Metal–ligand geometry relevant to proteins and in proteins: sodium and potassium

In previous papers [Harding (2001), Acta Cryst. D57, 401–411, and references therein] the geometry of metal–ligand interactions was examined for six metals (Ca, Mg, Mn, Fe, Cu, Zn) using the Protein Data Bank and compared with information from accurately determined structures of relevant small-molecule crystals in the Cambridge Structural Database. Here, the environments of Na⁺ and K⁺ ions found in protein crystal structures are examined in an equivalent way. Target M⁺...O distances are proposed and the agreement with observed distances is summarized. The commonest interactions are with water molecules and the next commonest with main-chain carbonyl O atoms.