Initial acyl chain segments of gluco- and phospholipids differ in orderingin both lamellar and reversed hexagonal phases

H-2-NMR studies of three synthetic phospholipids and two glucolipids from A choleplasma laidlawii, all having C2-deuterated acyl chains, have been perf ormed. In the lamellar liquid crystalline (L-alpha) phase the phospholipids generate three Pake doublets with large differences in magnitude, while on ly two Pake doublets of similar magnitude are recorded from the glucolipids because of overlap between the two outer splittings. In an L-alpha phase w ith one phospholipid and one glucolipid a superposition of the individual 2 H-NMR spectra is obtained. These results point to a distinct difference in the conformations of the initial segments of the acyl chains in the two lip id classes, and to the fact that the two acyl chains in the glucolipids hav e more similar average conformations of the initial segments. Furthermore, the results imply that the two lipids retain their respective acyl chain an d glycerol backbone conformations in the mixtures. We report, for the first time, that separate quadrupole splittings are obtained from the sn-1 and e ach of the sn-2 deuterons present in phospholipids forming a reversed hexag onal (H-II) phase. In contrast, glucolipids generate just one splitting in an H-II phase. These results indicate that both the phospholipids and the g lucolipids essentially retain their respective conformations of the acyl ch ains and the glycerol backbone upon a transition from an L-alpha to an H-II phase. It is speculated that the different appearance of the spectra obtai ned from the glucolipids is achieved by slightly tilting the glycerol backb one relative to the normal of the bilayer surface. The reason for the tilti ng may be that the non-ionic, less hydrophilic, glucolipid headgroups are s omewhat more deeply positioned in the interfacial polarity gradient of the bilayer than the ionic phosphate-containing headgroups.