Adhesion between cerebroside bilayers

The structure, hydration properties, and adhesion energy of the membrane gl ycolipid galactosylceramide (GalCer) were studied by osmotic stress/X-ray d iffraction analysis.(1) Fully hydrated GalCer gave a repeat period of 67 An gstrom, which decreased less than 2 Angstrom with application of applied os motic pressures as large as 1.6 x 10(9) dyn/cm(2). These results, along wit h the invariance of GalCer structure obtained by a Fourier analysis of the X-ray data, indicated that there was an extremely narrow fluid space (less than the diameter of a single water molecule) between fully hydrated cerebr oside bilayers. Electron density profiles showed that the hydrocarbon chain s from apposing GalCer monolayers partially interdigitated in the center of the bilayer. To obtain information on the adhesive properties of GalCer bi layers, we incorporated into the bilayer various mole ratios of the negativ ely charged lipid dipalmitoylphosphatidylglycerol (DPPG) to provide known e lectrostatic repulsion between the bilayers. Although 17 and 20 mol % DPPG swelled (disjoined) the GalCer bilayers by an amount predictable from elect rostatic double-layer theory, 5, 10, 13, and 15 mol % DPPG did not disjoin the bilayers. By calculating the magnitude of the electrostatic pressure ne cessary to disjoin the bilayers, we estimated the adhesion energy for GalCe r bilayers to be about -1.5 erg/cm(2), a much larger value than that previo usly measured for phosphatidylcholine bilayers. The observed discontinuous disjoining with increased electrostatic pressure and this relatively large value for adhesion energy indicated the presence of an attractive interacti on, in addition to van der Waals attraction, between cerebroside bilayers. Possible attractive interactions are hydrogen bond formation and hydrophobi c interactions between the galactose headgroups of apposing GalCer bilayers .