Characterization of cholesterol-containing DOPG bilayers in the presence of the saponins aescin and glycyrrhizin: A NMR and scattering study

Biological lipid membranes can be mimicked by small unilamellar vesicles (SUVs), which e.g. consist of the negatively charged phospholipid 1,2-dioleoyl-sn-glycero-3-phosphatidylglycerol (DOPG). Due to the unsaturated hydrocarbon chains and hence the low main phase transition temperature, a DOPG membrane is always fluid-like in aqueous solution. By addition of cholesterol to the DOPG model membrane, the membrane stiffness is found to increase. These cholesterol-containing DOPG SUVs are characterized in the presence of varying amounts of the saponins aescin and glycyrrhizin at a temperature of 30°C by diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY NMR), small-angle neutron scattering (SANS), small- and wide-angle X-ray scattering (SAXS, WAXS), neutron spin echo spectroscopy (NSE), and cryogenic transmission electron microscopy (cryo-TEM). All methods reveal that cholesterol is incorporated into the long-term stable SUVs. Upon saponin addition up to a molar ratio of 1:1 no significant modifications of the SUV size parameters are detected. However, NSE reveals a slight alteration in the membrane elasticity. In sum, the DOSY NMR and scattering results clearly show the coexistence of DOPG-cholesterol SUVs and saponin unimers or micelles and an interaction from the outside of the DOPG vesicles seems to promote the change in the membrane rigidity.