Global small-angle X-ray scattering data analysis for multilamellar vesicles: the evolution of the scattering density profile model

Induction of the Interdigitated Gel Phase of Hydrated Dipalmitoylphosphatidylcholine Bilayers by the Artificial Sweetener Sucralose

Recent research indicates that high doses of sucralose content can weaken the immune response in mice. To better understand the interaction between cell membranes and sucralose, we studied model biomembranes composed of dipalmitoylphosphatidylcholine bilayers in a sucralose solution. Calorimetry measurements showed that the effect of sucralose on the phase behavior is biphasic. Pretransitions and main transitions are decreased at low sucralose concentrations, while the main transition is increased at high concentrations. Pretransitions cannot be detected above the concentration at which the direction of change in the main transition temperature reverses. X-ray diffraction measurements revealed that sucralose at concentrations higher than 0.2 M induces the interdigitated gel (LβI) phase below the main transition temperature. Fluorescence Prodan measurements suggested that the sucralose solution is slightly more hydrophobic than the sucrose solution. This could be one reason why sucralose induces the LβI phase.

Modulating a model membrane of sphingomyelin by a tricyclic antidepressant drug

Tricyclic medicine such as amitriptyline (AMT) hydrochloride, initially developed to treat depression, is also used to treat neuropathic pain, anxiety disorder, and migraines. The mechanism of functioning of this type of drugs is ambiguous. Understanding the mechanism is important for designing new drug molecules with higher pharmacological efficiency. Hence, in the present study, biophysical approaches have been taken to shed light on their interactions with a model cellular membrane of brain sphingomyelin in the form of monolayer and multi-lamellar vesicles. The surface pressure-area isotherm infers the partitioning of a drug molecule into the lipid monolayer at the air water interface, providing a higher surface area per molecule and reducing the in-plane elasticity. Further, the surface electrostatic potential of the lipid monolayer is found to increase due to the insertion of drug molecule. The interfacial rheology revealed a reduction of the in-plane viscoelasticity of the lipid film, which, depends on the adsorption of the drug molecule onto the film. Small-angle X-ray scattering (SAXS) measurements on multilamellar vesicles (MLVs) have revealed that the AMT molecules partition into the hydrophobic core of the lipid membrane, modifying the organization of lipids in the membrane. The modified physical state of less rigid membrane and the transformed electrostatics of the membrane could influence its interaction with synaptic vesicles and neurotransmitters making higher availability of the neurotransmitters in the synaptic cleft.

Small-angle X-ray and neutron scattering applied to lipid-based nanoparticles: Recent advancements across different length scales

Lipid-based nanoparticles (LNPs), ranging from nanovesicles to non-lamellar assemblies, have gained significant attention in recent years, as versatile carriers for delivering drugs, vaccines, and nutrients. Small-angle scattering methods, employing X-rays (SAXS) or neutrons (SANS), represent unique tools to unveil structure, dynamics, and interactions of such particles on different length scales, spanning from the nano to the molecular scale. This review explores the state-of-the-art on scattering methods applied to unveil the structure of lipid-based nanoparticles and their interactions with drugs and bioactive molecules, to inform their rational design and formulation for medical applications. We will focus on complementary information accessible with X-rays or neutrons, ranging from insights on the structure and colloidal processes at a nanoscale level (SAXS) to details on the lipid organization and molecular interactions of LNPs (SANS). In addition, we will review new opportunities offered by Time-resolved (TR)-SAXS and -SANS for the investigation of dynamic processes involving LNPs. These span from real-time monitoring of LNPs structural evolution in response to endogenous or external stimuli (TR-SANS), to the investigation of the kinetics of lipid diffusion and exchange upon interaction with biomolecules (TR-SANS). Finally, we will spotlight novel combinations of SAXS and SANS with complementary on-line techniques, recently enabled at Large Scale Facilities for X-rays and neutrons. This emerging technology enables synchronized multi-method investigation, offering exciting opportunities for the simultaneous characterization of the structure and chemical or mechanical properties of LNPs.

A dilute nematic gel produced by intramicellar segregation of two polyoxyethylene alkyl ether carboxylic acids

MOTIVATION

Surfactants like C8E8CH2COOH have such bulky headgroups that they cannot show the common sphere-to-cylinder transition, while surfactants like C18:1E2CH2COOH are mimicking lipids and form only bilayers. Mixing these two types of surfactants allows one to investigate the competition between intramicellar segregation leading to disc-like bicelles and the temperature dependent curvature constraints imposed by the mismatch between heads and tails.

EXPERIMENTS

We establish phase diagrams as a function of temperature, surfactant mole ratio, and active matter content. We locate the isotropic liquid-isotropic liquid phase separation common to all nonionic surfactant systems, as well as nematic and lamellar phases. The stability and rheology of the nematic phase is investigated. Texture determination by polarizing microscopy allows us to distinguish between the different phases. Finally, SANS and SAXS give intermicellar distances as well as micellar sizes and shapes present for different compositions in the phase diagrams.

FINDINGS

In a defined mole ratio between the two components, intramicellar segregation wins and a viscoelastic discotic nematic phase is present at low temperature. Partial intramicellar mixing upon heating leads to disc growth and eventually to a pseudo-lamellar phase. Further heating leads to complete random mixing and an isotropic phase, showing the common liquid-liquid miscibility gap. This uncommon phase sequence, bicelles, lamellar phase, micelles, and water-poor packed micelles, is due to temperature induced mixing combined with dehydration of the headgroups. This general molecular mechanism explains also why a metastable water-poor lamellar phase quenched by cooling can be easily and reproducibly transformed into a nematic phase by gentle hand shaking at room temperature, as well as the entrapment of air bubbles of any size without encapsulation by bilayers or polymers.

Structure of symmetric and asymmetric lipid membranes from joint SAXS/SANS

Small-angle X-ray and neutron scattering (SAXS/SANS) techniques excel in unveiling intricate details of the internal structure of lipid membranes under physiologically relevant temperature and buffer conditions, all without the need to resort to bulky labels. By concurrently conducting and analyzing neutron and X-ray data, these methods harness the complete spectrum of contrast and resolution from various components constituting lipid membranes. Despite this, the literature exhibits only a sparse presence of applications compared to other techniques in membrane biophysics. This chapter serves as a primer for conducting joint SAXS/SANS analyses on symmetric and asymmetric large unilamellar vesicles, elucidating fundamental elements of the analysis process. Specifically, we introduce the basics of interactions of X-rays and neutrons with matter that lead to the scattering contrast and a description of membrane structure in terms of scattering length density profiles. These profiles allow fitting of the experimentally observed scattering intensity. We further integrate practical insights, unveiling strategies for successful data acquisition and providing a comprehensive assessment of the technique's advantages and drawbacks. By amalgamating theoretical underpinnings with practical considerations, this chapter aims to dismantle barriers hindering the adoption of joint SAXS/SANS approaches, thereby encouraging an influx of studies in this domain.

