Structure of an adsorbed dimyristoylphosphatidylcholine bilayer measured with specular reflection of neutrons

Leaflet-Specific Structure and Dynamics of Solid and Polymer Supported Lipid Bilayers

Polymer-supported or tethered lipid bilayers serve as versatile platforms for mimicking plasma membrane structure and dynamics, yet the impact of polymer supports on lipid bilayers remains largely unresolved. In this study, we introduce a novel methodology that combines graphene-induced energy transfer (GIET) with line-scan fluorescence lifetime correlation spectroscopy (lsFLCS) to examine the structural and dynamic properties of lipid bilayers. Our findings reveal that polymer supports markedly influence both the structural parameters, such as the membrane height from the substrate, its thickness, as well as dynamic properties, including leaflet-specific diffusion coefficients and interleaflet coupling. These findings highlight the complex interplay between a polymer support and the lipid bilayers. By resolving leaflet-specific diffusion and heights of the two leaflets from the substrate, this study emphasizes the potential of GIET-lsFLCS for probing membrane dynamics and structure. These insights significantly advance the understanding and application of polymer-supported membranes across diverse research contexts.

Unveiling the Interactions between Amino Acids-Based Surfactants and Lipid Bilayers: A Small Angle Neutron Scattering and Reflectivity Study

N-Acyl amino acids are biodegradable anionic amphiphilic molecules made up of linear fatty acids as hydrophobic tails and amino acids as polar heads, which are promising for their applicability in different technological fields. In the light of widening their use, a deeper understanding of their interactions with biological membranes is required, especially to further assess their toxicological profile. We investigated the interaction between N-decanoyl amino acid surfactants and phospholipid bilayers as simple in vitro models for biological membranes in comparison to sodium dodecyl sulfate using neutron scattering techniques. The information from small angle neutron scattering (SANS, q range from 0.008 to 0.25 Å-1) focusing on liposome-to-surfactant interactions and neutron reflectivity (NR, Q range measured at three incident angles θ = 0.35, 0.65, and 1.5°) focusing on lipid bilayer-to-surfactant interactions was combined to provide a detailed characterization. All amino acid surfactants (C10-alanine, C10-glycine, C10-leucine, C10-methionine, C10-serine, and C10-proline) exhibited a similar behavior in terms of incorporation in liposomes and lipid removal as well as adsorption profiles in bilayers up to their critical micelle concentration (CMC). Notably, bilayer destabilization occurred for all surfactants (except for C10-serine and C10-alanine) at a concentration between CMC and 2× CMC. Such a result demonstrates the exceptional ability of C10-serine and C10-alanine to integrate into bilayers without disruption up to concentrations as high as ∼3-4× CMC. These findings support the lower cytotoxic effect of C10-serine and C10-alanine surfactants, observed in previous studies, and provide new insights on the mechanism of interaction of N-decanoyl amino acids with lipid membranes.

Diffusion of a single colloid on the surface of a giant vesicle and a droplet

The study of interactions between biomimetic membranes and micron-sized particles is crucial for understanding various biological processes. Here, we control microparticle spontaneous engulfment by giant lipid vesicles by tuning particle surface charge, exploring regimes from negligible to strong adhesion. We focus our attention on dissipative phenomena at the micron- and nanoscales, occurring when a particle is wrapped by a lipid vesicle bilayer or when the particle diffuses at the lipid-monolayer interface of a droplet. For particles wrapped by membrane bilayers, we highlight the influence of the particle penetration depth and the impact of substructures on particle friction. Our work is complemented by hydrodynamic simulations that take into account the effects of the shape of the membrane wrapping the particle and the water gap separating the lipid bilayer membrane from the particle on translational particle drag. We show, however, that a purely hydrodynamic model is not suitable to describe the friction of a particle diffusing at the interface of an aqueous microdroplet in oil, stabilized by a single lipid layer. In hydrodynamic models, dissipation is solely described by the surface shear viscosity of the interface and the bulk fluid viscosity, but in this partial wetting configuration, an additional source of dissipation is required to account for fluctuations at the contact line. Hence, through experimental and numerical studies, we demonstrate that the dissipation contributions for the two geometries are fundamentally different.

Melittin-Induced Structural Transformations in DMPG and DMPS Lipid Membranes: A Langmuir Monolayer and AFM Study

In this study, we explore the interactions between melittin, a cationic antimicrobial peptide, and model lipid membranes composed of the negatively charged phospholipids 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) and 1,2-dimyristoyl-sn-glycero-3-phosphoserine (DMPS). Using the Langmuir monolayer technique and atomic force microscopy (AFM), we reveal novel insights into these interactions. Our key finding is the observation of the ripple phase in the DMPS bilayer on mica, a phenomenon not previously reported for negatively charged single bilayers. This discovery is significant given the critical role of phosphatidylserine (PS) in cancer biology and the potential of melittin as an anticancer agent. We also highlight the importance of subphase composition, as melittin interacts preferentially with lipids in the liquid-condensed phase; thus, selecting the appropriate subphase composition is crucial because it affects lipid behavior and consequently melittin interactions. Our results show that melittin incorporates into lipid monolayers in both liquid-expanded and liquid-condensed phases, enhancing membrane fluidity and disorder, but is expelled from DMPS in the solid phase. AFM imaging further reveals that melittin induces substantial structural changes in the DMPG membrane and forms the ripple phase in the DMPS bilayers. Despite these alterations, melittin does not cause pore formation or membrane rupture, suggesting strong electrostatic adsorption on the membrane surface that prevents penetration. These findings highlight the differential impacts of melittin on lipid monolayers and bilayers and underscore its potential for interacting with membranes without causing disruption.

Structural and Spectroscopic Characterization of Supported Sarcoplasmic Reticulum Membranes on Solid Substrates

Sarcoplasmic reticulum (SR) membranes from rabbit muscle were deposited on silicon substrates and characterized by the combination of spectral ellipsometry (SE), high energy specular X-ray reflectivity (XRR), specular neutron reflectivity (NR), and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Following the optimization of the preparative conditions by SE, the detailed structures in the direction perpendicular to the membrane were probed by XRR. ATR-FTIR data showed strong signals from amide I and amide II bands of the native SR membranes containing a large amount of Ca2+-ATPase, which could not be achieved by the reconstitution in artificial lipid membranes. The treatment with protease led to a significant decrease in the amide peaks, and the XRR data confirmed the modulation of the membrane structures. The obtained data show the potential of the in situ combination of reflectivity and vibrational spectroscopy of native supported membranes in order to unravel both structure and dynamics of complex biological membranes.

