Small-Angle Neutron Scattering Study of Lateral Phase Separation in Dimyristoylphosphatidylcholine-Cholesterol Mixed Membranes

The small-angle neutron scattering (SANS) technique developed previously is used to study the lateral phase separation in dimyristoylphosphatidylcholine (DMPC)-cholesterol mixed vesicles in the Lα (35 °C) and (7 °C) phase of DMPC. To increase the sensitivity of the previous method, we apply the so-called inverse contrast variation technique where contrast matching is performed at a constant H20/D20 ratio by varying the ratio of DMPC with deuterated and protonated hydrocarbon chains. Phase boundaries can be determined to an accuracy of ±0.5 mol %. In parallel experiments phase separation in the Lβ, phase was also studied by freeze-fracture electron microscopy. For DMPC in the Lα phase complete miscibility is clearly established up to cholesterol molar fractions of xc = 0.14. Strong evidence is provided that this is also the case up to xc ≈ 0.45. Cholesterol is no longer soluble above this limit and precipitates as small crystallites. For the phase (7 °C) phase boundaries are clearly established at xc1 = 0.08 and xc2 = 0.24, and very strong evidence is provided for two additional boundaries at xc3 = 0.435 and xc4 ≈ 1.0. At 0 ≤ xc < xc1 the mixture forms a tilted solid solution in both the Lβ' and Pβ' phase while at xc1 ≤ xc ≤ xc2 this phase coexists with a nontilted mixture containing 24 mol % cholesterol. At xc2 ≤ xc ≤ xc3 a second region exists where mixtures containing 24 and 43.5 mol % cholesterol coexist within the plane of the membrane. To fulfill the phase rule, xc2 must correspond to a stoichiometric mixture which would reconcile the concepts of phase separation and complex formation. Our freeze-fracture studies do not provide evidence for a decrease of the repeat distance of the ripple phase at xc ≤ 0.08 and for a regular ripple phase with defined ripple distance within the first coexistence region.