Chapter 9 The Biologically Relevant Lipid Mesophases as “Seen” by X-Rays

Optical and molecular features of negatively curved surfaces created by POPE lipids: A crucial role of the initial conditions

Membrane fusion is closely related to plasma membrane domains rich in cone-shaped phosphatidylethanolamine (PE) lipids that can reverse membrane curvature under certain conditions. The phase transition of PE-based lipid membranes from the lamellar fluid phase (Lα) to the inverse hexagonal phase (HII) is commonly taken as a general model in reconstructing the membrane fusion pathway, and whose structural features have been mostly described so far using structural and microscopic techniques. The aim of this paper is to decipher the optical and molecular features of Lβ → Lα and especially of Lα → HII transition of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) lipids at pH = 7.0 when they are initially prepared in the form of both multi- and unilamellar liposomes (MLVs and LUVs). The distinction between optical properties of MLS- and LUVs-derived HII phase, provided from turbidity-sensitive temperature-dependent UV-Vis spectra, was attributed to different formation mechanisms of HII phase. Most importantly, from FTIR spectroscopic data of POPE lipids in Lβ (15 °C), Lα (50 °C) and HII (85 °C) phases we identified the changes in molecular features of POPE lipids during phase transitions. Among the latter, by far the most significant is different hydration pattern of POPE lipids in MLVs- and LUVs-derived HII phase which extends from the polar-apolar interface all the way to the terminal amino group of the POPE lipid, along with the changes in the conformation of glycerol backbone as evidenced by the signature of α-methylene groups. Molecular dynamics simulations confirmed higher water penetration in HII phase and provided insight into hydrogen bonding patterns.

Impact of macromolecular crowding on the mesomorphic behavior of lipid self-assemblies

Using LAURDAN fluorescence we observed that water dynamics measured at the interface of DOPC bilayers can be differentially regulated by the presence of crowded suspensions of different proteins (HSA, IgG, Gelatin) and PEG, under conditions where the polymers are not in direct molecular contact with the lipid interface. Specifically, we found that the decrease in water dipolar relaxation at the membrane interface correlates with an increased fraction of randomly oriented (or random coil) configurations in the polymers, as Gelatin > PEG > IgG > HSA. By using the same experimental strategy, we also demonstrated that structural transitions from globular to extended conformations in proteins can induce transitions between lamellar and non-lamellar phases in mixtures of DOPC and monoolein. Independent experiments using Raman spectroscopy showed that aqueous suspensions of polymers exhibiting high proportions of randomly oriented conformations display increased fractions of tetracoordinated water, a configuration that is dominant in ice. This indicates a greater capacity of this type of structure for polarizing water and consequently reducing its chemical activity. This effect is in line with one of the tenets of the Association Induction Hypothesis, which predicts a long-range dynamic structuring of water molecules via their interactions with proteins (or other polymers) showing extended conformations. Overall, our results suggest a crucial role of water in promoting couplings between structural changes in macromolecules and supramolecular arrangements of lipids. This mechanism may be of relevance to cell structure/function when the crowded nature of the intracellular milieu is considered.

Design of Lipid-Based Nanocarriers via Cation Modulation of Ethanol-Interdigitated Lipid Membranes

Short-chain alcohols (i.e., ethanol) can induce membrane interdigitation in saturated-chain phosphatidylcholines (PCs). In this process, alcohol molecules intercalate between phosphate heads, increasing lateral separation and favoring hydrophobic interactions between opposing acyl chains, which interpenetrate forming an interdigitated phase. Unraveling mechanisms underlying the interactions between ethanol and model lipid membranes has implications for cell biology, biochemistry, and for the formulation of lipid-based nanocarriers. However, investigations of ethanol-lipid membrane systems have been carried out in deionized water, which limits their applicability. Here, using a combination of small- and wide-angle X-ray scattering, small-angle neutron scattering, and all-atom molecular dynamics simulations, we analyzed the effect of varying CaCl₂ and NaCl concentrations on ethanol-induced interdigitation. We observed that while ethanol addition leads to the interdigitation of bulk phase 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers in the presence of CaCl₂ and NaCl regardless of the salt concentration, the ethanol-induced interdigitation of vesicular DPPC depends on the choice of cation and its concentration. These findings unravel a key role for cations in the ethanol-induced interdigitation of lipid membranes in either bulk phase or vesicular form.

