Binding of Ca(II) and Sr(II) to the sugar-empty, 5F- Trp-labeled receptor: 19F NMR spectra. Samples contained 200–500 μM 5F-Trp-labeled receptor depleted of metal ions and sugar, with 100 mM KCl, 10 mM Tris, pH 7.1 with HCl, 10% D2O, and (A) 20 mM EDTA, (B) 0.5 mM CaCl2, or (C) 2.0 mM SrCl2. Other sample and spectral parameters at 470 MHz were as in Figure 1.