Kinetic Studies of Liposome Solubilization by Sodium Dodecyl Sulfate Based on a Dynamic Light Scattering Technique

Abstract

The vesicle to micelle transformations induced by the action of surfactants in phospholipid vesicles are currently attracting much interest. These processes leads to the solubilization of lipid vesicles, and they represent good models for the study of solubilization of cell membranes.1 A number of studies have been devoted to using techniques of light scattering2-6 and cryo-TEM7-9 to clarify the principles governing these transformations. In general, there is agreement that a growth of vesicles occurred in the initial interaction steps followed by the formation of a number of complex lipid-surfactant aggregates associated with the vesicle to micelle transformations. Thus, Edwards and Almgren,7 Silvander et al.,8 and Gustafsson et al.,9 reported open bilayer fragments in coexistence with different micellar structures as intermediate aggregates in the interaction of varios anionic and cationic surfactants with phosphatidylcholine (PC) liposomes. However, recent studies using vesicles prepared from nonionic and oppositely charged surfactants proposed rapid and simpler mechanisms for vesicle to micelle transformations as single-step processes.10-14 In earlier papers we investigated the structural changes resulting in the interaction of alkyl sulfates with PC liposomes.15-17 Although kinetic studies of formation of vesicles and micelles as independent processes have been reported,18,19 kinetics of vesicle solubilization by surfactants remained elusive in spite its obvious importance. Hence, we present here a kinetic study of the vesicle to micelle structural transformations that take place in the solubilization of PC liposomes by sodium dodecyl sulfate (SDS). To this end, a dynamic light-scattering (DLS) technique (Ar laser source, useful in systems in which small and large particles coexisted) has been used. The use of this technique in solubilization kinetic studies opens up new avenues in the knowledge of the mechanisms that occur in this process. The anionic surfactant SDS has been selected given its frequent use in simplified membrane models such as PC liposomes20-22 and those formed by stratum corneum lipids,23,24 due to its irritating effect in biological membranes.25-27