FLEXOELECTRICITY OF CHARGED AND DIPOLAR BILAYER-LIPID MEMBRANES STUDIED BY STROBOSCOPIC INTERFEROMETRY

Curvature-induced surface polarization (flexoelectricity) of bilayer lipid membranes (BLMs), prepared from a charged phospholipid (phosphatidylserine, PS), was studied by real-time stroboscopic interferometry. In this method, one side of the BLM was subjected to an oscillating hydrostatic pressure and the resultant microvolt-range, trans-BLM ac electric potential difference was determined along with that of the amplitude of the oscillation of the two principal membrane curvatures. Flexoelectric coupling coefficients (f) for PS BLMs were determined as a function of the frequency of the oscillating hydrostatic pressure in the range of 100-800 Hz at pH values of 4.0, 7.0, and 10.0. The obtained f values for PS BLMs were compared to those which were previously determined for weakly charged (egg lecithin, PC, and phosphatidylethanolamine, PE) and purely dipolar (glycerol monooleate, GMO) BLMs. The f values of strongly and weakly charged BLMs were similar (f congruent-to 10(-18)C). However, a strikingly different f value was found for dipolar GMO BLMs (f congruent-to 10(-20)C). These results are discussed in terms of two molecular mechanisms contributing to the flexoelectric polarization of a BLM: (i) curvature-induced variation of the surface potential (leaving the two halves of the BLM electrically neutral) and (ii) curvature-induced shift of the surface charge equilibrium (giving rise to charge separation across the whole BLM thickness). Surface potential variations, as measured in monolayer experiments by others, can only account for f values in the range of 10(-20)C (i.e., for purely dipolar BLMs). Large flexoelectric coefficients of charged BLMs can only be rationalized in terms of the curvature-induced shift of surface charge equilibrium, most notably due to the area variation of the degree of dissociation of the charged lipid head groups.