201 - Binding of Cations to Phosphatidylserine Vesicles

The binding of Ca²⁺ and Mg²⁺ to phosphatidylserine (PS) vesicles is studied experimentally and analyzed. The surface potential and charge density are found for any given concentration of both monovalent and divalent cations in solution from a modified Gouy-Chapman equation. An analytical expression is given for the amount of cations concentrated in the double-layer region, which is treated distinctly from the amount of tightly bound cations. The explanation for the binding data of Ca²⁺ and Mg²⁺ to PS required taking into account the binding of Na⁺ to PS. The computed binding of Na⁺ to PS is in accord with recent results of NMR and vesicle aggregation studies. The calculated surface potentials of PS in the presence of Ca²⁺ and Na⁺ are in a reasonable agreement with the previously measured zeta-potentials. Our results indicate that Ca²⁺ has a ten-fold greater intrinsic binding constant than Mg²⁺ for PS vesicles.