Excitable and ion-selective artificial membranes : II. Non-bilayer lipidic membranes

Abstract

The bilayer membranes described in the previous paper are not the only selective and excitable models of living membranes. A wide variety of lipidic derivatives can be used and the bilayer structure is unnecessary. The incorporation in such model membranes of specific additives is also unnecessary. Excitability and selectivity appear to be common features of many lipidic components provided they possess some acidic groups. Ionic permeability of these membranes, especially for organic cations such as dyes or drugs, depends upon an ion-exchange process. Analogies between these model membranes and the axon membranes are presented: electrical excitability, and impedance and its temperature variation. The dielectric constant of these membranes appears to be small. Thus the fixed acidic groups in the membranes and the cations can form ion pairs. The dissociation of ion pairs controls the membrane conductance. This dissociation is described by classical equations in which the main factors are: ion size, temperature, and dielectric constant. It is shown that several features of the above model membranes can be accounted for by the ion-pairing concept. For example, it is shown that selectivity can be related to cation size. Conductance variation with temperature can depend upon a low dielectric constant. Excitability, that is conductance increase under an electric field, can result from a slight augmentation of dielectric constant which could be produced by a small flux of water brought about by electroosmosis. The ion-pairing concept accounts also for the photodissociation (i.e. conductance variations) of the above model membranes when they are in contact with a solution containing a cationic dye. The ion-pairing concept thus appears to be a possible alternative to the “pore” concept described in the previous paper.