Surface-differentiated model phospholipid bilayers

Article abstract of DOI:10.1021/ja00031a033

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 ～ 4.5 × 10^-2 s^-1) than the endovesicular PNPB groups [k ～ 7 × 10^-6 s^-1 (DPPC) or 7 × 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) ～ 6 min at 65 °C and (DOPC) ～ 20 min at 65 °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.