Ion‐Binding to Phospholipids

Abstract

The binding of Ca²⁺ to monolayers and bilayers of phosphatidylserine has been investigated as a function of pH, ionic strength (NaCl concentration) and Ca²⁺ concentration using surface and colloid chemical techniques. The molar ratio of lipid to bound calcium decreases to 2 as the Ca²⁺ concentration is increased to about 0.1 mM. At [Ca²⁺] > 0.1 mM a 1:1 complex is formed. The apparent binding constant K_a ranges from about ∼ 10⁶-−10⁴ l/mol depending on the Ca²⁺ concentration. After allowing for electrostatic effects and neighbour group interactions, the intrinsic binding constant K_i of the phosphorylserine polar group at pH 7 (I= 0.01 M), where it carries a net negative charge of one, is ∼ 10⁴ l/mol; consistent values for K_i were obtained using several independent approaches. K_a for Ca²⁺ binding decreases with increasing NaCl concentration because the monovalent cations compete with Ca²⁺+ for the same binding site. Na⁺ and K+ are equally effective in displacing ⁴⁵Ca²⁺ adsorbed to monolayers of phosphatidylserine, both with respect to the kinetics and the equilibrium of the displacement. K_a for the reaction between phosphatidylserine and monovalent cations is about 10³-fold smaller than that of Ca²⁺. An investigation of the binding of Mn²⁺ to phosphatidylserine by both surface chemical and nuclear magnetic resonance methods shows that this cation has a similar binding constant to that of Ca²⁺. The Ca²⁺ -binding capabilities of monolayers containing only carboxyl groups (i.e. arachidic acid) and phosphodiester groups (i.e. dicetyl phosphate) have also been determined; the apparent pK for the –COOH group in monolayers is ⋧ 9 and that for the phosphodiester group is < 4. Since these groups do not retain the same pK values when they are in close proximity in the phosphorylserine group, the relative contributions of the two groups to the binding of Ca²⁺ to phosphatidylserine, is not obvious.