A SANS Study of High Pressure Phase Transitions in Model Biomembranes

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Abstract
The phase behaviour and thermotropic and barotropic phase transitions of different model biomembranes (DPPC, DMPC, DEPC and POPC*)), which consist of different hydrocarbon chain length and degree of unsaturation, have been investigated by small‐angle neutron scattering (SANS) as a function of temperature and pressure up to about 65°C and 3 kbar, respectively. A pressure induced structural phase transition from a liquid‐crystalline to a gel state is observed for the whole temperature range studied. The pressure of this transition increases with increasing temperature with a slope dTm/dP of about 21°C/kbar, which is common for all the different lipids investigated. The SANS experiments indicate that the effect of the two trans double bonds in DEPC and the cis double bond in the sn‐2 hydrocarbon chain of POPC enhances the conformational disorder in the hydrocarbon chains not only in the liquid‐crystalline, but also in the gel state. For DEPC and POPC the prefered structural conformation in the gel state is probably not tilted as it is in the gel state of the saturated phospholipids DMPC and DPPC. Only for DPPC multilamellar vesicles, an additional gel phase at higher pressures has been observed, which can be ascribed to an interdigitation of the hydrocarbon chains of opposing chains in the lipid bilayer system.