Neutron reflection from supported lipid membranes

HIV and influenza fusion peptide interactions with (dis)ordered lipid bilayers: Understanding mechanisms and implications for antimicrobial and antiviral approaches

The interactions of viral fusion peptides from influenza (E4K and Ac-E4K) and human immunodeficiency virus (gp41 and Ac-gp41) with planar lipid bilayers and monolayers was investigated herein. A combination of surface-sensitive techniques, including quartz crystal microbalance with dissipation (QCM-D), Langmuir-Blodgett area-pressure isotherms with Micro-Brewster angle microscopy, and neutron reflectometry, was employed. Differences in the interactions of the viral fusion peptides with lipid bilayers featuring ordered and disordered phases, as well as lipid rafts, were revealed. The HIV fusion peptide (gp41) exhibited strong binding to the DOPC/DOPS bilayer, comprising a liquid disordered phase, with neutron reflectometry (NR) showing interaction with the bilayer's headgroup area. Conversely, negligible binding was observed with lipid bilayers in a liquid ordered phase. Notably, the influenza peptide (E4K) demonstrated slower binding kinetics with DOPC/DOPS bilayers and distinct interactions compared to gp41, as observed through QCM-D. This suggests different mechanisms of interaction with the lipid bilayers: one peptide interacts more within the headgroup region, while the other is more involved in transmembrane interactions. These findings hold implications for understanding viral fusion mechanisms and developing antimicrobials and antivirals targeting membrane interactions. The differential binding behaviours of the viral fusion peptides underscore the importance of considering membrane composition and properties in therapeutic strategy design.

Influence of adhesion-promoting glycolipids on the structure and stability of solid-supported lipid double-bilayers

Glycolipids have a considerable influence on the interaction between adjacent biomembranes and can promote membrane adhesion trough favorable sugar-sugar "bonds" even at low glycolipid fractions. Here, in order to obtain structural insights into this phenomenon, we utilize neutron reflectometry in combination with a floating lipid bilayer architecture that brings two glycolipid-loaded lipid bilayers to close proximity. We find that selected glycolipids with di-, or oligosaccharide headgroups affect the inter-bilayer water layer thickness and appear to contribute to the stability of the double-bilayer architecture by promoting adhesion of adjacent bilayers even against induced electrostatic repulsion. However, we do not observe any redistribution of glycolipids that would maximize the density of sugar-sugar contacts. Our results point towards possible strategies for the investigation of interactions between cell surfaces involving specific protein-protein, lipid-lipid, or protein-lipid binding.

Biophysical studies of lipid nanodomains using different physical characterization techniques

For the past 50 years, evidence for the existence of functional lipid domains has been steadily accumulating. Although the notion of functional lipid domains, also known as "lipid rafts," is now widely accepted, this was not always the case. This ambiguity surrounding lipid domains could be partly attributed to the fact that they are highly dynamic, nanoscopic structures. Since most commonly used techniques are sensitive to microscale structural features, it is therefore, not surprising that it took some time to reach a consensus regarding their existence. In this review article, we will discuss studies that have used techniques that are inherently sensitive to nanoscopic structural features (i.e., neutron scatting, nuclear magnetic resonance, and Förster resonance energy transfer). We will also mention techniques that may be of use in the future (i.e., cryoelectron microscopy, droplet interface bilayers, inelastic x-ray scattering, and neutron reflectometry), which can further our understanding of the different and unique physicochemical properties of nanoscopic lipid domains.

Cation and buffer specific effects on the DNA-lipid interaction

Knowledge of DNA - lipid layer interactions is key for the development of biosensors, synthetic nanopores, scaffolds, and gene-delivery systems. These interactions are strongly affected by the ionic composition of the solvent. We have combined quartz crystal microbalance (QCM) and ellipsometry measurements to reveal how pH, buffers and alkali metal chloride salts affect the interaction of DNA with lipid bilayers (DOTAP/DOPC 30:70 in moles). We found that the thickness of the DNA layer adsorbed onto the lipid bilayer decreased in the order citrate > phosphate > Tris > HEPES. The effect of cations on the thickness of the DNA layer decreased in the order (K⁺ > Na⁺ > Cs⁺ ∼ Li⁺). Rationalization of the experimental results requires that adsorption, due to cation specific charge screening, is driven by the simultaneous action of two mechanisms namely, the law of matching water affinities for kosmotropes (Li⁺) and ion dispersion forces for chaotropes (Cs⁺). The outcome of these two opposing mechanisms is a ^"bell-shaped" specific cations sequence. Moreover, a superimposed buffer specificity, which goes beyond the simple effect of pH regulation, further modulated cation specificity. In summary, DNA-lipid bilayer interactions are maximized if citrate buffer (50 mM, pH 7.4) and KCl (100 mM) are used.

Solid and Liquid Surface-Supported Bacterial Membrane Mimetics as a Platform for the Functional and Structural Studies of Antimicrobials

Increasing antibiotic resistance has provoked the urgent need to investigate the interactions of antimicrobials with bacterial membranes. The reasons for emerging antibiotic resistance and innovations in novel therapeutic approaches are highly relevant to the mechanistic interactions between antibiotics and membranes. Due to the dynamic nature, complex compositions, and small sizes of native bacterial membranes, bacterial membrane mimetics have been developed to allow for the in vitro examination of structures, properties, dynamics, and interactions. In this review, three types of model membranes are discussed: monolayers, supported lipid bilayers, and supported asymmetric bilayers; this review highlights their advantages and constraints. From monolayers to asymmetric bilayers, biomimetic bacterial membranes replicate various properties of real bacterial membranes. The typical synthetic methods for fabricating each model membrane are introduced. Depending on the properties of lipids and their biological relevance, various lipid compositions have been used to mimic bacterial membranes. For example, mixtures of phosphatidylethanolamines (PE), phosphatidylglycerols (PG), and cardiolipins (CL) at various molar ratios have been used, approaching actual lipid compositions of Gram-positive bacterial membranes and inner membranes of Gram-negative bacteria. Asymmetric lipid bilayers can be fabricated on solid supports to emulate Gram-negative bacterial outer membranes. To probe the properties of the model bacterial membranes and interactions with antimicrobials, three common characterization techniques, including quartz crystal microbalance with dissipation (QCM-D), surface plasmon resonance (SPR), and neutron reflectometry (NR) are detailed in this review article. Finally, we provide examples showing that the combination of bacterial membrane models and characterization techniques is capable of providing crucial information in the design of new antimicrobials that combat bacterial resistance.

