for pulmonary drug delivery using surfactant a a carrier

Intratracheal administration of corticosteroids using a natural pulmonary surfactant as a delivery vehicle has recently received significant attention in hopes of treating premature newborns with or at high risk for chronic lung disease. As a new practice, both the surfactant preparation used as the carrier and the corticosteroid delivered as the anti-inflammatory agent, and their mixing ratios, have not been standardized and optimized. Given the concern that corticosteroids delivered via a pulmonary surfactant may compromise its surface activity and thus worsen lung mechanics, the present study was carried out to characterize the biophysical interaction between a natural surfactant preparation, Infasurf, and two commonly used inhaled corticosteroids, budesonide and beclomethasone dipropionate (BDP). Based on surface activity measurements by the Langmuir balance and lateral film structure studied by atomic force microscopy, our findings suggest that when Infasurf is used as a carrier, a budesonide concentration less than 1 wt% of surfactant or a BDP concentration up to 10 wt % should not significantly affect the biophysical properties of Infasurf, thus being feasible for pulmonary delivery. Increasing corticosteroid concentration beyond this range leads to early collapse of the surfactant film due to increased film fluidization. Our study further suggests that different affinities to the surfactant films are responsible for the different behavior of budesonide and BDP. In addition to the translational value in treating chronic lung disease, this study may also have implications in inhaled steroid therapy to treat asthma.

Influence of hydrophobic alkylated gold nanoparticles on the phase behavior of monolayers of DPPC and clinical lung surfactant

The effect of hydrophobic alkylated gold nanoparticles (Au NPs) on the phase behavior and structure of Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC) and Survanta, a naturally derived commercial pulmonary surfactant that contains DPPC as the main lipid component and hydrophobic surfactant proteins SP-B and SP-C, has been investigated in connection with the potential implication of inorganic NPs in pulmonary surfactant dysfunction. Hexadecanethiolate-capped Au NPs (C(16)SAu NPs) with an average core diameter of 2 nm have been incorporated into DPPC monolayers in concentrations ranging from 0.1 to 0.5 mol %. Concentrations of up to 0.2 mol % in DPPC and 16 wt % in Survanta do not affect the monolayer phase behavior at 20 °C, as evidenced by surface pressure-area (π-A) and ellipsometric isotherms. The monolayer structure at the air/water interface was imaged as a function of the surface pressure by Brewster angle microscopy (BAM). In the liquid-expanded/liquid-condensed phase coexistence region of DPPC, the presence of 0.2 mol % C(16)SAu NPs causes the formation of many small, circular, condensed lipid domains, in contrast to the characteristic larger multilobes formed by pure lipid. Condensed domains of similar size and shape to those of DPPC with 0.2 mol % C(16)SAu NPs are formed by compressing Survanta, and these are not affected by the C(16)SAu NPs. Atomic force microscopy images of Langmuir-Schaefer-deposited films support the BAM observations and reveal, moreover, that at high surface pressures (i.e., 35 and 45 mN m(-1)) the C(16)SAu NPs form honeycomb-like aggregates around the polygonal condensed DPPC domains. In the Survanta monolayers, the C(16)SAu NPs were found to accumulate together with the proteins in the liquid-expanded phase around the circular condensed lipid domains. In conclusion, the presence of hydrophobic C(16)SAu NPs in amounts that do not influence the π-A isotherm alters the nucleation, growth, and morphology of the condensed domains in monolayers of DPPC but not of those of Survanta. Systematic investigations of the effect of the interaction of chemically defined NPs with the lipid and protein components of lung surfactant on the physicochemical properties of surfactant films are pertinent to understanding how inhaled NPs impact pulmonary function.

