CATION-BINDING BY THE PHENOLATE GROUP IN SMALL MOLECULES AND PROTEINS

The geometry of interactions of metal ions with the phenolate group in proteins and small molecules has been examined using coordinates list ed in structural databases. Cations are found to avoid the sp2 lone pa ir directions of the ligand oxygen; in small-molecule structures most of the cations are clustered close to the aromatic plane in the region between the sp2 and the C-O vectors, whereas in proteins the ions mov e out of the plane. Such a spatial distribution is different from that observed for hydrogen-bonded neighbors interacting with a neutral tyr osine residue. The environment of a tyrosinate anion is such that it h as a cation on one side of its plane and a hydrogen-bond donor on the other. Metal binding can disturb the normal conformation of the tyrosi ne side chain.