Determination of Water Permeability Through Stress-Free Lipid Membranes Using Time-Resolved Small Angle Neutron Scattering

The lipid bilayer, the basic unit of biological membranes, is an essentially impermeable barrier. Because of this, the environment inside the cell can be significantly different from the environment surrounding it, such as pH, ion compositions and biomolecular concentrations. Since several cellular functions require the fast transport of large or charged molecules through the membrane, the cell membrane contains specialized proteins that specifically transport these across the membrane. Notwithstanding permeation happens via passive diffusion as well. This type of transport, we imagine, is slow, even for small molecules, such as water. Indeed, the cell has active water transporters (aquaporin) to move water through its membranes faster. The time and energy required to move water across the lipid bilayer passively is still debated, however. Several techniques have been used to measure water permeation such as NMR, fluorescence, and micropipetting. Here we take advantage of contrast between H2O and D2O to investigate, through time-resolved SANS, the water permeability of stress-free membranes composed of different saturated lipids. The principle is to follow the scattering intensity of a hydrogenated vesicle solution, initially prepared in H2O, when diluted with D2O. The resulting changes in scattering are a result of the intake of D2O into the vesicle's interior without any induced stress on the membrane. Our results show that water permeation in membranes in the gel state is faster, by at least an order of magnitude, than previously reported. The water permeation changes due to cholesterol in liquid ordered phases is also discussed.