Composition effects on photooxidative membrane destabilization by TiO2 nanoparticles

Membrane interactions and photooxidative membrane destabilization of titanium dioxide (TiO₂) nanoparticles were investigated, focusing on the effects of membrane composition, notably phospholipid headgroup charge and presence of cholesterol. For this, we employed a battery of state-of-the-art methods for studies of bilayers formed by zwitterionic palmitoyloleoylphosphatidylcholine (POPC) containing also polyunsaturated palmitoylarachidonoylphosphocholine (PAPC), as well as its mixtures with anionic palmitoyloleoylphosphatidylglycerol (POPG) and cholesterol. It was found that the TiO₂ nanoparticles display close to zero charge at pH 7.4, resulting in aggregation. At pH 3.4, in contrast, the 6 nm TiO₂ nanoparticles are well dispersed due to a strongly positive ζ-potential. Mirroring this pH dependence, TiO₂ nanoparticles were observed to bind to negatively charged lipid bilayers at pH 3.4, but much less so at pH 7.4. While nanoparticle binding has some destabilizing effect alone, illumination with ultraviolet (UV) light accentuates membrane destabilization, a result of oxidative stress caused by generated reactive oxygen species (ROS). Neutron reflectivity (NR), quartz crystal microbalance (QCM), and small-angle X-ray scattering (SAXS) results all demonstrate that membrane composition strongly influences membrane interactions and photooxidative destabilization of lipid bilayers. In particular, the presence of anionic POPG makes the bilayers more sensitive to oxidative destabilization, whereas a stabilizing effect was observed in the presence of cholesterol. Also, structural aspects of peroxidation were found to depend strongly on membrane composition, notably the presence of anionic phospholipids. The results show that membrane interactions and UV-induced ROS generation act in concert and need to be considered together to understand effects of lipid membrane composition on UV-triggered oxidative destabilization by TiO₂ nanoparticles, e.g., in the context of oxidative damage of bacteria and cells.

Optimization of Slipids Force Field Parameters Describing Headgroups of Phospholipids

The molecular mechanics force field Slipids developed in a series of works by Jämbeck and Lyubartsev (J. Phys. Chem. B 2012, 116, 3164-3179; J. Chem. Theory Comput. 2012, 8, 2938-2948) generally provides a good description of various lipid bilayer systems. However, it was also found that order parameters of C-H bonds in the glycerol moiety of the phosphatidylcholine headgroup deviate significantly from NMR results. In this work, the dihedral force field parameters have been reparameterized in order to improve the agreement with experiment. For this purpose, we have computed energies for a large amount of lipid headgroup conformations using density functional theory on the B3P86/cc-pvqz level and optimized dihedral angle parameters simultaneously to provide the best fit to the quantum chemical energies. The new parameter set was validated for three lipid bilayer systems against a number of experimental properties including order parameters, area per lipid, scattering form factors, bilayer thickness, area compressibility and lateral diffusion coefficients. In addition, the order parameter dependence on cholesterol content in the POPC bilayer was investigated. It is shown that the new force field significantly improves agreement with the experimental order parameters for the lipid headgroup while keeping good agreement with other experimentally measured properties.

Magainin 2 and PGLa in Bacterial Membrane Mimics II: Membrane Fusion and Sponge Phase Formation

We studied the synergistic mechanism of equimolar mixtures of magainin 2 (MG2a) and PGLa in phosphatidylethanolamine/phosphatidylglycerol mimics of Gram-negative cytoplasmic membranes. In a preceding article of this series, we reported on the early onset of parallel heterodimer formation of the two antimicrobial peptides already at low concentrations and the resulting defect formation in the membranes. Here, we focus on the structures of the peptide-lipid aggregates occurring in the synergistic regime at elevated peptide concentrations. Using a combination of calorimetric, scattering, electron microscopic, and in silico techniques, we demonstrate that the two peptides, even if applied individually, transform originally large unilamellar vesicles into multilamellar vesicles with a collapsed interbilayer spacing resulting from peptide-induced adhesion. Interestingly, the adhesion does not lead to a peptide-induced lipid separation of charged and charge-neutral species. In addition to this behavior, equimolar mixtures of MG2a and PGLa formed surface-aligned fibril-like structures, which induced adhesion zones between the membranes and the formation of transient fusion stalks in molecular dynamics simulations and a coexisting sponge phase observed by small-angle x-ray scattering. The previously reported increased leakage of lipid vesicles of identical composition in the presence of MG2a/PGLa mixtures is therefore related to a peptide-induced cross-linking of bilayers.