Miscibility behavior and nanostructure of monolayers of the main phospholipids of Escherichia coli inner membrane

We report a thermodynamic study of the effect of calcium on the mixing properties at the air-water interface of two phospholipids that mimic the inner membrane of Escherichia coli: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol. In this study, pure POPE and POPG monolayers and three mixed monolayers, χ(POPE) = 0.25, 0.5, and 0.75, were analyzed. We show that for χ(POPE) = 0.75, the values of the Gibbs energy of mixing were negative, which implies attractive interactions. We used atomic force microscopy to study the structural properties of Langmuir-Blodgett monolayers that were transferred onto mica substrate at lateral surface pressures of 25 and 30 mN m(-1). The topographic images of pure POPE and POPG monolayers exhibited two domains of differing size and morphology, showing a step height difference within the range expected for liquid-condensed and liquid-expanded phases. The images captured for χ(POPE) = 0.25 were featureless, and for χ(POPE) = 0.5 small microdomains were observed. The composition that mimics quantitatively the proportions found in the inner membrane of E. coli , χ(POPE) = 0.75, showed large liquid condensed domains in the liquid expanded phase. The extension of each domain was quantitatively analyzed. Because calcium is used in the formation of supported bilayers of negatively charged phospholipids, the possible influence of the nanostructure of the apical on the distal monolayer is discussed.

Fluid and condensed ApoA-I/phospholipid monolayers provide insights into ApoA-I membrane insertion

Apolipoprotein A-I (ApoA-I) is a protein implicated in the solubilization of lipids and cholesterol from cellular membranes. The study of ApoA-I in phospholipid (PL) monolayers brings relevant information about ApoA-I/PL interactions. We investigated the influence of PL charge and acyl chain organization on the interaction with ApoA-I using dipalmitoyl-phosphatidylcholine, dioleoyl-phosphatidylcholine and dipalmitoyl-phosphatidylglycerol monolayers coupled to ellipsometric, surface pressure, atomic force microscopy and infrared (polarization modulation infrared reflection-absorption spectroscopy) measurements. We show that monolayer compressibility is the major factor controlling protein insertion into PL monolayers and show evidence of the requirement of a minimal distance between lipid headgroups for insertion to occur, Moreover, we demonstrate that ApoA-I inserts deepest at the highest compressibility of the protein monolayer and that the presence of an anionic headgroup increases the amount of protein inserted in the PL monolayer and prevents the steric constrains imposed by the spacing of the headgroup. We also defined the geometry of protein clusters into the lipid monolayer by atomic force microscopy and show evidence of the geometry dependence upon the lipid charge and the distance between headgroups. Finally, we show that ApoA-I helices have a specific orientation when associated to form clusters and that this is influenced by the character of PL charges. Taken together, our results suggest that the interaction of ApoA-I with the cellular membrane may be driven by a mechanism that resembles that of antimicrobial peptide/lipid interaction.

Comparative study of clinical pulmonary surfactants using atomic force microscopy

Clinical pulmonary surfactant is routinely used to treat premature newborns with respiratory distress syndrome, and has shown great potential in alleviating a number of neonatal and adult respiratory diseases. Despite extensive study of chemical composition, surface activity, and clinical performance of various surfactant preparations, a direct comparison of surfactant films is still lacking. In this study, we use atomic force microscopy to characterize and compare four animal-derived clinical surfactants currently used throughout the world, i.e., Survanta, Curosurf, Infasurf and BLES. These modified-natural surfactants are further compared to dipalmitoyl phosphatidylcholine (DPPC), a synthetic model surfactant of DPPC:palmitoyl-oleoyl phosphatidylglycerol (POPG) (7:3), and endogenous bovine natural surfactant. Atomic force microscopy reveals significant differences in the lateral structure and molecular organization of these surfactant preparations. These differences are discussed in terms of DPPC and cholesterol contents. We conclude that all animal-derived clinical surfactants assume a similar structure of multilayers of fluid phospholipids closely attached to an interfacial monolayer enriched in DPPC, at physiologically relevant surface pressures. This study provides the first comprehensive survey of the lateral structure of clinical surfactants at various surface pressures. It may have clinical implications on future application and development of surfactant preparations.

