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 (almost-equal-to 60% of the ester cleaved) was associated with the Cu(II)-catalyzed hydrolysis of the exovesicular ester; the slower one (the remaining almost-equal-to 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.