FIGURE 4.

The effects of Ba (open diamonds) and Mg (open triangles) on the surface potentials of vesicles formed from mixtures of 70% egg PC and 30% brain PS. Each point represents the mean of the surface potential measurements from five to seven vesicle preparations. The curves are the theoretical predictions of the Stern equation with K₁ = 0.6 M⁻¹, the same corresponding K₃ values derived from Fig. 3, and optimized fits for K₂, the 1:1 association constant of the divalent cation with PS: K₂ = 1.8 M⁻¹ for Ba; 1.9 M⁻¹ for Mg. As a reference, the dashed line is the theoretical prediction for surface potential assuming K₂ = 0.