Direct Measurement of Polyethylene Glycol Induced Depletion Attraction between Lipid Bilayers

Abstract

Although polyethylene glycol (PEG) is widely used for aggregating or fusing cells, the forces responsible for these interactions have remained elusive. Through a variety of techniques including quasi-elastic light scattering, surface force measurements, and 31P-NMR, we have established 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 18 500, respectively) is ineffective. For the first time, direct force measurements between lipid bilayers in solutions of 8000−10000 molecular weight reveal the existence of an attractive osmotic force due to a polymer depleted layer near the bilayer surface. Lower molecular weight PEG does not have a large enough size (Flory radius, RF) to generate a significant depletion force, while higher molecular weight PEG adsorbs sufficiently on the bilayer surfaces to eliminate the depletion attraction and produces a repulsive steric barrier to aggregation. The measured forces can be ...