Physical Properties and Membrane Packing in Hybrid Archaeosomes

Abstract

In this study, we have characterized the physical properties of liposomes made of archaeal bipolar tetraether lipids (BTL) and “conventional” monopolar diester lipids (termed as hybrid archaeosomes). We used the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius as the BTL component of the hybrid archaeosomes. Hybrid archaeosomes were made by extrusion with the particle size ranging from 100-300 nm. Dynamic light scattering and zeta potential measurements showed that the presence of PLFE greatly stabilizes liposomes. The steady-state polarization of DPH fluorescence in PLFE/PC mixtures decreases with increasing temperature (18-56oC) in a molar ratio dependent manner. At low temperatures, hybrid archaeosomes have membrane packing, as inferred from DPH polarization, similar to that in gel state of disaturated PC bilayers. At the high temperature end, membrane packing of the hybrid archaeosomes is significantly tighter than that of PC fluid state. A similar conclusion can be drawn from the studies using Laurdan's generalized polarization. Additionally, due to tight membrane packing, drug release from hybrid archaeosome is much slower than from liposomes composed of only diester lipids. Confocal fluorescence microscopy showed that these hybrid archaeosomes can be taken up by live cells (e.g., MCF-7 breast cancer cells) and deliver entrapped drug molecules to the cell's nucleus, but with a rate significantly lower than that obtained from conventional diester liposomes.