Quantitative Comparison against Experiments Reveals Imperfections in Force Fields' Descriptions of POPC-Cholesterol Interactions

Cholesterol is a central building block in biomembranes, where it induces orientational order, slows diffusion, renders the membrane stiffer, and drives domain formation. Molecular dynamics (MD) simulations have played a crucial role in resolving these effects at the molecular level; yet, it has recently become evident that different MD force fields predict quantitatively different behavior. Although easily neglected, identifying such limitations is increasingly important as the field rapidly progresses toward simulations of complex membranes mimicking the in vivo conditions: pertinent multicomponent simulations must capture accurately the interactions between their fundamental building blocks, such as phospholipids and cholesterol. Here, we define quantitative quality measures for simulations of binary lipid mixtures in membranes against the C-H bond order parameters and lateral diffusion coefficients from NMR spectroscopy as well as the form factors from X-ray scattering. Based on these measures, we perform a systematic evaluation of the ability of commonly used force fields to describe the structure and dynamics of binary mixtures of palmitoyloleoylphosphatidylcholine (POPC) and cholesterol. None of the tested force fields clearly outperforms the others across the tested properties and conditions. Still, the Slipids parameters provide the best overall performance in our tests, especially when dynamic properties are included in the evaluation. The quality evaluation metrics introduced in this work will, particularly, foster future force field development and refinement for multicomponent membranes using automated approaches.

Phenylalanine and Tryptophan-Based Surfactants as New Antibacterial Agents: Characterization, Self-Aggregation Properties, and DPPC/Surfactants Vesicles Formulation

Cationic surfactants based on phenylalanine (C_nPC₃NH₃Cl) and tryptophan (C_nTC₃NH₃Cl) were synthesized using renewable raw materials as starting compounds and a green synthetic procedure. The synthesis, acid-base equilibrium, aggregation properties, and antibacterial activity were investigated. Conductivity and fluorescence were used to establish critical micelle concentrations. Micellization of C_nPC₃NH₃Cl and C_nTC₃NH₃Cl occurred in the ranges of 0.42-16.2 mM and 0.29-4.6 mM, respectively. Since those surfactants have some acidic character, the apparent pK_a was determined through titrations, observing increasing acidity with increasing chain length and being slightly more acidic with the phenylalanine than the tryptophan derivatives. Both families showed promising antibacterial efficacy against eight different bacterial strains. Molecular docking studies against the enzyme peptidoglycan glycosyltransferase (PDB ID:2OQO) were used to investigate the potential binding mechanism of target surfactant molecules. According to small angle X-ray scattering (SAXS) results, the surfactants incorporate into DPPC (Dipalmitoyl Phosphatidyl Choline) bilayers without strong perturbation up to high surfactant concentration. Some of the C₁₂TC₃NH₃Cl/DPPC formulations (40%/60% and 20%/80% molar ratios) exhibited good antibacterial activity, while the others were not effective against the tested bacteria. The strong affinity between DPPC and surfactant molecules, as determined by the DFT (density functional theory) method, could be one of the reasons for the loss of antibacterial activity of these cationic surfactants when they are incorporated in vesicles.

Investigating the cut-off effect of n-alcohols on lipid movement: a biophysical study

Cellular membranes are responsible for absorbing the effects of external perturbants for the cell's survival. Such perturbants include small ubiquitous molecules like n-alcohols which were observed to exhibit anesthetic capabilities, with this effect tapering off at a cut-off alcohol chain length. To explain this cut-off effect and complement prior biochemical studies, we investigated a series of n-alcohols (with carbon lengths 2-18) and their impact on several bilayer properties, including lipid flip-flop, intervesicular exchange, diffusion, membrane bending rigidity and more. To this end, we employed an array of biophysical techniques such as time-resolved small angle neutron scattering (TR-SANS), small angle X-ray scattering (SAXS), all atomistic and coarse-grained molecular dynamics (MD) simulations, and calcein leakage assays. At an alcohol concentration of 30 mol% of the overall lipid content, TR-SANS showed 1-hexanol (C6OH) increased transverse lipid diffusion, i.e. flip-flop. As alcohol chain length increased from C6 to C10 and longer, lipid flip-flop slowed by factors of 5.6 to 32.2. Intervesicular lipid exchange contrasted these results with only a slight cut-off at alcohol concentrations of 30 mol% but not 10 mol%. SAXS, MD simulations, and leakage assays revealed changes to key bilayer properties, such as bilayer thickness and fluidity, that correlate well with the effects on lipid flip-flop rates. Finally, we tie our results to a defect-mediated pathway for alcohol-induced lipid flip-flop.

Ceratonia siliqua L. Pod Extract: From Phytochemical Characterization to Liposomal Formulation and Evaluation of Behaviour in Cells

The formulation of plant extracts in phospholipid vesicles is a promising strategy to exploit their biological properties while solving problems related to poor solubility in water, high instability, and low skin permeation and retention time. In this study, Ceratonia siliqua ripe pods were used for the preparation of a hydro-ethanolic extract, which showed antioxidant properties owing to the presence of biologically active compounds identified by liquid chromatography-mass spectrometry (e.g., hydroxybenzoic acid and flavonoid derivatives). To improve the applicability of the extract in therapy, a topical formulation based on liposomes was explored. The vesicles were characterized by small size (around 100 nm), negative charge (-13 mV), and high entrapment efficiency (>90%). Furthermore, they displayed both spherical and elongated shapes, with oligolamellar structure. Their biocompatibility was demonstrated in cells, including erythrocytes and representative skin cell lines. The antioxidant activity of the extract was proved by the scavenging of free radicals, the reduction of ferric ions, and the protection of skin cells from oxidative damage.

