A mechanism of liposome electroformation

Abstract

External electric fields can induce or prevent liquid swelling and liposome formation on solid surfaces. These effects depend on the type of lipid and surface, the medium parameters (temperature, osmolarity, ionic strength), the dried lipid layer thickness, the type and parameters of the electric field (dc or ac, amplitude, frequency, current), the situation of the lipid (on the very electrode surface or an another surface) and the time of exposure. This paper presents new data and theoretical estimates which allow us to suggest a possible mechanism of liposome electroformation. The new experimental results with negatively charged egg lecithin (EggL⁻) and neutral synthetic phosphatidylcholine (PC) are that (1) cholesterol in mixtures with EggL⁻ inhibits liposome formation, (2) sodium chloride leads to a decrease in the size and number of liposomes; liposomes do not form in solution more concentrated than 10 mM NaCl, (3) dextran also decreases the size and number of liposomes; they do not form in solution of dextran concentration higher than 2.5 mM, (4) dc electric fields can overcome the effect of dextran and lead to swelling even in 2.5 mM dextran solutions, (5) dc fields are most effective if applied in the first 30 s of lipid swelling, the increase of the period of the field action does not lead to a significant effect on the liposome yield, (6) a similar effect was also observed with (PC), but the critical period of time was a bit shorter — 10 s. The balance of forces acting on a lamellae of hydrating lipid shows that a possible pathway of liposome formation can include at least three basic stages: (1) separation of interacting membranes — the hydration and electrostatic interactions yield the main driving forces for this process, (2) instability of bending which can result from negative membrane tension due to surface and line tension and (3) the bending itself and closing of the membranes — the kinetics of this process and the instability of bending largely determine the liposome size distribution. External electric fields can affect any of these stages by at least two mechanisms: (1) direct electrostatic interaction and (2) redistribution of the counter-ions between the membranes. The interplay between the van der Waals, hydration and electrostatic forces is the major determinant of the mechanisms of liposome electroformation.