Surface topography of membrane domains

Membrane Catalyzed Formation of Nucleotide Clusters and Their Role in the Origins of Life: Insights from Molecular Simulations and Lattice Modeling

One of the mysteries in studying the molecular "Origin of Life" is the emergence of RNA and RNA-based life forms, where nonenzymatic polymerization of nucleotides is a crucial hypothesis in formation of large RNA chains. The nonenzymatic polymerization can be mediated by various environmental settings, such as cycles of hydration and dehydration, temperature variations, and proximity to a variety of organizing matrices, such as clay, salt, fatty acids, lipid membrane, and mineral surface. In this work, we explore the influence of different phases of the lipid membrane toward nucleotide organization and polymerization in a simulated prebiotic setting. Our molecular simulations quantify the localization propensity of a mononucleotide, uridine monophosphate (UMP), in distinct membrane settings. We perform all-atom molecular dynamics (MD) simulations to estimate the role of the monophasic and biphasic membranes in modifying the behavior of UMPs localization and their clustering mechanism. Based on the interaction energy of mononucleotides with the membrane and their diffusion profile from our MD calculations, we developed a lattice-based model to explore the thermodynamic limits of the observations made from the MD simulations. The mathematical model substantiates our hypothesis that the lipid layers can act as unique substrates for "catalyzing" polymerization of mononucleotides due to the inherent spatiotemporal heterogeneity and phase change behavior.

Nanoscale Structure of Lipid-Gemini Surfactant Mixed Monolayers Resolved with AFM and KPFM Microscopy

Drug delivery vehicles composed of lipids and gemini surfactants (GS) are promising in gene therapy. Tuning the composition and properties of the delivery vehicle is important for the efficient load and delivery of DNA fragments (genes). In this paper, we studied novel gene delivery systems composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-3-phosphocholine (DPPC), and GS of the type N,N-bis(dimethylalkyl)-α,ω-alkanediammonium dibromide at different ratios. The nanoscale properties of the mixed DOPC-DPPC-GS monolayers on the surface of the gene delivery system were studied using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). We demonstrate that lipid-GS mixed monolayers result in the formation of nanoscale domains that vary in size, height, and electrical surface potential. We show that the presence of GS can impart significant changes to the domain topography and electrical surface potential compared to monolayers composed of lipids alone.

Unidirectional Freezing of Polymer Solution Droplets

Ice templating provides a means of generating textures with a well-defined topography. Recent applications involve the freezing of water droplets, with or without colloids, on flat or textured surfaces. An interesting feature of water droplets freezing on a substrate is the formation of a pointy tip at a constant angle, regardless of the substrate temperature, surface energy, or droplet volume. Here, by adding the polymer to water, we demonstrate how to manipulate and even prevent the formation of such an icy tip. We find that the sharpness of the tip decreases with increasing polymer concentration until completely disappearing above the overlap concentration, while the total freezing time increases concomitantly. Building on these observations, we combined simple geometrical arguments with heat flux measurements to model and connect the spatial and temporal evolution of polymer droplets under unidirectional freezing. Together our results provide new ways to control the shape of frozen droplets for ice templating or microstructure fabrication, with applications in tissue engineering, separation membranes, and soft robotics.

HSAFM-MIREBA - Methodology for Inferring REsolution in biological applications

Due to a lack of requirement for any direct labelling of the target molecule, high speed atomic force microscopy (HS-AFM) is a potentially powerful procedure for the assessment of biological processes involving macromolecules. When the sample is static the AFM device can be purposefully setup to recover high-resolution information about the feature in question. However, when the feature to be studied moves an appreciable amount during the course of the measurement, the obtained image will be blurred. Encountering such blurred observations prompts the experimenter to sacrifice higher resolution images for higher scanning speeds by tuning available experimental parameters (such as the scanned image area, the image pixel size, the resonance frequency of the cantilever and/or the diameter of the AFM tip). The present work describes a software tool, HSAFM-MIREBA (High Speed Atomic Force Microscopy - Methodology for Inferring REsolution in Biological Applications) that allows for pre-experimental optimization of such parameters through iterative rounds of simulation of both the dynamic surface process and the HS-AFM measurement (based on the particular set of governing parameters). A representative set of five dynamic biological processes that describe a range of diffusive and directed motions (which can themselves be tuned by altering characteristic governing parameter sets) are provided.

The Membrane Phase Transition Gives Rise to Responsive Plasma Membrane Structure and Function

Several groups have recently reported evidence for the emergence of domains in cell plasma membranes when membrane proteins are organized by ligand binding or assembly of membrane proximal scaffolds. These domains recruit and retain components that favor the liquid-ordered phase, adding to a decades-old literature interrogating the contribution of membrane phase separation in plasma membrane organization and function. Here we propose that both past and present observations are consistent with a model in which membranes have a high compositional susceptibility, arising from their thermodynamic state in a single phase that is close to a miscibility phase transition. This rigorous framework naturally allows for both transient structure in the form of composition fluctuations and long-lived structure in the form of induced domains. In this way, the biological tuning of plasma membrane composition enables a responsive compositional landscape that facilitates and augments cellular biochemistry vital to plasma membrane functions.