Phase segregation of untethered zwitterionic model lipid bilayers observed on mercaptoundecanoic-acid-modified gold by AFM imaging and force mapping

Planar supported lipid bilayers (SLBs) are often studied as model cell membranes because they are accessible to a variety of surface-analytic techniques. Specifically, recent studies of lipid phase coexistence in model systems suggest that membrane lateral organization is important to a range of cellular functions and diseases. We report the formation of phase-segregated dioleoylphosphatidylcholine (DOPC)/sphingomyelin/cholesterol bilayers on mercaptoundecanoic-acid-modified (111) gold by spontaneous fusion of unilamellar vesicles, without the use of charged or chemically modified headgroups. The liquid-ordered (l(o)) and liquid-disordered (l(d)) domains are observed by atomic force microscopy (AFM) height and phase imaging. Furthermore, the mechanical properties of the bilayer were characterized by force-indentation maps. Fits of force indentation to Sneddon mechanics yields average apparent Young's moduli of the l(o) and l(d) phases of 100 +/- 2 and 59.8 +/- 0.9 MPa, respectively. The results were compared to the same lipid membrane system formed on mica with good agreement, though modulus values on mica appeared higher. Semiquantitative comparisons suggest that the mechanical properties of the l(o) phase are dominated by intermolecular van der Waals forces, while those of the fluid l(d) phase, with relatively weak van der Waals forces, are influenced appreciably by differences in surface charge density between the two substrates, which manifests as a difference in apparent Poisson ratios.

Adsorption of bile salts and pancreatic colipase and lipase onto digalactosyldiacylglycerol and dipalmitoylphosphatidylcholine monolayers

It is increasingly recognized that changes in the composition of the oil-water interface can markedly affect pancreatic lipase adsorption and function. To understand interfacial mechanisms determining lipase activity, we investigated the adsorption behavior of bile salts and pancreatic colipase and lipase onto digalactosyldiacylglycerol (DGDG) and dipalmitoylphosphatidylcholine (DPPC) monolayers at the air-water interface. The results from Langmuir trough and pendant drop experiments showed that a DGDG interface was more resistant to the adsorption of bile salts, colipase, and lipase compared to that of DPPC. Atomic force microscopy (AFM) images showed that the adsorption of bile salts into a DPPC monolayer decreased the size of the liquid condensed (LC) domains while there was no visible topographical change for DGDG systems. The results also showed that colipase and lipase adsorbed exclusively onto the mixed DPPC-bile salt regions and not the DPPC condensed phase. When the colipase and lipase were in excess, they fully covered the mixed DPPC-bile salt regions. However, the colipase and lipase coverage on the mixed DGDG-bile salt monolayer was incomplete and discontinuous. It was postulated that bile salts adsorbed into the DPPC monolayers filling the gaps between the lipid headgroups and spacing out the lipid molecules, making the lipid hydrocarbon tails more exposed to the surface. This created hydrophobic patches suitable for the binding of colipase and lipase. In contrast, bile salts adsorbed less easily into the DGDG monolayer because DGDG has a larger headgroup, which has strong intermolecular interactions and the ability to adopt different orientations at the interface. Thus, there are fewer hydrophobic patches that are of sufficient size to accommodate the colipase on the mixed DGDG-bile salt monolayer compared to the mixed DPPC-bile salt regions. The results from this work have reinforced the hypothesis that the interfacial molecular packing of lipids at the oil-water interface influences the adsorption of bile salts, colipase, and lipase, which in turn impacts the rate of lipolysis.

Single-step process to reconstitute cell membranes on solid supports

A new technique is presented to create supported lipid bilayers from whole cell lipids without the use of detergent or solvent extraction. In a modification of the bubble collapse deposition (BCD) technique, an air bubble is created underwater and brought into contact with a population of cells. The high-energy air/water interface extracts the lipid component of the cell membrane, which can subsequently be redeposited as a fluid bilayer on another substrate. The resulting bilayers were characterized with fluorescence microscopy, and it was found that both leaflets of the cell membrane are transferred but the cytoskeleton is not. The resulting supported bilayer was fluid over an area much larger than a single cell, demonstrating the capacity to create large, continuous bilayer samples. This capability to create fluid, biologically relevant bilayers will facilitate the use of high-resolution scanning microscopy techniques in the study of membrane-related processes.

