Phase transition and electric oscillations in synthesized lipid membranes

Dynamic characteristics of a model membrane constructed from a porous filter impregnated with synthesized lipids, dioleyl phosphate (DOPH), are studied. This model membrane shows a change in electric properties with the salt concentration. It is shown that a relaxation from the low-capacitance to high-capacitance state exhibits a sigmoid curve with time delay, while the transition from the high- to low-capacitance state is a simple exponential type. A hysteresis loop is time-dependent. A theoretical model of phase transition between oil droplets, spherical micelles and bilayers explains the experimental results. When the membrane is placed under the salt-concentration gradient and sometimes under the electric current and the pressure difference, the oscillation of electric potential across the membrane appears spontaneously. The burst as often found in neurons is also observed. The theoretical model proposed is able to describe the above oscillations, and shows that nonlinear coupling of phase transition of DOPH molecules with salt flow plays an essential role on the oscillations.