Comparison of the effect of two Quillaja bark saponin extracts on DPPC and DPPC/cholesterol Langmuir monolayers.

The present study aims at comparing the effect of two commercially available Quillaja bark saponin (QBS) products on model Langmuir monolayers. Pure dipalmitoylphosphatidylcholine (DPPC) and mixed DPPC/cholesterol Langmuir monolayers were used as crude models of erythrocyte membranes in order to better understand a hemolytic activity of QBS. Both QBS products are mixtures of several saponins and non-saponin fractions, only few of which can be assigned an exact chemical structure, as shown by an HPLC analysis. Noticeable differences in the saponin profiles, and most importantly-in the content of non-saponin fractions (tannins and phenolic compounds) are probably responsible for their different adsorption behavior at the water/air interface. The lipids Langmuir monolayers were initially spread on pure water and compressed to surface pressure of 32.5 mN/m, which is believed to provide the lipid packing similar to that in real biological membranes. The water subphase was subsequently exchanged with the respective QBS solutions in the concentration range 5 × 10(-4) to 2 × 10(-1)wt%. In order to assess the resistance of the model lipid monolayers to QBS, a combination of surface pressure relaxation and surface dilatational rheology was employed. Both QBS are shown to penetrate the lipid layers without removing them, but their effect on the lipid layers' relaxation and viscoelastic dilational properties is different. The differences virtually disappear when cholesterol is present in the monolayer, especially for the DPPC/Cholesterol molar ratio between 10:9 and 4:1 (mol/mol). Despite several jumps of surface pressure, the lipid layers containing cholesterol can resist penetration of QBS.