Compositional aspects of lipid hydration.

The effect of various lipids such as cerebrosides, gangliosides and dipalmitoyl lecithin (DPPC) on the lowering of the melting temperature of water, was determined by differential scanning calorimetry (DSC). The lowering of the melting temperature, and the number of water molecules per lipid molecule which apparently do not undergo melting, increase with the increase of the size and charge of the polar group and with the unsaturation of the hydrocarbon chains. Freezing curves show supercooling to about -20 degrees C. It was found that the number of apparently unfreezable water molecules is about four for glucocerebroside from Gaucher's spleen and about eight or nine for galactocerebroside from bovine brain. In gangliosides from bovine brain the following number of water molecules/lipid molecule are apparently unfreezable: 22-30 in GM1, 33-40 in GD1a + GD1b while a fraction of gangliosides containing 75% GQ1b and 25% GT1b affects up to 60 molecules of water/molecule of lipid. A zwitterionic DPPC molecule removes apparently six to seven water molecules from freezing. There is no indication that the apparently unfreezable water molecules are in a distinct state. It is suggested that they freeze at very low temperatures producing a flat tail preceding the transition peak which cannot be discerned from the base line.