Mechanism of the spontaneous transfer of phospholipids between bilayers

A fluorescent phospholipid, 1-palmitoyl-2-pyrenedecanoylphosphatidylcholine, was used to study the mechanism of spontaneous phospholipid transfer between single-walled phospholipid vesicles. The half-time for transfer of this molecule between vesicles of dimyristoylphosphatidylcholine at 36 degrees C is 13 h if flip-flop is negligible or 24 h if flip-flop is faster than intervesicle exchange. The half-time is unaffected by the concentration of acceptor vesicles, which indicates that transfer of label takes place by diffusion of monomers or micelles through the aqueous phase rather than by collision of vesicles. These results are compared with previous studies of spontaneous lipid transfer.