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Specificity of Na+ binding to phosphatidylserine vesicles from A 23Na NMR relaxation rate study
Authors:Robert Kurland  Carolyn Newton  Shlomo Nir  Demetrios Papahadjopoulos
Affiliation:1. Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14214 U.S.A.;2. Department of Experimental Pathology, Roswell Park Memorial Institute, Buffalo, NY 14263 U.S.A.
Abstract:23Na NMR relaxation rate measurements show that Na+ binds specificially to phosphatidylserine vesicles and is displaced partially from the binding site by K+ and Ca2+ but to a considerably less extent by tetraethylammonium ion. The data indicate that tetraethylammonium ion affects the binding of Na+ only slightly, by affecting the surface potential through its presence in the double layer, without competing for a phosphatidylserine binding site. Values for the intrinsic binding constant for the Na+-phosphatidylserine complex that would be consistent with the competition experiments (and the dependence of the relaxation rate on concentration of free Na+) fall in the range 0.4–1.2 M?1 with a better fit towards the higher values. We conclude that in the absence of competing cations in solution an appreciable fraction of the phosphatidylserine sites could be associated with bound Na+ at 0.1 M Na+ concentration.
Keywords:Phosphatidylserine  (Vesicle)  tetraethylammonium ion  TNS  toluidinylnaphthalene sulfonate
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