“Bicellar” Lipid Mixtures as used in Biochemical and Biophysical Studies

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

MOTIVATION

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

EXPERIMENTS

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

FINDINGS

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

Efficient Drug Loading Method for Poorly Water-Soluble Drug into Bicelles through Passive Diffusion

The bicelle, a type of solid lipid nanoparticle, comprises phospholipids with varying alkyl chain lengths and possesses the ability to solubilize poorly water-soluble drugs. Bicelle preparation is complicated and time-consuming because conventional drug-loading methods in bicelles require multiple rounds of thermal cycling or co-grinding with drugs and lipids. In this study, we proposed a simple drug-loading method for bicelles that utilizes passive diffusion. Drug-unloaded bicelles were placed inside a dialysis device and incubated in a saturated solution of ketoconazole (KTZ), which is a model drug. KTZ was successfully loaded into bare bicelles over time with morphological changes, and the final encapsulated concentration was dependent on the lipid concentration of the bicelles. When polyethylene glycol (PEG) chains of two different lengths (PEG2K and 5K) were incorporated into bicelles, PEG2k and PEG5k bicelles mitigated the morphological changes and improved the encapsulation rate. This mitigation of morphological changes enhanced the encapsulated drug concentration. Specifically, PEG5k bicelles, which exhibited the greatest prevention of morphological changes, had a lower encapsulated concentration after 24 h than that of PEG2k bicelles, indicating that PEGylation with a longer PEG chain length improved the loading capacity but decreased the encapsulation rate owing to the presence of a hydration layer of PEG. Thus, PEG with a certain length is more suitable for passive loading. Moreover, loading factors, such as temperature and vehicles used in the encapsulation process, affected the encapsulation rate of the drug. Taken together, the passive loading method offers high throughput with minimal resources, making it a potentially valuable approach during early drug development phases.

Morphological control and modern applications of bicelles

Bicellar systems have become popularized as their rich morphology can be applied in biochemistry, physical chemistry, and drug delivery technology. To the biochemical field, bicelles are powerful model membranes for the study of transmembrane protein behavior, membrane transport, and environmental interactions with the cell. Their morphological responses to environmental changes reveal a profound fundamental understanding of physical chemistry related to the principle of self-assembly. Recently, they have also drawn significant attention as theranostic nanocarriers in biopharmaceutical and diagnostic research due to their superior cellular uptake compared to liposomes. It is evident that applications are becoming broader, demanding to understand how the bicelle will form and behave in various environments. To consolidate current works on the bicelle's modern applications, this review will discuss various effects of composition and environmental conditions on the morphology, phase behavior, and stability. Furthermore, various applications such as payload entrapment and polymerization templating are presented to demonstrate their versatility and chemical nature.

Effects of lipid saturation on bicelle to vesicle transition of a binary phospholipid mixture: a molecular dynamics simulation study

Controlling the transition from lipid bicelles to vesicles is essential for producing engineered vesicles. We perform coarse-grained molecular dynamics (CGMD) simulations of unsaturated/saturated lipid mixtures to clarify the effects of lipid unsaturation on vesiculation at the molecular scale. The results demonstrate that vesiculation depends on the concentration of unsaturated lipids and the degree of unsaturation. The probability of vesiculation increases linearly with the apparent unsaturated lipid concentration at a low degree of unsaturation. Higher degrees of unsaturation lead to phase segregation within the binary bicelles, reducing the probability of vesiculation. A comparison between CGMD simulations and the conventional theory of vesiculation shows that the theoretical predictions of binary lipid systems must explicitly include phase segregation effects. Furthermore, simulations with biased lipid distributions reveal that vesiculation is facilitated by the preconcentration of unsaturated lipids in the core region of the bicelle but is then temporally limited as the unsaturated lipids move to the bicelle edges. These findings advance theoretical and experimental studies on binary lipid systems and promote the development of tailor-made vesicles.

Fragmentation of DMPC Membranes by a Wedge-Shaped Amphiphilic Cyclodextrin into Bicellar-like Aggregates

Small bilayer lipid aggregates such as bicelles provide useful isotropic or anisotropic membrane mimetics for structural studies of biological membranes. We have shown previously by deuterium NMR that a wedge-shaped amphiphilic derivative of trimethyl βcyclodextrin anchored in deuterated DMPC-d27 bilayers through a lauryl acyl chain (TrimβMLC) is able to induce magnetic orientation and fragmentation of the multilamellar membranes. The fragmentation process fully detailed in the present paper is observed with 20% cyclodextrin derivative below 37 °C, where pure TrimβMLC self-assembles in water into large giant micellar structures. After deconvolution of a broad composite ²H NMR isotropic component, we propose a model where the DMPC membranes are progressively disrupted by TrimβMLC into small and large micellar aggregates depending whether they are extracted from the outer or inner layers of the liposomes. Below the fluid-to-gel transition of pure DMPC-d27 membranes (T_c = 21.5 °C), the micellar aggregates vanish progressively until complete extinction at 13 °C, with a probable release of pure TrimβMLC micelles leaving lipid bilayers in the gel phase doped with only a small amount of the cyclodextrin derivative. Bilayer fragmentation between T_c and 13 °C was also observed with 10% and 5% of TrimβMLC, with NMR spectra suggesting possible interactions of micellar aggregates with fluid-like lipids of the P_β' ripple phase. No membrane orientation and fragmentation was detected with unsaturated POPC membranes, which are able to accommodate the insertion of TrimβMLC without important perturbation. The data are discussed in relation to the formation of possible DMPC bicellar aggregates such as those known to occur after insertion of dihexanoylphosphatidylcholine (DHPC). These bicelles are in particular associated with similar deuterium NMR spectra exhibiting identical composite isotropic components which were never characterized before.

Stable Discoidal Bicelles: Formulation, Characterization, and Functions

Bicellar mixtures have been used as alignable membrane substrates under a magnetic field applicable for the structural characterization of membrane-associated proteins. Recently, it has shown that bicelles can serve as nanocarriers to effectively deliver hydrophobic therapeutic molecules to cancer cells with a three- to ten-fold enhancement compared to that of liposomes of a chemically identical composition. In this chapter, detailed preparation protocol, common structural characterization methods, the structural stability, the cellular uptake and a few unique functions of bicellar nanodiscs are discussed.

Expanding the Toolbox for Bicelle-Forming Surfactant–Lipid Mixtures

Bicelles are disk-shaped models of cellular membranes used to study lipid–protein interactions, as well as for structural and functional studies on transmembrane proteins. One challenge for the incorporation of transmembrane proteins in bicelles is the limited range of detergent and lipid combinations available for the successful reconstitution of proteins in model membranes. This is important, as the function and stability of transmembrane proteins are very closely linked to the detergents used for their purification and to the lipids that the proteins are embedded in. Here, we expand the toolkit of lipid and detergent combinations that allow the formation of stable bicelles. We use a combination of dynamic light scattering, small-angle X-ray scattering and cryogenic electron microscopy to perform a systematic sample characterization, thus providing a set of conditions under which bicelles can be successfully formed.

