Bilayer dimensions and hydration of glycolipids

X-ray diffraction measurements are available on a wide range of glycolipid multilamellar assemblies in excess water, but not at the defined water contents that are needed to derive bilayer dimensions. For lamellar crystalline phases and gel phases with untilted chains, or where the tilt angle is known, the cross-sectional area per chain from wide-angle diffraction can be used to determine the area per lipid molecule at the bilayer surface. Using the lipid molecular volume from densitometry, it is then possible to obtain the bilayer thickness and hence, from the lamellar repeat spacing, the water layer thickness and degree of hydration of the lipid polar groups. This is done here by using the available data for bilayer-forming diacyl and dialkyl glycosylglycerols, and for certain glycosphingolipids. The lamellar crystalline phases of these glycolipids are largely anhydrous, and the degree of hydration of the lamellar gel phases is much lower than that of the corresponding phosphoglycerolipid gel phases. A point of current uncertainty is whether the chains in the gel phases of diacyl glycoglycerolipids are appreciably tilted, unlike their dialkyl counterparts.