STRUCTURE OF MIXED MULTILAYERS OF PALMITOYLOLEOYLPHOSPHATIDYLCHOLINE AND OLIGO(OXYETHYLENE GLYCOL) MONODODECYL ETHER DETERMINED BY X-RAY AND NEUTRON-DIFFRACTION

The structure of oriented multilayers of the zwitterionic phospholipid palmitoyloleoylphosphatidylcholine containing nonionic surfactants (o ligo(oxyethylene glycol) monododecyl ethers, C(12)E(n) with n = 2, 4, and 6) was studied by X-ray and neutron diffraction. Investigations we re done at T = 20 degrees C in the lamellar liquid-crystalline phase w ith surfactant/lipid molar ratios R(A/L) = 0.5 and 1 and hydration lev els corresponding to 85 and 97% relative humidity. The transbilayer di stribution of the surfactant was determined by neutron diffraction usi ng selectively deuterated C(12)E(n). The incorporation of surfactant c auses a decrease of the repeat distance and of the thickness of the hy drocarbon core of the membrane. The effect increases with the number o f oxyethylene units and with the concentration of the surfactant as we ll as with the relative humidity. The alpha-methylene group of the sur factants is anchored near the boundary of the hydrophobic core. The ox yethylene moieties are mainly located in the polar membrane/water inte rface region. The impact of molecular disorder on the diffraction data is discussed.