Short and long-range forces involved in cation-induced aggregation of chromaffin granule membranes

The kinetics and equilibria of cation-induced aggregation of chromaffin granule (CG) membranes are analyzed. The following mechanistic observations are made 1. (1) Cation-induced dimerization of CG membranes is brought about by reducing the membrane surface potential and hence by reducing the electrostatic repulsion between membranes. This can be accomplished by either monovalent or divalent cations. 2. (2) The dimerization reaction is nearly diffusion-controlled. The activation energy of the process is low. 3. (3) The kinetics are analyzed in terms of an "encounter complex" model formulated for comparison with the delay times involved in Ca²⁺ -triggered exocytotic release. The half time for aggregation of vesicles with distances of separation between 26 Å and 126 .Å is calculated to be on the order of 3 msec. 4. (4) The role of van der Waals attraction, electrostatic repulsion and dipolar repulsion is considered. Calculations show that dipolar repulsion of phosphatidyl choline polar head groups should outweigh van der Waals attraction for short distances of separation. 5. (5) Phospholipid vesicles prepared from CG membrane lipids also show cation-induced aggregation. However, the behavior is qualitatively and quantitatively dissimilar to that of the CG membrane. The rates of aggregation are two orders of magnitude slower. This is shown to be due to a negative entropy of activation resulting from the necessity of matching the surfaces of the vesicles. Vesicle approaches involving high energies (enthalpies) of activation are excluded.