FIGURE 2.

(A) Simulation systems. Simulation system with a K⁺ ion solvated by water. To mimic the effect of a nonpolar membrane region, the water molecules are prevented to enter between −10 and +10 Å using a half-harmonic potential. (B) Simulation system with a 41-residue fragment of the voltage sensor of the KvAP channel, starting from Pro⁹⁹ (near the center of the S3 helix, shown in red) to Ser¹³⁹ (near the C-terminus of the S4 helix, shown in blue). To mimic the effect of a nonpolar membrane region, the water molecules are prevented to enter between −10 and +10 Å using a half-harmonic potential. The structure of the S3-S4 segment is directly taken from the x-ray structure of the voltage sensor (PDB id 1ORS). The important charged residues are displayed as sticks and the position of their center of charge are (z = 0 is at the center of the membrane): Glu¹⁰⁷ (10.8 Å), Arg¹¹⁷ (14.1 Å), Arg¹²⁰ (9.7 Å), Arg¹²³ (10.0 Å), Arg¹²⁶ (4.4 Å), Arg¹³³ (−4.2 Å), and Lys¹³⁶ (−12.7 Å).