Nucleotide chain length and the morphology of complexes with cationic amphiphiles: 31 P-NMR observations

31 P-NMR and UV spectroscopies were used to study the interactions between cationic amphiphile-containing lipid bilayers and either a phosphorothioate oligonucleotide (OligoS) ( n =21) or polyadenylic acid (PolyA) ( n ≈18 000). Multilamellar vesicles (MLVs) were composed of 1-palmitoyl-2-oleoyl- sn -glycero-3-phosphocholine (POPC) or 1,2-dioleoyl- sn -glycero-3-phosphoethanolamine (DOPE) in binary mixture with either of the cationic lipids, N -[1-(2,3-dioleoyloxy)propyl]- N ′, N ′, N ′-trimethylammonium chloride (DOTAP) or cetyltrimethylammonium bromide (CTAB). A UV-difference assay showed that OligoS binding ceased above a 1:1 anion/cation ratio, while PolyA binding continued until a 2:1 ratio was reached, indicating a ‘flat’ conformation for bound OligoS, but not necessarily for PolyA. Cross-polarization 31 P-NMR of the nucleotide chains bound to 100% DOTAP MLVs produced spectra virtually identical to those of dry powders of OligoS or PolyA, indicating effective immobilization of the surface-bound nucleotide chains. Hahn echo 31 P-NMR showed that MLVs composed of binary mixtures of POPC with DOTAP or CTAB retained a lamellar bilayer architecture upon adding nucleotide chains. At less than stoichiometric anion/cation ratios little or no signal attributable to free nucleotide chains was visible. A narrow signal at the chemical shift expected for phosphorothiodiesters or phosphodiesters became visible at greater levels of added OligoS or PolyA, respectively, indicating the presence of mobile nucleotide chains. Salt addition caused complete desorption of the nucleotide chains. When POPC was replaced with DOPE, binding of OligoS or PolyA produced non-bilayer lipid phases in the presence of DOTAP, but not in the presence of CTAB.