Twelfth School on Biophysics of Membrane Transport
School Proceedings, Poland, May 4-13, 1994
Primo¾ Peterlin, France Sev¹ek, Sa¹a Svetina and Bo¹tjan ®ek¹
Instutute of Biophysics, Lipičeva 2, 61105 Ljubljana, Slovenia
External electric field deforms a phospholipid vesicle into a rotational ellipsoid, with the rotational axis parallel to the field. The extent of deformation is dependent on the field strength, its frequency, the size and the relative volume of the vesicle, the membrane tension, the membrane elastic constants, and the electric properties of both the membrane and the surrounding medium.
A class of degenerate stomatocyte (cup-shaped) vesicles was studied, where the cup is invaginated entirely into itself, thus forming a minor inner spherical vesicle within the vesicle observed, with which it is joined via a tight neck. When exposed to external electric field, the inner vesicle tends to diminish as the field strength is increasing, and at certain threshold field level the neck linking the two vesicles opens abruptly and a single simple rotational ellipsoid is formed. The process was found to be reversible to large extent, i.e. by decreasing the field the neck has closed again.
The field strength needed for such process has been estimated within the bilayer couple model. The equilibrium vesicle shape is determined by the minimum of the total vesicle energy, consisting of the energy due to electric field, and the membrane bending energy. The latter is independent on the field strength, and is increasing as the spherical vesicle is elongated into a prolate rotational ellipsoid. It thus opposes the ellipsoidal deformation. The energy due to electric field, on the contrary, depends strongly on the field strength, and favours ellipsoidal deformation. Clearly, the two effects balance each other at certain level of deformation, and this equilibrium deformation is dependent on the field strength.
What is the field strength needed to open the neck? We can think of a gedanken experiment, and increase the deformation to the maximal value, where the invaginated vesicle vanishes completely. The process is energy consuming, but at the final point the energy difference might be paid off, since the constant term stating for the bending energy for the invaginated vesicle vanishes, too. We can see that as field strength increases, the equilibrium shape becomes more and more deformed, and lesser and lesser energy would be needed to deform the vesicle from the equilibrium shape into a fully extended one. At some point, this energy becomes as low as the bending elastic energy of the invaginated vesicle is. Further increase in field strength makes the fully extended shape more energetically favourable than the ``vesicle within a vesicle'', so this is the point where the transition occurs.
School Proceedings, Part 2, Twelfth School on Biophysics of Membrane Transport, Ko¶cielisko-Zakopane, Poland, May 4-13, 1994. Ed. by Stanis³aw Przestalski, Janina Kuczera and Halina Kleszczyńska, Department of Physics and Biophysics, Agricultural University of Wroc³aw, p. 236.