Coupled Response of Membrane Hydration with Oscillating Metabolism in Live Cells: An Alternative Way to Modulate Structural Aspects of Biological Membranes?

We propose that active metabolic processes may regulate structural changes in biological membranes via the physical state of cell water. This proposition is based on recent results obtained from our group in yeast cells displaying glycolytic oscillations, where we demonstrated that there is a tight coupling between the oscillatory behavior of glycolytic metabolites (ATP, NADH) and the extent of the dipolar relaxation of intracellular water, which oscillates synchronously. The mechanism we suggest involves the active participation of a polarized intracellular water network whose degree of polarization is dynamically modulated by temporal ATP fluctuations caused by metabolism with intervention of a functional cytoskeleton, as conceived in the long overlooked association-induction hypothesis (AIH) of Gilbert Ling. Our results show that the polarized state of intracellular water can be propagated from the cytosol to regions containing membranes. Since changes in the extent of the polarization of water impinge on its chemical activity, we hypothesize that metabolism dynamically controls the local structure of cellular membranes via lyotropic effects. This hypothesis offers an alternative way to interpret membrane related phenomena (e.g., changes in local curvature pertinent to endo/exocytosis or dynamical changes in membranous organelle structure, among others) by integrating relevant but mostly overlooked physicochemical characteristics of the cellular milieu.

Caveolin-1 Endows Order in Cholesterol-Rich Detergent Resistant Membranes

Cholesterol-enriched functional portions of plasma membranes, such as caveolae and rafts, were isolated from lungs of wild-type (WT) and caveolin-1 knockout (Cav-1 KO) mice within detergent resistant membranes (DRMs). To gain insight into their molecular composition we performed proteomic and lipid analysis on WT and Cav-1 KO-DRMs that showed predicted variations of proteomic profiles and negligible differences in lipid composition, while Langmuir monolayer technique and small and wide-angle X-ray scattering (SAXS-WAXS) were here originally introduced to study DRMs biophysical association state. Langmuir analysis of Cav-1 containing DRMs displayed an isotherm with a clear-cut feature, suggesting the coexistence of the liquid-ordered (L_o) phase typical of the raft structure, namely “cholesterol-rich L_o phase”, with a phase fully missing in Cav-1 KO that we named “caveolin-induced L_o phase”. Furthermore, while the sole lipid component of both WT and KO-DRMs showed qualitatively similar isotherm configuration, the reinsertion of recombinant Cav-1 into WT-DRMs lipids restored the WT-DRM pattern. X-ray diffraction results confirmed that Cav-1 causes the formation of a “caveolin-induced L_o phase”, as suggested by Langmuir experiments, allowing us to speculate about a possible structural model. These results show that the unique molecular link between Cav-1 and cholesterol can spur functional order in a lipid bilayer strictly derived from biological sources.

Polar-Nonpolar Interfaces of Inverse Bicontinuous Cubic Phases in Phytantriol/Water System are Parallel to Triply Periodic Minimal Surfaces

We investigated two distinct lyotropic liquid crystal inverse bicontinuous cubic phases of phytantriol/water mixtures by small-angle X-ray crystallography of the single-crystal regions. Reconstructed electron density maps revealed hydrophilic head and hydrophobic tail regions of the phytantriol bilayer membranes and water regions. The bilayer membranes are shown to be located on the D and gyroid triply periodic minimal surfaces. To investigate the structures of the polar-nonpolar interfaces, we optimized two models: a parallel surface model and a constant mean curvature surface model. The parallel surface model agreed well with the X-ray data, and the R factors, which show the degree of agreement between those structural models and the data, were less than 0.04. In stark contrast, the constant mean curvature surface model deviated significantly from the data, and the R factors were around 0.15. We therefore conclude that the polar-nonpolar interface of the inverse bicontinuous cubic phase of the phytantriol/water system is close to a parallel surface to a triply periodic minimal surface.

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.