High Resolution Membrane Structures within Hybrid Lipid-Polymer Vesicles Revealed by Combining X-Ray Scattering and Electron Microscopy

Hybrid vesicles consisting of phospholipids and block-copolymers are increasingly finding applications in science and technology. Herein, small angle X-ray scattering (SAXS) and cryo-electron tomography (cryo-ET) are used to obtain detailed structural information about hybrid vesicles with different ratios of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and poly(1,2-butadiene-block-ethylene oxide) (PBd₂₂ -PEO₁₄ , M_s  = 1800 g mol^-1 ). Using single particle analysis (SPA) the authors are able to further interpret the information gained from SAXS and cryo-ET experiments, showing that increasing PBd₂₂ -PEO₁₄ mole fraction increases the membrane thickness from 52 Å for a pure lipid system to 97 Å for pure PBd₂₂ -PEO₁₄ vesicles. Two vesicle populations with different membrane thicknesses in hybrid vesicle samples are found. As these lipids and polymers are reported to homogeneously mix, bistability is inferred between weak and strong interdigitation regimes of PBd₂₂ -PEO₁₄ within the hybrid membranes. It is hypothesized that membranes of intermediate structure are not energetically favorable. Therefore, each vesicle exists in one of these two membrane structures, which are assumed to have comparable free energies. The authors conclude that, by combining biophysical methods, accurate determination of the influence of composition on the structural properties of hybrid membranes is achieved, revealing that two distinct membranes structures can coexist in homogeneously mixed lipid-polymer hybrid vesicles.

Phenolic Fingerprint, Bioactivity and Nanoformulation of Prunus spinosa L. Fruit Extract for Skin Delivery

The nanoformulation of plant extracts in phospholipid vesicles is a promising strategy to exploit the biological properties of natural bioactive substances and overcome drawbacks such as poor aqueous solubility, chemical instability, low skin permeation and retention time, which strongly limit their topical application. In this study, Prunus spinosa berries were used for the preparation of a hydro-ethanolic extract, which showed antioxidant and antibacterial properties owing to the presence of phenolic compounds. Two types of phospholipid vesicles were developed to improve the applicability as topical formulations. Liposomes and Penetration Enhancer-containing Vesicles were characterized for mean diameter, polydispersity, surface charge, shape, lamellarity, and entrapment efficiency. Additionally, their safety was assayed with different cell models, including erythrocytes and representative skin cell lines.

Small Angle X-Ray Scattering Data Analysis and Theoretical Modelling for the Size and Shape Characterization of Drug Delivery Systems Based on Vitamin E TPGS Micelles

We developed a simple two-dimensional/two-components theoretical model that describes the structure and functionality of a VitE-TPGS system of micelles assuming a hydrophobic inner core and an outer hydrated hydrophilic shell. We then conceptually applied the developed methodology to a simple system of VitE-TPGS micelles unloaded and loaded with an active pharmaceutical ingredient, eltrombopag, to verify if the model could reliably monitor the size change of the micelle upon loading. The fit of laboratory Small Angle X-Ray Scattering data against such model allows us to extract absolute values of the micelles size under a spherical shape hypothesis as well as the distribution within the system between components and level of hydration. The intensity scale of the SAXS experimental data needs to be normalized to a reference standard (pure water) to get absolute scattered intensities. The mathematical model which has been developed under a general hypothesis of ellipsoidal micelles, is applied to our experimental data under the simplified spherical assumption, which suitably fits our experimental data.

Computational methods for the analysis of solution small-angle X-ray scattering of biomolecules: ATSAS

The ATSAS software suite provides a comprehensive set of programs for the processing, analysis and modeling of small-angle scattering data, tailored for but not limited to data acquired on biological macromolecules. In this review the major components and developments in the ATSAS package are described, with a focus on user driven application. Data reduction, analysis and modeling approaches and strategies will be introduced and discussed. At the time of writing the latest package, ATSAS 3.1, is freely available for academic users at: https://www.embl-hamburg.de/biosaxs/software.html.

Partial Volumes of Phosphatidylcholines and Vitamin E: α-Tocopherol Prefers Disordered Membranes

Despite its discovery over 95 years ago, the biological and nutritional roles of vitamin E remain subjects of much controversy. Though it is known to possess antioxidant properties, recent assertions have implied that vitamin E may not be limited to this function in living systems. Through densitometry measurements and small-angle X-ray scattering we observe favorable interactions between α-tocopherol and unsaturated phospholipids, with more favorable interactions correlating to an increase in lipid chain unsaturation. Our data provide evidence that vitamin E may preferentially associate with oxygen sensitive lipids─an association that is considered innate for a viable membrane antioxidant.

Conjugation-Free Multilamellar Protein-Lipid Hybrid Vesicles for Multifaceted Immune Responses

Various lipid-based nanocarriers have been developed for the co-delivery of protein antigens with immunological adjuvants. However, their in vivo potency in vaccine delivery is limited by structural instability, which causes off-target delivery and low cross-presentation efficacies. Recent works employ covalent cross-linking to stabilize the lipid nanostructures, though the immunogenicity and side effects of chemically modified protein antigens and lipids can cause a long-lasting safety issue. Here robust "conjugation-free" multilamellar protein antigen-lipid hybrid nanovesicles (MPLVs) are introduced through the antigen-mediated self-assembly of unilamellar lipid vesicles for the co-delivery of protein antigens and immunologic adjuvants. The nanocarriers coated with monophosphoryl lipid A and hyaluronic acids elicit highly increase antigen-specific immune responses in vitro and in vivo. The MPLVs increase the generation of immunological surface markers and cytokines in mouse-derived bone-marrow dendritic cells compared to soluble antigens with adjuvants. Besides, the vaccination of mice with the MPLVs significantly increase the production of anti-antigen antibody and interferon-gamma via the activation of CD4⁺ and CD8⁺ T cells, respectively. These findings suggest that MPLVs can serve as a promising nanovaccine delivery platform for efficient antigen cross-presentation through the efficient co-delivery of protein antigens with adjuvants.

Protein-induced metamorphosis of unilamellar lipid vesicles to multilamellar hybrid vesicles

Protein encapsulation into nanocarriers has been extensively studied to improve the efficacy and stability of therapeutic proteins. However, the chemical modification of proteins or new synthetic carrier materials are essential to achieve a high encapsulation efficiency and structural stability of proteins, which hinders their clinical applications. New strategies to physically incorporate proteins into nanocarriers feasible for clinical uses are required to overcome the current limitation. Here we report the spontaneous protein-induced reorganization of 'pre-formed' unilamellar lipid vesicles to efficiently incorporate proteins within multilamellar protein-lipid hybrid vesicles without chemical modification. Epidermal growth factor (EGF) binds to the surface of cationic unilamellar lipid vesicles and induces layer-by-layer self-assembly of the vesicles. The protein is spontaneously entrapped in the interstitial layers of a multilamellar structure with extremely high loading efficiency, ~99%, through polyionic interactions as predicted by molecular dynamics simulation. The loaded protein exhibits much higher structural, chemical, and biological stability compared to free protein. The method is also successfully applied to several other proteins. This work provides a promising method for the highly efficient encapsulation of therapeutic proteins into multilamellar lipid vesicles without the use of specialized instruments, high energy, coupling agents, or organic solvents.

