Analysis of Quil A–phospholipid mixtures using drift spectroscopy

Aescin - a natural soap for the formation of lipid nanodiscs with tunable size

The saponin β-aescin from the seed extract of the horse chestnut tree Aesculus hippocastanum has demonstrated a beneficial role in clinical therapy which is in part related to its strong interaction with biological membranes. In this context the present work investigates the self-assembly of nm-sized discoidal lipid nanoparticles composed of β-aescin and the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The discoidal lipid nanoparticles reassemble from small discs into larger discs, ribbons and finally stacks of sheets upon heating from gel-phase to fluid phase DMPC. The morphological transition of the lipid nano-particles is mainly triggered by the phospholipid phase state change. The final morphology depends on the phospholipid-to-saponin ratio and the actual temperature. The study is conducted by small-angle X-ray scattering (SAXS) and transmission (TEM) and freeze fracture electron microscopy (FFEM) are used to cover larger length scales. Two different models, representing a disc and ribbon-like shape are applied to the SAXS data, evaluating possible geometries and molecular mixing of the nano-particles. The stacked sheets are analysed by the Caillé theory.

Aescin-Cholesterol Complexes in DMPC Model Membranes: A DSC and Temperature-Dependent Scattering Study

The saponin aescin, a mixture of triterpenoid saponins, is obtained from the seeds of the horse chestnut tree Aesculus hippocastanum. The β-form employed in this study is haemolytically active. The haemolytic activity results from the ability of aescin to form strong complexes with cholesterol in the red blood cell membrane. In this study, we provide a structural analysis on the complex formation of aescin and cholesterol when embedded in a phospholipid model membrane formed by 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). In this work, the temperatures investigated extend from DMPC’s L_β′ to its L_α phase in dependence of different amounts of the saponin (0–6 mol% for calorimetric and 0–1 mol% for structural analyses) and the steroid (1–10 mol%). At these aescin contents model membranes are conserved in the form of small unilamellar vesicles (SUVs) and major overall structural modifications are avoided. Additionally, interactions between aescin and cholesterol can be studied for both phase states of the lipid, the gel and the fluid state. From calorimetric experiments by differential scanning calorimetry (DSC), it could be shown that both, the steroid and the saponin content, have a significant impact on the cooperative phase transition behaviour of the DMPC molecules. In addition, it becomes clearly visible that the entire phase behaviour is dominated by phase separation which indeed also depends on the complexes formed between aescin and cholesterol. We show by various methods that the addition of cholesterol alters the impact of aescin on structural parameters ranging from the acyl chain correlation to vesicle-vesicle interactions. While the specific saponin-phospholipid interaction is reduced, addition of cholesterol leads to deformation of SUVs. The analyses of the structures formed were performed by wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS).

Temperature dependent self-organization of DMPC membranes promoted by intermediate amounts of the saponin aescin

The plant-derived biosurfactant aescin is naturally present in many plants and is used for treatment of disorders such as varicose veins and inflammation of veins. The hemolytic activity of this saponin is attributed to its interaction with cholesterol in the red blood cell membrane. This work investigates the phase and aggregation behavior of saponin-containing model membranes consisting of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The aescin concentrations studied range from 1 mol% to 7 mol% with respect to the total lipid content. The methods of choice to elucidate the structural picture are small-angle scattering of X-rays (SAXS) and neutrons (SANS) and cryogenic transmission electron microscopy (cryo-TEM). SANS and SAXS revealed that at lower aescin contents vesicular structures are conserved and vesicles tend to aggregate already at aescin contents of around 1 mol%. Aggregation and vesicle deformation effects are found to be stronger when the phospholipids are in the L [Formula: see text] phase. With increasing aescin content, mixed structures, i.e. aggregated and deformed vesicles and solubilized bilayer fragments, are present. This was proven for a sample with 4 mol% aescin by cryo-TEM. An increasing aescin amount leads to membrane decomposition and free standing bilayers which tend to build stacks at high temperature. These stacks are characterized by SAXS using the modified Caillé theory. Analyses and model dependent fitting reveal formation of well-defined structures beginning at 7 mol% aescin.

