Liposome fluidization and melting point depression by compressed and liquid n-alkanes

Fluidization and melting point depression of aqueous dipalmitoylphosphatidylcholine (DPPC, Tm ≈ 315 K) liposomes by compressed/supercritical n-alkanes (C2 and C3; 0.8–20.7 MPa) was quantified and compared to fluidization by liquid n-alkanes (C5–C10) using fluorescence anisotropy. The adsorption of ethane and propane into DPPC bilayers resulted in significant depression of the melting temperature (up to ΔTm = −16.2 K at 7.0 MPa ethane and propane). Pressure-dependent fluidization by gaseous n-alkanes (1.8–20.7 MPa) and pressure-induced reversal of this fluidization was observed at temperatures corresponding to gel (295 K) and fluid (333 K) phases at atmospheric pressure. The ability to manipulate the degree of bilayer ordering, melting temperature, and width of the phase transition region is discussed in terms of the n-alkane concentration in the bilayer, effect of n-alkane molecular size, and van’t Hoff enthalpy of the gel–fluid phase transition. The permeability, phase, and solvent properties of the bilayer can be altered with pressurized ethane or propane, which may provide enhanced control for liposome formation and processing.