Nucleotide insertion in cationic bilayers

The interaction between 2'-deoxyadenosine 5'-monophosphate (DMP) and cation ic liposomes made up of dioctadecyldimethylammonium bromide (DODAB) in wate r 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 beco mes close to zero, demonstrating that the electrostatic attraction essentia lly drives the DODAB/DMP complexation. Over the millimolar range of DMP con centrations (0.4-1.5 mM), upon nucleotide addition, turbidity of the liposo me dispersion (0.08 mM DODAB) steeply increases as a function of time in co ntrast with the much smaller flocculation rates upon NaCl addition over a m uch higher range of NaCl concentrations (40-210 mM NaCl). The nucleotide be haves 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, liposo me electrophoretic mobility decreases as a function of DMP concentration bu t the liposome/DMP complex remains positively charged, even at the highest DMP concentrations tested. DMP-induced rupture of liposomes containing [C-1 4]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 maxim al adsorption, DMP insertion in the cationic bilayer is the most reasonable explanation for the remaining positive charge on the vesicle, the 2:1 DODA B:DMP molar ratio, and leakage of internal contents from the liposome.