Nuclear Magnetic Resonance Studies of the Adsorption of Divalent Cations to Phospholipid Bilayer Membranes
Thermodynamic aspects of the adsorption of divalent cations to phospholipid bilayer membranes can be reasonably well described by the Gouy-Chapman-Stern theory (S. McLaughlin, this volume). To proceed beyond this thermodynamic description requires molecular information about the structure of the divalent cation-phospholipid complexes. Because most experimental techniques, i.e., equilibrium dialysis (Portis et al., 1979) or ion-sensitive electrodes (McLaughlin et al., 1981), measure only the loss of divalent cations from the aqueous medium, they give no information on the bound complexes. For example, they cannot be used to determine which groups in the phospholipid molecule provide ligands for the divalent cation, or to distinguish between inner-sphere complexes, where the ligand is inserted into the first coordination sphere of the divalent cation, and outer-sphere complexes, where the ligand and the fully hydrated cation form an “ion pair” (Basolo and Pearson, 1967; Hewkin and Prince, 1970; Ahland, 1972; Beck, 1968). This review briefly discusses how nuclear magnetic resonance (NMR) can provide this type of molecular information and can also be used to quantitatively test one of the major assumptions of the Gouy-Chapman-Stern theory.
KeywordsNuclear Magnetic Resonance Divalent Cation Nuclear Magnetic Resonance Signal Alkaline Earth Cation Phospholipid Molecule
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