Change in Morphology of Dimyristoylphosphatidylcholine/Bile Salt Derivative Bicelle Assemblies with Dodecylmaltoside in the Disk and Ribbon Phases

Bicelles, composed of a mixture of long and short chain lipids, form nanostructured molecular assemblies that are attractive lipid-membrane mimics for in vitro studies of integral membrane proteins. Here we study the effect of a third component, the single chain detergent n-dodecyl-β-d-maltoside (DDM) on the morphology of bicelles composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO) below (10 °C) and above (38 °C) the phase transition. In the absence of DDM, bicelles convert from ellipsoidal disks at 10 °C to extended ribbon-like structures at 38 °C. The addition of DDM reshapes the ellipsoidal disc to a circular one and the flattened ribbon to a circular-cylinder worm-like micelle. Knowledge of the influence of the single chain detergent DDM on bicelle nanoscale morphology contributes toward comprehending lipid membrane self-organization and to the goal of optimizing lipid mimics for membrane biology research.

Mechanisms of membrane protein crystallization in 'bicelles'

Despite remarkable progress, mainly due to the development of LCP and ‘bicelle’ crystallization, lack of structural information remains a bottleneck in membrane protein (MP) research. A major reason is the absence of complete understanding of the mechanism of crystallization. Here we present small-angle scattering studies of the evolution of the “bicelle” crystallization matrix in the course of MP crystal growth. Initially, the matrix corresponds to liquid-like bicelle state. However, after adding the precipitant, the crystallization matrix transforms to jelly-like state. The data suggest that this final phase is composed of interconnected ribbon-like bilayers, where crystals grow. A small amount of multilamellar phase appears, and its volume increases concomitantly with the volume of growing crystals. We suggest that the lamellar phase surrounds the crystals and is critical for crystal growth, which is also common for LCP crystallization. The study discloses mechanisms of “bicelle” MP crystallization and will support rational design of crystallization.

Changes Experienced by Low-Concentration Lipid Bicelles as a Function of Temperature

Differential scanning calorimetry (DSC) of dipalmitoyl phosphatidylcholine (DPPC), dihexanoyl phosphatidylcholine, and dipalmitoyl phosphatidylglycerol bicelles reveals two endothermic peaks. Based on analysis of small angle neutron scattering and small angle X-ray scattering data, the two DSC peaks are associated with the melting of DPPC and a change in bicellar morphology─namely, either bicelle-to-spherical vesicle or oblate-to-spherical vesicle. The reversibility of the two structural transformations was examined by DSC and found to be consistent with the corresponding small angle scattering data. However, the peak that is not associated with the melting of DPPC does not correspond to any structural transformation for bicelles containing distearoyl phosphatidylethanolamine conjugated with polyethylene glycol. Based on complementary experimental data, we conclude that membrane flexibility, lipid miscibility, and differential solubility between the long- and short-chain lipids in water are important parameters controlling the reversibility of morphologies experienced by the bicelles.

Insights into the mechanism of high lipid–detergent crystallization of membrane proteins

Obtaining well diffracting crystals of membrane proteins is often challenging, but chances can be improved by crystallizing them in lipidic conditions that mimic their natural membrane environments. One approach is the high lipid–detergent (HiLiDe) method, which works by mixing the target protein with high concentrations of lipid and detergent prior to crystallization. Although this approach is convenient and flexible, understanding the effects of systematically varying lipid/detergent ratios and a characterization of the lipid phases that form during crystallization would be useful. Here, a HiLiDe phase diagram is reported for the model membrane protein MhsT, which tracks the precipitation and crystallization zones as a function of lipid and detergent concentrations, and is augmented with data on crystal sizes and diffraction properties. Additionally, the crystallization of SERCA1a solubilized directly with native lipids is characterized as a function of detergent concentration. Finally, HiLiDe crystallization drops are analysed with transmission electron microscopy, which among other features reveals liposomes, stacked lamellae that may represent crystal precursors, and mature crystals with clearly discernible packing arrangements. The results emphasize the significance of optimizing lipid/detergent ratios over broad ranges and provide insights into the mechanism of HiLiDe crystallization.

Versatile Phospholipid Assemblies for Functional Synthetic Cells and Artificial Tissues

The bottom-up construction of a synthetic cell from nonliving building blocks capable of mimicking cellular properties and behaviors helps to understand the particular biophysical properties and working mechanisms of a cell. A synthetic cell built in this way possesses defined chemical composition and structure. Since phospholipids are native biomembrane components, their assemblies are widely used to mimic cellular structures. Here, recent developments in the formation of versatile phospholipid assemblies are described, together with the applications of these assemblies for functional membranes (protein reconstituted giant unilamellar vesicles), spherical and nonspherical protoorganelles, and functional synthetic cells, as well as the high-order hierarchical structures of artificial tissues. Their biomedical applications are also briefly summarized. Finally, the challenges and future directions in the field of synthetic cells and artificial tissues based on phospholipid assemblies are proposed.

Effects of fluidity and charge density on the morphology of a bicellar mixture - A SANS study

The spontaneously formed structures of physiologically relevant lipid model membranes made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and 1,2-hexanoyl-sn-glycero-3-phosphocholine have been evaluated in depth using small angle neutron scattering. Although a common molar ratio of long- to short- chain phospholipids (~4) as reported in many bicellar mixtures was used, discoidal bicelles were not found as the major phase throughout the range of lipid concentration and temperature studied, indicating that the required condition for the formation of bicelle is the immiscibility between the long- and short- chain lipids, which were in the gel and L_α phases, respectively, in previous reports. In this study, all lipids are in the L_α phase. The characterization outcome suggests that the spontaneous structures tie strongly with the physical parameters of the system such as melting transition temperature of the long-chain lipid, total lipid concentration and charge density of the system. Multilamellar vesicles, unilamellar vesicles, ribbons and perforated lamellae can be obtained based on the analysis of the small angle neutron scattering results, leading to the construction of structural diagrams. This report provides the important map to choose suitable lipid systems for the structural study of membrane-associated proteins, design of theranostic nanocarriers or other related research fields.