The Fluid-Mosaic model of cell membranes: A brief introduction, historical features, some general principles, and its adaptation to current information

The Fluid-Mosaic Membrane (FMM) model was originally proposed as a general, nanometer-scale representation of cell membranes (Singer and Nicolson, 1972). The FMM model was based on some general principles, such as thermodynamic considerations, intercalation of globular proteins into a lipid bilayer, independent protein and lipid dynamics, cooperativity and other characteristics. Other models had trimolecular structures or membrane globular lipoprotein units. These latter models were flawed, because they did not allow autonomous lipids, membrane domains or discrete lateral dynamics. The FMM model was also consistent with membrane asymmetry, cis- and trans-membrane linkages and associations of membrane components into multi-molecular complexes and domains. It has remained useful for explaining the basic organizational principles and properties of various biological membranes. New information has been added, such as membrane-associated cytoskeletal assemblies, extracellular matrix interactions, transmembrane controls, specialized lipid-protein domains that differ in compositions, rotational and lateral mobilities, lifetimes, functions, and other characteristics. The presence of dense, structured membrane domains has reduced significantly the extent of fluid-lipid membrane areas, and the FMM model is now considered to be more mosaic and dense than the original proposal.

Brillouin Spectroscopy of Binary Phospholipid-Cholesterol Bilayers

Multicomponent lipid bilayers are used as models for searching the origin of spatial heterogeneities in biomembranes called lipid rafts, implying the coexistence of domains of different phases and compositions within the lipid bilayer. The spatial organization of multicomponent lipid bilayers on a scale of a hundred nanometers remains unknown. Brillouin spectroscopy providing information about the acoustic phonons with the wavelength of several hundred nanometers has an unexplored potential for this problem. Here, we applied Brillouin spectroscopy for three binary bilayers composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol. The Brillouin experiment for the oriented planar multibilayers was realized for two scattering geometries involving phonons for the lateral and normal directions of the propagation. The DPPC-DOPC mixtures known for the coexistence of the solid-ordered and liquid-disordered phases had bimodal Brillouin peaks, revealing the phase domains with sizes more than a hundred nanometers. Analysis of the Brillouin data for the binary mixtures concluded that the lateral phonons are preferable for testing the lateral homogeneity of the bilayers, while the phonons spreading across the bilayers are sensitive to the layered packing at the mesoscopic scale.

Structural and Dynamical Studies of a Lipid-Nanoclay Composite Layer at the Air-Water Interface

We modulate the adsorption affinities of nanoclay particles for the air-water interface by changing the cationic surface charge composition of the lipid monolayer and thereby tune the attractive electrostatic interaction between the positively charged lipid layer and the zwitterionic nanoclay particles in the water subphase. Our findings emphasize the significance of electrostatic interaction between lipids and the nanoclay, as well as its impact on the structural and viscoelastic features of the composite layer. We use surface pressure (Π)-mean molecular area (A) isotherms, atomic force microscope (AFM), Brewster angle microscopy (BAM), and energy dispersive X-ray spectrsocopy (EDXS) measurements to analyze the structure phases of lipid and lipid-nanoclay composite interfacial layer. The Π-A isotherm curve shows that the lipid-nanoclay composite layer has a larger lift-off area than the neat lipid layer, indicating that nanoparticles adsorb at the lipid layer via electrostatic interaction between lipid and nanoclay molecules. The surface density of the adsorbed nanoclay particles increases with an increase in the composition of the cationic lipid molecules. The stress relaxation response of the composite layer, measured using step compression measurements, exhibits exponential decay and ubiquitous dependence on the cationic dimyristoy-trimethylammonium propane (DMTAP) composition in the lipid layer with crossover to faster relaxation dynamics at DMTAP > 0.75. The power-law study of the frequency-dependent dynamic viscoelastic responses of the interfacial layer, measured using the barrier oscillation method, reveals a transition from glass-like response from neat lipid layer to gel-like dynamic response for the lipid-nanoclay composite layer. A solid-like behavior is evident for all the interface layers with dilation elastic modulus (E') > dilational viscous modulus (E″); however, the dynamic response of the neat layer is largely frequency-independent, whereas lipid-nanoclay composite layers with DMTAP > 0.75 reveal a frequency-dependent dynamic responses. The frequency-dependent power-law exponent of E', E″ increases on increasing the fractional composition of cationic DMTAP from 0.1 to 1.0, which forms a saturated interface of laponite particles and behaves as a viscoelastic gel in 2D.

Conformational state diagram of DOPC/DPPCd62/cholesterol mixtures

Raman spectra of aqueous suspensions of vesicles composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), deuterated 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPC_d62), and cholesterol (Chol) were studied at room temperature to determine the conformational states of the phospholipid hydrocarbon chains. Deuteration of DPPC_d62 allowed us to characterize the conformational states of DOPC and DPPC_d62 independently. The parameters of Raman peaks, which are sensitive to the conformational order, were studied in a wide range of compositions. It was found that the DOPC molecules are conformationally disordered for all compositions. The conformational state of the DPPC_d62 molecules changes with composition. Their conformational state is influenced by cholesterol-induced partial disordering and DOPC solvation, transforming the DPPC molecules into the disordered state. The conformational state diagram from the Raman experiment was compared with outcomes from the differential scanning calorimetry (DSC) experiment. The Raman spectra also revealed that the DPPC molecules coexist in the disordered and all-trans ordered states for the DOPC/DPPC_d62/Chol mixtures except for the pure liquid-disordered phase.

