DIRECT MEASUREMENT OF POLYETHYLENE-GLYCOL INDUCED DEPLETION ATTRACTION BETWEEN LIPID BILAYERS

Although polyethylene glycol (PEG) is widely used for aggregating or f using cells, the forces responsible for these interactions have remain ed elusive. Through a variety of techniques including quasi-elastic li ght scattering, surface force measurements, and P-31-NMR, we have esta blished that while PEG of molecular weight 8000-10000 is effective in causing the aggregation of vesicles, PEG of lower or higher molecular weight (1000 and 18500, respectively)is ineffective. For the first tim e, direct force measurements between lipid bilayers in solutions of 80 00-10000 molecular weight reveal the existence of an attractive osmoti c force due to a polymer depleted layer near the bilayer surface. Lowe r molecular weight PEG does not have a large enough size (Flory radius , RF) to generate a significant depletion force, while higher molecula r weight PEG adsorbs sufficiently on the bilayer surfaces to eliminate the depletion attraction and produces a repulsive steric barrier to a ggregation. The measured forces can be quantitatively described in ter ms of current theories of colloidal and polymer interactions. These fi ndings suggest that the differential osmotic pressure produced by the depletion layer is responsible for vesicle aggregation and that fusion is promoted when the depletion pressure is strong enough to locally d estabilize two membranes by possibly thinning them at their point of c losest approach. The results provide a physicochemical basis for using PEG of certain molecular weights as fusogens for cells, liposomes, an d vesicles.