A HYDROLYTIC REPORTER OF CU(II) AVAILABILITY IN ARTIFICIAL LIPOSOMES

The dioctadecyl ammonium surfactant 1, functionalized with a p-nitroph enyl ester of picolinic acid, has been synthesized and its hydrolysis in covesicular blends of nonfunctional surfactants dihexadecyldimethyl ammonium bromide, 3, dioctadecyldimethylammonium bromide, 4, and racem ic 1,2-bis(palmitoyloxy)-3-(trimethylammonium)propyl bromide, 5, studi ed after addition of Cu(II) ions at pH = 5.0 and different temperature s. The cleavage of 1 gives the p-nitrophenoxide surfactant 2, which sh ows a strong absorption band at 400 nm. Clear biphasic kinetics were o bserved for all vesicular systems at a temperature below the gel-to-li quid crystal phase transition temperature (T(c)) of the membrane: the first, faster process (almost-equal-to 60% of the ester cleaved) was a ssociated with the Cu(II)-catalyzed hydrolysis of the exovesicular est er; the slower one (the remaining almost-equal-to 40% of the ester cle aved) was associated with the uncatalyzed hydrolysis of the endovesicu lar ester. Above T(c) only a monoexponential process was observed. Var iable temperature experiments allowed one to conclude that the cationi c vesicles studied are totally impermeable to Cu(II) ions either below or above their T(c) which controls the rate of the transbilayer movem ents of the lipids.