On the forces between phospholipid bilayers

Calculations for the free energy of interaction between phospholipid vesicles have been performed and applied to the question of vesicle aggregation. Calculations of van der Waals interactions have been performed by a computationally simple procedure based on an effective pair summation, which yields results close to those calculated with the Lifshitz theory. The equivalent (composition and distance dependent) Hamaker coefficients are in the range of 2 × 10^-14-10^-13 erg. Calculations of electrostatic energies of repulsion are based on detailed binding studies of calcium and magnesium to phosphatidylserine (PS). The binding of sodium to PS has also been considered. Total potential curves are computed for several vesicle systems at various concentrations of monovalent and divalent cations. Experimentally measured rates of aggregation of sonicated PS vesicles are shown to be explained by considering the existing potential barriers and hydrodynamic corrections. Predictions for both aggregation and stability, for various cationic environments, are shown to be verified by experiments. It is shown that the dimerization reaction of sonicated vesicles is expected to require surface contact; whereas, for large vesicles, aggregation may preferentially occur in a secondary minimum, with separations of several nanometers.