WATER PERMEABILITY OF POLYUNSATURATED LIPID-MEMBRANES MEASURED BY O-17 NMR

Diffusion-controlled water permeation across bilayers of polyunsaturat ed phospholipids was measured by O-17 nuclear magnetic resonance. In 1 00-nm extruded liposomes containing 50 mM MnCl2, water exchange betwee n internal and external solutions was monitored via changes in the lin ewidth of the O-17 water resonance of external water. Liposome size an d shape were characterized by light scattering methods and determinati on of liposome trapped volume. At 25 degrees C, the following water pe rmeability coefficients were determined: 18:0-18:1n-9 PC, 155 +/- 24 m u m/s; 18:0-18:3n-3 PC, 330 +/- 88 mu m/s; and 18:0-22:6n-3 PC, 412 +/ - 91 mu m/s. The addition of 1 M ethanol reduced permeability coeffici ents to 66 +/- 15 mu m/s for 18:0-18:1n-9 PC and to 239 +/- 67 mu m/s for 18:0-22:6n-3 PC. Furthermore, the addition of 50 mol% 18:1n-9-18:1 n-9 PE reduced the water permeability from 122 +/- 21 mu m/s for pure 18:1n-9-18:1n-9 PC to 74 +/- 15 mu m/s for the mixture. The significan t increase in water permeation for membranes with polyunsaturated hydr ocarbon chains correlates with looser packing of polyunsaturated lipid s at the lipid-water interface and the suggested deeper penetration of water into these bilayers. Ethanol may block water diffusion pathways by occupying points of water entry into bilayers at the interface. Th e addition of dioleoylphosphatidylethanolamine increases lipid packing density and, consequently, reduces permeation rates.