Figure A1.

Force between a K⁺ ion and a water molecule as a function of distance. (a) The negative oxygen atom of the water molecule points towards the positive K⁺ ion. As long as the distance between the center of K⁺ and of the water molecule is larger than 0.248 nm (about 90% of the sum of the radius of K⁺ and the effective radius of H₂O), the total coulomb force between the K⁺ and the three charges of the water molecule is attractive with a maximum magnitude of almost 10⁻⁹ N. Within the repulsion zone an extremely strong repulsive force sets in, however. This configuration occurs when the water molecule is part of a hydration shell of the K⁺ ion. When this configuration occurs within the selectivity filter, which is often the case when a water molecule from the inner cell cavity follows a K⁺ ion into the selectivity filter, the water molecule becomes attached to the K⁺ and moves with it through the selectivity filter. (b) One positive hydrogen atom of the water molecule points towards the positive K⁺ ion. Here the total Coulomb force between the K⁺ and the water molecule is always repulsive, but at a distance of 0.5 nm it has fallen off to only about 2 × 10⁻¹⁰ N. However, if the water molecule and the K⁺ approach closer than the repulsion zone and the electron shells begin to overlap, the very strong repulsion force sets in.