Supercooled Water and Ice Included in Multilamellar L-α, Distearoyl Phosphatidylcholine in Low-Temperature Regions below 0°C

Bilayer, supercooled water and ice states in multilamellar phospholipids are investigated in the low-temperature region from 0°C to -100°C using L-α, distearoyl phosphatidylcholine (DSPC) having water contents of 44 wt% and 15 wt%, by differential scanning calorimeter and X-ray diffraction methods. Multilamellar DSPC contains 44 wt% water in not only the water layer, but also as excess water in the form of stock regions; on the other hand, multilamellar DSPC containing 15 wt% water has no excess water. The difference in water content between these multilamellar DSPCs causes characteristic changes of the H2O molecules and of the phospholipid bilayer in low-temperature regions below 0°C, e.g., ice crystallization inside or outside the multilamellae, changes in the chain packing conformation and changes in the long-range distance. It is confirmed that the supercooled water in the water layer inside multilamellar DSPC containing 15 wt% water continues to exist down to about -45°C during the cooling process and then is frozen into heterogeneous ice or ice nuclei below about -45°C. In the case of multilamellar DSPC with 44 wt% water, although changes of excess water from supercooled water to homogeneous ice with hexagonal symmetry are observed at about -20°C, the water layer inside the multilamellae remains supercooled down to about -45°C. Dehydration of polar head groups due to the appearance of ice in the water layer at around -45°C promotes some movement of polar head groups, thereby inducing changes in the chain packing conformation.