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

Abstract

²H-NMR studies of three synthetic phospholipids and two glucolipids from Acholeplasma laidlawii, all having C2-deuterated acyl chains, have been performed. In the lamellar liquid crystalline (L_α) phase the phospholipids generate three Pake doublets with large differences in magnitude, while only two Pake doublets of similar magnitude are recorded from the glucolipids because of overlap between the two outer splittings. In an L_α phase with one phospholipid and one glucolipid a superposition of the individual ²H-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 lipid classes, and to the fact that the two acyl chains in the glucolipids have more similar average conformations of the initial segments. Furthermore, the results imply that the two lipids retain their respective acyl chain and glycerol backbone conformations in the mixtures. We report, for the first time, that separate quadrupole splittings are obtained from the sn-1 and each of the sn-2 deuterons present in phospholipids forming a reversed hexagonal (H_II) phase. In contrast, glucolipids generate just one splitting in an H_II phase. These results indicate that both the phospholipids and the glucolipids essentially retain their respective conformations of the acyl chains and the glycerol backbone upon a transition from an L_α to an H_II phase. It is speculated that the different appearance of the spectra obtained from the glucolipids is achieved by slightly tilting the glycerol backbone relative to the normal of the bilayer surface. The reason for the tilting may be that the non-ionic, less hydrophilic, glucolipid headgroups are somewhat more deeply positioned in the interfacial polarity gradient of the bilayer than the ionic phosphate-containing headgroups.