Grazing incidence X-ray diffraction and Brewster angle microscopy studies on domain formation in phosphatidylethanolamine/cholesterol monolayers imitating the inner layer of human erythrocyte membrane.

In this work the properties of monomolecular films composed of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (SOPE) and cholesterol, differing in lipid proportion, were investigated in the context of domain formation in the inner leaflet of membrane. To perform comprehensive analysis of the studied model systems the Langmuir monolayer experiments were performed in combination with Brewster angle microscopy (BAM) and Grazing incidence X-ray diffraction (GIXD) techniques. The analysis of the collected data proved non-ideal behavior of the investigated films. It was found that cholesterol at its lower concentration in the system (10%) is of disturbing influence on SOPE film. Further addition of cholesterol into phospholipids film (33, 50, and 67% of cholesterol) induces an ordering effect on SOPE acyl chains and provokes the formation of sterol-poor and sterol-rich domains which stoichiometry is independent of monolayer composition. The foregoing findings allow one to conclude that in cytosolic leaflet of membrane the lipids may segregate into domains of various cholesterol contents which depending on their composition may play different roles in membrane functioning.