Temperature scanning simultaneous small- and wide-angle X-ray scattering studies on glycolipid vesicles: areas, expansion coefficients and hydration

Abstract

The polymorphic behaviour of glycolipid model membranes has been characterized by monitoring simultaneously wide- and small angle synchrotron X-ray scattering as a function of temperature along with temperature scanning differential density (DSD) measurements. We estimated the area per lipid and calculated the total volume of the lipid, V T, which includes the hydration shell as well as the interlamellar water. This volume was compared with the partial specific volume of the lipid, v̄ obtained from the DSD studies. The difference was attributed to the presence of water between the lamellar stacks. On this basis estimates of the number of water molecules associated with the sugar head group were made. The main results can be summarized as follows: The average area occupied by two chains within the bilayer plane changes from about 37 Å 2 in the L C phase to about 57 Å 2 in the L α phase at the respective main transition temperatures of the lipids. In the gel phase at T 1/2 an area of about 42 Å 2 is required for two lipid chains. We found that the highly ordered L C phase of the glycolipids is nearly unhydrated, while in the L β phase about seven H 2O molecules and in the L α phase about 14 H 2O molecules are observed per sugar moiety. Hydration increases slightly with temperature in the L β phase. This can be derived from differences in the expansion coefficients of V T and v̄.