Attraction between Lipid Bilayer Membranes in Concentrated Solutions of Nonadsorbing Polymers: Comparison of Mean-Field Theory with Measurements of Adhesion Energy

Recent experimental advances have made quantitation of weak membrane attraction possible in concentrated solutions of macromolecules. Here, we report direct measurements of the free energy potential for adhesion of synthetic lipid bilayers in aqueous solutionsof dextran (polyglucose) over a wide range ofvolume fraction (0.01-0.1) and molecular weight (10000-150000). These polymer solutions are well-modeled by a Flory interaction parameter of 0.43, characteristic of a “good“ solvent. Controlled assembly of two giant bilayer vesicles was used to evaluate the free energy potential for formation of adhesive contact. The adhesion energy for neutral(phosphatidylcholine) bilayers in 0.1 M NaCl was found to steadily increase (from 0.01 to >0.2 erg/cm2) with polymer volume fraction-without any indication of a plateau-and with little dependence of polymer size. The distance dependence of the polymer-induced field was tested by incorporation of electric(phosphatidylserine) charges in the lipid bilayer surfaces to stabilize adhesion at large membrane separations. With the use of fluorescently labeled polymer, attempts were made tomeasure the concentration of polymer in the gap between adherent neutral bilayers; the result was negative, which indicated a significant reduction between the surfaces. Hence, we examined a thermodynamic mean-field theory for adhesion based on interaction of surface-depletion layers. Our analysis shows that (for equilibrium exchange of the polymer between gap and bulk regions) the attractive stress onthe membrane surfaces is simply the osmotic pressure reduction at the midpoint of the gap relative to the bulk region. Calculations of adhesion energies based on the mean-field theory agree very well with the concentration dependence of neutral bilayer adhesion for large molecular weights and with the attenuation of adhesion energy due to electric double-layer repulsion between charged bilayers in polymer solutions fixed at a specific concentration. © 1988, American Chemical Society. All rights reserved.

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Author David Needham Thomas Lord Department of Mechanical Engineering and Materia ...