Adsorption of Ca 2+ and La 3+ to bilayer membranes: Measurement of the adsorption enthalpy and binding constant with titration calorimetry

Abstract

The adsorption of Ca 2+ and La 3+ ions to the surface of lipid bilayer membranes was studied with high sensitivity titration calorimetry. Ca 2+ adsorbs to mixed phosphatidylcholine/phosphatidylglycerol membranes with a reaction enthalpy of Δ H ≈ 0.1–0.2 kcal/mol. La 3+ binds to sonified phosphatidylcholine vesicles with a reaction enthalpy of Δ H ≈ + 1.8 kcal/mol. Adsorption of La 3+ to phosphatidylcholine bilayers imparts a net positive charge to the membrane surface which makes the binding of further La 3+ increasingly more difficult. From the decreasing amplitudes in the calorimetric titration experiment a La 3+ adorption constant of K ≈ (4.1 ± 1.1)· 10 3 M −1 was evaluated. Electrostatic effects were corrected for by means of the Gouy-Chapman theory. The adsorption constant of Ca 2+ was determined previously as K ≈ 10–20 M −1 using the same binding model. Since the reaction enthalpies of Ca 2+ and La 3+ adsorption are endothermie, the adsorption of both metal ions to the membrane surface is driven by a distinct change in entropy.