Capillary-Assisted Assembly of Polymer Gel-Supported Lipid Bilayers

The polymer-supported lipid bilayer represents an attractive supramolecular assembly in numerous biophysical and bioanalytical applications. The assembly of polymer-supported membranes with a polymer layer thickness of just a few nanometers is now well-established, but bilayer properties in such a membrane architecture are still influenced by the nearby solid substrate. Polymer-supported lipid bilayer systems with a several micrometers thick polymer layer will overcome this shortcoming. However, formation of a fluid lipid bilayer on a fully hydrated, micrometer thick polymer film using traditional methods (e.g., vesicle fusion and lipid monolayer deposition techniques) remains a challenging task due to the rather unfavorable interfacial conditions for bilayer formation in such a system. Here, we report for the first time on the facile capillary-assisted formation of a lipid bilayer on the surface of a fully hydrated, several micrometers thick polyacrylamide (PAA) gel, in which forced molecular crowding of lipids at the air-water interface of the capillary results in monolayer instability and collapse, thereby forming a lipid bilayer on the top of the polymer gel inside the capillary. Stable bilayer attachment on the surface of the polymer gel can be achieved via physisorption or specific chemical linkages (tethering) on both cross-linked and non-cross-linked PAA films. Unlike the traditional solid-supported lipid bilayer (SLB), the lipid lateral diffusion in the polymer gel-supported lipid bilayer is not anymore perturbed by a solid substrate. Instead, more like a plasma membrane, it is mainly influenced by the properties of the underlying polymer and the nature/distribution of polymer-bilayer attachments. Polymer gel-supported lipid bilayers built using the capillary-assisted assembly approach show attractive self-healing properties, resulting in superior long-term stability relative to the SLB. We hypothesize that the described capillary-assisted assembly method can be applied to a wide range of polymeric materials and lipid compositions, opening exciting opportunities as an advanced model membrane system.

Structural and Mechanical Characterization of DNA-Tethered Membranes on Graphene

Lipid membranes that are separated from the surface of graphene by DNA tethers were prepared by surface functionalization with pyrene coupled to single-stranded DNA (ssDNA), followed by self-assembly of the mixture of ssDNA-functionalized phospholipid and the matrix phospholipids. The formation of uniform membranes was confirmed by fluorescence microscopy, and the structures of the systems before and after hybridization in the direction perpendicular to the global plane of the membranes were investigated using high-energy X-ray reflectivity. The thickness values of the DNA spacers (15 and 37 bp) calculated from the best-fit results were less than the expected thicknesses of the double-stranded DNA (dsDNA) chains taking the upright conformation, indicating that the DNA spacers are tilted with respect to the direction normal to the surface. The Young's moduli of the DNA-tethered membranes obtained by AFM nanoindentation showed higher values than the membranes with no DNA tethers, which suggests that the DNA layer resists against the compression, lifting up the membrane. Intriguingly, the presence of DNA tethers caused no increase in the yield depth. The smaller thickness values as well as the unchanged yield depth suggest that the dsDNA chains can tilt and rotate, which can be attributed to the flexible pyrene-DNA junction.

CHARMM-GUI Multicomponent Assembler for modeling and simulation of complex multicomponent systems

Atomic-scale molecular modeling and simulation are powerful tools for computational biology. However, constructing models with large, densely packed molecules, non-water solvents, or with combinations of multiple biomembranes, polymers, and nanomaterials remains challenging and requires significant time and expertise. Furthermore, existing tools do not support such assemblies under the periodic boundary conditions (PBC) necessary for molecular simulation. Here, we describe Multicomponent Assembler in CHARMM-GUI that automates complex molecular assembly and simulation input preparation under the PBC. In this work, we demonstrate its versatility by preparing 6 challenging systems with varying density of large components: (1) solvated proteins, (2) solvated proteins with a pre-equilibrated membrane, (3) solvated proteins with a sheet-like nanomaterial, (4) solvated proteins with a sheet-like polymer, (5) a mixed membrane-nanomaterial system, and (6) a sheet-like polymer with gaseous solvent. Multicomponent Assembler is expected to be a unique cyberinfrastructure to study complex interactions between small molecules, biomacromolecules, polymers, and nanomaterials.

Fluorophore position of headgroup-labeled Gb3 glycosphingolipids in lipid bilayers

Fluorescent lipid probes are an invaluable tool for investigating lipid membranes. In particular, localizing certain receptor lipids such as glycosphingolipids within phase-separated membranes is of pivotal interest to understanding the influence of protein-receptor lipid binding on membrane organization. However, fluorescent labeling can readily alter the phase behavior of a lipid membrane because of the interaction of the fluorescent moiety with the membrane interface. Here, we investigated Gb3 glycosphingolipids, serving as receptor lipids for the protein Shiga toxin, with a headgroup attached BODIPY fluorophore separated by a polyethylene glycol (PEG) spacer of different lengths. We found that the diffusion coefficients of the fluorescently labeled Gb3 species in 1,2-dioleoyl-sn-glycero-3-phosphocholine/Gb3 (98:2, n/n) supported lipid bilayers are unaltered by the PEG spacer length. However, quenching as well as graphene-induced energy transfer experiments indicated that the length of the PEG spacer (n = 3 and n = 13) alters the position of the BODIPY fluorophore. In particular, the graphene-induced energy transfer technique provided accurate end-to-end distances between the fluorophores in the two leaflets of the bilayer thus enabling us to quantify the distance between the membrane interface and the fluorophore with sub-nanometer resolution. The spacer with three oligo ethylene glycol groups positioned the BODIPY fluorophore directly at the membrane interface favoring its interaction with the bilayer and thus may disturb lipid packing. However, the longer PEG spacer (n = 13) separated the BODIPY moiety from the membrane surface by 1.5 nm.

Ostwald Ripening of Triacylglycerol Droplets Embedded in Glass-Supported Phospholipid Bilayers

Lipid droplets are fat storage organelles that consist of a neutral lipid core surrounded by a phospholipid monolayer. Because of their important biological functions, reconstituting model lipid droplets in synthetic phospholipid membranes is of great interest. In the present study, we investigated the incorporation of triacylglycerol droplets into glass-supported phospholipid bilayers by using fluorescence microscopy. We adsorbed triolein emulsions onto a glass surface that was partially covered with planar bilayers. After adsorption, triolein droplets were found to be immobilized in the bilayer membrane. The volume of each bound droplet varied over time. Large droplets grew, whereas small droplets shrank. Additionally, data on fluorescence recovery after photobleaching obtained for a phospholipid probe indicate that phospholipids on and near triolein droplets were fully mobile. Furthermore, photobleaching data obtained for a triacylglycerol probe indicate that triolein molecules diffused between different droplets along the planar bilayer. These results demonstrate Ostwald ripening, where triolein molecules in a small droplet dissolved in the bilayer, diffused laterally, and eventually bound to the interfaces of larger droplets. We investigated the ripening rate by using the average of the cube root of the fluorescence emission obtained for individual droplets. The ripening slowed after the addition of trilinolein to the triolein phase. Finally, we investigated the time dependence of the size distributions of the triolein droplets. The distribution was initially nearly unimodal and subsequently became bimodal.

