Specificity of Na⁺ binding to phosphatidylserine vesicles from A ²³Na NMR relaxation rate study

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.