Frequency dependence of electric-field-induced orientation of myelin tubes

Orientation induced by an alternating electric field was studied for myelin tubes of egg phosphatidylcholine (egg-PC) in water and in KCl electrolyte solution. The orientation was also studied for myelin tubes of egg-PC/cholesterol mixtures in water. The orientation effect was measured by the bending curvature of myelin tubes with hairpin-like deformation at frequencies between 10 kHz and 20 MHz. The bending deformation of egg-PC myelin tubes in water decreased abruptly with a decreasing frequency within the low-frequency range below 100 kHz, and shrinkage of myelin tubes was often observed. The bending deformation decreased as the frequency was increased in a higher frequency range. The profile of the frequency dependence for egg-PC in KCl solution was similar to that in water but shifted towards a higher frequency. At low KCl concentrations below 3 mM, the relaxation frequency increased proportionally with increasing an KCl concentration, which was due to a proportional increase in the conductivity of the surrounding medium. Similar profiles of the frequency dependence were observed for egg-PC/cholesterol mixtures but with no shift in the relaxation frequency. These data in the high-frequency range fitted well with calculations based on theoretical equations for the electric-field-induced orientation of nonspherical particles. The conductivity of myelin tubes was estimated to be in the order of 10(-4) S/m in water and of 10(-3) S/m at the low KCl concentrations. The bending modulus of a bilayer membrane was estimated to be (1.0 +/- 0.5) x 10(-19) J for egg-PC in water. No change in the bending modulus was observed for egg-PC in KCl solution. However, for egg-PC/cholesterol mixtures in water, the bending modulus abruptly increased to (1.8 +/- 0.5) x 10(-19) J at a cholesterol concentration of 40 mol%.