Short note Surface potential determination in planar lipid bilayers: A simplification of the conductance-ratio method Fernando Abdulkader, Manoel Arcisio-Miranda, Rui Curi, Joaquim Procopio ⁎ Departmento de Fisiologia e Biofísica, Instituto de Ciências Biomédicas, Universidade de São Paulo, São Paulo, Brazil Received 21 August 2006; received in revised form 28 December 2006; accepted 10 January 2007 Abstract One of the methods available for the measurement of surface potentials of planar lipid bilayers uses the conductance ratio between a charged and a neutral bilayer doped with ionophores to calculate the surface potential of the charged bilayer. We have devised a simplification of that method which does not require the use of an electrically neutral bilayer as control. The conductance of the charged bilayer is measured before and after the addition of divalent cations (Ba 2+ ) to the bathing solution. Ba 2+ ions screen fixed surface charges, decreasing the surface potential. If the membrane is negatively charged the screening has the effect of decreasing the membrane conductance to cations. The resulting conductance ratio is used to calculate the surface potential change, which is fed into an iterative computer program. The program generates pairs of surface potential values and calculates the surface charge density for the two conditions. Since the surface charge density remains constant during this procedure, there is only one pair of surface potentials that satisfies the condition of constant charge density. Applying this method to experimental data from McLaughlin et al. [McLaughlin, S.G.A., Szabo, G. and Eisenman, G., Divalent ions and the surface potential of charged phospholipid membranes, J. Gen. Physiol., 58 (1971) 667–687.] we have found very similar results. We have also successfully used this method to determine the effect of palmitic acid on the surface potential of asolectin membranes. © 2007 Elsevier B.V. All rights reserved. Keywords: Surface potential; Surface charge density; Membrane conductance; Planar lipid bilayers Electrical surface potentials are characteristically present at the lipid-solution interface of biological membranes and in many biomimetic membrane systems. In pure lipid bilayers, these potentials are due to the negative net charges from charged polar headgroups. These charges attract cations from the solution, changing their local concentration relative to the bulk solution. The importance of surface potentials has been recognized since the works of Frankenhäuser and Hodgkin on the squid axon [1]. Surface potentials are relevant to the comprehension of issues such as fatty acid membrane kinetics [2], modulation of ion channel conductance by phospholipids [3,4] and soluble protein interactions with lipid bilayers [5] and others. Surface charge density and surface potential of a membrane immersed in an electrolitic solution are related by the Grahame equation [6]: r ¼ 1 272 X i C i exp -z i F w s RT   -1 " # 1=2 ð1Þ where σ is the surface charge density in charges per Å 2 , C i is the concentration of the ith ionic species in the bulk solution, ψ s is the surface potential and z i is the valence, R and T are respec- tively, the gas constant and absolute temperature. McLaughlin et al. [6] showed that the conductance of bilayer membranes induced by nonactin (an alkali metal ionophore) is reduced in the presence of divalent cations. These cations screen the surface charges, decreasing the surface potential but, to a J. Biochem. Biophys. Methods 70 (2007) 515 – 518 www.elsevier.com/locate/jbbm Abbreviations: σ, surface charge density; C i , concentration of the ith ionic species in the bulk solution; ψ s , surface potential; G, bilayer electrical conductance. ⁎ Corresponding author. Departamento de Fisiologia e Biofísica, Universidade de São Paulo, Av. Prof. Lineu Prestes, 1524 - CEP 05508-900, São Paulo - Brazil. Tel.: +55 11 3091 7284; fax: +55 11 3091 7285. E-mail address: procopio@icb.usp.br (J. Procopio). 0165-022X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jbbm.2007.01.003