X-ray diffraction evidence for the presence of discrete water layers on the surface of membranes

X-ray diffraction spacings in multilayered membranes obtained from frog sciatic nerves were found to increase in discrete steps of approx. 5 A during swelling. These observed jumps in the repeat period suggest that the lipid bilayers exist in distinct states of hydration, and perhaps the swelling occurs by step-wise addition of water layers between the polar head groups. Our analysis and statistical tests of this hypothesis are presented.