EXCHANGE OF MONOOLEOYLPHOSPHATIDYLCHOLINE AS MONOMER AND MICELLE WITHMEMBRANES CONTAINING POLY(ETHYLENE GLYCOL)-LIPID

Surface-grafted polymers, such as poly(ethylene glycol) (PEG), provide an effective steric barrier against surface-surface and surface-macro molecule interactions. In the present work, we have studied the exchan ge of monooleoylphosphatidylcholine (MOPC) with vesicle membranes cont aining 750 mol wt surface-grafted PEG (incorporated as PEG-lipid) from 0 to 20 mol% and have analyzed the experimental results in terms of t hermodynamic and stationary equilibrium models. Micropipette manipulat ion was used to expose a single lipid vesicle to a flow of MOPC soluti on (0.025 mu M to 500 mu M). MOPC uptake was measured by a direct meas ure of the vesicle area change, The presence of PEG(750) lipid in the vesicle membrane inhibited the partitioning of MOPC micelles (and to s ome extent microaggregates) into the membrane, while even up to 20 mol % PEG-lipid, it did not affect the exchange of MOPC monomers both into and out of the membrane. The experimental data and theoretical models show that grafted PEG acts as a very effective molecular scale ''filt er'' and prevents micelle-membrane contact, substantially decreasing t he apparent rate and amount of MOPC taken up by the membrane, thereby stabilizing the membrane in a solution of MOPC that would otherwise di ssolve it.