anaklasis: a compact software package for model-based analysis of specular neutron and X-ray reflectometry data sets

A new software package (anaklasis) for model-based analysis of specular neutron and X-ray reflectivity is introduced. Key features include a user-friendly compact interfacial model definition scheme and a complete set of methods for co-refining data and estimating parameter uncertainty.

anaklasis constitutes a set of open-source Python scripts that facilitate a range of specular neutron and X-ray reflectivity calculations, involving the generation of theoretical curves and the comparison/fitting of interfacial model reflectivity against experimental data sets. The primary focus of the software is twofold: on one hand to offer a more natural framework for model definition, requiring minimum coding literacy, and on the other hand to include advanced analysis methods that have been proposed in recent work. Particular attention is given to the ability to co-refine reflectivity data and to the estimation of model-parameter uncertainty and covariance using bootstrap analysis and Markov chain Monte Carlo sampling. The compactness and simplicity of model definition together with the streamlined analysis do not present a steep learning curve for the user, an aspect that may accelerate the generation of reproducible, easily readable and statistically accurate reports in future neutron and X-ray reflectivity related literature.

Lipid bilayer degradation induced by SARS-CoV-2 spike protein as revealed by neutron reflectometry

SARS-CoV-2 spike proteins are responsible for the membrane fusion event, which allows the virus to enter the host cell and cause infection. This process starts with the binding of the spike extramembrane domain to the angiotensin-converting enzyme 2 (ACE2), a membrane receptor highly abundant in the lungs. In this study, the extramembrane domain of SARS-CoV-2 Spike (sSpike) was injected on model membranes formed by supported lipid bilayers in presence and absence of the soluble part of receptor ACE2 (sACE2), and the structural features were studied at sub-nanometer level by neutron reflection. In all cases the presence of the protein produced a remarkable degradation of the lipid bilayer. Indeed, both for membranes from synthetic and natural lipids, a significant reduction of the surface coverage was observed. Quartz crystal microbalance measurements showed that lipid extraction starts immediately after sSpike protein injection. All measurements indicate that the presence of proteins induces the removal of membrane lipids, both in the presence and in the absence of ACE2, suggesting that sSpike molecules strongly associate with lipids, and strip them away from the bilayer, via a non-specific interaction. A cooperative effect of sACE2 and sSpike on lipid extraction was also observed.

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.

Charge Effects Provide Ångström-Level Control of Lipid Bilayer Morphology on Titanium Dioxide Surfaces

Interfaces between molecular organic architectures and oxidic substrates are a central feature of biosensors and applications of biomimetics in science and technology. For phospholipid bilayers, the large range of pH- and ionic strength-dependent surface charge densities adopted by titanium dioxide and other oxidic surfaces leads to a rich landscape of phenomena that provides exquisite control of membrane interactions with such substrates. Using neutron reflectometry measurements, we report sharp, reversible transitions that occur between closely surface-associated and weakly coupled states. We show that these states arise from a complex interplay of the tunable length scale of electrostatic interactions with the length scale arising from other forces that are independent of solution conditions. A generalized free energy potential, with its inputs only derived from established measurements of surface and bilayer properties, quantitatively describes these and previously reported observations concerning the unbinding of bilayers from supporting substrates.

Biophysical analysis of the plant-specific GIPC sphingolipids reveals multiple modes of membrane regulation

The plant plasma membrane (PM) is an essential barrier between the cell and the external environment, controlling signal perception and transmission. It consists of an asymmetrical lipid bilayer made up of three different lipid classes: sphingolipids, sterols, and phospholipids. The glycosyl inositol phosphoryl ceramides (GIPCs), representing up to 40% of total sphingolipids, are assumed to be almost exclusively in the outer leaflet of the PM. However, their biological role and properties are poorly defined. In this study, we investigated the role of GIPCs in membrane organization. Because GIPCs are not commercially available, we developed a protocol to extract and isolate GIPC-enriched fractions from eudicots (cauliflower and tobacco) and monocots (leek and rice). Lipidomic analysis confirmed the presence of trihydroxylated long chain bases and 2-hydroxylated very long-chain fatty acids up to 26 carbon atoms. The glycan head groups of the GIPCs from monocots and dicots were analyzed by gas chromatograph-mass spectrometry, revealing different sugar moieties. Multiple biophysics tools, namely Langmuir monolayer, ζ-Potential, light scattering, neutron reflectivity, solid state 2H-NMR, and molecular modeling, were used to investigate the physical properties of the GIPCs, as well as their interaction with free and conjugated phytosterols. We showed that GIPCs increase the thickness and electronegativity of model membranes, interact differentially with the different phytosterols species, and regulate the gel-to-fluid phase transition during temperature variations. These results unveil the multiple roles played by GIPCs in the plant PM.

