Mode of interaction of ganglioside Langmuir monolayer originated from echinoderms: three binary systems of ganglioside/DPPC, ganglioside/DMPE, and ganglioside/cholesterol

The surface pressure (pi)-area (A), the surface potential (DeltaV)-A, and the dipole moment (mu( perpendicular))-A isotherms were obtained for monolayers made from a ganglioside originated from echinoderms [Diadema setosum ganglioside (DSG-1)], dipalmitoylphosphatidylcholine (DPPC), dimyristoylphosphatidylethanolamine (DMPE), cholesterol (Ch), and their combinations. Monolayers spread on several different substrates were investigated at the air/water interface by the Wilhelmy method, ionizing electrode method, fluorescence microscopy (FM) and atomic force microscopy (AFM). Surface potentials (DeltaV) of pure components were analyzed using the three-layer model proposed by Demchak and Fort [R.J. Demchak, T. Fort, J. Colloid Interface Sci. 46 (1974) 191-202]. The new finding was that DSG-1 was stable and showed a liquid-expanded film and that its monolayer behavior of DeltaV was sensitive for the change of the NaCl concentration in the subphase. Moreover, the miscibility of DSG-1 and three major lipids in the two-component monolayers was examined by plotting the variation of the molecular area and the surface potential as a function of the DSG-1 molar fraction (X(DSG-1)), using the additivity rule. From the A-X(DSG-1) and DeltaV(m)-X(DSG-1) plots, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined at the discrete surface pressure. The PMA and APSP with the mole fraction were extensively discussed for the miscible system. The miscibility was also investigated from the two-dimensional phase diagrams. Furthermore, a regular surface mixture, for which the Joos equation was used for the analysis of the collapse pressure of two-component monolayers, allowed calculation of the interaction parameter (xi) and the interaction energy (-Deltavarepsilon) between them. The observations using fluorescence microscopy and AFM image also provide us the miscibility in the monolayer state.