EPR Determination of Membrane Potentials

TLDR: The boundary potential is defined as 1/10,1/1;, the potential difference between the site of ion adsorption and the corre­ sponding bulk aqueous phase (see Figure 1).

Abstract: A large number of membrane processes are electrically active; that is, they involve the movement of charge within or through the membrane. Membrane electrical potentials are primary variables that must be measured in investigating such processes. A possible electrostatic potential profile across a membrane that delineates an interior and exterior space is shown in Figure l. �I/I, the transmembrane potential, is the potential difference between the bulk aqueous phases on either side of the membrane and results from separation of mobile charge across the membrane. The surface poten­ tials, 1/1'0,1/1.;, are the electrostatic potentials at the surfaces of the membrane relative to the potential of the bulk aqueous solution, and arise from fixed charge at the membrane-solution interface. Recently, Andersen et al (3) have described an internal membrane potential that arises from the adsorption of hydrophobic ions like tetraphenylborate in a region within the membrane low dielectric. This potential is termed the boundary potential and in this review is defined as 1/10,1/1;, the potential difference between the site of ion adsorption and the corre­ sponding bulk aqueous phase (see Figure 1). An important characteris­ tic of the boundary potential is that it arises from the charge density within a low dielectric and thus cannot be completely screened by high ionic strength (3). The potentials shown here ignore contributions from dipoles within the membrane structure and include only variable poten­ tials arising from charge separations involving mobile ions.