Interaction of the Saponin Aescin with Ibuprofen in DMPC Model Membranes

In the present work, we study the interaction of the saponin aescin with the nonsteroidal anti-inflammatory drug (NSAID) ibuprofen at concentrations of 1.2-2.5 mM. These amounts are higher than those usually used for medication (10-300 μM) to show possible structures and formulations for orally absorbed drug delivery systems. It is shown how the interaction of both substances, separately or together, alters the thermotropic phase behavior of the 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) bilayer in the presence of different amounts of aescin, ranging from 20 μM to 1 mM. The methods of choice are differential scanning calorimetry (DSC), and additionally wide-angle (WAXS) and small-angle X-ray scattering (SAXS). We found that these two additives, aescin and ibuprofen, alter the temperature-dependent structural appearance of the DMPC membrane depending on the aescin and drug content. The presence of the saponin and the drug become visible on different length scales, i.e., ranging from a global structural change to inner-membrane interactions. DSC reveals that the drug and saponin alter the cooperativity of the DMPC phase transition in a concentration-dependent manner. Furthermore, there is a significant difference between the drug-containing compared to the drug-free systems. By WAXS, we could resolve that aescin reverses the strong impact of ibuprofen on the diffraction peak of DMPC. Both molecules interact strongly with the phospholipid headgroups. This becomes visible in a changing area per lipid and shifting phase transition to higher temperatures. SAXS experiments reveal that the addition of ibuprofen leads to major morphological changes in the phospholipid bilayer. SAXS experiments performed on representative samples do not only cover the drug-saponin interaction within the bilayer from the structural perspective but also confirm the visually observed macroscopic concentration and temperature-dependent phase behavior. Vesicular shape of extruded samples is conserved at low aescin contents. At intermediate aescin content, aggregation between vesicles occurs, whereby the strength of aggregation is reduced by ibuprofen. At high aescin contents, DMPC bilayers are solubilized. The kind of formed structures depends on temperature and drug content. At low temperature, separated bilayer sheets are formed. Their size increases with ibuprofen in a concentration-dependent manner. At high temperature, the drug-free system reorganizes into stacked sheets. Whereas sheets at 5 mol % ibuprofen close to vesicles, the ones with 10 mol % of the drug increase massively in size. Altogether, ibuprofen was found to rather enhance than inhibit structural and thermotropic membrane modifications induced by the aescin on the DMPC model membrane.

DMPC vesicle structure and dynamics in the presence of low amounts of the saponin aescin

Vesicle shape and bilayer parameters are studied by small-angle X-ray (SAXS) and small-angle neutron (SANS) scattering in the presence of the saponin aescin. Bilayer dynamics is studied by neutron spin-echo (NSE) spectroscopy.

Aescin Incorporation and Nanodomain Formation in DMPC Model Membranes

The saponin aescin from the horse chestnut tree is a natural surfactant well-known to self-assemble as oriented-aggregates at fluid interfaces. Using model membranes in the form of lipid vesicles and Langmuir monolayers, we study the mixing properties of aescin with the phase-segregating phospholipid 1,2-dimyristoyl-sn-glycero-phosphocholine (DMPC). The binary membranes are experimentally studied on different length scales ranging from the lipid headgroup area to the macroscopic scale using small-angle X-ray scattering (SAXS), photon correlation spectroscopy (PCS), and differential scanning calorimetry (DSC) with binary bilayer vesicles and Langmuir tensiometry (LT) with lipid monolayers spread on the surface of aescin solutions. The binary interaction was found to strongly depend on aescin concentration in two well differentiated concentration regimes. Below 7 mol %, the results reveal phase segregation of nanometer-sized aescin-rich domains in an aescin-poor continuous bilayer. Above this concentration, aescin-aescin interactions dominate, which inhibit vesicle formation but lead to the formation of new membrane aggregates of smaller sizes. From LT studies in monolayers, the interaction of aescin with DMPC was shown to be stronger in the condensed phase than in the liquid expanded phase. Furthermore, a destructuring role was revealed for aescin on phospholipid membranes, similar to the fluidizing effect of cholesterol and nonsteroidal anti-inflammatory drugs (NSAIDs) on lipid bilayers.

