Study of water permeability through phospholipid vesicle membranes by 17O NMR

Vesicle suspensions of up to 5% egg lecithin and 2.5% cholesterol have been found to have no effect on the NMR relaxation times of 17O from water. Addition of 1-5 mM Mn2+ to an equimolar vesicle suspension of egg lecithin and cholesterol permitted resolution of the free induction decay into two exponential components, a fast one arising from the external water and a slow one arising from the intravesicular fluid. From the rates of relaxation the mean life time of the water molecules within the vesicles was calculated to be 1+/- 0.1 ms at 22 degrees C. The size of the vesicle was estimated from electron micrographs to be about 500 A in diameter. These data yield an equilibrium water permeability, Pw, of about 8 mus-1 for the vesicle membranes. From the temperature dependence of Pw an activation energy of 12+/-2 kcal/mol was obtained. The longitudinal relaxation time (T1) of water within vesicles remained the same as in pure water.