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 Lβ′ to its Lα 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).
Highlights
O OO (a) β-aescin (b) cholesterol (c) DMPC degradation of the endothelial cells that separate the blood from the surrounding tissue[1,10]
In earlier studies we showed that the effect of aescin incorporation into a DMPC membrane is especially visible below Tm
This work investigates structual parameters of DMPC model membranes in small unilamellar vesicles (SUVs) modified by the presence of aescin-cholesterol complexes
Summary
O OO (a) β-aescin (b) cholesterol (c) DMPC degradation of the endothelial cells that separate the blood from the surrounding tissue[1,10]. A different key feature of some saponins including aescin is the haemolytic activity It results from the interaction through complexation with cholesterol in the red blood cell membrane[13,14,15,16,17,18]. By using this phospholipid, interactions in both the gel and fluid phases are experimentally accessible. Using DSC we confirm that there is a strong interaction between saponin and phospholipid and its complexation with cholesterol and show how the cooperative phase transition is altered depending on saponin and steroid content. Cholesterol enhances aggregation and leads to structural deformation of SUVs
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