Cationic amphiphile interactions with polyadenylic acid as probed via 2H-NMR

2H-NMR spectroscopy was used to investigate the effects of polyadenylic acid (PolyA) on three aminomethyl-deuterated cationic amphiphiles: specifically, N-[1-(2,3-dioleoyloxy)propyl]-N',N',N'-trimethylammonium chloride (DOTAP-gamma-d3), 3beta-[N-(N',N',N'-trimethylaminoethane)carbamoyl] cholesterol (TC-CHOL-gamma-d3), and cetyltrimethylammonium bromide (CTAB-gamma-d9). When mixed with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and incorporated into lipid bilayer membranes, each of the cationic amphiphiles yielded 2H-NMR spectra consisting of a motionally averaged Pake powder pattern. The 2H-NMR quadrupolar splitting generally increased with increasing mole fraction of cationic amphiphile in the lipid bilayer with the exception of CTAB-gamma-d9. Adding PolyA caused the quadrupolar splitting to increase progressively in every case, until a 1:1 cation:anion charge ratio was achieved, after which the quadrupolar splitting changed no further. Deuterium NMR relaxation time measurements showed a parallel increase in T(qe)2 with increasing PolyA. The size of these changes produced by PolyA increased in the order: TC-CHOL < DOTAP < CTAB. NaCl addition reversed much, but not all, of the PolyA-related changes in 2H-NMR quadrupolar splittings and T(qe)2 relaxation times. A UV-based PolyA-membrane binding assay showed that salt addition caused PolyA desorption, and that the salt concentration required to do so increased in the order: TC-CHOL < DOTAP < CTAB. The results are consistent with an electrostatic binding of PolyA to the cationic lipid bilayer surface, accompanied by formation of a stoichiometric charge complex between PolyA and the cationic amphiphile, in which the cationic amphiphile retains considerable motional freedom. The strength of the complex increases in the order: TC-CHOL < DOTAP < CTAB.