Structural insights into fusion mechanisms of small extracellular vesicles with model plasma membranes

Extracellular vesicles (EVs) are a potent intercellular communication system. Such small vesicles transport biomolecules between cells and throughout the body, strongly influencing the fate of recipient cells. Due to their specific biological functions they have been proposed as biomarkers for various diseases and as optimal candidates for therapeutic applications. Despite their extreme biological relevance, their mechanisms of interaction with the membranes of recipient cells are still hotly debated. Here, we propose a multiscale investigation based on atomic force microscopy, small angle X-ray scattering, small angle neutron scattering and neutron reflectometry to reveal structure-function correlations of purified EVs in interaction with model membrane systems of variable complex compositions and to spot the role of different membrane phases on the vesicle internalization routes. Our analysis reveals strong interactions of EVs with the model membranes and preferentially with the borders of protruding phase domains. Moreover, we found that upon vesicle breaking on the model membrane surface, the biomolecules carried by/on EVs diffuse with different kinetics rates, in a process distinct from simple fusion. The biophysical platform proposed here has clear implications on the modulation of EV internalization routes by targeting specific domains at the plasma cell membrane and, as a consequence, on EV-based therapies.

Substrate-dependent arrangements of the subunits of the BAM complex determined by neutron reflectometry

In Gram-negative bacteria, the β-barrel assembly machinery (BAM) complex catalyses the assembly of β-barrel proteins into the outer membrane, and is composed of five subunits: BamA, BamB, BamC, BamD and BamE. Once assembled, - β-barrel proteins can be involved in various functions including uptake of nutrients, export of toxins and mediating host-pathogen interactions, but the precise mechanism by which these ubiquitous and often essential β-barrel proteins are assembled is yet to be established. In order to determine the relative positions of BAM subunits in the membrane environment we reconstituted each subunit into a biomimetic membrane, characterizing their interaction and structural changes by Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) and neutron reflectometry. Our results suggested that the binding of BamE, or a BamDE dimer, to BamA induced conformational changes in the polypeptide transported-associated (POTRA) domains of BamA, but that BamB or BamD alone did not promote any such changes. As monitored by neutron reflectometry, addition of an unfolded substrate protein extended the length of POTRA domains further away from the membrane interface as part of the mechanism whereby the substrate protein was folded into the membrane.

Understanding the Structure and Dynamics of Complex Biomembrane Interactions by Neutron Scattering Techniques

The membrane is one of the key structural materials of biology at the cellular level. Composed predominantly of a bilayer of lipids with embedded and bound proteins, it defines the boundaries of the cell and many organelles essential to life and therefore is involved in almost all biological processes. Membrane-specific interactions, such as drug binding to a membrane receptor or the interactions of an antimicrobial compound with the lipid matrix of a pathogen membrane, are of interest across the scientific disciplines. Herein we present a review, aimed at nonexperts, of the major neutron scattering techniques used in membrane studies: small-angle neutron scattering, neutron membrane diffraction, neutron reflectometry, quasielastic neutron scattering, and neutron spin echo. Neutron scattering techniques are well suited to studying biological membranes. The nondestructive nature of cold neutrons means that samples can be measured for long periods without fear of beam damage from ultraviolet, electron, or X-ray radiation, and neutron beams are highly penetrating, thus offering flexibility in samples and sample environments. Most important is the strong difference in neutron scattering lengths between the two most abundant forms of hydrogen, protium and deuterium. Changing the relative amounts of protium/deuterium in a sample allows the production of a series of neutron scattering data sets, enabling the observation of differing components within complex membrane architectures. This approach can be as simple as using the naturally occurring neutron contrast between different biomolecules to study components in a complex by changing the solution H₂O/D₂O ratio or as complex as selectively labeling individual components with hydrogen isotopes. This review presents an overview of each experimental technique with the neutron instrument configuration, related sample preparation and sample environment, and data analysis, highlighted by a special emphasis on using prominent neutron contrast to understand structure and dynamics. This review gives researchers a practical introduction to the often enigmatic suite of neutron beamlines, thereby lowering the barrier to taking advantage of these large-facility techniques to achieve new understandings of membranes and their interactions with other molecules.

Solid-supported lipid bilayers - A versatile tool for the structural and functional characterization of membrane proteins

The cellular membrane is central to the development of single-and multicellular life, as it separates the delicate cellular interior from the hostile environment. It exerts tight control over entry and exit of substances, is responsible for signaling with other cells in multicellular organisms and prevents pathogens from entering the cell. In the case of bacteria and viruses, the cellular membrane also hosts the proteins enabling invasion of the host organism. In a very real sense therefore, the cellular membrane is central to all life. The study of the cell membrane and membrane proteins in particular has therefore attracted significant attention. Due to the enormous variety of tasks performed by the membrane, it is a highly complex and challenging structure to study. Ideally, membrane components would be studied in isolation from this environment, but unlike water soluble proteins, the amphiphilic environment provided by the cellular membrane is key to the structure and function of the cell membrane. Therefore, model membranes have been developed to provide an environment in which a membrane protein can be studied. This review presents a set of tools that enable the comprehensive characterization of membrane proteins: electrochemical tools, surface plasmon resonance, neutron scattering, the surface forces apparatus and atomic force microscopy are discussed, with a particular focus on experimental technique and data evaluation.