Nanodesign of new self-assembling core-shell gellan-transfersomes loading baicalin and in vivo evaluation of repair response in skin

Gellan nanohydrogel and phospholipid vesicles were combined to incorporate baicalin in new self-assembling core-shell gellan-transfersomes obtained by an easy, scalable method. The vesicles were small in size (~107 nm) and monodispersed (P.I. ≤ 0.24), forming a viscous system (~24 mPa/s) as compared to transfersomes (~1.6 mPa/s), as confirmed by rheological studies. Gellan was anchored to the bilayer domains through cholesterol, and the polymer chains were distributed onto the outer surface of the bilayer, thus forming a core-shell structure, as suggested by SAXS analyses. The optimal carrier ability of core-shell gellan-transfersomes was established by the high deposition of baicalin in the skin (~11% in the whole skin), especially in the deeper tissue (~8% in the dermis). Moreover, their ability to improve baicalin efficacy in anti-inflammatory and skin repair tests was confirmed in vivo in mice, providing the complete skin restoration and inhibiting all the studied inflammatory markers.

Toward the Fabrication of Advanced Nanofiltration Membranes by Controlling Morphologies and Mesochannel Orientations of Hexagonal Lyotropic Liquid Crystals

Water scarcity has been recognized as one of the major threats to human activity, and, therefore, water purification technologies are increasingly drawing attention worldwide. Nanofiltration (NF) membrane technology has been proven to be an efficient and cost-effective way in terms of the size and continuity of the nanostructure. Using a template based on hexagonal lyotropic liquid crystals (LLCs) and partitioning monomer units within this structure for subsequent photo-polymerisation presents a unique path for the fabrication of NF membranes, potentially producing pores of uniform size, ranging from 1 to 5 nm, and large surface areas. The subsequent orientation of this pore network in a direction normal to a flat polymer film that provides ideal transport properties associated with continuous pores running through the membrane has been achieved by the orientation of hexagonal LLCs through various strategies. This review presents the current progresses on the strategies for structure retention from a hexagonal LLCs template and the up-to-date techniques used for the reorientation of mesochanels for continuity through the whole membrane.

Adjuvants Based on Synthetic Mycobacterial Cord Factor Analogues: Biophysical Properties of Neat Glycolipids and Nanoself-Assemblies with DDA

Synthetic mycobacterial cord factor analogues, e.g., trehalose 6,6'-dibehenate (TDB), are highly promising adjuvants due to their strong immunopotentiating capabilities, but their biophysical properties have remained poorly characterized. Here, we report the synthesis of an array of synthetic TDB analogues varying in acyl chain length, degree of acylation, and headgroup display, which was subjected to biophysical characterization of neat nondispersed self-assembled nanostructures in excess buffer and as aqueous dispersions with cationic dimethyldioctadecylammonium (DDA) bromide. The array comprised trehalose mono- (TMX) and diester (TDX) analogues with symmetrically shortened acyl chains [denoted by X: arachidate (A), stearate (S), palmitate (P), myristate (Myr), and laurate (L)] and an analogue with a short hydrophilic polyethylene glycol (PEG) linker inserted between the trehalose headgroup of TDS and the acyl chains (PEG-TDS). All dispersions were liposomes, but in contrast to the colloidally stable and highly cationic TDX-containing liposomes, the zeta-potential was significantly reduced for DDA/TMX and DDA/PEG-TDS liposomes, suggesting a charge-shielding effect, which compromises the colloidal stability. An increased d-spacing was observed for the lamellar phase of neat TDB analogues in excess buffer (TDS < TMS < PEG-TDS), confirming that the charge shielding is caused by an extended molecular configuration of the more flexible headgroup. Differential scanning calorimetry showed highly cooperative phase transitions for all tested dispersions albeit the monoesters destabilized the lipid bilayers. Langmuir experiments demonstrated that incorporation of TDXs and PEG-TDS stabilized DDA monolayers due to improved hydrogen bonding and reduced intermolecular repulsions. In conclusion, data suggest that the DDA/TDS dispersions exhibit favorable physicochemical properties rendering these DDA/TDS liposomes an attractive vaccine adjuvant, and they emphasize that not only the receptor binding and immune activation but also the biophysical properties of immunopotentiator formulations should be collectively considered when designing adjuvants with optimal safety, efficacy, and storage stability.