Restoring structural parameters of lipid mixtures from small-angle X-ray scattering data

Small-angle X-ray scattering (SAXS) is widely utilized to study soluble macromolecules, including those embedded into lipid carriers and delivery systems such as surfactant micelles, phospho-lipid vesicles and bilayered nanodiscs. To adequately describe the scattering from such systems, one needs to account for both the form factor (overall structure) and long-range-order Bragg reflections emerging from the organization of bilayers, which is a non-trivial task. Presently existing methods separate the analysis of lipid mixtures into distinct procedures using form-factor fitting and the fitting of the Bragg peak regions. This article describes a general approach for the computation and analysis of SAXS data from lipid mixtures over the entire angular range of an experiment. The approach allows one to restore the electron density of a lipid bilayer and simultaneously recover the corresponding size distribution and multilamellar organization of the vesicles. The method is implemented in a computer program, LIPMIX, and its performance is demonstrated on an aqueous solution of layered lipid vesicles undergoing an extrusion process. The approach is expected to be useful for the analysis of various types of lipid-based systems, e.g. for the characterization of interactions between target drug molecules and potential carrier/delivery systems.

Increasing complexity in small-angle X-ray and neutron scattering experiments: from biological membrane mimics to live cells

Small-angle X-ray and neutron scattering are well-established, non-invasive experimental techniques to interrogate global structural properties of biological membrane mimicking systems under physiologically relevant conditions. Recent developments, both in bottom-up sample preparation techniques for increasingly complex model systems, and in data analysis techniques have opened the path toward addressing long standing issues of biological membrane remodelling processes. These efforts also include emerging quantitative scattering studies on live cells, thus enabling a bridging of molecular to cellular length scales. Here, we review recent progress in devising compositional models for joint small-angle X-ray and neutron scattering studies on diverse membrane mimics - with a specific focus on membrane structural coupling to amphiphatic peptides and integral proteins - and live Escherichia coli. In particular, we outline the present state-of-the-art in small-angle scattering methods applied to complex membrane systems, highlighting how increasing system complexity must be followed by an advance in compositional modelling and data-analysis tools.

Structural Characterization of Biomaterials by Means of Small Angle X-rays and Neutron Scattering (SAXS and SANS), and Light Scattering Experiments

Scattering techniques represent non-invasive experimental approaches and powerful tools for the investigation of structure and conformation of biomaterial systems in a wide range of distances, ranging from the nanometric to micrometric scale. More specifically, small-angle X-rays and neutron scattering and light scattering techniques represent well-established experimental techniques for the investigation of the structural properties of biomaterials and, through the use of suitable models, they allow to study and mimic various biological systems under physiologically relevant conditions. They provide the ensemble averaged (and then statistically relevant) information under in situ and operando conditions, and represent useful tools complementary to the various traditional imaging techniques that, on the contrary, reveal more local structural information. Together with the classical structure characterization approaches, we introduce the basic concepts that make it possible to examine inter-particles interactions, and to study the growth processes and conformational changes in nanostructures, which have become increasingly relevant for an accurate understanding and prediction of various mechanisms in the fields of biotechnology and nanotechnology. The upgrade of the various scattering techniques, such as the contrast variation or time resolved experiments, offers unique opportunities to study the nano- and mesoscopic structure and their evolution with time in a way not accessible by other techniques. For this reason, highly performant instruments are installed at most of the facility research centers worldwide. These new insights allow to largely ameliorate the control of (chemico-physical and biologic) processes of complex (bio-)materials at the molecular length scales, and open a full potential for the development and engineering of a variety of nano-scale biomaterials for advanced applications.

The antioxidant vitamin E as a membrane raft modulator: Tocopherols do not abolish lipid domains

The antioxidant vitamin E is a commonly used vitamin supplement. Although the multi-billion dollar vitamin and nutritional supplement industry encourages the use of vitamin E, there is very little evidence supporting its actual health benefits. Moreover, vitamin E is now marketed as a lipid raft destabilizing anti-cancer agent, in addition to its antioxidant behaviour. Here, we studied the influence of vitamin E and some of its vitamers on membrane raft stability using phase separating unilamellar lipid vesicles in conjunction with small-angle scattering techniques and fluorescence microscopy. We find that lipid phase behaviour remains unperturbed well beyond physiological concentrations of vitamin E (up to a mole fraction of 0.10). Our results are consistent with a proposed line active role of vitamin E at the domain boundary. We discuss the implications of these findings as they pertain to lipid raft modification in native membranes, and propose a new hypothesis for the antioxidant mechanism of vitamin E.

Molecular Structure of Sphingomyelin in Fluid Phase Bilayers Determined by the Joint Analysis of Small-Angle Neutron and X-ray Scattering Data

We have determined the fluid bilayer structure of palmitoyl sphingomyelin (PSM) and stearoyl sphingomyelin (SSM) by simultaneously analyzing small-angle neutron and X-ray scattering data. Using a newly developed scattering density profile (SDP) model for sphingomyelin lipids, we report structural parameters including the area per lipid, total bilayer thickness, and hydrocarbon thickness, in addition to lipid volumes determined by densitometry. Unconstrained all-atom simulations of PSM bilayers at 55 °C using the C36 CHARMM force field produced a lipid area of 56 Ų, a value that is 10% lower than the one determined experimentally by SDP analysis (61.9 Ų). Furthermore, scattering form factors calculated from the unconstrained simulations were in poor agreement with experimental form factors, even though segmental order parameter (S_CD) profiles calculated from the simulations were in relatively good agreement with S_CD profiles obtained from NMR experiments. Conversely, constrained area simulations at 61.9 Ų resulted in good agreement between the simulation and experimental scattering form factors, but not with S_CD profiles from NMR. We discuss possible reasons for the discrepancies between these two types of data that are frequently used as validation metrics for molecular dynamics force fields.

Deciphering the Peculiar Behavior of β-Lapachone in Lipid Monolayers and Bilayers

β-Lapachone (β-Lap) is a promising anticancer drug whose applications have been limited so far because of its poor solubility and stability. Its encapsulation in liposomes has been proposed to overcome these issues. However, surface pressure measurements show that β-Lap exhibits atypical interfacial behavior when mixed with lipids. Although the drug does not seem to be retained in lipid monolayers as deduced from the π-A isotherms, small changes in compressibility moduli suggest that β-Lap actually interacts with lipids, either disorganizing or rigidifying their monolayers. Thermal and structural analyses of lipid bilayers confirm the existence of β-Lap/lipid interactions and show that the drug inserts between hydrophobic chains, close to the polar headgroup in DPPC bilayers and deeper in the acyl chains in POPC bilayers. Molecular dynamics simulations allow a comprehensive description of the drug position and orientation in DOPC and POPC bilayers in the presence or absence of cholesterol.