Effect of Anionic Lipids on Mammalian Plasma Cell Membrane Properties

The effect of lipid composition on models of the inner leaflet of mammalian cell membranes has been investigated. Grazing incidence X-ray diffraction and X-ray and neutron reflectivity have been used to characterize lipid packing and solvation, while electrochemical and infrared spectroscopic methods have been employed to probe phase behavior in an applied electric field. Introducing a small quantity of the anionic lipid dimyristoylphosphatidylserine (DMPS) into bilayers of zwitterionic dimyristoylphosphatidylethanolamine (DMPE) results in a significant change in the bilayer response to an applied field: the tilt of the hydrocarbon chains increases before returning to the original tilt angle on detachment of the bilayer. Equimolar mixtures, with slightly closer chain packing, exhibit a similar but weaker response. The latter also tend to incorporate more solvent during this electrochemical phase transition, at levels similar to those of pure DMPS. Reflectivity measurements reveal greater solvation of lipid layers for DMPS > 30 mol %, matching the greater propensity for DMPS-rich bilayers to incorporate water. Taken together, the data indicate that the range of 10-35 mol % DMPS provides optimum bilayer properties (in flexibility and function as a barrier), which may explain why the DMPS content of cell membranes tends to be found within this range.

Spatz: the time-of-flight neutron reflectometer with vertical sample geometry at the OPAL research reactor

The Spatz neutron beam instrument is the second time-of-flight neutron reflectometer to be installed at the OPAL research reactor. The instrument was formerly the V18 BioRef reflectometer at the BER-II reactor in Berlin and was transferred to Australia in 2016. Subsequently the instrument was re-installed in the neutron guide hall of the OPAL reactor at the end position of the CG2B cold-neutron guide and recommissioned. The instrument performance has not been compromised by the move, with reflectivity achieved down to 10-7 and good counting statistics within a reasonable time frame using a wavelength range of 2-20 Å. Several different samples at the solid-air interface and the solid-liquid interface have been measured to demonstrate the instrument's capabilities.

Characterization of Phase Separated Planar Lipid Bilayer Membrane by Fluorescence Ratio Imaging and Scanning Probe Microscope

The lipid membrane forms nanodomains (rafts) and shows heterogeneous properties. These nanodomains relate to significant roles in various cell functions, and thus the analysis of the nanodomains in phase-separated lipid membranes is important to clarify the function and role of the nanodomains. However, the lipid membrane possesses small-sized nanodomains and shows a small height difference between the nanodomains and their surroundings at certain lipid compositions. In addition, nanodomain analysis sometimes requires highly sensitive and expensive apparatus, such as a two-photon microscope. These have prevented the analysis by the conventional fluorescence microscope and by the topography of the scanning probe microscope (SPM), even though these are promising methods in macroscale and microscale analysis, respectively. Therefore, this study aimed to overcome these problems in nanodomain analysis. We successfully demonstrated that solvatochromic dye, LipiORDER, could analyze the phase state of the lipid membrane at the macroscale with low magnification lenses. Furthermore, we could prove that the phase mode of SPM was effective in the visualization of specific nanodomains by properties difference as well as topographic images of SPM. Hence, this combination method successfully gave much information on the phase state at the micro/macro scale, and thus this would be applied to the analysis of heterogeneous lipid membranes.

Electrically Controlling and Optically Observing the Membrane Potential of Supported Lipid Bilayers

Supported lipid bilayers are a well-developed model system for the study of membranes and their associated proteins, such as membrane channels, enzymes, and receptors. These versatile model membranes can be made from various components, ranging from simple synthetic phospholipids to complex mixtures of constituents, mimicking the cell membrane with its relevant physiochemical and molecular phenomena. In addition, the high stability of supported lipid bilayers allows for their study via a wide array of experimental probes. In this work, we describe a platform for supported lipid bilayers that is accessible both electrically and optically, and demonstrate direct optical observation of the transmembrane potential of supported lipid bilayers. We show that the polarization of the supported membrane can be electrically controlled and optically probed using voltage-sensitive dyes. Membrane polarization dynamics is understood through electrochemical impedance spectroscopy and the analysis of an equivalent electrical circuit model. In addition, we describe the effect of the conducting electrode layer on the fluorescence of the optical probe through metal-induced energy transfer, and show that while this energy transfer has an adverse effect on the voltage sensitivity of the fluorescent probe, its strong distance dependency allows for axial localization of fluorescent emitters with ultrahigh accuracy. We conclude with a discussion on possible applications of this platform for the study of voltage-dependent membrane proteins and other processes in membrane biology and surface science.

Revealing local molecular distribution, orientation, phase separation, and formation of domains in artificial lipid layers: Towards comprehensive characterization of biological membranes

Lipids, together with molecules such as DNA and proteins, are one of the most relevant systems responsible for the existence of life. Selected lipids are able to assembly into various organized structures, such as lipid membranes. The unique properties of lipid membranes determine their complex functions, not only to separate biological environments, but also to participate in regulatory functions, absorption of nutrients, cell-cell communication, endocytosis, cell signaling, and many others. Despite numerous scientific efforts, still little is known about the reason underlying the variability within lipid membranes, and its biochemical significance. In this review, we discuss the structural complexity of lipid membranes, as well as the importance to simplify studied systems in order to understand phenomena occurring in natural, complex membranes. Such systems require a model interface to be analyzed. Therefore, here we focused on analytical studies of artificial systems at various interfaces. The molecular structure of lipid membranes, specifically the nanometric thickens of molecular bilayer, limits in a major extent the choice of highly sensitive methods suitable to study such structures. Therefore, we focused on methods that combine high sensitivity, and/or chemical selectivity, and/or nanometric spatial resolution, such as atomic force microscopy, nanospectroscopy (tip-enhanced Raman spectroscopy, infrared nanospectroscopy), phase modulation infrared reflection-absorption spectroscopy, sum-frequency generation spectroscopy. We summarized experimental and theoretical approaches providing information about molecular structure and composition, lipid spatial distribution (phase separation), organization (domain shape, molecular orientation) of lipid membranes, and real-time visualization of the influence of various molecules (proteins, drugs) on their integrity. An integral part of this review discusses the latest achievements in the field of lipid layer-based biosensors.