Microfluidic Platform for the Continuous Production and Characterization of Multilamellar Vesicles: A Synchrotron Small-Angle X-ray Scattering (SAXS) Study

A microfluidic platform combined with synchrotron small-angle X-ray scattering (SAXS) was used for monitoring the continuous production of multilamellar vesicles (MLVs). Their production was fast and started to evolve within less than 0.43 s of contact between the lipids and the aqueous phase. To obtain nanoparticles with a narrow size distribution, it was important to use a modified hydrodynamic flow focusing (HFF) microfluidic device with narrower microchannels than those normally used for SAXS experiments. Monodispersed MLVs as small as 160 nm in size, with a polydispersity index (PDI) of approximately 0.15 were achieved. The nanoparticles produced were smaller and had a narrower size distribution than those obtained via conventional bulk mixing methods. This microfluidic platform therefore has a great potential for the continuous production of monodispersed NPs.

Conserved Molecular Superlattices in a Series of Homologous Synthetic Mycobacterial Cell-Wall Lipids Forming Interdigitated Bilayers

Synthetic analogues of the cell-wall lipid monomycoloyl glycerol (MMG) are promising as next-generation vaccine adjuvants. In the present study, the thermotropic phase behavior of an array of synthetic MMG analogues was examined by using simultaneous small- and wide-angle X-ray scattering under excess water conditions. The MMG analogues differed in the alkyl chain lengths and in the stereochemistry of the polar glycerol headgroup or of the lipid tails (native-like versus alternative compounds). All MMG analogues formed poorly hydrated lamellar phases at low temperatures and inverse hexagonal (H₂) phases at higher temperatures prior to melting. MMG analogues with a native-like lipid acid configuration self-assembled into noninterdigitated bilayers whereas the analogues displaying an alternative lipid acid configuration formed interdigitated bilayers in a subgel (L_c') state. This is in contrast to previously described interdigitated phases for other lipids, which are usually in a gel (L_β) state. All investigated MMG analogues displayed an abrupt direct temperature-induced phase transition from L_c' to H₂. This transition is ultimately driven by the lipid chain melting and the accompanying change in molecular shape. No intermediate structures were found, but the entire array of MMG analogues displayed phase coexistence during the lamellar to H₂ transition. The structural data also showed that the headgroups of the MMG analogues adopting the alternative lipid acid configuration were ordered and formed a two-dimensional molecular superlattice, which was conserved regardless of the lipid tail length. To our knowledge, the MMG analogues with an alternative lipid acid configuration represent the first example of a lipid system showing both interdigitation and superlattice formation, and as such could serve as an interesting model system for future studies. The MMG analogues are also relevant from a subunit vaccine perspective because they are well-tolerated and display promising immunopotentiating activity. The structural characterization described here will serve as a prerequisite for the rational design of nanoparticulate adjuvants with specific and tailored structural features.

Nano-Self-Assemblies Based on Synthetic Analogues of Mycobacterial Monomycoloyl Glycerol and DDA: Supramolecular Structure and Adjuvant Efficacy

The mycobacterial cell-wall lipid monomycoloyl glycerol (MMG) is a potent immunostimulator, and cationic liposomes composed of a shorter synthetic analogue (MMG-1) and dimethyldioctadecylammonium (DDA) bromide represent a promising adjuvant that induces strong antigen-specific Th1 and Th17 responses. In the present study, we investigated the supramolecular structure and in vivo adjuvant activity of dispersions based on binary mixtures of DDA and an array of synthetic MMG-1 analogues (MMG-2/3/5/6) displaying longer (MMG-2) or shorter (MMG-3) alkyl chain lengths, or variations in stereochemistry of the polar headgroup (MMG-5) or of the hydrophobic moiety (MMG-6). Synchrotron small-angle X-ray scattering experiments and cryo transmission electron microscopy revealed that DDA:MMG-1/2/5/6 dispersions consisted of unilamellar and multilamellar vesicles (ULVs/MLVs), whereas a coexistence of both ULVs and hexosomes was observed for DDA:MMG-3, depending on the DDA:MMG molar ratio. The studies also showed that ULVs were formed, regardless of the structural characteristics of the neat MMG analogues in excess buffer [lamellar (MMG-1/2/5) or inverse hexagonal (MMG-3/6) phases]. Immunization of mice with a chlamydia antigen surface-adsorbed to DDA:MMG-1/3/6 dispersions revealed that all tested adjuvants were immunoactive and induced strong Th1 and Th17 responses with a potential for a central effector memory profile. The MMG-1 and MMG-6 analogues were equally immunoactive in vivo upon incorporation into DDA liposomes, despite the reported highly different immunostimulatory properties of the neat analogues in vitro, which were attributed to the different nanostructural characteristics. This clearly demonstrates that optimal formulation and delivery of MMG analogues to the immune system is of major importance and challenges the use of in vitro screening assays with nondispersed compounds to identify potential new vaccine adjuvants.

