Ionic Probes of Membrane Structures*

In this paper we have examined the possibility of identifying those membrane structural variables (polar head groups and the nature of hydrocarbon tails) that modulate membrane ionic permeability. Altering the bilayer lipid composition produces variations in physical parameters (surface potential, partition coefficient, and mobility) governing the conductance mediated by neutral carriers of anions and cations. Specifically, the effects of the charged polar head groups are shown to be understandable in terms of the surface potential they produce through the formation of a diffuse double layer, whereas the effects of the viscosity may be demonstrated by "freezing" the membrane. The effects of membrane composition on membrane conductance are illustrated by a third, less well understood, example of how cholesterol alters bilayer conductances. The results indicate the possibility of using positive and negative permeant species as probes of membrane structures.