Synthetic Cell as a Platform for Understanding Membrane-Membrane Interactions

In the pursuit of understanding life, model membranes made of phospholipids were envisaged decades ago as a platform for the bottom-up study of biological processes. Micron-sized lipid vesicles have gained great acceptance as their bilayer membrane resembles the natural cell membrane. Important biological events involving membranes, such as membrane protein insertion, membrane fusion, and intercellular communication, will be highlighted in this review with recent research updates. We will first review different lipid bilayer platforms used for incorporation of integral membrane proteins and challenges associated with their functional reconstitution. We next discuss different methods for reconstitution of membrane fusion and compare their fusion efficiency. Lastly, we will highlight the importance and challenges of intercellular communication between synthetic cells and synthetic cells-to-natural cells. We will summarize the review by highlighting the challenges and opportunities associated with studying membrane-membrane interactions and possible future research directions.

A Study of the Effects of Plasma Surface Treatment on Lipid Bilayers Self-Spreading on a Polydimethylsiloxane Substrate under Different Treatment Times

Plasma-treated poly(dimethylsiloxane) (PDMS)-supported lipid bilayers are used as functional tools for studying cell membrane properties and as platforms for biotechnology applications. Self-spreading is a versatile method for forming lipid bilayers. However, few studies have focused on the effect of plasma treatment on self-spreading lipid bilayer formation. In this paper, we performed lipid bilayer self-spreading on a PDMS surface with different treatment times. Surface characterization of PDMS treated with different treatment times is evaluated by AFM and SEM, and the effects of plasma treatment of the PDMS surface on lipid bilayer self-spreading behavior is investigated by confocal microscopy. The front-edge velocity of lipid bilayers increases with the plasma treatment time. By theoretical analyses with the extended-DLVO modeling, we find that the most likely cause of the velocity change is the hydration repulsion energy between the PDMS surface and lipid bilayers. Moreover, the growth behavior of membrane lobes on the underlying self-spreading lipid bilayer was affected by topography changes in the PDMS surface resulting from plasma treatment. Our findings suggest that the growth of self-spreading lipid bilayers can be controlled by changing the plasma treatment time.

Reflectometry and molecular dynamics study of the impact of cholesterol and melatonin on model lipid membranes

The effect of melatonin and/or cholesterol on the structural properties of a model lipid bilayer prepared from 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) has been investigated both experimentally and via molecular dynamics (MD) simulations. Neutron reflectometry experiments performed with single supported membranes revealed changes in lipid bilayer thickness upon the introduction of additional components. While the presence of cholesterol led to an increase in membrane thickness, the opposite effect was observed in the case of melatonin. The results obtained are in a good agreement with MD simulations which provided further information on the organization of components within the systems examined, indicating a mechanism underlying the membranes' thickness changes due to cholesterol and melatonin that had been observed experimentally. Cholesterol and melatonin preferentially accumulate in different membrane regions, presumably affecting the conformation of lipid hydrophobic moieties differently, and in turn having distinct impacts on the structure of the entire membrane. Our findings may be relevant for understanding the effects of age-related changes in cholesterol and melatonin concentrations, including those in the brains of individuals with Alzheimer's disease.

The dynamic face of lipid membranes

Cell membranes - primarily composed of lipids, sterols, and proteins - form a dynamic interface between living cells and their environment. They act as a mechanical barrier around the cell while selectively facilitating material transport, signal transduction, and various other functions necessary for the cell viability. The complex functionality of cell membranes and the hierarchical motions and responses they exhibit demand a thorough understanding of the origin of different membrane dynamics and how they are influenced by molecular additives and environmental cues. These dynamic modes include single-molecule diffusion, thermal fluctuations, and large-scale membrane deformations, to name a few. This review highlights advances in investigating structure-driven dynamics associated with model cell membranes, with a particular focus on insights gained from neutron scattering and spectroscopy experiments. We discuss the uniqueness of neutron contrast variation and its remarkable potential in probing selective membrane structure and dynamics on spatial and temporal scales over which key biological functions occur. We also present a summary of current and future opportunities in synergistic combinations of neutron scattering with molecular dynamics (MD) simulations to gain further understanding of the molecular mechanisms underlying complex membrane functions.

Influence of Substrate Hydrophilicity on Structural Properties of Supported Lipid Systems on Graphene, Graphene Oxides, and Silica

Pristine graphene, a range of graphene oxides, and silica substrates were used to investigate the effect of surface hydrophilicity on supported lipid bilayers by means of all-atom molecular dynamics simulations. Supported 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayers were found in close-contact conformations with hydrophilic substrates with as low as 5% oxidation level, while self-assembled monolayers occur on pure hydrophobic graphene only. Lipids and water at the surface undergo large redistribution to maintain the stability of the supported bilayers. Deposition of bicelles on increasingly hydrophilic substrates shows the continuous process of reshaping of the supported system and makes intermediate stages between self-assembled monolayers and supported bilayers. The bilayer thickness changes with hydrophilicity in a complex manner, while the number of water molecules per lipid in the hydration layer increases together with hydrophilicity.

Unravelling the structural complexity of protein-lipid interactions with neutron reflectometry

Neutron reflectometry (NR) is a large-facility technique used to examine structure at interfaces. In this brief review an introduction to the utilisation of NR in the study of protein-lipid interactions is given. Cold neutron beams penetrate matter deeply, have low energies, wavelengths in the Ångstrom regime and are sensitive to light elements. High differential hydrogen sensitivity (between protium and deuterium) enables solution and sample isotopic labelling to be utilised to enhance or diminish the scattering signal of individual components within complex biological structures. The combination of these effects means NR can probe buried structures such as those at the solid-liquid interface and encode molecular level structural information on interfacial protein-lipid complexes revealing the relative distribution of components as well as the overall structure. Model biological membrane sample systems can be structurally probed to examine phenomena such as antimicrobial mode of activity, as well as structural and mechanistic properties peripheral/integral proteins within membrane complexes. Here, the example of the antimicrobial protein α1-purothionin binding to a model Gram negative bacterial outer membrane is used to highlight the utilisation of this technique, detailing how changes in the protein/lipid distributions across the membrane before and after the protein interaction can be easily encoded using hydrogen isotope labelling.

