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.