Complexation of phospholipids and cholesterol by triterpenic saponins in bulk and in monolayers

The interactions between three triterpene saponins: α-hederin, hederacoside C and ammonium glycyrrhizate with model lipids: cholesterol and dipalmitoylphosphatidylcholine (DPPC) are described. The oleanolic acid-type saponins (α-hederin and hederacoside C) were shown to form 1:1 complexes with lipids in bulk, characterized by stability constants in the range (4.0±0.2)·10(3)-(5.0±0.4)·10(4) M(-1). The complexes with cholesterol are generally stronger than those with DPPC. On the contrary, ammonium glycyrrhizate does not form complexes with any of the lipids in solution. The saponin-lipid interactions were also studied in a confined environment of Langmuir monolayers of DPPC and DPPC/cholesterol with the saponins present in the subphase. A combined monolayer relaxation, surface dilational rheology, fluorescence microscopy and neutron reflectivity (NR) study showed that all three saponins are able to penetrate pure DPPC and mixed DPPC/cholesterol monolayers. Overall, the effect of the saponins on the model lipid monolayers does not fully correlate with the lipid-saponin complex formation in the homogeneous solution. The best correlation was found for α-hederin, for which even the preference for cholesterol over DPPC observed in bulk is well reflected in the monolayer studies and the literature data on its membranolytic activity. Similarly, the lack of interaction of ammonium glycyrrhizate with both lipids is evident equally in bulk and monolayer experiments, as well as in its weak membranolytic activity. The combined bulk and monolayer results are discussed in view of the role of confinement in modulating the saponin-lipid interactions and possible mechanism of membranolytic activity of saponins.

The amphiphilic nature of saponins and their effects on artificial and biological membranes and potential consequences for red blood and cancer cells

Saponins, amphiphiles of natural origin with numerous biological activities, are widely used in the cosmetic and pharmaceutical industry. Some saponins exhibit relatively selective cytotoxic effects on cancer cells but the tendency of saponins to induce hemolysis limits their anticancer potential. This review focused on the effects of saponin activity on membranes and consequent implications for red blood and cancer cells. This activity seems to be strongly related to the amphiphilic character of saponins that gives them the ability to self-aggregate and interact with membrane components such as cholesterol and phospholipids. Membrane interactions of saponins with artificial membrane models, red blood and cancer cells are reviewed with respect to their molecular structures. The review considered the mechanisms of these membrane interactions and their consequences including the modulation of membrane dynamics, interaction with membrane rafts, and membrane lysis. We summarized current knowledge concerning the mechanisms involved in the interactions of saponins with membrane lipids and examined the structure activity relationship of saponins regarding hemolysis and cancer cell death. A critical analysis of these findings speculates on their potential to further develop new anticancer compounds.

Environmentally friendly formulations of alachlor and atrazine: preparation, characterization, and reduced leaching

Atrazine and alachlor formulations were designed by encapsulating the herbicide molecules into phosphatidylcholine (PC) vesicles, which subsequently were adsorbed on montmorillonite. PC and montmorillonite are classified as substances of minimal toxicological risk by the U.S. EPA. PC enhanced alachlor and atrazine solubilities by 15- and 18-fold, respectively. A 6 mM PC:5 g/L clay ratio was found as optimal for PC adsorption on the clay. Active ingredient contents of the PC-clay formulations ranged up to 8.6% for atrazine and 39.5% for alachlor. Infrared spectroscopy showed hydrophobic interactions of herbicide molecules with the alkyl chains of PC, in addition to hydrophilic interactions with the PC headgroup. Release experiments in a sandy soil showed a slower rate from the PC-clay formulations than the commercial ones. Soil column experiments under moderate irrigation and bioactivity experiments indicate that a reduction in the recommended dose of alachlor and atrazine can be accomplished by using PC-clay formulations.