Heating-Induced DMPC/Glycyrrhizin Bicelle-to-Vesicle Transition: A X-Ray Contrast Variation Study

In this study, we investigated the conversion of lipid bicelles into vesicles in the case of a system composed of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and the saponin glycyrrhizin in the presence of sucrose. Glycyrrhizin is a biosurfactant present in the licorice root and possesses a triterpenic hydrophobic backbone and a hydrophilic headgroup built from two sugar molecules. The aim of this study is to determine the initial bicelle size at temperatures below the lipid's main phase transition temperature T_m and, based on these results, characteristics of the temperature-induced bicelle-to-vesicle transition. Moreover, the influence of the heating rate on this transition is followed. The general picture concluded from photon correlation spectroscopy and small angle X-ray scattering was confirmed by additional imaging with cryogenic transmission electron microscopy. Small angle X-ray scattering was especially used to determine size parameters of the existing structures. To enhance the contrast for X-rays, a buffer containing 25 wt% sucrose was used. It was found that larger vesicles were formed from smaller precursor particles and that monodisperse precursors are required for formation of very monodisperse vesicles upon temperature increase. At high glycyrrhizin contents and above a critical heating rate of ∼5°C min^-1, the polydispersity of these vesicles is decoupled from both parameters, glycyrrhizin content and heating rate. However, the vesicle size stays tunable by the glycyrrhizin content and increases upon increasing the glycyrrhizin concentration. Therefore, vesicles of defined size and with a rather low polydispersity of ∼12-14% can be formed.