A quantitative determination of lipid bilayer deposition efficiency using AFM

The efficacy of a number of different methods for depositing a dimyristoylphosphatidylcholine (DMPC) lipid bilayer or DMPC-cholesterol (3 : 1) mixed bilayer onto a silicon substrate has been investigated in a quantitative manner using atomic force microscopy (AFM) image analysis to extract surface coverage. Complementary AFM-IR measurements were used to confirm the presence of the lipids. For the Langmuir-Blodgett/Schaefer deposition method at temperatures below the chain-melting transition temperature (T m), a large number of bilayer defects resulted when DMPC was deposited from a water subphase. Addition of calcium ions to the trough led to smaller, more frequent defects, whereas addition of cholesterol to the lipid mixture led to a vast improvement in bilayer coverage. Poor coverage was achieved for deposition at temperatures above T m. Formation of the deposited bilayer from vesicle fusion proved a more reliable method for all systems, with formation of near-complete bilayers within 60 seconds at temperatures above T m, although this method led to a higher probability of multilayer formation and rougher bilayer surfaces.

Why Do Tethered-Bilayer Lipid Membranes Suit for Functional Membrane Protein Reincorporation?

Membrane proteins (MPs) are essential for cellular functions. Understanding the functions of MPs is crucial as they constitute an important class of drug targets. However, MPs are a challenging class of biomolecules to analyze because they cannot be studied outside their native environment. Their structure, function and activity are highly dependent on the local lipid environment, and these properties are compromised when the protein does not reside in the cell membrane. Mammalian cell membranes are complex and composed of different lipid species. Model membranes have been developed to provide an adequate environment to envisage MP reconstitution. Among them, tethered-Bilayer Lipid Membranes (tBLMs) appear as the best model because they allow the lipid bilayer to be decoupled from the support. Thus, they provide a sufficient aqueous space to envisage the proper accommodation of large extra-membranous domains of MPs, extending outside. Additionally, as the bilayer remains attached to tethers covalently fixed to the solid support, they can be investigated by a wide variety of surface-sensitive analytical techniques. This review provides an overview of the different approaches developed over the last two decades to achieve sophisticated tBLMs, with a more and more complex lipid composition and adapted for functional MP reconstitution.

Characterizing the Structure and Interactions of Model Lipid Membranes Using Electrophysiology

The cell membrane is a protective barrier whose configuration determines the exchange both between intracellular and extracellular regions and within the cell itself. Consequently, characterizing membrane properties and interactions is essential for advancements in topics such as limiting nanoparticle cytotoxicity. Characterization is often accomplished by recreating model membranes that approximate the structure of cellular membranes in a controlled environment, formed using self-assembly principles. The selected method for membrane creation influences the properties of the membrane assembly, including their response to electric fields used for characterizing transmembrane exchanges. When these self-assembled model membranes are combined with electrophysiology, it is possible to exploit their non-physiological mechanics to enable additional measurements of membrane interactions and phenomena. This review describes several common model membranes including liposomes, pore-spanning membranes, solid supported membranes, and emulsion-based membranes, emphasizing their varying structure due to the selected mode of production. Next, electrophysiology techniques that exploit these structures are discussed, including conductance measurements, electrowetting and electrocompression analysis, and electroimpedance spectroscopy. The focus of this review is linking each membrane assembly technique to the properties of the resulting membrane, discussing how these properties enable alternative electrophysiological approaches to measuring membrane characteristics and interactions.

Stability of Lipid Coatings on Nanoparticle-Decorated Surfaces

Lipid membranes supported on solid surfaces and nanoparticles find multiple applications in industrial and biomedical technologies. Here, we explore in silico the mechanisms of the interactions of lipid membranes with nanostructured surfaces with deposited nanoparticles and explain the characteristic particle size dependence of the uniformity and stability of lipid coatings observed in vitro. Simulations are performed to demonstrate the specifics of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid membrane adhesion to hydrophilic and hydrophobic nanoparticles ranging in size from 1.5 to 40 nm using an original coarse-grained molecular dynamics model with implicit solvent and large simulation boxes (scales up to 280 × 154 × 69 nm³). We find that one of the major factors that affects the uniformity and stability of lipid coatings is the disjoining pressure in the water hydration layer formed between the lipid membrane and hydrophilic solid surface. This effect is accounted for by introducing a special long-range lipid-solid interaction potential that mimics the effects of the disjoining pressure in thin water layers. Our simulations reveal the physical mechanisms of interactions of lipid bilayers with solid surfaces that are responsible for the experimentally observed nonmonotonic particle size dependence of the uniformity and stability of lipid coatings: particles smaller than the hydration layer thickness (<2-3 nm) or larger than ∼20 nm are partially or fully enfolded by a lipid bilayer, whereas particles of the intermediate size (5-20 nm) cause membrane perforation and pore formation. In contrast, hydrophobic nanoparticles, which repel the hydration layer, tend to be encapsulated within the hydrophobic interior of the membrane and coated by the lipid monolayer. The proposed model can be further extended and applied to a wide class of systems comprising nanoparticles and nanostructured substrates interacting with lipid and surfactant bilayers and monolayers.

The Antifungal Mechanism of Amphotericin B Elucidated in Ergosterol and Cholesterol-Containing Membranes Using Neutron Reflectometry

We have characterized and compared the structures of ergosterol- and cholesterol-containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes before and after interaction with the amphiphilic antifungal drug amphotericin B (AmB) using neutron reflection. AmB inserts into both pure POPC and sterol-containing membranes in the lipid chain region and does not significantly perturb the structure of pure POPC membranes. By selective per-deuteration of the lipids/sterols, we show that AmB extracts ergosterol but not cholesterol from the bilayers and inserts to a much higher degree in the cholesterol-containing membranes. Ergosterol extraction by AmB is accompanied by membrane thinning. Our results provide new insights into the mechanism and antifungal effect of AmB in these simple models of fungal and mammalian membranes and help understand the molecular origin of its selectivity and toxic side effects.

Bottom Effect in Atomic Force Microscopy Nanomechanics

In this work, the influence of the rigid substrate on the determination of the sample Young's modulus, the so-called bottom-effect artifact, is demonstrated by an atomic force microscopy force-spectroscopy experiment. The nanomechanical properties of a one-component supported lipid membrane (SLM) exhibiting areas of two different thicknesses are studied: While a standard contact mechanics model (Sneddon) provides two different elastic moduli for these two morphologies, it is shown that Garcia's bottom-effect artifact correction yields a unique value, as expected for an intrinsic material property. Remarkably, it is demonstrated that the ratio between the contact radius (and not only the indentation) and the sample thickness is the key parameter addressing the relevance of the bottom-effect artifact. The experimental results are validated by finite element method simulations providing a solid support to Garcia's theory. The amphiphilic nature of the investigated material is representative of several kinds of lipids, suggesting that the results have far reaching implications for determining the correct Young's modulus of SLMs. The generality of Garcia's bottom-effect artifact correction allows its application to every kind of supported soft film.

