Fig. 2. (A) Zero-current membrane potential of the bilayer in the presence of Omp35 and Omp36 porins measured as a function of the salt (KCl, NaCl and MgCl2) gradient across the membrane. The respective salt concentration at the beginning was 10 mM on both sides and then gradually titrated at the cis (potential extracellular) side to reach the desired concentration. Average results of three experiments with different membranes at 20 °C and pH 7 are shown. (B) For 1 M MgCl2 the permeability ratio of anions to cations (PCl/PMg) is estimated for all four proteins from MD simulations performed without an external field. (C) The top three binding sites for Mg2+ ions which have been identified by MD simulations for all four proteins are denoted as sites 1, 2 and 3 (magenta spheres). The position of the binding sites inside the constriction region is depicted by the respective sphere, whereas the size of the sphere represents the probability of occurrence of these sites, i.e., the larger the sphere, the higher the probability. In the case of OmpF, one binding site of Mg2+ ions was observed in the crystal structure (yellow sphere) and is identical to site 3 observed in the simulations. The acidic residues inside the constriction region are highlighted by van der Waals spheres.
