Liquid-liquid domains in bilayers detected by wide angle X-ray scattering

Water Hydrogen-Bond Mediated Layer by Layer Alignment of Lipid Rafts as a Precursor of Intermembrane Processes

Lipid rafts, which are dynamic nanodomains in the plasma membrane, play a crucial role in intermembrane processes by clustering together and growing in size within the plane of the membrane while also aligning with each other across different membranes. However, the physical origin of layer by layer alignment of lipid rafts remains to be elucidated. Here, by using fluorescence imaging and synchrotron X-ray reflectivity in a phase-separated multilayer system, we find that the alignment of raft-mimicking Lo domains is regulated by the distance between bilayers. Molecular dynamics simulations reveal that the aligned state is energetically preferred when the intermembrane distance is small due to its ability to minimize the volume of surface water, which has fewer water hydrogen bonds (HBs) compared to bulk water. Our results suggest that water HB-driven alignment of lipid rafts plays a role as a precursor of intermembrane processes such as cell-cell fusion, virus entry, and signaling.

Novel non-helical antimicrobial peptides insert into and fuse lipid model membranes

This research addresses the growing menace of antibiotic resistance by exploring antimicrobial peptides (AMPs) as alternatives to conventional antibiotics. Specifically, we investigate two linear amphipathic AMPs, LE-53 (12-mer) and LE-55 (16-mer), finding that the shorter LE-53 exhibits greater bactericidal activity against both Gram-negative (G(-)) and Gram-positive (G(+)) bacteria. Remarkably, both AMPs are non-toxic to eukaryotic cells. The heightened effectiveness of LE-53 is attributed to its increased hydrophobicity (H) compared to LE-55. Circular dichroism (CD) reveals that LE-53 and LE-55 both adopt β-sheet and random coil structures in lipid model membranes (LMMs) mimicking G(-) and G(+) bacteria, so secondary structure is not the cause of the potency difference. X-ray diffuse scattering (XDS) reveals increased lipid chain order in LE-53, a potential key distinction. Additionally, XDS study uncovers a significant link between LE-53's upper hydrocarbon location in G(-) and G(+) LMMs and its efficacy. Neutron reflectometry (NR) confirms the AMP locations determined using XDS. Solution small angle X-ray scattering (SAXS) demonstrates LE-53's ability to induce vesicle fusion in bacterial LMMs without affecting eukaryotic LMMs, offering a promising strategy to combat antibiotic-resistant strains while preserving human cell integrity, whereas LE-55 has a smaller ability to induce fusion.

Insight into structural biophysics from solution X-ray scattering

The current challenges of structural biophysics include determining the structure of large self-assembled complexes, resolving the structure of ensembles of complex structures and their mass fraction, and unraveling the dynamic pathways and mechanisms leading to the formation of complex structures from their subunits. Modern synchrotron solution X-ray scattering data enable simultaneous high-spatial and high-temporal structural data required to address the current challenges of structural biophysics. These data are complementary to crystallography, NMR, and cryo-TEM data. However, the analysis of solution scattering data is challenging; hence many different analysis tools, listed in the SAS Portal (http://smallangle.org/), were developed. In this review, we start by briefly summarizing classical X-ray scattering analyses providing insight into fundamental structural and interaction parameters. We then describe recent developments, integrating simulations, theory, and advanced X-ray scattering modeling, providing unique insights into the structure, energetics, and dynamics of self-assembled complexes. The structural information is essential for understanding the underlying physical chemistry principles leading to self-assembled supramolecular architectures and computational structural refinement.

Viscosity-Sensitive Membrane Dyes as Tools To Estimate the Crystalline Structure of Lipid Bilayers

Lipid membranes are crucial for cellular integrity and regulation, and tight control of their structural and mechanical properties is vital to ensure that they function properly. Fluorescent probes sensitive to the membrane's microenvironment are useful for investigating lipid membrane properties; however, there is currently a lack of quantitative correlation between the exact parameters of lipid organization and a readout from these dyes. Here, we investigate this relationship for "molecular rotors", or microviscosity sensors, by simultaneously measuring their fluorescence lifetime to determine the membrane viscosity, while using X-ray diffraction to determine the membrane's structural properties. Our results reveal a phase-dependent correlation between the membrane's structural parameters and mechanical properties measured by a BODIPY-based molecular rotor, giving excellent predictive power for the structural descriptors of the lipid bilayer. We also demonstrate that differences in membrane thickness between different lipid phases are not a prerequisite for the formation of lipid microdomains and that this requirement can be disrupted by the presence of line-active molecules. Our results underpin the use of membrane-sensitive dyes as reporters of the structure of lipid membranes.

Novel Helical Trp- and Arg-Rich Antimicrobial Peptides Locate Near Membrane Surfaces and Rigidify Lipid Model Membranes

Antibiotics are losing effectiveness as bacteria become resistant to conventional drugs. To find new alternatives, antimicrobial peptides (AMPs) are rationally designed with different lengths, charges, hydrophobicities (H), and hydrophobic moments (μH), containing only three types of amino acids: arginine, tryptophan, and valine. Six AMPs with low minimum inhibitory concentrations (MICs) and <25% toxicity to mammalian cells are selected for biophysical studies. Their secondary structures are determined using circular dichroism (CD), which finds that the % α-helicity of AMPs depends on composition of the lipid model membranes (LMMs): gram-negative (G(-)) inner membrane (IM) >gram-positive (G(+)) > Euk33 (eukaryotic with 33 mol% cholesterol). The two most effective peptides, E2-35 (16 amino acid [AA] residues) and E2-05 (22 AAs), are predominantly helical in G(-) IM and G(+) LMMs. AMP/membrane interactions such as membrane elasticity, chain order parameter, and location of the peptides in the membrane are investigated by low-angle and wide-angle X-ray diffuse scattering (XDS). It is found that headgroup location correlates with efficacy and toxicity. The membrane bending modulus KC displays nonmonotonic changes due to increasing concentrations of E2-35 and E2-05 in G(-) and G(+) LMMs, suggesting a bacterial killing mechanism where domain formation causes ion and water leakage.

