Generation of membrane potential (ΔΨ) by NqrB-D397N reconstituted into proteolipsomes. The essay buffer contained 5 µM oxonol VI, 200 µM NADH, 100 µM CoQ-1, 300 mM sorbitol, 50 mM Tris-HCl (pH 7.5), and 1 mM EDTA, along with the cation(s) of interest (A) using 20 mM NaCl (black), 20 mM LiCl (red), and 20 mM KCl (blue) and (B) (top) NaCl titration (1 to 50 mM) and (bottom) effect of KCl (100 mM) on the membrane potential in the presence of 2 mM NaCl and 50 mM NaCl.