The Effect of the Osmotically Active Compound Concentration Difference on the Passive Water and Proton Fluxes across a Lipid Bilayer

The molecular details of the passive water flux across the hydrophobic membrane interior are still a matter of debate. One of the postulated mechanisms is the spontaneous, water-filled pore opening, which facilitates the hydrophilic connection between aqueous phases separated by the membrane. In the paper, we provide experimental evidence showing that the spontaneous lipid pore formation correlates with the membrane mechanics; hence, it depends on the composition of the lipid bilayer and the concentration of the osmotically active compound. Using liposomes as an experimental membrane model, osmotically induced water efflux was measured with the stopped-flow technique. Shapes of kinetic curves obtained at low osmotic pressure differences are interpreted in terms of two events: the lipid pore opening and water flow across the aqueous channel. The biological significance of the dependence of the lipid pore formation on the concentration difference of an osmotically active compound was illustrated by the demonstration that osmotically driven water flow can be accompanied by the dissipation of the pH gradient. The application of the Helfrich model to describe the probability of lipid pore opening was validated by demonstrating that the probability of pore opening correlates with the membrane bending rigidity. The correlation was determined by experimentally derived bending rigidity coefficients and probabilities of lipid pores opening.

Interaction of Tau construct K18 with model lipid membranes

One of the hallmarks of Alzheimer's disease (AD) is the formation of neurofibrillary tangles, resulting from the aggregation of the tubulin associated unit protein (Tau), which holds a vital role in maintaining neuron integrity in a healthy brain. The development of such aggregates and their deposition in the brain seem to correlate with the onset of neurodegeneration processes. The misfolding and subsequent aggregation of the protein into paired helical filaments that further form the tangles, lead to dysfunction of the protein with neuronal loss and cognitive decline. The aggregation of the protein then seems to be a causative factor of the neurodegeneration associated with AD. The hypothesis of an involvement of the membrane in modulating the misfolding and assembly of Tau into paired helical filaments attracts increasing interests. To provide more insight about how lipids can modulate the interactions with Tau, we have conducted a comprehensive Atomic Force Microscopy (AFM) study involving supported lipid bilayers of controlled compositions with the Tau microtubule-binding construct K18. Particularly, the effects of zwitterionic and negatively charged phospholipids on the interaction have been investigated. Deleterious solubilization effects have been evidenced on fluid zwitterionic membranes as well as an inability of K18 to fragment gel phases. The role of negative lipids in the aggregation of the peptide and the particular ability of phosphatidylinositol-4,5-bisphosphate (PIP₂) in inducing K18 fibrillization on membranes are also reported.

Insertion and activation of functional Bacteriorhodopsin in a floating bilayer

The proton pump transmembrane protein bacteriorhodopsin was successfully incorporated into planar floating lipid bilayers in gel and fluid phases, by applying a detergent-mediated incorporation method. The method was optimized on single supported bilayers by using quartz crystal microbalance, atomic force and fluorescence microscopy techniques. Neutron and X-ray reflectometry were used on both single and floating bilayers with the aim of determining the structure and composition of this membrane-protein system before and after protein reconstitution at sub-nanometer resolution. Lipid bilayer integrity and protein activity were preserved upon the reconstitution process. Reversible structural modifications of the membrane, induced by the bacteriorhodopsin functional activity triggered by visible light, were observed and characterized at the nanoscale.

Breakdown of classical paradigms in relation to membrane structure and functions

Updates of the mosaic fluid membrane model implicitly sustain the paradigms that bilayers are closed systems conserving a state of fluidity and behaving as a dielectric slab. All of them are a consequence of disregarding water as part of the membrane structure and its essential role in the thermodynamics and kinetics of membrane response to bioeffectors. A correlation of the thermodynamic properties with the structural features of water makes possible to introduce the lipid membrane as a responsive structure due to the relaxation of water rearrangements in the kinetics of bioeffectors' interactions. This analysis concludes that the lipid membranes are open systems and, according to thermodynamic of irreversible formalism, bilayers and monolayers can be reasonable compared under controlled conditions. The inclusion of water in the complex structure makes feasible to reconsider the concept of dielectric slab and fluidity.

Nanoscale Probing of Cholesterol-Rich Domains in Single Bilayer Dimyristoyl-Phosphocholine Membranes Using Near-Field Spectroscopic Imaging

Cholesterol is believed to induce the formation of membrane domains, "rafts", which are implicated in a range of natural and pathologic membrane processes. Therefore, it is important to understand the role that cholesterol plays in the formation of these structures. Here, we use label-free spectroscopic imaging to investigate cholesterol fractioning in supported bilayer membranes at nanoscale. Scattering-type scanning near-field optical microscopy (s-SNOM) was used to visualize the formation of cholesterol-induced domains in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes. Our results revealed the coexistence of phase separated domains in DMPC lipids with 10 mol % cholesterol content, whereas a mostly homogeneous bilayer was found at low (5 mol %) and high (15 mol %) cholesterol content. Near-field nano-FTIR spectroscopy was used to identify the cholesterol-rich domains based on their qualitative chemical compositions. It was determined that cholesterol binds to phosphodiester and alkyl glycerol ester moieties, likely via hydrogen bonding of the alcohol to either of the ester oxygens. The results also confirm the existence of an ideal cholesterol-lipid mixture ratio (∼15:85) with a geometrically defined packing. At lower cholesterol content there is phase separation between liquid ordered and almost neat DMPC domains. Thus, the liquid ordered phase exists at an energy minimum at a given lipid-cholesterol ratio.

