Hydration of DOPC bilayers by differential scanning calorimetry.

The phase diagram of the unsaturated lipid dioleoylphosphatidylcholine (DOPC) in aqueous multibilayer dispersions has been constructed from a series of differential scanning calorimetry (DSC) thermograms over the temperature range from -40 to +10 degrees C, covering a range of hydration levels from the monohydrate to excess free water. Both the lipid chain melting transition and the ice melting point are found to be hydration dependent. From their respective variations it is found that the bilayer in the gel phase binds approximately 9 H2O per lipid, while the liquid-crystalline state has a saturation limit near 20 H2O. The water transition exhibits a hydration-dependent melting point depression, which can be explained in terms of newly incorporated water between the bilayer surfaces upon melting of the acyl chains, and which is reminiscent of the events that occur at the pre-transition for saturated lipids. From the melting point depression, the thermodynamic activity of the interbilayer water can be calculated and thus the repulsive hydration force characterized quantitatively. We evaluate a (non-isothermal) hydration force decay constant around 2.8 H20, which demonstrates that this DSC approach is well-suited for quantitatively characterizing the hydration properties of unsaturated lipid dispersions at low temperature.