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.