This review discusses recent progress in physicochemical understanding of the action of the saponin $\beta$ -aescin (also called $\beta$ -escin), the biologically active component in the seeds of the horse chestnut tree Aesculus hippocastanum. $\beta$ -Aescin is used in pharmacological and cosmetic applications showing strong surface activity. In this review, we outline the most important findings describing the behavior of $\beta$ -aescin in solution (e.g., critical micelle concentration ( $cmc$ ) and micelle shape) and special physicochemical properties of adsorbed $\beta$ -aescin monolayers at the air–water and oil–water interface. Such monolayers were found to posses very special viscoelastic properties. The presentation of the experimental findings is complemented by discussing recent molecular dynamics simulations. These simulations do not only quantify the predominant interactions in adsorbed monolayers but also highlight the different behavior of neutral and ionized $\beta$ -aescin molecules. The review concludes on the interaction of $\beta$ -aescin with phospholipid model membranes in the form of bilayers and Langmuir monolayers. The interaction of $\beta$ -aescin with lipid bilayers was found to strongly depend on its $cmc$ . At concentrations below the $cmc$ , membrane parameters are modified whereas above the $cmc$ , complete solubilization of the bilayers occurs, depending on lipid phase state and concentration. In the presence of gel-phase phospholipids, discoidal bicelles form; these are tunable in size by composition. The phase behavior of $\beta$ -aescin with lipid membranes can also be modified by addition of other molecules such as cholesterol or drug molecules. The lipid phase state also determines the penetration rate of $\beta$ -aescin molecules into lipid monolayers. The strongest interaction was always found in the presence of gel-phase phospholipid molecules.

This work investigates the temperature-dependent micelle formation as well as the micellar structure of the saponin aescin. The critical micelle concentration ( $cmc$ ) of aescin is determined from the concentration-dependent autofluorescence (AF) of aescin. Values between $cm{c}_{\mathrm{aescin},\mathrm{AF}}$ (10 ${}^{\circ }$ C) = 0.38 ± 0.09 mM and $cm{c}_{\mathrm{aescin},\mathrm{AF}}$ (50 ${}^{\circ }$ C) = 0.32 ± 0.13 mM were obtained. The significance of this method is verified by tensiometry measurements. The value determined from this method is within the experimental error identical with values obtained from autofluorescence ( $cm{c}_{\mathrm{aescin},\mathrm{T}\left(\mathrm{WP}\right)}$ (23 ${}^{\circ }$ C) = 0.33 ± 0.02 mM). The structure of the aescin micelles was investigated by small-angle X-ray scattering (SAXS) at 10 and 40 ${}^{\circ }$ C. At low temperature, the aescin micelles are rod-like, whereas at high temperature the structure is ellipsoidal. The radii of gyration were determined to ≈31 Å (rods) and ≈21 Å (ellipsoid). The rod-like shape of the aescin micelles at low temperature was confirmed by transmission electron microscopy (TEM). All investigations were performed at a constant pH of 7.4, because the acidic aescin has the ability to lower the pH value in aqueous solution.

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).

Phosphatidylglycerols represent a large share of the lipids in the plasmamembrane of procaryotes. Therefore, this study investigates the role of charged lipids in the plasma membrane with respect to the interaction of the antiviral saponin glycyrrhizin with such membranes. Glycyrrhizin is a natural triterpenic-based surfactant found in licorice. Vesicles made of 1,2-dioleoyl-sn-glycero-3-phospho-rac-(1’-glycerol) (DOPG)/glycyrrhizin are characterized by small-angle scattering with neutrons and X-rays (SANS and SAXS). Small-angle scattering data are first evaluated by the model-independent modified Kratky–Porod method and afterwards fitted by a model describing the shape of small unilamellar vesicles (SUV) with an internal head-tail contrast. Complete miscibility of DOPG and glycyrrhizin was revealed even at a ratio of lipid:saponin of 1:1. Additional information about the chain-chain correlation distance of the lipid/saponin mixtures in the SUV structures is obtained from wide-angle X-ray scattering (WAXS).

This work investigates the conversion of bicelles into larger sheets or closed vesicles upon dilution and temperature increase for a system composed of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and the saponin aescin. Due to its peculiar amphiphilic character, aescin is able to decompose DMPC bilayers into smaller, rim-stabilized bicelles. Aspects of the transition process are analyzed in an aescin content- and temperature-dependent manner by photon correlation spectroscopy (PCS), turbidimetry and small-angle neutron scattering (SANS). Both the conversion of bicelles into vesicles induced by temperature increase and the decomposition process upon cooling are presumably related to the main phase transition temperature T_m of DMPC. Therefore, not only conventional DMPC, but also chain-deuterated d54-DMPC was used due to its significantly lower T_m-value compared to the conventional DMPC. It will be demonstrated that the reconversion of vesicle structures (present at low aescin content) into bicelles shows a strong hysteresis effect whereas this is not observed for the reconversion at high aescin amounts, at which for high temperature still bicelle structures are present. The results indicate formation of a trapped state, correlated with the lipid's T_m and the decomposition of vesicles into bicelles is only possible if the lipid membrane entirely adopts the rigid phase state.