CONTROL OF PERMEATION OF LANTHANIDE IONS ACROSS PHOSPHATE-FUNCTIONALIZED LIPOSOMAL MEMBRANES

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.