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

Abstract

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).

The Biosurfactant β-Aescin: A Review on the Physico-Chemical Properties and Its Interaction with Lipid Model Membranes and Langmuir Monolayers

“…They found that the most prominent effect of β-aescin in the subphase occurs on cholesterol-containing monolayers confirming the strong affinity of β-aescin and cholesterol molecules. These results are in concordance with the effects of β-aescin on DMPC bilayers in the presence of cholesterol as discussed previously [55] (Section 6).…”

“…The effect of cholesterol addition on the structural parameters of β-aescin-containing phospholipid bilayers composed of DMPC was investigated by Sreij et al [55]. In this work, the cholesterol contents studied were 1, 5, and 10 mol% and the β-aescin contents ranged from 0 to 6 mol% with respect to the amount of DMPC in solution.…”

“…An increasing β-aescin concentration enhances phase segregation and shows in the formation of a second peak at lower temperature. The experimental procedure is given elsewhere[21,48,55]. Adapted from Ref [48].…”

Effect of Cholesterol and Ibuprofen on DMPC-β-Aescin Bicelles: A Temperature-Dependent Wide-Angle X-ray Scattering Study

“…In lipidic environment consisting of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC, Figure 1b, main phase transition temperature T m (DMPC) = 23.6 • C [30]), self-assembled and thermodynamically stable structures form, which depend on β-aescin concentration and lipid-to-saponin ratio [9]. Depending on β-ascin concentration phase segregation into saponin-rich and saponin-poor regions occurs, as proven by differential scanning calorimetry (DSC) [30][31][32]. Far below cmc(β-aescin), stable unilamellar vesicles are obtained by extrusion [33,34].…”

“…The phase behaviour of the β-aescin-DMPC mixtures can be modified through the addition of additive molecules, such as the steroid cholesterol ( Figure 1c) [32] or the NSAID ibuprofen ( Figure 1d) [31]. Cholesterol is one major compound in mammalian cells and its concentration can be as high as 50% in lipid bilayers [39].…”