Cholesterol enhances the negative impact of vaping additives on lung surfactant model systems

Aims: Vaping has given rise to e-cigarette or vaping product use-associated lung injury. Model lung surfactant films were used to assess the impact of vape additives (vitamin E, vitamin E acetate, tetrahydrocannabinol, cannabidiol). This work builds upon our previous findings, by incorporating cholesterol, to understand the interplay between the additives and the sterol in surfactant function. Materials & methods: Compression-expansion cycles of lipid monofilm at the air-water interface and Brewster angle microscopy allowed elucidating the effects of vape additives. Results & conclusion: Vape additives at 5 mol% inhibited proper lipid packing and reduced film stability. Cholesterol enhanced the additive effects, resulting in significantly destabilized films and altered domains. The observed impact could signify dysfunctional lung surfactant and impaired lung function.

Critical Phenomena in Plasma Membrane Organization and Function

Lateral organization in the plane of the plasma membrane is an important driver of biological processes. The past dozen years have seen increasing experimental support for the notion that lipid organization plays an important role in modulating this heterogeneity. Various biophysical mechanisms rooted in the concept of liquid-liquid phase separation have been proposed to explain diverse experimental observations of heterogeneity in model and cell membranes with distinct but overlapping applicability. In this review, we focus on the evidence for and the consequences of the hypothesis that the plasma membrane is poised near an equilibrium miscibility critical point. Critical phenomena explain certain features of the heterogeneity observed in cells and model systems but also go beyond heterogeneity to predict other interesting phenomena, including responses to perturbations in membrane composition.

Thermodynamic and structural study of DMPC-alkanol systems

The thermodynamic and structural behaviors of lamellar dimyristoylphosphatidylcholine-alkanol (abbreviation DMPC-CnOH, n = 8-18 is the even number of carbons in the alkyl chain) systems were studied by using DSC and SAXD/WAXD methods at a 0-0.8 CnOH : DMPC molar ratio range. Up to n≤ 10 a significant biphasic effect depending on the main transition temperature t_m on the CnOH concentration was observed. Two breakpoints were revealed: turning point (TP), corresponding to the minimum, and threshold concentration (c_T), corresponding to the end of the biphasic tendency. These breakpoints were also observed in the alkanol concentration dependent change in the enthalpy of the main transition ΔH_m. In the case of CnOHs with n > 10 we propose a marked shift of TP and c_T to very low concentrations; consequently, only increase of t_m is observed. A partial phase diagram was constructed for a pseudo-binary DMPC-C12OH system. We suggest a fluid-fluid immiscibility of the DMPC-C12OH system above c_T with a consequent formation of domains with different C12OH contents. At a constant CnOH concentration, the effects of CnOHs on ΔH_m and bilayer repeat distance were found to depend predominantly on the mismatch between CnOH and lipid chain lengths. Observed effects are suggested to be underlined by a counterbalancing effect of interchain van der Waals interactions and headgroup repulsion.

Structural Characterization of Natural Yeast Phosphatidylcholine and Bacterial Phosphatidylglycerol Lipid Multilayers by Neutron Diffraction

Eukaryotic and prokaryotic cell membranes are difficult to characterize directly with biophysical methods. Membrane model systems, that include fewer molecular species, are therefore often used to reproduce their fundamental chemical and physical properties. In this context, natural lipid mixtures directly extracted from cells are a valuable resource to produce advanced models of biological membranes for biophysical investigations and for the development of drug testing platforms. In this study we focused on single phospholipid classes, i.e. Pichia pastoris phosphatidylcholine (PC) and Escherichia coli phosphatidylglycerol (PG) lipids. These lipids were characterized by a different distribution of their respective acyl chain lengths and number of unsaturations. We produced both hydrogenous and deuterated lipid mixtures. Neutron diffraction experiments at different relative humidities were performed to characterize multilayers from these lipids and investigate the impact of the acyl chain composition on the structural organization. The novelty of this work resides in the use of natural extracts with a single class head-group and a mixture of chain compositions coming from yeast or bacterial cells. The characterization of the PC and PG multilayers showed that, as a consequence of the heterogeneity of their acyl chain composition, different lamellar phases are formed.

Low-flux scanning electron diffraction reveals substructures inside the ordered membrane domain

Ordered/disordered phase separation occurring in bio-membranes has piqued researchers’ interest because these ordered domains, called lipid rafts, regulate important biological functions. The structure of the ordered domain has been examined with artificial membranes, which undergo macroscopic ordered/disordered phase separation. However, owing to technical difficulties, the local structure inside ordered domains remains unknown. In this study, we employed electron diffraction to examine the packing structure of the lipid carbon chains in the ordered domain. First, we prepared dehydrated monolayer samples using a rapid-freezing and sublimation protocol, which attenuates the shrinkage of the chain-packing lattice in the dehydration process. Then, we optimised the electron flux to minimise beam damage to the monolayer sample. Finally, we developed low-flux scanning electron diffraction and assessed the chain packing structure inside the ordered domain formed in a distearoylphosphatidylcholine/dioleoylphosphatidylcholine binary monolayer. Consequently, we discovered that the ordered domain contains multiple subdomains with different crystallographic axes. Moreover, the size of the subdomain is larger in the domain centre than that near the phase boundary. To our knowledge, this is the first study to reveal the chain packing structures inside an ordered domain.