Information gain from isotopic contrast variation in neutron reflectometry on protein-membrane complex structures

A framework is applied to quantify information gain from neutron or X-ray reflectometry experiments [Treece, Kienzle, Hoogerheide, Majkrzak, Lösche & Heinrich (2019). J. Appl. Cryst. 52, 47-59], in an in-depth investigation into the design of scattering contrast in biological and soft-matter surface architectures. To focus the experimental design on regions of interest, the marginalization of the information gain with respect to a subset of model parameters describing the structure is implemented. Surface architectures of increasing complexity from a simple model system to a protein-lipid membrane complex are simulated. The information gain from virtual surface scattering experiments is quantified as a function of the scattering length density of molecular components of the architecture and the surrounding aqueous bulk solvent. It is concluded that the information gain is mostly determined by the local scattering contrast of a feature of interest with its immediate molecular environment, and experimental design should primarily focus on this region. The overall signal-to-noise ratio of the measured reflectivity modulates the information gain globally and is a second factor to be taken into consideration.

Recent Advances in Hybrid Biomimetic Polymer-Based Films: from Assembly to Applications

Biological membranes, in addition to being a cell boundary, can host a variety of proteins that are involved in different biological functions, including selective nutrient transport, signal transduction, inter- and intra-cellular communication, and cell-cell recognition. Due to their extreme complexity, there has been an increasing interest in developing model membrane systems of controlled properties based on combinations of polymers and different biomacromolecules, i.e., polymer-based hybrid films. In this review, we have highlighted recent advances in the development and applications of hybrid biomimetic planar systems based on different polymeric species. We have focused in particular on hybrid films based on (i) polyelectrolytes, (ii) polymer brushes, as well as (iii) tethers and cushions formed from synthetic polymers, and (iv) block copolymers and their combinations with biomacromolecules, such as lipids, proteins, enzymes, biopolymers, and chosen nanoparticles. In this respect, multiple approaches to the synthesis, characterization, and processing of such hybrid films have been presented. The review has further exemplified their bioengineering, biomedical, and environmental applications, in dependence on the composition and properties of the respective hybrids. We believed that this comprehensive review would be of interest to both the specialists in the field of biomimicry as well as persons entering the field.

Fluorescent Lipo-Beads for the Sensitive Detection of Phospholipase A2 and Its Inhibitors

Phospholipase A₂ (PLA₂) is a membrane lytic enzyme that is present in many organisms. Human PLA₂ has emerged as a potential biomarker as well as a therapeutic target for several diseases including cancer, cardiovascular diseases, and some inflammatory diseases. The current study focuses on the development of lipo-beads that are very reactive and highly sensitive to PLA₂. To develop the best supported lipid bilayer formulation, several lipid combinations were investigated using 10 μm porous silica beads. The reactivity of PLA₂ was monitored via the decrease in particle fluorescence because of the release of entrapped fluorescent dye from the particle pores or the disintegration of a fluorescent lipid constituted on the bilayer upon lipid hydrolysis using flow cytometry. The enzyme binding studies indicate that lipo-beads with bulky fluorescent tags in the lipid head group and anionic lipids produce a more pronounced response. The kinetic studies suggest that these lipo-beads are very reactive with PLA₂ and can generate a detectable signal in less than 5 min. The enzyme inhibition studies were also conducted with two known PLA₂ inhibitors, varespladib and quercetin. We find that quercetin can hydrolyze the supported membrane, and thus inhibition of PLA₂ is not observed; however, varespladib has shown significant PLA₂ inhibition on lipo-beads. We have demonstrated that our lipo-bead-based approach can detect annexin-3, a known disease biomarker, as low as 10 nM within 5 min after incubation.

A Lipid Photoswitch Controls Fluidity in Supported Bilayer Membranes

Supported lipid bilayer (SLB) membranes are key elements to mimic membrane interfaces on a planar surface. Here, we demonstrate that azobenzene photolipids (azo-PC) form fluid, homogeneous SLBs. Diffusion properties of azo-PC within SLBs were probed by fluorescence microscopy and fluorescence recovery after photobleaching. At ambient conditions, we find that the trans-to-cis isomerization causes an increase of the diffusion constant by a factor of two. Simultaneous excitation with two wavelengths and variable intensities furthermore allows to adjust the diffusion constant D continuously. X-ray reflectometry and small-angle scattering measurements reveal that membrane photoisomerization results in a bilayer thickness reduction of ∼0.4 nm (or 10%). While thermally induced back-switching is not observed, we find that the trans bilayer fluidity is increasing with higher temperatures. This change in diffusion constant is accompanied by a red-shift in the absorption spectra. Based on these results, we suggest that the reduced diffusivity of trans-azo-PC is controlled by intermolecular interactions that also give rise to H-aggregate formation in bilayer membranes.

Mechanism and Kinetics of Lipid Bilayer Formation in Solid-State Nanopores

Solid-state nanopores provide a highly versatile platform for rapid electrical detection and analysis of single molecules. Lipid bilayer coating of the nanopores can reduce nonspecific analyte adsorption to the nanopore sidewalls and increase the sensing selectivity by providing possibilities for tethering specific ligands in a cell-membrane mimicking environment. However, the mechanism and kinetics of lipid bilayer formation from vesicles remain unclear in the presence of nanopores. In this work, we used a silicon-based, truncated pyramidal nanopore array as the support for lipid bilayer formation. Lipid bilayer formation in the nanopores was monitored in real time by the change in ionic current through the nanopores. Statistical analysis revealed that a lipid bilayer is formed from the instantaneous rupture of individual vesicle upon adsorption in the nanopores, differing from the generally agreed mechanism that lipid bilayer forms at a high vesicle surface coverage on a planar support. The dependence of the lipid bilayer formation process on the applied bias, vesicle size, and concentration was systematically studied. In addition, the nonfouling properties of the lipid bilayer coated nanopores were demonstrated during long single-stranded DNA translocation through the nanopore array. The findings indicate that the lipid bilayer formation process can be modulated by introducing nanocavities intentionally on the planar surface to create active sites or changing the vesicle size and concentration.

Binding and dynamics of melatonin at the interface of phosphatidylcholine-cholesterol membranes

The characterization of interactions between melatonin, one main ingredient of medicines regulating sleeping rhythms, and basic components of cellular plasma membranes (phospholipids, cholesterol, metal ions and water) is very important to elucidate the main mechanisms for the introduction of melatonin into cells and also to identify its local structure and microscopic dynamics. Molecular dynamics simulations of melatonin inside mixtures of dimyristoylphosphatidylcholine and cholesterol in NaCl solution at physiological concentration have been performed at 303.15 K to systematically explore melatonin-cholesterol, melatonin-lipid and melatonin-water interactions. Properties such as the area per lipid and thickness of the membrane as well as selected radial distribution functions, binding free energies, angular distributions, atomic spectral densities and translational diffusion of melatonin are reported. The presence of cholesterol significantly affects the behavior of melatonin, which is mainly buried into the interfaces of membranes. Introducing cholesterol into the system helps melatonin change from folded to extended configurations more easily. Our results suggest that there exists a competition between the binding of melatonin to phospholipids and to cholesterol by means of hydrogen-bonds. Spectral densities of melatonin reported in this work, in overall good agreement with experimental data, revealed the participation of each atom of melatonin to its complete spectrum. Melatonin self-diffusion coefficients are of the order of 10-7 cm2/s and they significantly increase when cholesterol is addeed to the membrane.

