Condensation effect of cholesterol, stigmasterol, and sitosterol on dipalmitoylphosphatidylcholine in molecular monolayers

The effects of the incorporation of cholesterol, sitosterol, and stigmasterol into dipalmitoylphosphatidylcholine (DPPC) monolayers were investigated by Langmuir monolayer film technique in this study. The compressibility coefficient was assessed at various surface pressures. The observations indicated that sitosterol and stigmasterol interacted less effectively than cholesterol with the phospholipid. Nevertheless, these sterol molecules could all cause condensation effect on DPPC monolayers. Attractive interactions between DPPC and sterol molecules, or hydrophobic effect, was found to play an important role, testified by negative excess molecular areas at particularly low surface pressures and negative partial molecular area of three sterols at low surface pressures. The minimum extreme points for the excess area were all located at around 0.3 mol fractions for the three sterols at 30 mN/m, which suggest a 2:1 ratio of DPPC/sterol in the ordered structures.