Lipid phase transition in saccharide-coated cholate-containing liposomes: coupling to the surrounding matrix

We performed FTIR measurements on cholate-containing liposomes (CCL) embedded in saccharide (trehalose or sucrose) matrixes with different contents of residual water. We obtained information on the CCL phase transition following the thermal evolution (310-70 K) of the IR spectrum of the carbonyl moieties of phospholipids in the frequency range 4225-4550 cm(-1). Furthermore, we simultaneously followed the thermal evolution of the water association band, which gave information on the behavior of the surrounding water-saccharide matrix. The analysis revealed a small sub-band of the water association band present in CCL but not in cholate-free liposomes, the thermal evolution of which is tightly coupled to that of the spectrum of the carbonyl moieties of phospholipids. We suggest that this band arises from water molecules, which are inserted within the lipidic structure, in the region located at the border between the hydrophilic and the hydrophobic moieties of phospholipids in the presence of cholic acid. Such water molecules could be responsible for the peculiar flexibility and hydrophilicity of CCL. Following Giuffrida et al. (J. Phys. Chem. B 2003, 107, 13211-13217), we also performed a Spectra Distance analysis, which enabled us to detect an overall liposomes-matrix structural coupling.