The Location of the Antimicrobial Peptide Maculatin 1.1 in Model Bacterial Membranes

Maculatin 1.1 (Mac1) is an antimicrobial peptide (AMP) from the skin secretions of Australian tree frogs. In this work, the interaction of Mac1 with anionic phospholipid bilayers was investigated by NMR, circular dichroism (CD) spectroscopy, neutron reflectometry (NR) and molecular dynamics (MD). In buffer, the peptide is unstructured but in the presence of anionic (DPC/LMPG) micelles or (DMPC/DMPG/DHPC) bicelles adopts a helical structure. Addition of the soluble paramagnetic agent gadolinium (Gd-DTPA) into the Mac1-DPC/LMPG micelle solution showed that the N-terminus is more exposed to the hydrophilic Gd-DTPA than the C-terminus in micelles. ²H and ³¹P solid-state NMR showed that Mac1 had a greater effect on the anionic lipid (DMPG). A deuterium labeled Mac1 used in NR experiments indicated that the AMP spanned across anionic (PC/PG) bilayers, which was compatible with MD simulations. Simulations also showed that Mac1 orientation remained transmembrane in bilayers and wrapped on the surface of the micelles regardless of the lipid or detergent charge. Thus, the peptide orientation appears to be more susceptible to curvature than charged surface. These results support the formation of transmembrane pores by Mac1 in model bacterial membranes.

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.

Design and use of model membranes to study biomolecular interactions using complementary surface-sensitive techniques

Cellular membranes are complex structures and simplified analogues in the form of model membranes or biomembranes are used as platforms to understand fundamental properties of the membrane itself as well as interactions with various biomolecules such as drugs, peptides and proteins. Model membranes at the air-liquid and solid-liquid interfaces can be studied using a range of complementary surface-sensitive techniques to give a detailed picture of both the structure and physicochemical properties of the membrane and its resulting interactions. In this review, we will present the main planar model membranes used in the field to date with a focus on monolayers at the air-liquid interface, supported lipid bilayers at the solid-liquid interface and advanced membrane models such as tethered and floating membranes. We will then briefly present the principles as well as the main type of information on molecular interactions at model membranes accessible using a Langmuir trough, quartz crystal microbalance with dissipation monitoring, ellipsometry, atomic force microscopy, Brewster angle microscopy, Infrared spectroscopy, and neutron and X-ray reflectometry. A consistent example for following biomolecular interactions at model membranes is used across many of the techniques in terms of the well-studied antimicrobial peptide Melittin. The overall objective is to establish an understanding of the information accessible from each technique, their respective advantages and limitations, and their complementarity.

Deposition of Cellulose Nanocrystals onto Supported Lipid Membranes

The deposition of cellulose nanocrystals (CNCs) on a supported lipid bilayer (SLB) was investigated at different length scales. Quartz crystal microbalance with dissipation monitoring (QCM-D) was used to probe the bilayer formation and to show for the first time the CNC deposition onto the SLB. Specifically, classical QCM-D measurements gave estimation of the adsorbed hydrated mass and the corresponding film thickness, whereas complementary experiments using D₂O as the solvent allowed the quantitative determination of the hydration of the CNC layer, showing a high hydration value. Scanning force microscopy (SFM) and total internal reflection fluorescence microscopy (TIRF) were used to probe the homogeneity of the deposited layers, revealing the structural details at the particle and film length scales, respectively, thus giving information on the effect of CNC concentration on the surface coverage. The results showed that the adsorption of CNCs on the supported lipid membrane depended on lipid composition, CNC concentration, and pH conditions, and that the binding process was governed by electrostatic interactions. Under suitable conditions, a uniform film was formed, with thickness corresponding to a CNC monolayer, which provides the basis for a relevant 2D model of a primary plant cell wall.

An introduction to neutron reflectometry

Specular neutron reflectivity is a neutron diffraction technique that provides information about the structure of surfaces or thin films. It enables the measurement of the neutron scattering length density profile perpendicular to the plane of a surface or an interface, and thereby gives access to the profile of the chemical composition of the film. The wave-particle duality allows to describe neutrons as waves; at an interface between two media of different refractive indexes, neutrons are partially reflected and refracted by the interface. Interferences can occur between waves reflected at the top and at the bottom of a thin film at an interface, which gives rise to interference fringes in the reflectivity profile directly related to its thickness. The characteristic sizes that can be probed range from 5Å to 2000 Å. Neutron-matter interaction directly occurs between neutron and the atom nuclei, which enable to tune the contrast by isotopic substitution. This makes it particularly interesting in the fields of soft matter and biophysics. This course is composed of two parts describing respectively its principle and the experimental aspects of the method (instruments, samples). Examples of applications of neutron reflectometry in the biological domain are presented by Y. Gerelli in the book section “Applications of neutron reflectometry in biology”.

Protocol for Investigating the Interactions Between Intrinsically Disordered Proteins and Membranes by Neutron Reflectometry

Several intrinsically disordered proteins (IDPs) exhibit high affinity for lipid membranes. Among the different biophysical methods to probe protein-lipid interaction, neutron reflectometry (NR) can provide direct and structural detailed information on the location of the IDP with respect to the membrane. Supported lipid bilayers are commonly used as cell membrane models in such experiments. NR measurements can be collected on the supported lipid bilayer before and after the interaction with the IDP to characterize whether the protein molecules are mainly located on the membrane surface (interaction with the lipid headgroups), are penetrating into the hydrophobic region of the membrane (interaction with the lipid acyl chains), or are not interacting at all with the membrane. The lipid composition of the supported lipid bilayer can easily be tuned; hence the NR experiments can be designed to investigate selective IDP-lipid interactions.This chapter will describe the fundamental steps for performing an NR experiment and the subsequent data analysis aimed at characterizing IDP-lipid bilayer interactions. The specific case of an intrinsically disordered region (IDR) from the membrane protein Na⁺/H⁺ exchanger isoform 1 (NHE1) will be used as an example, but the same protocol can be easily adapted to other IDPs.

