Thermodynamic behavior of D-sphingosine/cholesterol monolayers and the topography observed by AFM

Lipid rafts are of a dynamic microdomain structure found in recent years, enriched in sphingolipids, cholesterol and particular proteins. The change of structure and function of lipid rafts could result in many diseases. In this work, the monolayer behavior of mixed systems of D-sphingosine with cholesterol was investigated in terms of the mean surface area per molecule (A _m), excess molecular area (ΔA _ex), surface excess Gibbs energy (ΔG _ex), interaction parameter (ω), activity coefficients (f ₁ and f ₂) as well as elasticity (C _s ⁻¹) of formed films. The deposited Langmuir-Blodgett (LB) monolayers were investigated with atomic force microscopy (AFM). Thermodynamic analysis indicates ΔA _ex and ΔG _ex in the binary systems with negative deviations from the ideal behavior, suggesting attractive interaction between molecules. The stability, elasticity and activity coefficients show a marked dependence on the mole faction of D-sphingosine. The results of observation by AFM show that the single D-sphingosine molecular film took on small granule structure. When mixing the D-sphingosine and cholesterol at different ratios, the mixed films transform from the chains structure to larger slice and net coexisting structure with the increasing of the cholesterol content. In the end, pure cholesterol forms more aggregated structure. AFM experiments effectively support the above findings and interpretation. Supported by the National Natural Science Foundation of China (Grant No. 20772077) and the National Basic Research Program of China (Grant No. 104167)