Comparison of differently bound molecules in the gel and subgel phases of a phospholipid bilayer system

The thermal behavior associated with the melting of ice was investigated by differential scanning calorimetry (DSC) for the gel and L-subgel phases of dipalmitoylphosphatidylcholine (DPPC)-water system of varying water contents up to a full hydration. By calorimetric analysis previously developed by us, the numbers of differently bound water molecules were estimated, and used to construct water distribution diagrams (i.e., a plot of the cumulative numbers of these water molecules vs. water content) for the two phases. A comparison of the diagrams revealed the critical role of interlamellar water which changes from freezable to non-freezable one in a conversion of the gel to the L-subgel phase by the thermal annealing.