Liposomes from phosphatidyl nucleosides: An NMR investigation

We describe an NMR analysis of liposomes obtained from two phosphatidyl nucleosides, namely 5'-(1-palmitoyl-2-oleoyl-sn-glycero(3)phosp (1) and 5'-(1-palmitoyl-2-oleoyl-sn-glycero(3)ph inosine (2). This analysis is mostly based on 1D- and 2D-H-1-N-MR at high field and is aimed at investigating intramolecular interactions between groups within the same lipid molecule and intermolecular interactions between neighboring lipid molecules in the liposome aggregate. Particular care was taken to choose mixing times which reduce the possibility of spin diffusion effects. The presence of particular cross peaks in 2D-H-1-NOESY experiments permits us to assess the presence of syn- and anti-conformations of the aromatic head groups both in the monomeric nucleotides and in the liposomes. The lack of any particular ring effect in liposomes suggests that no stacking interactions of the nucleobases are present. On the other hand, the presence of other distinct cross peaks indicates short interatomic distances between a given aromatic base and the sugar moiety of an adjacent phosphatidyl nucleoside headgroup. These data permit one to draw a structural model of the liposome surface which accounts for the poor binding characteristics of phosphatidyl nucleoside liposomes toward complementary oligonucleotide strands.