Interactions of lipid vesicles with solvent in heavy and light water

The phase-transition properties of lipid vesicles prepared from 1,2-distearoyl-, 1,2-dipalmitoyl-, and 1,2-dimyristoyl-L-3-glycerylphosphatidylcholines in D/sub 2/O and H/sub 2/O were compared. The phase-transition temperatures were 0.2 to 0.4/sup 0/C higher in D/sub 2/O than in H/sub 2/O. In general, the D/sub 2/O solvent effect caused increases in the enthalpy (..delta..H), entropy (..delta..S), and free energy (..delta..G) of the melting, with the enhancement ranging from 230 to 1170 cal/mol, from 0.7 to 2.0 eu, and from 4 to 12 cal/mol respectively. These observations are consistent with the premises that water is present around the hydrocarbon tails of lipid vesicles, and the melting of the difference in the amount of the more structured water around the tails in gel and liquid-crystalline phases is a major contribution to ..delta..H. Some recent findings regarding the phase-transition properties of mixed-chain and fluoro-substituted phosphatidylcholines are also consistent with this mechanism. 1 figure, 3 tables.