Non-isothermal potential of phospholipid bilayer films. Influence of cholesterol and macrocyclic carrier effects.

The effect of cholesterol on the ion selective behavior of phoshpolipid (phosphatidylcholine or phosphatidylethanolamine) bilayer films is studied through the measurement of the membrane non-isothermal potential. It is shown how the mixed phosphatidylcholine-cholesterol membrane can be either cation of anion permselective according to the film composition (cationic behavior is met in the 0--10% cholesterol composition range while anionic selectivity appears in the 20--50% range). On the contrary, mixed phosphatidylethanolamine-cholesterol membranes show the absence of ionic selectivity already met with pure phosphatidylethanolamine films. The presence of a cationic carrier as Dibenzo-18-crown-6 in the film transforms all the studied films (cationic, anionic and no selective bilayers) into ideally cationic selective membranes. These results are discussed on the basis of the current ideas on the charge distribution through the bilayer membranes. Moreover, the role of the permeating ions as potential determining species is stressed.