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 approximately 0.1-0.2 kcal/mol. La3+ binds to sonified phosphatidylcholine vesicles with a reaction enthalpy of delta H approximately +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+ adsorption constant of K approximately (4.1 +/- 1.1) x 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 approximately 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.
Lehrmann
R
R
Department of Biophysical Chemistry, Biocenter of the University of Basel, Switzerland.
Seelig
J
J
D000327:Adsorption; D002118:Calcium / Q000378:metabolism*; D002151:Calorimetry; D004563:Electrochemistry; D007811:Lanthanum / Q000378:metabolism*; D008051:Lipid Bilayers; D013816:Thermodynamics
D008051:Lipid Bilayers; D007811:Lanthanum; D002118:Calcium