A molecular dynamics study proposing the existence of statistical structural heterogeneity due to chain orientation in the POPC-cholesterol bilayer

We performed molecular dynamics simulations of a lipid bilayer consisting of POPC and cholesterol at temperatures from 283 to 308K and cholesterol concentrations from 0 to 50% mol/mol. The purpose of this study was to look for the existence of structural differences in the region delimited by these parameters and, in particular, in a region where coexistence of liquid disordered and liquid ordered phases has been proposed. Our interest in this range of concentration and temperature responds to the fact that polyene ionophore activity varies considerably along it. Two force fields, CHARMM36 and Slipids, were compared in order to determine the most suitable. Both force fields predict non-monotonic behaviors consistent with the existence of phase transitions. We found the presence of lateral structural heterogeneity, statistical in nature, in some of the bilayers occurring in this range of temperatures and sterol concentrations. This heterogeneity was produced by correlated ordering of the POPC tails and not due to cholesterol enrichment, and lasts for tens of nanoseconds. We relate these observations to the action of polyenes in these membranes.

Vesicle Adhesion and Fusion Studied by Small-Angle X-Ray Scattering

We have studied the adhesion state (also denoted by docking state) of lipid vesicles as induced by the divalent ions Ca²⁺ or Mg²⁺ at well-controlled ion concentration, lipid composition, and charge density. The bilayer structure and the interbilayer distance in the docking state were analyzed by small-angle x-ray scattering. A strong adhesion state was observed for DOPC:DOPS vesicles, indicating like-charge attraction resulting from ion correlations. The observed interbilayer separations of ∼1.6 nm agree quantitatively with the predictions of electrostatics in the strong coupling regime. Although this phenomenon was observed when mixing anionic and zwitterionic (or neutral) lipids, pure anionic membranes (DOPS) with highest charge density σ resulted in a direct phase transition to a multilamellar state, which must be accompanied by rupture and fusion of vesicles. To extend the structural assay toward protein-controlled docking and fusion, we have characterized reconstituted N-ethylmaleimide-sensitive factor attachment protein receptors in controlled proteoliposome suspensions by small-angle x-ray scattering.

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.

Flexible lipid nanomaterials studied by NMR spectroscopy

Advances in solid-state nuclear magnetic resonance spectroscopy inform the emergence of material properties from atomistic-level interactions in membrane lipid nanostructures.

Bile Salts Caught in the Act: From Emulsification to Nanostructural Reorganization of Lipid Self-Assemblies

Bile salts (BSs) are important for the digestion and absorption of fats and fat-soluble vitamins in the small intestine. In this work, we scrutinized, with small-angle X-ray scattering (SAXS), the crucial functions of bile salts beyond their capacity for the interfacial stabilization of submicrometer-sized lipid particles. By studying a wide compositional range of BS-lipid dispersions using two widely applied lipids for drug-delivery systems (one a monoglyceride being stabilizer-sensitive and the other an aliphatic alcohol being relatively stabilizer-insensitive), we identified the necessary BS to lipid ratios to guarantee full emulsification. A novel ad hoc developed global small-angle-X-ray scattering analysis method revealed that the addition of BS hardly changes the bilayer thicknesses in bicontinuous phases, while significant membrane thinning is observed in the coexisting fluid lamellar phase. Furthermore, we show that a BS strongly decreases the average critical packing parameter. At increasing BS concentration, the order of phases formed is (i) the bicontinuous diamond cubic ( Pn3 m), (ii) the bicontinuous primitive cubic ( Im3 m), and (iii) the fluid lamellar phase ( L_α). These distinctive findings on BS-driven "emulsification" and "membrane curvature reduction" provide new molecular-scale insights for the understanding of the interfacial action of bile salts on lipid assemblies.

Global Small-Angle X-ray Scattering Data Analysis of Triacylglycerols in the Molten State (Part I)

The study of triacylglycerols (TAGs) in their molten state is of fundamental importance for a deeper understanding of the TAG crystallization processes, being highly relevant for both manufacturing and medical applications. Although different models have been proposed to explain the nanostructured nature of the fluid state of TAGs, none of them are fully satisfactory. In this paper, we propose a new model consisting of positionally uncorrelated lamellar TAG assemblies embedded in an isotropic medium that assist as prenucleating structures. This model was validated by applying a novel global fitting method, resulting in an excellent agreement with the small-angle X-ray scattering data. A deeper analysis of the scattering patterns at different temperatures, both in cooling and heating directions, allowed us further to detect the crystalline traces of TAGs even after heating to 40 °C and record, on cooling, the onset of crystallization at 30-25 °C. The application of the presented novel model not only explains the outstandingly structured fluid of molten TAGs, but also lays the basis for analyzing first the crystallization steps in greater detail, which is outlined in our follow-up paper "Global Small-Angle X-ray Scattering Data Analysis of Triacylglycerols in the α-Phase (Part II)".

High-performance small- and wide-angle X-ray scattering (SAXS/WAXS) experiments on a multi-functional laboratory goniometer platform with easily exchangeable X-ray modules

Small-angle X-ray scattering (SAXS) is a well-established, versatile technique for the analysis of nanoscale structures and dimensions, e.g., in liquid dispersions, thin solid objects or powder samples. When combined with wide-angle X-ray scattering (WAXS), complementary information about the atomic structure can be obtained. SAXS experiments traditionally require dedicated instruments to achieve the desired angular resolution, sensitivity, stability, and speed of measurement. Here we demonstrate how a multi-functional laboratory goniometer platform, as widely being used for powder X-ray diffraction and for a variety of related techniques, can be configured with pre-aligned X-ray modules that enable advanced SAXS/WAXS experiments, without compromising the exceptional versatility of the instrument. Line and point collimation setups, as well as quick and easy switching between them, are readily possible. Key components are a detachable, evacuated beam path and a high-resolution, low-noise hybrid pixel area detector, in combination with a hardware interface design that allows to configure the instrument with different X-ray modules without the need for re-alignment. Software for SAXS data reduction and analysis was developed. The good SAXS/WAXS performance and the derived analytical results were verified on various test samples, such as gold nanoparticles, colloidal silica, liposomes, dilute protein solutions, and solid polymer samples. It is believed that this novel approach to SAXS/WAXS instrumentation will help to make this powerful structure analysis technique more widely accessible and affordable for multi-user laboratories.