Versatile formation of supported lipid bilayers from bicellar mixtures of phospholipids and capric acid

Originally developed for the structural biology field, lipid bicelle nanostructures composed of long- and short-chain phospholipid molecules have emerged as a useful interfacial science tool to fabricate two-dimensional supported lipid bilayers (SLBs) on hydrophilic surfaces due to ease of sample preparation, scalability, and versatility. To improve SLB fabrication prospects, there has been recent interest in replacing the synthetic, short-chain phospholipid component of bicellar mixtures with naturally abundant fatty acids and monoglycerides, i.e., lauric acid and monocaprin. Such options have proven successful under specific conditions, however, there is room for devising more versatile fabrication options, especially in terms of overcoming lipid concentration-dependent SLB formation limitations. Herein, we investigated SLB fabrication by using bicellar mixtures consisting of long-chain phospholipid and capric acid, the latter of which has similar headgroup and chain length properties to lauric acid and monocaprin, respectively. Quartz crystal microbalance-dissipation, epifluorescence microscopy, and fluorescence recovery after photobleaching experiments were conducted to characterize lipid concentration-dependent bicelle adsorption onto silicon dioxide surfaces. We identified that uniform-phase SLB formation occurred independently of total lipid concentration when the ratio of long-chain phospholipid to capric acid molecules ("q-ratio") was 0.25 or 2.5, which is superior to past results with lauric acid- and monocaprin-containing bicelles in which cases lipid concentration-dependent behavior was observed. Together, these findings demonstrate that capric acid-containing bicelles are versatile tools for SLB fabrication and highlight how the molecular structure of bicelle components can be rationally finetuned to modulate self-assembly processes at solid-liquid interfaces.

Bicelles Rich in both Sphingolipids and Cholesterol and Their Use in Studies of Membrane Proteins

How the distinctive lipid composition of mammalian plasma membranes impacts membrane protein structure is largely unexplored, partly because of the dearth of isotropic model membrane systems that contain abundant sphingolipids and cholesterol. This gap is addressed by showing that sphingomyelin and cholesterol-rich (SCOR) lipid mixtures with phosphatidylcholine can be cosolubilized by n-dodecyl-β-melibioside to form bicelles. Small-angle X-ray and neutron scattering, as well as cryo-electron microscopy, demonstrate that these assemblies are stable over a wide range of conditions and exhibit the bilayered-disc morphology of ideal bicelles even at low lipid-to-detergent mole ratios. SCOR bicelles are shown to be compatible with a wide array of experimental techniques, as applied to the transmembrane human amyloid precursor C99 protein in this medium. These studies reveal an equilibrium between low-order oligomer structures that differ significantly from previous experimental structures of C99, providing an example of how ordered membranes alter membrane protein structure.

Heating-Induced DMPC/Glycyrrhizin Bicelle-to-Vesicle Transition: A X-Ray Contrast Variation Study

In this study, we investigated the conversion of lipid bicelles into vesicles in the case of a system composed of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and the saponin glycyrrhizin in the presence of sucrose. Glycyrrhizin is a biosurfactant present in the licorice root and possesses a triterpenic hydrophobic backbone and a hydrophilic headgroup built from two sugar molecules. The aim of this study is to determine the initial bicelle size at temperatures below the lipid's main phase transition temperature T_m and, based on these results, characteristics of the temperature-induced bicelle-to-vesicle transition. Moreover, the influence of the heating rate on this transition is followed. The general picture concluded from photon correlation spectroscopy and small angle X-ray scattering was confirmed by additional imaging with cryogenic transmission electron microscopy. Small angle X-ray scattering was especially used to determine size parameters of the existing structures. To enhance the contrast for X-rays, a buffer containing 25 wt% sucrose was used. It was found that larger vesicles were formed from smaller precursor particles and that monodisperse precursors are required for formation of very monodisperse vesicles upon temperature increase. At high glycyrrhizin contents and above a critical heating rate of ∼5°C min^-1, the polydispersity of these vesicles is decoupled from both parameters, glycyrrhizin content and heating rate. However, the vesicle size stays tunable by the glycyrrhizin content and increases upon increasing the glycyrrhizin concentration. Therefore, vesicles of defined size and with a rather low polydispersity of ∼12-14% can be formed.

Supported Lipid Bilayer Formation from Phospholipid-Fatty Acid Bicellar Mixtures

Supported lipid bilayers (SLBs) are versatile cell membrane-mimicking biointerfaces for various applications such as biosensors and drug delivery systems, and there is broad interest in developing simple, cost-effective methods to achieve SLB fabrication. One promising approach involves the deposition of quasi-two-dimensional bicelle nanostructures that are composed of long-chain phospholipids and either short-chain phospholipids or detergent molecules. While a variety of long-chain phospholipids have been used to prepare bicelles for SLB fabrication applications, only two short-chain phospholipids, 1,2-dihexanoyl-sn-glycero-3-phosphocholine and 1,2-diheptanoyl-sn-glycero-3-phosphocholine (collectively referred to as DHPC), have been investigated. There remains an outstanding need to identify natural alternatives to DHPC, especially ones that are more affordable, to improve fabrication prospects and application opportunities. Herein, we explored the potential to fabricate SLBs from bicellar mixtures composed of long-chain phospholipids and lauric acid (LA), which is a low-cost, naturally abundant fatty acid that is widely used in soapmaking and various industrial applications. Quartz crystal microbalance-dissipation (QCM-D) experiments were conducted to track bicelle adsorption onto silica surfaces as a function of bicelle composition and lipid concentration, along with time-lapse fluorescence microscopy imaging and fluorescence recovery after photobleaching (FRAP) experiments to further characterize lipid adlayer properties. The results identified optimal conditions where it is possible to efficiently form SLBs from LA-containing bicelles at low lipid concentrations while also unraveling mechanistic insights into the bicelle-mediated SLB formation process and verifying that LA-containing bicelles are biocompatible with human cells for surface coating applications.

Targeted Delivery of Adamantylated Peptidoglycan Immunomodulators in Lipid Nanocarriers: NMR Shows That Cargo Fragments Are Available on the Surface

We present an in-depth investigation of the membrane interactions of peptidoglycan (PGN)-based immune adjuvants designed for lipid-based delivery systems using NMR spectroscopy. The derivatives contain a cargo peptidoglycan (PGN) dipeptide fragment and an adamantyl group, which serves as an anchor to the lipid bilayer. Furthermore, derivatives with a mannose group that can actively target cell surface receptors on immune cells are also studied. We showed that the targeting mannose group and the cargo PGN fragment are both available on the lipid bilayer surface, thereby enabling interactions with cognate receptors. We found that the nonmannosylated compounds are incorporated stronger into the lipid assemblies than the mannosylated ones, but the latter compounds penetrate deeper in the bilayer. This might be explained by stronger electrostatic interactions available for zwitterionic nonmannosylated derivatives as opposed to the compounds in which the charged N-terminus is capped by mannose groups. The higher incorporation efficiency of the nonmannosylated compounds correlated with a larger relative enhancement in immune stimulation activities upon lipid incorporation compared to that of the derivatives with the mannose group. The chirality of the adamantyl group also influenced the incorporation efficiency, which in turn correlated with membrane-associated conformations that affect possible intermolecular interactions with lipid molecules. These findings will help in improving the development of PGN-based immune adjuvants suitable for delivery in lipid nanoparticles.

