EVALUATION OF DIVALENT-CATION BINDING TO PHOSPHATIDYLSERINE MEMBRANESBY AN ANALYSIS OF CONCENTRATION-DEPENDENCE OF SURFACE-POTENTIAL

Electrostatic considerations predict that the binding of divalent cati ons to singly charged acidic lipids is likely to proceed with a 1:2 io n-to-lipid stoichiometry, but the fact that divalent cations at simila r to 0.1 M concentrations reverse the sign of the negative surface pot ential of membranes provides evidence for 1:1 stoichiometry, In this w ork, analytic expressions are obtained for 1:1 and 1:2 binding constan ts and for the fractions of corresponding complexes based on extension s of the Gouy-Chapman-Stern theory and of Cohen-Cohen theory (J. A. Co hen and M. Cohen, Biophys. J. 36, 623, 1981), This approach relies on the ion concentration and the slope of the surface potential vs concen tration curve at the charge reversal point. Application of this theory to previously published electrophoresis data on phosphatidylserine ve sicles (S. McLaughlin, N. Mulrine, T. Gresalfi, G. Vaio, and A. McLaug hlin, J. Gen. Physiol. 77, 445, 1981), varying the thickness of the sh earing layer (delta) from 0 to 2 Angstrom, showed that (a) the 1:1 bin ding constants do not depend on delta and are similar to those determi ned in the original paper; (b) the 1:2 binding constants are extremely sensitive to delta: for Mg2+, Ca2+, Sr2+, and Ba2+, respectively, K-2 2 approximate to 25-35, 85-120, 110-150, and 190-270 M(-1) at delta = 0 and K-22 approximate to 2-3, 3-5, 5-7, and 10-15 M(-1) at delta = 2 Angstrom; (c) the fraction of lipid molecules involved in 1:2 complexe s (theta) also strongly depends on delta: the values of theta calculat ed either at the charge reversal point or at the saturation of adsorpt ion were 0.60-0.88 at delta = 0 and 0.22-0.55 at delta = 2 Angstrom. T hese results imply that the thickness of the shearing layer is a criti cal parameter for which the exact value is required to gain informatio n on the ion binding mechanism from electrophoresis data. The present theory may also be applied to experimental results on surface potentia l vs concentration curves obtained by other methods. (C) 1995 Academic Press, Inc.