Kinetics and structural aspects of reconstitution of phosphatidylcholine vesicles by dilution of phosphatidylcholine-sodium cholate mixed micelles

Abstract

Dilution of mixed micellar dispersions of egg phosphatidylcholine (PC) and sodium cholate beyond a critical value results in formation of cholate-containing PC vesicles. The structure of the resultant vesicles and some mechanistic aspects of this process have been investigated by the use of light scattering and nuclear magnetic resonance techniques. The main findings and conclusions are the following: Both the state of aggregation (micellar or vesicular) and the apparent equilibrium size distribution of micelles or vesicles obtained by dilution of the PC-cholate mixed micellar dispersions are a function of the cholate to PC molar ratio in the mixed aggregates (micelles or vesicles). When this effective ratio (Re) is higher than 0.4, the dispersion is micellar, and the size of the mixed micelles increases with decreasing Re; when Re less than 0.3, the dispersion is essentially vesicular, and the mean hydrodynamic radius of the vesicles is an increasing function of Re; in dispersions with 0.3 less than Re less than 0.4, mixed micelles and vesicles coexist. Addition of cholate to vesicular dispersions, to Re values below 0.3, results in vesicle size growth through a concentration-independent lipid-exchange mechanism. Addition of cholate to higher Re values results in micellization (solubilization) of the vesicles. On the other hand, dilution of vesicular dispersions does not affect the size of the vesicles. Apparent equilibration of a mixed micellar dispersion following dilution to Re values below 0.3 is slow (many hours). The overall process involves a series of three subsequent categories of steps: (i) a rapid (approximately 1-2 min) prevesiculation equilibration of micellar sizes.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical Properties of Phosphatidylcholine Vesicles Containing Small Amount of Sodium Cholate and Consideration on the Initial Stage of Vesicle Solubilization.

“…No significant change occur in the turbidity curve of EPC SUV in the region of ReϽ0.23, whilst the turbidity for DMPC SUV increases considerably in the region of 0ϽReϽ0.08, followed by a gradual increase in the region of 0.08ϽReϽ0. 16. The average sizes of DMPC SUV increased from 30 (Reϭ0) to 80-100 nm (Reϭ0.14) accompanied with this turbidity increase, which agreed with the results reported by Malloy and Binford.…”

“…The molar ratio of Na-chol to PC in the membrane (Re) was employed as a measure of detergent concentration. [15][16][17][18] The Re value was directly measured by equilibrium dialysis method. That is, the vesicle suspension was incubated with an appropriate amount of Na-chol for 1 d using a dialysis cell, which was composed of two compartments and separated by a dialysis membrane.…”

The mechanism of vesicle formation

“…Therefore it was the first method for which a model of vesicle formation was proposed (Lasic, 1982a,b) and for which an experimental search for the mechanism and intermediate structure was performed (Schurtenberger et al, 1984;Almog et al, 1986;Cornell et al, 1986). Later it will be shown that the same mechanism can be applied to the other preparation techniques as well (Lasic, 1987 (Lasic, 1982b) at the (shielded) edges of micelles of the system is proportional to the total circumference (S) of disk-like micelles, since:…”

“…After a model of vesicle formation by detergent depletion was proposed (Lasic, 1982a,b) several groups tried to confirm this model in which a disk-like micelle is predicted as an intermediate structure in the vesicle formation process (Schurtenberger et al, 1984;Fromherz & Ruppel, 1985;Almog et al, 1986;Cornell et al, 1986) (see the next section). Later this model was also generalized to explain other preparation techniques (Lasic, 1983(Lasic, , 1987 and the mechanisms of the processes involved 1988 in the vesicle formation by all these methods were qualitatively described.…”

“…Later this model was generalized to encompass the other preparation techniques which, as a consequence, could be at least qualitatively understood (Lasic, 1983(Lasic, , 1985(Lasic, , 1987. Several research groups also tried to prove this model experimentally (Schurtenberger et al, 1984;Fromherz & Ruppel, 1985;Almog et al, 1986;Cornell et al, 1986;Wrigglesworth et al, 1987). The aim of this article is to review, after a brief introduction of vesicle preparation methods, the present state of the understanding of the vesicle formation.…”