Improving the quality of oriented membrane protein spectra using heat-compensated separated local field experiments

Oriented sample solid-state NMR (OS-ssNMR) spectroscopy is a powerful technique to determine the topology of membrane proteins in oriented lipid bilayers. Separated local field (SLF) experiments are central to this technique as they provide first-order orientational restraints, i.e., dipolar couplings and anisotropic chemical shifts. Despite the use of low-E (or E-free) probes, the heat generated during the execution of 2D and 3D SLF pulse sequences causes sizeable line-shape distortions. Here, we propose a new heat-compensated SE-SAMPI4 (hcSE-SAMPI4) pulse sequence that holds the temperature constant for the duration of the experiment. This modification of the SE-SAMPI4 results in sharper and more intense resonances without line-shape distortions. The spectral improvements are even more apparent when paramagnetic relaxation agents are used to speed up data collection. We tested the hcSE-SAMPI4 pulse sequence on a single-span membrane protein, sarcolipin (SLN), reconstituted in magnetically aligned lipid bicelles. In addition to eliminating peak distortions, the hcSE-SAMPI4 experiment increased the average signal-to-noise ratio by 20% with respect to the original SE-SAMPI4.

Small-Angle X-ray Scattering Studies on the Structure of Disc-Shaped Bicelles Incorporated with Neutral PEGylated Lipids

In this study, small-angle X-ray scattering (SAXS) is successfully employed to investigate the structure of the DPPC/diC7PC disc-shaped bicelles incorporated with different amounts of C16-PEG2000-Ceramide lipids. The incorporation of the C16-PEG2000-Ceramide lipids could provide an antifouling capability to the bicelle for biomedical applications. However, traditionally it is believed that most of the incorporated PEGlylated lipids should lie in the rim of the disc-shaped bicelle. In this study, high sensitivity SAXS reveals the distribution of the added C16-PEG2000-Ceramide lipids in both the planar region and in the rim of the bicelle. The PEG brushes of C16-PEG2000-Ceramide lipids form a second shell outside the lipid headgroup shell of the bicelle. A double shell disc bicelle model is used in analyzing the SAXS data. The lipid density of C16-PEG2000-Ceramide in the rim is found to be about 1.7 times the C16-PEG2000-Ceramide lipid density in the planar region for all three C16-PEG2000-Ceramide concentrations, 1, 2, and 3 mM. Moreover, the bicelle core radius can be predicted well using the actual molecular ratio of lipids in the planar region to the lipids in the rim of the bicelles in the model calculation.

Effects of Amphipathic Polypeptides on Membrane Organization Inferred from Studies Using Bicellar Lipid Mixtures

SP-B_63-78, a lung surfactant protein fragment, and magainin 2, an antimicrobial peptide, are amphipathic peptides with the same overall charge but different biological functions. Deuterium nuclear magnetic resonance has been used to compare the interactions of these peptides with dispersions of 1,2-dimyristoyl- sn-glycero-3-phophocholine (DMPC)/1,2-dihexanoyl- sn-glycero-3-phophocholine (DHPC) (4:1) and DMPC/1,2-dimyristoyl- sn-glycero-3-phopho-(1'-rac-glycerol) (DMPG)/DHPC (3:1:1), two mixtures of long-chain and short-chain lipids that display bicellar behavior. This study exploited the sensitivity of a bicellar system structural organization to factors that modify partitioning of their lipid components between different environments. In small bicelle particles formed at low temperatures, short-chain components preferentially occupy curved rim environments around bilayer disks of the long-chain components. Changes in chain order and lipid mixing, on heating, can drive transitions to more extended assemblies including a magnetically orientable phase at intermediate temperature. In this work, neither peptide had a substantial effect on the behavior of the zwitterionic DMPC/DHPC mixture. For bicellar mixtures containing the anionic lipid DMPG, the peptide SP-B_63-78 lowered the temperature at which magnetically orientable particles coalesced into more extended lamellar structures. SP-B_63-78 did not promote partitioning of the zwitterionic and anionic long-chain lipid components into different environments. Magainin 2, on the other hand, was found to promote separation of the anionic lipid, DMPG, and the zwitterionic lipid, DMPC, into different environments for temperatures above 34 °C. The contrast between the effects of these two peptides on the lipid mixtures studied appears to be consistent with their functional roles in biological systems.

Protein Rotational Dynamics in Aligned Lipid Membranes Probed by Anisotropic T1ρ NMR Relaxation

A membrane-bound form of Pf1 coat protein reconstituted in magnetically aligned DMPC/DHPC bicelles was used as a molecular probe to quantify for the viscosity of the lipid membrane interior by measuring the uniaxial rotational diffusion coefficient of the protein. Orientationally dependent ¹⁵N NMR relaxation times in the rotating frame, or T_1ρ, were determined by fitting individually the decay of the resolved NMR peaks corresponding to the transmembrane helix of Pf1 coat protein as a function of the spin-lock time incorporated into the 2D SAMPI4 pulse sequence. The T_1ρ relaxation mechanism was modeled by uniaxial rotational diffusion on a cone, which yields a linear correlation with respect to the bond factor sin⁴θ_B, where θ_B is the angle that the NH bond forms with respect to the axis of rotation. Importantly, the bond factors can be independently measured from the dipolar couplings in the separated local-field SAMPI4 spectra. From this dependence, the value of the diffusion coefficient D_|| = 8.0 × 10⁵ s^-1 was inferred from linear regression of the experimental T_1ρ data even without any spectroscopic assignment. Alternatively, a close value of D_|| = 7.7 × 10⁵ s^-1 was obtained by fitting the T_1ρ relaxation data for the assigned NMR peaks of the transmembrane domain of Pf1 to a wavelike pattern as a function of residue number. The method illustrates the use of single-helix transmembrane peptides as molecular probes to assess the dynamic parameters of biological membranes by NMR relaxation in oriented lipid bilayers.

Fabrication Procedures and Birefringence Measurements for Designing Magnetically Responsive Lanthanide Ion Chelating Phospholipid Assemblies

Bicelles are tunable disk-like polymolecular assemblies formed from a large variety of lipid mixtures. Applications range from membrane protein structural studies by nuclear magnetic resonance (NMR) to nanotechnological developments including the formation of optically active and magnetically switchable gels. Such technologies require high control of the assembly size, magnetic response and thermal resistance. Mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and its lanthanide ion (Ln³⁺) chelating phospholipid conjugate, 1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine-diethylene triaminepentaacetate (DMPE-DTPA), assemble into highly magnetically responsive assemblies such as DMPC/DMPE-DTPA/Ln³⁺ (molar ratio 4:1:1) bicelles. Introduction of cholesterol (Chol-OH) and steroid derivatives in the bilayer results in another set of assemblies offering unique physico-chemical properties. For a given lipid composition, the magnetic alignability is proportional to the bicelle size. The complexation of Ln³⁺ results in unprecedented magnetic responses in terms of both magnitude and alignment direction. The thermo-reversible collapse of the disk-like structures into vesicles upon heating allows tailoring of the assemblies' dimensions by extrusion through membrane filters with defined pore sizes. The magnetically alignable bicelles are regenerated by cooling to 5 °C, resulting in assembly dimensions defined by the vesicle precursors. Herein, this fabrication procedure is explained and the magnetic alignability of the assemblies is quantified by birefringence measurements under a 5.5 T magnetic field. The birefringence signal, originating from the phospholipid bilayer, further enables monitoring of polymolecular changes occurring in the bilayer. This simple technique is complementary to NMR experiments that are commonly employed to characterize bicelles.

