Cover picture: Cover picture: State-dependent binding of a cationic ligand in the Na⁺ channel. (Top left) When a water molecule (space-filled) occupies the selectivity filter, the ligand's ammonium group (red dot) often occurs in the focus of macrodipoles of the green-colored pore helices. (Top right) When a Na⁺ ion (yellow) occupies the selectivity filter, it repels the ammonium group, which often faces the cytoplasm. (Bottom) Representative complexes show the space-filled ligand and residues contributing to the interaction energy (sticks). See the article by Tikhonov and Zhorov on page 1557.