Structural properties of a phosphatidylcholine-cholesterol system as studied by small-angle neutron scattering: ripple structure and phase diagram

Small-angle neutron scattering has been used to study structural features of lamellar bilayer membranes of dimyristoylphosphatidylcholine (DMPC) and DMPC mixed with various amount of cholesterol. The studies were recorded at a fixed hydration level of 17% ²H₂O, i.e. just below saturation. Bragg reflections gives information on the ripple structure and on the bilayer periodicity. The crystalline L_c phase, which was stabilized after long time storage at low temperature, exhibits major small angle scattering when cholesterol is mixed into the membrane. The intermediate P_β′ gel-phase, which is characteristic by the rippled structure, is dramatically stabilized by the introduction of cholesterol. The ripple structure depends significantly both on the cholesterol content and on the temperature. At high temperatures, T>15°C, the inverse ripple periodicity varies basically linearly with cholesterol content, and approach zero (i.e. periodicity goes to infinite) at 20 mol% cholesterol, approximately. At lower temperatures the correlation is more complex. The data indicate additional phase boundaries below 2 mol% and at approx. 8 mol%. Secondary rippled structures are observed in the low temperature L_β′-phase for cholesterol content below approx. 8 mol%. The data gives detailed insight into the phosphatidylcholine cholesterol phase diagram, which is discussed on the basis of a simple model in which the cholesterol complexes are fixed to the defect stripes of the rippled structure.