Kinetics of water diffusion across phospholipid membranes. 1H- and 17O-NMR relaxation studies.

Nuclear relaxation measurements of 1H and 17O of water have been applied to study the kinetics of water diffusion across vesicular lipid membranes. Differentiation between the intra- and extravesicular media was achieved by entrapping Mn2+ inside the vesicles. The water permeability of egg phosphatidylcholine vesicles was found to be 2.9 x 10(-3) cm/s at 25 degrees C, with an activation energy of 10.5 kcal/mol which remains constant through the temperature range 0-65 degrees C. The water permeability across vesicular bilayers of L-alpha-dipalmitoyl phosphatidylcholine exhibited a sharp change through the lipid phase transition. The permeability in the lipid crystalline phase (45 degrees C) was found to be 7.2 x 10(-3) cm/s with an activation energy of 7.2 kcal/mol. Below the transition at the gel phase (35 degrees C) a permeability of 1.0 x 10(-3) cm/s was determined. The results indicate that water diffuses through lipid membranes in the liquid crystalline phase in a similar fashion to its diffusion in hydrocarbon liquids. However, when the lipids undergo a phase transition to the gel state, this similarity does not hold any more and water diffusion becomes much more restricted than in hydrocarbon liquids. The change in water permeability through the phase transition was correlated with the changes observed in the lipid segmental motion determined from 13C T1 measurements.