Suppression of Lα/Lβ Phase Coexistence in the Lipids of Pulmonary Surfactant

To determine how different constituents of pulmonary surfactant affect its phase behavior, we measured wide-angle x-ray scattering (WAXS) from oriented bilayers. Samples contained the nonpolar and phospholipids (N&PL) obtained from calf lung surfactant extract (CLSE), which also contains the hydrophobic surfactant proteins SP-B and SP-C. Mixtures with different ratios of N&PL and CLSE provided the same set of lipids with different amounts of the proteins. At 37°C, N&PL by itself forms coexisting L_α and L_β phases. In the L_β structure, the acyl chains of the phospholipids occupy an ordered array that has melted by 40°C. This behavior suggests that the L_β composition is dominated by dipalmitoyl phosphatidylcholine (DPPC), which is the most prevalent component of CLSE. The L_β chains, however, lack the tilt of the L_β' phase formed by pure DPPC. At 40°C, WAXS also detects an additional diffracted intensity, the location of which suggests a correlation among the phospholipid headgroups. The mixed samples of N&PL with CLSE show that increasing amounts of the proteins disrupt both the L_β phase and the headgroup correlation. With physiological levels of the proteins in CLSE, both types of order are absent. These results with bilayers at physiological temperatures indicate that the hydrophobic surfactant proteins disrupt the ordered structures that have long been considered essential for the ability of pulmonary surfactant to sustain low surface tensions. They agree with prior fluorescence micrographic results from monomolecular films of CLSE, suggesting that at physiological temperatures, any ordered phase is likely to be absent or occupy a minimal interfacial area.

Direct label-free imaging of nanodomains in biomimetic and biological membranes by cryogenic electron microscopy

The nanoscale organization of biological membranes into structurally and compositionally distinct lateral domains is believed to be central to membrane function. The nature of this organization has remained elusive due to a lack of methods to directly probe nanoscopic membrane features. We show here that cryogenic electron microscopy (cryo-EM) can be used to directly image coexisting nanoscopic domains in synthetic and bioderived membranes without extrinsic probes. Analyzing a series of single-component liposomes composed of synthetic lipids of varying chain lengths, we demonstrate that cryo-EM can distinguish bilayer thickness differences as small as 0.5 Å, comparable to the resolution of small-angle scattering methods. Simulated images from computational models reveal that features in cryo-EM images result from a complex interplay between the atomic distribution normal to the plane of the bilayer and imaging parameters. Simulations of phase-separated bilayers were used to predict two sources of contrast between coexisting ordered and disordered phases within a single liposome, namely differences in membrane thickness and molecular density. We observe both sources of contrast in biomimetic membranes composed of saturated lipids, unsaturated lipids, and cholesterol. When extended to isolated mammalian plasma membranes, cryo-EM reveals similar nanoscale lateral heterogeneities. The methods reported here for direct, probe-free imaging of nanodomains in unperturbed membranes open new avenues for investigation of nanoscopic membrane organization.

Laterally Resolved Small-Angle Scattering Intensity from Lipid Bilayer Simulations: An Exact and a Limited-Range Treatment

When combined, molecular simulations and small-angle scattering experiments are able to provide molecular-scale resolution of structure. Separately, scattering experiments provide only intermingled pair correlations between atoms, while molecular simulations are limited by model quality and the relatively short time scales that they can access. Their combined strength relies on agreement between the experimental spectra and those computed by simulation. To date, computing the neutron spectra from a molecular simulation of a lipid bilayer is straightforward only if the structure is approximated by laterally averaging the in-plane bilayer structure. However, this neglects all information about lateral heterogeneity, e.g., clustering of components in a lipid mixture. This paper presents two methods for computing the scattering intensity of simulated bilayers with in-plane heterogeneity, enabling a full treatment of both the transverse and lateral bilayer structure for the first time. The first method, termed the Dirac Brush, computes the exact spectra including spurious artifacts resulting from using information from neighboring periodic cells to account for the long-range structure of the bilayer. The second method, termed PFFT, applies a mean-field treatment in the field far from a scattering element, resulting in a correlation range that can be tuned (eliminating correlations with neighboring periodic images), but with computational cost that prohibits obtaining the exact (Dirac Brush) spectra. Following their derivation, the two methods are applied to a coarse-grained molecular simulation of a bilayer inhomogeneity, demonstrating the contributions of lateral correlations to the resulting spectra.

Selective Interaction of Colistin with Lipid Model Membranes

Although colistin's clinical use is limited due to its nephrotoxicity, colistin is considered to be an antibiotic of last resort because it is used to treat patients infected with multidrug-resistant bacteria. In an effort to provide molecular details about colistin's ability to kill Gram-negative (G(-)) but not Gram-positive (G(+)) bacteria, we investigated the biophysics of the interaction between colistin and lipid mixtures mimicking the cytoplasmic membrane of G(+), G(-) bacteria as well as eukaryotic cells. Two different models of the G(-) outer membrane (OM) were assayed: lipid A with two deoxy-manno-octulosonyl sugar residues, and Escherichia coli lipopolysaccharide mixed with dilaurylphosphatidylglycerol. We used circular dichroism and x-ray diffuse scattering at low and wide angle in stacked multilayered samples, and neutron reflectivity of single, tethered bilayers mixed with colistin. We found no differences in secondary structure when colistin was bound to G(-) versus G(+) membrane mimics, ruling out a protein conformational change as the cause of this difference. However, bending modulus K_C perturbation was quite irregular for the G(-) inner membrane, where colistin produced a softening of the membranes at an intermediate lipid/peptide molar ratio but stiffening at lower and higher peptide concentrations, whereas in G(+) and eukaryotic mimics there was only a slight softening. Acyl chain order in G(-) was perturbed similarly to K_C. In G(+), there was only a slight softening and disordering effect, whereas in OM mimics, there was a slight stiffening and ordering of both membranes with increasing colistin. X-ray and neutron reflectivity structural results reveal colistin partitions deepest to reach the hydrocarbon interior in G(-) membranes, but remains in the headgroup region in G(+), OM, and eukaryotic mimics. It is possible that domain formation is responsible for the erratic response of G(-) inner membranes to colistin and for its deeper penetration, which could increase membrane permeability.