The Diverse Range of Possible Cell Membrane Interactions with Substrates: Drug Delivery, Interfaces and Mobility

The cell membrane has gained significant attention as a platform for the development of bio-inspired nanodevices due to its immune-evasive functionalities and copious bio-analogs. This review will examine several uses of cell membranes such as (i) therapeutic delivery carriers with or without substrates (i.e., nanoparticles and artificial polymers) that have enhanced efficiency regarding copious cargo loading and controlled release, (ii) exploiting nano-bio interfaces in membrane-coated particles from the macro- to the nanoscales, which would help resolve the biomedical issues involved in biological interfacing in the body, and (iii) its effects on the mobility of bio-moieties such as lipids and/or proteins in cell membranes, as discussed from a biophysical perspective. We anticipate that this review will influence both the development of novel anti-phagocytic delivery cargo and address biophysical problems in soft and complex cell membrane.

Structural insights on biologically relevant cationic membranes by ESR spectroscopy

Cationic bilayers have been used as models to study membrane fusion, templates for polymerization and deposition of materials, carriers of nucleic acids and hydrophobic drugs, microbicidal agents and vaccine adjuvants. The versatility of these membranes depends on their structure. Electron spin resonance (ESR) spectroscopy is a powerful technique that employs hydrophobic spin labels to probe membrane structure and packing. The focus of this review is the extensive structural characterization of cationic membranes prepared with dioctadecyldimethylammonium bromide or diC14-amidine to illustrate how ESR spectroscopy can provide important structural information on bilayer thermotropic behavior, gel and fluid phases, phase coexistence, presence of bilayer interdigitation, membrane fusion and interactions with other biologically relevant molecules.

Methods for Generating Highly Magnetically Responsive Lanthanide-Chelating Phospholipid Polymolecular Assemblies

Mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and its lanthanide ion (Ln³⁺) chelating phospholipid conjugate, 1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine-diethylene triaminepentaacetate (DMPE-DTPA), assemble into highly magnetically responsive polymolecular assemblies such as DMPC/DMPE-DTPA/Ln³⁺ (molar ratio 4:1:1) bicelles. Their geometry and magnetic alignability is enhanced by introducing cholesterol into the bilayer in DMPC/Cholesterol/DMPE-DTPA/Ln³⁺ (molar ratio 16:4:5:5). However, the reported fabrication procedures remain tedious and limit the generation of highly magnetically alignable species. Herein, a simplified procedure where freeze thawing cycles and extrusion are replaced by gentle heating and cooling cycles for the hydration of the dry lipid film was developed. Heating above the phase transition temperature T_m of the lipids composing the bilayer before cooling back below the T_m was essential to guarantee successful formation of the polymolecular assemblies composed of DMPC/DMPE-DTPA/Ln³⁺ (molar ratio 4:1:1). Planar polymolecular assemblies in the size range of hundreds of nanometers are achieved and deliver unprecedented gains in magnetic response. The proposed heating and cooling procedure further allowed to regenerate the highly magnetically alignable DMPC/Cholesterol/DMPE-DTPA/Ln³⁺ (molar ratio 16:4:5:5) species after storage for one month frozen at -18 °C. The simplicity and viability of the proposed fabrication procedure offers a new set of highly magnetically responsive lanthanide ion chelating phospholipid polymolecular assemblies as building blocks for the smart soft materials of tomorrow.

Stable Discoidal Bicelles: A Platform of Lipid Nanocarriers for Cellular Delivery

Bicellar mixtures have been used as alignable membrane substrates for the structural characterization of membrane-associated proteins. Most recently, it has been shown that bicelles can serve as nanocarriers to effectively deliver hydrophobic molecules to cancer cells with a 3- to 10-fold enhancement compared to that of chemically identical liposomes. In this chapter, a detailed preparation protocol, common structural characterization methods, the structural stability and the cellular uptake of bicellar nanodisks are discussed.

Crystallization of Membrane Proteins: An Overview

Membrane proteins are crucial components of cellular membranes and are responsible for a variety of physiological functions. The advent of new tools and technologies for structural biology of membrane proteins has led to a significant increase in the number of structures deposited to the Protein Data Bank during the past decade. This new knowledge has expanded our fundamental understanding of their mechanism of function and contributed to the drug-design efforts. In this chapter we discuss current approaches for membrane protein expression, solubilization, crystallization, and data collection. Additionally, we describe the protein quality-control assays that are often instrumental as a guideline for a shorter path toward the structure.

Impact of purification conditions and history on A2A adenosine receptor activity: The role of CHAPS and lipids

The adenosine A2A receptor (A2AR) is a much-studied class A G protein-coupled receptor (GPCR). For biophysical studies, A2AR is commonly purified in a detergent mixture of dodecylmaltoside (DDM), 3-(3-cholamidopropyl) dimethylammoniopropane sulfonate (CHAPS), and cholesteryl hemisuccinate (CHS). Here we studied the effects of CHAPS on the ligand binding activity and stability of wild type, full-length human A2AR. We also tested the cholesterol requirement for maintaining the active conformation of the receptor when solubilized in detergent micelles. To this end, the receptor was purified using DDM, DDM/CHAPS, or the short hydrocarbon chain lipid 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC, di-6:0PC). After solubilization in DDM, DDM/CHAPS, or DHPC micelles, although A2AR was found to retain its native-like fold, its binding ability was significantly compromised compared to DDM or DDM/CHAPS with CHS. It therefore appears that although cholesterol is not needed for A2AR to retain a native-like, α-helical conformation, it may be a critical component for high affinity ligand binding. Further, this result suggests that the conformational differences between the active and inactive protein may be so subtle that commonly used spectroscopic methods are unable to differentiate between the two forms, highlighting the need for activity measurements. The studies presented in this paper also underline the importance of the protein's purification history; i.e., detergents that interact with the protein during purification affect the ligand binding properties of the receptor in an irreversible manner.

