A monolayer study on three binary mixed systems of dipalmitoyl phosphatidyl choline with cholesterol, cholestanol and stigmasterol

Abstract

The monolayer behavior of three mixed systems of dipalmitoyl phosphatidyl choline (DPPC) with sterols; cholesterol (Ch), stigmasterol (Stig), and cholestanol (Chsta) formed at the interface of air/water (phosphate buffer solution at 7.4 with addition of NaCl) was investigated in terms of surface pressure ( π) and molecular occupation surface area ( A) relation. A series of π– A curves at every 0.1 mol fraction of each sterol for the three combinations of mixed systems were obtained at 25.0 °C. On the basis of the π– A curves, the additivity rule in regard to A versus sterol mole fraction ( X st) was examined at discrete surface pressures such as 5, 10, 15, 20, 25, 30 mN m −1, and then from the obtained A– X st curves the partial molecular areas (PMA) were determined. The A– X st relation exhibited a marked negative deviation from ideal mixing in the pressure range below 10 mN m −1, i.e. in the expanded liquid film region (below the transition pressure of DPPC). The PMA of Ch at π=5 mN m −1, for example, was found to be conspicuously negative in the range of X Ch=0–0.2 (about −0.4 nm 2 per molecule) and slightly positive (ca. 0.1 nm 2 per molecule) in the range X Ch=0.2 to 0.4. Above X Ch=0.5, Ch’s PMA was almost the same as the surface area of pure Ch, while DPPC’s PMA was reduced to 60% of that of the pure system. Excess Gibbs energy (Δ G (ex)) as a function of X st was estimated at different pressures. Applying the regular solution theory to thermodynamic analysis of Δ G (ex), the activity coefficients ( f 1 and f 2) of DPPC and the respective sterols as well as the interaction parameter ( I p) in the mixed film phase were evaluated; the results showed a marked dependence on X st. Compressibility C s and elasticity C s −1 were also examined. These physical parameters directly reflected the mechanical strength of formed monolayer film. Phase diagrams plotting the collapse pressure ( π c) against X st were constructed, and the π c versus X st curves were examined for the respective mixed systems in comparison with the simulated curves of ideal mixing based on the Joos equation. Comparing the monolayer behavior of the three mixed systems, little remarkable difference was found in regard to various aspects. In common among the three combinations, the mole fraction dependence in monolayer properties was classified into three ranges: 0< X st<0.2, 0.2< X st<0.4 and 0.5< X st<1. How the difference in the chemical structure of the sterols influenced the properties was examined in detail.