Lateral proton conduction in mixed monolayers of phosphatidylethanolamine and cetyltrimethylammonium bromide

Proton conduction is known to be facilitated along phospholipid monolayers spread on aqueous phases. This property was monitored with mixed cetyltrimethylammonium bromide/phosphatidyl-ethanolamine monolayers. The film was shown to be metastable by surface pressure and fluorescence measurements. The detergent was leaving the interface for the bulk phase. Nevertheless, a fraction of the detergent remained in the lipid matrix, as shown by the binding of the fluorescent probe 8-anilino-1-naphthalensulfonate. Its dissociation constant decreased, and the nature of its binding site was affected, as shown by a shift of its emission spectrum. Apart from film expansion, the properties of the film were affected only at the water/membrane interface. Proton conduction was prevented only when the surface concentration of the detergent was larger than a critical value. Such an effect could be due either to the disruption in the continuity of the conducting hydrogen-bond network or to an electrostatic repulsion of the protons by the interface.