Miscibility Behavior of Sphingomyelin with Phytosterol Derivatives by a Langmuir Monolayer Approach

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  • Sakamoto Seiichi
    Department of Biophysical Chemistry, Faculty of Pharmaceutical Sciences, Nagasaki International University
  • Nakahara Hiromichi
    Department of Biophysical Chemistry, Faculty of Pharmaceutical Sciences, Nagasaki International University
  • Shibata Osamu
    Department of Biophysical Chemistry, Faculty of Pharmaceutical Sciences, Nagasaki International University

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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 (XPSM). 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 (πeq), from a liquid-expanded to liquid-condensed state, and collapse (πc) pressure shows that the two-component 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 XPSM 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.

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