Lipid-based Liquid Crystalline Films and Solutions for the Delivery of Cargo to Cells

A major challenge in the delivery of cargo (genes and/or drugs) to cells using nanostructured vehicles is the ability to safely penetrate plasma membranes by escaping the endosome before degradation, later releasing the payload into the cytoplasm or organelle of interest. Lipids are a class of bio-compatible molecules that self-assemble into a variety of liquid crystalline constructs. Most of these materials can be used to encapsulate drugs, proteins, and nucleic acids to deliver them safely into various cell types. Lipid phases offer a plethora of structures capable of forming complexes with biomolecules, most notably nucleic acids. The physichochemical characteristics of the lipid molecular building blocks, one might say the lipid primary structure, dictates how they collectively interact to assemble into various secondary structures. These include bilayers, lamellar stacks of bilayers, two-dimensional (2D) hexagonal arrays of lipid tubes, and even 3D cubic constructs. The liquid crystalline materials can be present in the form of aqueous suspensions, bulk materials or confined to a film configuration depending on the intended application (e.g. bolus vs surface-based delivery). This work compiles recent findings of different lipid-based liquid crystalline constructs both in films and particles for gene and drug delivery applications. We explore how lipid primary and secondary structures endow liquid crystalline materials with the ability to carry biomolecular cargo and interact with cells.

Influence of high hydrostatic pressure on solid supported DPPC bilayers with hyaluronan in the presence of Ca2+ ions

The molecular mechanisms responsible for outstanding lubrication of natural systems, like articular joints, have been the focus of scientific research for several decades. One essential aspect is the lubrication under pressure, where it is important to understand how the lubricating entities adapt under dynamic working conditions in order to fulfill their function. We made a structural investigation of a model system consisting of two of the molecules present at the cartilage interface, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and hyaluronan, at high hydrostatic pressure. Phospholipid layers are found at the cartilage surfaces and are able to considerably reduce friction. Their behavior under load and varied solution conditions is important as pressures of 180 bar are encountered during daily life activities. We focus on how divalent ions, like Ca2+, affect the interaction between DPPC and hyaluronan, as other investigations have indicated that calcium ions influence their interaction. It could be shown that already low amounts of Ca2+ strongly influence the interaction of hyaluronan with DPPC. Our results suggest that the calcium ions increase the amount of adsorbed hyaluronan indicating an increased electrostatic interaction. Most importantly, we observe a modification of the DPPC phase diagram as hyaluronan absorbs to the bilayer which results in an Lα-like structure at low temperatures and a decoupling of the leaflets forming an asymmetric bilayer structure.

Mechanism of Acid-Triggered Cargo Release from Lipid Bilayer-Coated Mesoporous Silica Particles

Lipid bilayer-coated mesoporous silica nanoparticles are unique core-shell nanomaterials currently being developed as drug delivery vehicles. To improve cargo loading and biocirculation, the pore structure and surface chemistry of the particle have been modified and well characterized. However, an understanding of cargo release mechanisms from cellular uptake pathways remains largely unexplored. Here, we present a study of the release mechanism of lipid bilayer-coated silica particles induced by endosomal-like pH change from 7.4 to 5.0. We found that this relatively small pH change produces rapid deformation of the supported lipid bilayer that ultimately results in holes in the membrane. Using a combination of dye release studies, wide-field and confocal fluorescence microscopies, and surface area modeling analysis, we determined that small blister-like structures are formed, which lead to lateral membrane displacement and hole formation. Possible mechanisms for the blister formation, which include curvature effects and interfacial interactions, are discussed.

Electrodes for Membrane Surface Science. Bilayer Lipid Membranes Tethered by Commercial Surfactants on Electrochemical Sensors

Commercial surfactants, which are inexpensive and abundant, were covalently grafted to flat and transparent electrodes, and it appears to be a simple functionalization route to design biomembrane sensors at large-scale production. Sparsely tethered bilayer lipid membranes (stBLM) were stabilized using such molecular coatings composed of diluted anchor-harpoon surfactants that grab the membrane with an alkyl chain out of a PEGylated-hydrogel layer, which acts as a soft hydration cushion. The goal of avoiding the synthesis of complex organic molecules to scale up sensors was achieved here by grafting nonionic diblock oligomers (Brij58 = C _xH_{2 x+1}(OCH₂CH₂) _nOH with x = 16 and n = 23) and PEO short chains ((OCH₂CH₂) _nOH with n = 9 and n = 23) from their hydroxyl (-OH) end-moiety to a monolayer of -Ar-SO₂Cl groups, which are easy to form on electrodes (metals, semiconducting materials, ...) from aryl-diazonium salt reduction. A hybrid molecular coating on gold, with scarce Ar-SO₂-Brij58 and PEO oligomers, was used to monitor immobilization and fusion kinetics of DOPC small unilamellar vesicles (SUV) by both quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) techniques. Using flat and transparent thin chromium film electrodes, we designed biosensors to couple surface sensitive techniques for membranes, including X-ray reflectivity (XRR), atomic force microscopy (AFM) with subnanometer resolution, and optical microscopy, such as fluorescence recovery after photobleaching measurements (FRAP), in addition to electrochemistry techniques (cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)). The advantages of this biomembrane-sensing platform are discussed for research and applications.

Dynamics of heroin molecule inside the lipid membrane: a molecular dynamics study

Heroin, or diamorphine (C₂₁H₂₃NO₅), is an opium product used for various pharmaceutical and euphoric purposes. In this work, the molecular dynamics simulation study of the heroin inside the two lipid bilayers, dipalmitoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) are presented. The whole study was conducted at three different temperatures. The location of the heroin drug, the nature of the diffusion, rotational correlation function and structural variation inside both lipid bilayers is studied. Moreover, the free energy of the solvation of the drug inside both lipid bilayers is calculated. It is found that during the whole molecular dynamics study, the drug locates at the center of both lipid membranes. The effect of the temperature is not seen at the drug location. The nature of the diffusion of the heroin drug is anomalous. The radius of gyration is calculated to study the structural variations of the heroin molecule inside both lipid bilayers. It is found that the heroin molecule does not change its structure at three temperatures. From the rotational correlation function, it is seen that the drug is more hindered for rotation inside the DPPC lipid bilayer as compared to the DMPC lipid bilayer. It is applicable for all three temperatures. The rotational correlation time of the drug is decreased while the temperature of the system is increased. In the case of DMPC, there is an abrupt change in rotational correlation time while the phase is changed.

