An unexpected all-metal aromatic tetranuclear silver cluster in human copper chaperone Atox1.

Metal clusters, such as iron-sulfur clusters, play key roles in sustaining life and are intimately involved in the functions of metalloproteins. Herein we report the formation and crystal structure of a planar square tetranuclear silver cluster when silver ions were mixed with human copper chaperone Atox1. Quantum chemical studies reveal that two Ag 5s 1 electrons in the tetranuclear silver cluster fully occupy the one bonding molecular orbital, with the assumption that this Ag 4 cluster is Ag 4 2+ , leading to extensive electron delocalization over the planar square and significant stabilization. This bonding pattern of the tetranuclear silver cluster represents an aromatic all-metal structure that follows a 4 n + 2 electron counting rule ( n = 0). This is the first time an all-metal aromatic silver cluster was observed in a protein.