Dehydration of permeating cations along the pore. (A) Snapshot of dehydrated Na⁺ bound to D55, having four water molecules in its hydration shell. Two water molecules and their oxygen are replaced by two contributing oxygens from the D55 sidechain. (B) Number of Na⁺ coordinating oxygens fluctuates over 50-ns trajectory as Na⁺ is dehydrated by pore-lining residues.