Packing of ganglioside-phospholipid monolayers: An X-ray diffraction and reflectivity study

Using synchrotron grazing-incidence x-ray diffraction (GIXD) and reflectivi ty, the in-plane and out-of-plane structure of mixed ganglioside-phospholip id monolayers was investigated at the air-water interface. Mixed monolayers of 0, 5, 10, 20, and 100 mol% ganglioside GM, and the phospholipid dipaimi toylphosphatidylethanolamine (DPPE) were studied in the solid phase at 23 d egreesC and a surface pressure of 45 mN/m. At these concentrations and cond itions the two components do not phase-separate and no evidence for domain formation was observed. X-ray scattering measurements reveal that GM, is ac commodated within the host DPPE monolayer and does not distort the hexagona l in-plane unit cell or out-of-plane two-dimensional (2-D) packing compared with a pure DPPE monolayer. The oligosaccharide headgroups were found to e xtend normally from the monolayer surface, and the incorporation of these g lycolipids into DPPE monolayers did not affect hydrocarbon tail packing (fl uidization or condensation of the hydrocarbon region). This is in contrast to previous investigations of lipopolymer-lipid mixtures, where the packing structure of phospholipid monolayers was greatly altered by the inclusion of lipids bearing hydrophilic polymer groups. Indeed, the lack of packing d isruptions by the oligosaccharide groups indicates that protein-GM, interac tions, including binding, insertion, chain fluidization, and domain formati on (lipid rafts), can be studied in 2-D monolayers using scattering techniq ues.