Crystallization of water in multilamellar vesicles

The two-step crystallization of water in multilamellar vesicles (MLVs) of phosphatidylcholines has been investigated. The main crystallization occurs near -15 degrees C and involves bulk water. Contrary to unilamellar vesicles, a sub-zero phase transition is observed for MLVs at -40 degrees C that corresponds to the crystallization of interstitial water, as proved by Fourier transform infrared absorption and differential scanning calorimetry (DSC) experiments. Furthermore, by means of the DSC method and, more specifically, using the enthalpy change values Delta H(sub) at the sub-zero transition, the number of water molecules per 1,2-dipalmitoylphosphatidylcholine (DPPC) molecule giving rise to this transition has been estimated for different H(2)O/DPPC molar ratios. The curve of the molecular fraction of water molecules involved in the sub-zero transition versus the H(2)O/DPPC molar ratio exhibits a maximum for H(2)O/DPPC equal to 27 (40% in mass of water) and tends towards zero for H(2)O/DPPC ratio values approaching that of the swelling limit of the membrane. A smaller enthalpy value of the sub-zero transition is found for 1-oleoyl-2-palmitoyl-3-phosphatidylcholine (OPPC) than for DPPC. This may be explained by the decrease of interstitial water's quantity when the lipid contains an unsaturated chain. When troxerutin, a hydrophilic drug, is added to the DPPC multilayers, the decrease of Delta H(sub) and melting enthalpy of bulk water is attributed to a decrease of the entropy of the liquid phase owing to the network of water molecules surrounding troxerutin molecules. In all cases, the experiments revealed that the sub-zero transition occurs only in the presence of excess water with respect to the swelling limit of membranes. This evidence could be, at least qualitatively, related to an increase of membrane pressure on interstitial water subsequent to bulk water crystallization.