Structural Modifications of DPPC Bilayers upon Inclusion of an Antibacterial Cationic Bolaamphiphile

The emergence of antibiotic-resistant bacterial strains has fostered fundamental research to develop alternative antimicrobial strategies. Among the several systems proposed so far, the association complexes (nanoplexes) formed by transcription factor decoys (TFDs), i.e., short oligonucleotides targeting a crucial bacterial transcription factor, and a bolaform cationic amphiphile, 10,10'-(dodecane-1,12-diyl)-bis-(9-amino-1,2,3,4-tetrahydroacridinium) chloride (12-bis-THA), have demonstrated their potential in vitro and in vivo. The application of these nanoplexes is hampered by a scarce colloidal stability, which can be addressed by including the bolaamphiphile in a liposomal carrier, which is then associated to the TFD. The present study reports an investigation on the effects of 12-bis-THA on the structure of synthetic lipid bilayers to assess the morphology of the mixed assemblies, gain insight into the location of the host within the bilayer, and determine the loading capacity of the carrier. Our results demonstrate that 12-bis-THA promptly inserts within 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) bilayers, bending its C-12 spacer chain to adopt a conelike shape and shifting the gel-liquid crystalline transition of the chains to lower temperatures. The host liposomal structure is retained for a bolaamphiphile concentration of up to 3.2% mol to DPPC, whereas higher concentrations lead to the destabilization by means of a detergency-like mechanism, with the simultaneous existence of different lamellar-based structures, such as liposomes, bicelles, and rafts, in which DPPC and 12-bis-THA could be present in different molar ratios. Overall, these results shed light on the interaction of the bolaamphiphile with a lipid bilayer and provide valuable insight to better formulate the antimicrobial amphiphile in liposomal carriers to circumvent the colloidal instability of nanoplexes.

Exploring the biophysical properties of phytosterols in the plasma membrane for novel cancer prevention strategies

Cancer is a global problem with no sign that incidences are reducing. The great costs associated with curing cancer, through developing novel treatments and applying patented therapies, is an increasing burden to developed and developing nations alike. These financial and societal problems will be alleviated by research efforts into prevention, or treatments that utilise off-patent or repurposed agents. Phytosterols are natural components of the diet found in an array of seeds, nuts and vegetables and have been added to several consumer food products for the management of cardio-vascular disease through their ability to lower LDL-cholesterol levels. In this review, we provide a connected view between the fields of structural biophysics and cellular and molecular biology to evaluate the growing evidence that phytosterols impair oncogenic pathways in a range of cancer types. The current state of understanding of how phytosterols alter the biophysical properties of plasma membrane is described, and the potential for phytosterols to be repurposed from cardio-vascular to oncology therapeutics. Through an overview of the types of biophysical and molecular biology experiments that have been performed to date, this review informs the reader of the molecular and biophysical mechanisms through which phytosterols could have anti-cancer properties via their interactions with the plasma cell membrane. We also outline emerging and under-explored areas such as computational modelling, improved biomimetic membranes and ex vivo tissue evaluation. Focus of future research in these areas should improve understanding, not just of phytosterols in cancer cell biology but also to give insights into the interaction between the plasma membrane and the genome. These fields are increasingly providing meaningful biological and clinical data but iterative experiments between molecular biology assays, biosynthetic membrane studies and computational membrane modelling improve and refine our understanding of the role of different sterol components of the plasma membrane.

All n-3 PUFA are not the same: MD simulations reveal differences in membrane organization for EPA, DHA and DPA

Eicosapentaenoic (EPA, 20:5), docosahexaenoic (DHA, 22:6) and docosapentaenoic (DPA, 22:5) acids are omega-3 polyunsaturated fatty acids (n-3 PUFA) obtained from dietary consumption of fish oils that potentially alleviate the symptoms of a range of chronic diseases. We focus here on the plasma membrane as a site of action and investigate how they affect molecular organization when taken up into a phospholipid. All atom MD simulations were performed to compare 1-stearoyl-2-eicosapentaenoylphosphatylcholine (EPA-PC, 18:0-20:5PC), 1-stearoyl-2-docosahexaenoylphosphatylcholine (DHA-PC, 18:0-22:6PC), 1-stearoyl-2-docosapentaenoylphosphatylcholine (DPA-PC, 18:0-22:5PC) and, as a monounsaturated control, 1-stearoyl-2-oleoylphosphatidylcholine (OA-PC, 18:0-18:1PC) bilayers. They were run in the absence and presence of 20mol% cholesterol. Multiple double bonds confer high disorder on all three n-3 PUFA. The different number of double bonds and chain length for each n-3 PUFA moderates the reduction in membrane order exerted (compared to OA-PC, S¯CD=0.152). EPA-PC (S¯CD=0.131) is most disordered, while DPA-PC (S¯CD=0.140) is least disordered. DHA-PC (S¯CD=0.139) is, within uncertainty, the same as DPA-PC. Following the addition of cholesterol, order in EPA-PC (S¯CD=0.169), DHA-PC (S¯CD=0.178) and DPA-PC (S¯CD=0.182) is increased less than in OA-PC (S¯CD=0.214). The high disorder of n-3 PUFA is responsible, preventing the n-3 PUFA-containing phospholipids from packing as close to the rigid sterol as the monounsaturated control. Our findings establish that EPA, DHA and DPA are not equivalent in their interactions within membranes, which possibly contributes to differences in clinical efficacy.

Impact of the biomolecular corona on the structure of PEGylated liposomes

Driven by the promises of gene therapy, PEGylated cationic liposomes (CLs) have been investigated for decades, but their use in the clinical setting is far from established. Such a dichotomy is due to several factors that have been ignored over the last two decades. The hardest challenge seems to occur when PEGylated CLs come into contact with a physiological environment (e.g. the blood). Recent evidence has demonstrated that PEGylation does not completely prevent protein binding (as believed so far), but a biomolecular shell, termed "biomolecular corona" (BC), covers the liposome surface. Here we show that the formation of a BC not only affects the surface properties of PEGylated CLs, but also, and significantly, their bilayer structure thus impairing their ability to safely deliver their cargo to the target site. Therefore, a mechanistic understanding of the structures emerging from liposome-protein interactions may represent a truly new paradigm for the clinical translation of PEGylated CLs.

