DETECTION OF PHASE-SEPARATION IN FLUID PHOSPHATIDYLSERINE PHOSPHATIDYLCHOLINE MIXTURES

The nonideal mixing of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (16:0,18:1)PS, and 1,2-didodecenoyl-sn-glycero-3-phosphocholine, (12: 1,12:1)PC, in fluid lamellar model membranes was studied by measuring binding of aqueous Ca2+ ions and by x-ray diffraction. A region of two -phase coexistence was found by invariance of the aqueous concentratio n and by the appearance of two sets of lamellar spacings. The phases w ere identified as fluid from the diffuse x-ray diffraction in the wide -angle region. The width of the two-phase coexistence region was great er at higher ionic strength. In 800 mM KCl, the phase boundaries were at PS mole fraction 0.5 and 0.8. In 100 mM KCl, the phase boundaries w ere at PS mole fraction 0.52 and 0.62. Monte Carte simulations of the lateral distributions of these PS/PC mixtures show pronounced clusteri ng of the lipids.