DMPC vesicle structure and dynamics in the presence of low amounts of the saponin aescin

Abstract

Vesicle shape and bilayer parameters are studied by small-angle X-ray (SAXS) and small-angle neutron (SANS) scattering in the presence of the saponin aescin. We confirm successful incorporation of aescin molecules by analysis of the radii of gyration R_G and study furthermore the impact of aescin incorporation on bilayer thickness parameters from the neutron and X-ray perspective. Additionally, the bending elasticity (κ) of these 1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicle bilayers is studied in the presence of aescin. Neutron spin-echo spectroscopy (NSE) allows to detect subtle changes in the dynamics and κ of lipid membranes. Changes of κ are detectable at temperatures below and above the main phase transition temperature T_m of the lipid. The impact of aescin is much more significant below T_m. It has been found that below T_m the addition of aescin to the vesicles decreases the value of κ and softens the bilayer. Above T_m the value of κ increases with increasing aescin content and the bilayer becomes more rigid. Altogether, we demonstrate by analysis of the structure and dynamics of the vesicles that the impact of aescin strongly depends on the lipid state. Below T_m the membrane becomes fluidized and softer, above T_m solidified and stiffer compared to a DMPC membrane without additive at similar conditions.