Near-field scanning optical microscopy: a tool for nanometric exploration of biological membranes

Near-field scanning optical microscopy (NSOM) is an emerging optical technique that enables simultaneous high-resolution fluorescence and topography measurements. Here we discuss selected applications of NSOM to biological systems that help illustrate the utility of its high spatial resolution and simultaneous collection of both fluorescence and topography. For the biological sciences, these attributes seem particularly well suited for addressing ongoing issues in membrane organization, such as those regarding lipid rafts, and protein-protein interactions. Here we highlight a few NSOM measurements on model membranes, isolated biological membranes, and cultured cells that help illustrate some of these capabilities. We finish by highlighting nontraditional applications of NSOM that take advantage of the small probe to create nanometric sensors or new modes of imaging.

Molecular recognition imaging using tuning fork-based transverse dynamic force microscopy

We demonstrate simultaneous transverse dynamic force microscopy and molecular recognition imaging using tuning forks as piezoelectric sensors. Tapered aluminum-coated glass fibers were chemically functionalized with biotin and anti-lysozyme molecules and attached to one of the prongs of a 32kHz tuning fork. The lateral oscillation amplitude of the tuning fork was used as feedback signal for topographical imaging of avidin aggregates and lysozyme molecules on mica substrate. The phase difference between the excitation and detection signals of the tuning fork provided molecular recognition between avidin/biotin or lysozyme/anti-lysozyme. Aggregates of avidin and lysozyme molecules appeared as features with heights of 1-4nm in the topographic images, consistent with single molecule atomic force microscopy imaging. Recognition events between avidin/biotin or lysozyme/anti-lysozyme were detected in the phase image at high signal-to-noise ratio with phase shifts of 1-2 degrees. Because tapered glass fibers and shear-force microscopy based on tuning forks are commonly used for near-field scanning optical microscopy (NSOM), these results open the door to the exciting possibility of combining optical, topographic and biochemical recognition at the nanometer scale in a single measurement and in liquid conditions.

Functional importance of the NH2-terminal insertion sequence of lung surfactant protein B

Lung surfactant protein B (SP-B) is required for proper surface activity of pulmonary surfactant. In model lung surfactant lipid systems composed of saturated and unsaturated lipids, the unsaturated lipids are removed from the film at high compression. It is thought that SP-B helps anchor these lipids closely to the monolayer in three-dimensional cylindrical structures termed "nanosilos" seen by atomic force microscopy imaging of deposited monolayers at high surface pressures. Here we explore the role of the SP-B NH(2) terminus in the formation and stability of these cylindrical structures, specifically the distribution of lipid stack height, width, and density with four SP-B truncation peptides: SP-B 1-25, SP-B 9-25, SP-B 11-25, and SP-B 1-25Nflex (prolines 2 and 4 substituted with alanine). The first nine amino acids, termed the insertion sequence and the interface seeking tryptophan residue 9, are shown to stabilize the formation of nanosilos while an increase in the insertion sequence flexibility (SP-B 1-25Nflex) may improve peptide functionality. This provides a functional understanding of the insertion sequence beyond anchoring the protein to the two-dimensional membrane lining the lung, as it also stabilizes formation of nanosilos, creating reversible repositories for fluid lipids at high compression. In lavaged, surfactant-deficient rats, instillation of a mixture of SP-B 1-25 (as a monomer or dimer) and synthetic lung lavage lipids quickly improved oxygenation and dynamic compliance, whereas SP-B 11-25 surfactants showed oxygenation and dynamic compliance values similar to that of lipids alone, demonstrating a positive correlation between formation of stable, but reversible, nanosilos and in vivo efficacy.