A time-resolved study on the interaction of oppositely charged bicelles--implications on the charged lipid exchange kinetics

Time-resolved small-angle X-ray scattering was applied to study charged lipid exchange between oppositely charged disc-shaped bicelles. The exchange of charged lipids gradually reduces the surface charge density and weakens the electrostatic attraction between the oppositely charged bicelles which form alternately stacked aggregates upon mixing. Initially, at a high surface charge density with almost no free water layer between the stacked bicelles, fast exchange kinetics dominate the exchange process. At a later stage with a lower surface charge density and a larger water gap between the stacked bicelles, slow exchange kinetics take over. The fast exchange kinetics are correlated with the close contact of the bicelles when there is almost no free water layer between the tightly bound bicelles with a charged lipid exchange time constant as short as 20-40 min. When the water gap becomes large enough to have a free water layer between the stacked bicelles, the fast lipid exchange kinetics are taken over by slow lipid exchange kinetics with time constants around 200-300 min, which are comparable to the typical time constant of lipid exchange between vesicles in aqueous solution. These two kinds of exchange mode fit well with the lipid exchange models of transient hemifusion for the fast mode and monomer exchange for the slow mode.

Temperature-resistant bicelles for structural studies by solid-state NMR spectroscopy

Three-dimensional structure determination of membrane proteins is important to fully understand their biological functions. However, obtaining a high-resolution structure has been a major challenge mainly due to the difficulties in retaining the native folding and function of membrane proteins outside of the cellular membrane environment. These challenges are acute if the protein contains a large soluble domain, as it needs bulk water unlike the transmembrane domains of an integral membrane protein. For structural studies on such proteins either by nuclear magnetic resonance (NMR) spectroscopy or X-ray crystallography, bicelles have been demonstrated to be superior to conventional micelles, yet their temperature restrictions attributed to their thermal instabilities are a major disadvantage. Here, we report an approach to overcome this drawback through searching for an optimum combination of bicellar compositions. We demonstrate that bicelles composed of 1,2-didecanoyl-sn-glycero-3-phosphocholine (DDPC) and 1,2-diheptanoyl-sn-glycero-3-phosphocholin (DHepPC), without utilizing additional stabilizing chemicals, are quite stable and are resistant to temperature variations. These temperature-resistant bicelles have a robust bicellar phase and magnetic alignment over a broad range of temperatures, between -15 and 80 °C, retain the native structure of a membrane protein, and increase the sensitivity of solid-state NMR experiments performed at low temperatures. Advantages of two-dimensional separated-local field (SLF) solid-state NMR experiments at a low temperature are demonstrated on magnetically aligned bicelles containing an electron carrier membrane protein, cytochrome b5. Morphological information on different DDPC-based bicellar compositions, varying q ratio/size, and hydration levels obtained from (31)P NMR experiments in this study is also beneficial for a variety of biophysical and spectroscopic techniques, including solution NMR and magic-angle-spinning (MAS) NMR for a wide range of temperatures.

Kinetics of lipid mixing between bicelles and nanolipoprotein particles

Nanolipoprotein particles (NLPs), also known as nanodiscs, are lipid bilayers bounded by apolipoprotein. Lipids and membrane proteins cannot exchange between NLPs. However, the addition of bicelles opens NLPs and transfers their contents to bicelles, which freely exchange lipids and proteins. NLP-bicelle interactions may provide a new method for studying membrane protein oligomerization. The interaction mechanism was investigated by stopped flow fluorometry. NLPs with lipids having fluorescence resonance energy transfer (FRET) donors and acceptors were mixed with a 200-fold molar excess of dihexanoyl phosphatidylcholine (DHPC)/dimyristoyl phosphatidylcholine (DMPC) bicelles, and the rate of lipid transfer was monitored by the disappearance of FRET. Near or below the DMPC phase transition temperature, the kinetics were sigmoidal. Free DHPC and apolipoprotein were ruled out as participants in autocatalytic mechanisms. The NLP-bicelle mixing rate showed a strong temperature dependence (activation energy = 28 kcal/mol). Models are proposed for the NLP-bicelle mixing, including one involving fusion pores.

Thermal stabilization of bicelles by a bile-salt-derived detergent: a combined ³¹P and ²H nuclear magnetic resonance study

The properties of bicelles composed of mixtures of long-chain lipids dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG), stabilized by zwitterionic bile salt analogue 3-[(3-cholamidopropyl)dimethyl-d6-ammonio]-2-hydroxy-1-propanesulfonate (CHAPSO-d6), deuterated at both amino methyls, were investigated by a combination of (31)P and (2)H NMR, focusing on the behavior of CHAPSO as a function of temperature. For compositions of molar ratio q = [DMPC + DMPG]/[CHAPSO] = 3, R = [DMPG]/[DMPC + DMPG] = 0, 0.01 and 0.10 and lipid concentration CL = 25 wt % lipid at temperatures of between 30 and 60 °C, magnetic alignment was readily achieved as assessed via both (31)P NMR of the phospholipids and (2)H NMR of CHAPSO-d6. Increasing temperature yielded higher values for the chemical shift anisotropy of the former and the quadrupole splitting of the latter, consistent with the progressive migration of CHAPSO from edge regions into planar regions of the bicellar assemblies. However, relative to dihexadecyl phosphatidylcholine (DHPC), CHAPSO exhibited lower miscibility with DMPC, although the presence of DMPG enhanced this miscibility. At 65 °C, thermal instability became evident in the appearance of a separate isotropic component in both (31)P and (2)H NMR spectra. This isotropic phase was CHAPSO-enriched but less so as a function of increasing DMPG. These findings indicate that the enhanced thermal stability of CHAPSO- versus DHPC-containing bicelles arises from a combination of the larger surface area that edge CHAPSO is able to mask, mole for mole, and its relative preference for edge regions, plus, possibly, specific interactions with DMPG.

[70]Fullerenes assist the formation of phospholipid bicelles at low lipid concentrations

The incorporation of neutral [70]fullerenes (C70) led to bicelle formation in a relatively low lipid concentration range from neutral lipid mixtures (DMPC/DHPC). Furthermore, C70 addition resulted in the formation of large bicelles with a radius of ca. 100 nm, in contrast to C70-free bicelles that were formed from anionic lipid mixtures (DMPC/DHPC/DMPG). The stabilization of these bicelles was attributed to C70 incorporation into the membranes.

Characterization of the morphology of fast-tumbling bicelles with varying composition

Small, fast-tumbling bicelles are frequently used in solution NMR studies of protein-lipid interactions. For this purpose it is critical to have information about the organization of the lipids within the bicelle structure. We have studied the morphology of small, fast-tumbling bicelles containing DMPC and DHPC as a function of temperature, lipid concentration, and the relative ratio (q value) of lipid (DMPC) to detergent (DHPC) amounts. Dynamic light scattering and cryo-transmission electron microscopy techniques were used to measure the size of the bicelles and to monitor the shape and dispersity of the particles in the samples. The stability and size of DMPC-containing bicelle mixtures were found to be highly dependent on temperature and the total lipid concentration for mixtures with q = 1 and q = 1.5. Stable DMPC/DHPC bicelles are only formed at low q values (0.5). Bicelle mixtures with q > 0.5 appear to be multidisperse containing more than one component, one with r(H) around 2.5 nm and one with r(H) of 6-8 nm. This is interpreted as a coexistence of small (possibly mixed micelles) bicelles and much larger bicelles. Incubating the sample at 37 °C increases the phase separation. Moreover, low total amphiphile concentrations and low q values lead to the formation of a temperature-independent morphology, interpreted as the formation of small particles in which the DHPC and DMPC are more mixed. On the basis of these results, we propose the existence of a critical bicelle concentration, a parameter that determines the existence of bilayered bicelles, which varies with q value. This polymorphism was not observed at any concentrations for q = 0.5 bicelles, for which a small but detectable temperature dependence was observed at high concentrations. The results demonstrate that q = 0.5 mixtures predominantly form "classical" bicelles, but that caution is needed when using fast-tumbling mixtures with q values higher than 0.5.

