Aggregation of Saponins and Bile Acids in Aqueous Solution

Abstract

Light scattering, viscometry , fluorescence spectroscopy, electron microscopy and equilibrium dialysis have been used to study the aggregates formed by the interactions of saponins and bile acids in aqueous solution. Purified saponins from three plant sources were used: commercial 'saponin white' (from Saponaria officinalis), quillaia saponin (from Quillaia saponaria), and saponin from soya beans. Alone, the saponins formed small micelles, dimers in the case of saponin white and soya saponin , and larger aggregates of about 50 molecules for quillaia saponin . With bile acids, the saponins formed large mixed micelles. The size and structure of these depended on the chemical structure of the saponin . Saponin white and quillaia saponin gave filamentous structures with the hydrophobic triterpene groups of the saponin forming elongated stacks interleaved with bile acid anions. Soya saponin formed very different micelles with bile acids. These had a loose, open structure with considerable interpenetration of water. These results help to explain the increased faecal excretion of bile acids which has been observed in response to foods rich in saponins.