DPPC Bilayers in Solutions of High Sucrose Content

Nanomolar PFOA Concentrations Affect Lipid Membrane Structure: Consequences for Bioconcentration Mechanisms

Independent methods show that sub-microMolar concentrations of perfluorooctanoic acid (PFOA), a member of the PFAS family of "forever chemicals", change the properties of DPPC vesicle bilayers. Specifically, calorimetry measurements show that PFOA at concentrations as low as 0.1 nM lowers DPPC's gel-liquid crystalline transition enthalpy by several J/g without changing the transition temperature (Tgel-LC), and dynamic light scattering (DLS) data illustrate that PFOA markedly broadens the size distribution of DPPC vesicles. Furthermore, DLS results from PFOA-containing, DPPC vesicle solutions also contain smaller objects having diameters of 30-50 nm. Close inspection of cryo-EM images reveals that DPPC vesicles formed in the presence of PFOA are multilamellar and the smaller objects have a clear bilayer structure similar to niosomes. A consequence of these PFOA-induced changes to DPPC bilayer structure is that the bilayers themselves are more susceptible to secondary solute accumulation. Time resolved emission measurements of Coumarin 152 (C152) report that C152 is 3-fold more likely to partition into the bilayer's acyl chain, hydrophobic interior when PFOA is present, and fluorescence lifetimes from C152 partitioned into the polar region of the lipid bilayer show evidence of PFOA-induced membrane hydration below Tgel-LC.

Effect of Monosaccharides Including Rare Sugars on the Bilayer Phase Behavior of Dimyristoylphosphatidylcholine

We observed bilayer phase transitions of dimyristoylphosphatidylcholine (DMPC) in aqueous solutions of four kinds of monosaccharides, namely, D-glucose, D-fructose, D-allose and D-psicose, using differential scanning calorimetry (DSC). D-allose (C3-epimer of D-glucose) and D-psicose (C3-epimer of D-fructose) are rare sugars. We performed DSC measurements using two types of sugar-containing sample dispersions of the DMPC vesicles: one is a normal sample dispersion with no concentration asymmetry between the inside and outside of the vesicles and the other is an unusual sample dispersion with a concentration asymmetry. DSC measurements using normal sample dispersions with different sugar concentrations revealed that the temperatures and transition enthalpies of the pre- and main transition of the DMPC bilayer membrane did not significantly depend on the sugar concentration for all monosaccharides. DSC measurements using the unusual sample dispersions demonstrated that the concentration asymmetry caused the splitting of the endothermic peak of the main transition similarly irrespective of the sort of monosaccharides present. From all these DSC results, we conclude that (i) most monosaccharide molecules exist in the bulk water phase, (ii) no specific interaction depending on the molecular structure of each monosaccharide directly occurs between the DMPC and each monosaccharide molecule, and (iii) almost all the effects of the monosaccharides observed in this study are understandable as the general colligative properties of solutions.

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

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

Phase behaviour and structural properties of SOPC model lipid system in a sucrose solution

Lipid membranes are an important component of the biological cell. The profound understanding of their structure and functionality, as well as, the influence of various biologically relevant admixtures on their main characteristics is of great importance for research and development in medicine and pharmacology. The effect of sugars on the behaviour of the membrane cell enjoys an ever-increasing interest as they are biologically significant substances. We have studied the influence of the disaccharide sucrose on the physicochemical properties of SOPC (1-stearoyl-2-oleoyl-sn- glycero-3-phosphocholine) lipid system aiming to gain better understanding of the mechanisms of the interaction between both substances. For that purpose, we have used differential scanning calorimetry and Fourier-transform infrared spectroscopy. Our results show that adding sugar up to 300 mM concentration substantially alters the thermodynamic and structural properties of SOPC. The DSC thermograms at heating reveal a general lowering of the SOPC transition temperature Tm from gel to liquid crystalline phase (main phase transition, ordered-disordered phase transition) in the presence of sugar. The corresponding peaks are smeared and harder to trace. In agreement with this, a gradual decrease of the enthalpy values up to 300 mM was measured. The IR spectroscopy study provided spectral evidence for two states of hydration of the phosphate groups in the sugar-SOPC model systems suggesting a mechanism of interaction where only part of the phospholipid headgroups are hydrogen bonded to the sugar molecules. The obtained results are in good agreement with various earlier data including results about the bending elasticity moduli, as well as, some theoretical simulations on the sugar-lipid interactions. The current results also reinforce the potential of sucrose to be used as a cell protector against drought at, both, high and low temperatures.

