Kinetic Studies of Liposome Solubilization by Sodium Dodecyl Sulfate Based on a Dynamic Light Scattering Technique O. Lo ´pez,* M. Co ´cera, R. Pons, N. Azemar, and A. de la Maza Departamento de Tensioactivos, Centro de Investigacio ´ ny Desarrollo (C.I.D.), Consejo Superior de Investigaciones Cientı ´ficas (C.S.I.C.), C/ Jordi Girona, 18-26, 08034 Barcelona, Spain Received February 24, 1998. In Final Form: May 21, 1998 Introduction 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 scattering 2-6 and cryo-TEM 7-9 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 associ- ated 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, re- cent 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 surfac- tants 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 mem- brane models such as PC liposomes 20-22 and those formed by stratum corneum lipids, 23,24 due to its irritating effect in biological membranes. 25-27 Materials and Methods PC was purified from egg lecithin (Merck, Darmstadt, Ger- many) using the method of Singleton 28 and was shown to be pure by TLC. Sodium dodecyl sulfate (SDS) was obtained from Merck and further purified by a column chromatographic method. 29 Tris(hydroxymethyl)aminomethane (TRIS buffer) obtained from Merck was prepared as 5.0 mM TRIS buffer adjusted to pH 7.4 with HCl and containing 100 mM of NaCl. Vesicle Preparation and Solubilization. Unilamellar PC liposomes of a defined size (about 200 nm) were prepared by extrusion of large unilamellar vesicles (through 800-200 nm polycarbonate membranes) previously obtained by reverse phase evaporation. 16,30 To study the kinetics of solubilization of PC liposomes by SDS, different surfactant concentrations in TRIS buffer (from 0.3 to 4.5 mM) were added to liposomes, the PC concentration remaining constant (0.5 mM). The study was carried out during 24 h using a dynamic light-scattering technique and paying special attention to the first 10 min to know in detail the initial steps of this process. Dynamic Light-Scattering Experiments. The hydrody- namic diameters (HDs) of pure PC vesicles, pure SDS micelles, and particles formed during the interaction of SDS with liposomes were determined by means of a dynamic light-scattering (DLS) technique using a photon correlator spectrometer (Malvern Autosizer 4700c PS/MV) equipped with an Ar laser source (wavelength 488 nm). To acquire the full range of decay time necessary to determine the signal from both the large and the small particles, a low sample time value (2 μs) and a dilatation of 3 with parallel subcorrelators were used. Quartz cuvettes were filled with the samples, and all the experiments were thermostatically controlled. DLS determinations were made with a reading angle of 90° in all cases. Measurements of the overall * To whom correspondence should be addressed. Telephone: (34- 3) 400.61.61. Telefax: (34-3) 204.59.04. (1) Lichtenberg, D. Biochim. Biophys. Acta 1985, 821, 470. (2) Edwards, K. Prog. Colloid Polym. Sci. 1990, 82, 190. (3) Paternostre, M.; Meyer, O.; Grabielle-Madelmont, C.; Lesieur, S.; Ghanam, M.; Ollivon, M. Biophys. J. 1995, 69, 2476. (4) Partearroyo, M. A.; Alonso A.; Gon ˜ i, F. M.; Tribout, M.; Paredes, S. J. Colloid Interface Sci. 1996, 178, 156. (5) Cladera, J.; Rigaud, J. L.; Villaverde, J.; Dun ˜ ach, M. Eur. J. Biochem. 1997, 243, 798. (6) Wenk, M. R.; Seelig, J. 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