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

This work investigates the temperature-dependent micelle formation as well as the micellar structure of the saponin aescin. The critical micelle concentration (<inline-formula> <math display="inline"> <semantics> <mrow> <mi>c</mi> <mi>m</mi> <mi>c</mi> </mrow> </semantics> </math> </inline-formula>) of aescin is determined from the concentration-dependent autofluorescence (AF) of aescin. Values between <inline-formula> <math display="inline"> <semantics> <mrow> <mi>c</mi> <mi>m</mi> <msub> <mi>c</mi> <mrow> <mi>aescin</mi> <mo>,</mo> <mi>AF</mi> </mrow> </msub> </mrow> </semantics> </math> </inline-formula>(10 <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mo>∘</mo> </msup> </semantics> </math> </inline-formula>C) = 0.38 &#177; 0.09 mM and <inline-formula> <math display="inline"> <semantics> <mrow> <mi>c</mi> <mi>m</mi> <msub> <mi>c</mi> <mrow> <mi>aescin</mi> <mo>,</mo> <mi>AF</mi> </mrow> </msub> </mrow> </semantics> </math> </inline-formula>(50 <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mo>∘</mo> </msup> </semantics> </math> </inline-formula>C) = 0.32 &#177; 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 (<inline-formula> <math display="inline"> <semantics> <mrow> <mi>c</mi> <mi>m</mi> <msub> <mi>c</mi> <mrow> <mi>aescin</mi> <mo>,</mo> <mi mathvariant="normal">T</mi> <mo>(</mo> <mi>WP</mi> <mo>)</mo> </mrow> </msub> </mrow> </semantics> </math> </inline-formula>(23 <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mo>∘</mo> </msup> </semantics> </math> </inline-formula>C) = 0.33 &#177; 0.02 mM). The structure of the aescin micelles was investigated by small-angle X-ray scattering (SAXS) at 10 and 40 <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mo>∘</mo> </msup> </semantics> </math> </inline-formula>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 &#8776;31 &#197; (rods) and &#8776;21 &#197; (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.