SURFACE-DIFFERENTIATED MODEL PHOSPHOLIPID-BILAYERS

Dipalmitoylphosphatidylcholine (DPPC) and dioleoylphosphatidylcholine (DOPC) were synthesized in which the head groups were covalently functionalized with p-nitrophenyl benzoate (PNPB) moieties. Liposomes composed of 1 part functional lipid and 7 parts nonfunctional DPPC or DOPC were created at pH 6 and subjected to an exovesicular/endovesicular 12/6 pH gradient. Under these conditions, the exovesicular PNPB groups saponified more rapidly (k approximately 4.5 X 10(-5) s-1) than the endovesicular PNPB groups [k approximately 7 X 10(-6) s-1 (DPPC) or 7 x 10(-5) s-1 (DOPC)], leading to surface-differentiated liposomes with exovesicular p-nitrophenylate labels and intact, endovesicular PNPB groups. The half-times for flip-flop reequilibration of the intact PNPB lipids between the endovesicular and exovesicular leaflets of the liposomal bilayers were (DPPC) approximately 6 min at 65-degrees-C and (DOPC) approximately 20 min at 65-degrees-C. The permeabilities of these liposomes to H+/OH- were also studied. Comparisons of the DPPC and DOPC flip-flop dynamics to those of analogous, cationic, ammonium ion lipids highlight the relative dynamic stability of the zwitterionic phospholipids in liposomal bilayers.