Electric field-induced transient birefringence and light scattering of synthetic liposomes

The dynamics of electric field-induced transient birefringence Delta n(t) and light scattering (detected as turbidity) of 190 nm diameter unilamellar vesicles of dioleoylphosphatidylcholine are investigated as a function of applied field strength E, length of the square pulse Delta t, lipid concentration, mean hydrodynamic diameter (D-h), ionic strength, and temperature. Generally, induced birefringence exclusively is observed at low lipid concentration and below certain threshold values of E and Delta t, whereas concomitant induced turbidity appears at high lipid concentration and above thresholds values of E and Delta t. Turbidity is monitored through the change in transmitted intensity Delta S-parallel to(t) and Delta S-perpendicular to(t) of light polarized parallel and perpendicular to the applied field E. The field-induced structural changes are reflected in double-exponential forward relaxation and triple-exponential reverse relaxation of the positive birefringence, and in non-exponential relaxations of Delta S-parallel to(t) and Delta S-perpendicular to(t). Under the field, the associated physical events are interpreted as elongation of the spherical bilayer shells in the direction of E, linear chain formation (pearling) of the induced dipolar liposomes parallel to E, and partial fusion of adjoining vesicles within the chains. Under conditions where electroporation can be detected, pore opening succeeds the elongation of the vesicles. After termination of the field, the vesicles return to their original time average spherical shape, the oriented chains randomize and disintegrate, and the fused structures are converted either to unilamellar or multilamellar vesicles. (C) 1999 Elsevier Science B.V. All rights reserved.