Phase Behavior and Lipid-Membrane Structure of Phospholipid-Glycosphingolipid Liposomes and the Thermal Unfolding of Insulin

Abstract

The thermodynamic phase behavior and lipid-membrane structure of fully hydrated uni- and multilamellar liposomes composed of dipalmitoylphosphatidylcholine (DPPC) surface-modified by a glycosphingolipid, sulfogalactosylceramide (sulfatide), have been investigated by means of differential scanning calorimetry and fluorescence polarization spectroscopy. The calorimetric and spectroscopic scans of the two-component liposomes demonstrate a distinct influence of increasing amounts of sulfatide on the lipidmembrane thermodynamics. This is manifested as a broad gel-fluid phasecoexistence region and a low-temperature two-phase region composed of highly ordered lipids. In addition, the pre-transition is abolished for small concentrations of sulfatide. A well-defined heat capacity peak, reflecting a thermotropic chain melting transition is observed for liposomes composed of pure sulfatide. On basis of the calorimetric and spectroscopic data a phase diagram has been established in the full temperature and composition plane. Dynamic light scattering measurements of liposome sizes reveal that the incorporation of sulfatide into the DPPC membrane matrix leads to a stabilization of the characteristic size of the extruded liposomes. Furthermore, incubation with insulin does not affect the liposome size and the aggregational behavior of the two-component sulfatide-DPPC liposomes. In accordance with this, the heat capacity curves demonstrate an insignificant influence on both the lipid membrane phase behavior and the thermal unfolding characteristics of insulin.