Curvature-electric effect in black lipid membranes

Upon periodical bending of a BLM, by means of oscillating hydrostatic pressure with sound frequency, the generation of an a.c. electric current with the same frequency can be observed under short circuit conditions. Previously, this phenomenon was attributed by us to a displacement current due to the oscillating flexoelectric polarization. The latter is proportional to the membrane curvature and depends on the lipid dipole moment and surface charge.The theory of this effect is outlined here. Earlier results concerning dipolar and quadrupolar contributions to the total current are presented and new expressions about charge contributions are derived for the two basic regimes of free and blocked lateral lipid exchange.Further, a systematic study of the frequency dependence of the amplitude and phase of the curvature-electric signal from a bacterial phosphatidylethanolamine/n-decane BLM is reported. Constant membrane curvature at each vibration frequency was assured by a calibration of the capacitance current observed with a small transmembrane voltage.The frequency dependence of the curvature-electric current amplitude was characterized by two regions: low frequency plateau and high frequency slope, the boundary between them being about 160 Hz. Such behaviour suggested a switching of the mechanism of membrane polarization from free to blocked lateral lipid exchange. Frequency dependence of the phase shift was characterized by low frequency and high frequency plateaus and a gradual transition between them. From phase measurements on initially curved membranes the sign of the membrane flexo-coefficient was found to be negative.The influence of some modifiers of the surface charge and surface dipole, as well as of the membrane conductivity, upon the value of the effect was studied. Surface charge was separately measured by the internal field compensation method under an ionic strength gradient. The membrane flexoelectric coefficient was evaluated and compared to the theoretical predictions. A conclusion was drawn that under the present experimental conditions the main contribution to the effect comes from the curvatureinduced shift of the surface charge equilibrium.Presented at the Tenth International Liquid Crystal Conference, 15–21 July 1984, York, UK