Electrostatic potential maps (EPMs). The figures show in pseudo colors the electrostatic potential obtained solving the Poisson-Boltzmann equation. The map corresponds to the electrostatic potential on the plane x = y = 0. The extracellular space is represented on the top of each figure and the intracellular space is below the bilayer. In blue are shown the positive potential (>20 mV) and in red are shown the negative potential (<−20 mV). The map was merged with a 3D representation of the α carbon trace of the BK protein. Amino acids Asp326, Glu329, Glu386, Glu389 are shown in CPK space-filling mode. Calculations were performed using the PBEQ module of CHARMM28, and figures were drawn using the program DINO: Visualizing Structural Biology (2002), http://www.dino3d.org. (A) The EPM for the WT hSlo structure. The plane cuts through the bilayer, the protein, and the water-filled vestibules. The electrostatic potential inside the protein and the lipid bilayer is negative (red) and neutral on the bulk of the aqueous phase (white). The water-filled vestibules of the channel have negative electrostatic potential (red to pink color). (B) The EPM for the neutralization mutant E386N/E389N, in this mutant the intracellular vestibule is no longer at negative potential (white). (C) The EPM for the mutant D326Q, E329N. Neutralization of these external negative charges produces only marginal changes in the electrostatic potential map of the BK pore.