A Model of Semimet Hemerythrin; NMR Spectroscopic Evidence of Valence Localization in Bis(mu-carboxylato)(mu-phenolato)diiron(II,III) Complexes in Solution

摘要

Hemerythrin (Hr), a dioxygen carrier found in sipunculid worms and other marine invertebrates, is considered the prototype of non-heme proteins with a diiron active site. Since the early 1980s the structures of four distinct forms of the protein havebeen solved by X-ray crystallography. In the deoxy form, the two Fe~(II) ions are bridged by one hydroxy and two carboxy-late groups. The coordination spheres of the two iron ions are quite different. One is bound by three histidine residues and is thushexacoordinate; the other is bound by only two histidines, thus leaving a coordination site accessible to dioxygen and other substrates. Oxygenation of Hr leads to deprotonation of the bridging hydroxo ligands, resulting in a (mu-oxo)diiron(III) unit with a terminal hydroperoxo ligand linked to the bridging oxo ligand through a hydrogen bond. The hydroperoxo ligand can be replaced by anions (e.g. azide) leading to the corresponding met forms, which have been structurally characterized. Apart from the [Fe~(II)Fe~(II)] and [Fe~(III)Fe~(III)] states, two mixed-valence forms [Fe~(II)Fe~(III)] have been detected. These semimet hemery-thrins are obtained either by oxidation of the deoxy form ((semimet)_0Hr) or by reduction of the met form ((semimet )_RHr). No X-ray structures are available for the semimet forms yet. but EXAFS and other spectroscopic methods have revealed a Fe~(II) (mu-OH)Fe~(III) unit. NMR experiments on (semimet)_RHr derivatives in solution have shown that the valence of the two ions is localized. Kurtz et al, and Solomon et al. have proposed that the two semimet forms differ in terms of the location of the valences of the iron atoms in the ligand environment as illustrated in Scheme 1.