Comparative Studies on the Influence of β\beta-Sitosterol and Stigmasterol on Model Sphingomyelin Membranes: A Grazing-Incidence X-ray Diffraction Study

Sterols are essential constituents of membranes, both in the plant world and in human organisms. Therefore, their activity on model lipid systems has systematically been studied. Despite intensive investigations, differences in the effect induced by beta-sitosterol (beta-sito) and stigmasterol (stigma) (two major phytosterols) are very controversial and still under debate. To compare the influence of these compounds on model membranes, we have performed grazing incidence X-ray diffraction (GIXD) experiments on phytosterol/sphingomyelin (Sph) monolayers. The analysis of the X-ray scattering and the resulting in-plane parameters provided information on the lateral organization of pure lipid films and the mixed systems. The obtained results prove a nonideal mixing between the investigated lipids in the monolayers and the existence of strong interactions between phytosterols and Sph. Both the plant sterols incorporated into sphingolipid film condense the monolayer and order Sph chains. The results of GIXD experiments, compared with those obtained previously from Langmuir monolayer studies allowed us to observe the comparable influence of beta-sito and stigma on model membrane organization