Self-Assembly of the Bio-Surfactant Aescin in Solution: A Small-Angle X-ray Scattering and Fluorescence Study

Abstract

This work investigates the temperature-dependent micelle formation as well as the micellar structure of the saponin aescin. The critical micelle concentration ( c m c ) of aescin is determined from the concentration-dependent autofluorescence (AF) of aescin. Values between c m c aescin , AF (10 ∘ C) = 0.38 ± 0.09 mM and c m c aescin , AF (50 ∘ 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 ( c m c aescin , T ( WP ) (23 ∘ 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 ∘ 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 Biosurfactant β-Aescin: A Review on the Physico-Chemical Properties and Its Interaction with Lipid Model Membranes and Langmuir Monolayers

“…Two separable fractions with an exact mass of 1130.5 g/mol were obtained. 1 H-NMR spectroscopy identified both fractions as cis-trans-isomers with either tiglic or angelic acid attached to the C-21 carbon atom of the triterpenic backbone [29,33,34]. The presence of xylose, which would lead to a different molar mass, was not proven.…”

“…In comparison to α-aescin, β-aescin shows a much higher pharmacological activity and β-aescin is the main active component in the pharmaceutically used horse chestnut tree extract [10,25]. [29]. The amphiphilic structure of β-aescin consists of a hydrophobic, aglyconic part and a hydrophilic, glyconic part.…”

“…β-Aescin is commercially available (CAS: 6805-41-0) and can e.g., be purchased from Merck/Sigma-Aldrich Company in purity of >95 %. The structures present in this mixture were determined by combination of analytical reversed-phase high performance liquid chromatography (RP-HPLC) and the analysis techniques mass spectrometry and 1 H-NMR spectroscopy [29]. Two separable fractions with an exact mass of 1130.5 g/mol were obtained.…”

“…The carboxylic group located in the glycone of β-aescin ( Figure 2) induces intrinsic pH-dependence of the β-aescin form. Dargel et al [29] recently determined the pK a value of β-aescin in water to be 4.7 ± 0.2. Therefore, it is assumed that β-aescin exists in its neutral form below and in a deprotonated ionized form above a pH value of 4.7.…”

“…Besides the numerous reports on the cmc-values of β-aescin, there are only two works elucidating the actual structure of β-aescin micelles in solution [29,39]. De Groot et al [39] report on rather flat and spherically shaped β-aescin micelles in pure water and ambient temperature ( Figure 13A) at unspecified pH.…”