Ionic strength dependence of localized contact formation between membranes: Nonlinear theory and experiment

Erythrocyte membrane surface or suspending phase properties can be experime ntally modified to give either spatially periodic local contacts or continu ous contact along the seams of interacting membranes. Here, for cells suspe nded in a solution of the uncharged polysaccharide dextran, the average lat eral separation between localized contacts in spatially periodic seams at e ight ionic strengths, decreasing from 0.15 to 0.065, increased from 0.65 to 3.4 mu m. The interacting membranes and intermembrane aqueous layer were m odeled as a fluid film, submitted to a disjoining pressure, responding to a displacement perturbation either through wave growth resulting in spatiall y periodic contacts or in perturbation decay, to give a plane continuous fi lm. Measured changes of lateral contact separations with ionic strength cha nge were quantitatively consistent with analytical predictions of linear th eory for an instability mechanism dependent on the membrane bending modulus . Introduction of a nonlinear approach established the consequences of the changing interaction potential experienced by different parts of the membra ne as the disturbance grew. Numerical solutions of the full nonlinear gover ning equations correctly identified the ionic strength at which the bifurca tion from continuous seam to a stationary periodic contact pattern occurred and showed a decrease in lateral contact and wave crest separation with in creasing ionic strength. The nonlinear approach has the potential to recogn ize the role of nonspecific interactions in initiating the localized approa ch of membranes, and then incorporate the contribution of specific molecula r interactions, of too short a range to influence the beginning of perturba tion growth. This new approach can be applied to other biological processes such as neural cell adhesion, phagocytosis, and the acrosome reaction.