Interplay between mucus mobility and alveolar macrophage targeting of surface-modified liposomes

Alveolar macrophages play a crucial role in the initiation and resolution of the immune response in the lungs. Pro-inflammatory M1 alveolar macrophages are an interesting target for treating inflammatory and infectious pulmonary diseases. One commune targeting strategy is to use nanoparticles conjugated with hyaluronic acid, which interact with CD44 overexpressed on the membrane of those cells. Unfortunately, this coating strategy may be countered by the presence on the surface of the nanoparticles of a poly(ethylene glycol) corona employed to improve nanoparticles' diffusion in the lung mucus. This study aims to measure this phenomenon by comparing the behavior in a murine lung inflammation model of three liposomal platforms designed to represent different poly(ethylene glycol) and hyaluronic acid densities (Liposome-PEG, Liposome-PEG-HA and Liposome-HA). In this work, the liposomes were obtained by a one-step ethanol injection method. Their interaction with mucin and targeting ability toward pro-inflammatory macrophages were then investigated in vitro and in vivo in a LPS model of lung inflammation. In vitro, poly(ethylene glycol) free HA-liposomes display a superior targeting efficiency toward M1 macrophages, while the addition of poly(ethylene glycol) induces better mucus mobility. Interestingly in vivo studies revealed that the three liposomes showed distinct cell specificity with alveolar macrophages demonstrating an avidity for poly(ethylene glycol) free HA-liposomes, while neutrophils favored PEGylated liposomes exempt of HA. Those results could be explained by the presence of two forces exercising a balance between mucus penetration and receptor targeting. This study corroborates the importance of considering the site of action and the targeted cells when designing nanoparticles to treat lung diseases.

The Role of Cryoprotective Agents in Liposome Stabilization and Preservation

To improve liposomes’ usage as drug delivery vehicles, cryoprotectants can be utilized to prevent constituent leakage and liposome instability. Cryoprotective agents (CPAs) or cryoprotectants can protect liposomes from the mechanical stress of ice by vitrifying at a specific temperature, which forms a glassy matrix. The majority of studies on cryoprotectants demonstrate that as the concentration of the cryoprotectant is increased, the liposomal stability improves, resulting in decreased aggregation. The effectiveness of CPAs in maintaining liposome stability in the aqueous state essentially depends on a complex interaction between protectants and bilayer composition. Furthermore, different types of CPAs have distinct effective mechanisms of action; therefore, the combination of several cryoprotectants may be beneficial and novel attributed to the synergistic actions of the CPAs. In this review, we discuss the use of liposomes as drug delivery vehicles, phospholipid–CPA interactions, their thermotropic behavior during freezing, types of CPA and their mechanism for preventing leakage of drugs from liposomes.

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.

