It is known that sterols are key components of cellular membranes, which can modulate membrane properties and induce influence on the organization of membrane phospholipids. Howerer, the influence of these sterols on membrane properties sees to be unclear. It is worth studying. The interactions between β-sitosterol and stigmasterol and dipalmitoyl phosphatidycholine (DPPC) in mixed monolayers were studied by langmuir technique and atomic force microscopy (AFM). The influence of β-sitosterol and stigmasterol effects on properties of DPPC membrane on phase transition from the liquid expanded (LE) to liquid condensed (LC) state were investigated and the condensing and ordering potency of binary monolayers containing β-sitosterol and stigmasterol were thoroughly analyzed. Based on the analysis of mixing values as well as AFM images it was evidenced that the effect of the plant sterol depends on the composition of model membrane. The results show that the addition of the plant sterols into model membrane modifies the condensation, ordering and interactions in the system. Moreover, at their higher concentration X_sterols=0.8, the values of the excess molecular area and excess Gibbs enerqy displayed negative values, which indicate that between molecules in the mixed monolayer exist stronger attraction than in the respective one-component films and suggest β-sitosterol and stigmasterol in the mixed system have the similar effects on its properties. Examination of the excess free energy of mixing reveals an enhanced stability of binary monolayers. At lower content, X_sterols=0.2, 0.4, the values of the excess molecular area and excess Gibbs energy suggested stronger attractive interaction between stigmasterol and DPPC, while an intermolecular stronger repulsion interaction between β-sitosterol and DPPC. It shows that stigmasterol was found to modify the properties of model membrane more strongly than β-sitosterol. It reflects that side-chain can reduce packing effectiveness, increased tilt of the molecules with respect to the monolayer plane, and the presence of the double bond reduces the hydrophobicity of the branched hydrocarbon tail of stigmasterol, which could cause strols to have long axis parallel to the interface. The result of AFM supported the theoretic analysis.

Key words: phytosterol, interaction, phase transition, condensation, atomic force microscopy (AFM)