Definition of the rate constants of blocking (k_OB) and unblocking (k_BO) under the assumption that the location of Mg²⁺ as found in crystal structure also applies to the electrophysiological studies here (Yamashita et al., 2008). The assignment periplasmic = trans results from the most plausible interpretation of the difference in accessibility described in Figs. 8 and 9 for the dominant F+ type insertion (flickering at positive potentials). (A) Crystal structure obtained by Yamashita et al. (2008). (PDB code 2ZFG, the water-coordinated Mg²⁺, and two detergent molecules have been removed for clarity.) Enrofloxacin (yellow bipolar ellipse) is given at the periplasmic side. Here, the issue of coloring the involved residues is showing the association of residues to the binding domains; thus, conventional coloring of amino acids has not been used. The binding domain of the (H₂O)₆-coordinated Mg²⁺ ion, including the residues D113, E117 (yellow), and L115 (red), is on the L3 loop (black) of the constriction zone CR. Residues on L3 are also involved in the periplasmic bindings site Mini Below for enrofloxacin (purple and red). The binding site Mini Above (blue) does not play a role in the fast blocking events investigated here. (B) For the definition of the rate constants that are involved when enrofloxacin is given at the external side, a cartoon is used. It highlights those features of the Ompf structure in A which are important for the understanding of the fast flickering events. The blue sphere presents (H₂O)₆-Mg²⁺, and the purple ring the binding site Mini Below.