Elastic behavior of model membranes with antimicrobial peptides depends on lipid specificity and d-enantiomers

In an effort to provide new treatments for the global crisis of bacterial resistance to current antibiotics, we have used a rational approach to design several new antimicrobial peptides (AMPs). The present study focuses on 24-mer WLBU2 and its derivative, D8, with the amino acid sequence, RRWVRRVRRWVRRVVRVVRRWVRR. In D8, all of the valines are the d-enantiomer. We use X-ray low- and wide-angle diffuse scattering data to measure elasticity and lipid chain order. We show a good correlation between in vitro bacterial killing efficiency and both bending and chain order behavior in bacterial lipid membrane mimics; our results suggest that AMP-triggered domain formation could be the mechanism of bacterial killing in both Gram-positive and Gram-negative bacteria. In red blood cell lipid mimics, D8 stiffens and orders the membrane, while WLBU2 softens and disorders it, which correlate with D8's harmless vs. WLBU2's toxic behavior in hemolysis tests. These results suggest that elasticity and chain order behavior can be used to predict mechanisms of bactericidal action and toxicity of new AMPs.

The actions of volatile anesthetics: a new perspective

This article reviews recent work in applying neutron and X-ray scattering towards the elucidation of the molecular mechanisms of volatile anesthetics. Experimental results on domain mixing in ternary lipid mixtures, and the influence of volatile anesthetics and hydrostatic pressure are placed in the contexts of ion-channel function and receptor trafficking at the postsynaptic density.

Membrane Lipid Nanodomains

Lipid membranes can spontaneously organize their components into domains of different sizes and properties. The organization of membrane lipids into nanodomains might potentially play a role in vital functions of cells and organisms. Model membranes represent attractive systems to study lipid nanodomains, which cannot be directly addressed in living cells with the currently available methods. This review summarizes the knowledge on lipid nanodomains in model membranes and exposes how their specific character contrasts with large-scale phase separation. The overview on lipid nanodomains in membranes composed of diverse lipids (e.g., zwitterionic and anionic glycerophospholipids, ceramides, glycosphingolipids) and cholesterol aims to evidence the impact of chemical, electrostatic, and geometric properties of lipids on nanodomain formation. Furthermore, the effects of curvature, asymmetry, and ions on membrane nanodomains are shown to be highly relevant aspects that may also modulate lipid nanodomains in cellular membranes. Potential mechanisms responsible for the formation and dynamics of nanodomains are discussed with support from available theories and computational studies. A brief description of current fluorescence techniques and analytical tools that enabled progress in lipid nanodomain studies is also included. Further directions are proposed to successfully extend this research to cells.

Structure of Lung-Mimetic Multilamellar Bodies with Lipid Compositions Relevant in Pneumonia

The hierarchical assembly of lipids, as modulated by composition and environment, plays a significant role in the function of biological membranes and a myriad of diseases. Elevated concentrations of calcium ions and cardiolipin (CL), an anionic tetra-alkyl lipid found in mitochondria and some bacterial cell membranes, have been implicated in pneumonia recently. However, their impact on the physicochemical properties of lipid assemblies in lungs and how it impairs alveoli function is still unknown. We use small- and wide-angle X-ray scattering (S/WAXS) and solid-state nuclear magnetic resonance (ssNMR) to probe the structure and dynamics of lung-mimetic multilamellar bodies (MLBs) in the presence of Ca²⁺ and CL. We conjecture that CL overexpressed in the hypophase of alveoli strongly affects the structure of lung-lipid bilayers and their stacking in the MLBs. Specifically, S/WAXS data revealed that CL induces significant shrinkage of the water-layer separating the concentric bilayers in multilamellar aggregates. ssNMR measurements indicate that this interbilayer tightening is due to undulation repulsion damping as CL renders the glycerol backbone of the membranes significantly more static. In addition to MLB dehydration, CL promotes intrabilayer phase separation into saturated-rich and unsaturated-rich lipid domains that couple across multiple layers. Expectedly, addition of Ca²⁺ screens the electrostatic repulsion between negatively charged lung membranes. However, when CL is present, addition of Ca²⁺ results in an apparent interbilayer expansion likely due to local structural defects. Combining S/WAXS and ssNMR on systems with compositions pertinent to healthy and unhealthy lung membranes, we propose how alteration of the physiochemical properties of MLBs can critically impact the breathing cycle.