Reconstitution of SNARE proteins into solid-supported lipid bilayer stacks and X-ray structure analysis

SNAREs are known as an important family of proteins mediating vesicle fusion. For various biophysical studies, they have been reconstituted into supported single bilayers via proteoliposome adsorption and rupture. In this study we extended this method to the reconstitution of SNAREs into supported multilamellar lipid membranes, i.e. oriented multibilayer stacks, as an ideal model system for X-ray structure analysis (X-ray reflectivity and diffraction). The reconstitution was implemented through a pathway of proteomicelle, proteoliposome and multibilayer. To monitor the structural evolution in each step, we used small-angle X-ray scattering for the proteomicelles and proteoliposomes, followed by X-ray reflectivity and grazing-incidence small-angle scattering for the multibilayers. Results show that SNAREs can be successfully reconstituted into supported multibilayers, with high enough orientational alignment for the application of surface sensitive X-ray characterizations. Based on this protocol, we then investigated the effect of SNAREs on the structure and phase diagram of the lipid membranes. Beyond this application, this reconstitution protocol could also be useful for X-ray analysis of many further membrane proteins.

In vivo analysis of the Escherichia coli ultrastructure by small-angle scattering

The flagellated Gram-negative bacterium Escherichia coli is one of the most studied microorganisms. Despite extensive studies as a model prokaryotic cell, the ultrastructure of the cell envelope at the nanometre scale has not been fully elucidated. Here, a detailed structural analysis of the bacterium using a combination of small-angle X-ray and neutron scattering (SAXS and SANS, respectively) and ultra-SAXS (USAXS) methods is presented. A multiscale structural model has been derived by incorporating well established concepts in soft-matter science such as a core-shell colloid for the cell body, a multilayer membrane for the cell wall and self-avoiding polymer chains for the flagella. The structure of the cell envelope was resolved by constraining the model by five different contrasts from SAXS, and SANS at three contrast match points and full contrast. This allowed the determination of the membrane electron-density profile and the inter-membrane distances on a quantitative scale. The combination of USAXS and SAXS covers size scales from micrometres down to nanometres, enabling the structural elucidation of cells from the overall geometry down to organelles, thereby providing a powerful method for a non-invasive investigation of the ultrastructure. This approach may be applied for probing in vivo the effect of detergents, antibiotics and antimicrobial peptides on the bacterial cell wall.

Complex biomembrane mimetics on the sub-nanometer scale

Biomimetic lipid vesicles are indispensable tools for gaining insight into the biophysics of cell physiology on the molecular level. The level of complexity of these model systems has steadily increased, and now spans from domain-forming lipid mixtures to asymmetric lipid bilayers. Here, we review recent progress in the development and application of elastic neutron and X-ray scattering techniques for studying these systems in situ and under physiologically relevant conditions on the nanometer to sub-nanometer length scales. In particular, we focus on: (1) structural details of coexisting liquid-ordered and liquid-disordered domains, including their thickness and lipid packing mismatch as a function of a size transition from nanoscopic to macroscopic domains; (2) membrane-mediated protein partitioning into lipid domains; (3) the role of the aqueous medium in tuning interactions between membranes and domains; and (4) leaflet-specific structure in asymmetric bilayers and passive lipid flip-flop.

High-resolution structure of coexisting nanoscopic and microscopic lipid domains

We studied coexisting micro- and nanoscopic liquid-ordered/liquid-disordered domains in fully hydrated multilamellar vesicles using small-angle X-ray scattering. Large domains exhibited long-range out-of-plane positional correlations of like domains, consistent with previous reports. In contrast, such correlations were absent in nanoscopic domains. Advancing a global analysis of the in situ data allowed us to gain a deep insight into the structural and elastic properties of the coexisting domains, including the partitioning of cholesterol in each domain. In agreement with a previous report, we found that the thickness mismatch between ordered and disordered domains decreased for nanoscopic domains. At the same time, we found also the lipid packing mismatch to be decreased for nano-domains, mainly due to the liquid-disordered domains becoming more densely packed when decreasing their size.

Depletion with Cyclodextrin Reveals Two Populations of Cholesterol in Model Lipid Membranes

Recent results provide evidence that cholesterol is highly accessible for removal from both cell and model membranes above a threshold concentration that varies with membrane composition. Here we measured the rate at which methyl-β-cyclodextrin depletes cholesterol from a supported lipid bilayer as a function of cholesterol mole fraction. We formed supported bilayers from two-component mixtures of cholesterol and a PC (phosphatidylcholine) lipid, and we directly visualized the rate of decrease in area of the bilayers with fluorescence microscopy. Our technique yields the accessibility of cholesterol over a wide range of concentrations (30-66 mol %) for many individual bilayers, enabling fast acquisition of replicate data. We found that the bilayers contain two populations of cholesterol, one with low surface accessibility and the other with high accessibility. A larger fraction of the total membrane cholesterol appears in the more accessible population when the acyl chains of the PC-lipid tails are more unsaturated. Our findings are most consistent with the predictions of the condensed-complex and cholesterol bilayer domain models of cholesterol-phospholipid interactions in lipid membranes.

Modulation of Elasticity and Interactions in Charged Lipid Multibilayers: Monovalent Salt Solutions

We have studied the electrostatic screening effect of NaCl solutions on the interactions between anionic lipid bilayers in the fluid lamellar phase using a Poisson-Boltzmann-based mean-field approach with constant charge and constant potential limiting charge regulation boundary conditions. The full DLVO potential, including the electrostatic, hydration and van der Waals interactions, was coupled to thermal bending fluctuations of the membranes via a variational Gaussian Ansatz. This allowed us to analyze the coupling between the osmotic pressure and the fluctuation amplitudes and compare them both simultaneously with their measured dependence on the bilayer separation, determined by the small-angle X-ray scattering experiments. High-structural resolution analysis of the scattering data revealed no significant changes of membrane structure as a function of salt concentration. Parsimonious description of our results is consistent with the constant charge limit of the general charge regulation phenomenology, with fully dissociated lipid charge groups, together with a 6-fold reduction of the membranes' bending rigidity upon increasing NaCl concentration.

Effect of quercetin and resveratrol co-incorporated in liposomes against inflammatory/oxidative response associated with skin cancer

The present investigation reports the development of liposomes for the co-delivery of naturally occurring polyphenols, namely quercetin and resveratrol. Small, spherical, uni/bilamellar vesicles were produced, as demonstrated by light scattering, cryo-TEM, SAXS. The incorporation of quercetin and resveratrol in liposomes did not affect their intrinsic antioxidant activity, as DPPH radical was almost completely inhibited. The cellular uptake of the polyphenols was higher when they were formulated in liposomes, and especially when co-loaded rather than as single agents, which resulted in a superior ability to scavenge ROS in fibroblasts. The in vivo efficacy of the polyphenols in liposomes was assessed in a mouse model of skin lesion. The topical administration of liposomes led to a remarkable amelioration of the tissue damage, with a significant reduction of oedema and leukocyte infiltration. Therefore, the proposed approach based on polyphenol vesicular formulation may be of value in the treatment of inflammation/oxidative stress associated with pre-cancerous/cancerous skin lesions.

