ADSORPTION OF CA2+ AND LA3+ TO BILAYER-MEMBRANES - MEASUREMENT OF THE ADSORPTION ENTHALPY AND BINDING CONSTANT WITH TITRATION CALORIMETRY

The adsorption of Ca2+ and La3+ ions to the surface of lipid bilayer membranes was studied with high sensitivity titration calorimetry. Ca2+ adsorbs to mixed phosphatidylcholine/ phosphatidylglycerol membranes with a reaction enthalpy of Delta H approximate to 0.1-0.2 kcal/mol. La3+ binds to sonified phosphatidylcholine vesicles with a reaction enthalpy of Delta H approximate to + 1.8 kcal/mol. Adsorption of La3+ to phosphatidylcholine bilayers imparts a net positive charge to the membrane surface which makes the binding of further La3+ increasingly more difficult. From the decreasing amplitudes in the calorimetric titration experiment a La3+ adorption constant of K approximate to (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 Ca2+ was determined previously as K approximate to 10-20 M(-1) using the same binding model. Since the reaction enthalpies of Ca2+ and La3+ adsorption are endothermic, the adsorption of both metal ions to the membrane surface is driven by a distinct change in entropy.