Solubilization of egg phosphatidylcholine liposomes by sodium taurocholate: partition behavior and morphology

Solubilization process of egg yolk phosphatidylcholine (EggPC) liposomes by sodium taurocholate (TC) was performed using membrane dialysis technique. Turbidity, apparent particle size, Cl⁻ permeability, membrane fluidity measured by electron spin resonance spectroscopy (ESR), and structural changes observed by freeze-fracture electron microscopy were investigated. The concentrations of TC were analyzed by high-performance liquid chromatography to determine the incorporation of TC in membrane phase and apparent partition coefficient. When TC concentration in water phase (D _w) was lower than 1.41 mM, or effective ratio of TC to EggPC (R _e) was less then 0.14, the partition coefficient was independent of the TC concentration and was 90 M⁻¹, where the membrane permeability of Cl⁻ was rather small. Upon increasing the concentrations in the range of 1.41 mM≤D _w<3.10 mM (0.14≤R _e<0.21), the apparent partition coefficient decreased and membrane permeability markedly increased. The coexistence of small vesicles and bilayer fragments were observed in this region. At 3.10 mM≤D _w<3.53 mM (0.21≤R _e<0.59), the steep decrease in turbidity was accompanied with the gradual increase in apparent partition coefficient, suggesting the formation of mixed micelles. The freeze-fracture electron micrographs confirmed the transformation of saturated bilayers to wormlike mixed micelles. When a further increase in TC (D _w≥3.53 or R _e≥0.59) was applied, the turbidity and apparent particle sizes became small. Transformation of the wormlike structure to ellipsoid mixed micelles was observed. The ESR spectra showed that TC/EggPC mixed micelles still retain a somewhat orderly structure.