SIMULTANEOUS ELECTRICAL AND OPTICAL INTERFEROMETRIC MEASUREMENTS OF PRESSURE-POTENTIAL-INDUCED AND APPLIED-POTENTIAL-INDUCED BILAYER LIPID-MEMBRANE DEFORMATION

Hydrostatic-pressure- and transmembrane-potential-induced deformations of glyceryl monooleate (GMO) and bovine brain phosphatidylserine (PS) bilayer lipid membranes (BLMs) were investigated by simultaneous optical and electrical measurements. The investigation of a large number of samples established subtle variations in the observable parameters (appearance of concentric optical interference fringes, capacitance, voltage-dependent capacitance, resistivity, and voltage-dependent resistivity changes as a function of hydrostatic pressure and applied potential) of separately formed, but otherwise identical BLMs. Simultaneous optical and electrical measurements were performed on given BLM preparations, each of which had relatively small Plateau-Gibbs borders and "survived" for 6 h or longer. Application of 4-21 mN m-2 hydrostatic pressure (P(hp)) resulted in the appearance of concentric optical interference fringes (some of which were transient) which allowed the calculation of translational (lateral) displacements (F(t)) and curvature changes (F(c)). Plots of F(c) values against P(hp) gave good straight lines from which interfacial tensions (gamma) of GMO and PS BLMs were calculated to be 0.22 +/- 0.02 and 0.29 +/- 0.03 mN m-1, respectively. Treatment of the P(hp)-induced F(c) changes also allowed the calculation of the curvature elastic modulus, kappa-R(c), from which, assuming constant BLM mass density, the surface elastic density modulus, kappa-S (kappa-S = gamma), and the thickness elastic modulus, kappa-delta-h, were also assessed. This mechanical approach (method M) gave kappa-R(c) = 3.99 x 10(-21) J, kappa-S = 2.92 x 10(-4) N m-1, and kappa-delta-h = 5.53 x 10(4) N m-2 for PS and kappa-R(c) = 2.89 x 10(-21) j, kappa-S = 2.22 x 10(-4) N m-1, and kappa-delta-h = 5.46 x 10(4) N m-2 for GMO and BLMs. Voltage-dependent capacitance measurements provided an alternative approach (electrical approach, method E) to kappa-delta-h, kappa-S, and kappa-R(c). Method E gave kappa-R(c) = 2.89 x 10(-21) j, kappa-S = 2.08 x 10(-4) N m-1, and kappa-delta-h = 4.01 x 10(4) N m-2 for PS and kappa-R(c) = 4.84 x 10(-21) J, kappa-S = 7.62 x 10(-4) N m-1, and kappa-delta-h = 21.5 x 10(4) N m-2 for GMO BLMs. The observed voltage-dependent, gamma, and elastic moduli changes were rationalized in terms of surfactant and solvent exchanges between the bilayer and its Plateau-Gibbs reservoir.