Coordination Modes of Water-Derived Bridging Ligands in Diiron(Ⅲ) Complexes: Stabilization of an Oxo-Hydroxo Bridge by Hydrogen Bonding

摘要

Metalloproteins such as hemerythrin (Hr), ribonucleotide reductase (RR), methane monooxygenase (MMO), and purple acid phosphatases (PAP), including uteroferrin (UF), feature dinuclear iron species multiply bridged by carboxylato residues and, in stable or resting states of the protein, by a single water-derived ligand, such as hydroxo or oxo. However, at intermediate stages of the enzymatic cycle or during (re)formation of the active site, the form and number of water-derived ligands may vary depending on the protein. The coordination spheres of the two iron centers differ due to unsymmetric coordination of histidine (Hr), glutamic acid (MO, RR, PAP), and tyrosine (PAP) residues. Such asymmetry plays important roles in providing selective substrate binding (e.g., a nonbridging peroxo species in oxyHr and possibly MMO; control of oxidation states (e.g., PAP), and stereochemical control of intermediates in the catalytic cycle (e.g., PAP and MMO). Currently there is intense interest in dinuclear iron model systems that have polyfunctional (N_xO_y) ligation, different coordination of the two iron centers, and unusual bridging modes of water-derived ligands and in species which mimic protein-substrate/ inhibitor intermediates (e.g., PAP—tetrahedral oxoanion complex).