Physics of HIV

This review summarizes over a decade of investigations into how membrane-binding proteins from the HIV-1 virus interact with lipid membrane mimics various HIV and host T-cell membranes. The goal of the work was to characterize at the molecular level both the elastic and structural changes that occur due to HIV protein/membrane interactions, which could lead to new drugs to thwart the HIV virus. The main technique used to study these interactions is diffuse X-ray scattering, which yields the bending modulus, K_C, as well as structural parameters such as membrane thickness, area/lipid and position of HIV peptides (parts of HIV proteins) in the membrane. Our methods also yield information about lipid chain order or disorder caused by the peptides. This review focuses on three stages of the HIV-1 life cycle: 1) infection, 2) Tat membrane transport, and 3) budding. In the infection stage, our lab studied three different parts of HIV-1 gp41 (glycoprotein 41 fusion protein): 1) FP23, the N-terminal 23 amino acids that interact non-specifically with the T-cell host membrane to cause fusion of two membranes, and its trimer version, 2) CRAC (cholesterol recognition amino acid consensus sequence), on the MPER (membrane proximal external region) near the membrane-spanning domain, and 3) LLP2 (lentiviral lytic peptide 2) on the CTT (cytoplasmic C-terminal tail). For Tat transport, we used membrane mimics of the T-cell nuclear membrane as well as simpler models that varied charge and negative curvature. For membrane budding, we varied the myristoylation of the MA₃₁ peptide as well as the negatively charged lipid. These studies show that HIV peptides with different roles in the HIV life cycle affect differently the relevant membrane mimics. In addition, the membrane lipid composition plays an important role in the peptides' effects.

Liquid-liquid phase coexistence in lipid membranes observed by natural abundance 1H-13C solid-state NMR

We demonstrate that 1H-13C solid-state MAS NMR is suitable to detect liquid disordered/liquid ordered phase coexistence in a DOPC/DPPC/cholesterol mixture with natural abundance of isotopes as an alternative to 2H NMR. Such methodology is potentially applicable to study lipid phase coexistence phenomena in biological matter with high lipid content, e.g. lung surfactant or myelin, for which isotopic labeling is not possible.

Effect of the presence of cholesterol in the interfacial microenvironment on the modulation of the alkaline phosphatase activity during in vitro mineralization

Mineralization of the skeleton starts within cell-derived matrix vesicles (MVs); then, minerals propagate to the extracellular collagenous matrix. Tissue-nonspecific alkaline phosphatase (TNAP) degrades inorganic pyrophosphate (PP_i), a potent inhibitor of mineralization, and contributes P_i (Phosphate) from ATP to initiate mineralization. Compared to the plasma membrane, MVs are rich in Cholesterol (Chol) (∼32%) and TNAP, but how Chol influences TNAP activity remains unclear. We have reconstituted TNAP in liposomes of dipalmitoylphosphatidylcholine (DPPC) or dioleoylphosphatidylcholine (DOPC) combined with Chol or its derivatives Cholestenone (Achol) and Ergosterol (Ergo). DPPC plus 36% sterols in liposome increased the catalytic activity of TNAP toward ATP. The presence of Chol also increased the propagation of minerals by 3.4-fold. The catalytic efficiency of TNAP toward ATP was fourfold lower in DOPC proteoliposomes as compared to DPPC proteoliposomes. DOPC proteoliposomes also increased biomineralization by 2.8-fold as compared to DPPC proteoliposomes. TNAP catalyzed the hydrolysis of ATP more efficiently in the case of the proteoliposome consisting of DOPC with 36% Chol. The same behavior emerged with Achol and Ergo. The organization of the lipid and the structure of the sterol influenced the surface tension (γ), the TNAP phosphohydrolytic activity in the monolayer, and the TNAP catalytic efficiency in the bilayers. Membranes in the Lα phase (Achol) provided better kinetic parameters as compared to membranes in the Lo phase (Chol and Ergo). In conclusion, the physical properties and the lateral organization of lipids in proteoliposomes are crucial to control mineral propagation mediated by TNAP activity during mineralization.

Cellular structures arising from viscoelastic phase separation in binary mixtures of thermotropic liquid crystals

Viscoelastic effects are known to influence pattern formation during phase separation in dynamically asymmetric mixtures. Evidence is shown for such an effect in some binary mixtures composed of liquid crystals made of rod-like (R) and bent-core (BC) molecules. The difference in dynamics at phase separation manifests itself in the form of cellular structures (CSs). This is mainly driven by dissimilarities in flow and rotational viscosities of the two types of molecules which differ in size and shape. The heterogeneous structure has been characterized by optical and confocal microscopy along with X-ray diffraction studies and found to be composed of coexisting liquid crystalline phases. The striking resemblance to CSs of biological systems further enriched by topological defects is unique to this system. The morphology and stability of the CSs are dictated by the smectic ordering influenced by the relative concentration and mutual orientation of the R and BC molecules. This type of phase separation process can also be utilized to form functional ordered assemblies of nanoparticles embedded in a liquid crystal matrix.

Cholesterol Partition and Condensing Effect in Phase-Separated Ternary Mixture Lipid Multilayers

The cholesterol partitioning and condensing effect in the liquid-ordered (Lo) and liquid-disordered (Ld) phases were systematically investigated for ternary mixture lipid multilayers consisting of 1:1 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-phosphocholine with varying concentrations of cholesterol. X-ray lamellar diffraction was used to deduce the electron density profiles of each phase. The cholesterol concentration in each phase was quantified by fitting of the electron density profiles with a newly invented basic lipid profile scaling method that minimizes the number of fitting parameters. The obtained cholesterol concentration in each phase versus total cholesterol concentration in the sample increases linearly for both phases. The condensing effect of cholesterol in ternary lipid mixtures was evaluated in terms of phosphate-to-phosphate distances, which together with the estimated cholesterol concentration in each phase was converted into an average area per molecule. In addition, the cholesterol position was determined to a precision of (±0.7Å) and an increase of disorder in the lipid packing in the Lo phase was observed for total cholesterol concentration of 20∼30%.

