Phosphatidylnucleosides that self-assemble in water to form closed liposomes have been investigated by spectroscopic methods to detect whether complementary base recognition can be achieved in spherical bilayer structures. We have prepared and characterized liposomes from 5‘-(1,2-dioleoyl-sn-glycero(3)phospho)adenosine (DOP-Ade), 5‘-(1,2-dioleoyl-sn-glycero(3)phospho)uridine (DOP-Uri), 5‘-(1,2-dioleoyl-sn-glycero(3)phospho)cytidine (DOP-Cyt), their mixtures, and liposomes formed by 5‘-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)adenosine (POP-Ade) or 5‘-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)uridine (POP-Uri). The 1:1 mixture of DOP-Ade and DOP-Uri liposomal solutions shows UV absorption and circular dichroism properties characteristic for base pairing, since a hypochromic effect, detected in the absorption region of the aromatic bases, is coupled to a CD intensity increase of the same band. The same effect is detected for POP-Ade liposomes mixed with POP-Uri liposomes. The hypochromic effect can be observed after three days from the mixing, and the spectroscopic feature is the same as that observed for freshly prepared liposomes formed by the mechanical 1:1 mixture of the two complementary lipids. Liposomes formed from noncomplementary phosphatidiylnucleosides, i.e. DOP-Ade and DOP-Cyt, fail to give these spectroscopic changes, supporting a specific and stoichiometric base interaction governed by Watson−Crick complementarity. The possible relevance for the origin of life of this recognition mechanism, involving bilayer self-assembling structures, is discussed.

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We describe an NMR analysis of liposomes obtained from two phosphatidyl nucleosides, namely 5′-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)cytidine (1) and 5′-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)-inosine (2). This analysis is mostly based on 1D-and 2D-1 H-NMR 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-1 H-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.

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