Self-Assembled Fluid Phase Floating Membranes with Tunable Water Interlayers

We present a reliable method for the fabrication of fluid phase, unsaturated lipid bilayers by self-assembly onto charged Self-Assembled Monolayer (SAM) surfaces with tunable membrane to surface aqueous interlayers. Initially, the formation of water interlayers between membranes and charged surfaces was characterized using a comparative series of bilayers deposited onto charged, self-assembled monolayers by sequential layer deposition. Using neutron reflectometry, a bilayer to surface water interlayer of ∼8 Å was found between the zwitterionic phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membrane and an anionic carboxyl terminated grafted SAM with the formation of this layer attributed to bilayer repulsion by hydration water on the SAM surface. Furthermore, we found we could significantly reduce the technical complexity of sample fabrication through self-assembly of planar membranes onto the SAM coated surfaces. Vesicle fusion onto carboxyl-terminated monolayers yielded high coverage (>95%) bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) which floated on a 7-11 Å solution interlayer between the membrane and the surface. The surface to membrane distance was then tuned via the addition of 200 mM NaCl to the bulk solution immersing a POPC floating membrane, which caused the water interlayer to swell reversibly to ∼33 Å. This study reveals that biomimetic membrane models can be readily self-assembled from solution onto functionalized surfaces without the use of polymer supports or tethers. Once assembled, surface to membrane distance can be tailored to the experimental requirements using physiological concentrations of electrolytes. These planar bilayers only very weakly interact with the substrate and are ideally suited for use as biomimetic models for accurate in vitro biochemical and biophysical studies, as well as for technological applications, such as biosensors.

The Role of Temperature and Lipid Charge on Intake/Uptake of Cationic Gold Nanoparticles into Lipid Bilayers

Understanding the molecular mechanisms governing nanoparticle-membrane interactions is of prime importance for drug delivery and biomedical applications. Neutron reflectometry (NR) experiments are combined with atomistic and coarse-grained molecular dynamics (MD) simulations to study the interaction between cationic gold nanoparticles (AuNPs) and model lipid membranes composed of a mixture of zwitterionic di-stearoyl-phosphatidylcholine (DSPC) and anionic di-stearoyl-phosphatidylglycerol (DSPG). MD simulations show that the interaction between AuNPs and a pure DSPC lipid bilayer is modulated by a free energy barrier. This can be overcome by increasing temperature, which promotes an irreversible AuNP incorporation into the lipid bilayer. NR experiments confirm the encapsulation of the AuNPs within the lipid bilayer at temperatures around 55 °C. In contrast, the AuNP adsorption is weak and impaired by heating for a DSPC-DSPG (3:1) lipid bilayer. These results demonstrate that both the lipid charge and the temperature play pivotal roles in AuNP-membrane interactions. Furthermore, NR experiments indicate that the (negative) DSPG lipids are associated with lipid extraction upon AuNP adsorption, which is confirmed by coarse-grained MD simulations as a lipid-crawling effect driving further AuNP aggregation. Overall, the obtained detailed molecular view of the interaction mechanisms sheds light on AuNP incorporation and membrane destabilization.

Model-independent recovery of interfacial structure from multi-contrast neutron reflectivity data

An indirect Fourier transform/simulated annealing method exploits the information content of multiple solvent contrast neutron reflectivity data and permits the model-independent recovery of interfacial structure at the air/liquid and solid/liquid interface.

Neutron specular reflectivity at soft interfaces provides sub-nanometre information concerning the molecular distribution of thin films, while the application of contrast variation can highlight the scattering from different parts of the system and lead to an overall reduction in fitting ambiguity. Traditional modelling approaches involve the construction of a trial scattering length density profile based on initial speculation and the subsequent refinement of its parameters through minimization of the discrepancy between the calculated and measured reflectivity. In practice this might produce an artificial bias towards specific sets of solutions. On the other hand, direct inversion of reflectivity data, despite its ability to provide a unique solution, is subject to limitations and experimental complications. Presented here is an integrated indirect Fourier transform/simulated annealing method that, when applied to multiple solvent contrast reflectivity data and within the limits of finite spatial resolution, leads to reliable reconstructions of the interfacial structure without the need for any a priori assumptions. The generality of the method permits its straightforward application in common experimental contrast-variation investigations at the solid/liquid and air/liquid interface.

Interactions of NIPAM nanogels with model lipid multi-bilayers: A neutron reflectivity study

In dermal drug delivery, the influence of the chemical structure of the carriers on their penetration mechanisms is not yet fully understood. This is a key requirement in order to design highly efficient delivery systems. In this study, neutron reflectivity is used to provide insights into the interactions between thermoresponsive N-isopropylacrylamide based nanogels, cross-linked with 10%, 20% and 30% N,N'-methylenebisacrylamide, and skin lipid multi-bilayers models. Ceramide lipid multi-bilayers and ceramide/cholesterol/behenic acid mixed lipid multi-bilayers were used for this work. The results indicated that in both multi-bilayers the lipids were depleted by the nanogels mainly through hydrophobic interactions. The ability of nanogels to associate with skin lipids to form water-dispersible complexes was found to be a function of the percentage cross-linker. An enhanced depletion of lipids was further observed in the presence of benzyl alcohol, a well-known skin penetration enhancer.

Following the Morphological Disruption by an Electrolyte of a Buried Interface

A challenge of broad interest in both materials science and biology is the study of interfaces that are buried within a structure, particularly multilayer structures. Despite the enormous costs of corrosion and many decades of corrosion research, details of the mechanisms of various sorts of corrosion are still not clear, in part due to the difficulty in interrogating the interface between the corroding metal and an organic coating, which is typically used to mitigate corrosion. Generally, the performance of such coatings is evaluated by visual inspection after exposure or by modeling impedance data, which is a process not straightforwardly connected to physical interface structures. "Rocking-curve" X-ray scattering measurements provide a means of probing such interfaces due to the ability of X-rays to penetrate materials. Here, variations in the morphology of an interface between a protective coating and a metal substrate due to exposure to an electrolyte are derived from analysis of rocking-curve data in conjunction with atomic force microscopy imaging of the outer coating surface. The interfaces of cross-linked epoxy coatings with aluminum are irreversibly changed after 12 h of contact between the electrolyte solution and the face of the coating. The character of this change varies with the molecule used to cross-link the coating. Since X-ray off-specular scattering is sensitive to changes on the nanometer scale, it is also able to register interface degradation on time scales shorter than those probed by many other techniques, potentially expediting the evaluation of coatings for protection against degradation of the interface.