Physicochemical and Biological Characterization of Novel Membrane-Active Cationic Lipopeptides with Antimicrobial Properties

We have designed and synthesized new short lipopeptides composed of tetrapeptide conjugated to fatty acids with different chain lengths. The amino acid sequence of the peptide moiety included d-phenylalanine, two residues of l-2,4-diaminobutyric acid and l-leucine. To explore the possible mechanism of lipopeptide action, we have provided a physicochemical characterization of their interactions with artificial lipid membranes. For this purpose, we have used monolayers and bilayers composed of lipids representative of Gram-negative and Gram-positive bacterial membranes. Using surface pressure measurements and atomic force microscopy, we were able to monitor the changes occurring within the films upon exposure to lipopeptides. Our experiments revealed that all lipopeptides can penetrate the lipid membranes and affect their molecular ordering. The latter results in membrane thinning and fluidization. However, the effect is stronger in the lipid films mimicking Gram-positive bacterial membranes. The results of the physicochemical characterization were compared with the biological activity of lipopeptides. The effect of lipopeptides on bacterial growth was tested on several strains of bacteria. It was revealed that lipopeptides show stronger antimicrobial activity against Gram-positive bacteria. At the same time, all tested compounds display relatively low hemolytic activity.

Asymmetric bilayers mimicking membrane rafts prepared by lipid exchange: Nanoscale characterization using AFM-Force spectroscopy

Sphingolipids-enriched rafts domains are proposed to occur in plasma membranes and to mediate important cellular functions. Notwithstanding, the asymmetric transbilayer distribution of phospholipids that exists in the membrane confers the two leaflets different potentials to form lateral domains as next to no sphingolipids are present in the inner leaflet. How the physical properties of one leaflet can influence the properties of the other and its importance on signal transduction across the membrane are questions still unresolved. In this work, we combined AFM imaging and Force spectroscopy measurements to assess domain formation and to study the nanomechanical properties of asymmetric supported lipid bilayers (SLBs) mimicking membrane rafts. Asymmetric SLBs were formed by incorporating N-palmitoyl-sphingomyelin (16:0SM) into the outer leaflet of preformed 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Cholesterol SLBs through methyl-β-cyclodextrin-mediated lipid exchange. Lipid domains were detected after incorporation of 16:0SM though their phase state varied from gel to liquid ordered (Lo) phase if the procedure was performed at 24 or 37 °C, respectively. When comparing symmetric and asymmetric Lo domains, differences in size and morphology were observed, with asymmetric domains being smaller and more interconnected. Both types of Lo domains showed similar mechanical stability in terms of rupture forces and Young's moduli. Notably, force curves in asymmetric domains presented two rupture events that could be attributed to the sequential rupture of a liquid disordered (Ld) and a Lo phase. Interleaflet coupling in asymmetric Lo domains could also be inferred from those measurements. The experimental approach outlined here would significantly enhance the applicability of membrane models.

Altering model cell membranes by means of localized magnetic heating

Isolated iron oxide magnetic nanoparticles (MNPs), 12 nm in diameter, coated with oleic acid molecules as capping agents have been deposited by the Langmuir-Blodgett (LB) method onto a model cell membrane incorporating 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and Cholesterol (Chol) in the 1:1 ratio, which was also fabricated by the LB technique. Atomic Force Microscopy (AFM) experiments showed that the application of an alternating magnetic field results in the embedding of the MNPs through the phospholipidic layer. These experimental results reveal that the heating of individual MNPs may induce a local increase in the fluidity of the film with a large control of the spatial and temporal specificity.

Lipid exchange enhances geometric pinning in multicomponent membranes on patterned substrates

Experiments on supported lipid bilayers featuring liquid ordered/disordered domains have shown that the spatial arrangement of the lipid domains and their chemical composition are strongly affected by the curvature of the substrate. Furthermore, theoretical predictions suggest that both these effects are intimately related with the closed topology of the bilayer. In this work, we test this hypothesis by fabricating supported membranes consisting of colloidal particles of various shapes lying on a flat substrate. A single lipid bilayer coats both colloids and substrate, allowing local lipid exchange between them, thus rendering the system thermodynamically open, i.e. able to exchange heat and molecules with an external reservoir in the neighborhood of the colloid. By reconstructing the Gibbs phase diagram for this system, we demonstrate that the free-energy landscape is directly influenced by the geometry of the colloid. In addition, we find that local lipid exchange enhances the pinning of the liquid disordered phase in highly curved regions. This allows us to provide estimates of the bending moduli difference of the domains. Finally, by combining experimental and numerical data, we forecast the outcome of possible experiments on catenoidal and conical necks and show that these geometries could greatly improve the precision of the current estimates of the bending moduli.

The Effect of Charge on Melittin-Induced Changes in Membrane Structure and Morphology

The binding of melittin to a range of phospholipid bilayers was studied using dual polarisation interferometry and atomic force microscopy. The phospholipid model membranes included zwitterionic dimyristylphosphatidylcholine (DMPC), together with mixtures of DMPC/dimyristylphosphatidylglycerol (DMPG) and DMPC/DMPG/cholesterol. Melittin caused significant disruption on all bilayers, but differences in morphological changes during binding were different on each membrane. Overall, the results demonstrate that the process of membrane disruption follows distinct structural changes for different lipid mixtures irrespective of the strength of binding to the membrane surface.

