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

Abstract

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 Xray 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 À1 , 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.

Changes Experienced by Low-Concentration Lipid Bicelles as a Function of Temperature

“…Moreover, the broad peak at ∼38 °C should not be confused with the pre-transition of DPPC, which normally exhibits a much lower excess enthalpy than that of the main transition. , Furthermore, multiple DSC heating scans at a higher scan rate (2 °C/min) is presented in Figure B, yielding qualitatively similar outcomes to the second heat scan in Figure A, with the exception that the DPPC melting transitions are broader due to the faster scan rate. Note that different heating rates can yield different sizes of vesicles as reported in literature. , However, the kinetic study is beyond the current scope of this report.…”

“…Other models can be used to describe the SAXS or SANS data of bicelles but will not be discussed here. 22,23,49 In the C3SS model, the bilayer is parsed into three layers, that is, the inner (t shell1 ) and outer (t shell3 ) phosphate headgroups, and a hydrophobic hydrocarbon layer (t shell2 ). Because DPPC is in the fluid phase, the contrast between methylene and methyl regions is not significant, thus reducing the number of fitting parameters.…”

“…Note that different heating rates can yield different sizes of vesicles as reported in literature. 49,50 However, the kinetic study is beyond the current scope of this report.…”

“…In the 5LCSD model, t shell2 , t shell1 , and t core refer, respectively, to the thicknesses of the two phosphate headgroups, the two ordered methylene regions, and the less ordered methyl core region within the bilayer. Other models can be used to describe the SAXS or SANS data of bicelles but will not be discussed here. ,, In the C3SS model, the bilayer is parsed into three layers, that is, the inner ( t shell1 ) and outer ( t shell3 ) phosphate headgroups, and a hydrophobic hydrocarbon layer ( t shell2 ). Because DPPC is in the fluid phase, the contrast between methylene and methyl regions is not significant, thus reducing the number of fitting parameters.…”

Stable DOPG/Glycyrrhizin Vesicles with a Wide Range of Mixing Ratios: Structure and Stability as Seen by Scattering Experiments and Cryo-TEM

“…Previous work on the interaction of glycyrrhizin with model membranes composed of the phospholipid 1,2-dimyristoyl- sn -glycero-3-phosphocholine (DMPC) clearly revealed an interaction of the saponin and the lipid [ 30 ]. At saponin amounts higher than ≈40 mol%, the DMPC model membrane is completely decomposed into smaller, free-standing bilayer fragments in the form of bicelles.…”