Stability of diether C(25,25) liposomes from the hyperthermophilic archaeon Aeropyrum pernix K1.

Temperature and pH effects were studied for stability, structural organization, fluidity and permeability of vesicles from a polar lipid methanol fraction isolated from the Aeropyrum pernix. We determined the permeability of C(25,25) liposomes using fluorescence intensity of released calcein. At pH 7.0 and 9.0, and from 85°C to 98°C, only 10% of entrapped calcein was released. After 10 h at 90°C, calcein release reached 27%, independent of pH. Fluorescence anisotropy measurements of hydrophobic probe 1,6-diphenyl-1,3,5-hexatriene revealed gradual changes up to 60°C. At higher temperatures, the anisotropy did not change significantly. Fluorescence alone did not provide detailed and direct structural information about these C(25,25) liposomes, so we used electron paramagnetic resonance spectroscopy (EPR) and differential scanning calorimetry (DSC). From EPR spectra, mean membrane fluidity determined according to maximal hyperfine splitting and empirical correlation times showed continuous increases with temperature. Computer simulation of EPR spectra showed heterogeneous membranes of these C(25,25) liposomes: at low temperatures, they showed three types of membrane regions characterized by different motional modes. Above 65°C, the membrane becomes homogeneous with only one fluid-like region. DSC thermograms of C(25,25) liposomes reveal a very broad and endothermic transition in the temperature range from 0°C to 40°C.