Boundary Lubrication, Hemifusion, and Self-Healing of Binary Saturated and Monounsaturated Phosphatidylcholine Mixtures ⧫

A wide range of phosphatidylcholine (PC) lipids with different degrees of unsaturation has been identified in the human synovial fluid and on the cartilage surface. The outstanding lubricity of the articular cartilage surface has been attributed to boundary layers comprising complexes of such lipids, though to date, only lubrication by single-component PC-lipid-based boundary layers has been investigated. As distinguishable lubrication behavior has been found to be related to the PC structures, we herein examined the surface morphology (on mica) and the lubrication ability of binary PC lipid mixtures, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), using atomic force microscopy (AFM) and a surface force balance (SFB). These two PC lipids are among the most abundant saturated and unsaturated PC components in synovial joints. Small unilamellar vesicles (SUVs) prepared from DPPC-POPC mixtures (8:2, 5:5, and 2:8, molar ratios) ruptured and formed bilayers on mica. The normal and shear forces between two DPPC-POPC bilayer-coated mica surfaces across the corresponding SUV dispersions show good boundary lubrication (friction coefficients ≤ ca. 10^-4) up to contact stresses of 8.3 ± 2.2 MPa for 8:2 DPPC-POPC and 5.0 ± 1.7 MPa for the others. Hemifusion induced at high normal pressures was observed, probably because of the height mismatch of two components. Reproducible successive approaches after hemifusion indicate rapid self-healing of the mica-supported bilayers in the presence of the SUVs reservoir. This work is a first step to provide insight concerning the lubrication, wear, and healing of the PC-based boundary layers, which must consist of multicomponent lipid mixtures, on the articular cartilage surface.

Visualization and measurement of the local absorption coefficients of single bilayer phospholipid membranes using scanning near-field optical microscopy

Here we report the results of shear-mode thicknesses and absorption coefficient measurements made on neat membranes using scanning near-field optical microscopy (SNOM). Biomimic neat membranes composed of two different types of phoshpholipid molecules: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2- dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were found to exhibit different absorption coefficients under the SNOM. The localization of the lipids could be identified and correlated to the morphology of the membrane domains indicating that SNOM can be an effective and accurate approach for the label-free characterization of the structure-function relationships in cell membranes.

Nanoscale Substrate Roughness Hinders Domain Formation in Supported Lipid Bilayers

Supported lipid bilayers are model membranes formed at solid substrate surfaces. This architecture renders the membrane experimentally accessible to surface-sensitive techniques used to study their properties, including atomic force microscopy, optical fluorescence microscopy, quartz crystal microbalance, and X-ray/neutron reflectometry, and allows integration with technology for potential biotechnological applications such as drug screening devices. The experimental technique often dictates substrate choice or treatment, and it is anecdotally recognized that certain substrates are suitable for a particular experiment, but the exact influence of the substrate has not been comprehensively investigated. Here, we study the behavior of a simple model bilayer, phase-separating on a variety of commonly used substrates, including glass, mica, silicon, and quartz, with drastically different results. The distinct micron-scale domains observed on mica, identical to those seen in free-floating giant unilamellar vesicles, are reduced to nanometer-scale domains on glass and quartz. The mechanism for the arrest of domain formation is investigated, and the most likely candidate is nanoscale surface roughness, acting as a drag on the hydrodynamic motion of small domains during phase separation. Evidence was found that the physicochemical properties of the surface have a mediating effect, most likely because of the changes in the lubricating interstitial water layer between the surface and bilayer.

Fluid Phase Coexistence in Biological Membrane: Insights from Local Nonaffine Deformation of Lipids

Lateral heterogeneities in biomembranes play a crucial role in various physiological functions of the cell. Such heterogeneities lead to demixing of lipid constituents and formation of distinct liquid domains in the membrane. We study lateral heterogeneities in terms of topological rearrangements of lipids to identify the liquid-liquid phase coexistence in model membranes. Using ideas from the physics of amorphous systems and glasses, we calculate the degree of nonaffine deformation associated with individual lipids to characterize the liquid-ordered (L_o) and liquid-disordered (L_d) regions in model lipid bilayers. We explore the usage of this method on all-atom and coarse-grained lipid bilayer trajectories. This method is helpful in defining the instantaneous L_o-L_d domain boundaries in complex multicomponent bilayer systems. The characterization is also used to highlight the effect of line-active molecules on the phase boundaries and domain mixing. Overall, we propose a framework to explore the molecular origin of spatial and dynamical heterogeneity in biomembrane systems, which can be exploited not only in computer simulations but also in experiments.

Polymer Coating and Lipid Phases Regulate Semiconductor Nanorods' Interaction with Neuronal Membranes: A Modeling Approach

The interplay between nanoparticles (NPs) and cell membranes is extremely important with regard to using NPs in biology applications. With the aim of unraveling the dominating factors on the molecular scale, we have studied the interaction between polymer-coated semiconductor nanorods (NRs) made of cadmium selenium/cadmium sulfur and model lipid membranes. The zeta potential (ζ) of the NRs was tuned from having a negative value (-24 mV) to having a positive one (+11 mV) by changing the amine content in the polymer coating. Supported lipid bilayers (SLBs) and lipid monolayers (LMs) were used as model membranes. Lipid mixtures containing anionic or cationic lipids were employed in order to change the membrane ζ from -77 to +49 mV; lipids with saturated hydrophobic chains were used to create phase-separated gel domains. NR adsorption to the SLBs was monitored by quartz crystal microbalance with dissipation monitoring; interactions with LMs with the same lipid composition were measured by surface pressure-area isotherms. The results showed that the NRs only interact with the model membrane if the mutual Δζ is higher than 70 mV; at the air-water interface, positively charged NRs remove lipids from the anionic lipid mixtures, and the negative ones penetrate the space between the polar heads in the cationic mixtures. However, the presence of gel domains in the membrane inhibits this interaction. The results of the Derjaguin-Landau-Verwey-Overbeek model frame indicate that the interaction occurs not only due to electrostatic and van der Waals forces, but also due to steric and/or hydration forces.

