Penetration of DPPC Membranes by Quillaja Bark Saponin

Despite a growing number of present and prospective applications of saponin-rich extracts from the bark of Quillaja saponaria Molina tree (Quillaja Bark Saponin, QBS), details of their interaction with biological membranes are still missing. Many saponins are known to lyse erythrocytes (haemolysis), and this property is often linked with their affinity to cholesterol [1]. Nevertheless, in some cases strong effect of saponins on cholesterol-free membranes have been observed [2], suggesting that saponins may interact also with other bilayer components, e.g., phospholipids.

In this contribution the interaction between a model phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and QBS will be described. Surface pressure relaxation and surface dilatational rheology complemented with neutron reflectivity (NR) were employed for this purpose. In contrast to synthetic surfactants of similar surface activity (SDS, CTAB, Triton X-100), QBS does not destroy DPPC monolayers, but penetrates them, improving their surface dilatational elastic properties even in the highly compressed solid state. The dilational visco-elasticity modulus increases from 204 mN/m for pure DPPC up to 310 mN/m for the QBS-penetrated layers, while it drops to near zero values in the case of the synthetic surfactants. The estimated maximum insertion pressure of QBS into DPPC monolayers exceeds 50 mN/m.