Reversible formation of nanodomains in monolayers of DPPC studied by kinetic modeling

Dipalmitoylphosphatidylcholine (DPPC) is the most abundant component in pulmonary surfactants and is believed to be responsible for maintaining low surface tension in alveoli during breathing. In this work, a kinetic model is introduced that describes the phase separation in DPPC films that produces the liquid-condensed (LC) and liquid-expanded (LE) fractions, which differ according to the area density of DPPC. The phase separation in an initially homogeneous film has been investigated numerically. Furthermore, explicit simulations of periodic compression-expansion cycles are reported. In this process, a moderate change of the surface area resulted in a dramatic change in the total amount of LC fraction, as well as in the surface morphology. Depending on the extent of the film's compression, the simulated surface morphologies comprised individual nanosized LC domains embedded in the LE fraction, interconnected networks of such domains, or continuous LC films with nanopores. Equilibration of the total area of the LC nanodomains occurred over a few milliseconds, indicating that the rate of the LE-LC phase transformation is sufficient for maintaining low surface tension during breathing, and that nanoscale LC domains are likely to play a major role in this process. Unlike the total content of the LC fraction, which stabilized quickly, the average size of LC nanodomains showed a tendency to increase slowly, at a rate determined by the diffusivity of DPPC. The computed average domain size seems to be compatible with published experiments for DPPC films. The numeric results also elucidate the distinction between thermodynamically determined and kinetically limited structural properties during phase separation in the major structure-forming component of pulmonary surfactants.

Fuming method for micropatterning structures on Langmuir-Blodgett films

Lipid monolayers of L-alpha-dipalmitoylphosphatidylcholine (DPPC) are used to pattern substrates using the Langmuir-Blodgett (LB) technique. Lipid monolayers are deposited onto freshly cleaved mica surfaces or glass capillaries under conditions that lead to distinct patterns in the film. Exposure of the supported monolayer to ethyl 2-cyanoacrylate fumes leads to preferential polymerization in the more hydrated regions of the patterned monolayer. This method enables surfaces to be micropatterned where the lateral features are controlled by the structure present in the underlying LB film, and the vertical feature size is controlled by the length of the fuming process. Atomic force microscopy (AFM) measurements confirm that the original structure in the LB film is preserved following fuming and that the lateral and vertical feature sizes can be controlled from nanometers to micrometers. This method, therefore, provides a rapid and versatile approach for micropatterning both flat and curved surfaces on a variety of substrates.

Molecular packing tunes the activity of Kluyveromyces lactis beta-galactosidase incorporated in Langmuir-Blodgett films

Functional consequences of constraining beta-Gal in bidimensional space were studied at defined molecular packing densities and constant topology. Langmuir-Blodgett films, LB15 and LB35 composed of dipalmitoyl phosphatidylcholine and K. lactis beta-Gal, were obtained by transferring Langmuir films (L) initially packed at 15 and 35 mN/m, respectively, to alkylated glasses. The beta-Gal-monolayer binding equilibrium, mainly the adsorption rate and affinity, depended on the initial monolayer's surface pressure (lower for higher pi i). At pi i = 15 and 35 mN/m, the surface excess (Gamma) followed downward parabolic and power-law tendencies, respectively, as a function of subphase protein concentration. Gamma values in L roughly reflected the protein surface density chemically determined in LBs (0-7.5 ng/mm2 at pi i = 0-35 mN/m and [beta-Gal] subphase = 0-100 microg/mL). The beta-Gal-catalyzed hydrolysis of o-nitrophenyl-galactopyranoside showed a Michaelian kinetics in solution as well as in LB15. KM, KM,LB15, Vmax, and Vmax,LB15 were 5.15 +/- 2.2 and 9.25 +/- 6 mM and 39.63 and 0.0096 +/- 0.0027 micromol/min/mg protein, respectively. The sigmoidal kinetics observed with LB35 was evaluated by Hill's model (K0.5 = 9.55 +/- 0.4 mM, Vmax,35 = 0.0021 micromol/min/mg protein, Hill coefficient n = 9) and Savageau's fractal model (fractal constant K f = 9.84 mM; reaction order for the substrate gs = 9.06 and for the enzyme ge = 0.62). Fractal reaction orders would reflect the fractal organization of the environment, demonstrated by AFM images, more than the molecularity of the reaction. Particular dynamics of the protein-lipid structural coupling in each molecular packing condition would have led to the different kinetic responses.