Structural Investigations of Protein-Lipid Complexes Using Neutron Scattering

Neutron scattering has significant benefits for examining the structure of protein-lipid complexes. Cold (slow) neutrons are nondamaging and predominantly interact with the atomic nucleus, meaning that neutron beams can penetrate deeply into samples, which allows for flexibility in the design of samples studied. Most importantly, there is a strong difference in neutron scattering length (i.e., scattering power) between protium ([Formula: see text], 99.98% natural abundance) and deuterium ([Formula: see text] or D, 0.015%). Through the mixing of H₂O and D₂O in the samples and in some cases the deuterium labeling of the biomolecules, components within a complex can be hidden or enhanced in the scattering signal. This enables both the overall structure and the relative distribution of components within a complex to be resolved. Lipid-protein complexes are most commonly studied using neutron reflectometry (NR) and small angle neutron scattering (SANS). In this review the methodologies to produce and examine a variety of model biological membrane systems using SANS and NR are detailed. These systems include supported lipid bilayers derived from vesicle dispersions or Langmuir-Blodgett deposition, tethered bilayer systems, membrane protein-lipid complexes and polymer wrapped lipid nanodiscs. The three key stages of any SANS/NR study on model membrane systems-sample preparation, data collection, and analysis-are described together with some background on the techniques themselves.

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.

Structure and Composition of Native Membrane Derived Polymer-Supported Lipid Bilayers

Over the last two decades, supported lipid bilayers (SLBs) have been extensively used as model systems to study cell membrane structure and function. While SLBs have been traditionally produced from simple lipid mixtures, there has been a recent surge in compositional complexity to better mimic cellular membranes and thereby bridge the gap between classic biophysical approaches and cell experiments. To this end, native cellular membrane derived SLBs (nSLBs) have emerged as a new category of SLBs. As a new type of biomimetic material, an analytical workflow must be designed to characterize its molecular composition and structure. Herein, we demonstrate how a combination of fluorescence microscopy, neutron reflectometry, and secondary ion mass spectrometry offers new insights on structure, composition, and quality of nSLB systems formed using so-called hybrid vesicles, which are a mixture of native membrane material and synthetic lipids. With this approach, we demonstrate that the nSLB formed a continuous structure with complete mixing of the synthetic and native membrane components and a molecular stoichiometry that essentially mirrors that of the hybrid vesicles. Furthermore, structural investigation of the nSLB revealed that PEGylated lipids do not significantly thicken the hydration layer between the bilayer and substrate when on silicon substrates; however, nSLBs do have more topology than their simpler, purely synthetic counterparts. Beyond new insights regarding the structure and composition of nSLB systems, this work also serves to guide future researchers in producing and characterizing nSLBs from their cellular membrane of choice.

Towards biomimics of cell membranes: Structural effect of phosphatidylinositol triphosphate (PIP3) on a lipid bilayer

Phosphoinositide (PIP) lipids are anionic phospholipids playing a fundamental role for the activity of several transmembrane and soluble proteins. Among all, phosphoinositol-3',4',5'-trisphosphate (PIP₃) is a secondary signaling messenger that regulates the function of proteins involved in cell growth and gene transcription. The present study aims to reveal the structure of PIP-containing lipid membranes, which so far has been little explored. For this purpose, supported lipid bilayers (SLBs) containing 1,2-dioleoyl-sn-glycero-3-phospho-(1'-myo-inositol-3',4',5'-trisphosphate (DOPIP₃) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were used as mimics of biomembranes. Surface sensitive techniques, i.e. Quartz Crystal Microbalance with Dissipation monitoring (QCM-D), Atomic Force Microscopy (AFM) and Neutron Reflectometry (NR), provided detailed information on the formation of the SLB and the location of DOPIP₃ in the lipid membrane. Specifically, QCM-D and AFM were used to identify the best condition for lipid deposition and to estimate the total bilayer thickness. On the other hand, NR was used to collect experimental structural data on the DOPIP₃ location and orientation within the lipid membrane. The two bilayer leaflets showed the same DOPIP₃ concentration, thus suggesting the formation of a symmetric bilayer. The headgroup layer thicknesses of the pure POPC and the mixed POPC/DOPIP₃ bilayer suggest that the DOPIP₃-headgroups have a preferred orientation, which is not perpendicular to the membrane surface, but instead it is close to the surrounding lipid headgroups. These results support the proposed PIP₃ tendency to interact with the other lipid headgroups as PC, so far exclusively suggested by MD simulations.

Neutron reflectivity for soft matter

Specular neutron reflectivity is a technique enabling the measurement of coherent neutron scattering length density profile perpendicular to the plane of a surface or interface, and thereby the profile of chemical composition. The characteristic sizes that are probed range from around 5Å up 5000 Å. It is a scattering technique that averages information over the entire surface and it is therefore not possible to obtain information on correlations in the plane of the interface. The specific properties of neutrons (possibility of tuning the contrast by isotopic substitution, negligible absorption, low energy of the incident neutrons) makes it particularly interesting in the fields of soft matter and biophysics. This course is composed of three parts describing respectively its principle, the experimental aspects (diffractometers, samples), and some scientific examples of neutron reflectometry focusing on the use of contrast variation to probe polymeric systems.

