Alkaline earth cation binding to large and small bilayer phosphatidylserine vesicles: A calorimetric and potentiometric study

Abstract

A study is reported of the interaction of alkaline earth metal ions with bovine brain phosphatidylserine vesicles by potentiometric and calorimetric titration methods. Two sizes of vesicle, 25 nm and 100 nm, were studied in 0.1 M NaCl at pH 7.4. The small vesicles were found to bind Ca 2+ more strongly than the large vesicles. Ca 2+ and Ba 2+, but not Sr 2+ or Mg 2+, cause crystallization of the phospholipid acyl hydrocarbon chains with coincident destruction of the vesicles. Ca 2+ -induced crystallization of the hydrocarbon chains occurs at approximately the same point on the Ca 2+ binding isotherm at which charge neutralization of the vesicle surface occurs. Thermodynamic data for binding of Ca 2+ and Mg 2+ to small vesicles and for binding of Ca 2+ to large vesicles are reported. Potentiometric titration data were used to obtain equilibrium constants for M 2+ binding and the calorimetric titration data were used to obtain ΔH values. The potentiometric and calorimetric titration data are mutually supportive in establishing the stoichiometry and nature of the reactions occurring.