Stability of bicelles: a simulation study

Aqueous mixtures of long-tailed lipids (e.g., dimyristoylphosphatidylcholine - DMPC) and detergents can sometimes form membrane disks called bicelles. Bicelles have found applications as an embedding medium for membrane proteins in the context of NMR studies and protein crystallization. However, the parameters that determine the thermodynamic stability of bicelles are not well understood. Here we report a coarse-grained simulation study of the relationship between lipid-aggregate morphology and the composition and temperature of the surfactant mixture. In agreement with experiments, we find that bicellar mixtures are destabilized at higher temperatures and detergents are present at membrane edges as well as in flat membranes with a strong preference for the edges. In addition, our results suggest that the free-energy difference between bicelles and the perforated lamellar phase is typically very small for molecules without intrinsic curvature and charge. Cone shaped surfactant molecules tend to favor the formation of bicelles; however, none of the systems that we have studied provide unambiguous evidence for the existence of thermodynamically stable bicelles in mixtures of uncharged lipids with long and short tails. We speculate that small changes in the properties of the system (charge, dopants) may make bicelles thermodynamically stable.

Formation of divalent ion mediated anionic disc bicelle-DNA complexes

Disc-shaped bicelles are formed by mixing long-chain lipids with short-chain lipids at suitable molar ratios and they have a relatively uniform size, typically around a few tens of nanometers in diameter. Different from the typically formulated cationic or anionic liposome–DNA complexes, which are used as nonviral vectors for improving the transfection efficiency of gene therapy, a novel way of packing the DNA can be developed by using the much smaller disc-like bicelles. We demonstrate that anionic lipid bicelle-ion–DNA (AB–DNA) complexes can be formed with the help of divalent ions. Multi-stacked AB–DNA complexes can be formed with diameters of around 50–100 nm and lengths of around 50–150 nm as revealed by TEM. Using the anionic lipid–DNA complexes has the advantage of lower cytotoxicity than using cationic lipids. The interaction of DNA with anionic bicelles was investigated by SAXS. It was found that the anionic bicelle could not form stable complexes with DNA at low calcium ion concentrations, such as 1 mM. The AB–DNA complexes can be formed in the investigated range of 10 mM to 100 mM calcium ion concentrations. However, for an equal anionic lipid charge and DNA charge system, an ion-membrane phase (multilamellar vesicles) would gradually appear as the calcium ion concentration is increased above a critical concentration. It indicates that DNA could be packed closer at above the critical divalent ion concentration. If more DNA is added to such a two-phase coexistence system (originally with the total anionic lipid charge equal to that of DNA), the ion-membrane phase could be transformed into the AB–DNA complexes. As a result, more DNA can be packed in the form of AB–DNA complexes at above the critical calcium ion concentration.

Morphological characterization of DMPC/CHAPSO bicellar mixtures: a combined SANS and NMR study

Spontaneously forming structures of a system composed of dimyristoyl phosphatidylcholine (DMPC) and 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO) were studied by small-angle neutron scattering (SANS), (31)P NMR, and stimulated echo (STE) pulsed field gradient (PFG) (1)H NMR diffusion measurements. Charged lipid dimyristoyl phosphatidylglycerol (DMPG) was used to induce different surface charge densities. The structures adopted were investigated as a function of temperature and lipid concentration for samples with a constant molar ratio of long-chain to short-chain lipids (= 3). In the absence of DMPG, zwitterionic bicellar mixtures exhibited a phase transition from discoidal bicelles, or ribbons, to multilamellar vesicles either upon dilution or with increased temperature. CHAPSO-containing mixtures showed a higher thermal stability in morphology than DHPC-containing mixtures at the corresponding lipid concentrations. In the presence of DMPG, discoidal bicelles (or ribbons) were also found at low temperature and lower lipid concentration mixtures. At high temperature, perforated lamellae were observed in high-concentration mixtures (≥7.5 wt %) whereas uniform unilamellar vesicles and bicelles formed in low-concentration mixtures (≤2.5 wt %), respectively, when the mixtures were moderately and highly charged. From the results, spontaneous structural diagrams of the zwitterionic and charged systems were constructed.

Cholesterol-diethylenetriaminepentaacetate complexed with thulium ions integrated into bicelles to increase their magnetic alignability

Lanthanides have been used for several decades to increase the magnetic alignability of bicelles. DMPE-DTPA (1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine-diethylenetriaminepentaacetate) is commonly applied to anchor the lanthanides into the bicelles. However, because DMPE-DTPA has the tendency to accumulate at the highly curved edge region of the bicelles and if located there does not contribute to the magnetic orientation energy, we have tested cholesterol-DTPA complexed with thulium ions (Tm(3+)) as an alternative chelator to increase the magnetic alignability. Differential scanning calorimetric (DSC) measurements indicate the successful integration of cholesterol-DTPA into a DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) bilayer. Cryo transmission electron microscopy and small-angle neutron scattering (SANS) measurements show that the disklike structure, that is, bicelles, is maintained if cholesterol-DTPA·Tm(3+) is integrated into a mixture of DMPC, cholesterol, and DMPE-DTPA·Tm(3+). The size of the bicelles is increased compared to the size of the bicelles obtained from mixtures without cholesterol-DTPA·Tm(3+). Magnetic-field-induced birefringence and SANS measurements in a magnetic field show that with addition of cholesterol-DTPA·Tm(3+) the magnetic alignability of these bicelles is significantly increased compared to bicelles composed of DMPC, cholesterol, and DMPE-DTPA·Tm(3+) only.