Thermotropic effects of PEGylated lipids on the stability of HPPH-encapsulated lipid nanoparticles (LNP)

In this work, we demonstrate the enhanced thermal and steric stability of lipid-based formulations in the presence of encapsulated HPPH that have demonstrated potential cancer applications in previously presented in vivo studies. Differential scanning calorimeter (DSC) was used to study the phase transitions, and domain formations, and to qualify the thermodynamic properties associated with change in lipid bilayer behavior due to the presence of PEGylated at varying concentrations and sizes, and the encapsulated HPPH molecules. Thermal instability was quantified by dramatic changes in calculated enthalpy, and the shape of the melting peak or calculated half width of melting peak. This systematic study focused on understanding the effects of varying molecular mass and concentrations of PEG polymers in the photopolymerizable lipid DC_{8, 9}PC lipid bilayer matrix for four weeks at room temperature of 25 °C. The major findings include increased thermal stability of the lipid bilayer due to the presence of PEG-2 K and the HPPH that resulted from the van der Waals forces between various molecular species, and the change in bilayer curvature confirmed via mathematical correlations. It is demonstrated that the encapsulation of therapeutics in lipid formulations can alter their overall thermal behavior, and therefore, it is imperative to consider calorimetric effects while designing lipid-based vaccines. The presented research methodologies and findings presented can predict the stability of lipid-based vaccines that are under development such as COVID-19 during their storage, transport, and distribution.

Budding and Fission of Nanovesicles Induced by Membrane Adsorption of Small Solutes

Membrane budding and fission are essential cellular processes that produce new membrane compartments during cell and organelle division, for intracellular vesicle trafficking as well as during endo- and exocytosis. Such morphological transformations have also been observed for giant lipid vesicles with a size of many micrometers. Here, we report budding and fission processes of lipid nanovesicles with a size below 50 nm. We use coarse-grained molecular dynamics simulations, by which we can visualize the morphological transformations of individual vesicles. The budding and fission processes are induced by low concentrations of small solutes that absorb onto the outer leaflets of the vesicle membranes. In addition to the solute concentration, we identify the solvent conditions as a second key parameter for these processes. For good solvent conditions, the budding of a nanovesicle can be controlled by reducing the vesicle volume for constant solute concentration or by increasing the solute concentration for constant vesicle volume. After the budding process is completed, the budded vesicle consists of two membrane subcompartments which are connected by a closed membrane neck. The budding process is reversible as we demonstrate explicitly by reopening the closed neck. For poor solvent conditions, on the other hand, we observe two unexpected morphological transformations of nanovesicles. Close to the binodal line, at which the aqueous solution undergoes phase separation, the vesicle exhibits recurrent shape changes with closed and open membrane necks, reminiscent of flickering fusion pores (kiss-and-run) as observed for synaptic vesicles. As we approach the binodal line even closer, the recurrent shape changes are truncated by the fission of the membrane neck which leads to the division of the nanovesicle into two daughter vesicles. In this way, our simulations reveal a nanoscale mechanism for the budding and fission of nanovesicles, a mechanism that arises from the interplay between membrane elasticity and solute-mediated membrane adhesion.

Membrane-Sugar Interactions Probed by Low-Frequency Raman Spectroscopy: The Monolayer Adsorption Model

Small sugars are known to stabilize biological membranes under extreme conditions of freezing and desiccation. The proposed mechanisms of stabilization suggest membrane-sugar interactions to be either attractive or repulsive. To obtain new insight into the problem, we use a recently developed low-frequency Raman scattering approach which allows detecting membrane mechanical vibrations. For model membranes of palmitoyl-oleoyl-glycero-phosphocholine (POPC) hydrated in aqueous sucrose and trehalose solutions, we studied the Raman peak between 12 and 15 cm^-1 that is attributed to an eigenmode of the normal mechanical vibrations of a lipid monolayer. For both sugars, similar results were obtained. With an increase in sugar concentration in solution, the frequency position of the peak was found to decrease by ∼13% which was interpreted as a consequence of the membrane thickening due sugar monolayer adsorption on the membrane surface. The concentration dependence of the peak frequency position was satisfactorily described by a Langmuir monolayer adsorption model. It is concluded that, at small sugar concentrations (less than 0.2 M), the membrane-sugar interactions are attractive, while at higher concentrations (more than 0.4 M) the attraction disappears. The data obtained show that one sugar molecule on the surface interacts with approximately 3-4 polar lipid heads.

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.

