States of aggregation and phase transformations in mixtures of phosphatidylcholine and octyl glucoside

Abstract

The result of mixing varying concentrations of the nonionic detergent octyl glucoside (OG) with small unilamellar vesicles (SUV) of egg phosphatidylcholine (PC) made by sonication depends on the ratio between OG and PC in the mixed aggregates. When this molar ratio (Re) is lower than 1.4, the detergent partitions between the PC vesicles and the aqueous medium with a partition coefficient of K = 0.033 mM-1. As a consequence of introduction of OG into the bilayers, the vesicles grow in size. The resultant vesicles have a mean diameter that is an increasing function of Re and is independent of the total PC concentration. Experiments in which the vesicles were loaded with high molecular weight dextran prior to being exposed to OG suggest that the mechanism responsible for the size growth involves lipid transfer rather than fusion. Mixtures with Re values within the range of 1.4-3.2 separate into two macroscopic phases: The lower phase is clear but very viscous. It contains constant OG and PC concentrations and is characterized by an Re value of 3.2, independent of the composition of the whole dispersion. The upper phase contains vesicles of varying concentrations of OG and PC, but a constant Re of 1.4. When the saturating level of 1.4 OG molecules per PC molecule is approached, the concentration of OG monomers in the aqueous medium reaches the value of 16.6 +/- 0.3 mM, which is the apparent cmc of OG in the lipid-containing medium. OG-PC mixed micelles contain at least 3.2 OG molecules per PC molecule. The mixed micelles present at Re = 3.2 apparently have the shape of oblate ellipsoids with a minor axis of about 2 nm and two major axes of about 25 nm. The surface area of the mixed micelles at this point is just sufficient for them to undergo conversion into the smallest possible spherical vesicles of a radius of 12 nm. At Re values above 3.2, the major axis of the mixed micelles becomes smaller as Re increases, while at values of Re below 3.2 the micelles would have been expected to grow very rapidly with decreasing Re. This may explain the partial vesicle closure occurring below Re = 3.2.

The micelle to vesicle transition of lipids and detergents in the presence of a membrane protein: towards a rationale for 2D crystallization

“…It has been shown that the process of liposome formation is in many respects the reverse of solubilization [21], the latter being described by a three-stage model [22]. Interestingly, in a number of studies on the solubilization of liposomes by detergents, a maximum of the turbidity was measured at the intermediate stage [23,24], similar to the maximum of the ~.-curve in our experiments.…”

Incorporation of cholesterol in sphingomyelin- phosphatidylcholine vesicles has profound effects on detergent-induced phase transitions

“…Vesicle ↔ micelle transitions have been studied in some detail by turbidity measurements (7,8), nuclear magnetic resonance (8,9), and cryotransmission electron microscopy (10,11). These studies were generally performed with phosphatidylcholine (PC) as the phospholipid [often with PC from egg yolk, which contains 16:0 acyl chains at the sn -1 position and mainly unsaturated (18:1 Ͼ 18:2 Ͼ 20:4) acyl chains at the sn -2 position].…”

Process of Destruction of Large Unilamellar Vesicles by a Zwitterionic Detergent, CHAPS: Partition Behavior between Membrane and Water Phases

“…This result agrees with those of a previous study by Almog et al showing that the liposome suspension was essentially vesicular within the upward turbidity curve in octylglucoside-EggPC system. 9) In the region of the downward turbidity curve, the EggPC content in the nonvesicular portion increased after centrifugation, defining the destruction of vesicles (data not shown).…”

“…In the past decades much attention has been paid to the interaction of detergents with phospholipid membranes. [9][10][11] Octylglucoside, Triton X100, and bile acid derivatives were most widely studied. [12][13][14] Although CHAPS has been widely used in membrane biology, little is known of the details of the interaction of CHAPS and vesicles.…”