Delivery of siRNA Complexed with Palmitoylated α-Peptide/β-Peptoid Cell-Penetrating Peptidomimetics: Membrane Interaction and Structural Characterization of a Lipid-Based Nanocarrier System

Proteolytically stable α-peptide/β-peptoid peptidomimetics constitute promising cell-penetrating carrier candidates exhibiting superior cellular uptake as compared to commonly used cell-penetrating peptides (CPPs). The aim of the present study was to explore the potential of these peptidomimetics for delivery of small interfering RNA (siRNA) to the cytosol by incorporation of a palmitoylated peptidomimetic construct into a cationic lipid-based nanocarrier system. The optimal construct was selected on the basis of the effect of palmitoylation and the influence of the length of the peptidomimetic on the interaction with model membranes and the cellular uptake. Palmitoylation enhanced the peptidomimetic adsorption to supported lipid bilayers as studied by ellipsometry. However, both palmitoylation and increased peptidomimetic chain length were found to be beneficial in the cellular uptake studies using fluorophore-labeled analogues. Thus, the longer palmitoylated peptidomimetic was chosen for further formulation of siRNA in a dioleoylphosphatidylethanolamine/cholesteryl hemisuccinate (DOPE/CHEMS) nanocarrier system, and the resulting nanoparticles were found to mediate efficient gene silencing in vitro. Cryo-transmission electron microscopy (cryo-TEM) revealed multilamellar, onion-like spherical vesicles, and small-angle X-ray scattering (SAXS) analysis confirmed that the majority of the lipids in the nanocarriers were organized in lamellar structures, yet coexisted with a hexagonal phase, which is important for efficient nanocarrier-mediated endosomal escape of siRNA ensuring cytosolic delivery. The present work is a proof-of-concept for the use of α-peptides/β-peptoid peptidomimetics in an efficient delivery system that may be more generally exploited for the intracellular delivery of biomacromolecular drugs.

Generation of geometrically ordered lipid-based liquid-crystalline nanoparticles using biologically relevant enzymatic processing

High-symmetry lipid nanoparticles with internal bicontinuous cubic phase structure (cubosomes) are prepared from a simple emulsion containing a mixture of a nondigestible lipid (phytantriol) and a digestible short-chained triglyceride using enzymatic lipolysis of the incorporated short-chained triglyceride. The lipolytic products partition away from the nondigestible lipid, resulting in crystallization of the cubic-phase internal structure. Time-resolved small-angle X-ray scattering revealed the kinetics of the disorder-to-order transition, with cryo-transmission electron microscopy showing an absence of liposomes. The new approach offers a new "sideways" method for the generation of lipid-based nanostructured materials that avoids the problems of top-down and bottom-up approaches.

Control and analysis of oriented thin films of lipid inverse bicontinuous cubic phases using grazing incidence small-angle X-ray scattering

Lipid cubic phases are complex nanostructures that form naturally in a variety of biological systems, with applications including drug delivery and nanotemplating. Most X-ray scattering studies on lipid cubic phases have used unoriented polydomain samples as either bulk gels or suspensions of micrometer-sized cubosomes. We present a method of investigating cubic phases in a new form, as supported thin films that can be analyzed using grazing incidence small-angle X-ray scattering (GISAXS). We present GISAXS data on three lipid systems: phytantriol and two grades of monoolein (research and industrial). The use of thin films brings a number of advantages. First, the samples exhibit a high degree of uniaxial orientation about the substrate normal. Second, the new morphology allows precise control of the substrate mesophase geometry and lattice parameter using a controlled temperature and humidity environment, and we demonstrate the controllable formation of oriented diamond and gyroid inverse bicontinuous cubic along with lamellar phases. Finally, the thin film morphology allows the induction of reversible phase transitions between these mesophase structures by changes in humidity on subminute time scales, and we present time-resolved GISAXS data monitoring these transformations.