Preparation and Characterization of Solid-Supported Lipid Bilayers Formed by Langmuir-Blodgett Deposition: A Tutorial

The structure, phase behavior, and properties of cellular membranes are derived from their composition, which includes phospholipids, sphingolipids, sterols, and proteins with various levels of glycosylation. Because of the intricate nature of cellular membranes, a plethora of in vitro studies have been carried out with model membrane systems that capture particular properties such as fluidity, permeability, and protein binding but vastly simplify the membrane composition in order to focus in detail on a specialized property or function. Supported lipid bilayers (SLB) are widely used as archetypes for cellular membranes, and this instructional review primarily focuses on the preparation and characterization of SLB systems formed by Langmuir deposition methods. Typical characterization methods, which take advantage of the planar orientation of SLBs, are illustrated, and references that go into more depth are included. This invited instructional review is written so that nonexperts can quickly gain in-depth knowledge regarding the preparation and characterization of SLBs. In addition, this work goes beyond traditional instructional reviews to provide expert readers with new results that cover a wider range of SLB systems than those previously reported in the literature. The quality of an SLB is frequently not well described, and details such as topological defects can influence the results and conclusions of an individual study. This article quantifies and compares the quality of SLBs fabricated from a variety of gel and fluid compositions, in correlation with preparation techniques and parameters, to generate general rules of thumb to guide the construction of designed SLB systems.

Microwave Spectroscopic Detection of Human Hsp70 Protein on Annealed Gold Nanostructures on ITO Glass Strips

Conductive indium-tin oxide (ITO) and non-conductive glass substrates were successfully modified with embedded gold nanoparticles (AuNPs) formed by controlled thermal annealing at 550 °C for 8 h in a preselected oven. The authors characterized the formation of AuNPs using two microscopic techniques: scanning electron microscopy (SEM) and atomic force microscopy (AFM). The analytical performances of the nanostructured-glasses were compared regarding biosensing of Hsp70, an ATP-driven molecular chaperone. In this work, the human heat-shock protein (Hsp70), was chosen as a model biomarker of body stress disorders for microwave spectroscopic investigations. It was found that microwave screening at 4 GHz allowed for the first time the detection of 12 ng/µL/cm² of Hsp70.

Real-time dynamics of carbon nanotube porins in supported lipid membranes visualized by high-speed atomic force microscopy

In-plane mobility of proteins in lipid membranes is one of the fundamental mechanisms supporting biological functionality. Here we use high-speed atomic force microscopy (HS-AFM) to show that a novel type of biomimetic channel-carbon nanotube porins (CNTPs)-is also laterally mobile in supported lipid membranes, mimicking biological protein behaviour. HS-AFM can capture real-time dynamics of CNTP motion in the supported lipid bilayer membrane, build long-term trajectories of the CNTP motion and determine the diffusion coefficients associated with this motion. Our analysis shows that diffusion coefficients of CNTPs fall into the same range as those of proteins in supported lipid membranes. CNTPs in HS-AFM experiments often exhibit 'directed' diffusion behaviour, which is common for proteins in live cell membranes.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

A spectroscopic sensing platform for MARCKS protein monolayers

We developed a highly sensitive silicon platform, suitable to assess the molecular organization of protein samples. Prototype platforms were obtained using different electrochemical protocols for the electrodeposition of Ag-nanoparticles onto the hydrogenated silicon surface. A platform with high Surface Enhanced Raman Scattering efficiency was selected based on the surface coverage and the number density of particles size distribution. The performance of the platform was determined by studying the interaction of Myristoylated Alanine-Rich C Kinase Substrate (MARCKS) protein with the substrate according to its molecular organization. The chemical and structural characteristics of MARCKS molecules were examined under two configurations: i) a disordered distribution given by a MARCKS solution drop deposited onto the platform and, ii) a compact monolayer transferred to the platform by the Langmuir-Blodgett method. Raman spectra show vibrational bands of Phenylalanine and Lysine residues specific for the protein effector domain, and evidence the presence of alpha helix structure in both configurations. Moreover, we distinguished the supramolecular order between the compact monolayer and random molecular distribution. The platforms containing Ag-nanoparticles are suitable for studies of protein structure and interactions, advancing a methodological strategy for our long term goal, which is to explore the interaction of proteins with model membranes.

