A monolayer study on phase behavior and morphology of binary mixtures of sulfatides with DPPC and DPPE

Sulfatides are important constituents of brain myelin membranes and it is thought to be involved in lateral domain formation in biological membranes. In this work, the interaction of mixed systems of sulfatide with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1, 2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), two of the major components in biological membranes, was investigated using the monolayer technique at the air–water interface. Based on the regular solution theory, the miscibility of the two binary systems in the mixed monolayer was evaluated in terms of mean surface area per molecule (Am), excess molecular area (ΔA(ex)), surface excess Gibbs energy (ΔG(ex)), interaction parameter (ω) as well as activity coefficients (f1 and f2) of formed films. Thermodynamic analysis indicates in the two binary systems with negative deviations from the ideal behavior. Accordingly, the values of the Gibbs energy of mixing, sulfatide-DPPC form stable mixtures at Xsul = 0.4 (Xsul is molar ratio of sulfatide in binary mixture) for all the selected pressures. As for sulfatide/DPPE system, at π = 5 and 30 mN m−1, the minimum for the Gibbs energy of mixing was found at Xsul = 0.6 and 0.2 respectively. But the minimum appeared at Xsul = 0.4 for other surface pressures. The activity coefficients (f1 and f2) of mixed monolayers were evaluated which show a marked dependence on the mole faction of sulfatide Xsul. AFM images could support the above findings as well as interpretation.