Ion‐Binding to Phospholipids

The binding of Ca 2+ to monolayers and bilayers of phosphatidylserine has been investigated as a function of pH, ionic strength (NaCl concentration) and Ca 2+ concentration using surface and colloid chemical techniques. The molar ratio of lipid to bound calcium decreases to 2 as the Ca 2+ concentration is increased to about 0.1 mM. At [Ca 2+ ] > 0.1 mM a 1:1 complex is formed. The apparent binding constant K a ranges from about ∼ 10 6 ‐−10 4 l/mol depending on the Ca 2+ 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 ∼ 10 4 l/mol; consistent values for K i were obtained using several independent approaches. K a for Ca 2+ binding decreases with increasing NaCl concentration because the monovalent cations compete with Ca 2+ + for the same binding site. Na + and K+ are equally effective in displacing 45 Ca 2+ 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 10 3 ‐fold smaller than that of Ca 2+ . An investigation of the binding of Mn 2+ to phosphatidylserine by both surface chemical and nuclear magnetic resonance methods shows that this cation has a similar binding constant to that of Ca 2+ . The Ca 2+ ‐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 p K 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 Ca 2+ to phosphatidylserine, is not obvious.