Membrane solubilization with and reconstitution from surfactant solutions: a comparison of phosphatidylserine and phosphatidylcholine interactions with octyl glucoside

Capabilities of liposomes for topological transformation

Dynamic behaviors of liposomes caused by interactions between liposomal membranes and surfactant were studied by direct real-time 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.

The vesicle-to-micelle transition of phosphatidylcholine vesicles induced by nonionic detergents: effects of sodium chloride, sucrose and urea

The vesicle-to-micelle transition of egg phosphatidylcholine LUVs induced by octylglucoside was studied in buffers with 0-4 M sodium chloride, sucrose or urea. We used both light scattering and fluorescent probes to follow the lipid-detergent complexes in these buffers. The vesicle-to-micelle transition process was fundamentally the same in each solute. However, the detergent-to-lipid ratio required for micelle formation shifted in ways that depended on the aqueous solute. The partitioning of octylglucoside between the vesicles and the aqueous phase was primarily determined by the change in its critical micelle concentration (cmc) induced by each solute. Specifically, the cmc decreased in high salt and sucrose buffers but increased in high concentrations of urea. Cmc for two additional nonionic detergents, decyl- and dodecyl-maltoside, and three zwittergents (3-12, 3-14 and 3-16) were determined as a function of concentration for each of the solutes. In all cases NaCl and sucrose decreased the solubility of the detergents, whereas urea increased their solubilities. The effects clearly depended on acyl chain length in urea-containing solutions, but this dependence was less clear with increasing NaCl and sucrose concentrations. The contributions of these solutes to solubility and to interfacial interactions in the bilayers, pure and mixed micelles are considered.

Partition coefficient of a surfactant between aggregates and solution: application to the micelle-vesicle transition of egg phosphatidylcholine and octyl beta-D-glucopyranoside

The mechanism of the solubilization of egg phosphatidylcholine containing 10% (M/M) of egg phosphatidic acid unilamellar vesicles by the nonionic detergent, octyl beta-D-glucopyranoside, has been investigated at both molecular and supramolecular levels by using fluorescence and turbidity measurements. In the lamellar region of the transition, the solubilization process has been shown to be first a function of the initial size before reaching an equilibrium aggregation state at the end of this region (the onset of the micellization process). The analysis during the solubilization process of the evolution of both the fluorescence energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-phosphatidylethanolamine (NBD-PE) and N-(lissamine rhodamine B sulfonyl)-phosphatidylethanolamine (Rho-PE) and the fluorescence of 6-dodecanoyl-2-dimethylaminoaphtalene (Laurdan) has allowed us to determine the evolution of the detergent partitioning between the aqueous and the lipidic phases, i.e., the evolution of the molar fraction of OG in the aggregates (XOG/Lip) with its monomeric detergent concentration in equilibrium ([OG]H2O), throughout the vesicle-to-micelle transition without isolating the aqueous medium from the aggregates. The curve described by XOG/Lip versus [OG]H2O shows that the partition coefficient of OG is changing throughout the solubilization process. From this curve, which tends to a value of 1/(critical micellar concentration), five different domains have been delimited: two in the lamellar part of the transition (for 0 < [OG]H2O < 15.6 mM), one in the micellization part, and finally two in the pure micellar region (for 16.5 < [OG]H2O < 21 mM). The first domain in the lamellar part of the transition is characterized by a continuous variation of the partition coefficient. In the second domain, a linear relation relates XOG/Lip and [OG]H2O, indicating the existence of a biphasic domain for which the detergent presents a constant partition coefficient of 18.2 M-1. From the onset to the end of the solubilization process (domain 3), the evolution of (XOG/Lip) with [OG]H2O can be fitted by a model corresponding to the coexistence of detergent-saturated lamellar phase with lipid-saturated mixed micelles, both in equilibrium with an aqueous phase, i.e., a three-phase domain. The micellar region is characterized first by a small two-phase domain (domain 4) with a constant partition coefficient of 21 M-1, followed by a one-phase mixed-micellar domain for which XOG/Lip no longer linearly depends on [OG]H2O. The results are discussed in terms of a phase diagram.