A Hydrolytic Reporter of Cu(II) Availability in Artificial Liposomes

The dioctadecyl ammonium surfactant 1, functionalized with a p-nitrophenyl ester of picolinic acid, has been synthesized and its hydrolysis in covesicular blends of nonfunctional surfactants dihexadecyldimethylammonium bromide, 3, dioctadecyldimethylammonium bromide, 4, and racemic 1,2-bis(palmitoyloxy)-3-(trimethylammonium)propyl bromide, 5, studied after addition of Cu(II) ions at pH = 5.0 and different temperatures. The cleavage of 1 gives the p-nitrophenoxide surfactant 2, which shows a strong absorption band at 400 nm. Clear biphasic kinetics were observed for all vesicular systems at a temperature below the gel-to-liquid crystal phase transition temperature (T_c) of the membrane: the first, faster process (≈60% of the ester cleaved) was associated with the Cu(II)-catalyzed hydrolysis of the exovesicular ester; the slower one (the remaining ≈40% of the ester cleaved) was associated with the uncatalyzed hydrolysis of the endovesicular ester. Above T_c only a monoexponential process was observed. Variable temperature experiments allowed one to conclude that the cationic vesicles studied are totally impermeable to Cu(II) ions either below or above their T_c which controls the rate of the transbilayer movements of the lipids.