Bilayer thickness in unilamellar extruded egg yolk phosphatidylcholine liposomes: a small-angle neutron scattering study

Using the small-angle neutron scattering and the Kratky-Porod plot, the bilayer thickness dL=41.7±0.3 A has been determined in unilamellar egg yolk phosphatidylcholine liposomes dispersed in heavy water and prepared by extrusion through 500 A pores in polycarbonate filters. The peak-peak distance across the bilayer obtained from the data of small-angle X-ray scattering on the liquid-crystalline Lα phase of fully hydrated egg yolk phosphatidylcholine is 37.4 A ≤dPP≤38.1 A. Using this value and the bilayer hydrocarbon region thickness of dHC=28.4±0.6 A, the bilayer thickness 47.4 A≤dL≤48.1 A, the phospholipid molecule surface area 64.0≤AL≤64.6 A at the bilayer/water interface, the number of water molecules 9.17≤nh≤9.57 penetrated into the polar region of the bilayer per one phospholipid and the mean molecular volume 28 A≤VW≤30 A of penetrated water have been obtained from the small-angle X-ray scattering repeat distance of the liquid-crystalline Lα phase of egg yolk phosphatidylcholine hydrated by 13<n<20 water molecules per one lipid. (author)