Orientation of Tie-Lines in the Phase Diagram of DOPC:DPPC:Cholesterol Molel Biomembranes

Abstract

We report the direction of tie-lines of coexisting phases in a ternary diagram of DOPC:DPPC:Cholesterol lipid bilayers, which has been a system of interest in the discussion of biological rafts. For coexisting Ld and Lo phases we find that the orientation angle α of the tie-lines increases as the cholesterol concentration increases and it also increases as temperature increases from T=15°C to T=30°C. Results at lower cholesterol concentrations support the existence of a different 2-phase coexistence region of Ld and So phases and the existence of a 3-phase region separating the two 2-phase regions. Our method uses the X-ray lamellar D-spacings observed in oriented bilayers as a function of varying hydration. Although this method does not obtain the ends of the tie-lines, it gives precise values (±1°) of their angles α in the ternary phase diagram.View Large Image | View Hi-Res Image | Download PowerPoint Slide