Structure of Carbon Nanotube Porins in Lipid Bilayers: An in Situ Small-Angle X-ray Scattering (SAXS) Study

Carbon nanotube porins (CNTPs), small segments of carbon nanotubes capable of forming defined pores in lipid membranes, are important future components for bionanoelectronic devices as they could provide a robust analog of biological membrane channels. In order to control the incorporation of these CNT channels into lipid bilayers, it is important to understand the structure of the CNTPs before and after insertion into the lipid bilayer as well as the impact of such insertion on the bilayer structure. Here we employed a noninvasive in situ probe, small-angle X-ray scattering, to study the integration of CNT porins into dioleoylphosphatidylcholine bilayers. Our results show that CNTPs in solution are stabilized by a monolayer of lipid molecules wrapped around their outer surface. We also demonstrate that insertion of CNTPs into the lipid bilayer results in decreased bilayer thickness with the magnitude of this effect increasing with the concentration of CNTPs.

Hexagonal Substructure and Hydrogen Bonding in Liquid-Ordered Phases Containing Palmitoyl Sphingomyelin

All-atom simulation data are presented for ternary mixtures of palmitoyl sphingomyelin (PSM), cholesterol, and either palmitoyl oleoyl phosphatidyl choline or dioleoyl phosphatidyl choline (DOPC). For comparison, data for a mixture of dipalmitoyl phosphatidyl choline (DPPC), cholesterol, and DOPC are also presented. Compositions corresponding to the liquid-ordered phase, the liquid-disordered phase, and coexistence of the two phases are simulated for each mixture. Within the liquid-ordered phase, cholesterol is preferentially solvated by DOPC if it is available, but if DOPC is replaced by POPC, cholesterol is preferentially solvated by PSM. In the DPPC mixtures, cholesterol interacts preferentially with the saturated chains via its smooth face, whereas in the PSM mixtures, cholesterol interacts preferentially with PSM via its rough face. Interactions between cholesterol and PSM have a very particular character: hydrogen bonding between cholesterol and the amide of PSM rotates the tilt of the amide plane, which primes it for more robust hydrogen bonding with other PSM. Cholesterol-PSM hydrogen bonding also locally modifies the hexagonal packing of hydrocarbon chains in the liquid-ordered phase of PSM mixtures.

Bending Rigidities and Interdomain Forces in Membranes with Coexisting Lipid Domains

To precisely quantify the fundamental interactions between heterogeneous lipid membranes with coexisting liquid-ordered (Lo) and liquid-disordered (Ld) domains, we performed detailed osmotic stress small-angle x-ray scattering experiments by exploiting the domain alignment in raft-mimicking lipid multibilayers. Performing a Monte Carlo-based analysis allowed us to determine with high reliability the magnitude and functional dependence of interdomain forces concurrently with the bending elasticity moduli. In contrast to previous methodologies, this approach enabled us to consider the entropic undulation repulsions on a fundamental level, without having to take recourse to crudely justified mean-field-like additivity assumptions. Our detailed Hamaker-coefficient calculations indicated only small differences in the van der Waals attractions of coexisting Lo and Ld phases. In contrast, the repulsive hydration and undulation interactions differed significantly, with the latter dominating the overall repulsions in the Ld phase. Thus, alignment of like domains in multibilayers appears to originate from both, hydration and undulation repulsions.

Thickness Mismatch of Coexisting Liquid Phases in Noncanonical Lipid Bilayers

Lipid composition dictates membrane thickness, which in turn can influence membrane protein activity. Lipid composition also determines whether a membrane demixes into coexisting liquid-crystalline phases. Previous direct measurements of demixed lipid membranes have always found a liquid-ordered phase that is thicker than the liquid-disordered phase. Here we investigated noncanonical ternary lipid mixtures designed to produce bilayers with thicker disordered phases than ordered phases. The membranes were composed of short, saturated (ordered) lipids; long, unsaturated (disordered) lipids; and cholesterol. We found that few of these systems yield coexisting liquid phases above 10 °C. For membranes that do demix into two liquid phases, we measured the thickness mismatch between the phases by atomic force microscopy and found that not one of the systems yields thicker disordered than ordered phases under standard experimental conditions. We found no monotonic relationship between demixing temperatures of these ternary systems and either estimated thickness mismatches between the liquid phases or the physical parameters of single-component membranes composed of the individual lipids. These results highlight the robustness of a membrane's liquid-ordered phase to be thicker than the liquid-disordered phase, regardless of the membrane's lipid composition.

Computational and experimental approaches for investigating nanoparticle-based drug delivery systems

Most therapeutic agents suffer from poor solubility, rapid clearance from the blood stream, a lack of targeting, and often poor translocation ability across cell membranes. Drug/gene delivery systems (DDSs) are capable of overcoming some of these barriers to enhance delivery of drugs to their right place of action, e.g. inside cancer cells. In this review, we focus on nanoparticles as DDSs. Complementary experimental and computational studies have enhanced our understanding of the mechanism of action of nanocarriers and their underlying interactions with drugs, biomembranes and other biological molecules. We review key biophysical aspects of DDSs and discuss how computer modeling can assist in rational design of DDSs with improved and optimized properties. We summarize commonly used experimental techniques for the study of DDSs. Then we review computational studies for several major categories of nanocarriers, including dendrimers and dendrons, polymer-, peptide-, nucleic acid-, lipid-, and carbon-based DDSs, and gold nanoparticles. This article is part of a Special Issue entitled: Membrane Proteins edited by J.C. Gumbart and Sergei Noskov.

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.

Hydrostatic Pressure Promotes Domain Formation in Model Lipid Raft Membranes

Neutron diffraction measurements demonstrate that hydrostatic pressure promotes liquid-ordered (Lo) domain formation in lipid membranes prepared as both oriented multilayers and unilamellar vesicles made of a canonical ternary lipid mixture for which demixing transitions have been extensively studied. The results demonstrate an unusually large dependence of the mixing transition on hydrostatic pressure. Additionally, data at 28 °C show that the magnitude of increase in Lo caused by 10 MPa pressure is much the same as the decrease in Lo produced by twice minimum alveolar concentrations (MAC) of general anesthetics such as halothane, nitrous oxide, and xenon. Therefore, the results may provide a plausible explanation for the reversal of general anesthesia by hydrostatic pressure.

