Base complementarity and nucleoside recognition in phosphatidylnucleoside vesicles

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.