Calcium binding to phosphatidylcholine bilayers as studied by deuterium magnetic resonance. Evidence for the formation of a calcium complex with two phospholipid molecules

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Abstract
The binding of Ca2+ to bilayer membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was investigated with atomic absorption spectroscopy and deuterium resonance, leading to the following conclusions. Atomic absorption spectroscopy allowed the determination of the amount of Ca2+ bound to the membrane surface (Cb) at low Ca2+ concentrations (3-100 mM). Simultaneous measurements of the deuterium magnetic resonance spectra of POPC with specifically deuterated choline head groups revealed a linear relationship between the quadrupole splitting and the amount of bound Ca2+. With this calibration, the amount of bound Ca2+ could be determined from the deuterium spectra under conditions where atomic absorption spectroscopy was technically not feasible, i.e., in the concentration range of 0.1-5 M CaCl2. The Ca2+ binding isotherm exhibited saturation behavior. The quadrupole splitting at the saturation limit corresponded to a binding stoichiometry of 1 Ca2+ per 2 POPC molecules. The surface charge density (.sigma.) could be evaluated from the amount of bound Ca2+ and the surface area per POPC molecule. By employing the Gouy-Chapman theory, it was then possible to determine the surface potential (.psi.0) and the Ca2+ concentration immediately at the lipid-water interface (CI). With this set of experimental parameters, various models for the mode of Ca2+ binding were tested. A simple partition equilibrium or a Langmuir absorption model could be ruled out. A very good fit to the experimental data was obtained by applying the law of mass action in the form Cb/(1-2Cb)2 = KCI in which K is the only adjustable parameter. This model independently supports the above conclusion of a Ca2+ to phospholipid stoichiometry of 1:2. For POPC in the liquid-crystalline state, this model predicts Ca2+ binding constants of K = 13.8 M-1 (0.1 M NaCl, 25.degree. C) and 7.0 M-1 (no NaCl, 40.degree. C).

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