Electrostatic potential difference along the axis of the pore. Ordinate represents the change in local electrostatic potential, in mV, observed when the charge of the different residues is turned off. Error bars are the standard deviation of the potential calculated over 100 different structures sampled during a 1-ns MD simulation. Abscissa is the z coordinate in Angstrom units. z = 5 is defined as the geometric center of the selectivity filter. The entrance of the extracellular mouth of the pore is at z = 17 Å and the intracellular mouth is at z = −15.5 Å. The magenta line describes the electrostatic potential differences calculated for the neutralization of the acidic amino acids located at the inner opening of the channel. The light green line is the calculation for the neutralization of residue Glu386. The cyan line is the result of the neutralization of Glu389. The red line is the electrostatic potential associated with turning off the charges of residues Asp326 and Glu329. Green and blue lines represent the individual contributions of Asp326 and Glu329, respectively.