Effect of Freeze-Thawing on Phospholipid/Surfactant Mixed Bilayers

Abstract

The effect of freeze-thawing of egg phosphatidylcholine (PC) and octyl glucoside (OG) on vesicle growth and on liposomal formation by the subsequent removal of the OG was investigated. When increasing concentrations of OG were mixed with PC vesicles of 10 or 20 mM PC, vesicle growht was observed until the ratio of OG/PC reached about 3.0, and then micellization occurred at a higher concentration of OG, as determined by turbidity change and microscopic observation. On the other hand, a marked increase in turbidity was observed after freeze-thawing of the samples when the OG/PC retio was less than 3.0. The frozen and thawed mixed bilayer composed of OG/PC=2 formed closed vesicles having solute-trapping ability as determined by fluorescence microscopy. Interstingly, the trapping volume of liposomes generated after removal of OG from freeze-thawed samples was higher for those from mixed micelles than for those from mixed vesicles composed of OG and PC. When increasing concentrations of OG were mixed with PC vesicles of 0.8 or 2 mM PC, micellization started when the OG was at about its critical micelle concentration (cmc), although marked turbidity was observed when the OG/PC ratio was 2.0 after freeze-thawing. These data suggest that freeze-thawing affects the bilayer-micelle transition and liposomal formation after removal of OG even at concentrations of detergent lower than its cmc.