Electrostatics maps for the open conformation of the MscS barrel. The model obtained first through extrapolated motion was relaxed in a 4-ns unrestrained simulation in a POPC bilayer at 10 dyne/cm followed by 1-ns symmetric annealing (see Materials and methods). The channel with the softly restrained backbone was then additionally preequilibrated with 1.5 M KCl at zero voltage. The maps were averaged over 4 ns. (A) Distribution of electric potential due to the structural charges on the protein, totally unscreened by the medium. (B) The equilibrium densities of ions calculated inside the column (4 × 4 Å) positioned axially through the pore. The densities in the column do not reflect ion concentrations near the walls in wider vestibules. (C) Electrostatic potential in the system in the presence of lipids, water, and ions. Note that the color scale in C reflects a 40 times smaller variation of voltage compared with A. The scale is centered such that white color on C corresponds to the averaged potential in the aqueous bulk.