Van der Waals interactions between cell surfaces.

Van der Waals energies of interaction between model cell surfaces are calculated for various distances of separation, layer thicknesses and compositions of cell surfaces and intercellular media. In these calculations the cell peripheries are considered to consist of two layers: (1) A phospholipid-cholesterol-protein plasma membrane and (2) a surface coat, which consists of protein, sugar and water. The required Van der Waals parameters of sugars, phospholipids and cholesterol are derived from refractive indices of their solutions in the visible and ultraviolet regions. Polarizabilities and Van der Waals parameters of these substances are determined and shown to be almost independent of concentration of solutions. Resulting isotropic polarizabilities differ by less than five percent from values obtained by the addition of bond polarizabilities. The magnitude of Van der Walls interactions between cell surfaces has been found to vary with composition according to the following sequence: water less than phospholipid less than cholesterol, protein less than sugar. A decrease in the concentration of a given substance in the cell surface at the expense of a corresponding increase in the concentration of a substance preceding it in this sequence lowers the magnitude of attractive interactions, whereas a similar change in the extracellular medium would have an opposite effect. A consideration of experimentally found variations in composition of cell surfaces results in calculated values of Hamaker's coefficients between 8 X 10(-15) ergs and 6 X 10(-14) ergs at 50 A distance of separation, which corresponds to free energies per unit area of 210-1600 kT/mu2.