Comparison of the interaction of dihydrocholesterol and cholesterol with sphingolipid or phospholipid Langmuir monolayers.

We report here a study of the interaction of dihydrocholesterol (DChol) with palmitoyl-oleoyl-phosphatidylcholine (POPC) or sphingomyelin (SM) in Langmuir monolayers. DChol and cholesterol (Chol) have very close chemical structures, and DChol is often used in place of Chol because of its better stability. Surface pressure measurements and experiments of desorption induced by beta-cyclodextrin show that POPC-DChol monolayers behave similarly to POPC-Chol ones: condensing effects of DChol and Chol on POPC and desorption percentages are in the same range. Moreover Brewster angle microscopy (BAM) experiments performed on these monolayers show that on the whole they are both homogenous. The analysis of mean molecular areas versus DChol percentage shows that this sterol is also able to induce SM condensation at low surface pressure. The condensation of SM molecules is particularly strong at 30 mol% of DChol. At higher surface pressure, the condensation efficiency of DChol decreases and monolayers behave more ideally, even if an inflection point is always observed at 30 mol% of DChol. However, desorption percentages, clearly lower than those obtained with POPC-DChol monolayers, show that DChol is kept at the interface. At last BAM images show also differences in the behaviour of SM-DChol and SM-Chol monolayers. These differences could be due to the different compressibility and conformation of the A/B rings in the two sterols and the rigidity of the sphingosine chain. They suggest that the use of DChol in place of Chol has to be done carefully in the presence of SM.