Controlling anisotropic drug diffusion in lipid-Fe3O4 nanoparticle hybrid mesophases by magnetic alignment

We present a new strategy to control the anisotropic diffusion of hydrophilic drugs in lyotropic liquid crystals via the dispersion of magnetic nanoparticles in the mesophase, followed by reorientation of the mesophase domains via an external magnetic field. We select a lipid reverse hexagonal phase doped with magnetic iron oxide nanoparticles and glucose and caffeine as model hybrid mesophase and hydrophilic drugs, respectively. Upon cooling through the disorder-order phase transition of the hexagonal phase and under exposure to an external moderate magnetic field (1.1 T), both the nanoparticles and the hexagonal domains align with their columnar axes along the field direction. As a result, the water nanochannels of the inverted hexagonal domains also align parallel to the field direction, leading to a drug diffusion coefficient parallel to the field direction much larger than what was measured perpendicularly: in the case of glucose, for example, this difference in diffusion coefficients approaches 1 order of magnitude. Drug diffusion of the unaligned reverse hexagonal phase, which consists of randomly distributed domains, shows values in between the parallel and transversal diffusion values. This study shows that modifying the overall alignment of anisotropic mesophases via moderate external fields is a valuable means to control the corresponding transport tensor of the mesophase and demonstrates that the orientation of the domains plays an important role in the diffusion process of foreign hydrophilic molecules.

Self-assembled nanostructures of fully hydrated monoelaidin-elaidic acid and monoelaidin-oleic acid systems

In recent years, there has been a surge of interest in exploring the effect of trans-fatty acids (TFAs) on biological membrane properties. The research studies are motivated by an increasing body of evidence suggesting that the consumption of TFAs increases the risk of developing negative health effects such as coronary heart disease and cancer. The ultimate goal of studying the lipid-fatty acid interactions at the molecular level is to predict the biological role of fatty acids in cells. In this regard, it is interesting to elucidate the effect of loading TFAs and their counterpart cis-fatty acids (CFAs) on the physical properties of lipid model membranes. Here, the present study focuses on discussing the following: (1) the effect of mixing monoelaidin (ME, TFA-containing lipid) with its counterpart monoolein (MO, CFA-containing lipid) on modulating the fully hydrated self-assembled structure, and (2) the influence of solubilizing oleic acid (OA) and its trans counterpart elaidic acid (EA) on the fully hydrated ME system. The ME model membrane was selected due to its sensitivity to variations in lipid composition and temperature. Synchrotron small-angle X-ray scattering (SAXS) was applied for studying the temperature-dependent structural behavior of the fully hydrated ME/MO-based system prepared with an equal ME/MO weight ratio and also for characterizing the fully hydrated OA- and EA-loaded ME systems. Wide-angle X-ray (WAXS) experiments were also performed for characterizing the formed crystalline lamellar phases at ambient temperatures. The results demonstrate the significant influence of the partial replacement of ME by MO on the phase behavior. The addition of MO induces the lamellar-nonlamellar phase transitions at ambient temperatures and promotes the formation of the inverted type hexagonal (H(2)) phase above 72 °C. The fully hydrated ME/EA and ME/OA systems with their rich polymorphism exhibit an interesting temperature-dependent complex behavior. The experimental findings show that the temperature-induced phase transitions are dictated by the solubilized fatty acid concentration and its configuration. Both EA and OA have a significant impact on the fully hydrated ME system. Similar to previous published studies, OA induces a significantly stronger mean negative membrane curvature as compared to EA. The two phase diagrams are discussed in terms of water-lipid and lipid-fatty acid interactions, membrane bending, and lipid packing concepts. A newly observed interesting epitaxial relationship for the lamellar-hexagonal phase transition in the EA-loaded ME system is illustrated and discussed in detail.