Effect of Cholesterol on the Structure of a Five-Component Mitochondria-Like Phospholipid Membrane

Cellular membranes have a complex phospholipid composition that varies greatly depending on the organism, cell type and function. In spite of this complexity, most structural data available for phospholipid bilayers concern model systems containing only one or two different phospholipids. Here, we examine the effect of cholesterol on the structure of a complex membrane reflecting the lipid composition of mitochondrial membranes, with five different types of headgroups (phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), phosphatidylserine (PS) and cardiolipin (CL)) and a variety of hydrocarbon tails. This particular system was chosen because elevated cholesterol contents in mitochondrial membranes have been linked to a breaking down of Bax-mediated membrane permeabilization and resistance to cancer treatments. High resolution electron density profiles were determined by X-ray reflectivity, while the area per phospholipid chain, Apc, and the chain order parameter, SX-ray, were determined by wide-angle X-ray scattering (WAXS). We show that chain order increases upon the addition of cholesterol, resulting in both a thickening of the lipid bilayer and a reduction in the average surface area per phospholipid chain. This effect, well known as cholesterol’s condensation effect, is similar, but not as pronounced as for single-component phospholipid membranes. We conclude by discussing the relevance of these findings for the insertion of the pro-apoptotic protein Bax in mitochondrial membranes with elevated cholesterol content.

Communication: Orientational self-ordering of spin-labeled cholesterol analogs in lipid bilayers in diluted conditions

Lipid-cholesterol interactions are responsible for different properties of biological membranes including those determining formation in the membrane of spatial inhomogeneities (lipid rafts). To get new information on these interactions, electron spin echo (ESE) spectroscopy, which is a pulsed version of electron paramagnetic resonance (EPR), was applied to study 3β-doxyl-5α-cholestane (DCh), a spin-labeled analog of cholesterol, in phospholipid bilayer consisted of equimolecular mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine. DCh concentration in the bilayer was between 0.1 mol.% and 4 mol.%. For comparison, a reference system containing a spin-labeled 5-doxyl-stearic acid (5-DSA) instead of DCh was studied as well. The effects of "instantaneous diffusion" in ESE decay and in echo-detected (ED) EPR spectra were explored for both systems. The reference system showed good agreement with the theoretical prediction for the model of spin labels of randomly distributed orientations, but the DCh system demonstrated remarkably smaller effects. The results were explained by assuming that neighboring DCh molecules are oriented in a correlative way. However, this correlation does not imply the formation of clusters of cholesterol molecules, because conventional continuous wave EPR spectra did not show the typical broadening due to aggregation of spin labels and the observed ESE decay was not faster than in the reference system. So the obtained data evidence that cholesterol molecules at low concentrations in biological membranes can interact via large distances of several nanometers which results in their orientational self-ordering.

Two-photon Laurdan studies of the ternary lipid mixture DOPC:SM:cholesterol reveal a single liquid phase at sphingomyelin:cholesterol ratios lower than 1

The ternary lipid mixture DOPC:eggSM:cholesterol in excess water has been studied in the form of giant unilamellar vesicles using two-photon fluorescence microscopy. Previous publications based on single-photon fluorescence microscopy had reported heterogeneous phase behavior (phase coexistence) in the region of the triangular phase diagram corresponding to SM:cholesterol molar ratios <1. We have examined this region by two-photon microscopy of Laurdan-labeled mixtures and have found that, under our conditions, only a single liquid phase exists. We have shown that macroscopic phase separation in the above region can be artifactually induced by one-photon excitation of the fluorescent probes and ensuing photooxidation and is prevented using two-photon excitation. The main effect of increasing the concentration of cholesterol in mixtures containing 30 mol % SM was to increase the rigidity of the disordered domains. Increasing the concentration of SM in mixtures containing 20 mol % cholesterol gradually augmented the rigidity of the ordered domains, while the disordered domains reached minimal order at a SM:cholesterol 2.25:1 molar ratio, which then increased again. Moreover, the detailed measurement of Laurdan generalized polarization across the whole phase diagram allowed the representation, for both the single- and two-phase regions, of the gradual variation of membrane lateral packing along the diagram, which we found to be governed largely by SM:cholesterol interactions.

Lipid domains in model membranes: a brief historical perspective

All biological membranes consist of a complex composite of macromolecules and macromolecular assemblies, of which the fluid lipid-bilayer component is a core element with regard to cell encapsulation and barrier properties. The fluid lipid bilayer also supports the functional machinery of receptors, channels and pumps that are associated with the membrane. This bilayer is stabilized by weak physical and colloidal forces, and its nature is that of a self-assembled system of amphiphiles in water. Being only approximately 5 nm in thickness and still encapsulating a cell that is three orders of magnitude larger in diameter, the lipid bilayer as a material has very unusual physical properties, both in terms of structure and dynamics. Although the lipid bilayer is a fluid, it has a distinct and structured trans-bilayer profile, and in the plane of the bilayer the various molecular components, viz different lipid species and membrane proteins, have the capacity to organize laterally in terms of differentiated domains on different length and time scales. These elements of small-scale structure and order are crucial for the functioning of the membrane. It has turned out to be difficult to quantitatively study the small-scale structure of biological membranes. A major part of the insight into membrane micro- and nano-domains and the concepts used to describe them have hence come from studies of simple lipid bilayers as models of membranes, by use of a wide range of theoretical, experimental and simulational approaches. Many questions remain to be answered as to which extent the result from model studies can carry over to real biological membranes.

