Pulsed field-gradient-spin echo NMR studies of water diffusion in a phospholipid model membrane

Water diffusion in the lamellar phase of egg phosphatidylcholine (egg PC)-water was studied by H-1 NMR using the pulsed field gradient-spin echo method. The curvature of diffusion plots obtained with egg PC-water mixtures indicates that water diffusion is highly anisotropic with respect to lipid lamellae. This was confirmed by measurements made on macroscopically aligned egg PC-water as a function of orientation that categorically establish D-parallel to/D-perpendicular to >> 1, where the respective subscripts refer to parallel and perpendicular to the lipid bilayer. A smooth, monotonic dependence on water concentration was observed for water diffusion in aligned egg PC-water, varying at 25 degrees C from D-parallel to = 1.2 x 10(-10) m(2)s(-1) at n = 4.9 mol water/lipid to D-parallel to = 4.0 x 10(-10) m(2)s(-1) at n = 18.6 mol water/lipid. The diffusion is approximately a factor of 10 slower than in pure water because of water binding and restriction to translational motion within the aqueous layer. No evidence for a sudden drop in water diffusion coefficient at a specific water content, as previously reported with egg PC-water mixtures (Lange and Gary Bobo. 1974. J. Gen. Physiol 63: 690-706), was detected. a Morphological reorganization of lamellar domains, which in random orientational distribution comprise lipid-water mixtures, is the likely explanation. The study of aligned lipid-water systems is manifestly preferable.