Porphyrins and Corrins

Considerable attention has been directed to dehalogenation mediated by corrinoids and porphyrins in the presence of a chemical reductant (references in Gantzer and Wackett 1991 Glod et al. 1997 Workman et al. 1997). Illustrations are provided by the dechlorination and elimination reactions carried out by titanium(III) citrate and hydroxocobala-min (Bosma et al. 1988 Glod et al. 1997). The involvement of corrinoids and porphyrins is consistent with the occurrence of analogous mechanisms for biological reactions that 

The reactions of alkyl halides with Fe(II) deuteroporphyrin IX have been examined (Wade and Castro 1973). Three classes of reaction were observed (i) hydrogenolysis, (ii) elimination to alkenes, and (iii) coupling of alkyl free radicals. Further discussion has been given in Castro (1998). 

Experiments have been carried out to mimic the reactions of model systems for coenzyme F430 that is involved in the terminal step in the biosynthesis of methane, and that is able to dechlorinate CCI4 successively to CHCI3 and CH2CI2 (Krone et al. 1989). Nickel(I) isobacteriochlorin anion was generated electrolytically and used to examine the reactions with alkyl halides in dimethylformamide (Helvenston and Castro 1992). The three classes of reaction were the same as those observed with Fe(II) deuteroporphyrin IX that have already been noted. 

Experiments that were carried out to compare nanoscale catalysts composed of Ee, Ni, and Co complexes of several porphyrins, or cyanocobalamin have already been noted (Dror etal.2005). 

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Uroporphyrinogen I (16c), a constitutional isomer of uroporphyrinogen III, also plays no direct role in porphyrin and corrin biosynthesis, but this unnatural substrate is methylated to give 17c10c f in the presence of SAM by the methyl transferase of some bacteria. A constitutional type I dihydroisobacteriochlorin can be obtained by methylation of uroporphyrinogen I with methylase Ml. Methyltransferase M1 is able to methylate the unnatural precorrin once more to give the trimethylpyrrocorphin type I.IOc 1... 

Marks TS, JD Allpress, A Maule (1989) Dehalogenation of lindane by a variety of porphyrins and corrins. Appl Environ Microbiol 55 1258-1261. 

In a wider context, extracellular redox mediators have been implicated in a number of reductions. The specific role of reductive dehalogenation by porphyrins and corrins has been discussed in Chapter 1. 

There are large numbers of naturally occurring representatives, especially of pyrrole that include the important polypyrroles (porphyrins and corrins), and the nitropyrrole antibiotics such as pyr-rolomycins and pyrroxamycin. Derivatives of furan have been used as fungicides and A-vinylpyr-rolidone is an important monomer for the production of blood plasma extenders and for cosmetic applications. On account of the similarity in the pathways for the aerobic degradation of monocyclic furan, thiophene, and pyrrole, all of them are considered here. Anaerobic degradation of furans is discussed in Part 2 of this chapter. 

The macrocycle types discussed so far tend to form very stable complexes with transition metal ions and, as mentioned previously, have properties which often resemble those of the naturally occurring porphyrins and corrins. The complexation behaviour of these macrocycles contrasts in a number of ways with that of the second major category of cyclic ligands - the crown polyethers. 

As we will see in subsequent chapters, many metalloproteins have their metal centres located in organic cofactors (Lippard and Berg, 1994), such as the tetrapyrrole porphyrins and corrins, or in metal clusters, such as the Fe-S clusters in Fe-S proteins or the FeMo-cofactor of nitrogenase. Here we discuss briefly how metals are incorporated into porphyrins and corrins to form haem and other metallated tetrapyrroles, how Fe-S clusters are synthesized and how copper is inserted into superoxide dismutase. 

Corphin is the F-430 cofactor found in methyl-coenzyme M reductase, a nickel-containing enzyme that participates in the conversion of carbon dioxide to methane in methanogenic bacteria. The nickel ion in F-430 is coordinated by the tetrahydrocorphin ligand, which contains structural elements of both porphyrins and corrins. 

A tetrapyrrole (related to porphyrins and corrins) containing a nickel ion. This cofactor, corphin, is a crucial component of methyl-coenzyme M reductase, a bacterial enzyme participating in the formation of methane. 

Porphyrins and corrins form part of the prosthetic groups in a large variety of vitally important natural products. As a result of intense interest in the structure, function and mechanistic details of the biological systems containing porphyrins and chlorins, the whole field has developed in a most remarkable way over the past 50 or more years. There can be few other research areas which possess such a rich, eventful and informative historical literature, and yet continue to produce important and exciting revelations in current journals. 

Presumably the biosynthetic pathways of the porphyrins and corrins are related to some extent, and the direct template cyclization of a tetrapyrrole has now been described to give a mixture of 10 diastereoisomers of tetradehydrocorrin complexes (equation 54).271 In the pursuit of chemical and biosynthetic similarities and differences between corrins and porphyrins, it is necessary to... 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

It is of interest to compare the porphyrin and corrin macrocycles, and to consider the reactivities of porphyrin- and corrin-bound metal ions. These may be related to the size of the cavity. The corrin macrocycle provides a smaller cavity, into which cobalt fits tightly in the B12 coenzymes. Thus the cobalt does not move relative to the corrin plane, and remains coplanar with the four nitrogen donor atoms. The cobalt remains low-spin, and reactivity is controlled by the loss of the axial ligands, giving five- and four-coordinate species. 

Gene-engineering synthesis of porphyrins and corrins 92T2559. 

Other important macrocycles are corrins, which are involved in vitamin B12, and corroles, which are intermediate between porphyrins and corrins. The corrins are... 

Cancer, Topics in Chemical Biology Photosynthesis, Electron Transfer Chemistry in Porphyrin and Corrin Biosynthesis... 

Since the initial disclosure of the basic corrin structure, there has been a considerable body of effort devoted toward the synthesis of macrocycles related to this chromophore that may be considered as being intermediates between porphyrin and corrin. These macrocycles, namely the dehydrocorrins (e.g., tetradehydrocor-rin 2.3) and the corroles (e.g., 2.4), represent interesting classes of contracted porphyrins that warrant specific mention here. The interest in these molecules derives in part from the fact that they could represent milestones along the biosynthetic pathway leading to vitamin B12. They are, however, also of interest from a non-biological perspective. Simply stated, this is because corrole-type macrocycles possess unique electronic and chemical characteristics, the study of which can help one to understand better the chemistry of all porphyrin analogs. 

F-430 A tetrapyrrole structure containing nickel, a component of the ENZYME methyl-coenzyme M REDUCTASE, that is involved in the formation of methane in METHANOGENIC bacteria. The highly reduced macro-cyclic structure, related to porphyrins and corrins, is termed a corphin. 

We discuss briefly now how metals are incorporated into porphyrins and corrins to form haem and other metallated tetrapyrroles, and how Fe—S clusters are synthesised. 

The biologically important tetrapyrroles contain four pyrrole rings, which are linked by CH2 or CH bridges. One differentiates between linear tetrapyrroles (bilirubinoids) and cyclic tetrapyrroles (porphyrins and corrins). 

The corresponding unsubstituted compounds are known as biladiene and bilin [44]. Porphyrins and corrins are discussed in chapter 8.3. 

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