MIXED MONOLAYERS OF PHOSPHATIDYLETHANOLAMINE AND (LAURYLAMIDO)-N,N'-DIMETHYLPROPYLAMINE OXIDE AT THE AIR-WATER-INTERFACE - LATERAL PROTON CONDUCTION ALONG A MIXED STRUCTURE OF CONDUCTING AND NONCONDUCTING ENTITIES

A localized lateral proton pathway is present along the phospholipid p olar heads and bound water molecules when the lipids are spread in mon olayers at the air/water interface. Mixed films of (laurylamido)-N,N'- dimethylpropylamine oxide (LAPAO) and phosphatidylethanolamine (PE) ar e studied by surface pressure and fluorescence measurements. The polar head of this amphiphilic detergent shows no global electric charge bu t a N+ <-- O- dipole. The localization of this detergent at the air/wa ter interface changes when the mixed monolayer is compressed. The mixe d monomolecular film is metastable, and a leakage of the amphiphile is observed even at very low surface pressure. Whatever the initial LAPA O/PE molar ratio spread at the air/water interface, when the monolayer is compressed, all molecules of LAPAO are expelled from the film to t he bulk at a surface pressure of 20 mN/m. Nevertheless, some of them r emain adsorbed at the interface. For mixed monomolecular films with a surface pressure of 5 mN/m, a facilitated lateral proton conduction is present. An increase in the percentage of LAPAO in the spread LAPAO/P E mixture modulates the lateral proton conduction when larger than a c ritical value. Mixed LAPAO/PE monolayers at 5 mN/m, with a spread rati o LAPAO/PE larger than 19/1 (mol/mol), present a slowing down of the k inetics in proton conduction. No alteration of the structure or the dy namics of the film can bc observed under these conditions. The effect of LAPAO on preferential lateral proton conduction must be explained b y a subtle change in the organization of the membrane/water interface.