CHEMICAL APPROACHES TO BRIDGED BIOLOGICAL METAL ASSEMBLIES

Biological metal centers that consist of two fragments covalently connected by one more bridging atoms or groups are becoming increasingly recognized by physicochemical properties and protein crystallography. As a class, bridged biological metal assemblies pose challenging problems in chemical synthesis; at least some are potentially subject to further structural, electronic, and reactivity characterization provided the assembly itself or a close molecular simulation thereof can be prepared. Synthetic analogue approaches to three bridged assemblies are summarized here: nitrogenase FeMo/V cofactor, the catalytic site of sulfite/nitrite reductase, and the heterometal CuFe site in cytochrome c oxidase. The cofactor cluster is approached by the clusters [MFe4S6(PEt3)4L] (M = Mo, V; L = Cl-, RS-), in which a cuboidal Fe4S3 unit is linked to the M site by three mu2-S atoms. The sulfite/nitrite site analogue consists of an Fe4S4 cluster linked to a heme group through an unsupported mu2-S bridge. The binuclear site in the oxidized form of cytochrome c oxidase has been investigated by synthesis of the unit [Fe(III)-O-Cu(II)] in a molecular heme complex. The cyanide-inhibited form of the enzyme has been simulated by the preparation of a series of heme complexes containing the bridge unit [Fe(III)-CN-Cu(II)] in which the Fe atom is six-coordinate and low-spin.