Silver and gold in the Protein Data Bank

The structural features of the silver and gold sites in protein crystal structures extracted from the Protein Data Bank have been investigated. It is observed that both cations have nearly always low oxidations states (+1) and low coordination numbers, adopt standard stereochemistries, and interact preferentially (particularly gold) with sulfur donor atoms of cysteine and methionine side-chains. Interestingly, gold cation have been very often refined with occupancy minor than 1.0 and are very often "naked", in the sense that no donor atoms are sufficiently close to the metal cation. This apparently strange observation points out towards the need to develop specific and efficient validation tools for these elements when they are coordinated to proteins.