Effects of gas pressure on the phase transition temperature of liposomal bilayer membranes

The main phase transition temperatures (T-m) for dimyristoyl phosphatidylcholine (DMPC) and dipalmitoyl phosphatidylcholine (DPPC) were determined under pressure of various gases. The gases used are pure ones such as O-2, N-2, H-2, He, Ar, and mixed ones (air, 20.0% O-2 (He base), 10.1% N2O (N-2 base) and 4.69% H-2 (N-2 base)). The behaviors of the T-m by pressurization were classified into two groups. In the one group, T-m increased linearly with increasing pressure, while it decreased up to 10 - 20 MPa in the other group. The depression of the latter group comes from the large solubilities of the gases in water. Especially, the effect of nitrous oxide on the depression was marked and this suggests the strong influence of the action of dissolved gases on the frame of bilayer membranes.