Phase separation in phosphatidylinositol/phosphatidylcholine mixed monolayers

Phosphatidylinositol is a minor component of lipid membranes yet plays a major role in cellular functions. To investigate whether PI forms domains in such a system, pure and mixed monolayers of distearoylphosphatidylcholine (DSPC) and PI have been compared using surface-pressure isotherms, grazing incidence X-ray diffraction (GIXD), Brewster angle microscopy (BAM) and scanning force microscopy (SFM). The mixed (up to 20 mol% PI) monolayer isotherms exhibited a plateau region corresponding to the collapse pressure of PI. The phase separation of the mixed monolayers into a DSPC condensed phase and a PI fluid phase was confirmed by BAM and SFM measurements. DSPC forms a condensed monolayer with centred rectangular structure and tilt towards nearest neighbours (NN) while PI formed a fluid monolayer with no lateral structure at all pressures investigated. Mixed DSPC/PI monolayers containing 20 mol% PI also exhibited a centred rectangular lattice with NN tilt. At surface pressures below the collapse of the fluid, PI phase, the addition of 20 mol% PI has very little effect on the structure of the condensed phase indicating that the phases almost completely demixed