Mixed micelles and other structures in the solubilization of bilayer lipidmembranes by surfactants

Authors
Citation
M. Almgren, Mixed micelles and other structures in the solubilization of bilayer lipidmembranes by surfactants, BBA-BIOMEMB, 1508(1-2), 2000, pp. 146-163
Citations number
65
Categorie Soggetti
Biochemistry & Biophysics
Journal title
BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES
ISSN journal
00052736 → ACNP
Volume
1508
Issue
1-2
Year of publication
2000
Pages
146 - 163
Database
ISI
SICI code
0005-2736(20001123)1508:1-2<146:MMAOSI>2.0.ZU;2-E
Abstract
The solubilization of lipid bilayers by surfactants is accompanied by morph ological changes of the bilayer and the emergence of mixed micelles. From a phase equilibrium perspective, the lipid/surfactant/water system is in a t wo-phase area during the solubilization: a phase containing mixed micelles is in equilibrium with bilayer structures of the lamellar phase. In some ca ses three phases are present, the single micelle phase replaced by a concen trated and a dilute solution phase. In the case of non-ionic surfactants, t he lipid bilayers reach saturation when mixed micelles, often flexible rod- like or thread-like, start to form in the aqueous solution, at a constant c hemical potential of the surfactant. The composition of the bilayers also r emains fixed during the dissolution. The phase behavior encountered with ma ny charged surfactants is different. The lamellar phase becomes destabilize d at a certain content of surfactant in the membrane, and then disintegrate s, forming mixed micelles, or a hexagonal phase, or an intermediate phase. Defective bilayer intermediates, such as perforated vesicles, have been fou nd in several systems, mainly with charged surfactants. The perforated memb ranes, in some systems, go over into thread-like micelles via lace-like str uctures, often without a clear two-phase region. Intermediates in the form of disks, either micelles or bilayer fragments, have been observed in sever al cases. Most noteworthy are: the planar and circular disks found in syste ms containing a large fraction of cholesterol in the bilayer. Bile salts ar e a special class of surfactants that seem to break down the bilayer at low additions. Originally, disk-like mixed micelles were conjectured, with pol ar membrane lipids building the disk, and the bile salts covering the hydro phobic rim. Later work has shown that flexible cylinders are the dominant i ntermediates also in these systems, even if the disk-like structures have b een re-established as transients in the transformation from mixed micelles to vesicles. (C) 2000 Elsevier Science B.V. All rights reserved.