Response to hydrostatic pressure of bicellar dispersions containing an anionic lipid: pressure-induced interdigitation

Bicellar dispersions of chain perdeuterated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC-d54), 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG), and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), with molar ratios of 3:1:1, were studied using variable-pressure (2)H NMR spectroscopy at hydrostatic pressures up to 125 MPa. Upon warming of the dispersions, spectra at ambient pressure indicated a progressive coalescence from small bilayered disks undergoing isotropic reorientation to more extended micellar structures in which spectra indicated anisotropic reorientation and, under some conditions, magnetic orientation and finally to randomly oriented lamellae or multilamellar vesicles. Temperatures for the onsets of anisotropic reorientation and random lamellar orientation increased with pressure at rates of 0.22 and 0.15 °C/MPa, respectively. In the 3.5-T magnetic field used for this work, magnetic orientation within the intermediate phase was not observed at 83 MPa or higher pressures. Comparison of spectra obtained at fixed pressure showed significant asymmetry between behaviors upon warming and cooling. For samples of DMPC-d54/DMPG/DHPC (3:1:1), but not DMPC-d54/DHPC (4:1), a persistent interdigitated phase was formed after repeated cooling from high temperature at 83 MPa. This is likely a metastable phase and might reflect kinetic trapping of the short-chain lipid component, DHPC, in a nonequilibrium spatial distribution as temperature is lowered at high pressure. Bicellar dispersions typically behave differently upon warming and cooling, and these observations could provide some insight into the observed behaviors in such systems. This work also suggests the possibility of trapping bicellar dispersions in persistent nonequilibrium morphologies.

High anisotropy of flow-aligned bicellar membrane systems

In recent years, multi-lipid bicellar systems have emerged as promising membrane models. The fast orientational diffusion and magnetic alignability made these systems very attractive for NMR investigations. However, their alignment was so far achieved with a strong magnetic field, which limited their use with other methods that require macroscopic orientation. Recently, it was shown that bicelles could be aligned also by shear flow in a Couette flow cell, making it applicable to structural and biophysical studies by polarized light spectroscopy. Considering the sensitivity of this lipid system to small variations in composition and physicochemical parameters, efficient use of such a flow-cell method with coupled techniques will critically depend on the detailed understanding of how the lipid systems behave under flow conditions. In the present study we have characterized the flow alignment behavior of the commonly used dimyristoyl phosphatidylcholine/dicaproyl phosphatidylcholine (DMPC/DHPC) bicelle system, for various temperatures, lipid compositions, and lipid concentrations. We conclude that at optimal flow conditions the selected bicellar systems can produce the most efficient flow alignment out of any lipid systems used so far. The highest degree of orientation of DMPC/DHPC samples is noticed in a narrow temperature interval, at a practical temperature around 25 °C, most likely in the phase transition region characterized by maximum sample viscosity. The change of macroscopic orientation factor as function of the above conditions is now described in detail. The increase in macroscopic alignment observed for bicelles will most likely allow recording of higher resolution spectra on membrane systems, which provide deeper structural insight and analysis into properties of biomolecules interacting with solution phase lipid membranes.

Interaction of the C-terminal peptide of pulmonary surfactant protein B (SP-B) with a bicellar lipid mixture containing anionic lipid

The hydrophobic lung surfactant SP-B is essential for respiration. SP-B promotes spreading and adsorption of surfactant at the alveolar air-water interface and may facilitate connections between the surface layer and underlying lamellar reservoirs of surfactant material. SP-B_63–78 is a cationic and amphipathic helical peptide containing the C-terminal helix of SP-B. ²H NMR has been used to examine the effect of SP-B_63–78 on the phase behavior and dynamics of bicellar lipid dispersions containing the longer chain phospholipids DMPC-d₅₄ and DMPG and the shorter chain lipid DHPC mixed with a 3∶1∶1 molar ratio. Below the gel-to-liquid crystal phase transition temperature of the longer chain components, bicellar mixtures form small, rapidly reorienting disk-like particles with shorter chain lipid components predominantly found around the highly curved particle edges. With increasing temperature, the particles coalesce into larger magnetically-oriented structures and then into more extended lamellar phases. The susceptibility of bicellar particles to coalescence and large scale reorganization makes them an interesting platform in which to study peptide-induced interactions between lipid assemblies. SP-B_63–78 is found to lower the temperature at which the orientable phase transforms to the more extended lamellar phase. The peptide also changes the spectrum of motions contributing to quadrupole echo decay in the lamellar phase. The way in which the peptide alters interactions between bilayered micelle structures may provide some insight into some aspects of the role of full-length SP-B in maintaining a functional surfactant layer in lungs.

Interactions of a cationic surfactant--(benzyloxymethyl) dodecyldimethylammonium chloride with model biomembrane systems

Phospholipids are the main components of biological membranes. The aim of the present study was to determine the influence of a cationic surfactant on phospholipid structure and dynamics. Fourier transform infrared (FTIR) and dielectric relaxation (DRS) spectroscopies as well as small-angle X-ray scattering (SAXS) with synchrotron radiation have been used to analyse the structure of fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) in the presence of a quaternary ammonium surfactant: (benzyloxymethyl) dodecyldimethylammoniumchloride (BzMDDAC). The presence of the surfactant caused changes in the temperature of the DMPC phase transition, as revealed using FTIR and DRS measurements. This change results from the disappearance of the multilamellar phase of DMPC and the formation of the unilamellar (most likely bicellar) phase, as indicated by the SAXS results.

Dependence of bicellar system phase behavior and dynamics on anionic lipid concentration

Bicellar dispersions of chain perdeuterated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC-d54) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) were prepared with the molar fraction of DHPC held fixed at 20% and varying amounts of DMPC replaced by the anionic lipid 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG). (2)H NMR spectra were examined to assess the effect of added DMPG on mixture phase behavior and morphology. Quadrupole echo decay and quadrupole-Carr-Purcell-Mieboom-Gill echo train measurements provided information about slow motions contributing to echo decay in the high temperature phases. The spectra and quadrupole echo decay properties of DMPC-d54/DHPC (4:1) and DMPC-d54/DMPG/DHPC (3:1:1) were qualitatively similar. With increasing DMPG concentration, the transition between the magnetically orientable phase and the higher temperature phase became increasingly distinct, and the spectral shape and echo decay characteristics of the high temperature bicellar phase became increasingly similar to those of DMPC-d54 in the liquid crystalline phase. The observation that DMPG changes spectra in the orientable phase incrementally while increasing the distinction between the orientable and high temperature bicellar phases provides new insights into how DMPG influences bicellar mixture morphology.

Alignment of bicelles studied with high-field magnetic birefringence and small-angle neutron scattering measurements

Birefringence measurements at high magnetic field strength of up to 33 T were used to detect magnetically induced alignment of bicelles composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), cholesterol, and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-diethylenetriaminepentaacetate (DMPE-DTPA) with complexed lanthanide ions. These birefringence measurements together with a small-angle neutron scattering (SANS) analysis in a magnetic field showed parallel alignment of the bicelles if the lanthanide was thulium (Tm(3+)), and perpendicular alignment with dysprosium (Dy(3+)). With the birefringence measurements, the order parameter S can be determined as a function of the magnetic field strength, if the magnetic alignment reaches saturation. Additional structural information can be obtained if the maximum induced birefringence is considered. The degree of alignment of the studied bicelles increased with decreasing temperature from 40 to 5 °C and showed a new bicellar structure comprising a transient hole formation at intermediate temperatures (20 °C) during heating from 5 to 40 °C.