Binding affinity of monovalent cations to KtrB examined by ITC. (a–d) Cation to buffer control titrations are shown above each (e–h) cation to protein titration. For the cation to protein titrations, the upper panels show the raw heat exchange data, associated with Na⁺ (e), K⁺ (f), Rb⁺ (g), or Cs⁺ (h) binding to detergent-solubilized KtrB in 200 mM LiCl buffered with 20 mM Tris-HCl, pH 7.5. The lower panels giving the integrated injection heat pulses, normalized per mole of injection, reveal different binding curves fitted by one-site binding model. Individual K_d values derived from fitting ± SE are indicated. Each graph represents an example of three independent experiments. Mean data ± SEM are summarized in Table 1.