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.