P. Scrimin et al., CONTROL OF PERMEATION OF LANTHANIDE IONS ACROSS PHOSPHATE-FUNCTIONALIZED LIPOSOMAL MEMBRANES, Journal of the American Chemical Society, 120(6), 1998, pp. 1179-1185
Unilamellar vesicles of anionic, phosphate-functionalized lipid 1 (DPG
PNP, 1,2-di-O-hexadecanoyl-rac-glyceryl p-nitrophenyl phosphate, sodiu
m salt) have been prepared both by sonication and by extrusion. The ph
osphate head groups are hydrolytically stable at pH 7 but can be speci
fically cleaved, at 25 degrees C, on the exo surface by adding, to pre
formed aggregates, lanthanide(III) cations which are unable to permeat
e across the bilayer. The exovesicular specificity disappears when the
cleavage is performed above the phase transition temperature of the v
esicles not because of facilitated permeation of the cations but becau
se of the increased rate of flip-flop of the lipid. Addition of lipoph
ilic amines (N-hexadecyl-N,N',N'-trimethylethylenediamine, TMED C-16;
N-hexadecylhistamine, C-16-His, hexadecylamine, C16NH2) and, to a less
er extent, the cationic surfactant cetyltrimethylammonium bromide, CTA
Br, induces the cleavage of the remaining fraction of lipid in exo-cle
aved vesicles. The additives affect the permeability of the bilayer, l
ikely creating ''leaky patches'' in the vesicular membrane and, in the
case of the amines, also transporting the cations across the bilayer
by forming lipophilic complexes.