When a transmembrane channel isn't, or how biophysics and biochemistry (mis)communicate

Annexins are a family of soluble proteins that bind to acidic phospholipids such as phosphatidylserine in a calcium-dependent manner. The archetypical member of the annexin family is annexin A5. For many years, its function remained unknown despite the availability of a high-resolution structure. This, combined with the observations of specific ion conductance in annexin-bound membranes, fueled speculations about the possible membrane-spanning forms of annexins that functioned as ion channels. The channel hypothesis remained controversial and did not gather sufficient evidence to become accepted. Yet, it continues to draw attention as a framework for interpreting indirect (e.g., biochemical) data. The goal of the mini-review is to examine the data on annexin-lipid interactions from the last ~30 years from the point of view of the controversy between the two lines of inquiry: the well-characterized peripheral assembly of the annexins at membranes vs. their putative transmembrane insertion. In particular, the potential role of lipid rearrangements induced by annexin binding is highlighted.

Coupling neutron reflectivity with cell-free protein synthesis to probe membrane protein structure in supported bilayers

The structure of the p7 viroporin, an oligomeric membrane protein ion channel involved in the assembly and release of the hepatitis C virus, was determined from proteins expressed and inserted directly into supported model lipid membranes using cell-free protein expression. Cell-free protein expression allowed (i ) high protein concentration in the membrane, (ii ) control of the protein's isotopic constitution, and (iii ) control over the lipid environment available to the protein. Here, we used cell-free protein synthesis to directly incorporate the hepatitis C virus (HCV) p7 protein into supported lipid bilayers formed from physiologically relevant lipids (POPC or asolectin) for both direct structural measurements using neutron reflectivity (NR) and conductance measurements using electrical impedance spectroscopy (EIS). We report that HCV p7 from genotype 1a strain H77 adopts a conical shape within lipid bilayers and forms a viroporin upon oligomerization, confirmed by EIS conductance measurements. This combination of techniques represents a novel approach to the study of membrane proteins and, through the use of selective deuteration of particular amino acids to enhance neutron scattering contrast, has the promise to become a powerful tool for characterizing the protein conformation in physiologically relevant environments and for the development of biosensor applications.

Structural characterization of membrane-bound human immunodeficiency virus-1 Gag matrix with neutron reflectometry

The structural characterization of peripheral membrane proteins represents a tremendous challenge in structural biology due to their transient interaction with the membrane and the potential multitude of protein conformations during this interaction. Neutron reflectometry is uniquely suited to address this problem because of its ability to structurally characterize biological model systems nondestructively and under biomimetic conditions that retain full protein functionality. Being sensitive to only the membrane-bound fraction of a water-soluble peripheral protein, neutron reflectometry obtains a low-resolution average structure of the protein-membrane complex that is further refined using integrative modeling strategies. Here, the authors review the current technological state of biological neutron reflectometry exemplified by a detailed report on the structure determination of the myristoylated human immunodeficiency virus-1 (HIV-1) Gag matrix associated with phosphoserine-containing model membranes. The authors found that the HIV-1 Gag matrix is able to adopt different configurations at the membrane in a pH-dependent manner and that the myristate group orients the protein in a way that is conducive to PIP2-binding.

Lipid phase behavior studied with a quartz crystal microbalance: A technique for biophysical studies with applications in screening

Quartz crystal microbalance (QCM) is emerging as a versatile tool for studying lipid phase behavior. The technique is attractive for fundamental biophysical studies as well applications because of its simplicity, flexibility, and ability to work with very small amounts of material crucial for biomedical studies. Further progress hinges on the understanding of the mechanism, by which a surface-acoustic technique such as QCM, senses lipid phase changes. Here, we use a custom-built instrument with improved sensitivity to investigate phase behavior in solid-supported lipid systems of different geometries (adsorbed liposomes and bilayers). We show that we can detect a model anesthetic (ethanol) through its effect on the lipid phase behavior. Further, through the analysis of the overtone dependence of the phase transition parameters, we show that hydrodynamic effects are important in the case of adsorbed liposomes, and viscoelasticity is significant in supported bilayers, while layer thickness changes make up the strongest contribution in both systems.

Neutron Reflectometry reveals the interaction between functionalized SPIONs and the surface of lipid bilayers

The safe application of nanotechnology devices in biomedicine requires fundamental understanding on how they interact with and affect the different components of biological systems. In this respect, the cellular membrane, the cell envelope, certainly represents an important target or barrier for nanosystems. Here we report on the interaction between functionalized SuperParamagnetic Iron Oxide Nanoparticles (SPIONs), promising contrast agents for Magnetic Resonance Imaging (MRI), and lipid bilayers that mimic the plasma membrane. Neutron Reflectometry, supported by Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) experiments, was used to characterize this interaction by varying both SPION coating and lipid bilayer composition. In particular, the interaction of two different SPIONs, functionalized with a cationic surfactant and a zwitterionic phospholipid, and lipid bilayers, containing different amount of cholesterol, were compared. The obtained results were further validated by Dynamic Light Scattering (DLS) measurements and Cryogenic Transmission Electron Microscopy (Cryo-TEM) images. None of the investigated functionalized SPIONs were found to disrupt the lipid membrane. However, in all case we observed the attachment of the functionalized SPIONs onto the surface of the bilayers, which was affected by the bilayer rigidity, i.e. the cholesterol concentration.

Nanostructural determination of a lipid bilayer tethered to a gold substrate

Tethered lipid bilayer membranes (tBLM) are planar membranes composed of free lipids and molecules tethered to a solid planar substrate providing a useful model of biological membranes for a wide range of biophysical studies and biotechnological applications. The properties of the tBLM depend on the free lipids and on the chemistry of the tethering molecules. We present a nanoscale characterization of a tBLM composed of deuterated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (d-DMPC) free lipids, benzyl disulfide undecaethylene glycol phytanol (DLP) tethering molecules, and benzyl disulfiide tetraethylene glycol polar spacer molecules (PSM) used to control the areal density of tethering molecules through coadsorption. The use of selected isotopic substitution provides a way to distinguish the conformation and location of the tethered lipids from the free lipids and to elucidate how the two components influence the structure of the tBLM. These findings provide useful information to optimise the insertion of transmembrane proteins into the tethered bilayer system.