Membrane remodeling and mechanics: Experiments and simulations of α-Synuclein

We review experimental and simulation approaches that have been used to determine curvature generation and remodeling of lipid bilayers by membrane-bending proteins. Particular emphasis is placed on the complementary approaches used to study α-Synuclein (αSyn), a major protein involved in Parkinson's disease (PD). Recent cellular and biophysical experiments have shown that the protein 1) deforms the native structure of mitochondrial and model membranes; and 2) inhibits vesicular fusion. Today's advanced experimental and computational technology has made it possible to quantify these protein-induced changes in membrane shape and material properties. Collectively, experiments, theory and multi-scale simulation techniques have established the key physical determinants of membrane remodeling and rigidity: protein binding energy, protein partition depth, protein density, and membrane tension. Despite the exciting and significant progress made in recent years in these areas, challenges remain in connecting biophysical insights to the cellular processes that lead to disease. This article is part of a Special Issue entitled: Membrane Proteins edited by J.C. Gumbart and Sergei Noskov.

Santosomes as natural and efficient carriers for the improvement of phycocyanin reepithelising ability in vitro and in vivo

New biocarriers, named santosomes, were formulated using Santolina insularis essential oil and hydrogenated phosphatidylcholine. They were modified by adding propylene glycol, a hydrophylic penetration enhancer, and loaded with phycocyanin, a protein found in cyanobacteria, which possesses antioxidant and antiinflammatory properties. The essential oil was expected to modify the bilayer structure and improve the delivery and efficacy of the protein due to a synergistic effect of the phospholipid and S. insularis terpenes. Santosomes were small in size (∼118nm), unilamellar and with polyhedral shape. SAXS patterns showed that phycocyanin strongly interacted with the polar heads of the vesicle bilayer. Phycocyanin-loaded vesicles did not show any toxic effect in vitro: cell viability was ∼100% in endothelial cells and ∼120% in keratinocytes, at all the concentrations tested. In addition, phycocyanin-loaded vesicles protected the cells against free radical damage. In vivo studies were performed to evaluate the ability of santosomes to inhibit chemically-induced oedema and inflammation in mice. Results demonstrated that the application of phycocyanin-loaded santosomes produced an evident amelioration of the skin lesion, confirming their great potential for wound healing.

Atomistic resolution structure and dynamics of lipid bilayers in simulations and experiments

Accurate details on the sampled atomistic resolution structures of lipid bilayers can be experimentally obtained by measuring C-H bond order parameters, spin relaxation rates and scattering form factors. These parameters can be also directly calculated from the classical atomistic resolution molecular dynamics simulations (MD) and compared to the experimentally achieved results. This comparison measures the simulation model quality with respect to 'reality'. If agreement is sufficient, the simulation model gives an atomistic structural interpretation of the acquired experimental data. Significant advance of MD models is made by jointly interpreting different experiments using the same structural model. Here we focus on phosphatidylcholine lipid bilayers, which out of all model membranes have been studied mostly by experiments and simulations, leading to the largest available dataset. From the applied comparisons we conclude that the acyl chain region structure and rotational dynamics are generally well described in simulation models. Also changes with temperature, dehydration and cholesterol concentration are qualitatively correctly reproduced. However, the quality of the underlying atomistic resolution structural changes is uncertain. Even worse, when focusing on the lipid bilayer properties at the interfacial region, e.g. glycerol backbone and choline structures, and cation binding, many simulation models produce an inaccurate description of experimental data. Thus extreme care must be applied when simulations are applied to understand phenomena where the interfacial region plays a significant role. This work is done by the NMRlipids Open Collaboration project running at https://nmrlipids.blogspot.fi and https://github.com/NMRLipids. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

On scattered waves and lipid domains: detecting membrane rafts with X-rays and neutrons

In order to understand the biological role of lipids in cell membranes, it is necessary to determine the mesoscopic structure of well-defined model membrane systems. Neutron and X-ray scattering are non-invasive, probe-free techniques that have been used extensively in such systems to probe length scales ranging from angstroms to microns, and dynamics occurring over picosecond to millisecond time scales. Recent developments in the area of phase separated lipid systems mimicking membrane rafts will be presented, and the underlying concepts of the different scattering techniques used to study them will be discussed in detail.

In situ determination of structure and fluctuations of coexisting fluid membrane domains

Biophysical understanding of membrane domains requires accurate knowledge of their structural details and elasticity. We report on a global small angle x-ray scattering data analysis technique for coexisting liquid-ordered (L_o) and liquid-disordered (L_d) domains in fully hydrated multilamellar vesicles. This enabled their detailed analysis for differences in membrane thickness, area per lipid, hydrocarbon chain length, and bending fluctuation as demonstrated for two ternary mixtures (DOPC/DSPC/CHOL and DOPC/DPPC/CHOL) at different cholesterol concentrations. L_o domains were found to be ∼10 Å thicker, and laterally up to 20 Å²/lipid more condensed than L_d domains. Their bending fluctuations were also reduced by ∼65%. Increase of cholesterol concentration caused significant changes in structural properties of L_d, while its influence on L_o properties was marginal. We further observed that temperature-induced melting of L_o domains is associated with a diffusion of cholesterol to L_d domains and controlled by L_o/L_d thickness differences.

The component group structure of DPPC bilayers obtained by specular neutron reflectometry

Specular neutron reflectometry was measured on a floating bilayer system consisting of 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine deposited over a 1,2-dibehenoyl-sn-glycero-3-phosphocholine bilayer at 25 and 55 °C. The internal structure of lipid bilayers was described by a one-dimensional neutron scattering length density profile model, originally developed for the evaluation of small-angle scattering data. The reflectivity data from the supported bilayer were evaluated separately and used further as constraints in modeling the floating bilayer reflectivity curves. The model reflectivity curves successfully describe the experimental reflectivities of the supported bilayer in the gel phase and the floating bilayer system in the liquid-crystalline phase. The results yield an internal structure of a deposited bilayer and a floating bilayer on the level of component groups of lipid molecules. The obtained structure of the floating d62-diC16:0PC bilayer displays high resemblance of the bilayer structure in the form of unilamellar vesicles. At the same time, however, the results show differences in comparison to unilamellar vesicle bilayers, most likely due to the undulations of supported bilayers.