Liposome Fluidization and Melting Point Depression by Pressurized CO₂ Determined by Fluorescence Anisotropy

Abstract

The influence of CO2 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 CO2-based processing of liposomes and microbial sterilization. Fluidization and melting point depression of aqueous dipalmitoylphosphatidylcholine (DPPC) liposomes by pressurized CO2 (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 (Tm) occurs at approximately 315 K. Isobaric measurements (PCO2 =1.8, 7.0, and 13.9 MPa) of DPH anisotropy demonstrate substantial melting point depression (DeltaTm = -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 CO2 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 CO2 pressure.

Membrane Damage Induced by Supercritical Carbon Dioxide in Rhodotorula mucilaginosa

“…It was reported that the florescence polarization were inversely proportional to membrane fluidity. The depression of membrane fluidity induced by SCCO 2 was demonstrated in a model cellular membrane, which was aqueous dipalmitoylphosphatidylcholine (DPPC) liposomes, and in a thermophilic bacterium model (Clostridium thermocellum) [15]. Their results showed that the decline of membrane fluidity also occurred in a lower extent, the same as our finding.…”

Preparation and evaluation of cyclosporin A-containing proliposomes: a comparison of the supercritical antisolvent process with the conventional film method

“…The stability of proliposomes prepared with the SAS process might be explained by the static repulsion of the carbonic acids incorporated into the bilayer membrane. 37 In general, liposomes have some stability issues with long-term storage. In this regard, proliposomes containing CsA are of particular interest as a new drug delivery system that may address the stability problems associated with liposomes.…”

Characterization and stability studies of a novel liposomal cyclosporin A prepared using the supercritical fluid method: comparison with the modified conventional Bangham method

“…These results indicated that liposomes prepared by the SCF-CO 2 method were relatively more stable compared with those prepared by the conventional method, which was in accordance with the results by Otake et al, 31 Aburai et al, 52 and Kadimi et al 26 The stability of liposomes prepared with SCF-CO 2 might be explained by the static repulsion of the carbonic acids incorporated into the bilayer membrane. 53 On the other hand, the chemical instability of conventional liposomes might be caused by the hydrolysis of ester bonds and/or oxidation of polyunsaturated acyl chains of lipids.…”