Iron tetrapyrrolic complexes

Stolzenberg and coworkers have used electrogenerated nickel(I) tetrapyrrole complexes for the catalytic reduction of dichloromethane and methyl iodide [364], alkyl halides [365-367], and aryl halides [367], and Lexa and coworkers [368] have discussed the catalytic reduction of frm75 -l,2-dibromocyclohexane to cyclohexene by electrogenerated nickel(I), cobalt(I), and iron(I) porphyrin complexes. 

While most of the tetrapyrrole pigments in the RC (FeS) type function as light harvesting, six central tetrapyrroles and three iron-sulphur complexes appear to constitute the photochemical heart of the reaction center. 

As we will see in subsequent chapters, many metalloproteins have their metal centres located in organic cofactors, like the tetrapyrrole porphyrins and corrins, in metal clusters like the Fe—S clusters in iron—sulfur proteins, or in even more complex cofactors, like the FeMo-cofactor of nitrogenase. We discuss their structures briefly here before moving on in a selected number of cases to the way in which metal insertion into the cofactor is engineered. 

These enzymes are homodimeric periplasmic proteins (encoded by nirB) that contain two different heme groups per monomer. One is a c-type heme bound via a Cys-X-X-Cys-His motif and has a role in electron transfer the other, a noncovalently linked d heme, is an unusual ferric-dioxoisobacteriochlorin, restricted to this class of enzyme and forms the active site. The biosynthetic pathway of heme di is unique among the tetrapyrroles in having 0x0, methyl, and acrylate side chains, involving some seven nir genes. The Em values of heme d model complexes are 200mV lower than iron porphyrins. 

Our knowledge of the stereochemistry of porphyrins and related tetrapyrrole macrocycles has expanded rapidly since the first reported x-ray structure determination in 1959 The structures of metallotetrapyrrole complexes are of interest because of the common occurrence of this type of macrocycle in biological systems. As is well known, foremost among these are the heme proteins (iron derivatives), the various photosynthetic pigments (magnesium complexes), the vitamin Bn coenzyme (cobalt corrinoids), and coenzyme F430 (nickel corphinoids) of the methanogenic bacteria. 

Compounds of iron complexed in a porphyrin (tetrapyrrole) ring differ in side chain composition. Haems are the prosthetic groups of cytochromes and are found in most oxygen carrier proteins. 

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