Characterization of bupivacaine-loaded formulations based on liquid crystalline phases and microemulsions: the effect of lipid composition

This report details the structural characterization and the in vitro drug-release properties of different local anesthetic bupivacaine (BUP)-loaded inverted-type liquid crystalline phases and microemulsions. The effects of variations in the lipid composition and/or BUP concentration on the self-assembled nanostructures were investigated in the presence of the commercial distilled glycerol monooleate Myverol 18-99K (GMO) and medium-chain triglycerides (MCT). Synchrotron small-angle X-ray scattering (SAXS) and rotating dialysis cell model were used to characterize the BUP formulations and to investigate the in vitro BUP release profiles, respectively. The evaluation of SAXS data for the BUP-loaded GMO/MCT formulations indicates the structural transition of inverted-type bicontinuous cubic phase of the symmetry Pn3m → inverted-type hexagonal (H(2)) phase → inverted-type microemulsion (L(2)) with increasing MCT content (0-40 wt %). In the absence of MCT, the solubilization of BUP induces the transition of Pn3m → H(2) at pH 7.4; whereas a transition of Pn3m → (Pn3m + H(2)) is detected as the hydration is achieved at pH 6.0. To mimic the drug release and transport from in situ formed self-assembled systems after subcutaneous administration, the release experiments were performed by injecting low viscous stimulus-responsive precursors to a buffer in the dialysis cell leaving the surface area between the self-assembled system and the release medium variable. Our results suggest that the pH-dependent variations in the lipidic partition coefficient, K(l/w), between the liquid crystalline nanostructures and the surrounding buffer solution are significantly affecting BUP release rates. Thus, a first step toward understanding of the drug-release mechanism of this drug-delivery class has been undertaken tackling the influence of drug ionization as well as the type of the self-assembled nanostructure and its release kinetics under pharmaceutically relevant conditions.

Mesostructured silica aerosol particles: comparison of gas-phase and powder deposit X-ray diffraction data

We report on the characterization of mesostructured aerosol silica particles in the gas phase using in situ synchrotron small-angle X-ray scattering (SAXS) in order to unveil the influence of the basic production parameters. The investigated system was based on tetraethylorthosilicate (TEOS) as the inorganic precursor and on cetyltrimethyl-ammonium bromide (CTAB) as the surfactant. The heating temperature, surfactant to silicate ratio, and particle flow rate were thoroughly investigated, and for this purpose, an in-house-built aerosol reactor equipped with a special X-ray observation chamber was used. Complementary fine structural analysis was applied on dried deposits of the silica aerosols comprising direct Fourier transforms as well as simple two-phase model fits. This resulted in robust estimates for the silica wall thickness and surfactant core radius of the hexagonally ordered mesostructure. The particle shape and size distribution were examined by scanning electron microscopy (SEM). The quality of the inner nanostructure was revealed from an analysis of the peak width. The comparison of data from the gas phase and powder deposit shows that, in general, slower drying conditions (heating temperature about 80 °C) and a medium surfactant to Si ratio (about 0.14) lead to nanostructures of the best quality in terms of well-defined long-range organization.

Effects of pressure and temperature on the self-assembled fully hydrated nanostructures of monoolein-oil systems

Synchrotron small-angle X-ray scattering (SAXS) was applied for studying the effects of hydrostatic pressure and temperature on the structural behavior of fully hydrated tetradecane (TC)-loaded monoolein (MO) systems. Our main attention focused on investigating the impact of isobaric and isothermal changes on the stability of the inverted type discontinuous Fd3m cubic phase as compared to the inverted type hexagonal (H(2)) liquid crystalline phase. The present results show that compressing the TC-loaded Fd3m phase under isothermal conditions induces a significant increase of its lattice parameter: it approximately increases by 1 A per 75 bar. Further, the Fd3m phase is more pressure-sensitive as compared to the Pn3m and the H(2) phases. At ambient temperatures, we observed the following structural transitions as pressure increases: Fd3m --> H(2) --> Pn3m. Our findings under isobaric conditions reveal more complicated structural transitions. At high pressures, we recorded the interesting temperature-induced structural transition of (Pn3m + L(alpha)) --> (Pn3m + L(alpha) + H(2)) --> (L(alpha) + H(2)) --> H(2) --> Fd3m --> traces of Fd3m coexisting with L(2). At high pressures and low temperatures, the TC molecules partially crystallize as indicated by the appearance of an additional diffraction peak at q = 3.46 nm(-1). This crystallite disappears at high temperatures and also as the system gets decompressed. The appearance of the Pn3m and the L(alpha) phases during compressing the fully hydrated MO/TC samples at high pressures and low temperatures is generally related to a growing hydrocarbon chain condensation, which leads to membrane leaflets with less negative interfacial curvatures (decreasing the spontaneous curvatures |H(0)|). Both the effects of pressure and temperature are discussed in detail for all nonlamellar phases on the basis of molecular shape and packing concepts.