Nucleotide insertion in cationic bilayers

The interaction between 2'-deoxyadenosine 5'-monophosphate (DMP) and cationic liposomes made up of dioctadecyldimethylammonium bromide (DODAB) in water is described. At maximal adsorption, the molar ratio DODAB/DMP is 2:1 and electrophoretic mobility for the liposomes attains a minimum at a positive value. At 5 mM ionic strength, maximal DMP adsorption on the liposome becomes close to zero, demonstrating that the electrostatic attraction essentially drives the DODAB/DMP complexation. Over the millimolar range of DMP concentrations (0.4-1.5 mM), upon nucleotide addition, turbidity of the liposome dispersion (0.08 mM DODAB) steeply increases as a function of time in contrast with the much smaller flocculation rates upon NaCl addition over a much higher range of NaCl concentrations (40-210 mM NaCl). The nucleotide behaves as a hydrophobic anion with an affinity for the membrane that is much higher than that exhibited by a simple anion as chloride. In water, liposome electrophoretic mobility decreases as a function of DMP concentration but the liposome/DMP complex remains positively charged, even at the highest DMP concentrations tested. DMP-induced rupture of liposomes containing [C-14]sucrose was evaluated from dialysis of DMP/liposomes mixtures. In water, DMP-induced leakage of radioactive liposomal contents suggests that the DMP/bilayer interaction is not superficial. Although the interaction preserves the positive liposome charge, it does not preserve its integrity. At maximal adsorption, DMP insertion in the cationic bilayer is the most reasonable explanation for the remaining positive charge on the vesicle, the 2:1 DODAB:DMP molar ratio, and leakage of internal contents from the liposome.