Topographic analysis by atomic force microscopy of proteoliposomes matrix vesicle mimetics harboring TNAP and AnxA5

Atomic force microscopy (AFM) is one of the most commonly used scanning probe microscopy techniques for nanoscale imaging and characterization of lipid-based particles. However, obtaining images of such particles using AFM is still a challenge. The present study extends the capabilities of AFM to the characterization of proteoliposomes, a special class of liposomes composed of lipids and proteins, mimicking matrix vesicles (MVs) involved in the biomineralization process. To this end, proteoliposomes were synthesized, composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-l-serine (DPPS), with inserted tissue-nonspecific alkaline phosphatase (TNAP) and/or annexin V (AnxA5), both characteristic proteins of osteoblast-derived MVs. We then aimed to study how TNAP and AnxA5 insertion affects the proteoliposomes' membrane properties and, in turn, interactions with type II collagen, thus mimicking early MV activity during biomineralization. AFM images of these proteoliposomes, acquired in dynamic mode, revealed the presence of surface protrusions with distinct viscoelasticity, thus suggesting that the presence of the proteins induced local changes in membrane fluidity. Surface protrusions were measurable in TNAP-proteoliposomes but barely detectable in AnxA5-proteoliposomes. More complex surface structures were observed for proteoliposomes harboring both TNAP and AnxA5 concomitantly, resulting in a lower affinity for type II collagen fibers compared to proteoliposomes harboring AnxA5 alone. The present study achieved the topographic analysis of lipid vesicles by direct visualization of structural changes, resulting from protein incorporation, without the need for fluorescent probes.

Ions Modulate Stress-Induced Nanotexture in Supported Fluid Lipid Bilayers

Most plasma membranes comprise a large number of different molecules including lipids and proteins. In the standard fluid mosaic model, the membrane function is effected by proteins whereas lipids are largely passive and serve solely in the membrane cohesion. Here we show, using supported 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayers in different saline solutions, that ions can locally induce ordering of the lipid molecules within the otherwise fluid bilayer when the latter is supported. This nanoordering exhibits a characteristic length scale of ∼20 nm, and manifests itself clearly when mechanical stress is applied to the membrane. Atomic force microscopy (AFM) measurements in aqueous solutions containing NaCl, KCl, CaCl₂, and Tris buffer show that the magnitude of the effect is strongly ion-specific, with Ca²⁺ and Tris, respectively, promoting and reducing stress-induced nanotexturing of the membrane. The AFM results are complemented by fluorescence recovery after photobleaching experiments, which reveal an inverse correlation between the tendency for molecular nanoordering and the diffusion coefficient within the bilayer. Control AFM experiments on other lipids and at different temperatures support the hypothesis that the nanotexturing is induced by reversible, localized gel-like solidification of the membrane. These results suggest that supported fluid phospholipid bilayers are not homogenous at the nanoscale, but specific ions are able to locally alter molecular organization and mobility, and spatially modulate the membrane’s properties on a length scale of ∼20 nm. To illustrate this point, AFM was used to follow the adsorption of the membrane-penetrating antimicrobial peptide Temporin L in different solutions. The results confirm that the peptides do not absorb randomly, but follow the ion-induced spatial modulation of the membrane. Our results suggest that ionic effects have a significant impact for passively modulating the local properties of biological membranes, when in contact with a support such as the cytoskeleton.

Lipid Bilayer Membrane in a Silicon Based Micron Sized Cavity Accessed by Atomic Force Microscopy and Electrochemical Impedance Spectroscopy

Supported lipid bilayers (SLBs) are widely used in biophysical research to probe the functionality of biological membranes and to provide diagnoses in high throughput drug screening. Formation of SLBs at below phase transition temperature (Tm) has applications in nano-medicine research where low temperature profiles are required. Herein, we report the successful production of SLBs at above-as well as below-the Tm of the lipids in an anisotropically etched, silicon-based micro-cavity. The Si-based cavity walls exhibit controlled temperature which assist in the quick and stable formation of lipid bilayer membranes. Fusion of large unilamellar vesicles was monitored in real time in an aqueous environment inside the Si cavity using atomic force microscopy (AFM), and the lateral organization of the lipid molecules was characterized until the formation of the SLBs. The stability of SLBs produced was also characterized by recording the electrical resistance and the capacitance using electrochemical impedance spectroscopy (EIS). Analysis was done in the frequency regime of 10-2-10⁵ Hz at a signal voltage of 100 mV and giga-ohm sealed impedance was obtained continuously over four days. Finally, the cantilever tip in AFM was utilized to estimate the bilayer thickness and to calculate the rupture force at the interface of the tip and the SLB. We anticipate that a silicon-based, micron-sized cavity has the potential to produce highly-stable SLBs below their Tm. The membranes inside the Si cavity could last for several days and allow robust characterization using AFM or EIS. This could be an excellent platform for nanomedicine experiments that require low operating temperatures.

1H NMR Shows Slow Phospholipid Flip-Flop in Gel and Fluid Bilayers

We measured the transbilayer diffusion of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in large unilamellar vesicles, in both the gel (L_β') and fluid (L_α) phases. The choline resonance of headgroup-protiated DPPC exchanged into the outer leaflet of headgroup-deuterated DPPC-d13 vesicles was monitored using ¹H NMR spectroscopy, coupled with the addition of a paramagnetic shift reagent. This allowed us to distinguish between the inner and outer bilayer leaflet of DPPC, to determine the flip-flop rate as a function of temperature. Flip-flop of fluid-phase DPPC exhibited Arrhenius kinetics, from which we determined an activation energy of 122 kJ mol^-1. In gel-phase DPPC vesicles, flip-flop was not observed over the course of 250 h. Our findings are in contrast to previous studies of solid-supported bilayers, where the reported DPPC translocation rates are at least several orders of magnitude faster than those in vesicles at corresponding temperatures. We reconcile these differences by proposing a defect-mediated acceleration of lipid translocation in supported bilayers, where long-lived, submicron-sized holes resulting from incomplete surface coverage are the sites of rapid transbilayer movement.

