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

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.