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.
Highlights
Saponins are a class of naturally occurring bio-surfactants which share the same basic structure
At pH 7.4, aescin is deprotonated at its glucuronic acid group and nicely dissolves in aqueous buffer
The critical micelle concentration of aescin was determined at constant pH of 7.4 from the concentration-dependent autofluorescence of aescin in a temperature-dependent manner
Summary
Saponins are a class of naturally occurring bio-surfactants which share the same basic structure. The high variability in the type of sugar as well as the number of these chains leads to a high diversity of saponins [5,6]. Saponins are supplied to the human body during ingestion, and depending on the dose, have different effects on the body. Amongst others, they have an anti-inflammatory effect [9,10] and show anti-fungal, anti-bacterial, and anti-yeast activity [7,11]. As a result of the diverse effects of saponins, they are used in different pharmaceuticals. Saponins have applications in the cosmetic, food, and agriculture sectors due to their ability to stabilize emulsions, and to solubilize bulky hydrophobic molecules [3]
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