X-ray diffraction of lipid model membranes

In this chapter the use of X-ray diffraction to study the structure of lyotropic phases and lipid model membranes is described. Determination of the phase symmetry and lattice parameters from small-angle X-ray scattering (SAXS), and of the nature of the hydrocarbon chain packing from wide-angle X-ray scattering (WAXS), are discussed. Methods by which the sign of the interfacial curvature of non-lamellar phases may be determined are then presented. Finally, the calculation of electron density profiles from the intensities of the observed Bragg peaks is described, for the lamellar phase and for the inverse hexagonal phase.

Positron annihilation lifetime spectroscopy (PALS): a probe for molecular organisation in self-assembled biomimetic systems

Positron annihilation lifetime spectroscopy (PALS) has been shown to be highly sensitive to conformational, structural and microenvironmental transformations arising from subtle geometric changes in molecular geometry in self-assembling biomimetic systems.

A detailed analysis of partial molecular volumes in DPPC/cholesterol binary bilayers

We examined the volumetric behavior of the dipalmitoylphosphatidylcholine (DPPC)/cholesterol binary bilayer system with high accuracy and more cholesterol concentrations to reveal the detailed molecular states in the liquid-disordered (Ld) phase, the liquid-ordered (Lo) phase and the gel phase. We measured the average specific volume of the binary bilayer at several temperatures by the neutral flotation method and calculated the average volume per molecule to estimate the partial molecular volumes of DPPC and cholesterol in each phase. As a result, we found that the region with intermediate cholesterol concentrations showed a more complicated behavior than expected from simple coexistence of Ld and Lo domains. We also measured fluorescence decay of trans-parinaric acid (tPA) added into the binary bilayer with more cholesterol concentrations to get further insight into the cholesterol-induced formation of the Lo phase. On the basis of these results we discuss the molecular interaction between DPPC and cholesterol molecule in the Lo phase and the manner of Ld/Lo phase coexistence.

HIV-1 Tat membrane interactions probed using X-ray and neutron scattering, CD spectroscopy and MD simulations

We report the effect on lipid bilayers of the Tat peptide Y47GRKKRRQRRR57 from the HIV-1 virus transactivator of translation (Tat) protein. Synergistic use of low-angle X-ray scattering (LAXS) and atomistic molecular dynamic simulations (MD) indicate Tat peptide binding to neutral dioleoylphosphocholine (DOPC) lipid headgroups. This binding induced the local lipid phosphate groups to move 3Å closer to the center of the bilayer. Many of the positively charged guanidinium components of the arginines were as close to the center of the bilayer as the locally thinned lipid phosphate groups. LAXS data for DOPC, DOPC/dioleoylphosphoethanolamine (DOPE), DOPC/dioleoylphosphoserine (DOPS), and a mimic of the nuclear membrane gave similar results. Generally, the Tat peptide decreased the bilayer bending modulus KC and increased the area/lipid. Further indications that Tat softens a membrane, thereby facilitating translocation, were provided by wide-angle X-ray scattering (WAXS) and neutron scattering. CD spectroscopy was also applied to further characterize Tat/membrane interactions. Although a mechanism for translation remains obscure, this study suggests that the peptide/lipid interaction makes the Tat peptide poised to translocate from the headgroup region.

Membrane order parameters for interdigitated lipid bilayers measured via polarized total-internal-reflection fluorescence microscopy

Incorporating ethanol in lipid membranes leads to changes in bilayer structure, including the formation of an interdigitated phase. We have used polarized total-internal-reflection fluorescence microscopy (pTIRFM) to measure the order parameter for Texas Red DHPE incorporated in the ethanol-induced interdigitated phase (LβI) formed from ternary lipid mixtures comprising dioleoylphosphatidylcholine, cholesterol and egg sphingomyelin or dipalmitoylphosphatidylcholine. These lipid mixtures have 3 co-existing phases in the presence of ethanol: liquid-ordered, liquid-disordered and LβI. pTIRFM using Texas Red DHPE shows a reversal in fluorescence contrast between the LβI phase and the surrounding disordered phase with changes in the polarization angle. The contrast reversal is due to changes in the orientation of the dye, and provides a rapid method to identify the LβI phase. The measured order parameters for the LβI phase are consistent with a highly ordered membrane environment, similar to a gel phase. An acyl-chain labeled BODIPY-FL-PC was also tested for pTIRFM studies of ethanol-treated bilayers; however, this probe is less useful since the order parameters of the interdigitated phase are consistent with orientations that are close to random, either due to local membrane disorder or to a mixture of extended and looping conformations in which the fluorophore is localized in the polar headgroup region of the bilayer. In summary, we demonstrate that order parameter measurements via pTIRFM using Texas Red-DHPE can rapidly identify the interdigitated phase in supported bilayers. We anticipate that this technique will aid further research in the effects of alcohols and other additives on membranes.

Line active molecules promote inhomogeneous structures in membranes: theory, simulations and experiments

We review recent theoretical efforts that predict how line-active molecules can promote lateral heterogeneities (or domains) in model membranes. This fundamental understanding may be relevant to membrane composition in living cells, where it is thought that small domains, called lipid rafts, are necessary for the cells to be functional. The theoretical work reviewed here ranges in scale from coarse grained continuum models to nearly atomistic models. The effect of line active molecules on domain sizes and shapes in the phase separated regime or on fluctuation length scales and lifetimes in the single phase, mixed regime, of the membrane is discussed. Recent experimental studies on model membranes that include line active molecules are also presented together with some comparisons with the theoretical predictions.