Temperature dependent self-organization of DMPC membranes promoted by intermediate amounts of the saponin aescin

The plant-derived biosurfactant aescin is naturally present in many plants and is used for treatment of disorders such as varicose veins and inflammation of veins. The hemolytic activity of this saponin is attributed to its interaction with cholesterol in the red blood cell membrane. This work investigates the phase and aggregation behavior of saponin-containing model membranes consisting of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The aescin concentrations studied range from 1 mol% to 7 mol% with respect to the total lipid content. The methods of choice to elucidate the structural picture are small-angle scattering of X-rays (SAXS) and neutrons (SANS) and cryogenic transmission electron microscopy (cryo-TEM). SANS and SAXS revealed that at lower aescin contents vesicular structures are conserved and vesicles tend to aggregate already at aescin contents of around 1 mol%. Aggregation and vesicle deformation effects are found to be stronger when the phospholipids are in the L [Formula: see text] phase. With increasing aescin content, mixed structures, i.e. aggregated and deformed vesicles and solubilized bilayer fragments, are present. This was proven for a sample with 4 mol% aescin by cryo-TEM. An increasing aescin amount leads to membrane decomposition and free standing bilayers which tend to build stacks at high temperature. These stacks are characterized by SAXS using the modified Caillé theory. Analyses and model dependent fitting reveal formation of well-defined structures beginning at 7 mol% aescin.

Transport of solid bodies along tubular membrane tethers

We study the crucial role of membrane fluctuations in maintaining a narrow gap between a fluid membrane tube and an enclosed solid particle. Solvent flows can occur in this gap, hence giving rise to a finite particle mobility along the tube. While our study has relevance for how cells are able to transport large organelles or other cargo along connecting membrane tubes, known as tunneling nanotubes, our calculations are also framed so that they can be tested by a specific in vitro experiment: A tubular membrane tether can be pulled from a membrane reservoir, such as an aspirated Giant Unilamellar Vesicle (GUV), e.g. using a conjugated bead that binds to the membrane and is held in a laser trap. We compute the subsequent mobility of colloidal particles trapped in the tube, focusing on the case when the particle is large compared to the equilibrium tube radius. We predict that the particle mobility should scale as ∼ σ^−2/3, with σ the membrane tension.

Probing the Interface Structure of Adhering Cells by Contrast Variation Neutron Reflectometry

Cellular adhesion is a central element in tissue mechanics, biological cell-cell signaling, and cell motility. In this context, the cell-substrate distance has been investigated in the past by studying natural cells and biomimetic cell models adhering on solid substrates. The amount of water in the membrane substrate gap, however, is difficult to determine. Here, we present a neutron reflectivity (NR) structural study of confluent epithelial cell monolayers on silicon substrates. In order to ensure valid in vitro conditions, we developed a cell culture sample chamber allowing us to grow and cultivate cells under proper cell culture conditions while performing in vitro neutron reflectivity measurements. The cell chamber also enabled perfusion with cell medium and hence allowed for contrast variation in situ by sterile exchange of buffer with different H₂O-to-D₂O ratio. Contrast variation reduces the ambiguity of data modeling for determining the thickness and degree of hydration of the interfacial cleft between the adherent cells and the substrate. Our data suggest a three-layer interfacial organization. The first layer bound to the silicon surface interface is in agreement with a very dense protein film with a thickness of 9 ± 2 nm, followed by a highly hydrated 24 ± 4 nm thick layer, and a several tens of nanometers thick layer attributed to the composite membrane. Hence, the results provide clear evidence of a highly hydrated intermediate region between the composite cell membrane and the substrate, reminiscent of the basal lamina.

Single Virion Tracking Microscopy for the Study of Virus Entry Processes in Live Cells and Biomimetic Platforms

The most widely-used assays for studying viral entry, including infectivity, cofloatation, and cell-cell fusion assays, yield functional information but provide low resolution of individual entry steps. Structural characterization provides high-resolution conformational information, but on its own is unable to address the functional significance of these conformations. Single virion tracking microscopy techniques provide more detail on the intermediate entry steps than infection assays and more functional information than structural methods, bridging the gap between these methods. In addition, single virion approaches also provide dynamic information about the kinetics of entry processes. This chapter reviews single virion tracking techniques and describes how they can be applied to study specific virus entry steps. These techniques provide information complementary to traditional ensemble approaches. Single virion techniques may either probe virion behavior in live cells or in biomimetic platforms. Synthesizing information from ensemble, structural, and single virion techniques ultimately yields a more complete understanding of the viral entry process than can be achieved by any single method alone.

Bayesian determination of the effect of a deep eutectic solvent on the structure of lipid monolayers

A novel reflectometry analysis method reveals the structure of lipid monolayers at the air-DES interface.

Tunable cell-surface mimetics as engineered cell substrates

Most recent breakthroughs in understanding cell adhesion, cell migration, and cellular mechanosensitivity have been made possible by the development of engineered cell substrates of well-defined surface properties. Traditionally, these substrates mimic the extracellular matrix (ECM) environment by the use of ligand-functionalized polymeric gels of adjustable stiffness. However, such ECM mimetics are limited in their ability to replicate the rich dynamics found at cell-cell contacts. This review focuses on the application of cell surface mimetics, which are better suited for the analysis of cell adhesion, cell migration, and cellular mechanosensitivity across cell-cell interfaces. Functionalized supported lipid bilayer systems were first introduced as biomembrane-mimicking substrates to study processes of adhesion maturation during adhesion of functionalized vesicles (cell-free assay) and plated cells. However, while able to capture adhesion processes, the fluid lipid bilayer of such a relatively simple planar model membrane prevents adhering cells from transducing contractile forces to the underlying solid, making studies of cell migration and cellular mechanosensitivity largely impractical. Therefore, the main focus of this review is on polymer-tethered lipid bilayer architectures as biomembrane-mimicking cell substrate. Unlike supported lipid bilayers, these polymer-lipid composite materials enable the free assembly of linkers into linker clusters at cellular contacts without hindering cell spreading and migration and allow the controlled regulation of mechanical properties, enabling studies of cellular mechanosensitivity. The various polymer-tethered lipid bilayer architectures and their complementary properties as cell substrates are discussed.