1ST OBSERVATION OF THE CONVERSE FLEXOELECTRIC EFFECT IN BILAYER-LIPID MEMBRANES

Converse flexoelectric effects (i.e., voltage-generated curving) have been demonstrated in uranyl aetate (UA)stabilized, phosphatidylserine (PS) bilayer lipid membranes (BLMs) by real-time stroboscopic interferometric measurements. Subjecting a 5.4-nm-thick PS membrane, bathed in 0. 1 M KCl and 1.0 x 10(-3) M UA solution, to 300-mV transmembrane ac potentials at frequencies ranging from 100 to 450 Hz resulted in the periodic flexing of the membrane at the same frequencies. The peak-to-peak amplitude of the flexing of the BLM [2c(m) = 1/2(c(x1) + c(y1)) - 1/2(c(x2) + c(y2)), where c(x) and c(y) are BLM curvatures along the two orthogonal axes, horizontal and vertical, and the subscripts 1 and 2 refer to the two extreme deviations of the BLM during the electrically generated vibrations] has been determined by the evaluation of the interferometric fringes. Substitution of the obtained c(m) values into 2fE = (sigmar2/2 + rhogpir6/a(s))c(m) + 2nuetar4c3(m) (wheref is the tlexoelectric coefficient, E is the electric field, E = U/d, U = 300 mV-the zero-to-peak amplitude, d is the thickness of the BLM = 5.4 nm, sigma is the lateral BLM tension = 5 mN/m, r is the radius of the BLM = 143 mum, rho is the electrolyte density = 1 X 10(3) kg/m3, g is the acceleration of gravity = 9.8 m/s2, nu is the frequency of the ac potential, eta is the electrolyte viscosity = 1 X 10(-3) J s/m3, and a(s) is the area electrolyte surface on one side of the BLM = 0.785 CM2) afforded values for the flexoelectric coefficient at each nu. Two domains of converse flexocoefficient values have been found, one at low nu (below 300 Hz) and one at high nu (above 300 Hz). The average value between 300 and 450 Hz which was obtained for the converse flexoelectric coefficient, (12.6 +/- 0.7) X 10(-18) C, agreed reasonably well with that reported previously for the direct flexoelectric coefficient of an UA-modified BLM, (12-24) x 10(-18) C, demonstrating the validity of the Maxwell relation. Thus, both manifestations of flexoelectricity in BLMs have now received experimental confirmation.