Physical properties of glycosyl diacylglycerols. 2. X-ray diffraction studies of a homologous series of 1,2-Di-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols

X-ray diffraction methods were used to characterize the thermotropic polymorphism exhibited by aqueous dispersions of a homologous series of 1,2-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols. Upon cooling from temperatures at which the acyl chains of these lipids are melted, all of these compounds form structures that exhibit both low-angle and wide-angle diffraction patterns consistent with the formation of lamellar L beta gel phases. After a suitable protocol of low-temperature annealing, complex diffraction patterns consistent with the formation of highly ordered, lamellar, crystal-like phases are obtained. These patterns are similar for all of the compounds studied, suggesting that the unit cell structure is invariant. The assumption that the unit cell structure is invariant permits the assignment of phases to the diffraction orders, thereby making possible the construction of electron density profiles. These electron density profiles indicate that the crystal-like phases of these lipids are poorly hydrated structures with the hydrocarbon chains inclined at 35 degrees to the bilayer normal. The diffraction patterns of the crystal-like phases of these lipids changed abruptly at the calorimetrically determined phase transition temperatures to those characteristic of either lamellar liquid crystalline phases (N less than or equal to 17) or inverted nonbilayer phases. With these X-ray diffraction data we demonstrate that, at elevated temperatures, the shorter chain homologues (N less than or equal to 16) form cubic phases of the Pn3m space group, whereas the longer chain compounds form inverted hexagonal phases.