Structural Properties of Asymmetric Mixed-Chain Phosphatidylethanolamine Films

By use of Langmuir isotherms and atomic force microscopy (AFM) experiments, we compare the structural properties of an asymmetric 1-palmitoyl-2-oleoyl-glycero-3-phosphoethanolamine (PE 181/160) film sampled at 21 degrees C on mica plates with those of a film composed of a mixture of the related saturated phospholipid (PE 160/160) and unsaturated phospholipid (PE 181/181). From PE 181/160, the Pi-A curve obtained presents a plateau (35 mN/m) where AFM images allowed us to determine a nucleation process, which governs the growth of condensed domains. This surface transition is interpreted as reflecting hydrophobic interactions mainly between the alkyl chains of the molecules. Moreover Pi-A curves of a mixture (v/v) of the two related saturated and unsaturated phospholipids (PE 160/160 and PE 181/181) show a "double plateau" (50 and 54 mN/m) and we have characterized a partial surface demixing at every surface pressure. Finally we show that only PE 181/160 is able to form bilayers above the collapse pressure. Copyright 1999 Academic Press.