Simple sugars shape giant vesicles into multispheres with many membrane necks

Simple sugars such as glucose and sucrose are ubiquitous in all organisms. One remarkable property of these small solutes is their ability to protect biomembranes against dehydration damage. This property, which reflects the underlying sugar-lipid interactions, has been intensely studied for lipid bilayers interacting with a single sugar at low hydration. Here, we use giant vesicles to investigate fully hydrated lipid membranes in contact with two sugars, glucose and sucrose. The vesicles were osmotically balanced, with the same total sugar concentration in the interior and exterior aqueous solutions. However, the two solutions differed in their composition: the interior solution contained only sucrose whereas the exterior one contained primarily glucose. This sugar asymmetry generated a striking variety of multispherical or "multi-balloon" vesicle shapes. Each multisphere involved only a single membrane that formed several spherical segments, which were connected by narrow, hourglass-shaped membrane necks. These morphologies revealed that the sugar-lipid interactions generated a significant spontaneous curvature with a magnitude of about 1 μm^-1. Such a spontaneous curvature can be generated both by depletion and by adsorption layers of the sugar molecules arising from effectively repulsive and attractive sugar-lipid interactions. All multispherical shapes are stable over a wide range of parameters, with a substantial overlap between the different stability regimes, reflecting the rugged free energy landscape in shape space. One challenge for future studies is to identify pathways within this landscape that allow us to open and close the membrane necks of these shapes in a controlled and reliable manner. We will then be able to apply these multispheres as metamorphic chambers for chemical reactions and nanoparticle growth.

A machine learning study of the two states model for lipid bilayer phase transitions

Machine learning algorithms can identify fluid and gel conformation states of individual lipid molecules.

Discerning the Structure Factor of Charged Micelles in Water and Supercooled Solvent by Contrast Variation X-ray Scattering

Sodium dodecyl sulfate (SDS) is a well-known anionic surfactant that forms micelles in various solvents including supercooled sugar-urea melt. Here, we explore the application of contrast variation small-angle X-ray scattering (SAXS) in discerning the structure and interactions of SDS micelles in aqueous solution and in a room-temperature supercooled solvent. The SAXS patterns can be analyzed in terms of a core-shell ellipsoid model. For aqueous SDS micelles, at low volume fractions, the features due to intermicellar interaction, S(q), in the SAXS pattern are poorly resolved because of the prominent contribution from shell scattering. Increasing the electron density of the solvent by the addition of the urea or fructose-urea mixture (at a weight ratio of 6:4) permits the systematic variation of shell scattering without influencing the structure drastically. For a 10% solution of SDS in water, the contribution from the shell can be completely masked by the addition of 40% urea or fructose-urea mixture. The fructose-urea mixture is a preferred additive as it can vary the scattering length density over a wide range and serves as a matrix to form supercooled micelles. The structural parameters of micelles in supercooled fructose-urea melt are obtained from contrast variation SAXS, small-angle neutron scattering, and high-resolution transmission electron microscopy.

Lyophilization of Liposomal Formulations: Still Necessary, Still Challenging

Nowadays, the freeze-drying of liposome dispersions is still necessary to provide a solid dosage form intended for different routes of administration (i.e., parenteral, oral, nasal and/or pulmonary). However, after decades of studies the optimization of process conditions remains still challenging since the freezing and the dehydration destabilize the vesicle organization with the concomitant drug leakage. Starting from the thermal properties of phospholipids, this work reviews the main formulation and process parameters which can guarantee a product with suitable characteristics and increase the efficiency of the manufacturing process. In particular, an overview of the cryo- and/or lyo-protective mechanisms of several excipients and the possible use of co-solvent mixtures is provided. Attention is also focused on the imaging methods recently proposed to characterize the appearance of freeze-dried products and liposome dispersions upon reconstitution. The combination of such data would allow a better knowledge of the factors causing inter-vials variability in the attempt to improve the quality of the final medicinal product.