Miscibility behavior of sphingomyelin with phytosterol derivatives by a Langmuir monolayer approach

The miscibility behavior of palmitoyl sphingomyelin (PSM) with phytosterol derivatives of β-sitosterol (SITO), β-sitosteryl glucoside (SG), and β -sitosteryl glucoside palmitate (SGP) was systematically investigated using Langmuir monolayers. The surface pressure (π)-molecular area (A) and surface potential (ΔV)-A isotherms for binary PSM/SITO, PSM/SG, and PSM/SGP systems on 0.02 M Tris buffer with 0.13 M NaCl (pH 7.4) were measured as a function of the molar fraction of PSM (X_PSM). The surface potentials (ΔV) of the pure components were analyzed using the three-layer model proposed by Demchak and Fort [J. Colloid Interface Sci. 46 (1974) 191-203]. The contributions of the hydrophilic D-glucose moiety, hydrophobic palmitoyl group, and sphingomyelin group to the vertical component of the dipole moment (μ_⊥) were evaluated. The thermodynamic quantities, based on the π-A isotherms, revealed that PSM interacts attractively with all three phytosterol derivatives in the following order: SITO > SGP > SG. In addition, the two-dimensional phase diagram constructed based on the phase transition pressure (π^eq), from a liquid-expanded to liquid-condensed state, and collapse (π^c) pressure shows that the twocomponent systems are all miscible with each other. The manner of miscibility between PSM/SG and PSM/ SGP systems is found to be opposite in the large X_PSM region. The interaction between the same molecules (PSM-PSM or SG-SG) is stronger than that of the different molecules (PSM-SG) in the PSM/SG system, and vice versa in the PSM/SGP system. These results suggest that the incorporation of a D-glucose and palmitoyl group to a SITO molecule dramatically changes the miscibility behavior with PSM. Observations using fluorescence microscopy imaging also provide insights into miscibility behavior in the monolayer state.