Protoporphyrin iron complexes

Iron as a cofactor in catalysis is receiving increasing attention. The most common oxidation states of iron are Fe2+ and Fe3+. Iron complexes are nearly all octahedral, and practically all are paramagnetic (as a result of unpaired electrons in the 3d orbital). The most common form of iron in biological systems is heme. Heme groups (Fe2+) and hema-tin (Fe3+) most frequently involve a complex with protoporphyrin IX (fig. 10.19). They are the coenzymes (prosthetic... 

Iron porphines constitute the active site of hemoproteins and as such are among the most important and most widely studied series of metal complexes. The syntheses of iron protoporphyrin (heme) and its derivatives are important for the reconstitution of hemoproteins (particularly when one wishes to incorporate 57Fe for Mossbauer studies), as well as for model studies. In addition to the naturally occurring porphyrins, iron complexes of both 5,10,15,20-tetraphenyl-21//,23//-porphine (H2tpp) and octaethyl-21//,23//-porphine (H2oep) are widely used in model studies of the natural systems. 

Dimethyl protoporphyrinato(2-)iron(IIl), either as the chloride or the p-oxo dimer, is fairly soluble in chloroform and dichloromethane. The iron complex is much more stable toward light than the parent protoporphyrin but is still susceptible to acid-catalyzed hydration. The ester can be hydrolyzed to yield pure protohemin by using the alcoholic-KOH procedure.1,2 The optical spectra of these and other iron porphyrins have been reviewed recently.7... 

Several different sequences of reactions may be postulated for the conversion of the magnesium complexes of pheophytlns (chlorophylls) and Iron complexes of protoporphyrin IX and related porphyrins (hemes and hemlns) Into the nickel and vanadium porphyrins found In petroleum. One possible reason for the Isolation of only the nickel and oxovanadlum metalloporphyrlns Is that only they were resistant to degradation. While studies of Hodgson do Indicate that complexatlon of vanadium and nickel do Impart added thermal stability to porphyrins (12). Berezin has found that complexatlon of other metal Ions such as cobalt and copper also Imparts added thermal stability (13. 14). In addition, Hodgson s study Indicates that relatively little thermal degradation of the metalloporphyrlns has taken place In most crude oils (which would lead to unbound vanadium and nickel), One would expect that if little degradation of these metalloporphyrlns has occurred, complete disappearance of other metalloporphyrlns by thermal degradation Is an unreasonable assumption. 

Porphyrins and their derivatives play critical roles in many biological functions. Some of the most remarkable examples are protoporphyrin IX and its iron complex that constitutes the heme prosthetic group, and the magnesium complexes of pheophytin a and bacteriopheophytin a that are known as chlorophyll a and bacteriochlorophyll a, respectively. These natural compounds are illustrated in Fig. 1 together with the structure of porphin,... 

Among the respiratory electron carriers are three b-type cytochromes, cytochromes c and cl, and cytochromes a and a3. Cytochromes b, c, and cl all contain the same heme found in hemoglobin and myoglobin - iron complexed with protoporphyrin IX (Figure 7,4). In cytochromes c and cl, but not b, this heme is linked covalently to the protein component via thioether bonds formed between two of the vinyl side chains and two cysteine residues (Figure 15.6a). 

Y. Sorek, H. Cohen, and D. Meyerstein, Kinetics of the 8-Hydroxy Elimination Reactions from the Protoporphyrin Iron(III)-CHRCH20H Complexes in Aqueous Solutions. A Pulse Radiolysis Study, J. Chem. Soc., Faraday Trans. 1, 82 (1986) 3431. 

Clezy s modification of the MacDonald synthesis using a 5,5 -diformyl-pyrro-ketone instead of a diformyl-pyrromethane has been extended to the synthesis of the a-oxy-derivative of mesoporphyrin IX and to the y-oxy-derivative of protoporphyrin IX (Scheme 3). The iron complex of the latter undergoes oxidative ring opening in pyridine solution to form biliverdin IX-y (25), the parent compound of a series of three green pigments found in certain types of butterflies and caterpillars this provides a model for their probable mode of biosynthesis. 

The active site in a cytochrome P-450 is a haem unit. An iron protoporphyrin(IX) complex (Fignre 29.7b) is covalently bound to the protein through an Fe cystrinc bond. This has been confirmed crystallographically for cytochrome P-450 complexed with (15)-camphor (Figure 29.10). The active site contains a 5-coordinate Fe(ni) centre, schematically represented by structure 29.16. In its rest state, cytochrome P-450 contains a low-spin Fe(III) centre. Carbon monoxide adducts of cytochromes P-450 absorb at 450 nm and this is the origin of the name of the enzyme. It is proposed that the catalytic cycle for the conversion of RH to ROH follows the sequence of steps ... 

The structure of one of the most important of the porphyrins, protoporphyrin-9 (as the iron complex), is diown in Fig. 1. 

Protoporphyrin is the most important natural porphyrin, and its iron complex, in union with protein, appears as haemoglobin, myoglobin, catalase, cytochrome, and other respiratory pigments. 

Hemin is the complex between protoporphyrin and iron in the +3 oxidation state. Iron is in the +2 state in the heme of hemoglobin. The molecule has the following structure ... 

Heme (C34H3204N4Fe) represents an iron-porphyrin complex that has a protoporphyrin nucleus. Many important proteins contain heme as a prosthetic group. Hemoglobin is the quantitatively most important hemoprotein. Others are cytochromes (present in the mitochondria and the endoplasmic reticulum), catalase and peroxidase (that react with hydrogen peroxide), soluble guanylyl cyclase (that converts guanosine triphosphate, GTP, to the signaling molecule 3, 5 -cyclic GMP) and NO synthases. 

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