Analytical approaches to study domain formation in biomimetic membranes

Novel characterization methods open new horizons in the study of membrane mixtures.

Structure and Nanomechanics of Model Membranes by Atomic Force Microscopy and Spectroscopy: Insights into the Role of Cholesterol and Sphingolipids

Biological membranes mediate several biological processes that are directly associated with their physical properties but sometimes difficult to evaluate. Supported lipid bilayers (SLBs) are model systems widely used to characterize the structure of biological membranes. Cholesterol (Chol) plays an essential role in the modulation of membrane physical properties. It directly influences the order and mechanical stability of the lipid bilayers, and it is known to laterally segregate in rafts in the outer leaflet of the membrane together with sphingolipids (SLs). Atomic force microscope (AFM) is a powerful tool as it is capable to sense and apply forces with high accuracy, with distance and force resolution at the nanoscale, and in a controlled environment. AFM-based force spectroscopy (AFM-FS) has become a crucial technique to study the nanomechanical stability of SLBs by controlling the liquid media and the temperature variations. In this contribution, we review recent AFM and AFM-FS studies on the effect of Chol on the morphology and mechanical properties of model SLBs, including complex bilayers containing SLs. We also introduce a promising combination of AFM and X-ray (XR) techniques that allows for in situ characterization of dynamic processes, providing structural, morphological, and nanomechanical information.

Oriented Membrane Protein Reconstitution into Tethered Lipid Membranes for AFM Force Spectroscopy

Membrane proteins act as a central interface between the extracellular environment and the intracellular response and as such represent one of the most important classes of drug targets. The characterization of the molecular properties of integral membrane proteins, such as topology and interdomain interaction, is key to a fundamental understanding of their function. Atomic force microscopy (AFM) and force spectroscopy have the intrinsic capabilities of investigating these properties in a near-native setting. However, atomic force spectroscopy of membrane proteins is traditionally carried out in a crystalline setup. Alternatively, model membrane systems, such as tethered bilayer membranes, have been developed for surface-dependent techniques. While these setups can provide a more native environment, data analysis may be complicated by the normally found statistical orientation of the reconstituted protein in the model membrane. We have developed a model membrane system that enables the study of membrane proteins in a defined orientation by single-molecule force spectroscopy. Our approach is demonstrated using cell-free expressed bacteriorhodopsin coupled to a quartz glass surface in a defined orientation through a protein anchor and reconstituted inside an artificial membrane system. This approach offers an effective way to study membrane proteins in a planar lipid bilayer. It can be easily transferred to all membrane proteins that possess a suitable tag and can be reconstituted into a lipid bilayer. In this respect, we anticipate that this technique may contribute important information on structure, topology, and intra- and intermolecular interactions of other seven-transmembrane helical receptors.

Effects of GPI-anchored TNAP on the dynamic structure of model membranes

Tissue-nonspecific alkaline phosphatase (TNAP) plays a crucial role during skeletal mineralization, and TNAP deficiency leads to the soft bone disease hypophosphatasia. TNAP is anchored to the external surface of the plasma membranes by means of a GPI (glycosylphosphatidylinositol) anchor. Membrane-anchored and solubilized TNAP displays different kinetic properties against physiological substrates, indicating that membrane anchoring influences the enzyme function. Here, we used Electron Spin Resonance (ESR) measurements along with spin labeled phospholipids to probe the possible dynamic changes prompted by the interaction of GPI-anchored TNAP with model membranes. The goal was to systematically analyze the ESR data in terms of line shape changes and of alterations in parameters such as rotational diffusion rates and order parameters obtained from non-linear least-squares simulations of the ESR spectra of probes incorporated into DPPC liposomes and proteoliposomes. Overall, the presence of TNAP increased the dynamics and decreased the ordering in the three distinct regions probed by the spin labeled lipids DOPTC (headgroup), and 5- and 16-PCSL (acyl chains). The largest change was observed for 16-PCSL, thus suggesting that GPI-anchored TNAP can give rise to long reaching modifications that could influence membrane processes halfway through the bilayer.

Phase transition behaviors of the supported DPPC bilayer investigated by sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM)

The phase transition behaviors of a supported bilayer of dipalmitoylphosphatidyl-choline (DPPC) have been systematically evaluated by in situ sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM).

Helium Ion Microscopy Visualizes Lipid Nanodomains in Mammalian Cells

Cell membranes are composed of 2D bilayers of amphipathic lipids, which allow a lateral movement of the respective membrane components. These components are arranged in an inhomogeneous manner as transient micro- and nanodomains, which are believed to be crucially involved in the regulation of signal transduction pathways in mammalian cells. Because of their small size (diameter 10-200 nm), membrane nanodomains cannot be directly imaged using conventional light microscopy. Here, direct visualization of cell membrane nanodomains by helium ion microscopy (HIM) is presented. It is shown that HIM is capable to image biological specimens without any conductive coating and that HIM images clearly allow the identification of nanodomains in the ultrastructure of membranes with 1.5 nm resolution. The shape of these nanodomains is preserved by fixation of the surrounding unsaturated fatty acids while saturated fatty acids inside the nanodomains are selectively removed. Atomic force microscopy, fluorescence microscopy, 3D structured illumination microscopy, and direct stochastic optical reconstruction microscopy provide additional evidence that the structures in the HIM images of cell membranes originate from membrane nanodomains. The nanodomains observed by HIM have an average diameter of 20 nm and are densely arranged with a minimal nearest neighbor distance of ≈ 15 nm.