On the interpretation of reflectivity data from lipid bilayers in terms of molecular-dynamics models

Neutron and X-ray reflectivity of model membranes is increasingly used as a tool for the study of membrane structures and dynamics. As the systems under study become more complex, and as long, all-atom molecular-dynamics (MD) simulations of membranes become more available, there is increasing interest in the use of MD simulations in the analysis of reflectometry data from membranes. In order to perform this, it is necessary to produce a model of the complete interface, including not only the MD-derived structure of the membrane, but also the supporting substrate and any other interfacial layers that may be present. Here, it is shown that this is best performed by first producing a model of the occupied volume across the entire interface, and then converting this into a scattering length density (SLD) profile, rather than by splicing together the separate SLD profiles from the substrate layers and the membrane, since the latter approach can lead to discontinuities in the SLD profile and subsequent artefacts in the reflectivity calculation. It is also shown how the MD-derived membrane structure should be corrected to account for lower than optimal coverage and out-of-plane membrane fluctuations. Finally, the method of including the entire membrane structure in the reflectivity calculation is compared with an alternative approach in which the membrane components are approximated by functional forms, with only the component volumes being extracted from the simulation. It is shown that using only the fragment volumes is insufficient for a typical neutron data set of a single deuteration measured at several water contrasts, and that either weighting the model by including more structural information from the fit, or a larger data set involving a range of deuterations, are required to satisfactorily define the problem.

Direct comparison of elastic incoherent neutron scattering experiments with molecular dynamics simulations of DMPC phase transitions

Neutron scattering techniques have been employed to investigate 1,2-dimyristoyl-sn -glycero-3-phosphocholine (DMPC) membranes in the form of multilamellar vesicles (MLVs) and deposited, stacked multilamellar-bilayers (MLBs), covering transitions from the gel to the liquid phase. Neutron diffraction was used to characterise the samples in terms of transition temperatures, whereas elastic incoherent neutron scattering (EINS) demonstrates that the dynamics on the sub-macromolecular length-scale and pico- to nano-second time-scale are correlated with the structural transitions through a discontinuity in the observed elastic intensities and the derived mean square displacements. Molecular dynamics simulations have been performed in parallel focussing on the length-, time- and temperature-scales of the neutron experiments. They correctly reproduce the structural features of the main gel-liquid phase transition. Particular emphasis is placed on the dynamical amplitudes derived from experiment and simulations. Two methods are used to analyse the experimental data and mean square displacements. They agree within a factor of 2 irrespective of the probed time-scale, i.e. the instrument utilized. Mean square displacements computed from simulations show a comparable level of agreement with the experimental values, albeit, the best match with the two methods varies for the two instruments. Consequently, experiments and simulations together give a consistent picture of the structural and dynamical aspects of the main lipid transition and provide a basis for future, theoretical modelling of dynamics and phase behaviour in membranes. The need for more detailed analytical models is pointed out by the remaining variation of the dynamical amplitudes derived in two different ways from experiments on the one hand and simulations on the other.

Small Angle X-ray and Neutron Scattering: Powerful Tools for Studying the Structure of Drug-Loaded Liposomes

Nanovectors, such as liposomes, micelles and lipid nanoparticles, are recognized as efficient platforms for delivering therapeutic agents, especially those with low solubility in water. Besides being safe and non-toxic, drug carriers with improved performance should meet the requirements of (i) appropriate size and shape and (ii) cargo upload/release with unmodified properties. Structural issues are of primary importance to control the mechanism of action of loaded vectors. Overall properties, such as mean diameter and surface charge, can be obtained using bench instruments (Dynamic Light Scattering and Zeta potential). However, techniques with higher space and time resolution are needed for in-depth structural characterization. Small-angle X-ray (SAXS) and neutron (SANS) scattering techniques provide information at the nanoscale and have therefore been largely used to investigate nanovectors loaded with drugs or other biologically relevant molecules. Here we revise recent applications of these complementary scattering techniques in the field of drug delivery in pharmaceutics and medicine with a focus to liposomal carriers. In particular, we highlight those aspects that can be more commonly accessed by the interested users.

Synthesis of Perdeuterated 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine ([D82 ]POPC) and Characterisation of Its Lipid Bilayer Membrane Structure by Neutron Reflectometry

1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), an unsaturated acyl chain containing lipid, is often the predominant lipid in eukaryotic cell membranes in which it is crucial for the fluidity of membranes under physiological conditions. Commercially available, partially deuterated [D₃₁ ]1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine ([D₃₁ ]POPC) does not provide sufficient isotopic contrast for detailed structural studies of multicomponent membranes through neutron techniques. Herein, a relatively straightforward and generic chemical deuteration method is discussed for the asymmetric synthesis of perdeuterated [D₃₁ ]1-palmitoyl-[D₃₃ ]2-oleoyl-sn-[D₅ ]glycero-[D₁₃ ]3-phosphocholine ([D₈₂ ]POPC) that also allows selective deuteration of any of its constituent groups. Neutron reflectivity of a [D₈₂ ]POPC-supported bilayer was used to experimentally determine the neutron scattering length density profile of the lipid. The acyl chains of [D₈₂ ]POPC are closely contrast-matched to heavy water, whereas the very high scattering length density of the deuterated glycerophosphocholine head groups provides good contrast to membrane-binding agents in both deuterated and non-deuterated solvent environments.