Dynamic behaviors of liposomes caused by interactions between liposomal membranes and surfactant were studied by direct realtime observation by using high-intensity dark-field microscopy. Solubilization of liposomes by surfactants is thought to be a catastrophic event akin to the explosion of soap bubbles in the air; however, the actual process has not been clarified. We studied this process experimentally and found that liposomes exposed to various surfactants exhibited unusual behavior, namely continuous shrinkage accompanied by intermittent quakes, release of encapsulated liposomes, opening up, and inside-out topological inversion. L iposomes (which are closed membrane vesicles) have been well studied as simplified models of biological membranes (1-5) and are now used in a number of applications (5, 6), for example, as carriers of drug or DNA delivery or as artificial membranes for reconstructing membranous enzyme activities (7-9). Recently, many important phenomena affecting lipid bilayers, including their detergent solubilization, have been explored by using liposomes; such studies promote a better understanding of the biophysical properties of bilayer membranes and moreover will improve the handling of membrane proteins when they are isolated from or reconstructed into lipid bilayers (10-13). However, studies of intermediate stages in the detergent solubilization of liposomes are only now in progress (14-17), and the interaction mechanism between membranes and surfactants has remained unclear. Therefore, real-time approaches by using optical microscopy to study the dynamic behavior of liposomes are very important.High-intensity dark-field microscopy has enabled us to obtain real-time high-contrast images of giant unilamellar liposomes in aqueous solutions (18)(19)(20)(21)(22). In this study, we used such techniques to characterize the interactions between liposomal membranes and surfactants. Eight kinds of liposomes and various types of surfactants ( Fig. 1) were mixed in all possible combinations in a mixing chamber to generate a concentration gradient of each surfactant for microscope specimens, and morphological changes of liposomes exposed to those surfactants were monitored (18, 23). In the absence of surfactant, liposomal membranes were spherical, and thermal fluctuations of their spherical shape were largely suppressed by the surface tension of their membranes. Hereafter, this morphological state of liposomes will be called tense. In this study, we found several unusual behaviors of liposomes (which are published as supplemental data on the PNAS web site, www.pnas.org).
Materials and MethodsPreparation and Observation of Liposomes. To prepare giant unilamellar liposome, liposome (total 1 mM lipid concentration) was made of phosphatidylcholine (PC) or of PC and one of seven other lipids (1:1, mol͞mol) in Hepes buffer (10 mM HepesNaOH, pH 7.0), as described previously (18,21,22). Lipids were dissolved in a chloroform͞methanol solution, 98:2 (vol͞vol), and mixed. The organic solvent was evaporated unde...