Liposome Fluidization and Melting Point Depression by Pressurized CO<INF>2</INF> Determined by Fluorescence Anisotropy

The influence of CO<INF>2</INF> on the bilayer fluidity of liposomes, which are representative of model cellular membranes, was examined for the first time at the elevated pressures (up to 13.9 MPa) associated with CO<INF>2</INF>-based processing of liposomes and microbial sterilization. Fluidization and melting point depression of aqueous dipalmitoylphosphatidylcholine (DPPC) liposomes by pressurized CO<INF>2</INF> (present as an excess phase) were studied by steady-state fluorescence anisotropy using the membrane probe 1,6-diphenyl-1,3,5-hexatriene (DPH). Isothermal experiments revealed reversible, pressure-dependent fluidization of DPPC bilayers at temperatures corresponding to near-gel (295 K) and fluid (333 K) phases at atmospheric pressure, where the gel-to-fluid phase transition (T<INF>m</INF>) occurs at ~315 K. Isobaric measurements (P<INF>CO</INF><INF></INF><INFINF>2</INFINF> = 1.8, 7.0, and 13.9 MPa) of DPH anisotropy demonstrate substantial melting point depression (ΔT<INF>m</INF> = −4.8 to −18.5 K) and a large broadening of the gel−fluid phase transition region, which were interpreted using conventional theories of melting point depression. Liposome fluidity is influenced by CO<INF>2</INF> accumulation in the hydrocarbon core and polar headgroup region, as well as the formation of carbonic acid and/or the presence of buffering species under elevated CO<INF>2</INF> pressure.