Anisotropic Water Diffusion in Macroscopically Oriented Lipid Bilayers Studied by Pulsed Magnetic Field Gradient NMR

The anisotropy, D(parallel)/D( perpendicular ), of water diffusion in fully hydrated bilayers of dimyristoylphosphatidylcholine at 29 degrees C has been measured by pulsed magnetic field gradient (pfg) NMR. By using NMR imaging hardware to produce magnetic field gradients in an arbitrary direction with respect to a stack of macroscopically aligned lipid bilayers, translational diffusion of water was measured as a function of the angle between the direction of the magnetic field gradient and the normal of the lipid membrane. The observed diffusion coefficient is found to depend strongly on this angle. The anisotropy cannot be accurately determined due to the very small value of D( perpendicular ), but a lower limit of about 70 can be estimated from the observed diffusion coefficients. The results are discussed in terms of the relatively low permeability of water across the lipid bilayer, instrumental limitations, and/or possible defects in the lamellae.